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Ion implantation for high temperature corrosion protection
ELSEVIER
Surface
Ion implantation
and Coatings
Technology 1OOGlOl ( 1998) 196&201
for high temperature corrosion protection M.F. Stroosnijder
Institute for Advanced Materials.
Joint Research Centre, European Commission. 21020 Ispra. (VA), Italy
Abstract Ion implantation has only limited potential as a corrosion protective treatment in high temperature technological applications. Far more important is the indirect contribution of ion implantation to corrosion science as a research tool to study the influence of various elements on the corrosion behaviour of materials. It can be a very efficient technique in the frame of screening tests for evaluating the effect of possible alloying additions in materials. Additionally, it can also play a role to study the underlying corrosion mechanisms. In the present paper this is illustrated with some high temperature corrosion studies. Notably, its use for studying the mechanisms underlying the so-called reactive element effect; for developing more corrosion resistant TiAl-based intermetallic alloys and to address mechanistic questions concerned with mixed oxidation/sulphidation. It is concluded that ion implantation is a powerful tool in high temperature corrosion studies, and can thus be very helpful for material development. 0 1998 Elsevier Science S.A. Keywords:
Ion implantation;
Corrosion;
Oxidation;
Reactive elements; Chromium;
1. Introduction Metallic components used for structural components at high temperature serve in a wide variety of environ-
ments depending on the type of industrial process and process parameters. Oxygen, sulphur, carbon and nitrogen are the aggressive reactive species most frequently encountered. Various types of corrosion can be anticipated within different industrial processes. Oxidation resistance depends mainly on the potential of the alloys to form and maintain a protective oxide scale of Cr,O,, Al,O, or SO, with low rates of growth. The role of this scale is to isolate the bare alloy from the environment and thereby to limit the corrosive attack. Oxides of other high temperature alloy constituents, such as Fe, Ni and Co, or mixed oxides are generally lessprotective. In a large number of applications other reactive species,in addition to oxygen, are present and attack by other gaseous species must be taken into account. These speciesare mostly incapable of forming protective scales and may even form low melting or volatile compounds with the alloy. The fact that oxide layers generally form a barrier against other reactive gaseous species means that the resistance of alloys to mixed gaseous attack depends on their ability, also under these conditions, to form a protective oxide scale. Even if a protective scale has formed, superimposed 0257-8972/98/$19.00
0 1998 Elsevier
PII SO257-8972(97)00613-O
Science
B.V. All rights reserved.
Intermetallic
alloys; Titanium
aluminide
erosive, thermal and/or mechanical loading, as generally encountered under operating conditions, may undermine the scale stability causing it to crack and spall, thereby leading to accelerated corrosive attack of the underlying alloy. In the case of corrosion protection arising from an artificially applied layer, spallation of this layer might be disastrous. Additionally the spalled fragments may causesevereerosion, blocking of gasflows or contamination of products. Cracking and spallation of protective oxide scales under practical conditions, therefore, is a subject of increasing concern. In this context the beneficial role of small additions (less than 1%) of reactive elements, such as yttrium, cerium and other rare earths, has received considerable attention, seee.g. Refs. [l-3] and literature therein. In ion implantation high energy ions are generated in an ion accelerator and implanted into the alloy surface. Penetration depths are typically of the order of O.OlLl pm, while the concentration distribution has a maximum with values up to several tens of percent. Ion implantation offers several advantages, such as: thin layers are produced, a wide range of elements can be implanted, low process temperatures and a good reproducibility of the process. The absence of a definite interface solves the problem of adhesion. The radiation damage, which is also generated during implantation and which might have an important additional effect at
A4.F Stroosnijder
/ Surj&z
and Coatings
low temperatures is rapidly annealed at moderate to high temperatures. The major drawbacks of ion implantation are besides the limited depth, the necessity of working under vacuum and the need for ion sources capable of supplying high ion doses, which make the process expensive. Because of this, there is no real application for high temperature corrosion resistance yet and possible applications can only be expected for small components in which quality and durability are essential and as a means of conservation of extremely expensive and rare alloying elements. However, far more important for corrosion science is the role of ion implantation as a research tool to study the influence of various elements on the corrosion behaviour of materials. The literature on ion implantation in high temperature oxidation studies prior to 1983 was reviewed by Bennett [4]. Since then most ion implantation studies dealt with the reactive element effect and these were summarized in 1989 by Bennett and Tuson [5]. It is not the intention of this paper to present a general survey, but to illustrate the use of ion implantation in some high temperature corrosion studies. This will include its use for studying the mechanisms underlying the so-called reactive element effect; for developing more corrosion resistant TiAlbased intermetallic alloys and to address mechanistic questions concerned with mixed oxidation/sulphidation.
2. High temperature corrosion studies 2.1. Effect of reactive elements upon oxidation Ion implantation has been used extensively and successfully for studying the mechanisms underlying the so-called reactive element effect [5-l 11. In general it is found that the influence of these elements are similar whether the reactive element is implanted, applied as a coating or present as an alloy or oxide dispersoid addition. The effects are on the initial nucleation, growth and adherence of the oxide scale. In a study the beneficial effect of yttrium ion implantation on the oxidation behaviour in air of powder metallurgically (PM ) produced chromium was investigated [9-l 11. In tests up to 600 h in duration, it was shown that corrosive attack was significantly reduced, typically by a factor 4, despite the shallow implantation depth (less than about 0.1 urn). Also the corrosion of PM chromium containing yttria as a dispersoid, which is smaller than of Cr, was reduced further by yttrium implantation. On examining the initial stages of oxidation as the yttrium-implanted chromium was rapidly heated to 900 “C, the short range structures of two yttrium based oxides, Y,O, and YCrO,, were observed by glancing angle X-ray techniques [ 111. The latter structure is believed to represent the structure around yttrium segre-
Technology
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( 1998) 196-201
197
gdted to the boundaries of the Cr,O, grains. The YCrO, originates from the Y,O, phase formed during the initial oxidation period. The oxide scale on the unimplanted Cr after 600 h of exposure (Fig. 1) is dominated by large columnar grains which grow with a preferred crystallographic orientation. Cross-sectional examinations revealed that the oxide scale consisted of large columnar grains. The oxide scale on the yttrium-implanted region of the chromium (Fig. 1) shows a different morphology. The oxide grains were comparatively small and a large fraction of the oxide scale had a convoluted character. Glancing angle X-ray diffraction indicated in this case no preferred orientation of the oxide grains. The scale formed on the unimplanted region of the Cr-l%Y,O, also exhibited large areas of columnar grains, but additionally regions with significantly thinner scales consisting of smaller oxide grains were observed. Crosssectional examinations on this material revealed that the oxide thickness was very inhomogeneous over the sample surface (Fig. 2). The oxide scale formed on the yttriumimplanted Cr-l%Y,O, was very thin. The structure after longer exposure times was similar to that of the yttrium-implanted chromium. Oxygen tracer experiments [9] have shown that outward cation transport dominates the oxide growth, the expected classical behaviour for Cr [ 121. The overall corrosion of Cr-l%Y,O, appears to be similar to that of pure chromium, but the corrosion rate is reduced. Similar experiments have suggested that inward transport of oxygen is the dominating oxide growth process for the yttrium-implanted materials [9]. However, a certain amount of outward transport was observed to occur. This might cause the growth of new oxide within the existing oxide scale, as a result of the simultaneous diffusion of cations and anions along the same diffusion paths, e.g. grain boundaries. Consequently, lateral scale
Fig. 1. Surface scale morphology near the boundary of unimplanted (left) and 10” Y ions cm m2 implanted (right) chromium after exposure for 600 h in air at 900 “C.
20 urn Fig. 2. Cross-sectional scanning electron micrograph after exposure for 150 h in air at 900 C.
of Cr-I
%YL03
growth occurs. This could result in for the convoluted oxide morphology, observed on the implanted materials (Fig. 1). Although several different mechanisms have been proposed to explain the reactive element effects, the segregation of yttrium to grain boundaries is almost certain to play an important role. As has been suggested in the literature [5,13,14], the segregation of yttrium to the grain boundaries, possibly with a local structure of YCrO, in the present case, as proposed in a previous study [Ill, plays a significant role and may even explain all of the reactive element effects. The most plausible effect will be that of a change in the anion and cation transport rates along the grain boundaries owing to the presence of yttrium, as was also inferred by two-stage oxidation experiments [9]. This directly changes the overall oxidation kinetics. It is interesting to note that the material containing 1% Y,O, appears on some areas to have the same good corrosion resistance as observed for the yttriumimplanted chromium. The fact that the overall behaviour of Cr-l%Y,O, is not as good is believed to be due to the rather coarse and inhomogeneous character of the yttria dispersion in the material ( Fig. 3). This would prevent the yttria particles acting as an efficient source for yttrium segregation. Thus it appears that the behaviour of the Cr-l%Y,O, material can be further improved. 2.2. TiAI-bused internzetdics Intermetallic compounds are being considered as suitable structural materials for high temperature applications. Especially alloys based on TiAl are promising for their high temperature strength in combination with low density [ 15,161. However, application of these materials at temperatures exceeding about 700 “C is limited, among others, by their poor oxidation resistance. In
Fig. 3. Scanning electron microscopy morphology of Cr- I %YzO, after a sputter etch treatment to reveal the yttria dispersoid (light) distribution.
spite of their high aluminium content, TiAl-based intermetallics do generally not form long-lasting protective alumina scales [ 17- 191. After longer exposure times, the scales, which are initially rich in alumina, deteriorate and scales consisting of mixed alumina and titania predominate, with high growth rates similar to those of pure titania. For this reason the addition of other elements by alloying to improve the corrosion resistance has been studied by several authors. The situation is rather complicated since elements which might be beneficial for high temperature corrosion, might impede the mechanical properties. Additionally, the published corrosion data show significant scatter and even lead to contradicting conclusions. This is possibly partly caused by differences in alloy purity, because several alloying elements have been demonstrated to posses a significant elIect on the corrosion properties even if they are present in small quantities. Hence, there is need for a fast screening procedure of possible alloying elements with respect to their impact on the corrosion properties of these materials. Ion implantation can serve as such a research tool and has been used successfully in this frame [ 20-241. In a recent study [22] the addition by ion implantation of a wide range of elements on the oxidation behaviour in air at 800 “C of a Ti-48Al-2Cr (at.%) was studied. Fig. 4 shows the mass change as a function of time of the unimplanted Ti-48Al-2Cr and material implanted with various elements. The mass change data for the implanted material presented in Fig. 4 are not corrected for the oxide growth on the non-implanted part of the specimen surface, which was approximately 20%. Thus, the mass change per unit area due to oxide growth on the implanted surface should be even lower. Implantation of Ti-48AlL2Cr with Ti ions did not alter the oxidation behaviour significantly. Implantation with Cr ions resulted in a slow transient oxidation, but at
M.F.
Stroosnijder
/ Surfucr
and Coatings
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2.3. Complex environments
‘O-7 1 Ti48Al2Cr
0
40
80 Time,
120
160
h
Fig. 4. Mass gain vs. time of unimplanted and Ti-, Cr-, Al-, Si-implanted ( IO” ions cm-‘) Ti-48Alk2Cr upon oxidation 800 “C. The values for the implanted samples have not been for the non-implanted sample faces, approximately 20% of surface area.
Nb-, and in air at corrected the total
later stages the oxidation rate increased and became almost the same as for the non-implanted material. Implantation with Al ions increases the Al concentration in the surface layer of the material, hence promoting the formation of a protective alumina layer. However, the increase of Al is technologically not desired for loss of ductility by the formation of brittle phases. In particular the addition of Nb and Si improved significantly the corrosion resistance. The outer corrosion scale formed on unimplanted Ti-48Al-2Cr after 150 h of exposure was dominated by large titania crystals, whereas a rather smooth scale, which was rich in alumina with only locally some titania-rich nodules, was formed on the niobium-implanted region of the material (Fig. 5). The beneficial effect of Nb and Si on the oxidation behaviour was confirmed by studies using materials in which these elements were added by alloying [25-271.
Fig. 5. Surface scale morphology near the boundary of unimplanted (left) and IO” Nb ions cm-’ implanted (right) Ti-48AlL2Cr after exposure for I50 h in air at 800 “C.
It should be noted that most ion implantation studies have concerned oxidation. There is a considerable scope also for using the technique to address mechanistic questions concerned with more complex corrosion conditions, for example mixed oxidizing/sulphidizing conditions, which to date, have received little attention [28& 321. Corrosion in gaseous environments containing sulphur is an essential problem in various technical high temperature processes, since sulphidation can significantly reduce the service life of metallic components. Whereas in strongly oxidizing conditions adequate corrosion resistance can be achieved, this is more difficult in environments characterized by a low oxygen and high sulphur activity. In these atmospheres the simultaneous nucleation of sulphides as well as oxides is observed. The alloy is able to resist corrosive attack under these conditions providing the oxides overgrow or undercut the initially formed sulphides and remain as a stable oxide layer [ 33,341. As discussed before, considerable attention has focused on the beneficial effects of small additions of reactive elements on high temperature oxidation resistance. Although various effects are known to occur and the exact mechanism(s) are still the subject of discussion, the nucleation and further growth of the oxide products seem to play an important role. Because of this aspect it is of interest to know whether reactive elements are also able to change the nucleation and growth of corrosion products formed in mixed sulphidizing/oxidizing environments. The effect of the addition of cerium by ion implantation on the corrosion behaviour of a commercial Fe-32Ni-20Cr (wt.%) austenitic steel, Alloy 800H, was examined in an environment, which was simultaneously oxidizing and sulphidizing at 700 “C [28,30,31]. In tests up to 1000 h in duration, it was shown that corrosive attack was significantly reduced, typically by a factor of 3, if the implantation dose was sufficiently high. The corrosion behaviour of the unimplanted Alloy 800H in this environment shows a fairly classical pattern. Owing to reaction kinetics, simultaneous nucleation of oxides and sulphides occurs on the alloy [33,34]. Both sulphur and oxygen inward transport can occur through these initial corrosion products. The activities of oxygen and sulphur, in relation to the stability of oxides and sulphides at the scale/matrix interface, may favour the enrichment of oxides beneath the scale. This can lead eventually to a continuous oxide sub-scale, which will increase the corrosion resistance. The observation of continuing localized internal and external corrosion (Fig. 6), however, indicates that the oxides do not form a fully uniform protective layer. Although the overall chemistry of the attack of the Ce-implanted Alloy 800H was similar to that of the unimplanted alloy, the improved corrosion resistance of
00
I 2
1 4
I 6
Sputtering Fig. 6. Surface scale morphology near the boundary of unimplanted (top) and IO” Ce ions cm -’ implanted (bottom) Alloy 800H after exposure for 15 min in an oxidizing/sulphidizing environment at 700°C.
the alloy resulting from the Ce implantation was manifested by thinner corrosion scales (Fig. 7) and a more uniform scale morphology with less localized corrosion attack (Fig. 6). A further three differences could be observed. The first was a greater tendency of the initial corrosion products to grow laterally with a higher coverage of the underlying alloy. Secondly, the uniform corrosion products had a higher Cr/Fe ratio and a relative higher oxygen content below the sulphide scale (Fig. 7). The third observation was that the implanted Ce was concentrated on top of the oxide layer, indicating that the oxide layer had grown preferentially by inward oxygen transport. As discussed above, the oxide layer which formed beneath the sulphides is expected to play a decisive role, since corrosion protection by sulphides is extremely poor. In general it is well known that Cr-rich oxide layers confer greater protection than that provided by iron-rich spine1 phases [ 33,35,36]. The mechanism(s) governing the improvement by the Ce implantation unfortunately could not be fully established. Since a significant influence of physical effects of ion implantation has been ruled out [31], the beneficial effect of cerium was essentially a chemical one, as was confirmed by similarities obtained when CeO, was applied to the surface of Alloy 800H by electrophoretic deposition [30]. The primary effect of the Ce implantation was to change the nucleation and growth of the initial corrosion products, resulting in a more uniform and more Cr-rich sulphide layer. The increase of the more stable sulphide-forming element (i.e. Cr) in the sulphides formed owing to the presence of reactive elements is in agreement with the literature [37]. It is not clear whether the improved uniformity and the increased Cr content of the sulphide layer are related to each other. Both factors can be argued to have a beneficial effect on the quality of the underlying oxide layer [ 30,3 I]. Since the oxygen activity at the scale/alloy
I 8 time,
Alloy 800H + 10’7Ce 700°C/200 h S-O-C gas
60
I 10
I 12
I 1L
16
12
1L
16
ks
ions/cm2
50
0
2
4
6 Sputtering
8
10
time.
ks
Fig. 7. Element distribution profiles using secondary neutral mass spectroscopy of (a) Alloy 800H and (b) Alloy 800H implanted with after exposure for 200 h in an oxidizing/sulphidizing lO”Ceionscm~ environment at 700 C.
interface will be lower than that at the outer scale interface with the atmosphere, it can be supposed that a more thermodynamically stable, i.e. more Cr rich (and thus more protective) oxide will be formed than on the unimplanted alloy surface in contact with this environment. A more uniform sulphide layer will promote a more continuous oxide subscale than that formed on the less well-covered unimplanted material, thus providing a more even protection. A further aspect is that diffusion in Cr-rich sulphides is known to be slower than that in the Fe-rich sulphides [38]. Thus it can be argued that the inward diffusion of oxygen might be slower in the case of a Cr-rich sulphide, thus lowering the oxygen activity below the sulphide. This will affect the underlying oxide, which will be richer in the more stable element, i.e. Cr. In addition to this indirect
M.F.
Stroosnijdrr
/ Surfbce
and Coatings
influence of Ce via the sulphide scale on quality of the oxide layer, it is known that under oxidizing conditions materials containing reactive elements show a stronger tendency to form Cr-richer oxides [l-3,8,36].
3. Concluding remarks Ion implantation has only limited potential as corrosion protective treatment in high temperature technological applications. However, the presented examples indicate that ion implantation is a powerful research tool in high temperature corrosion studies. The technique can be an efficient way in the frame of screening tests for evaluating the effect of possible alloying additions in materials. Additionally, it can play a role to elucidate the underlying corrosion mechanisms. The significance and further potential of this indirect contribution of ion implantation to corrosion science and material development should not be underestimated.
Acknowledgement The author is grateful to all those whose work made this review possible and apologizes for omissions.
has any
References [l] [2] [3] [4]
[5] [6] [7] [8] [9] [lo]
F.H. Stott, G.C. Wood, J. Stringer, Oxid. Met. 44 (1995) 113. B.A. Pint, Oxid. Met. 45 (1996) 1. Y. Saito, B. &ray, T. Maruyama, J. Phys. IV 3 (1993) 217. M.J. Bennett, The role of ion implantation in high temperature corrosion studies, in: High Temperature Corrosion, NACE-6, Houston, TX, 1983, pp. 1455154. M.J. Bennett, A.T. Tuson, Mater. Sci. Eng. A 116 (1989) 79. W.J. Quadakkers. J. Jedlinski, K. Schmidt, M. Krasovec, G. Borchardt, H. Nickel, Appl. Surf. Sci. 47 ( 1991) 261. K. Przybylski, A.J. Garratt-Reed, G.J. Yurek, J. Electrochem. sot. 135 (1988) 135. P.Y. Hou, J. Stringer, Oxid. Met. 29 (1988) 45. M.F. Stroosnijder, J.D. Sunderkiitter, M.J. Cristobal, H. Jenett. K. Isenbtigel, M.A. Baker, Surf. Coat. Technol. 83 (1996) 205. M.F. Stroosnijder, M.J. Cristobal. J.D. Sunderkbtter, Mater. Sci. Forum 251-254 (1997) 259.
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[ 1 l] M.J. Cristobal, P.N. Gibson, M.F. Stroosnijder, Corros. Sci. 38 (1996) 805. [ 121 P. Kofstad, High Temperature Corrosion, Elsevier, Barking, 1988. [ 131 K. Przybylski, A.J. Garret-Reed, G.J. Yurek, J. Electrochem. Sot. 135 (1988) 509. [ 141 C.M. Cotell. G.J. Yurek, R.J. Hussey, D.F. Mitchell, M.J. Graham, Oxid. Met. 38 (1990) 201. [ 151 Y.W. Kim, J. Miner. Met. Mater. Sot. 46 ( 1994) 30. [ 161 M. Yamaguchi, Mater. Sci. Technol. 8 ( 1992) 299. [17] A. Rahmel, W.J. Quadakkers, M. Schtitze, Mater. Corros. 46 (1995) 271. [18] G.H. Meier, F.S. Petit, S. Hu, J. Phys. IV 3 (1993) 395. [ 191 M.F. Stroosnijder, J.D. Sunderkotter, V.A.C. Haanappel. in: R.Y. Lin, Y.A. Chang, C.T. Liu (Eds.). Design Fundamentals of Composites, Intermetallics and Metal-Ceramics Systems. TMS, Wdrrendale, PA, 1996. pp. 2877293. [20] A. Gil, B. Rajchel, N. Zheng, W.J. Quadakkers. H. Nickel. J. Mater. Sci. 30 (1995) 5793. [21] M.F. Stroosnijder, N. Zheng. W.J. Quadakkers, R. Hofman, A. Gil, F. Lanza, Oxid. Met. 46 (1996) 19. [22] M.F. Stroosnijder. H.J. Schmutzler, V.A.C. Haanappel. J.D. Sunderkotter, Mater. Corros. 48 ( 1997) 40. [23] S. Taniguchi, T. Shibata. T. Saeki, H. Zhang, X. Liu, Mater. Trans. Jpn. Inst. Met., submitted. [24] J.D. Sunderkbtter, V.A.C. Haanappel. M.F. Stroosnijder. Microsc. Oxid., in press. [25] J.D. Sunderkiitter, H.J. Schmutzler, V.A.C. Hadnappel, R. Hofman, W. Glatz, H. Clemens, M.F. Stroosnijder, Intermetallics 5 (1997) 525. [26] S. Becker, M. Schtitze. A. Rahmel, Oxid. Met. 39 (1993) 93. [27] D.W. McKee. S.C. Huang, Corros. Sci. 33 (1992) 1899. [28] M.F. Stroosnijder, J.F. Norton, V. Guttmann, M.J. Bennett, J.H.W. de Wit, Mater. Sci. Eng. A 116 (1989) 103. [29] V. Srinivasan, D.E. Goodman, A.R. Kraus, Mater. Sci. Eng. A 121 (1989) 525. [30] M.F. Stroosnijder, M.J. Bennett, V. Guttmann. J.F. Norton, J.H.W. de Wit, Oxid. Met. 35 (1991) 19. [31] M.F. Stroosnijder, J.F. Norton. Surf. Interface Anal. 22 (1994) 436. [32] V. Srinivasan, A.W. McCormick, A.K. Rai, Oxid. Met. 34 (1990) 401. [33] R.E. Lobnig, H.J. Grabke, Corros. Sci. 30 (1990) 1045. [34] M.F. Stroosnijder. W.J. Quadakkers, High Temp. Technol. 4 (1986) 141. [35] M. Loudjani, J.C. Pivin, C. Roques-Carmes, P. Lacombe. J.H. Davidson. Metall. Trans. A 13 (1982) 1299. [36] M.F. Stroosnijder, V. Guttmann, T. Fransen, J.H.W. de Wit, Oxid. Met. 33 (1990) 371. [37] D. Saxena. S. Prakash. M.L. Mehta, I.P. Saraswat. Oxid. Met. 28 (1987) 127. [38] K.N. Strafford. W.Y. Chan, J.F. Norton, in: D.B. Meadowcroft, M.1. Manning (Eds.), Corrosion Resistant Materials for Coal Conversion Systems, Applied Science, Barking, 1983, pp. 517-549.
Surface
Ion implantation
and Coatings
Technology 1OOGlOl ( 1998) 196&201
for high temperature corrosion protection M.F. Stroosnijder
Institute for Advanced Materials.
Joint Research Centre, European Commission. 21020 Ispra. (VA), Italy
Abstract Ion implantation has only limited potential as a corrosion protective treatment in high temperature technological applications. Far more important is the indirect contribution of ion implantation to corrosion science as a research tool to study the influence of various elements on the corrosion behaviour of materials. It can be a very efficient technique in the frame of screening tests for evaluating the effect of possible alloying additions in materials. Additionally, it can also play a role to study the underlying corrosion mechanisms. In the present paper this is illustrated with some high temperature corrosion studies. Notably, its use for studying the mechanisms underlying the so-called reactive element effect; for developing more corrosion resistant TiAl-based intermetallic alloys and to address mechanistic questions concerned with mixed oxidation/sulphidation. It is concluded that ion implantation is a powerful tool in high temperature corrosion studies, and can thus be very helpful for material development. 0 1998 Elsevier Science S.A. Keywords:
Ion implantation;
Corrosion;
Oxidation;
Reactive elements; Chromium;
1. Introduction Metallic components used for structural components at high temperature serve in a wide variety of environ-
ments depending on the type of industrial process and process parameters. Oxygen, sulphur, carbon and nitrogen are the aggressive reactive species most frequently encountered. Various types of corrosion can be anticipated within different industrial processes. Oxidation resistance depends mainly on the potential of the alloys to form and maintain a protective oxide scale of Cr,O,, Al,O, or SO, with low rates of growth. The role of this scale is to isolate the bare alloy from the environment and thereby to limit the corrosive attack. Oxides of other high temperature alloy constituents, such as Fe, Ni and Co, or mixed oxides are generally lessprotective. In a large number of applications other reactive species,in addition to oxygen, are present and attack by other gaseous species must be taken into account. These speciesare mostly incapable of forming protective scales and may even form low melting or volatile compounds with the alloy. The fact that oxide layers generally form a barrier against other reactive gaseous species means that the resistance of alloys to mixed gaseous attack depends on their ability, also under these conditions, to form a protective oxide scale. Even if a protective scale has formed, superimposed 0257-8972/98/$19.00
0 1998 Elsevier
PII SO257-8972(97)00613-O
Science
B.V. All rights reserved.
Intermetallic
alloys; Titanium
aluminide
erosive, thermal and/or mechanical loading, as generally encountered under operating conditions, may undermine the scale stability causing it to crack and spall, thereby leading to accelerated corrosive attack of the underlying alloy. In the case of corrosion protection arising from an artificially applied layer, spallation of this layer might be disastrous. Additionally the spalled fragments may causesevereerosion, blocking of gasflows or contamination of products. Cracking and spallation of protective oxide scales under practical conditions, therefore, is a subject of increasing concern. In this context the beneficial role of small additions (less than 1%) of reactive elements, such as yttrium, cerium and other rare earths, has received considerable attention, seee.g. Refs. [l-3] and literature therein. In ion implantation high energy ions are generated in an ion accelerator and implanted into the alloy surface. Penetration depths are typically of the order of O.OlLl pm, while the concentration distribution has a maximum with values up to several tens of percent. Ion implantation offers several advantages, such as: thin layers are produced, a wide range of elements can be implanted, low process temperatures and a good reproducibility of the process. The absence of a definite interface solves the problem of adhesion. The radiation damage, which is also generated during implantation and which might have an important additional effect at
A4.F Stroosnijder
/ Surj&z
and Coatings
low temperatures is rapidly annealed at moderate to high temperatures. The major drawbacks of ion implantation are besides the limited depth, the necessity of working under vacuum and the need for ion sources capable of supplying high ion doses, which make the process expensive. Because of this, there is no real application for high temperature corrosion resistance yet and possible applications can only be expected for small components in which quality and durability are essential and as a means of conservation of extremely expensive and rare alloying elements. However, far more important for corrosion science is the role of ion implantation as a research tool to study the influence of various elements on the corrosion behaviour of materials. The literature on ion implantation in high temperature oxidation studies prior to 1983 was reviewed by Bennett [4]. Since then most ion implantation studies dealt with the reactive element effect and these were summarized in 1989 by Bennett and Tuson [5]. It is not the intention of this paper to present a general survey, but to illustrate the use of ion implantation in some high temperature corrosion studies. This will include its use for studying the mechanisms underlying the so-called reactive element effect; for developing more corrosion resistant TiAlbased intermetallic alloys and to address mechanistic questions concerned with mixed oxidation/sulphidation.
2. High temperature corrosion studies 2.1. Effect of reactive elements upon oxidation Ion implantation has been used extensively and successfully for studying the mechanisms underlying the so-called reactive element effect [5-l 11. In general it is found that the influence of these elements are similar whether the reactive element is implanted, applied as a coating or present as an alloy or oxide dispersoid addition. The effects are on the initial nucleation, growth and adherence of the oxide scale. In a study the beneficial effect of yttrium ion implantation on the oxidation behaviour in air of powder metallurgically (PM ) produced chromium was investigated [9-l 11. In tests up to 600 h in duration, it was shown that corrosive attack was significantly reduced, typically by a factor 4, despite the shallow implantation depth (less than about 0.1 urn). Also the corrosion of PM chromium containing yttria as a dispersoid, which is smaller than of Cr, was reduced further by yttrium implantation. On examining the initial stages of oxidation as the yttrium-implanted chromium was rapidly heated to 900 “C, the short range structures of two yttrium based oxides, Y,O, and YCrO,, were observed by glancing angle X-ray techniques [ 111. The latter structure is believed to represent the structure around yttrium segre-
Technology
100-101
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gdted to the boundaries of the Cr,O, grains. The YCrO, originates from the Y,O, phase formed during the initial oxidation period. The oxide scale on the unimplanted Cr after 600 h of exposure (Fig. 1) is dominated by large columnar grains which grow with a preferred crystallographic orientation. Cross-sectional examinations revealed that the oxide scale consisted of large columnar grains. The oxide scale on the yttrium-implanted region of the chromium (Fig. 1) shows a different morphology. The oxide grains were comparatively small and a large fraction of the oxide scale had a convoluted character. Glancing angle X-ray diffraction indicated in this case no preferred orientation of the oxide grains. The scale formed on the unimplanted region of the Cr-l%Y,O, also exhibited large areas of columnar grains, but additionally regions with significantly thinner scales consisting of smaller oxide grains were observed. Crosssectional examinations on this material revealed that the oxide thickness was very inhomogeneous over the sample surface (Fig. 2). The oxide scale formed on the yttriumimplanted Cr-l%Y,O, was very thin. The structure after longer exposure times was similar to that of the yttrium-implanted chromium. Oxygen tracer experiments [9] have shown that outward cation transport dominates the oxide growth, the expected classical behaviour for Cr [ 121. The overall corrosion of Cr-l%Y,O, appears to be similar to that of pure chromium, but the corrosion rate is reduced. Similar experiments have suggested that inward transport of oxygen is the dominating oxide growth process for the yttrium-implanted materials [9]. However, a certain amount of outward transport was observed to occur. This might cause the growth of new oxide within the existing oxide scale, as a result of the simultaneous diffusion of cations and anions along the same diffusion paths, e.g. grain boundaries. Consequently, lateral scale
Fig. 1. Surface scale morphology near the boundary of unimplanted (left) and 10” Y ions cm m2 implanted (right) chromium after exposure for 600 h in air at 900 “C.
20 urn Fig. 2. Cross-sectional scanning electron micrograph after exposure for 150 h in air at 900 C.
of Cr-I
%YL03
growth occurs. This could result in for the convoluted oxide morphology, observed on the implanted materials (Fig. 1). Although several different mechanisms have been proposed to explain the reactive element effects, the segregation of yttrium to grain boundaries is almost certain to play an important role. As has been suggested in the literature [5,13,14], the segregation of yttrium to the grain boundaries, possibly with a local structure of YCrO, in the present case, as proposed in a previous study [Ill, plays a significant role and may even explain all of the reactive element effects. The most plausible effect will be that of a change in the anion and cation transport rates along the grain boundaries owing to the presence of yttrium, as was also inferred by two-stage oxidation experiments [9]. This directly changes the overall oxidation kinetics. It is interesting to note that the material containing 1% Y,O, appears on some areas to have the same good corrosion resistance as observed for the yttriumimplanted chromium. The fact that the overall behaviour of Cr-l%Y,O, is not as good is believed to be due to the rather coarse and inhomogeneous character of the yttria dispersion in the material ( Fig. 3). This would prevent the yttria particles acting as an efficient source for yttrium segregation. Thus it appears that the behaviour of the Cr-l%Y,O, material can be further improved. 2.2. TiAI-bused internzetdics Intermetallic compounds are being considered as suitable structural materials for high temperature applications. Especially alloys based on TiAl are promising for their high temperature strength in combination with low density [ 15,161. However, application of these materials at temperatures exceeding about 700 “C is limited, among others, by their poor oxidation resistance. In
Fig. 3. Scanning electron microscopy morphology of Cr- I %YzO, after a sputter etch treatment to reveal the yttria dispersoid (light) distribution.
spite of their high aluminium content, TiAl-based intermetallics do generally not form long-lasting protective alumina scales [ 17- 191. After longer exposure times, the scales, which are initially rich in alumina, deteriorate and scales consisting of mixed alumina and titania predominate, with high growth rates similar to those of pure titania. For this reason the addition of other elements by alloying to improve the corrosion resistance has been studied by several authors. The situation is rather complicated since elements which might be beneficial for high temperature corrosion, might impede the mechanical properties. Additionally, the published corrosion data show significant scatter and even lead to contradicting conclusions. This is possibly partly caused by differences in alloy purity, because several alloying elements have been demonstrated to posses a significant elIect on the corrosion properties even if they are present in small quantities. Hence, there is need for a fast screening procedure of possible alloying elements with respect to their impact on the corrosion properties of these materials. Ion implantation can serve as such a research tool and has been used successfully in this frame [ 20-241. In a recent study [22] the addition by ion implantation of a wide range of elements on the oxidation behaviour in air at 800 “C of a Ti-48Al-2Cr (at.%) was studied. Fig. 4 shows the mass change as a function of time of the unimplanted Ti-48Al-2Cr and material implanted with various elements. The mass change data for the implanted material presented in Fig. 4 are not corrected for the oxide growth on the non-implanted part of the specimen surface, which was approximately 20%. Thus, the mass change per unit area due to oxide growth on the implanted surface should be even lower. Implantation of Ti-48AlL2Cr with Ti ions did not alter the oxidation behaviour significantly. Implantation with Cr ions resulted in a slow transient oxidation, but at
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2.3. Complex environments
‘O-7 1 Ti48Al2Cr
0
40
80 Time,
120
160
h
Fig. 4. Mass gain vs. time of unimplanted and Ti-, Cr-, Al-, Si-implanted ( IO” ions cm-‘) Ti-48Alk2Cr upon oxidation 800 “C. The values for the implanted samples have not been for the non-implanted sample faces, approximately 20% of surface area.
Nb-, and in air at corrected the total
later stages the oxidation rate increased and became almost the same as for the non-implanted material. Implantation with Al ions increases the Al concentration in the surface layer of the material, hence promoting the formation of a protective alumina layer. However, the increase of Al is technologically not desired for loss of ductility by the formation of brittle phases. In particular the addition of Nb and Si improved significantly the corrosion resistance. The outer corrosion scale formed on unimplanted Ti-48Al-2Cr after 150 h of exposure was dominated by large titania crystals, whereas a rather smooth scale, which was rich in alumina with only locally some titania-rich nodules, was formed on the niobium-implanted region of the material (Fig. 5). The beneficial effect of Nb and Si on the oxidation behaviour was confirmed by studies using materials in which these elements were added by alloying [25-271.
Fig. 5. Surface scale morphology near the boundary of unimplanted (left) and IO” Nb ions cm-’ implanted (right) Ti-48AlL2Cr after exposure for I50 h in air at 800 “C.
It should be noted that most ion implantation studies have concerned oxidation. There is a considerable scope also for using the technique to address mechanistic questions concerned with more complex corrosion conditions, for example mixed oxidizing/sulphidizing conditions, which to date, have received little attention [28& 321. Corrosion in gaseous environments containing sulphur is an essential problem in various technical high temperature processes, since sulphidation can significantly reduce the service life of metallic components. Whereas in strongly oxidizing conditions adequate corrosion resistance can be achieved, this is more difficult in environments characterized by a low oxygen and high sulphur activity. In these atmospheres the simultaneous nucleation of sulphides as well as oxides is observed. The alloy is able to resist corrosive attack under these conditions providing the oxides overgrow or undercut the initially formed sulphides and remain as a stable oxide layer [ 33,341. As discussed before, considerable attention has focused on the beneficial effects of small additions of reactive elements on high temperature oxidation resistance. Although various effects are known to occur and the exact mechanism(s) are still the subject of discussion, the nucleation and further growth of the oxide products seem to play an important role. Because of this aspect it is of interest to know whether reactive elements are also able to change the nucleation and growth of corrosion products formed in mixed sulphidizing/oxidizing environments. The effect of the addition of cerium by ion implantation on the corrosion behaviour of a commercial Fe-32Ni-20Cr (wt.%) austenitic steel, Alloy 800H, was examined in an environment, which was simultaneously oxidizing and sulphidizing at 700 “C [28,30,31]. In tests up to 1000 h in duration, it was shown that corrosive attack was significantly reduced, typically by a factor of 3, if the implantation dose was sufficiently high. The corrosion behaviour of the unimplanted Alloy 800H in this environment shows a fairly classical pattern. Owing to reaction kinetics, simultaneous nucleation of oxides and sulphides occurs on the alloy [33,34]. Both sulphur and oxygen inward transport can occur through these initial corrosion products. The activities of oxygen and sulphur, in relation to the stability of oxides and sulphides at the scale/matrix interface, may favour the enrichment of oxides beneath the scale. This can lead eventually to a continuous oxide sub-scale, which will increase the corrosion resistance. The observation of continuing localized internal and external corrosion (Fig. 6), however, indicates that the oxides do not form a fully uniform protective layer. Although the overall chemistry of the attack of the Ce-implanted Alloy 800H was similar to that of the unimplanted alloy, the improved corrosion resistance of
00
I 2
1 4
I 6
Sputtering Fig. 6. Surface scale morphology near the boundary of unimplanted (top) and IO” Ce ions cm -’ implanted (bottom) Alloy 800H after exposure for 15 min in an oxidizing/sulphidizing environment at 700°C.
the alloy resulting from the Ce implantation was manifested by thinner corrosion scales (Fig. 7) and a more uniform scale morphology with less localized corrosion attack (Fig. 6). A further three differences could be observed. The first was a greater tendency of the initial corrosion products to grow laterally with a higher coverage of the underlying alloy. Secondly, the uniform corrosion products had a higher Cr/Fe ratio and a relative higher oxygen content below the sulphide scale (Fig. 7). The third observation was that the implanted Ce was concentrated on top of the oxide layer, indicating that the oxide layer had grown preferentially by inward oxygen transport. As discussed above, the oxide layer which formed beneath the sulphides is expected to play a decisive role, since corrosion protection by sulphides is extremely poor. In general it is well known that Cr-rich oxide layers confer greater protection than that provided by iron-rich spine1 phases [ 33,35,36]. The mechanism(s) governing the improvement by the Ce implantation unfortunately could not be fully established. Since a significant influence of physical effects of ion implantation has been ruled out [31], the beneficial effect of cerium was essentially a chemical one, as was confirmed by similarities obtained when CeO, was applied to the surface of Alloy 800H by electrophoretic deposition [30]. The primary effect of the Ce implantation was to change the nucleation and growth of the initial corrosion products, resulting in a more uniform and more Cr-rich sulphide layer. The increase of the more stable sulphide-forming element (i.e. Cr) in the sulphides formed owing to the presence of reactive elements is in agreement with the literature [37]. It is not clear whether the improved uniformity and the increased Cr content of the sulphide layer are related to each other. Both factors can be argued to have a beneficial effect on the quality of the underlying oxide layer [ 30,3 I]. Since the oxygen activity at the scale/alloy
I 8 time,
Alloy 800H + 10’7Ce 700°C/200 h S-O-C gas
60
I 10
I 12
I 1L
16
12
1L
16
ks
ions/cm2
50
0
2
4
6 Sputtering
8
10
time.
ks
Fig. 7. Element distribution profiles using secondary neutral mass spectroscopy of (a) Alloy 800H and (b) Alloy 800H implanted with after exposure for 200 h in an oxidizing/sulphidizing lO”Ceionscm~ environment at 700 C.
interface will be lower than that at the outer scale interface with the atmosphere, it can be supposed that a more thermodynamically stable, i.e. more Cr rich (and thus more protective) oxide will be formed than on the unimplanted alloy surface in contact with this environment. A more uniform sulphide layer will promote a more continuous oxide subscale than that formed on the less well-covered unimplanted material, thus providing a more even protection. A further aspect is that diffusion in Cr-rich sulphides is known to be slower than that in the Fe-rich sulphides [38]. Thus it can be argued that the inward diffusion of oxygen might be slower in the case of a Cr-rich sulphide, thus lowering the oxygen activity below the sulphide. This will affect the underlying oxide, which will be richer in the more stable element, i.e. Cr. In addition to this indirect
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influence of Ce via the sulphide scale on quality of the oxide layer, it is known that under oxidizing conditions materials containing reactive elements show a stronger tendency to form Cr-richer oxides [l-3,8,36].
3. Concluding remarks Ion implantation has only limited potential as corrosion protective treatment in high temperature technological applications. However, the presented examples indicate that ion implantation is a powerful research tool in high temperature corrosion studies. The technique can be an efficient way in the frame of screening tests for evaluating the effect of possible alloying additions in materials. Additionally, it can play a role to elucidate the underlying corrosion mechanisms. The significance and further potential of this indirect contribution of ion implantation to corrosion science and material development should not be underestimated.
Acknowledgement The author is grateful to all those whose work made this review possible and apologizes for omissions.
has any
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