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FAA Advisory Circular (AC) 43-4A, Corrosion Con- trol for Aircraft. The advisory circular is an extensive handbook, which deals with the sources of corrosion particular to aircraft structures, as well as steps the aircraft maintenance technician can take in the course of maintaining aircraft that have been attacked by cor- rosion.
Metal corrosion is the deterioration of the metal by chemical or electrochemical attack. This type of dam- age can take place internally as well as on the sur- face. As in the rotting of wood, this deterioration may change the smooth surface, weaken the interior, or damage or loosen adjacent parts.
Water or water vapor containing salt combines with oxygen in the atmosphere to produce the main source of corrosion in aircraft. Aircraft operating in a marine environment, or in areas where the atmosphere con- tains industrial fumes that are corrosive, are particu- larly susceptible to corrosive attacks. [Figure 6-1]
If left unchecked, corrosion can cause eventual struc- tural failure. The appearance of corrosion varies with the metal. On the surface of aluminum alloys and magnesium, it appears as pitting and etching, and is
Corrosion Control
Many aircraft structures are made of metal, and the most insidious form of damage to those structures is corrosion. From the moment the metal is manufac- tured, it must be protected from the deleterious effects of the environment that surrounds it. This protection can be the introduction of certain elements into the base metal, creating a corrosion resistant alloy, or the addition of a surface coating of a chemical conver- sion coating, metal or paint. While in use, additional moisture barriers, such as viscous lubricants and pro- tectants may be added to the surface.
The introduction of airframes built primarily of com- posite components has not eliminated the need for careful monitoring of aircraft with regard to corro- sion. While the airframe itself may not be subject to corrosion, the use of metal components and acces- sories within the airframe means the aircraft mainte- nance technician must be on the alert for the evidence of corrosion when inspecting any aircraft.
This chapter provides an overview to the problems associated with aircraft corrosion. For more in-depth information on the subject, refer to the latest edition of
Figure 6-1. Seaplane operations.
Electron conductor metal
Electron conductor metal
Continuous liquid path (electrolyte)
Continuous liquid path (electrolyte)
Unbroken paint film
No contact between electrolyte and anode and cathode
Anodic area
Cathodic area
Anodic area Cathodic area
E l e c t r o nfl ow
Currentflow
Figure 6-3. Electrochemical attack.
often combined with a gray or white powdery deposit. On copper and copper alloys, the corrosion forms a greenish film; on steel, a reddish corrosion byproduct commonly referred to as rust. When the gray, white, green, or reddish deposits are removed, each of the surfaces may appear etched and pitted, depending upon the length of exposure and severity of attack. If these surface pits are not too deep, they may not significantly alter the strength of the metal; however, the pits may become sites for crack development, par- ticularly if the part is highly stressed. Some types of corrosion burrow between the inside of surface coat- ings and the metal surface, and can spread until the part fails.
Types of Corrosion
There are two general classifications of corrosion that cover most of the specific forms: direct chemi- cal attack and electrochemical attack. In both types of corrosion, the metal is converted into a metallic com- pound such as an oxide, hydroxide, or sulfate. The corrosion process always involves two simultaneous changes: The metal that is attacked or oxidized suffers what may be called anodic change, and the corrosive agent is reduced and may be considered as undergo- ing cathodic change.
Figure 6-2. Direct chemical attack in a battery compartment.
sion will spread under the surface coating and cannot be recognized by either the roughening of the surface or the powdery deposit. Instead, closer inspection will reveal the paint or plating is lifted off the surface in small blisters which result from the pressure of the underlying accumulation of corrosion products. [Fig- ure 6-5]
Filiform corrosion gives the appearance of a series of small worms under the paint surface. It is often seen on surfaces that have been improperly chemically treated prior to painting. [Figure 6-6]
Dissimilar Metal Corrosion Extensive pitting damage may result from contact between dissimilar metal parts in the presence of a conductor. While surface corrosion may or may not be taking place, a galvanic action, not unlike electro- plating, occurs at the points or areas of contact where the insulation between the surfaces has broken down or been omitted. This electrochemical attack can be very serious because in many instances the action is taking place out of sight, and the only way to detect it prior to structural failure is by disassembly and inspection. [Figure 6-7]
The contamination of a metal’s surface by mechani- cal means can also induce dissimilar metal corrosion. The improper use of steel cleaning products, such as steel wool or a steel wire brush on aluminum or mag- nesium, can force small pieces of steel into the metal being cleaned, which will then further corrode and
become anodic and the less active metal to become cathodic, thereby establishing conditions for corro- sion. These are called local cells. The greater the dif- ference in electrical potential between the two metals, the greater will be the severity of a corrosive attack, if the proper conditions are allowed to develop.
The conditions for these corrosion reactions are the presence of a conductive fluid and metals having a difference in potential. If, by regular cleaning and surface refinishing, the medium is removed and the minute electrical circuit eliminated, corrosion cannot occur. This is the basis for effective corrosion control. The electrochemical attack is responsible for most forms of corrosion on aircraft structure and compo- nent parts.
Forms of Corrosion
There are many forms of corrosion. The form of cor- rosion depends on the metal involved, its size and shape, its specific function, atmospheric conditions, and the corrosion producing agents present. Those described in this section are the more common forms found on airframe structures.
Surface Corrosion
Surface corrosion appears as a general roughening, etching, or pitting of the surface of a metal, frequently accompanied by a powdery deposit of corrosion prod- ucts. Surface corrosion may be caused by either direct chemical or electrochemical attack. Sometimes corro-
Figure 6-5. Surface corrosion.
ruin the adjoining surface. Carefully monitor the use of nonwoven abrasive pads, so that pads used on one type of metal are not used again on a different metal surface.
Intergranular Corrosion
This type of corrosion is an attack along the grain boundaries of an alloy and commonly results from a lack of uniformity in the alloy structure. Aluminum
alloys and some stainless steels are particularly sus- ceptible to this form of electrochemical attack. [Figure 6-8] The lack of uniformity is caused by changes that occur in the alloy during heating and cooling during the material’s manufacturing process. Intergranular corrosion may exist without visible surface evidence. Very severe intergranular corrosion may sometimes cause the surface of a metal to “exfoliate.” [Figure 6-9] This is a lifting or flaking of the metal at the
Figure 6-6. Filiform corrosion.
Figure 6-7. Dissimilar metal corrosion.
- Soil and atmospheric dust.
- Oil, grease, and engine exhaust residues.
- Salt water and salt moisture condensation.
- Spilled battery acids and caustic cleaning solutions.
- Welding and brazing flux residues.
It is important that aircraft be kept clean. How often and to what extent an aircraft should be cleaned depends on several factors, including geographic location, model of aircraft, and type of operation.
Preventive Maintenance
Much has been done to improve the corrosion resis- tance of aircraft: improvements in materials, surface treatments, insulation, and in particular, modern protective finishes. All of these have been aimed at reducing the overall maintenance effort, as well as improving reliability. In spite of these improvements, corrosion and its control is a very real problem that requires continuous preventive maintenance. During any corrosion control maintenance, consult the Mate- rial Safety Data Sheet (MSDS) for information on any chemicals used in the process.
Corrosion preventive maintenance includes the fol- lowing specific functions:
- Adequate cleaning
- Thorough periodic lubrication
ized abrasion occurs. [Figure 6-10] The presence of water vapor greatly increases this type of deteriora- tion. If the contact areas are small and sharp, deep grooves resembling brinell markings or pressure indentations may be worn in the rubbing surface. As a result, this type of corrosion (on bearing surfaces) has also been called false brinelling.
Factors Affecting Corrosion
Many factors affect the type, speed, cause, and seri- ousness of metal corrosion. Some of these factors can be controlled and some cannot.
Climate
The environmental conditions under which an aircraft is maintained and operated greatly affect corrosion characteristics. In a predominately marine environ- ment (with exposure to sea water and salt air), mois- ture-laden air is considerably more detrimental to an aircraft than it would be if all operations were con- ducted in a dry climate. Temperature considerations are important because the speed of electrochemical attack is increased in a hot, moist climate.
Foreign Material
Among the controllable factors which affect the onset and spread of corrosive attack is foreign material that adheres to the metal surfaces. Such foreign material includes:
Figure 6-10. Fretting corrosion.
- Detailed inspection for corrosion and failure of protective systems
- Prompt treatment of corrosion and touchup of damaged paint areas
- Keeping drain holes free of obstructions
- Daily draining of fuel cell sumps
- Daily wipe down of exposed critical areas
- Sealing of aircraft against water during foul weather and proper ventilation on warm, sunny days
- Maximum use of protective covers on parked aircraft
After any period during which regular corrosion pre- ventive maintenance is interrupted, the amount of maintenance required to repair accumulated corrosion damage and bring the aircraft back up to standard will usually be quite high.
Inspection
Inspection for corrosion is a continuing problem and should be handled on a daily basis. Overemphasiz- ing a particular corrosion problem when it is discov- ered and then forgetting about corrosion until the next crisis is an unsafe, costly, and troublesome practice. Most scheduled maintenance checklists are complete enough to cover all parts of the aircraft or engine, and no part of the aircraft should go uninspected. Use these checklists as a general guide when an area is to be inspected for corrosion. Through experience it will be learned that most aircraft have trouble areas where, despite routine inspection and maintenance, corrosion will set in.
In addition to routine maintenance inspections, amphibians or seaplanes should be checked daily and critical areas cleaned or treated, as necessary.
Corrosion Prone Areas
Discussed briefly in this section are most of the trou- ble areas common to all aircraft. However, this cover- age is not necessarily complete and may be amplified and expanded to cover the special characteristics of the particular aircraft model involved by referring to the applicable maintenance manual.
Exhaust Trail Areas
Both jet and reciprocating engine exhaust deposits are very corrosive and give particular trouble where gaps, seams, hinges, and fairings are located downstream from the exhaust pipes or nozzles. [Figure 6-11]
Deposits may be trapped and not reached by nor- mal cleaning methods. Pay special attention to areas around rivet heads and in skin lap joints and other crevices. Remove and inspect fairings and access plates in the exhaust areas. Do not overlook exhaust deposit buildup in remote areas, such as the empen- nage surfaces. Buildup in these areas will be slower and may not be noticed until corrosive damage has begun.
Battery Compartments and Battery Vent Openings Despite improvements in protective paint finishes and in methods of sealing and venting, battery compart- ments continue to be corrosion prone areas. Fumes from overheated electrolyte are difficult to contain and will spread to adjacent cavities and cause a rapid corrosive attack on all unprotected metal surfaces. Battery vent openings on the aircraft skin should be included in the battery compartment inspection and maintenance procedure. Regular cleaning and neu- tralization of acid deposits will minimize corrosion from this cause.
Bilge Areas These are natural sumps for waste hydraulic fluids, water, dirt, and odds and ends of debris. Residual oil quite often masks small quantities of water that settle to the bottom and set up a hidden chemical cell.
Instead of using chemical treatments for the bilge water, current float manufacturers recommend the diligent maintenance of the internal coatings applied to the float’s interior during manufacture. In addition to chemical conversion coatings applied to the surface of the sheet metal and other structural components, and to sealants installed in lap joints during construc- tion, the interior compartments are painted to protect
Figure 6-11. Exhaust nozzle area.
Corrosion Removal
In general, any complete corrosion treatment involves the following: (1) cleaning and stripping of the cor- roded area, (2) removing as much of the corrosion products as practicable, (3) neutralizing any residual materials remaining in pits and crevices, (4) restoring protective surface films, and (5) applying temporary or permanent coatings or paint finishes.
The following paragraphs deal with the correction of corrosive attack on aircraft surface and compo- nents where deterioration has not progressed to the point requiring rework or structural repair of the part involved.
Surface Cleaning and Paint Removal The removal of corrosion necessarily includes removal of surface finishes covering the attacked or suspected area. To assure maximum efficiency of the stripping compound, the area must be cleaned of grease, oil, dirt, or preservatives. This preliminary cleaning operation is also an aid in determining the extent of the spread of the corrosion, since the stripping operation will be held to the minimum consistent with full exposure of the corrosion damage. Extensive corrosion spread on any panel should be corrected by fully treating the entire section.
The selection of the type of materials to be used in cleaning will depend on the nature of the matter to be removed. Modern environmental standards encour- age the use of water-based, non-toxic cleaning com- pounds whenever possible. In some locations, local or state laws may require the use of such products, and prohibit the use of solvents that contain volatile
Corrosion of metal skins joined by spot welding is the result of the entrance and entrapment of corrosive agents between the layers of metal. This type of corro- sion is evidenced by corrosion products appearing at the crevices through which the corrosive agents enter. More advanced corrosive attack causes skin buckling and eventual spot weld fracture. Skin buckling in its early stages may be detected by sighting along spot welded seams or by using a straightedge. The only technique for preventing this condition is to keep potential moisture entry points, including seams and holes created by broken spot welds, filled with a seal- ant or a suitable preservative compound.
Miscellaneous Trouble Areas
Helicopter rotor heads and gearboxes, in addition to being constantly exposed to the elements, contain bare steel surfaces, many external working parts, and dis- similar metal contacts. Inspect these areas frequently for evidence of corrosion. The proper maintenance, lubrication, and the use of preservative coatings can prevent corrosion in these areas.
All control cables, whether plain carbon steel or cor- rosion resistant steel, should be inspected to determine their condition at each inspection period. In this pro- cess, inspect cables for corrosion by random cleaning of short sections with solvent soaked cloths. If exter- nal corrosion is evident, relieve tension and check the cable for internal corrosion. Replace cables that have internal corrosion. Remove light external corrosion with a nonwoven abrasive pad lightly soaked in oil or, alternatively, a steel wire brush. When corrosion products have been removed, recoat the cable with preservative.
Bare steel hinge pin
Al Alloy extrusions
Hidden corrosion occurs here. Joint freezes and lugs break off when hinge is actuated.
Figure 6-13. Hinge corrosion points.
abrasive pads intended for paint stripping may also prove to be useful in removing the loosened paint.
- Remove the loosened paint and residual stripper by washing and scrubbing the surface with water and a broom, brush or fresh nonwoven abrasive pad. If water spray is available, use a low to medium pressure stream of water directly on the area being scrubbed. If steam-cleaning equipment is available and the area is sufficiently large, cleaning may be accomplished using this equipment together with a solution of steam-cleaning compound. On small areas, any method may be used that will assure complete rinsing of the cleaned area. Use care to dispose of the stripped residue in accordance with environmental laws.
Corrosion of Ferrous Metals
One of the most familiar types of corrosion is ferrous oxide (rust), generally resulting from atmospheric oxidation of steel surfaces. Some metal oxides protect the underlying base metal, but rust is not a protective coating in any sense of the word. Its presence actu- ally promotes additional attack by attracting moisture from the air and acting as a catalyst for additional cor- rosion. If complete control of the corrosive attack is to be realized, all rust must be removed from steel surfaces.
Rust first appears on bolt heads, hold-down nuts, or other unprotected aircraft hardware. [Figure 6-14] Its presence in these areas is generally not dangerous and has no immediate effect on the structural strength of any major components. The residue from the rust may also contaminate other ferrous components, promot- ing corrosion of those parts. The rust is indicative of a need for maintenance and of possible corrosive attack in more critical areas. It is also a factor in the gen- eral appearance of the equipment. When paint failures occur or mechanical damage exposes highly stressed steel surfaces to the atmosphere, even the smallest amount of rusting is potentially dangerous in these areas and must be removed and controlled.
Rust removal from structural components, followed by an inspection and damage assessment, must be done as soon as feasible. [Figure 6-15]
Mechanical Removal of Iron Rust The most practicable means of controlling the cor- rosion of steel is the complete removal of corrosion products by mechanical means and restoring corro- sion preventive coatings. Except on highly stressed
organic compounds (VOCs). Where permitted, dry cleaning solvent (P-D-680) may be used for remov- ing oil, grease, or soft preservative compounds. For heavy-duty removal of thick or dried preservatives, other compounds of the solvent emulsion type are available.
The use of a general purpose, water rinsable stripper can be used for most applications. There are other methods for paint removal that have minimal impact upon the aircraft structure, and are considered “envi- ronmentally friendly.”
Wherever practicable, chemical paint removal from any large area should be accomplished outside (in open air) and preferably in shaded areas. If inside removal is necessary, adequate ventilation must be assured. Synthetic rubber surfaces, including aircraft tires, fabric, and acrylics, must be thoroughly pro- tected against possible contact with paint remover. Care must be exercised in using paint remover. Care must also be exercised in using paint remover around gas or watertight seam sealants, since the stripper will tend to soften and destroy the integrity of these sealants.
Mask off any opening that would permit the stripping compound to get into aircraft interiors or critical cavi- ties. Paint stripper is toxic and contains ingredients harmful to both skin and eyes. Therefore, wear rubber gloves, aprons of acid repellent material, and goggle- type eyeglasses. The following is a general stripping procedure:
- Brush the entire area to be stripped with a cover of stripper to a depth of 1 ⁄ 32 to 1 ⁄ 16 inch. Any paintbrush makes a satisfactory applicator, except that the bristles will be loosened by the effect of paint remover on the binder, and the brush should not be used for other purposes after being exposed to paint remover.
- Allow the stripper to remain on the surface for a sufficient length of time to wrinkle and lift the paint. This may be from 10 minutes to several hours, depending on both the temperature and humidity, and the condition of the paint coat being removed. Scrub the surface with a bristle brush saturated with paint remover to further loosen finish that may still be adhering to the metal.
- Reapply the stripper as necessary in areas where the paint remains tightly adhered or where the stripper has dried, and repeat the above process. Only nonmetallic scrapers should be used to assist in removing persistent paint finishes. Nonwoven
Chemical Removal of Rust
As environmental concerns have been addressed in recent years, interest in noncaustic chemical rust removal has increased. A variety of commercial prod- ucts, which actively remove the iron oxide without chemically etching the base metal, are available and should be considered for use. Generally speaking, if at all possible, the steel part should be removed from the airframe for treatment, as it can be nearly impos- sible to remove all residues. The use of any caustic rust removal product will require the isolation of the part from any nonferrous metals during treatment, and will probably require inspection for proper dimensions.
Chemical Surface Treatment of Steel
There are approved methods for converting active rust to phosphates and other protective coatings. Other commercial preparations are effective rust con- verters where tolerances are not critical and where thorough rinsing and neutralizing of residual acid is possible. These situations are generally not applicable to assembled aircraft, and the use of chemical inhibi- tors on installed steel parts is not only undesirable but also very dangerous. The danger of entrapment of corrosive solutions and the resulting uncontrolled attack, which could occur when such materials are used under field conditions, outweigh any advantages to be gained from their use.
Removal of Corrosion from Highly Stressed Steel Parts
Any corrosion on the surface of a highly stressed steel part is potentially dangerous, and the careful removal of corrosion products is required. Surface scratches or change in surface structure from over- heating can also cause sudden failure of these parts. Corrosion products must be removed by careful pro-
Figure 6-16. Nonwoven abrasive pads.
cessing, using mild abrasive papers such as rouge or fine grit aluminum oxide, or fine buffing compounds on cloth buffing wheels. Nonwoven abrasive pads can also be used. It is essential that steel surfaces not be overheated during buffing. After careful removal of surface corrosion, reapply protective paint finishes immediately.
Corrosion of Aluminum and
Aluminum Alloys
Corrosion on aluminum surfaces is usually quite obvious, since the products of corrosion are white and generally more voluminous than the original base metal. Even in its early stages, aluminum corrosion is evident as general etching, pitting, or roughness of the aluminum surfaces.
NOTE: Aluminum alloys commonly form a smooth surface oxidation that is from 0.001 to 0.0025 inch thick. This is not considered detrimental; the coat- ing provides a hard shell barrier to the introduction of corrosive elements. Such oxidation is not to be confused with the severe corrosion discussed in this paragraph.
General surface attack of aluminum penetrates rela- tively slowly, but is speeded up in the presence of dissolved salts. Considerable attack can usually take place before serious loss of structural strength develops.
At least three forms of attack on aluminum alloys are particularly serious: (1) the penetrating pit-type corro- sion through the walls of aluminum tubing, (2) stress corrosion cracking of materials under sustained stress, and (3) intergranular corrosion which is characteristic of certain improperly heat-treated aluminum alloys.
In general, corrosion of aluminum can be more effec- tively treated in place compared to corrosion occurring on other structural materials used in aircraft. Treatment includes the mechanical removal of as much of the corrosion products as practicable, and the inhibition of residual materials by chemical means, followed by the restoration of permanent surface coatings.
Treatment of Unpainted Aluminum Surfaces Relatively pure aluminum has considerably more corrosion resistance compared with the stronger alu- minum alloys. To take advantage of this character- istic, a thin coating of relatively pure aluminum is applied over the base aluminum alloy. The protection obtained is good, and the pure-aluminum clad surface (commonly called “Alclad”) can be maintained in a polished condition. In cleaning such surfaces, how-
ever, care must be taken to prevent staining and mar- ring of the exposed aluminum and, more important from a protection standpoint, to avoid unnecessary mechanical removal of the protective Alclad layer and the exposure of the more susceptible aluminum alloy base material. A typical aluminum corrosion treat- ment sequence follows:
- Remove oil and surface dirt from the aluminum surface using any suitable mild cleaner. Use caution when choosing a cleaner; many commercial consumer products are actually caustic enough to induce corrosion if trapped between aluminum lap joints. Choose a neutral Ph product.
- Hand polish the corroded areas with fine abrasives or with metal polish. Metal polish intended for use on clad aluminum aircraft surfaces must not be used on anodized aluminum since it is abrasive enough to actually remove the protective anodized film. It effectively removes stains and produces a highly polished, lasting surface on unpainted Alclad. If a surface is particularly difficult to clean, a cleaner and brightener compound for aluminum can be used before polishing to shorten the time and lessen the effort necessary to get a clean surface.
- Treat any superficial corrosion present, using an inhibitive wipe down material. An alternate treatment is processing with a solution of sodium dichromate and chromium trioxide. Allow these solutions to remain on the corroded area for 5 to 20 minutes, and then remove the excess by rinsing and wiping the surface dry with a clean cloth.
- Overcoat the polished surfaces with waterproof wax.
Aluminum surfaces that are to be subsequently painted can be exposed to more severe cleaning procedures and can also be given more thorough corrective treat- ment prior to painting. The following sequence is generally used:
- Thoroughly clean the affected surfaces of all soil and grease residues prior to processing. Any general aircraft cleaning procedure may be used.
- If residual paint films remain, strip the area to be treated. Procedures for the use of paint removers and the precautions to observe were previously mentioned in this chapter under “Surface Cleaning and Paint Removal.”
- Treat superficially corroded areas with a 10 percent solution of chromic acid and sulfuric acid. Apply the solution by swab or brush. Scrub the corroded area with the brush while it is still damp. While
chromic acid is a good inhibitor for aluminum alloys, even when corrosion products have not been completely removed, it is important that the solution penetrate to the bottom of all pits and underneath any corrosion that may be present. Thorough brushing with a stiff fiber brush should loosen or remove most existing corrosion and assure complete penetration of the inhibitor into crevices and pits. Allow the chromic acid to remain in place for at least 5 minutes, and then remove the excess by flushing with water or wiping with a wet cloth. There are several commercial chemical surface treatment compounds, similar to the type described above, which may also be used.
- Dry the treated surface and restore recommended permanent protective coatings as required in accordance with the aircraft manufacturer’s procedures. Restoration of paint coatings should immediately follow any surface treatment performed. In any case, make sure that corrosion treatment is accomplished or is reapplied on the same day that paint refinishing is scheduled.
Treatment of Anodized Surfaces As previously stated, anodizing is a common surface treatment of aluminum alloys. When this coating is damaged in service, it can only be partially restored by chemical surface treatment. Therefore, any cor- rosion correction of anodized surfaces should avoid destruction of the oxide film in the unaffected area. Do not use steel wool or steel wire brushes. Do not use severe abrasive materials.
Nonwoven abrasive pads have generally replaced alu- minum wool, aluminum wire brushes, or fiber bristle brushes as the tools used for cleaning corroded anod- ized surfaces. Care must be exercised in any cleaning process to avoid unnecessary breaking of the adjacent protective film. Take every precaution to maintain as much of the protective coating as practicable. Oth- erwise, treat anodized surfaces in the same manner as other aluminum finishes. Chromic acid and other inhibitive treatments can be used to restore the oxide film.
Treatment of Intergranular Corrosion in Heat‑Treated Aluminum Alloy Surfaces As previously described, intergranular corrosion is an attack along grain boundaries of improperly or inadequately heat-treated alloys, resulting from pre- cipitation of dissimilar constituents following heat treatment. In its most severe form, actual lifting of metal layers (exfoliation, see Figure 6-9) occurs.
Treatment of Installed Magnesium Castings
Magnesium castings, in general, are more porous and more prone to penetrating attack than wrought mag- nesium skins. For all practical purposes, however, treatment is the same for all magnesium areas. Engine cases, bellcranks, fittings, numerous covers, plates, and handles are the most common magnesium castings.
When attack occurs on a casting, the earliest prac- ticable treatment is required if dangerous corrosive penetration is to be avoided. In fact, engine cases submerged in saltwater overnight can be completely penetrated. If it is at all practicable, parting surfaces should be separated to effectively treat the existing attack and prevent its further progress. The same gen- eral treatment sequence in the preceding paragraph for magnesium skin should be followed.
If extensive removal of corrosion products from a structural casting is involved, a decision from the man- ufacturer may be necessary to evaluate the adequacy of structural strength remaining. Specific structural repair manuals usually include dimensional tolerance limits for critical structural members and should be referred to, if any question of safety is involved.
Treatment of Titanium and
Titanium Alloys
Attack on titanium surfaces is generally difficult to detect. Titanium is, by nature, highly corrosion resis- tant, but it may show deterioration from the presence of salt deposits and metal impurities, particularly at high temperatures. Therefore, the use of steel wool, iron scrapers, or steel brushes for cleaning or for the removal of corrosion from titanium parts is prohibited.
If titanium surfaces require cleaning, hand polishing with aluminum polish or a mild abrasive is permis- sible, if fiber brushes only are used and if the surface is treated following cleaning with a suitable solution of sodium dichromate. Wipe the treated surface with dry cloths to remove excess solution, but do not use a water rinse.
Protection of Dissimilar Metal
Contacts
Certain metals are subject to corrosion when placed in contact with other metals. This is commonly referred to as electrolytic or dissimilar metals corrosion. Con- tact of different bare metals creates an electrolytic action when moisture is present. If this moisture is salt
water, the electrolytic action is accelerated. The result of dissimilar metal contact is oxidation (decomposi- tion) of one or both metals. The chart shown in Figure 6-18 lists the metal combinations requiring a protec- tive separator. The separating materials may be metal primer, aluminum tape, washers, grease, or sealant, depending on the metals involved.
Contacts Not Involving Magnesium All dissimilar joints not involving magnesium are protected by the application of a minimum of two coats of zinc chromate or, preferably, epoxy primer in addition to normal primer requirements. Primer is applied by brush or spray and allowed to air dry 6 hours between coats.
Contacts Involving Magnesium To prevent corrosion between dissimilar metal joints in which magnesium alloy is involved, each surface is insulated as follows:
At least two coats of zinc chromate or, preferably, epoxy primer are applied to each surface. Next, a layer of pressure sensitive vinyl tape 0.003 inch thick is applied smoothly and firmly enough to prevent air bubbles and wrinkles. To avoid creep back, the tape is not stretched during application. When the thick- ness of the tape interferes with the assembly of parts, where relative motion exists between parts, or when service temperatures above 250 °F are anticipated, the use of tape is eliminated and extra coats (minimum of three) of primer are applied.
Corrosion Limits
Corrosion, however slight, is damage. Therefore, cor- rosion damage is classified under the four standard types, as is any other damage. These types are: (1) neg- ligible damage, (2) damage repairable by patching, (3) damage repairable by insertion, and (4) damage necessitating replacement of parts.
The term “negligible,” as used here, does not imply that little or nothing should be done. The corroded surface should be cleaned, treated, and painted as appropriate. Negligible damage, generally, is corro- sion that has scarred or eaten away the surface pro- tective coats and begun to etch the metal. Corrosion damage extending to classifications of “repairable by patching” and “repairable by insertion” should be repaired in accordance with the applicable structural repair manual. When corrosion damage exceeds the damage limits to the extent that repair is not possible, the component or structure should be replaced.
Processes and Materials Used in
Corrosion Control
Metal Finishing
Aircraft parts are almost always given some type of surface finish by the manufacturer. The main purpose is to provide corrosion resistance; however, surface finishes may also be applied to increase wear resis- tance or to provide a suitable base for paint.
In most instances, the original finishes described in the following paragraphs cannot be restored in the field due to unavailable equipment or other limita- tions. However, an understanding of the various types of metal finishes is necessary if they are to be properly maintained in the field and if the partial restoration techniques used in corrosion control are to be effective.
Surface Preparation
Original surface treatments for steel parts usually include a cleaning treatment to remove all traces of dirt, oil, grease, oxides, and moisture. This is neces- sary to provide an effective bond between the metal
surface and the final finish. The cleaning process may be either mechanical or chemical. In mechanical cleaning, the following methods are employed: wire brush, steel wool, emery cloth, sandblasting, or vapor blasting.
Chemical cleaning is preferred over mechanical since none of the base metal is removed by cleaning. There are various chemical processes now in use, and the type used will depend on the material being cleaned and the type of foreign matter being removed.
Steel parts are pickled to remove scale, rust, or other foreign matter, particularly before plating. The pick- ling solution can be either muriatic (hydrochloric) or sulfuric acid. Cost wise, sulfuric acid is preferable, but muriatic acid is more effective in removing cer- tain types of scale.
The pickling solution is kept in a stoneware tank and is usually heated by means of a steam coil. Parts not to be electroplated after pickling are immersed in a lime bath to neutralize the acid from the pickling solution.
Green areas indicate dissimilar metal contact
Contacting
Metals
Aluminium alloyCalcium plate
Zinc plate
Carbon and alloy steels
Lead Tin coating Copper and alloysNickel and alloysTitanium and alloysChromium plate Corrosion resisting steel
Magnesium alloys
Aluminium alloy Calcium plate Zinc plate Carbon and alloy steels Lead Tin coating Copper and alloys Nickel and alloys Titanium and alloys Chromium plate Corrosion resisting steel Magnesium alloys
Figure 6-18. Dissimilar metal contacts that will result in electrolytic corrosion.
tion. For example, some chemicals used in surface treatments will react violently if inadvertently mixed with paint thinners. Chemical surface treatment mate- rials must be handled with extreme care and mixed exactly according to directions.
Chromic Acid Inhibitor
A 10 percent solution by weight of chromic acid, activated by a small amount of sulfuric acid, is par- ticularly effective in treating exposed or corroded alu- minum surfaces. It may also be used to treat corroded magnesium.
This treatment tends to restore the protective oxide coating on the metal surface. Such treatment must be followed by regular paint finishes as soon as practi- cable, and never later than the same day as the latest chromic acid treatment. Chromium trioxide flake is a powerful oxidizing agent and a fairly strong acid. It must be stored away from organic solvents and other combustibles. Either thoroughly rinse or dispose of wiping cloths used in chromic acid pickup.
Sodium Dichromate Solution
A less active chemical mixture for surface treatment of aluminum is a solution of sodium dichromate and chromic acid. Entrapped solutions of this mixture are less likely to corrode metal surfaces than chromic acid inhibitor solutions.
Chemical Surface Treatments
Several commercial, activated chromate acid mix- tures are available under Specification MIL-C- for field treatment of damaged or corroded aluminum surfaces. Take precautions to make sure that sponges or cloths used are thoroughly rinsed to avoid a pos- sible fire hazard after drying.
Protective Paint Finishes
A good, intact paint finish is the most effective bar- rier between metal surfaces and corrosive media. The most common finishes include catalyzed polyure- thane enamel, waterborne polyurethane enamel, and two-part epoxy paint. As new regulations regarding the emission of volatile organic compounds (VOCs) are put into effect, the use of waterborne paint systems have increased in popularity. Also still available are nitrate and butyrate dope finishes for fabric-covered aircraft. In addition, high visibility fluorescent materi- als may also be used, along with a variety of miscel- laneous combinations of special materials. There may also be rain erosion resistant coatings on metal lead-
ing edges, and several different baked enamel finishes on engine cases and wheels.
Aircraft Cleaning
Cleaning an aircraft and keeping it clean are extremely important. From an aircraft maintenance technician’s viewpoint, it should be considered a regular part of aircraft maintenance. Keeping the aircraft clean can mean more accurate inspection results, and may even allow a flight crewmember to spot an impending com- ponent failure. A cracked landing gear fitting covered with mud and grease may be easily overlooked. Dirt can hide cracks in the skin. Dust and grit cause hinge fittings to wear excessively. If left on the aircraft’s outer surface, a film of dirt reduces flying speed and adds extra weight. Dirt or trash blowing or bounc- ing around the inside of the aircraft is annoying and dangerous. Small pieces of dirt blown into the eyes of the pilot at a critical moment can cause an accident. A coating of dirt and grease on moving parts makes a grinding compound that can cause excessive wear. Salt water has a serious corroding effect on exposed metal parts of the aircraft, and should be washed off immediately.
There are many different kinds of cleaning agents approved for use in cleaning aircraft. It is impractical to cover each of the various types of cleaning agents since their use varies under different conditions, such as the type of material to be removed, the aircraft fin- ish, and whether the cleaning is internal or external.
In general, the types of cleaning agents used on air- craft are solvents, emulsion cleaners, soaps, and syn- thetic detergents. Their use must be in accordance with the applicable maintenance manual. The types of cleaning agents named above are also classed as light or heavy duty cleaners. The soap and synthetic detergent type cleaners are used for light duty clean- ing, while the solvent and emulsion type cleaners are used for heavy duty cleaning. The light duty clean- ers, which are nontoxic and nonflammable, should be used whenever possible. As mentioned previously, cleaners that can be effectively rinsed and neutralized must be used, or an alkaline cleaner may cause cor- rosion within the lap joints of riveted or spot-welded sheet metal components.
Exterior Cleaning There are three methods of cleaning the aircraft exte- rior: (1) wet wash, (2) dry wash, and (3) polishing. Polishing can be further broken down into hand pol- ishing and mechanical polishing. The type and extent
of soiling and the final desired appearance determine the cleaning method to be used.
Wet wash removes oil, grease, or carbon deposits and most soils, with the exception of corrosion and oxide films. The cleaning compounds used are usu- ally applied by spray or mop, after which high pres- sure running water is used as a rinse. Either alkaline or emulsion cleaners can be used in the wet wash method.
Dry wash is used to remove airport film, dust, and small accumulations of dirt and soil when the use of liquids is neither desirable nor practical. This method is not suitable for removing heavy deposits of carbon, grease, or oil, especially in the engine exhaust areas. Dry wash materials are applied with spray, mops, or cloths, and removed by dry mopping or wiping with clean, dry cloths.
Polishing restores the luster to painted and unpainted surfaces of the aircraft, and is usually performed after the surfaces have been cleaned. Polishing is also used to remove oxidation and corrosion. Polishing mate- rials are available in various forms and degrees of abrasiveness. It is important that the aircraft manufac- turer’s instructions be used in specific applications.
The washing of aircraft should be performed in the shade whenever possible as cleaning compounds tend to streak the surface if applied to hot metal, or are per- mitted to dry on the area. Install covers over all open- ings where water or cleaners might enter and cause damage. Pay particular attention to instrument system components such as pitot-static fittings and ports.
Various areas of aircraft, such as the sections hous- ing radar and the area forward of the cockpit that are finished with a flat-finish paint, should not be cleaned more than necessary and should never be scrubbed with stiff brushes or coarse rags. A soft sponge or cheesecloth with a minimum of manual rubbing is advisable. Any oil or exhaust stains on the sur- face should first be removed with a solvent such as kerosene or other petroleum base solvent. Rinse the surfaces immediately after cleaning to prevent the compound from drying on the surface.
Before applying soap and water to plastic surfaces, flush the plastic surfaces with fresh water to dissolve salt deposits and wash away dust particles. Plastic surfaces should be washed with soap and water, pref- erably by hand.
Rinse with fresh water and dry with a chamois, syn- thetic wipes designed for use on plastic windshields, or absorbent cotton. In view of the soft surface, do not rub plastic with a dry cloth since this is not only likely to cause scratches, but it also builds up an electrostatic charge that attracts dust particles to the surface. The charge, as well as the dust, may be removed by patting or gently blotting with a clean, damp chamois. Do not use scouring powder or other material that can mar the plastic surface. Remove oil and grease by rubbing gently with a cloth wet with soap and water. Do not use acetone, benzene, carbon tetrachloride, lacquer thinners, window cleaning sprays, gasoline, fire extin- guisher or deicer fluid on plastics because they soften the plastic and will cause crazing. Finish cleaning the plastic by coating with a plastic polish intended for aircraft windows and windshields. These polishes can minimize small surface scratches and will also help keep static charges from building up on the surface of the windows.
Surface oil, hydraulic fluid, grease, or fuel can be removed from aircraft tires by washing with a mild soap solution. After cleaning, lubricate all grease fit- tings, hinges, and so forth, where removal, contami- nation, or dilution of the grease is suspected during washing of the aircraft.
Interior Cleaning Keeping the interior of the aircraft clean is just as important as maintaining a clean exterior surface. Corrosion can establish itself on the inside structure to a greater degree because it is difficult to reach some areas for cleaning. Nuts, bolts, bits of wire, or other metal objects carelessly dropped and neglected, com- bined with moisture and dissimilar metal contact, can cause electrolytic corrosion.
When performing structural work inside the aircraft, clean up all metal particles and other debris as soon as possible. To make cleaning easier and prevent the metal particles and debris from getting into inacces- sible areas, use a drop cloth in the work area to catch this debris.
A vacuum cleaner can be used to pick up dust and dirt from the interior of the cockpit and cabin.
Aircraft interior present certain problems during cleaning operations. The following is taken from The National Fire Protection Association (NFPA) Bulletin #410F, Aircraft Cabin Cleaning Operation.
