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CHAPTER 6. CORROSION, INSPECTION & PROTECTION

SECTION 1. GENERAL

6-1. GENERAL. The purpose of this chap-ter is to provide information that will helpmaintenance personnel prevent, control, iden-tify, and treat various types of corrosion. (Re-fer to AC 43-4A, Corrosion Control For Air-craft for a more in-depth study on the detectionand treatment of corrosion.)

a. Corrosion is a natural occurrence thatattacks metal by chemical or electrochemicalaction and converts it back to a metallic com-pound.

b. Four conditions must exist beforeelectrochemical corrosion can occur. (See fig-ure 6-1.) They are:

(1) A metal subject to corrosion (An-ode);

(2) A dissimilar conductive material(Cathode), which has less tendency to corrode;

(3) Presence of a continuous, conduc-tive liquid path (Electrolyte); and

(4) Electrical contact between the anodeand the cathode (usually in the form of metal-to-metal contact such as rivets, bolts, and cor-rosion).

c. Elimination of any one of these condi-tions will stop electrochemical corrosion. (Seefigure 6-2.)

NOTE: Paint can mask the initialstages of corrosion. Since corrosionproducts occupy more volume thanthe original metal, painted surfacesshould be inspected often for irregu-larities such as blisters, flakes, chips,and lumps.

6-2. FACTORS INFLUENCING COR-ROSION.

a. Some factors which influence metalcorrosion and the rate of corrosion are:

(1) Type of metal;

(2) Heat treatment and grain direction;

(3) Presence of a dissimilar, less corro-dible metal;

(4) Anodic and cathodic surface areas(in galvanic corrosion);

(5) Temperature;

(6) Presence of electrolytes (hard water,salt water, battery fluids, etc.);

(7) Availability of oxygen;

(8) Presence of biological organisms;

(9) Mechanical stress on the corrodingmetal; and,

(10) Time of exposure to a corrosiveenvironment.

(11) Lead/graphite pencil marks on air-craft surface metals.

b. Most pure metals are not suitable foraircraft construction and are used only in com-bination with other metals to form alloys.Most alloys are made up entirely of smallcrystalline regions, called grains. Corrosioncan occur on surfaces of those regions which

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are less resistant and also at boundaries be-tween regions, resulting in the formation ofpits and intergranular corrosion. Metals have awide range of corrosion resistance. The mostactive metals, (those which lose electrons eas-

ily), such as magnesium and aluminum, cor-rode easily. The most noble metals (thosewhich do not lose electrons easily), such asgold and silver, do not corrode easily.

FIGURE 6-1. Simplified corrosion cell showing conditions which must exist for electrochemical corrosion.

FIGURE 6-2. Elimination of corrosion by application of an organic film to metal surface.

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c. Corrosion is quickened by high-temperature environments that acceleratechemical reactions and increase the concentra-tion of water vapor in the air.

d. Electrolytes (electrically-conductingsolutions) form on surfaces when condensa-tion, salt spray, rain, or rinse water accumulate.Dirt, salt, acidic gases, and engine exhaustgases can dissolve on wet surfaces, increasingthe electrical conductivity of the electrolyte,thereby increasing the rate of corrosion.

e. When some of the electrolyte on ametal surface is partially confined, (such asbetween faying surfaces or in a deep crevice)the metal around this area corrodes more rap-idly. This type of corrosion is called an oxygenconcentration cell. Corrosion occurs morerapidly because the reduced oxygen content ofthe confined electrolyte causes the adjacentmetal to become anodic to other metal surfaceson the same part that are immersed in electro-lyte or exposed to air.

f. Slime, molds, fungi, and other livingorganisms (some microscopic) can grow ondamp surfaces. Once they are established, thearea usually remains damp, increasing the pos-sibility of corrosion.

g. Manufacturing processes such as ma-chining, forming, welding, or heat treatmentcan leave residual stress in aircraft parts andcan cause cracking in a corrosive environment.

6-3. COMMON CORROSIVE AGENTS.Substances that cause corrosion are called cor-rosive agents. The most common corrosiveagents are acids, alkalies, and salts. The at-mosphere and water, the two most commonmedia for these agents, may also act as corro-sive agents.

a. Any acid will severely corrode mostof the alloys used in airframes. The most de-

structive are sulfuric acid (battery acid), halo-gen acids (hydrochloric, hydrofluoric, and hy-drobromic), nitrous oxide compounds, and or-ganic acids found in the wastes of humans andanimals.

b. Alkalies, as a group, are not as corro-sive as acids. Aluminum and magnesium al-loys are exceedingly prone to corrosive attackby many alkaline solutions unless the solutionscontain a corrosion inhibitor. Substances par-ticularly corrosive to aluminum are washingsoda, potash (wood ashes), and lime (cementdust).

c. The major atmospheric corrosiveagents are oxygen and airborne moisture.Corrosion often results from the direct actionof atmospheric oxygen and moisture on metalnd the presence of additional moisture oftenaccelerates corrosive attack, particularly onferrous alloys. The atmosphere may also con-tain other corrosive gases and contaminants,particularly industrial and marine salt spray.

d. The corrosiveness of water dependson the type and quantity of dissolved mineraland organic impurities and dissolved gasses(particularly oxygen) in the water. One char-acteristic of water that makes it corrosive is itsconductivity. Physical factors, such as watertemperature and velocity also have a directbearing on its corrosiveness.

6-4. MICRO-ORGANISMS.

a. Bacteria may be either aerobic or an-aerobic. Aerobic bacteria require oxygen tolive. They accelerate corrosion by oxidizingsulfur to produce sulfuric acid. Bacteria livingadjacent to metals may promote corrosion bydepleting the oxygen supply or by releasingmetabolic products. Anaerobic bacteria, on theother hand, can survive only when free oxygen

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is not present. The metabolism of these bacte-ria requires them to obtain part of their suste-nance by oxidizing inorganic compounds, suchas iron, sulfur, hydrogen, and carbon monox-ide. The resultant chemical reactions causecorrosion.

b. Fungi are the growths of micro-organisms that feed on organic materials.While low humidity does not kill microbes, itslows their growth and may prevent corrosiondamage. Ideal growth conditions for most mi-cro-organisms are temperatures between68 and 104 °F (20 and 40 °C) and relative hu-midity between 85 and 100 percent.

c. Damage resulting from microbialgrowth can occur when any of three basicmechanisms, or a combination of these, isbrought into play. First, fungi have a tendencyto hold moisture, which contributes to otherforms of corrosion. Second, because fungi areliving organisms, they need food to survive.This food is obtained from the material onwhich the fungi are growing. Third, these mi-cro-organisms secrete corrosive fluids that at-tack many materials, including some that arenot fungi nutrient.

d. Microbial growth must be removedcompletely to avoid corrosion. Microbialgrowth should be removed by hand with a firmnon-metallic bristle brush and water. Removalof microbial growth is easier if the growth iskept wet with water. Microbial growth mayalso be removed with steam at 100 psi. Pro-tective clothing must be used when usingsteam for removing microbial growth.

6-5. 6-10. [RESERVED.]

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SECTION 2. TYPES OF CORROSION

6-11. GENERAL. All corrosive attacks be-gin on the surface of the metal making theclassification of corrosion by physical appear-ance a convenient means of identification.(See figure 6-3.)

FIGURE 6-3. Corrosion attack.

6-12. GENERAL SURFACE CORRO-SION. General surface corrosion (also re-ferred to as Uniform Etch or Uniform AttackCorrosion) is the most common form of corro-sion and results from a direct chemical attackon a metal surface and involves only the metalsurface. (See figure 6-4.) General surface cor-rosion usually occurs over a wide area and ismore or less equal in dispersion. On a pol-ished surface, this type of corrosion is firstseen as a general dulling of the surface, and ifallowed to continue, the surface becomesrough and possibly frosted in appearance. Thediscoloration or general dulling of metal cre-ated by exposure to elevated temperatures isnot to be considered general surface corrosion.

FIGURE 6-4. General surface corrosion.

6-13. PITTING CORROSION. Pittingcorrosion is one of the most destructive andintense forms of corrosion. It can occur in anymetal but is most common on metals that formprotective oxide films, such as aluminum andmagnesium alloys. It is first noticeable as awhite or gray powdery deposit, similar to dust,which blotches the surface. When the depositis cleaned away, tiny holes or pits can be seenin the surface. (See figures 6-5(a) and 6-5(b).)These small surface openings may penetratedeeply into structural members and causedamage completely out of proportion to its sur-face appearance.

FIGURE 6-5(a). Pitting corrosion (external view).

FIGURE 6-5(b). Pitting corrosion (magnified cross sec-tion).

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6-14. CONCENTRATION CELL COR-ROSION. Concentration cell corrosion, (alsoknown as Crevice Corrosion) is corrosion ofmetals in a metal-to-metal joint, corrosion atthe edge of a joint even though the joined met-als are identical, or corrosion of a spot on themetal surface covered by a foreign material.Metal ion concentration cells and oxygen con-centration cells are the two general types ofconcentration cell corrosion. (See figure 6-6.)

a. Metal Ion Concentration Cells. Thesolution may consist of water and ions of themetal which is in contact with water. A highconcentration of the metal ions will normallyexist under faying surfaces where the solutionis stagnant, and a low concentration of metalions will exist adjacent to the crevice which iscreated by the faying surface. An electricalpotential will exist between the two points; thearea of the metal in contact with the low con-centration of metal ions will be anodic andcorrode, and the area in contact with the highmetal ion concentration will be cathodic andnot show signs of corrosion.

b. Oxygen Concentration Cells. Thesolution in contact with the metal surface willnormally contain dissolved oxygen. An oxy-gen cell can develop at any point where theoxygen in the air is not allowed to diffuse intothe solution, thereby creating a difference inoxygen concentration between two points.Typical locations of oxygen concentration

cells are under gaskets, wood, rubber, andother materials in contact with the metal sur-face. Corrosion will occur at the area of lowoxygen concentration (anode). Alloys such asstainless steel are particularly susceptible tothis type of crevice corrosion.

6-15. ACTIVE-PASSIVE CELLS. Metalswhich depend on a tightly adhering passivefilm, usually an oxide, for corrosion protectionare prone to rapid corrosive attack by active-passive cells. Active-passive cells are often re-ferred to as a type of concentration cell corro-sion. However, the active-passive cell is actu-ally two forms of corrosion working in con-junction. The corrosive action usually starts asan oxygen concentration cell. As an example,salt deposits on the metal surface in the pres-ence of water containing oxygen can create theoxygen cell. The passive film will be brokenbeneath the salt crystals. Once the passivefilm is broken, the active metal beneath thefilm will be exposed to corrosive attack. (Seefigure 6-7.) Rapid pitting of the active metalwill result. This reaction can become locallyintense due to several factors. First the reac-tion is augmented by the affected area, sincethe proportion of the exposed base metal issmall compared to the surrounding non-reactive metal. This effectively concentratesthe focal point of the reaction, often resultingin deep pits in a short time and a greater rate ofcorrosion.

FIGURE 6-6. Concentration cell corrosion.

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6-16. FILIFORM CORROSION. Filiformcorrosion is a special form of oxygen concen-tration cell which occurs on metal surfaceshaving an organic coating system. It is recog-nized by its characteristic worm-like trace ofcorrosion products beneath the paint film.(See figure 6-8.) Polyurethane finishes are es-pecially susceptible to filiform corrosion. Fili-form occurs when the relative humidity of theair is between 78 and 90 percent and the sur-face is slightly acidic. This corrosion usuallyattacks steel and aluminum surfaces. Thetraces never cross on steel, but they will crossunder one another on aluminum which makesthe damage deeper and more severe for alumi-num. If the corrosion is not removed, the areatreated, and a protective finish applied, the cor-rosion can lead to inter-granular corrosion, es-pecially around fasteners and at seams. Fili-form corrosion can be removed using glassbead blasting material with portable abrasiveblasting equipment or sanding. Filiform corro-sion can be prevented by storing aircraft in anenvironment with a relative humidity below70 percent, using coating systems having a lowrate of diffusion for oxygen and water vapors,and by washing the aircraft to remove acidiccontaminants from the surface.

6-17. INTERGRANULAR CORROSION.Inter-granular corrosion is an attack on thegrain boundaries of a metal. A highly magni-fied cross section of any commercial alloyshows the granular structure of the metal. Itconsists of quantities of individual grains, andeach of these tiny grains has a clearly

FIGURE 6-8. Filiform corrosion.

defined boundary which chemically differsfrom the metal within the grain. The grainboundary and the grain center can react witheach other as anode and cathode when in con-tact with an electrolyte. (See figure 6-9.)Rapid selective corrosion of the grain bounda-ries can occur. High-strength aluminum alloyssuch as 2014 and 7075 are more susceptible tointer-granular corrosion if they have been im-properly heat-treated and then exposed to acorrosive environment.

6-18. EXFOLIATION CORROSION. Ex-foliation corrosion is an advanced form of in-ter-granular corrosion and shows itself by lift-ing up the surface grains of a metal by theforce of expanding corrosion products occur-ring at the grain boundaries just below the sur-face. (See figure 6-10.) It is visible evidenceof inter-granular corrosion and is most oftenseen on extruded sections where grain thick-ness are usually less than in rolled forms.

FIGURE 6-7. Active-passive cell.

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FIGURE 6-9. Inter-granular Corrosion of 7075-T6 aluminum adjacent to steel fastener.

FIGURE 6-10. Exfoliation corrosion.

6-19. GALVANIC CORROSION. Gal-vanic corrosion occurs when two dissimilarmetals make contact in the presence of anelectrolyte. (See figure 6-11.) It is usuallyrecognizable by the presence of a build-up ofcorrosion at the joint between the metals.

6-20. STRESS CORROSION CRACKING.This form of corrosion involves a constant orcyclic stress, acting in conjunction

with a damaging chemical environment. Thestress may be caused by internal or externalloading.

a. Internal stress may be trapped in apart of structure during manufacturing proc-esses such as cold working or by unequalcooling from high temperatures. Most manu-facturers follow up these processes with astress relief operation. Even so, sometimesstress remains trapped. The stress may be ex-ternally introduced in part structure by rivet-ing, welding, bolting, clamping, press fit, etc.If a slight mismatch occurs, or a fastener isover-torque, internal stress will be present.

b. Internal stress is more important thandesign stress, because stress corrosion is diffi-cult to recognize before it has overcome thedesign safety factor. The level of stress variesfrom point to point within the metal. Stressesnear the yield strength are generally necessaryto promote stress corrosion cracking. (See fig-ure 6-12.) However, failures may occur atlower stresses. Specific environments havebeen identified which cause stress corrosioncracking of certain alloys.

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FIGURE 6-11. Galvanic corrosion of magnesium adjacent to steel fastener.

FIGURE 6-12. Stress corrosion cracking.

(1) Salt solutions and sea water causestress corrosion cracking of high-strength,heat-treated steel and aluminum alloys.

(2) Methyl alcohol-hydrochloric acidsolutions will cause stress corrosion crackingof some titanium alloys.

(3) Magnesium alloys may stress-corrode in moist air.

(4) Stress Corrosion may be reduced by

• applying protective coatings,• stress relief heat treatments,• using corrosion inhibitors, or• controlling the environment.

6-21. FATIGUE CORROSION. Fatiguecorrosion involves cyclic stress and a corrosiveenvironment. Metals may withstand cyclicstress for an infinite number of cycles so longas the stress is below the endurance limit of themetal. Once the limit has been exceeded, themetal will eventually crack and fail from metalfatigue. However, when the part or structureundergoing cyclic stress is also exposed to acorrosive environment, the stress level for fail-ure may be reduced many times. Thus, failureoccurs at stress levels that can be dangerouslylow depending on the number of cycles as-signed to the life-limited part.

a. Fatigue corrosion failure occurs intwo stages. During the first stage the combinedaction of corrosion and cyclic stress damagesthe metal by pitting and crack formations tosuch a degree that fracture by cyclic stress willoccur, even if the corrosive environment iscompletely removed.

b. The second stage is essentially a fa-tigue stage in which failure proceeds by propa-gation of the crack (often from a corrosion pitor pits). It is controlled primarily by stressconcentration effects and the physical proper-ties of the metal. Fracture of a metal part dueto fatigue corrosion, generally occurs

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at a stress level far below the fatigue limit ofan uncorroded part, even though the amount ofcorrosion is relatively small.

6-22. FRETTING CORROSION. Frettingcorrosion,(also known as wear corrosion orfriction oxidation) can occur at the interface oftwo highly-loaded surfaces which are not sup-posed to move against each other. However,vibration may cause the surfaces to rub to-gether resulting in an abrasive wear known asfretting. (See figure 6-13.) The protectivefilm on the metallic surfaces is removed bythis rubbing action. With continued rubbing,metal particles sheared from the surface of themetal combine with oxygen to form metal ox-ide. As these oxides accumulate, they causedamage by abrasive action and increased localstress. The most common example of frettingcorrosion is the smoking rivet found on enginecowling and wing skins. This is one corrosionreaction that is not driven by an electrolyte,and in fact, moisture may inhibit the reaction.

Application of a lubricant or installation of afretting-resistant material between the two sur-faces can reduce fretting corrosion.

FIGURE 6-13. Fretting corrosion.

6-23. 6-28. [RESERVED.]

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SECTION 3. CORROSION PROTECTION MEASURES FOR BASIC MATERIALS

6-29. GENERAL. In the repair of aircraft,apply corrosion proofing of the same type orequivalent to that originally applied unless therepair would result in increased susceptibilityto corrosion, in which case use additional cor-rosion protection measures. The following is alist of the most commonly-used corrosion-proofing techniques.

6-30. ANODIZING AND RELATEDPROCESSES. In anodizing, aluminum alloysare placed in an electrolytic bath causing a thinfilm of aluminum oxide to form on the surfaceof the aluminum. This is resistant to corrosionand affords a good paint base. However, otherprocesses, which do not provide as good a cor-rosive protection as anodizing, are good paintbases. The processes are:

a. Alkaline cleaning followed by chromicacid dip;

b. Alcoholic phosphoric acid cleaner; and

c. Alkaline dichromate treatment.

6-31. PLATING. Steels are commonlyplated with other metals to prevent corrosion.Plating is accomplished by placing the articlein an electrolytic bath. Metals used in platingvary in the corrosion protection they affordsteel. For instance, in platings that corrode be-fore steel, such as zinc or cadmium, slightbreaks or cracks throughout the plating willnot result in rusting of the exposed steel. Withthe surface metal corroded, the steel is pro-tected. However, when the steel corrodesfaster than the plate metal, such as chromium,the amount of protection depends on the tight-ness of the plating. Post-plate bake treatmentto relieve hydrogen embrittlement is a neces-sary part of replating procedures for

high-strength steel parts. High-strength nutsand bolts are susceptible to failure from hydro-gen embrittlement. Because of the potentialfailures of embrittled parts, careful controlover the heat treatment, grinding, preplatecleaning, plating, and post-plate baking ofhigh-strength parts is necessary.

6-32. PHOSPHATE RUST-PROOFING.This process is commercially known asParkerizing, Bonderizing, Granodizing, etc.The coating placed on the part is used to pro-tect steel parts after machining and beforepainting.

6-33. CHROME-PICKLE TREATMENT.Magnesium parts which have been immersedor brushed with a solution of nitric acid andsodium dichromate will be protected for tem-porary storage. The coating will also serve asa bond for subsequent organic finishes. Sealedchrome-pickle treatment is used on magnesiumparts for long term protection. Diluted chro-mic acid is a touch-up treatment. It is lesscritical to apply and can be applied over previ-ously-applied thin chromate films.

6-34. DICHROMATE TREATMENT.This treatment consists of boiling magnesiumparts in a solution of sodium dichromate. Itprovides good paint base and protective quali-ties on all standard wrought magnesium alloysexcept the magnesium-thorium alloys. Acidpickling of the magnesium surface prior to ap-plication of the dichromate treatment is re-quired if maximum corrosion resistance of thefinish is expected.

6-35. STANNATE IMMERSION TREAT-MENT. Stannate immersion treatment depositsa layer of tin. It is a protective paint base formagnesium alloy parts which contain insertsand fasteners of a dissimilar metal such as

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brass, copper, or steel. This treatment cannotbe used with parts containing aluminum insertsor fasteners because the high alkalinity of thebath attacks the aluminum.

6-36. GALVANIC ANODIZING TREAT-MENT. An electrolytic process that providesa paint base and corrosion-preventive film onmagnesium alloys containing manganese.

6-37. CLADDING. Aluminum alloys whichare susceptible to corrosion are frequently cladwith pure aluminum. Slight pits, scratches, orother defects through the cladding materialmust be avoided, since the aluminum alloycore will corrode rapidly.

6-38. METAL SPRAYING. Metal is meltedand sprayed on the surface to be protected.The surface must be properly prepared andthoroughly cleaned to prevent peeling of thesprayed coat.

6-39. SHOT-PEENING. Shot-peening andother treatments, by which the surface can beplaced in compression, are effective in pre-venting stress corrosion.

6-40. ORGANIC COATINGS. Zinc chro-mate primer, enamels, chlorinated rubber com-pounds, etc., are organic coatings commonlyused to protect metals.

6-41. DOPE PROOFING. When dopedfabrics are applied over an organic finishedmetal structure, the dope will have a tendencyto loosen the finish on the metal. For this rea-son, organic coatings on the metal are usuallycovered with a dope-proof paint, with metalfoil, or with cellulose tape to prevent the dopefrom soaking through.

6-42. TUBE INTERIORS. Protect the inte-riors of structural steel and aluminum tubingagainst corrosion. A small amount of waterentrapped in a tube can corrode entirelythrough the tube thickness in a short period.Coat the tube interior by flushing with hot lin-seed oil, paralketone, or other approved corro-sion inhibitor. The flushing liquid is usuallyintroduced through small holes drilled in thetubing. Allow the flushing liquid to drain andplug the holes with a screw or by other meansto prevent entry of moisture. Air and water-tight sealing of the tubing will also give ade-quate protection against corrosion if the tubingis internally dry before being sealed.

6-43. 6-49. [RESERVED.]

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SECTION 4. CORROSION PREVENTIVE MAINTENANCE

6-50. GUIDELINES: ALL AIRCRAFT.Corrosion prevention depends on a compre-hensive prevention and control plan, imple-mented from the start of operation of an air-craft, which includes:

a. Adequately-trained personnel in

(1) recognition of corrosion-inducingconditions;

(2) corrosion identification techniques;

(3) corrosion detection, cleaning, andtreating; and

(4) lubrication and preservation of air-craft structure and components.

b. Inspection for corrosion on a sched-uled basis.

c. Thorough cleaning, inspection, lubri-cation, and preservation at prescribed intervals.

d. Prompt corrosion treatment after de-tection.

e. Accurate record-keeping and report-ing of material or design deficiencies to themanufacturer and the Federal Aviation Ad-ministration (FAA).

f. Use of appropriate materials, equip-ment, and technical publications.

g. Maintenance of the basic finish sys-tems.

h. Keeping drain holes and passagesopen and functional. Sealants, leveling com-pounds, miscellaneous debris, or corrosion in-hibitors should not block drain paths.

i. Replacing deteriorated or damagedgaskets and sealants (using non-corrosive typesealants) to avoid water intrusion and entrap-ment that leads to corrosion.

j. Minimizing the exposure of aircraft toadverse environments by keeping the aircraftin a hangar.

k. Periodic and frequent inspection ofareas where there are foamed plastics or otherabsorbent material.

l. Daily draining of fuel cavities to re-move accumulated water and other foreignmatter.

m. Daily wipe-down of exposed criticalsurfaces of hydraulic cylinders.

6-51. GUIDELINES: AIRCRAFT OP-ERATING OVER SALT WATER. In addi-tion to the inspection and treatment prescribedabove, the following treatment shall be ap-plied:

a. Remove all traces of salt water and saltwater residue by thoroughly washing the air-craft with fresh water.

(1) After drying, coat the propeller,hubs, blades and other unpainted or unpro-tected parts of the engine and its installationparts by spraying or rubbing lightly with corro-sion preventive compound, SpecificationMIL-C-16173, Grade 4.

(2) Apply this mixture on parts thatmove or require some lubrication and on allfittings subject to corrosion such as landinggear retracting plungers, control surfacehinges, control cables, exposed rivets andbolts, and other similar parts not protected by

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paint. Apply with a cloth or a soft brushsoaked in the mixture.

(3) Wipe off excess mixture. When ap-plying the mixture take care that as little aspossible is deposited on exhaust pipes or col-lector rings to avoid a fire hazard when the en-gine is started. Keep the ignition wires, pro-peller anti-icer feed hose, tires, and other rub-ber parts free of the mixture.

b. Where maximum corrosion protectionis desired on stationary parts, use exterior sur-face corrosion preventive compound, Specifi-cation MIL-C-16173, grade I.

c. Wipe the exposed portion of the land-ing gear shock strut piston with a cloth soakedin the applicable hydraulic fluid.

d. Most parts of landing gear wheels aremade from magnesium or aluminum alloyswhich corrode rapidly unless carefully pro-tected. When the aircraft operates near saltwater and off coral beaches, the corrosion canbe very rapid. Inspect wheels to determine thepaint condition.

e. Refinish portions of a wheel wherepaint has deteriorated, peeled, or chipped.

f. Except for friction and bearing sur-faces, apply a protective coating to all parts ofwheels and brake assemblies.

6-52. 6-62. [RESERVED.]

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SECTION 5. VISUAL CORROSION INSPECTION GUIDE FOR AIRCRAFT

6-63. GENERAL. This guide provides ageneral inspection checklist for those parts orsurfaces that can be visually inspected withoutdisassembly of the aircraft. It is intended foruse in establishing corrosion inspection areasfor which the manufacturer has not provided arecommended corrosion inspection program.The manufacturer’s recommended corrosioninspection program will take precedence overthis guideline. These inspections should beaccomplished in conjunction with other pre-ventive maintenance.

6-64. EXHAUST TRAIL AREAS.

a. Visually inspect paint in areas of theexhaust trails for damage.

b. Visually inspect under fairings,around rivet heads, and in skin crevices, forcorrosion in areas of engine exhaust trail.

6-65. BATTERY COMPARTMENTSAND BATTERY VENT OPENINGS.

a. Inspect battery compartment forelectrolyte spillage, corrosion, and condition ofprotective paint.

b. Inspect area around battery vent forcorrosion.

6-66. LAVATORIES AND GALLEYS. Inspection areas around lavatories, sinks, andranges for spillage and corrosion. Pay par-ticular attention to floor area and the area be-hind and under lavatories, sinks, and ranges.

6-67. BILGE AREAS.

a. Inspect bilge areas for waste hydraulicfluids, water, dirt, loose fasteners, drill chips,and other debris.

b. Remove any foreign material frombilge and inspect for corrosion.

6-68. WHEEL WELLS AND LANDINGGEAR. Inspect wheel well area and landinggear components for damage to exterior finishcoating and corrosion. Particular attentionshould be given to exposed surfaces of struts,oleos, arms, links, and attaching hardware;axle interiors, exposed position indicatorswitches and other electrical equipment; crev-ices between stiffeners, ribs, and lower skinsurfaces; magnesium wheels, particularlyaround bolt heads, lugs, and wheel web areas;and exposed rigid tubing at “B” nuts and fer-rules under clamps, and tubing identificationtapes.

6-69. EXTERNAL SKIN AREAS.

a. Inspect external skin surfaces fordamage to protective finishes and corrosion.

b. Inspect around fasteners for damageto protective finishes and corrosion.

c. Inspect lap joints for bulging of skinsurface, which may indicate the presence ofcorrosion between the faying surfaces. Skincracks and/or dished or missing fastener headsmay also indicate severe corrosion in bondedjoints.

d. Inspect area around spot welds forbulges, cracks, or corrosion.

e. Inspect piano type hinges for corro-sion. When piano hinges are inspected theyshould be lubricated and actuated through sev-eral cycles to ensure complete penetration ofthe lubricant.

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f. Inspect thick alloy skin surfaces forpitting, intergranular corrosion, and exfoliationof the metal. Look for white or gray depositsaround countersunk fastener heads and raisedareas or bumps under the paint film.

g. Inspect composite skins for corrosionof attachment fasteners.

6-70. WATER ENTRAPMENT AREAS. Inspect area around edge of drain holes for cor-rosion and ensure that drain holes are notblocked by debris.

6-71. ENGINE FRONTAL AREAS.

a. Inspect reciprocating engine cylinderfins, engine cases, and cooling air ducts fordamage to exterior finish and corrosion.

b. Inspect radiator cooler cores for cor-rosion.

6-72. ELECTRONIC PACKAGE COM-PARTMENTS.

a. Inspect circuit-breakers, contactpoints, and switches for evidence of moistureand corrosive attack.

b. Treatment of corrosion in electricaland electronic components should be done byor supervised by qualified personnel familiarwith the function of the unit involved.

6-73. FLEXIBLE HOSE ASSEMBLIES.

a. Inspect hose assemblies for chafing,weather-checking, hardening, discoloration,evidence of fungus, torn weather protectivecoatings or sleeves, and corrosion of fittings.

b. Replace any defective, damaged,twisted, or bulging hoses.

6-74. SANDWICH PANELS. Inspectedges of sandwich panels for damage to thecorrosion protection finish or sealant and forcorrosion.

6-75. CONTROL CABLES.

a. Inspect control cables for bare spots inthe preservative coating and corrosion.

b. If external corrosion is found, relievetension on the cable and check internal strandsfor corrosion. Cables with corrosion on inter-nal strands should be replaced. External cor-rosion should be removed by a clean, dry,coarse rag or fiber brush. A preservativeshould be applied after removal of externalcorrosion.

6-76. INTEGRAL FUEL CELLS.

a. Inspect top coat finish for breaks,peeling, lifting of surface, or other damage.

b. Inspect aircraft structure for top coatfinish damage from pitting or intergranularcorrosion.

6-77. ELECTRICAL CONNECTORS.

a. Inspect electrical connectors forbreaks in potting compound and corrosion ofpins and wires.

b. If the electrical connector is sus-pected of having moisture intrusion, disas-semble the connector, clean the connector, andinspect it for corrosion.

6-78. 6-88. [RESERVED.]

9/8/98 AC 43.13-1B

Par 6-89 Page 6-17

SECTION 6. CORROSION REMOVAL PROCEDURES

6-89. GENERAL. General safety precau-tions for handling materials with hazardousphysical properties are outlined in the follow-ing paragraphs. They also address emergencyprocedures for immediate treatment of person-nel who have inadvertently come into contactwith harmful materials. All personnel respon-sible for using or handling hazardous materialsshould be thoroughly familiar with the infor-mation in the following paragraphs.

6-90. SAFETY PRECAUTIONS.

a. Chemical. When required to use orhandle solvents, special cleaners, paint strip-pers (strong alkalies and acids), etchants (cor-rosion removers containing acids), or surfaceactivation material (Alodine 1200), observethe following safety precautions:

(1) Avoid prolonged breathing of sol-vent or acid vapors.

(2) Never add water to acid. Alwaysadd acid to water.

(3) Mix all chemicals per the manufac-turer’s instructions.

(4) Clean water for emergency useshould be available in the immediate work areabefore starting work.

(5) Avoid prolonged or repeated contactwith the skin of solvents, cleaners, etchants(acid), or conversion coating material (Alodinesolution). Rubber or plastic gloves should beworn. Goggles or plastic face shields and suit-able protective clothing should be worn

when cleaning, stripping, etching, or conver-sion coating overhead surfaces.

(6) When mixing alkalies with water orother substance, use containers that are madeto withstand heat generated by this process.

(7) Wash any paint stripper, etchant, orconversion coating material immediately frombody, skin, or clothing.

(8) Materials splashed into the eyesshould be promptly flushed out with water, andmedical aid obtained immediately.

(9) Do not eat or keep food in areaswhere poisons may be absorbed. Always washhands before eating or smoking.

(10) Verify that the area within 50 feetof any cleaning or treating operations wherelow flash point (140 °F or below) materials arebeing used, is clear and remains clear of allpotential ignition sources.

(11) Suitable fire-extinguishing equip-ment should be available to the clean-ing/treating area.

(12) Equipment should be effectivelygrounded where any flammable materials arebeing used.

(13) If materials (acid, alkali, paint re-mover, or conversion coatings) are spilled onequipment and/or tools, treat immediately byrinsing with clean water, if possible, and /orneutralizing acids with baking soda and alka-lies with a weak (5 percent) solution of aceticacid in water.

AC 43.13-1B 9/8/98

Page 6-18 Par 6-90

(14) In confined location, do not usesolvents with a low flash point, (below 100 �F)such as Methyl Ethyl Ketone (MEK) and ace-tone.

(15) All equipment should be cleanedafter work has been completed.

(16) Check and follow all applicable re-strictions and requirements on the use of sol-vents, primers, and top coats.

(17) Check and follow all applicable re-strictions and requirements for use and dis-posal of waste material.

b. Blasting. The following precautionsshould be taken when using any type of blast-ing equipment:

(1) Operators should be adequatelyprotected with complete face and head cover-ing equipment, and provided with purebreathing air.

(2) Static-ground the dry abrasiveblaster and the material to be blasted.

(3) Magnesium cuttings and smallshavings can ignite easily and are an extremehazard. Fires of this metal must be extin-guished with absolutely dry talc, calcium car-bonate, sand, or graphite by applying the pow-der to a depth of 1/2 inch over the metal.

(4) Titanium alloys and high-tensile-strength steel create sparks during dry abrasiveblasting. Care should be taken to ensure thathazardous concentrations of flammable vaporsdo not exist.

6-91. CORROSION CONTROL WORKPROCEDURES. The effectiveness of corro-sion control depends on how well basic workprocedures are followed. The following com-mon work practices are recommended:

a. If rework procedures or materials areunknown, contact the aircraft manufacturer orFAA authorized Designated Engineering Rep-resentative (DER) before proceeding.

b. The work areas, equipment, and com-ponents should be clean and free of chips, grit,dirt, and foreign materials.

c. Do not mark on any metal surface witha graphite pencil or any type of sharp, pointedinstrument. Temporary markings (defined asmarkings soluble in water or methyl chloro-form) should be used for metal layout work ormarking on the aircraft to indicate corroded ar-eas.

d. Graphite should not be used as a lu-bricant for any component. Graphite is ca-thodic to all structural metals and will generategalvanic corrosion in the presence of moisture,especially if the graphite is applied in dryform.

e. Footwear and clothing should be in-spected for metal chips, slivers, rivet cuttings,dirt, sand, etc., and all such material removedbefore walking or working on metal surfacessuch as wings, stabilizers, fuel tanks, etc.

f. Do not abrade or scratch any surfaceunless it is an authorized procedure. If sur-faces are accidentally scratched, the damageshould be assessed and action taken to removethe scratch and treat the area.

g. Coated metal surfaces should not bepolished for aesthetic purposes. Buffingwould remove the protective coating and abrightly polished surface is normally not ascorrosion resistant as a non-polished surfaceunless it is protected by wax or paint.

9/8/98 AC 43.13-1B

Par 6-91 Page 6-19 (and 6-20)

h. Protect surrounding areas whenwelding, grinding, or drilling, to prevent con-tamination with residue from these operations.In those areas where protective covering can-not be used, remove the residue by cleaning.

i. Severely corroded screws, bolts, andwashers should be replaced. When a protec-tive coating, such as a cadmium plating onbolts, or screws, is damaged, immediately ap-ply an appropriate protective finish to preventadditional corrosion damage.

6-92. 6-112. [RESERVED.]

9/8/98 AC 43.13-1B

Par 6-113 Page 6-21

SECTION 7. BASIC CORROSION REMOVAL TECHNIQUES

6-113. GENERAL. When active corrosionis found, a positive inspection and rework pro-gram is necessary to prevent any further dete-rioration. The following methods of assessingcorrosion damage and procedures for reworkof corroded areas could be used during cleanupprograms. In general, any rework would in-volve the cleaning and stripping of all finishfrom the corroded area, removal of corrosionproducts, and restoration of surface protectivefilm.

a. Repair of corrosion damage includesremoval of all corrosion and corrosion prod-ucts. When the corrosion damage is severeand exceeds the damage limits set by the air-craft or parts manufacturer, the part must bereplaced.

b. If manufacturer information andlimits are not available, then a DER must beconsulted before the aircraft or part is returnedto service.

6-114. PREPARATIONS FOR REWORK.All corrosion products should be removedcompletely when corroded structures are re-worked. Before starting rework of corrodedareas, carry out the following:

a. Document corrosion damage.

b. Position the aircraft in a wash rack orprovide washing apparatus for rapid rinsing ofall surfaces.

c. Connect a static ground line from theaircraft to a grounding point.

d. Prepare the aircraft for safe groundmaintenance.

(1) Remove battery(s), liquid oxygengenerator container (if installed), and externalhydraulic and electric power.

(2) Install all applicable safety pins,flags, and jury struts.

e. Protect the pitot-static ports, louvers,airscoops, engine opening, wheels, tires, mag-nesium skin panels, and airplane interior frommoisture and chemical brightening agents.

f. Protect the surfaces adjacent to reworkareas from chemical paint strippers, corrosionremoval agents, and surface treatment materi-als.

6-115. FAIRING OR BLENDING RE-WORKED AREAS. All depressions result-ing from corrosion rework should be faired orblended with the surrounding surface. Fairingcan be accomplished as follows:

a. Remove rough edges and all corrosionfrom the damaged area. All dish-outs shouldbe elliptically shaped with the major axis run-ning spanwise on wings and horizontal stabi-lizers, longitudinally on fuselages, and verti-cally on vertical stabilizers. (Select the properabrasive for fairing operations from table 6-1.)

b. In critical and highly stressed areas,all pits remaining after the removal of corro-sion products should be blended out to preventstress risers that may cause stress corrosioncracking. (See figure 6-14.) On a non-criticalstructure, it is not necessary to blend out pitsremaining after removal of corrosion productsby abrasive blasting, since this results in un-necessary metal removal.

AC 43.13-1B 9/8/98

Page 6-22 Par 6-115

TABLE 6-1. Abrasives for corrosion removal.

METALS OR RESTRICTIONS OPERATION ABRASIVE PAPER OR CLOTH ABRASIVE ALUMINUM STAINLESS PUMICE 350 ABRASIVEMATERIALS TO

BEPROCESSED

ALUMINUMOXIDE

SILICONCARBIDE

GARNETFABRIC OR

PADSTEEL MESH OR

FINERWHEEL

FERROUSALLOYS

CORROSIONREMOVAL

OR FAIRING

150 GRITOR FINER

180 GRITOR FINER

FINE TOULTRA FINE

X X X X

FINISHING 400 X X X

ALUMINUM AL-LOYS

EXCEPT CLAD

DO NOT USESILICONCARBIDE

CORROSIONREMOVAL

OR FAIRING

150 GRITOR FINER

7/0 GRITOR FINER

VERY FINEAND

ULTRA FINEX X X

ALUMINUM ABRASIVE FINISHING 400 X X

CLADALUMINUM

SANDINGLIMITED TO THE

REMOVAL OFMINOR

CORROSIONREMOVAL

OR FAIRING

240 GRITOR FINER

7/0 GRITOR FINER

VERY FINEAND

ULTRA FINEX X

SCRATCHES FINISHING 400 X

MAGNESIUMALLOYS

CORROSIONREMOVAL

OR FAIRING

240 GRITOR FINER

VERY FINEAND

ULTRA FINEX X X

FINISHING 400 X X

TITANIUM CLEANINGAND

FINISHING

150 GRITOR FINER

180 GRITOR FINER X X X

c. Rework depressions by formingsmoothly blended dish-outs, using a ratioof 20:1, length to depth. (See figure 6-15.) Inareas having closely spaced multiple pits, in-tervening material should be removed tominimize surface irregularity or waviness.(See figure 6-16.) Steel nut-plates and steelfasteners should be removed before blendingcorrosion out of aluminum structure. Steel orcopper particles embedded in aluminum canbecome a point of future corrosion. All corro-sion products must be removed during blend-ing to prevent reoccurrence of corrosion.

6-116. CORROSION REMOVAL BYBLASTING. Abrasive blasting is a processfor cleaning or finishing ferrous metals by di-recting a stream of abrasive particles againstthe surface of the parts. Abrasive blasting isused for the removal of rust and corrosion andfor cleaning prior to painting or plating. Thefollowing standard blast-cleaning practicesshould be adopted.

a. The part to be blast-cleaned should beremoved from the aircraft, if possible. Other-wise, areas adjacent to the part should bemasked or protected from abrasive impinge-ment and system (hydraulic, oil, fuel, etc.)contamination.

b. Parts should be dry and clean of oil,grease, or dirt, prior to blast cleaning.

c. Close-tolerance surfaces, such asbushings and bearing shafts, should bemasked.

d. Blast-clean only enough to remove cor-rosion coating. Proceed immediately with sur-face treatments as required.

6-117. CLEANERS, POLISHES, ANDBRIGHTENERS. It is important that aircraftbe kept thoroughly clean of contaminating de-posits such as oil, grease, dirt, and other for-eign materials.

9/8/98 AC 43.13-1B

Par 6-117 Page 6-23

FIGURE 6-14. Typical example of acceptable cleanup of corrosion pits.

a. Materials. Do not use harmful clean-ing, polishing, brightening, or paint-removingmaterials. Use only those compounds thatconform to existing government or establishedindustry specifications or that have been spe-cifically recommended by the aircraft manu-facturer. Observe the product manufacturer’srecommendations concerning use.

b. Chemical Cleaners. Chemicals mustbe used with great care in cleaning assembledaircraft. The danger of entrapping corrosivematerials in faying surfaces and crevicescounteracts any advantages in their speed andeffectiveness. Use materials that are relativelyneutral and easy to remove.

c. Removal of spilled battery acid. Thebattery, battery cover, battery box and adjacentareas will be corroded if battery acid spillsonto them. To clean spilled battery acid, brushoff any salt residue and sponge the area withfresh water. For lead-acid batteries, sponge thearea with a solution of 6 ounces of sodium

bicarbonate (baking soda) per gallon of freshwater. Apply generously until bubbling stopsand let solution stay on the area for 5 to6 minutes, but do not allow it to dry. Fornickel-cadmium batteries, sponge the area witha solution of 6 ounces of monobasic sodiumphosphate per gallon of fresh water. Spongearea again with clean fresh water and dry sur-face with compressed air or clean wipingcloths.

6-118. STANDARD METHODS. Severalstandard mechanical and chemical methods areavailable for corrosion removal. Mechanicalmethods include hand sanding using abrasivemat, abrasive paper, or metal wool; and pow-ered mechanical sanding, grinding, and buff-ing, using abrasive mat, grinding wheels,sanding discs, and abrasive rubber mats. Themethod used depends upon the metal and de-gree of corrosion. The removal method to useon each metal for each particular degree ofcorrosion is outlined in the following section.

AC 43.13-1B 9/8/98

Page 6-24 Par 6-118

FIGURE 6-15. Blendout of corrosion as a single depression.

FIGURE 6-16. Blendout of multiple pits in a corroded area.

6-119. 6-131. [RESERVED.]

9/8/98 AC 43.13-1B

Par 6-132 Page 6-25

SECTION 8. ALUMINUM AND ALUMINUM ALLOYS

6-132. GENERAL. Aluminum and alumi-num alloys are the most widely used materialfor aircraft construction. Aluminum appearshigh in the electro-chemical series of elementsand corrodes very easily. However, the forma-tion of a tightly-adhering oxide film offers in-creased resistance under most corrosive condi-tions. Most metals in contact with aluminumform couples that undergo galvanic corrosionattack. The alloys of aluminum are subject topitting, intergranular corrosion and inter-granular stress corrosion cracking. In somecases the corrosion products of metal in con-tact with aluminum are corrosive to aluminum.Therefore, aluminum and its alloys must becleaned and protected.

6-133. SPECIAL TREATMENT OFANODIZED SURFACES. Anodizing is themost common surface treatment of aluminumalloy surfaces. The aluminum sheet or castingis made the positive pole in an electrolyte bathin which chromic acid or other oxidizingagents produce a supplemental protective ox-ide film on the aluminum surface. The ano-dized surface coating offers the alloy a greatdeal of protection as long as it is not damaged.Once the film is damaged, it can only be par-tially restored by chemical surface treatment.Therefore exercise care to avoid breaking ofthe protective film, particularly at the edges ofthe sheet.

6-134. REPAIR OF ALUMINUM ALLOYSHEET METAL. After extensive corrosionremoval the following procedures should befollowed:

a. If water can be trapped in blended ar-eas, chemical conversion coat in accordancewith MIL-C-81706 and fill the blended area

with structural adhesive or sealant to the samelevel and contour as the original skin. Whenareas are small enough that structural strengthhas not been significantly decreased, no otherwork is required prior to applying the protec-tive finish.

b. When corrosion removal exceeds thelimits of the structural repair manual, contact aDER or the aircraft manufacturer for repair in-structions.

c. Where exterior doublers are installed,it is necessary to seal and insulate them ade-quately to prevent further corrosion.

d. Doublers should be made from alclad,when available, and the sheet should be ano-dized (preferred) or a chemical conversion coatapplied after all cutting, drilling, and counter-sinking has been accomplished.

e. All rivet holes should be drilled, coun-tersunk, surface treated, and primed prior toinstallation of the doubler.

f. Apply a suitable sealing compound inthe area to be covered by the doubler. Applysufficient thickness of sealing compound to fillall voids in the area being repaired.

g. Install rivets wet with sealant. Suffi-cient sealant should be squeezed out into holesso that all fasteners, as well as all edges of therepair plate, will be sealed against moisture.

h. Remove all excess sealant after fasten-ers are installed. Apply a fillet sealant beadaround the edge of the repair. After the sealanthas cured apply the protective paint finish tothe reworked area.

AC 43.13-1B 9/8/98

Page 6-26 Par 6-135

6-135. CORROSION REMOVALAROUND COUNTERSUNK FASTENERSIN ALUMINUM ALLOY. Intergranular cor-rosion in aluminum alloys often originates atcountersunk areas where steel fasteners areused.

a. When corrosion is found around afixed fastener head, the fastener must be re-moved to ensure corrosion removal. All cor-rosion must be removed to prevent further cor-rosion and loss of structural strength. To re-duce the recurrence of corrosion, the panelshould receive a chemical conversion coating,be primed, and have the fasteners installed wetwith sealant.

b. Each time removable steel fastenersare removed from access panels, they shouldbe inspected for condition of the plating. Ifmechanical or plating damage is evident, re-place the fastener. One of the following fas-tener installation methods should be used:

(1) Brush a corrosion-preventive com-pound on the substructure around and in thefastener hole, start the fastener, apply a bead ofsealant to the fastener countersink, set andtorque the fastener within the working time ofthe sealant (this is the preferred method).

(2) Apply the corrosion preventivecompound to the substructure and fastener, setand torque the fastener.

(3) Apply a coating of primer to thefastener, and while wet with primer, set andtorque the fastener.

6-136. EXAMPLES OF REMOVINGCORROSION FROM ALUMINUM ANDALUMINUM ALLOYS.

a. Positively identify the metal as alumi-num.

b. Clean the area to be reworked. Strippaint if required.

c. Determine extent of corrosion damage.

d. Remove light to moderate corrosionwith one of the following.

(1) Non-Powered Corrosion Removal.

(a) The removal of corrosion prod-ucts by hand can be accomplished by use ofaluminum grit and silicon carbide abrasive,such as non-woven, non-metallic, abrasive mat(Spec. MIL-A-9962), abrasive cloth, and pa-per. Aluminum wool, fiber bristle brushes, andpumice powder are also acceptable methods.

(b) Stainless steel brush (Spec.H-B-178, type III, class 2) may be used as longas the bristles do not exceed 0.010 inch in di-ameter. After use of this brush the surfaceshould be polished with 60 grit aluminum ox-ide abrasive paper, then with 400 grit alumi-num oxide paper. Care should be exercised inany cleaning process to avoid breaking theprotective film.

(c) Steel wool, emery cloth, steelwire brushes (except stainless steel brush)copper alloy brushes, rotary wire brushes, orsevere abrasive materials should not be usedon any aluminum surface.

(2) Chemical Corrosion Removal.

(a) The corrosion removal compoundaluminum pretreatment MIL-C-38334, an acidmaterial, may be used to remove corrosionproducts from aluminum alloy materials oritems (e.g., skins, stringer, ribs in wings, tub-ing, or ducts). MIL-C-38334 is available intwo types:

1 Type I Liquid concentrate mate-rials should be diluted in accordance with the

9/8/98 AC 43.13-1B

Par 6-136 Page 6-27

manufacture’s instructions before use. Type Ihas a 1 year shelf life; therefore it shall not beused after 1 year from the date of manufacture.

2 Type II Powdered concentratematerials should be dissolved in the volume ofwater specified on the kit. These materialshave an indefinite shelf life in the dry state.Once mixed, they should be used within90 days.

(b) Mix MIL-C-38334 in wood,plastic, or plastic-lined containers only. Wearacid-resistant gloves, protective mask andprotective clothing when working with thisacid compound. If acid contacts the skin oreyes, flush immediately with water.

(c) Apply MIL-C-38334 solution byflowing, mopping, sponging, brushing, orwiping. When applying the solution to largeareas, begin the application at the lowest areaand work upward, applying the solution with acircular motion to disturb the surface film andensure proper coverage. If pumping is re-quired, pumps, valves, and fittings should bemanufactured from 18-8 stainless steel orplastic.

CAUTION: When working withMIL-C-38334, keep the solution awayfrom magnesium surfaces. The solu-tion must be confined to the area be-ing treated. All parts and assembliesincluding cadmium-plated items andhinges susceptible to damage fromacid should be masked and/or pro-tected. Also mask all openings leadingto the primary structure that couldtrap the solution and doors or otheropenings that would allow the solution(uncontrolled) to get into the aircraftor equipment interior. It is a goodpractice to keep a wet rag on hand atall times, for removal of spills orsplashes.

(d) Allow the solution to remain onthe surface for approximately 12 minutes andthen rinse away with clean tap water. For pit-ted or heavily-corroded areas the compoundwill be more effective if applied warm(140 °F) followed by vigorous agitation with anon-metallic acid-resisting brush or aluminumoxide abrasive nylon mat. Allow sufficientdwell time, 12 to 15 minutes, before rinsing.After each application examine the pits and/orcorroded area to determine if another applica-tion is required with a 4 to 10 power magnify-ing glass. (Select the power depending on thedistance available to make the inspection.)Corrosion still on the area will appear as apowdery crust slightly different in color thanthe uncorroded base metal. Darkening of areadue to shadows and reaction from the acid re-mover should not be considered.

(e) Once the corrosion has been re-moved and the area well-rinsed with cleanwater, a chromate conversion coating such asMIL-C-81706 or MIL-C-5541 alodine 1200,must be applied immediately thereafter.

e. Remove moderate to heavy corrosionwith one of the following.

(1) Powered Corrosion Removal.

(a) Where the problem is severeenough to warrant the use of power tools, apneumatic drill motor driving either an alumi-num-oxide-impregnated nylon abrasive wheel,flap brush or rubber grinding wheel may beused with an abrasive value to approximately120 grit, as needed. Corrosion-removal acces-sories, such as flap brushes or rotary files,should be used on one type of metal only. Forexample, a flap brush used to remove alumi-num should not be used to remove magnesium,steel, etc. Pneumatic sanders may be usedwith disk and paper acceptable for use on alu-minum.

AC 43.13-1B 9/8/98

Page 6-28 Par 6-136

(b) When mechanically removingcorrosion from aluminum, especially aircraftskin thinner than 0.0625 inch, extreme caremust be used. Vigorous, heavy, continuousabrasive grinding can generate enough heat tocause metallurgical change. If heat damage issuspected, hardness tests or conductivity testsmust be accomplished to verify condition ofthe metal. The use of powered rotary filesshould be limited to heavy corrosion andshould not be used on skin thinner than0.0625 inch.

(2) Blasting.

(a) Abrasive blasting may be used onaluminum alloys using glass beads (Spec.MIL-G-9954) sizes 10 to 13, or grain abrasive(Spec. MIL-G-5634) types I and III may beused as an alternate method of removing cor-rosion from clad and non-clad aluminum al-loys. Abrasive blasting should not be used toremove heavy corrosion products. Direct pres-sure machines should have the nozzle pressureset at 30 to 40 psi for clad aluminum alloysand 40 to 45 psi for non-clad aluminum alloys.Engineering approval should be obtained priorto abrasive blasting metal thinner than0.0625 inch.

(b) When using abrasive blasting onaluminum alloys, do not allow the blast streamto dwell on the same spot longer than15 seconds. Longer dwell times will cause ex-cessive metal removal. Intergranular exfolia-tion corrosion is not to be removed by abrasiveblasting; however, blasting may be used withpowered corrosion removal to determinewhether all exfoliation corrosion has been re-moved.

f. Inspect the area for remaining corro-sion. Repeat procedure if any corrosion re-mains.

NOTE: If corrosion remains after thesecond attempt, use a strongermethod, e.g., chemical to mechanical.

g. Using a blend ratio of 20:1 (length todepth) blend and finish the corrosion reworkarea with progressively finer abrasive paperuntil 400-grit paper is used.

h. Clean reworked area using dry cleaningsolvent. Do not use kerosene or any other pe-troleum base fuel as a cleaning solvent.

i. Determine depth of faired depressionsto ensure that rework limits have not been ex-ceeded.

j. Apply chemical conversion coating,MIL-C-81706, immediately after reworking. If48 hours or more have elapsed since the con-version coating was first applied and theprimer or final paint system has not yet beenapplied, then reapply the conversion coatingbefore continuing.

NOTE: These solutions should not beallowed to come in contact with mag-nesium or high-strength steels(180,000 psi). Do not permit solutionsor materials to contact paint thinner,acetone or other combustible material:FIRE MAY RESULT.

k. Apply paint finish to area.

6-137. 6-147. [RESERVED.]

9/8/98 AC 43.13-1B

Par 6-148 Page 6-29

SECTION 9. MAGNESIUM AND MAGNESIUM ALLOYS

6-148. GENERAL. Magnesium and mag-nesium alloys are the most chemically activeof the metals used in aircraft construction andare the most difficult to protect. However, cor-rosion on magnesium surfaces is probably theeasiest to detect in its early stages. Since mag-nesium corrosion products occupy severaltimes the volume of the original magnesiummetal destroyed, initial signs show a lifting ofthe paint films and white spots on the magne-sium surface. These rapidly develop intosnow-like mounds or even white whiskers.The prompt and complete correction of thecoating failure is imperative if serious struc-tural damage is to be avoided.

6-149. TREATMENT OF WROUGHTMAGNESIUM SHEETS AND FORGINGS.Corrosive attack on magnesium skins will usu-ally occur around the edges of skin panels, un-derneath hold-down washers, or in areasphysically damaged by shearing, drilling, abra-sion, or impact. Entrapment of moisture underand behind skin crevices is frequently a con-tributing factor. If the skin section can be eas-ily removed, this should be accomplished toensure complete inhibition and treatment.

a. Complete mechanical removal of cor-rosion products should be practiced whenpractical. Mechanical cleaning should nor-mally be limited to the use of stiff bristlebrushes and similar nonmetallic cleaning tools.

b. Any entrapment of steel particlesfrom steel wire brushes, steel tools, or con-tamination of treated surfaces, or dirty abra-sives, can cause more trouble than the initialcorrosive attack. The following proceduralsummary is recommended for treatment of cor-roded magnesium areas when accomplishedunder most field conditions.

c. When aluminum insulating washersare used and they no longer fasten tightly tomagnesium panels, corrosion is likely to occurunder the washers if corrective measures arenot taken.

(1) When machine screw fasteners areused, aluminum insulating washers must beremoved from all locations to surface treat themagnesium panel.

(2) Where permanent fasteners otherthan machine screws are used, the insulatingwasher and fastener must be removed.

(3) When located so water can betrapped in the counter-bored area where thewasher was located, use sealants to fill thecounterbore. If necessary, fill several areasadjacent to each other. It may be advantageousto cover the entire row of fasteners with a stripof sealant.

6-150. REPAIR OF MAGNESIUMSHEET METAL AFTER EXTENSIVECORROSION REMOVAL. The same gen-eral instructions apply when making repairs inmagnesium as in aluminum alloy skin, exceptthat two coats of epoxy primer may be re-quired on both the doubler and skin beingpatched instead of one. Where it is difficult toform magnesium alloys in the contour, alumi-num alloy may be utilized. When this is done,it is necessary to ensure effective dissimilarmetal insulation. Vinyl tape will ensure posi-tive separation of dissimilar metals, but edgeswill still have to be sealed to prevent entranceof moisture between mating surfaces at allpoints where repairs are made. It is recom-mended that only non-corrosive type sealant beused, since it serves a dual purpose of materialseparation and sealing.

AC 43.13-1B 9/8/98

Page 6-30 Par 6-151

6-151. IN-PLACE TREATMENT OFMAGNESIUM CASTINGS. Magnesiumcastings, in general, are more porous and moreprone to penetrating attack than wrought mag-nesium skin. However, treatment in the fieldis, for all practical purposes, the same for allmagnesium. Bellcranks, fittings, and numer-ous covers, plates, and handles may also bemagnesium castings. When attack occurs on acasting, the earliest practical treatment is re-quired to prevent dangerous corrosive penetra-tion. Engine cases in salt water can developmoth holes and complete penetration over-night.

a. If at all practical, faying surfaces in-volved shall be separated to treat the existingattack effectively and prevent its further prog-ress. The same general treatment sequence asdetailed for magnesium skin should be fol-lowed. Where engine cases are concerned,baked enamel overcoats are usually involvedrather than other top coat finishes. A good airdrying enamel can be used to restore protec-tion.

b. If extensive removal of corrosionproducts from a structural casting is involved,a decision from the aircraft manufacturer or aDER may be necessary to evaluate the ade-quacy of structural strength remaining. Referto the aircraft manufacturer if any questions ofsafety are involved.

6-152. EXAMPLE OF REMOVINGCORROSION FROM MAGNESIUM. Ifpossible, corroded magnesium parts shall beremoved from aircraft. When impossible toremove the part, the following procedure willbe used.

a. Positively identify metal as magnesium.

b. Clean area to be reworked.

c. Strip paint if required.

d. Determine the extent of corrosiondamage.

e. Remove light to moderate corrosion byone of the following means.

(1) Non-Powered Corrosion Removal.

(a) Non-powered removal can be ac-complished using abrasive mats, cloth, and pa-per with aluminum oxide grit (do not use sili-con carbide abrasive). Metallic wools andhand brushes compatible with magnesium suchas stainless steel and aluminum, may be used.

(b) When a brush is used the bristlesshould not exceed 0.010 inch in diameter.After using a brush, the surface should be pol-ished with 400 grit aluminum oxide abrasivepaper, then with 600 grit aluminum oxideabrasive paper.

(c) Pumice powder may be used toremove stains or to remove corrosion on thinmetal surfaces where minimum metal removalis allowed.

(2) Chemical Corrosion Removal.

(a) Chemical corrosion removal onmagnesium alloys is usually done with achromic acid pickle solution. Chemical corro-sion removal methods are not considered ade-quate for areas that have:

1 Deep pitting,

2 Heavy corrosion and corrosionby products,

3 Previously had corrosion re-moved by mechanical means, or

4 Previously been sand blasted.

9/8/98 AC 43.13-1B

Par 6-152 Page 6-31

(b) Do not use this method for partscontaining copper and steel-based inserts (un-less the inserts are masked off) and where itmight come into contact with adhesive bondedskins or parts.

(3) The following solution may be usedto remove surface oxidation and light corro-sion products from magnesium surfaces.

(a) Solution Composition and Op-eration:

1 Chromium Trioxide. 24 oz.

2 (O-C-303, Type II). Water toMake 1 gal. Reaction Time 1 to 15 min.

3 Operation Temperature. (Solu-tion can be operated at room temperature for alonger reaction time if desired.) 190 to 202 °F.

4 Container Construction. Lead-lined steel, stainless steel, or 1100 aluminum.

(b) Mask off nearby operatingmechanisms, cracks and plated steel to keepthe solution from attacking them.

(c) Apply chromic acid solution care-fully to the corroded area with an acid-resistantbrush.

(d) Allow the solution to remain onthe surface for approximately 15 minutes.Agitation may be required.

(e) Thoroughly rinse the solutionfrom the surface with plenty of clean water.

(f) Repeat the preceding sequence asnecessary until all corrosion products havebeen removed and the metal is a bright metal-lic color.

f. Remove moderate to heavy corrosion byone of the following means.

(1) Powered Corrosion Removal.

(a) Powered corrosion removal canbe accomplished using pneumatic drill motorwith either an aluminum-oxide-impregnatedabrasive wheel, flap brush, or rubber grindingwheel with an abrasive value to approximately120 grain size.

(b) Also a rotary file with fine flutescan be used for severe or heavy corrosionproduct buildup on metals thicker than0.0625 inch. If a flap brush or rotary file isused, it should only be used on one type ofmetal. Do not use either a hand or rotary car-bon steel brush on magnesium.

(c) Pneumatic sanders are acceptableif used with disk or paper of aluminum oxide.When using sanders, use extra care to avoidover heating aircraft skins thinner than0.0625 inch.

(d) Do not use rotary wire brushes onmagnesium.

WARNING: Cuttings and smallshavings from magnesium can igniteeasily and are an extreme fire hazard.Fires of this metal must be extin-guished with absolutely dry talc, cal-cium carbonate, sand, or graphite byapplying the powder to a depth of1/2 inch over the metal.

(2) Blasting. Abrasive blasting is anapproved method of corrosion removal onmagnesium alloys of a thickness greater than0.0625 inch. Remove heavy corrosion prod-ucts by hand brushing with a stainless steel orfiber brush followed by vacuum abrasive

AC 43.13-1B 9/8/98

Page 6-32 Par 6-152

blasting with glass beads, (Spec. MIL-G-9954)sizes 10-13; or grain abrasive (Spec.MIL-G-5634), types I or III at an air pressureof 10 to 35 psi (if suction equipment is used,use a 50 percent higher pressure). Upon com-pletion of blasting, inspect for the presence ofcorrosion in the blast area. Give particular at-tention to areas where pitting has progressedinto intergranular attack. This is necessary be-cause abrasive blasting has a tendency to closeup streaks of intergranular corrosion ratherthan remove them if the operator uses an im-proper impingement angle. If the corrosionhas not been removed in a total blasting timeof 60 seconds on any one specific area, othermechanical methods of removal should beutilized.

CAUTION: When blasting magne-sium alloys, do not allow the blaststream to dwell on the same spotlonger than 15 seconds. Longer dwelltimes will cause excessive metal re-moval.

g. Inspect the reworked area to ensurethat no corrosion products remain. If corrosionproducts are found, repeat method used andre-inspect.

h. Fair depressions resulting from reworkusing a blend ratio of 20:1. Clean rework areausing 240 grit abrasive paper. Smooth with300 grit and finally polish with 400 grit abra-sive paper.

i. Determine depth of faired depressionsto ensure that rework limits have not been ex-ceeded. Refer to the manufacture’s specifica-tions.

j. Clean reworked area using a solvent toprovide a water-break-free surface. Do not usekerosene or another petroleum base fuel as acleaning solvent.

k. Apply Chromic Acid Brush-on Pre-treatment.

(1) Chemical pretreatment such as thefollowing chromic acid solution (Conversioncoat conforming to Spec. MIL-M-3171,type VI) provides a passive surface layer withan inhibitive characteristic that resists corro-sive attack and also provides a bond for subse-quent coatings. Properly-applied magnesiumpretreatment tend to neutralize corrosion me-dia in contact with the surface.

(2) The chromic acid brush-on pre-treatment may be applied to all magnesiumparts that require touch-up. This treatment isgenerally used in refinishing procedures orwhen parts and assemblies are too large to beimmersed. This treatment is less critical to ap-ply than the other brush-on treatments. It isrelatively inexpensive and not as harmful whentrapped in faying surfaces.

(a) Solution Composition and Op-eration:

1 Distilled Water 1 gal.

2 Chromic Acid (CrO3) 1.3 oz.

3 (99.5 pure), Calcium Sulfate1 oz. (CaSo4.2H2O)

4 Operating Temp. 70-90 °F.

5 Container: Stainless Steel, Alu-minum, Vinyl, Polyethylene, or Rubber.

NOTE: Good application requiresproper preparation of the chromicacid coating solution and cleaning ofthe surface where the solution will beapplied. A water-break test is recom-mended if the cleanliness of the sur-face is in doubt.

9/8/98 AC 43.13-1B

Par 6-152 Page 6-33 (and 6-34)

(b) Add chemicals to water in the or-der shown.

(c) Stir vigorously for at least15 minutes, either mechanically or by air agi-tation, to ensure that the solution is saturatedwith calcium sulfate. (Let solution stand for15 minutes before decanting.)

(d) Prior to use, decant solution(avoid transfer of undissolved calcium sulfate)into suitable containers (polyethylene or glass).

(e) Apply solution by brush, swab, orflow on using low-pressure spray (non-atomizing) until the metal surface becomes adull color (the color can vary from green-brown, brassy, yellow-brown to dark-brown).For good paint adhesion, a dark-brown colorfree of powder is considered best. The colormay vary in using different vendors’ materials.

NOTE: Too long an exposure to thebrush-on solution produces coatingsthat will powder and impair adhesionof applied paint finish/films.

(f) Observe the coating closely dur-ing the treatment for color changes, rinsed withcold running water when the desired condi-tion/color is reached and air dried. Preparationand use of test panels made of the same mate-rial and under the same conditions, beforestarting the actual treating operation may beused as to determine the application time re-quired to produce the required coating. Agood coating is uniform in color/density, ad-heres well and is free of loose powder.

l. Apply primer and top coat finish

m. Remove masking and protective cov-erings.

6-153. 6-163. [RESERVED.]

9/8/98 AC 43.13-1B

Par 6-164 Page 6-35

SECTION 10. FERROUS METALS

6-164. GENERAL. One of the most famil-iar kinds of corrosion is red iron rust. Red ironrust results from atmospheric oxidation of steelsurfaces. Some metal oxides protect the un-derlying base metal, but red rust is not a pro-tective coating. Its presence actually promotesadditional attack by attracting moisture fromthe air and acts as a catalyst to promote addi-tional corrosion.

a. Red rust first shows on bolt heads,hold down nuts, and other unprotected aircrafthardware. Red rust will often occur undernameplates that are secured to steel parts. Itspresence in these areas is generally not dan-gerous. It has no immediate effect on thestructural strength of any major components.However, it shows a general lack of mainte-nance and may indicate attack in more criticalareas.

b. When paint failures occur or me-chanical damage exposes highly-stressed steelsurfaces to the atmosphere, even the smallestamount of rusting is potentially dangerous andshould be removed immediately.

6-165. SPECIAL TREATMENT OFHIGH-STRENGTH STEEL. (High-strengthsteels heat treated above Rockwell C40,180,000 psi tensile strength). Any corrosionon the surface of a highly-stressed steel part ispotentially dangerous, and the careful removalof corrosion products is mandatory. Surfacescratches or change in surface structure fromoverheating can cause sudden failure of theseparts. The removal of corrosion products isrequired and will be performed carefully andcompletely.

a. Acceptable methods include carefuluse of mild abrasive mats, cloths, and papers,

such as fine grit aluminum oxide, metallicwool, or fine buffing compounds.

b. Undesirable methods include the useof any power tool because the danger of localoverheating and the formation of notches thatcould lead to failure. The use of chemical cor-rosion removers is prohibited, without engi-neering authorization, because high-strengthsteel parts are subject to hydrogen embrittle-ment.

6-166. SPECIAL TREATMENT OFSTAINLESS STEEL. Stainless steels are oftwo general types: magnetic and nonmagnetic.

a. Magnetic steels are of the ferritic ormartensitic types and are identified by num-bers in the 400-series. Corrosion often occurson 400-series stainless steels and treatment isthe same as specified in high-strength steels.(See paragraph 6-165.)

b. Non-magnetic stainless steels are ofthe austenitic type and are identified by num-bers in the 300-series. They are much morecorrosion resistant than the 400-series steels,particularly in a marine environment.

(1) Austenitic steels develop corrosionresistance by an oxide film, which should notbe removed even though the surface is discol-ored. The original oxide film is normallyformed at time of fabrication by passivation. Ifthis film is broken accidentally or by abrasion,it may not restore itself without repassivation.

(2) If any deterioration or corrosiondoes occur on austenitic steels, and the struc-tural integrity or serviceability of the part is af-fected, it will be necessary to remove the part.

AC 43.13-1B 9/8/98

Page 6-36 Par 6-167

6-167. EXAMPLE OF REMOVINGCORROSION FROM FERROUS MET-ALS. If possible, corroded steel parts shouldbe removed from the aircraft. When impracti-cal to remove the part, follow the procedurebelow.

a. Prepare the area for rework.

b. Positively identify the metal as steeland establish its heat-treated value.

c. Clean the area and strip paint if re-quired.

NOTE: Use of acid-based strippers,chemical removers, or chemical con-version coatings are not permitted onsteel parts without engineeringauthorization.

d. Determine extent of corrosion damage.

e. Remove residual corrosion by handsanding with mild abrasive mats, cloths, andpapers, such as fine aluminum oxide grit.

f. Remove heavy deposits of corrosionproducts by approved mechanical methods forthat particular form of steel and/or stainlesssteel.

g. Inspect the area for remaining corro-sion. Repeat procedure if any corrosion re-mains and the structural integrity of the part isnot in danger, and the part meets the reworklimits established by the manufacturer or FAAauthorized DER.

h. Fair depressions using a blend ratioof 20:1. Clean area using 240-grit paper.Smooth area with 300-grit paper and give finalpolish with 400-grit paper.

i. Determine depth of faired depressionto ensure that rework limits have not been ex-ceeded.

j. Clean reworked area with dry cleaningsolvent. Do not use kerosene.

k. Apply protective finish or specific or-ganic finish as required.

NOTE: Steel surfaces are highly-reactive immediately following corro-sion removal; consequently, primercoats should be applied within 1 hourafter sanding.

l. Remove masking and protective cover-ings.

6-168. 6-178. [RESERVED.]

9/8/98 AC 43.13-1B

Par 6-179 Page 6-37

SECTION 11. OTHER METALS AND ALLOYS

6-179. NOBLE METAL COATINGS -CLEANUP AND RESTORATION. Silver,platinum, and gold finishes are used in aircraftassemblies because of their resistance to ordi-nary surface attack and their improved electri-cal or heat conductivity. Silver-plated elec-trodes can be cleaned of brown or black sulfidetarnish, by placing them in contact with a pieceof magnesium sheet stock while immersed in awarm water solution of common table saltmixed with baking soda or by using a finegrade abrasive mat or pencil eraser followedby solvent cleaning. If assemblies are in-volved, careful drying and complete displace-ment of water is necessary. In general, clean-ing of gold or platinum coatings is not recom-mended in the field.

6-180. COPPER AND COPPER ALLOYSare relatively corrosion resistant, and attack onsuch components will usually be limited tostaining and tarnish. Such change in surfacecondition is not dangerous and should ordinar-ily have no effect on the function of the part.However, if it is necessary to remove suchstaining, a chromic acid solution of 8 to 24ounces per gallon of water containing a smallamount of battery electrolyte (not to exceed50 drops per gallon) is an effective brighteningbath. Staining may also be removed using afine grade abrasive mat or pencil eraser fol-lowed by solvent cleaning.

a. Immerse the stained part in the coldsolution. Surfaces can also be treated in placeby applying the solution to the stained surfacewith a small brush.

b. Avoid any entrapment of the solutionafter treatment. Clean the part thoroughlyfollowing treatment with all residual solutionremoved.

c. Serious copper corrosion is evident bythe accumulation of green-to-blue copper saltson the corroded part. Remove these productsmechanically using a stiff bristle brush, brasswire brush, 400-grit abrasive paper or beadblast with glass beads, (specificationMIL-G-9954, size 13). Air pressure whenblasting should be 20 to 30 psi for direct pres-sure machines. Do not bead blast braided cop-per flexible lines. Reapply a surface coatingover the reworked area. Chromic acid treat-ment will tend to remove the residual corro-sion products.

WARNING: Brushing, sanding, andabrasive blasting of copper and cop-per alloys can be dangerous due to thecreation of toxic airborne particles.Take necessary precautions to ensuresafety.

6-181. TITANIUM AND TITANIUM AL-LOYS are highly corrosion resistant becausean oxide film forms on their surfaces uponcontact with air.

a. When titanium is heated, differentoxides having different colors form on the sur-face. A blue oxide coating will form at 700 to800 °F; a purple oxide at 800 to 950 °F; and agray or black oxide at 1000 °F or higher.These coatings are protective discolorationsand should not be removed.

b. Corrosive attack on titanium surfacesis difficult to detect. It may show deteriorationfrom the presence of salt deposits and metalimpurities at elevated temperatures so periodicremoval of surface deposits is required. How-ever, if corrosion develops on titanium, it usu-ally occurs as pitting. Acceptable methods forcorrosion removal are:

AC 43.13-1B 9/8/98

Page 6-38 Par 6-181

(1) Stainless steel wool or hand brush.

(2) Abrasive mats, cloths, and paperswith either aluminum oxide or silicon carbidegrit.

(3) Dry abrasive blasting using glassbeads (spec. MIL-G-9954) sizes 10-13 orAluminum oxide (spec. MIL-G-21380, type I,grades A or B) at a blast pressure of 40 to50 psi (if using suction equipment use50 percent higher pressure).

WARNING: Dry abrasive blasting oftitanium alloys creates sparking. En-sure that hazardous concentrations offlammable vapors are not present.

(4) Hand polish with aluminum polishand soft cloth.

c. Titanium surfaces are susceptible tohydrogen embrittlement that can induce stresscorrosion and associated pitting. Therefore,chemicals such as fire-resistant hydraulic flu-ids must be controlled. Chlorinated hydrocar-bon solvents and chemical corrosion removersare prohibited from use on titanium and tita-nium alloys.

6-182. 6-192. [RESERVED.]

9/8/98 AC 43.13-1B

Par 6-193 Page 6-39 (and 6-40)

SECTION 12. PLATED PARTS

6-193. CHROMIUM AND NICKEL-PLATED PARTS. Nickel and chromiumplatings are used extensively as protective andwear-resistant coatings over high-strength steelparts (landing gear journals, shock strut pis-tons, etc.). Chromium and nickel plate provideprotection by forming a somewhat imperviousphysical coat over the underlying base metal.When breaks occur in the surface, the protec-tion is destroyed.

a. The amount of reworking that can beperformed on chromium and nickel-platedcomponents is limited.

b. The rework should consist of lightbuffing to remove corrosion products and pro-duce the required smoothness. The buffingshould not take the plating below the mini-mum allowable thickness.

c. Whenever a chromium or nickel-platedcomponent requires buffing, coat the area witha corrosion-preventive compound, if possible.

d. When buffing exceeds the minimumthickness of the plating, or the base metal hassustained corrosive attack, the componentshould be removed and replaced.

e. The removed component can be re-stored to serviceable condition by having theold plating completely stripped and replated inaccordance with acceptable methods andspecifications.

6-194. CADMIUM AND ZINC-PLATEDPARTS. Cadmium plating is used extensivelyin aircraft construction as a protective finish

over both steel and copper alloys. Protectionis provided by a sacrificial process in whichthe cadmium is attacked rather than the un-derlying base material. Properly functioningcadmium surface coatings may show mottling,ranging from white to brown to black spots ontheir surfaces. These show the sacrificial pro-tection being offered by the cadmium coat, andunder no condition should such spotting beremoved merely for appearance sake. In fact,cadmium will continue to protect even whenactual breaks in the coating develop and baresteel or exposed copper surfaces appear.

a. When the breakdown of the cad-mium plating occurs and the initial appear-ance of corrosion products on the base metaldevelops, some mechanical cleaning of thearea may be necessary but shall be limited toremoval of the corrosion products from theunderlying base material.

b. Under no condition should such acoating be cleaned with a wire brush. If pro-tection is needed, a touch-up with primer or atemporary preservative coating should be ap-plied. Restoration of the plate coating cannotbe done in the field.

c. Zinc coatings offer protection in anidentical manner to cadmium, and the correc-tive treatment for failure is generally the sameas for cadmium-plated parts. However, theamount of zinc on aircraft structures is verylimited and usually does not present a mainte-nance problem.

6-195. 6-205. [RESERVED.]

9/8/98 AC 43.13-1B

Par 6-206 Page 6-41

SECTION 13. CORROSION PROOFING OF LAND PLANESCONVERTED TO SEA PLANES

6-206. GENERAL. A special problem isencountered in the conversion of land planes toseaplanes. In general, land planes do not re-ceive corrosion proofing to the same extent asdo seaplanes. Corrosion-proofing standardsfor land planes converted to seaplanes are di-vided into two classes, necessary minimumprecautions and recommended precautions.Regardless of such precautions, it is imperativethat the exterior surfaces of seaplanes bewashed with clear fresh water immediatelyfollowing extended water operation, or at leastonce a day when operated in salty or brackishwater. Wash interior surfaces of seaplanes ex-posed to spray, taking care to prevent damageto electrical circuits or other items subject toinjury.

6-207. NECESSARY MINIMUM PRE-CAUTIONS. The following procedures areconsidered the minimum to safeguard the air-worthiness of converted aircraft and are not inthemselves intended to maintain airworthinessfor an indefinite period.

a. Unless already protected, treat ex-posed fittings or fittings that can be reachedthrough inspection openings with two coats ofzinc chromate primer, paralketone, nonwater-soluble heavy grease, or comparable materials.This applies to items such as wing-root fit-tings, wing-strut fittings, control-surfacehinges, horns, mating edges of fittings, and at-tached bolts.

b. Coat non-stainless control cables withgrease or paralketone or other comparableprotective coating, if not replaced with corro-sion-resistant cables.

c. Inspect all accessible sections of air-craft structure. Clean structural parts showing

corrosion and refinish if corrosion attack is su-perficial. If a part is severely corroded, replacewith an adequately corrosion-proofed part.

6-208. RECOMMENDED PRECAUTIONS.Recommended precautions are those which aresuggested as a means of maintaining such air-craft in condition for safe operation over ex-tended periods.

a. Provide additional inspection openingsto assist in detecting corrosion. Experiencehas shown openings to allow inspection of thelower and rearward portion of the fuselage tobe particularly desirable.

b. Incorporate additional provisions forfree drainage and ventilation of all interiors toprevent collection of moisture (scoop-type ma-rine drain grommets).

c. Protect the interior of structural steeltubing. This may be done by air and watertightsealing or by flushing with hot linseed oil andplugging the openings. Inspect tubing formissing sealing screws, presence of entrappedwater, local corrosion around sealing screws,welded clusters, and bolted fittings, which maybe indicative of entrapped moisture.

d. Slit the fabric of fabric-covered aircraftlongitudinally on the bottom of the fuselageand tail structure for access to these sections.Coat the lower structural members with zincchromate primer (two coats); follow by a coatof dope-proof paint or wrap with cellophanetape and rejoin the fabric. This precaution isadvisable within a few months after start ofoperation as a seaplane.

e. Spray the interior of metal-coveredwings and fuselages with an adherent corro-sion inhibitor.

AC 43.13-1B 9/8/98

Page 6-42 Par 6-208

f. Place bags of potassium or sodium di-chromate in the bottom of floats and boat hullsto inhibit corrosion.

g. Prevent the entry of water by sealing,as completely as possible, all openings inwings, fuselage, control-surface members,openings for control cables, tail-wheel wells,etc.

6-209. 6-219. [RESERVED.]

9/8/98 AC 43.13-1B

Par 6-220 Page 6-43

SECTION 14. HANDLING AND CARE OF AIRCRAFT RECOVEREDFROM WATER IMMERSION.

6-220. GENERAL. Aircraft recovered frompartial or total immersion in standing water orflash floods require an in-depth inspection andcleaning of both the exterior and interior areas.Water-immersion increases the probability ofcorrosive attack, it removes lubricants, deterio-rates aircraft materials, and destroys electricaland avionics components.

a. Sea water, because of salt content, ismore corrosive than fresh water. However,fresh water may also contain varying amountsof salt and, as drying occurs, the salt concen-tration is increased and corrosive attack accel-erated.

b. Prompt action is the most importantfactor following recovery of an aircraft fromwater-immersion. Components of the aircraftwhich have been immersed, such as thepowerplant, accessories, airframe sections,actuating mechanisms, screws, bearings,working surfaces, fuel and oil systems, wiring,radios, and radar should be disassembled, asnecessary, and the contaminants completelyremoved.

6-221. INITIAL FRESH WATER ORDETERGENT WASH. As soon as possibleafter the aircraft is recovered from water-immersion, thoroughly wash all internal andexternal areas of the aircraft using a wa-ter/detergent solution as follows:

a. Mix liquid detergent (MIL-D-16791,type I) and isopropyl alcohol (TT-I-735) in ra-tio of eight parts detergent, to 20 parts of alco-hol. Add the detergent/alcohol mixture to72 parts of tap water and mix thoroughly. Foruse, add one part of the preceding concentrateto nine parts of tap water (warm water if avail-able) and mix thoroughly.

b. If the above specified deter-gent/alcohol materials are not available, usewater-emulsion cleaning compound(MIL-C-43616). Add one part compound tonine parts water. If the MIL cleaning com-pound is not available, use any available mildhousehold detergent solution with fresh tapwater.

6-222. RECIPROCATING ENGINESAND PROPELLERS. Remove the propellerfrom the engine and the engine from the air-craft. The exterior of the engine and propellershould be washed with steam, or fresh water,preferably hot.

a. Major accessories, engine parts, etc.,should be removed and all surfaces flushedwith fresh water, preferably hot. If facilitiesare available, immerse the removed parts, timepermitting, in hot water or hot oil, 180 °F, for ashort time. Soft water is preferred. Changethe water frequently. All parts must be com-pletely dried by air blast or other means. If noheat-drying facility is available, wipe thecleaned parts with suitable drying cloths.

b. The constant-speed propeller mecha-nism should be disassembled, as required, topermit complete decontamination. Clean partswith steam or fresh water, preferably hot. Drythe cleaned parts in an oven, but if a heat-drying facility is not available, wipe thecleaned parts with suitable drying cloths.

6-223. AIRFRAME. The salvable compo-nents of the fuselage, wings, empennage, sea-plane and amphibian hulls and floats, andmovable surfaces should be processed as fol-lows:

AC 43.13-1B 9/8/98

Page 6-44 Par 6-223

a. The fabric from fabric-covered sur-faces should be removed and replaced.

b. Clean the aircraft interior and exteriorusing steam under pressure with steam clean-ing compound. Direct the steam into all seamsand crevices where corrosive water may havepenetrated. Avoid steam cleaning electricalequipment, such as terminal boards and re-lays.

c. Areas that have been steam cleanedshould be rinsed immediately with either hotor cold fresh water.

d. Touch up all scratches and scars onpainted surfaces using zinc chromate primer orpreservative.

e. Undrained hollow spaces or fluid en-trapment areas should be provided temporarydraining facilities by drilling out rivets at thelowest point. Install new rivets after drainage.

f. Remove and replace all leather, fabricupholstery, and insulation. Plastic or rubberfoam that cannot be cleaned of all corrosivewater must be replaced.

g. All drain plugs or drive screws in tu-bular structures should be removed and thestructure blown out with compressed air. Ifwater has reached the tubular interiors, care-fully flush with hot fresh water and blow outwater with compressed air. Roll the structureas necessary to remove water from pockets.Fill the tubes with hot linseed oil, approxi-mately 180 °F, drain oil and replace drainplugs or drive screws.

h. Clean sealed wood, metallic, and othernon-metallic areas, excluding acrylic plastics,with warm water. Replace wood, metalite, andother porous materials exposed to water-immersion unless surfaces are adequatelysealed to prevent penetration by water. Virtu-ally all solvents and phenolic type cleaningagents are detrimental to acrylics and will ei-ther soften the plastic or cause crazing.

i. Remove instruments and radios and ap-plicable cables and plumbing, and repair andinspect as necessary.

6-224. 6-234. [RESERVED.]