8/19/13Flow-Accelerated Corrosion: A Critical Issue Revisited - Print this page Close Flow-Accelerated Corrosion: A Critical Issue Revisited By Brad Buecker, Contributing Editor Sudden failures of high-pressure, high-temperature feedwater piping by flow-accelerated-corrosion (FAC) continue to claim lives in the utility industry, which is, of course, the ultimate catastrophe. The conventional condensate/feedwater treatment of oxygen-scavenging/pH-conditioning, used for many years by utilities throughout the country, has proven to be the culprit. The reducing environment generated by this chemistry will induce FAC, where gradual thinning of pipe or tube walls in a very localized area leads to sudden and catastrophic failure. The following discussion of FAC, and methods to prevent it, is taken from a seminar given at the 2007 Electric Utility Chemistry Workshop.1 Fundamental Feedwater Chemistry Mild steel is the universal material for condensate/feedwater piping as it offers excellent strength at economic value. However, iron is just one of several metals that exhibit amphoteric behavior, in that the corrosion rate is minimized within a narrow pH range and increases as conditions move either to acidic or strongly alkaline territory. Figure 1 outlines the general corrosion pattern for iron and shows that in acid or alkaline conditions, corrosion rates can be quite significant. www.power-eng.com/articles/print/volume-111/issue-7/features/flow-accelerated-corrosion-a-critical-issue-revisited._printArticle.html 1/7 some experts recommend ammonia as the best pH-conditioning chemical. As outlined above. For a feedwater system of all steel metallurgy the optimum pH range is 9. Admiralty brass exhibits minimum corrosion within a pH range of 8.0 to 9. This mechanism also occurs on boiler waterwall tubes. Similarly. a commonly recommended pH range for mixed-metallurgy systems was 8.0 and 9.8 to 9. When a steam generator is placed on line. So the question becomes.com/articles/print/volume-111/issue-7/features/flow-accelerated-corrosion-a-critical-issue-revisited.html 2/7 . steel develops a protective coating of iron oxide known as magnetite (Fe3O4). and where subsequent oxygen concentrations in the condensate and feedwater remain at levels within a low part-per-billion (ppb) concentration. in high-pressure utility boilers where the steam is quite pure. Ammonia or organic amines are the pH-conditioning chemicals of choice. to prevent corrosion of the piping and heater tubes themselves._printArticle. decomposition of amines can potentially introduce unwanted organic acids and CO2 to the turbine. www.1. Copper-nickel alloys.power-eng. are most stable around a pH of 9. Formation of protective oxide layers is a very important concept. For this reason. However. most notably in ironbased materials.Print this page Click here to enlarge image The “safe” range for condensate/feedwater systems is much more restricted than that shown on the graph and is typically held between the pH values of 9. copper alloys develop a protective layer of cuprous oxide (Cu2O). Optimizing chemistry in the condensate/feedwater system is critical for two reasons.) Corrosion control in mixed-metallurgy systems is more complicated. Feedwater piping and heat exchanger tubes exhibit minimal corrosion at a mildly alkaline pH.3. and second to minimize the formation and transport of corrosion products that travel to the boiler and beyond. (These conditions are different in oxygenated treatment systems.6. as will be described in a later section.6. The two primary corrosion control issues in condensate/feedwater systems involve pH and oxygen. First.2 to 9. but this recommendation was recently raised to 9.8/19/13 Flow-Accelerated Corrosion: A Critical Issue Revisited . excursions of pH outside a relatively narrow range induce corrosion. It is these control issues that influence FAC. particularly the 90-10 material.5 to 9.3. Amines decompose to produce ammonia in feedwater. “What is the best pH for a system containing carbon-steel piping and copper-alloy heat exchanger tubes?” In years past. Flow Accelerated Corrosion The use of a pH-conditioner and oxygen scavenger-particularly the latter-constitutes what is known as an “allvolatile treatment (reducing)” AVT(R) program. Click here to enlarge image So.com/articles/print/volume-111/issue-7/features/flow-accelerated-corrosion-a-critical-issue-revisited. Figure 2 Oxygen pitting of an economizer tube Photo courtesy of Mel Esmacher. virtually all conventional utility steam generators have been designed with condenser air removal compartments. mechanical oxygen removal was supplemented with chemical treatment.html 3/7 . which reacts with oxygen as follows: Hydrazine proved advantageous because it does not add any dissolved solids to the feedwater. It was thought-incorrectly as we shall see-that the total absence of oxygen was best for corrosion control in the feedwater network. The latter constitutes the primary difficulty.Print this page Dissolved Oxygen Corrosion and Treatment Issues Uncontrolled oxygen ingress into a steam generator can cause problems._printArticle. Into the 1980s (in the United States at least). A primary benefit of hydrazine is that it will passivate oxidized areas of piping and tube materials as follows: Hydrazine residuals were typically maintained at relatively low levels of perhaps 20 to 100 ppb. The workhorse for many years was hydrazine (N2H4). Thus. it reacts with oxygen in a one-to-one weight ratio and it is supplied in liquid form at 35 percent concentration. Hydrazine or its “safer” alternatives are now no longer recommended unless the feedwater system includes copper-alloy heater tubes.8/19/13 Flow-Accelerated Corrosion: A Critical Issue Revisited . GE Water Systems. In the 1980s and 1990s.power-eng. researchers began to discover that www. as Figure 2 illustrates. conventional wisdom called for complete removal of oxygen from feedwater. deaerators in the condensate/feedwater system and chemical feed systems to inject not only the pH-conditioner but also an oxygen scavenger/metal passivator into the feedwater. first consider the nature of the protective magnetite (Fe3O4) layer._printArticle. and tees. To understand the problem. The compound is actually a joint mixture of FeO and Fe2O3 that often exhibits a rippled pattern.Print this page AVT(R) was the cause of previously unspecified problems.) The ammonia does not attack the magnetite directly.com/articles/print/volume-111/issue-7/features/flow-accelerated-corrosion-a-critical-issue-revisited. at low ammonia concentrations in a reducing environment. the complete removal of oxygen during normal operation can be very troublesome. which is responsible for magnetite dissolution. It is the lower pH. reducers. FAC develops at flow disturbances and direction changes. (Figure 3.power-eng. steam attemperating lines: essentially all flow-disturbed areas touched continuously by feedwater.8/19/13 Flow-Accelerated Corrosion: A Critical Issue Revisited . for example. is a direct function of the ammonia concentration. Iron exists in a +2 (ferrous) www.html 4/7 . Click here to enlarge image While it is well documented that dissolved oxygen can induce serious boiler and feedwater corrosion. This explains why corrosion can be much higher at an NH3 concentration of 0. feedwater and economizer elbows. like condensate and feedwater. Most notable is the dissolution of magnetite at a temperature range and chemical conditions common to the feedwater network. A special note should be made that pH in high-purity water. especially during unit shutdown. in strongly reducing environments.1 ppm than in any other case. Print this page oxidation state in FeO and +3 (ferric) in Fe2O3.com/articles/print/volume-111/issue-7/features/flow-accelerated-corrosion-a-critical-issue-revisited. GE Water Systems Click here to enlarge image As is clearly evident. Catastrophic failures occur when the affected area can no longer withstand the inside pressure. FAC has also been a problem in heat recovery steam generator (HRSG) waterwall tubes that have many tightradius elbows. Photo courtesy of Mel Esmacher. Figure 4 illustrates a side-view of FAC. of 1. The low-pressure circuits of HRSGs often operate near the temperature of highest corrosion potential (Figure 3). the pipe wall gradually thins. the corrosion has the texture of an orange peel. This material is resistant to attack. A particular difficulty with HRSGs is that the two or three semi-independent waterwall circuits make chemistry control rather difficult. The ferrous ions are those that are susceptible to FAC and in affected zones the ions migrate out of the magnetite matrix.power-eng.8/19/13 Flow-Accelerated Corrosion: A Critical Issue Revisited . Sudden ruptures of high-temperature feedwater lines have killed approximately 10 utility workers at several stations in the last decade or so. particularly at plants that once used or continue to use AVT(R). The reducing environment continually regenerates ferrous iron.html 5/7 . which exacerbates FAC potential._printArticle. One extra-cost solution for controlling FAC in HRSGs in the design phase is to specify tube material. From a straight-on view. Figure 4 Feedwater pipe thinned by FAC. FAC Prevention www. whose constant migration weakens the wall structure and eventually reduces pipe strength to the point of sudden failure.25 percent chrome steel. at least in elbows. It is imperative that potentially affected areas be checked ultrasonically for wall thinning. 20 to 70 ppb of ammonia will produce this pH. this program is only for systems with all-ferrous metallurgy. Pure feedwater in oncethrough units is usually a given. The issue can be addressed mechanically by utilizing 1. ammonia is injected for pH control. OT was developed in Germany some 30 years ago for replacement of AVT(R) in once-through steam generating units._printArticle. What the researchers found is that in condensate/feedwater networks where condenser air in-leakage is minor and where condensate dissolved oxygen levels stay at or below 10 ppb.Print this page Often. The program was adopted by other European utilities and has gained large acceptance at oncethrough utilities in the U.25-chrome steel in affected areas. as was mentioned earlier for HRSG elbows. zones of physical separation between water and steam will develop. particularly deaerators and feedwater heater drains.2 μS/cm. Ammonia is added to raise the pH within a range of 8. Two-phase FAC is virtually impossible to control chemically. Iron particulates are the primary product that carries over to the boiler. www. This program is known as neutral water treatment (NWT). Such parameters are typical in once-through units anyway. Two variations of oxygenated treatment are most popular. discontinued feed of the oxygen scavenger allows the FeOOH protective layer to form naturally.power-eng. as these steam generating systems are always equipped with condensate polishers.S. Common injection points are just after the condensate polisher and again at the deaerator outlet. The chemistry of oxygenated treatment is interesting and explains why the program has become popular. With oxygen injection however. In CWT programs. but by a less intensive mechanism. it may be difficult to keep dissolved/suspended iron concentrations in the feedwater below 2 ppb and excursions above 10 ppb are not uncommon. One difference from OT. oxygen is dosed to maintain a 30 to 150 ppb residual. oxygen is deliberately introduced to the condensate and feedwater system. This compact layer is more stable than magnetite and releases very little dissolved iron or suspended iron-oxide particles to the fluid.6. Thus. Typically. OT cannot be used in systems that contain copper-alloy feedwater heater tubes.html 6/7 . The idea continues to be establishing a FeOOH layer on the feedwater piping. the base layer of magnetite becomes covered and interspersed with an even tighter film of ferric oxide hydrate (FeOOH). as it is a condensate/feedwater treatment and thus can function regardless of boiler design. Excursions in dissolved oxygen concentration and cation conductivity-particularly the former-indicate excess air in-leakage within the condenser. is that the pH should be maintained within a range of 9.2 to 9. Chemicals in the feed solution that prevent FAC may settle in one phase leaving the other without protection. which are most common in the U. The keys to an OT program are controlled oxygen feed and high-purity condensate. At points in the steam generating system.8/19/13 Flow-Accelerated Corrosion: A Critical Issue Revisited . An offshoot of OT is a program developed by EPRI known as all-volatile treatment (oxidizing). chemical methods are effective for controlling FAC. or AVT(O). An issue that has come to light regarding OT programs is that of two-phase FAC. FAC can occur when all chemistry parameters are seemingly in acceptable ranges. In an OT program.15 μS/cm. Oxygenated treatment (OT) is a feedwater treatment that also serves to protect the boiler. In the first. The treatment requires the controlled injection of oxygen into the condensate/feedwater system. More often. In conventional AVT(R) programs.S. where cation conductivity can be maintained ≤0.5. A properly orchestrated OT program should lower feedwater iron concentrations to less than 1 ppb. As with OT..0 to 8. An operating guideline is cation conductivity ≤0. Sometimes only mechanical means are suitable. as makeup and feedwater must be highly pure because contaminants would enter the steam directly. This is known as combined water treatment (CWT). however. OT has also been applied to a number of drum units throughout the world. as copper corrosion would be much too severe. oxygen is injected alone without any pH-conditioning chemicals.com/articles/print/volume-111/issue-7/features/flow-accelerated-corrosion-a-critical-issue-revisited. OT and AVT(O) are not acceptable. Ill. Thus.html 7/7 . and he is the author of three books on steam generation topics published by PennWell Publishing.com/content/pe/en/articles/print/volume-111/issue-7/features/flow-acceleratedcorrosion-a-critical-issue-revisited. S. A general rule-of-thumb is that the oxygen scavenger should be fed to maintain an ORP within a range of -350 to -300 millivolts (mV). and S. However. Buecker. So. The technique of oxidation-reduction potential (ORP) monitoring is becoming popular for mixed-metallurgy condensate/feedwater chemistry.8/19/13 Flow-Accelerated Corrosion: A Critical Issue Revisited . However. Buecker has written more than 70 articles on steam generation. Human life cannot. Tulsa. most commonly silver/silver chloride. in pre-engineering from Springfield College in Illinois and a BS in Chemistry from Iowa State University. May 15-17. chemists have found that this guideline should not be considered an absolute. and coordinate the optimum ORP range with minimized copper and iron concentrations. References: 1.A. This corresponds to a range of -150 to -100 mV for a standard hydrogen electrode (SHE)._printArticle. He has previous experience as a chemical cleaning services engineer. from the 27th Annual Electric Utility Chemistry Workshop. 2007. To access this Article. B. AIChE. Champaign. 2001.power-eng. May 8-10. This can be quite difficult when relying upon standard dissolved oxygen and oxygen scavenger analyses. which influences corrosion..power-eng.Print this page Increased air in-leakage also introduces excess carbon dioxide. The important point is that plant personnel establish chemistry to prevent severe copper corrosion but also to prevent FAC. for a unit on AVT(O) any air in-leakage difficulties that raise condensate dissolved oxygen levels much above a mid-teen ppb concentration should be investigated and corrected as quickly as possible. He is a member of the ACS. flue gas desulfurization (FGD) engineer and analytical chemist for City Water Light & Power. 2. Author: Brad Buecker is an air quality control specialist at a large Midwestern power plant. Ill. Buecker has an A. Springfield. In short. as they would initiate excessive copper corrosion. Champaign. ASME and NACE. Okla. “Practical Application of Oxidation Reduction Potential (ORP) to Control Oxygen Scavenger Injection to Fossil Power [Systems]”. Feedwater heater tubes can be replaced. operation with complete absence of oxygen leads to FAC. For mixed-metallurgy systems.html www.com/articles/print/volume-111/issue-7/features/flow-accelerated-corrosion-a-critical-issue-revisited. water treatment and FGD chemistry. the correct program is to feed an oxygen scavenger. on-line ORP monitors measure the electrochemical potential of the solution versus a standard electrode. as a water and wastewater system supervisor and as a consulting chemist for an engineering firm. Shulder.2 A better plan is to set up comprehensive tests that include dissolved iron and copper analyses. Shulder. He also serves as a results engineer. “The Basics of Power Plant Cycle Chemistry”. go to: http://www. Ill. but at reduced concentrations to minimize FAC. in the proceedings for the 21st Annual Electric Utility Chemistry Workshop.
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