FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 6. Corrosion, Inspection, and Protection | 3. Corrosion Protection Measures for Basic Materials | 6-31. Plating
9/8/98
AC 43.13-1B
SECTION 3. CORROSION PROTECTION MEASURES FOR BASIC MATERIALS
6-29. GENERAL. In the repair of aircraft,
apply corrosion proofing of the same type or
equivalent to that originally applied unless the
repair would result in increased susceptibility
to corrosion, in which case use additional cor
rosion protection measures. The following is a
list of the most commonly-used corrosion-
proofing techniques.
6-30. ANODIZING AND RELATED
PROCESSES. In anodizing, aluminum alloys
are placed in an electrolytic bath causing a thin
film of aluminum oxide to form on the surface
of the aluminum. This is resistant to corrosion
and affords a good paint base. However, other
processes, which do not provide as good a cor
rosive protection as anodizing, are good paint
bases. The processes are:
a. Alkaline cleaning followed by chromic
acid dip;
b. Alcoholic phosphoric acid cleaner; and
c. Alkaline dichromate treatment.
6-31. PLATING. Steels are commonly
plated with other metals to prevent corrosion.
Plating is accomplished by placing the article
in an electrolytic bath. Metals used in plating
vary in the corrosion protection they afford
steel. For instance, in platings that corrode be
fore steel, such as zinc or cadmium, slight
breaks or cracks throughout the plating will
not result in rusting of the exposed steel. With
the surface metal corroded, the steel is pro
tected. However, when the steel corrodes
faster than the plate metal, such as chromium,
the amount of protection depends on the tight
ness of the plating. Post-plate bake treatment
to relieve hydrogen embrittlement is a neces
sary part of replating procedures for
high-strength steel parts. High-strength nuts
and bolts are susceptible to failure from hydro
gen embrittlement. Because of the potential
failures of embrittled parts, careful control
over the heat treatment, grinding, preplate
cleaning, plating, and post-plate baking of
high-strength parts is necessary.
6-32. PHOSPHATE RUST-PROOFING.
This process is commercially known as
Parkerizing, Bonderizing, Granodizing, etc.
The coating placed on the part is used to pro
tect steel parts after machining and before
painting.
6-33. CHROME-PICKLE TREATMENT.
Magnesium parts which have been immersed
or brushed with a solution of nitric acid and
sodium dichromate will be protected for tem
porary storage. The coating will also serve as
a bond for subsequent organic finishes. Sealed
chrome-pickle treatment is used on magnesium
parts for long term protection. Diluted chro
mic acid is a touch-up treatment. It is less
critical to apply and can be applied over previ
ously-applied thin chromate films.
6-34. DICHROMATE TREATMENT.
This treatment consists of boiling magnesium
parts in a solution of sodium dichromate. It
provides good paint base and protective quali
ties on all standard wrought magnesium alloys
except the magnesium-thorium alloys. Acid
pickling of the magnesium surface prior to ap
plication of the dichromate treatment is re
quired if maximum corrosion resistance of the
finish is expected.
6-35. STANNATE IMMERSION TREAT-
MENT. Stannate immersion treatment deposits
a layer of tin. It is a protective paint base for
magnesium alloy parts which contain inserts
and fasteners of a dissimilar metal such as
Par 6-29
Page 6-11