The Ability of PVD Chromium (III) Oxide Films to Inhibit the Corrosion-Driven Cathodic Delamination of Organic Coatings from the Surface of Mild Steel

Abstract

This paper describes a systematic study into the role of metallic chromium and chromium (III) oxide films of varying thickness in slowing or preventing the corrosion-driven cathodic disbondment of an organic overcoat applied to chromium/oxide coated steel. A graded wedge of chromium metal or chromium (III) oxide is applied to a steel substrate using physical vapour deposition (PVD) in conjunction with a stepping motor driven sliding shutter arrangement. An overcoat of electrically insulating polyvinyl butyral (PVB) lacquer is then applied and this acts as model organic coating. Corrosion is initiated from an artificially created coating defect, penetrating to the steel substrate, though the introduction of an aqueous sodium chloride electrolyte. Scanning Kelvin probe (SKP) potentiometry is then used to follow the resulting kinetics of PVB coating delamination in air at high relative humidity. The variation in layer thickness across the PVD wedge allows for a high-throughput investigation of the effect of the layer thickness for both metallic chromium and chromium (III) oxide on the rate of corrosion driven cathodic coating delamination, as shown schematically in the figure below. Varying chromium metal layer thickness between 100 nm and 1000 nm produced very little change in the rate of PVB film delamination and the delamination kinetics remained parabolic throughout. Parabollic kinetics are consistent with rate limitation by the mass transport of (sodium) cations through electrolyte layer forming beneath the delaminated coating. Conversely, increasing the thickness of chromium (III) oxide from 10nm to 200 nm (applied over a 500 nm metallic chromium layer), resulted in a significant decrease in rates of PVB delamination. At chromium oxide layer thicknesses ≥ 120 nm delamination kinetics became linear (zero order with respect to time). Linear kinetics are consistent with rate limitation by cathodic electron transfer to oxygen across the chromium (III) oxide layer. For chromium (III) oxide thicknesses of 200 nm PVB delamination was prevented over the entire 48 hour experimental time period. This level of inhibition is comparable to the protection provided by electroplated hexavalent chromium/ hydrated chromium oxide (ECCS) material used in packaging steel products. These finding indicate the ability of PVD to produce corrosion resistant coatings of controlled thickness and composition and provides relevant information regarding the likely relative importance of metallic metal and chromium (III) oxide in potential alternatives to coatings produced using hexavalent chromium. Figure I. Schematic representation of: a) the process by which a wedge of chromium metal or chromium (III) oxide is formed using a sliding shutter mechanism and b) how the PVB overcoated wedge sample is oriented with respect to the cathodic delamination cell. Figure 1