Environment-Enhanced Fatigue Crack Propagation and Inhibition in Aluminum Alloys under Full Immersion and Atmospheric Exposure

This study provides a first examination of environmentally assisted fatigue crack propagation (EFCP) under atmospheric conditions, and confirms that addition of molybdate to both bulk solution and thin film chloride electrolytes effectively inhibits EFCP in age-hardened aluminum alloys similar to or better than chromate.  EFCP inhibition is explained by a mechanically stable crack tip passive film incorporating Mo species for improved passivity which hinders production and uptake of embrittling hydrogen.  Inhibition under full immersion is promoted by reduced loading frequency and stress intensity range, and potentials at or anodic to free corrosion.  These variables affect crack tip strain rate and repassivation kinetics that govern stability of the crack tip passive film.  For low R loading, molybdate inhibition behavior is similar to that for chromate inhibition: a critical loading frequency exists below which the passive film is sufficiently stable for inhibition to occur and inhibition increases with increasing molybdate concentration.  For high R loading, inhibition does not depend on molybdate concentration and at sufficiently low frequency the environmental contribution to EFCP is effectively eliminated producing growth rates typical of fatigue in high vacuum.  Molybdate inhibition depends on potential with da/dN more strongly reduced and crack arrest promoted at ultra low frequencies when a modest anodic polarization is applied.  Most research to date has been performed under full immersion or in moist-air.  These simplified environments may not accurately simulate atmospheric conditions that include contaminants which cause deliquescence of a thin film electrolyte.  Experiments demonstrate enhanced Paris regime da/dN and reduced fatigue life for a peak aged Al-Cu-Li alloy exposed to a relatively thick Cl^- containing thin film electrolyte.  Successful inhibition is observed when molybdate is present in this surface electrolyte.  Quantification of the thin film environment effect on fatigue is complicated by heterogeneous salt deposition, surface wetting, and electrochemical potential measurement challenges.This research is supported by the Alcoa Technical Center and OSD/DoD Corrosion Pilot Program on Collaborative University Research