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...


BRUSH PLATING, A REPRESENTATIVE JOB
Upon repairing damaged cadmium plating on a landing gear during a routine maintenance inspection, the operator discovered several damaged areas on the landing gear that were probably caused by runway debris. The damage was in the form of several small dings and scratches that penetrated both the paint and the underlying cadmium plating. This localized damage was a good candidate for repair by brush cadmium plating because it could be done without any disassembly of the landing gear.

Graphic
Sketch of repair area

Carrying out the job
The individual areas to be plated were solvent cleaned to remove any traces of oil or grease that could impair adhesion of the cadmium plating. They were then mechanically abraded (by hand) with aluminum oxide sand paper to slightly feather the defect area and to clean the area to be plated (see photo above).

Because the surface was painted, the only masking that was required was to catch the small amount of runoff of the plating solution.

The plating tool was wrapped with cotton batting and cotton tubegauze and placed into a small container of the cadmium-plating solution to soak for a few minutes prior to plating. The positive lead was plugged into the anode, and electrical contact from the power pack was made by attaching the negative lead to a conductive area near the damage.

With the power pack voltage set at 20V, the deposit was applied by rubbing the saturated anode on the defect area in a circular motion until a visible layer of cadmium appeared. The voltage was then reduced to 10V at which the remaining calculated ampere-hours were passed. The area was water rinsed. And then the other defects were plated in a similar manner.

Once the plating was completed, the chromate conversion coating was applied by dabbing a saturated cotton pad on the plated areas for 30 seconds. The entire area was rinsed and allowed to air dry. At this point the repaired areas were ready for the touchup paint to be applied.

BRUSH ANODIZING, A REPRESENTATIVE JOB
During a routine maintenance inspection on a F50 Jet Engine inlet assembly, a mechanic discovered a 3 inch long scratch on the inside skin of the engine inlet. The base material was a 2024-T3 aluminum that had been tank chromic acid anodized.

The typical repair for damage of this nature was to replace the entire section of skin; however, brush chromic acid anodizing allowed the repair to be made to the localized area. Requirements for the repair were corrosion protection and a good cosmetic appearance.

Coating requirements
The minimum coating weight required by MIL-A-8625E for a Type 1, Class 2 chromic acid anodized coating is 500mg/ft2 .

The finished coating should provide a very close color match to the original coating.

Equipment requirements
Brush chromic acid anodizing must be conducted at approximately 100 F, and therefore a small heating system was required to maintain the operating temperature of the anodizing solution or gel. A gel was used for this particular application to eliminate any chances of solution leaking into unwanted areas. An internally hot water heated tool was used to apply the gel, and white Scotchbrite® was used as a cover material for the tool.

The direct current required for this anodizing job was supplied by a special anodizing power pack with a high resolution voltmeter, ammeter and ampere-hour meter, and with an output of at least 40 volts.

Carrying out the job
Since obtaining a good color match was an important part of the job, sample panels of the 2024-T3 were anodized at various voltages and were then compared to the actual part (varying the voltage used while anodizing the 2024-T3 has considerable effect on the appearance of the coating) and the operator determined that 38 volts provided the best color match.

The 42 minute anodizing time required to achieve the minimum 500mg/ft2 was arrived found in the following table:

The defect and adjacent areas were solvent cleaned to remove any traces of oil or grease that can degrade the adhesion of the masking material. Because the damage to the anodized coating was a single, sharply-defined scratch, the area was carefully masked with AeroNikl® tape as closely as possible to the scratch and with as few tape overlaps as possible.

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