Photoinhibition of Pitting on Nickel and Alloy CDA 715 in Buffered Chloride-Containing Solutions

Based upon extensive experimental work and theoretical prediction, it has become apparent that localized corrosion on certain metal surfaces in aqueous media can be inhibited by illumination with electromagnetic radiation. In the present work, irradiation of nickel in a chloride (Cl–)-containing borate buffer solution of pH 9.2 with white light (power density of 180 W/m2, 200-nm to 900-nm wavelength) yielded a pitting potential 34 mV ± 8 mV more positive than in the dark. A similar anodic shift in pitting potential (by 47 mV ± 9 mV) was found for alloy CDA 715 (70% Cu-30% Ni, UNS C71500) in the same medium and under the same conditions of illumination. The number of pits initiated under illumination for the same observation time was reduced by a factor of 3 ± 1 for both materials. The effect of light on passivity breakdown was explained in terms of the point defect model (PDM) for the growth and breakdown of anodic films. Current observations indicated illumination of the passive film on nickel with white light of a power density of 180 W/m2 suppressed the electric field by a factor of 2 to 4. The PDM predicted that, if the electric field could be suppressed by a factor > 85, practically useful photoinhibition of the nucleation of localized corrosion on nickel and alloy CDA 715 might be realized.