Corrosion behavior of surface films on Boron-implanted high purity iron and stainless steels

Materials Science and Engineering, 69 (1985) 297-301

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Corrosion Behavior of Surface Films on Boron-implanted High Purity Iron and Stainless Steels* H. J. KIM, W. B. CARTER and R. F. HOCHMAN

Metallurgy Program, School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100 (U.S.A.) E. I. MELETIS

Metal Science and Technology, Illinois Institute of Technology Research Institute, Chicago, IL 60616 (U.S.A.) (Received September 17, 1984)

ABSTRACT

Boron (dose, 2 X 1017 ions cm -2) was implanted into high purity iron, A I S 1 3 1 6 austenitic stainless steel and A I S 1 4 4 0 C martensitic stainless steel at 40 keV. The film structure o f implanted samples was examined and characterized by contrast and diffraction analyses utilizing transmission electron microscopy. The effect o r B + ion implantation on the corrosion behavior was studied using the p o t e n t i o d y n a m i c polarization technique. Tests were performed in deaerated 1 N HeS04 and O.1 M NaCl solutions. Scanning electron microscopy was used to examine the morphology o f the corroded surfaces after testing.

1. INTRODUCTION

Ion implantation of materials has been studied in relation to surface-sensitive corrosion properties [1-7]. This modification, in general, can be achieved b y ion implanting either a passivating element [1, 2, 4, 5] or a non-passivating element [3, 6, 7] to produce a thin alloyed surface layer. Recently the present authors [7 ] studied the effect of nitrogen implantation on the corrosion behavior o f high purity iron and martensitic stainless steel. It was shown in this study that improvements in the corrosion resistance were the result of a supersaturation of nitrogen as well as nitride introduced into the surface of these materials. *Paper presented at the International C o n f e r e n c e o n Surface M o d i f i c a t i o n of Metals b y Ion Beams, Heidelberg, F.R.G., September 17-21, 1984. 0025-5416/85/$3.30

In recent years the modification of surface properties has also been attained b y altering the surface structure to the amorphous state, using ion implantation [8, 9]. A metal in the amorphous condition is basically at a higher thermodynamic energy than in its normal crystalline condition and is therefore expected to be more reactive to a corrosive environment. However, the amorphous structure does n o t contain crystalline defects such as grain boundaries, dislocations and impurity aggregates typically f o u n d in crystalline materials [ 9 , 1 0 ] . Therefore the amorphous state has been reported to have improved pitting resistance [10]. In the present work, b o r o n was implanted into high purity iron and stainless steels in an a t t e m p t to produce a supersaturated borided and/or amorphous structure in the surface region. These surfaces were characterized and the corrosion properties were evaluated.

2. EXPERIMENTAL DETAILS

The substrate materials studied were iron of 99.999% purity, AISI 316 austenitic stainless steel and AISI 440C martensitic stainless steel. The materials were in the form of rods 10 mm in diameter. The as-received condition for the three materials was cold worked for iron and AISI 316 stainless steel and double tempered at 163 °C for AISI 440C stainless steel. Disc specimens, 2.5 mm thick and 9 mm in diameter, were machined from the rods. One side of each disc was wet ground and metallographically polished with I pm diam o n d paste. The polished surfaces were im© Elsevier Sequoia/Printed in The Netherlands

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planted with 2 X 10 i7 B ÷ ions c m -2 at an accelerating voltage of 40 keV at room temperature. The surface structure of the as-Implanted samples was examined and characterized by contrast and diffraction analyses utilizing transmission electron microscopy (TEM). TEM foils were electropolished from one side without perforation and then implanted on the polished side. The implanted foils were electropolished again from the unimplanted side until perforation occurred. Samples were then subjected to TEM examination. The effect of the boron implantation on the corrosion behavior of the three materials was investigated using the potentiodynamic polarization technique. A polarizing scan of 1 mV s-1 from 300 mV below the open-circuit

potential in the noble direction to the transpassive or breakdown potential was used. Scanning in the cathodic region was attempted to remove or reduce the oxide film formed during or after implantation. Polarization experiments were performed in deaerated solutions of 1 N H2SO4 and 0.1 M NaC1. After testing, specimen surfaces were examined using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis.

3. RESULTS AND DISCUSSION

Figures l(a) and 1(c) show the TEM microstructures of unimplanted and B+-ion-im planted AISI 440C martensitic stainless steel; the corresponding selected area diffraction

Fig. 1. TEM micrographs o f AISI 440C stainless steel: (a) unimplanted sample; (b) selected area diffraction pattern of (a); (c) B+-ion-implanted sample; (d) selected area diffraction pattern o f (c). The diffraction spots present are related to the unaffected carbide.

299

patterns are given in Figs. l i b ) and l i d ) . With boron implantation a large change in structure was observed. This is shown by the diffuse ring in the diffraction pattern. This is indica-

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tive of an amorphous phase or a supramicrocrystalline state. B+-ion-implanted high purity iron was also determined to be in the amorphous state, but boron implantation in AISI 316 austenitic stainless steel did not form the amorphous phase for the implantation characteristic used in this study. This result is in contrast with the study by Chen et aI. [6], in which boron implantation in AISI 316L stainless steel did produce an amorphous layer at the implanted surfaces. Figure 2 gives the potentiodynamic polarization curves of unimplanted and B+-ion implanted samples tested in a deaerated 1 N H2SO4 solution. As shown in the figure, boron implantation in high purity iron and AISI

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Fig. 2. P o t e n t i o d y n a r n i c polarization curves of samples in a 1+ N. H2SO 4 solution: (a) u n i m p l a n t e d . . . . ( ) a n d B -zon-]mplanted (- - - ) hzgh purzty zron; (b) u n i m p l a n t e d ( - - ) and B*-ion-implanted ( - - - ) AISI 316 stainless steel; (c) u n i r n p l a n t e d ( ) and B+-ion-implanted (- - - ) AISI 440C stainless steel.

Fig. 3. SEM micrographs of corroded surfaces on (a) u n i m p l a n t e d and (b) B+-ion-implanted AISI 440C stainless steel after testing in a 1 N H2SO 4 solution.

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440C stainless steel has resulted in a significant reduction in the current densities at all potentials (measured with respect to a saturated calomel electrode (SCE)). The open-circuit potentials were lowered by 150 mV and

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170 mV respectively. In contrast, B÷-ion implanted AISI 316 stainless steel shows a higher critical current density and a larger primary passive range compared with the unimplanted steel. The transpassive potential is also elevated but the passive current density was reduced by implantation. SEM examination of the corroded surfaces after polarization revealed, as shown in Fig. 3, areas without defined grain boundaries. The corrosion behavior is probably a result of the amorphous surface condition as described by other workers [11, 12]. Figure 4 displays the potentiodynamic polarization curves of unimplanted and B ÷ion-implanted samples tested in a 0.1 M NaC1 solution. Boron implantation elevates the

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Fig. 4. P o t e n t i o d y n a m i c polarization curves o f samples in a 0.1 M NaCI solution: (a) u n i m p l a n t e d ( ) and B÷-ion-implanted (- - - ) high purity iron; (b) unimplanted ( ) and B+-ion-impLanted (- - - ) AISI 316 stainless steel; (c) u n i m p l a n t e d ( ) and B~-ion implanted (- - - ) AISI 440C stainless steel.

Fig. 5. SEM micrographs o f corroded surfaces o f (a) u n i m p l a n t e d and (b) B*-ion-implanted AISI 4 4 0 C stainless steel after testing in a 0.1 M NaCI solution.

301 o p e n - c i r c u i t p o t e n t i a l o f high p u r i t y i r o n a n d increases t h e c u r r e n t d e n s i t y c o n t i n u o u s l y . B + - i o n - i m p l a n t e d A I S I 3 1 6 stainless steel has a higher passive c u r r e n t d e n s i t y a n d b o r o n implantation lowers the open-circuit potential o f A I S I 4 4 0 C stainless steel. F i g u r e 5 s h o w s SEM m i c r o g r a p h s o f t h e c o r r o d e d s u r f a c e o f A I S I 4 4 0 C stainless steel a f t e r p o l a r i z a t i o n in a 0.1 M NaCI s o l u t i o n . I m p l a n t e d A I S I 4 4 0 C stainless steel displays a f e w small pits a n d was essentially p r o t e c t e d from pitting attack. 4. CONCLUSIONS B o r o n i m p l a n t a t i o n i n t o high p u r i t y iron a n d A I S I 4 4 0 C m a r t e n s i t i c stainless steel at 40 keV a n d r o o m t e m p e r a t u r e w i t h a f l u e n c e o f 2 × 1017 ions c m -2 results in t h e f o r m a t i o n o f i m p l a n t e d s u r f a c e s o f a m o r p h o u s phase. H o w e v e r , b o r o n i m p l a n t a t i o n in A I S I 3 1 6 a u s t e n i t i c stainless steel s u b j e c t e d t o t h e s a m e i m p l a n t a t i o n c o n d i t i o n s did n o t . T h e m o d i f i e d s u r f a c e films o n high p u r i t y i r o n a n d A I S I 4 4 0 C stainless steel s h o w e d m a r k e d l y i n c r e a s e d c o r r o s i o n r e s i s t a n c e in a d e a e r a t e d 1 N H2SO4 s o l u t i o n . B o r o n i m p l a n t a t i o n o f A I S 1 4 4 0 C stainless steel significantly increases t h e p i t t i n g c o r r o s i o n r e s i s t a n c e in a d e a e r a t e d 0.1 M NaC1 s o l u t i o n ; h o w e v e r , t h a t o f high p u r i t y i r o n a n d A I S I 3 1 6 stainless steel decreases. ACKNOWLEDGMENTS T h e a u t h o r s wish to a c k n o w l e d g e s u p p o r t o f t h e N a t i o n a l A e r o n a u t i c s a n d S p a c e Administration under Contract NAS 8-350-48 (studies o f A I S I 4 4 0 C stainless steel) a n d the N a t i o n a l I n s t i t u t e o f D e n t a l R e s e a r c h under Training Grant DE 07053.

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