Influence of Uniaxial Deformation on Surface Morphology and Corrosion Performance of Trivalent Chromium Based Coatings for Packaging Steels

Abstract

The packaging steel industry has many products for the consumer market which are deformed to a required shape. Products produced in this way undergo different types of deformation, including uniaxial, biaxial and plane strain. In a situation where the steel is coated and then deformed, the coating must still offer a substantial level of protection for inhibiting corrosion. An In Situ scanning kelvin probe (SKP) experiment was used to investigate the kinetics of organic coating disbondment on uniaxially deformed trivalent chromium coated steel. The studied material was a model trivalent chromium coated steel prototype, designed for over coatings with a lacquer or polymer laminate. The samples were uniaxially deformed to 5%, 10%, 20%, 25% strain using a Hounsfield/Tinius Olsen tensile tester prior to any further experimentation. The strain percentages selected corresponded to the yield point elongation zone, strain-hardening zone and early and late necking zones respectably. A model Poly(vinyl) butyral (PVB) coating was applied to the uniaxially deformed samples. A defect was created in the PVB coating and the sample exposed to aqueous sodium chloride electrolyte in 95% relative humidity. The disbondment kinetics of the polymer overcoat were measured every hour using the SKP, for up to 120 hours. The disbondment kinetics have been quantified as a plot of distance (µm) vs time (tdel− ti), see Figure 1. The linear plots suggest as the strain percentage is increased the distance of delamination over a 24 hour period increases. Scanning electron microscope (SEM) images, shown in Figure 2, display cracking of the trivalent chromium coating. The availability of the underlying iron is estimated by studying the surface controlled hydrogen evolution curves. Increasing the uniaxial strain produced higher values for hydrogen evolution. It is proposed that the increase in hydrogen evolution levels can be related to an increase in the exposure of iron caused by the cracking formations found on the surface. The cracking shown on the surface of the trivalent chromium coated steel (Figure 2) is also suggested to cause the increase in cathodic disbondment rates found during the SKP investigations. Cracking of the surface and exposure of iron is suggested to allow for a continuous cathodic front, causing an ingress of electrolyte underneath the PVB model coating. This investigation displays a link between surface cracking, caused by deformation, and the delamination rates of organic coatings on trivalent chromium coated steels. Figure 1