3-239. ANODIZING PROCESS FOR INSPECTION OF ALUMINUM ALLOY PARTS . Parts for which anodic coating is
applicable in accordance with MIL-A-8625 Type I, can be anodized for the inspection of defects as cited in Specification
3-240. The parts are examined visually for indications of cracks, forging laps or other defects. Parts inspected by this
method shall be limited to sheet stock and surface defect of forgings. This method of inspection is not acceptable for
inspection of parts subject to internal defects, i.e. inclusion in castings and forging or any part subject to internal stress,
3-241. Defects are indicated by darkening of cracked or void areas after the anodic treatment. Insufficient rinsing in cold
water after anodizing produces stains which may be confused with defects. In case of doubt strip film from part and
reanodize. If the indications do not reappear the defects shall be considered absent and part should not be rejected for
For additional general information on inspection and testing see Section VI of this technical order.
3-242. ALUMINUM ALLOY EFFECTS ON SCRATCHES ON CLAD ALUMINUM ALLOY . The purpose of the following
information on the effects of scratches on aluminum alloys is to assist in eliminating controversy in depots and field
inspection, regarding serviceability of aluminum alloy, sheet, skin and aircraft structural parts which have been scratched,
abraded or discolored from the stand point of corrosion resistance and fatigue strength.
3-243. In some instances, serviceable aluminum alloy parts and sheets, have been disposed of due to lack of knowledge
by inspection personnel as to the effect of various depth scratches on the strength and corrosion resistance of the clad
alloy. Also, attempts have been made to remove scratches from aircraft skin by sanding, buffing, or polishing resulting in
removal of much of the cladding material and causing decrease in strength and corrosion resistance.
3-244. ALLOWABLE DEFECTS.
The following surface defects are those which do not affect the strength or corrosion resistance.
Scratches which penetrate the surface layer of clad aluminum alloy sheets or parts but do not extend
beneath the cladding are not serious or detrimental.
The presence of small corroded areas will not materially affect the strength of clad unless the corroded
pitted area extends through the cladding down to or into the bare metal. Clean corroded areas thoroughly by authorized
methods (See Par 3-253).
Stains are not grounds for rejection since they affect neither the strength nor the corrosion resistance.
No attempt will be made to remove scratches or other surface defects by sanding or buffing
since the protective layer of cladding will be removed by such operations.
3-245. HARMFUL SCRATCHES. Scratches which extend through the cladding and penetrate the core material act as
notches and create stress concentrations which will cause fatigue failure if the part is highly stressed or subjected to
repeated small stress reversals. However, sheets so scratched may be utilized for non-stressed applications.
3-246. INSPECTION. Assemblies fabricated from clad aluminum-alloy sheets will not be rejected by inspection
personnel, unless the defect is of sufficient depth to adversely affect the mechanical properties or cover sufficient area to
impair the corrosion resistance of the assembly. Scratches or abrasions which penetrate the cladding will not affect
corrosion resistance. Scratches resulting from the normal handling and processing of clad aluminum-alloy sheet rarely
extend through the cladding and penetrate the core.
3-247. TEST FOR DEPTH OF SCRATCHES. Since it is very difficult to measure the depth of a scratch on a sheet
without cross sectioning the sheet, it has been found convenient (on clad material) to use a "spot" test to determine
whether or not a scratch extends through the cladding.
3-248. On alloys except 7075 and 7178 the "spot" test is made by placing a drop of caustic solution (10% by weight of
sodium hydroxide, NaOH, in water) on a portion of the scratch, and allowing it to react for 5 minutes. The caustic
solution will then be rinsed off the sheet with water, and the spot allowed to dry. If a black residue remains in the base of
the scratch at the spot tested, it indicates that the scratch extends to the core. If no black color is visible and only a white
residue remains in the base of the scratch, it indicates that the scratch does not penetrate through the cladding.