Stress Corrosion Cracking

Soil-Side Chloride-Induced Stress Corrosion Cracking Under Insulation

A length of TP 304 stainless steel pipe in service as a high pressure steam condensate return in an outdoor underground environment (beneath a sidewalk along a roadway) suffered cracking near its welded ends, while the main body showed no visual signs of failure.  The client reported that the system pipes were provided with factory-installed insulating jackets except for one foot of length at each end to permit welding.  After welding, additional insulating jackets were installed in the field to protect the joints.  The location and appearance of the cracks in the pipe specimen indicated that the seam between the factory-installed and the field-installed jackets played a role in this failure.

Stress Corrosion Cracking Under Insulation
Figure 1.  A length of TP 304 pipe suffered cracking in service as a high-pressure steam condensate return.  Left: the as-received specimen, weld at far end.  Right: after forcing open a main longitudinal crack.

Bulk alloy analysis confirmed that the pipe alloy composition met the specification for stainless steel 304.  Using scanning electron microscopy/energy dispersive x-ray spectroscopy (SEM/EDX) analysis chlorine, likely as chlorides) was found mixed with corrosion products in sometimes high concentrations.  Extensive wide shallow pitting was noted on the exterior surfaces near the weld, in the area formerly covered by the field-applied insulation jacket, and some distance under the factory-installed jacket.  Multiple large cracks, both radial and longitudinal, were visible at the exterior in the area formerly covered by the factory-applied insulation jacket, near the seam with the field-applied jacket.  The wide shallow pitting indicated moisture penetration with dissolved corrosives on the soil-side under the factory-installed insulation jacket.  In addition to the visible cracks, inspection of prepared samples removed from the pipe specimen revealed a vast number of non-penetrating transgranular cracks extending from the soil-side inward.  Such cracks were found in abundance at any location examined within roughly two feet of the welded end.

Figure 2.  Upper left: Exterior surface wide shallow pitting along the main longitudinal crack.  Evidence of chlorides was found in material harvested from the damaged exterior surface (see Figure 3).  Upper right: Interior surface with penetration pits and corrosion along the main longitudinal crack.  Lower left: The main longitudinal crack face was corroded in stages suggesting penetration from the outside inward.  Lower right: A longitudinal sawcut showed numerous feathery stress corrosion cracks emanating from the exterior surface inward through the pipe wall.

The combined observations and analytical results presented below strongly suggest chloride-induced stress corrosion cracking as the main failure mechanism, initiating at the exterior surfaces in the vicinity of the welds and under the insulation near the seam between the factory-installed and field-installed jackets.

Figure 3.  Example SEM image and EDX spectrum of swabbed material collected from the pipe exterior surface.  Note the presence of alloy corrosion products (Fe, Cr, Ni), and additional elements including Cl.  Chlorides, for example from road salts, can accelerate corrosion of stainless steel and promote localized attack.
Figure 4.  Mounted, polished, and etched surfaces of cracked (left) and healthy (right) metal samples are shown.  Microstructure is as expected for an austenitic stainless steel 304 in both cases.  The crack path, especially of the finer cracks, was primarily transgranular rather than intergranular.  In combination with other results this favors stress corrosion cracking over other possible mechanisms such as sensitization, which can occur near welds in the presence of chlorides but is primarily intergranular in its crack propagation.