Zinc plating is the most common of the electrolytic deposits of steel. The coating made with this method results in a moderate corrosion protection and pleasant appearance with low cost. The coating is also frequently used under painting to improve the corrosion properties and paint adherence.
Pinnoitus Helin’s zinc coating solutions:
- We use cyanide-free alkaline zinc bath
- Barrel and rack
- Maximum dimensions of the object to be zinc coated (mm): 2200x1000x400
Passivations:
- blue passivation, Cr3+
- black passivation, Cr3+
- thick-film passivation, Cr3+
- post dip (sealer)
- all RoHS compliant
Standards:
- Electroplated coatings of zinc with supplementary treatments on iron or steel (ISO 2081:2008)
- Fasteners — Electroplated coatings (ISO 4042:1999)
- Corrosion tests in artificial atmospheres. Salt spray tests (ISO 9227:2006)
Zinc, as a less precious metal, gives cathodic protection to steel; therefore small scratches in the coating do not cause immediate rusting of the steel. In modelling the corrosion durability, the coating is given a time limit it has to keep the object free of red rust. The most common model is neutral salt spray test according to standard ISO 9227:2006.
Most commonly the electroplated coating is passivated to slow down zinc oxidation. Passivation protects zinc from white rust and zinc itself protects the base material from red rust. Nowadays, especially passivations conforming to new RoHS and ELV directives are often protected with post dip (sealer) that results remarkable improvements in corrosion values. The durability of the coating against white rust is defined in the final test according to how long the passivation (and post dip) last in the salt spray. The drawings can include the required durability, e.g. 200/400 h, against white rust and red rust.
These days almost all bath types in use are either acid or alkaline. The appearance of acid zinc is shinier than that of alkaline zinc. On the other hand, the alkaline bath is more capable of spreading the zinc coating in different parts of the object than the acidic bath. Cyanide bath has disappeared except in few exceptions.