The influence of erodent characteristics on the erosion of selected coatings at ambient and elevated temperature

Surface and Coat ings Te chn ology, 54/55 (1992) 32-38

32

The influence of erodent characteristics on the erosion of selected coatings at ambient and elevated temperature Patrick N. Walsh and Robert C. Tucker, Jr. Union Carbide Coatings Seroice Corpo ration , J500 Polco Street. Indianapolis , IN 46224 (USA )

Abstract The erosion of a Super D-Oun.... a D-Oun*. and a plasma chromium carbide coating of interest to the power generation industry was compared with that of 400-series stainless steels in tests with chromite erodents of differing particle size. and with alumina eroden!. Some tests were conducted at 550°C with particle velocities of 305 m s - 1 and some at room temperature with particle velocities near 49 m 5- 1. The 400-series stainless steels were found to exhibit maximum eros ion rates at impingement angles close to 30° when attacked by chromite fractions with particles no larger than [20 microns , but not when attacked by larger chromite particles or by alumina. The ratios of the erosion rates of the coatings to those of the steel at low angles were also low when the larger chromite or the alumina were used, but attained values of 9 : 1 or greater with the smaller chrornite; the highest ratios wcre obtained at room temperature, with the smallest chromite. For all conditions. the Super D-Oun coating eroded more slowly than the D-Oun coating. which eroded more slowly than the plasma coating . The erosion resistance of all the coatings was enhanced by pre-oxidation.

1. Introduction Erosion of construction materials by solid particles is recognized as a significant economic factor in many industries. Turbine engines, particularly the compressor sections of gas turbines and the control stages of steam turbines, are cases in point. The former have to survive ingestion of sand and/or dirt, depending on their locations and local conditions. The latter must resist damage by impacting boiler scale that has broken loose and become entrained in the steam path. Power recovery turbines in other industries also have to contend with solid particles in the gas stream. In refineries, for example, dust from catalyst beds often becomes incorporated in the gas flow and erodes the turbine blades. Loss of efficiency due to changes in size and configuration of blades and vanes, and the increase in down-time required for their replacement, are at least as important in these power applications as the cost of replacing eroded parts. In addition, there are many other less critical applications where the cost of replacing eroded parts is a significant economic factor; one such is the fans used to induce draft in some coal-fired boiler systems. In all of the cases of erosion damage cited, hard coatings applied to the gas-path or steam-path surfaces have provided adequate protection from erosion to justify the cost of applying the coatings. Union Carbide Coatings Service Corporation has developed materials "'Super D-Oun and D-Oun are trademarks of Union Carbide Coatings Service Technology Corporation.

0257-8972/92/$5.00

and supplied coatings that have been successful in all these cases. In some, the coatings are still in the field trial stage; in others, they have been adopted as bill-ofmaterials as the result of successful performance in such trials. In every instance, however, trials in the field installation have been necessary, as might be expected. However, there are limits to the amount of time, space and personnel that can be allotted to evaluate new materials. Selecting coating materials for new applications therefore requires testing in the laboratory prior to testing in the field, and testing in a way that rates. materials relative to each other in the same order as the application. It is widely agreed that laboratory tests can never entirely replicate the conditions that will apply in the actual application. This makes the choice of which test to use to measure the likely suitability of an application a matter of judgement. One purpose of this paper is to illustrate, for a particular case, certain factors that were found to be significant in discriminating between materials. The case is the selection of materials to coat steam turbine components. The successive steps in a program to identify evidence of the suita:bility of two coatings for this application will be described. In particular, the way particle size and composition were found to affect rates, and particularly relative rates, of high temperature erosion will be illustrated. Preliminary stages of this program have been reported previously [1]. That work was concerned strictly with the materials, detonation gun coatings composed of chromium carbides and either Ni-Cr or Fe-Cr-Al-Y

© 1992 - Union Ca rbide Coat ings Service Technology Corporation

P. N . Walsh, R. C. Tucker, Jr. I The influence of erodent characteristics

alloys, and the effect of pre-oxidation on them. It was shown that oxidation (at 550 °C and 650 °C for n100 h) of coatings of these compositions, prior to testing, increased their hardness and their resistance to erosion by alumina at room temperature, but had little effect on their erosion at 500 °C and 700 "C, Improvement of high temperature erosion resistance had been demonstrated earlier by Wlodek [3J for a plasma coating containing Fe-Cr-AI- Y. Wlodek used a chromite erodent, judging it to be more similar to the magnetite boiler scale that is the erodent in steam turbines than alumina. At the 1990 UPGC [2J, measurements on the erosion by chromite of some as-deposited D -Gun and Super D-Gun coatings and of a heat-treated plasma coating similar to Wlodek's were presented. That report provides the point of departure for the present considerations.

2. Materials A sub-group of the materials described in ref. 1 was used in the extended study described here. The materials selected are described in Table I. The codes in Table 1 will be used to designate the coatings throughout this discussion. Attachment of an asterisk to the letter code indicates the coating was hea t-treated at 550 °C for n h in air before testing. The materials studied here are intended to be used as coatings in steam turbines where they will necessarily be exposed for thousands of hours at temperatures near 550 °C. It is therefore pertinent to determine what effect exposure at this temperature will have on their erosion resistance. In the work described in ref. 2, only the plasma coating (coating B) was heat-treated before testing. For the current study, however, all the coatings were tested after preliminary exposure in air for 72 h at 550 °C. The extensive oxidation of splat boundaries in coating B that resulted from this treatment is shown in Fig. 4 of ref. 2. The pre-oxidation has a much smaller TABLE I. Ch aracteristics of co atings Code

A B*

C

N ame

Composition

How coated

80 Chromium carbide 20 (80Ni- 20Cr) 85 Chromium carb ide 15 Fe-Cr-AI- Y 85 Chromium carbide 15 Alloy 7 18

D-Gun Plasm a torch Super D-Gun

*Indicates the coating was hea t-treated befor e testing. Coa tin g B was only used in the heat- treated condition. Heat-treated versions of coatings A and C were included in some test s; they are indicated as A* and C* in the text andfigurcs. RUCAR is a registered trademark of Union Carbide Corporation. ™SD G is a trademark of Union Carbide Coatings Service Technology Co rporation.

33

effect on the microstructure of the detonation gun and Super D-Gun coatings. The almost negligible change that occurs in coating A is illustrated in Figs. l(c) and 2(c) of ref. 1. Similarly, coating C undergoes very slight microstructural change as a result of heat treatment.

3. Erosion tests at 550 °C 3.1. Apparatus and procedure The elevated temperature testing was done at the University of Cincinnati under the direction of Dr W. Tabak off. The apparatus used has been described in ref. 4 and also in ref. 2. In brief, the surface to be eroded is exposed to a flow of heated gas in which the erodent particles are entrained. The design and operation of the apparatus permit both the sample and the erodent to come to a preselected temperature before the erodent strikes the target. The erodent also reaches the velocity of the gas stream before impact. All the new tests described in this paper were carried out at 550 °C at a particle velocity of305 m s- 1 (1000 ft s -1). Half the tests described in ref. 2 had been carried out under these conditions, making direct comparison possible. 3.2 . Erosion with - 200 micron chromite The tests employing chrornite with a maximum particle size of 200 microns , and the erodent itself, arc descr ibed in ref. 2. The results obtained for some of the materials of interest to the present discussion, coatings A, C, and B*, and the uncoated type 410 stainless steel arc illustrated in Fig. 1. (Other materials and other test conditions were also reported in ref. 2.) Of particular note in Fig. 1 is the difference in angular dependence of the plasma coating from that of the two detonation gun coatings. The former exhibits much more nearly the classical dependence on angle of erosion of a brittle material, with a maximum rate of erosion occurring near the 90° angle of impingement. Type 410 stainless steel exhibits the behavior expected for a ductile metal, exhibiting a maximum erosion rate near 30°. The erosion of the two as-coated detonation gun coatings exhibits only a small dependence on angle , though it increases slowl y with the highest rates being noted at the 90° angle of incidence. The most interesting facet of Fig. 1, however , is the small difference in erosion rate at angles less than 30° between the uncoated stainless steel and even the best coating. This ratio ranges from 1.6 to 2.6 at a 15° angle of impingement, and from 1.3 to 1.6 at 30°. This is much lower than expected, particularly in the light of reports from utilities that coatings of the kind being examined here were already providing useful life increases in steam turbines operating at the same temperatures and velocities as were employed in these tests. Furthermore,

P. N . Walsh. R . C. Tu ckel'. Jr . I T he Influence of erodent cbaracteristics

34

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Wlodek [3J had observed that a plasma coating quite similar in composition to the one being tested had reduced the rate of erosion of type 422 stainless steel by up to a factor of 10 at similar velocities and temperatur~s. Comparison of the size distribution of the chrornite particles in our study and that of Wlodek (curves labeled - 200 micron and - 30 micron, respectively, in Fig. 2) suggested that the size of the chromite particles might be critical to resolving the discrepancy. A program to study this' factor was therefore started. 3 .3. Ero sion with - 120 micron chromite Wlodek [3J had used a commercial "chromite flour" wit h a maximum particle size of approxim atel y 30 microns, The material purchased by us under the same des ignation turned out to be quite different, howe ver. Its size distribution is shown in Fig . 2 as -120 micron. As was the case with the larger chromite used earlier, the - 120 micron chrom ite contained a very wide range of particle sizes. While this is still a much coarser material than that used by Wlodek, it is significan tly lower in large particle content than that used in the series of tests just described abo ve. A second round of testing in the University of Cincinnati's high temperature apparatus was therefore undertaken, using this grade of chromite. Experiments were limited to incidence angles of 15° and 30° as the questio n of whether relative rates were dependent on erodent size could best be answered there, Furthermore, these low angles are most significant to steam turbine operations.

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Testing at 550 DC and 305 m s - 1 particle velocity with the - 120 micron cut of chromite resulted, as might be expected, in lower eros ion rates for all the coatings. The extent of the change for the as-deposited coatings can be seen in Table 2. The rates generated by th e -120 micron erodent fall systema tica lly below those measured with - 200 micron material, but exhibit approximately the same average incre ase in rate between 15° and 30° as the older data. The same was no t true for the stainless steel, however. As shown in Table 2, the rate of erosion of the type 422 stainless steel with -120 micron chromite more than doubled between 15° and 30°, whereas the erosion of the type 410 stai nless steel with - 200 micron chromite increased by about only 20% between the two angl es, The erosion ra tes measured in this phase of the in vestigation are presented in Fig. 3 as ratios of the rate of erosion of type 422 stainless steel to that of each material. Two things can be noted in this figure. First, TABLE 2. Erosion rates (mg g- I of erod ent) in tes ts with - 200

micron and - 120 m icro n chromite Angle

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P. N. Walsh, R. C. Tucker, Jr.

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35

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COA TlNG Fig. 3. Ratio of rate of erosion of type 422 stainless steel to that of coatings, for two angles: -120 micron chromite; 550°C, 305 m S-I.

the relative rate of erosion of the uncoated stainless steel has increased to the point that the steel is seen to erode at a rate ranging from six times the rate of coating A up to 14 times the rate of heat-treated coating C*, at a 15° incidence angle. The relative rates at 30° fall in the range of six times that of coating A to almost ten times that of coating C*. Second, for the two coatings tested both as-coated and after heat treatment, the heat treatment provides a significant increase in erosion resistance. For both coating A and coating C, this increase amounts to more than 50%.

with -120 micron chromite. As a result, no further increase in the ratio of metal erosion to coating erosion was noted. In fact, the ratio of the rate of erosion of type 422 stainless steel to that of coating C'" dropped back into the 9-10 range from the nearly 14 that was observed with -120 micron powder (Fig. 5). 15'

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A further extension of the work described was initiated by screening the -120 micron chromite "flour" through a 325 mesh (44 micron) screen. The product proved to contain a few percent of particles greater than 44 microns, as can be seen in Fig. 2, and is best characterized as a - 60 micron powder. Experiments were again carried out in the Cincinnati apparatus at 550°C and 305 m s -1 particle velocity. In this exercise, measurements were made at angles ranging from 15° to 90°. The measured rates are shown.in Fig. 4. The absolute values of the rates of erosion by - 60 micron chromite were about two thirds of those generated by -120 micron chromite, in most cases; for coatings A* and C*, virtually no decrease in the rate at a 15° incidence angle was noted. The rate of erosion of type 422 stainless steel also decreased, again by about one third, from what it was

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p, N, Walsh. R. C, Tucker. Jr. / The influence oj erodent charactel'istics

36

The results of the - 60 micron tests are illustrated in Figs. 6 and 7, which summarize the metal-coating erosion rate ratios for the three series of experiments. It is apparent that the effect of particle size on relative erosion rates has diminished to almost zero with the - 60 micron experiments. For all the coatings, the ratio of erosion rate of type 422 stainless steel to that of the coating has either decreased with the substitution of - 60 micron chromite for -120 micron, or has remained approximately the same. Furthermore, the shape of the dependence of erosion rate on incidence angle has changed for at least some of the materials studied. Comparison of Figs. 1 and 4 shows that the stainless steel has taken on more of the shape expected for a ductile metal as particle size changed from - 200 micron to - 60 micron. Notably, the plasma coating has also changed significantly, Whereas (Fig, 1) it had risen steadily between 15° and 90° when eroded with - 200 micron chromite, with the smaller erodent (Fig.4), its rate of erosion is nearly flat between 45° and 90°. In this, it very much resembles the detonation gun and Super D-Gun coatings, These coatings have also flattened somewhat, though they were not highly angle-dependent at high angles even with the - 200 micron erodent.

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The erosion of the same coatings by the same size fractions of chromite was studied at room temperature while the high temperature test program was in progress, The room temperature measurements were done using the apparatus shown in Fig, 8 in which a fixed mass of erodent is aspirated into the air stream and directed at the sample, Particle velocity is approximately 49 m S-l (162 ± 19 ft s - 1), Tests were conducted at 20° and 90°. In the tests at 20 1200 g of chromite impinged on each specimen; at 90°, only 600 g was required to get the appropriate weight losses. The erosion rate was deter· 0

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P. N. Walsh, R. C. Tucker, Jr. / The influence of erodent characteristics

mined by dividing the loss of mass of the sample by the mass of erodent. Tests were run using -120 micron and - 60 micron chromite, at impingement angles of 20 and 90 0 • The erosion rates measured at 20° are shown in Fig. 9, again in the form of the ratio of erosion rate of type 422 stainless steel to that of the coating in question measured under the same conditions. It is obvious that the relative rate so defined increases in every case as the chromite size decreases. The same proved to be true for all materials measured at the 90 0 impingement angle, Fig. 10. Notably, the diminution of the size effect that was evident in the high temperature tests does not appear in the room temperature test. In the latter, the relative rates measured with - 60 micron chromite are virtually all substantially higher than those resulting from attack by -120 micron material. 4.1. Erosion by 50 micron alumina Some of the coatings that were being extensively tested with chromite were, at about the same time, subjected to erosion by 50 micron alumina particles in the apparatus shown in Fig. 8. The results are shown in Fig. 11, again in the form of the ratio of the erosion rate of type 422 stainless steel to that of the coating under the same conditions. In this case, coating C had been heat-treated in air for 8h at 718°C (I324°F) and is therefore

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38

P. N. Walsh, R. C. Tucker, Jr. / The influence of erodent characteristics

designated C**; this heat treatment has generally tended to improve the erosion resistance of coating C somewhat more than the 72 h at 550°C heat treatment applied to all the samples discussed previously. It is obvious from Figs. 9-11 that the tests with alumina would rank the improvement provided by coatings much lower than the tests using chromite. The same has frequently been observed in our laboratory using the ASTM 076-83 test procedure employing 27 micron alumina as the erodent. In this test, the erosion rate for both hard and soft steels and for other alloys is invariably found to be lower at 30° than at 90°, by a factor of about 2. This is obviously not in accord with the usual picture of metallic erosion. This test also regularly ranks metals, even annealed 1018 steel, as superior in 30° erosion resistance to all but the most erosion-resistant coatings, even though the same coatings are in service providing real and useful erosion protection to working turbine and fan components. Alumina thus seems not to be a desirable erodent to use in a program aimed at estimating the benefit that a coating might bring to an erosion-prone system; chromite of appropriate size might be better.

Room temperature erosion by 50 micron alumina yielded ratios of metal-to-coating erosion rates that were as low as those measured at 550°C with the largest chromite fraction used. It is concluded that neither alumina nor the - 200 micron chromite material gives a realistic picture of the improvement that coatings provide to steels in many commercially significant situations. Use of -120 micron or - 60 micron chromite gives a more realistic picture. Within the limits of the present investigation, tests at room temperature with the preferred size fractions of chromite impelled at relatively low velocities ranked the materials about the same as the more realistic tests conducted at 550°C and 305 m s -1 particle velocity.

Acknowledgments The authors are indebted to T. A. Jordan for preparation of the samples, and to Dr Widen Tabakoff, whose cooperation in the high temperature testing is much appreciated.

5. Summary and conclusions References The erosion of a number of coatings and 400-series stainless steels has been measured using chromite fractions differing principally in the maximum particle size. The ratio of the rate of erosion of the steel to that of the coatings, at low incidence angles, was small when the maximum erodent size was 200 microns. It increased markedly when the maximum erodent size was reduced to 120 microns, and stabilized or decreased slightly when the maximum size was further reduced to 60 microns. In concurrent room temperature tests, the ratio of steel erosion to that of the coatings was higher when - 60 micron chromite was substituted for -120 micron chromite,

P. N. Walsh and R. C. Tucker, Jr., The effects of pre-oxidation on the high temperature erosion resistance of several chromium-carbidebased coatings, presented at Int. Can! Metallurgical Coatings and Thin Films, San Diego, CA, 1989. 2 P. N. Walsh and W. Tabakoff, Comparative erosion resistance of coatings intended for steam turbine componellts, in C. P. Bellanca (ed.), PWR-Vol. 10, Advances in Steam Turbine Technologyfor Power Generation, ASME, 1990. 3 S. T. Wlodek, Erosion resistant coatings for steam turbines, Report EPRI CS-5415, Electric Power Research Institute, Palo Alto, CA,

1987. 4 W. Tabakoff, Turbomachinery alloys affected by solid particles, ASME paper 88-GT-295, presented at the Gas Turbine and Aeroengine Congress, Amsterdam, 1988.