Fastener Corrosion: Definition and Causes
Corrosion, especially in fasteners, is a complicated material phenomenon, and many academic and engineering textbooks are devoted solely to this subject. These resources detail the precise chemical mechanisms and detailed prevention methods. However, in general, the bulk of fastener corrosion can be categorized into two categories.
- Electro Chemical Corrosion: This is the more common fastener corrosion category. In simplest terms, it happens when two dissimilar metals are in physical contact with the electrolytic medium. This term is a fancy way to say the operational environment of the fastener system, such as open air, fresh water, or another liquid medium.
2. Direct Chemical Attack Corrosion: This is a more extreme corrosion category and happens when the material is soluble in the operative environment. This only occurs in the most extreme applications and environments. As such, it is not a very common type of corrosion, but it is worth mentioning for machine design disciplines so you can ensure that the fasteners in your application will not experience this corrosion mechanism.
Furthermore, corrosion is inherently a surface phenomenon. This means it begins on the surface and propagates down into the material. This is important for fasteners because they have a large amount of surface area compared to the comparable part volume. There is a significant surface area for corrosion, given the thread areas and shanks. Furthermore, fasteners are often used in critical machine design joints, which must be robust and withstand adverse product environments over long operational lifetimes.
How to Prevent Fastener Corrosion in Your Machine Design
With the background definitions in hand, the methods for preventing fastener corrosion begin where corrosion starts: on the surface of the fastener. Many different surface treatments are available and can be specified for fasteners. Where fasteners are specified standard carbon steels. The following are specialized surface treatments on carbon steels:
- Hot Dipped Galvanizing
Hot dip galvanizing is where metallic (most commonly steel) fasteners are dipped entirely in a hot bath of molten Zinc. The zinc forms a thick molecular bond of zinc upon hardening, covering all the exposed steel fastener surfaces. This process is most commonly used on carbon steel products.
This process affords a few critical benefits for the steel fastener. First, the metallurgically bonded zinc creates a physical barrier between the underlying steel fastener and the corrosive environment encountered by mechanical fasteners. However, zinc also protects the steel by essentially becoming a “sacrificial” coating to protect the underlying steel from corrosion.
2. Zinc Plated Steel
Zinc plating is a process where fasteners are placed in a molten zinc bath with an electric current running through the tub. The electric current serves as the bonding agent, plating the Zinc to the fastener surfaces to create the protective coating. The plating process produces a pleasant aesthetic surface coating. The big difference between this process and the previously mentioned hot dipped galvanization process is that Zinc plating creates a much thinner surface coating on the fasteners. For example, Zinc plating is typically 1/5th the thickness of the same protective layer that can be achieved with Hot dipped galvanization. This follows that zinc plating is less effective than hot-dipped galvanized products where long corrosion lifetimes are required in corrosive environments such as outdoor applications. However, the thinner coating of the zinc plating process may be necessary for certain mechanical joints and fittings.
Furthermore, some fasteners can be specified as corrosion-resistant base material, such as 300 series stainless steel, which negates the need for surface treatments. The most commonly used stainless steel alloys for rust prevention are type 304 or type 316 stainless steel. These two steel alloys are similar in appearance. However, type 316 steel has a higher concentration of molybdenum, making it more expensive but more resistant to chemical attack corrosion.
To learn more about stainless steel fasteners, read our blog “Stainless Steel Fasteners: Types and Use Environments.”
A firm understanding of fastener corrosion causes and preventions will enable you to incorporate them directly into your machine designs. MISUMI offers a wide range of corrosion-resistant fasteners available to fit the most corrosive applications. Explore our selection of fasteners here.