Taking Plating Technology to the Airplane
By Derek Vanek
Amultitude of coatings are available today to protect or to enhance the performance of aircraft components. Uses include corrosion protection, increasing wear resistance, improving electrical conductivity, enhancing lubricity, increasing hardness, or improving the adhesive bond between cemented parts. A majority of these coatings and deposits are originally applied to the individual components at facilities that use large tanks to electroplate or anodize.
Once the components are put into service and are subject to normal wear and tear, refinishing may be required. This usually consists of removing the damaged or worn component from the aircraft and sending it to the plating shop to be stripped and then replated or reanodized — even if the damage or wear is localized. In many cases disassembly is relatively simple, but in some instances it can be very time consuming — bordering on impractical.
One such instance is the damage that can occur to the chromic acid anodized coatings on aircraft skins. For example, the wing droop leading edge on the Falcon Jet HU-25A is chromic acid anodized using the Bengough Process. The coating may be damaged from in-service use or it may even be damaged during a routine maintenance operation.
In one case, an operator was removing corrosion products from cadmium-plated steel fasteners. In the process, they completely removed the anodized coating for up to 3/8 of an inch away from roughly 1200 fasteners that ran the length of the wings. The $150,000 option was to disassemble, strip, reanodize, and reassemble. The more attractive option was to selectively, brush chromic acid anodize the individual areas right on the wing.
Brush 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.
How does it work?
Brush plating and anodizing operations, in their simplest forms, resemble painting. The operator soaks or dips the tool in a solution and then brushes or rubs it against the surface of the material that is to be finished. The tools are covered with an absorbent material that holds solutions so they can be applied to the work surface.
A portable power pack provides a source of direct current for all the processes. The power pack has at least 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. Movement is required to ensure a quality finish.
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.
The adhesion of brush electroplates is excellent and comparable to that of good tank plating on a wide variety of materials including steel, cast iron, stainless steel, copper, high temperature nickel-base materials, etc. When plating on these materials, the adhesion requirements of federal and military specifications are easily met. Limited, but occasionally useful, adhesion is obtained on metals that are difficult to plate such as titanium, tungsten, and tantalum.
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.
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.
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.
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.
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.
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.
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.
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.
The water heated anodizing tool was wrapped with white Scotchbrite® and was saturated with the chromic acid anodizing gel by forcing it into the cover material with a spatula. The gel is thixotropic and becomes more fluid when it is actually being used. The tool was then set aside to warm up to the 95 to 105 F operating temperature required for the job.
Electrical contact was then made on a noncoated, conductive area several feet away from the defect area. The power pack was set to 38 volts, and the defect was then anodized by slowly rubbing the saturated tool over the defect area for 42 minutes. When the anodizing process was completed, the area was rinsed and unmasked.
The scratch itself was only slightly lighter in color that the adjacent anodized coating. An organic dye was used to slightly darken the scratch to a near perfect match with the existing anodized coating.