Diffusion, metallic coatings

Mansford, R. E., Sprayed and Diffused Metal Coatings , Metal Ind., London, 93, 413 (1958)... 

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). 

Each type of metallic coating process has some sort of hazard, whether it is thermal energy, the reactivity of molten salt or metal baths, particulates in the air from spray processes, poisonous gases from pack cementation and diffusion, or electrical hazards associated with arc spray or ion implantation. 

In metallurgical practice, sodium uses include preparation of powdered metals removal of antimony, tin, and sulfur from lead modification of the stmcture of siHcon—aluminum alloys appHcation of diffusion alloy coatings to substrate metals (162,163) cleaning and desulfurizing alloy steels via NaH (164) nodularization of graphite in cast iron deoxidation of molten metals heat treatment and the coating of steel using aluminum or zinc. 

In addition to the analysis of relatively thick metallic coatings and diffusion profiles, dc GD-OES has also been successfully applied to the analysis of thin protective layers (thickness typically <100 nm), e.g. phosphate and chromate layers on steel [4.185]. 

Diffusion Coating-application of a metallic coating. The chemical composition of the metal is modified by diffusing the coating into the substrate at the metal s melting temperature. 

The more important metallic coating methods such as electroplating, spraying, hot-dipping and diffusion have already been considered. There are, however, several other methods which have certain advantageous features and therefore find exclusive application in specific fields. These include new techniques such as laser hardening and alloying. ... 

Figure 12-14 Effusion of gases, (a) A molecular interpretation of effusion. Molecules are in constant motion occasionally they strike the opening and escape, (b) Latex balloons were filled with the same volume of He fellow), N2 (blue), and O2 (red). Lighter molecules, such as He, effuse through the tiny pores of the latex balloons more rapidly than does N2 or O2. The silver party balloon is made of a metal-coated polymer with pores that are too small to allow rapid He effusion, (c) If a bell jar full of hydrogen is brought down over a porous cup full of air, rapidly moving hydrogen diffuses into the cup faster than the oxygen and nitrogen in the air can effuse out of the cup. This causes an increase in pressure in the cup sufficient to produce bubbles in the water in the beaker.

Coating (immersion, electrolytic metal deposition, diffusion deposition, hot-dip metal coating, evaporation coating, and metal spray coating)... 

---