Localized corrosion of Al90Fe5Gd5 and Al87Ni8.7Y4.3 alloys in the amorphous, nanocrystalline and crystalline states: resistance to micrometer-scale pit formation

The role of isothermal aging on the localized corrosion behavior of Al₉₀Fe₅Gd₅ and Al₈₇Ni_8.7Y_4.3 alloys was characterized in 0.6 M NaCl solution. The pitting (E_pit) and repassivation (E_rp) potentials were both increased ∼400 mV by the presence of transition and rare earth metal additions in supersaturated solid solution and amorphous structure. A statistical distribution in E_pit observed on small electrodes was due, in part, to the sensitivity of this critical potential to the presence of a population of critical surface flaws that serve as pit initiation sites. Mechanistic insight on the spacing of critical flaws was enabled by varying the tested electrode surface area. E_rp was not dependent on electrode surface area due to the similarity of pit depths in all electrode sizes. The critical potentials were also characterized after heat treating the amorphous ribbons isothermally at 150 °C for 25 h and 550 °C for 1 h. The former produced Al-rich nanocrystals embedded in the remaining amorphous matrix while the latter produced a fully crystalline condition containing intermetallic phases. Notably, the improved resistance to the formation of micrometer-scale pits was not lost compared with the fully amorphous condition when small Al-rich nanocrystals were present in an amorphous matrix. However, improvement in pitting corrosion resistance was completely lost in the fully crystallized condition as indicated by values for E_pit and E_rp that were similar to those of high purity, polycrystalline aluminum.