99345 the basics and not so basics of water corrosion processes altered by flow changes

Paper No.

345

THE BASICS AND NOT SO BASICS OF WATER CORROSION

PROCESSES ALTERED BY FLOW CHANGES

Thomas M. Laronge and M. A. (Andy) WardThomas M. Laronge, Inc.

10411 N.E. Fourth Plain Road, Suite 149P. O. BOX 4448

Vancouver, WA 98662-0448

ABSTRACT

Corrosion and electrochemical material wastage processes take on many differentforms. Most of these forms are dramatically affected by the relative flow rates of thereactants in the corroding cell. Some forms of corrosion are termed flow-assisted, flow-accelerated, erosion-corrosion, etc. Old processes, aging infrastructure issues, and aninflux of study dollars has brought these corrosion processes to the forefront of the modernworld. This manuscript discusses the basics of the flow effects in systems which corrodeby water.

INTRODUCTION

Corrosion rates of materials in water systems are impacted by changes in water flowrates. One of the most important factors affecting corrosion reactions in the presence of aflowing liquid is the linear velocity of the flowing fluid at the interface of the metal or metaloxide surface and the flowing fluid. This linear velocity is not, of necessity, equal to thebulk flow rate. Rather, this is the fluid flow rate measured in a known direction at the fluid-metal or fluid-metal oxide interface. The units of linear velocity are those of distancedivided by time, i.e., foot per second or meters per second, etc. Another important factor isthe turbulence which occurs at the interface of the metal oxide surface and the flowingfluid.

Erosion, as opposed to corrosion, may be defined as material loss resulting fromabrasion or physical degradation. In a fluid, the velocity at which erosion is said to begin is

Copyright@l 999 by NACE international. Requests for permission to publish this manuscript in any form, in part or in whole must be made in writing to NACEInternational, Conferences Division, P.O. Box 218340, Houeton, Texas 77218-8340. The material presented and the views expressed in thispaper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.Joao Silva - Invoice INV-351861-9H0QTF, downloaded on 7/17/2010 4:20:50 PM - Single-user license only, copying and networking prohibited.

termed the crRica/ ve/ocity. At velocities equal to or greater than the critical velocity,erosion processes proceed. When corrosive fluids, Le., e/ectro/ytes such as water areflowing, the loss of material can occur as a result of both erosion processes and corrosionprocesses when the critical velocity is exceeded. At velocities below the critical velocity,only corrosion processes can be responsible for material losses.

The intent of this paper is to review traditional material wastage literature and definethe types of corrosion that typically progress in flowing electrolytes. We, therefore, discussthe differences between generaI corrosion, flow-accelerated corrosion, erosion-corrosion,liquid impact-induced erosion, cavitation-erosion, and flashing-induced erosion.

CORROSION

Corrosion may be defined as wastage resulting from oxidation of a material byelectrochemical reaction with its environment. The corrosion processes are oftenrepresented as a combination of an anodic half-cell reaction and a cathodic half-cellreaction. The reason for this is simply that oxidation can not occur without a concurrentreduction. The common anodic half-cell reaction for iron corrosion that occurs in water atmost temperatures and pressures is shown in equation (l).

Fe + Fez+ + 2e- (1)

The corresponding cathodic half-cell reaction for iron corrosion that occurs insystems that contain water is shown in equation (2).

1/202 + H20 + 2e- + 2(OH)- (2)

One overall reaction for the corrosion of iron in water is the sum of equation (1) PIUSequation (2) or:

Fe + 1/202 + H20 + Fe*+ + 2(OH)- (3)

Recall that chemistty is an empirical science. Under most conditions, the chemistryof iron in oxygenated water continues through another oxidation-reduction reaction to formthe iron oxide, rust or hematite. These first two processes, when combined, are typicallyshown as the result of the Schikorr reaction pathway. The overall reaction is (4):

3Fe+2 + 4H20 + Fe30x + 8+ + 2e- (4)

Water in the Schikorr reaction serves as the electrolyte. An e/ectro/yte is a fluidwhich facilitates the physical transport of ionically charged mass. Mass which leaves theanode of a corrosion cell is usually positively charged. Mass which leaves the cathode of acorrosion cell is usually negatively charged. Both mass and charge are conserved duringthe process of corrosion.

THE IMPACT OF FLOW ON CORROSION AT FLOW RATESBELOW THE CRITICAL VELOCITY

At flow rates below a critical velocity, the corrosion processes which degradematerial are most commonly termed general corrosion, /oca/ized corrosion. and f/ow-acce/crated corrosion (FAC).

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General Corrosion

Generai corrosion may be defined as metal loss through oxidation that occursrelatively uniformly at a surface. Corrosion is typically measured in units of an averagesurface penetration per Unit time such as roils/year, 1 mil = 0.001 inch = 25.4 ~mo Whenthe general corrosion of iron occurs in wet air, the corrosion rate of the iron follows a nearparabolic rate reduction with respect to time, as illustrated in Figure 1. The corrosion ratesactually decreases with time as the oxide layer grows.2 This corrosion rate reduction overtime happens because the rate of oxygen diffusion decreases as the oxide layer grows.Oxygen diffusion is the mass transfer of oxygen through the porous oxide surface layer tothe base metal-metal oxide interface. Oxygen diffusion is required for ferrous corrosionand copper corrosion to occur at low temperatures, as oxygen is required for the cathodicreaction in systems that contain water. The steady state or average corrosion rate is ofteninversely proportional to the oxide layer thickness.

There are many examples of general corrosion that occur in systems where waterflows. Generai corrosion may occur in the following:

. Municipal Water Systems.

. Cooling Water Systems.

. Clarified Makeup Water Systems.

. Once-through Cooling Water Systems.

. Closed Cooling Water Systems.

. Wastewater Effluent Systems.

In fact, wherever the corroding surface is penetrated uniformly, general corrosion is usuallythe result.

General corrosion rates are often influenced by total dissolved solids, oxygenconcentration, saturation indexes, corrosion inhibitor concentration, temperature,pressure, surface area, etc. General corrosion rates typically increase proportionally withincreasing temperature and with increasing oxygen concentration.

Common techniques used to control general corrosion rates include the following:adding corrosion inhibitors; mechanically and/or chemically deaerating water to reduceoxygen corrosion; controlling the water chemistry to maintain a supersaturated conditionwith respect to calcium carbonate or another surface-scale forming compound; theapplication of linings or coatings; painting; and the use of anodic, cathodic orelectrochemical protection. To have a general corrosion cell process, four conditions mustexist. There must be an anode where oxidation occurs (Le., loss of electrons), a cathodewhere reduction occurs (Le., gaining of electrons), an electrolyte (Le., usually a liquid forfacilitating movement of charged masses), and an electrically conductive path for currentflow.

Localized Corrosion

Localized corrosion differs from general corrosion in that the metal loss occurs overrelatively small, concentrated surface areas. The rate of localized corrosion is not typicallyaltered by changes in water flow rate and will not further be discussed in this manuscript.


Therefore, pitting corrosion, underdeposit corrosion and crevice corrosion are oftenvelocity independent.

Flow-Accelerated Corrosion (FAC)

FAC may be defined as the result of a material loss that occurs through the alternatedissolving of the normally protective oxide layer that typically forms on the surface of manymaterials, followed by the reformation of the protective layer. The latter step is a trueoxidation-reduction reaction and hence corrosion.

Historically, the terms f/ow-asskted corrosion and flow-induced corros~on are usedto define restricted velocity ranges in a flow-accelerated corrosion cell.3 We elect to useonly one term, namely, FAC or flow-accelerated corrosion.

One unusual and identifying characteristic of FAC of iron and low carbon steel isthat the magnetite layer, normally protective iron oxide, is dissolved by the flowing waterstream.4 It is very important to note that the removal of the magnetite occurs only bychemical dissolution. Water flow is responsible for transporting the dissolved iron awayfrom the magnetite and into the bulk water where contact with constantly changingchemistry promotes dissolution. Unlike the erosion-corrosion process, there is noevidence of magnetite removal by excessive velocity abrading the magnetite layer duringFAC.5

FAC is known to occur under at Ieast two different phase conditions. Single-phaseFAC occurs in systems containing water. Two-phase FAC occurs in systems containingwater and steam. The appearance of the corroded carbon steel differs between single-phase FAC and two-phase FAC. In single-phase FAC where the corrosion damage hasbeen high, the typical metal surface appearance is scalloped similar to an orange peel.This scalloped appearance is a result of overlapping horseshoe-shaped pits which aretypically aligned so that the closed portion of the horseshoe points upstream. In two-phaseFAC, the typical appearance is tiger striping. The tiger striping appearance results from theblack corroded area and the oxide layer being colored blue or red. This blue or red coloredportion of the surface is typically protected by a thick oxide film.G This example of two-phase FAC assumes the FAC results in a flowing mixture of predominately steam with arelatively small amount of liquid water at an iron or mild steel surface.

When FAC occurs, the corrosion rate tends to have a constant value as illustrated inFigure 2. This relationship occurs because the protective layer reaches a near steady statecondition where the rate of layer formation nearly equals the rate of layer dissolution.Figure 2 does not show activity at velocity equal to zero. Water flow must be present towash away the soluble iron from the magnetite. When stagnant conditions exist, onlyconvection could wash away the soluble iron.

FAC of carbon steel is typically caused by low oxygen concentrations incombination with other operating conditions. It typically occurs in boiler feedwater piping,boiler feedwater heat exchangers, condensate piping in high purity waters in power plants,etc.

The FAC rate of carbon steel is controlled by the rate of dissolution of magnetite.The critical factors that affect the FAC of carbon steel include temperature, flow rate,critical velocity, pH, oxygen concentration and system metallurgy.


FAC of carbon steel can occur in many waters including, but not limited to,demineralized water orsteam-water mixtures. FACistypicaily found at temperatures of212 F to 482 F (100 C to 250 C). The maximum FAC rates for carbon steel indemineralized water aresometimes stated to beinthe temperature range of 300 OFto 350F (=150 C to 180 C).7 There is some question about the magnitude of FAC withtemperature in demineralized water. For example, one source clearly states that themaximum rate of FAC under these conditions occurs in a temperature range of 265 F to300 F (=130 C to 150 C) for single-phase FAC and from 300 F to 390 F (=150 C to 200C) for two-phase FAC.8 These temperature ranges appear to correlate with thetemperatures where magnetite volubility is maximized. The FAC rate decreases at higheror lower temperatures because magnetite volubility decreases.

The FAC rate increases as velocity and turbulence increase. There is no knownpractical threshold velocity below which FAC will not occur.g Increased turbulence andincreasing velocities enhance the interaction of the soluble magnetite layer with water.However, bulk water velocities of greater than 8 to 10 feet per second (2.4 to 3.0 meters persecond) may be sufficient to create a condition where FAC rates on iron or carbon steel aresignificant.5

FAC of metals is often found in areas where there are flow disturbances whichchange the fluid velocity near the surface of pipe or other related wet contact surfaces.Areas where FAC are commonly found include pipe elbows, pipe tees, pipe locatedimmediately downstream of control valves, orifices, flow meters, chemical feed nozzles,construction taps, etc. FAC has also been found in straight piping runs. However, thehighest FAC rates are often located immediately downstream of flow disturbances thattend to create local areas of increased turbulence at the metal surface.3

The pH range where FAC of carbon steel is most often detected is neutral to slightlyalkaline, roughly 7.0 to 9.5 pH units. FAC of carbon steel seldom, if ever, occurs atconditions above 10.0 pH units. Other forms of corrosion can occur at high pH values.

Oxygen concentration plays an important role in the FAC of carbon steel. FAC ofcarbon steel occurs when the oxygen concentration is low, often below 15 parts per billion(ppb) to 20 ppb. FAC of carbon steel typically occurs in high purity demineralized water inthe aforementioned temperatures and pH ranges. At these conditions, the volubility ofmagnetite (Fe~OJ is greater than the volubility of hematite (FezOJ. Also, the limitedamount of oxygen present tends to preclude the oxidation of magnetite to hematite.

There are two possible known mechanisms where oxygen can react to formhematite and reduce FAC. One possible mechanism is the reaction of oxygen withmagnetite at the magnetite-water interface. The other possible mechanism is the reactionof oxygen with dissolved iron to form hematite, which can plug the pores of small cracks inexisting magnetite layers. Either of these mechanisms can reduce the corrosion of carbonsteel that occurs in oxygenated boiler feedwater.

Techniques to minimize or limit FAC or carbon steel other than those alreadymentioned are said to include the following: utilize metallurgy that contains at least 1 YOchromium; add oxygen to boiler feedwater and to condensate streams that have very lowconcentrations of dissolved oxygen; size piping that contains water and water-steammixtures so that fluid velocities are minimized; and maintain operating temperaturesoutside of the range where FAC occurs, Le., 212 F to 482 F (100 C to 250 C).


In the relatively new use of oxygenated boiler feedwater treatment, the oxygenconcentration is maintained at 30 ppb or greater.5 Oxygenated boiler feedwater treatmentrequires that the boiler feedwater have cation conductivity of 0.15 to 0.30 pS/cm at 25 Cand all ferrous metallurgy in the pre-boiler system. These conditions are more commonlyencountered in utility boiler systems than in industrial boiler systems.

In some utility boiler systems, the use of hydrazine (NZHJ or other chemical oxygenscavengers has been eliminated in an effort to reduce FAC of carbon steel in boilerfeedwater and condensate systems. In other utility boiler systems, the concentration ofhydrazine or other chemical oxygen scavengers has been maintained at lowconcentrations, typically at 20 ppb hydrazine or itsmeasurable concentrations with low concentrationsscavengers.

In utility boiler systems where hydrazine andfeedwater system, FAC of carbon steel is reportedhydrazine when the oxygen concentration is above

equivalent. Oxygen can be present atof hydrazine or other chemical oxygen

oxygen are both present in the boilerto be controllable in the presence of5 ppb to 7 ppb. These oxvaen levels

are not considered oxygenated boiler teedwater trea~rhent. However, this p~a~tice hasreduced FAC in some utility systems that are not using oxygenated boiler feedwatertreatment.

THE IMPACT OF FLOW ON CORROSION AT FLOW RATES EQUALTO AND ABOVE THE CRITICAL VELOCITY

Once a critical velocity is reached or exceeded, erosion becomes involved in thecorrosion process. At a critical velocity and above, the protective oxide film is removed byerosion caused by the abrasive action of the flowing fluid. There is no singular criticalvelocity. Rather, there is a potentially different critical velocity for each eroding system.Most critical fluid velocities have not been accurately measured so that very little practicaldata exists in the technical literature.

Erosion-Corrosion

Erosion-corrosion may be defined as material loss which occurs as a combination ofthe alternating abrasive action of flowing fluids removing the surface layer and thesubsequent corrosion of the exposed base material surface. Erosion-corrosion occurswhen the surface shear stress is sufficient to remove, by abrasion, the protective surfacefilm from the base materials, usually metal. The overall rate of metal loss is a function ofboth the erosion of the surface layer and the corrosion of the metal. The rate of erosion-corrosion becomes more nearly linear with flow as the removal of the surface layerbecomes more frequent.

Examples of erosion-corrosion include the following: inlet end erosion-corrosion orimpingement attack which occurs in heat exchanger tubes that have excessive inletvelocities; piping systems that have suspended particles which can erode the protectivefilm; and piping systems with excessive velocities that can wear away the protective film.

The occurrence of erosion-corrosion requires that a critical surface shear stress beexceeded for the material in use. The surface shear stress can be calculated usingequation (5) for turbulent flows. The shear stress will be expressed in foot-pounds/second2or in Joules/second* depending upon whether the variables are expressed in English Units


or S. 1. Units, respectively. The value of shear stress is symbolized with the lower caseGreek letter tau (@.

Therefore, z = 1/8 pfV2 (5)

where: P is the fluid density, typically in pounds/cubic foot or grams/cubiccentimeter.

f is the Darcy friction factor, dimensionless, but dependent uponReynolds Number, etc.

v is the average flow velocity, typically in feetLsecond ormeters/second.

Techniques to control erosion-corrosion include operating the system at velocitiesbelow those that would cause the critical surface shear stress to be exceeded for themetallurgy/corrosion system in use, and the use of erosion resistant materials.

Liquid Impact-Induced Erosion

Liquid impact-induced erosion k defined as material loss or damage caused by theimpact of liquid droplets in two-phase flow. Liquid impact-induced erosion occurs in two-phase flow when the impact of the liquid droplets is sufficient to cause crater formation inthe material surface due to plastic deformation. The material loss or damage is caused bythe net force of the liquid droplets impacting the material surface at sufficiently highvelocities. These impacts cause loss of surface films that result in exposing the baredalloys to enhanced oxidation or enhanced corrosion with their environments.

Liquid impact-induced erosion can occur in wet steam lines when the velocities aresufficiently high to cause plastic deformation of the steam piping, fittings, primary steamseparators, and secondary steam separators.

The occurrence of liquid impact-induced erosion requires that a system specificthreshold velocity be exceeded. The threshold velocity for carbon steel in wet steamservice is approximately 300 feetkecond or 90 meters/second. The threshold impactvelocity for copper in wet steam service is nearly 100 feethecond or 30 meters/second.2These threshold velocities for liquid impact-induced erosion are much higher than thosefor erosion-corrosion because of the inherent differences in the two processes.

Techniques to eliminate or minimize liquid impact-induced erosion include properline sizing to insure that threshold velocities are not exceeded for the specific materials inuse, proper materials selection, removing liquid droplets, etc.

Cavitation-Erosion

Cavitat/on-erosion is defined as the damage of a material due to the collapse ofbubbles, or cavities, in a liquid at the solid-liquid interface.

Cavitation-erosion is normally a mechanical process that occurs by the repeatedformation and collapsing of bubbles, or cavities, in a liquid stream. Bubbles are formedwhen local pressure falls below the vapor pressure of the liquid. Cavitation bubbles


typically rupture within five to ten pipe diameters of their points of formation. Therefore,cavitation-erosion damage typically occurs just downstream of the cavitation source.12

Cavitation-erosion can occur in pumps, turbines, valves, piping components, etc.The occurrence of cavitation-erosion requires that the local vapor pressure of the fluid begreater than the local operating pressure. Cavitation-erosion can be prevented andminimized by designing systems so that localized pressures are always greater than thevapor pressure of the fluid at its operating temperature.

Flashing-Induced Erosion

F/ashing-induced erosion k the abrasion damage to materials caused by suddenvelocity increases that occur after a sudden loss of pressure in a system. Flashing-induced erosion occurs when there is a sudden loss of pressure in a liquid system. Theresulting sudden increase in fluid velocity can accelerate the liquid phase, the liquiddroplets, or liquid films. If these velocities are sufficient to exceed the system specificcritical velocity, flashing-induced erosion can occur.13

Flashing-induced erosion can occur downstream of valves, in drain lines and ventlines. Sudden drops in pressure, which can create high velocity in downstream piping,cause the occurrence of flashing-induced erosion.

Techniques to control or minimize flashing-induced erosion include the following:cooling liquid streams before a large pressure drop occurs; sizing the downstream Iinesufficiently to limit the downstream velocity; reducing the pressure in multiple stagesrather than in one pressure reduction stage; etc.

CONCLUSIONS

The flow effect of system components or structures which corrode in water can bedifferentiated by their flow velocity regimes. Two major categories exist. These aresystems where a critical flow velocity is not reached and systems where a critical flowvelocity is reached or exceeded. When the critical velocity is not reached, the flow effectson the corrosion processes can be categorized into general corrosion and flow-accelerated corrosion. FAC occurs when a materials protective film, typically magnetitein the case of carbon steel in water, is dissolved by the flowing water stream at a rate equalto or greater than its rate of formation.

When the critical velocity is reached or exceeded, the corrosion processincorporates erosion damage. The resulting combinations of erosion and corrosion includethe distinct processes of erosion-corrosion, cavitation-erosion, liquid impact-inducederosion, and flashing-induced erosion. The combined effects of erosion damage andcorrosion can significantly increase the damage to materials relative to the singular effectsof either mechanism.

The material scientist must carefully and thoroughly characterize those systemsobserved in an effort to discern the correct material wastage mechanism. Sometimesmultiple material wastage mechanisms may further complicate this determination.

In order to identify the specific method of material degradation, we recommend thata detailed knowledge of at least the following parameters be assembled and analyzed:


1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

l

l

l

l

l

l

l

l

l

l

l

Type(s) of Material(s).Temperature(s) of System(s).Pressure(s) of System(s).Fluid Velocities.Dissolved Oxygen Concentration.pH of Fluids.Single or Multiple Phase Flow.Chemistry of SurFace Layer(s).Volubility of Surface Layer(s).Does base metal loss occur physically or by electrochemical oxidation-reduction.Composition of the Fluid Phase(s).

REFERENCES

M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, (Houston,TX:NACE International, 1974) p. 310.

Flow-Acce Ierated Co rrosion in Power Plants EPRI, (Pleasant Hill, CA:EPRl-TR-10661, 1996), p. 3-11.

V. K. Chexal, R. B. Dooley, D. P. Munson and R. M. Tilley, Control of Flow-Accelerated Corrosion in Fossil, Co-generation and Industrial Steam Plants, IWCpaper no. IWC-97-57, (Pittsburgh, PA:lnternational Water Conference, 1997).

Flow-Accelerated Corrosion in Power Plants EPRI, (Pleasant Hill, CA:EPRl-TR-10661, 1996), p. 3-2.

R. D. Port, Flow Accelerated Corrosion, Corrosion/98 Paper No. 721, (Houston,TX:NACE International, 1998).

Flow-Accelerated Corrosion in Power Plants EPRI, (Pleasant Hill, CA:EPRl-TR-10661, 1996), p. 3-15.

Ibid., p. 2-3.

Ibid., p. 4-27.

Ibid., p. 2-1.

[bid., p. 4-19.

Ibid., p. 3-5.

Ibid., p. 3-8.

Ibid., p. 3-7.


APPENDIX A

GLOSSARY OF TERMS

Abras/on -- One mechanism for wearing away of a surface by contact with its environmentin the absence of corrosion or an oxidation-reduction chemical reaction.

Cavitation-Erosion -- One mechanism for degrading material when bubbles or cavities in aliquid collapse at their intetiace with the material.

Corrosion -- The degradation of a material by electrochemical oxidation-reductionreaction with its environment.

Erosion-Corrosion -- One mechanism for degrading material through the repeatedly,combined effects of the abrasive action of flowing fluids removing the materialssurface layer and the subsequent corrosion of the base metal exposed to itsenvironment.

Flashing-Induced Erosion -- One mechanism for degrading material when sudden fluidvelocity increases as a result of a sudden loss of pressure in a system withsubsequent abrasion of the material sutface at the fluid-material interface.

F/ow-Acce/crated Corrosion -- One mechanism for alternately degrading material bydissolving a protective layer baring the material surface to contact with itsenvironment and altering the surface chemistry to reform a protective layer.

F/o w-Assisted Corrosion -- One mechanism for alternately degrading material bydissolving a protective layer baring the material surface to contact with itsenvironment and altering the surface chemistry to reform a protective layer. It isgenerally accepted that flow-assisted corrosion is called flow-acceleratedcorrosion.

Genera/ Corrosion -- The results of material degradation through oxidation-reductionchemistry that occurs relatively uniform at a material surface.

Liquid Impact-Induced Erosion -- A mechanism for material damage following the impactof liquid droplets in two-phase flow, such as liquid water in steam, causing erosion ata material contact surface.

Localized Corrosion -- The result of material degradation through oxidation-reductionchemistry that occurs relatively non-uniformly at a material surface.


GENERAL

CORROSION

RATE L

ATIME(days)

FIGURE 1 - General corrosion ratewith respect to time.

FAC

RATE

(MPY)

C3 ~ 3xFlow

C2 + 2xFlow

cl ~ lxFlow

0 ~(days)

Note: C3 = 3/2c2 = 2CI

FIGURE 2- FAC rates at various flow rates withrespect to time - Note the material loss is amultiplier of systems flow rate.


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