Investigating coating adherence on S275JR carbon steel under cyclic corrosion

This study investigates the surface integrity of coated low-carbon steel in simulated marine splash zones, addressing the need for standard surface roughness guidelines to improve coating adhesion. Using laboratory test equipment (designed and developed as per ISO 11130), bare and coated S275JR grade carbon steel specimens are subjected to cyclic wet-dry conditions for 90 days in 3.5% NaCl solution, with corrosion rates assessed via analysis of mass-loss taken at five intervals. The surface roughness is varied by sandblasting, with three different grit abrasives, to produce fine, medium and coarse blasted profiles (ISO 8501-1), followed by spray painting to a Nominal Dry Film Thickness (NDFT) of 450 µm (ISO 12944-9). Coated specimens are scribed as per ASTM D1654 and rust creepage due to loss of adhesion around the scribe line is measured. Results for uncoated specimens show an accelerating corrosion trend, following a power law with an exponent of 1.24. FTIR analysis reveals rust phases such as lepidocrocite (γ-FeOOH), goethite (α-FeOOH), akaganeite (β-FeOOH) and magnetite (Fe₃O₄). Control (non-blasted) coated specimens (Ra: 1–2 µm and Rz DIN: 5–10 µm) exhibit severe cathodic delamination followed by a peeling and flaking effect. In contrast, medium-blasted coated specimens (Ra: 10–13 µm and Rz DIN: 50–65 µm) show improved adhesion due to enhanced mechanical interlocking bonds at the coating-substrate interface.