A scanning vibrating electrode study of chromated-epoxy primer on steel and aluminum

Chromated-epoxy primers are often used for corrosion control of iron and aluminum structural alloys. However, due to environmental concerns and adverse health effects surrounding such use of chromates, there is an intensive effort to find suitable replacements for chromate-based coatings. This effort is hampered by lack of a detailed understanding of how the chromate-based coatings function. In this work, the scanning vibrating electrode technique (SVET), also known as the current density probe, was used to measure current flowing at chromated-epoxy coated steel and aluminum substrates. The coatings were scribed to simulate a defect through the coating to the metal substrate surface. A plain (nonchromated) epoxy coating was also studied for comparison. The SVET was used to map the current flowing in and around the defect while the sample was immersed in either 3% NaCl (steel) or in dilute Harrison solution (aluminum). Both steel and aluminum substrates coated with the chromated epoxy exhibited a significant delay before the onset of corrosion within the scribe, compared with substrates coated with nonchromated epoxy. Furthermore, the current density maps for steel suggest that the reduction reaction may occur at the surface of the chromated-epoxy coating. With the nonchromated-epoxy coating, the reduction reaction was always confined to the defect area. (C) 2000 The Electrochemical Society. S0013-4651(00)01-028-4. All rights reserved.