Aqueous corrosion of tantalum- and carbon-implanted low carbon steel

Materials Science and Engineering, 69 (1985) 261-271

261

Aqueous Corrosion of Tantalum- and Carbon-implanted Low Carbon Steel* H. FERBER and G. K. WOLF

Physikalisch-Chemisches Institut, Universitiit Heidelberg, Heidelberg (F. R. G.) H. SCHMIEDEL

Eduard-Zintl-Institut, Technische Hochschule, Darmstadt (F.R.G.) G. DEARNALEY

Nuclear Physics Division, Atomic Energy Research Establishment, HarweU, Oxon. 0 X l l ORA (Gt. Britain) (Received September 17, 1984)

ABSTRACT

The effect o f tantalum implantation (5 × 1016-1017 ions cm-2; 8 0 - 1 5 0 k e V ) on the electrochemical behaviour of a low carbon steel in an acetate buffer solution o f p H 5.6 has been investigated. Samples had been irradiated in the 300 k e V accelerator at the Gesellschaft fi~r Schwerionenforschung (GSI), Darmstadt, and in the Cockcroft-Walton implantation machine at the A t o m i c Energy Research Establishment (AERE), Harwell. Tantalum-implanted specimens produced at GSI yielded potentiokinetic polarization curves similar to those obtained earlier for chromium-implanted iron with respect to the hydrogen evolution kinetics, the critical current densities required for passivation and the passive current densities. All effects were more pronounced after implantation o f higher doses o f tantalum. Successive sweeps indicated the possibility o f preferential (selective) dissolution o f iron. Tantalum implantations carried out at A E R E repeatedly resulted in specimens with differently coloured surfaces, which then showed a much stronger inhibition o f anodic as well as cathodic reaction kinetics. Implantations with Ti + and W + ions gave similar results. Carbon analysis using Auger electron spectroscopy and the 12C(d, p ) nuclear reaction m e t h o d showed the presence o f carbon layers a few tens o f nanometres thick on these samples. *Paper presented at the International Conference on Surface Modification of Metals by Ion Beams, Heidelberg, F.R.G., September 17-21, 1984. 0025-5416/85/$3.30

Further experiments carried o u t in an a t t e m p t to explain the above p h e n o m e n a were (a) pre-implantation surface contamination with organic compounds, (b ) successive b o m b a r d m e n t with Ta +, C + and 0 + ions, (c) implantation o f tantalum in residual gas atmospheres containing organic c o m p o u n d s and C02, and (d) implantation o f tantalum into carbon (or hydrocarbon) layers deposited onto steel. The results o f these experiments illustrate the marked difference between the corrosion behaviour o f samples enriched in bulk carbon and the corrosion behaviour o f samples with surface layers containing carbon.

1. INTRODUCTION The possibility of improving the properties of metals with respect to their corrosion behaviour in aqueous solutions by the m e t h o d of ion implantation has aroused increasing interest since the first experiments of Ashw o r t h et al. [ 1 - 3 ] which were r e p o r t e d between 1976 and 1978. The first corrosion studies on implanted iron included Cr+ ion implantations, and it was shown t hat t he resulting F e - C r surface alloys exhibited a corrosion behaviour comparable with t hat of Fe-Cr bulk alloys. Ashworth et al. included tantalum implantation into iron in their investigations and f o u n d t h a t the influence of tantalum on the active dissolution of iron was greater than t hat of chromium. An effect on the cathodic h y d r o g e n evolution kinetics was also reported, which was attributed to the presence of Ta205 on the mixed surface. The T a - F e system is especially interesting because © Elsevier Sequoia/Printed in The Netherlands

262 metastable alloys which are not obtainable by conventional alloying techniques are formed, and the electrochemistry of these novel alloys can n o w be investigated. Takahashi e t al. [4] carried out implantations of chromium and other metals in iron and using the multisweep m e t h o d showed that beneficial effects on the corrosion behaviour of iron lasted only for a limited number of sweeps because of fast selective dissolution of the iron. Our research group in Heidelberg [5, 6] has been involved in the investigation of the corrosion behaviour of pure iron after implantation of several ion species. We found that implantation of gold and platinum strongly enhanced the hydrogen evolution kinetics of pure iron electrodes, thus also enhancing active corrosion, whereas lead and mercury inhibited the cathodic reaction. These measurements were performed in a 0.5 M H2SO4 electrolyte, and because of the high reaction rates changes in the anodic dissolution due to the formation of the thin metastable surface alloys could not be detected. Potentiodynamic polarization experiments using an acetate buffer solution, however, yielded similar beneficial effects on the anodic dissolution reaction (active corrosion) as reported by Ashworth et al. [1-3] and Takahashi et al. [4], especially for chromium, nickel and mercury implantation. Clayton et al. [7] used AISI 52100 steel targets for tantalum and chromium implantation experiments. They attributed the enhanced corrosion resistance that was found to the formation of a passive film consisting of mainly amorphous hydrated Ta205 and hydrated oxides of chromium and iron. As early as 1970, Covino e t al. [8] indicated the problems which can be caused by carbon during the implantation process. Implanting chromium into iron they found surface contamination of carbon and silicon on their samples, with carbon distributed throughout the implanted layer. However, they did n o t observe an effect of the carbon on the electrochemical behaviour of iron. The important role of carbon in implantation experiments was also discussed by Knapp e t al. [9]. They b o m b a r d e d iron with titanium and detected an amorphous surface alloy growing inwards from the surface. They could correlate the growth of this amorphous layer

with the a m o u n t of carbon on and in the sample. In the ion beam mixing experiments performed by Chan et al. [10] with AISI 52100 steel samples plated with chromium layers, an enhanced pitting resistance of the steel was reported. These beneficial effects were reduced, however, by carbon contamination during irradiation with rare gas and Cr + ions which led to the formation of carbides. Singer [ 11 ] reported more detailed experiments concerned with the carbon uptake of samples (vacuum carburization) during implantation with titanium. He measured carbon profiles in AISI 52100 steel by means of Auger electron spectroscopy (AES) and correlated the uptake of carbon with the dose of the implanted ions. He suggested that other elements such as tantalum, tungsten and hafnium with a high carbon affinity should produce similar effects when implanted into metals. Our research group has investigated the beneficial effects of the implantation of tantalum into a low carbon steel. After the first encouraging results of Ashworth et al. [2] and our earlier observation of strong corrosion inhibition effects in samples implanted with tantalum at the Atomic Ene~3y Research Establishment (AERE), Harwell, we have been looking more closely at the electrochemistry of tantalum-implanted steel and especially at the role of the carbon present after irradiation, either in the bulk or on the surface of implanted electrodes.

2. EXPERIMENTAL DETAILS We maae use of a low carbon tool steel with a carbon content of approximately 0.3 wt.%. Cylindrical specimens were cut from 8 mm rods on a lathe and polished on one face, firstly with abrasive paper and then with diam o n d paste d o w n to 1 ttm. The area of the electrodes in contact with the electrolyte during the electrochemical measurements was 0.5 cm 2. The remaining surface of the cylinders was covered with a t w o - c o m p o n e n t adhesive. The test solution was a sodium acetate-acetic acid buffer solution of pH 5.6 made from anatytical reagen~ grade chemicals. Before immersion in the electrolyte the electrodes were cleaned with hot acetone.

263 The implantations were carried out partly in the 300 keV accelerator at the Gesellschaft fi]r Schwerionenforschung (GSI), Darmstadt, and partly in the Cockcroft-Walton implantation machine at AERE, Harwell. The energies of the implanted ions were in the range 80150 keV for tantalum, titanium and tungsten and between 40 and 50 keV for carbon and oxygen. During the implantations the beam was swept across the samples in the x and y directions to obtain uniform distribution of the dopants across the target surface. The vacuum in the target chamber was typically of a value near 5 × 10 -6 mbar during implantations at AERE and in the region of 2 X 10 -~3 × 10 -~ mbar during implantations at GSI. The beam line in the GSI machine was equipped with turbomolecular pumps only, whereas oil diffusion pumps were used in the Cockcroft-Walton machine. The corrosion was investigated using potentiodynamic polarization and, although usually more than one positive-going plus one negative-going sweep (equal to 1 cycle) was recorded, evaluation of the curves was mainly concentrated on the first cycle. The electrodes were immersed under a negative overpotential (--1000 mV with respect to a standard hydrogen electrode (SHE)) and then prepolarized to a voltage corresponding to a cathodic current of 100 mA cm -2 for 10 min to reduce surface oxide layers. After a period of 5 min, during which the corrosion potential was observed and registered, the initial sweep and several subsequent sweeps were recorded in the potential region between --1000 and + 1 5 0 0 mV (SHE) starting from --1000 mV (SHE). The sweep rate was always 10 mV s-1. The complete measurement set-up consisted of the electrochemical cell with working electrode, counterelectrode and reference electrode, including a gas inlet for deaeration with oxygen-free nitrogen. The electronic equipment included a potentiostat, a voltage scan generator, a logarithmic converter and the x - y recorder. The whole arrangement has been described elsewhere [12]. AES experiments were performed at the Technical University, Darmstadt, on a 10 keV Varian Auger electron spectrometer equipped with a 3 keV sputter gun for depth profiling. Carbon detection was carried out using the 12C(d, p) nuclear reaction m e t h o d by the Nuclear Physics Division, AERE, Harwell.

3. RESULTS AND DISCUSSIONS Implantations performed with the two accelerators mentioned above lead to substantially different corrosion behaviours of the steel samples. Therefore the results and a discussion of these results are presented separately for the two different accelerators.

3.1. Tantalum implantation in the 300 k e V accelerator at the GeseUschaft fi~r Schwerionenforschung, Darmstadt (moderate inhibition effects) Figure 1 shows a comparison of the current density-potential plot for unimplanted steel with the curve obtained with a pure tantalum electrode. Both electrodes were treated exactly the same way as all implanted samples including cathodic prepolarization. The steel curve presented in Fig. 1 served as a standard for the implanted electrodes to be compared with. Hydrogen evolution at the starting potential was about 8-10 mA cm -2, and the corrosion potential had a value of --450 mV (SHE); the critical current density was slightly higher than 10 mA cm -2 and the average passive current density was around 100 pA cm -2. Consecutive sweeps showed little change in hydrogen evolution, but icrit increased rapidly as a consequence of strong selective dissolution of iron. This effect is not found for pure iron and is attributed to the presence of iron carbide in the steel. Therefore, it was very important to compare only the corresponding sweeps of different electrodes. For comparison, we normally used the first cycle only. The tantalum curve in Fig. 1 shows the outstanding electrochemical stability of this metal and explains its choice as a candidate for the formation of a corrosion-resistant metastable alloy. After the cathodic pretreatment the a m o u n t of hydrogen evolution is nearly as high as for steel, but after the first positive-going sweep it has already decreased by more than two orders of magnitude; on the oxidized electrode, hydrogen evolution is markedly reduced. There is almost no dissolution in the anodic region; the electrode is passive and currents result only from oxide layer formation and double-layer charging. Anodic current densities are increasingly reduced during subsequent sweeps.

264 -looo

o

.1,~o

~ (r.V (SH,E) )

-3

A t / /

-S

-6

Fig. 1. Potentiokinetic polarization curves for unimplanted steel (curve SO) and pure tantalum {curve Ta0) in acetate buffer solution (pH 5.6): - - - , first positive-going sweep; - - -, first negative-going sweep.

-2

-1C~01~...~

0

1

I

l

÷ 1000

~ (r#/ (SHE)) ,

-3

\ \ -4

s I

\

anode \

\

Ij

Fig. 2. Three typical potentiokinetic polarization curves for steel implanted with 5 × 1016 Ta + ions 100 keV.

F i g u r e 2 gives t h e e l e c t r o c h e m i c a l results o f a set o f t h r e e steel s a m p l e s i m p l a n t e d w i t h T a + ions (dose, 5 × 1016 ions c m - 2 ; e n e r g y , 1 5 0 k e V ) w h i c h are r e p r e s e n t a t i v e o f all t h e i r r a d i a t i o n s carried o u t at G S I w i t h t h e s a m e parameters. The temperature of the samples

e m -2

at

was held at 60 °C d u r i n g all i m p l a n t a t i o n experiments by means of a flow of hot water t h r o u g h t h e s a m p l e holder. Because o f t h e relatively high s p u t t e r r a t e o f t a n t a l u m b e a m s i n c i d e n t on iron, the ret a i n e d d o s e o f t a n t a l u m in t h e t a r g e t a f t e r

265

implantation can be expected to be around 2.5 X 10 le ions cm -2, according to recent investigations of Grabowski et al. [13]. Figure 2 illustrates the following effects of tantalum implantation. Hydrogen evolution is not changed drastically. A slight enhancement can be attributed to the formation of lattice defects on the electrode. This is usually also observed during rare gas ion implantation into metals, icrit is reduced by t w o orders of magnitude compared with the corresponding value for the standard steel. The passive region indicates only a slight influence of the irradiation. The first negativegoing sweep shows that with this dose there was no significant influence of Ta205 formation on the hydrogen evolution. After only a few cycles, icrit regains the value obtained during the first sweep for unimplanted steel. This rapid increase indicates pronounced selective dissolution of iron. However, it has to be pointed out that, when the value of icrit for implanted specimens has attained the initial value for icrit for unimplanted steel, the corresponding icrit value for unimplanted steel after the same number of sweeps would be significantly higher still, so that a reduction in the iron dissolution still prevails. _i, 1C

I

.r"~

oI

I

Figure 3 shows the effect of increasing the tantalum dose to 1 X 1017 ions cm -2 with otherwise unchanged implantation parameters. The retained dose can be estimated to a b o u t (4-5) X 1016 ions cm -2 [13]. Hydrogen evolution kinetics show no additional effects in the first positive-going sweep and the additional reductions in icrit and ip~s are expressed by factors of a b o u t 2 and 4 respectively. The presence of an increased amount of tantalum, however, leads to a reduction by one order of magnitude for the hydrogen evolution current at the end of the first negativegoing sweep. Although hydrogen evolution was further reduced in the subsequent cycles, icrit w a s constantly increasing because of the process of selective dissolution of iron, even in the presence of tantalum.

3.2. Tantalum implantations at the Atomic Energy Research Establishment, Harwell (strong inhibition effects) Many of the samples which were implanted with tantalum in the Cockcroft-Walton machine at AERE with the same implantation parameters as those produced at GST yielded exactly the same results as those reported above, but we also repeatedly obtained • looo !

¢ (my

(,SHF_J)

tog i (A crY)

-3

\ \ \

\

-z,

\ \

\

\

\

\

-5

~11neg.going Sweep S/Ta+

Fig. 3. Effect of an increased t a n t a l u m dose (1017 ions cm-2; 150 keV) (curve S/Ta + 1017) on the electrochemical behaviour of the steel samples: curve So, u n i m p l a n t e d s t e e l ; - - - , curve 1 neg. going sweep S/Ta +, first negativegoing sweep for the steel i m p l a n t e d with an increased t a n t a l u m dose.

266

samples which looked and behaved completely differently from those described in Section 3.1. The surfaces were totally or partially stained with colours ranging from light brown to golden brown and violet to dark blue. The surface layers responsible for the coloured appearance were mechanically stable and even hydrogen pretreatment (cathodic reduction with i = 100 mA cm -2 for 10 min) could n o t damage them. Potentiokinetic polarization results indicated very little electrochemical activity. Cathodic hydrogen evolution as well as anodic iron dissolution were strongly reduced in comparison with values for "clean" tantalum-implanted samples. Figure 4 shows t w o typical polarization curves obtained for this kind of specimen: one was Ta ÷ ion implanted and the other was W + ion implanted (doses, 10 z7 and 5 × 1016 ions cm -2 respectively at E = 100 keV; colours, dark blue and golden brown respectively). Both types of specimen and also the implantation of Ti ÷ ions gave similar results. The amount of hydrogen evolution as well as icrit w e r e three orders of magnitude lower

and the passive region nearly two orders of magnitude lower than for unimplanted steel. These marked effects can no longer be attributed to the implanted ions alone. Although one of the first assumptions was that some surface contamination (namely a carbon or hydrocarbon layer) was the reason for the effects observed, other possibilities such as the formation of carbides by means of carburization or the formation of an amorphous layer on the surface of the samples could not be excluded. In order to learn more about the protective surface layers, a number of samples were investigated using AES and depth profiles of iron, tantalum (tungsten) and carbon recorded by stepwise sputtering with argon.

3.3. Auger electron spectroscopy experiments and depth profiles The depth profiles obtained by AES profiling for four characteristic samples (details of which are given in Table 1) will be reported. Figure 5 shows the results of the AES profile measurements. Sample l a has a carbon content of about 40 at.% on the surface together with an iron

-1000

[ogi ~ S °

-4

- 6

,~

/

Fig. 4. Strong inhibition effects d e m o n s t r a t e d by p o t e n t i o k i n e t i c polarization curves of t w o steel electrodes i m p l a n t e d at A E R E with 1017 Ta + ions crn -2 (curve Ta +) and 5 × 1016 W + ions cm -2 (curve W +) (E = 100 keV) c o m p a r e d with the curve for u n i m p l a n t e d steel (curve So).

267 TABLE 1 I r r a d i a t i o n details a n d r e s u l t a n t c o l o u r s of t h e f o u r c h a r a c t e r i s t i c samples

Sample

Resultant colour

Irradiation details Ion

Dose

Energy

(ions c m -2)

(keV)

Temperature

(°C)

la

Ta+

5 x 1016

100

60

lb

Ta +

5 x 1016

100

60

2 3 4

Ta + W+ Ta +

5 x 1016 5 x 1016 1017

100 100 100

120 60 60

content of about 30 at.%. Tantalum is present after the first removal of surface material by sputtering. The Auger signal for carbon indicates that the carbon is in the form of carbide as soon as the tantalum appears. This was the case in all the Auger experiments described here. (The signal from carbon in carbides can be easily distinguished from t h a t of elemental carbon, and different sensitivity factors for elemental carbon and carbon in carbide form were taken into account during the evaluation of the profiling experiments.) Tantalum exhibits an implantation profile as expected and carbon does not extend much deeper into the matrix than tantalum does. This is a typical result for the implantation of an ion with high carbon affinity into iron or low carbon steel. This result is in contrast with the result for sample l b , which was obtained within the stained zone on sample 1. The surface in this area consists of carbon only and it takes several sputtering steps before an iron signal appears. Obviously a carbon (or hydrocarbon) layer of some thickness has grown on top of this sample. In the region of the tantalum peak, all three elements are present in nearly equal concentrations and the carbon Auger signal is from a carbide; beyond the tantalum range the retained carbon remains rather high and its signal still indicates that it is present as a carbide. It is known that argon implantation alone can lead to carbide formation in a matrix with high carbon affinity. So we cannot distinguish whether the detected carbide was already present before argon sputtering or was only created during this process. It should also be noted that the tantalum pro-

P a r t l y s t a i n e d light b r o w n ; profile t a k e n o u t s i d e t h e b r o w n area P a r t l y s t a i n e d light b r o w n ; profile t a k e n inside t h e b r o w n area Colourless Golden brown Dark blue

files taken inside and outside the stained area do not indicate any marked difference in the retained tantalum doses. Therefore, the nonuniformity of the surface layer cannot be regarded as a consequence of non-uniform implantation. However, it could be explained by the existence of a temperature gradient across the sample during the implantation. It is evident that, in the stained as well as in the unstained area, tantalum has been implanted into the bulk iron. Therefore it must be concluded that the carbon layer has grown during or after implantation only. Specimens with a similar surface appearance as sample 1, if homogeneously stained, usually exhibited strongly suppressed electrode kinetics. Sample 2 was irradiated at 120 °C. Such samples never showed any colour and the corrosion inhibition was always rather slight. The depth profile indicates that at this temperature the formation of a carbon layer is less favoured. Although the carbon surface content was nearly 100 at.%, the percentage of iron increased quickly with sputter time; again we found a region where all three elements were present in nearly equal concentrations and the carbon signal indicated carbide formation. The result of electrochemical measurements at such electrodes suggested a definite correlation between the presence of surface carbon layers and strong corrosion inhibition. Sample 3 irradiated at 60 °C with W+ ions again shows a substantial carbon layer. Only after prolonged sputtering did iron appear in higher concentrations. It has to be mentioned at this point, however, that sputter times in the different AES experiments (samples 1-4)

2

o0Qt:lo.,

/

i

10

c

zu

3U sputter time

Ta

40 (min)

sputter time

C

/, ~ (rain) sputter time

Fe

(d)

10

50

1

1 ~o0 at.=/,

(b)

10

3

c

5

±

7

9 sputter time

(~nin)

sputter time

Fe C

Fig. 5. Results of AES profile measurements on four steel samples irradiated at AERE : (a) sample l a ; (b) sample 1 b; (c) sample 2" (d) sample 3 ; (e) sample 4. Irradiation details of the samples are given in Table 1.

(e)

5O

8

(c)

10

5O

d

1 ~0 at.=/o

(a)

50

o

1 ~0 atYo OO

269 cannot be directly compared with one another because of difficulties in maintaining equally high sputter currents throughout different profile detections. The results for sample 3 (like those for sample l b ) show that tungsten was implanted into the iron and not into the carbon layer present after implantation. The electrochemical behaviour of a sample comparable with sample 3 is shown in Fig. 4, curve W + (strong inhibition). Sample 4 was implanted with 1017 Ta ÷ ions cm -2 at 60 °C and the electrochemical behaviour of a sample produced during the same implantation experiment is also included in Fig. 4 (curve Ta+). The depth profile measurement shows a substantial carbon layer on the sample surface which, unlike the situation in all the other cases reported, should have been present at least partially before the implantation of Ta + ions because of the finding that tantalum has been implanted partly into this carbon layer. The m a x i m u m of the tantalum profile is found in the region of the Fe-C interface. The carbon signal again indicates carbide formation. From these results a rather crude estimation of the total carbon layer thickness can be derived. When the predicted range parameters for an element such as tantalum implanted into carbon are taken into account, the total carbon layer thickness can be estimated to be not less than 25 nm but possibly significantly higher. The presence of high carbon contamination on or in samples implanted at AERE was also detected by means of the 12C(d, p) nuclear reaction m e t h o d by the Nuclear Physics Division, AERE, Harwell. In one case, 8 X :10~6 atoms of carbon were f o u n d in an implanted sample, but the m e t h o d was not capable of distinguishing between bulk or surface carbon. So far we do n o t know in which form the carbon is present in the surface layer, but we observed t h a t these layers had a high mechanical stability and could not be destroyed even by strong bubble formation during cathodic hydrogen evolution. Preliminary experimental results obtained recently showed that carbon layers produced on steel by glow discharge deposition from an ethylene atmosphere for comparison with carbon layers obtained during implantation did n o t exhibit such a mechanical and electrochemical stability. However, we observed that after ion implantation into such layers using tantalum

as well as xenon beams their corrosion behaviour was comparable with that of the strongly inhibited samples described above. 4. ATTEMPTS TO INTRODUCE CARBON DURING TANTALUM IMPLANTATION After we had obtained the experimental results presented above, we tried to introduce carbon directly into our samples in various better-defined ways.

4.1. Contamination of samples with organic compounds prior to tantalum implantation The possibility of organic contamination of pre-implanted samples by use of impurityenriched organic solvents must be regarded as highly improbable. Experiments to try to produce such contaminations with various organic compounds (such as anthracene and paraffin) illustrated t h a t it was impossible to obtain a uniform distribution of the; contaminants on metal samples without application of special methods. Electrochemical measurements of tantalum-implanted samples with non-homogeneous organic surface contamination did n o t show any relevant inhibition effects. 4.2. Implantations in residual gas atmospheres deliberately enriched with carbon We implanted Ta + ions into steel in residual gas atmospheres with high pressures in the region of 10 -a mbar, obtained by backfilling the target chamber with the vapour of different organic compounds (CH4, C~H6, acetone) and CO2. None of these experiments yielded targets with coloured surfaces. Potentiokinetic polarization experiments showed no reproducible additional corrosion protection caused by carbon uptake of the samples apart from the normal tantalum effect. The process of carburization as described by Singer [11], which must be assumed to take place during tantalum implantation under the above-described conditions, obviously did not have the same marked influence on the electrochemical behaviour of our steel electrodes which had carbon-containing surface layers. 4.3. Successive implantation of C+, Ta+ and 0 + ions During these experiments we obtained results which fell into two distinct categories,

270

with differences similar to those between tantalum-implanted specimens with carbon surface layers and tantalum-implanted specimens w i t h o u t carbon surface layers. Although all samples were irradiated in the same machine (the 300 keV accelerator at GSI), in some cases Ta+-ion - and C+-ion-implanted samples showed a slight staining after implantation and exhibited considerably reduced electrochemical activity. These effects were strongest if we implanted C + ions as well as O + ions prior to the Ta + ions. In the absence of visible beam spots, however, we could not detect any effect of the implanted carbon in addition to the usual effect of tantalum implantation described in Section 3.1. All the results obtained during the experiments (Section 4) indicate that carbon which was distributed in the bulk of the implanted specimens (even if present in the form of carbides) is n o t capable of further enhancing the already improved corrosion resistance of tantalum-implanted steel samples. Only in cases where vacuum and residual gas conditions obviously led to surface carbon contamination were such additional effects observed.

5. CONCLUSIONS

(1) The effect of tantalum implantation on the electrochemical behaviour of a low carbon steel is similar to that found by other researchers for pure iron. The corrosion behaviour was considerably influenced, the main effect being on the anodic active dissolution of the iron (icrit which was suppressed by one to two orders of magnitude). Multisweep measurements, however, indicated that the process of selective dissolution of iron takes place. (2) Steel samples with subsurface carbon enrichment produced by C ÷ ion implantation or carbon uptake from highly contaminated residual gas atmospheres during ion implantation (carburization) showed no detectable change in their electrochemical behaviour. (3) A limited number of implanted specimens, however, showed an extraordinarily high corrosion resistance and were investigated with AES. These samples were all shown to have substantial carbon layers on

the surface to which the strong inhibition effects were attributed. These layers had been formed before as well as during the implantation and had a thickness of approximately a few tens of nanometres. With respect to hydrogen evolution, samples with carbon surface layers behaved in a similar way to pure carbon electrodes, which exhibit only little cathodic activity. The remaining anodic dissolution currents indicate that, at least below a thickness of 100 nm, the carbon layers are still porous. The present authors think that such protective carbon layers formed under the conditions of ion implantation with optimized experimental parameters could be of interest for the corrosion protection of metals.

ACKNOWLEDGMENTS

We would like to thank H. Wirth (MaxPlanck-Institut fi~r Kernforschung, Heidelberg) for the preparation of samples consisting of carbon layers deposited onto steel, the personnel at the 300 keV accelerator at GSI, Darmstadt, and those researchers in the Nuclear Physics Division, AERE, Harwell, who were involved in the different implantation experiments. Parts of this work were supported by the Bundesministerium fi~r Forschung und Technologie, Bonn, under Contract Fe-KKs 2.5/9. REFERENCES 1 V. Ashworth, W. A. Grant and R. P. M. Procter, Corros. Sci., 16 (1976) 775. 2 V. Ashworth, D. Baxter, W. A. Grant and R. P. M. Procter, Corros. Sci., 17 (1977) 347. 3 V. Ashworth, W. A. Grant, R. P. M. Procter and E. J. Wright, Corros. Sci., 18 (1978) 681. 4 K. Takahashi, Y. Okabe and M. Iwaki, in R. E. Benenson, E. H. Kaufmann, G. L. Miller and W. W. Scholz (eds.), Proc. 2nd Int. Conf. on Ion Beam Modification o f Material~ Albany, NY, 1980, in Nucl. Instrum. Methods, 182-183 (1981) 1009-1015. 5 H. Ferber, H. Kasten, G. K. Wolf, W. J. Lorenz, H. Sehweickert and H. Folger, Corros. Sci., 20 (1980) 117. 6 G. K. Wolf and H. Ferber, in B. Biasse, G. Destefanis and J. P. Gaillard (eds.), Proc. 3rd Int. Conf. on Ion Beam Modification o f Materials, Grenoble, 1982, in Nucl. Instrum. Methods, 209-210 (1983) 197. 7 C. R. Clayton, W. K. Chan, J. K. Hirvonen, G. K. Hubler and J. R. Reed, in E. McCafferty, C. R. Clayton and J. Oudar (eds.), Proc. Int. Symp. o n

271

Fundamental Aspects of Corrosion Protection by Surface Modification, Washington, DC, October 9-14, 1983, inSpec. Publ. 84-3, 1984, p. 17 (Corrosion Division, Electrochemical Society, Pennington, NJ). 8 B. S. Covino, B. D. Sartwell and P. B. Needham, J. Electrochem. Soc., 125 (1978) 370. 9 J. A. Knapp, D. M. Follstaedt and S. T. Picraux,

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Destefanis and J. P. Gaillard (eds.), Proc. 3rd Int. Conf. on Ion Beam Modification of Materials, Grenoble, 1982, in Nucl. Instrum. Methods, 209210 (1983) 857. 11 I. L. Singer, J. Vac. Sci. Technol. A, 1 (2) 419. 12 G. K. Wolf, in J. K. Hirvonen (ed.), Ion Implantation, Treatise Mater. ScL Technol., 18 (1980) 373. 13 K. S. Grabowski, F. D. Correll and F. R. Vozzo, in J. W. Mayer (ed.), Proc. 4th lnt. Conf. on Ion

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