Materials Performance

MAR 2017

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.

Issue link: http://mp.epubxp.com/i/792600

Contents of this Issue

Navigation

Page 31 of 80

Continued f rom page 26 Continued on page 30 The gas separation system with the absorption tower. Photo courtesy of Sigrún Nanna Karlsdóttir. The specimen holder with three disk coupons and one U-bend coupon for each material. Photo courtesy of Sigrún Nanna Karlsdóttir. When H 2 S and CO 2 gases are dissolved in geothermal fluid, the corrosive nature of the gases and other process factors, such as the pressure, flow rate, and pH level of the liquid containing the dissolved gases, can cause corrosion in geothermal equipment and wells that may lead to failure. In addi- tion to general and localized corrosion, H 2 S also can cause hydrogen-induced cracking (HIC), stress corrosion cracking (SCC), and sulfide stress corrosion cracking (SSCC). Karlsdóttir comments that the gas separa- tion station in the initial pilot plant had considerable uniform and pitting corrosion problems that stemmed from the construc- tion material used—carbon steel (CS). Because the gas separation station is a cru- cial component of the abatement process, it was extremely important that the gas separation station in the larger-scale plant would perform without major corrosion issues. This meant that a more suitable material needed to be selected for the plant's construction. Materials Selection The material chosen for the construc- tion of the absorption tower and piping in the larger-scale gas separator was austen- itic stainless steel (SS) (UNS S31603). Karls- dóttir comments that austenitic SS is the most widely used class among SS. Austen- itic steels contain Cr and Ni, which render a compact, passive surface film that resists corrosion in corrosive environments. UNS S31603 also contains 2 to 3% Mo, which makes it more resistant to pitting corro- sion. It also contains a lower amount of C (<0.03%) to decrease the risk of intergranu- lar attack in the as-welded condition or in short periods of high-temperature exposure ( from 788 to 1,490 °F [427 to 816 °C]). Austenitic SS is commonly used in the oil and gas industry where H 2 S is present, Karlsdóttir says. She notes that some research has been done on SCC and SSCC of UNS S31603 in environments that con- tain H 2 S, CO 2 , and chlorides, and both H 2 S and chlorides were found to promote and accelerate general corrosion and SCC. UNS S31603 in geothermal environments was also studied, and pitting corrosion was observed in laboratory experiments in geo- thermal brine at 140 °F (60 °C) with a pH of 4.7, and SCC was also reported at 140 °F. The temperature in the abatement system's larger-scale gas separation station, how- ever, is considerably lower at ~20 °C, and chloride ions are not expected to be in the environment. However, oxygen is present, which can accelerate corrosion in a geo- thermal environment. Corrosion Study To evaluate the corrosion resistance of the absorption tower, which is in contact with high concentrations of H 2 S, Karlsdóttir and her colleagues at the University of Ice- land and the Innovation Center Iceland conducted a study 4 on the corrosion behav- ior of UNS S31603 while being exposed to the H 2 S cleaning process in the absorption tower. Karlsdóttir was the chief investigator of the corrosion testing in the tower. The project investigated the steel's resistance to uniform and pitting corrosion as well as its susceptibility to SCC. Other materials—CS (UNS S235JR), austenitic SS (UNS S30403), and duplex SS (UNS S31803)—were also investigated for comparison. The round corrosion coupons used for testing, 45 mm in diameter with an 11-mm hole in the center, were cut from 3-mm thick plates of UNS S235JR, UNS S30403, UNS S31603, and UNS S31803. Before test- ing, the specimens were cleaned and weighed. Three coupons for each material tested were threaded onto a specimen holder, with a 15-mm space between them and separated with a plastic material (poly- oxymethylene). To study the likelihood of UNS S31603 cracking in the high H 2 S con- centration, the ASTM G30-97(2016) 5 U-bend test method was used. Test speci- mens, 90-mm wide by 15-mm long by 3-mm thick, were cut from UNS S31603 plate, with the longitudinal direction parallel to the rolling direction of the steel plate. All speci- mens were bent around a predetermined radius of 15 mm with a custom-made U-bend machine capable of providing single-stage stressing. The specimen holder was inserted in a valve located near the middle of the tower, 29 NACE INTERNATIONAL: VOL. 56, NO. 3 MATERIALS PERFORMANCE MARCH 2017 Testing Materials for Corrosion in an Icelandic Geothermal Environment

Articles in this issue

Links on this page

Archives of this issue

view archives of Materials Performance - MAR 2017