CORROSION IN ALUMINUMAIRPLANES

We are fortunate to have the material"aluminum" which is strong enough tosatisfy our structural requirements, lightenough to allow a reasonable usefulload in a finished craft, and easilyworked with hand tools to fashion theaircraft of our choice. The one problemwith this material, however, is the abilityto literally destroy itself. And it will ifproper precautions are not taken. Fortu-nately, these precautions are relativelysimple and can be applied during fabri-cation and maintenance with very littleadditional effort. It is this extra effortwhich will be described here; but firstwe need to know a little more about thisprocess called "corrosion". . . what it is,where it comes from, and what to doabout it.

Basically corrosion is the deteriora-tion of the metal which occurs after itsexposure to certain environmental con-ditions or, in some cases, stress ormechanical action. Failure to do some-thing about it in its early stages can leadto some very expensive repairs or, evenworse, a structural failure. By studyingthe various types of corrosion we canbecome familiar with its characteristicsand, therefore, learn how to identify it,treat or repair it, and even prevent it.

The most common type is called Uni-form Surface corrosion, and is causedby simply exposing the material to oxy-gen in the air. This type of corrosion willoccur more rapidly if exposed to fumes,acid, pollutants, or just plain water. Asa matter of fact, the surface is more vul-nerable to soft water or rain water thanit is to hard water. Salt water is alsoextremely corrosive and should bewashed off immediately after exposure.Some of these elements can be so se-vere that even a good finish is not suffi-cient to protect the metal without con-stant cleaning or maintenance. In theearly stages, this type of corrosionstarts by giving the metal a dark or dullappearance. As it progresses, the sur-face starts to feel rough or sandy andwill begin to show small deposits ofwhite powder. The corrosion can startat the edge of the metal and progressinward under the finish and not benoticed until it starts to blister. Anothercommon place for it to start is at a jointor skin lap where moisture becomes72 JANUARY 1992

By LANDIS G. KETNEREAA 12749

120 Sopwith Dr.Vero Beach, FL 32968

Surface corrosion starting to form underfinish next to riveted seam.

trapped. This phenomenon occurs fre-quently under urethane finishes wherein time it contracts, thereby pulling awayfrom tight corners formed by skin lapsor rivets and leaves a microscopic tun-nel in which moisture can accumulate.That is why the first sign of it may be arising or blistering of the paint along askin lap or an individual rivet.

On the exterior of the airplane thiscondition is easily spotted since it isreadily visible during pre-flight inspec-tions. Inside the aircraft is a differentstory. Every access panel, fairing, andtip should be removed and a thoroughinspection conducted with a strong lightand mirror. Any place water or moisturecan accumulate is a likely candidate.Areas such as the inside of wings andfuselages assembled from formed ribsand frames usually have drainage pro-vided by reliefs at the ends of the bendsin the sheet metal. Any place that doesnot, such as along extruded angles orat trailing edges of surfaces, etc.,should have drain holes. Don't neglectthe underside of top surfaces or otherareas where moisture cannot accumu-late, no surface is immune, especially ifthey are unpainted.

One of the first places corrosion willform on an airplane is the area just in-

Fretting corrosion where baffle contacted cylinder fins. Note blackened surface.

Accumulation of white powder on aluminum trimstrip caused by water leaking throughedge of window on steel screw and washer resulting in dissimilar metals corrosion.

side openings in the skin required forpushrods, cables, hardware access,etc. These areas are exposed to mois-ture during flight from the slipstreamand on the ground from rain or snow,or just plain fresh air, especially in awindy climate. Corrosion will not onlyform around the opening on the insideof the skin, but will also be found on theskin opposite. One way to prevent thisis to cover the hole with a piece of rub-ber slotted to allow the movement ofthe surface control.

More frequent inspection is requiredaround battery boxes or toilet installa-tions. Battery fumes, acid, and urine areamong the most corrosive elementsyour aircraft will be exposed to. Both ofthese installations should be paintedwith a special finish for that purpose.Make sure that the vents to these areasare open and functioning properly.Check the outside of the airplane aft ofwhere these vents protrude through theskin.

Inside the cabin behind upholsterypanels, under the carpet, or anywherethere is an abundance of soundproofingoften hides corrosion deposits. It iscommon for water to leak around win-dows or along fairings directly into thecabin. You may not notice it if thesoundproofing absorbs most of it. To

prevent this, soundproofing should bemade of a nonhygroscopic materialsuch as glass fiber or mineral wool.Avoid the use of felt padding in anyareas subject to moisture; it makes anexcellent wick. Don't overlook doors,make sure the inside is clean and con-tains drain holes in the bottom chan-nels.

Another area of concern is im-mediately aft of the exhaust stack. Thisarea should contain extra heavy finishand be cleaned frequently to avoid theeffects of the extremely corrosiveexhaust gasses. Some aircraft, espe-cially twins, contain a shield of stainlesssteel on the nacelle and flaps im-mediately aft of the exhaust outlets. Payclose attention to the rivet heads andthe inside of any access panels andwheel wells in these areas.

Up to now the conditions describedhere for uniform surface corrosion inmost cases can be cleaned, treated,and refinished to prevent any further de-terioration. If allowed to progress, it willresult in erosion of the metal and severepitting which will require either repair orreplacement.

Very similar to uniform surface corro-sion is filiform corrosion which, in-stead of covering an area, usually pro-gresses in an erratic line which appears

as worm-like tracks under the finish.This type usually occurs next to rivetheads or skin laps due to moisture trap-ped there just like the surface corrosiondescribed above, and again thisphenomenon occurs most frequentlyunder urethane finishes for the samereason. This type of corrosion shouldcommand immediate attention as it canbe worse than it appears. If the metalunder the rivet is deteriorating andcaught soon enough, a simple replace-ment of the rivet or one the next sizelarger may be all that is required, in ad-dition to the usual cleaning and refinish-ing. If not, you may end up repairing orreplacing a skin panel.

Another type of corrosion frequentlyfound in aircraft, particularly larger shipswhich contain a lot of extrudedaluminum structure, is intergranularcorrosion. This type is caused by anirregularity in the manufacturing pro-cess either during the initial productionof the metal or during the heat treatmentprocess. In both finished and unfinishedsurfaces the first indication of this willbe a thickening or rising of the surface.When it progresses to the point ofbreaking the surface it appears as if thematerial is coming apart in layers. Itlooks similar to the edge of a very wellworn pulp novel. This action is calledexfoliation, which is why this type is alsosometimes referred to as exfoliationcorrosion.

Since some of the main structuralmembers of our aircraft, such as wingspars, struts, landing gear, or enginemount fittings, etc., are made of ex-truded material, this type of corrosionrequires closer and more immediate at-tention. Once started it spreads orgrows very rapidly. Almost without ex-ception, by the time this type of corro-sion is spotted it has already penetratedtoo deeply to simply clean and refinish.When treating this type, it is necessaryto penetrate well into the surroundingarea to assure its complete removal. Bythis time, you have usually reduced thesize of the section to the point that eithera structural repair or replacement is re-quired.

Another type of corrosion found fre-quently in our aircraft occurs where twoparts are joined together and resultsfrom the action of these two parts oneach other. This is called fretting cor-rosion and is probably most seenaround rivet heads in a highly stressedarea. The action between the rivet,especially flush rivets, and the metal itsecures causes the formation of amicroscopically fine powder surround-ing the area. This powder trailing in theslipstream causes the phenomenonknown as "smoking rivets." On apainted airplane, you will notice that thefinish also contains a minute crack

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Fin spar with surface corrosion which originated from dissimilar metals, steel nutplates and aluminum spar.

around the circumference of the rivetwhich allows moisture to penetrate andstart a surface or filiform corrosion. Ifcaught soon enough, a thorough clean-ing and replacement of the rivet may beall that is required. Whether the frettingwas caused by high stresses, or just aloose rivet, it is always a good policy toreplace it with the next larger size when-ever possible. Fretting may also be anindication of an improperly driven rivet,in which case a thorough examinationof all the rivets in that area is in order.

Other areas to look for fretting iswhere there is motion or chafing be-tween two parts, such as the internaledges of piano hinges, around quarterturn fasteners in access panels, controlsurface joints, etc. You may also find itin areas not intended for motion, suchas landing gear fittings bolted to spars,engine mount structure aft of the fire-wall, and major airframe assembly fit-tings. Paint around some of these areasmay actually hide the first indications ofthis condition, but just as in the case ofa loose rivet, the first indication may bethe trail of black powder emitting fromthe edge. On major structural compo-nents this condition can be preventedby assuring that all mating surfaces arein proper alignment and have a goodpositive fit whether they are bolted or74 JANUARY 1992

riveted together. In the case of a boltedjoint it is extremely important that thecorrect torque is applied.

Fretting corrosion can also be iden-tified by the displacement of metal andis sometimes referred to as false brinel-ling. A common place to find this typeis on propeller spinner bulkheads whichare sandwiched between the crankshaftflange and the propeller mounting base.An imprint of the propeller base orcrankshaft flange is often found on thesurface of the bulkhead with the dis-placed metal rolled up next to it. Thesame thing can happen to the forwardbulkhead with the imprint of the mount-ing bolts imbedded into it. If not re-placed, a combination of fretting andstress corrosion will lead to cracks orcomplete failure. Some of this can beprevented or prolonged by making surethat the propeller mounting flange hasa well rounded edge. On the forwardbulkhead either round the edges of thewashers under the bolt heads or add asecondary plate under the bolt headswith a rolled edge around the circumfer-ence. Eliminating the sharp edges pre-vents the imprinting when the materialvibrates against it.

Cowlings, landing gear doors, or anyother areas subject to extreme vibrationare also candidates for fretting corro-

sion. In many cases it can be preventedby simply using proper lubrication. Or inthe case of parting surfaces, add a gas-ket or anti-chafing material.

What may appear at first to be frettingcorrosion may actually be stress corro-sion which is a gradual breakdown ofthe metal under consistent highly loadedconditions. An accelerated exampleof this would be if you took a piece ofsheet metal and bent it back and forthon the same bend line several timesuntil it broke. An examination of thebroken edge will reveal a stretchedlooking area with a lighter or chalky ap-pearance. This is not a condition whichwe think of as similar to the other typesmentioned here, but nevertheless it isconsidered a type of corrosion since itis a deterioration of the material struc-ture.

Unless there is a complete failure ofa part, this condition is not always easilydiscovered. Cracks emitting from rivetsto the edge of a skin are usually obvi-ous, but in heavier members they maybe covered by paint or dirt. Or the metalmay be weakening but has not yetreached the point of failure. I recall thatseveral years ago I watched the re-moval of a skin from under an engineinstallation which was painted on theoutside, but not on the inside, and

showed absolutely no sign of anycracks on either surface. Imagine oursurprise when, as the rivets were beingremoved, the skin came off in severalpieces as long irregular cracks ap-peared.

Prevention of stress corrosion usuallyhas to start with the design of the par-ticular structural area involved. Compo-nents of this area should be of sufficientsize and of the proper alloy to absorbnot only the applied loads, but a satis-factory margin of safety as well. Sincemost of our applications will be after thefact, we have no real control over thisexcept that when we find this condition,check with the original designer to seeabout reinforcing the area. In the caseof a certified aircraft, the manufacturershould be contacted. If this is not possi-ble, the area should be treated as amajor repair. In either case, a Malfunc-tion or Defect Report should be submit-ted to the FAA.

One caution that can be taken duringconstruction or repair of an aircraft toprevent stress corrosion is to neverforce a part or component into positionas this will probably induce a preloadwhich will only become worse as vibra-tion or normal stresses are applied.Also, when installing a sheet metalpanel, start riveting from the center ofeach of the four sides and work towardthe corners. This not only prevents pre-loading, but helps to prevent wrinklesand oil cans as well.

Although many airplanes are built al-most entirely of aluminum, there is anecessity for the use of other metal al-loys which, when mated to aluminum,cause a form of corrosion known as dis-similar metal corrosion. Due to theunique composition of each metal,when bare surfaces contact each otherand are exposed to moisture in the airor a direct application of water, a smallelectric current is caused to flow be-tween them, just like a battery. And justlike a battery, the white powder whichyou see on your car battery around theterminal connections will also formaround the area of contact of dissimilarmetals. While this condition usually ap-pears worse than it is, it should not beignored as it can spread very rapidlyand cause severe erosion if notchecked.

Perhaps the most common place onthe airplane to find this type of corrosionis on the control surface bracketsmounted to the spars of the surface,and the trailing edges of the wings,stabilizers, and fins. The airplane struc-ture is aluminum, and usually thesebrackets are steel. The area aroundthem is almost always open, exposingthem directly to moisture. Even with acoat of paint, these areas are known tocorrode very severely. They should be

Aft spinner bulkhead with dissimilar metal corrosion where bulkhead contacted pro-peller flange and fretting corrosion around perimeter of corroded area.

checked at every preflight inspectionand given immediate attention at thefirst sign of corrosion.

Correction or prevention of this typeof corrosion is basically simple, and thatis to keep the mating surfaces of bothmaterials well primed and painted, orcadmium plate the steel before painting.Adding a light coat of cosmoline or evengrease will help. Another good preven-tion is to add a gasket. However, if thisis done be sure the gasket material can-not absorb moisture or your problemswill be even more severe. Since assem-bly hardware is almost always steel,there is always the possibility of an in-teraction between them and the alumi-num structure. The cadmium platingon the hardware will usually preventthis but not always, particularly if theplating is worn off or starts corroding orrusting on its own. A good procedurefollowed by many aviation mechanics isto apply a coating or grease to everybolt as it is installed in the airplane. Irecall working on an airplane built dur-ing WW-II which contained a number ofaluminum forgings and required analuminum washer between the forgingand any mating hardware. The theoryhere was that any corrosion wouldoccur between the steel bolt and thealuminum washer and would leave the

forging intact, and the washer could beeasily replaced.

Perhaps the most common type andmost easily identified type of corrosionoccurs on steel parts in the form of justplain rust. Unlike the types of aluminumusually found in aircraft structures, mostof the steel alloys must have surfaceprotection or they will just plain rustaway. There are, of course, some corro-sion resistant or stainless steel applica-tions but they are extremely limited be-cause stainless alloys are generallyheavier, do not contain the structuralqualities required, and are often harderto work. With few exceptions, such asslainless exhaust shields, all steel partsmust be either painted or plated. Anyrust found during inspections should beimmediately cleaned and painted.

There are, of course, other metal al-loys sometimes used in aircraft con-struction such as magnesium which hasbeen commonly used to fabricate con-trol surfaces, and copper or brass whichis found more often on older aircraft.Corrosion of these materials and thetreatment for it, in most cases, are simi-lar to that of the alloys described here.

The procedures for prevention andtreatment of corrosion are basically thesame except when building an aircraft,all the materials are new and clean and

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Filiform corrosion on a severely neglected propeller.

totally accessible. Whereas to treat acorroded area on an existing airplane,it will have to be cleaned, stripped, re-moved, cleaned again, refinished andperhaps repaired. And, according toMurphy's Law, it will be in the most inac-cessible areas. For a new airplane,every piece of metal on it should containsome form of corrosion protection:either paint, plating, alodine, anodizing,cosmoline, etc., to name a few. Which-ever method you use should be chosenvery carefully with consideration forwhere in the airplane the particularpart will be located. For parts with themost exposure, you may even wantto apply a combination of finishes. Forwhatever product you choose, obtain allthe manufacturer's information avail-able and follow it very carefully. This isimportant not only for a quality aircraft,but for your own well being. Some ofthese types of materials are extremelytoxic. Don't take anything for granted.

Unfortunately, with some homebuiltsand many certified aircraft, the man-ufacturers decided that treatment forthe prevention of corrosion just was notnecessary. These are the problem air-craft, and more extensive proceduresare required. When the corrosion is firstdiscovered it should be treated as soonas possible or it will spread. If it ispainted, the finish must be removed toexpose the affected area and for somedistance beyond to make sure that youhave it all. Every visible trace of the cor-rosion must be removed or it will con-tinue to form or grow even under thenew finish. Extreme conditions may re-quire removal by mechanical means.Blasting is best, either with glass beads,76 JANUARY 1992

walnut shells or similar material. Forminor applications use aluminum oxidepapers Scotch Brite pads, or aluminumwool. Avoid carborundum abrasives,emery cloth or paper, or anything madeof iron oxides. Never use steel wool.

It is important to remember that be-fore starting to apply any finishing pro-cess that the metal must be superclean, regardless whether it is a repairor new material. Complacency here canbe costly if a new finish must be re-placed within a year of its application.As soon as you are satisfied that yourmetal is clean, apply the etching pro-cess immediately and continue on withprimer and final finish.

Inspection of the aircraft for corrosionand its subsequent treatment can beaccomplished by the aircraft owner aslong as a repair is not required. Do notneglect areas such as the propeller,landing gear, and powerplant. The lead-ing edge of the propeller is always sub-ject to corrosion as a result of erosionof the finish. If it appears severe, sendit to a certified propeller repair shop forrefinish. The landing gear containscombinations of several metals andshould always be checked very care-fully. Nicks or erosion of the plating onstruts is very common and should beaddressed to prevent rust and assureproper operation. In the powerplantarea you have baffles and exhaustshrouds which are very vulnerable tofretting and stress corrosion.

Another area not to be overlooked isthe control system. Stainless steel ca-bles are highly recommended over car-bon steel. Carbon steel cables must becoated with paralketone or similar pre-

servative to prevent rust. Pay close at-tention to sections of cable that passthrough fairleads which can rub off theprotective coating. Also check all areaswhich are exposed to the elements.Bends around a pulley more than 90degrees can cause an internal stresscorrosion from the continual action oftrying to unravel itself. Examine the ca-bles in these areas by compressing andflexing a short distance at a time or passa rag along the cable length to see if itsnags any broken strands. Brokenstrands and rust or corrosion are goodreasons to replace an otherwise accept-able looking cable.

None of the procedures mentionedhere are really very difficult. There aremany fine publications available forboth corrosion control and painting. Ipersonally have found the followingvery helpful:

1. Aircraft Painting Manual, AviationMaintenance Foundation, Inc.

2. Aircraft Corrosion Control, AviationMaintenance Foundation, Inc.

3. AC 43-4 Advisory Circular on Cor-rosion Control for Aircraft, Federal Avia-tion Adminstration.

4. AC 43.13-1 Acceptable Mehotds,Techniques and Practices - Aircraft In-spection and Repair, Federal AviationAdministration.

5. AC 65-9 Airframe and PowerplantMechanics General Handbook, FederalAviation Administration.

6. Body Maintenance, Light PlaneMaintenance magazine.

7. Refinishing Metal Aircraft, Christy,TAB Books #2291.