Improvement in Corrosion Resistance of Aluminium Alloy by Permanganate-based Conversion Coating

ABSTRACT

This investigation on permanganate-based conversion coating describes a chromate-free coating and efforts at reducing the environmental foot prints of metal finishing industries. A golden coloured coating on aluminium specimen was obtained from the permanganate coating (PMC) bath with nominal pH of 7.9. Results from scanning electron microscopy (SEM) examination showed that the conversion coatings are continuous, and the surfaces of the specimens treated in the coating bath are composed of irregularly shaped and mud-cracked coating materials. The coatings, as revealed from energy dispersive spectroscopy (EDX) analyses in the SEM are composed essentially of aluminium, oxygen and manganese compounds which are likely to be hydrated. Accelerated corrosion tests in near neutral 3.5% sodium chloride solution and natural exposure tests revealed that the replacement of chromate with permanganate improved paint adhesion and corrosion resistance on aluminium substrates. Furthermore, the results indicated that the corrosion resistance of the permanganate-based conversion coating matched those from the traditional chromate coating bath, but for the corrosion resistance, after application of a top coat of lacquer, the former was adjudged better than the later.

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