Why are there so many different types of “galvanized” bolts? Not sure which one to ask for or specify? Not all galvanizing is the same. Read on to discover why these finishes are intended for different applications, and to decide which option best fits the needs of your project.
Hot dip galvanized. Hot dip galvanizing (often abbreviated as HDG) is the traditional method of applying zinc coating to bolts for maximum corrosion protection. The process involves immersing the fasteners in a molten bath of zinc until the zinc and steel bond in a metallurgical reaction. Commonly used in outdoor and marine applications, specified by ASTM A153 or F2329. Due to the coating thickness, over-tapped nuts are required for assembly.
Mechanically Galvanized. Mechanical galvanizing is a process of applying a dry zinc powder to fasteners, which results in a much more consistent and smooth finish, albeit not metallurgically bonded to the steel itself like hot dip galvanizing. Not recommended for applications where corrosion resistance is a major concern. Subject to ASTM B695.
Electro-Galvanized. Zinc plating, often just referred to as “Zinc” or “electrogalvanized”, is a low-cost coating option, and best used for indoor or temporary applications. Zinc is applied in a liquid bath charged with DC current. The plating can be applied in a variety of colors, most often in a silver color. Subject to ASTM F1941, zinc plated bolts do not require oversized nuts.
Zinc/Aluminum coating. A newer addition to the fastener industry, Magni 565 is a chrome-free duplex coating that combines an inorganic zinc-rich basecoat with an aluminum-rich organic topcoat. It can be applied using a dip-spin or spray application method and is rated for over 1,000 hours of salt spray. Allfasteners stocks A490 structural hex bolts with this coating, which is not subject to hydrogen embrittlement concerns like hot dip galvanizing. Specified by ASTM F2833, Magni coated bolts do require oversized nuts.
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