Specifying Steel Corrosion Protection

Steel ConstructionJournal of the Australian Steel Institute Volume 45 Number 1 – December 2011 Specifying corrosion protection on steel ISSN 0049-2205 PRINT POST APPROVED PP255003/01614 STEEL CONSTRUCTION—EDITORIAL A fundamental consideration impacting on cost and long term sustainable outcomes is the assessment of the working environment and evaluation of the necessary protective systems to ensure that a steel structure will meet its lifecycle requirements for functionality and aesthetics. The following two papers by Dr Rob Francis provide a holistic view into corrosion protection issues to be considered by structural engineers and specifiers using steel. The first paper provides background on the development and use of Australian atmospheric corrosivity standard AS 4312, an important reference which drives the selection of appropriate corrosion protection. The new standard documents a range of zones throughout Australia and assigns each a corrosivity level, helping to standardize corrosion protection materials and simplify selection without the cost penalties of over-protection. This paper is based on one presented at the Australasian Corrosion Association Conference held in November 2007. The second paper looks at defining coating specifications to meet the project requirements, which include the corrosivity environment in which the project is located. The paper provides a balanced analysis of the major types of coating systems available and their range of applicability and is therefore an important reference in a subject area that is often confusing, particularly to younger engineers. Dr Rob Francis is a corrosion and coating specialist with Aurecon's Materials Technology Group in Melbourne. He has over 20 years of industrial and research experience in general corrosion and protective coatings and is Chairman of the Standards Australia committee which prepared AS/NZS 2312 on protection of structural steel by coatings. Dr. Peter Key National Technical Development Manager, Australian Steel Institute AUSTRALIAN STEEL INSTITUTE The Australian Steel Institute (ASI) seeks to achieve industry and professional development through regular technical seminars, publishing technical materials and making these available through its bookshop and online, and providing information through its web site http://steel.org.au. It operates for its members the largest steel technical library in the Southern Hemisphere and provides lectures at colleges and universities as well as hosting a range of committees providing direction and assistance to ASI outputs. 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STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 1 designers and specifiers need to be aware of the corrosiveness of the environment in which they are working when selecting materials. placing it in a standard gives it much greater credibility and easy availability. fasteners and other items exposed to the exterior environment. that will be exposed to a specific environment. Many standards and literature from manufacturers. 2 DEVELOPMENT OF A CORROSIVITY STANDARD Information on atmospheric corrosivity in Australia has been available in AS/NZS 2312 [1] for many years. such as those for selecting protective coatings. etc. thicker coating system in a severe industrial or marine environment than in a mild interior environment. This paper provides an overview of the standard. Clearly an understanding of the factors that influence aggressiveness of the environment to coatings and materials is of great benefit to many within and outside the corrosion protection industry. This will simplify selection of such products. requires the specifier to determine the environment as the first stage in coating selection. The categories in AS/NZS 2312 have been accepted throughout industry. This paper looks at the development of a standard for determining corrosivity zones in Australia. coatings and other corrosion protection systems. In recent years. and AS/NZS 2312 has moved to adopt the ISO categories to enable the corrosion protection industry to use internationally recognised corrosion zones. South Melbourne. This standard takes the 2 STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 . There are a number of grades of alloys such as stainless steel where selection of the optimum grade often depends on the exposure environment. AS 4312—2008 Atmospheric corrosivity zones in Australia . Australia SUMMARY Engineers. 1 INTRODUCTION Knowing the corrosivity of the atmosphere is of critical importance to many in the corrosion control industry. contain guidelines to enable the user to determine atmospheric corrosivity. The success of the system has led to separation of the corrosivity section from AS/NZS 2312 in the form of a recent standard. designed for selection of protective coatings for steelwork exposed to the atmosphere. and should enable specifiers and users in many fields to gain an awareness of atmospheric corrosivity and select optimum materials. While such information may be available in the scientific literature or manufacturers’ literature. International Standards have been developed to categorise corrosivity. coated sheet metal products and fasteners. a brief overview of the zones. coating specifiers will normally specify a more complex. This standard contains a description of corrosion zones and an appendix to assist the user in determining the correct zone. For example. the information it contains and how it can be used. Identification of the correct environment is important to ensure that the user selects adequate corrosion protection. Victoria. This standard is believed to be the only one of its type in the world. AS 4312 Atmospheric corrosivity zones in Australia has been developed to standardise corrosivity zones which can apply for a range of corrosion protection materials and coatings. This standard. shown by the fact that the AS/NZS 2312 categories and approach have been adopted by many other Australian standards.AS 4312: AN AUSTRALIAN ATMOSPHERIC CORROSIVITY STANDARD by R A Francis Aurecon. machinery. coatings. and make certain that users are using the most accurate and up-to-date data available. Repair and maintenance requirements in an industrial plant are usually more urgent and more complicated in a severe environment than in a mild environment. how the standard is used and how it relates to corrosivity discussions in other Australian standards. from specialist practitioners such as paint and material specifiers through to a wide range of engineers and others responsible for buildings. structures. without the cost penalties of over protection. even though the corrosion rate of steel may be the same. This category is unique to Australian standards. a Tropical (T) category is included to account for the effect of a tropical environment on some paint coatings. There are two main methods to determine their corrosivity. which does not relate to corrosion rate of steel. but expands it significantly to make categorisation easier. This provides consistency with AS/NZS 2312 and ISO 12944.3–25 25–50 50–80 80–200 80–200 – Typical environment dry indoors arid/urban inland coastal or industrial sea-shore (calm) sea-shore (surf) severe industrial tropical inland STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 3 . it also makes it easier to reference when developing standards that require consideration of atmospheric corrosivity. ISO 9223 [2] and ISO 9224 [3] define corrosion zones. ISO 9223 Classification of atmospheric corrosivity Method 1: Classification in terms of time of wetness and pollution Method 2: Classification based on corrosion rate measurement ISO 9223 Corrosivity categories ISO 9225 Measurement of pollution ISO 9224 Guiding values of corrosion rate for each category ISO 9226 Determination of corrosion rate of standard specimens Figure 1: Family of ISO Corrosivity standards Whichever method is used to determine corrosivity. ISO 9225 [4] determines the zone by measuring the time of wetness. but rather that this newer standard provide much more detailed information for the user if and when required. As a separate standard. as shown in Table 1. The starting point for determination of corrosion zones is ISO Standard 9223 and related standards. chloride concentration and SO 2 levels. and provide much more information to the user.3 1. AS 4312 expands these categories slightly. The C5 zone is split into C5M and C5I (Marine and Industrial) to account for the differing effects of marine and industrial environments on some coatings. It is not intended that the discussion on corrosivity and categories be removed from the individual standards. The relationship between these various standards is shown in Figure 1. Table 1: Corrosivity categories according to AS 4312 and ISO 9223 AS 4312 Category C1 C2 C3 C4 C5M C5I T ISO 9223 Category C1 C2 C3 C4 C5 C5 – Corrosivity very low low medium high very high – marine very high – industrial – Steel corrosion rate (µm/yr) <1. In addition. ISO 9226 [5] uses corrosion rates of metals to determine the corrosion zone.2 [6]. atmospheres are divided into one of five categories from C1 to C5 in ISO 9223 in increasing severity of the environment.approach and information from AS/NZS 2312. they are potentially in one of the more severe categories. a zone may be designated as C3 according to steel corrosion rate.5 kilometres when conditions are severe. There are. Regions right on the coast with rough seas will be in the C5M (very severe marine) category as there is significant salt deposition. The next region inland from rough seas is the C4 region. The surveys carried out by George King. but as most of the population of the country live within 50 kilometres of the sea. especially for Melbourne. and extends to around one kilometre inland.1 to 0. and such areas would normally be in Category C2.0 km 1. References are given so that the user can investigate the original work if so desired. 3 CORROSIVITY SURVEYS Surveys of corrosivity in Australia have been carried out for many years. salt deposition and SO2 concentration available in this country to use these levels to determine corrosion zone.0 to 20/50* km > 20/50* km Notes: (1) (2) Rough seas within 20/50* kilometres C5M C5M C4 C3 C2 (T) No rough seas within 20/50* kilometres C4 C3 C3 C2 (T) C2 (T) T = Use Tropical category for sites in tropical region of the country. It is not the intention of this paper to list or review the work that has been done.5 kilometres inland. Table 2: Selection of corrosivity category according to distance from shoreline Distance from shoreline 0 to 0. Furthermore. On sheltered bays. have been especially beneficial for the development of this standard. It points out that proximity to the coast is the single most important factor.1 km 0. as shown in Table 2. AS 4312 overcomes this limitation by only categorising zones according to steel corrosion rates. If there is no marine influence. John Moresby and others at CSIRO. and that measured from corrosion rate of exposed metal specimens. there is little data on time of wetness.3 kilometres.While the ISO standards provide an established and well recognised method of categorising corrosivity. For example. However. although it does not specifically forbid use of one of the other methods. the marine influence has disappeared within a kilometre or so of the shoreline. they are not without their weaknesses and limitations. Newcastle and the state of South Australia. This covers a vast proportion of the Australian land mass.1 kilometre of the shoreline) on sheltered bays are in C4. As a basic rule. The corrosivity category will not only depend on distance from the shoreline. simplified to show only the corrosion categories. of course. as the standard contains a list of readily available technical papers in Appendix C. These are discussed in detail by King and Duncan [7]. One of these weaknesses is the poor correlation between corrosivity zone as calculated from time of wetness and pollution levels. generally of the order of 0. However. The difference is one corrosivity category. but may be as much as 0. Marine influence can fall away within a kilometre of the coast on a sheltered bay when winds and topography are favourable. then corrosion rates are generally low. salt deposition and resulting high corrosion rates can be found up to fifty kilometres inland in some areas. as determined by corrosion rates on different metals. or two in the case of the region from 0. Table 2 shows the importance of distinguishing between local rough or quiet seas. This work has been included in the standard.5* km 0. * Use the higher figure when winds are strong and/or topography conducive to salt travel inland.3/0. does not correlate. Corrosion rates at various sites around the country from numerous other surveys are included in a table in the standard. Another is that corrosivity zone. most of the experimental work done in Australia is with steel so this is unlikely to be a major issue.5* to 1. The corrosion zones in Australia are described in the standard. any site more than 50 kilometres from the coast will be in the Moderate C2 category. This extends inland by a small distance.1 to 0. 4 STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 . Regions right on the coast (within 0. and the significant amount of data available is one of the reasons that a standard such as this can be produced for such a large country. such as south eastern South Australia. but also whether seas are rough or quiet.3/0. but C2 or C4 according to zinc corrosion rate. along with their effect on corrosivity. unlike many other parts of STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 5 . The important factors are listed in Table 3. It should be noted that SO 2 pollution (‘acid rain’) is considered as a micro-environmental factor in Australia. The survey carried out for South Australia shows that in such situations it would be best to estimate zones as somewhere between these two extremes.some regions where the distinction between rough and quiet seas is not clear. In the gulf regions of South Australia for example. and the research work listed in Appendix C of the standard. the environment generated by normal weather patterns. These are discussed below. These are discussed in detail in the standard. Figure 2 summarises the maps given in the standard. The standard contains maps of major regions of Australia where surveys have been carried out. Figure 2: Corrosivity zones in some Australian centres according to AS 4312 4 MICRO-ENVIRONMENTS AND DESIGN FACTORS The corrosivity surveys used to delineate corrosivity zones determine the effect of the macro-environment at a given site. that is. Adelaide has surf beaches to the south. In addition. but the seas to the north of the metropolitan area are relatively benign. maps are included of the Sydney. but quieter further north. No attempt has been made to determine the extent of the very severe marine zone on these maps. Delineation of zones in these three regions is based on estimates and behaviour of regions where surveys have been carried out. the seas are rough at the southern end. However. It should be stressed that the borders are only estimates for these regions. indicating the major zones. can interact to convert a mildly corrosive site into a more severe one. Brisbane and Perth metropolitan areas. which cannot be resolved on this scale. Newcastle and South Australia. and users should be aware of limitations. The important factors influencing macro-environmental corrosivity are identified as time of wetness and salt deposition. namely Melbourne. and design features of the structure under consideration. the standard notes that micro-environmental or micro-climatic factors at a site. As most engineering structures will have flat ponding surfaces. copper and aluminium. 2205 6 STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 . Unlike ISO 9223. It must be recognised that this is a very simplified approach. exposure to rain or washing can increase corrosion rates. are required as the environment becomes more severe. More highly alloyed stainless steels. presence of welding scale and maintenance regimes. It refers to the relevant Australian standards for details. other corrosive gases Chemical salts. Australia has been fortunate that its fossil fuels have historically been low in sulphur. Higher PRE presents better resistance to pitting and crevice corrosion. 5 METALS OTHER THAN STEEL Section 2 of the standard contains a brief summary of the atmospheric corrosion properties of metals other than steel. The table shows that micro-environmental factors will generally increase corrosivity. farms. the most important forms of corrosion in severe environments. the standard does not accept a numerical relationship between steel corrosion rates and corrosion rate of zinc. regions sheltered from rain washing generally show higher corrosion rates. The standard does not cover the useful ‘Pitting Resistance Equivalent (PRE)’ which can be used to select stainless steels.3 x % Mo + 16 x % N Table 3: Micro-environmental factors influencing corrosivity Feature Micro-environmental • Industrial pollutants • Chemicals • Abrasion and impact Design • Regions sheltered from rain or regular washing • Prolonged surface wetness Cause SO 2 . Table 4: Selection of stainless steels based on PRE number Corrosivity category C1 C2 C3 C4 C5 Corrosivity description Very low Low Medium High Very high PRE <15 15–20 20–25 25–30 >30 Example stainless steels 409. and actual selection requires consideration of factors such as surface finish. farming wastes Wind erosion. handling Effect on corrosivity Increase Increase Increase Build up of salts and Under canopies. farming activities Dusty regions. 304 444. fertilisers. 316 2304 904L. and that generally their corrosion rate will increase as the environment becomes more severe. The PRE formula depends on the amount of chromium (Cr). and furthermore that the influence of such pollutants has dropped over the past 30 or so years [8]. zinc and aluminium. The standard notes that these metals have much lower corrosion rates than steel. The major micro-environmental factors all increase corrosivity. but in regions with little atmospheric contamination (Category C2). for example. traffic. PRE can be estimated using the following formula: PRE = % Cr + 3.the world where it is considered a macro-environmental factor. and example stainless steels. including stainless steel. if not influenced by the other factors. and the standard recommends moving up to at least the next corrosivity category if any of these influence the structure. Corrosivity of the environment is important in selecting such alloys for atmospheric use. 3CR12 430. bridge pollutants soffits Increased time-of-wetness Ponding areas. shaded regions Increase in severe environments Decrease in mild environments Increase Table 4 provides a rough rule-of-thumb giving required PRE. copper. molybdenum (Mo) and nitrogen (N) in the stainless steel. This allows comparison between different grades with respect to pitting and crevice corrosion. However. livestock Examples Around and within fossil fuel burning industries Industrial activities. moving up to the next category will be the usual procedure. for the various corrosivity categories. especially with regard to chloride environments. For other periods of time. If surveys have not been carried out at the site. Even where surveys have been carried out. giving their corrosion rate. or at an analogous site. it is a relatively easy task to determine corrosivity zone. LaQue. considerable variation in results can arise when carrying out surveys at a given site. Some of these variables are described below:  When using steel. The user must be aware that proximity to the sea is critical as indicated in Table 2. the corrosivity zone is C3. Two year corrosion rates are reckoned to be about 20 per cent less than one year rates. use of a low copper alloy steel (known as BISRA steel) overcomes the effects of slight variations in chemical composition of the steel on corrosion rate. and this has been used in most surveys in Australia. such as a list of work carried out in Australia in Appendix C of the standard. These include specimen orientation. the corrosion rate determined with this material needs to be ‘converted’ to an equivalent corrosion rate of mild steel. other estimates must be performed. If. for example. but again there is no accepted figure.1 Where surveys have been carried out. although the borders between them are arbitrary. noting their influence as either major or minor. height above sea level. Firstly. There are many experimental variations which can significantly influence the corrosion rate obtained. concluded: “The factors that influence atmospheric corrosion of test specimens are so many and so diverse that one must conclude that results of tests of this sort can have only a limited quantitative status” [10]. This appendix refers to the recent work of Melchers and Jeffries [9] which showed a variation of corrosion rate from 300 to 600 microns per year at a severe marine site just by varying some of these factors. Surveys are often carried out over periods of time other than one year. The standard contains maps of the Melbourne and Newcastle metropolitan areas. after investigating factors that influence atmospheric corrosivity. the semi-quantitative ISO categories are probably sufficiently accurate to distinguish between significant environmental corrosivity groups. Perhaps the most important change has been a reduction in the amount of atmospheric pollution over the past fifty years STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011   7 . Corrosivity figures from such investigations must be considered only as approximate and small differences are not significant. surface finish and others. It should be noted that the standard is not designed for those carrying out such surveys although it has much useful information. There are two main problems with using experimental results. distance from the sea and corrosivity zone. the user needs to be aware of problems using experimentally determined corrosion rates. These are: (a) (b) Where surveys have been done. However. and the southern region of South Australia. Appendix B of the standard discusses these factors. then the zone can be estimated by analogy. Zones using corrosivity surveys (c) 6.6 HOW THE STANDARD IS USED There are three methods of determining the corrosivity zone for a given site in Australia. but BISRA steel tends to show a rate of 20 to 40 per cent less than mild steel. The second important factor is that corrosivity at a given site can and will change over time. adapted from the CSIRO surveys. according to Table 1. Appendix A of the standard contains a list of 89 locations around the country where surveys have been carried out. the corrosivity zone can be determined from the steel corrosion rate. the one year steel corrosion rate is 40 microns per year at the site of interest then. However. If the site has similar climatic and geographic features (including proximity to the sea) to a site where a survey has been carried out. and a given town or suburb near the coast could be in any one of three different corrosivity categories. then the corrosivity zone will need to be determined from first principles. There is no accepted conversion constant. In the 1994 version of the standard.or so. putting it in the ISO C3 category. The prevailing winds are from the south-east and the temperature. AS/NZS 2312 has generally been the major source of corrosivity information in an Australian standard. that is. but rather a starting point only. The map in the standard is a best estimate based on survey results and estimated behaviour. 8 STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 . Brisbane is rather difficult to predict. It is then best to work through the categories. 6. for example. a conservative approach would be required.3 Zones from first principles If a survey has not been done. the standard quotes the early work of Egan [11] who carried out surveys at various industrial sites in 1971. It is clear that adoption of zones based on corrosion rates has had a major effect on the way that corrosion environments are now recognised. However. noting that the escarpment at the west of the city would be a natural boundary between the C3 and C2 regions where it is relatively close to the coast. a range with steel corrosion rates varying from 25 to 200 microns per year. The crucial factor would be distance from the coast. both largely covered by current C2. but more corrosive than the sheltered Melbourne region. as well as the ISO classification. the map of Perth in the standard is a best estimate from survey results and estimated behaviour. It is less corrosive than regions from the Gold Coast south. the user will need to determine the classification from first principles. Similarly. For example. Many of these sites would be expected to have changed corrosivity.2 Zones by analogy to surveys If an actual survey has not been carried out. C4 and C5 of the current standard. and the appendix contains a table relating classifications in earlier versions of the standard to later versions. For those areas within 50 kilometres of the coast. as it is more humid. the next alternative would be to attempt to find a site or area with similar environmental conditions resulting in a similar corrosivity. For critical applications. working through the zones in a systematic way. 6. and there is no existing site analogous to the desired site. as there is no breaking surf. If the site is more than 50 kilometres from the coast. The main change that has occurred over the years is recognition of the significant variation in corrosivity within a ‘marine’ environment. but against this has the sheltering effect of the Stradbroke and Moreton islands offshore minimising salt deposition. The map of Sydney in the standard has been drawn based on the Newcastle experience. and work down from this until the most reasonable estimate can be made. Egan quotes a steel corrosion rate of 30 microns per year for the Adelaide suburb of Woodville. putting it in the ISO C2 category. the next decision to be made is whether the nearest seas are considered best as rough or quiet. using the more severe category if there is any doubt. and the corrosivity would now be expected to be at a background level. The standard provides some information in Section 4. For example. or Tropical in the northern part of the country. Polluting plants have closed down or reduced emissions. For example. 7 COMPARISON WITH OTHER STANDARDS AND CLASSIFICATIONS Appendix D in the standard is a brief summary of corrosivity information in other Australian standards. the corrosivity is C2. The figures in the standard must not be seen as fixed. there was a mild and a moderate category. Both have since closed. Furthermore. Furthermore. climate changes now believed to be taking place will mean changes to time of wetness. and any sites where pollution was a major contributor to corrosivity will show reduced rates for more recent surveys. it should be possible for the user to come up with a realistic estimate. while a survey carried out by CSIRO only ten years later [12] showed a corrosivity nearly half this for the same suburb. similar to the approach described above. winds and other factors which could see changes to corrosivity of a given site. rainfall and humidity are similar. it is reasonable to assume that places along the coast of NSW from the Victorian border to the Gold Coast will have similar behaviour to that determined at Newcastle. Other places along the east coast could be expected to show similar behaviour. There is a flow chart which should assist. looking at the most severe category first. the Newcastle corrosivity map shows “islands” of high corrosivity around the steelworks and Boolaroo smelter. With the figures obtained from surveys carried out across the continent. there was one ‘marine’ category which would cover corrosivities given in C3. 5* km 0. A summary of the classifications in AS 3600 and related AS 4312 categories is given in Table 5. how it is used. simply and accurately determine corrosivity. and does not recognise the extremely high corrosivity found within a few hundred metres of rough seas. 8 CONCLUSIONS This paper has described the Australian standard on atmospheric corrosivity. It has described the development of the standard. the information contained.1 to 0. can be related to environmental corrosivity. whether the structure is less than or more than one kilometre from the coast. It is expected in the future that this new standard will become the central reference for other standards. such as strength and cover to reinforcement. a high strength concrete with significant cover to reinforcement is required to minimise risk of chloride diffusion through the concrete causing rusting of the reinforcement and spalling of the concrete. with possible serious safety. maintenance and economic consequences. C4. These define various exposure classifications which are intended to relate to required properties of concrete. AS 4312. It also notes that the recent development of standards with corrosion resistance performance requirements.0 km 1. While degradation of concrete does not directly relate to steel corrosivity.5* to 1. such as for lintels [13] and self tapping screws [14]. providing one consistent and up-to-date reference point. It does not distinguish between rough sea and quiet seas.0 to 20/50* km > 50 km (tropical) > 50 km (industrial) > 50 km (temperate) > 50 km (arid) AS 4312 Rough seas within 20/50* kilometres C5M C5M C4 C3 Tropical C2 (C3. It is hoped that the concrete industry will note the content of this new standard and make the required changes. C5 if severe) C2 C2 AS 3600 exposure classification for reinforced concrete B2 B2 B2 B1 B1 B1 A2 A1 Note: * Use the higher figure when winds are strong and/or topography conducive to salt travel inland. C4. In an aggressive marine environment. and how it relates to other Australian standards with discussions on corrosivity. the main reason for this classification is to prevent corrosion of reinforcement. such as anodising and coated steel products. This standard should enable those making decisions on selection of corrosion control strategies for atmospheric exposure to quickly. STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 9 .1 km 0. which is influenced by the same factors as corrosion of steel in the atmosphere. Table 5: Corrosivity category according to distance from shoreline for AS 4312 and AS 3600 Distance from shoreline 0 to 0.3/0.3/0.The Appendix also notes that the classification used in AS/NZS 2312 has generally been adopted by other standards. for example. This shows that the concrete standards consider that there is a need for only two marine environments. 9 ACKNOWLEDGEMENT The author thanks the members of Standards Australia committee MT14/5 for their contributions to the standard. C5 if severe) C2 C2 AS 4312 No rough seas within 20/50* kilometres C4 C3 C3 C2 (T) Tropical C2 (C3. One standard with a corrosivity classification not mentioned in the new standard is AS 3600 [15] and related standards concerning concrete. Ignorance of these facts must mean that many structures in marine environments are either under-designed or over-designed. King.E. Australasian Corrosion Engineering . 1998. AS 3600—2001. ISO 12944. Australasian Corrosion Association. F.A. ‘Corrosion of metals and alloys—Corrosivity of atmospheres—Guiding values for the corrosivity categories’. J. D. ‘Built in components for masonry construction—Lintels and shelf angles (durability requirements)’. G. 11.10 [1] REFERENCES Standards Australia/Standards New Zealand. and Melchers. ‘Corrosion of metals and alloys—Corrosivity of atmospheres—Measurement of pollution’. AS/NZS 2699. Industrie Chimique Belge. ‘Corrosion of metals and alloys—Corrosivity of atmospheres—Determination of corrosion rate of standard specimens’. R. ISO 9223:1992. pp. ‘Self drilling screws for the building and construction industries—Corrosion resistance requirements’. 1981. paper 044. Corrosion & Materials . vol. no. 1177–1185.J. 1964. International Standards Organization. Standards Australia. AS 4312—2008. G. AS 3566. 1971. 15. no. Standards Australia/Standards New Zealand. ‘Concrete structures’. 2001. 6. 8–14 & 22–25. International Standards Organization. 9–16. 1. Hobart. no. R. ‘Precautions in the interpretation of corrosion tests in marine environments’. International Standards Organization. F. 23. Jeffrey. and Duncan.3:2002. and King. no. Standards Australia. pp.2—2002. pp. vol.R. AS/NZS 2312:2002. vol. pp. Corrosion and Prevention 2006. ‘Some apparent limitations in using the ISO atmospheric corrosivity categories’.J. ‘Industrial pollution and its impact on corrosion and corrosion mitigation practices’. LaQue. ISO 9225:1992. 6. International Standards Organization. Standards Australia. Bartlett. 2006. Corrosion Australasia . Corrosion and Prevention 2001 . [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] 10 STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 . ISO 9226:1992. Martin.2:1998.G. 10–15. ‘Effect of environmental factors on the corrosion of steels’. Australasian Corrosion Association.L. ‘Corrosion of metals and alloys—Corrosivity of atmospheres—Classification’. 4. ‘Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings’. ‘Atmospheric corrosivity zones in Australia’. International Standards Organization. ‘Paints and varnishes—Corrosion protection of steel structures by protective paint systems—Part 2: Classification of environments’. ‘Corrosivity measurements at some Australian cities’. ‘Early observations of corrosion losses for steels at a severe marine atmospheric site’. paper 028. Newcastle. ISO 9224:1992. Egan. K.A. so mistakes can be made. we were still seeing coating specifications for structural steel along the lines of ‘wire brush the surface and apply two coats of good quality paint’. we have environmental and health and safety regulations to obey and coating contractors and paint company representatives to deal with.PRODUCING COATING SPECIFICATIONS THAT WORK by R A Francis Aurecon. little advancement over that used by Noah. This paper describes some of the factors that the specifier must consider when selecting coatings to protect steelwork. This standard uses the ISO corrosivity categories which are becoming widely used around the world for selecting methods of corrosion control for structures and other items exposed to the atmosphere. Recently. Indeed. maximum durability. Victoria. We require coatings to withstand environments other than heavy rainfall and wear and tear by animals. Unlike Noah. that is. and that document should be an essential reference for anyone selecting and specifying coatings for steel in our part of the world. A coating which lasts a few months in a severe marine environment may last decades in a mild environment. Genesis 6:14 Noah’s ark may be the earliest example of the selection and written specification of a quality coating and there is no doubt that the resultant coating performed the task required of it. In addition. and some typical specification clauses. a new standard [3] has provided more details on determining local corrosivity in Australia. A good coating specification is essential if steel structures are to continue to function as designed. South Melbourne. Perhaps most importantly. We require coatings to last longer than 40 days and 40 nights. conditions are rather different 6000 years on. until relatively recently. Australia God said to Noah: make yourself an ark . However. The important environmental factors influencing corrosion in atmospheric environments were discussed in an earlier paper [2]. It follows the content of AS/NZS 2312 [1]. the Almighty generally is not called upon to select coatings and write the specification. The corrosion categories are summarised in Table 1. STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 11 . However. . there are many other factors that must be considered and some of these are described below. (a) Environment The environment is perhaps the single most important factor affecting durability of a coating system. acidic environments can be corrosive to zinc coating systems which have excellent durability in neutral environments. 1 FACTORS INFLUENCING COATING SELECTION The most important characteristic of a coating system for protecting a steel structure against corrosion would normally be the ability to provide protection to the substrate for as long as possible. including provision of corrosion maps of major centres. . and coat it with pitch inside and out. provide visual impact. Shop or site application Many structures are fabricated and coated in a shop. there are many situations where colour or gloss or both are required and such coatings are not appropriate. 2 CONTENT OF A COATING SPECIFICATION Before selecting coatings and looking at some individual clauses in a specification. such as a list of contractor submittals. then only a limited range of products is available. (d) Surface preparation and coating application methods Most of the best quality primers must be applied to a blast cleaned surface. etc Areas which become inaccessible can be coated Inspection is easier and more thorough There will not be problems working around other trades. etc and gloss may enhance public image. The range of coatings that can be site-applied is less than for those designed for shop application. Similarly. requirements for a kick-off meeting.3–25 25–50 50–80 80–200 80–200 – Typical environment dry indoors arid/urban inland coastal or mild industrial sea-shore (calm) sea-shore (surf) severe industrial inland tropical (b) Colour and gloss Many of the most corrosion resistant coatings. Such an approach will normally provide optimum durability because of the following advantages:      Coating is carried out under controlled conditions meaning less contamination from smoke. many modern high performance coatings must be applied by spray. brush or roller can only be used for small areas with such coatings. If blast cleaning is not permitted for environmental or other reasons. At best. concerns with overspray or other reasons. 12 STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 . galvanizing and inorganic zincs are only available in matt grey. Section 7 of AS/NZS 2312:2002 covers these and other issues in more detail. with only some touch up required on site. it is necessary to present an overview of the content of a typical coating specification. Some coatings. coatings would normally be applied in a shop. then selection will be from a limited number of coating products. If spraying is not permitted for environmental or OH&S regulations. There may be other more general sections. Even when considering coatings applied to minimise the effects of corrosion. visibility. heat reflection. etc. dust. such as metal spray.3 1. In such cases. such as galvanizing. The following are sections which would normally be present in a typical protective paint specification. The specifier must review these before selecting coatings. transport arrangements for shopapplied coatings and warranty and guarantee requirements. taken to site and erected.Table 1: Corrosivity categories according to AS 4312 and AS/NZS 2312 AS 4312 category C1 C2 C3 C4 C5M C5I T AS/NZS 2312 category A B C D E-M E-I F Corrosivity very low low medium high very high – marine very high – industrial – Steel corrosion rate (µm/yr) <1. Some reasons for requiring colour. The main concerns with shop-applied coatings are that the item must be small enough to be able to be transported and there will always be a problem with site touch up. include:     (c) Colour Colour Colour Colour is often necessary for logos and identification can brighten dark areas and hide ugly areas is required for safety. even for nominally protective coatings. are only applied in a shop. so fewer ‘extras’ Coating should not be affected by bad weather. Rather than discussing durability in all environments as given in [1]. This includes major centres such as Sydney. friction grip surfaces. This provides details of how the coatings should be applied. These are selected for very severe environments where long life is required and the high cost can be justified. It is normally produced after the document is completed. users or other standards. This provides a concise but accurate description of the items that will be coated. such as ultra high build epoxy. References. Sites further inland will have proportionally greater durability and sites more corrosive will have reduced durability compared to the figures given. This environment is found from approximately 0. Some covered in AS/NZS 2312 include:   Metal or thermal spray zinc or aluminium systems. 3. Wollongong. Newcastle and the Gold Coast. Such corrosivity is also found on sheltered bays. This lists the coatings used in the selected coating system. This section will provide a list of standards and other documents referred to in the specification. 7. the life to first maintenance is given only for the ‘C4: High’ corrosivity (AS/NZS 2312 Category D) environment. 3 COATING SYSTEMS FOR STRUCTURAL STEEL There is an almost infinite number of combinations of primer. It should also contain items that will not be coated. Inspection requirements. such as Melbourne or Brisbane right on the coast. 4. This provides detailed requirements regarding surface preparation requirements for the selected coating system. Very thick coatings. and the factors that must be considered if they are selected. Chlorinated rubber coatings have good durability and are easy to maintain. such as flange faces. Coating application. These are generally used for very severe environments such as splash zones or severe chemical exposure. They would normally only be used for mild environments. Scope. along with 48 additional systems for other environments. 5. Continuously galvanized or electro-galvanized products that have significantly thinner zinc coatings than that achieved with hot dip galvanizing. They would not normally be specified for atmospheric environments. often one-twentieth or less. The section may also contain some general clauses. 2. 4 HOT DIP GALVANIZING Zinc metal is perhaps the most important weapon in the fight against corrosion. polyester or vinyl ester. This section contains important definitions and abbreviations that may need clarifying or defining for those outside the coatings industry. intermediate and top coats. along with variations in thickness. Therefore. 6. Surface preparation. but their high solvent content means they have very limited availability.   The durability of the systems as given in [1] is provided in the discussion. is clearly not possible. There are many other coating systems in addition to those described below. selecting an optimum coating system is not easy. Definitions and abbreviations. This lists the tests required for surface preparation and coating application and the required records. and proportionally lower durability. A full discussion of all these. etc. The following sections describe some commonly recommended atmospheric coating systems. Coating materials. pigment types and application properties. let alone the many other systems which may be suggested by suppliers. It corrodes at a much lower rate than steel. AS/NZS 2312 has about 70 metallic and paint coating systems for atmospheric exposure. This is the life to first major maintenance.1. such as most sites along the east coast of Australia from the Victorian border to Fraser Island.1 to 1 kilometre inland from a surf beach. noting that normally some minor touch up will be required before this time. so an intact coating of reasonable thickness will provide good STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 13 . Different batches of the same grade of steel can vary in silicon content giving different appearance and. different zinc thicknesses. The other important variable is the silicon content of the steel.2 of AS/NZS 2312:2002. It has many advantages over paint coatings. and these do not have to be separately specified. It can provide complete protection to complex shapes.2% gives a bright. so called duplex systems. However. The heat of the molten zinc can cause distortion of complex shapes. at additional cost. zinc coatings last a long time and when they do break down. Unlike most paints. it has excellent adhesion and is highly abrasion resistant. A comprehensive Australian standard [4] is available and a simple sentence on a drawing along the lines of ‘Hot dip galvanize the item to AS/NZS 4680’ is often sufficient. edges have good thick coating coverage and no cure time is required. note that the quality of the finish permitted in AS/NZS 4680 is pretty ordinary (for example. bare spots up to 40 square centimetres area are permitted) and architectural finishes will require details of the required finish. More importantly. If a thicker than normal galvanized coating is required. but if colour is required paint systems should be specified. the longer it takes to heat up resulting in a thicker coating). protecting it by cathodic protection. the specifier should communicate requirements to the galvanizer. However.durability to steelwork except in the most aggressive environments. The HDG industry usually designates coating thickness in terms of grams per square metre rather than microns (100 grams per square metre is 14 microns). The durability of HDG in various corrosivity zones is given in Table 5. showing very good durability. they are only available in grey colour and are not suitable in acidic environments. reducing the need for maintenance. The rough finish is unlikely to be acceptable in architectural applications. etc AS/NZS 2312 C4 durability (years) 5 – 15 10 – 25 15 – 25 25+ AS/NZS 2312 lists a number painted HDG systems. The galvanizer has little control over zinc thickness. they do not blister or undercut. This standard covers surface preparation. Longer residence times and abrasive blast cleaning. The single most important coating for protecting steel against corrosion is probably hot dip galvanizing (HDG). it is only a factory-applied process and there is a limit to the size of an item which can be galvanized.5 to 3 3 to 6 >6 >6. Specifiers cannot request a zinc coating thickness. may be required if the section thickness and silicon content are not adequate. the zinc will corrode in preference to the steel. Thickness depends mainly on steel section thickness (the thicker the section. Surface treatments needed to achieve long-term adhesion appear somewhat arbitrary and painting galvanizing is not worth the risk. Table 2: Hot dip galvanizing systems System HDG390 HDG500 HDG600 HDG900 Average coating thickness (µm) 55 70 85 125 Article thickness (mm) 1. high Si. coating the steel by dipping in a bath of molten zinc after thorough cleaning. summarised for the C4 environment in Table 2. galvanizing and inspection. It is process rather than operator controlled. Galvanizing alone provides very good durability. This process. As a result. A big advantage to the specifier with HDG is the ease of specification. and touch up and repair will always be visible and usually of reduced durability. thicker coating. more importantly. A silicon content of around 0. Along with other zinc coatings. and vent and drain holes may be required. but thin coating while silicon contents less than or greater than this give a dull grey. and inspection and quality assurance requirements are normally much simpler. there have also been many examples where the paint has severely disbonded from the galvanizing after only a few years. 14 STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 . Painting the galvanizing provides colour as well as additional durability. has been successfully protecting steel for over 100 years. if the coating is damaged exposing steel. While there have been many successful examples of painted galvanizing performing well. but should be avoided in marine environments. IZS should be considered as a complementary coating system to galvanizing [5]. it is hard and tough and provides cathodic protection if damaged. water-borne and solvent-borne. A relatively thin coating of the order of 70 to 100 microns can provide better protection in severe environments than many organic coating systems two or three times as thick. either epoxy zinc or inorganic zinc. STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 15 . the most widely specified systems include a top coat of polyurethane.15 Type 4 which covers properties such as minimum zinc content. acceptable proprietary products would normally be listed. adhesion and toughness. Polyurethane provides excellent gloss. These systems are listed in Table 3. but selection should really be on prevailing environmental conditions. colour and durability. Variations to this system are shown in Figure 1. Table 3: Inorganic zinc silicate coating systems System IZS1 IZS2 IZS3 AS 4848. so normally require topcoating. but chalk. IZS2 and IZS3 are water-borne. although the durability of the solvent-borne system is rather conservative. Mid coats are based on epoxies which also have excellent durability. Although the standard requires the product to meet the requirements of AS 3750. it can be used on items of any size and applied on site as well as in a shop. indicating the sort of decisions that the specifier must make. The water-borne is considered to have slightly better durability. required durability. There are a number of possible systems. Although it is often considered as a competitor. depending on environment. water-borne would be selected for application under dry conditions. For most new work. slightly higher than IZS1 so with durability probably closer to IZS2. it continues to harden and cure over time.1 Surface preparation Sa2½ Sa2½ Sa2½ Sa2½ Coating 75 µm solvent-borne IZS 75 µm water-borne IZS 125 µm water-borne IZS 100 µm solvent-borne IZS AS/NZS 2312 C4 durability (years) 5 – 10 15 – 25 25+ – 6 COLOUR (POLYURETHANE) COATING SYSTEMS Where colour is required. Some coating suppliers provide cheaper. Generally.5 INORGANIC ZINC SILICATE SYSTEMS Inorganic zinc silicate (IZS) is one of the best liquid coatings that can be used to protect steelwork. low zinc products which are acceptable for interior and mild environments. Primers can be zinc-rich. There are two main types of inorganic zincs. and it becomes denser and more protective. is a widely-specified system. Inorganic zinc is different from other paint coatings in that. ideal for most environments. It can only be applied to properly blast cleaned steel and application and curing can be challenging. whether it is maintenance or new work and whether there are restrictions on blasting or spraying. AS 4848. application of a single coat of solvent-borne inorganic zinc can be specified by a single sentence such as: ‘Apply a single coat of solvent-borne inorganic zinc according to AS 4848. windy conditions. but is a little more forgiving in application. The water-borne is harder and faster drying. designated PUR in AS/NZS 2312.1 [6] covers surface preparation and application of a solvent-borne system to a minimum thickness of 100 microns. despite initially producing a porous film. Like galvanizing. the solvent-borne under more humid conditions. but must be applied under dry.1’ Again. It is the only paint coating that can be used for friction grip joints. or zinc-free epoxy. PUR4 consisting of 75 microns of zinc-rich primer. Unlike galvanizing. this covers surface preparation and inspection so these do not have to be separately specified. AS/NZS 2312 has three IZS systems: IZS1 is a solvent-borne system applied to 75 microns. The solvent-borne must have a certain minimum humidity to cure. As with hot dip galvanizing. 125 microns of epoxy mid coat and 50 microns polyurethane top coat. If spraying cannot be used.PUR2 Sa2½ 75 Epoxy primer 50 PU No zinc Lower cost Two coat PUR2a* Sa2½ 75 Zinc primer 50 PU Maintain Can’t blast Two coat No zinc PUR1 St2 125 Epoxy mastic 50 PU Lower cost.4 [7] Class Sa2½ with a zinc-rich primer. As epoxies are easier to apply and top coat. Two coat PUR7 Sa2½ 75 Zinc primer 75 HB epoxy 75 HB PU Can’t spray PUR4 Sa2½ 75 Zinc primer 125 HB epoxy 50 PU Maintain Can’t blast. Either inorganic zinc or epoxy zinc could be specified.  16 STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 . in a multi-coat system. and do not have curing limitations of inorganic zincs. Epoxy mastic would also be the primer specified for spot repair. again to 75 microns. If use of zinc is restricted. there is little difference between performance of epoxy zincs and inorganic zincs. hand or power tool cleaning to ISO 8501-1 [8] St2 is specified. although this can be increased to 200 microns for very severe environments. but recent findings indicate that. Three coats will always provide better protection than two as there is better coverage of critical areas such as edges and corners. such as in some refineries or if the environment is acidic. A mid coat of 125 microns of standard epoxy is normally sufficient. Such primers may contain zinc phosphate inhibitive pigment or no corrosion resistant pigments. Mid or intermediate coats: A mid coat is normally applied to build up thickness and ensure good coverage. for shorter life or to reduce costs. Can’t spray No zinc PUR6 St 2 75 Epoxy mastic 75 HB epoxy 75 HB PU No zinc Higher cost & durability PUR3 Sa2½ 75 Epoxy primer 125 HB epoxy 50 PU PUR5 Sa2½ 75 Zinc primer 200 HB epoxy 50 PU Notes: (1) PU = polyurethane (2) HB = High build (3) Numbers indicate nominal thickness in microns (4) * PUR2a is not listed in AS/NZS 2312 Figure 1: Relationship between polyurethane topcoat systems in AS/NZS 2312 Looking at the stages in the various systems:  Surface preparation and priming: For most new work. 75 microns if brush application is required (primers should not be applied by roller). even if spot preparation can be carried out by blast cleaning. If blast cleaning cannot be carried out. A mid coat is not required in less severe environments. Epoxy mastic is normally applied to a minimum dry film thickness of 125 microns if applied by spray. the surface would be blast cleaned to AS 1627. they would normally be recommended. a zinc-free epoxy primer would be specified. the mid coat would be specified to 75 microns. as it is likely to be compatible with existing weathered coatings. noting its reduced durability because of limited practical experience. Requires blasting and spray application. zinc-rich primers provide far better protection to edges and damaged areas and should be specified wherever possible in atmospheric applications. There is no free isocyanate in cured product and such coatings have no known hazard once the coating is cured. and painters must be careful to minimise exposure. However. their long term performance has been disappointing. high build polyurethanes which can be applied up to 75 microns by brush or roller are preferred. free isocyanate in the curing agent is a hazardous material. Where spraying is not possible. The top coat is normally specified as 50 microns. such coatings can be safely applied. they do not have the long-term experience of the polyurethanes and are more expensive. Similar to PUR4 but for brush/ roller application. PUR6 St2 75 µm spot epoxy mastic 75 µm zinc primer 75 µm epoxy primer 75 µm HB epoxy 75 µm HB polyurethane 225 2–5 For repair of above systems. as a new coating. Generally. Requires blasting and spray application. blasting rather than hand or power tool cleaning should be used and three coat systems are better than two coat systems. health concerns have led to a desire for safer coatings. However. STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 17 . However. during application. 125 µm epoxy mastic 50 µm polyurethane For repair of above systems. coating suppliers now recommend that this product simply replace polyurethane as a 50 or 75 micron coating in a system such as PUR4 or PUR5. Most of the polyurethane systems described above have catalysed acrylic equivalents in AS/NZS 2312 (the ACC systems) but there is little reason to consider them. One problem with polyurethane is that. Brush or roller application. Mid coat could be epoxy mastic. showing reduced durability and greater chalking than the polyurethane equivalents. for best durability. Table 4: Polyurethane topcoat systems Ref No. they are difficult to apply with problems such as very fast drying (although slow curing) and issues with intercoat adhesion. *Not listed in AS/NZS 2312 Lower durability version of PUR3. For new work or where full removal of existing coating is required. Very good durability. but for some colours a second coat is required for opacity. Surface preparaation Sa2½ Sa2½ Sa2½ Sa2½ First coat Second coat Third coat Total thickness (µm) 325 250 250 225 C4 Durability (years) 15 – 25 10 – 15 10 – 15 5 – 10 PUR5 PUR4 PUR3 PUR7 75 µm zinc primer 75 µm zinc primer 75 µm epoxy primer 75 µm zinc primer 200 µm high build epoxy 50 µm polyurethane 125 µm high build epoxy 50 µm polyurethane 125 µm high build epoxy 50 µm polyurethane 75 µm HB epoxy 75 µm HB polyurethane Excellent durability. Requires blasting and spray application. no blasting but spraying allowed. It is a two coat system with a polysiloxane top coat of 125 microns over 75 microns of zinc rich primer. colour and gloss. However. The industry is well aware of the OH&S issues and with proper ventilation and the correct use of personal protective equipment. Non zinc primer for acidic conditions. The latest top coat technology is the polysiloxane coating. Table 4 lists the polyurethane systems and gives relative durability of each. Although not indicated by the durability figures. PUR2a* Sa2½ PUR2 PUR1 Sa2½ St2 50 µm polyurethane 50 µm polyurethane 125 125 175 – 5 – 10 2–5 Lower durability version of PUR4. or restrictions on zinc. Catalysed or two-pack acrylics were introduced in the 1980s as an alternative isocyanate-free coating and became popular in Australia. and have shown excellent performance over many years. Furthermore. AS/NZS 2312 only lists one polysiloxane system. 7 ALTERNATIVE COLOUR COATING SYSTEMS Polyurethane top coat systems are widely specified and used. Top coats: A thin top coat of polyurethane is applied to provide colour and gloss retention as epoxies exposed to atmospheric environments tend to chalk. which appears to have even better durability and gloss retention than polyurethanes. colour and gloss. However. Sa2½ 75 µm epoxy primer or 200 µm high build epoxy 275 zinc primer High durability. as measured by replica tape (Method A in AS/NZS 3894 Part 5). Notes: (1) NR = Not recommended (2) WB IZS = Water-borne inorganic zinc silicate. at additional cost. Note this system has lower durability than 75 microns of water-based inorganic zinc by itself (IZS2). A typical specification clause for blast cleaning is along the lines of: ‘ The surface shall be blast cleaned with steel grit or garnet to AS 1627 Part 4 Class Sa2½ with an angular profile between 40 microns and 75 microns. C4 Durability (years) NR 5 – 10 10 – 15 ALK1 ALC2 EHB3 EHB4 EHB6 10 – 25 8 SURFACE PREPARATION Selection of the optimum coating system is crucial. are easier to clean and do not lose thickness from chalking. ease of maintenance and good economy as well as good durability. Although epoxies chalk and discolour with exposure to UV. This flaky pigment adds to moisture resistance. two coat system where appearance not critical. It should be stressed that. Requires blasting and spraying. The top coat is purely for decorative purposes and actually inhibits the long term protection that can be achieved by uncoated inorganic zincs. appearance and colour retention are not critical requirements. Surface First coat Second coat Third coat Total preparthickness ation (µm) St2 40 µm alkyd primer 40 Simple system for use in a non corrosive environment. Requires blasting and spraying. they can be used without a top coat. These are easy to apply. Sa2½ 75 µm WB IZS primer 40 µm acrylic latex 40 µm acrylic latex 155 Water based system with reasonable durability. One system which does have good durability is a single coat of acrylic latex over 75 microns of water-based inorganic zinc silicate over a blast cleaned surface (ACL2). their durability is considerably reduced compared with the above systems. Requires blasting and spraying. provides additional protection against UV light and gives a lustrous metallic finish. A single coat of alkyd primer (ALK1) may be all that is required for internal steel beams which are never exposed to the weather. and they are rarely specified for structural steelwork except in mild environments. but the specification must make the surface preparation requirements clear. The AS/NZS 2312 systems covered in this section are summarised in Table 5. However. EHB6 is a two-coat MIO epoxy systems with very good durability. Table 5: Other Recommended AS/NZS 2312 Colour Paint Systems Ref No.Accelerated testing suggests such systems should give better durability and gloss retention than the polyurethane equivalents. or indeed as a top coat where colour and gloss retention are not critical. They have ease of application. AS/NZS 2312 also lists single pack alkyd (ALK) and water-borne acrylic latex (ACL) systems. The ultimate epoxy atmospheric systems use micaceous iron oxide (MIO) pigment in the top or intermediate coat or both. High build epoxies applied up to 200 microns or more over 75 microns of zinc-rich (EHB4) or zinc-free primer (EHB3) over blast cleaned steel can provide an economic coating system with good durability. but one is usually sufficient unless colour has poor opacity. This is a water-based system so can meet the low Volatile Organic Compound (VOC) requirements of ‘Green Star’ buildings without compromising durability. relatively cheap and come in a wide range of colours. even where gloss. ACL2 does require 2 coats. it can be difficult to apply and requires low humidity and windy conditions for proper curing. Sa2½ 75 µm zinc primer 125 µm epoxy MIO 125 µm epoxy MIO 325 Very high durability system where a MIO finish is acceptable. especially when blast cleaning is required. polyurethane systems are often specified as they hold less dirt.’ 18 STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 . Issues of concern to the specifier include:  Blast cleaning for atmospheric work is normally specified to Class Sa2½.2.6 of [1] provides a summary of some of the issues. such as salts.’ STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 19 . A completely clean surface (Class Sa3 or white metal) is normally only required for critical applications such as tank or pipeline linings. but three important aspects will be reviewed. grease and related contamination. These do not interfere with coating adhesion or durability. The manufacturer’s data sheet will give cleanliness requirements. Surface profile is a measure of the height of the peaks to the valleys of the blasted surface. this is normally only required for maintenance work in severe environments where salt contamination has been a problem. handling. Class Sa2 leaves quite a bit of contamination and may be suitable for mild environments and less exacting coatings. have environmental and OH&S issues as they may contain heavy metals. underground or underwater structures or similar environments. However. Most of the high durability coating systems described above must be carefully applied by skilled applicators to properly prepared surfaces. removal of dust after blasting and the time limit between blasting and priming will also require specification. but Class Sa2½ should be specified. but is still used in many other countries. This is almost completely clean but a few stains of adherent contamination are allowed. The profile must be jagged or angular for optimum adhesion. ISO 8501-1 also contains descriptions of the classes of blast and is sometimes used in specifications. If not given in the data sheet. Table 6 shows typical profiles for atmospheric coating systems. INSPECTION AND QUALITY ASSURANCE The best coating system is wasted if it is not properly applied. and rounded profile produced by shot is not normally acceptable. A profile range. Clauses on removal of oil. are sometimes specified. The clause limits abrasives to steel grit or garnet. A full discussion of items requiring specification is outside the scope of this paper. However. but replica tape is most accurate and the only method that provides a hard copy for QA purposes. even if Sa2 is permitted. removal of fabrication defects. very roughly related to coating thickness is required for most heavy duty coatings. It is not normally required for new work.Such a clause covers two main requirements: the visual cleanliness and the surface profile (or anchor pattern). Garnet can be used in the field as well as in shops. There are a number of other abrasives available. and catalysed coatings shall be used prior to expiration of pot life. but is not specified for lesser cleanliness grades such as hand or power tool cleaned surfaces. such as some slag. There are a number of methods for measuring profile. Section 4. thinning and application of all materials shall be in accordance with the manufacturer’s recommendations. Sand as an abrasive was banned in Australia many years ago as it can cause silicosis. Steel grit is only used in shops where it is cleaned and recycled. Garnet and steel grit are both effective. There are conflicts in the industry over acceptable levels. 9 APPLICATION. ‘Storage. Table 6: Typical profile requirements for different primer coating types Primer coating type Epoxy (zinc) primer Inorganic Zinc High Build Epoxy Nominal DFT (microns) 75 to 100 75 to 125 150 to 250 Recommended profile (microns) 30 – 50 30 – 50 50 – 75   Limits on non visible contamination. never specify a lesser standard than permitted by the manufacturer. All coatings shall be used prior to expiration of shelf life. rough surface with minimal environmental impact. methods of test and the effect of subsequent coatings. low dust abrasives and will provide a clean. surface preparation and application are more critical than the coatings used. In some ways. A lesser standard of cleaning. mixing. sometimes called “near white”. but some. Profile is not related to cleanliness. 2-10. a clause such as this is normally a mandatory part of any coating specification. for example you may require that the paint is sprayed for good finish. They may not include all temperature and humidity restrictions that are desirable. AS 4312—2008. vol. good QA processes and satisfied customers.A. AS/NZS 4680:2006. A freshly blasted surface may rust if exposed to high humidity or dew point conditions. 32. humidity and dew point must be monitored and controlled during surface preparation and coating application. the surface temperature less than three degrees above the dew point or under other unfavourable weather conditions. most paints will dry too slowly or too quickly if applied at temperatures which are too low or too high. the coating shall not be applied if the ambient temperature is below 10° C or surface temperature above 45° C. The Painting Contractor Certification Program (PCCP) [9] is a scheme that accredits contractors who can demonstrate that they can meet certain minimum performance requirements. ‘Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings’. vol. The manufacturer’s data sheet should give this information and Table 8. Francis.A. Data sheets rarely specify the requirement for stripe coats on edges. 1996. ‘Atmospheric corrosivity zones in Australia’. Checking on these is not easy. Class 5 covers removal of hazardous material such as lead paint. pp. 3.’ Good quality coating contractors are essential for any coating work. whereas the data sheet may allow any method of application. welds and other critical regions. pp. no. [2] [3] [4] [5] 20 STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 . 11 [1] REFERENCES Standards Australia/Standards New Zealand. and such certification is normally mandatory for such work. R. well-maintained equipment. ‘An update on the corrosion process and protection of structural steelwork’. 2-11. 3. Classes 1 to 4 cover application of coatings of increasing complexity in a shop or on site. You may wish to override some TDS requirements. ‘Preference shall be given to contractors who are registered with the Painting Contractor’s Certification Program (PCCP). ‘Hot-dip galvanized (zinc) coatings on fabricated ferrous articles’. no. Steel Construction. ‘No surface preparation or coating application shall take place if the relative humidity is greater than 85%. ‘Inorganic zinc or galvanizing: Choosing the ideal corrosion protection for structural steel’. In addition. Standards Australia. but making sure such contractors are selected is not easy.A clause such as this means that you do not have to specify all these requirements. 10 CONCLUSIONS Structural steel will usually require a protective coating system if it is to provide years of good service. 1998. A freshly painted surface will more often than not be damaged by the same conditions. 30. Steel Construction .1 in [1] provides useful information. Good contractors will have trained workers. Following such advice should enable steel structures to remain corrosion-free for many years. This paper has provided:    Some of the factors that must be considered when selecting a protective coating system Some recommended coating systems from AS/NZS 2312 Some typical clauses from a protective coating specification. Restrictions such as these would normally be included in most specifications. AS/NZS 2312:2002. In addition. Standards Australia/Standards New Zealand. among many other qualities. R. The manufacturer’s technical data sheet (TDS) then becomes part of the specification. unless the work is well protected from such conditions.’ Temperature. However. Francis. There are five classes of certification. ‘Metal finishing—Preparation and pretreatment of surfaces— Abrasive blast cleaning of steel’. Standards Australia.apas.au/pccp/ [7] [8] [9] STEEL CONSTRUCTION VOLUME 45 NUMBER 1 – DECEMBER 2011 21 . http://www. ‘Application specifications for coating systems.[6] Standards Australia. ‘Preparation of steel substrates before application of paints and related products—Visual assessment of surface cleanliness—Part 1: Rust grades and preparation grades of uncoated steel substrates and of steel substrates after overall removal of previous coatings’. AS 1627. Part 1: Single coat inorganic (ethyl) zinc silicate—Solvent-borne’. International Standards Organization. AS 4848.1—2006.gov. ISO 8501-1:2007.4—2005. Bisalloy +64 9 525 9414 GB Galvanizing Service www.au OneSteel Australian Tube Mills www.brice.onesteel.au 08 9551 6666 Fletcher Building www.com. 4 Queen Street Bentley WA 6102 PDC Global Pty Ltd 48 Kishorn Road Applecross WA 6153 Universal Drafting Pty Ltd Suite 2.donhad.pacificsteel. 16-22 Bremner Road Rothwell QLD 4022 Innovative Steel Detailing Suite 10.rigbyjones.au +64 6 356 8253 Korvest Galvanisers www.grahamgroup.com/apac Dexion Australia www.au BlueScope Steel www.com Brice Metals www.au Midalia Steel www.au National Galvanizing Industries Pty Ltd www.au Aus Steel www.com.com.au 03 8368 1555 03 9746 9193 22 STEEL CONSTRUCTION VOLUME 45 NUMBER 1 .austubemills.com.bluescopesteel.au Litesteel Technologies www.com Orrcon Pty Ltd www.au Metalcorp Steel www.ingal.au Pacific Steel Group www.kingfieldgalvanizing. 68 Jessica Blv Minyama QLD 4575 Steelcad Drafting Pty Ltd 8/63 Annerley Road Woolloongabba QLD 4102 The Drawing Office 88 Springfield Road Maryborough QLD 4650 Time Line Drafting Suite 1.com.rondo.A.metalcorpsteel. 8 Hasler Road Osborne Park WA 6017 Westplan Drafting Unit 3/11 Robinson Road Rockingham WA 6168 NEW ZEALAND Steel Pencil 554 Main Street Palmerston North 4410 Fielders Steel Roofing www.com.au Ferrocut www.com Metaland www.com.com. 39 Stanley Street Bankstown NSW 2200 02 9708 6500 Enterprise Drafting Company (EDC) 982 Hunter Street Newcastle West NSW 2302 02 4927 6969 Multicad Pty Ltd Shop 14.au Menghello Galvanizing www.au Industrial Galvanizing Corporation www.metaland.onesteel. 104-106 Ferntree Gully Road Oakleigh VIC 3166 03 9544 3877 Engineering Design Resource 68 Hotham Street Traralgon VIC 3844 Fabcad Drafting Suite 3 59 Church Street Morwell VIC 3840 G.onesteel.bisalloy.com.com.com/litesteel Macrack www.com.nz Premier Steel www.ferrocut.com Fletcher Insulation Group www.M.natgalv.midaliasteel. 231 Margaret Street Toowoomba QLD 4350 SOUTH AUSTRALIA Australian Steel Detailers 3/ 147 Goodwood Road Goodwood SA 5034 VICTORIA Balpara Pty Ltd Unit 8.au BlueScope Lysaght www.com. Steel 557 Mount Derrimut Road Derrimut VIC 3030 Hybrid Steel Engineering 48 Wilson Road Melton VIC 3338 PlanIT Design Group Unit 2A 14-16 Garden Boulevard Dingley VIC 3172 CADstruction Drafting Suite 4.co.aussteel.au Intercast and Forge www.hartway.au Atlas Steel www.com.com.com.au Graham Group (NSW) www.au Kingspan Insulated Panels www. Annett’s Arcade 9-11 Orient Street Batemans Bay NSW 2536 QUEENSLAND BDS VirCon 80 Tribune Street South Brisbane QLD 4101 Brice Engineers Pty Ltd 7-8 Brice Court Mt Louisa QLD 4814 Cubic Steel Pty Ltd 11/79 Lawson Street Brisbane QLD 4170 Draftology Pty Ltd Suite 11.dexion.orrcon. Level 3 445 Upper Edward Street Spring Hill QLD 4004 Hempsall Steel Detailing Pty Ltd Unit 7.DECEMBER 2011 . First Floor 896 Albany Highway East Victoria Park WA 6101 Formation Design Systems Suite B.horan.au Horan Steel www.com OneSteel Limited www.au Commando www.meneghello.arcreo.au Rondo Building Services www.kingspan.au 08 9592 2499 Kingfield Galvanising www.com.cmcaustralia.ASI STEEL DETAILER MEMBERS NEW SOUTH WALES Elmasry Steel Design and Detailing Suite 3. 1A Pakenham Street Fremantle WA 6160 Multiplan Drafting Pty Ltd Unit 12.com.au Mantamesh/ Delta Shelving www.au 03 5173 7600 03 5133 0733 Dematic www.com OneSteel Market Mills www.com.au Hartway Galvanizers www.ingaleps.au 07 5444 7600 07 3844 3955 07 4121 4321 07 4659 8633 Bisalloy Steels www.au Ingal EPS www.intercast.tasmaninsulation.com.au Australian Reinforcing Company (ARC) www.com Fletcher Challenge Steel .au WESTERN AUSTRALIA 08 9472 7457 08 9335 1522 02 4472 1611 08 9356 5993 07 3503 5800 08 9315 6600 07 4774 8322 08 9242 8944 07 3399 5164 07 3831 3775 07 3204 1054 ASI STEEL MANUFACTURER.apcgroup.fletcherbuilding.fielders.com.gbgalv.com.com.macrack.au Donhad www.com Molnar Engineering www.molnarhoists.com 08 8271 6555 BlueScope Distribution www.com.korvest.atlassteels.au CMC Coil Steels www.com.net. 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Ascot Place Belmont WA 6104 Fitti Steel Fabrication 11 Erceg Road Yangebup WA 6965 Fremantle Steel Fabrication Co. 99 Mount Street.org.au Website steel.au . NSW 2060 Phone (02) 9931 6666 Facsimile (02) 9931 6633 Email enquiries@steel. North Sydney.Level 13.org.