The most common method for corrosion protection of fasteners is electro-zinc plating (FZB). Zinc alloys provide increased protection against corrosion and also improve other properties such as heat resistance. Zinc coatings have high wear resistance and great resistance to mechanical impact. Since zinc is a base metal, it acts as a sacrificial material for the steel, meaning that the zinc corrodes instead of the steel. This protects the steel even if the coating is damaged. Under normal conditions, zinc corrodes about ten times slower than steel.
Electro-zinc plating is a relatively simple and cost-effective surface treatment method. The zinc layer is usually shiny, but with satin zinc, a matte, aluminum-like surface can be achieved. Zinc plating also provides a decorative appearance, where a bright zinc-plated surface can resemble a chrome-plated or nickel-plated surface. In electrolytic zinc plating, which takes place through electrolysis in a water bath, 8–20 μm of zinc is applied depending on the desired corrosion protection. The protective effect is directly linked to the thickness of the layer.
Electro-zinc plating is performed on screws up to strength class 10.9. At higher strength classes, the risk of hydrogen embrittlement is high, and surface treatment is therefore not performed using electrolytic methods. For screws with a strength above 8.8, a special procedure is used that includes stress-relieving treatment before surface treatment and hydrogen de-embrittlement after surface treatment.
To further improve corrosion protection and prevent the formation of white spots (white rust) on the zinc in damp environments, the zinc-plated components are treated with passivation. Passivation has replaced chromating due to health and environmental aspects.
