2021 Volume 62 Issue 11 Pages 1677-1680
The equiatomic CoCrFeMnNi alloy and the equiatomic quaternary alloys with no addition of one element of the CoCrFeMnNi alloy were fabricated, and the roles of the alloying elements in corrosion resistance were clarified. In 1 M H2SO4, the passivity of the CoCrFeMnNi alloy was mainly due to Cr. The Co and Fe additions also contributed the decrease in the passivity current density. While Ni addition was found to suppress active dissolution, Mn addition increased the active dissolution rate. The quaternary alloy without Mn indicated superior pitting corrosion resistance in 0.1 M NaCl. To improve the corrosion resistance of the CoCrFeMnNi alloy, Mn was replaced with Mo, and the effect of the Mo content on the pitting corrosion resistance and the role of Co addition were assessed in 1 M HCl.
Fig. 5 Potentiodynamic polarization curves of Fe–20Cr–20Ni–20Co–xMo (x = 6, 10, 20) and Fe–20Cr–20Ni–6Mo in deaerated 1 M HCl at 353 K.
The concept behind high-entropy alloys is recognized as a new strategy for alloy design. The alloying of five or more dissimilar elements increases the mixing entropy, resulting in the formation of a solid solution single-phase (or multi-phase) alloy, allowing the roles of alloying elements to be easily clarified.1,2) Due to the difficulty of forming a solid solution over a wide range in binary and ternary systems, conventional explorations based on the knowledge of conventional binary and ternary alloy systems have been limited. High-entropy alloys are considered a break through to these limitations.
The equiatomic Co20Cr20Fe20Mn20Ni20 high-entropy alloy, the so-called “Cantor alloy”, was first reported in 2004.2) In austenitic stainless steels, Fe, Cr, and Ni are major constituents, and Mn is usually added in the industrial steelmaking process. In biomedical applications, Co-base alloys are widely used. The clarifying of the roles of the alloying elements of Cantor alloy is expected to contribute to the design of corrosion-resistant alloys. Many studies have been conducted to compare the corrosion resistance of Cantor alloys with conventional corrosion resistant alloys.3–5) Luo et al. compared the corrosion behavior of Cantor alloy and Fe–18Cr–8Ni.5) They demonstrated that the selective dissolution of elements in the Cantor alloy is not evident during passivation in 0.1 M H2SO4, whereas it was evident in the case of Fe–18Cr–8Ni, and that the passive film on Cantor alloy is enriched in Fe and Mn but depleted in Cr. It was concluded that the lower pitting corrosion resistance of Cantor alloy could be attributed to the lower content of Cr in the passive film.5) However, few systematic studies have been conducted on the roles of the other alloying elements in the corrosion resistance of Cantor alloys. Studies on the role of Co are notably missing from the literature. In this study, the corrosion resistance of Cantor alloy and quaternary alloys with no addition of one element of Cantor alloy was compared, and then Mn was replaced with Mo to improve the corrosion resistance.
The equiatomic Co20Cr20Fe20Mn20Ni20 alloy and equiatomic quaternary alloys with no addition of one element of the Co20Cr20Fe20Mn20Ni20 alloy were fabricated by arc-melting. In addition to these, alloys with x (mass%) of Fe–20Cr–20Co–20Ni–xMo set to 6, 10, and 20 were prepared. The Fe–20Cr–20Ni–6Mo and Fe–18Cr–8Ni alloys were fabricated by arc-melting. Pure Co, Cr, Fe, Mn, and Ni were used as starting materials (each with a purity of at least 99.9%). The ingot (ca. 30 g) was re-melted at least three times in a Ti-gettered Ar atmosphere. Hot-rolling was performed at 1523 K, and heat-treatment was performed at 1473 K for 2 h, followed by water-quenching. In the case of Fe–18Cr–8Ni, heat-treatment was performed at 1273 K for 20 h to reduce delta-ferrite. Using a scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy system, it was confirmed that the chemical compositions of the prepared specimens were as intended. For electrochemical measurements, the specimen surface was ground and finally polished with a diamond paste of 1 µm.
2.2 Polarization measurementsPotentiodynamic polarization curves were measured in deaerated 1 M H2SO4 (298 K), deaerated 1 M HCl (353 K), and naturally aerated 0.1 M NaCl (298 K). The size of the working electrode area was 1.0 cm × 1.0 cm, and a conventional three electrode method was used. The scan rate of the electrode potential was set at 23 mV min−1. In this paper, all potentials refer to the Ag/AgCl (3.33 M KCl) electrode.
2.3 Surface analysisAuger electron spectroscopy (AES) was performed with an accelerating voltage of 5 kV and a beam current of 5 nA. The specimen surface was etched by an Ar+ beam with a tilt of 30 degrees and an accelerating voltage of 2 kV. The sputtering rate was estimated to be 1.13 nm min−1 based on a thermal-oxidized SiO2 film on Si.
The potentiodynamic anodic polarization of equiatomic Co20Cr20Fe20Mn20Ni20 alloy (hereafter referred to as CoCrFeMnNi) and Fe–18Cr–8Ni in deaerated 1 M H2SO4 at 298 K are shown in Fig. 1. For reference, Fe–18Cr–8Ni can be expressed as Fe73Cr19Ni8. In the case of both alloys, active dissolution occurred at around −0.3 V, and the maximum current densities were almost the same. Passive regions are seen from approx. 0 to 1 V. The passive current density of CoCrFeMnNi alloy was slightly lower than that of Fe–18Cr–8Ni. Above 1 V, large increases in current density were measured for both alloys, and differences in polarization behavior were observed. These differences appear to be due to the presence or absence of Mn (the details will be presented later in Fig. 4). It has been reported that Mn is likely to prevent transpassive dissolution and result in a decrease in the current density at high potentials.4) However, below 1 V, no significant difference was found in the polarization behavior of the two alloys despite the large difference in the alloy compositions.
Comparison of the potentiodynamic polarization curves of equiatomic CoCrFeMnNi alloy and Fe–18Cr–8Ni in deaerated 1 M H2SO4 at 298 K.
Figure 2 shows the Auger electron spectroscopy (AES) depth profiles on the as-polished specimens and the specimens polarized up to 0.5 V in 1 M H2SO4, respectively. In the case of the as-polished specimens, the specimens were stored after polishing for 24 h in air and then set in an AES vacuum chamber. In the case of the polarized specimens, potentiodynamic polarization was done up to 0.5 V using the same procedure as that for collecting the data shown in Fig. 1, and then the specimens were promptly set in an AES vacuum chamber. According to the figure, the composition of the alloy matrix in the CoCrFeMnNi alloy is different from that of the equiatomic alloy. This is likely to be the low accuracy of quantification due to the closeness of the AES peak positions of Cr, Fe, Mn, Co, and Ni. However, it was thought that the degree of enrichment of each element in the surface film and the film thickness could be evaluated. The thickness of the oxide film was determined by the depth at half maximum position of the oxygen profile. The cation fractions in the oxide films on the as-polished and the polarized specimens are shown in Fig. 3 for comparison. Unlike the polarization behavior, a significant difference can be seen in the compositions of the oxide films. In the oxide film both on the as-polished and the polarized Fe–18Cr–8Ni, the large amount of Fe is the main component of cations, while it is not possible to determine the main component of the cations in the case of CoCrFeMnNi alloy either before or after polarization. Despite the significant differences in the oxide films, the results clearly indicate no large difference in polarization behavior. In addition to this, no selective dissolution of CoCrFeMnNi alloy was confirmed despite alloying with elements with different corrosion resistance. This is consistent with the findings by Luo et al.5)
AES depth profiles of (a), (b) equiatomic CoCrFeMnNi alloy and (c), (d) Fe–18Cr–8Ni: (a), (c) the as-polished specimens and (b), (d) the specimens after potentiodynamic polarized up to 0.5 V in deaerated 1 M H2SO4 at 298 K.
Comparisons of the cation fractions in the oxide films on the as-polished and the polarized specimens of equiatomic CoCrFeMnNi alloy and Fe–18Cr–8Ni shown in Fig. 2.
Figure 4 shows the potentiodynamic polarization of CoCrFeMnNi alloy and the equiatomic quaternary alloys with no addition of one element of CoCrFeMnNi alloy in 1 M H2SO4 and 0.1 M NaCl. As presented in Fig. 4(a), the passive region from 0 to 1 V disappeared on the alloy without Cr, suggesting that the passivity of the CoCrFeMnNi alloy is mainly due to the existence of Cr. It can also be seen that the additions of Co and Fe contribute to passivity since the passive current density of the alloy without Co and that without Fe is higher than that of the CoCrFeMnNi alloy. The large amount of Co in the oxide film on the CoCrFeMnNi alloy shown in Figs. 2 and 3 suggests that Co is likely to contribute the decrease in the passive current density. Also, the results suggest that Fe is required in the oxide film for the improved corrosion resistance of the CoCrFeMnNi alloy. No effect due to Cr, Co, and Fe was seen on the dissolution rate of CoCrFeMnNi alloy in the active dissolution region around −0.3 V. However, the results in Fig. 4(b) indicate that the Ni in the CoCrFeMnNi alloy results in a reduction in active dissolution: the alloy without Ni exhibits an extremely large current density around −0.3 V. In the case of the alloy without Mn addition, the active dissolution rate was lower. Based on the above results, it can be concluded that no addition of Mn improves the corrosion resistance of the CoCrFeMnNi alloy.6,7)
Potentiodynamic polarization curves of the equiatomic CoCrFeMnNi alloy and quaternary alloys with no addition of one element of the CoCrFeMnNi alloy: (a), (b) deaerated 1 M H2SO4 at 298 K and (c) naturally aerated 0.1 M NaCl at 298 K.
In 0.1 M NaCl, the equiatomic quaternary alloy without Mn indicates superior pitting corrosion resistance, as shown in Fig. 4(c). This is consistent with the finding that the Mn addition decreases the protective ability of the passive films on stainless steels.6) In the cases of the alloys without Ni, Co, Fe, or Cr, the pitting potentials were lower than those of the CoCrFeMnNi alloy. Because these elements contributed to a decrease in the current density in either the passive region or the active region, as shown in Fig. 4(a) and 4(b), it is presumed that metastable pits readily grow on the alloys without Cr, Co, Fe, or Ni.
3.3 Pitting corrosion resistance of Fe–Cr–Ni–Co–Mo alloysThe replacement of Mn with Mo has been reported to have the benefit of improving the corrosion resistance of the CoCrFeMnNi alloy.8–10) To ascertain the effect of the Mo content on the pitting corrosion resistance of Fe–Cr–Ni–Co–Mo alloys, the alloys with x (mass%) of Fe–20Cr–20Co–20Ni–xMo set to 6, 10, and 20 were fabricated. Figure 5 shows the effect of Mo content on the potentiodynamic polarization behavior for these alloys in 1 M HCl at 353 K. The polarization curve of Fe–20Cr–20Ni–6Mo is presented to consider the effect of Co on pitting corrosion resistance. The experiments were conducted twice for each of the Fe–Cr–Ni–Co–Mo alloys to ensure reproducibility. In the cases of Fe–20Cr–20Ni–20Co–20Mo and Fe–20Cr–20Ni–20Co–10Mo, no pitting was observed on the electrode areas after polarization; therefore, the large increases in current density around 0.9 V were determined to be due to transpassive dissolution and/or the electrolysis of the electrolyte. In the case of Fe–20Cr–20Ni–20Co–6Mo, because the generation of pits was observed on the working electrode area after polarization, pitting also contributes to the increase in current density around 0.9 V. Notably, in the case of the Co-free alloy of Fe–20Cr–20Ni–6Mo, the pitting potential decreased to approx. 0.55 V. It was found that Co contributes to the increase in the pitting corrosion resistance of Fe–20Cr–20Ni–20Co–6Mo. The addition of Co to Fe–Cr–Ni–Mo alloys is thought to be effective to improve the pitting corrosion resistance.
Potentiodynamic polarization curves of Fe–20Cr–20Ni–20Co–xMo (x = 6, 10, 20) and Fe–20Cr–20Ni–6Mo in deaerated 1 M HCl at 353 K.
Figure 6 shows the AES depth profiles of the as-polished specimen of Fe–20Cr–20Ni–20Co–6Mo and Fe–20Cr–20Ni–6Mo. The Cr content in the oxide films is almost the same in these alloys. However, the existence of Co is observed in the oxide film on Fe–20Cr–20Ni–20Co–6Mo. Based on the AES analysis, it is likely that Co increases the protective ability of the alloy surface. The chemical composition of Fe–20Cr–20Ni–6Mo is equivalent to Type 312L stainless steel (Fe–20Cr–18Ni–6Mo–Cu–N) of the JIS G 4304 standard. This implies that Co can be used for improving the corrosion resistance of commercial stainless steels in severe environments such high temperature HCl. When considering the corrosion resistance of ternary Fe–Cr–Ni or quaternary Fe–Cr–Ni–Mo systems, the contribution of Cr or Mo is significant. However, further research on the five-element system, or indeed systems with more elements, may well reveal a new element besides Cr or Mo, such as Co in this study, that contributes to corrosion resistance.
AES depth profiles of as-polished specimen surfaces: (a) Fe–20Cr–20Ni–20Co–6Mo and (b) Fe–20Cr–20Ni–6Mo.
This work was supported by a Grant-in-Aid for Scientific Research on the innovation area “Science of New-Class of Materials Based on Elemental Multiplicity and Heterogeneity (Grant Number: JP 18H05452)” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT, Japan), and by JSPS KAKENHI Grant Number JP 17H01331.
