Cleaning of Castings

Cleaning of the surface is the most important prerequisite of any coating process. Suitable levels of cleanliness and surface roughness are established by various mechanical and nonmechanical methods. Foundries deliver castings that have been shot or grit blasted (see the discussion on blast cleaning below). Supplementary nonmechanical cleaning may be needed to reach interior passages or to remove heat-treating scale or machining oil.

The choice of cleaning process depends not only on the types of soils to be removed but also on the characteristics of the coating to be applied. The cleaning process must leave the surface in a condition that is compatible with the coating process. For example, if a casting is to be treated with phosphate and then painted, the cleaning process must remove all oils and oxide scale because these inhibit good phosphating.

If castings are heat treated before they are coated, the choice of heat treatment conditions can influence the properties of the coating, particularly a metallic or conversion coating. In most cases, heat treatment should be done in an atmosphere that is not oxidizing. Oxides and silicates formed during heat treating must be removed before most coating processes.

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