Comparison of Age-Hardenable Al Alloy Corrosion Fatigue Crack Growth Susceptibility and the Effect of Testing Environment

A comparison of literature data and laboratory results on transgranular corrosion fatigue (CF) crack growth (also known as environmental fatigue crack propagation) of age-hardenable Al alloys used in aerospace applications shows that different Al alloys exhibit differing sensitivities to CF (measured as the crack growth rate in 3.5% NaCl normalized by the crack growth rate in lab air). In particular, AA2xxx and Cu-containing AA6xxx Al alloys appear to possess an inherent resistance to the environmental acceleration to fatigue crack growth, whereas AA7xxx alloys are more susceptible. It is hypothesized that AA2xxx and high Cu AA6xxx Al alloys have an intrinsic resistance to the environment with regard to CF, and as a result of crack wake surface Cu enrichment catalyzing the cathodic reaction(s) within the crack environment, the adverse acidic environment, which is typical and expected of AA7xxx alloys, is self-mitigated. This increase in crack solution pH for AA2xxx and high-Cu AA6xxx Al alloys reduces environmental sensitivity and CF susceptibility by reducing crack tip H uptake through stabilization of a crack tip passive film. This implies that, while addition of Cu to Al alloys is generally accepted as deleterious to localized corrosion, it may be advantageous to environmentally assisted cracking and CF in particular. The results of a literature review are centered on CF crack growth kinetics (da/dN) generated under conventional experimental environments (full immersion in aqueous sodium chloride solutions, uncontrolled laboratory-moist air, or pure water vapor), but these laboratory test environments do not well replicate in-service conditions for aerospace Al alloys. Limited research and literature findings suggest that conventional laboratory testing environments for CF do not rigorously or holistically assess the effect of the environment on fatigue and da/dN for in-service aerospace Al alloys. These findings highlight the importance of continued research efforts to investigate the effects of temperature and atmospheric conditions on CF crack growth susceptibility difference between aerospace Al alloys.