Assessment of zinc oxide-polyurethane internal coating for protection of ASTM A106 steel in seawater piping

This study evaluates the effectiveness of zinc oxide–polyurethane internal coatings on ASTM A106 carbon steel pipes exposed to a 3.5% NaCl solution. The testing was carried out using immersion testing, potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and pull-off adhesion testing to assess corrosion resistance, corrosion rate, and coating adhesion strength. Corrosion rate results obtained from PDP testing were compared with those from immersion testing, indicating that short-term electrochemical testing does not fully reflect the long-term degradation effects observed in immersion tests. Coated specimens generally exhibited lower corrosion rates, with optimal performance achieved at specific zinc oxide concentrations. However, Specimen E (92.5% polyurethane – 7.5% zinc oxide) showed macrocracking after prolonged exposure, indicating that excessive zinc oxide content may lead to coating failure and exposure of the metal surface to corrosion. Pull-off adhesion testing and SEM analysis revealed that the addition of zinc oxide improved the mechanical strength of the coating, as demonstrated by Specimen C (97.5% polyurethane – 2.5% zinc oxide), which exhibited the highest tensile strength. In contrast, agglomeration due to high zinc oxide content in Specimens D (95% polyurethane – 5% zinc oxide) and E resulted in decreased adhesion strength and reduced corrosion protection. The uncoated specimen (Specimen A) exhibited a corrosion rate of 0.17139 mm/year, while Specimen D achieved a rate of 0.03352 mm/year, corresponding to the highest inhibition efficiency of 99.98% based on PDP results. These findings underscore the importance of optimizing zinc oxide concentration to achieve a balance between corrosion resistance, mechanical integrity, and coating durability in marine environments.