Galvanized steel coils are mainly produced by the continuous galvanizing process, that is, the rolled steel plates are continuously immersed in a plating bath containing molten zinc to make galvanized steel coils. This kind of steel coil is made by hot-dip method. After coming out of the tank, it is heated to about 500°C immediately to form an alloy film of zinc and iron, which has good paint adhesion and weldability.
Recently, there have been many claims for damage to exported steel coils, and the accident situations are basically similar. I conducted research on this and found that they mainly have the following things in common:
1. Transportation method: container, bulk ship;
2. Packaging method: The inner packaging is wrapped with plastic film and moisture-proof paper; the outer packaging is sheathed with steel plates, and the coils and edges on both sides are wrapped with protective sleeves;
3. Damage situation: If there is no abnormality in the outer packaging, the steel coil contained in it will rust and produce white or black spots.
Can black or white spots on galvanized steel coils be considered a type of rust damage?
There are three types of galvanizing processes: electro-galvanizing, hot-dip galvanizing and coated galvanizing. The more commonly used one is hot-dip galvanizing. White spots on galvanized steel coils are generally formed by the oxidation of the galvanized layer (the oxide of zinc is white), which is rust damage (white rust); black spot defects are composed of densely gathered black spots in the form of flakes. Galvanized steel There are many reasons for dark spots on curls, such as:
1) The purity of the zinc in the galvanized layer is not enough, and metal impurities form electrodes on the zinc surface;
2) Black spots caused by passivation treatment. The galvanized layer contains high impurities such as copper, iron, and arsenic. After passivation treatment, black spots are formed;
3) The insufficient aluminum content in the galvanized layer causes gray-black spots on the iron-zinc alloy without a pure zinc layer on the surface of the galvanized sheet;
4) Abrasion black spots, caused during transportation and stacking.
Among them, abrasion black spots are caused by dislocation between steel layers during transportation. During the transportation process of hot-dip galvanized coils, there are stress concentration areas between the steel coils and the fixed brackets and between the steel coils. When the friction coefficient is certain, when the stress exceeds the bonding force between the outer passivation film and the matrix, an external stress will occur. The passivation film of the first layer is damaged, which in turn causes the inner zinc layer to be exposed. The oxygen content is insufficient in the closed environment, resulting in under-oxidation products, which appear as black spots macroscopically.
During shipping, will the galvanized steel coils be affected by damage to the outer packaging and produce black or white spots?
The function of the zinc coating of galvanized steel coils is to prevent the internal steel coils from being corroded and rusted. In humid air, a dense basic zinc carbonate film will form on the zinc surface, thereby preventing the zinc and steel inside from being further oxidized. Since zinc is easily soluble in acid and alkali, zinc has poor corrosion resistance in gases containing sulfur dioxide, hydrogen sulfide and marine gases, and the galvanized layer is easily corroded. Therefore, damage to the outer packaging during shipping causes the galvanized layer to be directly exposed to marine moisture gas, which can easily lead to corrosion of the galvanized layer and further corrode the internal steel coil.
How to seal-pack galvanized steel coils?
The zinc steel coil should first be wrapped with anti-rust paper and plastic film, install inner and outer peripheral buffer corner protectors, and then install the inner peripheral protective plate (the standard is a direct layer of inner peripheral protective plate, but it is best to add one inside Layer inner peripheral buffer guard plate), then install the outer peripheral buffer material guard plate, then install the outer peripheral guard plate (metal) and the end round guard plate, then install the inner and outer peripheral metal corner protectors, and finally tie them firmly with straps, circumferentially and There are no less than three radial roots.
.png)
.png)
