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

Intact epoxy primers are expected to protect steel structures for more than 20 years, provided that no residual corrosion products or oils remain after the prescribed surface preparation. However, systematic comparisons of waterborne (WB) and solventborne (SB) coatings under controlled contamination and corrosion conditions, representative of practical situations, are lacking. This study therefore provides a direct comparison of WB and SB epoxy primers applied to clean, corroded, engine-oil-contaminated, and silicone-oil-contaminated steel substrates. Adhesion performance and post-failure interfacial characteristics were evaluated using pull-off testing, failure-mode segmentation, optical and scanning electron microscopy (SEM) examinations of coating cross-sections, X-ray photoelectron spectroscopy (XPS), and attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy.
On clean steel, sandblasting increased adhesion strength by a factor of 2–5, reaching 14 MPa for WB and 17 MPa for SB. Residual corrosion products (remaining after light abrasive cleaning) caused the largest adhesion loss, reducing strength by 70–90% to below 1 MPa on smooth steel, 1–4 MPa for WB coatings on sandblasted steel, and 5–10 MPa for SB coatings on sandblasted steel. After pull-off (adhesive break), XPS measurements showed that Fe³⁺-rich hydroxylated corrosion species remained on the steel-side fracture surface. Combined with pull-off and imaging results, this finding is consistent with corrosion-derived interfacial weakening as a major contributor to the adhesion loss, although XPS alone could not establish the precise fracture path.
The presence of engine oil on the substrate (film deposited from a 0.1 wt% solution) reduced adhesion to 6–9 MPa and 3–6 MPa when 0.5 wt% was used, whereas the presence of silicone oil reduced adhesion to 5–8 MPa (0.1 wt%) and approximately 2 MPa (1.0 wt%). Contaminated substrates exhibited predominantly adhesive coating failure.
Summarizing, although SB coatings achieved the highest adhesion on clean steel, this advantage was not consistently retained under oil contamination. These findings show that the adhesion benefit of sandblasting can be overshadowed by the presence of residual corrosion products or oils and that coating type alone is not a reliable indicator of tolerance for poorly prepared steel surfaces.