Q74. Information regarding corrosion removal
limitations can be found in what publications?
CHEMICAL SURFACE TREATMENT
LEARNING OBJECTIVE: Define the purpose
for chemically preparing a surface for priming
and painting.
Chemical conversion coatings increase a surface’s
resistance to corrosion and improve paint bonding on
the surface.
The metal to be treated must be cleaned to a water
breakfree surface (fig. 4-30). Metal surfaces not free
of water breaks must be recleaned with a solution of 1
part MIL-C-43616 or MIL-C-25769 aircraft cleaning
compound to 16 parts of water, and then rinsed with
water. Surfaces that have been waxed, particularly
with silicone wax, may require special cleaning. After
cleaning and removal of surface oxides, aluminum
should be treated with MIL-C-81706 and magnesium
with MIL-M-3171, type VI, chemical conversion
coating material.
214.248
Figure 4-30.—Water break comparison.
CAUTION
Personnel must wear protective clothing,
rubber gloves, and chemical goggles when
using a solution of MIL-C-81706 and MIL-
M-3171 or serious injury could result.
Apply these chemical conversion coatings
immediately after cleaning the surface to a water
breakfree surface and while the surface is still wet.
Apply these coatings by brush, nonatomizing spray, or
sponge stick moistener. The sponge stick moistener is
particularly useful for small areas.
Soluble salt residues that remain on the surface
after treatment accelerate corrosion and can cause
blistering of paint finishes. Thus, complete rinsing
with fresh water following the chemical treatment is
very important. Flush the chemical with free-flowing
water only. Allow the chemical conversion coated
surface to dry (usually 30 minutes) before painting. Do
NOT wipe the surface with a damp cloth or brush, as
this will degrade or remove the chemical conversion
coating.
Chemical conversion coatings are often damaged
during aircraft maintenance, or they may be
contaminated by grease, oil, or other foreign matter.
Therefore, the treated surface should be painted soon
after treating to obtain the best results.
CHEMICAL CONVERSION OF
ALUMINUM ALLOYS
The procedure to be used for the chemical
conversion of aluminum alloys is as follows: Apply
the conversion coating material, MIL-C-81706 (Form
V [powdered] is preferred, Form III [premixed] is an
alternate), until you obtain a golden iridescent color.
Immediately rinse the chemical from the surface with
large amounts of fresh water when you obtain the
proper color conversion. This rinsing stops the
chemical action and minimizes solution entrapment.
Failure to rinse may accelerate corrosion and reduce
paint bonding. If a long period of contact before
rinsing is allowed, a powdery, coated surface may be
the result.
CHEMICAL CONVERSION OF
MAGNESIUM ALLOYS
The procedure for the chemical conversion of
magnesium alloys is as follows: Apply the conversion
4-45
