Inhibition of Corrosion Fatigue in 7075 Aluminum Alloys

High purity aluminum-zinc-magnesium alloys (7075 type) were tested in fatigue in argon, dry air and four aqueous solutions. Sodium chloride was used as a crack growth accelerator and sodium nitrate as a crack growth inhibitor. Measured growth rates in the presence of chlorine ions were ten times greater than that in dry air and almost one hundred times that in argon. Sufficient quantities of sodium nitrate added to distilled water and to aqueous sodium chloride solution reduced the fatigue crack growth rate to that observed in dry air. Analysis by scanning electron microscopy of the fracture surfaces features correlated well with macroscopic growth rate data. In argon, no fatigue striation features were observed. Ductile striations were present on samples tested in dry air. Both distilled water and aqueous sodium chloride solution promoted a brittle fatigue striation morphology, while addition of sodium nitrate caused a reversion to the ductile striation type observed in dry air. Competitive adsorption between chlorine and nitrate ions is suggested as the mechanism of inhibiting fatigue crack growth.