What is hot-dip galvanizing?
Hot-dip galvanizing is a process in which a corrosion-resistant zinc coating is bonded to the surface of anchor bolts that will be exposed to moisture. After undergoing a thorough cleaning and preparation process, anchor bolts are “dipped” in a bath of 840° F molten zinc. While the bolts are submerged, the zinc bonds to the steel creating a cost-effective, corrosion-resistant coating that will provide dozens of years of service life for the anchor bolts. The process itself is covered under ASTM specification F2329 and adds between 2 and 6 mils of zinc to the surface of the anchor bolts. Contrary to popular belief, threads of galvanized anchor bolts are not cut undersize; they are threaded the same as bolts that do not undergo the galvanizing process. Instead, galvanized nuts are tapped oversize so they fit on the threads of the galvanized anchor bolts.
Galvanizing Threaded Parts
The challenging aspect of galvanizing anchor bolts is avoiding excess zinc in the threaded portion of the anchor bolt. As the racks or baskets of anchor bolts are removed from the galvanizing tank, the zinc begins to rapidly solidify. While the zinc is still in a liquid state, a specialized centrifuge “spins” the excess zinc out of the threads. “Chasing” the threads of an anchor bolt (rethreading after galvanizing) is a common practice used by bolt manufacturers and galvanizing companies that don’t have the specialized systems necessary to process threaded parts, and is prohibited under specification F2329.
Environmental Impact of Hot-Dip Galvanizing
Hot-dip galvanizing uses many chemicals and emits fumes that can be harmful to the environment. Some galvanizers will also add lead to their zinc, which makes it easier to galvanize but is a toxic heavy metal. Consider buying galvanized anchor bolts from a company that is environmentally conscious. These galvanizers will “scrub” the fumes that come off the chemical preparation tanks before emitting the air back into the atmosphere and choose not to add lead to their zinc. Environmentally conscious galvanizing companies will also recycle zinc ash and dross and dispose of chemicals in an environmentally friendly manner, and will not dump any water into the public sewer system.
Hot-Dip Galvanizing Process
The following steps are involved in properly preparing and galvanizing anchor bolts to meet the requirements of ASTM specification F2329.
Wheelabrating is similar to sandblasting. In this process, steel shot is fired at the anchor bolts as they are tumbled. This process removes scale on the surface of the steel as a result of rolling the round bar, heat treating, and hot forging the head. Zinc will not adhere to bolts with excess scale. Some anchor bolts without heavy scale skip this preparation stage.
Anchor bolts are submerged in large tanks of caustic soda that removes cutting oil and other organic contaminants that accumulate during the manufacturing process. Anchor bolts are rinsed with water after being removed from the caustic soda.
Anchor bolts are then submerged in large tanks of sulfuric acid that remove light mill scale from anchor bolts that have not been wheelabrated and etches the surface of the steel. Anchor bolts are rinsed with water after being removed from the sulfuric acid.
Anchor bolts are submerged in large tanks of flux which is a chemical that helps bond the zinc to the steel. This is the final preparation process prior to galvanizing.
Anchor bolts, depending on their size and configuration, are placed in baskets or racks and submerged in molten zinc for 2 – 4 minutes. During this time, the zinc bonds to the anchor bolts, creating a strong, corrosion-resistant coating. Upon being removed from the galvanizing tank, the anchor bolts are spun to remove the excess zinc. The galvanized anchor bolts are quenched in water to cool them so they can be immediately handled, inspected, and packaged.
Want to learn more?
Check out Portland Bolt’s YouTube channel for videos of bolt manufacturing and the hot-dip galvanizing process.