Molybdate inhibition of corrosion fatigue crack propagation in precipitation hardened Al–Cu–Li

Abstract

Addition of molybdate (MoO42-) to aqueous chloride solution effectively inhibits environmental fatigue crack propagation (EFCP) in peak aged Al–2.6Cu–1.6Li (wt pct, C47A-T86) which also exhibits alloy-induced inhibition in pure chloride solution. MoO42- inhibits EFCP at frequencies below an upper bound and eliminates the effect of environment at sufficiently low loading frequencies by yielding crack growth rates equivalent to those for fatigue in ultra-high vacuum. Ion-assisted inhibition is attributed to MoO42- stabilizing a crack tip passive film which reduces H production and uptake due to a diffusion barrier film, reduced crack acidification by hydrolysis, and buffered pH. Inhibition is governed by the balance between passive film rupture by crack tip strain and repassivation. As such, inhibition is promoted by reduced loading frequency and potentials at or anodic to free corrosion; each of which favors passivity over film rupture. Alloy-induced inhibition is destabilized by addition of molybdate. This is likely due to molybdate inhibiting localized corrosion of Al2CuLi precipitates in the crack wake which otherwise leads to Cu enrichment required for alloy-induced EFCP inhibition in C47A-T86. The inhibiting effect of molybdate for this Al–Cu–Li alloy parallels chromate and molybdate inhibition of EFCP in 7075-T651, establishing molybdate as a viable chromate replacement inhibitor.