I am not an expert on thermal spraying and I am not sure what is meant by "does . . . metalizing inside . . . a vessel create hydrogen as it sacrifices itself in preference to the steel?" But if I understand the processes used, it seems there would be little chance of this being an issue. Substrate temperatures are low when using wire flame spray or two-wire electric arc thermal spray technology so existing weldments wouldn't be affected if proper welding procedures were used originally. Let me know if you find out something to the contrary.