A crack was found in 2020 on a 324 mm (12-inch) OD above ground pipeline externally uncoated but insulated with mineral wool with aluminum cladding. The steel grade of the pipeline is CSA Z245.1 Gr 414 Category II with wall thickness of 7.9 mm (0.310-inch) operating since 2013. The product that the pipeline transports is casing gas and the pipeline is operated at a temperature of 150–200 °C. The circumferential crack initiated on the outer diameter (OD) and grew through-wall. Detailed fractographic analysis showed a mixed mode of fracture: predominantly intergranular separation with a small amount of cleavage corresponding to {001} lattice plane, which is confirmed by mated fracture surfaces and the presence of {001} etching pits. Striation-like features were found on the fracture surface; however, these striation-like features are not due to fatigue. Metallographic cross section analysis also showed the crack paths are mainly intergranular with some transgranular that is consistent with micro-fractographic analysis. Classic high-pH stress corrosion cracking (SCC) mechanism cannot explain the presence of transgranular cracking, and other evidence observed such as the lack of corrosion products at the crack tip, lack of corrosion damage on the grain boundary and the lack of SCC colony on the OD surface of the pipe. These findings led to the potential role of hydrogen in the cracking mechanism observed in the above ground insulated pipeline.


Fracture surface morphology is useful in determining the mechanisms involved in cracking. Features in the fracture surface of steel like micro-void coalescence or dimples are often related to ductile crack mode, while features like cleavage, and rock-candy showing grain boundary cracking are commonly related to brittle crack mode. Some fracture surface may have a predominant feature, while others may have presence of mixed features. Types of stress corrosion cracking (SCC) are commonly identified by the features present in the fracture surface. Features showing transgranular quasi-cleavage cracking are often related to near-neutral pH SCC, while features that show rock-candy intergranular cracking are often related to high-pH SCC [1]. Fracture surface morphology provides clues related to the history of the failed specimens, and prevalently used by investigators to identify failure mechanism. Therefore, careful evaluation of these fracture surface features is essential in identifying the correct failure mechanism. However, tiny details present in the fracture surface are most of the times overlooked.

This content is only available via PDF.
You can access this article if you purchase or spend a download.