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. Addition of molybdate (MoO42- ) to bulk-low chloride solution effectively inhibits EFCP in peak aged 7075, comparable to that of CrO42-. The effectiveness of inhibition depends strongly on loading variables: Delta K, R, and frequency as explained qualitatively by mechanical instability of a crack tip passive film that otherwise hinders production and uptake of embrittling hydrogen. The critical loading frequency (and crack tip strain rate), below which film stability and inhibition occur, increases with increasing inhibitor concentration, but only for low stress ratio loading, perhaps due to occluded crack transport and reaction considerations. Molybdate could be a beneficial replacement for chromate and a candidate for inhibitor release from a tailored coating.