The corrosion resistance of hot-dip galvanized bolts depends not only on high-quality zinc raw materials but also on the zinc coating thickness on the bolt surface, which is a key factor. So, is there a relevant national standard regulating the zinc coating thickness of hot-dip galvanized bolts? What is its implementation standard?
First, it should be clarified: the thickness of the conventional electrogalvanized coating for bolts is about 7 microns, with a certain positive and negative tolerance allowed for different specifications. The maximum standard thickness in the industry usually does not exceed 15 microns. Excessively thick coatings can easily cause thread adhesion or coating peeling, making effective electroplating impossible. For hot-dip galvanized bolts, the zinc coating thickness has clear national standard basis (such as GB/T 30790 "Technical Requirements and Test Methods for Hot-Dip Galvanized Coatings on Steel and Iron Products"). Long-term practical verification shows that when the hot-dip galvanized coating thickness is less than 40 microns, the corrosion resistance will decrease significantly. Therefore, 40 microns is the minimum thickness standard for the zinc coating. The maximum thickness needs to be matched according to the bolt specifications and types, and there are differences in the thickness implementation standards between hot-dip galvanized hex head bolts and hex socket head cap screws.
Hot-Dip Galvanized Hex Head Bolts
The hot-dip galvanized thickness of hex head bolts needs to be adjusted according to the thread specifications:
Starting from the M8 specification (the standard thread pitch of M8 is 1.25 millimeters), due to the small thread gap, the zinc coating thickness should be controlled at around 50 microns. Excessively thick coatings will fill the thread gap, leading to assembly difficulties;
For hex head bolts of M20 and above specifications, the hot-dip galvanized thickness can reach more than 70 microns, with a maximum of 120 microns, which can not only ensure corrosion resistance but also not affect the thread fit accuracy.
Hot-Dip Galvanized Hex Socket Head Cap Screws
The zinc coating thickness standard for hex socket head cap screws needs to be appropriately lowered. The core reason is that they adopt a hole-type installation structure, and excessively thick coatings will prevent the installation tool from fitting in:
For M8 hex socket head cap screws, the zinc coating thickness is controlled between 40-50 microns, and in principle, shall not exceed 50 microns;
For hex socket head cap screws of M20 and above specifications, the zinc coating thickness can be controlled at 60-70 microns. While meeting the corrosion resistance requirements, it ensures the smooth embedding of the installation tool.
For hot-dip galvanized bolts, in addition to thickness, the density of the zinc coating is the core to ensure adhesion. If the adhesion is poor, when the zinc coating is damaged, the bolt substrate is easily exposed, forming corrosion points, which will affect the corrosion resistance stability of hot-dip galvanized bolts. Hot-dip galvanized bolts are convenient to produce and can be shipped in large quantities, meeting users' bulk procurement needs without delayed delivery. However, due to the relatively simple process of hot-dip galvanized bolts, there has been a problem of low surface finish. Some bolts may have reticulated spots. This phenomenon is mainly caused by the difference in pickling time control of bolt materials (pickling time directly affects the uniformity of zinc liquid adhesion), and higher temperatures may also make the bolt surface appear gray. However, such surface phenomena will not affect the rust resistance. Therefore, users of hot-dip galvanized bolts do not need to overemphasize the surface finish. As long as there is no white delamination on the surface of hot-dip galvanized bolts, there is no quality problem and they can be used with confidence.
