CORROSION REMOVAL, ASSESSMENT AND REPROTECTION:

 

Due to the high cost of modern aircraft, operators are expecting them to last much longer than perhaps even the manufacturer anticipated. As a result, the manufacturers have taken more care in the design of the aircraft, to improve the corrosion resistance of aircraft. This improvement includes the use of new materials and improved surface treatments and protective finishes. The use of preventative maintenance has also been emphasised more than previously.

Preventative maintenance, relative to corrosion control, should include the:

•  Adequate and regular cleaning of the aircraft
•  Periodic lubrication (often after the cleaning) of moving parts
•  Regular and detailed inspection for corrosion and failure of protective treatments
•  Prompt treatment of corrosion and touch-up of damaged paint
•  Keeping of drain holes clear
•  Draining of fuel cell sumps
•  Daily wiping down of most critical areas
•  Sealing of aircraft during foul weather and ventilation on sunny days
•  Use of protective covers and blanks. General treatments for corrosion removal include:
•  Cleaning and stripping of the protective coating in the corroded area
•  Removal of as much of the corrosion products as possible
•  Neutralisation of the remaining residue
•  Checking if damage is within limits
•  Restoration of protective surface films
•  Application of temporary or permanent coatings or paint finishes.
  
  

Cleaning and Paint Removal 

It is essential that the complete suspect area be cleaned of all grease, dirt or preservatives. This will aid in determining the extent of corrosive spread. The selection of cleaning materials will depend on the type of matter to be removed.

 

Solvents such as trichloroethane (trade name ‘Genklene’) may be used for oil, grease or soft compounds, while heavy-duty removal of thick or dried compounds may need solvent/emulsion-type cleaners.
 

General-purpose, water-removable stripper is recommended for most paint stripping. Adequate ventilation should be provided and synthetic rubber surfaces such as tyres, fabrics and acrylics should be protected (remover will also soften sealants). Rubber gloves, acid-repellent aprons and goggles, should be worn by personnel involved with paint removal operations.

 

The following represents a typical paint stripping procedure:

• Brush the area with stripper, to a depth of approximately 0.8 mm – 1.6 mm (0.03 in – 0.06 in). Ensure that the brush is only used for paint stripping
• Allow the stripper to remain on the surface long enough for the paint to wrinkle. This may take from 10 minutes to several hours
• Re-apply the stripper to those areas which have not stripped. Non-metallic scrapers may be used to assist the stripping action
• Remove the loosened paint and residual stripper by washing and scrubbing the surface with water and a broom or brush. Water spray may assist, or the use of steam cleaning equipment may be necessary.

Note: Strippers can damage composite resins and plastics, so every effort should be made to 'mask' these vulnerable areas.

Aluminium and Aluminium Alloys
Corrosion attack, on aluminium surfaces, gives obvious indications, since the products are white and voluminous. Even in its early stages, aluminium corrosion is evident as general etching, pitting or roughness.

Aluminium alloys form a smooth surface oxidation, which provides a hard shell, that, in turn, may form a barrier to corrosive elements. This must not be confused with the more serious forms of corrosion.
 

General surface attack penetrates slowly, but is speeded up in the presence of dissolved salts. Considerable attack can take place before serious loss of strength occurs. Three forms of attack, which are particularly serious, are:

• Penetrating pit-type corrosion through the walls of tubing
• Stress corrosion cracking under sustained stress
• Intergranular attack, characteristic of certain improperly heat-treated alloys.
  

Treatment involves mechanical or chemical removal of as much of the corrosion products as possible and the inhibition of residual materials by chemical means.

Obviously great care must be taken, not to remove too much of the protective aluminium layer by mechanical methods, as the core alloy metal may be exposed, therefore, where heavy corrosion is found, on clad aluminium alloys, it must be removed by chemical methods wherever possible.

Corrosion-free areas must be masked off and the appropriate remover (usually a phosphoric acid-based fluid) applied, normally with the use of a stiff (nylon) bristled brush, to the corroded surface, until all corrosion products have been removed. Copious amounts of clean water should, next, beused to flood the area and remove all traces of the acid, then the surface should be dried thoroughly.
 

Note: A method of checking that the protective aluminium coating remains intact is by the application of one drop of diluted caustic soda to the cleaned area. If the alclad has been removed, the aluminium alloy core will show as a black stain, whereas, if the cladding is intact, the caustic soda will cause a white stain.

 

The acid must be neutralised and the area thoroughly washed and dried before a protective coating (usually Alocrom 1200 or similar) is applied to the surface. Further surface protection may be given by a coat of suitable primer, followed by the approved top coat of paint.

EASA PART66 / SARI MODULE 7 ESSAY QUESTION:

During an inspection, it was found that a certain Aluminium structure was etched and pitted due to the surface corrosion. Write down the complete procedure for removal of this corrosion also ensure that the protective Alclad layer is not damaged.

(Note: write your answer in the comment section below)