The pitting corrosion of amorphous Ni63Cr12.5Fe4Si8B12.5 alloy in chloride solution

The pitting corrosion of amorphous Ni63Cr12.5Fe4Si8B12.5 alloy in chloride solution

Corrosion Science, Vol. 34. No. 10, pp. 1697-1706, 1993 0010-938X/93 $6.0(I + 0.00 Pergamon Press Ltd Printed in Great Britain. THE PITTING CORROSI...

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Corrosion Science, Vol. 34. No. 10, pp. 1697-1706, 1993

0010-938X/93 $6.0(I + 0.00 Pergamon Press Ltd

Printed in Great Britain.

THE PITTING CORROSION OF AMORPHOUS Ni63Cr12.sFe4SisB12.5 A L L O Y I N C H L O R I D E SOLUTION Yu Zuo* and RONALDM. LATANISION Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.

Abstract--The pitting corrosion behavior of amorphous Ni63Cr12.sFe4SisB12.5 alloy in NaCI solution was studied using electrochemical methods. Hemispherical pits were found to initiate along mechanic polishing lines on the surface. Eleetropolishing improved pitting resistance greatly. Potentiostatic anodic polarization tests showed an incubation time for pit nucleation after which the pits propagated very quickly. The poor pitting resistance of this amorphous alloy was ascribed to its repassivation characteristics. INTRODUCTION

THE GOODpitting corrosion resistance of some amorphous alloys has been known for years. The early work on this subject was done by Naka, Hashimoto and Masumoto. 1,2 Pitting was not observed on amorphous Fe-Cr-P-C and Fe-Cr-Ni-PC alloys in 1 N NaCl solution when the chromium contents of the alloys were > 8%. Devine 3"4 illustrated the immunity of amorphous Ni3sFe30Cr15P14B6 alloy to pitting corrosion in chloride solution. The great resistance of this alloy to pitting was attributed to its high degree of chemical homogeneity and lack of inclusions. Increasing the chromium content in Fe-Cr-Ni-P-B glassy alloys from 0 to 16 at% reduced the critical current density5 and pitting was not observed on the alloys polarized below the transpassive potential region. Cold rolling of the 16% Cr alloy caused a large active dissolution peak and a high passive current density. Many small pits ranging up to 10 ~tm in diameter were seen. The pits were never observed to penetrate the specimen. The effect of metallic alloying elements in glassy Fe-M-P-C alloys was investigated by Naka et al. 6 Pitting was not observed in 3% NaC1 solution for the alloys containing Cr, Mo, Ti and some other alloying elements. The excellent pitting resistance of Cr- and P-containing glassy alloys appears to result from the rapid formation of a protective film. It was suggested that the very homogeneous nature of the substrate, combined with a highly chromium-enriched passive film that is very protective, makes the initiation of pits very difficult except at transpassive potentials. Most of the published work on local corrosion of amorphous alloys dealt with Ni36Fe32Cr14PL2B6, an alloy with excellent corrosion resistance. There is still a lack of knowledge about the nucleation and propagation of pits in amorphous structures, or the effect of alloy composition on pitting behavior. This paper reports the pitting behavior of amorphous Ni63Crt2.sFe4SisB12.5 alloy in aqueous NaCI solution. *Present address: Department of Applied Chemistry, Beijing Institute of Chemical Technology, Beijing 100029, China. Manuscript received 2 February 1992; in amended forms 1 October 1992 and 9 February 1993. 1697

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Yu Zoo and R. M. LATANISION EXPERIMENTAL METHOD

Amorphous melt spun Ni63Cr12.sFe4Si8B12.5 (at%) alloy was obtained from Allied Signal Corporation, New Jersey, in the form of ribbon 2.5 cm wide and 0.0035 cm thick. X-Ray diffraction, TEM and electron diffraction tests were carried out to confirm the amorphous structure of the alloy.7 The samples were mechanically polished with 600# silicon carbide paper, to remove any possible surface irregularities that may be formed during the melt spinning process, then were passivated in 25% HNO 3 at 50°C for 1 h to reduce the tendency of crevice corrosion. Two layers of epoxy resin were coated on the sample, leaving an area of 1 cm 2 uncovered (both sides). Before electrochemical tests, the samples were polished again using 600# SiC paper and rinsed in acetone and de-ionized water. To compare the effect of different surface conditions on the pitting behavior of the alloy, some samples were electropolished. The sample was first polished using 600# paper, then was electropolished in 85% H3PO 3 + 15% CrO 3 with a current density of 1 A c m -2 for 30 s. After the electropolishing the polishing lines made by the abrasive paper were removed completely. The electrochemical tests were run in aqueous NaCI solutions of various concentrations and pH values which were adjusted by adding sulfuric acid or sodium hydroxide. Analytical grade reagents and deionized water were used. After the sample was exposed to the solution for 30 min, potentiodynamic polarization began. The sample was first polarized cathodically to -600 mV(SCE) for 1 min, then was polarized continuously in anodic direction. The potential scanning rate was 0.45 mV s-1 . When the current density reached 5 A m -2, the potential scanning was reversed to produce a cyclic scanning curve. All the potentials reported in this paper are relative to a saturated calomel electrode (SCE). The solution was open to the air. The occurrence of pitting corrosion was indicated by the presence of hysteresis in the polarization curve on backscanning the potential. After the tests, the surface of the samples were examined using an optical microscope and SEM. EXPERIMENTAL RESULTS AND DISCUSSION

Figure 1 shows the polarization curve of the amorphous NiraCr12.5Fe4SisB12.5 alloy in neutral 0.5 M NaC1 solution (curve 1). The material was spontaneously passivated in the solution and had a low passivation current density up to about 500 mV above which the current density increased quickly. When the potential scanning direction was reversed, the current density remained high, forming a hysteresis loop on the polarization curve. Many pits with different sizes were observed on the surface of the polarized sample. According to curve 1 in Fig. 1, the potential range between the breakdown potential, E b, and the protection potential, Epp, is wide. The protection potential Epp is relatively low, close to the corrosion potential, indicating

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that the alloy has a poor repassivation ability and that its crevice corrosion susceptibility may be high. The latter was confirmed. If the sample was not prepassivated in nitric acid solution or if the coating quality was not good, the current density increased at a much lower potential and severe local corrosion occurred along the interface between the epoxy coating and the exposed surface. For comparison, the polarization curve of the same alloy in 0.5 M Na2SO 4 solution is shown in Fig. 1 (curve 2). The potential at which the current density began to increase is much higher than in NaCI solution, and the backscanning curve exactly follows the forward curve. Also for comparison, the polarization curve of amorphous Ni36Fe32Cr]4Pl2B 6 alloy (Metglass 2826A) in 0.5 M NaCI solution is shown in Fig. 1 (curve 3). The transpassive potential of this alloy is much higher. Pitting was not observed on the alloy, as reported previously. ~5 Figure 2 shows the effect of solution chloride concentration on the breakdown potential of the Ni63Cr 12.sFe4Si8B ]2.5 alloy in pH 6 solution. There is a clear tendency that the breakdown potential of the alloy decreases as the NaC1 concentration increases. In 0.01 M NaC1 solution, the current hysteresis again occurred and pits were observed on the surface, although the breakdown potential is more oxidizing than in concentrated chloride solutions. The current density as a function of time at various constant anodic potentials is shown in Fig. 3. At potentials up to 200 mV, no current increase was observed up to 48 h. At higher potentials the current density first gradually approached the value of the steady passive current density at that potential, then increased very quickly after a certain period, indicating the beginning of pitting corrosion. The pitting breakdown potential of the alloy in 0.5 M NaCI solution is between 200 and 250 mV(SCE). Above 250 mV the pitting incubation time decreased as the applied potential increased. To investigate the effect of surface conditions of the specimens on the pitting corrosion behavior of the alloy, the polarization curves of the as-received surface, 600# paper polished surface and electropolished surface were measured. The results are shown in Fig. 4. The as-received surface (curve 1) showed a lower current density and a more negative breakdown potential than the 600# paper polished surface

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(curve 2). On the as-received surface, besides the pits, severe crevice corrosion was observed at the interface between the epoxy coating and the exposed surface. Along the interface crevice corrosion almost penetrated the ribbon. Hence it seems that the lower breakdown potential was mainly induced by crevice corrosion. During the melt spinning process, some surface irregularities may be formed on the wheel side of the ribbon. Compositional difference exists between the two surfaces of the ribbon. These may both facilitate corrosion. 4 The surface irregularities were removed by mechanical polishing and a more homogeneous surface was obtained for the 600# paper polished surface, which showed a higher breakdown potential. The electropolished sample showed a more positive breakdown potential which was about 100 mV higher than that of 600# paper polished surface, close to the transpassive potential of the alloy in Na2SO 4 solution without chloride. Figure 5 shows the surface morphology of a sample after it was polarized

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Pitting corrosion of Ni63CrI2.5Fe4SisB12.5

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potentiodynamically in 0.01 M NaCl solution. Many pits of various sizes are seen on the surface. The shape of the pits are hemispherical. Figure 5(b) shows a pit which has penetrated through the thickness of the ribbon sample. The thickness of the ribbon is 35/~m, and from the photo the diameter of the pit is about 80/~m. The propagating rate of the pit in different directions was almost the same. From Fig. 5(c), it can be seen that many small pits were nucleated along the polishing lines on the surface, and the pit density is very high. On the electropolished surface only a few big pits were seen. A pit that has penetrated the sample is shown in Fig. 6. The pit density on the smooth surface is very low, indicating the difficulty of pit nucleation. From these results, it seems that the roughness of the surface facilitated nucleation of the pits. On crystalline alloys, the location of pits is usually associated with inclusions, such as sulfides, oxides and other types of inclusions. 8"9 For the amorphous alloy studied, it appears that despite the amorphous structure, the chemical homogeneity and the lack of inclusions, the grooves left behind from mechanical surface treatment are a source for the initiation of pits. As described previously, under the same condition, amorphous Ni36Fe32Crl4Pl2B6 alloy remained passive, showing no pitting. The passivity of Ni36Fe32Cr14P12B 6 alloy and Ni63Cr12.sFe4SisB 12.5 alloy was compared in the following test: the samples were anodically polarized quickly from open circuit potential to 200 mV(SCE), a potential located in the passive region of the alloys. Then the current transients were recorded, as shown in Fig. 7. In both pH 6 and in pH 2 solutions, the passive current density of Ni36Fe32Cr14P12B6 alloy decayed more quickly than that of Ni63CrI2.sFe4SisB12.5 alloy, and the steady passive current density of the former was only about one tenth of that of the latter. Hence Ni3(,Fe32Cr14PlzB 6 alloy has a much stronger passivity than Ni63Cqz.sFe4SisBt2.~ alloy in chloride solution. Diegle 1° reported a similar comparison for passivity of Ni36Fe32Crt4P12B 6 and Ni36Fe32Cr14B12Si6 alloys. The former showed a considerably faster repassivation rate than the latter in chloride-containing 1 N H2SO 4. Since the

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Yu Zuo and R. M. LATANISION

content of chromium (the most important passivation-promoting element in the two alloys) is about the same, the difference in passivity may be explained by the effect of phosphorus. The synergism of Cr and P in passivation has been reported by Hashimoto and co-workers. 11,12They suggested that phosphorus serves the purpose of stimulating rapid initial dissolution of the unfilmed surface, leading subsequently to the formation of a more protective passive film. However, recent work by Moffat and Latanision 13'14 showed that phosphorus additions to chromium-phosphorus alloys simultaneously results in the acceleration of the cathodic reaction and the inhibition of the anodic process. These two effects result in spontaneous passivation of chromium. Boron exerts the same influence but less effectively. The influence of solution acidity on polarization behavior of the Ni63Cr12.sFe4Si8B12.5 alloy in 0.5 M NaCI solution is shown in Fig. 8. In the range of pH 2 to pH 6, the breakdown potential decreased with decrease of pH value. Both the peak current for anodic dissolution and the steady passive current density increased as the solution acidity increased. In pH 1.1 solution, the steady passive potential range was very narrow and the current density increased at lower potential, indicating an increase in the general dissolution of the passive film under the condition of high chloride concentration plus high acidity. Neither a hysteresis loop in the polarization curve nor pit initiation were observed in the pH 1.1 solution. Figure 9 shows the influence of the addition of Na2SO4 to NaC1 solution on the pitting behavior of Ni63Cr12.5FeaSisB12.5 alloy. The breakdown potential in 0.5 M NaC1 + 0.1 M Na2SO4 solution is higher than in 0.5 M NaCI solution. In 0.5 M NaCI + 0.3 M Na2SO 4 and 0.5 M NaCI + 0.5 M Na2SO 4 solutions, the polarization curves are almost the same as that in 0.5 M Na2SO 4 solution without chloride, and pitting was not observed. The addition of sulfate improved the pitting corrosion resistance of the amorphous alloy in chloride environment. The same behavior has been reported for crystalline steels. 15,16The result may be explained by the competitive adsorption of sulfate ions. It follows from the above results that there is no particular difference between the pitting behavior of amorphous alloy and crystalline alloy. Different models have 10 2

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been proposed to explain pitting behavior. Localized adsorption of aggressive anions on the surface of passivated metals is considered to be the first step in the pitting process. The facilitated release of cations and the migration of anions to microdepressions existing at defective sites, such as non-metallic inclusions, pores or flaws in the oxide film, induce the formation of an aggressive environment resulting from hydrolysis. The polishing lines left by 600# abrasive paper on the surface of the amorphous alloy studied are 1 or 2/~m in width and about the same in depth. In these grooves the accumulation of aggressive anions and the acidification of the electrolyte are more easy than elsewhere, inducing a high dissolution rate at the bottom of the grooves. However, given the same surface pretreatment, amorphous Ni36Fe32CrI4P12B6 alloy still showed excellent pitting resistance. Crevice corrosion tests of glassy Ni36Fe32CrI4P12B6 alloy were performed by several authors, 4's'17 for the purpose of differentiating between initiation-related and propagation-related corrosion behaviors. Artificial crevice cell experiments demonstrated that the susceptibility of this alloy to crevice corrosion was slight. The alloy exhibited an apparent strong tendency to remain covered with a passive film. Although localized anodic dissolution and acidification of the anolyte could be made to occur, quite noble potentials were required. The present Ni63Cr12.sFe4SisB12.5 alloy is susceptible to crevice corrosion as well as pitting corrosion. This result is consistent with the previous studies 13'14'17'1s that the resistance of the chromium- and phosphoruscontaining glassy alloys to pitting and crevice corrosion results mainly from the ability of spontaneous passivation, and the amorphous structure of the materials is not the determining factor. Therefore, the pitting susceptibility of Ni63CrI2.sFe4SisB12.5alloy can be attributed to its poor repassivation characteristics. At the bottom of the polishing grooves, the dissolution rate of the metals is greater than the repassivation rate. Once a pit is formed, it proceeds at high speed, leading to rapid penetration of the ribbon. The great difference in pitting resistance of amorphous Ni63Cr12.sFe4SisB12.5 alloy and Ni36Fe32Cr14Pi2B6 alloy may be explained by the difference in the protective quality of their passive films. The

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relationship among the composition, structure and dissolution and repassivation b e h a v i o r s of p a s s i v e films on a m o r p h o u s alloys will b e the s u b j e c t o f f u t u r e w o r k . CONCLUSIONS (1) In N a C I s o l u t i o n s , a m o r p h o u s Ni63CrI2.sFe4SisB1~.5 alloy shows pitting susceptibility. T h e p i t t i n g b r e a k d o w n p o t e n t i a l d e c r e a s e s as c h l o r i d e c o n c e n t r a t i o n a n d acidity o f t h e s o l u t i o n increase. (2) In n e u t r a l 0.5 M N a C I s o l u t i o n , t h e p i t t i n g p o t e n t i a l is l o c a t e d b e t w e e n 200 a n d 250 m V ( S C E ) . T h e r e is an i n c u b a t i o n t i m e for pitting after which t h e c u r r e n t d e n s i t y i n c r e a s e s v e r y q u i c k l y , i n d i c a t i n g t h e quick d e v e l o p m e n t o f the pits. (3) T h e a d d i t i o n o f N a 2 S O 4 to N a C i s o l u t i o n inhibits p i t t i n g c o r r o s i o n o f the a m o r p h o u s alloy. (4) T h e surface p r e t r e a t m e n t o f t h e s a m p l e has a significant influence on pitting b e h a v i o r . H e m i s p e r i c a l pits a r e f o u n d which i n i t i a t e d p r e f e r e n t i a l l y a l o n g t h e p o l i s h i n g lines on the surface. E l e c t r o p o l i s h i n g i n c r e a s e s t h e b r e a k d o w n p o t e n t i a l a n d d e c r e a s e s the pit d e n s i t y greatly. (5) C o m p a r e d with Ni36Fe32Cr14P12B 6 alloy, t h e p o o r pitting r e s i s t a n c e o f the a m o r p h o u s Ni63CrlE.sFeaSi8B12.5 alloy m a y b e a t t r i b u t e d to its p o o r r e p a s s i v a t i o n characteristics. Acknowledgement--This work is supported by the National Science Foundation under contract DMR 88-06460. We are grateful to Dr H. H. Liebermann and Dr W. Gao for the supply of materials.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. i1. 12. 13. 14. 15. 16. 17. 18.

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