Inhibition of Fatigue Crack Propagation in Age-Hardened Aluminum Alloys under Full Immersion and Atmospheric Exposure

This study demonstrates effective inhibition of environmental fatigue crack propagation (EFCP) in age-hardenable aluminum alloys through addition of a passivating ion (ion-assisted inhibition) and localized-alloy corrosion creating passivating conditions (self inhibition) in chloride solutions.  Additionally, it provides a first examination of EFCP inhibition under atmospheric conditions creating thin film electrolytes.  Because age-hardenable aluminum alloys are susceptible to EFCP through hydrogen environment embrittlement (HEE), both self and ion-assisted inhibition can be attributed to presence of a crack tip passive film reducing H production and uptake as explained by the film rupture-HEE mechanism.  Molybdate addition to chloride solution is capable of fully eliminating the detrimental effect of environment by yielding vacuum level crack growth rates for Al-Zn-Mg-Cu and Al-Cu-Mg/Li alloys.  Al-Cu-Mg/Li alloys, including 2024-T351, are capable of self inhibition; a behavior likely due to localized corrosion of anodic Cu-containing precipitates creating conditions which stabilize the native Al passive film. The stability of the crack tip passive film is governed by balance of passive film rupture and repassivation, controlled by crack tip strain rate and repassivation kinetics.  EFCP experiments under atmospheric conditions show that self inhibition is hindered by NaCl containing droplets on the surface.  However, when molybdate is present, ion-assisted inhibition is enhanced.  For Al-Cu-Li, molybdate inhibits EFCP up to 3 Hz for full immersion; but under atmospheric conditions, inhibition was seen up to 30 Hz.  These findings demonstrate that self healing of EFCP by release of ions is possible for age-hardenable aluminum alloys , and suggests that a molybdate or Mo-bearing coating can provide a vehicle for strong inhibition of EFCP by controlled release of MoO₄^2-.  Work is in progress at the US Air Force Academy to extend these results to inhibition of corrosion-to-fatigue damage under realistic airframe loading conditions.

Research supported by OSD/DoD Corrosion Pilot Program on Collaborative University Research