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WEARELSEVIER Wear 180 (1995) 91-99

Relationship between the effects of velocity andresistance in erosion-corrosion environm ents

temperaturesM.M . Stack, J. Chacon-Nava, F.H. Stott

alloy corrosionat elevated

Corrosion and Protection Cenhq UM IST, P.O. Bar 88, Sackvill e Street, Manchester M60 IQD, UKReceived 16 July 1993; accepted 31 August 1994

Abstract

Erosion

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92 M.M . Stack et al. I Wear 180 (1995) 91-99

corrosion rate, velocity, surface pre-treatments andparticle size were evaluated. As a result, a n under-standing of the effects of such variables in these en-vironments is beginning to emerge.

The results showed that erosion-corrosion rate hada distinctive pattern with temperature, increasing upto a critical temperature and decreasing with furtherincreases in temperature [l-5]. This curve shifted tohigher temperatures with increasing alloy corrosionresistance [2], and higher temperatures and wastagerates with increasing particle velocity [l]. An interestingfeature about suc h results is that some years earlierthe same effect of temperature and alloy corrosionresistance had been observed for erosion-corrosion inaqueous environments [6-81.

The effects of these erosion-co rrosion variab les influidized-bed environments have lead to the classifi-cation of various erosion-corrosion regimes. The tran-sitions between such regimes are best understood byconsidering the effect of temperature, Fig. 1. At lowtemperatures, erosion of the alloy substrate is thedominant process. As the temperature is increased, therate of formation of corrosion product increases. If thislayer which forms between successive erosion eventsis removed after impaction, a temperature is eventuallyreached where the removal of corrosion product is thedominant wastage mec hanism. This is the regime inwhich corrosion enhances the erosion-corrosion rate,and marks the transition to the erosionTemperatureFig. 1. Schematic diagram of the transitions through the erosion- One of the more interesting results from the ero-corrosion regimes as a function of temperature. sion-corrosion literature, Fig. 2, has been the effect of

corrosion product formed, and hence the so-called critical scale thickness required to effect the transitionto corrosion-dominated behaviour m ust be attainedat higher temperatures. Increases in velocity increasesthe impact energy, and the overall efficiency of theerosion event. Hence, the critical thickness needed toeffect the transition to corrosion-dominated behaviouris also attained at successively higher tempe ratures asvelocity is increa sed.

The effects of velocity in solid particle erosion arewell established. The generalized relationship whichrelates erosion (m ass loss/mass impacting particles) tovelocity is given by the follow ing equa tion:E=Kvf(B) (I)where e = erosion, k = consta nt, v = velocity, it = expo-nent and 0= impact angle.

The velocity exponents which are usually reportedfor the erosion of metals vary between 2 and 3 [9,10].For ceramics, the dependence of erosion on velocityis less clear, with high exponents being reported insome environments, and models for erosion of brittlematerials have been developed to account for thisobservation [11,12]. The velocity dependency for ce-ramics is determined by the mode of fracture, whetherductile or brittle. In the ductile regime wherethe ceramic is not significantly eroded, the exponentsare low [13]. Above a critical velocity, the erosionincreases considerably and the velocity exponents aremuch higher, closer to 3. It has been pointed out thatextending this ductile regime to higher velocities w illresult in a lower velocity dependence on the extent oferosion [13]. For example, if the hardness of the targettends to that of the particle, the velocity expon ent maydecrease. A similar situation may occur if the particlegeometry is changed from angular to rounded, or ifthe particle size is decreased significantly.

In erosion-co rrosion, howe ver, the effects of velocityare complicated by the fact that erosion conditionsdetermine the extent of corrosion on the surface. Forexample, the extent of scale growth rate is dependenton the time between impacts which may itself be afunction of the velocity. In addition, the effects ofvelocity in the various regimes are not well understood.A wide range of exponents has been reported inerosion-corrosion environments, ranging from 1 to 5[l-5,14,15]. In addition, there is no general consensuson why exponents should be high in one environmentand low in another. Attempts to evaluate velocity ex-ponents do not consider that the exponent may changeaccording to the erosion-corrosion regime. Generally,exponents tend to be evaluated over the whole velocityspectrum, despite apparent transitions through ero-sion-corrosion regimes.

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h TcmpWatur* bFig. 2. Schem atic diagram of the variation of erosion-corrosion ratewith temperature for two alloys of different corrosion resistances(corrosion rate of Alloy I> Alloy 2).alloy corrosion resistance on the erosion-corrosion rateversus temperature curve. The ranking order of theerosion< orrosion rate can reverse for alloys of differentcorrosion resistances as the temperature is increased.In addition, the literature shows that the velocity ex-ponen t for alloys of different corrosion resistanc e isdependent on the conditions [4]. Thus, the relationshipbetween corrosion resistance and velocity needs to beexamined in order to ascertain why corrosion rate mayalter the velocity exponent in such environments.

Hence , this pape r conside rs the effect of velocity inthe various erosion+rrosion regimes. The results willbe evaluated from a range of erosion-corrosion studiesinvolving solid particles in gaseous media at elevatedtemperatures. The factors which affect the derivationof the velocity expon ents are exam ined. The effects ofalloy corrosion resistance on such exponents are con-sidered, and some general provisos are outlined forinterpre ting the effect of velocity for alloys of differentcorrosion resistances in erosion

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dominated regime should be subdivided into two sub-regimes [4], corrosion-dominated l, where the erosionrate versusvelocity dependence was high and corrosion-dominated 2, where the velocity dependence was low.

Other studies on the effect of velocity at a range oftemperatures have shown that the design of the rigused for the erosion-corrosion tests can have a sig-nificant effect on the experimental results, and this hasbeen pointed out in previous work [2]. In some ex-perimental rigs, which have been used to simulateerosion-corrosion in fluidized beds, the velocity wasachieved by rotating the specimens through a bed ofparticles [2]. In others, the vertical drop app aratus[l], the particles impacted a stationary target. Thedifference between the two methods, was that in theformer, the particle flux increased with velocity, whereasin the latter case, the particle velocity w as independentof flux. For experiments in which flux was linked tovelocity [2], there was a significant difference in theresults obtained, Fig. 4, from those of the earlier w ork,Fig. 3, because the exponents were much higher thanthose of the previous study, i.e. 5.0 at 300 C. Thiswas attributed to the fact that in this case, the exponentreflected the increase in particle flux in a d d i t i o n toimpact energy as the velocity w as increased. Such aneffect would not have occurred in the previous studywhere flux was independent of velocity.

The results of an early study [14] for the erosion ofa low alloy steel (Fe-9Cr-1M o) at 850 C, Fig. 5, showan interesting pattern of erosion-corrosion rate versusvelocity. I n this study, the velocities were s ignificantlyhigher and particle size lower than in the previousstudies. It can be seen that the velocity dependencecan be divided into a number of regimes. A low exponent,0.78, was measured for the lower velocity region between0 and 25 m s-. The intermediate velocities wereassociated with higher exponents (i.e. 3.3 in the range

Rmporolure (C)Fig. 4. Variation of wastage w ith temperature for AI.51 347 [2] aftererosion-corrosion in a fluid&d bed environment (erodent 180 pmalumina).

0 I 1 I I I I I J0 10 20 30 40 so 60 70 80v0baw 01 paw-km mlr

Fig. 5. Effect of velocity on the thickness loss of 9Cr-1Mo steel aftererosion at 850 C at 90 impact angles. T=5 h (erodent 128 pmalumina) [14].

II-I -.ominor i Turbulentflow flOUrogimo I rOglm0

IIIIIIiI

VC Plow Velocity

1

Fig. 6. Schema tic d iagram of the variation of erosion-corrosion ratewith velocity in aqueou s erosion-corrosion conditions [6].

25-35 m s-l), and, subsequently, the exponent decreasedat the higher velocities above 35 m SK. Such a variationin the exponents derived was evidence of transitionsthrough several erosion-corrosion regimes, as velocitywas increased. These results were interesting becausesuch a pattern has also been seen for erosion in aqueousmedia, Fig. 6. Hence, this shows that the transitionsbetween erosion-corrosion regimes, observed at ele-vated temperatures for erosion by solid particles areanalogous to those for erosion-corrosion in aqueousenvironments.

The exponents derived for two sets of studies involvingas-received steels also showed some interesting trends,Figs. 7-8 [3,17]. The tests were carried out in laboratorysimulated fluidized-bed environments. For the erosionof a 722M 24T steel a t 500 C, it was shown that at

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0 0.05 0.1 0.15 0.2 0. 25 0.3 0.35 0. 4 0. 45Log voloci ty

Fig. 7. Effect of velocity on the wastage of 722M2 4T at 500 C [17].

VI4 0.2 i- / -I

0. 1 0.2 0.3 0.4 0.5Log velocity

Fig. 8. E ffect of velocity on the weight loss of Incoloy 8O OH influidized-bed environments at 500 C [3] (erodent 200 pm silica).

the lower velocities, the exponents were less than 1,Fig. 7. Such results were consistent with the earlierresults, Fig. 5, which reported low exponents in thehigh temperature corrosion-dominated regimes. Itwas interesting that for other work, Fig. 8, low exponentswere also reported for this regime. At the high velocities,Figs. 7-8, the exponents increased significantly. Thiswas due to a transition in the erosion-corrosion regimein addition to the flux/velocity dep endenc e for thesetests, outlined earlier.

It is worth noting that when a transition betweentwo erosion

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MM . Stack et al. / Wear 180 (1995) 91-99 91

Mild steelWeight loss (mg cm-2130

0 1 2 3(a) Velocity (m e-l)

310 SteelWeight lore (ma cm-21

20

10

7?? ,

, 6OOC

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Fig. 12. Variation of weight loss with velocity at 300 C and 600 C,after erosion in a fluidized bed environment (erodent 580 pm alumina):(a) mild steel; (b) stainless steel.

The reasons why exponents should be high in the erosion~orrosion-dominated and corrosion-domi-nated 1 regimes are not fully understood. One ex-planation is that the erosion constant K changes in thedifferent regimes, and the velocity exponents derivedreflect this change. Such an effect can be analyzed asfollows. If in the derivation of the exponent, K, andK2 are the erosion constants of the scale formed aftererosion at o1 and uuz respectively:

(2)

In the evaluation of the exponent, K2 is always assumedto be equal to K1. However, if a transition through anerosionK,. If this is the case, then the calculated nwill be less than the figure eva luated than if it wereassumed that K,=K,.It is interesting that the velocity dependence throughthe erosion

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Fig. 13. Schematic diagram of the anticipated transitions betweenthe erosion Alloy 2.

dominated regime to lower velocities, and, if thecorrosion rate of the environment increases, or theparticle im pact energy decreases, the corrosion-dom-inated regime w ill shift to higher velocities. Hence,increasing the temperature will shift this curve to highervelocities, as the transition to erosion-dominatedbehaviour is achieved at higher velocities. (An analogycan be made between this behaviour and the effect ofvelocity in Figs. l-2, w here increa ses in velocity shiftsthe so-called corrosion-dominated regimes to highertemperatures) Fig. 14 is a schematic diagram of theeffect of velocity for alloys of different corrosion re-sistances, where the corrosion rate of Alloy 1> Alloy2, based on the experimental results reported earlier,Figs. 10-1 1. It can be seen that in the low velocityrange 2)1-V2, the dependence is lowest for the leastcorrosion resistant alloy, Alloy 1. However, in the highervelocity range Q -V,, this behaviour reverses, as erosion-

dominated behaviour (where velocity dependence var-ies between 2-3) is achieved by the most corrosionresistant alloy, Alloy 2 at a lower velocity than forAlloy 1. A similar pattern to the above w ould be expectedif the effect of velocity w ere a ssesse d for the samealloy at a range of temperatures (Fig. 12), since in-creasing corrosion resistance by increasing alloying ad-ditions is analogous to reducing the erosion+orrosiontemperature. This pattern shows that the apparentinconsiste ncies in the velocity expon ent for alloys ofdifferent corrosion resistance can be related to thedifferent ve locity depend ence in the various ero-sion-corrosion regimes. These observations also suggestthat it may be erroneous to assum e that the velocityexpone nt observe d for alloys of different corrosionresistance remains unchanged even if the severity oferosion-corrosion conditions is varied. Hence, cautionmust be used in extrapolating velocity effects in oneset of erosion-co rrosion condition s to other conditionsfor alloys of different corrosion resistan ces.

The results also show, Figs. 11 and 1 2, that therank ing order of erosion-co rrosion rates of alloys ofdifferent corrosion resistances can reverse in differentvelocity conditions. This has important consequencesfor materials selection in erosion-corrosion environ-ments. The relative erosion*orrosion resistance of al-loys in one environment cannot be used arbitrarily topredict resistance in other environments, particularlyif parameters such as velocity are varied significantly.

4. Concluding remarks(i) The effect of velocity is depen dent on the ero-

sion-corrosion regime on the alloy surface. Factorswhich change this regime, suc h as temperature or alloycorrosion resistance, will alter the erosion

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of different corrosion resistan ce can reverse if thevelocity is chan ged significantly. Hence , the relativeerosion-corro sion resistanc e of alloys in one environ-ment cannot be used arbitrarily to predict resistancein other environments, particularly if parameters suchas velocity a re varied significantly.

Acknowledgements

The authors are grateful to CON ACY T Mexico forsponsorship of J. Chacon-N ava.

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M.M. Stack, F.H. Stott and G.C. Wood, J. de Phys. N , Colloq.C9, Suppl., 3 (1993) 687-694.M.M. Stack, F.H. Stott and G.C. Wood, Mat. Sci. Tech., 7(1991) 1128.G. Cragnilino, Pmt. Con5 CORROSION 87, 1987, paper 86.V.H. Heitman and W. Kastner, v%B fiafi werkrtechnik, 62 (3)(1982) 211.G. Ito, Y. Shimizu, T. Ishihara and T. Fugi, Proc. Con& ht.Nickel Power, Japan, 1972, paper 8.N. Gat and W. Tabakoff, Wear, 50 (1978) 85.I.M. Hutchings, Proc. 5th In t. Conf Erosion by Li quid and SolidImpact, Cambridge 1979, paper 36.A.G. Evans, M.E. Gulden and W.H.E. Rosenblatt, Proc. Roy.Sot. London, Ser. A, 361 (1978) 343.A.W. Ruff and S.M. Wiederhom, Erosion by solid particleimpact, Treatise in Mater. Sci. Technol., Vol. 16, Mater. Erosion,Academic Press, 1979.I.M. Hutchings, J. Phys. D: App. Phys., 25 (1992) (A212-A221).A.V. Levy, Wang Buquian, Yong Fa Man and Nancy Jee, Proc.7th Int. Conf on Erosion by Li quid and Solid Impact, Cambridge1987, paper 72.LG. Wright, V. Nagarajan and J. Stringer, Oxid. Met., 25 (1986)175.M.M. Stack, F.H. Stott and G.C. Wood, Wear, 162-164 (B)(1993) 706.P.M. Rogers, I.M. Hutchings and J.A. Little, in S.A. Meguid(ed.), Surface Engineering, 1991.A.J. Ninham, M.J. Entwisle, I.M. Hutchings and J.A. Little,Proc. 10th Int. Conf on FBC, 1989.M.M. Stack and J. Chacon-Nava, submitted for publication.