A coating is a treatment process, wherein the surface of components including - fasteners get treated. This surface treatment could be either mechanically or chemically. The techniques used to deposit a coating on the surface of a Fastener includes dipping, electroplating, and spraying.
Some specifications cover surface finishes or coating, for example, SAE J 1648-2019, SAE AMS 2506E-2018, ASTM F1137/F1137M-19, and ISO 4042:2018.
Generally, ceramic, zinc flake, and Xylan or PTFE treated fasteners are coated by spraying. Any conventional air spraying method covers the surface of these screws, followed by a curing process. The follow up curing process sets in the coat and increases the endurance of the finish.
The process to electroplate a fastener starts with its cleaning its surface in an alkaline detergent solution. After cleansing its surface, the screws get treated with an acid. The acid removes any rust or scales present on the surface. Thorough sterility is a prerequisite for this process as the molecular layers of oil or rust can prevent adhesion of any coating to the metal surface. While the main objective of applying a coat on the fastener, is its protection; there are instances during cleaning or pre-cleaning, wherein there is the absorption of some hydrogen content. If these hydrogen absorbed screws are a part of a sour-well environment, there is a risk of the fastener becoming susceptible to Hydrogen embrittlement; And the Fastener is rendered useless. Fasteners that belong to strength classes 10.9 and above, as well as USA grade 8, are most likely to witness hydrogen embrittlement. To prevent this, the coated undergoes baking, although complete elimination of Hydrogen embrittlement in screws is not a guarantee.
Electroplating is an action performed via the process of electrodeposition. During this process, screws immersed in a chemical bath. This chemical bath contains dissolved zinc. When direct current or DC, is applied, the zinc metal present at the anode starts to dissolve. The free zinc metal ions reach the cathode, leading to the formation of a thin layer of zinc coating on the screw. The thickness of the zinc plating layer will depend upon the time spent in the chemical plating bath, the voltage of the electric current, as well as the bath's chemical composition.
Some of the more common surface treatments include electroplating and galvanizing. There are several benefits to coating a fastener. Primary amongst them is resistance to corrosion. Alloys like stainless steel, carbon steel, alloy steel, aluminum, copper, etc. are susceptible to corrosion in certain conditions. For instance, oxidation corrodes stainless steel and alloy steel grades where the content of chromium is less than 10%. Galvanization protects several alloys against oxidation. During galvanization, the fastener gets coated with a thick layer of zinc. This thick layer prevents it from losing its corrosion resistance properties as the surface remains coated.
Using a coat could also increase thread lubrication of a fastener, which in turn, aids in the easy assembly of an application. Some applications require coated bolts, solely for the aesthetics it provides. In such cases, administering a copper plating on their surface gives its a pinkish-orange tone as the metal copper. Nickel fittings are electroplated using copper to improve its conductivity properties. Another aesthetically pleasing coat is Black Japanning. Well suited for decorative applications and furniture, wood screws dipped in black enamel. Cadmium electroplating produces a bright or a dull silver-grey surface, with excellent corrosion resistance properties. But using Cadmium to coat fasteners is not a regular practice, because Cadmium has a high level of toxicity for the environment while being an expensive coating technique. Chromium, on the other hand, produces a bright blue or white finish, while offering superior wear and corrosion resistance. The downside of this hard and lustrous chromium finish is that it is expensive. Molybdenum Disulfide is a finish applied to fasteners as a protection against abrasion for high-pressure loads.
Screws coated with phosphate have a dull grey appearance. While the layer used on fasteners is thin, the protection offered by them is lower as compared to zinc plated ones in mild environments. Phosphate conversion provides an excellent base for both lubrication and painting. Therefore, the automotive industry uses this coat either plain or in conjunction with an oil coat.