The Performance of Novel Cr(III) Based Coatings for Packaging Steel

Abstract

The main driver for this thesis was the understanding of the mechanisms involved in the corrosion of novel trivalent chromium coatings for packaging steels and comparison with traditional materials, namely electro chromium coated steel (ECCS).Tata Steel are currently developing trivalent chromium coated technology steel (TCCT) as a replacement for ECCS in steel packaging applications. ECCS offers excellent corrosion resistance when used in conjunction with a polymer overcoat. The understanding of corrosion performance of the novel TCCT when overcoated with an organic coating is currently limited. Thus, this work utilised experimental procedures to help understand the mechanism behind two forms of corrosion-driven organic coating failure; filiform corrosion and corrosion-driven cathodic delamination. In turn, this work also aimed to identify, and supply the steel industry with evidence of the TCCT coating weights that are important for optimum resistance to organic coating failure.Increasing the Cr (III) oxide coating weight led to an improvement in corrosion performance with regards to both filiform corrosion and cathodic delamination, with a Cr (III) oxide coating weight of approximately 7 mg.m -2 providing impressive corrosion resistance, comparable to that of the current standard, ECCS. The increased presence of defects on TCCT samples with lower Cr (III) oxide coating weights, as well as the impressive insulating properties of Cr (III) oxide, were thought to be contributing factors to the improved corrosion resistance observed with increasing coating weight.Furthermore, a lab-scale TCCT electroplating line was successfully replicated in the Swansea labs. Varying parameters including current density and temperature had a profound effect on the amount of Cr deposited. Hydrogen evolution was proposed to be one of the reasons for this variance in deposit.IV