Many engineers will put great thought into selecting the proper type of fastener for their design, but overlook the importance of finish. When selecting the best finish for fasteners, it is wise to consider the following factors:

Many exotic coatings have been developed for specific applications, and more are being produced every day. This article will not attempt to discuss all of these options, but will concentrate on the following, commonly available finishes:

The main safety concern when choosing a finish for fasteners is to avoid hydrogen embrittlement (HE). During HE, or hydrogen assisted-cracking, hydrogen is introduced to the surface of a fastener and hydrogen atoms diffuse through the metal. These atoms gradually recombine to form hydrogen molecules, creating pressure from within the fastener. As this pressures increases, the fastener loses ductility, toughness and tensile strength. This can lead to catastrophic failure of the fastener.

HE can occur during various manufacturing operations or operational use—any time fasteners come into contact with hydrogen. Fasteners can become susceptible to HE during forming, coating, plating, cleaning and finishing operations. The wrong finish on a high-strength fastener can promote HE. So, what is a “high-strength fastener”?

Fasteners are considered “at risk” for HE if their hardness exceeds 380 HV on the Vickers Pyramid scale. Some examples include:

To eliminate the risk of HE in these fasteners, acid pickling and electro-plating should be avoided if possible. Measures may be taken to reduce risk, such as baking immediately following electroplating, but these measures can only serve to reduce the risk, and cannot completely eliminate it.

The type of finish needed on a fastener depends on the expected service life of the fastener and the environment will it be operating in. Standard B633 from the American Society for Testing and Materials defines “service conditions” as follows:

The above recommendations assume a mostly natural environment. If the fasteners will be exposed to a more “industrial” environment where they will be exposed to dirt, grime and chemicals, contact Bossard for a recommendation.

Hydrogen embrittlement and corrosion aren’t the only factors to consider when selecting a fastener finish.

Handling damage, such as nicks and scrapes resulting from transportation or wrenching, can be a concern for some fastener finishes. Soft or brittle finishes will begin the corrosion cycle much sooner if care is not taken in the handling and assembly of these fasteners. Finish adherence to the fastener is also a factor in maintaining good corrosion protection during handling.

Getting a joint tight and keeping it tight all depends on proper clamp load. A well designed joint with the proper clamp load should not come loose. Fastener finish is important for joints assembled using torque control, which relies on a consistent joint friction to produce a predictable and repeatable clamp load. Some finishes have known friction values while others do not. If consistent and repeatable clamp load is important to the joints survival, then coatings with a known friction should be used.

Smaller-diameter fasteners and threaded fasteners with internal recess drives may not lend themselves well to certain finishes. The type of finish and the method of application may cause excess coating material in threads and recesses.

The following guide offers information on the most common fastener finishes to help make an informed decision on which one to use.

Electrodeposited zinc or “commercial” zinc

A final consideration is the presence of hexavalent chromium, also called chrome VI or hex chrome, in a fastener finish. Many countries now regulate the use of hex chrome, which was commonly used in fastener finishes in the past and can still be found in some areas today. All of the options above are available in “hex chrome free” versions, but it is important to specify this requirement if it applies.

For more information call Bossard at 319-277-5520 or visit www.bossard.com.

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