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Continued f rom page 29 Photographs of the disk specimens after testing for four weeks before cleaning (top row) and after cleaning (bottom row): (a, d) CS, (b, e) UNS S31603, and (c, f) UNS S31803. Photos courtesy of Sigrún Nanna Karlsdóttir. An image of the surface of the UNS S235JR CS specimen after four weeks of testing, taken with an optical microscope equipped with a digital camera. Large cracks and blisters are clearly visible. Photo courtesy of Sigrún Nanna Karlsdóttir. where the H 2 S concentration is midway between the lowest and highest concentra- tions, to get test results that represented an average for the tower, says Karlsdóttir. After four and 12 weeks of testing, the specimens were removed, cleaned, weighed, and eval- uated for corrosion. For each material, the microstructural and chemical composition of a specimen's surface and cross-section were analyzed with scanning electron microscopy (SEM) and x-ray energy disper- sive spectroscopy (EDS). The corrosion rates were calculated per ASTM G1-03 (2011). 6 The UNS S31603 and S31803 SS samples held up quite well in this experiment. "They performed as expected," Karlsdóttir com- ments. The corrosion rates were negligible and no corrosion damage was detected in the microstructural analysis of the cross- section of either the UNS S31603 or the S31803 SS samples after four and 12 weeks of testing. The UNS S31603 U-bend speci- mens did not experience SCC, and cracking or pitting was not detected in the tested specimens. The results of this test indicate that S31603 is a sufficient material choice for the absorption tower. "Because of the low temperature, UNS S31603 is a pretty safe selection for this system," Karlsdóttir says, adding that a longer testing time is advised to support that conclusion. She notes that SCC would be more likely to occur with UNS S31603 if the temperature was higher. In the UNS S30403 SS samples, stress corrosion cracks were starting to form. Since this SS grade does not contain Mo, Karlsdóttir explains, it is more susceptible to localized corrosion such as SCC and pit- ting when exposed to H 2 S. The UNS S235JR CS samples exhibited poor performance during the test, with a significant amount of corrosion damage and a high corrosion rate—4.2 mm/y and 3.0 mm/y for four and 12 weeks respec- tively, which is considerably higher than the acceptable limit of 0.1 mm/y. Although Karlsdóttir expected the CS to corrode, she was surprised at the degree of blistering and HIC that was observed— particularly for the four-week test, a rela- tively short testing time in the absorption tower. The researchers acknowledge that wet environments containing H 2 S and/or CO 2 along with oxygen contamination can be very aggressive to carbon and low- alloyed steels. Internal blistering, HIC, and stress oriented hydrogen-induced cracking (SOHIC) of CS in the presence of very high H 2 S concentrations have been associated with wet H 2 S environments. The study confirmed that the selection of UNS S31603 for the absorption tower was a good material pick, Karlsdóttir com- ments. "It verified that it was the right deci- sion not to go with a lower-grade material like UNS S30403 because cracking was detected, indicating its susceptibility to SCC in the system," she says. The research also shed light on what happened to the CS components in the first pilot plant: the availability and aggressiveness of the H 2 S along with the presence of oxygen caused the corrosion damage. "The environment really requires the use of UNS S31603," Karlsdóttir concludes. "It is not expensive compared to higher grades of corrosion- resistant steels. It was a good choice." More information on the study can be found in the CORROSION 2016 paper, "Cor- rosion Testing in H 2 S Abatement System at Hellisheidi Geothermal Power Plant in Ice- land," by S.N. Karlsdóttir, S.M. Hjaltason, and K.R. Ragnarsdottir. References 1 "Basic facts about geothermal and renewable energy in Iceland," Geothermal Energy, ON Power, http://www.onpower.is/geothermal- energy (February 16, 2017). 2 "Tackling the Challenge of H 2 S Emissions," ON Power, https://www.on.is/sites/on.is/files/ on_h2s_presentation-australia-new.pdf (February 16, 2017). 3 "The CarbFix Project," Orkuveita Reykjavíkur (Reykjavík Energy), https://www.or.is/ english/carbfix-project (February 16, 2017). 4 S.N. Karlsdóttir, S.M. Hjaltason, K.R. Ragnarsdottir, "Corrosion Testing in H 2 S Abatement System at Hellisheidi Geothermal Power Plant in Iceland," CORROSION 2016, paper no. 7568 (Houston, TX: NACE Interna- tional, 2016). 5 ASTM G30-97 (2016), "Standard Practice for Making and Using U-Bend Stress-Corrosion Test Specimens (West Conshohocken, PA: ASTM, 2016). 6 ASTM G1-03 (2011), "Standard Practice for Preparing, Cleaning, and Evaluating Corro- sion Test Specimens" (West Conshohocken, PA: ASTM, 2011). 30 MARCH 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 3 FEATURE ARTICLE