Corrosion Science, Vol. 33, No. 5, pp. 815-818, 1992 Printed in Great Britain.
0010-938X/92 $5.00 + 0.00 © 1992 Pergamon Press plc
PITTING CORROSION RESISTANCE OF SILICONIMPLANTED STAINLESS STEELS J. BASZKIEW1CZ,* M. KAMINSKI,* A. PODGORSKI,t J. JAGIELSKI~-and G. GAWLIKt *Institute of Materials Science and Engineering, Warsaw University of Technology, Narbutta 85, 02-524 Warsaw, Poland Hnstitute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland Abstract--The pitting corrosion resistance of three different types of stainless steel implanted with silicon is investigated using the potentiokinetic polarization technique. The specimens are tested in 3% NaCl and 0.1 N HCI solutions. Silicon ion implantation inhibits pitting corrosion of the steels in both aqueous media. The corrosion resistance depends on the silicon dose. Post implantation annealing only slightly alters the localized corrosion. INTRODUCTION THE ELECTROCHEMICAL properties of ion-implanted ferrous surfaces have b e e n mainly studied in o r d e r to i m p r o v e the corrosion resistance of the modified surface in a q u e o u s ambience. A n u m b e r of papers have dealt with the corrosion b e h a v i o u r of iron and different types of steel implanted with various ions (see for example Ref. 1 and the references therein). T h e corrosion research on steels carried out in recent years c o n c e r n e d the f o r m a t i o n of surface alloys by the implantation of such ions as: M o +, Cr +, Ti +, C r + + M o +, Ni +. T h e beneficial effects of the surface alloys on the a q u e o u s corrosion in various electrolytes have b e e n reported. 2-5 This e m b r a c e d both uniform corrosion attack and pitting. H o w e v e r , it has to be stressed that the uniform corrosion protection p r o v i d e d by the implanted layers was f o u n d to deteriorate rather rapidly, particularly when active dissolution has taken place. Protection against the pitting and the stress corrosion cracking seems to be m o r e i m p o r t a n t as far as the application of ion implantation for the i m p r o v e m e n t of corrosion resistance is c o n c e r n e d . T h e f o r m a t i o n of a m o r p h o u s surface layers on the steels implanted with high dose of p h o s p h o r u s or b o r o n ions has also b e e n reported. 6"7 C h e n et al. 6 d e m o n strated the r e m a r k a b l e corrosion resistance of such layers on a stainless steel in the acid solution e n v i r o n m e n t . TABLE 1 Steel A (18) B (18/8) C (18/8/2)
0.061 0.019 0.031
0.016 0.015 0.015
0.004 0.022 0.005
0.57 1.5 1.55
0.41 0.56 0.48
18.38 19.04 17.07
0.27 11.76 11.95
0.05 0.08 2.65
Manuscript received 16 October 1990. 815
J. BASZKIEWICZet al.
FIc. 1. The effect of the silicon dose on the anodic polarization of the steels: A (Fig. la) and C (Fig. lb) in 3% NaCI: (1) unimplanted; (2) 0.5 × 1017 at cm-2; (3) 1 X 1017 a t c m - 2 ; (4) 1.5 × 1017 at cm -2.
In the present work the localized corrosion behaviour of three different types of s t a i n l e s s s t e e l i m p l a n t e d w i t h s i l i c o n is e x a m i n e d u s i n g t h e p o t e n t i o k i n e t i c p o l a r i z ation technique. EXPERIMENTAL METHOD Three types of steel were used in the experiments, The compositions of the steels are listed in Table 1. All specimens were mechanically polished, degreased in acetone and dried prior to implantations. The samples were 100keV Si + implanted with doses ranging from 0.5 x 1017 at cm-2 up to 1.5 × 1017 at cm 2. All implantations were performed at RT. Post implantation annealing at 120°C for 6 h or 300°C for 1 h was applied to some of the samples. Annealing at 300°C was carried out in vacuum to avoid oxidation. The samples were examined by the potentiokinetic method. The aqueous solutions of 3% NaCI and 0.1 N HCI were used. To determine the corrosive potential, the measurements were preceded by keeping the samples in an appropriate solution for 16 h. The saturated calomel electrode was used as the reference. The value of the corrosion potential was the starting point for polarization. The anodic polarization sweep was carried out at the rate of 1000 mV h - 1 . The breakdown potential ( E b ) w a s determined assuming that the anodic current of the passive sample attained 50/~A cm -2 . Following electrochemical measurements, the samples were optically examined using a standard microscope and SEM. EXPERIMENTAL
A n o d i c polarization in N a C 1 T h e s t e e l s o f r e s p e c t i v e c o m p o s i t i o n s A a n d C ( T a b l e 1) a r e i n v e s t i g a t e d in 3 % N a C l . B o t h s t e e l s b e c a m e s e l f - p a s s i v a t e d i n t h i s s o l u t i o n as s o o n as t h e p o t e n t i a l is s t a b i l i z e d . P i t t i n g is o b s e r v e d a t t h e p o t e n t i a l s h i g h e r t h a n t h e b r e a k d o w n p o t e n t i a l . T h e e f f e c t o f s i l i c o n d o s e o n t h e p o t e n t i o d y n a m i c p o l a r i z a t i o n c u r v e s is s h o w n i n
Corrosion resistance of Si-implanted steels
H E -1
Elvl FIG. 2. The effect of the silicon dose on the anodic polarization of the steels: A (Fig. 2a), B (Fig. 2b) and C (Fig. 2c) in 0.1 N HCI: (1) unimplanted; (2) 0.5 x 1017 a t c m z; (3) 1 × 1017at cm-2; (4) 1.5 x 1017at cm-2.
Fig. 1. The E - log i characteristics of steels A and C are presented in Fig. l(a) and (b), respectively. No changes in the polarization properties of the steels implanted with the dose 0.5 × 1017 at cm -2 are revealed (Fig. 1). When the dose is raised up to 1 × 1017 cm -2 (steel A) and 1.5 × 1017 cm -2 (steel C), the breakdown potential is increased attaining 800 mV for the steel C (Fig. la,b).
Anodic polarization in HC1 The steel B is found to be active in the HC1 solution (Fig. 2b) and uniform corrosion occurs in this condition. Passivation is produced by increasing the potential as it is seen in Fig. 2(b). The reduction in the critical passivation current and the increasing of Eb are caused by the silicon implantation (Fig. 2b). The breakdown potential at the dose of 1 × 1017 at cm -e is 480 mV higher than Eb for the original sample. Uniform etching and pitting of the steel B sample are revealed after measurements.
J. BASZKIEWICZet al.
C 0 ¸
FIG. 3. The effect of post implantation annealing on the anodic polarization of the steel C in 0.1N HCh (1) unimplanted, 300°C for lh; (2) unimplanted, 120°C for 6h; (3) 1 × 1017at cm 2,120oC for 6 h; (4) 1 × 1017at cm -2, 300°C for 1 h. Steels A and C b e c a m e self-passivated in H C I as in NaCI. T h e silicon implantation of b o t h steels leads to an increase in the b r e a k d o w n potential. A dose effect is also revealed (Fig. 2a,c). T h e b r e a k d o w n potential of the steel C is f o u n d to be higher than E b of the steel A. T h e pits are o b s e r v e d on the surface of the samples w h e n the b r e a k d o w n potential value is exceeded. T h e samples o f the steel C, b o t h u n i m p l a n t e d and implanted with the dose 1 × 1017 cm -2, were a n n e a l e d at 120 or 300°C. T h e E - l o g i characteristics are shown in Fig. 3. T h e post implantation annealing only slightly influences the electrochemical properties of the steel C. CONCLUSIONS Silicon ion implantation inhibits the pitting corrosion of the steels of different c o m p o s i t i o n in very aggressive a q u e o u s media. T h e effect of the dose on the b r e a k d o w n potential has b e e n clearly revealed. T h e thermal t r e a t m e n t s only slightly alter the corrosion behaviour. REFERENCES 1. H. FERBERand G. K. WOLF,Mater. Sci. Engng 90, 213 (1987). 2. Y. F. WANG, C. R. CLAYTON,G. K. HUBLER,W. H. LUCKEand J. R. HIRVONEN,Thin Solid Films 63, 11 (1979). 3. S. B. AGARWAL,Y. F. WANG, C. R. CLAYTON,H. HERMANand J. K. HIRVONEN,Thin Solid Films 63, 19 (1979). 4. H. FERBER,G. K. WOLF, H. SCHM1EDELand G. DEARNALEY,Mater. Sci. Engng 69, 261 (1985). 5. G. K. HUBLERand E. MCCAFFERTY,Corros. Sci. 20, 103 (1980). 6. Q. M. CHEN, H. M. CHEN, X. D. BAI, J. Z. ZHANGand H. H. WANG,Nucl. Instr. Meth. 209/210, 867 (1983). 7. H.J. KIM,W. B. CARTER,R. F. HOCrtMANand E. I. MELETIS,Mater. Sci. Engng 69, 297 (1985).