The objective of this study is to develop underwater-feasible and high corrosion-proof surface treatment technology for an extended life span of offshore structures. Cavity formation is a prerequisite to underwater plasma spraying using a fluid-stabilized local dry wet method. It was examined that the effects of the slit angle of a cavity protection tube, the jet velocity of water curtain and the clearance between the protection tube end and the plate surface (below described as the clearance ) , etc. on the cavity formation. By using pure Ti wire as a spraying material, plasma-sprayed coating was formed underwater. As a result, when the clearance was 2mm, the protection tube having the slit angle of 75°, formed broader cavity than that having the angle of 45°, however the clearance of 6mm brought the opposite result. A cavity which has enough dimensions of spraying distance and diameter for the purpose of underwater spraying was formed under the conditions of the clearance of 2-6mm. In the macroscopic appearance, neither cracks nor peelings were observed in the underwater sprayed coating. X-ray analysis revealed that the underwater sprayed coating obtained was consisted mostly of Ti phase. Key words : thermal spraying, underwater, protection tube, water curtain, cavity formation

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