CORROSION FATIGUE CRACK PROPAGATION AND INHIBITION IN Al-Zn-Mg-Cu VS Al-Cu-Mg/Li ALLOYS

Age-hardenable aluminum alloys used in aerospace structures are susceptible to environment assisted fatigue crack propagation (EFCP), limiting component durability and safety. The objective is to quantitatively understand EFCP and its inhibition for important aerospace alloys: 7075-T651 (Al-Zn-Cu-Mg), C433-T3 (Al-Cu-Mg), and C47A-T86 (AlCu-Li). EFCP is understood through the hydrogen embrittlement mechanism (HEE) where crack tip dissolution creates H which is absorbed and promotes fatigue damage accumulation at a crack tip. For full immersion in aqueous chloride solution, crack growth rate (da/dN) typical of this mechanism is independent of loading frequency (f) or increases with decreasing f. This is observed for Al-Zn-Mg-Cu alloys; but for Al-Cu-Mg and Al-Cu-Li alloys, the opposite behavior is seen: decreasing da/dN with decreasing f. The Al-Cu-Mg/Li dependence is similar to Al-Zn-Cu-Mg when the crack growth inhibitor molybdate (MoO4) is added to chloride solution. MoO4 effectively inhibits EFCP in 7075-T651; understood by MoO4 promoting crack tip passive film formation, reducing H uptake. MoO4 inhibition increases with decreasing f which acts to increase crack tip passive film stability. Therefore, Al-Cu-Mg/Li alloys are capable of self inhibition because Al2CuMg and Al2LiCu intermetallic precipitates release Cu into the crack tip solution allowing for inhibition by increased passivity without passive inhibitor addition.