Study on Factors Affecting Cracking of Electroplated Zinc Bolts

The thickness of the coating bolt is 1424Lm, the surface of the large number of infiltrated coatings is darker, and some places even become purple-red. These phenomena indicate that the coating is impure. Analysis of the poor quality of the coating is almost impossible to observe the complete coating: from the energy spectrum test results, there is basically no pure galvanized layer but zinc-iron alloy, zinc hydrogen absorption is about 0.1001%-0.11%, iron The hydrogen absorption of the group metal is about 0.11%. Since the galvanized layer contains a large amount of iron, the amount of hydrogen absorption of the plating layer is large, which accelerates the diffusion of hydrogen into the metal matrix. Hydrogen permeates into the metal lattice to distort the lattice, creating large internal stresses and reducing the toughness of the metal matrix. Impurities such as silicon, sulfur, and calcium in the plating layer make the plating layer high in hardness and brittle. The microscopic defects of the bolt formed by the heat treatment exacerbate the hydrogen permeation process of the bolt.

When the work is subjected to an external force, the crack expands along a region where the amount of hydrogen permeated (such as grain boundaries, defects, etc.), and forms brittleness and intergranular fracture. In short: its fracture is a short-cycle fatigue fracture. There is a crack between the coating and the substrate, indicating that hydrogen permeation has occurred during pickling before electrolysis and when the cathode is de-oiled. The bolts are not thoroughly cleaned when they are out of the groove; the coating is rough and impure, indicating that there are too many metal impurities in the electrolyte, especially due to excessive iron content. Black streaks in the coating are also caused by improper handling. In order to avoid the occurrence of hydrogen embrittlement, the last process of electroplating zinc is to remove the hydrogen, and the hydrogen brittle fracture of the bolt is broken, indicating that the hydrogen removal treatment is not complete. It can be seen that there is a big problem in the galvanizing process.

The influence of machining factors has many cracks on the surface of the bolt, and most of them are along the direction of the furrow generated by machining. Machining defects form a crack source, which is one of the main reasons for reducing the quality of the coating and the strength of the bolt. The reason for the bolt breakage is the hydrogen permeation phenomenon caused by the pickling of the bolt before electrolysis, the electric discharge of the cathode, and the electroplating process. In particular, the surface defects of the bolt formed by the heat treatment and machining process catalyze the progress of the hydrogen permeation process and aggravate the stress concentration. In the subsequent dehydrogenation treatment, the partial hydrogen content of the bolt is too large, resulting in sudden hydrogen embrittlement fracture during operation. The galvanized layer is impure, the structure is not uniform, and the tempering is insufficient, which also accelerates the breakage of the bolt.