Degradation of organic coatings in a corrosive environment: a study by scanning Kelvin probe and scanning acoustic microscope

Degradation of pigmented epoxy coatings on a steel substrate during exposure to aerated 3% NaCl solution has been investigated using the scanning Kelvin probe (SKP) and scanning acoustic microscopy (SAM). A method for processing SKP potential maps has been used to visualize the distribution of anodic and cathodic current. It was observed that rust deposited at the defects, or later beneath the coating, acted as a cathodic reactant, leading to development of new anodes beneath the coating. Exposure of rusty samples to moist air (rather than salt solution) during SKP mapping is shown to lead to more noble potentials at the defect that influence the electrochemistry of under-film processes.