STRUCTURE-DEPENDENCE OF INTERGRANULAR CORROSION IN HIGH-PURITY NICKEL

Electrochemical studies were conducted in 2 N H2SO4 at 303 K in order to assess the applicability of the CSL/DSC model of interface structure to intergranular corrosion susceptibility at grain boundaries in high purity (99.999%) polycrystalline nickel. Susceptibility to the initiation of localized corrosion at grain boundaries was manifested through characteristic overpotentials for passive film breakdown. These characteristic overpotentials were found to (1) decrease with increasing bulk sulphur concentration (0.3-50 ppm), and (2) be strongly dependent on interface structure (CSL/DSC). Boundaries close (Δθ) to low ΣCSL relationships were observed to be most resistant to the initiation of localized corrosion. A limiting structural field was determined, not extending beyond Σ25, and restricted to an angular deviation limit defined by a relation of the type: Δθ = 15° Σ- 5 6. Results were determined to be consistent with a mechanism whereby susceptibility to intergranular corrosion is dictated by the (1) geometry, and (2) chemistry (i.e. solute concentration) of intrinsic grain boundary dislocations. © 1990.