Exfoliation Analysis on Corroded Thermal Sprayed Coating Materials by Acoustic Emission

This paper is to investigate the adhesiveness between substrate and coating layer and the effect of corrosion by acoustic emission method in tensile loading. The used powders are Zn and Al. There are coated on brass alloy substrate, respectively. AE signals of Zn and Al coating layer increase drastically in strain 2%. but that of Zn specimen have more than that of Al specimen. When the specimens executed under 3.5% NaCl solution for 500 hours, the salt solution penetrated into the surface of the substrate through pore of the coating layer. Corrosion production formed on the surface of substrate. the adhesiveness between coating layers is weaken by the polarization and corrosion itself. The AE events, count and energy of corroded coating specimens decreased in comparison with specimens without corrosion. The results are summarized as follows : 1. In the tensile tests, the times that it took for the cracks and exfoliations between the surface of the substrates and the thermal spray coatings to start and develop were different according to the thermal- sprayed materials, Zn, Al, and Amdry625. These phenomena were obvious at the strain rate 1 to 5% and few available data were found after the stage. 2. The specimens with Zn and Al showed the characteristics of cracking, while those with Amdry625 showed of exfoliating. It was judged that the thermal spray coatings on the specimen with Zn and Al have the cracks according to the changes of the tensile strength applied on the substrates, while those with Amdry625 have the exfoliations as a result of being adapted little to the tensile strength. 3. The anti-erosion specimens showed that the adhesive properties between the substrate and the thermal spray coating are strong in the order of Al, Zn, and Amdry, and erosion specimens showed that they are cracked or exfoliated easily even by the small energy in all the 3 cases because they have the comparatively weaker adhesive property. 4. Among the three immersed specimens, those with Zn and Al showed no corrosion phenomena on the surface of the substrates because they had the function of sacrifice anode, but that with Amdry625 showed the corrosion because it did not have that function