Corrosion Resistance

Almost every steel fastener produced is either plated or coated with a layer to reduce corrosion. Un-plated steel will rust, even indoors, due to moisture in the air and eventually lose its structural integrity. To combat this, manufacturers coat steel fasteners in a variety of coatings and other metals through processes like Electroplating and Chemical Vapor Deposition.

Corrosion resistance in fasteners is measured using a Salt Spray Test. This method tests the material using a 5% Na saltwater sprayed onto the product in a closed environment. By measuring the time it takes for oxides (rust) to form on the product, corrosion resistance can be determined.

There are many variables that affect rusting on a steel part, so this test is not exact. It is the convention in the fastener industry and therefore the metals and coatings below will be described by their hours of corrosion resistance in a Salt Spray Test. The following information is based on our 65+ years of experience in the fastener industry, always refer to your engineering team before implementing product changes.

Plating

The most common plating by far is Zinc Electroplating. This coating is a thin layer of zinc over the steel part, about .0002” thick. This provides a shiny-metallic finish and about 30 hours of corrosion resistance using the Salt Spray Test. .00015” thick Yellow Zinc (Zinc with Yellow Chromate) has about 72 hours of corrosion resistance.

Thicker layers of Zinc provide more protection. .0005” thick zinc electroplating provides about 96 hours of corrosion resistance.

Hot Dipped Galvanizing, where the parts are physically dipped into liquid zinc, has a thickness of about .01” and can provide corrosion resistance up to 1000 hours.

Steel parts can be plated with other metals, such as Nickel, Brass, Copper, and Tin. Each of these will provide some protection of the underlying steel, but eventually, all will rust.

Coatings

Synthetic coatings have been created to improve corrosion resistance over electroplating. There are many brand names for these coatings, and in general they provide 1000 hours or more of corrosion resistance. These coatings also come in various colors to match customer products.

At Uneeda, we often use Ruspert Coatings, which consists of a 3-layer coating.

  • Layer 1: Metallic Zinc
  • Layer 2: Anti-Corosion chemical film
  • Layer 3: Baked Ceramic surface

Another coating method that has become popular is Dacrotization. This method was developed by Diamond Shamrock Co., Ltd and is especially popular in deck screws that can resist corrosion in treated lumber for outdoor use. Dacrotized treatment is applied by dipping parts into a liquid bath and then spinning to remove any excess. The parts are then baked at high temperatures to give an extremely corrosion-resistant coating. See the image below for results after 2,000 hours of the Salt Spray Test.


*NOF METAL COATINGS ASIA PACIFIC CO., LTD.

Material

The most important component of corrosion resistance is the base material that a part is made from.

Low-Carbon steel is one of the most common material types as it is inexpensive and strong. Its tendency to rust requires some sort of plating or coating in most cases to protect from corrosion.

410 Stainless Steel is similar to low-carbon steel in its strength, and it provides a moderate amount of corrosion resistance. Passivation is often performed on 410SS which dips the material into an acid bath to remove any iron particles or other contaminants from the surface. Passivated 410SS parts will typically pass a 48 hours salt spray test. Some customers plate the 410SS Zinc to add corrosion resistance and improve drivability (Zinc acts as a lubricant). 400 series stainless steel is hardened through heat treatment.

18/8SS is a term for all 300-Series Stainless Steels, typically 302HQ or 304. 300 Series Stainless is much more corrosion resistant than Steel or 410, but is a weaker material for fasteners. 18/8 Stainless has higher levels of Chromium (~18%) and Nickel (~8%) and can pass 1000 hours of the salt spray test. 18/8 Stainless can be used in ACQ treated lumber without any additional plating or coating. These types of fasteners are hardened through cold working, where the fastener is formed from the wire without heating.

316 Stainless, which is often used in commercial kitchens and other high-touch environments, includes 2-3% Molybdenum which increases corrosion resistance. 316 Stainless is less prone to pitting and bleeding than either 410 or 18/8.

With all types of stainless steel, there may be some surface rust as Iron particles exposed to the elements begin to oxidize.  This surface rust will not penetrate as quickly through the part as it would in a steel part.

If you have any questions about what type of material, plating, or coating is right for your application, call us or email sales@uneedabolt.com!

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