Corrosion Inspection, Prevention, and Removal

8-11 protective systems 4. Prompt treatment of corrosion and touch up of damaged paint areas 5. Accurate record keeping and reporting of material or design deficiencies to the manufacturer and the FAA 6. Use of appropriate materials, equipment, technical publications, and adequately-training personnel 7. Maintenance of the basic finish systems 8. Keeping drain holes free of obstructions 9. Daily draining of fuel cell sumps 10. Daily wipe down of exposed critical areas 11. Sealing of aircraft against water during foul weather and proper ventilation on warm, sunny days 12. Replacing deteriorated or damaged gaskets and sealants to avoid water intrusion and/or entrapment 13. Maximum use of protective covers on parked aircraft After any period where regular corrosion preventive maintenance is interrupted, the amount of maintenance required to repair accumulated corrosion damage and bring the aircraft back up to standard is usually quite high.

Inspection

Inspection for corrosion is a continuing problem and must be handled daily. 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 complete enough to cover all parts of the aircraft or engine, thus no part of the aircraft goes uninspected. Use these checklists as a general guide when an area is to be inspected for corrosion. Through experience, one learns that most aircraft have trouble areas where, despite routine inspection and maintenance, corrosion still sets in.

All corrosion inspections start with a thorough cleaning of the area to be inspected. A general visual inspection of the area follows using a flashlight, inspection mirror, and a 5– l0X magnifying glass. The general inspection is to look for obvious defects and suspected areas. A detailed inspection of damage or suspected areas found during the general inspection follows. Visual inspection is the most widely used technique and is an effective method for the detection and evaluation of corrosion. Visual inspection employs the eyes to look directly at an aircraft surface or at a low angle of incidence to detect corrosion. Using the sense of touch is also an effective inspection method for the detection of hidden, well-developed corrosion. Other tools used during the visual inspection are mirrors, optical micrometers, and depth gauges.

Sometimes the inspection areas are obscured by structural members, equipment installations, or for some reason are awkward to check visually. Adequate access for inspection must be obtained by removing access panels and adjacent equipment, cleaning the area as necessary, and removing loose or cracked sealants and paints. Mirrors, borescopes, and fiber optics are useful in providing the means of observing obscure areas. In addition to visual inspection, there are several NDI methods, such as liquid penetrant, magnetic particle, eddy current, x-ray, ultrasonic, and acoustical emission, that may be of value in the detection of corrosion. These methods have limitations and must be performed only by qualified and certified NDI personnel. Eddy current, x-ray, and ultrasonic inspection methods require properly calibrated (each time used) equipment and a controlling reference standard to obtain reliable results.

In addition to routine maintenance inspections, amphibians or seaplanes must be checked daily and critical areas cleaned or treated, as necessary. Corrosion Prone Areas Discussed briefly in this section are most of the corrosion problem areas common to all aircraft. These areas should be cleaned, inspected, and treated more frequently than less corrosion prone areas. This information 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 8-19] Deposits may be trapped 8-12 and not reached by normal 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 empennage surfaces.

Buildup in these areas is 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 compartments continue to be corrosion prone areas. Fumes from overheated electrolyte are difficult to contain and spread to adjacent cavities, causing 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. If aircraft batteries with electrolytes, sulfuric acid, or potassium hydroxide are in use, their leakage will cause corrosion. Regular cleaning and neutralization of acid deposits minimizes corrosion from this cause. Consult the applicable maintenance manuals for the particular aircraft to determine the type of battery installed and the recommended maintenance.

Bilge Areas These are natural collection points 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 construction, the interior compartments are painted to protect the bilge areas. When seaplane structures are repaired or restored, this level of corrosion protection must be maintained.

Lavatories, Buffets, & Galleys These areas, particularly deck areas behind lavatories, sinks, and ranges, where spilled food and waste products may collect if not kept clean, are potential trouble spots. Even if some contaminants are not corrosive in themselves, they attract and retain moisture and, in turn, cause corrosive attack. Pay attention to bilge areas located under galleys and lavatories. Clean these areas frequently and maintain the protective sealant and paint finishes. Wheel Well and Landing Gear More than any other area on the aircraft, this area probably receives more punishment due to mud, water, salt, gravel, and other flying debris. [Figure 8-20] Because of the many complicated shapes, assemblies, and fittings, complete area paint film coverage is difficult to attain and maintain.

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 bolt heads, lugs, and wheel web areas, for the presence of entrapped water or its effects 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 that are typical water and debris traps 5. Axle interiors 6. Exposed surfaces of struts, oleos, arms, links, and attaching hardware (bolts, pins, etc.) Water Entrapment Areas Design specifications require that aircraft have drains installed in all areas where water may collect. Daily inspection of low point drains is a standard requirement. If this inspection is neglected, the drains may become ineffective because of accumulated debris, grease, or sealants.

8-13 more frequently than other areas. Engine Frontal Areas & Cooling Air Vents These areas are being constantly abraded with airborne dirt and dust, bits of gravel from runways, and rain erosion, leading to removal of the protective finish. Furthermore, cores of radiator coolers, reciprocating engine cylinder fins, etc., may not be painted due to the requirement for heat dissipation. Engine accessory mounting bases usually have small area of unpainted magnesium or aluminum on the machined-mounted surfaces. Inspection of these areas must 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 are maintained on seaplane and amphibian engine surfaces at all times.

Wing Flap & Spoiler Recesses Dirt and water may collect in flap and spoiler recesses unnoticed, because they are normally retracted. For this reason, these recesses are potential corrosion problem areas. Inspect these areas with the spoilers and flaps in the fully deployed position. 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 attention. 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 and can never be completely restored by touchup procedures. Any inspection for corrosion must 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 must include lubrication and actuation through several cycles to ensure complete lubricant penetration. Use water-displacing lubricants when servicing piano hinges. [Figures 8-21 and 8-22] 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 corrosion is evidenced by corrosion products appearing at the crevices where 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.

Electronic & Electrical Compartments Electronic and electrical compartments cooled by ram air or compressor bleed air are subjected to the same conditions common to engine and accessory cooling vents and engine frontal areas. While the degree of exposure is less, because a lower volume of air passing through and special design features incorporated to prevent water formation in enclosed spaces, this is still a trouble area that requires special attention. Circuit breakers, contact points, and switches are extremely sensitive to moisture and corrosive attack, thus inspection is required for these conditions as thoroughly as design permits.

If design features hinder examination of these items while in the installed condition, inspection is accomplished after component removal for other reasons. 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. 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 corrosion- resistant steel, are to be inspected to determine their condition at each inspection period. In this process, inspect cables for corrosion by random cleaning of short sections with solvent soaked cloths. If external corrosion is evident, relieve tension 8-14 Bare steel hinge pin Al alloy extrusions Hidden corrosion occurs here. Joint freezes and lugs break off when hinge is actuated.

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.

Corrosion Removal

In general, any complete corrosion treatment involves cleaning and stripping of the corroded area, removing as much of the corrosion products as practicable, neutralizing any residual materials remaining in pits and crevices, restoring protective surface films, and applying temporary or permanent coatings or paint finishes. Repair of corrosion damage includes removal of all corrosion and corrosion products. When the corrosion damage is severe and exceeds the damage limits set by the aircraft or parts manufacturer, the part must be replaced. The following paragraphs deal with the correction of corrosive attack on aircraft surface and components where deterioration has not progressed to the point requiring rework or structural repair of the part involved.

Several standard methods are available for corrosion removal. The methods normally used to remove corrosion are mechanical and chemical. Mechanical methods include hand sanding using abrasive mat, abrasive paper, or metal wool, and powered mechanical sanding, grinding, and buffing, using abrasive mat, grinding wheels, sanding discs, and abrasive rubber mats. However, the method used depends upon the metal and the degree of corrosion. Surface Cleaning and Paint Removal The removal of corrosion 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 is held to the minimum consistent with full exposure of the corrosion damage. Extensive corrosion spread on any panel is to be corrected by fully treating the entire section.

The selection of the type of materials to be used in cleaning depends on the nature of the matter to be removed. Modern environmental standards encourage the use of water-based, non-toxic cleaning compounds 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 organic compounds (VOCs). Where permitted, dry cleaning solvent (P-D-680) 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 soluble 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 “environmentally friendly.” Wherever practicable, chemical paint removal from any large area is to 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 be exercised in using paint remover, especially around gas or watertight seam sealants, since the stripper tends to soften and destroy the integrity of these sealants.

8-15 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. Therefore, wear rubber gloves, aprons of acid repellent material, and goggle type eyeglasses. The following is a general stripping procedure: 1. Brush the entire area to be stripped with a cover of stripper to a depth of 1⁄32" to 1⁄16". 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 must not be used for other purposes after being exposed to paint remover.

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 temperature, 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 where the paint remains tightly adhered or where the stripper has dried, and repeat the above process. Only nonmetallic scrapers are to be used to assist in removing persistent paint finishes. Nonwoven abrasive pads intended for paint stripping may also prove to be useful in removing the loosened paint.

4. 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 assures complete rinsing of the cleaned area. Use care to dispose of the stripped residue in accordance with environmental laws.

Fairing or Blending Reworked Areas All depressions resulting from corrosion rework must be faired or blended with the surrounding surface. Fairing can be accomplished as follows: 1. Remove rough edges and all corrosion from the damaged area. All dish-outs must be elliptically shaped with the major axis running spanwise on wings and horizontal stabilizers, longitudinally on fuselages, and vertically on vertical stabilizers. 2. In critical and highly stressed areas, all pits remaining after the removal of corrosion products must be blended out to prevent stress risers that may cause stress corrosion cracking. [Figure 8-23] On a non- critical structure, it is not necessary to blend out pits remaining after removal of corrosion products by abrasive blasting, since this results in unnecessary metal removal.

Rework depressions by forming smoothly blended dish-outs, using a ratio of 20:1, length to depth. [Figure 8-24] In areas having closely-spaced, multiple pits, intervening material must be removed to minimize surface irregularity or waviness. [Figure 8-25] Steel nut-plates and steel fasteners are to be removed before blending corrosion out of aluminum structure. Steel or copper particles embedded in aluminum can become a point of future corrosion. All corrosion products must be removed during blending to prevent reoccurrence of corrosion. 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 for additional corrosion. 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 8-26] 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, promoting 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 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. Rust removal from structural components, followed by an inspection and damage assessment, must be done as soon as feasible. [Figure 8-27] Mechanical Removal of Iron Rust The most practicable means of controlling the corrosion of steel is the complete removal of corrosion products by mechanical means and restoring 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 8-16 ACCEPTABLENOT ACCEPTABLE CROSS SECTION TOP VIEW TRUE PERSPECTIVE Location of corrosion pits Width of cleaned up area (10 times depth min) Length of cleaned up area (20 times depth min) Depth of cleaned up of corrosion LONGITUDINAL CORROSION DAMAGE BEFORE REWORK Pit has been cleaned up to the extent that all loose corrosion products have been removed.

Rough edges have been smoothed and all corrosion has been removed. However, depression has not been shaped. Dish-out after blending 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 cleaned in such a manner has rusted, it usually corrodes again more easily than it did the first time. The introduction of variations of the nonwoven abrasive pad has also increased the options available for the removal of surface rust. [Figure 8-28] Flap wheels, pads intended for use with rotary or oscillating power tools, and hand-held nonwoven abrasive pads all can be used alone or with light oils to remove corrosion from ferrous components.

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 products that actively remove the iron oxide without chemically etching the base metal are available and can be considered for use. If at all possible, the steel part is removed from the airframe for treatment, as it can be nearly impossible to remove all residue. The use of any caustic rust removal product requires the isolation of the part from any nonferrous metals during treatment and probably 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 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 8-17 Damage removed and surface smoothed with shallow elliptical dish-out Bottom of depression after corrosion removal Corrosion damage before removal LONGITUDINAL TRANSVERSE 5D MIN 10D MIN inhibitors on installed steel parts is not only undesirable, but also very dangerous. The danger of entrapment of corrosive solutions and the resulting uncontrolled attack, that 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 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. 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. The use of chemical corrosion removers is prohibited without engineering authorization, because high-strength steel parts are subject to hydrogen embrittlement.

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

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" thick. This is not considered detrimental. The 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 speeds 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: the penetrating pit-type corrosion through the walls of aluminum tubing, stress-corrosion cracking of materials under sustained stress, and intergranular corrosion, which is characteristic of certain improperly heat- treated aluminum alloys. In general, corrosion of aluminum can be more effectively 8-18 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 when compared with the stronger aluminum alloys. To take advantage of this characteristic, 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, however, care must be taken to prevent staining and marring of the exposed aluminum. More important from a protection standpoint, 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: 1. 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. 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 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. 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 be used. 2. If residual paint film remains, 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.” 3. 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 loosens or removes most existing corrosion and assures 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 that 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 must 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. 8-19 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, avoid destruction of the oxide film in the unaffected area when performing any corrosion correction of anodized surfaces. Do not use steel wool or steel wire brushes. Do not use severe abrasive materials.

Nonwoven abrasive pads have generally replaced aluminum wool, aluminum wire brushes, or fiber bristle brushes as the tools used 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 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 precipitation of dissimilar constituents following heat-treatment. In its most severe form, actual lifting of metal layers (exfoliation) occurs. [Figure 8-15] 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 must be evaluated prior to repair or replacement of the part. If the manufacturer’s limits do not adequately address the damage, a designated engineering representative (DER) can be brought in to assess the damage.

Corrosion of Magnesium Alloys Magnesium is the most chemically active of the metals used in aircraft construction and is 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 film and white spots on the magnesium surface. These rapidly develop into snow-like mounds or even “white whiskers.” [Figure 8-29] Reprotection involves the removal of corrosion products, the partial restoration of surface coatings by chemical treatment, and a reapplication of protective coatings.

Treatment of Wrought Magnesium Sheet & Forgings Magnesium skin corrosion usually occurs around edges of skin panels, underneath washers, or in areas physically damaged by shearing, drilling, abrasion, or impact. If the skin section can be removed easily, do so to assure complete inhibition and treatment. If insulating washers are involved, loosen screws sufficiently to permit brush treatment of the magnesium under the insulating washer. Complete mechanical removal of corrosion products is to be practiced insofar as practicable. Limit such mechanical cleaning to the use of stiff, hog bristle brushes and similar nonmetallic cleaning tools (including nonwoven abrasive pads), particularly if treatment is to be performed under field conditions. Like aluminum, under no circumstances are steel or aluminum tools; steel, bronze, or aluminum wool; or other cleaning abrasive pads used on different metal surfaces to be used in cleaning magnesium. Any entrapment of particles from steel wire 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 and are also addressed in FAA AC 43.13-1, Acceptable Methods, Techniques, and Practices— Aircraft Inspection and Repair. 2. Use a stiff, hog-bristle brush or nonwoven abrasive pad to break loose and remove as much of the corrosion products as practicable. Steel wire brushes, carborundum abrasives, or steel cutting tools must not be used. 3. Treat the corroded area liberally with a chromic acid solution that sulfuric acid has been added to. Work the solution 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 is 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 prone to penetrating attack than wrought magnesium skins. For 8-20 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 practicable 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, separate parting surfaces to effectively treat the existing attack and prevent its further progress. The same general treatment sequence in the preceding paragraph for magnesium skin is to 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 must be referred to if any question of safety is involved.

Treatment of Titanium & 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 permissible 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 dissimilar metal contact is oxidation (decomposition) of one or both metals. The chart shown in Figure 8-30 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 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" 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 negligible damage, damage repairable by patching, damage repairable by insertion, and damage necessitating replacement of parts. The term “negligible” does not imply that little or nothing is to be done. The corroded surface must be cleaned, treated, and painted as appropriate. Negligible damage, generally, is corrosion that has scarred or eaten away the surface protective coats and begun to etch the metal. Corrosion damage