Development of Al-Mg-Ca Thermal Spray Coatings for Corrosion Protection of Port Steel Structures

In Japan, nearly a half of the mooring quays are constructed using steel products. Corrosion mitigation of these port steel structures can be achieved by thermal spray coatings. These coatings, which are usually used Al-5Mg alloys, provide a physical barrier against the environment when intact, and cathodic protection to underlying steel when damaged. In this study, a high-Ca type Al-Mg-Ca thermal spray coating was developed to improve durability against damage by increasing the amount of Ca addition. The Al-Mg-Ca coating exhibited the same adhesion strength as the Al-Mg coating. Defects were artificially machined in order to expose the steel substrate, simulating mechanical damage or erosion of the coating. There was no difference in corrosion protection between the Al-Mg and Al-Mg-Ca thermal spray coatings in the combined cyclic corrosion test. However, the Al-Mg-Ca coating was superior to the Al-Mg coating in terms of durability for damage of 5 mm or more in width in the splash zone exposure test. EPMA analysis results show no Fe in the upper layer of corrosion products on 5 mm wide damage in the exposure specimen with high Ca type Al-Mg-Ca coatings.