Electrochemical Attack An electrochemical attack may be likened chemically to the electrolytic reaction which takes place in electroplating, anodizing, or in a drycell battery. The reaction in this corrosive attack requires a medium, usually water, which is capable of conducting a tiny current of electricity. When a metal comes in contact with a corrosive agent and is also connected by a liquid or gaseous path through which electrons may flow, corrosion begins as the metal decays by oxidation. During the attack, the quantity of corrosive agent is reduced and, if not renewed or removed, may completely react with the metal (become neutralized). Different areas of the same metal surface have varying levels of electrical potential and if connected by a conductor, such as salt water, will set up a series of corrosion cells and corrosion will All metals and alloys are electrically active and have a specific electrical potential in a given chemical environment. The constituents in an alloy also have specific electrical potentials which are generally different from each other. Exposure of the alloy surface to a conductive, corrosive medium causes the more active metal to become anodic and the less active metal to become cathodic, thereby establishing conditions for corrosion. These are called local cells. The greater the difference in electrical potential between the two metals, the greater will be the severity of a corrosive attack, if the proper conditions are allowed As can be seen, the conditions for these corrosion reactions are 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 component parts.
FORMS OF CORROSION There are many forms of corrosion. The form of corrosion 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 products. Surface corrosion may be caused by either direct chemical or electrochemical attack. Sometimes corrosion will spread under the surface coating and cannot be recognized by either the roughening of the surface or the powdery deposit. Instead, the paint or plating will be lifted off the surface in small blisters which result from the pressure of the underlying accumulation of corrosion products.
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 electroplating, occurs at the points or areas of contact where the insulation has broken down or been omitted. This electrochemical attack can be very serious because the action is, in many instances, taking place out of sight. and the only way to detect it prior to structural failure is by disassembly and inspection.
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 susceptible to this form of electrochemical attack. The lack of uniformity is caused by changes that occur in the alloy during heating and cooling. intergranular corrosion may exist without visible surface evidence. Very severe intergranular corrosion may sometimes cause the surface of a metal to "exfoliate." This is a lifting or flaking of the metal at the surface due to delamination of the grain boundaries caused by the pressure of corrosion residual product buildup.
This type of corrosion is difficult to detect in its original stage. Ultrasonic and eddy current inspection methods are being used with a great deal of success.
Stress Corrosion Stress corrosion occurs as the result of the combined effect of sustained tensile stresses and a corrosive environment. Stress corrosion cracking 172
is found in most metal systems; however, it is particularly characteristic of aluminum, copper, certain stainless steels, and high-strength alloy steels (over 240,000 p.s.i.). It usually occurs along lines of cold working and may be transgranular or intergranular in nature. Aluminum alloy bellcranks with pressed-in bushings, landing gear shock struts with pipe-thread type grease fittings, clevis pin joints, shrink fits, and overstressed tubing B-nuts are examples of parts which are susceptible to stress corrosion cracking.
Fretting Corrosion Fretting corrosion is a particularly damaging form of corrosive attack which occurs when two mating surfaces, normally at rest with respect to one another, are subject to slight relative motion.
It is characterized by pitting of the surfaces, and the generation of considerable quantities of finely divided debris. Since the restricted movements of the two surfaces prevent the debris from escaping very easily, an extremely localized abrasion occurs. The presence of water vapor greatly increases this type of deterioration. If the contact areas are small and sharp, deep grooves resembling called false brinelling.
FACTORS AFFECTING CORROSION Many factors affect the type, speed, cause, and seriousness 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 environment (with exposure to sea water and salt air), moisture-laden air is considerably more detrimental to an aircraft than it would be if all operations were conducted in a dry climate.
Temperature considerations are important because the speed of electrochemical attack is increased in a hot, moist climate.
Size and Type of Metal It is a well known fact that some metals will corrode faster than others. It is a less known fact that variations in size and shape of a metal can indirectly affect its corrosion resistance.
Poor heat Good heat treatment zone treatment zone Original surface Tapering exposes zone of poor heat treatment to corrosive environment FIGURE 6-59. The effect of machining thick heattreated wrought aluminum alloys.
Thick structural sections are more susceptible to corrosive attack than thin sections because variations in physical characteristics are greater.
When large pieces are machined or chemically milled after heat treatment, the thinner areas will have different physical characteristics than the thicker areas. (See figure 6-59.)
From a corrosion control standpoint, the best approach is to recognize the critical nature of the integrity and strength of major structural parts and to maintain permanent protection over such areas at all times to prevent the onset of deterioration.
Foreign Material Among the controllable factors which affect the onset and spread of corrosive attack is foreign material which adheres to the metal surfaces.
Such foreign material includes:
(1) Soil and atmospheric dust.
(2) Oil, grease, and engine exhaust residues.
(3) Salt water and salt moisture condensation.
(4) Spilled battery acids and caustic cleaning solutions.
(5) 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, such as location, model of aircraft, and type of operation.
PREVENTIVE MAINTENANCE Much has been done to improve the corrosion resistance of aircraft: improvement in materials, surface treatments, insulation, and protective finishes. All of these have been aimed at reducing 173
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.
Corrosion-preventive maintenance includes the following specific functions:
(I) An adequate cleaning.
(2) Thorough periodic lubrication.
(3) Detailed inspection for corrosion and failure of protective systems.
(4) Prompt treatment of corrosion and touchup of damaged paint areas.
(5) Keeping drainholes free of obstruction.
(6) Daily draining of fuel cell sumps.
(7) Daily wipe-down of exposed critical areas.
(8). Sealing of aircraft against water during foul weather and proper ventilation on warm, sunny days.
(9) Making maximum use of protective covers on parked aircraft.
After any period during which regular corrosion-preventive 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. Overemphasizing a particular corrosion problem when it is discovered and then forgetting about corrosion until the next crisis is an unsafe, costly, and troublesome practice.
Most scheduled maintenance checklists are conplete enough to cover all parts of the aircraft or engine, and no part of the aircraft should go unchecked. 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 corrosion will set in despite routine inspection and maintenance.
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 trouble areas common to all aircraft. However, this coverage 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 down the exhaust path and where deposits may be trapped and not reached by normal cleaning methods. Special attention should be paid to areas around rivet heads and in skin crevices. Fairings and access plates in the exhaust areas should be removed for inspection. Exhaust deposit buildup in remote areas such as the empennage surfaces should not be overlooked.
Buildup in these areas will be slower and sometimes completely absent, but it has become a problem on some currently operating aircraft.
Battery Compartments and Battery Vent Openings Despite improvements in protective paint finishes and in methods of sealing and venting, battery compartments continue to be corrosion problem 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 neutralization 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 which settle to the bottom and set up a hidden chemical cell. Seaplane and amphibian aircraft bilge areas are protected by small bags of potassium dichromate inhibitor suspended near the low point in each bilge compartment. These crystals dissolve in any waste water and tend to inhibit the attack on exposed metal surfaces.
Inspection procedures should include replacement of these bags when most of the chemical has been dissolved. Particular attention must be paid to areas located under galleys and lavatories and to human waste disposal openings on the aircraft exteriors. Human waste products and the 174
chemicals used in lavatories are very corrosive to the common aircraft metals. Clean these areas frequently and keep the paint touched up.
Wheel Well and Landing Gear This area probably receives more punishment due to mud, water, salt, gravel, and other flying debris than any other area on the aircraft.
Because of the many complicated shapes, assemblies, and fittings, complete area paint film coverage is difficult to attain. A partially applied preservative tends to mask corrosion rather than prevent it. Due to heat generated by braking action, preservatives cannot be used on some main landing gear wheels. During inspection of this area, pay particular attention to the following trouble spots:
1. Magnesium wheels, especially around boltheads, lugs, and wheel web areas, particularly for the presence of entrapped water or
2. Exposed rigid tubing, especially at B-nuts and ferrules, under clamps and tubing identification tapes.
3. Exposed position indicator switches and other electrical equipment.
4. Crevices between stiffeners, ribs, and lower skin surfaces, which are typical water and debris traps.
Water Entrapment Areas Design specifications require that aircraft have drains installed in all areas where water may collect. Daily inspection of low-point drains should be a standard requirement. If this inspection is neglected, the drains may become ineffective because of accumulated debris, grease, or sealants.
Engine Frontal Areas and Cooling Air Vents These areas are being constantly abraded with airborne dirt and dust, bits of gravel from runways, and rain erosion which tend to remove the protective finish. Inspection of these areas should include all sections in the cooling air path, with special attention to places where salt deposits may be built up during marine operations. It is imperative that incipient corrosion be inhibited and that paint touchup and hard film preservative coatings be maintained intact on seaplane and amphibian engine surfaces at all times.
Wing Flap and Spoiler Recesses Dirt and water may collect in flap and spoiler recesses and go unnoticed because they are normally retracted. For this reason these recesses are potential corrosion problem areas.
External Skin Areas External aircraft surfaces are readily visible and accessible for inspection and maintenance.
Even here, certain types of configurations or combinations of materials become troublesome under certain operating conditions and require special Relatively little corrosion trouble is experienced with magnesium skins if the original surface finish and insulation are adequately maintained. Trimming, drilling, and riveting destroy some of the original surface treatment which is never completely restored by touchup procedures. Any inspection for corrosion should include all magnesium skin surfaces with special attention to edges, areas around fasteners, and cracked, chipped, or missing paint.
Piano-type hinges are prime spots for corrosion due to the dissimilar metal contact between the steel pin and aluminum hinge. They are also natural traps for dirt, salt, and moisture. Inspection of hinges should include lubrication and actuation through several cycles to ensure complete lubricant penetration.
Corrosion of metal skin joined by spot welding is the result of the entrance and entrapment of corrosive agents between the layers of metal. This type of corrosion 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 sealant 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 dissimilar metal contacts. These areas should be inspected frequently for evidence of 175
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 corrosion-resistant steel, should be inspected to determine their condition at each inspection period. Cables should be inspected for corrosion by random cleaning of short sections with solventsoaked cloths. If external corrosion is evident, tension should be relieved and the cable checked for internal corrosion. Cables with internal corrosion should be replaced. Light external corrosion should be removed with a steel-wire brush. When corrosion produets have been removed, re-coat the cable with preservative.
CORROSION REMOVAL In general, any complete corrosion treatment involves the following: (1) Cleaning and stripping of the corroded area, (2) removing as much of the corrosion produets 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 surfaces and components 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. In order 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 corrosion spread, since went to that the stripping operation will be held to the minimuni 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. Drycleaning solvent may be used for removing 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 is recommended for most applications.
Wherever practicable, 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 protected against possible contact with paint remover. Care must also be exercised in using paint remover around gas or watertight seam sealants, since this material 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 cavities. Paint stripper is toxic and contains ingredients harmful to both skin and eyes.
Rubber gloves, aprons of acid-repellent material, and goggle-type eyeglasses should be worn if any extensive paint removal is to be accomplished.
The following is a general stripping procedure:
1. Brush the entire area to be stripped with a cover of stripper to a depth of ½2 to 6 inch.
Any paint brush 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 re-
2. 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.
3. Reapply the stripper as necessary in areas that remain tight or where the material has dried, and repeat the above process. Only nonmetallic scrapers may be used to assist in removing persistant paint finishes.
4. Remove the loosened paint and residual stripper by washing and scrubbing the sur face with water and a broom or brush. If water spray is available, use a low-to-medium pressure stream of water directly on the scrubbing broom or brush. If steam cleaning equipment is available and the area is sufficiently large, cleaning may be accomplished using this equipment together with a solution 176
of steam cleaning compound. On small areas, any method may be used that will assure complete rinsing of the cleaned area.
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 actually promotes additional attack by attracting moisture from the air and acting as a catalyst in causing additional corrosion to take place. As a result, all rust must be removed from steel surfaces, if complete control of the corrosive attack is to be realized.
Rust first shows on boltheads, holddown nuts, or other unprotected aircraft hardware. Its presence in these areas is generally not dangerous and has no immediate effect on the structural strength of any major components. However, it is indicative of a need for maintenance and of possible corrosive attack in more critical areas. It is also a factor in the general 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.
Mechanical Removal of Iron Rust The most practicable means of controlling the corrosion of steel is the complete removal of corsion products by mechanical means and restorng corrosion-preventive coatings. Except on highly stressed steel surfaces, the use of abrasive papers and compounds, small power buffers and buffing compounds, hand wire brushing, or steel wool are all acceptable cleanup procedures. However, it should be recognized that in any such use of abrasives, residual rust usually remains in the bottom of small pits and other crevices. It is practically impossible to remove all corrosion products by abrasive or polishing methods alone.
As a result, once a part has rusted it usually corrodes again more easily than it did the first Chemical Surface Treatment of Steel There are approved methods for converting active rust to phosphates and other protective coatings. Parco Lubrizing and the use of other phosphoric acid proprietary chemicals are examples of such treatments. However, these processes require shop-installed equipment and are impracticable for field use. Other commercial preparations are effective rust converters 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 inhibitors on installed steel parts is not only undesirable but 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 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 overheating can also cause sudden failure of these parts. Corrosion products must be removed by careful processing, using mild abrasive papers such as rouge or fine grit aluminum oxide, or fine buffing compounds on cloth buffing wheels. It is essential that steel surfaces not be overheated during buffing. After careful removal of surface corrosion, protective paint finishes should be re-applied immediately.
CORROSION OF ALUMINUM AND ALUMINUM Corrosion attack 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 which is from .001 in.
to .0025 in. thick. This is not considered detrimental as such a coating 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 relatively slowly, but is speeded up in the presence of dissolved salts. Considerable attack can usually take place before serious loss of structural strength
develops. However, at least three forms of attack on aluminum alloys are particularly serious: (1) The penetrating pit-type corrosion through the walls of aluminum tubing, (2) stress-corrosion cracking of materials under sustained stress, and
(3) the intergranular attack which is characteristic of certain improperly heat-treated aluminum al- In general, corrosion of aluminum can be more effectively treated in place than corrosion occuring 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 Alumium Surfaces Relatively pure aluminum has considerably more corrosion resistance compared with the stronger aluminum alloys. Advantage is taken of this by laminating a thin sheet of relatively pure aluminum over the base aluminum alloy. The protection obtained is good, and the alclad surface can be maintained in a polished condition. In cleaning such surfaces, however, care must be taken to prevent staining and marring 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 treatment sequence follows:
I. Remove oil and surface dirt with any suitable mild cleaner prior to abrasive cleaning of aluminum surfaces.
2. 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 high, lasting polish 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.
3. 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.
4. 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 treatment prior to painting. The following sequence is generally used:
1. Thoroughly clean the affected surfaces of all soil and grease residues prior to processing.
Any general aircraft cleaning procedure may
2. 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 Re-
3. Treat superficially corroded areas with a 10 percent solution of chromic acid and sulphuric 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, 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.
4. 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 re-applied 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 be only partially restored by chemical surface treatment.
Therefore, any corrosion correction of anodized surfaces should avoid destruction of the oxide film in the unaffected area. Avoid the use of steel wool, steel wire brushes, or severe abrasive materials.
Aluminum wool, aluminum wire brushes, or fiber bristle brushes are the approved tools for cleaning corroded anodized 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. Otherwise, treat anodized surfaces in the same manner as other aluminum finishes. Chromic acid and other inhibitive treatments tend 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 precipitation of dissimilar constituents following heat treatment. In its most severe form, actual lifting of metal layers (exfoliation) occurs.
More severe cleaning is a must when intergranular corrosion is present. The mechanical removal of all corrosion products and visible delaminated metal layers must be accomplished to determine the extent of the destruction and to evaluate the remaining structural strength of the component.
Corrosion depth and removal limits have been established for some aircraft. Any loss of structural strength should be evaluated prior to repair or replacement of the part.
CORROSION OF MAGNESIUM ALLOYS Magnesium is the most chemically active of the metals used in aircraft construction and is, therefore, the most difficult to protect. When a failure in the protective coating does occur, the prompt and complete correction of the coating failure is imperative if serious structural damage is to be avoided. Magnesium attack is probably the easiest type of corrosion to detect in its early stages, since magnesium corrosion products occupy several times the volume of the original magnesium metal destroyed. The beginning of attack shows as a lifting of the paint films and white spots on the magnesium surface. These rapidly develop into snowlike mounds or even "white whiskers." Reprotection involves the removal of corrosion products, the partial restoration of surface coatings by chemical treatment, and a re-application of protective coatings.
Treatment of Wrought Magnesium Sheet and Forgings Magnesium skin attack will usually occur around edges of skin panels, underneath holddown washers, or in areas physically damaged by shearing, drilling, abrasion, or impact. If the skin section can be removed easily, this should be done to assure complete inhibition and treatment. If insulating washers are involved, screws should be loosened, at least sufficiently to permit brush treatment of the magnesium under the insulating washer. Complete mechanical removal of corrosion products should be practiced insofar as practicable. Such mechanical cleaning should be limited to the use of stiff, hog-bristle brushes and similar nonmetallic cleaning tools, particularly if treatment is to be performed under field conditions. Any entrapment of steel particles from steelwire brushes or steel tools, or contamination of treated surfaces by dirty abrasives, can cause more trouble than the initial corrosive attack. Corroded magnesium may generally be treated as follows:
1. Clean and strip the paint from the area to be treated. (Paint stripping procedures were discussed earlier in this chapter.)
2. Using a stiff, hog-bristle brush, break loose and remove as much of the corrosion products as practicable. Steel-wire brushes, carborundum abrasives, or steel cutting tools should not be used.
3. Treat the corroded area liberally with a chromic acid solution, to which has been added sulphuric acid, and work into pits and crevices by brushing the area while still wet with chromic acid, again using a nonmetallic brush.
4. Allow the chromic acid to remain in place for 5 to 20 minutes before wiping up the excess with a clean, damp cloth. Do not allow the excess solution to dry and remain on the surface, as paint lifting will be caused by, such deposits.
5. As soon as the surfaces are dry, restore the original protective paint.
Treatment of Installed Magnesium Castings Magnesium castings, in general, are more porous and more prone to penetrating attack, than wrought magnesium skins. However, treatment is, for all practical purposes, the same for all magnesium areas. Engine cases, belleranks, fittings, "umerous covers, plates, and handles are the most common magnesium castings.
When attack occurs on a casting, the earliest practicable treatment is required if dangerous corrosive penetration is to be avoided. In fact, engine cases submerged in salt water 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 general 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 manufacturer 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 resistant, 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 permissable, 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. Contact 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 dissimilarmetal contact is oxidation (decomposition) of one or both metals. The chart shown in figure 6-60 lists the metal combinations requiring a protective 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 zine chromate 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 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 thickness 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, corrosion damage is classified under the four standard types, as is any other damage. These types are: (1) Negligible 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 corrosion which has scarred or eaten away the surface protective coats and begun to etch 180
CONTACTING Aluminum alloy Cadmium plate Carbon and alloy steels Copper and alloys Nickel and alloys Titanium and alloys Chromium plate Corrosion resisting steel Magnesium alloys METALS Zinc plate Lead Tin coating Aluminum alloy Cadmium 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 Shaded areas indicate dissimiliar metal contacts FIGURE 6-60. Dissimilar metal contacts that will result in electrolytic corrosion.
Corrosion damage extending to classifications of "repairable by patching» and "repairable by insertion" should be rapaired 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 resistance or to provide a suitable base for paint.
In most instances the original finishes cannot be restored in the field due to nonavailability of equipment or other limitations. 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 necessary 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 181
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 pickling solution can be either muriatic (hydrochloric) or sulfuric acid. Cost-wise, sulfuric acid is preferable, but muriatic acid is more effective in removing certain 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.
Electrocleaning is another type of chemical cleaning used to remove grease, oil, or organic matter. In this cleaning process, the metal is suspended in a hot alkaline solution containing special wetting agents, inhibitors, and materials to provide the necessary electrical conductivity. An electric current is then passed through the solution in a manner similar to that used in electroplating.
Aluminum and magnesium parts are also cleaned by using some of the foregoing methods.
Blast cleaning is not applicable to thin aluminum sheets, particularly alclad. Steel grits are not used on aluminum or corrosion-resistant metals.
Polishing, buffing, and coloring of metal surfaces play a very important part in the finishing of metal surfaces. Polishing and buffing operations are sometimes used when preparing a metal surface for electroplating, and all three operations are used when the metal surface requires a high-luster Electroplating Electroplating is the process of transferring metal from one object to another by chemical and electrical means. Several reasons for applying plated coatings are:
1. To protect the base metal (metal being plated) against corrosion. Tin, zinc, nickel, and cadmium are some of the metals used to form a protective coating on another metal by electrolytic action.
2. To protect the base metal against wear, caused by abrasion or fretting corrosion.
Chromium plating is extensively used for wear resistance on gages, dies, oleo pistons, and cylinder barrels. Nickel plating can also be used for this purpose.
3. To produce and retain a desired appearance (color and luster), as well as improve resistance to tarnish. Gold, nickel, or chromium plating can be used in this application.
4. To protect a base metal against some special chemical reaction; for example, copper plating is sometimes used to prevent certain parts of a component manufactured of steel from absorbing carbon during casehardening.
5. To increase the dimensions of a part. This process, known as "building up," may be applied to parts accidentally made undersize, or to worn parts. Nickel or chromium plating is commonly used for this purpose.
6. To serve as a base for further plating bright de bright deposits of nickel or nickel and operations, reduce buffing costs, and ensure chromium. Copper is commonly used for this purpose.
All electroplating processes are basically similar.
The equipment used consists of a tank or bath containing a liquid solution called an electrolyte, a control panel, and a source of direct current.
When a current is passed through the circuit, the plating material is the positive electrode or anode of the circuit. The part on which the plating is deposited is the negative electrode or cathode of the circuit. The source of power anode, cathode, and electrolyte form the plating electrical circuit and cause tiny particles of the plating material to be deposited on the surface of the part being plated. The process is continued until a plating of the required thickness is obtained. The electrolyte, anode, cathode, and current setting will vary with the type of plating material being used.
Some plating operations do not use anodes of the metal being deposited but obtain the metal from the electrolyte alone. Chromium plating is an example of this type plating. Lead anodes, instead of chromium anodes, which are unsatisfactory, are employed to complete the electrical circuit. The chromium for the plating comes from the chromic acid in the electrolyte.
Metal Spraying Metal spraying, or metallizing, is the surface application of molten metal on any solid base material. It is possible to spray aluminum, cadmium, copper, nickel, steel, or any of several metals using this process. In aircraft work the process is used primarily to spray a coat of pure aluminum on steel parts to improve their corrosion 182
The base material must be roughened (usually by sandblasting) and perfectly clean in order for the sprayed metal to adhere to the surface of the base material.
Metal-spraying equipment consists of a supply of oxygen and acetylene piped to the spray gun, which ends in a nozzle. At this point they can be ignited as in a welding torch. A supply of compressed air is also piped to the spray gun. This compressed air operates a feeding mechanism that draws the wire through the spray gun. The wire is melted by the hot oxyacetylene flame and is thrown against the surface being metallized by the compressed air.
CHEMICAL TREATMENTS Parco Lubrizing Parco Lubrizing is a chemical treatment for iron and steel parts which converts the surface to a nonmetallic oil-absorptive phosphate coating. It is designed primarily to reduce wear on moving parts.
The process is a modification of Parkerizing, and consists of a precleaning treatment in which vapor degreasing, acid pickle, or spray emulsion is used, followed by a 15-minute dip in a solution of water and 10 percent by volume of Parco Lubrite. This is followed by a water rinse and a dip in watersoluble oil. The phosphate surface soaks up oil and retains it.
Anodizing Anodizing is the most common surface treatment of nonclad aluminum alloy surfaces. The aluminum alloy sheet or casting is the positive pole in an electrolytic bath in which chromic acid or other oxidizing agent produces an aluminum oxide film on the metal surface. Aluminum oxide is naturally anodizing merely increases thickness and density of the natural oxide film.
When this coating is damaged in service, it can only be partially restored by chemical surface treatments.
faces, including corrosion removal, should avoid unnecessary destruction of the oxide film.
The anodized coating provides excellent resistance to corrosion. The coating is soft and easily scratched, making it necessary to use extreme caution when handling it prior to coating it with Aluminum wool, nylon webbing impregnated with aluminum oxide abrasive or fiber bristle brushes are the approved tools for cleaning anodized surfaces. The use of steel wool, steel wire brushes, or harsh abrasive materials on any aluminum surfaces is prohibited. Producing a buffed or wire brush finish by any means is also prohibited. Otherwise, anodized surfaces are treated in much the same manner as other aluminum finishes.
In addition to its corrosion-resistant qualities, the anodic coating is also an excellent bond for paint. In most cases parts are primed and painted as soon as possible after anodizing, The anodic coating is a poor conductor of electricity; therefore, if parts require bonding, the coating is removed where the bonding wire is to be attached.
• Alclad surfaces that are to be left unpainted require no anodic treatment; however, if the alclad surface is to be painted, it is usually anodized to provide a bond for the paint.
Alodizing is a simple chemical treatment for all aluminum alloys to increase their corrosion resistance and to improve their paint-bonding qualities. Because of its simplicity, it is rapidly replacing anodizing in aircraft work.
The process consists of precleaning with an acidic or alkaline metal cleaner that is applied by either dipping or spraying. The parts are then rinsed with fresh water under pressure for 10 to 15 seconds. After thorough rinsing, alodine is applied by dipping, spraying, or brushing. A thin, hard coating results which ranges in color from light, bluish-green with a slight iridescence on copperfree alloys to an olive green on copper-bearing alloys. The alodine is first rinsed with clear, cold or warm water for a period of 15 to 30 seconds. An additional 10- to 15-second rinse is then given in a Deoxylyte bath. This bath is to counteract alkaline material and to make the alodyzed aluminum surface slightly acid on drying.
Chemical Surface Treatment and Inhibitors As previously described, aluminum and magnesium alloys in particular are protected originally by a variety of surface treatments. Steels may have been Parco Lubrized or otherwise oxidized on the surface during manufacture. Most of these coatings can only be restored by processes which are completely impractical in the field. But, corroded areas where such protective films have been destroyed require some type of treatment prior to refinishing. The following inhibiting materials are particularly effective in the field treatment of aluminum, are beneficial to bare magnesium, and are of some value even on bare steel parts.
The labels on the containers of surface treatment chemicals will provide warnings if a material is toxic or flammable. However, the label might not be large enough to accommodate a list of all the 183