Diffusion coating

The purpose of diffusion coatings is not to produce a coating of another metal on the substrate, but to change the composition of the surface layers 

Zinc diffusion is used for protection against atmospheric corrosion. Aluminum diffusion is used to improve the oxidation resistance of low-carbon steels. 

Chromium diffusion applied to a low-carbon steel produces a surface that has the characteristics of ferritic stainless steel, such as AISI446 to a depth about 0.1 mm. When diffusion is applied to a high-carbon steel, a surface rich in chromium carbides is formed. This has a hardness greater than 1000 VHN, which provides good resistance to abrasion. 

Nickel alloys and stainless steels such as A1SI310 (25Cr/20Ni) diffusion treated with chromium enhance resistance to sulfur gases at high temperatures. The chromium-rich surface prevents the formation of nickel sulfide. 

The use of equipment close to the temperature at which the material was diffusion treated will result in continuing diffusion of chromium, aluminum etc., into the substrate, thus depleting chromium with consequent loss in oxidation and corrosion resistance. For aluminum, this effect is noticeable above 700°C in steels, and above 900 C in nickel alloys. For chromium, the effect is pronounced above 850°C for steels and above 950°C for nickel alloys. 

Nickel, chromium and zinc are commonly used as electrodeposits. Chromium, the hardest of these coatings, is applied for abrasion resistance 

Aluminum is the liighest melting point metal (660°C) applied by hot dipping. Aluminized steel can be used at temperatures up to SSO C without appreciable oxidation. Tliis steel has very good resistance to gases and vapors containing small quantities of sulfur dioxide and hydrogen sulfide [41,42,43]. 

aluminum, nickel alloys, cobalt alloys and tungsten carbide are applied for sprayed coatings, which are slightly porous. Flame-sprayed zinc coatings are used for corrosion protection of steel and provide similar properties for galvanized coatings. 

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S. Lyakhovich, ed.. Multicomponent Diffusion Coatings, Oxonian Press Pvt., New Delhi, India, 1987. 

There are other methods of preparation that iavolve estabhshing an active phase on a support phase, such as ion exchange, chemical reactions, vapor deposition, and diffusion coating (26). For example, of the two primary types of propylene polymerization catalysts containing titanium supported on a magnesium haUde, one is manufactured usiag wet-chemical methods (27) and the other is manufactured by ball milling the components (28). 

Zinc—Nickel. Steel has the best salt spray resistance when the nickel is 12—13% of the alloy. At increasing nickel contents, the deposit becomes more difficult to chromate and more noble, eventually becoming cathodic to steel. At those levels and above, corrosion resistance usually decreases and is dependent on a complete lack of porosity for protection of the steel. In efforts to replace cadmium and nickel—ca dmium diffused coatings in the aircraft industry, 2inc—nickel has insufficient wear properties for some appHcation, but is under study as an undercoat to various electroless nickel top coats (153). 

There are three basic types of coatings, thermai barrier coatings, diffusion coatings, and piasma sprayed coatings. The advancements in coating have aiso been essentiai in ensuring that the biade base metai is protected at these... 

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. 

Sulfur compounds, whether organic or inorganic in nature, cause sulfidation in susceptible materials. The sulfide film, which forms on the surface of much con-stmction materials at low temperatures, becomes friable and melts at higher temperatures. The presence of molten sulfides (especially nickel sulfide) on a metal surface promotes the rapid conversion to metal sulfides at temperatures where these sulfides are thermodynamically stable. High-alloy materials such as 25% Cr, 20% Ni alloys are widely used, but these represent a compromise between sulfidation resistance and mechanical properties. Aluminum and similar diffusion coatings can be of use. 

These properties of integration with the underlying metal and negligible dimensional change are the most important features which distinguish diffusion coatings from other types of protection. 

Diffusion coatings are formed as a result of interaction of two distinct processes the solute metal is brought into contact with the surface of the solvent, and this is followed by diffusion proper which consists in the gradual absorption of the solute into the lattice of the solvent. 

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. 

Chromising offers a good example of the types of reaction involved and of the general characteristics of diffusion coating. 

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. 

Table 12.4 Diffusion coatings obtainable on ferrous materials...

It must always be remembered that diffusion coatings are produced by a form of heat treatment and that, with the exception of low-temperature zinc diffusion (sherardising), the treated ferrous materials are usually in the annealed condition. Whenever the mechanical properties of the parts must be restored to their original level, a subsequent heat treatment is necessary . This does not as a rule present any difficulty with chromised or boronised steels. In order to prevent undue distortion and internal stresses during treatment and subsequent hardening, it is recommended that high-carbon and alloy steels should be processed in the normalised condition. 

Other diffusion coatings include manganising to produce austenitic or martensitic surface layers on steel. Mixed Mn/Cr diffusion coatings are readily produced by pack techniques. Carbide coatings based on Ti, V and Cr, singly or in combination, are mainly produced for their wear-resisting properties There are now several commercial boronising processes Con-... 

The present treatment has been limited to a general survey of diffusion coatings for a more complete account on specific aspects, the reader should consult the references. 

Up to 750°C, the performance of all aluminium diffusion coatings is considered to be very good, but above this temperature the results appear to be dependent on the coating thickness, diffusion treatment and the specific service environment. Sprayed aluminium coatings can be used up to 900°C after diffusion treatment. Hot-dipped coatings also benefit from additional diffusion treatment, and omission of silicon from the coating alloy improves performance at the elevated temperatures. 

Drewett, R., Diffusion Coatings for the Protection of Iron and Steel , Part I, Anti-Corrosion, 16 No. 4, 11-16, April (1969)... 

Diffusion Coating a coating produced by diffusion at elevated temperatures. 

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