MECHANICAL ZINC PLATING
Zinc can also be deposited on steel and other metals by means of a rotating drum filled with zinc powder, chemicals and glass balls of different diameters. When the products are introduced into this drum, the glass balls hammer the zinc onto the surface of these products, which creates a zinc layer comparable to zinc electroplating. Afterwards, the plated products are given a passivation layer, just as after the electroplating process. Mechanical zinc plating is less common for fasteners than zinc electroplating, but it has an advantage: there is no risk of hydrogen embrittlement. However, the buildup of the zinc layer must be tightly controlled in order to prevent thread interference. The mechanical plating process is standardized in ISO 12683.
Source: Thielco Metal Coating