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WEARWear186-187(1995)291-298

The erosion-corrosion of alloys under oxidizing-sulphidizing conditionsat high temperature

F.H. Stott, M.P. Jordan, S. Lekatos, M.M. Stack, G.C. WoodCorrosion and Protecti on Centr e, Uni versity of M anchester Insti tui e of Science and Technology, M anchester, UK

AbstractA whirling-arm erosion-corrosion rig has been designed and constructed to allow studies under gaseous conditions of high-sulphur, low-

oxygen activities at high tempera tures; these environments are pertinent in processes such as gasification of coal and catalytic c racking o f oil.The system can operate at particle impact velocities up to 30 m s-, particle impact fluxes up to 1 g cm- s- and temperatures up to 800 C.The particles are carried to the specimen chamber in a stream of nitrogen and mixed with hydrogen, hydrogen sulphide and, if necessary,water vapour to attain the required sulphur a nd oxygen activities. In this paper, results are presented for two comm ercial austenitic high-tempera ture alloys, 310 stainless steel and Alloy 8OO HT,during impact erosion by 25 pm alumina particles at velocities of lo-25 m s- andparticlefluxesin theranges0.06-0 .16 g cmm 2 SK and0.38-0.95 g crn-~-~ at 500 C. The gaseo us environment resulted in the developmentof sulphide sca les on the alloys during exposu res in the absence of the erodent particles. The erosion-corrosion dama ge was determined interms of mean thickness-change measurem ents obtained every 5 h and overall metal-recession rates obtained by cross-sectional examinationat the end of the 35 h or 70 h exposure periods. T he results are discussed in terms of the synergistic interactions of growth of metal sulphidesand removal of such phases by the impacting particles.Keywords: Alloys; Erosion; Corrosion; Oxidation; Sulphidation; High temperature

1. IntroductionThe synergistic interaction of mechanical dama ge due to

impact of metallic surfaces by fast-moving particles andchemical degradation due to high-tempera ture gaseous cor-rosion is not well understood. Under so me conditions, cor-rosion may enhance the rate of erosion dam age while, underother conditions, erosion may enhance the rate of corrosion.How ever, particularly for relatively low impact velocities byrelatively small particles, the formation of a corrosion-prod-uct scale can give some protection to a metallic componentagainst erosion dama ge. These interactions have led to theintroduction of terms such as erosion-enhanced oxidation[ 11, erosion+orrosion-dominated behaviour [ 21 and cor-rosion-dominated behaviour [ 2,3].

During th e past 10 years, considerable advances have beenmade in understanding the synergism between impact erosionand high-tempera ture gaseous corrosion, in which formationof an oxide scale is the usual m ode of corrosion (e.g. [ l-81 ) .Emp hasis has ben placed on oxidation in air or oxygensince many industrial process es operate under conditionswhe re alloys for metallic components are selected on the basisthat they develop a slow-growing oxide scale for protectionagainst further oxidation.0043-1648/95/$09.50 0 1995 Elsevier Science S.A. All rights reservedSSDIOO43-1648(95)07149-O

How ever, there are several modern industrial process es inwhich components are expose d to environments of high-sulphur, low-oxygen contents under potentially erosive con-ditions; these include systems to convert coal into a gaseousproduct to drive a gas turbine and catalytic cracke r systemsin the petrochem ical industry. Under such conditions, a pro-tective, slow-growing oxide sca le may not be able to be estab-lished, o r only with difficulty, or be maintained for longperiods, and much less protective sulphide scales may be ableto develop. Such scales on conventional iron-andiron-nickel-base high-temperature alloys are much less protective thanthe corresponding oxides and grow at faster rates, resultingin much more rapid rates o f metal loss, even if erosion playsno role. Indeed, in some coal-conversion systems, the tem-peratures of metallic components have to be kept relativelylow (below 600 C) , to prevent excessive sulphidation.

The aim of the present researc h has been to design andbuild an erosion-corrosion rig capable of operating at hightempera tures in gaseous environments that have high sulphurpartial pressures and low oxygen partial pressures, in orderto carry out basic studies of the interactions of oxidation/sulphidation and particle erosion. T he present pap er describesthe rig and includes the results from s ome of the initial studieson the degradation under particle-impact conditions of two

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standard com mercial high-tem perature alloys, 3 10 stainlesssteel and Alloy 800HT, at 500 C in an oxidizing/sulphidiz-ing gaseou s environm ent that was essentially sulp hidizing tothese materials.

2. Experimental2.1. Test rig

The erosion-corrosion rig has been built to allow tests tobe undertaken under moderate particle-loading conditions ingaseous environments of relatively low oxygen concentra-tions and high sulphur concentrations at temperature s to800 C. The practical difficulties a nd safety implications ofthese requirements have placed considerable restrictions onthe design of the equipment. The system was built by SevernFurnaces Ltd of Thornbury, Bristol and consists of three mainparts: a particle fluidizer, a gas-mixing device and an erosion-corrosion chamber (Fig. 1) .Particles in a cylindrical Pyrex glass vessel ( 100 mm indiameter) are filled with erodent particles to a depth of170 mm and fluidized by nitrogen passing through a sintereddisc at the bottom of the vessel an d by injection of a jet ofnitrogen via a vertical tube in the centre of the fluidizer. Theparticles and gas form a cloud in the upper part of the vesseland are drawn off via a small diameter tube and fed into themain gas stream above the preheater. The nitrogen gas flowrate to the disc and jet no zzle essentially controls the inletparticle loading into the erosion chamber. Excess gas fromthe fluidizer provides additional cooling to the electrical ter-minal boxes in the chamber. Flow meters are used to controlthe inlet comp osition of the reactive ga s (consisting of hydro-gen mixed with hydrogen-2% hydrogen sulphide for thepresent tests). The gas streams are mixed and then fed into

r

the stream of nitrogen plus erodent particles before enteringthe erosion cham ber v ia a carbon-block preheater. T he latterensures th at the gas and particles attain the temperature of thechamber. In order to minimize corrosion of the components,the outer m etal case of the chamber is kept below 100 Cwhile the volume of the hot zone is restricted to the graphitecylinder at the centre. The heating elements in the dead spaceoutside the cylinder are protected from the reactive gases bya nitrogen purge du ring a test.

Up to 10 specimens can be accommodated in the rig, with5 being located on each side of the central axis of a 200 mmlong specimen holder. During a test, the holder is rotated bya motor attached to the drive shaft. This moves the specimensthrough the stream of erodent particles that enter the chamberat the top. The relative impact velocities of the particles andthe specimens are determined by the locations of the speci-mens with respect to the axis of rotation of the holder and itsrotational speed. In these tests, the specimens were orientedso that they impacted the particle stream at an angle of 90.The gases and particles leave via the bottom of the chamberwhere the former is scrubbed with potassium carbonate toremove hydrogen sulphide and the latter are collected in acatchpot.

2.2. Materials

Resu lts are presented for the erosion-corrosion behaviourof two commercial high-temperature alloys: Alloy 8OOHTand 310 stainless steel, with compositions given in Table 1.The impacting face of the specimens was essentially flat andrectangular ( 10 X 8 mm ), while grooves in the specimensallowed them to be secured in the holder. The surface thatcontacted the erodent particles was ground to a 320 grit finishand ultrasonically cleaned in acetone im mediately before a

-

Speck hoiderN2

Fig. 1. Schematic diagram of the erosion-corrosion rig.

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Table 1Chemical compositions of the alloys (wt.%)

Fe Ni Cr Mn Si Al cu Ti CBal. 32.0 21.0 1.5 1.0 0.4 0.75 0.4 0.07Bal. 20.0 25.0 1.5 1.5 _ 0.04

Fig. 2. Scanning electron micrograph of erodent particles.test. The eroden t p articles w ere 25 p,rn alum ina, from U ni-versal Abrasives Ltd. They were angular in shape (Fig. 2).2.3. Test env i ro nmen ts

In this paper, the results of tests carried out under two inletparticle loadings (65 g mP3 and 380 g me3) in one gaseousenvironmen t at 500 C are reported. The equilibrium partialpressure of sulphur in the mixed g as was lo-* atm and thatof oxygen w as nominally 1O-3 5 atm, although it is unlikelythat gas-phase equilibrium was attained in these tests, owingto the high gas-flow rates. It had been intended to obtain thisenvironment by mixing hydrogen with hydrogen-2% hydro-gen sulphide and water vapour (as is usual for corrosiontests). However, although the hydrogen sulphide contentcould be attained accurately, it was not possible to controlthe oxygen content as closely, owing to the trace oxygenconcentration in the nitrogen carrier gas. In order to estimatethis concentration, several pure metals (nickel, iron chro-mium, cobalt, aluminium and molybdenum) were exposedto the gas (but containing no hydrogen sulphide) flowingthrough the rig at 500 C. From the products formed on thesetest coupons (ie chromium oxide on chromium, aluminiumoxide on alum inium, and no oxides on the other metals), itwas concluded that the partial pressure of oxygen wasbetween 10P3 and 10W u atm. In view of this, it was decidednot to add water vapour to the test gas, but to carry out theexperimen ts in the appropriate hydrogen-hydrogen sulphidegas mixture and rely on oxygen impurities in the nitrogen toprovide the required oxygen concentration. The particle load-ing in the inlet gas to the erosion chamber was controlled bythe flow rate of nitrogen and was determined by calibrationprocedures. The gas flow rates used in the tests for the twoparticle loadings are given in Table 2. The particle fluxes

Table 2Gas flow rates used to attain the two particle loadingsGas Gas flow rate ( cm3 min- )

Loading:380 g m-3

Loading:65 g m-3

Hydrogen 495 495Hydrogen-2% hydrogen sulphide 105 105Nitrogen fluidizer 3000 2000Nitrogen jet 1000 600Nitrogen + particles leaving the tluidizer 1500 1100

Table 3Particle fluxes used in the test programme for the two inlet particle loadingsImpact velocity(m s-)

Impacting particle flux (g cm- s- )

25.021.217.513.810.0

Loading: 38 0 g me3 Loading: 65 g m-30.95 0.160.81 0.140.67 0.110.52 0.090.38 0.06

were estimated from knowledge of the inlet particle loadingof the gas and the velocities of the specimens as they impactthe particles (Table 3).2.4. Exper im en t a l p rocedu res

For a test, 10 specimen s w ere fastened into the specim enholder in the chamber, which was sealed and purged withnitrogen at 2 1 min- for 45 min before introd uction of themixed hydrogen-hydrogen sulphide gas and the nitrogenstream containing the alumina particles. The preheater andchamber heater were switched on and the system was broughtto 500 C over a 2 h period. There was no significant differ-ence between the inlet gas temperature and the temperatureof the specimens in the erosion chamber. Following stabili-zation, the motor was started and the specimens were rotatedthrough the stream of particles at 2388 rev min- , giving therelative specim en velocities specified in Table 3. After 5 h,the motor was switched off and the system was cooled toroom temperature over an 8 h period. Thickness-changemeasurements (which included any thickness increase dueto formation of corrosion scale on the back surface of thespecimen that was not in contact with the erodent particles)were made, using a digital ball-ended m icrometer, accurateto + 1 pm, at 12 locations on each specimen and the average

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determined. The specimens were then returned to the cham berand the process repeated, for a total of 35 h or 70 h. At theend of the test, the specimens were exam ined in plan by X-ray diffraction and analytical scanning electron micro scopy.Subsequently, they were mounted and polished in cross-sec-tion and the thicknesses of residual metal a nd of residualscale were m easured in at least 24 locations, using an opticalmicroscope, to determine the average values. As there wasvery little internal corrosion after exposures at 500 C, theresidual metal thickness was taken as the distance from theoriginal back surface of the specimen to the metal-scale inter-face at the side of the specimen exposed to the erodent. Themean metal recession was the difference between the averagethickness of the original specimen (determined by microm-eter measurem ents) and the average residual m etal thickness.The dama ged regions were examined in the analytical scan-ning electron microscope.

3. Experimental results3.1. M easu remen ts o f eros i on -co r ro s i on dama ge

The average thickness changes (with respec t to the originalspecimen dimensions) as a function of time, measured at 5 hintervals, are plotted in Figs. 3 and 4 for the inlet particle

Ti me, h

Fig. 3. Plot of thickness change against time for specimens of Alloy 800HTduring erosion-corrosion at 500 C (inlet particle loading, 65 g m-3) at thevelocities indicated: 0, corrosion only; A, 17.5 m s-t; X, 10 m s-; 0,21.2 m s-l; V, 13.8 m s-; 0.25 m s-.

Fig. 4. Plot of thickness ch ange against time for specimens of 3 10 stainlesssteel during erosion-corrosion at 500 C (inlet particle loading, 65 g m-)at the velocities indicated: 0, corrosion only; A , 1 7.5 m s-; X , 10 m SK;??21.2ms-;V,13.8ms-;0,25ms-.

c -20f t

' a,4??2

- 30 I I I I0 10 20 30 LO 50 60 70Tome, h

Fig. 5. Plot of thickness change against time for specimens of Alloy 800HTduring erosion-conosion at 500 C (inlet particle loading, 380 g m-) atthe velocities indicated: 0, corrosion only; A, 17.5 m s-l; X, 10 m SK;0.21.2 m s-; V, 13.8 m s-; c3,25 m SK.

-300 10 20 301 LOTfme, h

50 _i___i 60 70

Fig. 6. Plots of thickness change against time for specimens of 310 stainlesssteel during erosion-corrosion at 500 C (inlet particle loading, 380 g m- 3,at the velocities indicated: 0, corrosion only; A, 17.5 m s-r; X, 10 m s-l;??21.2ms-;0,13.8ms-;0,25ms-

Fig. 7. Plots of metal recession as a function of velocity for erosionxorrosionat500C:(a)alloy800HTfor35h(65gm~3);(b)3l0stainlesssteelfor35h(65gm~3);(c)alloy800HTfor70h(380gm~3);(d)3l0stainlesssteelfor70h(380gm-3).

loading of 65 g m-3 and in Figs. 5 and 6 for that of380 g mP3, for impact velocities of 10-25 m s-; a positivevalue indicates a thickness increase. The corresponding datafor corrosion only are included for comparison. The meanvalues of metal recession, meas ured after the tests, are plottedas a function of velocity for the various conditions, in Fig. 7.

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However, it should be emphasized that the thickness-changevalue is the difference between the average value determinedfor the specimen prior to the test and that determined aftereach test period, usin g a microm eter. Since the surfaces ofthe exposed specimens were often irregular, particularly asmaterial w as removed by the particle im pacts, there is a sig-nificant scatter band associated with each stated value, evenusing 1 2 measurements to determine the mean. From obser-vations of the surfaces after the tests and calculations ofstandar d deviations, it is estimated that each value is withina scatter band of between + 2 p,m and f 3 p.m. The m etalrecession value is the difference between the average thick-ness value determined for the specimen prior to the test, usinga microm eter, and the average thickn ess of residual metaldetermined at the end of the test run, using an optical micro-scope, after mou nting in cross-section (ensuring that thespecimen was square to the polishing plane) and metallo-graphic preparation. This change in method, even using 24measurements, inherently must increase the scatter band foreach value, particularly as the scale-metal interfaces weresometimes irregular. From examination of these difficultiesand from calculation of standard deviations, the scatter bandsfor the metal recession data are between + 4 Frn and + 6 p.m.It should also be appreciated that the plots given in Fig. 7 aremerely convenient methods of presenting the data; they donot represent true relationships since a change in velocity isalso accomp anied by a change in particle flux (Table 3). Inaddition, the data for a loading of 65 g mW3are for 35 h totalexposure times while those for a loading of 380 g me3 arefor 70 h.As described later, the corrosion process was essentiallysulphidation, resulting in the relatively large thicknessincreases being recorded during the corrosion-only expo-sures, owing to development and growth of relativelyrapidly thickening sulphide scales. Although there weresome irregularities in the thickness-cha nge data, severaltrends were apparent following the erosion-corrosiontests.1. Under a loading of 65 g m- 3 for 3 10 stainless steel (Fig.

4), there was a greater thickness increase after a giventime up to 10 h with increasing velocity. T his indicatesthat the particle impacts did not result in substantialremoval of scale. Moreover, the trend for the thicknessincrease to be greater at higher velocities suggests thaterosion-enhanced sulphida tion may have occurred,although the differences were not much greater thanthe scatter band for the measu remen ts. After longertimes, smaller thickness increases than for thecorrosion-only specimen were observed at the highervelocities (21.2 and 25 m s- ), while, by 35 h, thethickness increases for all velocities were less thanthat for the corrosion-only specimen , indicating atransition to erosion-corrosion-do minated behaviour,involving significant removal of scale by the erosionaction.

Under a loading of 65 g mm 3 for Alloy 800HT (Fig. 3),the significantly larger thickness increases that occurredafter a given tim e up to 10 h for velocities of 1 0 and13.8 m s-, compared with that for corrosion only, werealso consistent with erosion-enhanced sulphida tion.Again, the thickness increases, particularly for the highervelocities, after 35 h were significantly less than thoseobserved after corrosion only, suggesting a transition toerosion-corrosion-dominant behaviour.Under a loading of 3 80 g m -3, the erosion damage wassignificantly greater than under the lower loading. Apartfrom a slightly larger increase during the initial 10 h at10 m s- , the thickness increases recorded for 3 10 stain-less steel were always less than those for the corrosion-only specimen (Fig. 6). Indeed, overall thickness losseswere eventually recorded for all velocities, with the extentincreasing with increasing velocity, as expected for ero-sion-corrosion-dominated behaviour.Under a loading of 380 g m-3, similar damage trends tothose for 310 stainless steel were observed for Alloy800HT (Fig. 5) ; however, the thickness change at10 m SK was always positive and, indeed, was slightlygreater than that for the corrosion-only specimen , evenafter 70 h.Under a loading of 65 g mP3, any differences in the meanmetal-recession values as a function of impact velocityfor either alloy were within experimental scatter of themeasurement method. A ll values were less than 6 brn(Fig. 7). However, there were much greater effects ofvelocity under a loading of 38 0 g mP3, with a rapidincrease to metal-recession values of about 30 pm after70hat25ms-.

3.2. Examination of the sur$aces after the testsIn the absence of erosion, both alloys formed sulphide

scales (Fig. 8 (a) ) , about 10 pm thick for Alloy 800HT and12 p,rn thick for 3 10 s tainless steel after 35 h, with no evi-dence for any oxides. These consisted essentially of an outerporous layer of (FeNi),& and an inner, more compact, layerof iron-chrom ium-rich sulphid es, probably FeCr$ ,, asexpected for such materials (Fig. 9(a)). There was someevidence for spallation of pieces of the outer layer after thefirst two 5 h cycles, but not after longer periods.

Following the erosion-corrosion tests, under a loading of65 g rne3, there w ere some obvious change s in the surfacemorphologies of the specimens. The very fine sulphide crys-tals observed in the absence of erosion (Fig. 8(a) ) were nolonger appare nt and typical craters, consistent with 90 impactdam age, were evident under both low-velocity (Fig. 8 (b) )and higher-velocity conditions (Fig. 8 (c) ) . Nonetheless,there were significant sulphide scales retained on the surfaces.At the lower velocities ( 10 m s-r), the scales on both alloyswere much more irregular in thickness than in the absence oferosion (Fig. 9(b) ) , but the mean values were very similar(about 10 Km for Alloy 800HT and I5 km for 310 stainless

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Fig. 8. Scanning electron micrographs of the surface of 310 stainless steelafter exposure for 35 h in the erosion-corrosion rig at 500 C (inlet particleloading, 65 g m-l): (a) corrosion only; (b) velocity 10 m s-; (c) velocity25 ms-.

Fig. 9. Scanning electron micrographs in cro ss section of the scales devel-oped on the alloys during exposure for 35 h at 500 C (inlet particle loading,65 g mem3): a) alloy 8OOHT, orrosion only; (b) alloy 8OOHTa t10 m s-;(c) alloy 800HTat 25 m s-l; (d) 310 stainless steel at 25 m SK.steel). At 25 m SK, the residual scales were a lso very irreg-ular (Figs. 9(c) and (d) ), but the mean thickne sses weresome what reduced (5-9 p,m for both alloys). It was alsoapparent that the residual scale at the higher velocities wasmainly iron-chromium-rich sulphide, w ith not much of theporous outer layer of (FeNi)& being retained. There wasno evidence for loss of scale by spallation in these tests. Also,remnants of erodent particles were never detected in the spec-imen surfaces.

After tests at the higher particle loading of 380 g rnd3,apart from A lloy 8OOH T exposed at 10 m s-, the residualscales retained on the specimens were very thin, consistentwith the considerably increased rate of dama ge and metal losscom pared with the results a t the lower loading. The meanscale thickness was less than l-2 p,rn in all cases. X-rayanalysis revealed that these scales we re an iron-chromium-rich sulphide, probably Fe Cr,$, on both alloys. There wasno indication of any remaining (FeNi) $& phase.

4. Discussion

From the relevant thermodynam ic phase stability diagram sfor the metal-oxygen-sulphur systems pertinent to the rele-vant alloying elem ents (iron, nickel and chromium ), ironsulphide, nickel sulphide and chromium oxide should be thestable pha ses with respect to the pure m etals in this environ-ment at 500 C (assuming that the gases are at thermody-namic equilibrium). Although such equilibrium is probablynot attained in the rig, the control exposure of chromium inthe gas (in the absence of hydrogen sulphide) indicates thatthe oxide is able to develop on the metal.

Exposure of both alloys to the gaseous environment in therig, without any erodent particles, results in the developmentof sulphide scales on the specimen surfaces. There is noindication that a Cr,O,-rich layer has been established at anystage. This is not unexpected since the rates of nucleation andgrowth of the oxide a re very much slower than those of iron-nickel sulphides, enabling grow th of the sulphide phases topredominate in the early stages.

Hence, these tests have essentially allowed a study of theinteractions of simultaneous sulphidation and impact erosionunder tw o particle loadings at velocities of 10-25 m s--l.Under the lower particle loading (65 g me3 in the inlet gas),there is relatively little difference (within the experimentalaccuracy of the measurem ents) in the extent of metal reces-sion at the end of the test as a function of impact velocity(Fig. 7). Although there is an apparent increase with increas-ing velocity for Alloy 8OO HT, this is within the scatter bandof the data. For instance, a metal recession of about 5 pm(rather than mean measured value of l-2 pm) would beexpected for a scale thickness of 10 pm, as was formed duringcorrosion only.

The mean thickness-change measurem ents are more sig-nificant and do show som e small trends (Figs. 3 and 4). Thus,after short times of exposure , there are larger increases inspecimen thickness in the presence of erodent particles thanduring corrosion only for both alloys, consistent with erosion-enhanced sulphidation. How ever, this trend does not persistand the mean thickness continues to increase less rapidly, oractually to decrea se, with longer exposure periods under theerosion conditions. The overall thickness increase at the endof the 35 h test period is less than that for corrosion-onlyexposure s in almost every case , with a general trend for thisvalue to be higher at lower impact velocities. Thus, as thescale thickens, there is a tendency towa rds erosion+orrosion-dominated behaviour. The main dama ge proce ss is removalof the outer sulphide scale. At the highest velocity, the resid-ual scales are some what thinner, in the range 5-9 p,rn for thetwo alloys, than after corrosion only ( 10-12 Fm) . However,the overall thickness-changedata suggest that the rate of scaleremoval is relatively small which perhaps accounts for theobservation that the extent of metal loss is not increasedsignificantly by erosion; the erosion conditions required forsignificant removal of scale, leading to increased metal reces-

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sion rates, have to be very severe under these corrosion con- to K / k ) while, ov erall, there is a linear rate of decrease in theditions. total thickness of the specimen.Under the higher particle loading (380 g mP3), the trendsare similar, except tha t the extent of scale removal is increasedconsiderably. Thus, there is a general trend for an increase inmean thickness in the early stages to be followed by adecrease on further exposure to the erodent particles (Figs. 5and 6). There is also some correlation between the erosiondamage and the impact velocity; the time to commencementof the thickness decrease is reduced and the rate of such adecrease is enhanced with increasing velocity. (The increasein llux that accompanies an increase in velocity m ay also bea contributory factor in this respect). Overall thicknessdecreases are observed eventually for almost all cases (exceptAlloy 800H T a t the lowest velocity). The effect of velocityis also shown in the metal-recession plots, with largerincreases being recorded with increasing velocity (Fig. 7).There are always thin residual sulphides retained on the sur-faces after the tests. Although these may have developedduring cooling after the particle stream was shut off (consis-tent with the damage process being removal of both metaland sulphide by the erodent particles), it is perhap s morelikely that they were present at the end of the erosion period(consistent with the damage process being formation andremoval of scale only). However, further work is needed todetermine the mechanisms of erosion-corrosion damageunder such conditions.

Both particle flux and impact velocity are important para-meters in influencing this type of erosion+orrosion behav-iour. The velocity determines the impact energy and, thus,the extent of damage on each impact while the particle fluxdetermines the number of impacts on the specimen surfacein a given time and the time interval between successiveimpac ts on a given spo t. The flux can have consequen ces interms of the time available for regrowth of scale to replacethat removed by previous impacts or for any damage sus-tained on a previous impact to be repaired, e.g. by an anneal-ing process. Hence, if removal of material follows anaccumulation of damage, e.g. a fatigue process, particle fluxis likely to be a more important parameter at high tempera-tures than at low temperatures.

During the early stages of exposure, there has been someevidence for erosion-enhanced sulphida tion. Here, if the scaleis damaged or even if pieces are removed from the outersurface by the impacting particles, accelerated scale growthin these affected areas may result in a thicker scale than inthe absence of erosion. However, the precise mechanism forsuch a process is not yet fully understood [ 11 .

It is also interesting that the mean thickness change againsttime plots follow very closely the trends predicted for ero-sion-corrosion, as discussed by Wright et. al. [9]. In sim-plistic terms, for a scale (su ch as an oxide or a sulphide) thatgrow s by a diffusion-controlled process, the growth followsa parabolic relationship with time i.e. the rate of scale thick-ening decreases with increasing thickness, y. If the scale isimpacted by erodent particles an d material is removed fromits outer surface at a linear rate, the overall rate of scalethickening follows a relationship of the type:dy K-=--dt Y k

Although the above discussion is based on very simpleassumptions and ignores many of the factors that complicatethe actual erosion+orrosion behaviour, such as the depend-ence of the erosion rate constant on the oxidation rate con-stant, the effects of erosion on the mechanisms of growth ofthe residual scale, the chang ing compo sition of the scale asthe outer regions are removed, the possibility of removal ofcomplete pieces of scale, etc, it provides a basis to accountfor the observed erosion-corrosion behaviour. Futur eresearch using this rig will address some of these mechanisticfeatures in much more detail. It will particularly compare theerosion characteristics during sulphidation/ero sion withthose during oxidation/erosion, emp hasising the adhesion/cohesion characteristics of the different scales and the inher-ent erosion resistance of their surfaces. An important differ-ence between exposures in sulphidizing gases and those inoxidizing gases for these two alloys is that both developrelatively fast-growing iron-n ickel-base sulphide scales inthe former environment and much slower growing Cr,O,-rich scales in the latter environment; there are considerabledifferences between these two types of scale in terms ofadhesion/cohesion with the substrate, growth rates and theability of the surface to resist multiple impact by erodentparticles. For instance, it is generally considered that, as thesulphides developed on such alloys have relatively low melt-ing points (in the region of 700-1000 C, depending on com-position), they can deform more easily and may resist theparticle impacts more effectively than the higher m eltingpoint oxide scales that may be more susceptible to failure byspallation under such conditions.

where K is the parabolic sulphidation rate constant and k isthe linear erosion rate constant. 5. ConclusionsHence, in a situation where the scale thickens by sulphi-

dation and thins by erosion, in the early stages, an increasein scale thickness, and thus in overall specimen thickness,should occur; however, eventually, the sulphidation rateshould be just fast enough to balance the rate of scale removal.Thereafter, the thickness of scale remains constant (and equal

1. An erosion

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4.

Preliminary tests have been carried out on two commercialaustentic alloys at 500 C in an essentially sulphidizingenvironment, using 25 km alumina erodent particles, atvelocities of 1O-25 m s - .Tests at a relatively low particle lo ading (giving particlefluxes in the region of 0.06-O . 16 g cm- s- ) resulted insome erosion-enhanced sulphida tion during the earlystages followed by a slight tendency for erosion-corrosion-dom inated behaviour in the later stages.However, increases in the overall rates of metal lossdue to erosion by the impacting particles, comparedwith those recorded for corrosion only, were withinthe experimental errors of the dam age measure-ments.Tests at a higher p article loading (giving particlefluxes in the region of 0.38-0.95 g cm-* SK) resultedin very significant overall rates of metal loss,which increased with increasing impact velocity. Therelationships between the extent of damage (asdetermined by thickness-change measurements)and exposure time approximately followed thoseexpected for erosion-corrosion-do minated behav-iour.

AcknowledgementsThe authors are grateful to Commission of European Com-

munity for support under the Joule Programme and the ShellInternational Petroleum Company for a grant towards thecosts of the erosion+orrosion rig.

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