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Stress Corrosion                               Corrosion Fatigue                               Fretting Corrosion                               Heat Treatment

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 the aluminum surfaces.  Note  :  Aluminum alloys commonly from a smooth surface oxidation which is from, .001 in. to .0025 in. thick.  This is not considered detrimental as such a coating provides a hard shell barrier to the introduction of corrosive elements.  Such oxidation is not to be confused with the severe corrosion discussed in this paragraph.  General surface attack of aluminum penetrates relatively slowly, but is speeded up in the presence of dissolved salts.  Considerable attack can usually take place before serious loss of structural strength develops.  However, at least three forms of attack on aluminum alloys are particularly serious   :   (1)  The penetrating pit-type corrosion through the walls of aluminum tubing,  (2)  stress - corrosion cracking of materials under sustained stress, and (3)  the intergranular attack which is characteristic of certain improperly heat-treated aluminum alloys.       In general, corrosion of aluminum can be more effectively treated in place than corrosion occuring on other structural materials used in aircraft.  Treatment includes the mechanical removal of as much of the corrosion products as practicable, and the inhibition of residual materials by chemical means, follow by the restoration of permanent surface coating. 

Treatment  of  Unpainted  Aluminum  Surfaces     Relatively 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.  The protection obtained is good, and the alclad surface can be maintained a polished condition.  In cleaning such surfaces, however, care must be taken to prevent staining and marring of the exposed aluminum and, more important from a protection standpoint, to avoid unnecessary mechanical removal of protective alclad layer and the exposure of the more susceptible aluminum alloy base material.  A typical aluminum corrosion treatment sequence follow  :   (1) Remove oil surface dirt with any suitable mild cleaner prior to abrasive cleaning of aluminum surfaces.  (2) Hand polish the corroded areas with fine abrasives or with metal polish.  Metal polish intended for use on clad aluminum aircraft surfaces must not be used on anodized aluminum since it is abrasive enough to actually remove the protective anodized film.  It effectively removes stains and produces a high, lasting polish on unpainted alclad.  If a surface 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.  (3)  Trest any superficial corrosion present, using an inhibitive wipe-down material.  An alternate treatment is processing with a solution of sodium dichromate and chromium trioxide.  Allow these solutions to remain on the corroded area for 5 to 20 minutes, and then remove the excess by rinsing and wiping the surface dry with a clean cloth.  (4) Overcoat the polished surfaces with 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.  The following sequence is generally used   :    (1)  Thoroughly clean the affected surfaces of all soil and grease residues prior to processing.  Any general aircraft cleaning procedure may be used.   (2)   Treat superficially corroded areas with a 10 percent solution of chromic acid and sulphuric acid.  Apply the solution by swab or brush.  Scrub the corroded area with the brush while it is still damp.  While chromic acid is a good inhibitor for aluminum alloys, even when corrosion products have not been completely removed, it is important that the solution penetrate to the bottom of all pits and underneath any corrosion that may be present.  Thorough brushing with a stiff fiber brush should loosen or remove most existing corrosion and assure complete penetration of the inhibitor into crevices and pits.  Allow the chromic acid to remain in place for least 5 minutes, then remove the excess by flushing with water or wiping with a wet cloth.  There are several commercial chemical surface treatment compounds, similar to the type described above, which may also be used.

    (3)   Dry the treated surface and restore recommended permanent protective coatings as required in accordance with the aircraft manufacturer's procedures.  Restoration of paint coatings should immediately follow any surface treatment performed.  In any case, make sure that corrosion treatment is accomplished or re-applied on the same day that paint refinishing is scheduled.

  Forms of Corrosion

Type of Corrosion

Corrosion Control

  Corrosion Removal

Preventive Maint.

Common Corrosive

List of Agent

Bugs

Inspection                                                           Corrosion Prone Areas                                                              Corrosion Limits

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