Pack diffusion coating of metals

Abstract

In the diffusion coating of aluminum onto brazed metal, the penetration of the aluminum into the brazing is reduced by conducting the diffusion coating with hydrated aluminum chloride, bromide or iodide as energizer, with the energizer preferably kept out of contact with the work being coated until the energizer volatilizes. This is particularly suited for aluminizing chromium-containing surfaces. Chromium diffusion coatings are less apt to form undesirable oxide inclusions when the diffusion coating is from a pack containing at least about 3% Ni 3 Al. Also the formation of undesirable alpha-chromium is reduced when the pack diffusion is carried out with a retort effectively not over 5 inches in height. Pack aluminizing where the aluminizing is inhibited by the presence of chromium in the pack makes a very effective top coating over platinum plated or platinum coated nickel-base superalloys. Aluminized nickel can also have its aluminum attacked and at least partially removed with aqueous caustic to leave a very highly active catalytic surface. Pack diffusion can also be arranged to simultaneously provide different coatings in different locations by using different pack compositions in those locations. An aluminizing pack containing a large amount of chromium provides a thinner aluminized case than an aluminizing pack containing less chromium and some silicon. Also a cobalt-chromium pack deposits essentially a chromized case when energized with a chloride, but deposits large amounts of cobalt along with chromium when energized with an iodide.

What is claimed: 
     
       1. In the process of diffusion coating a metal article, the improvement according to which a single diffusion coating heat .[.is arranged to coat different portions of the article in different ways by maintaining the different portions in contact with different diffusion coating packs.]. .Iadd.applies essentially the same coating in predetermined different thicknesses to adjacent portions of the article by packing one portion in a diffusion coating pack containing an inhibitor for the desired coating to inhibit the coating thickness formed on that portion as against the coating thickness formed on a different portion. .Iaddend. 
     
     
       2. The combination of claim 1 in which the metal article is a superalloy, the diffusion coating is an aluminum diffusion coating, the different coating packs are chromium-containing aluminum diffusion packs, one pack contains more chromium than aluminum, and another pack contains less chromium than the one pack, and also contains silicon in an amount from about 10% to about 20% of the aluminum by weight. .[.3. The combination of claim 1 in which the different packs differ in that they have different energizers, and the different portions of the metal are on opposite sides of a metal wall that effectively inhibits the intermingling of the 
     
     
        atmospheres at these portions..]. 4. The combination of claim .[.3.]. .Iadd.6 .Iaddend.in which cobalt and chromium are the essential diffusible metal ingredients of both packs, one pack is energized with a chloride energizer to cause diffusion coating of essentially only the chromium and the other with an iodide energizer to cause diffusion coating of more cobalt than chromium. .Iadd.5. The combination of claim 1 in which the metal article coated is a turbine engine vane or blade. .Iaddend..Iadd.6. In the process of diffusion coating a metal article, the improvement according to which a single diffusion coating heat applies significantly different coatings to different portions of the article from packs that differ from each other essentially only in the compositions of their energizers. .Iaddend..Iadd.7. The combination of claim 6 in which the metal article coated is a turbine engine vane or blade. .Iaddend.