Cracks in the pipe

The cracks in the austenitic stainless steel sample on the picture below are most likely a case of Stress Corrosion Cracking (SCC). SCC is the cracking induced from the combined influence of tensile stress and a corrosive environment (such as chlorides). The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses. The problem itself can be quite complex.

An austenitic stainless steel pipe sample prepared

for microscopic examination

Usually, most of the surface remains unattacked, but with fine cracks penetrating into the material. In the microstructure, these cracks can have an intergranular or a transgranular morphology. Macroscopically, SCC fractures have a brittle appearance. SCC is classified as a catastrophic form of corrosion, as the detection of such fine cracks can be very difficult and the damage cannot easily be predicted. 

SCC occurs typically in austenitic stainless steels. In nickel containing alloys with a nickel content higher than about 45%, SCC does not occur.

With thanks to: Gerard Janssen and Rijk Koster, mechanical engineers