Fasteners Coating & Surface Treatments

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:
  • Sacrificial Coating
  • Barrier Coating
The following are types of protection durations:
  • Short-term protection: Typically, thin electro deposited coatings provide enhanced shelf life, cosmetics, and limited corrosion protection. Short-term protection is generally used for storage purposes.
  • Long-term protection: Protection generated by sacrificial, barrier, or coating systems intended to last for an extended period (typically greater than four months).

Following Hierarchical Chart lists some of the more commonly used plating's and auxiliary finishes and their more useful engineering properties and limitations.

  1. Fastener Coatings & Surface Treatments
    1. TOP COATS
      1. ASTM
      1. ASTM
      2. ISO

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:
  • a) The plating material is usually the controlling factor for maximum service temperature.
  • b) Hydrogen embrittlement is possible with most common methods of plating, unless special manufacturing procedures are used. For example, ASTM B850 and ASTM F519.
  • c) The use of dissimilar materials can create galvanic corrosion issues.
  • d) Improper installation may contribute to stress relaxation and fatigue issues.

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:
  • Corrosion protection: Fluoropolymer coatings offer corrosion resistance where the film is not damaged or breached. However, they can be subject to extensive damage during installation (mechanical or UV exposure), thus exposing the substrate to corrosion.
  • Lubricity: Some fluoropolymer fastener coatings are used primarily as a dry film lubricant, offering consistent torque values. Typically, no additional lubricants, greases, or otherwise are required with fluoropolymer coatings due to the PTFE content.
  • Marine epoxy topcoats can be applied to fasteners after assembly for additional protection (specifically for offshore environments), but are not considered fastener class coatings due to limited corrosion resistance and thicknesses that may impair fit and function.