A449 Bolt coatings

The A449 specification can be manufactured with numerous coatings. The two most common coatings are hot-dip galvanizing and zinc plating. 

Hot-dip galvanizing is a process where fasteners are placed in a bath of molten zinc for a designated amount of time in order to allow the zinc to metallurgically bond with the steel. The specification covering hot-dip galvanizing of A449 fasteners is ASTM F2329. The process of galvanizing bolts is extensive and requires many operations. 

Hot-Dip Galvanizing

Preparation & Cleaning

  1. The bolts are dipped into a caustic soda solution for a certain amount of time to eliminate any oils from the previous manufacturing processes and any other organic materials that may be on the bolts prior to galvanizing. The bolts are then briefly rinsed in water.
  2. After the caustic soda solution, the bolts are lowered into a sulfuric acid bath to remove scale and etch the surface of the steel which allows the zinc to penetrate the outer layer of the bolt. The bolts are then rinsed again with water.
  3. The final stage of preparation & cleaning requires a flux solution in which the fasteners are submerged to enable the bonding of the zinc to the steel.

The Galvanizing Process

  1. The galvanizing process requires the fasteners to be fully submerged in molten zinc for a designated amount of time. This time is determined by the diameter of the material, the type of steel, along with a variety of other factors. The time that the fasteners are submerged will allow the zinc to metallurgically bond with the steel.
  2. The bolts are lifted out of the molten zinc and while they are still hot, they are spun in a centrifuge-like spinner to remove any excess zinc from the bolt and the threads. This is key to ensuring the nut fit and is the highest quality zinc coating for threaded fasteners available.
Bolts make the journey from plain finish to hot-dip galvanized at Portland Bolt & Manufacturing.

Zinc Plating

Zinc plating is a process where zinc is applied to the surface of the fastener by using a current of electricity. ASTM F1941 is the specification covering zinc plating for fasteners. This coating is much thinner than hot-dip galvanizing and is not recommended for outdoor applications. It is most commonly used for aesthetic purposes since the bolts are bright and uniform in color.