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8/19/13

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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.

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Flow-Accelerated Corrosion: A Critical Issue Revisited - Print this page

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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 9.0 and 9.6. When a steam generator is placed on line, and where subsequent oxygen concentrations in the condensate and feedwater remain at levels within a low part-per-billion (ppb) concentration, steel develops a protective coating of iron oxide known as magnetite (Fe3O4). This mechanism also occurs on boiler waterwall tubes. Similarly, copper alloys develop a protective layer of cuprous oxide (Cu2O). Formation of protective oxide layers is a very important concept. Optimizing chemistry in the condensate/feedwater system is critical for two reasons. First, to prevent corrosion of the piping and heater tubes themselves, and second to minimize the formation and transport of corrosion products that travel to the boiler and beyond. The two primary corrosion control issues in condensate/feedwater systems involve pH and oxygen. It is these control issues that influence FAC. As outlined above, excursions of pH outside a relatively narrow range induce corrosion, most notably in ironbased materials. Feedwater piping and heat exchanger tubes exhibit minimal corrosion at a mildly alkaline pH. For a feedwater system of all steel metallurgy the optimum pH range is 9.2 to 9.6. (These conditions are different in oxygenated treatment systems, as will be described in a later section.) Corrosion control in mixed-metallurgy systems is more complicated. Admiralty brass exhibits minimum corrosion within a pH range of 8.5 to 9. Copper-nickel alloys, particularly the 90-10 material, are most stable around a pH of 9.3. So the question becomes, “What is the best pH for a system containing carbon-steel piping and copper-alloy heat exchanger tubes?” In years past, a commonly recommended pH range for mixed-metallurgy systems was 8.8 to 9.1, but this recommendation was recently raised to 9.0 to 9.3. Ammonia or organic amines are the pH-conditioning chemicals of choice. Amines decompose to produce ammonia in feedwater. However, in high-pressure utility boilers where the steam is quite pure, decomposition of amines can potentially introduce unwanted organic acids and CO2 to the turbine. For this reason, some experts recommend ammonia as the best pH-conditioning chemical.
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8/19/13

Flow-Accelerated Corrosion: A Critical Issue Revisited - Print this page

Dissolved Oxygen Corrosion and Treatment Issues
Uncontrolled oxygen ingress into a steam generator can cause problems, as Figure 2 illustrates.

Figure 2 Oxygen pitting of an economizer tube Photo courtesy of Mel Esmacher, GE Water Systems.

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So, virtually all conventional utility steam generators have been designed with condenser air removal compartments, 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. The latter constitutes the primary difficulty. Into the 1980s (in the United States at least), conventional wisdom called for complete removal of oxygen from feedwater. It was thought-incorrectly as we shall see-that the total absence of oxygen was best for corrosion control in the feedwater network. Thus, mechanical oxygen removal was supplemented with chemical treatment. The workhorse for many years was hydrazine (N2H4), which reacts with oxygen as follows: Hydrazine proved advantageous because it does not add any dissolved solids to the feedwater, it reacts with oxygen in a one-to-one weight ratio and it is supplied in liquid form at 35 percent concentration. 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. Hydrazine or its “safer” alternatives are now no longer recommended unless the feedwater system includes copper-alloy heater tubes.

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. In the 1980s and 1990s, researchers began to discover that
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Flow-Accelerated Corrosion: A Critical Issue Revisited - Print this page

AVT(R) was the cause of previously unspecified problems. 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, like condensate and feedwater, is a direct function of the ammonia concentration. It is the lower pH, at low ammonia concentrations in a reducing environment, which is responsible for magnetite dissolution. This explains why corrosion can be much higher at an NH3 concentration of 0.1 ppm than in any other case. (Figure 3.) The ammonia does not attack the magnetite directly.

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While it is well documented that dissolved oxygen can induce serious boiler and feedwater corrosion, especially during unit shutdown, the complete removal of oxygen during normal operation can be very troublesome. FAC develops at flow disturbances and direction changes; for example, feedwater and economizer elbows, reducers, and tees, steam attemperating lines: essentially all flow-disturbed areas touched continuously by feedwater, in strongly reducing environments. To understand the problem, first consider the nature of the protective magnetite (Fe3O4) layer. The compound is actually a joint mixture of FeO and Fe2O3 that often exhibits a rippled pattern. Iron exists in a +2 (ferrous)
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Flow-Accelerated Corrosion: A Critical Issue Revisited - Print this page

oxidation state in FeO and +3 (ferric) in Fe2O3. The ferrous ions are those that are susceptible to FAC and in affected zones the ions migrate out of the magnetite matrix. The reducing environment continually regenerates ferrous iron, whose constant migration weakens the wall structure and eventually reduces pipe strength to the point of sudden failure. From a straight-on view, the corrosion has the texture of an orange peel. Figure 4 illustrates a side-view of FAC.

Figure 4 Feedwater pipe thinned by FAC. Photo courtesy of Mel Esmacher, GE Water Systems

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As is clearly evident, the pipe wall gradually thins. Catastrophic failures occur when the affected area can no longer withstand the inside pressure. Sudden ruptures of high-temperature feedwater lines have killed approximately 10 utility workers at several stations in the last decade or so. It is imperative that potentially affected areas be checked ultrasonically for wall thinning, particularly at plants that once used or continue to use AVT(R). FAC has also been a problem in heat recovery steam generator (HRSG) waterwall tubes that have many tightradius elbows. The low-pressure circuits of HRSGs often operate near the temperature of highest corrosion potential (Figure 3), which exacerbates FAC potential. A particular difficulty with HRSGs is that the two or three semi-independent waterwall circuits make chemistry control rather difficult. One extra-cost solution for controlling FAC in HRSGs in the design phase is to specify tube material, at least in elbows, of 1.25 percent chrome steel. This material is resistant to attack.

FAC Prevention
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Flow-Accelerated Corrosion: A Critical Issue Revisited - Print this page

Often, chemical methods are effective for controlling FAC. Sometimes only mechanical means are suitable. Oxygenated treatment (OT) is a feedwater treatment that also serves to protect the boiler. In an OT program, oxygen is deliberately introduced to the condensate and feedwater system. Two variations of oxygenated treatment are most popular. In the first, oxygen is injected alone without any pH-conditioning chemicals. This program is known as neutral water treatment (NWT). More often, ammonia is injected for pH control. This is known as combined water treatment (CWT). OT was developed in Germany some 30 years ago for replacement of AVT(R) in once-through steam generating units. The program was adopted by other European utilities and has gained large acceptance at oncethrough utilities in the U.S. The treatment requires the controlled injection of oxygen into the condensate/feedwater system. Common injection points are just after the condensate polisher and again at the deaerator outlet. In CWT programs, which are most common in the U.S., oxygen is dosed to maintain a 30 to 150 ppb residual. Ammonia is added to raise the pH within a range of 8.0 to 8.5. Typically, 20 to 70 ppb of ammonia will produce this pH. The chemistry of oxygenated treatment is interesting and explains why the program has become popular. In conventional AVT(R) programs, it may be difficult to keep dissolved/suspended iron concentrations in the feedwater below 2 ppb and excursions above 10 ppb are not uncommon. Iron particulates are the primary product that carries over to the boiler. With oxygen injection however, the base layer of magnetite becomes covered and interspersed with an even tighter film of ferric oxide hydrate (FeOOH). This compact layer is more stable than magnetite and releases very little dissolved iron or suspended iron-oxide particles to the fluid. A properly orchestrated OT program should lower feedwater iron concentrations to less than 1 ppb. The keys to an OT program are controlled oxygen feed and high-purity condensate, where cation conductivity can be maintained ≤0.15 μS/cm. Such parameters are typical in once-through units anyway, as makeup and feedwater must be highly pure because contaminants would enter the steam directly. Pure feedwater in oncethrough units is usually a given, as these steam generating systems are always equipped with condensate polishers. OT has also been applied to a number of drum units throughout the world, as it is a condensate/feedwater treatment and thus can function regardless of boiler design. OT cannot be used in systems that contain copper-alloy feedwater heater tubes, as copper corrosion would be much too severe. An issue that has come to light regarding OT programs is that of two-phase FAC. At points in the steam generating system, particularly deaerators and feedwater heater drains, zones of physical separation between water and steam will develop. Chemicals in the feed solution that prevent FAC may settle in one phase leaving the other without protection. Thus, FAC can occur when all chemistry parameters are seemingly in acceptable ranges. Two-phase FAC is virtually impossible to control chemically. The issue can be addressed mechanically by utilizing 1.25-chrome steel in affected areas, as was mentioned earlier for HRSG elbows. An offshoot of OT is a program developed by EPRI known as all-volatile treatment (oxidizing), or AVT(O). The idea continues to be establishing a FeOOH layer on the feedwater piping, but by a less intensive mechanism. 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, discontinued feed of the oxygen scavenger allows the FeOOH protective layer to form naturally. As with OT, this program is only for systems with all-ferrous metallurgy. One difference from OT, however, is that the pH should be maintained within a range of 9.2 to 9.6. An operating guideline is cation conductivity ≤0.2 μS/cm. Excursions in dissolved oxygen concentration and cation conductivity-particularly the former-indicate excess air in-leakage within the condenser.
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Increased air in-leakage also introduces excess carbon dioxide, which influences corrosion. Thus, 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. For mixed-metallurgy systems, OT and AVT(O) are not acceptable, as they would initiate excessive copper corrosion. However, operation with complete absence of oxygen leads to FAC. So, the correct program is to feed an oxygen scavenger, but at reduced concentrations to minimize FAC. This can be quite difficult when relying upon standard dissolved oxygen and oxygen scavenger analyses. The technique of oxidation-reduction potential (ORP) monitoring is becoming popular for mixed-metallurgy condensate/feedwater chemistry. In short, on-line ORP monitors measure the electrochemical potential of the solution versus a standard electrode, most commonly silver/silver chloride. 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). This corresponds to a range of -150 to -100 mV for a standard hydrogen electrode (SHE). However, chemists have found that this guideline should not be considered an absolute.2 A better plan is to set up comprehensive tests that include dissolved iron and copper analyses, and coordinate the optimum ORP range with minimized copper and iron concentrations. The important point is that plant personnel establish chemistry to prevent severe copper corrosion but also to prevent FAC. Feedwater heater tubes can be replaced. Human life cannot. References: 1. Buecker, B., and S. Shulder, “The Basics of Power Plant Cycle Chemistry”; from the 27th Annual Electric Utility Chemistry Workshop, May 15-17, 2007, Champaign, Ill. 2. S. Shulder, “Practical Application of Oxidation Reduction Potential (ORP) to Control Oxygen Scavenger Injection to Fossil Power [Systems]”; in the proceedings for the 21st Annual Electric Utility Chemistry Workshop, May 8-10, 2001, Champaign, Ill. Author: Brad Buecker is an air quality control specialist at a large Midwestern power plant. He has previous experience as a chemical cleaning services engineer, as a water and wastewater system supervisor and as a consulting chemist for an engineering firm. He also serves as a results engineer, flue gas desulfurization (FGD) engineer and analytical chemist for City Water Light & Power, Springfield, Ill. Buecker has written more than 70 articles on steam generation, water treatment and FGD chemistry, and he is the author of three books on steam generation topics published by PennWell Publishing, Tulsa, Okla. Buecker has an A.A. in pre-engineering from Springfield College in Illinois and a BS in Chemistry from Iowa State University. He is a member of the ACS, AIChE, ASME and NACE.

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