Lead Tin Alloy Coatings

Lead-tin alloy coatings are widely known as terne coatings, from the French word that describes their characteristically dull surface appearance. Generally, 3 to 15% Sn is added to the hot dip bath in order to facilitate wetting of the steel substrate. The solid solubility of tin in lead is very small, and it appears as patches in the lead overlay matrix (Fig. 16).

Fig. 16 Terne coating microstructure. Scanning electron microscope cross section

Terne-coated steel sheet has a long history of use in automotive fuel tanks and tubing because of its excellent weldability, solderability, and formability. Moreover, the low corrosion rate of terne allows it to function primarily as a barrier coating without the evolution of voluminous corrosion products that might otherwise clog fuel and hydraulic systems. Nickel terne is produced by applying the usual terne coating to steel sheet that has first been electroplated with a thin (1 g/m2) nickel layer in order to improve the barrier properties.

Owing to concerns about the effects of lead in the environment, the future of terne is uncertain. Work is now under way to find suitable replacement materials. Zinc-alloy coatings (possibly hot dip zinc-iron or electroplated zinc-nickel) with an organic topcoat are among the most likely candidates.

Continuous Electrodeposited Coatings for Steel Strip

Stavros G. Fountoulakis, Bethlehem Steel Corporation

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