Page 310 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

288 CORROSION CONTROL AND PREVENTION

4.23.4 Sealants
The potential for lap joints or joint crevice corrosion is eliminated by the application
of a sealant to the faying surfaces of the joints. A poysulfide sealant is applied to areas
such as skin-to-stringer and skin-to-shear tie joints in the lower lobe of the fuselage,
longitudinal and circumferential skin splices, skin doublers, the spar web-to-chord
and chord-to-skin joints of the wing and empennage, wheel well structure, and pres-
sure bulkheads. High-strength steel and titanium fasteners on the exterior of the air
plane and fasteners that penetrate the pressurized portion of the fuselage are installed
with a sealant. Fillet seals are used in severe corrosion environments.
Although the design aspects provide most of the corrosion protection for air-
planes, corrosion inhibitors are widely used to provide additional protection when
used periodically in service. Corrosion inhibitors are applied in areas such as the
lobe of the fuselage and most of the aluminum parts. The corrosion inhibitors are
petroleum-based compounds that either displace the water or serve as a coating. The
water-displacing inhibitors are sprayed onto a structure to penetrate faying surfaces
and keep water away from crevices. The application of these inhibitors must be
repeated at intervals of every few years. The more viscous heavy-duty inhibitors
are also sprayed, which form a much thicker film and have a lesser penetrating
ability. These thicker inhibitors are applied on airplane parts that are most prone to
corrosion.
The proper corrosion maintenance program should prevent or eliminate the con-
ditions favoring corrosion: (i) trapped moisture; (ii) wet insulation blankets; (iii)
plugged drain holes and passages; (iv) chipped or missing paint; (v) loss of protective
finish; (vi) corrosive cargo.
Most of the corrosion in aircraft can be avoided by proper and timely application
of sealants and corrosion inhibitors. When lavatories and galleys are removed for
maintenance or repair, utmost care should be taken when the sealants are applied.
Maintenance programs should be thorough such that necessary action can be taken
before the corrosion problem becomes uncontrollable. Nondestructive inspection
(NDI) techniques such as ultrasonic testing, eddy current testing, optical testing,
and radiographic testing may be used to detect flaws in the aircraft parts before they
become major defects. When corrosion is detected, it is removed by blending out.
Further developments in NDI techniques should lead to detection of smaller flaws
and corrosion that is hidden in the structure, which are beyond the capabilities of the
existing NDI techniques. Until recently, corrosion control of airplanes was based on
“find and fix.” In an effort to control corrosion in an economical manner, corrosion
is now being managed by a combination of selective blend-out and application of
corrosion-inhibitive or water-displacing compounds.

4.24 HAZARDOUS MATERIALS TRANSPORT (HAZMAT)

The corrosive materials that were most often involved in HAZMAT incidents in
1998 consisted of sodium hydroxide solutions, basic inorganic liquids, hydrochloric