The Effect of Atmospheric Corrosion on Environmental Fatigue Crack Propagation and Its Inhibition

Age-hardenable aluminum alloys used in aerospace structures are susceptible to environment assisted fatigue crack propagation (EFCP). There has been no controlled examination of thin film electrolyte, typically produced by atmospheric exposures, effects on EFCP or inhibition of such cracking. The goal of this research is to understand EFCP and inhibition for atmospheric conditions of important aerospace alloys: 7075-T651 (Al-Zn-Mg-Cu) and C47A-T8 (Al-Cu-Li). A candidate chromate (CrO4) replacement inhibitor, molybdate (MoO4), is examined. MoO4 effectively inhibits EFCP in 7075-T651 stressed during full immersion in chloride solution; as understood by hydrogen environment embrittlement and film stability where MoO4 promotes crack tip passivity, thus reducing H uptake. MoO4 inhibition is promoted by reduced loading frequency and stress intensity range and potentials at or anodic to free corrosion. The inhibiting effect of MoO4 parallels that of CrO4, but is shifted to lower frequencies suggesting the Mo-bearing passive film is less stable under crack tip deformation than the Cr-bearing. MoO4 can fully inhibit EFCP by reducing crack growth rate to that of ultra-high vacuum. For slightly anodic potentials, full crack arrest occurs. This research aims to improve fatigue life prediction for safe and economic operations of aircraft. Introduction Age-hardenable aluminum alloys such as AA7075 and C47A, which are Al-Zn-Mg-Cu and AlCu-Li alloys, are selected for military and commercial aerospace structures because of their corrosion resistance and high strength to weight ratio. One major form of degradation in these alloys is fatigue, damage that accumulates with loading to cause the structure to fail. This damage is enhanced by the environment in which the structure is stressed and is referred to as environment-assisted fatigue. Aluminum alloys in aerospace structures are susceptible to environment assisted fatigue crack propagation (EFCP) by hydrogen environment embrittlement (HEE). EFCP damage in water vapor and aqueous chloride solutions is attributed to the interaction between irreversible plastic deformation, tensile stresses, and atomic hydrogen (H) produced by chemical or electrochemical reactions all highly localized at the crack tip. Basically, HEE is initiated when O2 is depleted within the crack solution causing the crack tip to become anodic with respect to the external surface and crack wake. The anodic tip preferentially corrodes and dissolved Al (Al) reacts with water molecules creating H ions and through a cathodic reaction at the crack tip they become H atoms. The hydrogen atoms then enter the fatigue process zone (FPZ) ahead of the crack tip by diffusion through the lattice. Interactions between the H trapped in the FPZ and local tensile stresses and dislocation structure cause embrittlement, enhancing fatigue crack propagation rate (da/dN) relative to inert gas and vacuum. Much of the prior research conducted on EFCP was performed under full immersion in aqueous chloride solutions, in uncontrolled laboratorymoist air, or pure water vapor. EFCP in these environments depends not only on the mechanical driving force for cracking, quantified by stress intensity range (∆K=Kmax-Kmin) and maximum stress (Kmax or R=Kmax/Kmin); but also on an environmental/chemical driving force. For full immersion, the environmental driving force is: (a) loading frequency (f), (b) amount of H embrittling the alloy which is controlled by the overpotential at the crack tip (an electrochemical parameter which is a function of crack tip pH and crack tip potential (E)), and (c) passivity (creates a protective passive film which is a barrier to H adsorption). Although aluminum alloys develop a passive film in aqueous solution, chloride ions (Cl) weaken the native Al passive film (Al2O3) allowing atomic H to more readily enter the alloy for embrittlement. For water vapor, the chemical driving force is an environmental exposure parameter which is defined by the ratio pure water vapor pressure to loading frequency (PH2O/f). For in-service aircraft none of these simplified laboratory circumstances accurately describe atmospheric conditions . The atmosphere is neither made up of pure water vapor nor contains the small number of contaminates found in laboratory air. Rather, atmospheric exposure involves a complex number of constituents that include: nitrogen dioxide