Prediction of Thermal Spray Coatings Performance in Marine Environments by Combination of Laboratory and Field Tests

Cost-effective corrosion mitigation of offshore steel structures can be achieved by thermal spray coatings. These coatings, when comprised of Al, Zn and their alloys, provide a physical barrier against the environment when intact, and cathodic protection to underlying steel when damaged. Due to the complexity of marine environments, laboratory tests should be combined with field work in order to understand the corrosion protection offered by these coatings. The work presented here was carried out with thermal spray coatings of aluminum alloys (AA1050, AA1100, Al-5Mg) and Zn-15Al prepared by Twin Wire Arc Spray onto low carbon steel substrates. The resulting coatings were ~300 μm in thickness, and 5% of surface area defects were artificially machined in order to expose the steel substrate, simulating mechanical damage or erosion of the coating. Electrochemical data collected over a 90 days period showed a good correlation between laboratory and real marine environment results. Aluminum alloys showed better corrosion protection in fully immersed conditions, while zinc alloys performed better in atmospheric and splash zones. Overall, these results aim to improve design of thermal spray coatings to protect carbon steel in marine environments.