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3) Add another volume of water until quantity of solution is equal to
2/3 the total desired quantity.
4) Slowly add total volume of nitric acid to solution and mix thor-oughly.
5) Add water until total desired quantity of solution is reached and
stir until entire solution concentration is equal.
6) Remove corrosion by carefully applying the chromic acid solution
to the corroded area with acid resistant brush.
7) Allow the solution to remain on the surface approximately 15 min-utes; agitate surface with brush. Wipe dry.
8) Rinse thoroughly with water while scrubbing the brush and wipe dry.
9) Repeat the preceding sequence as necessary until all corrosion
products have been removed and a bright metallic color of the metal
appears.
10)Fair depressions resulting from the rework per par. 7.C.
11) Surface treatment after corrosion removal per 20-50-00.
10.Corrosion Removal Procedures for Carbon Steel
A. General. Carbon steel in its heat-treated form is used in areas where high
structural or aerodynamic loads occur on the airplane. Landing gear, flap
tracks, structural splices, terminal fittings and miscellaneous brackets
are some of the typical parts made from heat-treated carbon steel. Red iron
rust is one of the most familiar kinds of corrosion on carbon steel and gen-erally caused from formation of ferrous oxides due to atmospheric exposure.
Some surface metal oxides are purposely used to protect the underlying base
metal. Red iron rust is not one of these and actually attracts moisture from
the air and promotes additional corrosion. The red rust first shows on
unprotected airplane hardware such as bolts, nuts and exposed fittings.
Slight corrosion damage on highly stressed steel parts becomes potentially
dangerous and the rust shall be removed and controlled. Corroded steel parts
should be removed from the airplane for treatment if possible. The general
procedures for corrosion removal are provided in the following paragraphs.
WARNING: PERSONNEL USING THESE PROCEDURES SHALL OBSERVE THE SAFETY PRECAU-TIONS AND PROCEDURES OUTLINED IN PAR. 2.
B. Corrosion Removal -Mechanical
(1)Positively identify the metal as steel and determine its heat-treat
value.
(2)Protect adjacent area to prevent additional corrosion damage from corro-sion products removed during mechanical removal.
(3)If grease or soil is present, clean the rework area per par. 5.
(4)Strip paint per par. 6, if applicable.
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CAUTION: HAND-HELD POWER TOOLS MUST NOT BE USED ON HIGH-STRENGTH STEELS
HEAT-TREATED TO 220,000 PSI AND ABOVE. EXERCISE EXTREME CARE WHEN
USING TOOLS ON HIGH-STRENGTH STEEL NOT TO OVERHEAT THE STEEL SUR-FACE.
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