1.What effect does low carbon steel have on the surface condition of steel?
The structure is mainly ferrite, with uniform composition and low surface oxidation tendency. It is not easy to form thick oxide scale at room temperature. Even during heating, the oxidation products are mostly loose FeO, which is easy to remove during pretreatment, and a clean and uniform substrate surface can be obtained, providing a good foundation for the adhesion of the galvanized layer.
2.What effect does high carbon steel have on the surface condition of steel?
The proportion of pearlite in the structure is high, and carbide segregation may exist. At high temperatures, carbon reacts more easily with iron and oxygen to form a dense and hard composite oxide scale, which is difficult to completely remove during pickling. If the oxide scale remains, it will cause an "isolation layer" to form between the zinc layer and the substrate, directly causing blistering and shedding of the coating.
3.What effect does carbon content have on the iron-zinc alloy layer?
During hot-dip galvanizing, after the molten zinc liquid contacts the steel matrix, Fe atoms diffuse into the zinc liquid, and Zn atoms diffuse into the matrix at the same time to form an alloy layer.
In low-carbon steel, Fe atoms diffuse evenly and at a stable rate, the alloy layer grows in an orderly manner, with a moderate thickness, and is tightly combined with the surface pure zinc layer.
In high-carbon steel, carbon exists in the form of cementite at the grain boundary or in the matrix. The Fe atoms in Fe₃C have low diffusion activity, and carbon is adsorbed on the Fe-Zn interface, hindering the mutual diffusion of Fe and Zn atoms, resulting in uneven growth of the alloy layer: some areas are too thin due to diffusion obstruction, and some areas are overgrown due to low local carbon concentration, forming "coarse grains" or "nodular" defects, and even plating stratification.
4.What special effects does carbon content have on electrogalvanizing?
Electroplating zinc relies on electrode reactions. Too high a carbon content will reduce the conductivity of the steel, resulting in uneven current distribution and increased differences in coating thickness. In addition, carbides on the surface of high-carbon steel may become "inert points", hindering the reduction and deposition of zinc ions, forming pinholes or exposed bottoms.
5.What effect does carbon content have on coating performance?
Adhesion:
Low carbon steel has uniform structure, no obvious carbide segregation on the surface, the zinc layer can form a "metallurgical bond" with the substrate, and the adhesion can reach 50-100MPa.
High carbon steel has uneven distribution of carbides, and there are microscopic "hard spots" or stress concentration areas on the surface of the substrate. It is difficult for the zinc layer to wrap evenly, the adhesion is reduced, and it is easy to "peel" after hot and cold cycles or mechanical impact.
Corrosion resistance:
The corrosion resistance of the zinc layer depends on its integrity. After high carbon steel is galvanized, if there are defects in the alloy layer or poor adhesion of the coating, a "corrosion channel" will be formed, causing the substrate to rust prematurely. In addition, high carbon steel itself has poor corrosion resistance. Once the zinc layer is damaged, the rust rate will be faster than that of low carbon steel.
Appearance quality:
The surface of low carbon steel is smooth and uniform in color after galvanizing. High carbon steel is prone to "spotting" and "blackening" due to difficult pretreatment or uneven reaction, and additional post-treatment is required to cover up defects, increasing costs.