BEHAVIOURS OF ANODIZED SUPERPURE ALUMINIUM AND AA 5657 ALLOY IN HIGHLY ALKALINE SOLUTION

Abstract

Bright anodized aluminium-magnesium alloys stand out in applications as automotive trims due to their attractive surface characteristics. To evaluate the alkaline stability of aluminium automotive parts in car washing detergents with a high pH, the behaviours of anodized superpure aluminium and AA 5657 alloy in a highly alkaline test environment used by industry have been investigated. Characterization of the alloy revealed Al-Fe-(Si) intermetallic particles as the main second phase constituent. Porous-type anodic films were developed during anodizing of aluminium and the alloy in sulphuric acid, with differing current density-time responses and thicknesses of the anodic films. The films were examined by optical inspection, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis, and focused ion beam - scanning electron microscopy. The study compared the appearance and microstructure of anodized specimens before and after exposure to the highly alkaline solution, with the purpose of assessing the alkaline resistance and revealing the interaction mechanism between the films and solution. Further, the relationship between the alloy microstructure and the alkaline resistance, which has not been reported previously, was investigated. The findings revealed that the anodized AA 5657 alloy provides greater alkaline corrosion resistance than anodized superpure aluminium in the alkaline environment. The anodic film on the alloy possesses a slower thinning rate, by a factor of about 1.2, and the barrier layer and cell walls dissolve about 1.4 times slower in comparison with the film on the aluminium. The improvement is related to the incorporation of alloying element species that alter the composition and structure of the film. However, deep holes, of several microns diameter, are locally generated in the anodic film on the AA 5657 alloy by the alkaline exposure, which are associated with the Al-Fe-(Si) intermetallic particles. The relevance of the results of the study to the practical procedures of the standard alkaline corrosion test is also considered, including changing the specimen orientation, modifying the solution composition, etc. The microenvironment of the alkaline test was shown to have a significant influence on the dissolution of the anodic films during the alkaline rest. Importantly, no evidence has been found that the formation of the deep holes in the anodic film on the AA 5657 alloy after the corrosion test are related to the chlorine species in the standard test solution. It is proposed that the holes are formed due to preferential dissolution of film material formed at locations of the second phase particles.

Date of Award	31 Dec 2017
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Peter Skeldon (Supervisor) & Xiaorong Zhou (Supervisor)