Good Engineering Practice/Corrosion

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• CORROSION
• FACTORS INFLUENCING CORROSION
• COMMON CORROSIVE AGENTS
• TYPES OF CORROSION
• CORROSION PROTECTION MEASURES FOR BASIC MATERIALS
• CORROSION PREVENTIVE MAINTENANCE
• VISUAL CORROSION INSPECTION GUIDE
• CORROSION REMOVAL PROCEDURES
• BASIC CORROSION REMOVAL TECHNIQUES
• ALUMINIUM AND ALUMINIUM ALLOYS
• FERROUS METALS
• PLATED PARTS
• TITANIUM AND TITANIUM ALLOYS
• MAGNESIUM

CORROSION

1. Corrosion is a natural occurrence that attacks metal by chemical or electrochemical action and converts it back to a metallic compound.
2. Four conditions must exist before electrochemical corrosion can occur. They are:
1. A metal subject to corrosion (Anode);
2. A dissimilar conductive material (Cathode), which has less tendency to corrode;
3. Presence of a continuous, conductive liquid path (Electrolyte); and
4. Electrical contact between the anode and the cathode (usually in the form of metal-to metal contact such as rivets, bolts, and corrosion).

FACTORS INFLUENCING CORROSION

1. Some factors which influence metal corrosion and the rate of corrosion are:
1. Type of metal;
2. Heat treatment and grain direction;
3. Presence of a dissimilar, less corrodible metal;
4. Anodic and cathodic surface areas (in galvanic corrosion);
5. Temperature;
6. Presence of electrolytes (hard water, salt water, battery fluids, etc.);
7. Availability of oxygen;
8. Presence of biological organisms;
9. Mechanical stress on the corroding metal;
10. Time of exposure to a corrosive environment; and,
11. Lead/graphite pencil marks on aluminium alloy. Believe it or not!!
2. Most pure metals are not suitable for aircraft (Se7en) construction and are used only in combination with other metals to form alloys. Most alloys are made up entirely of small crystalline regions, called grains. Corrosion can occur on surfaces of those regions which are less resistant and also at boundaries between regions, resulting in the formation of pits and intergranular corrosion. Metals have a wide range of corrosion resistance. The most active metals, (those which lose electrons easily), such as magnesium and aluminium, corrode easily. The most noble metals (those which do not lose electrons easily), such as gold and silver, do not corrode easily. [Hence their use for switch contacts, for example].
3. Corrosion is quickened by high-temperature environments that accelerate chemical reactions and increase the concentration of water vapor in the air.
4. Electrolytes (electrically-conducting solutions) form on surfaces when condensation, salt spray, rain, or rinse water accumulate. Dirt, salt, acidic gases, and engine exhaust gases can dissolve on wet surfaces, increasing the electrical conductivity of the electrolyte, thereby increasing the rate of corrosion.
5. When some of the electrolyte on a metal surface is partially confined, (such as between faying surfaces (overlapping joints) or in a deep crevice) the metal around this area corrodes more rapidly. This type of corrosion is called an oxygen concentration cell. Corrosion occurs more rapidly because the reduced oxygen content of the confined electrolyte causes the adjacent metal to become anodic to other metal surfaces on the same part that are immersed in electrolyte or exposed to air.
6. Slime, molds, fungi, and other living organisms (some microscopic) can grow on damp surfaces. Once they are established, the area usually remains damp, increasing the possibility of corrosion.
7. Manufacturing processes such as machining, forming, welding, or heat treatment can leave residual stress in aircraft (Se7en) parts and can cause cracking in a corrosive environment.

COMMON CORROSIVE AGENTS

1. Any acid will severely corrode most of the alloys used in airframes (Se7ens). The most destructive are sulfuric acid (battery acid), halogen acids (hydrochloric, hydrofluoric, and hydrobromic), nitrous oxide compounds, and organic acids found in the wastes of humans and animals. (Eewh!)
2. Alkali's, as a group, are not as corrosive as acids. Aluminium and magnesium alloys are exceedingly prone to corrosive attack by many alkaline solutions unless the solutions contain a corrosion inhibitor. Substances particularly corrosive to aluminium are washing soda, potash (wood ashes), and lime (cement dust).
3. The major atmospheric corrosive agents are oxygen and airborne moisture. Corrosion often results from the direct action of atmospheric oxygen and moisture on metal and the presence of additional moisture often accelerates corrosive attack, particularly on ferrous alloys (those containing iron).** The atmosphere may also contain other corrosive gases and contaminants, particularly industrial and marine salt spray.
4. The corrosiveness of water depends on the type and quantity of dissolved mineral and organic impurities and dissolved gasses (particularly oxygen) in the water. One characteristic of water that makes it corrosive is its conductivity. Physical factors, such as water temperature and velocity also have a direct bearing on its corrosiveness.

TYPES OF CORROSION

1. Metal Ion Concentration Cells. The solution may consist of water and ions of the metal which is in contact with water. A high concentration of the metal ions will normally exist under faying surfaces where the solution is stagnant, and a low concentration of metal ions will exist adjacent to the crevice which is created by the faying surface. An electrical potential will exist between the two points; the area of the metal in contact with the low concentration of metal ions will be anodic and corrode, and the area in contact with the high metal ion concentration will be cathodic and not show signs of corrosion.
2. Oxygen Concentration Cells. The solution in contact with the metal surface will normally contain dissolved oxygen. An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse into the solution, thereby creating a difference in oxygen concentration between two points. Typical locations of oxygen concentration cells are under gaskets, wood, rubber, and other materials in contact with the metal surface. Corrosion will occur at the area of low oxygen concentration (anode). Alloys are particularly susceptible to this type of crevice corrosion.

1. Fatigue corrosion failure occurs in two stages. During the first stage the combined action of corrosion and cyclic stress damages the metal by pitting and crack formations to such a degree that fracture by cyclic stress will occur, even if the corrosive environment is completely removed.
2. The second stage is essentially a fatigue stage in which failure proceeds by propagation of the crack (often from a corrosion pit or pits). It is controlled primarily by stress concentration effects and the physical properties of the metal. Fracture of a metal part due to fatigue corrosion, generally occurs at a stress level far below the fatigue limit of an un-corroded part, even though the amount of corrosion is relatively small.

CORROSION PROTECTION MEASURES FOR BASIC MATERIALS

1. Alkaline cleaning followed by chromic acid dip;
2. Alcoholic phosphoric acid cleaner; and
3. Alkaline dichromate treatment.

CORROSION PREVENTIVE MAINTENANCE

1. recognition of corrosion-inducing conditions;
2. corrosion identification techniques;
3. corrosion detection, cleaning, and treating; and
4. lubrication and preservation of aircraft (Se7en) structure and components.
1. Inspection for corrosion on a scheduled basis.
2. Thorough cleaning, inspection, lubrication, and preservation at prescribed intervals.
3. Prompt corrosion treatment after detection.
4. Accurate record-keeping and reporting of material or design deficiencies to the manufacturer.
5. Use of appropriate materials, equipment, and technical publications.
6. Maintenance of the basic finish systems.
7. Keeping drain holes and passages open and functional. Sealants, leveling compounds, miscellaneous debris, or corrosion inhibitors should not block drain paths.
8. Replacing deteriorated or damaged gaskets and sealants (using non-corrosive type sealants) to avoid water intrusion and entrapment that leads to corrosion.
9. Minimizing the exposure of aircraft (Se7ens) to adverse environments by keeping the aircraft (Se7en) in a hangar (Garage).
10. Periodic and frequent inspection of areas where there are foamed plastics or other absorbent material.
11. Daily draining of fuel cavities to remove accumulated water and other foreign matter.
12. Daily wipe-down of exposed critical surfaces of hydraulic cylinders.

1. Remove all traces of salt water and salt water residue by thoroughly washing the aircraft with fresh water.
1. After drying, coat the unpainted or unprotected parts of the engine and its installation parts by spraying or rubbing lightly with corrosion preventive compound, WD-40
2. Apply this mixture on parts that move or require some lubrication and on all fittings subject to corrosion such as control surface hinges, control cables, exposed rivets and bolts, and other similar parts not protected by paint. Apply with a cloth or a soft brush soaked in the mixture.
3. Wipe off excess mixture. When applying the mixture take care that as little as possible is deposited on exhaust pipes or collector rings to avoid a fire hazard when the engine is started. Keep the ignition wires, propeller anti-icer feed hose, tires, and other rubber parts free of the mixture.
2. Where maximum corrosion protection is desired on stationary parts, use exterior surface corrosion preventive compound. (WAXOIL)
3. Wipe the exposed portion of the landing gear shock strut (Shock absorber) piston with a cloth soaked in the applicable hydraulic fluid.
4. Most parts of landing gear wheels are made from magnesium or aluminum alloys which corrode rapidly unless carefully protected. When the aircraft (Se7en) operates near salt water, the corrosion can be very rapid. Inspect wheels to determine the paint condition.
5. Refinish portions of a wheel where paint has deteriorated, peeled, or chipped.
6. EXCEPT for friction and bearing surfaces, apply a protective coating to all parts of wheels and brake assemblies.

VISUAL CORROSION INSPECTION GUIDE

1. Visually inspect paint in areas of the exhaust trails for damage.
2. Visually inspect under fairings, around rivet heads, and in skin crevices, for corrosion in areas of engine exhaust trail.

1. Inspect battery compartment for electrolyte spillage, corrosion, and condition of protective paint.
2. Inspect area around battery vent for corrosion.

1. Inspect bilge areas for waste hydraulic fluids, water, dirt, loose fasteners, drill chips, and other debris.
2. Remove any foreign material from bilge and inspect for corrosion.

1. Inspect wheel well area and landing gear components for damage to exterior finish coating and corrosion. Particular attention should be given to exposed surfaces of struts, oleos, arms, links, and attaching hardware; axle interiors, exposed position indicator switches and other electrical equipment; crevices between stiffeners, ribs, and lower skin surfaces; magnesium wheels, particularly around bolt heads, lugs, and wheel web areas; and exposed rigid tubing at "B" nuts and ferrules under clamps, and tubing identification tapes.

1. Inspect external skin surfaces for damage to protective finishes and corrosion.
2. Inspect around fasteners for damage to protective finishes and corrosion.
3. Inspect lap joints for bulging of skin surface, which may indicate the presence of corrosion between the faying surfaces. Skin cracks and/or dished or missing fastener heads may also indicate severe corrosion in bonded joints.
4. Inspect area around spot welds for bulges, cracks, or corrosion.
5. Inspect piano type hinges for corrosion. When piano hinges are inspected they should be lubricated and actuated through several cycles to ensure complete penetration of the lubricant.
6. Inspect thick alloy skin surfaces for pitting, intergranular corrosion, and exfoliation of the metal. Look for white or gray deposits around countersunk fastener heads and raised areas or bumps under the paint film.
7. Inspect composite skins for corrosion of attachment fasteners.

1. Inspect area around edge of drain holes for corrosion and ensure that drain holes are not blocked by debris.

1. Inspect engine cases for damage to exterior finish and corrosion.
2. Inspect radiator cooler cores for corrosion.

1. Inspect circuit-breakers, fuses, contact points, and switches for evidence of moisture and corrosive attack.
2. Treatment of corrosion in electrical and electronic components should be done by or supervised by qualified personnel familiar with the function of the unit involved.

1. Inspect hose assemblies for chafing, weather-checking, hardening, discoloration, evidence of fungus, torn weather protective coatings or sleeves, and corrosion of fittings.
2. Replace any defective, damaged, twisted, or bulging hoses.

1. Inspect edges of sandwich panels for damage to the corrosion protection finish or sealant and for corrosion.

1. Inspect top coat finish for breaks, peeling, lifting of surface, or other damage.
2. Inspect aircraft (Se7en) structure for top coat finish damage from pitting or intergranular corrosion.

1. Inspect electrical connectors for breaks in potting compound and corrosion of pins and wires.
2. If the electrical connector is suspected of having moisture intrusion, disassemble the connector, clean the connector, and inspect it for corrosion.

CORROSION REMOVAL PROCEDURES

1. Chemical. When required to use or handle solvents, special cleaners, paint strippers (strong alkalies and acids), etchants (corrosion removers containing acids), or surface activation material, observe the following safety precautions:
1. Avoid prolonged breathing of solvent or acid vapors.
2. Never add water to acid. Always add acid to water.
3. Mix all chemicals per the manufacturer's instructions.
4. Clean water for emergency use should be available in the immediate work area before starting work.
5. Avoid prolonged or repeated contact with the skin of solvents, cleaners, etchants (acid), or conversion coating material. Rubber or plastic gloves should be worn. Goggles or plastic face shields and suitable protective clothing should be worn when cleaning, stripping, etching, or conversion coating overhead surfaces.
6. When mixing alkalies with water or other substance, use containers that are made to withstand heat generated by this process.
7. Wash any paint stripper, etchant, or conversion coating material immediately from body, skin, or clothing.
8. Materials splashed into the eyes should be promptly flushed out with water, and medical aid obtained immediately.
9. Do not eat or keep food in areas where poisons may be absorbed. Always wash hands before eating or smoking.
10. Verify that the area within 50 feet of any cleaning or treating operations where low flash point (140 °F or below) materials are being used, is clear and remains clear of all potential ignition sources.
11. Suitable fire-extinguishing equipment should be available to the cleaning/treating area.
12. Equipment should be effectively grounded where any flammable materials are being used.
13. If materials (acid, alkali, paint remover, or conversion coatings) are spilled on equipment and/or tools, treat immediately by rinsing with clean water, if possible, and /or neutralizing acids with baking soda and alkalies with a weak (5 percent) solution of acetic acid in water.
14. In confined location, do not use solvents with a low flash point, (below 100 degF) such as Methyl Ethyl Ketone (MEK) and acetone.
15. All equipment should be cleaned after work has been completed.
16. Check and follow all applicable restrictions and requirements on the use of solvents, primers, and top coats.
17. Check and follow all applicable restrictions and requirements for use and disposal of waste material.

1. The effectiveness of corrosion control depends on how well basic work procedures are followed. The following common work practices are recommended:
1. The work areas, equipment, and components should be clean and free of chips, grit, dirt, and foreign materials.
2. Do not mark on any metal surface with a graphite pencil or any type of sharp, pointed instrument. Temporary markings (defined as markings soluble in water or methyl chloroform) should be used for metal layout work or marking on the aircraft (Seven) to indicate corroded areas.
3. Graphite should not be used as a lubricant for any component. Graphite is cathodic to all structural metals and will generate galvanic corrosion in the presence of moisture, especially if the graphite is applied in dry form.
4. Do not abrade or scratch any surface unless it is an authorized procedure. If surfaces are accidentally scratched, the damage should be assessed and action taken to remove the scratch and treat the area.
5. Coated metal surfaces should not be polished for aesthetic purposes. Buffing would remove the protective coating and a brightly polished surface is normally not as corrosion resistant as a non-polished surface unless it is protected by wax or paint.
6. Protect surrounding areas when welding, grinding, or drilling, to prevent contamination with residue from these operations. In those areas where protective covering can-not be used, remove the residue by cleaning.
7. Severely corroded screws, bolts, and washers should be replaced. When a protective coating, such as a cadmium plating on bolts, or screws, is damaged, immediately apply an appropriate protective finish to prevent additional corrosion damage.

BASIC CORROSION REMOVAL TECHNIQUES

1. Repair of corrosion damage includes removal of all corrosion and corrosion products. When the corrosion damage is severe and exceeds the damage limits set by the aircraft (Se7en) or parts manufacturer, the part must be replaced.

1. Document corrosion damage.
2. Provide washing apparatus for rapid rinsing of all surfaces.
3. Prepare the aircraft (Se7en) for safe maintenance.
1. Remove battery.
2. Place car on secure and adequate axle stands if required.
4. Protect the louvres, airscoops, engine opening, wheels, tyres, and cockpit from moisture and chemical brightening agents.
5. Protect the surfaces adjacent to re-work areas from chemical paint strippers, corrosion removal agents, and surface treatment materials.

1. Remove rough edges and all corrosion from the damaged area. All dish-outs should be elliptically shaped with the major axis running longitudinally. (Select the proper abrasive for fairing operations from the table)

1. In critical and highly stressed areas, all pits remaining after the removal of corrosion products should be blended-out to prevent stress risers that may cause stress corrosion cracking.
2. Re-work depressions by forming smoothly blended dish-outs, using a ratio of 20:1, length to depth. In areas having closely spaced multiple pits, intervening material should be removed to minimize surface irregularity or waviness. Steel nut-plates and steel fasteners should be removed before blending corrosion out of aluminium structure. Steel or copper particles embedded in aluminium can become a point of future corrosion. All corrosion products must be removed during blending to prevent reoccurrence of corrosion.

1. Materials. Do not use harmful cleaning, polishing, brightening, or paint-removing materials. Use only those compounds that conform to existing government or established industry specifications or that have been specifically recommended by the aircraft (Se7en) manufacturer. Observe the product manufacturer's recommendations concerning use.
2. Chemical Cleaners. Chemicals must be used with great care in cleaning assembled aircraft (Se7ens). The danger of entrapping corrosive materials in faying surfaces and crevices counteracts any advantages in their speed and effectiveness. Use materials that are relatively neutral and easy to remove.
3. Removal of spilled battery acid. The battery, battery cover, battery box and adjacent areas will be corroded if battery acid spills onto them. To clean spilled battery acid, brush off any salt residue and sponge the area with fresh water. For lead-acid batteries, sponge the area with a solution of 6 ounces of sodium bicarbonate (baking soda) per gallon of fresh water. Apply generously until bubbling stops and let solution stay on the area for 5 to 6 minutes, but do not allow it to dry.

ALUMINIUM AND ALUMINIUM ALLOYS

1. If water can be trapped in blended areas, chemical conversion coat and fill the blended area with structural adhesive or sealant to the same level and contour as the original skin. When areas are small enough that structural strength has not been significantly decreased, no other work is required prior to applying the protective finish.
2. Where exterior doublers are installed, it is necessary to seal and insulate them adequately to prevent further corrosion.
3. All rivet holes should be drilled, countersunk, surface treated, and primed prior to installation of the doubler.
4. Apply a suitable sealing compound in the area to be covered by the doubler. Apply sufficient thickness of sealing compound to fill all voids in the area being repaired.
5. Install rivets wet with sealant. Sufficient sealant should be squeezed out into holes so that all fasteners, as well as all edges of the repair plate, will be sealed against moisture.
6. Remove all excess sealant after fasteners are installed. Apply a fillet sealant bead around the edge of the repair. After the sealant has cured apply the protective paint finish to the reworked area.

1. When corrosion is found around a fixed fastener head, the fastener must be removed to ensure corrosion removal. All corrosion must be removed to prevent further corrosion and loss of structural strength. To reduce the recurrence of corrosion, the panel should receive a chemical conversion coating, be primed, and have the fasteners installed wet with sealant.
2. Each time removable steel fasteners are removed from access panels, they should be inspected for condition of the plating. If mechanical or plating damage is evident, replace the fastener. One of the following fastener installation methods should be used:
1. Brush a corrosion-preventive compound on the substructure around and in the fastener hole, start the fastener, apply a bead of sealant to the fastener countersink, set and torque the fastener within the working time of the sealant (this is the preferred method).
2. Apply the corrosion preventive compound to the substructure and fastener, set and torque the fastener.
3. Apply a coating of primer to the fastener, and while wet with primer, set and torque the fastener.

1. Positively identify the metal as aluminium. (Sounds like a real good first step)
2. Clean the area to be reworked. Strip paint if required.
3. Determine extent of corrosion damage.
4. Remove light to moderate corrosion with one of the following.
1. Non-Powered Corrosion Removal.
1. The removal of corrosion products by hand can be accomplished by use of aluminium oxide paper or cloth, such as non-woven, non-metallic, abrasive mat, abrasive cloth, and paper. Aluminium wool, fiber bristle brushes, and pumice powder are also acceptable methods.
2. Stainless steel brush may be used as long as the bristles do not exceed 0.010 inch in diameter. After use of this brush the surface should be polished with 60 grit aluminium oxide abrasive paper, then with 400 grit aluminium oxide paper. Care should be exercised in any cleaning process to avoid breaking the protective film.
3. Steel wool, emery cloth, steel wire brushes (except stainless steel brush) copper alloy brushes, rotary wire brushes, or severe abrasive materials should not be used on any aluminium surface.
2. Chemical Corrosion Removal.
1. The corrosion removal compound aluminium pre-treatment MIL-C-38334, an acid material, may be used to remove corrosion products from aluminium alloy materials or items (e.g., skins, stringer, ribs, tubing, or ducts). MIL-C-38334 is available in two types:
1. Type I Liquid concentrate materials should be diluted in accordance with the manufacture's instructions before use. Type I has a 1 year shelf life; therefore it shall not be used after 1 year from the date of manufacture.
2. Type II Powdered concentrate materials should be dissolved in the volume of water specified on the kit. These materials have an indefinite shelf life in the dry state. Once mixed, they should be used within 90 days.
2. Mix MIL-C-38334 in wood, plastic, or plastic-lined containers only. Wear acid-resistant gloves, protective mask and protective clothing when working with this acid compound. If acid contacts the skin or eyes, flush immediately with water.
3. Apply MIL-C-38334 solution by flowing, mopping, sponging, brushing, or wiping. When applying the solution to large areas, begin the application at the lowest area and work upward, applying the solution with a circular motion to disturb the surface film and ensure proper coverage. If pumping is required, pumps, valves, and fittings should be manufactured from 18-8 stainless steel or plastic.

1. Allow the solution to remain on the surface for approximately 12 minutes and then rinse away with clean tap water. For pitted or heavily-corroded areas the compound will be more effective if applied warm (140 °F) followed by vigorous agitation with a non-metallic acid-resisting brush or aluminium oxide abrasive nylon mat. Allow sufficient dwell time, 12 to 15 minutes, before rinsing. After each application examine the pits and/or corroded area to determine if another application is required with a 4 to 10 power magnifying glass. (Select the power depending on the distance available to make the inspection.) Corrosion still on the area will appear as a powdery crust slightly different in color than the uncorroded base metal. Darkening of area due to shadows and reaction from the acid re-mover should not be considered.
2. Once the corrosion has been removed and the area well-rinsed with clean water, a chromate conversion coating must be applied immediately thereafter.
1. Powered Corrosion Removal.
1. Where the problem is severe enough to warrant the use of power tools, a pneumatic drill motor driving either an aluminium-oxide-impregnated nylon abrasive wheel, flap brush or rubber grinding wheel may be used with an abrasive value to approximately 120 grit, as needed. Corrosion-removal accessories, such as flap brushes or rotary files, should be used on one type of metal only. For example, a flap brush used to remove aluminium should not be used to remove magnesium, steel, etc. Pneumatic sanders may be used with disk and paper acceptable for use on aluminium.
2. When mechanically removing corrosion from aluminum, especially aircraft skin thinner than 0.0625 inch, extreme care must be used. Vigorous, heavy, continuous abrasive grinding can generate enough heat to cause metallurgical change. If heat damage is suspected, hardness tests or conductivity tests must be accomplished to verify condition of the metal. The use of powered rotary files should be limited to heavy corrosion and should not be used on skin thinner than 0.0625 inch.

FERROUS METALS

1. Red rust first shows on bolt heads, hold down nuts, and other unprotected aircraft (Se7en) hardware. Red rust will often occur under nameplates that are secured to steel parts. Its presence in these areas is generally not dangerous. It has no immediate effect on the structural strength of any major components. However, it shows a general lack of maintenance and may indicate attack in more critical areas.
2. When paint failures occur or mechanical damage exposes highly-stressed steel surfaces to the atmosphere, even the smallest amount of rusting is potentially dangerous and should be removed immediately.

1. Magnetic steels are of the ferritic or martensitic types and are identified by numbers in the 400-series. Corrosion often occurs on 400-series stainless steels and treatment is the same as specified in high-strength steels.
2. Non-magnetic stainless steels are of the austenitic type and are identified by numbers in the 300-series. They are much more corrosion resistant than the 400-series steels, particularly in a marine environment.
1. Austenitic steels develop corrosion resistance by an oxide film, which should not be removed even though the surface is discolored. The original oxide film is normally formed at time of fabrication by passivation. If this film is broken accidentally or by abrasion, it may not restore itself without repassivation.
2. If any deterioration or corrosion does occur on austenitic steels, and the structural integrity or serviceability of the part is affected, it will be necessary to remove the part.

1. Prepare the area for re-work.
2. Positively identify the metal as steel and establish its heat-treated value.
3. Clean the area and strip paint if required.

1. Determine extent of corrosion damage.
2. Remove residual corrosion by hand sanding with mild abrasive mats, cloths, and papers, such as fine aluminum oxide grit.
3. Remove heavy deposits of corrosion products by approved mechanical methods for that particular form of steel and/or stainless steel.
4. Inspect the area for remaining corrosion. Repeat procedure if any corrosion remains and the structural integrity of the part is not in danger, and the part meets the rework limits established by the manufacturer.
5. Fair depressions using a blend ratio of 20:1. Clean area using 240-grit paper. Smooth area with 300-grit paper and give final polish with 400-grit paper.
6. Determine depth of faired depression to ensure that re-work limits have not been exceeded.
7. Clean reworked area with dry cleaning solvent. Do not use kerosene.
8. Apply protective finish or specific organic finish as required.

PLATED PARTS

1. The amount of re-working that can be performed on chromium and nickel-plated components is limited.
2. The re-work should consist of light buffing to remove corrosion products and produce the required smoothness. The buffing should not take the plating below the minimum allowable thickness.
3. Whenever a chromium or nickel-plated component requires buffing, coat the area with a corrosion-preventive compound, if possible.
4. When buffing exceeds the minimum thickness of the plating, or the base metal has sustained corrosive attack, the component should be removed and replaced.
5. The removed component can be restored to serviceable condition by having the old plating completely stripped and re-plated in accordance with acceptable methods and specifications.

1. When the breakdown of the cadmium plating occurs and the initial appearance of corrosion products on the base metal develops, some mechanical cleaning of the area may be necessary but shall be limited to removal of the corrosion products from the underlying base material.
2. Under no condition should such a coating be cleaned with a wire brush. If protection is needed, a touch-up with primer or a temporary preservative coating should be applied. Restoration of the plate coating cannot be done in the field.
3. Zinc coatings offer protection in an identical manner to cadmium, and the corrective treatment for failure is generally the same as for cadmium-plated parts. However, the amount of zinc on aircraft (Se7en) structures is limited and usually does not present a maintenance problem.

TITANIUM AND TITANIUM ALLOYS

1. Corrosive attack on titanium surfaces is difficult to detect. It may show deterioration from the presence of salt deposits and metal impurities at elevated temperatures so periodic removal of surface deposits is required. However, if corrosion develops on titanium, it usually occurs as pitting. Acceptable methods for corrosion removal are:
1. Stainless steel wool or hand brush.
2. Abrasive mats, cloths, and papers with either aluminum oxide or silicon carbide grit.
3. Hand polish with aluminum polish and soft cloth.
2. Titanium surfaces are susceptible to hydrogen embrittlement that can induce stress corrosion and associated pitting. Therefore, chemicals such as fire-resistant hydraulic fluids must be controlled. Chlorinated hydrocarbon solvents and chemical corrosion removers are prohibited from use on titanium and titanium alloys.

MAGNESIUM

1. Baked enamel overcoats are usually involved rather than other top coat finishes. A good air drying enamel can be used to restore protection.
2. If extensive removal of corrosion products from a structural casting is involved, a decision from the aircraft (Wheel) manufacturer may be necessary to evaluate the adequacy of structural strength remaining. Refer to the aircraft (Wheel) manufacturer if any questions of safety are involved.

1. Positively identify metal as magnesium.
2. Clean area to be re-worked.
3. Strip paint if required.
4. Determine the extent of corrosion damage.
5. Remove light to moderate corrosion by one of the following means.
1. Non-Powered Corrosion Removal.
1. Non-powered removal can be accomplished using abrasive mats, cloth, and paper with aluminum oxide grit (do not use silicon carbide abrasive). Metallic wools and hand brushes compatible with magnesium such as stainless steel and aluminum, may be used.
2. When a brush is used the bristles should not exceed 0.010 inch in diameter. After using a brush, the surface should be polished with 400 grit aluminium oxide abrasive paper, then with 600 grit aluminium oxide abrasive paper.
3. Pumice powder may be used to remove stains or to remove corrosion on thin metal surfaces where minimum metal removal is allowed.
2. Chemical Corrosion Removal. This section has been edited-out because of the potential danger to an untrained operator
6. Remove moderate to heavy corrosion by one of the following means.
1. Powered Corrosion Removal.
1. Powered corrosion removal can be accomplished using pneumatic drill motor with either an aluminium-oxide-impregnated abrasive wheel, flap brush, or rubber grinding wheel with an abrasive value to approximately 120 grain size.
2. Also a rotary file with fine flutes can be used for severe or heavy corrosion product buildup on metals thicker than 0.0625 inch. If a flap brush or rotary file is used, it should only be used on one type of metal. Do not use either a hand or rotary carbon steel brush on magnesium.
3. Pneumatic sanders are acceptable if used with disk or paper of aluminum oxide. When using sanders, use extra care to avoid over heating the metal.
4. Do not use rotary wire brushes on magnesium.