Cathodic Protection Levels Under Disbonded Coatings

Abstract

A theoretical and experimental study of potential gradients in crevices between a piece of steel and an insulating coating has revealed that the condition of the steel surface and the potential of the steel at the coating holiday have important effects on the potential gradient. Holiday potentials more negative than that required to form hydrogen bubbles can cause difficulty in controlling the potential under disbonded coatings, whereas good control of potential under the coating appears to be possible at less negative holiday potentials. The potential gradients are surprisingly insensitive to the shape of the crevice. A relatively simple mathematical relationship exists among the potential gradient, the current flowing in the electrolyte under the coating, and the current density on the metal.