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Why is zinc produced for die-casting purer than zinc used for galvanizing?

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Diagram of a zinc-lead blast furnace system showing ore and coke entering through charge hoppers, with labeled furnace, air preheater, blower, taphole for slag and lead bullion, and a connected lead-splash condenser using rotors to separate and cool lead and zinc, with pathways for molten metals and off-gas. — metallurgy A zinc-lead blast furnace and lead-splash condenser. This technology is utilized mainly for producing zinc while managing both lead and zinc from raw materials, a technique that was highly influential in 20th-century smelting.

Zinc used for die-casting must be of high purity (99.99 percent) to ensure the mechanical integrity and performance of the cast components. This high-purity requirement is essential because die-casting involves injecting molten alloys into steel dies under pressure to create complex, dimensionally stable parts; impurities can negatively affect the casting process and the final properties of the alloy.1

In contrast, galvanizing—the process of coating iron or steel to protect against corrosion—typically utilizes an ordinary grade of zinc that may contain up to 1.5 percent lead. Because the primary function of galvanizing is to provide a sacrificial layer that corrodes preferentially to protect the underlying steel, the presence of these impurities does not interfere with the coating's protective purpose.23