Effects of mechanical stress and cathodic protection on the performance of a marine organic coating on mild steel

Abstract

The separate and interactive effects of mechanical stress and cathodic protection on degradation behaviour of a marine epoxy-based organic coating on Q235 steel in 3.5% NaCl solution were investigated by employing electrochemical impedance spectroscopy technique. Results revealed that both tension stress (e.g., 120 and 200 MPa) and cathodic protection (e.g., −0.8, −1.0 and −1.2 V with respect to a saturated silver/silver chloride electrode) could greatly deteriorate coating protective performance, including barrier properties, water uptake and diffusion as well as delaminated area, and meanwhile their harmful impacts could become more pronounced with the prolongation of immersion time. The interactive effect of the two factors on coating degradation was shown to be associated with their respective contribution. In addition, the degradation rate for the coating at the potentials of −0.8 and −1.0 V depended largely on the main reaction rate of oxygen reduction and hydrogen evolution, and the predominant control procedure of the two reactions could be reversed by the critical tension stress of 120 MPa.