Influence of Electrochemical Conditions in a Defect on the Mode of Paint Corrosion Delamination from a Steel Surface

• Published in: Corrosion (Houston, Tex.) Vol. 66; no. 2; pp. C1 - C10
• Main Authors: Nazarov, A, Thierry, D
• Format: Journal Article
• Language: English
• Published: Houston NACE International 01.02.2010
• Subjects: Chemical speciation; Coatings; Corrosion; Corrosion potential; Defects; Delaminating; Efficiency; Electrochemistry; Electrolytes; Failure; Galvanized steel; Influence; Iron; Paints; Protective coatings; Residual stress; Sodium chloride; Steels
• ISSN: 0010-9312; 1938-159X

This study investigates the mechanism of atmospheric corrosion of carbon steel beneath polymeric coatings. In situ scanning Kelvin probe was applied to measure the potential around the artificial defect in the coating. Measurement of galvanic currents in a model cell (bare steel-coated steel) and current polarity provides information on the polarization of a metal-paint interface by the defect and on the mode of delamination. In addition, the redistribution of chemical species between the defect and coated areas was evaluated by a surface analytical technique after delamination of the coating. It was shown that the mode of corrosion de-adhesion of paint from the steel surface can differ depending on electrochemical conditions inside the defect. The grathent in the potential between the steel surface in the defect and under the intact coating determines the mechanism of underfilm corrosion. Deposition of thick water electrolyte film containing sodium chloride (NaCl) leads to cathodic delamination of the coating. In more "dry" conditions, during corrosion in humid air, polymeric coating around the defect is subject to anodic undermining. The potential in the defect is the function of efficiencies of anodic and cathodic reactions. Signs of the potential gradient across the defect and intact coating are different and defined by the potential of the corroding steel surface. [PUBLICATION ABSTRACT]