Modified diffusion coating of the interior of a steam boiler tube

Abstract

Diffusion coatings can be masked from portions of a workpiece by combination of layers, the outer one of which forms coherent strong shell that holds inner layer or layers in place. All ingredients of these layers can be materials such as nickel, nickel aluminide, chromium, chromic oxide (Cr.sub.2 O.sub.3) and inert diluent that do not contaminate superalloys or even low alloy or plain carbon steels. Such a holding shell can also be used to retain on a workpiece surface a layer that causes formation of a diffusion coating. Low alloy steel conduit can be internally chromized and/or externally chromized or aluminized to make it more desirable for use as high pressure steam boiler heat exchange tubing. Nickel articles aluminized with deep Ni.sub.2 Al.sub.3 layer that has outer skin more heavily aluminized can have aluminum correspondingly leached out from outer skin only.

Dependencies are inferred from the claim text.