Hot Dip Coatings

Hot dip coating consists of immersing the casting in a bath of molten metal. A flux-coated and/or chemically cleaned surface is necessary to achieve satisfactory results. Aluminum, tin, zinc, and their alloys can be applied from a molten bath. Hot dip coatings are preferred because they are thicker than electroplates and because an alloy layer is formed between the coating metal and the iron. This provides additional durability and adhesion. Castings of complex shape are easily coated by these processes, although air may become trapped in blind holes unless the castings are rotated. More detailed information on hot dip coating is available in the articles "Continuous Hot Dip Coatings" and "Batch Hot Dip Galvanized Coatings" in this Volume.

Hot dip zinc coating (galvanizing) is widely used on iron castings, particularly pipe, valves, and fittings. The uniform and adherent coating provides a barrier against corrosive attack and will further protect an iron casting by acting as a sacrificial anode or by undergoing preferential corrosion. Successful galvanizing depends on surface preparation. Pickling followed by dipping in a bath of zinc ammonium chloride or other flux is done prior to dipping in molten zinc. Excess zinc may be drained or centrifuged from the castings before quenching. Quenching improves the brightness of the coating. Iron castings of any type and any composition can be hot dip galvanized.

Hot dip tin coating (hot tinning) provides a protective, decorative, and nontoxic coating for food equipment, a bonding layer for babbitted bearings, or a precoated surface for soldering. Surface preparation is particularly important, and when maximum adherence is desired, such as when tinning is used to prepare a casting for the application of babbitt, electrolytic cleaning in a molten salt is preferred.

For the hot dip lead coating of iron castings, lead-base alloys are preferred over pure lead; with pure lead, bonding is mechanical rather than metallurgical. Tin is the element most widely used to enhance bonding. Lead coatings are noted for their resistance to fumes from sulfuric and sulfurous acids.

The aluminum coating (aluminizing) of iron castings imparts resistance to corrosion and heat. The coating oxidizes rapidly, thus passivating the surface. The resultant aluminum oxide is refractory in nature; it seals the surface and resists degradation at high temperatures. An aluminized surface has limited resistance to sulfur fumes, organic acids, salts, and compounds of nitrate-phosphate chemicals.

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