Stress Corrosion Cracking
Stress corrosion cracking (SCC)
is caused by the simultaneous effects of tensile stress and a specific
corrosive environment. Stresses may be due to applied loads, residual
stresses from the manufacturing process, or a combination of both.
Cross sections of SCC
frequently show branched cracks. This river branching pattern is unique
to SCC and is used in failure analysis to identify when this form of
corrosion has occurred.
The photo below shows SCC of
an insulated stainless-steel condensate line. Water wetted the
insulation and caused chlorides to leach from the insulation onto the
hot metal surface. This is a common problem on steam and condensate
lines. Control is by maintaining the jackets around the lines so that
moisture doesn't enter the insulation or is quickly drained off.
The next two photos show
intergranular SCC of an aluminum aerospace part. The intergranular
nature of the corrosion can be seen in the scanning electron microscope
image on the left and in the microscopic cross section on the right. The
arrows indicate the primary crack shown in both pictures. Note that
secondary cracks are also apparent. These secondary cracks are common in
stress corrosion cracking.
The failure above occurred on
an aluminum alloy subjected to residual stresses and salt water. Changes
in alloy heat treatment recommended by KSC
Materials Laboratory eliminated this problem. McDanels, S. J.,
"An Overview of Fatigue and Other Metallurgical Failure Modes
and Analysis at the Kennedy Space Center", Flammability
and Sensitivity of Materials in Oxygen-Enriched Atmospheres: Eight
Volume, ASTM STP 1319, W. T. Royals, T. C. Chou, and T.A.
Steinberg, Eds., American Society for Testing and Materials, 1997.
Several years ago, wide spread
use of plastic tubing was started in new house construction and for
repair of old systems. Flexible tubing was used to connect faucets to
the fixed metal piping. The picture below shows stress corrosion
cracking after eight years in this service. The tubing was bent and
stress cracks started at the outside tensile side of the tube. Flexible
plastic piping is now used less often in this service-especially for
hot water service.
(Courtesy of www.skyinc.net/~kpr)
of stress corrosion cracking are shown in the section on microbial