The A307 specification can be manufactured with various types of coatings. The two most common are hot-dip galvanizing and zinc plating.
Hot-dip galvanizing is a process where the fasteners are dipped in a bath of molten zinc for a designated amount of time to allow the zinc to metallurgically bond with the steel. The specification covering hot-dip galvanizing of A307 fasteners is ASTM F2329. The process to galvanize bolts is thorough and requires many steps.
Preparation & Cleaning
- The bolts are lowered into a caustic soda solution for a designated amount of time to remove any oils from the cutting process and other organic materials that may be on the bolts before galvanizing. The bolts are then quickly rinsed in water.
- After the caustic soda solution, the bolts are placed in sulfuric acid to remove scale and score the surface of the steel which will allow the zinc to penetrate the surface layer of the bolt. The bolts are then rinsed with water a second time.
- The final stage of preparation involves a flux solution in which the fasteners are submerged to facilitate the bonding of the zinc to the steel.
- The galvanizing process requires the bolts to be completely submerged in molten zinc for a designated amount of time depending on the diameter of the material, the grade of steel, and a variety of other factors. The time in which the fasteners are submerged will allow the zinc to metallurgically bond with the steel.
- The fasteners are lifted out of the molten zinc while they are still hot and spun in a centrifuge-like spinner to remove any excess zinc from the bolt and the threads. This is key to ensure the nut fit and is considered the highest quality zinc coating for threaded fasteners.
Zinc plating is a process where zinc is applied by using a current of electricity. The specification covering zinc-plated fasteners is F1941. This coating is much thinner than hot-dip galvanizing and is not usually recommended for outdoor applications. It is typically used for aesthetics since the bolts are bright and uniform in color.