Chromium coatings diffusion

The structure and composition of diffusion coatings depends of necessity on the metal or alloy from which the article is made. Thus, for example, it is not possible to speak of chromised coatings generally the material into which chromium is diffused must be specified. Some data on methods of application and properties of commercially chromised irons and steels are given in Table 12.4. 

A low deposition velocity in combination with a high vacancy supersaturation caused by the ion bombardment creates conditions for the intensive diffusion of aluminum atoms into a chromium coating. 

Another important function of metallic coatings is to provide wear resistance. Hard chromium, electroless nickel, composites of nickel and diamond, or diffusion or vapor-phase deposits of sUicon carbide [409-21-2], SiC , SiC tungsten carbide [56780-56-4], WC and boron carbide [12069-32-8], B4C, are examples. Chemical resistance at high temperatures is provided by aUoys of aluminum and platinum [7440-06-4] or other precious metals (10—14). 

The reactions work both ways, and constituents of the alloy being coated may be removed by the halide atmosphere even when interchange does not occur. For example, a nickel-chromium alloy may be superficially dechro-mised by nickel powder in a chloride atmosphere. Thus loss of important alloying constituents may have to be controlled during diffusion coating processes. 

Hack Diffusion. Pack diffusion or cementation processes are similar 10 pack carburizing, anil are used to coat iron, nickel, cuhali. and coppci with chromium, boron, zinc (Sheradi/ingl. aluminum, silicon, titanium, molybdenum, and other metals. 

---