A zinc coating is applied to the steel for rust protection.
镀锌层是用来防止钢铁生锈的。
英语百科
Galvanization
Galvanization, or galvanisation, (or galvanizing as it is most commonly called in that industry), is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which parts are submerged in a bath of molten zinc. Galvanizing protects in two ways: