Standard Test Methods for Cathodic Disbonding of Coated Steel

Abstract

4.1 Breaks or holidays in a coating applied over steel exposes the substrate to a potential corrosion cell. When the steel is subjected to cathodic protection by the polarization of the steel via sacrificial anodes or impressed current, the exposed steel at the holiday becomes the cathode in the corrosion cell. When the electrolyte is neutral or slightly alkaline, hydroxyl ions form from the reduction of oxygen and, when paired with a suitable cation from the electrolyte, form an alkaline solution. Depending on the strength of this alkaline solution and the concentration of the alkaline compound, this alkalinity may disrupt the adhesion between the coating and the steel, disbonding the coating from the steel. 4.2 Current density of the cathodic cell also can affect the degree of cathodic disbondment. The greater the current density generated by the concentration of electrons at the anode, the greater the number of hydroxyl ions formed, thus increasing the alkalinity available for disrupting the adhesion between the coating and the steel substrate. Likewise, the concentration of oxygen in the electrolyte will affect the concentration of hydroxyl ions formed at the cathode. 4.3 For these reasons it is often useful to measure pH, oxygen, and current density when conducting a cathodic disbondment test.