The aqueous corrosion characteristics of nickel aluminides

The aqueous corrosion characteristics of three nickel aluminides were studied by using (a) immersion corrosion tests, (b) cyclic potentiodynamic measurements and (c) electron spectroscopy for chemical analysis (ESCA) techniques.

The immersion mass-loss tests were conducted in 15 different solutions at 25°C and 95°C. The nickel aluminides were found to have good corrosion resistance in inorganic acids, organic acids and basic solutions except at a higher concentration of hydrochloric acid and in ferric chloride solution at 25°C. All three nickel aluminides were suitably resistance to corrosion in the organic acids (oxalic acid, acetic acid), sodium chloride solution, and bases (sodium hydroxide, ammonium hydroxide) at 95°C. The cyclic potentiodynamic polarization curves showed that the nickel aluminides exhibited active-passive behavior in 0.6M sodium chloride (pH=7), IN nitric acid and IN sodium hydroxide solutions at 25°C and 95°C. Hysteresis loops indicating susceptibility to localized corrosion were observed in 0.6M sodium chloride (pH=7) solution.

From the above results, it was noted that anodic dissolution of nickel aluminides significantly increased with increasing temperature and that the Cr-containing compositions had better corrosion resistance in several solutions than the Cr-free composition. Prior manufacturing procedures did not appear to influence the corrosion behavior of the nickel aluminides.

Finally, the compositions of passive films formed on the surface after 200-hour immersion in 0.6M sodium chloride solution were analyzed by the surface-sensitive ESCA technique. Under these conditions, the passive films of the nickel aluminides were found to be enriched with Al, Cr and Zr, at the expense of Ni.