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Nitrate Stress Corrosion Cracking of Process Piping

Background: A client contacted KnightHawk Materials Lab with an urgent need for a field evaluation and failure analysis of as a brand new installation, in service less than 2 months, that exhibited extreme widespread cracking of the welds, which caused a forced outage (shown in Figure 1).

Figure 1: Photograph of Multiple Cracks on a Girth Weld with Higher Magnification Inset

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Figure 2: Optical Micrographs of Intergranular Fracture of the Carbon Steel in the As-Polished (Left) and Etched (Right) Conditions

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Summary: The severe cracking of the welds throughout the installation was caused by nitrate stress corrosion cracking (SCC), as revealed by the intergranular nature of the fractures (Figure 1) and the orientation of the cracks transverse to the weld direction, both characteristic of nitrate SCC. The piping carried a process gas very high in NOx and H2O vapor. When the gas condensed onto the tubing, it formed nitric acid which led to nitrate SCC of the piping driven by the residual stresses in the welds.

Take Away: NOx fumes, if allowed to condense and combine with water, are extremely damaging for carbon steel installations, particularly those with welds that have not undergone a post weld heat treatment.