Significance and Use
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.
Scope
1.1 These test methods apply to procedures for determining the degree of disbondment of a coating from a steel substrate when placed in contact with an electrolyte and a potential is applied to the steel. Specimens may include coated steel pipe or coated flat or curved steel plate. The coating applied to the steel substrate shall be non-metallic and shall not show flow characteristics at the test temperature.
1.2 These test methods apply to specimens that are immersed in an electrolyte bath or specimens with an attached electrolyte cell at ambient room temperature, 21 °C to 25 °C (70 °F to 77 °F), conditions. If higher temperatures are required, use Test Method G42.
1.3 These test methods apply to methods of polarization including sacrificial anodes or impressed current applied to the steel by a rectifier.
1.4 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
