SURFACE COATINGS FOR FASTENERS
Many different surface coatings for fasteners exist. Most surface coatings consist of a thin metal layer applied to the fastener. Common surface coatings for fasteners therefore need to take into account the thickness of this layer on threaded parts. The clearance between male and female threads, the tolerance class, must be considered. When the coating layer exceeds the tolerance class limits, interference occurs, and the functionality of the thread is compromised.
The coatings that are discussed have been developed in order to protect steel fasteners from corroding quickly or to enhance the appearance of the fasteners. Beside these protective coatings for steel fasteners, there is also information about anti-seize and lubricating coatings for stainless steel fasteners, aluminum fasteners and more.
Surface corrosion of steel is an electrochemical reaction that occurs when steel is exposed to water or humidity. Since every metal has its own internal electrical potential, direct contact with moisture may create a corrosion cell.
To protect the steel from corroding, it is common practice to cover the surface of the steel with a protective layer. Considering the above information, the best protection would be an electrical isolating layer, like paint or plastic. As long as this layer is intact, the steel cannot make contact with the moisture, and thus no corrosion will occur. Polytetrafluoroethylene (PTFE; also known as Teflon®) is a type of plastic surface coating that protects, for example, specialized petrochemical fasteners. However, as these layers are relatively soft and are easily damaged, most surface coatings for fasteners are metals or at least contain a metal.
THE MOST COMMON SURFACE COATINGS TO PROTECT STEEL FASTENERS
- Electro zinc or zinc alloy plating
- Mechanical zinc plating
- Hot dip galvanization
- Zinc flake coatings
- Anti-seize coatings