Zinc casting

Aluminum Casting Copper and Alloy Casting Magnesium Casting Zinc Casting... 

Diphase Cleaning. Use of an organic solvent top layer over a mild aqueous cleaner solution was popular for aiding in the removal of buffing compounds, especially on zinc base die castings. Use of this diphase cleaning has declined with the decline in buffed zinc castings. 

The metals recommended for use with ethanol include carbon steel, stainless steel, and bronze [3.10]. Like methanol, metals such as magnesium, zinc castings, brass, and copper are not recommended. Aluminum can be used if the ethanol is very pure, otherwise it should be nickel-plated or suitably protected from corrosion by another means. The metals compatible with ethanol represent a much wider range than those for methanol and represent most of the metals currently used in fuel systems, so few changes would be anticipated when using ethanol. 

Zinc casting almost exclusively uses die pressure die-casting technique. In the EU, there are mainly two alloys in use, their compositions are given in Table 2.2. They are also referred to as Zamac, which in origin is a trade name. The basis of these alloys is pure zinc. 

Table 1.9 Ranking of Finishing Processes for Zinc Castings ... 

The suitability of zinc castings, especially ZA-8, ZA-12, and ZA-27, for use in engines and equipment has led to many tests—largely by Noranda (1993)—on their suitability with fuels, oils, and so on of different types. 

Noranda (1993). Designing Zinc Castings for Corrosion Resistance. Noranda, Toronto, 24 pp. 

Riiegg, W. (1970). Oberflachenbehandlung von Zinkdruckguss (Surface treatment of zinc castings) (in German). Juris Druck Verlag, Zurich, 493 pp. 

In aqueous corrosion, raising the temperature increases the dissolution of zinc in water. A marked increase occurs up to around 60°C followed, by a decrease at higher temperatures due to the decrease in oxygen supply and the formation of more compact and adherent scale. Intergranular corrosion of zinc casting alloys is a risk above 70 C in wet or humid conditions, such as in steam, when no protective layer can form and selective dissolution of the structure occurs. In hot hard waters, scale forms at water temperatures above 55 C. This scale has a coarse grain structure and does not adhere well to the zinc surfece. Corrosion of the zinc occurs locally because of the discontinuities in the scale or local electrochemical action. Above 60°C, zinc usually becomes cathodic to steel therefore, the steel will corrode to protect the zinc coating. 

It had been established previously that the Annex A1 of ASTM G 85, applied to copper-nickel-chromium plated zinc castings, would produce the type of failure experienced on such articles in service. However, the 16 to 20 h required to produce such failure were considered by many to be imde-sirably long. 

The mechanical properties of zinc casting alloys are shown in Table 23.6. 

Figure 9.50 Close-up of corrosion pits in a water jug (Fig. 6.8) made of a silver-plated zinc casting.

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