Coatings are applied to bolting to protect it from corrosive environments or to achieve/enhance desired properties, such as lubricity for consistent torqueing. There are different types of coatings that are used depending on the type of protection and duration of protection required.Coating Types:
Following Hierarchical Chart lists some of the more commonly used plating's and auxiliary finishes and their more useful engineering properties and limitations.
Metallic coatings are applied to fasteners for corrosion resistance, decorative purposes, and extended shelflife. Metallic coatings are primarily sacrificial in nature. Common electrodeposited metallic coatings for use on fasteners include zinc, cadmium, nickel-cobalt, and zinc-nickel. Corrosion resistance is dependent on the corrosion rate of the given coating system and the thickness of the coating. Hot dipped galvanizing is also a fastener coating, but is used less because of interference issues that arise due to inconsistent coating thickness and tight tolerances. Cermet coatings, typically aluminium filled, are used extensively as base coats, but require topcoats (typically fluoropolymer) for lubricity required for uniform torque. Cermet coatings could cause thread interference, and this should be considered ( reference API Specification 20E for more guidance). The electroplating process varies with regard to types and desired results. Most plating processes generate hydrogen. Post-baking after plating is used to diffuse internal hydrogen. If the post-bake temperature is not high enough, the hydrogen does not diffuse. If the post-bake temperature is too high, the plating can be compromised. If plating temperature exceeds the optimised operating limits of the bath or if improper temperature control during post-bake occurs, it can lead to premature bolt failures due to hydrogen embrittlement and loss of corrosion protection.The following precautions should be noted:
Nonmetallic coatings for fasteners typically are fluoropolymers offering a combination of corrosion resistance, wear resistance, and lubricity. They can be applied over phosphate-etched substrate where lubricity is required, or as a topcoat to zinc, cadmium, or zinc-nickel. In some instances, nonmetallic coatings can introduce clearance issues due to the thickness of the coating system.Some examples of nonmetallic coatings are: