The corrosion behavior of thermally sprayed Al-Zn alloy coatings immersed in NaCl aqueous solution was determined by using electrochemical impedance spectroscopy. Ohmic resistance and capacitance of the corrosion product film formed on the sprayed coating surface, and the corrosion resistance and electric double layer capacitance at the interface between the sprayed coating and the solution were evaluated. As immersion time in NaCl solution increased, the corrosion rate of the Al-2mass%Zn coating increased, but the corrosion rate for Al-6mass%Zn and Al-15mass%Zn coatings decreased due to the influence of the corrosion product.

New concept and method of testing for the interconnecting porosity of thermally sprayed nonconductive ceramic coatings is proposed. This process is useful to differentiate the open porosity from the closed porosity. Thermally sprayed ceramic coating with metallic substrate is plated, and the intergranular gaps in the coating are filled by deposited metal. Typically, 304 stainless steel substrate deposited with atmospheric plasma sprayed alumina coating is immersed in copper(II) sulphate bath, and electroplated. Inward of the alumina coating, plating solution penetrates toward the interface of coating/substrate via interconnected porosity, and attains the coating-substrate interface to deposit metallic copper. Deposit of copper is gradually grown up along the coating intergranular. The exposure of deposited copper on the coating surface can be visible. Because these phenomena occur only in the interconnected pore structure, it is easy to differentiate the interconnected porosity from the closed porosity. Also the proposed process suggests the unprecedented possibility of manufacturing method of three-dimensional structure of thermally sprayed ceramic coating.

The sealing effect of various sealants applied to sprayed coatings in sodium chloride aqueous solution was investigated with the galvanostatic technique in which the same quantity of electricity is fed to each test material coated with sealant film cut crossly. The amount of dissolved sprayed coating sealed with sealant can be obtained as the difference between the dissolved material volume at the crosscut part and that in the solution. From the result it was found that the higher the sealing effect of sealant the more aluminum is dissolved from the sprayed coating at the crosscut part, which is considered to mean in case of the sealant poor in sealability, sprayed coating is dissolved even where it is coated with sealant and electrolytic current is dispersed. This proves that the galvanostatic technique is effective in evaluating the sealing effect of sealants.

Hardness and microstructure of Cr 3 C 2 -NiCr coating formed by vacuum plasma spraying process (VPS coating) were investigated in compare with that formed by High Velocity Oxygen Fuel Spraying process (HVOF coating). The results are as follows. (1) The hardness of VPS coatings in as sprayed condition was HV1243 ± 80, which was much higher than that of HVOF coatings, HV958±44, and never went down under HV1100 even after heat treatment at 1273K for 3.6ks. (2) VPS coating presents dense lameller structures composed of Cr 2 C 3 , Cr 7 C 3 and γ-NiCr phase, while HVOF coating presents lameller structures with many fine gaps, composed of Cr 2 C 3 , Cr 7 C 3 , γ-NiCr phase and relatively large amounts of Cr 2 O 3 . (3) The reason why such high hardness was obtained in VPS coating, has been considered due to their dense lamella structures.

The work reported herein deals with the interaction among the thermally sprayed coatings and steel substrate in acid aqueous solutions investigated by using the electrochemical process. Thermally sprayed coatings on steel substrate for such as the tribological applications except for the sacrificial anode property have been extended. The environment of these machinery components is often utilized accompanied with the aqueous solutions. The galvanic corrosion occurred in aqueous solutions among the metals or alloys have been well known. In the actual spray process, the formation of penetrated pores or crevice defects are not generally avoided. In the aqueous solution environment, the penetrated defects cause the galvanic corrosion between coating and steel substrate, and tend to be occurred the coating spalling. In this work, the electrochemical process was employed to determine the corrosion phenomena. The preferable combination of sprayed coatings and steel substrate are discussed and the examples of design of the intermediate layers are proposed.