When a bolt is submerged in the plating tank, the electrolyte seeps into the gaps and is difficult to remove completely. After being left for some time, the electrolyte may drain out and react with the electroplated layer. This interaction can lead to corrosion of the coating, causing damage to the protective film and resulting in dark spots or patterns. This reduces both the coating's protective effect and its overall strength.
During the electro-galvanizing process, hydrogen evolution and hydrogen permeation are inevitable. As the bolt is plated, hydrogen and zinc are deposited together on the cathode. While most of the hydrogen escapes as gas bubbles, a small portion enters the zinc coating in atomic form and diffuses into the metal matrix along grain boundaries, leading to hydrogen permeation. This issue is more pronounced when the plating layer is thin or when there are defects in the coating that allow hydrogen to penetrate more easily.
The plating thickness on the inspected bolt reaches 1424 micrometers. Despite this, it still fails to prevent a significant amount of hydrogen from penetrating the surface, causing deep discoloration and even purple-red areas in some regions. This suggests that the plating is not pure, and the lattice structure is distorted, leading to high internal stresses and reduced toughness in the metal matrix. Impurities such as silicon, sulfur, and calcium in the coating increase its hardness but also make it brittle.
Additionally, microscopic defects introduced during heat treatment further promote hydrogen penetration. These defects contribute to different types of fractures, including brittle cracks along crystal planes, ductile fractures, and flattened regions. This indicates that when a bolt has such flaws, a crack initiation site is created. Under external stress, these cracks propagate through areas where hydrogen has accumulated—such as grain boundaries or other imperfections—leading to embrittlement and intergranular fracture.
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