CONTROL OF COSMETIC CORROSION OF AN A356 ALUMINIUM ALLOY

Abstract

Cosmetic corrosion is a form of localised corrosion that affects aluminium alloys. It is particularly prevalent in the automotive industry where it has a significant impact on aluminium alloy wheels. When the wheels, commonly made from an A356 alloy, experience this form of corrosion, they lose their desired aesthetic appeal. The development of this form of corrosion has been linked to the near-surface region of the substrate and the microstructure that is generated when aluminium is processed. In this study the impact of various methods of processing and formation of the near-surface deformed layer has been examined. Characterisation of the near-surface layer formed during machining revealed that the layer consisted of ultrafine equiaxed grains in the order of 50-100 nm in diameter and was observed to have maximum thickness of 3.65 μm. No oxide particles were observed in the deformed layer, and only Type B grains were present. The layer was electrochemically active and preferential dissolution of the layer was shown to occur during anodic polarisations in 1 wt% NaCl solution with a current density surge evident. When examined ex-situ in an electron microscope, corrosion of the layer was confirmed. When a sample was mechanically ground it is found that a near-surface deformed layer with a maximum thickness of approximately 2 μm, characterised by ultrafine equiaxed grains of 50-150 nm diameter, is introduced by the grinding process on the α-aluminium matrix that is intermittently interrupted by eutectic silicon particles. Preferential dissolution of the near-surface deformed layer occurs when exposed to NaCl solution, along with the trenching of aluminium matrix around the eutectic silicon particles. This trenching developed into pits with longer immersion times. Both fast, superficial and slow, blistering filaments were observed to develop on the surface of a coated ground substrate. Different levels of mechanically ground surface finishes, coarse to fine, led to different thicknesses of deformed layer being generated on the α-aluminium matrix. The thickness of the layer with a coarse finish was approximately 2 μm, whereas with a fine finish (320 SiC grit) the deformed layer was approximately 800 nm. The thicker layers produced a longer open circuit potential transient than the thinner counterparts. Each surface finish observed similar levels of filiform corrosion when tested in a humidity chamber. Furthermore, when the layers were heat treated, grain coarsening was observed. No secondary phase particles were seen to develop in the deformed layer when heat treated. Pre-treatment of ground panels saw a significant increase in filiform resistance of the substrate. Fast superficial filaments were no longer observed, with only a limited number of slow blisters developing on the surface. It was observed that the near-surface deformed layer was removed from the surface during the full cleaning stages of pre-treatment, leaving behind the bulk aluminium and the eutectic silicon particles.

Date of Award	16 Jul 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	George Thompson (Co Supervisor) & Xiaorong Zhou (Main Supervisor)