Taking Plating Technology to the Aircraft

Selective plating is a widely used tool in the aerospace industry for both repair and OEM applications.


Numerous coatings are available today to protect or to enhance the performance of aircraft components. Surface enhancements include corrosion protection, increasing wear resistance, improving electrical conductivity, enhancing lubricity, increasing hardness, or improving the adhesive bond between cemented parts.

Once the components are put into service and are subject to normal wear and tear, refinishing may be required. For tank plating applications, this consists of removing the damaged or worn component from the aircraft and sending it to the plating shop to be stripped and plated or anodized. In many cases disassembly is time consuming and impractical.

For example, an operator removed corrosion products from cadmium plated steel fasteners. During the process, the anodized coating was removed up to 3/8 of an inch from 1,200 fasteners that ran the length of the wings of the aircraft. The timely and expensive option was to disassemble, strip, reanodize, and reassemble the parts. The more attractive option was to selectively brush plate the individual areas directly on the wing.

Selective plating encompasses a family of portable electrochemical processes that are used on aircraft in both OEM and repair applications. This includes systems that are used for on-site electroplating, as well as portable anodizing and electropolishing. These systems are set apart from traditional tank finishing processes because they can be performed anywhere — in the shop or out in the hangar, and the parts can be plated or anodized without removing them from the aircraft.

Specifications and approvals

Brush plating is widely accepted for use on both military and commercial aircraft. Key specifications include AMS 2451 and MIL-STD 865, as well as numerous commercial specifications from Bell Helicopter, Boeing, Messier-Bugatti-Dowty Aerospace, Goodrich, and Pratt & Whitney; and prime approvals from Sikorsky, Honeywell, GE AE, and Rolls-Royce (See chart on page 15.)

How does it work?

Brush plating and anodizing operations somewhat resemble painting. The operator dips an absorbent tool in a solution and then brushes it against the surface of the area to be plated.

A portable power pack provides a source of direct current for all the processes. The power pack has a minimum of two leads. One lead is connected to the tool and the other is connected to the part being finished.

The direct current, supplied by the power pack, is used in a circuit that is completed when the tool is touching the work surface. The tool is always kept in motion whenever it is in contact with the work surface.

Work surface preparation is usually accomplished through a series of electrochemical operations. These preparatory steps are performed with the same equipment and tool types that are used for the final finishing operation. Good preparation of the work surface is as important as movement of the tools to produce a quality finish.

Adhesion:

The adhesion of brush electroplates is excellent on a wide variety of materials including steel, cast iron, stainless steel, copper, and high temperature nickel-base materials. Adhesion from brush plating is comparable to that of tank plating.

Metallographic structure:

Most brush plated deposits are metallurgically dense and free of defects. Some of the harder deposits, such as chromium, cobalt-tungsten, and hard nickel, are microcracked, much like hard tank chromium. A few deposits, such as cadmium and zinc are deliberately microporous. Brush plated deposits tend to be more fine grained than tank deposits.

Hardness:

The hardness of brush plated deposits lies within the broad range of the hardness levels obtained with tank deposits. Brush plated chromium is somewhat softer; however, a new green alternative to chromium, Nickel-Tungsten (80 percent Ni, 20 percent W) provides comparable hardness to chromium when heat treated and 10 times better wear.

Corrosion protection:

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