Corrosion of stainless steel, carbon steel, and aluminum
takes place to varying degrees under thermal insulation
in the presence of moisture. Water and/or steam may
enter the insulation space through penetration points in
the jacket, by capillary action (wicking) through or
around the insulating medium, or through damaged
steam coils. The presence of moisture can lead to general
corrosion, pitting, and/or cracking. Because the
exterior surfaces of the tank car shell and the interior
surfaces of the jacket are not normally inspected after
the original construction, such corrosion may go undetected
for a great portion of the tank car's expected life
unless the jacket is removed.
The potential for significant corrosion under insulation on
rail tank cars can be related to:
? operating temperature;
? constituents in the insulation that form corrosive
solutions when combined with moisture;
? the tendency of some insulation material to absorb
and/or retain moisture;
? the insulation's lack of adhesion to the substrate;
and
? contact with the commodity being transported.
Corrosion was first detected under foam insulation
applied to tank cars in the 1960s. Some urethane foams
were found to have a propensity to release acids upon
exposure to heat and moisture. Consequently, all
insulation systems used on tank cars are now subject to
corrosion testing in accordance with AAR(1)Specification
M-1002, Section 2.2.12.2 This corrosion test is actually
designed for foam-in-place (FIP) urethane. In addition to
corrosion testing for the insulation system, Section
2.2.12 requires that the exterior surfaces of carbon steel
tanks and the interior surfaces of carbon steel jackets be
given a protective coating.
This state-of-the-art report describes types of insulation
and coating systems used on tank cars to provide
corrosion control. Temperature limitations are provided
for each system.
Types of Insulation Materials
Following are descriptions of various types of insulation
materials that have been used on tank cars. They are
listed in descending order based on frequency of use.
Fiberglass: A light-density (1.4 to 3.5 lb/ft3 [2.2 to 5.6
kg/m3]) glass fiber with an organic binding resin, a semirigid
formed board, or a sheet (sheets are of various
densities). Available with various types of factoryattached
outer facings. Maximum service temperature
has been 450°F (232°C). These materials have very
high water absorption, 93% by volume.
Ceramic Fiber : A light- to medium-density (2.0 to 8.0
lb/ft3 [3.2 to 12.8 kg/m3]) spun or woven material composed
of ceramic fibers. Maximum service temperature
has been 3,200°F (1,760°C). These materials have extremely
high water absorption and are hygroscopic.