Zinc coatings diffusion

Zinc diffusion sherardisingY " is mainly used for protection of ferrous metals against atmospheric corrosion. It has, in some respects, properties related to other types of zinc coating such as galvanising, but owing to the small dimensional change involved, it is of particular value for the treatment of machined parts, bolts, nuts, etc. 

Figure 24. The dependence of the degree of reflection on the visible light wavelength for zinc coatings obtained from solution containing the dextrin/salicyl aldehyde mixture (solution Zn I). M mirror reflection, D diffuse reflection. (Reprinted from Ref.18 with permission from the Serbian Chemical Society.)...

The application of zinc coatings is generally carried out at temperatures lower than for other metallic diffusion coatings, and it is unlikely that the mechanical properties of the substrate will be affected, providing that the process temperature does not exceed 400°C. 

Cementation consists of tumbling the work in a mixture of metal powder and a flux at elevated temperatures, allowing the metal to diffuse into the base metal. Aluminum and zinc coatings can be prepared in this way. Diffusion coatings of chromium, nickel, titanium, aluminum, and so on, can also be prepared by immersing metal parts, under an inert atmosphere, in a bath of molten calcium containing some of the coating metal in solution [6]. 

Nowadays, the use of the reflection electron microscope (REM) or, recently, the tunnel electron microscope, as well as secondary ion mass spectrometry (SIMS), AES, electron-dispersive X-ray spectrometry, impedance spectroscopy, and so on, are yielding substantial increases in the knowledge of corrosion reactions in coatings and at their interface with metal or other substrates. As far as zinc or zinc-coated surfaces are concerned, problems of interfacial and intercoat adhesion, differential diffusion phenomena and electrolytic cell behavior on the substrate, and interreactions of zinc with conversion coatings (chromates, phosphates, silanes, silanols, etc.) have been analyzed, leading toward spectacular improvements in, for example, paint adhesion, absorption of conversion coatings and, in general, the protective action inside films as well as on their substrates. 

The zinc-coated samples are diffusion coatings starting from powder. The tests are apparently at different times/places. 

Table 3.15 Diffusion Annealing Conditions Securing the Production of the Most Corrosion-Resistant Zinc Coatings...

Sony Group. (1966). Automobile battery charger. British Patent 1,171,677, 6 pp. Sotskov, N., and Olshanskaya, O. (1991). Corrosion protection of fasteners by diffusion zinc coatings. Alumozinc Symposium, Dnepropetrovsk. Typescript, 10 pp. (in Russian). 

The sherardizing process involves diffusion of a zinc coating into steel by tumbling steel parts with zinc dust at high temperatures. 

The beneficial effects of Ce02 are also reported in the literature. It was found that sulphidation of FeAl alloys was retarded because the nanoparticles inhibit iron diffusion and act as traps for the sulphur ions [26]. Cabot and Foissy also demonstrated that silica layers stabilised with CeOz nanoparticles led to significant improvements in the corrosion resistance of zinc-coated steels [27]. Cerium oxides are also reported as having a pronounced stabilising effect on the passive state of steels and then-corrosion resistance [28]. 

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. 

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). 

Tin will protect copper from corrosion by neutral water. Pure tin is anodic to copper, and protects discontinuities by sacrificial corrosion. Both intermetallic phases are strongly cathodic to copper, and corrosion is stimulated at gaps in wholly alloyed coatings. An adequate thickness of tin is needed for long service, e.g. 25-50 xm. Another diffusion problem occurs with tin-plated brass. Zinc passes very quickly to the tin surface, where under conditions of damp storage zinc corrosion products produce a film... 

Temperatures well in excess of 400°C can be used for processing in this case much deeper coatings are obtained, but the iron content of the surface alloy is higher and the diffusion layer is very brittle and less corrosion-resistant. This effect is easily explained when it is remembered that the rate of interdiffusion is far more rapid when the temperature is above the melting point of zinc (420°C). 

Although zinc has an appreciable vapour pressure at the temperatures of treatment, it is unlikely that zinc vapour plays any significant part in the diffusion process and it is generally accepted that the mechanism relies almost exclusively on intimate contact of hnely divided zinc dust with the steel surface. In spite of this requirement, coatings of even thickness and composition are obtained on the most intricate shapes, on fine threads, inside blind holes, and in the bore of small-diameter tubes. Large articles of uniform section, e.g. rods, tubes, etc. can be coated by this process. 

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. 

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