Aircraft Metal and Covering Inspection

• Defects are events or occurrences that reduce an aircraft's serviceability.
• Maintenance schedules specify inspection areas and expected faults.
• Inspectors look for abnormalities in inspected items.

Metal Parts Inspection

• Inspect metal parts, bodies, casings, electrical, instrument, and radio installations, metal pipes, ducting, tubes, rods, and levers for:
• Cleanliness and external damage.
• Leaks and discharge.
• Overheating.
• Fluid ingress.
• Obstruction of drainage/vent holes or overflow pipe orifices.
• Correct seating of panels/fairings and fastener serviceability.
• Distortion, dents, scores, and chafing.
• Pulled/missing fasteners, rivets, bolts, or screws.
• Evidence of cracks or wear.
• Separation of adhesive bonding.
• Failures of welds/spot welds.
• Deterioration of protective treatment and corrosion.
• Security of attachments, fasteners, connections, locking, and bonding.

Coverings, Ducts, and Flexible Mountings Inspection

• Inspect coverings, ducting, flexible mountings, seals, electrical cable insulation, and windows for:
• Cleanliness.
• Cracks, cuts, chafing, kinking, twisting, crushing, contraction, or sufficient free length.
• Deterioration, crazing, loss of flexibility.
• Overheating.
• Fluid soakage.
• Attachment security, correct connections, and locking.

Cables, Chains, Pulleys, Rods, and Tubes Inspection

• Inspect cables, chains, pulleys, rods, and tubes for:
• Correct alignment (no fouling).
• Free movement, distortion, bowing.
• Scores, chafing, fraying, kinking.
• Wear evidence, flattening.
• Cracks, loose rivets, deterioration of protective treatment, and corrosion.
• Correctly positioned, undamaged, and secure electrical bonding.
• Secure attachments, end connections, and locking.

Electrical Components Inspection

• Inspect electrical components (actuators, alternators, generators, motors, relays, solenoids, contactors) for:
• Cleanliness and/or obvious damage.
• Overheating evidence.
• Corrosion and attachment/connection security.
• Scoring, worn brushes, or adequate spring tension (after cover removal).
• Overheating and fluid ingress.
• Burning or pitting of contacts.
• Contact security after cover removal.

Airframe Damage Causes

• Airframe damage can occur from moving part interference, ground equipment strikes, or hail.
• Ground servicing vehicles can cause dents or puncture the pressure hull.

Blocked Inlets and Exhausts

• Wildlife can nest in inlets/exhausts, causing damage.
• Blocked ducts can overheat equipment or damage engine parts.

Liquid Systems Checks

• Liquid levels are assessed using gauges and physical checks.
• Systems include oil tanks (engine, APU, IDGs), hydraulics, fuel, and potable water.
• Investigate lower-than-expected levels, considering normal fluid consumption rates.
• Do not replenish low hydraulic systems without finding the leak source.
• External leaks are usually fixed by replacing faulty components and testing per the AMM.
• Internal leaks require thorough component inspection and may cause slower/erratic service operation.

Gaseous Systems Checks

• Gaseous systems use gases like oxygen, nitrogen, and air.
• Oxygen system leaks are dangerous due to explosion risk if contacting oil/grease.
• After fixing a leak, recharge and leak test the system.
• Pneumatic systems should maintain consistent pressure; low pressure suggests compressor issues or leaks.
• Falling pressure between flights indicates a slow leak in the storage system.

Dimensions Indicating Serviceability

• Component measurements can indicate serviceability.
• Landing gear oleo shock struts' extension indicates correct inflation.
• Low extension suggests low pressure, requiring further checks (usually during line maintenance).

Tyre Inspection

• Tyre serviceability is indicated by tread groove depth.
• The AMM defines worn or damaged tyre characteristics.
• Defects include cuts, blisters, creep, and low pressure.
• Some tyres can be re-treaded if not severely damaged.
• Creep is tyre movement around the rim during braking, which can damage tubes.
• White marks across wheel and tyre indicate creep if misaligned.

Wheel Inspection

• Wheel defects often result from heavy landings or runway debris impact.
• Corrosion can result from impact damage.
• Sheared wheel bolts (on split wheels) can be an issue.
• Wheels are thoroughly inspected during tyre replacement.

Brake Inspection

• Brakes absorb energy as heat, causing rotor and stator wear.
• Excessive heat can cause stator material to break up.
• Inspections check for overheating signs and wear limits.
• Wear reduces brake pack thickness, monitored by measurement.
• Worn brake packs are overhauled when wear reaches a limit.
• Debris or scoring indicate pad break-up, requiring complete brake unit replacement.

Landing Gear Lock Inspection

• Landing gear locks prevent inadvertent undercarriage collapse.
• Usually fitted during extended ground stays and removed before flight.
• Flags remind personnel to remove locks before departure.

Indicator Inspection

• 'Blow-out' discs in fire extinguishing and oxygen systems indicate discharge.
• Ruptured discs can result from fire extinguisher operation or excessive pressure.

External Probe Inspection

• Probes (pitot/static, AOA) are vulnerable to physical damage.
• Inspect for physical damage, discoloration, or obstructions.

Handle and Latch Inspection

• Handles/latches wear from constant use, especially on cargo bays and baggage holds.
• Panel fasteners wear and must be secured.
• Positive closing/locking mechanisms and indication systems (painted lines, flush-fitting catches) are used.
• Worn fasteners should be repaired/replaced to prevent panel loss in flight.

Panel and Door Inspection

• Panels and doors can be damaged by excessive use.
• Frames can be damaged by items passing through them.
• Check inside doors/panels for dissimilar metal corrosion, especially around fastener holes.
• Remove doors/panels to assess corrosion extent.
• Different aircraft have specialist quick-release fasteners and doors.

Emergency Systems/Equipment Inspection

• Protective covers prevent switch operation.
• Frangible devices (e.g., thin copper wire) indicate system operation if broken.
• Any indication of tampering requires thorough investigation.

Pressurised Structure Inspection

• Consider cyclic loading/hoop stress (fatigue cracks around cut-outs).
• Look for subtle skin changes indicating hidden defects.
• Check serviceability of blow-out panels, doors, windows, and seals.

Sheet Metal Repair Inspection

• Check the surrounding area for hidden damage.
• Feel for deformation (wrinkles, buckling).
• Measure to ensure airframe symmetry.
• Thoroughly investigate corrosion when removing skin panels.

Door/Window/Cut-out Seal Inspection

• Ensure seals are in good condition on pressurised aircraft.
• Damaged blade seals should be inspected for aircraft serviceability.

Aerodynamic Cleanliness

• Important for flight characteristics and fuel consumption.
• Ensure fillet seals on skin joints are intact.
• Fitted components must have environmental seals.

Moving Part Inspection

• Check for defects from component operation (wear, binding, interference).
• Identify cracked/broken fittings, loose hinges, defective bearings, corrosion, and deformation (dents, kinks, bending).
• Attachments should be secure with required motion range.
• Retaining mechanisms (pins, clips, safety wiring) should be in good condition.
• Lubricate moving parts with approved lubricant.
• Inspect seals for damage, wear, or leaks.
• Parts should be clean and free from dirt, fluids, or old lubricants.
• Moving components should have correct tension, travel, and operate per the manual.

Fluid Line and Hose Inspection

• Check material, installation, fittings, and torque.
• Inspect for leaks, tears, cracks, dents, kinks, chafing, correct bend radius, security, corrosion, deterioration, obstructions, and foreign matter.

Wiring Inspection

• Ensure wiring is securely attached.
• Check for chafing/rubbing, burning/overheating, defective insulation, loose/corroded/broken terminals, heat/fluid degradation, and proper clamp installation.

Bolt Inspection

• Check for correct torque and intact tamper-proof paste.
• Identify elongation, deformation, shear, or tension damage.
• Fittings should be correctly installed, the right size/type, and corrosion-free.

Rivet Inspection

• Record any corrosion around rivets.
• Drill out rivets to check corrosion extent.
• Follow the maintenance manual for reinstallation.
• Check for gaps under rivet heads or looseness, indicating stretching from stress.

Filter, Screen and Fluid Inspection

• Check for cleanliness and contamination.
• Ensure inspection/replacement per the maintenance manual.
• Verify correct part and installation per the manual.

Powerplant Inspection

• Inspect engine mounts for security, movement/wear evidence, cracking, or corrosion.
• Check engine assembly mounting bolt torques.
• Visually and physically inspected for fitment security on spark plugs, ignition harness and pipes, hoses and mufflers .
• Inspect for oil/exhaust leaks, muffler cracks/wear, loose accessories, gearbox case cracks, and oil breather obstructions.
• Inspect the engine firewall for physical defects, impact damage, corrosion, or loose fitment.
• Mechanical controls/moving parts should operate properly with no wear and full range of motion.

Propeller Inspection

• Inspect for nicks, dents, cracks, and gouges after cleaning.
• Nicks on the leading edge require immediate attention due to stress concentration.
• Ensure cleanliness, lubrication, proper blade angles, tracking, and dimensions per the manual.
• Perform governor leak and operational checks.
• Visually and physically inspect control linkages for proper tension and installation.

Aircraft Inspection During Ground Runs

• Checks during a ground run include engine temperatures/pressures, static RPM, magneto drop, engine response to power changes, unusual noises, ignition switch operation, fuel shut-off/selector valves, idle speed/mixture settings, suction gauge, and fuel flow indicator operation.

Lifed Items

• Many parts have a specific service life.
• Manufacturers monitor major components and advise on replacement during major service.
• Other lifed items include fire bottles, extinguishers, first aid kits, oxygen bottles, and emergency oxygen generators.
• Light bulbs are tested frequently, and important lights have two bulbs for redundancy.

Corrosion Prone Locations

• External surfaces, magnesium, and aluminum alloy surfaces.
• Rivet lines, lap joints, fasteners, faying surfaces.
• Damaged or neglected protective coatings.

Exhaust Areas

• Fairings are located in gas turbine and piston engine exhaust paths which are subject to highly corrosive influences.
• Exhaust deposits may be trapped in fissures, crevices, seams, or hinges and are difficult to remove by ordinary cleaning methods.
• Remove critical area fairings for cleaning/examination during maintenance.
• Inspect and clean all fairings in other exhaust areas.
• Apply a chemical barrier to critical areas to reduce deposits and corrosive effects.

Engine Intakes and Cooling Air Vents

• The protective finish on engine frontal areas is abraded by dust and high-velocity air and eroded by rain.
• Heat-exchanger cores and cooling fins may also be vulnerable to corrosion.
• Pay attention to obstructions/crevices in the cooling air path, especially in corrosive environments.

Landing Gear

• Landing gear bays are exposed to flying debris and require frequent cleaning/touch-up.
• Inspect crevices, ribs, and lower-skin surfaces where debris can lodge.
• Examine landing gear assemblies, especially magnesium alloy wheels, paintwork, bearings, exposed switches, and electrical equipment.
• Regularly clean, apply water-dispersing treatment, and relubricate.

Bilge and Water Entrapment Area

• Inspect drains frequently, as they can be blocked by debris.
• Carefully inspect areas beneath galleys and toilets/washrooms for corrosion.
• The protection in these areas must be carefully inspected and renewed if necessary.

Recesses in Flaps and Hinges

• Potential corrosion areas are found at flap and speed brake recesses.
• Water and dirt may collect in them unnoticed because the moveable parts are normally in the ‘closed’ position.
• Thorough inspection of the components and their associated stowage bays is required at regular intervals.
• Hinges are vulnerable to dissimilar metal corrosion between steel pins and aluminium tangs.

Spot-Welded Skins

• Corrosive agents may become trapped between the metal layers of spot-welded skins.
• Moisture entering the seams may set up electrolytic corrosion that eventually corrodes the spot-welds or causes the skin to bulge.
• Spot-welding is not considered good practice on aircraft structures.

Sandwich Constructions

• Cavities, gaps, punctures or damaged places in honeycomb sandwich panels should be sealed to exclude water or dirt.
• Water should not be permitted to accumulate in the structure adjacent to sandwich panels.
• Inspection of honeycomb sandwich panels and box structures is difficult and generally requires that the structure be dismantled.

Electrical Equipment

• Sealing, venting and protective paint cannot wholly obviate the corrosion in battery compartments.
• Spray from electrolyte spreads to adjacent cavities and rapidly attacks unprotected surfaces.
• Inspection should also be extended to all vent systems associated with battery bays.
• Circuit-breakers, contacts and switches are extremely sensitive to the effects of corrosion and need close inspection.

Control Cables

• Loss of protective coatings on carbon steel control cables can, over time, lead to mechanical problems and system failure.
• Corrosion-resistant cables can also be affected by corrosive marine environments.
• Any corrosion found on the outside of a control cable should result in a thorough inspection of the internal strands, and if any damage is found the cable should be rejected.
• Cables should be carefully inspected in the vicinity of bell-cranks, sheaves and other places where the cables flex as there is a greater chance of corrosion getting inside the cables when the strands are moving around (or being moved by) these items.
• Manufacturers have taken more care in the design process to improve aircraft corrosion resistance.
• This improvement includes the use of new materials and improved surface treatments and protective finishes.

Preventative Maintenance for Corrosion Control

• Adequate and regular cleaning of the aircraft.
• Periodic lubrication of moving parts (often after cleaning).
• Regular and detailed inspection for corrosion and failure of protective treatments.
• Prompt treatment of corrosion and touch-up of damaged paint.
• Maintenance of clear drain holes.
• Drainage of fuel cell sumps.
• Daily wiping down of most critical areas.
• Sealing of aircraft during foul weather and ventilation on sunny days.
• Use of protective covers and blanks.

General Treatments for Corrosion Removal

• Cleaning and stripping the protective coating in the corroded area.
• Removing as much of the corrosion product as possible as soon as it is discovered.
• Neutralising the remaining residue.
• Checking if damage is within limits and choosing the action to be taken.
• Restoring protective surface films.
• Applying temporary or permanent coatings or paint finishes.

Paint Removal Due To Corrosion

• Corrosion under a film of paint cannot be thoroughly inspected without first removing all of the paint.
• The complete suspect area should be cleaned of all grease, dirt or preservatives.
• Prior to applying a paint remover, all areas not to be stripped should be masked with heavy aluminium foil.
• Water-rinseable paint remover of a syrupy consistency is usually best for aircraft surfaces.
• Strippers can damage composite resins and plastics, so every effort should be made to mask these vulnerable areas.

Aluminium and Aluminium Alloys

• Aluminium corrosion products are white and voluminous.
• Even in its early stages, aluminium corrosion is evident as general etching, pitting or roughness.
• Three particularly serious forms of attack are: penetrating pit-type corrosion, stress corrosion cracking, intergranular attack.
• Treatment involves mechanical or chemical removal of corrosion products, inhibition of residual materials, and restoration of surface coatings.

Alclad

• Alclad is a trademark of Alcoa, used as a generic term to describe corrosion-resistant aluminium sheet.
• A high-purity aluminium surface layer is metallurgically bonded to both sides of high-strength aluminium alloy core material.
• The cladding thickness on each side of the sheet is approximately 5%.
• Where heavy corrosion is found on clad aluminium alloys, it must be removed by chemical methods wherever possible.
• Copious amounts of clean water should next be used to flood the area and remove all traces of the chemical, and then the surface should be dried thoroughly.

Assessment for Serviceability for Alclad

• To test for sufficient remaining aluminium cladding after corrosion removal from Alclad, a solution of Caustic soda can be sparingly applied.
• If the coating is intact, a white stain will appear.
• Where the pure aluminium has been exposed, a black stain will appear.
• Following this integrity test of the alclad, a solution of Chromic anhydride is used to neutralise the effects of the solution.
• The maximum depth of metal removed may be determined by using a dial test indicator mounted in a steel block or a depth gauge as illustrated.
• The Structural Repair Manual (SRM) or Service Bulletin gives the maximum acceptable reduction in thickness.

Re-protection Process for Alclad

• Apply the ALOCROM 1200 (Alodine) as follows: Brush Alocrom is supplied as two liquids: Parts A and B.
• To make the working solution, mix equal volumes of Part A and Part B in a plastic container. Stir well. Mix only a sufficient amount for use within 24 hours. Any mix remaining after 24 hours must be disposed of in the approved manner.
• Thoroughly de-grease the area to be treated and apply the solution with a nylon brush or cotton cloth until the surface turns to a golden yellow colour. This takes 1–10 min, depending on the temperature.
• Rinse with clean water, then allow it to dry for a minimum of 2 hr.
• Finally, apply the specified primer, e.g. epoxy primer, and the appropriate final finish within 48 hr. Note: Etch primers should not be applied over Alocrom 1200. Care must be taken when mixing and applying Alocrom 1200. PVC gloves and eye shields should be worn. Cloths used with Alocrom must be washed before discarding, or they may create a fire hazard.

Aluminium Alloy Castings and Forgings, Milled Skin Panels, Etc.

• After degreasing and removing the paint finish, remove the corrosion. The mechanical methods of removing corrosion are preferred.
• Nylon scrubbers or Scotch-Brite® pads may be used to remove mild corrosion.
• Aluminium wool or aluminium wire brush may be used to remove more severe corrosion when the part is not in-situ.
• Abrasive papers may also be used for mild corrosion.
• Vacu-Blast abrasive blasting with glass beads smaller than 500 meshes can be used to remove corrosion from pits.
• After using abrasives or brushing, examine the metal to ensure that all traces of corrosion have been removed.
• Under no circumstances should you use a steel wire brush or steel wool since traces of the steel can become embedded in the aluminium and lead to severe corrosion.
• The corrosion pits should be transformed into saucer-shaped depressions which relieve stress concentrations. Care must be taken when using power-driven tools to avoid overheating.
• The SRM or Service Bulletin gives the required proportions for blended areas.

Assessment for Serviceability for Aluminium Alloy Castings and Forgings, Milled Skin Panels, Etc.

• The maximum depth of metal removed must be within the limits specified in the SRM, Service Bulletins, etc. Initial assessment is normally carried out after removal of the loose corrosion to determine whether or not the component can be salvaged.
• It is usually specified that no pitting is permissible. Where pitting may be blended out, the maximum depth and area will be specified, or the dimensions of the part may be required to remain within the drawing limits.
• In general, corrosion removal must not weaken a part to such an extent that it endangers the safety of the aircraft. If in doubt, the part must be repaired or replaced.

Ferrous Metals

• Atmospheric oxidation of iron or steel surfaces causes ferrous oxide rust to be deposited.
• Rust shows on bolt heads, nuts or any unprotected hardware.
• The most practical means of controlling corrosion of steel is complete mechanical removal of corrosion products.
• Abrasive papers, power buffers, wire brushes and steel wool are all acceptable methods of removing rust on lightly stressed areas.
• Residual rust usually remains in pits and crevices. 

High-Stressed Steel Components

• Corrosion on high-stressed steel components may be dangerous and should be removed carefully with mild abrasive papers or fine buffing compounds.
• Protective finishes should be re-applied immediately.
• Steels in aircraft structures are normally plated with zinc, cadmium or chrome.
• The aim is to remove only the minimum of plating during corrosion removal.
• The most effective and preferred method of removing corrosion products from ferrous surfaces is by mechanical means.
• Chemical corrosion removers fall into two categories: phosphoric acid- or alkaline-based.
• After using rust removers, the area must be thoroughly rinsed with clean water and dried.

Assessment for Serviceability for Ferrous Metals

• The procedure followed during the assessment of steel parts is the same as that described for non-clad aluminium alloys.

Re-protection for Ferrous Metals

• Where possible, cadmium-plated steel parts should be re-plated in accordance with the aircraft manufacturer’s instructions. Any special instructions given in the SRM or Service Bulletins must be followed.

Magnesium Alloys

• When magnesium corrodes, the corrosion products occupy more space than the metal.
• Therefore, magnesium corrosion typically raises paint or, if it forms between lap joints, swells the joint.
• Degrease and remove paint from the affected area.
• Since magnesium is anodic to almost all commonly used aircraft structure metals, corrosion should not be removed with metallic tools.
• Where there is no danger of trapping the solution, light corrosion can be cleaned off by swabbing with a magnesium approved corrosion preventative solution.
• Care should be taken to confine the solution to the corroded area as it can damage the existing chromate film.

Assessment for Serviceability for Magnesium Alloys

• The procedure followed during the assessment of magnesium parts is the same as that described for non-clad aluminium alloy castings and forgings.

Blend Ratios

• Minimum 10D Depth of depression.
• Minimum 5D Depth of depression.

Acid Spillage

• Acids corrode most metals used in aircraft construction.
• Aircraft batteries of the lead-acid type give off acidic fumes.
• Battery bays should be well ventilated.
• Surfaces in the area should be treated with anti-acid paint.
• Correct Health and Safety procedures must be followed when such spillages occur.
• The correct procedure in the event of an acid spillage is as follows:

1. Mop up as much of the spilled acid as possible using wet rags or paper wipes.
2. Flood the area with large quantities of clean water, taking care that electrical equipment is suitably protected from the water.
3. Neutralise the area with a solution of sodium bicarbonate with water.
4. Test the area using universal indicating paper (or litmus paper) to check if acid has been cleaned up.
5. Dry the area completely and examine it for signs of damaged paint or plated finish and signs of corrosion.
6. Remove corrosion, repair damage and restore surface protection as appropriate.

Alkali Spillage

• Alkali spillage is most likely to occur from the alternative nickel-cadmium (NICAD) or nickel-iron (Ni-Fe) types of batteries, which contain an electrolyte of potassium hydroxide (or potassium hydrate).
• Removal of the alkali spillage and subsequent protective treatment follow the same basic steps as outlined in acid spillage, with the exception that the alkali is neutralised with a solution of chromic acid crystals in water.

Metallic Mercury Corrosion on Aluminium Alloys

• Spilled mercury on aluminium is rapid in both pitting and intergranular attack and is very difficult to control.
• X-ray inspection may be an effective method of locating the small particles of spilled mercury because the dense mercury shows up readily on X-ray film.
• Leakage of mercury on the aircraft structure. May result in the following items, which can cause aircraft issues: Damaged containers, cartons, electronic tubes,
• Spillage of mercury or mercury compounds within an aircraft requires immediate action for its isolation and recovery.
• Isolation and recovery of mercury prevents possible corrosion damage and embrittlement of aluminium alloy structural components, stainless steels and un-plated brass components such as cable turnbuckle barrels.

Mercury Spill Precautions

• Avoid contact with surfaces suspected of being contaminated. Use wood or fibre sheets to support the body while working in the area.
• Wear wing socks (shoe protectors), protective (disposable) clothing, and latex or vinyl gloves in the contaminated area to avoid scratching metal surfaces.
• Do not wear clothing used in contaminated areas on jobs in uncontaminated areas.
• Dispose of wing socks and protective clothing in unused metal containers outside of the aircraft.
• Contact the Waste Management Service or similar authority of the local State Health Department for proper mercury disposal procedures.
• Have personal clothing cleaned.
• Wash shoes with soap and water.
• Clean all tools that have been used in the contaminated area with steam or hot water and soap.
• Discard any drill bits used on mercury-contaminated areas.
• Thoroughly clean any vacuum cleaners used in the clean-up process.
• Always wash hands, face and exposed skin thoroughly with soap and water after contacting mercury. Keep hands away from mouth.
• Do not eat, smoke or blow your nose without first washing your hands thoroughly.
• Appreciable amounts of mercury will vaporise at normal temperatures. Stagnant air will become dangerous to personal health.
• Do not use cleaning products, such as solvents, solids or polishes, on contaminated areas. Such materials may promote corrosion.
• If your hands become contaminated with mercury while working with cleaning equipment, do not touch any exposed metal in surrounding area, as you may contaminate it.
• Dont pick up Mercury by hand.
• Always provide good ventilation while cleaning up mercury.
• Aviod breathing mercury vapours.

Mercury Removal Procedure

• Removal procedure and precautions necessary when mercury is spilled are:

1. Use a high-capacity vacuum cleaner with a trap-type glass container attached to the large vacuum hose. The size of the pickup hose from the container should be about 1/4 in. in diameter to increase the amount of suction applied to the mercury. Due to the weight of mercury, the container will catch the mercury before it can enter the vacuum cleaner hose.
2. An all-rubber storage battery water syringe or a medicine dropper may be used to remove mercury if the trap-type glass container and vacuum cleaner are not available.
3. Cellulose tape may be used to pick up very tiny particles.

• General clean-up and inspection (using equipment available) should be accomplished immediately after spillage occurs or is detected.

Cladding

• Aluminium alloys can be protected from corrosion by coating them with a layer of pure aluminium
• This is known as cladding.
• In manufacturing of clad aluminium, pure aluminium is hot-rolled onto the surface of an aluminium alloy and accounts for 10% of the sheet’s total thickness (5% per side).

Surface Conversion Coating

• These films also have the benefit of acting as a base for paint finishes to adhere to.
• For aluminium alloys, there are two types of conversion coats.
• Applying an oxide film is performed in factories by an electrolytic process known as anodising.
• Magnesium alloys may also be anodised.
• The anodising process is an electrolytic treatment in which a part is bathed in a lead vat containing a solution of chromic acid and water to form an oxide film.
• When small parts are fabricated in the field, or when the protective anodising film has been damaged or removed, the part can have a protective film applied through chemical processing.
• This process is known as alodising.
• The part may be submersed in a solution of chromic acid and water, or it may be brushed or swabbed on the part. Alodising leaves a gold-coloured film on aluminium alloys.

The process for Alodine is as follows:

1. Chemically clean the area or part to achieve an unbroken water film finish.
2. While the surface is still wet with rinse water, brush or spray on a liberal coating of the Alodine solution.
3. After the Alodine has stood for the recommended time, the area should be flushed with fresh water and allowed to dry.

• The area is ready for painting when dry.
• Powder appearing on the surface after the material is dried indicates poor rinsing or a failure to keep the surface wet during processing, and the part must be re-treated.

Nickel or Chrome Plating

• One method of protecting metals from corrosion is chrome plating.
• Two types of chrome processes are used in aircraft construction: decorative and hard chrome.

Cadmium Plating

• The vast majority of all steel aircraft hardware is cadmium plated.
• Cadmium oxides are similar to aluminium oxides as they form a protective layer and are dense, airtight and watertight.

Galvanising

• Galvanising is a process in which steel is coated with a protective layer of zinc.
• Zinc is deposited onto metal by dipping the metal into molten zinc or by electroplating.

Metal Spraying

• Metal spraying is the process of spraying molten metal onto the steel surface by feeding wire through an acetylene flame, then blowing it onto the surface with compressed air.
• Aircraft engine cylinders are sometimes protected from corrosion by spraying molten aluminium on their surface.

Paint Finishing

• One of the most universally used corrosion control devices for metal surfaces is a coat of paint.
• The quality of materials used to cover the substrate should match the desired durability, the type of material to be covered and the desired look.

After surface pre-treatment has been completed, primer is applied to provide a good bond between the surface to be painted and finish coats.

Primer

• Zinc chromate has been the standard primer for aircraft because of its good corrosion resistance.
• Two-component epoxy primer is recommended.

Wash Primer

• High-volume production of all-metal aircraft has brought about the development of a wash primer, which provides a good bond between the metal and the finish and cures after half an hour. 
• Wash Primer require moisture to properly convert the acid into the protective film.
• These primers can be used on aluminium, magnesium, steel or fibreglass surfaces.

Epoxy Primer

• Epoxy primer is the most popular primer as it provides maximum corrosion protection.
• It can be applied over the top of wash primer to provide maximum protection.

Zinc Chromate

• Zinc chromate is an olive-green inhibiting primer.
• The synthetic resin base of a zinc chromate primer provides a good bond between the finish and the metal. 

Four basic types of finish coating are used on aircraft:

• Synthetic enamel, acrylic lacquer, polyurethane and acrylic urethanes.
• Synthetic Enamel
• Enamel paint is one of the older finishes for metal aircraft and has been commonly used for automobiles.

Acrylic Lacquer

• They are primed with a two-part wash primer, and as soon as the primer is entirely dry, they are sprayed with the acrylic lacquer.

Polyurethane

• One of the most durable and attractive finishes on high-speed, high-altitude aircraft is produced by the polyurethane system.

Acrylic Urethanes

• Acrylic urethane finishes have the advantages of both acrylic lacquer and polyurethane.

Painting

• Painting processes vary greatly and often depend on the type of material, the painting surface and the equipment used.
• Prior to painting, test the gun to ensure the desired pressure and spray pattern are set. Adjust the air pressure and fluid adjustment valves to obtain the required flow and spray pattern.
• Begin spraying the surfaces by first painting the edged or corners.
• Move the gun using a steady stroke parallel to the surface.
• The smooth surface of a quality finish helps reduce drag as well as protect the base material from corrosion and abrasion.

Corrosion Prevention

• Corrosion requires an anode, a cathode, an electrolyte (water) and an electrical contact.
• Corrosion is typically prevented by removing the electrical contact through the application of protective coatings.

Aircraft Surface Cleaning

• Regular cleaning not only protects the surface finish and structure by removing corrosive agents, dirt and grime, but makes thorough visual inspections for corrosion, cracks and other surface defects possible.
• Prior to aircraft washing, pitot tubes and static ports should be covered, and wheel and brake assemblies should be covered to keep out cleaning agents.

Non-Metal Cleaning

• Non-metallic components sometimes require different cleaning techniques than metallic components.
• Plastic should be rinsed with water before drying with a soft cloth to prevent scratching.
• Whenever these materials are spilt onto a tyre, they should be immediately wiped off with a dry towel. The tyre should then be washed with soap and water.
• Areas that should be cleaned gently and never subjected to stiff brushes or abrasive materials or Rubber de-ice boots, composite structures.

Post-Wash Procedure

• Lubricate landing gear, flight controls, hinges, etc. Pressure greasing forces out any water that may have entered the component and prevents corrosion.

Other Considerations

• Covered pitot probes need to be visible from the ground.
• Nose and main wheel bearings should be covered to prevent washing fluid from causing them to fail.