As it’s know, the surface finish on a fastener is as important as the underlying fastener material.  Fastener threads are made by a cutting, forming or machining process, and the surface treatment provide 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 crucial safety systems that must survive for long product lifespan.
Therefore, fasteners benefit a lot from a wide range of engineered surface finishes which are custom to each fastener application with the goal of adding optimal protection against corrosion and cracking.
In one word, surface treatments are applied to fasteners to add significant resistance to surface degradation and protect the fastener from unpredictable failure as a result of corrosion or cracking.
To determine the best surface finish for a particular fastener, it is necessary to keep in mind of the following factors.
1 – Applied Environment and Mechanical Application

The applied environment of the fastener will determine the degree of surface protection required.  The first 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 exposed in open air under a constant, static load? For fasteners subjected to highly corrosive mediums, Zinc surface coatings provide a durable level of protection against corrosion.  Zinc plating are the first choice when they are applied via hot-dip galvanizing process because this technique allows the zinc to form a metallurgical bond with the underlying fastener. This finish is widely 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.

Chrome Plated Pan Head Socket Screw

Zinc Plated SEM Screw
2 – Fastener Appearance

The other application consideration is the appearance effect required on the fasteners. Different fastener surface finish will have different appearances and thicknesses. While this is not the first consideration for fastener coating specifications, it is still something that must be considered.  For example, chrome and nickel plating offer the best appearance (e.g., shiny), if that is to be required.  On the other hand, Zinc coatings offer better corrosion resistance in comparison but are not as bright as other finish.  Black Oxide coatings provide a dark appearance that definitely reduces the reflective properties of the metal. These types of surface treatments are commonly specified for fasteners used in indoor applications. Lastly, each surface finish 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.

Black Oxide Flange Bolt
3 – Fastener Material
Most fasteners are fabricated from steel alloys, and therefore most surface treatments are applicable and custom to steel fasteners.  Steel fasteners can be suspect-able to oxidation and corrosion which means that a surface coating is essential.
Oppositely, non-steel fasteners such as stainless steel, brass, aluminum, or plastic have more limited surface treatment options.  For example, coatings are not typically applied to stainless steel fasteners because the stainless-steel base material offers a significant high performance in corrosion resistance compared to standard steel fasteners.

The other alternative fastener material to mention is Aluminum. The most common surface treatment for Aluminum fasteners is anodizing process. Anodizing is an electrolytic process really simply increases the natural aluminum oxide layer on the surface of the fastener. By this process, additional material isn't added to the surface. Anodizing largely 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.

Color Anodized Aluminum Hex Nut For Pinball

With this brief know-how to fastener surface finish selection, it’s important to remember that no matter the application, Peak Fasten will always choose the right process for you to meet every challenge. To explore more about the selection, please feel free to call us or drop an email to sales@peakfasten.com. All your query shall be responded properly within 12 hours.