7FAQ

Good Engineering Practice/Corrosion

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From the length of this article you might think that corrosion is not a straight forward subject. There are many variables, but some basic facts are easily understood so that you can use the appropriate technique to prevent corrosion forming on your pride-and-joy in the first place and to weed-out the bits that do appear.
Basically, as soon as metal is extracted from its hidey-hole in the ground, it tries to go back there as soon as possible. Some metals are veritably lazy in this respect and don't seem to mind the change of scenery for a while. Others however, can't wait to get back home! It's our job to keep all our metal where we want it - in the car. Who knows, metal might actually get to enjoy the ride as much as you do!
Your Se7en contains quite a lot of aluminium alloy, both in cast and sheet form. Some people seem to think aluminium doesn't corrode. Unfortunately it's one of the metals that can't wait to get back home. But in its rush to leave it gets TOO excited and piddles all over itself. An oxide film forms which blocks any further progress. This oxide film forms naturally, but it can also be artificially produced and even given some pretty colours - ANODISING!
This Article has been extracted from: Federal Aviation Administration (FAA) Advisory Circular AC43.13-1B "ACCEPTABLE METHODS, TECHNIQUES, AND PRACTICES - AIRCRAFT INSPECTION AND REPAIR, Dated 27/9/01[/

CORROSION

GENERAL. The purpose of this article is to provide information that will help you prevent, control, identify, and treat various types of 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).
Elimination of any one of these four conditions will stop electrochemical 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

Substances that cause corrosion are called corrosive agents. The most common corrosive agents are acids, alkali's, and salts. The atmosphere and water, the two most common media for these agents, may also act as 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

GENERAL. All corrosive attacks begin on the surface of the metal making the classification of corrosion by physical appearance a convenient means of identification.

GENERAL SURFACE CORROSION. General surface corrosion is the most common form of corrosion and results from a direct chemical attack on a metal surface and involves only the metal surface. General surface corrosion usually occurs over a wide area and is more or less equal in dispersion. On a polished surface, this type of corrosion is first seen as a general dulling of the surface, and if allowed to continue, the surface becomes rough and possibly frosted in appearance. The discoloration or general dulling of metal created by exposure to elevated temperatures is not to be considered general surface corrosion.

PITTING CORROSION. Pitting corrosion is one of the most destructive and intense forms of corrosion. It can occur in any metal but is most common on metals that form protective oxide films, such as aluminium and magnesium alloys. It is first noticeable as a white or gray powdery deposit, similar to dust, which blotches the surface. When the deposit is cleaned away, tiny holes or pits can be seen in the surface. These small surface openings may penetrate deeply into structural members and cause damage completely out of proportion to its surface appearance.

CONCENTRATION CELL CORROSION. Concentration cell corrosion, (also known as Crevice Corrosion) is corrosion of metals in a metal-to-metal joint, corrosion at the edge of a joint even though the joined metals are identical, or corrosion of a spot on the metal surface covered by a foreign material. Metal ion concentration cells and oxygen concentration cells are the two general types of concentration cell 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.

ACTIVE-PASSIVE CELLS. Metals which depend on a tightly adhering passive film, usually an oxide, for corrosion protection are prone to rapid corrosive attack by active-passive cells. Active-passive cells are often referred to as a type of concentration cell corrosion. However, the active-passive cell is actually two forms of corrosion working in conjunction. The corrosive action usually starts as an oxygen concentration cell. As an example, salt deposits on the metal surface in the presence of water containing oxygen can create the oxygen cell. The passive film will be broken beneath the salt crystals. Once the passive film is broken, the active metal beneath the film will be exposed to corrosive attack. Rapid pitting of the active metal will result. This reaction can become locally intense due to several factors. First the reaction is augmented by the affected area, since the proportion of the exposed base metal is small compared to the surrounding non-reactive metal. This effectively concentrates the focal point of the reaction, often resulting in deep pits in a short time and a greater rate of corrosion.

FILIFORM CORROSION. Filiform corrosion is a special form of oxygen concentration cell which occurs on metal surfaces having an organic coating (e.g. paint)system. It is recognized by its characteristic worm-like trace of corrosion products beneath the paint film. Polyurethane finishes are especially susceptible to filiform corrosion. Filiform occurs when the relative humidity of the air is between 78 and 90 percent and the surface is slightly acidic. This corrosion usually attacks steel and aluminium surfaces. The traces never cross on steel, but they will cross under one another on aluminium which makes the damage deeper and more severe for aluminium. If the corrosion is not removed, the area treated, and a protective finish applied, the corrosion can lead to intergranular corrosion, especially around fasteners and at seams. Filiform corrosion can be removed by sanding. Filiform corrosion can be prevented by storing aircraft (Se7ens) in an environment with a relative humidity below 70 percent, using coating systems having a low rate of diffusion for oxygen and water vapors, and by washing the aircraft (Se7en) to remove acidic contaminants from the surface.

INTERGRANULAR CORROSION. Intergranular corrosion is an attack on the grain boundaries of a metal. A highly-magnified cross section of any commercial alloy shows the granular structure of the metal. It consists of quantities of individual grains, and each of these tiny grains has a clearly defined boundary which chemically differs from the metal within the grain. The grain boundary and the grain center can react with each other as anode and cathode when in contact with an electrolyte. Rapid selective corrosion of the grain boundaries can occur.

EXFOLIATION CORROSION. Exfoliation corrosion is an advanced form of intergranular corrosion and shows itself by lifting up the surface grains of a metal by the force of expanding corrosion products occurring at the grain boundaries just below the surface. It is visible evidence of intergranular corrosion and is most often seen on extruded sections where grain thickness are usually less than in rolled forms.

GALVANIC CORROSION. Galvanic corrosion occurs when two dissimilar metals make contact in the presence of an electrolyte. It is usually recognizable by the presence of a build-up of corrosion at the joint between the metals.

FATIGUE CORROSION. Fatigue corrosion involves cyclic stress and a corrosive environment. Metals may withstand cyclic stress for an infinite number of cycles so long as the stress is below the endurance limit of the metal. Once the limit has been exceeded, the metal will eventually crack and fail from metal fatigue. However, when the part or structure undergoing cyclic stress is also exposed to a corrosive environment, the stress level for failure may be reduced many times. Thus, failure occurs at stress levels that can be dangerously low depending on the number of cycles assigned to the life-limited part.
  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.

FRETTING CORROSION. Fretting corrosion,(also known as wear corrosion or friction oxidation) can occur at the interface of two highly-loaded surfaces which are not supposed to move against each other. However, vibration may cause the surfaces to rub together resulting in an abrasive wear known as fretting. The protective film on the metallic surfaces is removed by this rubbing action. With continued rubbing, metal particles sheared from the surface of the metal combine with oxygen to form metal oxide. As these oxides accumulate, they cause damage by abrasive action and increased local stress. The most common example of fretting corrosion is the "smoking rivet" found on engine cowling and wing skins. This is one corrosion reaction that is not driven by an electrolyte, and in fact, moisture may inhibit the reaction. Application of a lubricant or installation of a fretting resistant material between the two surfaces can reduce fretting corrosion.

CORROSION PROTECTION MEASURES FOR BASIC MATERIALS

(This segment has had the items considered to be non-relevant edited out)

GENERAL. In the repair of aircraft (Se7ens), apply corrosion proofing of the same type or equivalent to that originally applied unless the repair would result in increased susceptibility to corrosion, in which case use additional corrosion protection measures. The following is a list of the most commonly-used corrosion-proofing techniques.

ANODIZING AND RELATED PROCESSES. In anodizing, aluminum alloys are placed in an electrolytic bath causing a thin film of aluminium oxide to form on the surface of the aluminium. This is resistant to corrosion and affords a good paint base. However, other processes, which do not provide as good a corrosive protection as anodizing, are good paint bases. The processes are:
  1. Alkaline cleaning followed by chromic acid dip;
  2. Alcoholic phosphoric acid cleaner; and
  3. Alkaline dichromate treatment.

PLATING. Steels are commonly plated with other metals to prevent corrosion. Plating is accomplished by placing the article in an electrolytic bath. Metals used in plating vary in the corrosion protection they afford steel. For instance, in platings that corrode before steel, such as zinc or cadmium, slight breaks or cracks throughout the plating will not result in rusting of the exposed steel. With the surface metal corroded, the steel is protected. However, when the steel corrodes faster than the plate metal, such as chromium, the amount of protection depends on the tightness of the plating. Post-plate bake treatment to relieve hydrogen embrittlement is a necessary part of replating procedures for high-strength steel parts. High-strength nuts and bolts are susceptible to failure from hydrogen embrittlement. Because of the potential failures of embrittled parts, careful control over the heat treatment, grinding, pre-plate cleaning, plating, and post-plate baking of high-strength parts is necessary. ('R' Clips?)

PHOSPHATE RUST-PROOFING. This process is commercially known as Parkerizing, Bonderizing, Granodizing, etc. The coating placed on the part is used to protect steel parts after machining and before painting.

CHROME-PICKLE TREATMENT. Magnesium parts which have been immersed or brushed with a solution of nitric acid and sodium dichromate will be protected for temporary storage. The coating will also serve as a bond for subsequent organic finishes. Sealed chrome-pickle treatment is used on magnesium parts for long term protection. Diluted chromic acid is a touch-up treatment. It is less critical to apply and can be applied over previously-applied thin chromate films.

DICHROMATE TREATMENT. This treatment consists of boiling magnesium parts in a solution of sodium dichromate. It provides good paint base and protective qualities on all standard wrought magnesium alloys except the magnesium-thorium alloys. Acid pickling of the magnesium surface prior to application of the dichromate treatment is required if maximum corrosion resistance of the finish is expected.

CLADDING. Aluminium alloys which are susceptible to corrosion are frequently clad with pure aluminium. Slight pits, scratches, or other defects through the cladding material must be avoided, since the aluminium alloy core will corrode rapidly.

ORGANIC COATINGS. Zinc chromate primer, enamels, chlorinated rubber com-pounds, etc., are organic coatings commonly used to protect metals.

TUBE INTERIORS. Protect the interiors of structural steel and aluminum tubing against corrosion. A small amount of water entrapped in a tube can corrode entirely through the tube thickness in a short period. Coat the tube interior by flushing with hot linseed oil, paralketone, or other approved corrosion inhibitor (WAXOYL). The flushing liquid is usually introduced through small holes drilled in the tubing. Allow the flushing liquid to drain and plug the holes with a screw or by other means to prevent entry of moisture. Air and water-tight sealing of the tubing will also give adequate protection against corrosion if the tubing is internally dry before being sealed.

CORROSION PREVENTIVE MAINTENANCE

GUIDELINES: ALL AIRCRAFT (Se7ens). Corrosion prevention depends on a comprehensive prevention and control plan, implemented from the start of operation of an aircraft (Se7en), which includes:
    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.

GUIDELINES: AIRCRAFT (Se7ens) OPERATING OVER (IN) SALT WATER (SPRAY). In addition to the inspection and treatment prescribed above, the following treatment shall be applied:
  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

GENERAL. This guide provides a general inspection checklist for those parts or surfaces that can be visually inspected without disassembly of the aircraft (Se7en). It is intended for use in establishing corrosion inspection areas for which the manufacturer has not provided a recommended corrosion inspection program. The manufacturer's recommended corrosion inspection program will take precedence over this guideline. These inspections should be accomplished in conjunction with other preventive maintenance.

EXHAUST TRAIL AREAS.
  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.

BATTERY COMPARTMENTS AND BATTERY VENT OPENINGS.
  1. Inspect battery compartment for electrolyte spillage, corrosion, and condition of protective paint.
  2. Inspect area around battery vent for corrosion.

BILGE AREAS. (Ahoy there matey!)
  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.

WHEEL WELLS AND LANDING GEAR.
  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.

EXTERNAL SKIN AREAS.
  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.

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

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

ELECTRONIC COMPARTMENTS.
  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.

FLEXIBLE HOSE ASSEMBLIES.
  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.

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

INTEGRAL FUEL CELLS.
  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.

ELECTRICAL CONNECTORS.
  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

GENERAL. General safety precautions for handling materials with hazardous physical properties are outlined in the following paragraphs. They also address emergency procedures for immediate treatment of personnel who have inadvertently come into contact with harmful materials. All personnel responsible for using or handling hazardous materials should be thoroughly familiar with the information in the following paragraphs.

SAFETY PRECAUTIONS.
  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.

CORROSION CONTROL WORK PROCEDURES.
  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

GENERAL. When active corrosion is found, a positive inspection and re-work program is necessary to prevent any further deterioration. The following methods of assessing corrosion damage and procedures for re-work of corroded areas could be used during cleanup programs. In general, any re-work would involve the cleaning and stripping of all finish from the corroded area, removal of corrosion products, and restoration of surface protective film.
  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.

PREPARATIONS FOR RE-WORK. All corrosion products should be removed completely when corroded structures are re-worked. Before starting re-work of corroded areas, carry out the following:
  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.

FAIRING OR BLENDING RE-WORKED AREAS. All depressions resulting from corrosion re-work should be faired or blended with the surrounding surface. Fairing can be accomplished as follows:
  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)
METALS OR MATERIALS TO BE PROCESSEDRESTRICTIONSOPERATIONABRASIVE PAPER OR CLOTHABRASIVE FABRIC OR PAD
FERROUS ALLOYSNoneCorrosion removal or fairingAluminium Oxide, 150 Grit or finer; 400 Grit finishing. 180 Grit or finer Silicon CarbideFine to Ultra Fine
ALUMINUM ALLOYSDO NOT USE SILICON CARBIDE ABRASIVECorrosion removal or fairingAluminium Oxide, 150 Grit or finer; 400 Grit finishing. 7/0 Grit or finer Garnet PaperVery Fine & Ulta Fine
MAGNESIUM ALLOYSNoneCorrosion removal or fairingAluminium Oxide, 240 Grit or finer; 400 Grit finishingVery Fine & Ulta Fine
TITANIUMNoneCleaning and finishingAluminium Oxide, 150 Grit or finer. 180 Grit or finer Silicon Carbiden/a
  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.

CLEANERS, POLISHES, AND BRIGHTENERS. It is important that aircraft (Se7ens) be kept thoroughly clean of contaminating deposits such as oil, grease, dirt, and other foreign materials.
  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.

STANDARD METHODS. Several standard mechanical and chemical methods are available for corrosion removal. Mechanical methods include hand sanding using abrasive mat, abrasive paper, or metal wool; and powered mechanical sanding, grinding, and buffing, using abrasive mat, grinding wheels, sanding discs, and abrasive rubber mats. The method used depends upon the metal and degree of corrosion. The removal method to use on each metal for each particular degree of corrosion is outlined in the following section.

ALUMINIUM AND ALUMINIUM ALLOYS

GENERAL. Aluminium and aluminium alloys are widely used in Se7en construction. Aluminium appears high in the electro-chemical series of elements and corrodes very easily. However, the formation of a tightly-adhering oxide film offers increased resistance under most corrosive conditions. Most metals in contact with aluminium form couples that undergo galvanic corrosion attack. The alloys of aluminium are subject to pitting, intergranular corrosion and intergranular stress corrosion cracking. In some cases the corrosion products of metal in contact with aluminum are corrosive to aluminum. There-fore, aluminium and its alloys must be cleaned and protected.

SPECIAL TREATMENT OF ANODIZED SURFACES. Anodizing is the most common surface treatment of aluminium alloy surfaces. The aluminium sheet or casting is made the positive pole in an electrolyte bath in which chromic acid or other oxidizing agents produce a supplemental protective oxide film on the aluminium surface. The anodized surface coating offers the alloy a great deal of protection as long as it is not damaged. Once the film is damaged, it can only be partially restored by chemical surface treatment. Therefore exercise care to avoid breaking of the protective film, particularly at the edges of the sheet.

REPAIR OF ALUMINIUM ALLOY SHEET METAL. After extensive corrosion removal the following procedures should be followed:
  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.

CORROSION REMOVAL AROUND COUNTERSUNK FASTENERS IN ALUMINIUM ALLOY. Intergranular corrosion in aluminium alloys often originates at countersunk areas where steel fasteners are used.
  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.

EXAMPLES OF REMOVING CORROSION FROM ALUMINIUM AND ALUMINIUM ALLOYS.
  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.
CAUTION: When working with MIL-C-38334, keep the solution away from magnesium surfaces. The solution must be confined to the area being treated. All parts and assemblies including cadmium-plated items and hinges susceptible to damage from acid should be masked and/or protected. Also mask all openings leading to the primary structure that could trap the solution and doors or other openings that would allow the solution (uncontrolled) to get into the aircraft (Se7en) or equipment interior. It is a good practice to keep a wet rag on hand at all times, for removal of spills or splashes.
      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

GENERAL. One of the most familiar kinds of corrosion is red iron rust. Red iron rust results from atmospheric oxidation of steel surfaces. Some metal oxides protect the underlying base metal, but red rust is not a protective coating. Its presence actually promotes additional attack by attracting moisture from the air and acts as a catalyst to promote additional corrosion.
  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.

SPECIAL TREATMENT OF STAINLESS STEEL. Stainless steels are of two general types: magnetic and non-magnetic.
  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.

EXAMPLE OF REMOVING CORROSION FROM FERROUS METALS. If possible, corroded steel parts should be removed from the aircraft (Se7en). When impractical to remove the part, follow the procedure below.
  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.
NOTE: Use of acid-based strippers, chemical removers, or chemical conversion coatings are not permitted on steel parts without engineering authorization.
  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.

NOTE: Steel surfaces are highly-reactive immediately following corrosion removal; consequently, primer coats should be applied within 1 hour after sanding.

PLATED PARTS

CHROMIUM AND NICKEL-PLATED PARTS. Nickel and chromium platings are used extensively as protective and wear-resistant coatings over high-strength steel parts (landing gear journals, shock strut pistons, etc.). Chromium and nickel plate provide protection by forming a somewhat impervious physical coat over the underlying base metal. When breaks occur in the surface, the protection is destroyed.
  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.

CADMIUM AND ZINC-PLATED PARTS. Cadmium plating is used in aircraft (Se7en) construction as a protective finish over steel. Protection is provided by a sacrificial process in which the cadmium is attacked rather than the underlying base material. Properly functioning cadmium surface coatings may show mottling, ranging from white to brown to black spots on their surfaces. These show the sacrificial protection being offered by the cadmium coat, and under no condition should such spotting be removed merely for appearance sake. In fact, cadmium will continue to protect even when actual breaks in the coating develop and bare steel surfaces appear.
  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

Titanium and titanium alloys are highly corrosion resistant because an oxide film forms on their surfaces upon contact with air.
When titanium is heated, different oxides having different colors form on the surface. A blue oxide coating will form at 700 to 800 °F; a purple oxide at 800 to 950 °F; and a gray or black oxide at 1000 °F or higher. These coatings are protective discolorations and should not be removed.
  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

Because of the properties of magnesium, it is recommended that extensive work on magnesium wheels be carried-out by the manufacturer

THIS SECTION IS INCLUDED FOR INFORMATION ONLY

GENERAL. Magnesium and magnesium alloys are the most chemically active of the metals used in aircraft (Se7en) construction (Some wheels) and are the most difficult to protect. However, corrosion on magnesium surfaces is probably the easiest to detect in its early stages. Since magnesium corrosion products occupy several times the volume of the original magnesium metal destroyed, initial signs show a lifting of the paint films and white spots on the magnesium surface. These rapidly develop into snow-like mounds or even white whiskers. The prompt and complete correction of the coating failure is imperative if serious structural damage is to be avoided.

IN-PLACE TREATMENT OF MAGNESIUM CASTINGS. Magnesium castings, in general, are porous. When attack occurs on a casting, the earliest practical treatment is required to prevent dangerous corrosive penetration. Engine cases in salt water can develop "moth holes" and complete penetration overnight.
  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.

EXAMPLE OF REMOVING CORROSION FROM MAGNESIUM. If possible, corroded magnesium parts shall be removed from aircraft (Se7ens). When impossible to remove the part, the following procedure will be used.
  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.

WARNING: Cuttings and small shavings from magnesium can ignite easily and are an extreme fire hazard. Fires of this metal must be extinguished with absolutely dry talc, calcium carbonate, sand, or graphite by applying the powder to a depth of 1/2 inch over the metal.

Tony Cummings

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