The effect of surface roughness on corrosion resistance of machined and epoxy coated steel

•The steel profile was important for the corrosion resistance of epoxy coating.•Corrosion resistance on grit blasted surfaces was high even at a roughness Rz = 30–40 μm.•Corrosion resistance was poor on surfaces with low ability for mechanical interlocking.•Roughness had to be increased at least six times to achieve similar corrosion resistance as on grit blasted surfaces.•High roughness may be a partial substitution for mechanical interlocking. By incorporating periodic micro-patterns on steel substrates, the effect of surface roughness on corrosion resistance of a two-component polyamine cured epoxy mastic coating has been studied. Machining was employed to pattern the surfaces with periodic peaks of varying peak-to-valley heights, Rz. The focus of the study was to find the surface parameter that contributes most to the stability of an organic coating in corrosive environment, and to evaluate if machining can be comparable to blast cleaning with respect to coating durability. A strong correlation between roughness (Rz) and corrosive delamination of coated surfaces was seen. Increasing Rz from 57 μm to 252 μm on surfaces with triangular peaks, increased the effective contact area by 40% and decreased delamination by 30%. By introducing tilted asperities at Rz = 224 μm while keeping the effective contact area in general unchanged, delamination decreased another 55%. Hence, an increased Rz is found to be only partially beneficial, and the profile shape is more significant than the roughness value per se. The results suggest that mechanical interlocking has a substantial influence on the interfacial stability of protective coatings in corrosive environments. By optimal selection of cutting parameters, machining can give surfaces where protective coatings have long lifetime.