Effect of metalloidal elements on corrosion resistance of amorphous iron-chromium alloys

Journal of Non.Crystalline Solids 28 (1978) 403-413 © North-Holland Publishing Company

EFFECT OF METALLOIDAL ELEMENTS ON CORROSION RESISTANCE OF AMORPHOUS I R O N - C H R O M I U M ALLOYS M. NAKA, K. HASHIMOTO and T. MASUMOTO The Research lnstitute for Iron, Steel and Other Metals, Tohoku University, Sendal, Japan

Received 11 November 1977

Corrosion and electrochemical behaviours of amorphous iron-chromium alloys containing different sets of glass-forming metalloidal elements, boron, carbon, silicon and phosphorus, have been investigated. The corrosion resistance in 0.1N H2SO4 increases in the order of alloys containing silicon, boron, carbon and phosphorus, and the corrosion resistance in 3% NaC1 increases in the order of alloys containing boron, silicon or carbon, and phosphorus. Difference in the corrosion resistance of amorphous alloys containing various metalloids has been interpreted in terms of the difference in the formation rate of the chromium-enriched protective film on the surface of alloys.

1. Introduction The present authors [ 1 - 7 ] have reported that amorphous F e - C r - P - C and F e - C r - N i - P - C alloys containing chromium o f 8 at% or more possess extremely high corrosion resistance in acidic and neutral solutions containing chloride and are immune to pitting and crevice corrosion. This superior corrosion-resistance character is attributed mainly to rapid formation of homogeneous protective passive film in which the concentration o f chromium is remarkably high. Metalloids in large amounts which accelerate the active dissolution of alloys, cause the rapid formation of passive f'dm and the enrichment o f chromium in the film. In contrast to thse alloy systems, it was found from our recent study [8] that the additidn of chromium to amorphous F e - B - C and F e - B - S i alloys is rarely effective in improving the corrosion resistance in various solutions. This implies that each individual metalloid has a different effect on the corrosion resistance o f amorphous alloys. The purpose of the present work is to investigate the effect of glass-forming metalloids on the corrosion resistance of amorphous i r o n - c h r o m i u m alloys.

2. Experimental The rotating-cylinder method [9] has been applied to the preparation of amorphous i r o n - c h r o m i u m alloy ribbon o f about 2 mm in width and 30/am in thick403

404

M. Naka et al. / Corrosion resistance o f amorphous Fe-Cr alloys

ness. This technique consists of impinging a stream of molten alloy on the outer surface of a rotating cylinder. The formation of amorphous structure has been confirmed by X-ray diffraction. The amorphous alloys prepared are F e - C r - 1 3 P - 7 X , where X is B, C or Si, and F e - C r - 1 3 B - 7 X , where X is B, C, Si or P. In order to unify the representation of alloy composition, F e - C r - 2 0 B alloys are expressed as F e - C r - 1 3 B - 7 B alloys. The number attached to respective elements denotes the nominal content in atomic percent. Alloys used in the present work contain chromium of 5 or 10 at%. Corrosion tests and electrochemical measurements were carried out in 0.1N H2SO 4 and 3% NaC1, prepared by using reagent-grade chemicals and deionized water. The corrosion rate was estimated from the loss in weight after immersion in the solutions at 30 -+ I°C. The polarization curves were measured in these solutions at room temperature by a potentiodynamic method with a potential sweep rate of 2.37 X 10 - 3 V s -1 from the cathodic region.

3. Results Figures la and lb show the corrosion rate in 0.1N H2SO4 for F e - 5 C r and F e - 1 0 C r amorphous alloys containing 13 at% boron as a major metalloid and 7 at%

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Fig. 1. Effects of additive metalloid elements X on corrosion rates of amorphous Fe-Cr-20B and Fe-Cr-13B-7X alloys in 0.1N H2SO4 at 30 -+ I°C. Chromium content: (a) 5 at%, (b) 10 at%.

M. Naka et al. / Corrosion resistance o f amorphous Fe-Cr alloys

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Fig. 2. Effects of additive metalloid elements X on corrosion rates o f a m o r p h o u s F e - C r - 1 3 P 7X alloys in 0.1N H2SO 4 at 30 -* I°C. C h r o m i u m content: (a) 5 at%, (b) 10 at%. Fig. 3. Effects of additive metalloid elements X on corrosion rates of a m o r p h o u s F e - C r - 2 0 B and F e - C r - 1 3 B - 7 X alloys in 3% NaCI at 30 -+ I°C. C h r o m i u m content: (a) 5 at%, (b) 10 at%.

X as a minor metalloid. Because of low corrosion resistance of the alloys with chromium of 5 at% the difference in their corrosion rates is not obvious (see fig. la). On the other hand, in alloys with 10 at% chromium, the effect of minor metalloids is more distinguishable (see fig. 1b). The corrosion resistance increases in the order of alloys containing silicon, boron, carbon and phosphorus. The order of the corrosion resistance of alloys in 0.1N H2SO4 remains unaltered when a major metalloid is changed from boron to phosphorus (see figs. 2a and 2b). However, from the comparison between fig. lb and fig. 2b, the corrosion rate of amorphous Fe-10Cr alloys is decreased by two or three orders of magnitude by the addition of phosphorus instead of boron as a major metalloid. In particular, the amorphous F e - C r - 1 3 P - 7 C alloys are extremely stable in 0.1N H2SO4. Figures 3a and 3b shows the effect of the minor metalloid additives on the corrosion rate of the Fe-5Cr and Fe-10Cr amorphous alloys containing 13 at% boron as a major metalloid in 3% NaC1. The amorphous F e - C r - 2 0 B alloy shows the highest corrosion rate and the replacement of 7 at% boron with phosphorous effectively improves corrosion resistance. Effects of carbon and silicon on the resistance are intermediate between those of boron and phosphorus. A similar effect of minor metalloids appears on the corrosion resiatance in 3%

406

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Fig. 4. Effects of additive metalloid elements, X on corrosion rates of amorphous F e - C r 13P-7X alloys in 3% NaCI at 30 * I°C. Chromium content: (a) 5 at%, (b) 10 at%.

NaC1 for amorphous F e - C r alloys with phosphorus as a major metalloid (see fig. 4). In particular, the F e - 1 0 C r alloy containing phosphorus and carbon as metalloids shows no weight loss in 3% NaC1.

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Fig. 5. Polarization curves of amorphous Fe-SCr-20B and Fe-5Cr-13B-7X (X is C, Si or P) alloys in 0.IN H2SO4.

M. Naka et al. / Corrosion resistance o f amorphous Fe-Cr alloys

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Fig. 6. Polarization curves of amorphous F e - 1 0 C r - 2 0 B and F e - 1 0 C r - 1 3 B - 7 X (X is C, Si or P) aUoys in 0.1N H2SO 4.

For further examination of corrosion behaviours of amorphous alloys, their potentiodynamic polarization curves have been measured in 0.1N H2SO4 and 3% NaC1. Fig. 5 shows polarization curves in 0.1N H2SO4 for the amorphous Fe-5Cr alloys containing 13 at% boron as a major metalloid. Because of the thinning of the specimen by a rapid dissolution, the anodic polarization curves of the amorphous Fe-5Cr-20B and Fe-5Cr-13B-7C alloys have not been completely measured. All amorphous alloys containing 10 at% chromium are anodically passivated in O.1N H2SO4 (see fig. 6). In general, the lower the peak current density in the active state and the higher the corrosion potential, the lower is the corrosion rate. In particular, the alloy containing 7 at% phosphorus as a minor metalloid exhibits the highest corrosion potential and the lowest peak current density in the active state. The amorphous Fe-5Cr alloys containing 13 at% phosphorus as a major metalloid are anodicaUy passicated in O.1N H2SO 4 (see fig. 7). An increase in the chromium content lowers the anodic current density of the alloys containing 13 at% phosphorus (see fig. 8). The alloy containing phosphorus and carbon shows the highest corrosion potential and the lowest anodic current density. Figure 9 shows polarization curves in 3% NaC1 for amorphous Fe-5Cr alloys with 13 at% boron as a major metalloid. Because of rapid dissolution in the active state, the anodic polarization curves of alloys other than those with phosphorus as a minor metalloid have not been measured. The corrosion potential increases in the order of alloys containing boron, carbon, silicon and phosphorus, and the corrosion rate of the alloys decreases in the same order. The amorphous Fe-lOCr alloys containing 13 at% boron as a major metalloid

408

M. Naka et al. / Corrosion resistance o f amorphous Fe-Cr alloys

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05 10 V (SCE)

1.5

2.0

Fig. 7. Polarization curves of amorphous F e - 5 C r - 1 3 P - 7 X (X is B, C or Si) alloys in 0.1N H2SO4.

have been passivated anodicaUy, except for the Fe-10Cr-20B alloy (see fig. 10). In particular, the amorphous Fe-10Cr-13B-7P- alloy shows spontaneous passivation. As shown in fig. 11, anodic polarization curves of the amorphous Fe-5Cr-

Potential

V (SCE)

Fig. 8. Polarization curves of amorphous F e - 1 0 C r - 1 3 P - 7 X (X is B, C or Si) alloys in 0.IN H2SO 4.

M. Naka et aL / Corrosion resistance o f amorphous Fe-Cr alloys

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Fig. 9. Polarization curves of amorphous Fe-5Cr-20B and Fe-5Cr-13B-7X (X is C, Si or P) alloys in 3% NaC1.

1 3 P - 7 X alloys containing phosphorus as a major metalloid have been measured through almost the entire passive region in 3% NaC1 solution and do not exhibit an abrupt rise in the anodic current density because of pitting corrosion. The amor-

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Fig. 10. Polarization curves of amorphous Fe-10Cr-20B and Fe-10Cr-13B-7X (X is C, Si or P) alloys in 3% NaC1.

410

hi, Naka et al. / Corrosion resistance o f amorphous Fe-Cr alloys

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Fig, 11. Polarization curves o f amorphous F e - 5 C r - 1 3 P - 7 X NaC1.

(X is B, C or Si) alloys in 3%

phous Fe-5Cr-13P-7C alloy shows the lowest peak current density and the highest corrosion potential. All amorphous Fe-10Cr alloys with 13 at% phosphorus are spontaneously passivated (see fig. 12).

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(X is B, C or Si) alloys in 3%

M. Naka et aL/ Corrosion resistance of amorphous Fe-Cr alloys

411

4. Discussion

The corrosion resistance of the amorphous Fe-Cr alloys increases in the order of alloys with silicon, boron, carbon and phosphorus in 0.1N H2SO4 and in the order of alloys containing boron, silicon or carbon, and phosphorus in 3% NaCl.When a major metalloid element comprising 13 at% is changed from boron to phosphorus, the corrosion rate of the amorphous Fe-Cr alloys decreases by two or three orders of magnitude. Accordingly, phosphorus is the most effective metalloid additive in improving the corrosion resistance of amorphous Fe-Cr alloys. Taking account of the easy formation of amorphous structure and the effect on the corrosion resistance, the addition of phosphorus and carbon to Fe-Cr alloys is recommended. Amorphous alloys with low corrosion rates generally exhibit high corrosion potentials and low anodic current densities. The raising of the corrosion potential takes place by the acceleration of cathodic reaction when the rate of anodic reactions remains unchanged. In fact, the acceleration of cathodic reaction facilitates spontaneous passivation, as may be seen in some of the amorphous alloys containing phosphorus. However, the corrosion potentials of many amorphous alloys with low corrosion rates and high cathodic current densities are in the active state and are higher compared with the corrosion potentials of the alloys.with high corrosion rates. Accordingly, the raising of the corrosion potential of the alloys with low corrosion rates results from the suppression of anodic reaction. This is one of the general characteristics of the alloys containing phosphorus. As the authors have reported previously [1-4], the amorphous F e - C r - 1 3 P - 7 C alloys containing more than 8 at% chromium has shown spontaneous passivation and no weight loss by immersion tests in several acidic and neutral solutions with and without chloride ions. Such a high corrosion resistance has been interpreted in terms of the rapid formation of a highly protective, uniform passive film in which chromium is remarkably enriched [5,6]. The high uniformity of the passive t'flm is ensured by the amorphous structure, that is, the chemically homogeneous single phase without localized defects such as grain boundaries, dislocations, precipitates, segregates, etc on which a stable passive film is unable to form and on which corrosion initiates [6]. The high homogeneity of the alloys is independent on the composition of amorphous alloys, and hence the homogeneity is not affected by the change in the metalloid additive. On the other hand, the formation rate of chromium-enriched passive film changes with metalloids. A large amount of phosphorus, which enhances active dissolution of alloys due to high chemical reactivity, gives rise to the rapid enrichment of chromium at the alloy-solution interface. When the corrosion potential is in the active state, the passive ffdmis not formed, but the enrichment of chromium oxyhydroide in the deposit film in the active state determines the corrosion resistance of the amorphous iron base alloys containing chromium [10]. The rate of formation of the chromium-enriched deposit film in the active state is dependent upon the rate of active dissolution. Conse-

412

M. Naka et al. / Corrosion resistance o f amorphous Fe-Cr alloys

quently, the difference in the effects o f additive metalloids on the corrosion resistance may be based on whether individual elements accelerate the active dissolution. For the amorphous F e - C r alloys without a large amount of phosphorus, chromium may not be highly enriched in the film. This will be reported subsequently

[13]. One of the superior characteristics o f amorphous alloys is the fact that an abrupt rise in the anodic current density owing to pitting corrosion has not been observed in acidic and neutral solutions containing chloride [1-3,10,11]. The critical content of chromium in the amorphous alloys to avoid pitting corrosion is considerably lower compared with crystalline stainless steels [12]. This suggests that the localized change in the quality of passive fdm by aggressive chloride ions may not be necessary or sufficient to cause pitting, and that pitting of crystalline alloys takes place on crystal defects such as grain boundaries, dislocations, precipitates, segregates etc. on which a stable passive film is hardly formed. Therefore, the amorphous alloys without such crystal defects are immune to pitting corrosion so long as the alloys possess sufficient corrosion resistance to form a stable passive fdm.

5. Conclusion Effects of metalloid additives on the corrosion resistance of amorphous F e - C r alloys have been investigated by immersion tests and electrochemical measurements. The corrosion resistance increased in the order of alloys containing silicon, boron, carbon and phosphorus in 0.1N H2SO4, and in the order of alloys with boron, silicon or carbon, and phosphorus in 3% NaC1. A combination of phosphorus and carbon as additive metalloids to amorphous F e - C r alloys has been recommended from the viewpoint o f their effect on the corrosion resistance as well as the easy formation of the amorphous structure. The difference in the influence of metalloid additives on the corrosion resistance of F e - C r alloys has been interpreted in terms o f the difference in the formation rate of protective surface fdm.

References [1 ] M. Naka, K. Hashimoto and T. Masumoto, J. Japan Inst. Metals 38 (1974) 835. [2] M. Naka, K. Hashimoto and T. Masumoto, Corrosion 32 (1976) 146. [3] K. Hashimoto and T. Masumoto, Mater. Sci. Eng. 23 (1976) 285. [4] K. Hashimoto, T. Masumoto and S. Shimodaira, Proc. Japan-U.S.A. Seminar on Passivity and Its Breakdown on Iron and Iron Base Alloys, ed. R.W. Staehle and H. Okada (National Association of Corrosion Engineers, Houston, Texas, 1976) p. 34. [5] K. Asami, K. Hashimoto, T. Masumoto and S. Simondaira, Corros. Sci. 16 (1976) 909. [6] K. Hashimoto, K. Osada, T. Masumoto and S. Shimodaira, Corros. Sci. 16 (1976) 71. [7] K. Hashimoto, M. Naka and T. Masumoto, Sci. Rep. Res. Inst. Tohoku Univ. A-26 (1976) 48.

M. Naka et aL / Corrosion resistance of amorphous Fe-Cr alloys

413

[8] M. Naka, K. Hashimoto and T. Masumoto, Sci. Rep. Res. Inst. Tohoku Univ. A-26 (1977) 283. [9] Y. Kobayashi, T. Masumoto et al., unpublished. [10] K. Hashimoto, J. Noguchi, M. Naka, K. Asami and T. Masumoto, "Passivity of Metals" Proc. 4th Int. Symp. on Passivity (1977). Electrochem. Soc., in press. [11] K. Hashimoto, M. Kasaya, K. Asami and T. Masumoto, Corros. Eng. 26 (1977) 445. [12] K. Sugimoto and Y. Sawada, Corros. Sci. 17 (1977) 425. [13] K. Hashimoto, M. Naka, K. Asami and T. Masumoto, Corrosion Eng. 27 (1978) 1106.