A “Toolbox” for Forensic Engineers

174 Forensic Materials Engineering: Case Studies
6.3 Corrosion Failures
Metals corrode because they are conductors of electricity. Thus when metal
atoms are in a chemical environment that allows or causes them to give up
electrons, they become positively charged ions that take part in chemical
reactions, provided an electrical circuit can be completed. The net effect is
that the metal component corrodes away where the electrons are given up
and the useful cross-sectional area is reduced. This can be concentrated
locally to form a pit or, sometimes, crack if a high level of tensile stress is
acting, or it can extend across a wide area to produce general wastage. The
load-carrying capacity is thus reduced and an eventual failure may occur
simply because a load in the upper part of the normal spectrum exceeds the
residual strength of the component.
The cause of accidents resulting from general wastage type of corrosion
is usually obvious, for example, where the chassis or a suspension member
of a vehicle has corroded away; there was one particular car where aluminum
alloy shock absorbers exposed to road spray literally flaked away over
a period of about 5 years in the British climate. Occasionally, however, the
corrosion may be found to have occurred in an area that is concealed, such
as the inside of a box section, or in an area that is inaccessible to routine
painting or protection, such as a gap between two close fitting members of
a structure. In a case some years ago, this was the reason for sudden collapse
of a footbridge over a railway line.
Localized corrosion that leads to pitting may provide sites for fatigue
initiation and, in addition, corrosive agents such as sea water may lead to
greatly enhanced growth of the fatigue crack. Even rainwater can cause serious
failures. Thus rainwater penetration of tubular cycle frames has caused
corrosion-fatigue failures, resulting in serious injuries to the riders. The
fatigue cracks had initiated at corrosion pits, although the immediately surrounding
areas were less severely pitted and only lightly rusted.
Pitting corrosion also occurs much faster in areas where microstructural
changes have occurred in welding operations. Some years ago an
educational institute with several large buildings heated by hot water distributed
through steel pipework from a single boiler house sued ten different
subcontractors on the basis that the welds were faulty. The pipework
had developed numerous pinhole leaks at the welds in pipes joined at
different times by different contractors throughout the entire campus. The
cause was not faulty welding but failure to control the boiler feed water,
which had led to severe pitting corrosion inside the pipes at the edges of
the weld beads.
Corrosion may be combated in a number of ways, ranging from cladding
with a protective sheath, applying a protective film to the surface like