Taking plating technology to the airplane

Taking Plating Technology to the Airplane By Derek Vanek February 1998 Amultitude of coatings are available today to protect or to enhance the performance of aircraft components. Uses include corrosion protection, increasing wear resistance...

Most adhesion evaluations have been made using destructive qualitative tests such as chisel or bend tests. These tests indicate that the adhesion and cohesion of brush plated deposits is about the same as the cohesive strength of the base material.

Quantitative tests have been run using ASTM Test Procedure C-653-79 "Standard Test Method for Adhesion or Cohesive Strength of Flame Sprayed Coating." Four samples were plated with a nickel neutral solution. The cement used to bond the plated sample to the testing apparatus failed during the test. Since the adhesive had a bond strength rated at approximately 11,300 psi, it was shown that the bond strength of the plated deposit is at least 11,300 psi. Even brush plated deposits with a fair adhesive rating survived this test. Therefore, have an adhesive bond and cohesive strength of at least 11,300 psi. Therefore, brush plated bonds are stronger than the bonds found with flame sprayed coatings.

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Metallographic Structure
The metallographic structure of an electroplate can be examined in an etched or unetched condition. In the unetched condition, most brush plated deposits are metallurgically dense and free of defects. Some of the harder deposits, such as chromium, cobalt-tungsten, and the hardest nickel are microcracked much like hard tank chromium. A few deposits are deliberately microporous, such as some of the cadmium and zinc deposits.

Microporosity does not affect the corrosion protection of these deposits since they are intended to be sacrificial coatings. The microporous structure offers an advantage over a dense deposit because it permits hydrogen to be baked out naturally at ambient temperatures or in a baking operation. Etched brush-plated deposits show grain structures that vary, but parallel those of tank deposits. However, brush-plated deposits tend to be more fine grained. Coarse grained, columnar structures, such as those found in Watts nickel tank deposits, have not been seen in brush-plated deposits.

The hardness of brush-plated deposits lies within the broad range of the hardnesses obtained with tank deposits. Brush-plated cobalt and gold, however, are harder than tank-plated deposits. Brush-plated chromium is softer, since tank-plated chromium is generally in the 750 to 1100 DPH range.

Corrosion Protection
Brush-plated cadmium, lead, nickel, tin, zinc, and zinc-nickel deposits on steel have been salt spray tested per ASTMB-117. When the results were compared with AMS and military specification requirements, the brush-plated deposits met or exceeded the requirements for tank electroplates.

Brush-anodized coatings have been tested and meet the performance requirements of MIL-A-8625E, AMS 2470, AMS 2468, AMS 2469, and BAC 5623.

Hydrogen Embrittlement
Cadmium and zinc-nickel plating solutions have been specifically developed for plating or touching up high-strength steel parts without the need for a post-plate bake.

Hydrogen embrittlement testing over the past 20 years has become progressively more difficult to pass. A no-bake, alkaline, brush-plating cadmium deposit has passed an aircraft manufacturer's test, which is perhaps the toughest imaginable. The test consisted of the following steps:

1. Prepare six notched tensile samples from SAE 4340, heat-treated to 260-280 Ksi with 0.010 inch radius notch.
2. Plate samples with 0.5 to 0.7 mil cadmium while under load at 75 percent of ultimate notched tensile strength.
3. Maintain the load for 200 hours.

The two most common uses of brush plating on aircraft components are applying sulfamate nickel to localized areas of engine components to improve the brazing process and plating cadmium onto localized areas of landing gear to repair damage caused by runway debris.

The brush-plated cadmium is a low hydrogen embrittling deposit and can be applied to localized areas for touchup of defective tank-plated cadmium deposits without a post-plating bake. It can be applied with the landing gear attached to the plane and with minimal to no masking. It is a fast and simple repair — solvent clean, mechanically abrade, and plate.

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