Materials Performance

JUN 2019

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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44 JUNE 2019 W W W.MATERIALSPERFORMANCE.COM environment, and a susceptible material. Generally, Type 304L austenitic SS is pro- tected from corrosion by a thin passivation layer (chromium oxide) and it does not cor- rode when exposed to a damp atmosphere. However, localized breakdown of the pas- sive layer can occur in the presence of chlo- ride ions, leading to corrosion pitting of the surface, and SCC can be initiated by applied stress (operating pressure) or residual stresses ( from fabrication/welding). In the present case, several possible sources of chloride must be considered: • Seawater spray—Random EDS analy- ses of surface deposits at different locations showed enhanced levels of multiple elements in varying propor- tions, including oxygen, chlorine, sul- fur, calcium, magnesium, sodium, silicon, and titanium. Some of the EDS results showed a combination of sodium, calcium, and chlorine, which provides possible evidence that sea- water spray played a role in the corro- sion of the pipe surface. However, some other EDS results showed large amounts of chlorine with relatively little sodium and calcium (i.e., the amounts of each element do not bal- ance in proportions that would be representative of sodium chloride and calcium chloride). Therefore, there must be another source of chlo- ride, most likely in the form of hydro- gen chloride (HCl). • Cooling water leaks that spray chlo- ride-containing water onto pipe sur- faces—This is unlikely as the damage at the plant is widespread and, as for seawater spray above, the EDS results point to another source of chloride. • Chlorine or hydrogen chloride used in processes at the subject plant or neigh- boring plants—It was observed dur- ing the study that the chemicals are not used in the process at the subject plant nor at the neighboring plant. • Pipe manufacturing process—SS are pickled with nitric acid (and hydro- f luoric acid [HF]) to remove mill scale, etc. Pickling with HCl should be avoided, and the possibility that this has been done in error is unlikely. Metallography showed that there were numerous cracks and the depth of the larg- est crack was >50% of the wall thickness. Rates of Cl-SCC propagation can vary from 0.6 to >30 mm/y. As the sample was only a few mm thick and the cracks have already propagated to mid-wall thickness, the pipe is at risk of leaking soon. Piping that shows similar corrosion damage at the external surface is likely to also be damaged by SCC cracks and is not fit for service, and there- fore should be replaced at the earliest opportunity. Bibliography Alyousif, O.M., and R. Nishimura. "On the Stress Corrosion Cracking and Hydrogen Embrittle- ment Behavior of Austenitic Stainless Steels in Boiling Saturated Magnesium Chloride Solutions." Intl. J. Corrosion 54 (2012): pp. 1-11. FIGURE 4 The locations analyzed using EDS with small pits that can be seen on areas of bare metal. FIGURE 5 Spectrum 1 EDS (noted in Figure 4). MATERIALS SELECTION & DESIGN

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