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Metallurgical investigation of premature failures for 316L austenitic stainless steel pipes

University of British Columbia

This thesis analyzed and investigated the premature failures of pipes made from type 316L austenitic stainless steels. Multiple leaks were observed in scattered locations of a piping network of around 10 km after only 4 months in service transferring ammonium sulfate solution. The initial investigation indicated that the piping network was constructed 3 years earlier. After the construction, the stainless steel pipes were hydrotested to ensure the joints integrity. However, the piping network was not properly drained and dried after the hydrotest which resulted in water stagnation for the complete idle period between construction and commissioning. Therefore, an electrochemical, chemical, mechanical and metallurgical testing and analyses were conducted to determine the damage mechanism which consequently caused these failures. I have conducted electrochemical tests on a 316L stainless steel electrode in chloridized ammonium sulfate solution to determine its corrosivity. The electrochemical tests showed that the corrosion rates of 316L SS in ammonium sulfate solution is very low. This conclusion was supported by other laboratory studies at higher temperature and by the industrial corrosion tables published online. Also, two spools from the piping network that experienced the failures were analyzed using stereoscope, optical microscope, scanning electron microscopy/energy dispersive spectrometry, X-ray fluorescence and carbon/sulfur analyzer, tensile testing and microhardness testing. The results of these tests indicated that the pipes were leaking at the 6 O’clock position near the weld and heat affected zone areas. The morphology of the attack illustrated a narrow opening with large sub-surface cavity and tunneling initiated from the internal surface of the pipes. The weld joints displayed weld defects in terms of root concavity and lack of penetration. The metallurgical investigation strongly suggests that the pipes failed due to Microbiologically Influenced Corrosion (MIC). During the idle period of 3 years, the stagnant untreated water in the closed system was an appropriate environment for bacterial growth leading to severe damage at the welding joints and the base metal.

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2018-05-09

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/65818

Materials Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/