Galvanized coatings typically contain a significant amount of iron, which increases the hydrogen absorption capacity of the coating. This leads to faster hydrogen diffusion into the metal substrate. Once hydrogen enters the metal lattice, it causes lattice distortion, generating internal stresses and reducing the material's toughness. Additionally, impurities such as silicon, sulfur, and calcium present in the coating increase its hardness and brittleness. Microscopic defects introduced during heat treatment further promote hydrogen penetration. The fracture patterns observed include brittle fractures along grain boundaries, ductile fractures, and flattened regions. These features suggest that when microscopic flaws are present, they act as initiation sites for cracks. Under external stress, these cracks propagate through areas where hydrogen has accumulated—such as grain boundaries or existing defects—leading to brittle and intergranular failure. In summary, the fracture is characteristic of a short-cycle fatigue failure. A crack between the coating and the substrate indicates that hydrogen permeation occurred during pickling before electroplating and during cathodic degreasing.
When bolts are removed from the plating bath, if they are not thoroughly cleaned, the coating may appear rough and impure, suggesting excessive metal impurities in the electrolyte, especially high levels of iron. Black streaks found in the coating of explosion-proof oil-cooled electric drums are often the result of improper plating techniques. To prevent hydrogen embrittlement, the final step in the zinc electroplating process should be hydrogen removal. However, if the bolt still fractures due to hydrogen embrittlement, it means that the hydrogen removal was incomplete. This highlights serious issues in the galvanizing process. Machining factors can also contribute to surface cracks on the bolt, many of which align with the direction of machining grooves. These machining defects serve as crack initiation points, significantly affecting both the quality of the coating and the mechanical strength of the bolt. The primary cause of bolt failure is hydrogen permeation that occurs during pre-electrolysis pickling, cathode discharge, and the electroplating process itself. Specifically, surface defects caused by heat treatment and machining accelerate hydrogen diffusion, increasing stress concentration. During subsequent dehydrogenation treatment, if residual hydrogen levels remain too high, sudden embrittlement and fracture can occur during operation. Additionally, an impure and non-uniform galvanized layer, combined with insufficient tempering, further contributes to the bolt's failure.
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