Ionic Inhibition of Environmental Fatigue Crack Growth in 7075-t6

The objective of this study is to quantify and understand the effectiveness of a hexavalent chrome replacement ion to inhibit environmentally assisted fatigue crack propagation (EFCP) in high strength aluminum alloys. Because of dynamic-cyclic stress (or strain) loading, fatigue degradation occurs at stress levels considerably lower than the tensile or yield strength; degradation is exacerbated when paired with a corrosive environment. Because CrO4 is carcinogenic and the hexavalent ionic state is not produced by oxidation of a metallic film in service, necessary for a “smart-coating” damage mitigation strategy, an effective replacement must be identified. Addition of molybdate (MoO4) to bulk-low chloride solution effectively inhibits EFCP in peak aged 7075, an Al-Zn-Mg-Cu alloy; comparable to that of CrO4. The effectiveness of inhibition depends strongly on loading variables: ∆K (stress intensity maximum – stress intensity minimum), R (stress intensity ratio), and loading frequency as explained qualitatively by mechanical instability of a crack tip passive film that hinders production and uptake of embrittling hydrogen. For low R loading only, the critical loading frequency below which film stability and inhibition occur increases with increasing inhibitor concentration. Molybdate could be a beneficial replacement for chromate and a candidate for inhibitor release from a coating.