Cathodic disbonding of an unpigmented epoxy coating on mild steel under semi- and full-immersion conditions

Cathodic disbonding of an unpigmented epoxy coating around a defect was studied with samples fully- and semi-immersed in 3.5 wt.% NaCl solution. Disbonding and the growth of blisters were monitored by scanning acoustic microscopy, which showed growth of blisters within the disbond for both type of samples. Scanning Kelvin probe potential maps suggest that blisters first form at local anodes. For semi-immersed samples, disbonding of the immersed part and the non-immersed part propagate with parabolic kinetics at identical rates, similar to fully-immersed samples. This implies that ion transport is along the coating–metal interface in both cases.

► Scanning acoustic microscopy detects blisters within cathodic disbond.
► “Disbonding” demonstrated to be only partial adhesion loss.
► Scanning Kelvin probe links blistering with local electrochemical activity.
► Semi-immersion tests show same behaviour above and below waterline.