Halide activated pack-cementation

Cockeram and Rapp have evaluated the kinetics of silicide coatings on Ti [103] and have used a halide-activated pack-cementation method to form boron- and germanium-doped silicide coatings on orthorhombic alloy substrates [104]. The coatings greatly decreased the cyclic oxidation kinetics and microhardness measurements did not indicate diffusion of oxygen into the substrate. 

R. Bianco, M.A. Harper, and R.A. Rapp, Codepositing Elements by Halide-Activated Pack Cementation, JOM, Nov 1991, p 68... 

In pack cementation, the part to be coated is placed in a retort and surrounded with a powdered pack consisting of the coating component and an activator the latter reacts with the coating component to form the carrier vapor, usually a halide or an inert diluent, to prevent the pack from sintering together and to permit vapor transport of the alloying component through the pack. 

The most common method of aluminizing is pack cementation, which has been a commercially viable process for many years. This process, which is shown schematically in Figure 10.1, involves immersing the substrate in a mixture of powders. The mixture contains a powder source of Al, either A1 metal or a suitable master alloy, a halide activator, and a filler, which is usually alumina and taken to be inert. A pack will usually contain 2-5% activator, 25% source and the rest filler. The purpose of the filler is to support the component and to provide a porous diffusion path for the gases generated by reaction between the source and activator. 

Sherardizing is a diffusion process in which the steel parts are heated in the presence of zinc dust or powder in inert medium. Aluminium oxide or sand in amount of 20 % is added to the zinc powder as inert filler and 1 to 2 % halide salts are used as activator. The thermochemical treatment can be carried out in retort, rotated drum or as a pack-cementation process at 350 to 500 °C for three to twelve hours. The structure of the obtained layer is the same as the structure on steel surface after hot-dip galvanizing with a thickness about 50 - 400 pm... 

Basic Principles. Pack cementation is a batch vapor-phase process that involves heating a closed/vented pack to an elevated temperature (e.g., 1050 °C, or 1920 °F) for a given time (e.g., 16 h) during which a diffu-sional coating is produced (Ref 18). The traditional pack consists of four components the substrate or part to be coated, the master alloy (i.e., a powder of the element or elements to be deposited on the surface of the part), a halide salt activator, and relatively inert filler powder. The master alloy, the filler, and halide activator are thoroughly mixed together, and the part to be coated is buried in this mixture in a retort (Ref 19). When the... 

Ecord [37]. The base metal is packed into a powdered mixture composed of the metal to be diffused, an inert filler, and an activator (usually a halide salt). The use of an activator allows that the temperature to be used is lower than that which would be required by a conventional cementation process. The sealed pack is then held at a given temperature in a furnace for an appropriate time. Parts to be coated by the fluidized bed method are suspended in a bed of the coating metal, which is fluidized by a halogen gas stream mixed with an inert carrier gas. This is similar to the flowing gaseous methods in which the gaseous metal halide (of the coating metal) is streamed over the metal to be coated. 

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