What are the corrosion characteristics of galvanized sheet?

There are two special corrosion conditions of galvanized sheet called black stain. One is that after being used for a period of time, due to the influence of lead and other elements in the zinc coating, the dull part of the zinc flower will be blackened. The blackened zinc coating still has a protective effect on the steel base, but has a certain impact on the service life. This situation has been greatly improved after the use of antimony instead of lead to promote the formation of zinc flowers.

In addition, during the transportation of the galvanized sheet, due to the small coiling tension and the looseness between the steel strip layers, the parts of the steel coil contacting the skid will rub against each other to produce oxidation. After the white oxide is rubbed off, the surface of the galvanized sheet will produce black spots. This situation will damage the passive film, reduce the thickness of the galvanized layer, greatly reduce the service life of the galvanized sheet, and also affect the appearance.

During the service life of the galvanized sheet, the oxidation of the galvanized layer on the surface will generate white rust. After a long time, the white rust on the surface further reacts with carbon dioxide and other impurities in the humid air to form black spots. When the galvanized sheet is used for a long time and the galvanized layer is severely corroded, the steel base loses the sacrificial anti-corrosion effect of zinc and begins to oxidize to form red rust. Once the steel base begins to oxidize, the corrosion rate becomes very fast, and the galvanized sheet ends its life.