Question about bolt coating
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I would like to ask the experts here. Recently, I came across a table showing pipeline grades used by foreigners. For ordinary ASTM bolts, three types of coatings are used: Teflon coating, galvanizing, and HGD. What exactly is HGD? What are these three types used for respectively? Thank you very much
HGD refers to hot-dip galvanizing. The three types of coatings—Teflon coating, galvanizing, and hot-dip galvanizing—are used in different environments and for various corrosion protection requirements: 1. Teflon coating: It offers excellent chemical resistance and a low coefficient of friction; it is suitable for environments where resistance to corrosion and minimal friction are required. 2. Zinc plating: Provides protection through electroplating; suitable for dry environments with little rainfall, and helps prevent rusting. 3. Hot-Dip Galvanizing (HGD): Provides a thicker and more corrosion-resistant protective layer than galvanizing; it is suitable for outdoor or humid environments, offering better protection against corrosion. .
HDG or H.D.G stands for hot-dip galvanizing, which is short for hot-dip galvanization； Hot-dip galvanizing involves reacting molten metal with the iron substrate to form an alloy layer, thereby bonding the substrate and the coating together. Hot-dip galvanizing involves first pickling the steel pipe to remove iron oxide from its surface; after pickling, it is cleaned in a tank containing an aqueous solution of ammonium chloride or zinc chloride, or a mixed solution of both, before being placed into the hot-dip plating tank. Hot-dip galvanizing offers advantages such as a uniform coating, strong adhesion, and a long service life.