Routinely overlooked, the surface coating on a fastener is as important as the underlying fastener material. Fastener threads are manufactured in a cutting or forming machining process, and the surface coatings add an essential layer of protection to the fastener shank and threads. Fastened joints are engineered for use in a wide range of applications and environments and are relied upon to maintain the structural integrity of fastened joints within an extremely diverse portfolio of engineered systems. These joints are sometimes critical safety systems that must survive for long product lifespans.
To this end, fasteners benefit greatly from a wide range of engineered surface coatings which are tailored to each fastener application with the goal of providing optimal protection against corrosion and cracking.
In a nutshell, surface coatings are applied to fasteners to add significant resistance to surface degradation and protect the fastener from premature failure as a result of corrosion or cracking.
To determine the best surface coating for a particular fastener, the following factors are vital to keep in mind.
1 – Service Environment and Mechanical Application
The service environment of the fastener will dictate the degree of surface protection required. The primary considerations are the environmental conditions of the fastened joint along with the loading conditions the fasteners will see during their useful life within the application. For example, is the fastener to be employed in a cyclic loading environment within a corrosive fluid, -or will it be used in open air under a constant, static load? The first consideration should always be the intended application. For fasteners subjected to highly corrosive mediums, Zinc surface coatings provide a durable level of protection against corrosion. Zinc coatings are most effective when they are applied via a process called hot-dip galvanizing because this technique allows the zinc to form a metallurgical bond with the underlying fastener. This coating is commonly applied to steel fasteners. Additionally, chromate coatings, when applied via an electroplating process, minimize friction within the joint and increase abrasion resistance. For example, this would be a useful coating for applications where the fastener is subjected to constant vibration.
2 – Fastener Appearance
The other application consideration is the decorative appearance required on the fasteners. Different fastener coatings will have different appearances and thicknesses. While this is probably a secondary consideration for fastener coating specifications, it is still something that must be considered. For example, chrome and nickel coatings offer the best appearance (e.g., shiny), if that is to be desired. Conversely, Zinc coatings offer better corrosion resistance in comparison but are not as bright as an appearance. Black Oxide coatings present a dark appearance that naturally reduces the reflective properties of the metal. These types of coatings are commonly specified for fasteners used in indoor decorative applications. Finally, each surface coating material and technique inherently have different coating thickness ranges which also must be considered if space is limited or there are other mating parts for which to account.
3 – Fastener Material
Most fasteners are fabricated from steel alloys, and therefore most surface coatings are applicable and tailored to steel fasteners. Steel fasteners can be suspectable to oxidation and corrosion which means that a surface coating is essential.
Conversely, non-steel fasteners such as stainless steel, brass, aluminum, or plastic have more limited coating options. For example, coatings are not typically applied to stainless steel fasteners because the stainless-steel base material offers a significant improvement in corrosion resistance compared to standard steel fasteners.
The other alternative fastener material of note is Aluminum. The most common surface treatment for Aluminum fasteners is a process called anodizing. Anodizing is an electrolytic process really simply increases the natural aluminum oxide layer on the surface of the fastener. In this sense, additional material isn’t added to the surface. Anodizing greatly increases the corrosion resistance of a fastener and increases the galling resistance of aluminum fasteners. Galling is more common in these fasteners because the alloy is more malleable than steel.
With this brief introduction to fastener surface coating selection, it’s important to remember that no matter the application, MISUMI has a wide range of surface coatings and fasteners available to meet every challenge. To explore the selection, visit our fasteners page.
