Failure mechanism of organic coatings overlaid on arc sprayed coatings

A general mechanism of corrosion degradation of organic coatings overlaid on Al and Zn-Al-Mg-RE (rare earth) coatings was investigated. The metallic coatings were prepared by the high velocity arc spraying (HVAS) technology. Artificial defects were introduced in the organic coatings and the samples were immersed in 5 wt-% NaCl aqueous solution. After 430 h, the different microstructures of corrosion products in the defects were studied by scanning electron microscopy. The microstructure of corrosion products of the HVAS Zn-Al-Mg-RE coating was more compact than that of the HVAS Al coating. The compact corrosion products were better barriers against chloride ions and water permeation, and the defects in the system were blocked by the corrosion products.