The Corrosion Resistance Of Al-Zn-Mg-X Alloy And Its Anodic Oxide Film

The 7xxx series high-strength aluminum alloys（Al-Zn-Mg-Cu）exhibited poor corrosion resistance when exposed to marine atmospheric corrosion environments.To address this issue,a new type of high-strength corrosion-resistant aluminum alloy,Al-Zn-Mg-Sc-Zr（AS alloy）,has been developed by replacing Cu with Sc to enhance the corrosion resistance of the aluminum alloy.However,even with the use of this alloy,the corrosion resistance was still insufficient,and it required the growth of a dense film layer through anodizing to meet the corrosion resistance requirements.Currently,the anodizing process of the AS alloy is not well understood,which hinders the process design of the new alloy’s anti-corrosion coating.In this study,a comparative analysis of the corrosion behavior of the traditional 7xxx series aluminum alloy,7050,and the new high-strength corrosion-resistant aluminum alloy,AS,was conducted using SEM,EDS,potentiodynamic polarization,and electrochemical impedance spectroscopy.The anodizing of both alloys was studied,and an anodizing process suitable for Al-Zn-Mg-X alloys was developed,with a focus on investigating the influence mechanism of the second phase on the growth process of the anodic oxide film in both alloys.The research results indicated that the main second phases in 7050 aluminum alloy was dispersed Al₇Cu₂Fe phases and Mg Zn₂precipitation phases,while the main second phases in the AS aluminum alloy was dispersed Al₃Fe phases and Mg Zn₂precipitation phases.Compared to 7050 alloy,the AS alloy exhibited higher corrosion potential,lower corrosion current,and better corrosion resistance.Both types of dispersed phases,Al₇Cu₂Fe and Al₃Fe,act as important triggers for corrosion in 7050 and AS alloys,respectively.These dispersed phases serve as cathodic sites,inducing corrosion in the vicinity of the matrix.The higher content,larger size,and dealloying effect of Al₇Cu₂Fe phase in 7050 alloy make it more prone to localized corrosion,which is the main reason for the significantly lower corrosion resistance compared to the AS alloy.Based on systematic process research,various compositions of anodic oxide films were prepared on the surfaces of the two aluminum alloys.An anodizing process suitable for the AS alloy was developed by adjusting the electrolyte,oxidation temperature,oxidation time,and electrical parameters,resulting in the successful production of corrosion-resistant anodic oxide films.The study showed that the anodizing of the AS alloy was best achieved using a borate-sulfate electrolyte system,with an optimal oxidation temperature of approximately10^oC and an optimal current density of approximately 2A/dm².After undergoing optimized anodizing treatment,both 7050 and AS alloys remained corrosion-free when immersed in a3.5 wt.%Na Cl solution for 60 and 160 days,respectively.Taking advantage of the differences in alloying elements and their distribution characteristics in the two substrates,the presence status and mechanisms of the second phases during the anodizing process were investigated using techniques such as linear polarization,potentiostatic polarization,and SEM observation.The study revealed that the presence of Mg Zn₂precipitation phases in both 7050 and AS alloys facilitates the growth of the anodic oxide film.During the initial stage of anodizing,the preferential dissolution of Mg Zn₂precipitation phases promoted the formation of micropores and enhanced the growth rate of the oxide film,thus positively affecting the corrosion resistance of the film.The Al₇Cu₂Fe dispersed phase on the surface of 7050 alloy comes into sufficient contact with the electrolyte during oxidation,leading to dissolution and detachment,causing the formation of microcracks and larger defects in the oxide film.This phase further influences the growth direction and pore size of the surrounding oxide film.As the film grows,the Al₇Cu₂Fe phase in the 7050alloy undergoes oxidation,resulting in internal defects within the film,which reduces the localized density of the film and contributes to its lower corrosion resistance.In contrast,the Al₃Fe dispersed phase in the AS alloy rapidly detaches during oxidation,but its small size and lack of microcracks have minimal impact on the microstructure of the film.The Al₃Zr phase in the AS alloy does not dissolve during the oxidation process but remains embedded within the film,tightly bonding with the interface,and does not compromise the integrity of the film.This study’s results confirm that the dispersed phase on the surface of aluminum alloys is an important factor that affects the density and integrity of the anodized oxide film.It provides a theoretical basis for pre-treatment methods aimed at removing/reducing the dispersed phase on the substrate surface to improve the corrosion resistance of the anodized oxide film..