Adhesion investigation of waterborne and solventborne epoxy coatings on oil-contaminated and surface-corroded steel substrates

Abstract

Epoxy coatings are widely used to protect steel structures; however, industrial substrates often retain residual corrosion products and organic contaminants after surface preparation. Waterborne (WB) epoxy coatings are generally considered less surface-tolerant than solventborne (SB) systems, although systematic comparisons under controlled contamination conditions are lacking. In this work, WB and SB epoxy systems were compared on clean, corroded, and oil-contaminated (engine and silicone) steel substrates. Adhesion was correlated with interfacial chemistry using pull-off testing, failure-mode quantification, image analysis, optical microscopy, and post-failure XPS and ATR-FTIR. On clean steel, sandblasting increased adhesion by 2–5 times, reaching 14 MPa (WB) and 17 MPa (SB). Corrosion caused the most severe reduction, decreasing adhesion by 70–90%, to below 1 MPa on smooth steel and to 1–4 MPa (WB) and 5–10 MPa (SB) on sandblasted steel. XPS showed a shift in fracture locus to a Fe3+-rich hydrated oxyhydroxide interphase, indicating that adhesion loss is governed by interfacial chemical transformation rather than roughness or wettability. Engine oil reduced adhesion to 5–7 MPa at 0.1 wt% and 2–3 MPa at 0.5 wt%, while silicone oil caused a more severe reduction, reaching 2 MPa at 1 wt%. Failure-mode analysis confirmed predominantly adhesive failure under contamination. Although SB coatings showed higher adhesion on clean substrates, this advantage was not consistently maintained under contamination. Overall, adhesion is governed by contamination-specific interfacial chemistry.