Protective coating on Mg AZ91D alloy – The effect of electroless nickel (EN) bath stabilizers on corrosion behaviour of Ni–P deposit

Abstract

The effects of the electroless nickel (EN) bath stabilizers; thiourea (TU) and maleic acid (MA), on the corrosion of the EN coatings on Cu and Mg substrates were investigated in an aggressive oxygenated chloride environment. Corrosion was limited to the external EN layer, and thus, the intermediate Cu layer as well as Mg AZ91D substrate remained intact. On the EN surface, two different types of corrosion morphologies were observed. Specimens prepared with TU showed preferential attack along particle boundaries, while pitting corrosion was observed for specimens prepared either without a stabilizer or with MA. The latter deposits showed higher corrosion resistances due, in part, to the formation of a P-enriched layer on the surface. The poorer corrosion performance of EN specimens prepared with TU was attributed to the presence of the particle boundaries and the trace impurities adsorbed on, or included in, them. The key impurity was sulphur incorporated during formation of deposits in TU-stabilized baths. It was clearly shown that the corrosion behaviour of the EN specimens was predominantly influenced by the morphological and surface compositional aspects of the deposits.