Aircraft Cleaning and Hydrogen Embrittlement C3

Cleaning for Inspection

• Use cleaning compounds, paint removers, and other chemicals recommended by the aircraft manufacturer or those that are MIL spec approved for application.
• Hydrogen embrittlement or corrosion can result from unapproved compounds.
• Steel is particularly susceptible to hydrogen embrittlement when using paint stripper.

Hydrogen Embrittlement

• Hydrogen embrittlement (HE), also known as hydrogen assisted cracking and hydrogen-induced cracking, results when a chemical reaction produces hydrogen gas absorbed into a metal.
• The process reduces flexibility, allowing the formation of cracks and stress corrosion.
• Steel such as spring steel or high-strength steel are prone to HE.
• Removing grime, dirt, exhaust residue oil, and grease deposits using appropriate cleaning agents is the first step to cleaning the aircraft.
• Heavy-duty degreasers is required for oily bellies, exhaust areas, and landing gear bays.
• Proper cleaning must take place, prior to inspection.

Inspection

• After washing and drying the aircraft, inspect the outside surfaces and pay attention to corrosion-prone areas.
• Utilize flashlights and work lights, along with a 10 power magnifying glass for suspected areas.
• Other tools used during visual inspection include mirrors, optical micrometers, and depth gauges.

Paint Removal

• To locate all corrosion, remove surface finishes covering the attacked or suspected area.
• The area must be cleaned of grease, oil, dirt, or preservatives for maximum stripping compound efficiency.
• Preliminary cleaning helps determine the extent of the corrosion spread, as the stripping operation is held to the minimum.
• Correct any corrosion spread on any panel by fully treating the entire section.
• Conduct chemical paint removal from large areas in open air, in shaded areas.
• Ensure adequate ventilation if inside removal is necessary.
• Protect synthetic rubber surfaces, including aircraft tires, fabric, and acrylics, thoroughly against contact with paint remover.
• Exercise care when using paint remover, especially around gas or watertight seam sealants, because the stripper softens and destroys the sealant's integrity.
• Follow all paint stripper instructions and properly clean the stripper off after the process to avoid further corrosion.
• Test the product on a small area first and ensure it works.

Removal and Control

• Ferrous oxide (iron oxide), or rust, generally results from atmospheric oxidation of steel surfaces.
• To completely control corrosive attack, all rust must be removed from steel surfaces.
• Exercise caution when using power tools to remove corrosion on steel parts, as overheating can lead to future failure.
• Sanding discs remove corrosion from both steel and aluminum parts
• Used sanding discs need to be thrown away to prevent cross-contamination between steel particles and aluminum components.

Sandblasting

• Sandblasting, also known as abrasive blasting, is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth or roughen the surface, shape or remove surface contaminants.
• While sandblasting is sometimes used on ferrous metals to remove rust, pay close attention to the aircraft manufacturer's instructions to prevent metal damage.
• Blasting with glass beads smaller than 500mesh can remove corrosion from pits.
• After sandblasting, use inspect all traces of corrosion with a 10X magnifying glass .

Chemical Removal of Rust (Iron Oxide)

• Commercial products that remove iron oxide without chemically etching the base metal are available.
• Use of chemical corrosion removers is prohibited without engineering authorization because high-strength steel parts are subject to hydrogen embrittlement.
• After removing and treating iron oxide (rust), prime the surface with zinc chromate primer and paint to protect it from further corrosion attacks.

Aluminum Alloys

• Treatment includes mechanically removing as much of the corrosion products, inhibiting residual materials chemically, and restoring permanent surface coatings.
• Scotchbrite pads remove corrosion from the surface of an aluminum component
• Rubber rotary discs or Scotchbrite rotary discs can also remove heavy corrosion.

Mechanical Removal

• Mechanical removal is performed by means of abrasive pads and discs; do not use steel wool, emery cloth, or wire brush to remove corrosion from aluminum surfaces.
• Abrasive blasting (sandblasting) with walnut shell or glass beads is acceptable on cast surfaces; blasting on thin skins can cause stretching.
• Exercise caution in any cleaning process to avoid breaking the adjacent protective film, such as paint or sealant.
• After removal, use a 10 power magnifying glass to inspect and ensure all corrosion is removed.

Magnesium

• Magnesium is the most chemically active of the metals used in aircraft construction, making it the most difficult to protect
• Prompt and complete correction of any coating failure is imperative to avoid serious structural damage.
• Magnesium alloys are protected similarly to aluminum, using a nonporous film produced electrically or chemically.
• Corrosion forms where the film has been broken, such as around drilled rivet holes and scratches, making a good coating of paint vital for corrosion protection.
• Magnesium attack is detectable in early stages, with magnesium corrosion products occupying several times the volume of the original magnesium metal destroyed.
• Initial attack presents as a lifting of the paint film and white spots on the magnesium surface.
• Use non-metallic cleaning tools to remove corrosion.
• Entrapped particles from steel wire brushes or steel tools, or contamination of treated surfaces by dirty abrasives, can cause more trouble than the initial corrosive attack.

Treating Corroded Magnesium

• Clean and strip the paint from the area needing treatment.
• Treat the corroded area liberally with a chromic acid solution to which sulfuric acid has been added.
• Work the solution into pits and crevices using a nonmetallic brush.
• Allow the chromic acid to remain in place for 5 to 20 minutes before wiping up the excess with a clean, damp cloth; do not allow the excess solution to dry.
• Restore the original protective paint as soon as the surfaces are dry.

Corrosion Inhibitors

• Corrosion prevention programs are essential for aircraft maintenance, especially on older aircraft.
• Many products are available for preventing and controlling corrosion, so follow the manufacturer's instructions and use the correct product.
• Corrosion inhibitors is a substance that effectively reduces the corrosion rate of a metal when added to an environment in a small concentration.
• Corrosion inhibitors are used for localized areas/ entire aircraft
• LPS 3 is a common inhibitor used frequently in commercial aviation.
• ACF-50 is another very common inhibitor, used in business jets, commuter aircraft, and some commercial aircraft.

Protective Coatings

• Alodine coating, is a type of conversion coating used to passivate (make less reactive) steel, aluminum, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys.
• As a corrosion inhibitor, the coating serves as a primer to improve adherence for paints and adhesives.
• It's also used as a decorative finish and to add resistance to abrasion and light chemical attack.
• Chromate conversion coatings are commonly seen on everyday items such as screws, hardware, and tools, and typically impart a distinctively iridescent, greenish-yellow color to otherwise white or gray metals.
• Aircraft skin commonly has this Alodine coating applied to improve the aluminum's corrosion resistance and to aid in paint adhesion.
• Cadmium plating is gold/yellow and a coating process offering corrosion resistance in high-salt environments; cadmium is a byproduct of zinc production.
• The cadmium plating is less noble (more corrosive) than the steel and will corrode first; the zinc oxide is nonporous, preventing oxygen from reaching the steel (sacrificial corrosion).

Galvanizing

• Galvanizing is the process of applying a protective zinc coating to steel or iron to prevent rusting.
• In hot-dip galvanizing, the most common method, parts are submerged in a bath of molten zinc.
• The zinc protects iron by corroding first (sacrificial corrosion).
• The galvanizing process leaves a crystalline pattern that is easy to identify.

Alclad (Cladding)

• Alclad is a corrosion-resistant aluminum sheet formed from high-purity aluminum surface layers metallurgically bonded to high-strength aluminum alloy core material.
• The pure aluminum corrodes first, forming a thin layer of non-porous material (sacrificial corrosion) to protect the aluminum alloy it covers.

Anodizing

• Anodizing is an electrolytic passivation process used to increase the natural oxide layer's thickness on metal parts, therefore corrosion resistance.
• The process is called anodizing because the part to be treated forms the anode electrode of an electrolytic cell.
• Anodizing is most commonly applied to protect aluminum alloys, but also exists for titanium, zinc, magnesium, and other metals.

Chrome Plating

• Chrome plating (Chromium Plating) is electroplating a thin chromium layer onto a metal object.
• The chromed layer enhances the item(s) by providing decorative effect, corrosion resistance, ease of cleaning, or increased surface hardness.

Metal Spraying

• Metal spraying involves protecting aircraft engine cylinders from corrosion by spraying molten aluminum on their surface.
• This process has been used for over 100 years, where molten metal or softened particles are applied to enhance surface properties.
• Corrosion protection from metal spraying is sacrificial, similar to cadmium and zinc coatings.

Phosphate Coatings

• These are used on steel parts for corrosion resistance, lubricity, or as a foundation for subsequent coatings or painting.
• It serves a conversion coating in which a dilute solution of phosphoric acid and phosphate salts is applied via spraying or immersion and chemically reacts with the surface to form a layer of insoluble, crystalline phosphates.
• Phosphate conversion coatings can be used on aluminum, zinc, cadmium, silver, and tin.

Stainless Steel

• Stainless steel, a steel alloy (inox steel), has a minimum of 11% chromium content by mass and a maximum of 1.2% carbon.
• Stainless steel's corrosion resistance increases with increasing chromium content and is commonly used on aircraft piston engine exhaust systems and mufflers.
• High-grade stainless steel (304 or 316) is non-magnetic; the higher the chromium content, the less magnetic the steel.

Paint

• Paint, or a "good coat of paint," is universally used for corrosion control on metal surfaces.
• For smooth surfaces (aluminum), the surface must be prepared for the paint to adhere.
• Zinc Chromate primer, used for years with lacquer and enamel paints, is green and an inhibiting primer.
• Its porous film allows water to cause chromate ions to release and hold on the metal surface, preventing electrolytic action and inhibiting corrosion.

Fairing or Blending Reworked Areas

• Depressions from corrosion rework must be faired or blended with the surrounding surface.
• Steps for fairing, remove rough edges and all corrosion from the damaged area.
• All dish-outs must be elliptically shaped with the major axis running spanwise on wings and horizontal stabilizers, longitudinally on fuselages, and vertically on vertical stabilizers.
• The removal of corrosion products must be blended out to prevent stress risers that may cause stress corrosion cracking but is not necessary on noncritical structures, and metal removal is not necessary.
• Rework depressions by forming smoothly blended dish-outs, using a ratio of 20:1 (length to depth).
• In areas with closely-spaced, multiple pits, remove intervening material to minimize surface irregularity or waviness.

Protection from Dissimilar Metal Corrosion

• Certain metals undergo corrosion when in contact with other metals, referred to as galvanic, electrolytic, or dissimilar metals corrosion.
• Contact of different bare metals creates galvanic action when moisture is present.
• Some metal combinations require a protective separator, which may be metal primer, aluminum tape, washers, grease, or sealant, depending on the metals involved.
• All dissimilar joints are protected by a minimum of two coats of zinc chromate or, preferably, epoxy primer.
• When using steel fasteners in an aluminum structure, treat drilled holes with Alodine and a zinc chromate primer, and install (wet) to enhance the bond and provide corrosion protection.
• For lap joints and cut edges, take ensure thorough priming and painting; sealing seams with a sealant prevents moisture infiltration for structure integrity.