The effect of surface roughness and topography on the organic coating/steel interface stability

Abstract

The study in this thesis, focused on the effect of surface roughness and topography on the organic coating/steel interface stability in humid air and corrosive conditions. A strong correlation between roughness, Rz, and corrosion resistance of coated surfaces was seen in corrosion tests, ISO 12944-9 (Paper I). Steel surfaces were patterned by machining to have periodic peaks of varying peak-to-valley heights, Rz. Grit blasted surfaces were used for reference. The coating was a two-component commercial polyamine cured epoxy mastic. Increasing the roughness on machined surfaces to 30 μm, decreased significantly the disbonding in corrosion resistance tests. By introducing tilted asperities while keeping the effective contact area in general unchanged, disbonding decreased even further. The results suggest that mechanical anchoring has a substantial influence on the protective coating/steel interface stability in corrosive conditions. For the next steps (Paper II and Paper III), steel surfaces were patterned by ultrafast laser structuring to have controlled periodic peaks of varying Rz and geometry. Grinded surfaces were used for reference. A model poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (PVB) coating was used. By the combination of the in-situ SKP technique and X-ray photoelectron spectroscopy (XPS), it was demonstrated that the steel surface oxidises underneath adhering coating, depending on oxide condition from the start. The changes in surface oxide under the coating were found to be independent of the topography or roughness of the steel substrate. On smooth surfaces however, the oxidation resulted in loss of adhesion. The in-situ SKP technique was also employed to investigate the cathodic disbonding (CD) in humid air and wet adhesion loss in inert atmosphere (3 ppm O2). CD and wet adhesion loss depended on surface roughness. A significant effect was found even when compensating for the increased effective contact area. Surfaces with features that enabled mechanical interlocking forces, had the best CD resistance and wet adhesion properties. Hence, the effect of surface roughness on CD rate and wet adhesion must partly be explained by mechanical adhesion forces.