Comparison of Corrosion-Fatigue Cracking of Al-Li Alloy AA 2090-T8E41 and Alloy AA 7075-T651 in Salt Water

The corrosion-fatigue cracking behavior of aluminum-lithium alloy AA(1) 2090-T8E41 (UNS(2) A92090) and alloy AA 7075-T651 (UNS A97505) in air and 3.5% NaCl solution were investigated, using a fracture mechanics approach. The 2090-T8E41 alloy, as fatigued in air, was found to exhibit a very tortuous fracture-surface morphology and high crack-closure levels that led to fatigue-crack growth rates about 10 times lower than counterpart growth rates for alloy 7075-T651. Such roughness-induced crack closure was greatly reduced for specimens tested in 3.5% NaCl solution, as the conjoint action of salt water corrosion and mechanical fretting greatly diminshed the fracture surface tortuosity; consequently, fatigue-crack growth rates were elevated approximately 6-fold relative to those in air. Fatigue-crack growth rates of alloy 7075-T651 in salt water in the intermediate stress intensity range (ΔK) region were 2 to 3 times higher than those in air. However, in the low ΔK region, the crack-growth rates in salt water were comparable or even lower than those in air. This unusual phenomenon was attributed to the corrosion product induced crack closure.