Duplex coatings, TSA, Zn sprayed coating, sealer, duplex, ships, oil&gas

SINTEF REPORTTITLE Coating systems for long lifetime: Thermally Sprayed Duplex Systems Final report AUTHOR(S) Ole Øystein Knudsen CLIENT(S) SINTEF Materials Technology Corrosion, J oining, and Surface Technology Address: NO-7465 Trondheim NORWAY Location: Richard Birkelands vei 3A Telephone: +47 73 59 27 80 Fax: +47 73 59 68 92 Enterprise No.: NO 948 007 029 MVA Spray Service, Statoil, Petrobras, Esso, Jotun, Carboline, International, Hempel, R&M, Bjørge Norcoat, NSL Gruppen, Rotorkontroll, Vegdirektoratet, Jernbaneverket REPORT NO. CLASSIFICATION CLIENTS REF. SINTEF A14189 Open Stein Paulsen CLASS. THIS PAGE ISBN PROJ ECT NO. NO. OF PAGES/APPENDICES Open 978-82-14-04760-8 243908 40 ELECTRONIC FILE CODE PROJ ECT MANAGER (NAME, SIGN.) CHECKED BY (NAME, SIGN.) I:\2400\prosjekt\243908\adm\notat_rapport\FinalReport OØK040625.doc Ole Øystein Knudsen Astrid Bjørgum FILE CODE DATE APPROVED BY (NAME, POSITION, SIGN.) 2010-01-20 Bård Tveiten, Research Director ABSTRACT Thermally sprayed zinc or aluminium in combination with organic coatings, TS Duplex Coating Systems, is a common method used for corrosion protection of bridges, ships and oil- and gas installations. These systems are supposed to provide a long lifetime (>20 years), and with that, be both cost effective and environment friendly. However, so far the potential market for these protective systems has not been reached. Potential users of such systems may have become sceptical because of some reported cases with rapid degradation. Particularly for systems were TS aluminium has been used, there have been problems. The main goals of this project have been to solve these problems, and to find an optimal protective system, which combines thermally sprayed metals with paint, and still have a relatively low price. KEYWORDS ENGLISH NORWEGIAN GROUP 1 Materials technology Materialteknologi GROUP 2 Corrosion Korrosjon SELECTED BY AUTHOR Duplex coatings Dupleks belegg Thermally sprayed metal Termisk sprøytet metal Organic coating Organisk belegg 2 TABLE OF CONTENTS Preface .........................................................................................................................................................3 List of reports from the project..................................................................................................................4 Publications and presentations from the project......................................................................................5 1 Summary and conclusions..................................................................................................................6 2 Background ......................................................................................................................................9 3 Approach ....................................................................................................................................10 4 State of the art ...................................................................................................................................11 4.1 Experiences with TSA duplex coatings ......................................................................................11 4.1.1 Sleipner Riser Platform......................................................................................................11 4.1.2 Åsgard A............................................................................................................................12 4.1.3 Risers at Jotun B................................................................................................................12 4.1.4 Nidelv bridge in Trondheim..............................................................................................13 4.1.5 Tromsø bridge....................................................................................................................13 4.2 Experiences with TSZ duplex coatings.......................................................................................14 4.2.1 Road bridges......................................................................................................................14 4.3 Failure or success – what are the determining factors? ..............................................................14 5 Rapid degradation of TSA duplex coatings – the mechanism......................................................16 5.1 Galvanic corrosion between duplex coating and bare steel ........................................................16 5.2 Properties of the thermally sprayed metal ..................................................................................16 5.3 TSA with sealer ..........................................................................................................................17 5.4 Recommendations for TS duplex coatings based on the degradation mechanism.....................18 6 Sealers ....................................................................................................................................22 6.1 Sealer penetration .......................................................................................................................22 6.1.1 Recommendations regarding sealers .................................................................................23 7 Testing of TS duplex coatings ..........................................................................................................24 7.1 Phase 1 – Effect of TSM, organic coating and structure of TSM...............................................24 7.1.1 Effect of composition and structure of TSM.....................................................................24 7.1.2 Effect of organic coating ...................................................................................................24 7.1.3 Reference systems .............................................................................................................25 7.2 Phase 2 – TSZ duplex coatings...................................................................................................28 7.2.1 Test results.........................................................................................................................28 7.2.2 Effect of paint film thickness.............................................................................................29 7.2.3 Cracking and flaking .........................................................................................................30 7.2.4 Low VOC coatings ............................................................................................................31 7.3 Properties of zinc coatings..........................................................................................................31 7.3.1 General experiences with zinc coatings.............................................................................31 7.3.2 Risk for zinc steel polarity reversal ...................................................................................32 7.3.3 Corrosion of zinc coatings exposed above 70°C...............................................................32 8 TS Duplex systems – expected lifetime............................................................................................33 9 Maintenance of duplex coatings.......................................................................................................34 9.1 Degradation of topcoat only – no corrosion of TSA...................................................................34 9.2 Corroded TSA.............................................................................................................................34 9.2.1 State of TSA after degradation ..........................................................................................34 9.2.2 Maintenance without removing the TSA...........................................................................34 9.2.3 Maintenance by removing the TSA...................................................................................35 9.3 Repair systems for TSA duplex coating .....................................................................................35 9.3.1 Experiences and test results...............................................................................................35 9.3.2 Recommendations .............................................................................................................37 9.4 Maintenance of TSZ duplex coatings .........................................................................................37 10 References ....................................................................................................................................39 3 List of reports from the project: 1. O.Ø. Knudsen, T.G. Eggen: "Duplex coating systems - State of art", SINTEF report STF24 F01328, Confidential (2001) 2. H. Vold: "Coating systems for long lifetime: Thermal sprayed duplex systems - Field exposure in sea water splash zone. DnV report BGN-R2701576, Restricted (2002) 3. J.E. Scheie, I. Lien: “Coating systems for long lifetime: Thermal sprayed duplex systems – Testing of 12 sealers on TSA and TSZ win accordance to ISO 7253, TI Report no. 410-02- 0019, Confidential (2002) 4. B.S. Tanem, O.Ø. Knudsen: “Penetration of sealer at the surface of thermally sprayed metal”, SINTEF report STF24 F02248, Confidential (2002) 5. G. Soleng, O.Ø. Knudsen: “Corrosion properties of various thermally sprayed metals”, SINTEF Report STF24 F02334, Confidential (2002) 6. O.Ø. Knudsen: “Mechanism for degradation of thermally sprayed duplex coatings”, SINTEF Report STF24 F02331, Confidential (2002) 7. A. Mikkelsen, O.Ø. Knudsen: ”Penetration of organic coating into TSM on samples prepared at R&M in May”, Memo, Confidential (2002) 8. O.Ø. Knudsen: "Duplex coatings – test results from phase 1". SINTEF report STF24 F03316, Confidential (2003). 9. O.Ø. Knudsen: "Maintenance of thermally sprayed duplex coatings", SINTEF report STF24 F03256 (2003). 10. B. Schei: "Coating systems for long lifetime: TS duplex systems – Field testing status report", DNV report no BGN-R2703356 (2003) 11. O.Ø. Knudsen: “Transport of ions through rubber coating”, SINTEF report STF24 F03344 (2003) 12. K. Kaltenborn and J. Scheie: "Duplex coating systems for long life time. Testing in accordance with NORSOK M 501 rev. 4 – cyclic test for 4200 hours" Report no. 410-03- 0226 Mt01 Rev.1 (2004) 13. K. Kaltenborn and J. Scheie: "Duplex coating systems for long life time. Testing in accordance with NORSOK M 501 rev. 4 – cyclic test for 4200 hours" Report no. 410-03- 0019 Mt01 Rev.3 (2004) 14. O.Ø. Knudsen: "Duplex coatings – Test results from phase 2: TSZ systems", SINTEF report STF24 F04256 (2004) 15. O.Ø. Knudsen: "Coating systems for long lifetime: Thermally Sprayed Duplex Systems. Final report", SINTEF report STF24 F04258, Confidential (2004) 16. Q. Dong: “Coating systems for long lifetime: TS duplex systems”, DNV report no 2008- 5320 (2008) 4 Publications and presentations from the project O.Ø. Knudsen: “Bruk av termisk sprøytet aluminium kombinert med organisk belegg for å redusere vedlikeholdskostnadene offshore. Hvorfor har dette gått galt?”, Materialdagen, Forus, 27 October 2004. O.Ø. Knudsen: "Metallisering/termisk sprøyting. Hvorfor ser vi tidlig nedbrytning av malt TSA?", Overflatedagene 2004. J.E. Scheie: "Metallisering/termisk sprøyting. Hva ser vi på lab av TSA og TSZ?", Overflatedagene 2004. O.Ø. Knudsen, T. Røssland: "Rapid Degradation of Painted TSA", Paper no. 04023, CORROSION/2004, 28/3 – 1/4 2004, New Orleans, NACE Houston TX O.Ø. Knudsen: "Hva kan vi forvente av duplex-systemer?" Presentation at Korrosjonsteknisk Forening, Bergen. 6 May 2003. R. Klinge: "Hva er riktig korrosjonsbeskyttelse i dag?", Presentation at Brukonferansen, Oslo, 30- 31 October 2002. O.Ø. Knudsen: Forskning innen organiske belegg ved SINTEF. Presentation at the annual meeting in Norsk Maling- og Lakkteknisk Forening, Voss, 25 October, 2002. O.Ø. Knudsen: Fra den korrosjonstekniske forskningsfronten. Presentation at Korrosjonsteknisk Forening, Haugesund, 19 September 2002 5 Preface This project called "Coating systems for long lifetime: Thermally Sprayed Duplex Systems" has been a joint industry project, financed by the Norwegian Research Council and a consortium of Norwegian and international companies. Spray Service AS has been contract partner. The project was initiated by "Forum for Overflate- og Beleggteknologi", and has lasted from 2001 to 2004. The project is a continuation of a project with the same title running at TI and SINTEF Marintek 1997 – 2000. The project has had a total budget of 4.43 MNOK, where 1.77 MNOK has been spent by the industry partners and 2.66 MNOK by the R&D performing companies. The Norwegian Research Council has supported the project by totally 1.27 MNOK. The industry consortium has consisted of Spray Service, Statoil, Esso, Petrobras, Jotun, Carboline, International, Hempel, R&M, Bjørge Norcoat, NSL Gruppen, Rotorkontroll, The Public Roads Administration and Jernbaneverket. Their contribution by finances, knowledge and experiences are gratefully acknowledged. The R&D has been performed jointly by: SINTEF, The National Institute of Technology and Det Norske Veritas. I would like to thank Jan Scheie, Isabel Lien, Kristian Kaltenborn at TI and Helge Wold and Birgith Schei at DNV for the cooperation. I would also like to thank all colleagues at SINTEF who have contributed to the project: Sten B. Axelsen (now Statoil), Tor Gunnar Eggen, Gro Soleng (now Optirock), Andre Mikkelsen, Erling Abusland and Bjørn Steinar Tanem. Some contributions to the project deserve special mentioning. I would like to thank my colleague Trond Rogne for taking the initiative to the project, for his contributions during the application process and later during the course of the project. Stein Paulsen at Spray Service has been contract partner for the project, and has taken care of the formal contact with the Research Council. Thank you for volunteering for the job and for good cooperation. Finally, special thanks go to Reidar Klinge. He has been a careful reviewer of all written deliveries from the project, in addition to generously sharing his long experience regarding thermally sprayed duplex coatings. All these contributions have been of great value to the project and are gratefully acknowledged. Trondheim 20 January, 2010 Ole Øystein Knudsen 6 1 Summary and conclusions Thermally sprayed zinc (TSZ) or aluminium (TSA) in combination with organic coatings, Thermally Sprayed (TS) Duplex Coating Systems, is a common method used for corrosion protection of bridges, ships and oil- and gas installations. These systems are supposed to provide a long lifetime (>20 years), and with that, be both cost effective and environment friendly. However, so far the potential market for these protective systems has not been reached. Potential users of such systems may have become sceptical because of some reported cases with rapid degradation. Particularly for systems were TS aluminium has been used, there have been problems. The main objectives with this project has been to solve these problems, and to find an optimal protective system, which combines thermally sprayed metals with paint, and still have a relatively low price. In the attempt to reach this objective the following more technical objectives were defined: 1. To find the corrosion mechanisms causing problems on painted TSA. 2. To find optimal sealers for thermally sprayed coats exposed in seawater or in marine atmosphere. 3. To determine which paint systems that are suitable for use on TS coats. 4. To optimise application parameters for TSA. 5. To optimise the application of, and the alloying constituent of aluminium and zinc in the new alloys. The experiences with TSZ duplex coating systems are very good. The Public Roads Administration has used TSZ duplex coating systems for more than 40 years, and has examples of bridges where the coating has performed excellent during three decades without maintenance. TSZ duplex coatings have also performed well in various field tests and accelerated tests performed at SINTEF, TI and DnV. With TSA duplex coating systems the experiences are mixed. Statoil has installations where such coating systems have degraded severely after only a few years of exposure. However, there are also examples where TSA duplex coating systems have performed well. The Public Roads Administration has good experiences with their TSA duplex systems in atmosphere. The reason may be choice of different organic coatings, where physically drying coatings have been used. Compared to offshore, the environment may also be somewhat less corrosive, though this has not been investigated. The performance in various test projects at SINTEF, TI and DnV has also varied from acceptable to very bad. Evidently, the degradation problems with TS duplex coating systems are limited to TSA duplex coatings. The degradation mechanism was investigated in a laboratory study. When duplex coatings are galvanically coupled to bare steel, a galvanic corrosion mechanism of the thermally sprayed metal starts. Cathodic oxygen reduction takes place at the bare steel, while anodic corrosion of the thermally sprayed metal takes place under the organic coating. In chloride containing environments, like in marine atmosphere, the chloride ions migrate in under the organic coating to balance the positive charge on the metal ions. So far the degradation mechanism is the same for TSA and TSZ duplex coatings. For TSA duplex coatings, aluminium chloride is formed under the organic coating as the aluminium corrodes. However, aluminium chloride is highly unstable in the presence of water. The aluminium chloride reacts with water and forms hydrochloric acid. The electrolyte under the organic coating will therefore be acidic (low pH). A new cathodic reaction, hydrogen evolution, will then start under the organic coating. Aluminium is not passive at pH < 4 and will corrode actively. 7 TSZ duplex coatings will not suffer the same rapid degradation. When chlorides migrate in under the organic coating on TSZ duplex coatings, zinc chloride is formed. Zinc chloride dissolves in water, but does not acidify the electrolyte, and hydrochloric acid is not formed. Cathodic hydrogen evolution under the organic coating will therefore not start, and the degradation of the TSZ duplex coating is not accelerated. In the mechanism outlined in this project, we have described a situation where the duplex coating is galvanically coupled to bare steel. The bare steel may be equipment in stainless steel mounted in the structure, mechanical damages in the duplex coating exposing the steel substrate or an area where the duplex coating has corroded away. Other metals that are efficient cathodes, e.g. copper and copper alloys, will have the same effect. The mechanism is similar to the traditional mechanism for crevice corrosion and pitting corrosion. Other initiation mechanisms may also exist. TSA with only a thin sealer will not be degraded by this mechanism. If the sealer is applied in a very thin coat, like it is supposed to, it will not be able to hold an aggressive electrolyte at the TSA surface. No crevice will be formed and acidification will probably never take place. Investigation of samples exposed offshore showed that aluminium oxide had been formed on top of the sealer, which means that aluminium ions probably penetrate the sealer. No significant corrosion of the TSA was found. Based on these findings the project has recommended that TSA should not be painted with thick organic coatings. Due to this recommendation the activities on optimising the application parameters and studies of TS alloys with a high content of aluminium was stopped or not started. TS coatings have a rough surface structure with pores and narrow crevices. In order to fill these pores a sealer is usually applied. Incomplete penetration of the organic coating will lead to pores in the interface between the thermally sprayed metal (TSM) and the organic coating where corrosion may initiate. To improve the penetration into the TSM surface structure a thin sealer with low viscosity should be applied. An investigation to evaluate the penetration of various sealer products into porous thermally sprayed metal coatings was therefore started, in order to help the selection of sealers to be used later in the project in combination with thicker organic coatings. The clear unpigmented sealers had the best penetrating properties. However, good penetration was also observed for some of the pigmented sealers. Flake shaped pigments and large pigment particles in the sealer seemed to inhibit the penetration. Generic type of the sealer did not seem to be the most important factor for pore formation. Good penetration was also found with waterborne sealers. Proper dilution of the sealer before application to decrease the viscosity seems to be important to improve the penetrating properties. Sealers applied with high film thickness had more pores than the ones applied in a thin coat. Too thin film will give insufficient amount of sealer to fill the larger pores in the TSM surface. Testing of TS duplex systems has been a large activity in the project. The testing was conducted in two phases. Phase 1 was started before we found the degradation mechanism for TSA duplex coatings, so a large number of TSA duplex systems were tested. In Phase 2 we focused on TSZ duplex systems. Coatings with zinc rich primers have been used as reference coating systems. The purpose with the testing in Phase 1 was to study the effect of type of TSM, type of organic coating and effect of porous TSM on corrosion properties of the coatings. The results showed that duplex coatings with TSZ performed well. None of the TSZ duplex coatings had more than 1 mm 8 scribe creep, neither in the NORSOK test nor in the field test. All the duplex coatings with thermally sprayed aluminium (both AlMg5 and 99.5% aluminium) showed severe degradati particularly in the 5 years field test. Porosity of the TSA does not seem to affect the result significantly. on, here was little difference between the TSZ duplex coatings, and most of them performed well, ere ne duplex coating with TS ZnAl15 was tested and performed equally well as the pure TSZ nd ater borne and high solid coatings gave good performance. The experimental results so far hermally sprayed zinc has been avoided on offshore installations, because in the highly corrosive f s to ted the ue to the severe degradation of TSA duplex coatings, developing efficient maintenance lex oat n nc he entire structure to Sa 2½ and application of a new coating system will probably give the best T both in the field test and in the NORSOK test. In Phase 2 of the project seven different duplex coating systems were prepared with TSZ and various organic coatings. The coating suppliers w asked to select organic coatings that will comply with future environmental legislation, e.g. high solid or waterborne products. The test results showed very little corrosion creep from scribe for the TSZ duplex samples. None of the TSZ duplex coatings showed any corrosion creep on the steel, and corrosion creep on the zinc was less than 1 mm. Hence, the results indicate that TSZ duplex coatings will have long lifetime. One of the TSZ duplex coatings showed general degradation in the field test, due to initiation of corrosion on the zinc in random spots. O duplex coatings. Hence, TS ZnAl15 duplex coatings should be regarded as equally resistant to degradation as TSZ duplex coatings. ZnAl15 has some advantages with respect to application a will in most cases be economically beneficial to the pure TSZ. W indicate that TSZ duplex coatings are less susceptible to degradation by corrosion than coating systems based on zinc rich primers. T environment zinc coatings usually only last for 5-10 years. Therefore only TSA have been used for metallizing offshore structures. TSZ duplex coatings therefore represent a new generic type o coatings on offshore installations. Painted TSZ will have different properties from bare TSZ, but some questions have been put forward regarding the use of TSZ duplex coatings offshore. In duplex coatings the zinc will be coated with an organic coating. As long as the coating adhere the zinc there can be no corrosion of the zinc. All results from this project indicate very low degradation rates for TSZ duplex coatings. Offshore TS duplex coatings may be exposed to temperatures up to 120°C. However, no reports of rapid degradation of TSZ coatings at eleva temperatures have been found. On the contrary, experimental results and practical experiences indicate that zinc coatings can be used without any problems up to 200°C. Polarity reversal between zinc and steel has been reported in some instances, where zinc becomes cathode and steel anode, i.e. causing corrosion of the steel. This has only been observed in water above 65°C with low chloride concentrations. Hence, there is no risk for polarity reversal in corrosive environments or below 65°C D procedures is important. A study of various options for maintenance, focusing on TSA dup coatings, was therefore conducted. The main conclusions from this study were: If only the topc is degraded, it is important not to damage the rest of the organic coating during the maintenance. Damages in the organic coating, exposing bare TSA may initiate rapid degradation. Gentle cleaning with water and soap and application of a new topcoat will give the best result. Whe corrosion of the TSA has started, but not propagated far, local cleaning of corroded areas and application of a repair system will probably temporarily stop or slow down the degradation. Zi rich primers may be applied on TSA. To remove the organic coating from the TSA, leaving the TSA more or less intact on the structure is in practice difficult. Degraded TSA duplex coatings often give a mixed surface consisting of intact coating, bare TSA and bare steel. Blast cleaning t 9 result and should be considered. If that is not an option, bare steel and TSA should be primed with a zinc rich primer. An epoxy mastic or equivalent barrier coat may be applied on the primer, before the entire structure is coated with a topcoat. Applying the barrier coat on the entire structure may decrease the chance for initiation of new degradation. 2 Background ms, consisting of TSM and a full organic coating system, have been used bridges in Norway. For offshore installations n ngs and their combination with paints. esults from the pre-project ”Coating System for long lifetime: TS Duplex Systems”, initiated by isk of , salt this project has been to develop new procedures and protective systems for corrosion rotection in aggressive atmosphere. For the owners of steel structures this means that the e in ese goals it is mandatory to develop an optimal protective system, which combines ermally sprayed aluminium, zinc or alloys of these metals with sealers and paint. The objective four coats of alkyd or lkyd/chlorinated rubber paints have been frequently used. During the last decade epoxymastic / gs with long lifetime, the following more technical ub-goals were defined: ealers for thermally sprayed coats exposed in marine atmosphere. . and zinc in the Duplex coating syste mainly for protecting offshore installations and road duplex systems with TSA have been applied. On road bridges duplex systems with TSZ has bee used most frequently, but TSA systems have also been applied on a few bridges. The experiences with TSZ duplex coating systems are very good. The Public Roads Administration has used TSZ duplex coating systems for more than 40 years, and has examples of bridges where the coating has performed excellent for more than three decades without maintenance. Within the oil- and gas industry severe problems have occurred on painted TSA. In fact the paint has reduced the lifetime of the protective system due to high local corrosion rates. Previous projects have investigated thermally sprayed coati R National Institute of Technology, Norway (TI) and MARINTEKs Laboratory for Material Technology, showed that some Duplex Systems with TSZ and paint performed well in the accelerated tests. However, good results were not achieved with TSA. The results showed r blistering and corrosion of aluminium when these systems are used in seawater or in marine spray containing atmosphere. The results did also reveal that these systems are vulnerable to mechanical damages. High underfilm corrosion rates around the scribe in the coatings were observed. The aim of p protection will be carried out cost effective and with less environmental impact. For material suppliers this means better understanding of the corrosion process and the paint performanc TS duplex systems. For applicators of the coatings the project will provide better application procedures. To achieve th th was to reach a lifetime (with lifetime of the duplex systems we understand the time to the first necessary maintenance of the paint) of the system of more than 20 years. In Norway, road bridges thermally sprayed with zinc in combination with a polyurethane coatings have also been used. In order to reach the goal of TS duplex coatin s 1. To find the corrosion mechanisms causing problems on painted TSA. 2. To find optimal s 3. To determine which paint systems that are suitable for use on TS coats 4. To optimise application parameters for TSA. 5. To optimise the application of, and the alloying constituent of aluminium new alloys. 10 3 Approach In order to fulfil the objectives of the project the following research activities have been carried out: State-of-Art: A study of previous experiences with TS duplex coatings on various installations and test results. Motivation: Collect experiences and get an overview of degradation problems with TS dupelx coatings. Sealers: An investigation of various sealers and their ability to penetrate into narrow crevices found in the surface of TS coatings. Motivation: Pores at the TSM/coating interface may serve as initiation points for corrosion. Improving sealer penetration may therefore decrease coating degradation by corrosion. Corrosion of various types of thermally sprayed metal: The corrosion rate of thermally sprayed aluminium, zinc and alloys of these were investigated. Motivation: By using TSM with high corrosion resistance the corrosion resistance of the TS duplex coating could probably be improved. Degradation mechanisms: The mechanism for rapid degradation of TSA duplex coatings was found. Motivation: Rapid degradation of TSA duplex coatings have been observed in some cases, which has decreased the confidence in duplex coatings. Finding the degradation mechanism was important in order to know how to avoid the degradation. Testing: A number of TS duplex coating systems have been tested in the NORSOK test and a field test, and compared to coating systems with zinc rich primers. Motivation: Understanding how duplex coating systems should be composed in order to obtain long lifetime. Maintenance: Give recommendations for maintenance of TS duplex coatings. Motivation: Decreasing maintenance costs by using the optimal cleaning method and applying the optimal repair coating system. 11 4 State of the art plex coatings have also been studied in research projects. The performance of such coating ystems have not always been as good as expected, and this has decreased the confidence in this thorough review of these experiences was given in a State-of-Art report (Knudsen and Eggen, duplex coatings ed nce. In order to achieve this a duplex coating system consisting of TSA and an organic oating was chosen. The specification for the coating system was: olyurethane topcoat 80 µm e om the SA lift the organic coating and blisters are formed. The blisters are particularly numerous around h is eople in Statoil have suggested that the degradation of the coating is connected to adhesion loss between TSA and the steel substrate. In order to obtain proper adhesion between TSA and steel you need:  A proper roughness and a sharp profile formed during grit blasting  During metal spraying the metal droplets must penetrate into the profile The impact angle both during grit blasting and metal spraying is important in order to obtain this. In both cases a high impact angle gives the best result. On complex structures it will be difficult to have the optimal impact angle in all areas, which may result in adhesion problems and corrosion. The distance between the spray gun and the steel substrate is also critical. If this is correct, the coating degradation observed is not connected to the organic coating system, but to the thermal spraying and/or the grit blasting. A number of offshore installations and road bridges are protected by TS duplex coating systems. TS du s type of protective coatings. The objective with this activity in the project was to find common denominators for the successful cases on one hand and the failures on the other. A 2002). A collection of field experiences with duplex coating systems is given in the following sections, followed by a discussion of similarities and differences. 4.1 Experiences with TSA 4.1.1 Sleipner Riser Platform The Sleipner Riser Platform was put in service in 1992. The platform was built with a design life of 50 years. Statoil wished to have a corrosion protection system on the platform with a minimum ne for maintena c TSA, arc spray AlMg5 200 µm Epoxy primer 50 µm Epoxymastic 200 µm P After seven years in service, Statoil was experiencing severe degradation of the coating system. Th coating system had started to develop blisters after relatively short time. Corrosion products fr T edges, corners and near details in stainless steel. A white salt is found under intact blisters, whic assumed to be aluminium hydroxide. Pictures of coating with degradation are shown in Figure 1. P 12 Figure 1: Coating degradation of TSA duplex syste ainless st nected to the st m at Sleipner Riser Platform. ructure. poxy tiecoat 20 µm ll was installed in 1999 and was in excellent condition after three years exposure. However, of the coating system. vel r coating was approximately 2 mm thick. Above this level, a coating consisting of TSA/polyester duplex coating was applied polyester duplex coating and a TSA/rubber duplex oating. The production at the Jotun field started in 1999. The risers were inspected during the summer 2003 and severe degradation of the duplex coatings was reported. In addition, subsequent corrosion of the risers had resulted in a material loss of up to 5-6 mm wall thickness. The degradation probably started in the TSA – polyester coating, and corrosion of the TSA later spread under the rubber coating as well. The corrosion products caused the polyester coating to crack, whereas the thick and somewhat elastic rubber coating developed Degradation near st eel con 4.1.2 Åsgard A On the Åsgard A platform a duplex coating system has been used on a wall protecting one of the decks from wind and sea waves. The coating system is as follows: TSA, arc spray Al 99.5 200 µm E Fluoropolymer topcoat 80 µm The wa three years is too short time toconclude anything about the performance 4.1.3 Risers at Jotun B Two risers at Jotun B were coated with TSA duplex coatings in the splash zone. From approximately 4 meters below the sea level and up to approximately 6-meters above the sea le the riser were coated with a TSA – rubber duplex coating. The rubbe 1 over a length of 0.5 meter. From here to the platform the risers were coated with the polyester. Hence, in the splash zone the risers had a TSA/ c 13 very large blisters, filled with corrosion products. The corrosion under the rubber coating had started at the top of the coating and spread downwards. Due to the rapid coating degradation and severe corrosion of the risers, maintenance of the coatings was performed later during the summer. The TSA duplex coating was removed by blast cleaning. The rubber coating was cut and peeled of, down to where no corrosion of the TSA could be found, and the exposed steel was blast cleaned. The intact TSA – rubber duplex coating was left on the riser. 4.1.4 Nidelv bridge in Trondheim Nidelv bridge was built in 1975/76 and coated with a TSA duplex coating system. The TSA was applied by arc spraying with a closed nozzle, which gave severe dusting problems. Investigation of cross sections of the metal coating showed that up to 50% of the coating consisted of dust particles (aluminium oxide). The organic coating consisted of a wash primer and four coats of alkyd/chlorinated rubber 1:1 pigmented with zinc-chromate. In spite of the dusting problems during application, the coating system has performed well during 30 years of exposure near the Trondheim harbour. .1.5 Tromsø bridge ø bridge is a concrete bridge that was opened to traffic in 1960. Concrete fenders protect the tructure was en into the seabed. Already during construction of the concrete rings, severe degradation of the TSA duplex coating system was found the tidal zone (Klinge 1976). The following coating system had been applied: : 225 µm 40 µm 435 µm ar ted in Australia. Pores, cracks or 4 Troms main columns of the bridge from ships passing under the bridge. A new concrete fender s built in 1975. The concrete fenders are standing on steel piles driv on the steel piles in TSA flamesprayed: 160 µm ashprimer: 10 µm W 3 coats vinyl tar @ 75 µm Topcoat: Total: The piles were coated with this system down to 1 m below lowest tide. The organic coating started to blister and inside the blisters there was a liquid with pH 3.5 – 4. The TSA was corroded away inside the blisters and the steel surface was exposed. The Association of Metal Sprayers has reported a similar case in Australia, on a bridge ne Adelaide. Within three weeks of exposure, severe degradation of the vinyl topcoat was found in the tidal zone. In this case intense galvanic corrosion between the TSA and the bare steel areas was assumed to cause the degradation. They have also reported about other cases where TSA and TSZ coatings have failed due to electrical contact to bare steel surfaces also immersed in water. It is reasonable to assume that the degradation of the coating on the fender structure of the Tromsø ridge was caused by a mechanism similar to what was repor b mechanical damages in the organic coating that lead to exposure of the TSA, may have been the initiating points for the degradation. When the seawater has access to the TSA, a galvanic cell between TSA and bare steel is established. Since we have a large bare steel area acting as cathode, and a small TSA area acting as anode (damages in the organic coating), the corrosion rate of the TSA will be very high. Severe coating degradation can then be expected within a few days or weeks. 14 4.2 Experiences with TSZ duplex coatings 4.2.1 Road bridges d n of the bridge in 1998, after 28 years of service without maintenance, no y the topcoat was degraded and a new topcoat was coating system consisted of: eplaced with alkyd/chlorinated rubber 1:1 mixtures. In 1977 the toxic ere replaced with harmless zinc phosphate, and the alkyd/chlorinated lkyd/chlorinated rubber 50 µm bber Alkyd 50 µm Alkyd 50 µm kyd 50 µm Alkyd 50 µm SZ/TSA, arc or flame 100 µm as .3 Failure or success – what are the determining factors? . The Public Roads Administration in Norway has used duplex coating systems with thermally spraye zinc for protecting bridges extensively since 1965. Many of these bridges are located near/at the coast and are therefore exposed in a highly corrosive marine environment. Their experiences with duplex coating systems are very good. E.g. Rombak Bridge was coated with a duplex system in 1970 and during an inspectio corrosion was found (Klinge 1999). Onl recommended for aesthetic reasons. The TSZ, flame sprayed 100 µm Phosphoric acid wash primer max 10 µm Alkyd, zinc chromate 2 x 50 µm Alkyd, MIO Al flake 2 x 50 µm Several other bridges have been coated with this or similar systems, showing the same good performance. In 1972 the alkyd coats were r zinc chromate pigments w rubber ratio was changed to 2:1. The duplex coating systems specified until recently were: System 1 TSZ/TSA, arc or flame 100 µm Phosphoric acid wash primer 10 µm System 2 TSZ/TSA, arc or flame 100 µm Phosphoric acid wash primer 10 µm A Alkyd/chlorinated ru 50 µm Alkyd/chlorinated rubber 50 µm Al Alkyd/chlorinated rubber 50 µm System 3 T Epoxy tiecoat/sealer 25-30 µm Epoxymastic 100-125 µm PU or PU/acryllic 60-100 µm In tenders where system 3 with TSA have been one of the possible solutions, this system h always been offered with a much higher price, and has never been chosen by the Public Roads Administration. 4 The experiences with TSZ duplex coating systems are very good. The Public Roads Administration has used TSZ duplex coating systems for more than 30 years, and has examples of bridges where the coating has performed excellent for this period of time without maintenance With TSA duplex coating systems the experiences are mixed. Statoil has installations where such coating systems have degraded severely after only a few years of exposure. However, there are also examples where TSA duplex coating systems have performed well. The Public Roads 15 Administration has good experiences with their TSA duplex systems, but these have been applied on two bridges only. o thick coats or too short overcoating intervals, which may cause solvent s paint systems However, the problem is limited to TSA duplex coatings and is therefore probably caused by properties of the TSA or the TSA/organic coating combination. The following theories for the poor performance of TSA duplex coating systems were suggested: 1. Unfilled pores or crevices in the TSA surface, which may - serve as initiation points for corrosion. - later formation of cracks and pinholes in subseqent paint layers . Adhesion loss between steel and TSA. 2 3. Application of to retention and pore formation. uitable ea 4. Application of uns lers and/or 16 5 Rapid degradation of TSA duplex coatings – the mechanism ct. eel Sta stainless steel ating. Aluminium is a poor d ion ilm is present on the TSZ duplex coatings were galvanically onnected to bare steel, as shown in Figure 2. The coating systems are described in Table 1. The are steel and the duplex coating were glued to a 1 mm thick PVC film and electrically connected rough an ampere meter. The samples were exposed in a salt spray cabinet, where they were ontinuously sprayed with synthetic seawater (ASTM D1141-90) for 900 hours. The galvanic urrent was automatically logged every six hours. Figure 3 shows the galvanic current measured in the experiment. Cathodic reduction of oxygen takes place at the bare steel, while anodic dissolution of the thermally sprayed metal takes place at the duplex sample, as shown in Figure 4. Initially the galvanic current is very high for the TSZ sample, but as Figure 3 shows, it decreases rapidly. Most likely this is due to formation of corrosion products or calcareous deposits on the bare steel surface, decreasing the cathodic reaction rate. For the TSA sample the current is initially low, but increasing. A passivating oxide film on the surface probably protects the TSA to some degree. The chlorides in the electrolyte attack the oxide, and corrosion initiates gradually, which causes the galvanic current to increase. Later, the galvanic current starts to decrease. As for the TSZ sample, formation of corrosion products or calcareous deposits most likely explains the reduction. Figure 3 shows that the galvanic current is about twice as high for the TSA sample as for the TSZ sample. In Table 2, the galvanic corrosion rate (mm/year laterally from the edge of exposed TSM) of the thermally sprayed zinc and aluminium is calculated, based on the galvanic current measured in the experiment. The corrosion rate calculated for the TSA is not significantly different from the TSZ, when the difference in thickness, density and valence between TSA and TSZ is included. Hence, there is no significant difference in galvanic corrosion between the two coating systems. Figure 5 shows pictures of the samples after the test. Contrary to the calculated galvanic corrosion rates, the pictures show that the TSA duplex coating is much more degraded than the TSZ duplex coating. Galvanic corrosion cannot explain the rapid degradation of this TSA duplex sample. The cathodic reaction responsible for the severe degradation of the TSA duplex coating must therefore take place under the organic coating. 5.2 Properties of the thermally sprayed metal The samples tested in the galvanic corrosion test were identical, except for the thermally sprayed metal. The properties of the thermally sprayed metal must therefore be important for the degradation process. In order to avoid the rapid degradation of TS duplex coatings in the future, it is important to know the degradation mechanism. The mechanism study was therefore an important part of the proje The first hypothesis that was tested was that galvanic corrosion between TSM and bare steel caused the rapid degradation. 5.1 Galvanic corrosion between duplex coating and bare st toil has reported that the coating degradation at Sleipner was most severe around unted into the structure, in galvanic contact with the duplex co mo cathode, but steel is a reasonably efficient cathode. When steel and aluminium are connecte together, the steel surface will act as cathode and the aluminium as anode. Galvanic corros y then start wherever the TSA is exposed, assu ma ming a conducting water f surface. This was tested in an experiment where TSA and c b th c c 17 The galvanic corrosion mechanism described in Figure 4 shows that chlorides are transported plex coatings, this umid that it is relatively stable. Aluminium chloride, on the other hand, ents. When dry aluminium chloride is added to water a violent s we found in the previous section. The overall corrosion reaction will be as there is a supply of water, the corrosion reaction steel mounted in the structure, The two upper images in Figure 6 shows cross sections of the left and the right side of the scribe ealer is shown as a thin bright line on top of the nalysed. under the organic coating as the thermally sprayed metal corrodes. For TSA du means that aluminium chloride (AlCl 3 ) should be formed under the organic coating, while for TSZ duplex coatings zinc chloride (ZnCl 2 ) should be formed. Aluminium chloride and zinc chloride have very different properties. Zinc chloride is a salt that may dissolve in h environments/water, but beside is not stable in humid environm reaction according to Eq. 1 starts, and hydrochloric acid is formed. (1) The reaction is more complicated than indicated here, but a detailed description is beyond our scope. During corrosion of TSA under the organic coating, hydrochloric acid and various aluminium   ydroxyl-chloride complexes are probably formed directly. Hence, for the TSA duplex coating hydrochloric acid will be formed, acidifying the environment under the organic coating. The low pH in the electrolyte under the organic coating has two effects. Firstly, the aluminium will corrode actively, since the protecting aluminium oxide is unstable at pH below 4. Secondly, hydrogen evolution from the acidic electrolyte will provide an effective cathodic reaction. Hence, the corrosion of the aluminium does not depend on any external athodic reaction, a AlCl 3 + 3 H 2 O Al(OH) 3 + 3 HCl c according to Eq. 2. (2) In the corrosion reaction the aggressive environment is regenerated, represented by aluminium hloride in the equation above. Hence, as long 2 Al + 6 HCl 2AlCl 3 + 3 H 2 c will maintain itself. The mechanism is similar to the traditional mechanism for localized corrosion of aluminium. In the mechanism outlined in Figure 4, we have described a situation where the duplex coating is galvanically coupled to bare steel. The galvanic corrosion initiates rapid degradation caused by e hydrochloric acid. The bare steel may be equipment in stainless th mechanical damage in the duplex coating down to the steel or an area where the duplex coating has corroded away. Other materials that may act as cathodes, e.g. copper and copper alloys, will have the same effect. 5.3 TSA with sealer TSA only coated with a thin sealer has given very good corrosion resistance and little coating degradation, even after very long exposure, as summarised in the introduction. In order to study corrosion of sealed TSA, a sample exposed offshore for five years was investigated. The sample was exposed with a scribe, and a cross section was made across the scribe. The cross section was studied by electron probe micro analysis (EPMA). The TSA had a thickness of about 200 m. The s TSA, as indicated in the upper right hand image. The lower images show the distribution of aluminium and oxygen in the two upper images. Bright contrast indicates a high concentration, while dark shades indicate low concentration. Black means no signal from the element that is a 18 The oxygen rich phase on top of the TSA is aluminium oxide. The thickness of the oxide varied between 20 and 50 m. The oxide is located partly on top of the sealer and partly penetrating it. Aluminium oxide is also found in the scribe, though much of the oxygen signal in the scribe is from corrosion products of the steel. Evidently the degradation mechanism for duplex coatings outlined above will not take place on sealed TSA. This is probably because the sealer is too thin to hold an aggressive electrolyte at the etal surface. Chlorides may be washed away by water on the coating surface. Since no aggressive electrolyte is formed, th plex coatings is caused y the chemical properties of aluminium. Galvanic corrosion is an initiation mechanism for the ating systems that were tested in the galvanic corrosion test. Code TSM Sealer opcoat m e TSA will remain passive. 5.4 Recommendations for TS duplex coatings based on the degradation mechanism The results from this investigation showed that the degradation of TSA du b degradation, but there may be others as well. Based on these findings, we recommend not to use TSA duplex coatings in corrosive environments in the future. Dupelx coating is here defined as thermally sprayed metal with thick, protective organic coatings. TSA with a thin sealer only provides excellent corrosion protection. able 1: Duplex co T Barrier coat T E2 200 µm TSA 35 µm epoxy 125 µm epoxymastic 100 µm polysiloxane M1 100 µm TSZ 35 µm epoxy 125 µm epoxymastic 100 µm polysiloxane Figure 2: Test set-up for duplex coating galvanically coupled to bare steel. The samples were exposed in a salt spray cabinet for 900 hours. The two duplex coating systems tested are described in Table 1. A Steel Duplex coating PVC (1 mm thick) 50 mm 5 mm Steel Paint TSM 19 -200 -100 0 TSZ -500 -400 -300 C u r r e n t [ µ A ] -600 -700 0 100 200 300 400 500 600 700 800 900 Time [hrs] Figure 3: Galvanic current between bare steel and TSA duplex coating (A) and TSZ dup coating (B). TSA lex able 2: Calculation of corrosion rate of the TSA and TSZ based on the current Sample m) Molecu gh o Corrosion /yr) T measurements shown in Figure 3. Galvanic current (µA) TSM thickness (µm) width (m Density (g/cm 3 ) wei (g/m lar t Valence l) rate (mm TSA 0 250 200 5 2.7 27 3+ 27 TSZ 100 100 50 7.1 65 2+ 30 Figure 4: Galvanic corrosion of TSA duplex coatings in electrical contact with bare steel. Organic coating TSA Steel Al Al 3+ + 3 e - O 2 + 2 H 2 O + 4 e - 4 OH - Cl - e - 20 TSA duplex coating Bare steel 1 cm TSZ duplex coating Bare steel Figure 5: Picture of TSZ and TSA duplex coatings galvanically coupled to bare steel, after 38 days of exposure in continuos salt spray. The TSZ duplex coating has protected approximately 2 cm of the bare steel cathodically. The protected distance is seen as the onset of red corrosion products. The TSA duplex coating is severely degraded, while the TSZ duplex coating is in much better condition. 21 Figure 6: Electron microscopy of cross section of sample coated with TSA and sealer exposed offshore for five years. The upper micrographs show SEM backscatter images of the left and right side of the scribe in the TSA sample. The other four images show the distribution of aluminium and oxygen on the left and e of the scribe. TSA Steel Embedding resin Sealer Scribe TSA right sid 22 6 Sealers Proper sealing of the porous surface will likely increase the lifetime of TS duplex coatings. Corrosion may initate in pores in the TSM surface. Hence a study of the ability of various sealers to penetrate pores and crevices was performed. The paint suppliers selected or developed new and TSZ. Cross sections of the sealed TS coatings were then studied in an optical microscope in order to characterise the penetration. m with this study was to find effective sealers for later application in duplex coatings. 6.1 Sealer penetration In this section the penetration of two of the sealers are shown in order to give an impression of the complexity of the TSM surface and the ability of sealers to fill the structure of the TSM. The complete study has been reported earlier (B.S. Tanem, O.Ø. Knudsen, 2002). Figure 7 shows an optical micrographs of sealer 2B, which was a unpigmented epoxy. The dark line defines the surface of the sealer. The sealer seems to fill out the surface towards TSA in a very good manner, even where the TSA/sealer interface is rather complex. Figure 7b shows that there are a few large pores in the sealer, but not all these pores are located at the TSA/sealer interface. There is a thin layer of sealer between the pore and the TSA and the pore will therefore probably not affect the corrosion properties of the coating system. sealers that were applied on TSA The ai Steel a TSA Sealer Sealer Sealer surface Large pore Sealer surface Embedding resin b Embedding resin Figure 7: Picture in a) shows optical micrograph of Sealer 2B, indicating that the sealer penetrates the surface towards TSA in a very good manner. There are a few large pores as shown in b), but some of these pores are located inside the sealer and not at the TSA/sealer interface. Cross sections of sealer 3C are shown in Figure 8. The optical micrographs show that the sealer is only partly able to fill the complex geometry of the TSA. As a result of this, many small and large pores appear at the TSA/sealer interface, especially where there are many pores in the TSA surface and where the sealer locally is thick (~100 µm). Pigments in the sealer might be partly responsible for the incomplete penetration. 23 Steel a Sealer TSA 100µm Embedding resin b TSA Small and large pores at TSA sealer interface Pigments Sealer 20 µm Embedding resin igure 8: Picture in a) shows optical micrograph of Sealer 3C, identifying F the sealer. e (8 sealers), waterborne poxy (2), silicone (1) and vinyl phenoxy copolymer (1). The penetration of the epoxy polyamide based sealers varied from excellent to poor, which shows that the binder itself is not decisive for the penetration. Both the waterborne sealers (2C and 2D) gave good penetration, which shows that good penetration can be obtained with waterborne sealers as well. The silicone and vinyl phenoxy copolymer based sealers gave pores in this test, but since only one sealer with these binders were tested, it is difficult to say anything in general about the pore filling properties of these binders. In general all organic binders have low surface energy, which imply that they wet metallic surfaces well. Some of the sealers were clear, some contained filler particles and some were pigmented with flake shaped pigments. We found that all the clear unpigmented sealers gave very good penetration and little pores. However, also some of the pigmented sealers gave rather good penetration. The sealers with large flake shaped igments were among the sealers that gave most ores. In the microscope we saw that pores were situated behind pigments that blocked complex etry in the TSM. We therefore conclude that pigmentation is not beneficial for the ess seen in a microscope will then vary from zero at the peaks in the thermally sprayed etal to several tens of microns between the peaks. For some of the samples we investigated the of a sealer to penetrate narrow crevices and complex eometry in the TSM surface. However, we have not measured the viscosity of the applied sealers (after dilution). The amount of solvent added before application is reported, but this is probably not a good parameter for discussing viscosity. The sealers that were least diluted were among the sealers that gave most pores. This is an indication that proper dilution of the sealer in order to decrease the viscosity improves the performance with respect to penetration properties. Pigments are present in the sealer. Small and large pores tend to appear at the TSA/sealer interface, especially where the TSA surface is complex, as shown in b), or where the sealer locally is thick (~100 µm). 6.1.1 Recommendations regarding sealers Sealers of four different generic types were tested: Epoxy polyamid e p p geom penetrating properties of the sealer. At least large flake shape pigments should be avoided. A sealer is supposed to be applied in a non-measurable film thickness, according to BS 5493. The film thickn m film thickness was according to this specification, while for others the films were thicker. Sealers applied in film thickness above 50 µm were among the products with most pores. Hence, there seems to be a correlation between amount of pores and applied film thickness. The viscosity probably also affect the ability g 24 7 Testing of TS duplex coatings The testing in Phase 1 was started before we found the degradation mechanism for TSA duplex coatings, so a large number of TSA duplex systems were tested. In Phase 2 we focused on TSZ duplex systems. The overall purpose with the testing has been to find duplex coating systems with long lifetime expectancy. 7.1 Phase 1 – Effect of TSM, organic coating and structure of TSM The purpose with the testing in Phase 1 was to study the effect of composition and structure of the metal coating and the generic type of the organic coating on the performance of the duplex coatings. Coatings with zinc rich primers were used as reference coating systems. The samples were tested in the NORSOK test (rev. 4) and in a 5 years field test. o ne (Al, porous Al, AlMg5, Zn, ZnAl15) ated rubber (Zn, Al) the results indicate that the porosity of the TSM is less important than anticipated. est y little mples. he results clearly demonstrate that TSA should not be painted with thick protective coatings, igure 10 shows how the various organic coatings performed when applied on TSA. In both tests ng 7.1.1 Effect of composition and structure of TSM Figure 9 shows how the various TSMs performed when tested with the same organic coating. Tw organic coating systems were tested with more than one TSM:  Epoxy polyamide + epoxymastic + polysiloxa  Washprimer + 4 coats of alkyd / chlorin In the NORSOK test the TSA duplex coatings performed surprisingly well, compared to what was expected, based on the degradation mechanism described earlier. However, in the field test more degradation of TSA duplex coatings was found. There was no significant difference between 99.5% Al and AlMg5, and both types of TSM evidently are subject to the same type of degradation, resulting in give rapid and severe degradation in corrosive environments. The samples with extra porous TSA did not degrade more than the other TSA duplex coatings. Hence, The field test was more discriminating with respect to type of TSM than the NORSOK test. In the NORSOK test the two best coating systems were both TSZ duplex coatings. The t indicates that TSZ duplex coatings are superior to TSA duplex coatings. In the field test ver degradation was found on the TSZ systems. The ZnAl15 coating performed similar to the pure TSZ. Corrosion creep was less than 1 mm for all TSZ systems. All the TSA systems had much more degradation, and up to 30 mm around the scribe was severely degraded on some sa T confirming the conclusions from the mechanism study. Duplex coatings with TSZ or TS ZnAl15 will give long lifetime, while TSA duplex coatings will degrade rapidly. 7.1.2 Effect of organic coating F there were significant differences between the various coatings, but the differences were not consistent between the two tests. In the NORSOK test most of the TSA duplex coatings passed the 3 mm average scribe creep qualification limit. However, in the field test significant degradation was found. Severe blisteri 25 around the scribe was found on all nine TSA duplex coatings shown in the diagram. In addition NORSOK testing of the coating systems with zinc rich primers. the NORSOK test are shown in the figure. All the systems have performed um scribe creep. The ep from the scribe. Including listering of the topcoat the degradation of the organic coating spread 4.5 mm from the scribe. moved, probably due to corrosion some of the TSA duplex coatings are suffering from general blistering. 7.1.3 Reference systems igure 11 shows the results from F Only the results from reasonably well, and neither of the systems have more than 3 mm maxim figure shows maximum scribe creep, while NORSOK qualification is based on average scribe creep. Average scribe creep is given in the report from TI, showing that except for A1, all systems had less than 1 mm average scribe creep [Kaltenborn 2003]. For system R4 one of the samples had large scribe creep locally on one of the samples, which explains the large standard deviation or this sample. System F1 had less than 3 mm average corrosion cre f b Very little degradation was found after 5 years exposure in the field test. None of the samples showed had more than 0.5 mm corrosion creep on the steel outside the scribe. For most of the amples up to 1 mm of the topcoat/intermediate coat could be re s of the zinc in the primer. Hence, all the reference systems have performed very well in the field test. 26 0 2 4 6 8 10 S c r i b e c r e e p ( m m ) 12 14 99.5 Al AlMg5 ZnAl15 TSZ TSA TSZ TSA porous NORSOK test Epoxy +epoxymastic +polysiloxane 4 x alkyd / CR Figure 9: Effect of Thermally sprayed metal on degradation of duplex coatings. Tw different organic coatings were tested. The bars show maximum co scribe (average of the three samples tested). o rrosion attack from 12 m ) 0 2 4 6 8 10 99.5 Al AlMg5 ZnAl15 TSZ TSA porous TSZ TSA B l i s t e r i n g a r o u n d s c r i b e ( m Field test, 5 years Epoxy +epoxymastic +polysiloxane 4 x alkyd / CR 27 0 2 4 6 8 10 12 14 B1 B2 C1 C2 D1 D2 E1 E2 R2 S c r i b e c r e e p ( m m ) NORSOK test pt for k 0 5 10 15 20 25 30 B1 B2 C1 C2 D1 D2 E1 E2 R2 B l i s t e r i n g a r o u n d s c r i b e ( m m Figure 10: Effect of type of organic coating on degradation of duplex coating in the ed). NORSOK test and five years field test. All coatings were applied on 200 µm TSA, exce R2 where the TSA thickness was only 100 µm. The bars show maximum corrosion attac from scribe (average of the three samples test ) Field test, 5 years 28 0 1 2 3 4 5 6 A1 A2 R3 R4 R5 R6 F1 S c r i b e c r e e p ( m m ) NORSOK test Figure 11: Systems with zinc rich primers. The bars show maximum corrosion attack from scribe (average of the three samples tested) in the NORSOK test. None of the samples showed any degradation after one year in the field test. 7.2 Phase 2 – TSZ duplex coatings Due to the risk for rapid degradation of TSA duplex coatings, focus in the project was shifted from TSA duplex systems to duplex coatings based on TSZ. In phase 2 of the project a set of duplex coated samples was prepared with TSZ and various organic coatings. The coating suppliers were asked to select organic coatings that will comply with future environmental legislation, e.g. high solid or waterborne products. Test results from laboratory testing (NORSOK M-501, Kaltenborn and Scheie 2004) and one year field exposure have been reported (O.Ø. Knudsen, 2004). Thermally sprayed zinc has been avoided on offshore installations, because in the highly corrosive nvironment zinc coatings usually only last for 5-10 years. Only TSA have been used for etallising offshore structures. TSZ duplex coatings therefore represent a new generic type of aviour 7.2.1 Test results Figure 12 shows the test results from the NORSOK test. Neither of the TSZ duplex coating systems showed significant corrosion creep from the scribe. System C2 showed white zinc corrosion products all over the coating surface, which means that zinc oxide penetrated the organic coating. The organic coating system was only specified to be 105 µm thick, and this was probably too thin. The zinc coating has a rough surface structure, and over peaks in the TSZ the organic coating may have been very thin, i.e. less than specified. e m coatings on offshore installations. Questions regarding steel/zinc polarity reversal and beh at temperatures up to 120°C have been put forward, which have been investigated by literature studies. 29 For system H4 the topcoat (50 µm oxirane ester) was flaking off, starting from the scribe, on all three parallels tested. Due to the 110 µm thick epoxy polyamide coat under, there was little corrosion of the zinc. For system I5 one of the parallels showed severe cracking of the topcoat, starting at the scribe and propagating more than 20 mm on either side of the scribe. The topcoat was also flaking off in this area. Some of the cracks also penetrated the epoxy barrier coat, and the zinc was corroding in the cracks. On the two other samples there were no cracking, but around the scribe the topcoat could be removed by a scalpel, indicating decreased intercoat adhesion. System H3, I6, J1 and J2 showed very little degradation after the NORSOK test. Scribe creep on the zinc was reported to be 0.1 to 0.4 mm, which is very good. As noted above, neither of the TSZ duplex samples showed any corrosion creep on the steel. After the 5 years field test very little degradation was found on the TSZ duplex coatings. None of the samples had any corrosion of the steel around the scribe. Most samples had about 0.5 - 1 mm corrosion of the TSZ around the scribe. Outside the scribe area no degradation was found. The nly exception was system I6, where the zinc started to corrode at spots all over the sample, and on. No thorough investigation about e cause of this degradation has been performed, so this should only be considered as o the corrosion products caused the coating to crack over these spots. The explanation for this degradation may have been incomplete penetration into the TSZ coating. During exposure these pores will be filled with water and the zinc may start to corrode. The I6 coating system was entirely water borne, which also may be part of the explanati th speculations. There was little correlation between the field test and the NORSOK test. 0 5 10 15 D e g r a d a t i o n f r o m 20 25 30 s c r i b e ( m m ) C2 H3 H4 I5 I6 J 1 J 2 Corrosion on steel Corrosion on zinc Degradation of organic coating h e c o a t i n g W h i t . 4 ystem the specified total film thickness was only 105 µm, consisting of 30 µm sealer and 75 µm topcoat, which evidently is too thin. The topcoat may have only just covered the largest peaks in the coarse structure of the thermally sprayed zinc. Due to the low film thickness of the sealer and t e r u s t p e n e t r a t i n g Figure 12: Test results from cyclic testing of TSZ duplex coatings (NORSOK M-501 rev est cycle). t 7.2.2 Effect of paint film thickness For system C2 zinc corrosion products penetrated the coating during the test. For this coating s 30 the coarse structure of the TSZ the sealer is not a continuous film. Hence, the organic coating system should be considered to be a one coat system, and not a two coat system. Coating films below 80 µm formed from solvent borne binders have in earlier studies been shown polymer film is not uniform in structure. It can rather be considered as consisting of microgels ). The film formation is not a omogeneous process. Formation of microgels starts at a number of different sites in the wet film. nt of ctive , es above a veral layers of microgels may be formed and block the conductive pathways. ultiple coats will have the same effect. 7.2.3 Cracking and flaking Cracking or flaking was found on two of the tested TSZ duplex coating systems, but only in the NORSOK test. These forms of degradation were probably not related to the duplex nature of the coating systems, but rather properties of the coats or the application. For system I5 the topcoat cracked on one of the parallels in the NORSOK test, starting at the scribe and propagating more than 50 mm. The crack penetrated the entire organic coating, causing the zinc to corrode inside the crack. The topcoat is a 100 µm polysiloxane. The same coating product was applied on system E1, E2, M1, M2, M3 and P1 in Phase 1 of the project (TI report from Phase 1: Kaltenborn and Scheie 2004), where we found cracking on all samples after the NORSOK test. For system H4 the topcoat (50 µm oxirane ester) was flaking off in the NORSOK test, starting at the scribe. At the end of the test the adhesion loss had spread about 20 mm on each side of the scribe. For system I5 the cracking was combined with flaking, and the two samples without y ince the adhesion loss seems to be spreading from the scribe, it seems likely that it is linked to th corrosion products from the zinc, probably a rbonate. This may have attacked the coating, e.g. by the the scribe. to have low ionic resistance (Mills and Mayne 1981). This was attributed to formation of conductive pathways through the organic coating. Mills also found that coating resistivity increases significantly when the film thickness increased above 80 µm, or when multiple coat films were tested. A (high molecular weight and high crosslinking density) connected via a low molecular weight and low cross-linked (LMW/LC) polymer fraction. The inhomogenity is due to a phase separation during film formation, according to Bascom (Bascom 1970 h As the microgels approach each other from their initiation sites, they are unable to merge into a homogeneous structure. When the microgels meet, the reactive groups in the polymer chains either already have reacted or are immobilized, so the polymerisation reaction is terminated. Unreacted or partly reacted resin is then left at the periphery. This unreacted or partly reacted resin is the LMW/LC fraction described above. The LMW/LC fraction takes up a large amou water, has a low resistance to ion transport, and is susceptible to water attack, e.g. hydrolysis and dissolution (Nguyen et al 1996). Nguyen has linked this inhomogenity to formation of condu pathways and transport of ions through coatings. If the microgels are in the order of 50 µm in size conductive pathways may penetrate the entire coating. When the film thickness increas certain value se M cracking also showed signs of poor adhesion between the polysiloxane and the intermediate epox coat. S processes at the scribe. The scribe was filled wi mixture of zinc oxide, hydroxide and ca slightly alkaline properties of zinc corrosion products. However, we would then have expected more degradation on the lower part of the samples, since the corrosion products were running down from the scribe. This was not the case. The degradation was the same at the upper side of 31 7.2.4 Low VOC coatings The coating suppliers were asked to select coating systems that are likely to comply with future ts gave olvent test and I6 in the field test. Thus, water based systems performed just as well s the best solvent based systems. Also the high solid systems applied with mist coats instead of d in s is ay in istance to atmospheric corrosion to the formation of an insoluble ntal conditions that interfere with the formation of such films may  he sphere on the initiation and growth of the protective carbonate film. evere than in most types of domestic or river water, which contains some scale-forming environmental legislation, e.g. high solid or waterborne systems. As far as I know, most of the applied coating products can be classified under one of these two categories. The test resul no indications that the water based systems were more susceptible to degradation than the s based systems. On the contrary, the water based systems performed very well, except for system C2 in the NORSOK a diluted sealers performed well. 7.3 Properties of zinc coatings 7.3.1 General experiences with zinc coatings The behavior of zinc and zinc coatings during atmospheric exposure has been closely examine tests conducted throughout the world. Precise comparison of corrosion behavior in atmosphere complex because of the many factors involved, such as prevailing wind direction, type and intensity of corrosive fumes, the amount of sea spray, and the relative periods of moisture or condensation and dryness. However, it is generally accepted that the corrosion rate of zinc is low; particularly in less corrosive environments. In rural atmospheres corresponding to corrosion class 2 or less, the corrosion rate is in the order of 0.1 µm/year. In moist industrial or marine atmosphere, corresponding to corrosion class 5, corrosion rates up to 15 µm/year has been reported. Hence, in very corrosive environments a 100 µm thick zinc coating may corrode aw less than 10 years. Zinc owes its high degree of res zinc carbonate film. Environme attack zinc quite rapidly. The important factors that control the rate at zinc corrodes in atmospheric exposures are: The duration and frequency of moisture  The rate at which the surface dries  Presence of industrial pollution or chlorides Atmospheric corrosion has been defined to include corrosion by air at temperatures between -18 to 70°C in the open and in enclosed spaces of all kinds. The corrosion rate of zinc is related to t effect of the atmo The corrosion of zinc in water is largely controlled by salinity, pH and impurities present in the water. As in the atmosphere, the corrosion resistance of a zinc coating in water depends on its initial ability to form a protective layer by reacting with the environment. In distilled water, which cannot form a protective scale to reduce the access of oxygen to the zinc surface, the attack is more s salts. Seawater is more aggressive than freshwater. For example, in UK waters the zinc corrosion rate normally lies in the range 10 - 15 µm/year for continuous immersion. 32 7.3.2 Risk for zinc steel polarity reversal In some cases it has been observed that the polarity has reversed between zinc and steel, i.e. steel as become anode and zinc the cathode. This situation has been reported for zinc coated steel 5°C (Corrosion and Corrosion Control, 1985). This reversal is ons of carbonates and nitrates, and inhibited by chlorides and f 7.3.3 Corrosion of zinc coatings exposed above 70°C efined to temperatures between -18°C and 70°C. Very little data are published about the properties of zinc coatings above 70°C, and most reported experiences are to G ting ermally sprayed zinc coatings do not, or to very small extent, form intermetallic phases with en claimed that thermally sprayed zinc coatings should not be used bove 60°C due to degradation processes starting at this temperature. This may be a problem on may be used on structures with temperature up information or documentation that support this gium, TSZ coatings may be used without ny problems at 120°C. h exposed in hot water above 6 favoured by high concentrati sulphates. The corrosion failure is explained by passivation of the zinc surface by the formation o a protective film consisting of zinc carbonates and possibly other zinc compounds. The dissolution of iron will occur at defects of the film. Formation of ZnO rather than Zn(OH) 2 may also contribute, where the ZnO may act as a semiconductor in aerated waters. The water quality is characterized by a pH above 9.0, and low concentrations of chloride and sulfate. Presence of aggressive ions such as chloride and sulfate will destroy or prevent the formation of the passive film. Hence, this will not be a problem in marine environments or at structures below 60°C. Atmospheric exposure has been d for ambient or room temperature. Studies of hot dip galvanized coatings (HDG) have shown that such coatings will withstand continuous exposure to temperatures of approximately 200°C and occasional excursions up 275°C without any effect on the coating. Above these temperatures there is a tendency for the outer zinc layer to separate, but the alloy-layer, which comprises the majority of the coating, remains intact. Adequate protection may often, therefore, be provided up to the melting point of the alloy layer (around 530°C). Exposure above 200°C results in phase transitions in HD coatings, and the amount of zinc iron intermetallic phases increase. This makes the zinc coa become brittle and susceptible to mechanical damage and cracking. Th iron during applications. Phase transitions during exposure at elevated temperature should therefore not be a problem. Earlier in the project it has be a offshore installations, where NORSOK System 1 to 120°C. However, we have not found any conclusion. According to Paul Delpire in Zinacor Bel a 33 8 TS Duplex systems – expected lifetime Compared to the reference systems with zinc rich primers, the TSA duplex coatings were significantly more degraded, while the TSZ duplex coatings were less degraded or equally li degraded. It should be noted that none of the duplex systems tested have given any corrosio the steel substrate. The coating degradation was solely affecting the TSM and the organic coating. However, based on offshore experiences degradation of TSA duplex coatings will devel when it first starts, and when the TSA have been consumed the steel starts to corrode. It is therefore reasonable to assume that the TSA duplex coatings tested here will degrade faster than the reference systems. Based on the test results, we expect the TSA duplex coatings to have shorter lifetime than this. ttle n of op rapidly ealed TSA has in many studies been shown to give excellent and long lasting protection. Hence, there must be a limit to the film thickness that can be applied on TSA without causing the ct of film thickness on degradation of omewhat thicker than a sealer and still have an acceptable sult. In order to avoid the rapid degradation the organic coating must probably have a er, g" hed ly will he TSZ duplex coatings tested in Phase 1 and 2 showed very good performance. Based on the , and to assume that the lifetime will be longer than for the reference ystems. The project was started with an objective to reach more than 20 years lifetime, which evelops linearly. The experiences from The Public Roads dministration indicate that 50 years lifetime is possible [Klinge 1999]. S degradation problem described earlier. In section 7.1.2 effe TSA duplex coatings were discussed. Based on the the experiences at Åsgard A, where a coating system consisting of TSA + 20 µm sealer + 80 µm topcoat seems to be performing well, it seems possible to apply a coating that is s re sufficiently open structure so that an aggressive electrolyte may penetrate the coating. Howev the lifetime of such a coating will probably not be longer than for sealed TSA, since the organic coating in a way must behave like a sealer. The only advantage with such a "lean duplex coatin will probably be a more coloured surface. During application the film thickness must be watc carefully in order to avoid too high film thickness, which may cause degradation. Maintenance of such a coating will also be difficult, since the total thickness of the organic coating then easi exceed a critical value for initiation of rapid degradation. T test results, they are therefore expected to have more than 20 years lifetime in highly corrosive atmosphere. In fact, since the degradation only has affected the TSZ and the organic coating not the steel, it is reasonable s very likely will be obtained with TSZ duplex coatings, based on the test results and the experiences from road bridges. Another question is whether we can assume lifetimes significantly longer than 20 years, e.g. approaching 50 years. An accelerated test and a 5 years field test do not give experimental evidence to conclude for certain about this. However, the low degree of degradation of the TSZ duplex coatings in the field test is promising, particularly if we can assume that degradation d A 34 9 Maintenance of duplex coatings Due to severe degradation, good maintenance procedures for TSA duplex coatings for oil & installations are needed. The task is extra challenging due to the limitations on surface cleaning methods and coating products. With TSZ duplex coatings there has been less degradation and the need for development of maintenance procedures is less urgent. The project has therefore focused on maintenance of TSA duplex coatings. The purpose with this part of the project was to give recommendations about maintenance of TSA duplex systems, with respect to cleaning meth and selection of repair coatings. The extent of coating degradation will influence the maintenance method. Recommendations were therefore given as function of the state of the coating. gas od d only ase tion of w coating. The first question that eeds answering is whether to remove the remaining TSA or not. The TSA represents a value and g TSA .2.2 Maintenance without removing the TSA he TSA in the duplex system represents a potential value if the rapid degradation can be prevented. A surface with a corroded TSA duplex coating will usually consist of areas with intact coating, areas with corroded TSA and areas with corroded steel, as discussed above. An attempt was made to remove only the organic coating at Sleipner R by blast cleaning during the summer 2003. However, this turned out to be difficult, and it was decided to remove the entire coating system, including the TSA, by blast cleaning. Ultra high pressure (UHP) water jetting will remove corrosion products, degraded and intact organic coating and some TSA. It was shown that UHP is able to remove the entire TSA coating, but this is a very slow process. Most of the TSA will usually remain on the surface. Hence UHP will give a surface consisting of bare steel, bare TSA and intact duplex coating. 9.1 Degradation of topcoat only – no corrosion of TSA If only the topcoat is degraded, i.e. the TSA is still totally covered by an organic coating an the visual appearance of the structure is affected, then it is important to avoid excessive cleaning of the surface, resulting in exposure of steel or TSA. Gentle cleaning with soap and water to remove salts and other contaminants, rinsing of the soap and drying before application of a new topcoat will give the best result. The additional film thickness of the organic coating will decre the probability of pores penetrating the coating, which will decrease the chance for initia corrosion of the TSA. This situation will not be discussed further in this report. 9.2 Corroded TSA In the case of heavy corrosion of the TSA under the organic coating, as experienced on e.g. Sleipner R, the situation is very different. A thorough cleaning with removal of loose paint and corrosion products is necessary before application of the ne n potential long term corrosion protection, but also potentially rapid degradation of the new coatin system. 9.2.1 State of TSA after degradation Due to the heavy corrosion of the thermally sprayed metal in TSA duplex coatings, one may ask how much aluminium is left. On offshore installation it has been reported that if the degradation is allowed to develop for sufficiently long time, the aluminium will be consumed and the steel substrate will start to corrode (Figure 1). Hence, when maintenance starts there will likely be a situation where in some areas the duplex coating is intact, in some areas there are remaining and in some areas bare steel is exposed. 9 T 35 micals. Paint removing chemicals are commercially is ethylene chloride but have other flammable chemicals, for example acetone, toluene, or g only acetone, toluene or methanol, or . .2.3 Maintenance by removing the TSA TSA duplex coating will provide a uniform surface with less restriction on e to P. products and the organic coating. However, the 00 cm 2 in two minutes (50 min/m 2 ). described earlier (Knudsen 2002) hydrochloric acid is rmed under the organic coating during degradation. A coating system that can absorb and acid would probably perform better than the previously tested systems. ave this ability. They will react with the omate), , al Another way of removing the paint is by che available in a wide range of compositions. In order to be effective the formulation must contain substances that are able to break bonds between the coating and the substrate. Protective coatings usually have cross linked binders, which makes them more demanding to remove. Efficiency and toxicity of paint removing chemicals are usually correlated, and many of the products contain harmful substances like organic solvents, caustic soda or strong acids. Most paint strippers are solvent based, and methylene chloride is the most commonly used chemical in paint strippers. Methylene chloride products come in two varieties. One type onflammable, while the other type is flammable. The flammable paint strippers have less n m methanol. There are also paint strippers available containin mixtures of these. N-methylpyrrolidone or dimethyl sulfoxide are also used. Paint strippers based on acids or alkaline solutions may attack the TSA as well as the organic coating, and are therefore not possible to use on duplex coatings. Some products are claimed to be environmentally friendly by the producers, e.g. water borne products containing weak organic acids and aromatic alcohols According to DnV some of these products have been effective in lab testing. However, the content of acids may exclude them, as mentioned above. 9 Removing the entire selection of the new coating system. The TSA may easily be removed by grit blasting, but du restrictions on blast cleaning offshore, it will be advantageous if UHP water jetting can be used. This was investigated in a test where a sample with a TSA duplex coating was cleaned with UH The water pressure was 200 bar, the flow was 11 litres/min, and the sample (20 x 20 cm) was cleaned for approximately 2 minutes. Electron microscopy showed that there was no aluminium left on the surface after the UHP water jetting. Hence, the UHP water jetting is capable of removing metallic TSA, as well as the corrosion method seems to be slow, cleaning only 4 9.3 Repair systems for TSA duplex coating 9.3.1 Experiences and test results According to the degradation mechanism fo neutralise hydrochloric Pigments like zinc, zinc oxide and zinc phosphate h hydrochloric acid and form zinc chloride. Another possibility is hexavalent chromium (chr known for its ability to form stabile complexes with chlorides (Williams 2003). Nidelv bridge in Trondheim, which is coated with a TSA duplex system, has an organic coating pigmented with inc chromate, which may partly explain the good performance of this coating system. However z hexavalent chromium is now abandoned in organic coatings. A zinc phosphate pigmented coating was tested in the first test series (reference system 2 – TSA + washprimer + 4 coats alkyd/chlorinated rubber). This system did not perform well in the NORSOK test and gave 5 – 11 mm scribe creep (Knudsen 2003). Can TSA be coated with zinc rich primers? There is a certain difference in corrosion potenti between the TSA and zinc epoxies, which may lead to galvanic corrosion. TSA has an open 36 circuit potential of about -1050 mV SCE, while zinc rich primers have potentials between -1000 r, tested two TSA duplex oating systems, one with a zinc rich epoxy primer and one with an epoxy primer. The barrier s in a vent the binder from penetrating the pores. The cribe creep progression rates shown in Figure 13 indicates that the zinc rich primer to some degree slow down the corrosion of the TSA. If the test is prolonged it seems that the zinc rich er. Hence, zinc rich primers may decrease and -800 mV SCE. The potential on the individual zinc particles are probably closer to -1000 mV SCE. The zinc may then be cathode and aluminium anode in case of galvanic corrosion. Howeve the potential difference is small and the zinc dust particles are to some degree insulated from the steel by the epoxy binder. In order to investigate whether galvanic corrosion is a problem, we have c coat was an epoxymastic and the topcoat was a polysiloxane. The zinc epoxy has been documented to be able to polarise steel below the protection potential for more than 100 day submersion test. The coating systems are described in . After NORSOK testing both coating systems have an average scribe creep of approximately 4 mm (Figure 13). During the first months of the test the zinc epoxy sample had more scribe creep than the epoxy primer. However, after the scribe creep first initiated on the epoxy primed samples, the corrosion has propagated faster than on the zinc epoxy samples. The rapid initiation of scribe creep on the zinc epoxy samples may be due to a galvanic effect, but a more likely explanation is insufficient penetration of the primer into the porous TSA surface. Zinc rich primers have a high content of fairly large zinc particles which may pre s primer will perform better than the epoxy prim corrosion to some degree. Table 3: TSA duplex coatings with and without a zinc rich primer tested in the NORSOK test. Except for the primer the two coating systems are identical. Sample TSA Thickness Primer Type DFT Barrier coat Type DFT Topcoat Type DFT EP 200 Epoxy 35 Epoxymastic 200 Polysiloxane 75 Zn EP 200 Zn Epoxy 75 Epoxymastic 200 Polysiloxane 75 4.5 4 0 0.5 1 1.5 2 2.5 3 3.5 0 1000 2000 3000 4000 5000 Hours S c r i b e c r e e p ( m m ) EP primer ZnEP primer Figure 13: Scribe creep test results in the NORSOK test as function of time. The coatings are described in Table 3. 37 9.3.2 Recommendations Blast cleaning in order to remove all remaining TSA will give a homogeneous surface and remove the chance for rapid corrosion of the TSA. Milder cleaning may give a surface consisting of bare steel, bare TSA and intact duplex coating. On bare steel a zinc rich primer will be beneficial. Zinc rich primers also seem to be beneficial on TSA, though we only have tested one coating system ss on for degradation. ore igure 14: Situation at a coating edge. On the drawing to the left the TSA is exposed at the coating edge. On the drawing to the right the TSA is totally insulated under the organic coating. The situation on the right will have better corrosion resistance. 9.4 Maintenance of TSZ duplex coatings The typical situation for TSZ duplex coatings exposed for many years in atmosphere is:  Degradation of the topcoat only and decreased visual appearance  Some corrosion of the zinc around mechanical damages in the paint  Little or no bare steel exposed Based on information from the Public Roads Administration and the paint suppliers the following is recommended for maintenance of TSZ duplex coatings:  In case only the topcoat is degraded, the structure should be cleaned without removing any ed with corrosion under surrounding organic coating, the surface must be cleaned and any corrosion products and loose paint removed. Remaining metallic zinc with zinc epoxy on TSA. Zinc rich primers should not be applied on remaining organic coating due to low adhesion. Intermediate- and topcoats may be applied on the entire surface. Additional film thickne already painted TSA will not make the situation worse. On the contrary, additional film thickness will contribute to insulate the TSA and decrease the risk Offshore experience and this project has shown that degradation usually initiates near bare steel and where the TSA is exposed, e.g. at damages in the coating or edges. The corrosion process and surface cleaning during maintenance will remove most of the TSA in degraded areas. After application of the maintenance coating system the remaining TSA is to a larger extent hidden under the organic coatings, compared to a new structure. Hence, the TSA is better insulated from bare steel. Rapid corrosion of the TSA will therefore not start until the maintained coating is degraded in such a way that TSA is exposed again. It may therefore take a while before degradation is initiated again, and probably longer than on the new built structure which has m spots with exposed TSA. The situation is described in Figure 14. F paint and a new topcoat applied. The new topcoat must be of a generic type that adheres to the original coating.  In case bare zinc is expos Organ ng TS Ste ic coati A el Organic coating e TSA Ste l Coating edge 38 is still providing corrosion protection and must not be removed. A repair coating system is and Sa 3 on their bridges. applied on exposed zinc.  If the steel is also exposed, a zinc rich primer may be applied on the steel and a repair system on top of that. Zinc rich primers should not be applied on remaining organic coating due to low adhesion. Thermal spraying of new zinc on clean steel is the best alternative, when that is possible. Thermal spraying requires blast cleaning to at least Sa 2½. 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