Threaded Rod 101: Finishes
Welcome to the second installment of our Threaded Rod 101 series. In this post, we will discuss threaded rod finishes and identify which work best for certain applications. Be sure to read our Threaded Rod 101 overview if you missed it.
What types of finishes are typically on threaded rod?
Corrosion protection is the main reason to have a finish or coating on steel products because they will corrode when exposed to the atmosphere. Other benefits include improving the appearance of the fastener and reducing friction. A finish is chosen based on corrosion protection, application conditions such as temperature, the presence of salt water, chemicals, or humidity, and the desired finish appearance. In a matter of minutes, a threaded rod can go from plain to coated by immersion in a specially-engineered chemical solution. Finishes are commonly categorized based on the industry and application. For example, contractors in the construction industry tend to prefer hot-dip galvanized anchor bolts because the iron in the steel metallurgically reacts with the molten zinc to form a tightly-bonded alloy coating that protects the bolt from damage and corrosion.
Before most finishes are applied, the threaded piece must first be cleaned to remove unwanted contaminants and oils that may have accumulated during the manufacturing process. Steel products are typically cleaned by submersion into a bath to remove unwanted contaminants. Once this is complete, the protective coating, or finish, is applied to protect the metal from the atmosphere.
There are several finishes to choose from; the finishes listed below are the most commonly used finishes throughout the threaded rod industry. Don’t forget – application is key in determining which finish would work best for your project!
Plain or uncoated threaded rod is quite common. The end product has no coating or finish; however, it still has a bit of lubrication leftover from the threading process. Plain threaded rod is commonly used in construction sites where the materials are hidden within structures and protected from weather elements.
Clear Zinc Electroplating
The clear zinc or zinc chromate finish is a type of coating used to protect steel. It serves as a corrosion inhibitor, retains electrical conductivity, and is often used for decorative purposes. The plating process includes cleaning, rinsing, and bathing the rod in a zinc solution with an electric current. Although the word “clear” is in name of the finish, the clear coating may have a slight blue tint after this process because of the chemical make-up. Commercial grade zinc plating, which is the most common on fasteners, will be between 2 and 5 microns thick. The benefit of electro-zinc plating is the coating’s consistent and predictable thickness.
Yellow Zinc Electroplating
The process for the yellow zinc or zinc dichromate finish is similar to the clear zinc finish. The only difference between them is the yellow chromate top coat used in the process. This gives the final product a gold tint. The robust look of the yellow chromate is often more appealing in certain industries such as electrical and plumbing, where fittings are exposed.
Hot-dip galvanizing is the process of dipping steel products into a kettle or vat of molten zinc. This finish provides excellent protection against corrosion and rust when used in outdoor applications such as street lights, signs, and on offshore drilling rigs or projects where salt spray is present. Considerations about hole sizes, over-tapping and thread recesses, and dissimilar metals in contact must be taken when using hot-dip galvanized threaded parts.
Black oxide is a unique finish created when metal is immersed in an alkaline aqueous salt solution of around 285°F. The ferrous alloy reacts with the hot oxide bath to give the part a black surface appearance. Black oxide does not produce noxious fumes when welded, which makes it a popular finish in the welding industry. It provides only moderate corrosion resistance, so it is preferred for indoor rather than atmospheric applications.
Fasteners used in extremely corrosive environments such as offshore projects or chemical plants need added protection against corrosion. Fluoropolymer coatings are blends of high-performance resins applied as a dry film lubricant. Some of the more commonly specified brands include Teflon® or Xylan®. They come in a wide variety of colors with the most popular being blue. Salt spray tests have proven this coating provides corrosion resistance for over 4,000 hours. In addition to its superior corrosion resistance, this coating has a slick/smooth application which provides easier assembly in the field and prevents seizing and galling.
RoHS stands for Restriction of Hazardous Substances and refers to the plating process of the fastener. RoHS restricts the use of 6 hazardous substances used in electronics and electrical items. Be sure to request RoHS compliant fasteners so an approved, compliant facility completes your plating needs.
At All America Threaded Products, we offer products with the following finishes:
- Clear Zinc Plated
- Zinc Yellow Plated
- Hot-Dip Galvanized
- Black Oxide
Looking for something you don’t see here? Contact your sales representative.
For further information on this topic feel free to ask in the comments section below or reach out to one of our experts.