10th National Pressure Vessel Academic Conference Paper ∣ Is the welded joint of galvanized steel sheet still resistant to corrosion?

The galvanized sheet is coated with a layer of zinc on the base steel plate, and the galvanized layer can effectively prevent the steel base from being corroded by the external environment. According to statistics, about 50% of the world's zinc production is used for galvanizing the surface of steel.

Anti-corrosion mechanism of galvanized sheet:
1. The galvanized layer has a physical shielding effect. The zinc layer exposed to the air will react with O2, H2O and CO2 in the atmosphere to form a dense oxide film, which acts as a barrier to isolate the steel from the external environment and protects it The role of inner steel.
2. In addition, zinc also has the function of cathodic protection, which plays the role of electrochemical protection on the base steel. When the surface galvanized layer is damaged, the galvanized layer and the substrate are exposed to a corrosive environment together to form a galvanic cell. The zinc acts as an anode to be oxidized, and the substrate steel is protected as a cathode. At the same time, the corroded zinc can generate dense oxidation to reduce the corrosion rate.
Because of its good corrosion resistance, galvanized sheet is widely used in the field of chemical process equipment as a connecting piece of equipment such as pressure vessels and pipelines.
However, it is precisely because of the existence of the galvanized layer, which brings great challenges to the welding process. After welding, the zinc layer at the welded joint of the galvanized sheet is damaged and a layer of black material will appear on the surface. The zinc layer around the weld is damaged to a certain extent. burn.

The literature conducted a comparative test of electrochemical corrosion on the base metal of galvanized sheet, the blackened area and the non-blackened area of welded joints, and found that welding has a certain influence on the corrosion resistance of galvanized sheet materials, and the blackened area of the joint will decrease. The corrosion resistance of the material, and the non-blackened area can improve the corrosion resistance of the material. After welding, the zinc layer at the joint is burnt, and the oxide film formed on the surface can slow down the corrosion reaction rate to a certain extent and delay the corrosion of the substrate.
The corrosion of the base metal of the galvanized sheet and the welded joint starts from the zinc layer. The zinc layer of the joint sample near the weld toe has a serious corrosion phenomenon. This is because the chemical properties of zinc are more active than iron, and the zinc at the weld toe is more active. The layer is damaged and more prone to corrosion. When the zinc layer is completely corroded, the welded joint does not begin to corrode. During the electrochemical corrosion process, the zinc layer protects the joint and the base carbon steel, and the morphology of the zinc layer after corrosion becomes rough and loose.