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.sub.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 it 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.
Aluminum coatings to reduce corrosion of steels and the like, are very effective when applied by pack diffusion below 1000.degree.F using a retort cup not over fifteen inches high, with anhydrous or hydrated energizer in a layer on top of the pack and out of contact with the workpieces. Such diffusion coatings are more uniform than corresponding coatings made with the pack energizer in a set of porous containers imbedded in the pack, even when using a retort not over 15 inches high and the porous containers are grouped together separately from all the workpieces. Keeping workpieces away from above and below the porous containers helps. Aluminum diffusion can also be effected from continuous coatings of leafing-type aluminum particles and such leafing coatings in very thin layers are more effective than coatings of non-leafing aluminum, with or without diffusion. The leafing aluminum coatings can be sprayed on from aqueous dispersion containing wetting agents and if desired a polyethylene glycol to help disperse the aluminum, as well as mixtures of phosphoric acid, chromic acid and magnesium, aluminum, calcium or zinc salts of these acids. A protective second coating of such mixtures can be applied as a cover layer over the layer containing the leafing aluminum, and this combination works best on a ferrous metal that has an aluminum diffusion coating, particularly a ferrous metal that contains less than 1% chromium and has such an aluminum diffusion coating. It also works very well on aluminum diffusion coatings from packs containing chromium, or chromium and silicon, in addition to the aluminum, and these alloys can be made by magnesothermic reduction of their mixed oxides or the like.
A uniquely surface-modified metallic part is provided by the utilization of microwave energy to promote diffusion of desired metals into the surface of the formed metallic part.
Inner and outer surfaces of structural components are aluminized by an aluminum gas diffusion process. For this purpose a gas mixture of a halogenous gas, aluminum monohalide gas, hydrogen, and negligible proportions of aluminum trihalide gas is caused to flow over the outer and inner surfaces of the component to be coated. The process is performed in a vessel in which at least two different temperature zones are maintained for keeping one or more aluminum sources at a higher temperature than the component to be coated. Especially gas turbine engine blades are protected against oxidation and corrosion by the so formed aluminum diffusion coatings on outer and inner surfaces of the blades.
Diffusion coating of internal surface of workpiece accessible only through very narrow passageway, is simply effected by coating that surface with uniform layer of particles of the metal to be diffused into it, and then holding the thus-prepared workpiece at diffusion-coating temperature in a diffusion-coating atmosphere.
A metal ring frame, having co-fired ceramic inserts, is hermetically sealed to a copper/molybdenum base using a brazing alloy having a melting temperature which is approximately the cross-over temperature for the TCE curves of the base and ring frame. Preferably the base contains at least 20% copper by weight. The brazing alloy is a silver/copper alloy such as 56% Ag, 22% Cu, 17% Zn and 5% Sn, or one with a higher brazing temperature such as 78/22 Ag/Cu, and is used for brazing both the inserts into the frame and the frame onto the base.
