Abstract
Non-heat-treatable wrought Al-Mg alloys are commonly used in marine structures that require light weight, moderate strength, weldability, and corrosion resistance. These alloys become susceptible to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) when an active β phase (Al3Mg2) precipitates on grain boundaries during prolonged thermal exposure. The Degree of Sensitization (DoS) is quantified using the standard nitric acid mass loss test. Coupled dissolution of β and H embrittlement is the hypothesized mechanism of IGSCC in sensitized Al-Mg. The primary objectives of this research are to characterize the interactive crack tip electrochemical and mechanical factors that govern IGSCC in Al-Mg alloy 5083-H131, and particularly to test this hypothesized mechanism. Research is focused on low temperature, long time sensitization with discrete grain boundary β precipitation. High resolution subcritical crack growth experiments established a critical DoS of about 10 mg/cm2 for IGSCC susceptibility in 5083-H131 stressed at near open circuit potential in neutral NaCl. Below this DoS, the alloy is essentially immune to IGSCC, while above this sensitization, crack growth rates increase as a unique function of DoS for several time-temperature conditions. This threshold is likely due to an acidic crack tip chemistry established by the dissolution of a critical volume of β, in combination with an increase in local stress for increased H concentration ahead of the crack tip due to β presence. As β volume increases and spacing decreases, these factors continue to increase crack growth rates. Crack growth measurements and elastic-plastic J-integral analysis establish that this alloy is susceptible to H embrittlement in the absence of β phase, provided that an acidic chemistry is artificially generated in the crack tip, as an analog to β dissolution. The role of β is verified using polarization to either promote or preclude β dissolution. Anodic polarization significantly enhances crack growth, while cathodic polarization suppresses IGSCC because both β and matrix dissolution are limited. Results demonstrate the strong benefit of modest cathodic polarization as a means to control IGSCC. The role of H is separated from β dissolution through hydrogen precharging of tensile specimens stressed in the absence of a β-dissolving crack tip electrolyte. Results demonstrate that, though β is not essential for hydrogen environment embrittlement, β presence promotes H damage. The coupled dissolution H-embrittlement mechanism for IGSCC in 5083-H131 is validated by demonstrating that: a) β distribution controls crack tip chemistry and H localization at the fracture process zone, b) crack tip electrochemistry provides the H production and uptake for grain boundary decohesion, and c) H-diffusion, rate-limited embrittlement is the dominant mechanism for crack growth between β. Unexpectedly rapid IGSCC rates can be explained based on enhanced H diffusion in this ligament as trapping impedance is reduced. The comprehensive, high-resolution characterization of corrosion and cracking phenomena generated in this research is essential for component prediction modeling, specifically with crack growth parallel to the rolling/forging plane.
