Corrosion of metal galvanised sheets

Corrosion of galvanized sheets
Corrosion of metal galvanised sheets occurs in three distinct stages
A brief initial stage of forming a surface protective layer
Long-term corrosion stage of zinc layer
Corrosion of steel in exposed areas after the zinc layer has been consumed.

The first corrosion that occurs is the oxidation of the surface galvanized layer, forming "white rust". After a while, the "white rust" on the surface further reacts with impurity gases such as carbon dioxide in the humid air to form "black spots". When galvanized sheets are used for a long time and the corrosion of the galvanized layer becomes serious, the steel base loses the "sacrificial anti-corrosion" effect of zinc and begins to oxidize, forming "red rust". Once the steel base begins to oxidize, the corrosion rate becomes very rapid and the galvanized sheet ends its life. There are two special corrosion conditions of metal galvanised sheets called "blackening".
One is that after the galvanized sheet has been used for a period of time, due to the influence of lead and other elements in the galvanized layer, the matte part of the zinc flower will turn black. The blackened galvanized layer still has a protective effect on the steel base. It just has a certain impact on lifespan. This situation has been greatly improved after the current use of lead was stopped and antimony was used to promote the formation of zinc splatter.

Another situation is that during transportation of galvanised sheet metal, due to the low coiling tension and loose steel strip layers, the parts of the steel coils that contact the pads will rub against each other and cause oxidation. After the white oxides are rubbed off and fall off, the galvanized sheets will become oxidized. Black spots are produced on the surface of the zinc plate, which will damage the passivation film, thin the galvanized layer, greatly reduce the service life of the galvanized plate, and also affect the appearance.

Galvanised steel plate have excellent corrosion resistance in dry environments or pollution-free environments. However, in a polluted environment, the service life will be greatly shortened, so it must be pre-coated and processed into colored plates before use. Generally speaking, the corrosion of galvanised iron sheets does not mean that all the zinc coating on the surface is corroded evenly, but the coating is corroded first in places where the bonding between the coating and the substrate is poor, resulting in severe local corrosion and loss of performance. From this point of view, the adhesion of the galvanized layer, especially the overall adhesion, is more important than the thickness of the galvanized layer. If the local adhesion of the galvanized layer is not good, no matter how thick the galvanized layer is, it will start to rust due to poor adhesion.

Surface treatment of galvanized sheets can improve corrosion resistance. The most commonly used process is passivation treatment. Due to environmental protection policies, the application of chromium passivation treatment is increasingly restricted, and chromium-free passivation treatment has become mainstream. Chromium-free passivation treatment is widely used in anti-corrosion treatment in the field of galvanizing because it can effectively delay corrosion on the zinc surface.