Why should galvanized fasteners not be used in compressed components?

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Section 6.6.7 of the current standard HG/T 20581-2011 \"Specifications for the Selection of Materials for Steel Chemical Process Vessels\" stipulates that bolts and studs coated with zinc or cadmium shall not be used in pressure-bearing components. The same provision is also included in 5.6.7 of the revised draft of HG20581. Electroplating surface treatment has become a commonly used method for fasteners due to its mature technology and controllable costs, including processes such as galvanizing, copper plating, zinc-nickel plating, zinc-iron plating, and chromium plating. However, fasteners with surface plating (especially high-strength fasteners) are at risk of hydrogen embrittlement. This is because, during cleaning prior to electroplating, acid washing is typically used to remove rust and scale, and hydrogen is generated during the acid washing process. Hydrogen is also produced during cathodic degreasing. During the electroplating process, hydrogen penetrates into the metal of the part serving as the cathode, along with the formation of the plating layer. Furthermore, for materials with different substrates, the degree of cathodic hydrogen absorption varies, such as Pd > Ti > Cr > Mn, etc. It can be seen that the hydrogen embrittlement problem of high-strength fasteners in electroplating mainly originates from within. Due to hydrogen’s high diffusion rate and its ability to pass through various tiny gaps, newly formed hydrogen atoms can penetrate into the pores of metal materials, resulting in significant internal stress. In many materials, especially those of high strength, stress generated by processing and welding, along with the presence of microcracks in the metal matrix itself, can lead to failure of the parts under external pressures when hydrogen is introduced. The high pressure caused by hydrogen can result in quality failures. Regarding cadmium plating, it should also be noted that the cadmium-plated layer has good corrosion resistance, especially in marine atmospheric environments, where its corrosion resistance is superior to that of other surface treatment methods. However, cadmium plating can cause environmental pollution, which leads to high costs associated with the treatment of waste liquids during the processing process. Therefore, it is not used in general industries, but only in certain specific environments, such as fasteners for oil drilling platforms and space shuttles.