Resistance to Electrochemical Reactions at Rust Layers Formed at the Defective Part of the Coating of Thermal Al-Si Alloy Spray Coated Steel Plates and the Adhesive Strength of the Coatings

Abstract

We considered the corrosion of a steel base at the through coating defective of thermal-sprayed aluminum（TAS） would be suppressed by adding Si to an Al spray material. The steel plate specimens that were thermally spray coated with Al-5Si or Al-12Si and were machined a disk-shaped artificial defect down to the base steel were prepared. For the sake of comparison, the specimens coated with the two types of Al with different purities or Al-5Mg were also prepared in a same manner. After subjecting to outdoor exposure in Okinawa, the charge transfer resistance at an interface between the rust layer and the base steel in the defective part was investigated by EIS. The charge transfer resistance at the interface for the specimen coated with the Al-5Si or the Al-12Si was higher than that coated with the Al or the Al-5Mg. It was estimated that the highly protective rust layers were formed by the action of Si and Al leaching from the coatings by atmospheric corrosion. The thermal spray coating of the Al-5Si and the Al-12Si was superior than that of the Al and the Al-5Mg in the respect of the tensile adhesive strength to the base steel.