A coating is essentially a type of surface treatment performed on fasteners. Today, with the advancement in technology, many are coated with modern yet smarter chemicals. They not only improve the corrosion resistance properties but also increases its service life while delivering better and smoother performance.
Traditionally - fasteners would be coated or plated with a layer of zinc. Either in the form of zinc electroplating or zinc coating in different colours. Using a coat of zinc would provide a limited protection level. It, however, is interesting to note that these methods would have a thinly coated layer of zinc on the surface of the Fastener, thereby getting stripped off by simple abrasions that could be caused during either fastening or unfastening or even transportation and storage. Using galvanization rectify this problem. Galvanization is a process that requires dip of products of different grades in zinc. This process ensures that the component receives a coat with a thick layer of zinc. In case, the Fastener was to get abrased during its use - the thick layer of zinc - that surrounds its surface protects the Fastener.
Frequently used in the automotive industry, a Phosphate coat is thin and dull, with a greyish tint on the surface of fasteners. The immersion of Fasteners takes place in a suspension that contains phosphoric acid. Though the chemical bath or solution makes provisions for low-intensity protection as compared to zinc coating in some environments, it proves to outstand itself as a medium for painting and lubrication. However, for applications that are at a high risk of corrosion, the use of a Phosphate coat is highly discouraged. For instance, this finish is not ideal to use in marine applications. Yet, they are a matter of preference for use in dry settings such as vehicles or interiors of buildings, etc.
Cadmium plating, although rarely used - has a high level of toxicity and environmental non-acceptability. It offers slightly better protection if compared to zinc while providing increased lubricity. The aircraft industry is one of the only users of this finish. Cadmium plating is a sacrificial coating for steel because - it readily takes a chromate conversion coating - which, in turn, improves the corrosion resistance. Above all, the platings galvanic features are compatible with aluminum.
Another method of coating is to passivate fasteners by altering the chemical structure of the Fasteners at or just below the surface, to make it more stable while ensuring that it does not react with other elements in an undesirable way. Methods to passivate Fasteners include - anodizing, bluing, or chromium. Passivation essentially is the formation of a thick, natural, self-healing oxide layer that improves the fasteners corrosion resistance and wear resistance, while providing a better adhesion for paint primers.
PTFE coatings (such as Xylan or Teflon) are a type of non-stick, fluoropolymer, high-temperature coatings. PTFE is a specific type of coat which has the highest operating temperature amongst all fluoropolymers at 500°F. In addition to having a low coefficient of friction, it has better chemical resistance as well as abrasion resistance. Due to its strong carbon-fluorine bonds, the coating tends to exhibit non-reactive properties. Thus, making it a viable option for applications where bolting material must hold up against volatile or corrosive chemicals.
Bolting material meant for marine environments are often ceramic coated. Though ceramic coating lacks resistance as stainless steels, they make up for it with their satisfactory resistance to brine or seawater.
Most coatings tend to have some levels of toxicity. Rilsan, an environmentally green product, is from renewable raw material that is - castor oil. Coating with Rislan ensures properties such as high corrosion resistance in the water, wastewater, and saltwater. Moreover, rislan coated bolting material have high chemical resistance along with resistance to abrasion, impact resistance, thermal as well as weathering and chalking.