Resolving Flow-Accelerated CorrosionProblems in the Industrial Steam Plant

J.RobinsonBetzDearbornTrevose, PA USA

T. DrewsBetzDearbornThe Woodlands, TX USA

Technical Paper

James O. RobinsonBetzDearborn

4636 Somerton RoadTrevose, PA 19053

Tom DrewsBetzDearborn

9669 Grogans Mill RoadThe Woodlands, TX 77380

ABSTRACTFlow-accelerated corrosion (FAC) and erosion corro-sion (EC) produce similar damage in industrial steamplants. While both are affected by velocity, geometry,and metallurgy, water chemistry also plays a role inFAC. The application of specific boiler water treatmentpolymers, reducing agents, and filming inhibitors hasbeen shown to reduce the rate of iron and copperreleased from boiler feedwater, economizer, boiler andcondensate systems.

INTRODUCTIONFlow-accelerated corrosion (FAC) and erosion corro-sion (EC), are often used interchangeably to describesimilar material degradation processes. As a result,confusion exists regarding the identification of FACand the differences between FAC and EC. Both typesof damage involve destruction of a protective oxidefilm on the surface of a material (usually a metal ormetal alloy). The elimination or removal of the oxidefilm is generally referred to as the "erosion" process.This is followed by electrochemical oxidation, or corro-sive attack of the underlying metal.1 Both processesinvolve a fluid that flows across or impinges on a metalsurface.

The differences between FAC and EC involve themechanism by which the protective film is removedfrom the metal surface. In the EC process, the oxidefilm is mechanically removed from a metallic sub-strate. This most often occurs under conditions of two-phase flow (i.e., water droplets in steam, solid particlesin water, or steam bubbles in water). It is also possi-ble, but less likely, for erosion to occur under single-phase flow conditions. For this to happen, the fluidvelocity must increase the surface shear stress to alevel that causes the oxide film to breakdown. In addi-

tion to shear stress, there must also be variations inthe fluid velocity.2

In the FAC process, the protective oxide film is notmechanically removed. Rather, the oxide is dissolvedor prevented from forming, allowing corrosion of theunprotected surface. Thus, flow-accelerated corrosionmay be defined as corrosion, enhanced by masstransfer, between a dissolving oxide film and a flowingfluid, that is unsaturated in the dissolving species.3Corrosion proceeds rapidly by electrochemical meansand the corrosion rate accelerates as the velocity ofthe fluid increases. Because the rate of protective filmremoval is dependent on the velocity of the fluid, thisdamage mechanism has also been referred to asvelocity-induced corrosion (VIC).4One way to think of FAC is to visualize the damagethat occurs to a susceptible material in a corrosive fluidenvironment as a "continuum." Minimum damageunder stagnant conditions is at one end of the contin-uum; maximum damage under extremely high flowrates is at the opposite end. The damage occurring atany point along this continuum is dependent to varyingdegrees on both the corrosivity and velocity of thefluid. In a stagnant fluid, the damage that occurs issimply corrosion, which involves growth of a corrosivefilm on the surface of a metal. With time, the corrosionrate decreases as the protective film thickens.

At very high fluid velocities, surface destruction canresult from erosion. In this case, the oxide film ismechanically removed and is unable to redevelop toany appreciable thickness. The corrosion componentunder these conditions can be considered negligible.The rate of material loss is very high, and the servicelife of the eroded component is usually short.

Along the continuum between these two extremes liedegradation mechanisms which are the result of vari-ous combinations of corrosive and flow-related dam-age. FAC is one of these mechanisms. It occurs atrelatively low velocities, involving laminar and turbu-lent flow, when a number of other environmental crite-ria are met. The protective surface film is dissolved bythe fluid as rapidly as it is formed. Thus, dissolution ofthe protective oxide film and electrochemical attackare occurring simultaneously, resulting in continuousloss of material.

In single-phase flow, the affected surface is character-ized by overlapping, "horseshoe-shaped" pits whichprovide a scalloped appearance, similar to an orange

RESOLVING FLOW-ACCELERATED CORROSION PROBLEMSIN THE INDUSTRIAL STEAM PLANT

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peel texture (Figure 1). The appearance is character-istic of surfaces that have suffered significant metalloss. Two-phase flow, however, yields a "tiger-striped"or streaked appearance, consisting of alternatingbands of black and red oxides (Figure 2).5Although FAC can occur with various fluids (e.g.,gases, liquid metals, molten salts, etc.) and materials(inorganic and organic), this paper discusses onlyFAC-related surface damage on iron and copper-based alloys in an aqueous environment.

LOCATIONS WHERE FAC HASOCCURREDFAC has caused damage and often failures in manyareas of the industrial steam plant. A feedwater linefailure in a Midwestern paper mill resulted in two fatal-ities which prompted OSHA to issue a bulletin warningothers of the possible dangers related to FAC and theneed to inspect equipment regularly to avoid addition-al failures.6

Less publicized, but more common, is the thinning ofsteam generating tubes in gas turbine exhaust, lowpressure, heat recovery steam generators (HRSGs).7Several of these units have required tube replace-ment, often with higher carbon steel alloy materialsuch as T-22. Another area within the boiler whereFAC has been seen frequently is in the steam separa-tion equipment. There, geometry as well as velocityand two-phase flow cause wastage of the systemmetal. Similar problems have been encountered insteam condensate systems where the failure ofelbows in lines carrying two-phase flow has occurred.

This was such a common problem for paper machinesiphon elbows that many of them have been replacedwith stainless steel. FAC in copper alloy air heatingcoils of pulp dryers has also been a problem.

FACTORS AFFECTING FACWhen carbon steel is exposed to oxygen-free water,the following reaction occurs:

Fe + 2H2O Fe2+ + 2OH- +H2 Fe(OH)2 + H2 (1)This reaction is then followed by the Schikorr reactionwhere precipitated ferrous hydroxide is converted intomagnetite:

3Fe(OH)2 Fe3O4 + 2H2O + H2 (2)Magnetite (Fe3O4) forms a protective surface layerwhich inhibits further oxidation of the steel. However,magnetite is slightly soluble in demineralized, neutralor slightly alkaline water (pH in the range of 7.0 to 9.2)and low dissolved oxygen concentration (

TemperatureThe second reaction [See (2) above] which producesthe protective magnetite film is also highly dependenton temperature. FAC has been found to occur whenthe temperature of the system falls within the range of212 to 482F (100 to 250C).10 In single-phase flow,the temperature at which the maximum rate of metalloss occurs varies from 265 to 300F (129 to 149C).11Under two-phase flow conditions, maximum damagefrom FAC occurs within the temperature range of 300to 390F (149 to 199C).12

MaterialAmong iron-based components, FAC has been foundto occur only in carbon and low alloy steels. Alloyingelements in steel improve the stability of the oxide filmand reduce its solubility. Although molybdenum andcopper effectively improve resistance to FAC in steel,chromium is considered to be the most beneficialalloying element. For example, steel with a chromiumcontent of 1% can be expected to have an extremelylow or even negligible FAC rate. Even lower amountsof chromium provide greatly increased the resistanceto FAC. It has been reported that as little as 0.1%chromium was sufficient to completely arrest FAC in aparticular carbon steel application.13

Water ChemistryThe role of pH and oxygen on FAC rates has beenwell documented by others. An increase of only 0.5pH units in the pH range of 9 to 10 can produce a 10-fold reduction in the rate of metal loss in carbon steelsystems.14 The addition of 50 ppb (mg/L) or more ofoxygen to pure water may reduce the rate of metalloss even more.15

Complexing Agents. Complexing agents such aschelants and polymers were shown by Godfrey et al.to affect FAC through economizers and boilers by themeasurement of Fe2+ in the effluent.16 Both the con-centration and type of complexing agent were shownto affect the rate of metal loss. For each of the com-plexing agents evaluated, the higher the concentra-tion, the higher the rate of metal loss. Of thecomplexing agents studied (at equivalent concentra-tions), the metal loss was least with polymethacrylate(PMA), followed by polyacrylate (PA), with the greatestloss occurring with ethylenediaminetetraacetate(EDTA).In 1994, Boyette et al. reported on the development ofa copolymer that controlled calcium and magnesiumboiler deposits as effectively as the acrylate andmethacrylate polymers but that had a much lower

chelating potential.17 In research economizer teststhere was significantly less metal loss with this copoly-mer, copoly (acrylic acid/ethylene gycol allyl ether),PEGAE, than the other boiler water treatment poly-mers. Figure 3 shows the data from these studiescomparing PEGAE to PMA and PA. The metal losswith PEGAE was half that with PMA which had pro-duced the lowest metal loss of all polymers previouslytested.

Reducing Agents. Reducing agents, more commonlyreferred to as oxygen scavengers, have been reportedto promote the formation of magnetite.18 However,there is much controversy over the ability of thesescavengers to influence metal corrosion.19 In 1969, itwas reported in West Germany that the addition of 80ppb (mg/L) of oxygen to salt-free (< 0.10micromhos/cm [mS/cm] cation conductivity) boilerfeedwater produced a rapid drop in iron concentrationthroughout low pressure and high pressure feedwaterpreheaters.20 In 1995, Dooley et al. reported four casehistories where hydrazine feed was discontinued as aprecursor to converting boiler units to oxygenatedtreatment. In each case, there were substantial reduc-tions in the feedwater iron levels measured at theeconomizer inlet after stopping hydrazine feed, prior toadding oxygen.21

By contrast, Page et al.22 and Cuisa et al.23 separate-ly reported on five industrial plants (an oil refinery, twopaper mills, a munitions plant, and a machine toolmanufacturer) where the application of organic reduc-ing agents substantially reduced condensate corrosionthereby decreasing iron levels returned to the power-house. Obviously, there are other factors such aswater purity, that determine whether the presence ofoxygen or reducing agents is beneficial for the protec-tion of the system metal against FAC.

Filming Inhibitors. Filming inhibitors have been stud-ied by various researchers. Filippov et al.24 investigat-ed the influence of octadecylamine (ODA) on the loss

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Figure 3: PEGAE polymer significantly reduces the amount of iron pickup from a model econo-mizer compared to other boiler water treat-ment polymers.

of metal from slotted test specimens exposed to highvelocity, 140C (284F) boiler feedwater. They foundthat 3 ppm (mg/L) of ODA in the feedwater reducedthe amount of metal loss by 75% in a 24-hour period.They further demonstrated that while the feed of ODAfor a six-hour period reduced the rate of metal loss,once the feed of ODA was discontinued, the rate ofmetal loss returned to the original rate for untreatedtest specimens.

Martynova et al.25 studied flow-dependent corrosion ofmild steel with 75% quality steam as a function of tem-perature. Similar to single-phase studies, the rate ofmetal loss increased with increasing temperaturebetween 100 and 170C (212 and 338F) and thendeclined as the temperature was increased further.The application of ODA to form a barrier film on themetal surface reduced the rate of metal loss dramati-cally in the temperature range of 115 to 220C (239 to428F). At 170C (338F), the temperature of maxi-mum metal loss, the application of ODA reduced therate of metal loss by 80%.

CASE HISTORIES

PEGAE Polymer Reduces Feedwater IronA Midwestern steel mill operates a 300,000 lb/h (136tonne/h), 265 psig (18.6 kg/cm2) boiler with hot lime,hot zeolite-softened makeup water. A methacrylate-based chemical treatment program was fed at the dis-charge of the boiler feedwater pumps. The feedwatertrain includes an economizer with an outlet samplingport for monitoring iron levels to the boiler. Althoughtreatment results were generally judged to be good,feedwater iron levels at the economizer outlet werehigher than desired, averaging about 100 ppb (mg/L). To reduce the amount of FAC and consequently thelevel of boiler feedwater iron, the methacrylate poly-mer was replaced with PEGAE polymer. Economizer

outlet iron levels decreased from a range of 50 to 250ppb (mg/L) to a range of 5 to 20 ppb (mg/L) as shownin Figure 4. This polymer has continued to producegood boiler results with reduced metal loss from thefeedwater system for over five years.

Reducing Agent Controls BoilerFeedwater Line FACA Southern pulp and paper mill with 1500 psig (105kg/cm2) boilers is particularly concerned with FAC infeedwater lines because previous operating experi-ence required them to replace portions of their feed-water system as a result of this phenomena. The millhas mixed bed-polished makeup water and electro-magnetic filter-polished condensate so that iron levelsat the economizer outlet are believed to be a reason-able measure of FAC in the feedwater circuit.Feedwater pH is maintained between 9.2 and 9.6 witha neutralizing amine. Traditionally, the mill usedhydrazine to control periodic intrusions of oxygen.Feedwater iron at the economizer outlet was collectedby a Sentry corrosion product sampling system. Thesamples were analyzed by inductively coupled plasma(ICP) for Fe2+ and found to vary from 4 to 6 ppb (mg/L). In an attempt to reduce metal wastage, the feed ofhydrazine was temporarily discontinued; however,there was no decrease observed in the level of ironcollected by the Sentry system. To determine if anoth-er treatment approach would be beneficial, hydroxy-lamine was applied to the system. Within a week, thefeedwater iron was reduced to 3 ppb (mg/L); within amonth, the feedwater iron was reduced to less than 1ppb (mg/L) (see Figure 5). For test purposes, hydrox-ylamine was replaced with hydrazine and the feedwa-ter iron returned to the previous 4 to 6 ppb (mg/L)levels. When the hydroxylamine feed was restored,the feedwater iron levels returned to less than 1 ppb(mg/L) and they have averaged about 0.5 ppb (mg/L)for the past two years.

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Figure 4: Change to PEGAE polymer on day 10 led todecreased iron in the boiler feedwater.

Figure 5: Boiler feedwater iron was reduced by the application of hydroxylamine feed initiated onOctober 22.

Filming Inhibitors are Effective onCopper AlloysCopper/nickel air heating coils are commonly used inpulp dryers. Breakdown of a protective copper oxidelayer and release of copper is a frequent problem withconventional neutralizing amine treatment.Consequently, the following case history is similar toexperiences in many other mills.

A Southern pulp mill produces 1500 psig (105 kg/cm2),900F (482C) steam. Prior to start-up, it was decidedto establish a target condensate pH of 8.8 to provideoptimal protection of the copper/nickel alloy air heatingcoils in the pulp dryer. The pH was to be controlledby the application of cyclohexylamine, one of the morestable neutralizing amines, to minimize ammonia gen-eration and any deleterious effects from the presenceof ammonia. Monitoring revealed that copper in thepulp dryer condensate averaged about 35 ppb (mg/L).A study was initiated to determine the impact of pH oncopper release. Adjustments in the feed of cyclohexy-lamine were made. After allowing two hours for thesystem to stabilize, condensate discharge sampleswere collected and analyzed for pH and copper levels.The results are presented in Figure 6. Increasing thepH beyond 8.5 had little effect on the copper releasewhich remained at about 35 ppb (mg/L).Filming amines have been used in other pulp dryers toreduce the rate of metal wastage. However, becauseof the potential for the formation of "gunk balls" withODA, the mill was reluctant to take this traditionalapproach.

Consequently, BetzDearborn Steamate FM1000, anethyoxylated organic filmer that does not formdeposits, was chosen for evaluation. Condensate pHwas maintained at about 8.5 and the filmer was inject-ed into the steam header, directly ahead of the pulpdryer. The results are shown in Figure 7. The filmerapplication was initiated in late September. Copperrelease, measured in the dryer condensate,

decreased from the previous 30 to 40 ppb (mg/L)range with neutralizing amine treatment to only 10 ppb(mg/L) or less almost immediately. This applicationhas continued for over three years. Copper levels inthe pulp dryer condensate have remained below 10ppb (mg/L) during normal operation. Copper surges of100 ppb (mg/L) or more during equipment start-uphave been practically eliminated.

SUMMARYFlow-accelerated corrosion (FAC) is affected by watervelocity, system geometry, and metallurgy. Waterchemistry can also play a very important role. A num-ber of case histories are presented that demonstratehow the addition of specialty chemical additives can, incertain instances, reduce the rate of iron and copperthrow from feedwater, economizer, boiler, and conden-sate systems.

REFERENCES1. Weed, R.H., Tvedt, T.J., Cotton, I.J., "Erosion-

Corrosion in Utility Systems," Power-GenConference, December, 1995, p. 1.

2. Flow-Accelerated Corrosion in Power Plants(Pleasant Hill, CA: EPRI TR-106611, 1996) pp. 3-4.

3. Burrill, K.A., "Modeling Flow-Accelerated Corrosionin CANDU," 4th International Symposium on Flow-Accelerated Corrosion, June, 1995, p.1.

4. Weed, p. 2.

5. Flow-Accelerated Corrosion in Power Plants, pp. 3-15.

6. OSHA Hazard Information Bulletin - "Potential forFeed Water Pipes in Electrical Power Generation

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Figure 6: Increasing condensate pH did not reduce cop-per release from the pulp dryer much below35 ppb (mg/L).

Figure 7: The addition of a filming inhibitor to the con-densate treatment reduced the pulp dryercopper release to less than 10 ppb (mg/L).

Facilities to Rupture Causing Hazardous Releaseof Steam and Hot Water," October 31, 1996.

7. Cotton, I. J., Weed, R. H., Kolarick, J., "HRSGWater-side Reliability - Design and OperatingConsiderations," Power-Gen, Orlando, FL,December 7-9, 1994.

8. Patulski, S.A., "Pleasant Prairie Unit 1 FeedwaterLine Failure," PWR-Vol. 28, 1995 Joint PowerGeneration Conference, Vol. 3, ASME, 1995, p.517.

9. Patulski, p. 517

10.Port, R.D., "Flow-Accelerated Corrosion,"Corrosion 98, NACE International, 1998, pp. 721/2.

11.Port, p. 721/2

12.Port, p. 721/2

13.Thailer, H.J., Dalal, K.J., Goyette, L.F., "Flow-Accelerated Corrosion in Steam Generators," PVP-Vol. 316, Plant Systems/Components AgingManagement, ASME, 1995, p. 134.

14.Heltmann, H. G., Kastner, W. "Erosion-Corrosion inWater-Steam Cycles - Causes andCountermeasures," VGB Kraftwerkstechnik 62,Number 3, March, 1982.

15.Henzel, N., Crosby, D. C., Eley, S. R.,"Erosion/Corrosion in Power Plants Single- andTwo-Phase Flow Experience, Prediction," NDEManagement.

16.Godfrey, M. R,, Chen, TZU-Yu, Eisner, I. E., "On-line Corrosion Monitoring in Boiler Systems,"Corrosion 92, New Orleans, LA

17.Boyette, S., Jolley, K., Henry, J., Michaels, L.,Olson, R., "UltrasperseTM: A New All-PolymerTreatment Program," International WaterConference, Pittsburgh, PA, 1994.

18.Huchler, L., "Passivation in Steam GeneratingSystems: The Known and the Unknown," NACECorrosion 94, Baltimore, MD, March, 1994.

19.Burgmayer, P. R., Cotton, I. J., Knowles, G.,"Oxygen Scavenging and Passivation in SteamGenerating Systems: Fiction, Folklore, and Fact,"NACE Corrosion 92, New Orleans, LA, April 1992.

20.Effertz, P.-H., "Combined Conditioning ofWater/Steam Cycle of Power-Generating Unitswith Continuous Flow Steam Generators UsingOxygen and Ammonia (Combined WaterTreatment CWT)."

21.Dooley, B., (Matt?hews), J., Taylor, J., "OptimumChemistry for All-Ferrous Feedwater Systems:Why Use an Oxygen Scavenger?," UltrapureWater, July/August, 1995, pp. 48-55.

22.Page, W. C., Totura, G. T., "Paper Mill CondensateQuality Improved by New Organic OxygenScavenger/Passivating Agent," Black LiquorRecovery Boiler Advisory Committee (BLRBAC),Atlanta, GA, April, 13, 1983.

23.Cuisa, D. G., Reid, R. W., "Preventing Return LineCorrosion in Condensate Systems," Plant EnergyManagement, October, 1982, pp. 52-54.

24.Filippov, G. A., Saltanov, G. A., Kukushkin, A. N.,Vasil'chenko, E. G., Chempik, E., Shindler, K.,"Increasing the Reliability and Efficiency of Steam-Water Power Generating Plant by Surface ActiveSubstances," Thermal Engineering, 29 (9), 1982.

25.Martynova, O. I., Povarov, O. A., Tomarov, G. V.,"Some Laws Governing Erosion-Corrosion ofMetals in Wet Steam," UDC 621.165.563.423.4,(1986), pp. 128-133.

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