There are a number of ways to coat fasteners with zinc
There are a number of ways to coat fasteners with zinc. The most common method is electroplating. It offers a thinner coating and better finish than hot-dip galvanized or mechanical plating techniques.
It also reduces the risk of hydrogen embrittlement by eliminating the chemical pickling step and by allowing for a shorter baking cycle to minimize the potential for stress relief. Typically, yellow chromate is added to protect against white spots of corrosion when exposed to moisture.
Zinc offers a high level of corrosion resistance and is the perfect coating to protect steel fasteners in harsh environments. It acts as a sacrificial coating, preferentially corroding itself rather than the metal underneath. This allows the underlying steel to remain structurally sound.
Various commercial zinc plating processes exist, including electro plating and galvanizing. Hot-dipped galvanized (HDG) fasteners have a thicker zinc coating and are the most suitable for outdoor environments where they will be exposed to excessive moisture and salt.
In addition to the standard zinc coating, some specialized fasteners are also coated with yellow chromate to further reduce the possibility of hydrogen embrittlement and increase their durability. Examples include split lock washers and two-way reversible center-lock nuts. While they require a baking process like standard zinc, this step can be minimized to significantly shorten the time required for the fasteners to become brittle and fail in service. This method is particularly effective for through-hardened parts such as studs and bolts.
There are several ways that zinc can be applied to fasteners for corrosion resistance, including through electro plating. This process involves immersing the fastener in a chemical bath containing dissolved zinc metal and applying direct current to create an electrical charge that deposits free metal ions onto the workpiece.
The resulting coating is thin, but still offers good corrosion protection in harsh environments. It also has moderate heat resistance and a high level of hardness.
A clear or bluish chromate finish can be added to the zinc layer for additional resilience against white oxidation spots, which can occur when moisture is present. The chromate passivates the zinc, which reduces the risk of hydrogen embrittlement, an issue that can occur with metallic coatings that contain cadmium, and it provides an alternative to using toxic hexavalent chromium in some applications.
Hot dip galvanizing is another method for applying a thicker layer of zinc to a fastener’s surface, which helps protect it from corrosion in harsh environments. Like electroplating, it requires a preventative bake to prevent hydrogen embrittlement for eight hours.
Zinc plated fasteners have an attractive finish and can be used in indoor environments where corrosion isn't a significant concern. A zinc coating acts as a sacrificial layer, protecting the iron and steel beneath it even if the coating becomes damaged by punctures or scratches.
Many different methods are used to apply zinc coatings to metals including electro plating, mechanical galvanizing, and thermal diffusion galvanizing. Each method has its own unique benefits and drawbacks.
For example, mechanical galvanizing uses a dip spin process that eliminates the need for baking and reduces the risk of hydrogen embrittlement in high strength fasteners. Similarly, electro plating uses electrical current to deposit a thin coating of zinc over the fastener. This type of application also allows for thinner coatings, making it suitable for smaller fasteners and use in interior or low intensity external environments. A clear chromate finish can then be applied to the fastener to protect it from white oxidation spots caused by moisture.
Many metal architectural structures are designed to be both functional and aesthetically pleasing. Zinc plating is an easy way to achieve both of these goals. The zinc coating protects the underlying steel or iron components from corrosion, while also adding a beauty that complements the design of most fasteners and bolts.
There are a variety of finishes and coatings for fasteners, including hot-dip galvanized, cadmium, and hexchrome chromate. However, the best choice for most applications is yellow plated zinc. This type of finish provides a high level of corrosion protection, plus it’s RoHS compliant.
The plating process involves electro deposition, or submerging the fastener in a solution of zinc and copper, then applying a direct current to create a thin layer of zinc on the surface of the metal. The resulting coat is more uniform than hot-dip galvanized and less likely to skew the fine tolerances of mechanical fasteners. Plating can also include a phosphate coating, which reduces friction and increases corrosion resistance.