FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 6. Corrosion, Inspection, and Protection | 9. Magnesium and Magnesium Alloys | 6-152. Example of Removing Corrosion From Magnesium
AC 43.13-1B
9/8/98
blasting with glass beads, (Spec. MIL-G-9954)
sizes 10-13; or grain abrasive (Spec.
MIL-G-5634), types I or III at an air pressure
of 10 to 35 psi (if suction equipment is used,
use a 50 percent higher pressure). Upon com
pletion of blasting, inspect for the presence of
corrosion in the blast area. Give particular at
tention to areas where pitting has progressed
into intergranular attack. This is necessary be
cause abrasive blasting has a tendency to close
up streaks of intergranular corrosion rather
than remove them if the operator uses an im
proper impingement angle. If the corrosion
has not been removed in a total blasting time
of 60 seconds on any one specific area, other
mechanical methods of removal should be
utilized.
CAUTION: When blasting magne-
sium alloys, do not allow the blast
stream to dwell on the same spot
longer than 15 seconds. Longer dwell
times will cause excessive metal re-
moval.
g. Inspect the reworked area to ensure
that no corrosion products remain. If corrosion
products are found, repeat method used and
re-inspect.
k. Apply Chromic Acid Brush-on Pre
treatment.
(1) Chemical pretreatment such as the
following chromic acid solution (Conversion
coat conforming to Spec. MIL-M-3171,
type VI) provides a passive surface layer with
an inhibitive characteristic that resists corro
sive attack and also provides a bond for subse
quent coatings. Properly-applied magnesium
pretreatment tend to neutralize corrosion me
dia in contact with the surface.
(2) The chromic acid brush-on pre
treatment may be applied to all magnesium
parts that require touch-up. This treatment is
generally used in refinishing procedures or
when parts and assemblies are too large to be
immersed. This treatment is less critical to ap
ply than the other brush-on treatments. It is
relatively inexpensive and not as harmful when
trapped in faying surfaces.
(a) Solution Composition and Op
eration:
1 Distilled Water 1 gal.
2 Chromic Acid (CrO3) 1.3 oz.
h. Fair depressions resulting from rework
using a blend ratio of 20:1. Clean rework area
using 240 grit abrasive paper. Smooth with
300 grit and finally polish with 400 grit abra
sive paper.
i. Determine depth of faired depressions
to ensure that rework limits have not been ex
ceeded. Refer to the manufacture’s specifica
tions.
j. Clean reworked area using a solvent to
provide a water-break-free surface. Do not use
kerosene or another petroleum base fuel as a
cleaning solvent.
3 (99.5 pure), Calcium Sulfate
1 oz. (CaSo4.2H2O)
4 Operating Temp. 70-90 °F.
5 Container: Stainless Steel, Alu
minum, Vinyl, Polyethylene, or Rubber.
NOTE: Good application requires
proper preparation of the chromic
acid coating solution and cleaning of
the surface where the solution will be
applied. A water-break test is recom-
mended if the cleanliness of the sur-
face is in doubt.
Page 6-32
Par 6-152