Corrosion Pre Finals

8/18/2019 Corrosion Pre Finals

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WIDE-AERO SCHOOL OF AVIATIONG/F BLK21 UNIWIDE COASTAL MALL, PARAÑAQUE CITY

CORROSION



Corrosion of aluminum and aluminum alloys

Corrosion attack on aluminum surfaces is usually quite obvious, since the products of

corrosion are white and generally more voluminous than the original base metal. Even in its

early stage, aluminum corrosion is evident as general etching, pitting, or roughness of thealuminum surfaces. Note : Aluminum alloys commonly from a smooth surface oxidation which

is from, .! in. to ."# in. thick. $his is not considered detrimental as such a coating provides

a hard shell barrier to the introduction of corrosive elements. %uch oxidation is not to beconfused with the severe corrosion discussed in this paragraph. &eneral surface attack of

aluminum penetrates relatively slowly, but is speeded up in the presence of dissolved salts.

'onsiderable attack can usually take place before serious loss of structural strength develops.

(owever, at least three forms of attack on aluminum alloys are particularly serious : )!* $he penetrating pit+type corrosion through the walls of aluminum tubing, )"* stress + corrosion

cracking of materials under sustained stress, and )* the intergranular attack which is

characteristic of certain improperly heat+treated aluminum alloys. -n general, corrosion of

aluminum can be more effectively treated in place than corrosion occuring on other structuralmaterials used in aircraft. $reatmentincludes the mechanical removal of as much of the

corrosion products as practicable, and the inhibition of residual materials by chemical means,follow by the restoration of permanent surface coating.

$reatment of npainted Aluminum %urfaces /elatively pure aluminum has considerably

more corrosion resistance compared with the stronger aluminum alloys. Avantage is taken of this

by laminating a thin sheet of relatively pure aluminum over the base aluminum alloy. $he protection obtained is good, and the alclad surface can be maintained a polished condition. -n

cleaning such surfaces, however, care must be taken to prevent staining and marring of the

exposed aluminum and, more important from a protection standpoint, to avoid unnecessary

mechanical removal of protective alclad layer and the exposure of the more susceptiblealuminum alloy base material. A typical aluminum corrosion treatment sequence follow : )!*

/emove oil surface dirt with any suitable mild cleaner prior to abrasive cleaning of aluminum

surfaces. )"* (and polish the corroded areas with fine abrasives or with metal polish. 0etal polish intended for use on clad aluminum aircraft surfaces must not be used on anodi1ed

aluminum since it is abrasive enough to actually remove the protective anodi1ed film. -t

effectively removes stains and produces a high, lasting polish on unpainted alclad. -f a surface in particularly difficult to clean, a cleaner and brightener compound for aluminum, can be used

before polishing to shorten the time and lessen the effort necessary to get a clean surface. )*

$rest any superficial corrosion present, using an inhibitive wipe+down material.An alternatetreatment is processing with a solution of sodium dichromate and chromium

trioxide. Allow these solutions to remain on the corroded area for # to " minutes, and thenremove the excess by rinsing and wiping the surface dry with a clean cloth. )2* 3vercoat the

polished surfaces with water proof wax.

Aluminum surfaces that are to be subsequently painted can be exposed to more severe

cleaning procedures and can also be given more thorough corrective treatment prior to painting.

$he following sequence is generally used : )!* $horoughly clean the affected surfaces of allsoil and grease residues prior to processing. Any general aircraft cleaning procedure may be



used. )"* $reat superficially corroded areas with a ! percent solution of chromic acid and

sulphuric acid. Apply the solution by swab or brush. %crub the corroded area with the brush

while it is still damp. 4hile chromic acid is a good inhibitor for aluminum alloys, even whencorrosion products have not been completely removed, it is important that the solution penetrate

to the bottom of all pits and underneath any corrosion that may be present. $horough brushing

with a stiff fiber brush should loosen or remove most existing corrosion and assure complete penetration of the inhibitor into crevices and pits. Allow the chromic acid to remain in place for

least # minutes, then remove the excess by flushing with water or wiping with a wet cloth. $here

are several commercial chemical surface treatment compounds, similar to the type describedabove, which may also be used.

)* 5ry the treated surface and restore recommended permanent protective coatings as

required in accordance with the aircraft manufacturer6s procedures. /estoration of paint coatings

should immediately follow any surface treatment performed. -n any case, make sure thatcorrosion treatment is accomplished or re+applied on the same day that paint refinishing is

scheduled.



Aviation Corrosion Inspections

Aircraft are built from a variety of materials. There is one group of materials out of

these, metal that is subject to deterioration due to corrosion. To a certain extent plastics

may also deteriorate but are not subject to corrosion like metals. While wood and fabricsuffer from the effects of aging and deterioration, fewer of these types of materials are

used in modern aircraft.

Corrosion in metal is basically caused by the addition of water between two different

metals this is also a problem with alloys !which is basically several metals combined" if

water is present. Corrosion is often difficult to detect but once the process begins the

deterioration and degradation of the metallic materials can be rapid. An example of this

is a #iper single engine aircraft that was inspected specifically for corrosion.

Twelve months later, at the next periodic inspection, severe corrosion was indicated.

The difference between the two occasions was that prior to the corrosion inspection the

aircraft was parked in a hangar and washed occasionally after the inspection the

aircraft was parked out in the weather and was washed constantly.

$etal once it is corroded is no longer the same alloy or compound it was made to be. %t

has little strength, becomes brittle and in some cases is soluble in water. Corrosion is

therefore a major problem that must be removed and controlled, if possible. Corrosion

needs to be detected and treated early, otherwise the only cure will be replacement of

the affected part with new material. &ike the old saying, as far as corrosion is concerned,



'prevention is better than cure(.

The Corrosion Process

$ost modern aircraft are manufactured from aluminum alloys. While this material islight and strong, it does have the drawback of being susceptible to fatigue and corrosion,

particularly in a salt)water environment. *ther alloys such as steel and magnesium are

often used in the manufacture of aircraft components. These are also readily corroded in

the presence of water. Water on these occasions may be as little as airborne particles on

a humid day or as much as total immersion or a soaking by rain. +ain is not pure water

but is in fact a solution of acidic compounds such as carbonic acid. %n some areas of the

world where pollution has become a problem rain can cover your aircraft with highly

acidic solutions, all without you realiing it.



Oxidization

All metals in the

presence of oxygen

will react in some

way in some cases

this reaction is

-uite violent. %n

the case of

aluminum, the

surface of the metal reacts with the oxygen in the air and forms a thin layer of aluminum

oxide this in fact prevents further contact between the metal and the oxygen in the air.

*nce this layer, which is soluble, is broken, more metal is oxidied. %f the oxide layer is

constantly broken or removed, then the oxidiation process continues until the majority

of metal is oxidied and no longer has any strength. $ost metals after processing begin

to corrode or oxidie in some way. They are inclined to turn back into the raw product

they came from i.e, iron turns into iron oxide, which is basically what iron ore consists

of. *xidiation is a natural process but it can be stopped or slowed.

Galvanic Reactions

Whenever two dissimilar metals are near each other in the presence of an acidic li-uid areaction takes place. The reaction causes one of the metals to slowly disintegrate, while

the other has its characteristics changed, in most cases What this means in an aircraft

structure is that when two metals are separated by a li-uid, such as water, there will be a

reaction between the two, causing corrosion. or example, when a steel undercarriage

component is bolted to the aluminum structure of an aircraft, there will be a corrosive

chemical reaction. This is very prevalent where the engine mounts contact the firewall of

an aircraft. Compounding the problem is the fact that most aircraft metals are alloys.

Alloys will always have two dissimilar metals present. %f through wear and tear or

damage !even minor" an acidic substance is placed between these metals, a galvanicreaction will take place. This means that the internal structure of the aircraft metal itself

will begin to corrode 'from the inside out(. %n most cases this internal reaction will be

unobservable until it is too late. The only alternative then is to replace the component,

which is a very expensive option. Any time corrosion is treated considerable time and

expense will be incurred, so the best way to deal with it is to prevent it. %f this is difficult,



then early detection and treatment is necessary. This is a simple explanation of what

corrosion is and what causes it.

$etal Coating Coating processes such as galvaniing, cadmium plating, chroming and

anodiing are widely used through aircraft. #ossibly the most common is aluminum

cladding processed metal such as A&C&A/ is used throughout aircraft manufacture and

repair. The process involves coating sheets of aluminum alloy with a thin layer of pure

aluminum this layer reacts with oxygen very -uickly forming aluminum oxide, which is

corrosion resistant. The major problems with all of these processes are that these are

very expensive and once the coating is worn the core metal is subject to corrosion. The

coatings are very difficult to repair and often the component re-uires complete

replacement once the coating is worn off. *ne further problem with these processes is

that during manufacture water can be trapped between the coating and the core metal,

giving rise to corrosion right from the start

object correctly could cause major problems in the future. +emember paint is not

applied to an aircraft just to look good but to protect the metallic components fromcorrosion. This means that the -uality of the preparation, the undercoating and the type

of painting process used are of vital importance.

#reparation 0efore painting commences, the area to

be protected must be prepared. This is the most

important part of the painting process and must

never be left out or condensed, /o not cut corners. Cutting corners may create future

expensive problems. #reparation re-uires that the area be 1stripped2 back to 1bare2 metal.

This area and the surrounding area should be inspected for corrosion, damage and

cleanliness. All corrosion must be removed prior to painting this may re-uire extensive

rework of the area. The area should then be treated with the appropriate preparation

compound. 3ome metals re-uire that all oxidiation be removed while other metals

re-uire that oxidiation be promoted. 4ou should address the approved data to

determine which is re-uired. *nce the area is prepared a primer should then be used to

1seal2 the area. 3ome primers and undercoats will actually absorb and retain water the



data will give you the information you need. Always use the primer and undercoat,

which is compatible with the metal surface and the paint you will be using. %n cases

where paint will not be applied special waterproof primer is used. This is usually olive

green in color. When two surfaces are to be in contact with each other it is important to

prepare and protect each surface prior to assembly. %n some cases the atmosphere in the building must be controlled for the process to be effective. #ainting and preparation

must never be performed outside. To be effective, paint must be even over the entire

area and must seal the area.

*ther Coatings Application of grease, oil, wax, polish or any other temporary protection

compound can be easy to apply and is relatively cheap. These compounds are all

temporary and must be replaced at regular intervals or whenever the underlying metal

is revealed

Some Simple Rules for Corrosion Prevention

!. /egularly inspect the aircraft for corrosion.

". /emove and treat all corrosion no matter how small.

. 0aintain the paint and other coatings in good condition.

2. 7eep the aircraft indoors, wherever possible.

#. /epair paint other protective coatings as soon as possible.

8. se the correct washing compounds.

9. se the correct polish.

. /emove all water from areas where it may collect inside and outside of the aircraft.;. <repare all areas in accordance with the approved data prior to application of permanent

corrosion preventative coatings.

!. -f an aircraft is to be stored, corrosion inhibiting compounds should be applied and

reapplied at regular intervals, these intervals are generally shorter in areas where there

may be salt or pollution in the air.

!!. %tored aircraft should be inspected regularly for corrosion and then treated immediately.

5ow to +epair Corroded Aluminum Alloy

Wheels



All! "#$$l% &'$ (')$ * +''%)

Aluminum alloy wheels are made from a combination of aluminum and other metals.

The aluminum makes the wheels lighter, stronger and more durable them most wheels,

and thus makes steering the +&' more precise. They require modest care to keep their

prized gleaming finish, but occasionally, even the bestcaredfor wheels will need

attention to remove the dirt and corrosion from driving on roads covered with asphalt,

salt and other materials that cause wear.

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Corrosion Pre Finals

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