September/October 2023 - 19

internal structure to proper treatment, according
to Bristol.
" The best tool to prevent corrosion is visual
inspections and NDT by experienced technicians.
NDT is used to find corrosion where one typically
cannot see visually. Rotor peening/shot peening
are used to prevent corrosion. Peening the metal
compresses the material which makes it more
difficult for the environmental elements to start
the corrosion process, " he said. " Depending on the
location, the material to be inspected depends on
the NDT inspection method. Each NDT method
can be different, for example X-rays can see corrosion
in layers of material, eddy current is used
to find small pits and subsurface corrosion, and
ultrasound can see material loss within a part. It
would be good practice, near the ocean or in an
area with high saltiness, to frequently rinse the
aircraft with fresh water. "
Corrosion-Related Repairs
Once corrosion is found, it needs to be reported and
documented to determine its allowable limits and
applicable repair process, affirmed Vega.
It is necessary to follow the reporting process as
outlined by the OEM manuals, explained Sharp.
" This process will provide what next steps to take
to remove the corrosion and what protections to
apply or repairs required to restore the area back to
a serviceable condition. Corrosion-related repairs
are typically provided by the OEM structural repair
manual or released by the structural engineering
teams for that specific aircraft, " he says.
The allowable limits indicate the amount of
corrosion that can be removed or cleaned, without
compromising the structural integrity of the
component, according to Vega.
" A component exhibiting corrosion within its
'allowable limits' is repaired by cleaning, disassembling,
and removing the corroded area, subsequently
inspected to confirm complete removal
of the corrosion. Subsequently, protective surface
films and protective coatings, primer and/or paint
are applied, " he says. " If, on the other hand, the
determined corrosion is beyond its 'allowable limits',
it is necessary to carry out a structural repair.
If a structural member, for example a floor beam,
is found to have corrosion beyond its allowable
limits, it must be repaired by removing or cutting
the affected area or section and replacing it with
the same material and additional reinforcement
splice pieces of the same material are installed to
bring it back to its original structural integrity. "
Typically, to remove corrosion, a surface conditioning
disc grinder is used for medium to light
surface corrosion, observes Bristol. " If, however,
the corrosion is intense, it will be necessary to
use more aggressive mechanical means such as
rotary files and then smooth the surface according
to the instruction manuals or technical order; in
fact, leaving the surface not adequately smooth
can result in the material being exposed to corrosion, "
he says.
The correct way to check the smoothness is to
use a smoothness gauge and typically the smoothness
of aluminium is 65 μin, according to Bristol.
" The corrosion should first be removed, typically
an NDT inspection is carried out to ensure all corrosion
is removed. One needs to be familiar with the
manuals to determine the location of the damage
on the wing or component. Typically, an aircraft
maintenance manual, component maintenance
manual, or structural repair manual would give
limitations, " he says. " It is not possible to measure a
pit to determine how severe the corrosion is, as the
pit could be the entry point to allow environmental
elements into the metal, so the small pit turns into
a channel within the metal, a corrosion cavern. "
Development of the Designs
Over the years aircraft OEMs have engineered and
implemented the use of non-metallic material and
corrosion-resistant metals to minimize corrosion
damage, affirms Vega. " Carbon fiber is one of the
materials that slowly taking over and replacing
aluminium alloy components, " he says.
With the implementation of new materials on
newer aircraft is born the responsibility to train
personnel to use proper tools to not affect the factory
coatings so not to allow the corrosion to start,
according to Bristol.
Today's built aircraft are assembled using better
developed materials which include identifying
the removal of some products that were found to
trap moisture and improving aircraft draining in
key areas, highlights Sharp. " Aircraft inspection
programs are continually being revised which may
include adjusting frequency of corrosion inspections,
release of improved corrosion inhibitors, and
OEM's sharing of recent discoveries using electronic
media via released news flashes, advisories, or
alerts, " he concludes.
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