Molybdate inhibition of environmental fatigue crack propagation in Al–Zn–Mg–Cu

Abstract

MoO 4 2 - effectively inhibits environmentally assisted fatigue crack propagation in 7075-T651 stressed during full immersion in low-chloride solution, as understood by hydrogen environment embrittlement and film rupture where MoO 4 2 - -enhanced passivity reduces H production and uptake due to reduced crack hydrolysis, buffered pH, and a diffusion-barrier film. Inhibition is governed by the balance between crack tip strain rate and repassivation kinetics which establish the stability of the passive film. Inhibition is promoted by reduced loading frequency, reduced stress intensity range, increased crack tip MoO 4 2 - concentration, and potentials at or anodic to free corrosion. The inhibiting effect of MoO 4 2 - parallels that of CrO 4 2 - , but molybdate effectiveness is shifted to a lower frequency regime suggesting the Al x Mo y O z passive film is less stable against crack tip deformation. For high R loading at sufficiently low frequencies MoO 4 2 - fully inhibits EFCP, quantified by reduced crack growth rate to that typical of ultra-high vacuum, reduction in crack surface facets typical of hydrogen embrittlement, and crack arrest. Chromate did not produce such complete inhibition. Methods exist to incorporate molybdate or Mo in self-healing coating systems, but the complex effects of mechanical and electrochemical variables must be understood for reliable-quantitative fatigue performance enhancement.