What Distributors Need to Know About Plating

There are three primary causes for why fastener distributors have issues with plating or coating their parts.

The most common cause of plating and coating “disasters” involves failure to inspect the parts for conformation prior to process, Carmen Vertullo of Carver Consulting told members of the Pacific-West Fastener Association.

At Pac-West’s autumn meeting held in Portland, OR, Vertullo told association members that many in the fastener industry have made this common mistake at some point.

“We all probably have a pallet of parts sitting somewhere because of this,” Vertullo said.

This happens because there is something wrong with the part, such as it not meet a specification, or because it was the wrong part to start with, meaning it meets a specification but the wrong one.

Distributors can prevent this by 1) checking the part, including quantity and size; and 2) inspecting the part to ensure it meets specifications.

“A plastic thread gauge is the most valuable inspection tool in the fastener industry,” Vertullo noted.

The second reason plating can go wrong involves a lack of understanding the need for, or the application of a spec or standard, including performance. This results in the wrong plating, the wrong plating spec or no spec, Vertullo explained.

A “distant” third reason for plating mishaps involves bad plating or coating.

“There is not a lot of risk with plating coming out wrong.”

Distributors apply finishes to fasteners for various reasons, Vertullo said.

Some want to change or improve the appearance of the part. Others seek to improve resistance to corrosion.

“In most folks minds, this is the primary reason for plating,” he noted.

Another reason to plate involves identification of the part.

The “most important and technical” reason for plating involves torque tension control, as classified by the “K factor.”

“Changing a coating has a tremendous impact on the performance of a fastener,” Vertullo told Pac-West members.

Common fastener coatings involve different processes.

Electroplating is carried out in a water based solution containing a chemical compound of the metal to be deposited. The parts to be plated are immersed in this bath and an electrical current passes through, causing the plating metal to deposit on the parts.

Electroplated cadmium used to be the most common plating until if fell out of favor due to its environmental impact. However, “CAD” plating is still used extensively in the aerospace industry.

“It’s the best coating ever,” Vertullo explained. “We should have learned how to use it safely instead of getting rid of it.”

Hot dip galvanizing involves the parts being dipped in molten zinc, a coating suited for larger fasteners that need protection in outdoor environments.

“Dip spin” coating (sometimes called “zinc flake”) is similar to a paint – a binder with metal flakes that can be applied by through a dip spin process or sprayed onto the part. This process is become more popular, especially in the automotive industry, Vertullo noted. Dip spin coatings can replace zinc in many applications, but Vertullo urged caution when using this process with stainless steel.

“Stainless steel and aluminum do not work well together,” he emphasized.

Problems related to plating include issues with thread fit. Plating affects the pitch diameter of the thread by an amount equal to four times the coating thickness, Vertullo explained.

“Plating lays down on the thread like snow on a roof.”

Another problem arising from plating is hydrogen embrittlement, which is the catastrophic failure of high hardness parts due to absorption of atomic hydrogen in the base metal during the electro-plating process. Hydrogen embrittlement causes the parts to fail under load, typically within 24 hours of installation.

However, this can be prevented by a program of susceptibility determination, proper post plating baking and testing, Vertullo said.

“Get some hydrogen embrittlement training,” he strongly advised.

Carmen Vertullo is a principal at Carver Consulting in El Cajon, CA. Tel: 619 440-5888 Email: carmenv@carverem.com Web: carverem.com