176
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 2, 1978
lnd Week, 43 (July 21, 1975). LaQue. F. L., Mat. Des. Eng , 99 (Jan 1963). LaQue.F. L.. CoDson, H. R.. "Corrosion Resistance of Metals and Allovs". , , 2nd ed, p 89, Reinhold, New York, N.Y., 1963. Mahan, B. H., "Elementary Chemical Thermodynamics", p 102, W. A. Benjamin, New York, N.Y.. 1963. Mazia, J., Met. Finish., 22 (Dec 1977). Moher, J. E., Met. Finish., 73 (May 1976). Parr, J. G., Chem. Eng.. 166 (July 6, 1964). Powers, R, A , , Roebuck, A , ,-,, ,~Encyc,opedia of ChemicalTechnology,t,vel, 6. D 300. 1964. Slabaugh, W. H., J Chem. Educ., 41, 218 (1974).
Uhlig, H. H., "Corrosion and Corrosion Control", pp 23, 28, Wiley, New York, N.Y., 1963.
Received for reuiew October 3,1977 Accepted March 9, 1978 Presented as p a r t of t h e Symposium on Interfacial Phenomena in Corrosion Protection a t the Division of Organic Coatings and Plastics Chemistry, 173rd National Meeting of t h e American Chemical Society, New Orleans, La., March 1977
Novel Corrosion Resistant Alloys by Ion Implantation V. Ashworth," W. A. Grant,' and R. P. M. Procler Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, Manchester, M60
IQD,England
Since corrosion is essentially a surface phenomenon, surface alloying will often serve to impart adequate corrosion resistance to a metal with intrinsically inferior properties. However, improved corrosion resistance is afforded generally only by alloying additions that remain in solid solution in the base metal. Thus the range of alloying additions that can be used in conventional surface alloying processes to produce corrosion-resistant alloy layers is limited to those which have reasonable solid solubility in the base metal. Ion implantation provides an alternative method for producing corrosion-resistant surface alloys. Furthermore, since the technique requires that the alloying element be ionized and accelerated into the base metal, the limitations on solid solubility imposed by equilibrium phase diagrams are no longer applicable. The paper discusses experimental results showing how the ease of passivation and resistance to environments containing chloride ions of iron, aluminum, and a type 304 stainless steel can be improved by producing novel surface alloys by ion implantation. Finally, some results are presented to show how ion implantation may be used as a research tool in a corrosion investigation.
Metallic corrosion has been defined as the passage of the metal into the chemically combined state (Shreir, 1963). It has been shown (Evans and Hoar, 1932) that in aqueous solutions the reaction occurs through an electrochemical mechanism in which the metal is oxidized (loses electrons in an anodic reaction) while solution species are reduced (gaining electrons in a cathodic reaction). Clearly the sites of these anodic and cathodic reactions must be linked by electronic and electrolytic pathways in the metal and the corrosive environment, respectively, if the requisite charge transfer is to occur. Further, it is a matter of experience that when corrosion takes place there is no discernible buildup of charge, from which it may be inferred that the rates of electron release (anodic process) and consumption (cathodic process) are equal. The existing methods of corrosion prevention and control are necessarily based on attempts to interfere with these prerequisites for corrosion. Thus corrosion control techniques fall into five broad categories, viz. materials selection, environmental control, electrochemical protection, the use of coatings, and design. Careful selection of materials may minimize corrosion, since for reasons that are readily explained by recourse to electrochemical thermodynamics and kinetics, not all metallic materials are equally susceptible to corrosion in a given environment. By contrast, it may be possible to modify the corrosive environment either by removing the agressive agent entirely or by making judicious additions that in the presence of the aggressive agent interfere with the electrochemical kinetics of oxidation or reduction and 1 Department of Electrical Engineering, University of Salford, Salford, England.
0019-7890/78/1217-0176$0.100/0
thereby inhibit the corrosion reaction. Alternatively, it may be possible to intervene in the corrosion reaction electrochemically, to stimulate either the anodic reaction, thereby promoting protective film formation (so-called anodic protection), or the cathodic reaction (cathodic protection) with a corresponding lowering of the metal oxidation rate. A too often neglected method of controlling corrosion is by means of good design. Bad design can completely negate subsequent good materials selection or protection schemes by fostering avoidable corrosion cells. Good design can permit the use of less expensive materials and protection schemes. Finally, corrosion may be controlled by the application of coatings that inter alia serve to separate the metal from the corrosive environment and may also have other beneficial effects. On a tonnage basis, organic coatings (paints, polymers etc.) protect more metal from corrosion than any other type. Inorganic coatings are less commonplace but find increasing application. As a class they range from the anodic oxide films produced on aluminum and the phosphate chemical conversion coatings on steel to the applied coatings based on vitreous enamels, glass, and ceramics. Both these coating types are effective largely because they reduce the interfacial conductivity and thereby provide a high resistance path between the anodic and cathodic reaction sites. Metallic coatings are somewhat different, and their use may be regarded as an exercise in metal substitution; Le., the coating presents to the environment a more corrosion resistant material than does the substrate itself. Metallic coatings that are less noble than the substrate material provide the added advantage that, if damaged, they confer temporary electrochemical protection on the substrate pending reassertion of
0 1978 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 2, 1978
the better corrosion resistance of the coating (as a result of the formation of a plug of corrosion product in the damaged zone). I t is, of course, possible to produce a material of markedly improved corrosion resistance by making appropriate alloying additions to a more corrodable metal. Further, since corrosion is a surface phenomenon, it is not necessary to produce bulk alloys to obtain corrosion resistance. Thus surface alloying techniques (diffusion coating, cladding etc.) find use where the mechanical and other properties of the base alloy are adequate or indeed more attractive, but where the demands of corrosion resistance favor the use of a more resistant alloy. In general, improved corrosion resistance by alloying is afforded only by those additions that remain in solid solution in the base metal. Thus the choice of potential alloying additions for both bulk and surface alloying is constrained by the need for reasonable solid solubility in the parent metal. This in turn depends on the limitations on solid solubility imposed by the equilibrium phase diagrams. The process of ion implantation involves the bombardment of a target material with energetic ions. These ions penetrate the target to a depth dependent on the energy of the ion and the nature of the ion and the target. In a metallic target the neutralized ions come to rest in the target lattice at interstitial and substitutional positions and remain there in metastable solid solution. Thus ion implantation may be used to overcome the equilibrium restraints on solid solubility associated with conventional alloying techniques and to produce novel, metastable, solid solution surface alloys. Clearly, any property that depends on the chemical composition of the para-surface layers, including corrosion resistance, may be affected by the formation of the novel alloy layers by ion implantation. This paper describes part of an ongoing program investigating the use of ion implantation to produce novel, corrosion-resistant surface alloys. Various implantations into pure iron and aluminum are discussed. The corrosion behavior has been assessed by electrochemical methods. Experimental Section Ion Implantation. The ion beam machines used in ion implantation have been described fully in a recent paper by Ashworth et al. (1976a). Briefly, a high-voltage ion source and an isotope separator are used to produce an intense, sharply focussed and resolved energetic ion beam. The beam (40 mm high X 2 mm wide) is swept sinusoidally across the target material in such a manner as to ensure uniform implantation of the ion into the target. In the absence of sputtering a Gaussian distribution of the implant ions within the target results. Where sputtering is found, the distribution becomes skewed with the peak concentration extending into the metal along a plateau but falling in a Gaussian manner thereafter. In the work reported here, to obtain a high concentration of the implant material near the specimen surface, low energy (20 keV) ions were used with high doses (usually 1 5 x 1014 ions mm-2). The dose rate, as measured by the beam current, was kept low (ca. 10l2ions mm-2 s-l) so that the implantation of a typical dose required several minutes, and the bulk temperature rise did not exceed 7 "C. During implantation the target chamber was maintained under vacuum (-5 x 10-7 Torr). The concentration profile of one implant species in iron was determined using a glancing-angle Rutherford backscattering technique (Ashworth et al., 1976a). The implantation conditions used gave peak implant species concentrations in the target of the order of 10 atomic %,. The best estimates of the projected range (Ashworth et al., 1976a) of the various implant species into iron and aluminum, based on published data (Grant et al., 1976; Lindhard et al., 1963; Williams and Grant, 1975) are given in Table I.
177
Table I. Projected Range of Implant Species into Pure Iron and Aluminum (Ion Energy 20 keV in Each Case) Implant species
Projected range in target, nm Fe A1
Ar+ Cr+
9
Ta+ Al+ Ni+ Mo+ Pb+
3.3
19 16
7
... ... ...
5
... 24 14 13
...
Electrochemical Assessment. Electrochemical polarization experiments were made using a potentiokinetic technique. In the case of pure iron a sweep rate of 0.8 mV/s was adopted and polarization was effected in a deaerated sodium acetate/acetic acid buffer solution (Clarke, 1928) of pH 7.3. The aluminum was polarized a t 0.47 mV/s in deaerated 0.01 M sodium sulfate solution (pH 7.0). These test solutions were chosen as being sufficiently bland to prevent complete dissolution of the implanted layer during polarization in the active dissolution region. A three-sweep polarization technique was adopted: positive-going from the immersion potential to a fixed potential limit, followed by negative-going to a fixed potential limit in the cathodic region, and finally a positive-going sweep to the first positive limit. Both the positive and negative potential limits were fixed by reference to the metal involved and the experimental evidence required. The mean thickness of the oxide films on unimplanted and Ar-implanted iron specimens was determined using electrometric reduction. The sodium boratehydrochloric acid buffer solution (pH 7.6) and experimental conditions employed by Oswin and Cohen (1957) were used here.
Results a n d Discussion Implantation into Iron. Pure iron was chosen as a target material for three reasons. First, its corrosion behavior is well understood. Secondly, pure materials were desirable to remove uncertainties introduced by the presence of impurity elements. Thirdly, since preliminary work had indicated that thickening of the natural oxide film was associated with ion implantation, i t was important that, in the first instance, any such film could be removed by electrochemical reduction so that the electrochemistry of the underlying implanted substrate could be examined. However, in the context of producing corrosion-resistant materials by ion implantation, the use of pure iron was regarded as an investigation of a model system only. The experimental program was divided into three parts. First, an investigation designed to identify any effects that were independent of the chemical nature of the species implanted, i.e., that could be attributed to the ion implantation process per se. Inert gas (Ar+) ions were implanted for this purpose. Secondly, an investigation to determine whether alloys of comparable composition produced by ion implantation and conventional means showed similar corrosion behavior; thus Cr+ ions were implanted. Thirdly, the technique was used to implant ions that could not, for reasons of solid solubility, be introduced by conventional means but where there was a reasonable expectation that their presence would contribute to improved corrosion resistance. Ta+ (a strong oxide former) and Pb+ (a metal with a very low hydrogen exchange current density) were used for this purpose. The results of the electrometric reduction experiments (Table 11) indicated that, by comparison with an unimplanted specimen carrying the natural air formed oxide film, an Arimplanted specimen carried a film in which the magnetite
178
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 2, 1978
Table 11. Thickness of Oxide Film on Unimplanted and Ar-Implanted Pure Irona Mean thickness of inner Fe304 layer, nm Unimplanted 1.3 Ar+-implanted 3.3 Ashworth et al. (1976b).
Mean thickness of outer y-FezOs layer, nm 1.1 1.1
No. of replicate experiments 5 5
content was more than doubled while the thickness of the haemetite layer was unaffected. I t has yet to be established unequivocally when the thickening of the oxide film occurs, although preliminary work suggests that it occurs immediately after the implanted specimen is removed from the ion beam machine. Although these results showed clearly that argon ion implantation thickened the air-formed oxide film on high purity iron, they gave no indication as to any effect of the implanted argon on the corrosion behavior of the unfilmed iron substrate. Although such an effect was expected to be minimal, owing to the inert nature of argon, an understanding of any effect of the process of ion implantation per se on corrosion behavior, and in particular on the response of specimens to potentiokinetic polarization, was a necessary precursor to studying the corrosion behavior of more complex alloys produced by ion implantation. Thus the electrochemical polarization curves for unimplanted and Ar-implanted pure iron were determined and are shown in Figures 1 and 2. A comparison of these figures shows that during the first positive-going sweep and the early stages of the negative-going sweep, there are significant differences in the potentiokinetic polarization behavior of unimplanted and argon implanted iron specimens. However, these differences can be attributed to the difference in the thickness and characteristics of the air-formed films on the two specimens. During the later stages of the negative-going sweep, when the air-formed films on both specimens have beem removed by cathodic reduction and during the second anodic sweep, there are no important or significant differences in the polarization curves other than the fact that the current density in the case of the Ar-implanted specimen is somewhat higher; this is attributed to the surface roughening and increased surface area associated with ion implantation and not to ion implantation per se. Thus the process of ion implantation itself had an effect on the electrode (film thickening) but, as expected, implantation of the inert gas argon was without chemical effect. Nevertheless, the fact that film thickening does occur during ion implantation has to be taken into account in assessing any apparent change in the corrosion or polarization behavior produced by other implant species. It also seems likely that similar film thickening was responsible for early reports of the apparently beneficial effect of implantation of inert gases on the corrosion resistance of other metals (Hondros and Bernard, 1962; Trillat, 1962). The polarization behavior of two different chromiumimplanted electrodes is shown in Figures 3 and 4. By comparison with Figure 1 it will be seen immediately that the critical current density for passivation is lowered by one and two orders of magnitude a t Cr doses of 5 X 1014 and 2 X l 0 l 5 ions mm+, respectively; the passive current density is reduced similarly. Further, unlike the case of Ar implantation (Figure 2), the inhibition of the anodic kinetics is not eliminated by cathodic polarization and consequent reduction of the oxide film. Thus the data indicate that implanted Cr has a signifi-
1 1 5 h 1
ob3
d3
dl
CUR3ENT CENSITY. pA?mm2
30
hi
10
Figure 1. Potentiokinetic polarization curves of unimplanted pure iron.
&I-
: 650-
2
/..."
450-
z. 5
250-,
....
. . I '
,.........
50-
.. ....
--
--
cx
Cc. -150-
E
sweep Is! kga!#ve-gotng sweep 2nd Pnsl!lve-gotng s w r q
151 P o s i t i v e - q i q
....,...
E
_ _ _ _ _ _ - --+-
-35W
Y
d
-750-750 -9%-9%
-11M 001
003
01
CURRENT 03
DENSITY, I O vA,rnAS
0
30
Figure 2. Potentiokinetic polarization curves of argon-implanted iron ( 5 x 1014 ions/mm*). cant and beneficial effect in the polarization behavior of pure iron (Ashworth et al., 1976~). This effect, which is particularly marked on the anodic kinetics, is clearly distinguishable from that of argon implantation and is generally what would be expected on the basis of the known corrosion behavior of conventional Fe-Cr alloys, namely the formation of more protective and more readily formed passive films. This view is confirmed by an examination of the polarization behavior of a conventional Fe-7.3% Cr alloy (Figure 5 ) . The general similarity between Figures 3 and 4 and Figure 5 is obvious. Indeed, when compared with the polarization behavior of a series of conventional Fe-Cr alloys, the specimens implanted with the lower Cr dose behaved very similarly to a 4.9% Cr alloy, while the higher dose specimens were similar to conventional alloys containing >13% Cr. Thus it is clear that the corrosion behavior, as judged by potentiokinetic polarization, of Fe-Cr surface alloys produced by ion implantation is generally similar to that of the corresponding conventional bulk alloys. This conclusion provides an essential basis for subsequent investigations of the corrosion behavior of surface alloys on iron which can only be produced by ion implantation and which cannot be prepared by conventional alloying techniques. Specifically, the evidence suggests that it may be anticipated that an ion introduced by ion implantation will exert its normal chemical effect when the surface alloy is exposed to a corrosive environment. ions mm-2 produce a maTantalum ion doses of 5 X terial that, not surprisingly, compares favorably with the equivalent Cr-dosed material, since both tantalum and chromium are strong oxide formers. The relevant polarization
179
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 2, 1978
is1 Porct~ve-gomg sweep
1st Ntgallvo-gomg sweep 2nd Positive-going sweep
.......
--
1st Posltlve-golng swprp 1st Negatrve-going sweep 2nd Pos!tivo-going sweep
> E M
-1150-
Ob1
Figure 3. Potentiokinetic polarization curves of low dose chromium-implanted iron (5 X 1014ions/mm2).
lst hsittve-plng s w r q 1st Negative-going r m p 2nd Posttwo-going-swrrp
003
O'cuRRENT o&,lTY
Vk9mrn2
30
32
IO
Figure 5. Potentiokinetic polarization curves of a conventional Fe7.3% Cr alloy.
.......
--
: 501 r
-11
N
Figure 4. Potentiokinetic polarization curves of high dose chromium-implanted iron (2 X 1015 ionsimm2).
curve is shown in Figure 6. Unexpectedly, higher T a doses did not improve the behavior further. However, using a glancing angle Rutherford back-scattering technique (Ashworth et al., 1976a) it was shown (Figure 7) that the retained dose of T a is not increased by doses greater than 5 X 1014ions mm-2; Le., the maximum surface concentration a t the implantation energy used had been reached. Since the surface Fe-Ta alloy produced is extremely thin (Table I), the retention of the improved corrosion resistance with respect to time was investigated. By monitoring the variation of current density with time of implanted and unimplanted specimens a t constant potential in the passive region (-175 mV) it was shown that the behavior of the former did not approach that of the latter until some 4.5 X lo4 s (-13 h) had elapsed (Ashworth et al., 1977). During this period the implanted specimen consistantly sustained lower current densities. Further during this time a metal loss of -1 pm had occurred; this is, of course, more than two orders of magnitude greater than the mean projected range of T a ions and demonstrates an unexpected persistence in the beneficial effect of the implanted Ta. It appears that this may well be due to selective dissolution of iron from the Fe-Ta surface alloy which leaves a surface relatively rich in tantalum. Using a very much higher implant energy (MeV), thicker alloy layers (km) can be produced wich the possibility of a proportionately longer useful life; the higher implant energy would also serve to increase the saturation dose. Lead implantation produced an alloy layer with a considerably reduced exchange current density for hydrogen evolution. Further a t P b doses of 2 X 10'5 ions mm-2 a reduced
on1
003
01
30
10
03
10
30
CURRENT CENSITY, VAlmm2
Figure 6. Potentiokinetic polarization curves of tantalum-implanted iron (5 x 1014 ions/mm*). I
I
X X X
5x1014 ,MSlmm2
xxx x
xx
X
xx
350 Channel
x
360
n m k r 1 S K o V per channel 1
Figure 7. Glancing angle R.B.S. curves for high and low dose tanta-
lum-implanted iron. critical current density for passivation and passive current were also recorded. Not unexpectedly, the corrosion rate of the implanted material, whether determined by Tafel extrapolation or linear polarization, was reduced (from 0.15 A rn+ for pure iron to 0.03 A m-2 in the implanted material). The persistence of the chemical effects of lead were also relatively long term. A constant potential experiment in this case still showed more sluggish cathodic kinetics even after 2 X 105 s (-55 h) immersion (Ashworth et al., 1978). The results of the T a and P b implantation into pure iron demonstrate that ion implantation can be used to produce novel solid solution surface alloys. Further, these novel alloys exhibit the corrosion behavior that would be predicted from the known corrosion and electrochemical behavior of the alloy
180
Ind. Eng. Chern. Prod. Res. Dev., Vol. 17, No. 2, 1978
addition in the pure state. However, as already noted, in the view of the present authors the pure iron system can only be regarded as a useful model when considering ion implantation and corrosion resistance. Since unalloyed iron has comparatively poor corrosion resistance and typically is used in bulk quantities, there appears little commercial scope for improving its long-term corrosion resistance significantly and economically by ion implantation. On the other hand, as will be seen below, there does appear to be considerable scope for using ion implantation to improve the specific corrosion resistance of more generally corrosion resistant ferrous alloys such as the stainless steels. Implantation into Aluminum. Due to the inert and protective nature of the thin oxide film which forms on aluminum, the inherent corrosion resistance of this metal in near neutral solution is much better than that of iron. Further improvement of the corrosion resistance of aluminum, particularly in the case of high strength aluminum alloys and with respect to localized corrosion, therefore offers the possibility, which was not available in the case of iron, of producing extremely useful and practical materials. On the other hand, the persistence of the oxide films on aluminum in contrast to those on iron which are readily removed by cathodic reduction, makes interpretation of the effect of ion implantation on the corrosion behavior of aluminum more complex. These properties of the oxide film on aluminum made it particularly important to bear in mind that a thickening of the air-formed oxide film had been shown to be associated with implantation into iron. Thus Ar was implanted into aluminum and the resultant material was used as the reference standard in examining the effects of implanting other elements into aluminum. It was not clear, however, whether the presence of the oxide film would permit implant species to exert their full effect. Thus Cr and Ni were implanted to examine whether they were able both to alter the effectiveness of aluminum oxide as a cathode and to exert their natural anodic behavior. Molybdenum, on the other hand, was implanted specifically to investigate the possibility of improving the resistance of aluminum to pitting attack in chloride-containing environments. Once again a three sweep potentiokinetic polarization technique, starting with a positive-going sweep from the immersion potential, was used. This procedure was originally adopted for the corresponding investigations of iron when it was observed that the air-formed oxide film was thickened by the process of ion implantation, even with chemically inert species. The first positive-goingsweep therefore related to iron bearing the air-formed oxide film, including any modifications introduced by ion implantation. During the negative-going sweep this film was cathodically reduced, so that the second positive-going sweep characterized the behavior of an initially film-free surface and any modifications of the iron substrate introduced by the implanted species. In the case of aluminum the first positive-goingsweep again characterizes the behavior of aluminum bearing the air-formed oxide film and, in the case of implanted specimens, includes any changes induced in this film by the process of ion implantation or the presence of the implanted species. However, the air-formed oxide film on aluminum is not cathodically reducible, although the nature of the film is undoubtedly altered by cathodic polarization. The cathodic reaction in the deaerated solution used in the present work is hydrogen evolution. This reaction results in an excess of OH- ions adjacent to the working electrode surface. The high local pH thus produced partially converts the air-formed oxide film to complex hydrated oxides. In addition, it also causes “cathodic corrosion” and dissolution, particularly at flaws and weak spots in the original air-formed oxide, with the formation of the soluble aluminate ion. In the case of aluminum, therefore, the second positive-going sweep
sweep 5 W P
......
--
sweep
-175
nmi
om3
001
003
IO
01
30
CURRENT DENSITY,
Figure 8. Potentiokinetic polarization curves of unimplanted pure aluminum.
y 4
-17
003
no1
/
i
1st PosItive-going 1st Nqative-ping 2nd Positive-going
no3
01
C M E M DENSITY, pNmmq
3
......
--
10
30
Figure 9. Potentiokinetic polarization curves of argon-implanted aluminum (2 x 1015 ions/mm2).
characterizes the behavior of aluminum bearing a solutionreinforced film, and in the case of implanted specimens, any effects of ion implantation. The potentiokinetic polarization curve for unimplanted and Ar-implanted aluminum are shown in Figures 8 and 9, respectively. I t will be seen that in the first positive-going, and the negative-going, sweeps the general features are the same. However, there is evidence in the higher immersion potential, the lower passive current density, etc., that the anodic kinetics were more sluggish on the implanted material. These data confirm that, as with iron, the process of ion implantation per se thickens the air-formed oxide film on aluminum. By contrast, the second positive-going sweep on argon implanted aluminum was virtually identical with that on the unimplanted material. This indicates that once the effects of the thickened air-formed film have been eliminated by cathodic polarization, the potentiokinetic polarization behavior of aluminum, like that of iron, is not effected by implantation of chemically inert species. It may be noted that very similar results were obtained by implanting A1 ions into aluminum (Al-Saffar et al., 1977). Implantation of Mo provided evidence that, even in the presence of an oxide film, an implanted ion is able to exert its normal chemical effect. Thus, when compared with Figure 9, Figure 10 shows that there is a marked increase in the corrosion potential, the passive range is extended, the passive currents are lowered, and the cathodic kinetics are stimulated. The higher corrosion potential and the more rapid cathodic
Ind. Eng. Chem. Prod. Res. Dev., Val. 17, No. 2, 1978
181
-,Is0
-17%
0 m,
OV1 C U R R E ~ ~ ~ E N S ( T Y , ~ : r i i m m '
0
0
Figure LO. Potentiokinetic polarization curves of molybdenumimplanted aluminum (2 X 10'5 ionsimm2). kinetics are almost certainly attributable to the higher hydrogen exchange current density on molybdenum than on oxide-covered aluminum. The lower passive currents and the extended passive range showed that, in comparison to unimplanted aluminum, hoth the air-formed and the solution reinforced films on Mo ion implanted aluminum are more protective and do not permit the substrate to anodize so readily. Very similar results, hut with less obvious inhibition of anodizing, were found using Cr implantation into aluminum. These effects on anodizing of implanted chromium and molybdenum ions are consistent with their chemical behavior in the bulk state. Of more practical importance is the effect of Mo implantation on the pitting resistance of aluminum. Bulk molyhdenum additions are made to stainless steel specifically to improve their resistance to pitting attack in structures containing chloride ions. Aluminum suffers the same attack rather more readily in the same environments. Unfortunately, molybdenum is insoluble in aluminum and so cannot be added by conventional alloying techniques with the object of improving the pitting resistance of aluminum. However, molybdenum was implanted into aluminum (dose 10'5 ions mm-2) and the resulting alloy polarized in a solution containing 1000 ppm of CI-. During three sweep polarization experiments pure aluminum suffered severe pitting attack while Ma-implanted aluminum did not (see Figures 11and 12). Again, therefore, the known chemical effect of the implanted ion, molybdenum, is exerted, even in a material in which it would not normally exhibit solid solubility, if a solid solution surface alloy is produced by ion implantation. Furthermore, a useful material, viz. a pit resistant aluminum, can be formed. The potentiokinetic polarization curves of nickel-implanted aluminum (Figure 13) differ very markedly from those of pure aluminum and also from those of all the other aluminum implanted specimens discussed above. Nevertheless they demonstrate very clearly the fact that the implant species behaves as though it were in solid solution and exerts its normal chemical behavior. First, the immersion and corrosion potentials a t --550 mV, are much higher than observed in other systems. This is again attributed to the fact that the hydrogen exchange current density on nickel (-10-3 FA mm@) is higher than that on oxide-covered aluminum. Secondly, on both the first and the second positive-going sweeps, secondary passivation is ohserved a t about 250 mV with nickel implanted specimens. That this secondary passivation behavior is due to the implanted Ni is clearly demonstrated by the potentiokinetic polarization curves for pure Ni in 0.01 M Na2S04 (Figure 14). These also exhibit an active/passive transition a t similar potentials and current densities. It is also important
Figure 11. Pits produced in pure aluminum polarized in 1000 ppm
ci- S0l"tl""
Figure 12. Surface of m ~ I ~ h d ~ " , , r n ~ i m p l a(dose n t e ~ I X lIVs ions/ mm2) pure aluminum polarized in 1000 ppm CIF solution.
to note that this behavior would not he expected if the nickel was not in metastable solid solution in the aluminum, hut rather was present as low volume fraction precipitates of NiAI3. The i,,,,, estimated for nickel-implanted aluminum by Tafel extrapolation of the hydrogen evolution reaction is -0.03 ,LAmm-2, similar to that for molybdenum and chromium-implanted aluminum and less than that for pure aluminum.
Conclusions The work reported in this paper shows that conventional and unconventional (novel) solid solution surface alloys can be produced by ion implantation. Apart from thickening the protective oxide film on the target material, there is clear evidence that ion implantation per se has no effect on the corrosion behavior of the target. By contrast, if chemically active species are implanted they behave as though they are in solid solution in the target and exert their normal chemical effect. Thus implantation of species that are normally insoluble in the target material (lead and tantalum in iron, molybdenum and nickel in aluminum) produced the beneficial effects on the behavior of the target that might have been anticipated from the known corrosion and electrochemical behavior of the implant species. Further, it appears that novel surface alloys produced by ion implantation can be designed by reference to the behavior of the implant species in the bulk form. Many metals have good corrosion resistance hy virtue of their protective oxide film. Nevertheless, they are often sus-
182
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 2, 1978
".I
1
4
105
......
'"''cuRRENP~~~ENNY
,"
pLm2
O3
Figure 13. P o t e n t i o k i n e t i c p o l a r i z a t i o n curves a l u m i n u m (2 x 1o'j ions/mm*).
IO
30
of n i c k e l - i m p l a n t e d
115c-
particular in improving the corrosion resistance of small, critical components for use in situations where the cost of implantation is small when compared to the cost of failure of the component, e.g., in the case of surgical implants and prostheses and in certain aerospace applications. In the longer term there appears to be no reason why technological developments should not occur that will enable the scope and application of ion implantation to be widened, for example to produce thicker surface alloy layers more cheaply or to accommodate more bulky and complex components. By any standards ion implantation must be regarded as a new technique for providing the engineer with novel materials in the form of previously inaccessible surface alloys; in this context the approach to selecting the alloy addition does not differ from that used in conventional alloying although the choice of alloy additions is very much wider. It must also be regarded as a new tool in the study of corrosion mechanisms. In particular, it has already been shown (Al-Saffar et al., 1978) that investigation of the corrosion behavior of specific novel alloys produced by ion implantation can result in advances in the understanding of the mechanisms of certain forms of general and localized corrosion.
1W-
2nd Positive-going sweep
Acknowledgments This work has been supported in part by the U.K. Science Research Council under Grant No. BlRG14264.7 and in part by the Air Force Office of Scientific Research (AFSC), U S . Air Force, under Grant No. AFOSR71-2115. Thanks are also due to Professor G. Carter and Professor G. C. Wood for provision of laboratory facilities.
-
Literature Cited
.Irn
001
01003
001
003
a3
1.0
30
I
CURRENT DENSITY, {dlrnrn2
AI-Saffar, A. H.. Ashworth, V., Grant, W. A,, Procter, R. P. M., "Proceedings, 6th European Congress on Metallic Corrosion", p 13, Society of Chemical Industry, London; 1977. AI-Saffar, A . H., Ashworth, V., Grant, W. A,, Procter, R. P. M., Corros. Sci., in Dress. 1978. r Ashworth, V., Grant, W. A,, Procter, R. P. M., Corros. Sci., 16, 661 (1976a). Ashworth. V.. Grant, W A., Procter, R. P. M.. Wellington, T. C., Corros. Sci., 16, 393 (1976b). Ashworth, V., Baxter, D..Grant, W. A,, Procter, R. P. M.. Corros. Sci., 16, 775 (1976~). Ashworth, V.. Baxter, D.,Grant, W. A,, Procter, R. P. M., Corros. Sci., 17, 947 (1977). Ashworth, V., Grant, W. A., Procter, R. P. M.. Wright, E. J., Corros. Sci., in press, 1978. Clarke, W. M., "The Determination of the Hydrogen Ion", Williams and Wilkins, Baltimore, Md., 1928. Evans, U. R., Hoar, T. P., Roc. Roy. SOC.London, Ser. A, 137, 343 (1932). Grant, W. A,. Williams, J. S., Dodds, D.,"Proceedings International Conference on Ion Beams for Surface Layer Analysis, Karlsruhe", Plenum Press, London, 1976. Hondros, E. D.,Bernard, J., "Le Bornbardement lonique". Colleque National de la Recherche Scientifique", p 21 1, Paris, 1962. Lindhard, J., Scharff, M.,Schiott, H. E., Kgl. Danske Vid. Selstr., Matt-Fys. M e d d , 33, No. 14 (1963). Oswin. H. G., Cohen, M.,J. Electrochem. Sac., 104, 9 (1957). Shreir, L. L., "Corrosion", 1st ed, Vol. 1, George Newnes, London, 1963. Trillat, J., "Le Bombardement lonique". Colleque National de la Recherche Scientifique, p 13, Paris, 1962. Williams, J. S . . Grant, W. A., Radiat. Eff., 25, 55 (1975). ~~~
Figure 14. P o t e n t i o k i n e t i c p o l a r i z a t i o n curves of u n i m p l a n t e d nickel.
ceptible to severe attack, in specific environments which results in a localized, rather than a general form of corrosion. Looking ahead, it appears that ion implantation might find its greatest value and most immediate application in improving the resistance of these corrosion resistant materials to such specific aggressive environments. The implantation of Mo into aluminum serves as a first example of this approach, since the specific problem of the pitting of aluminum in the presence of C1- ions has been overcome. I t may be envisaged that a similar approach might be used to improve the resistance of austenitic stainless steels, for example, to stress corrosion cracking or pitting attack; other similar systems will suggest themselves to the reader. Currently ion implantation must be regarded as a sophisticated alloying technique; the question therefore arises whether this method of surface alloying will find commercial application. In the authors' view this possibility does exist, in
~
Presented a t t h e S y m p o s i u m on I n t e r f a c i a l Phenomena in Corrosion P r o t e c t i o n h e l d a t t h e 1 7 3 r d N a t i o n a l M e e t i n g of t h e A m e r i c a n C h e m i c a l Society, New Orleans, La., M a r 21-23, 1977.
