Materials Performance

NOV 2018

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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24 NOVEMBER 2018 W W W.MATERIALSPERFORMANCE.COM FEATURE ARTICLE FIGURE 1 None of the post-test stereomicroscope images of the apex of the UNS R53400 and UNS N06625 U-bend specimens at 20X magnification showed obvious pitting. the world's highest-temperature resources for geothermal energy. 2 The temperature of the hypersaline brine associated with the field is typically >250 °C, which facilitates a very efficient flashed steam system. Accord- ing to the U.S. Department of Energy, the Salton Sea KGRA is home to the Imperial Valley Geothermal project, which consists of 10 generating plants with a combined net capacity of ~327 MW. 3 MacDonald and Grauman comment that in the Salton Sea KGRA, corrosion of casing and equipment fabricated of conven- tional alloys was unacceptably high and economically intolerable due to the high chloride content, low pH, and corrosive non-condensable gases contained in the brine. Because exposing possible corrosion- resistant alloy casing materials to actual operating environments is challenging and expensive, end users have relied on labora- tory experiments to prequalify materials. Since the early 1990s, titanium has been used in the Salton Sea KGRA to control the excessive corrosion experienced in produc- tion wells. Early corrosion testing in the geothermal brine, however, showed that unalloyed titanium was prone to localized attack under the most severe conditions in these brines. Some of the more corrosion- resistant titanium alloys, however, appeared to resist any type of corrosion attack. MacDonald and Grauman note that these results correlate well with other chloride media testing that show unalloyed titanium can be susceptible to crevice corrosion attack in seawater and other concentrated brines at temperatures exceeding 80 to 90 °C, but more corrosion-resistant titanium alloys can withstand corrosion attack in these same brines at temperatures greater than 315 °C. Over the last 30-plus years, corrosion testing of more than 10 different titanium alloys in actual and simulated hypersaline geothermal brines has tracked mass loss, crevice, pitting, and stress corrosion crack- ing behavior of these materials. Test condi- tions included brines with a composition typical of the Salton Sea geothermal brine— pH usually adjusted to create an acidic media in the range of 4 to 6; the presence of chloride (140,000 ppm), sodium (53,000 ppm), calcium (28,800 ppm), and potassium (16,500 ppm); partial pressures of corrosive gases such as CO 2 and H 2 S; and tempera- tures up to 304 °C. The outcome of these geothermal exposure tests, both in the field and in numerous laboratory exposures, showed that titanium always ranked at or near the top in material exposure performance. In their paper presented at CORROSION 2018, 1 MacDonald and Grauman discuss laboratory corrosion testing of UNS R53400 and UNS R56404 titanium alloys, as well as UNS N06625 nickel alloy, in a simulated hypersaline geothermal system such as the one found in the Salton Sea KGRA. The corrosion testing was performed at a commercial laboratory and used speci- mens supplied by MacDonald and Grau- man. The laboratory machined the speci- mens to unloaded dimensions of 100 by 9 by 3 mm and then loaded them according to ASTM G30 4 by bending them 180 degrees into U-bends with UNS N10276 nickel alloy bolts. Ceramic shoulder washers were used TABLE 1. SIMULATED GEOTHERMAL BRINE COMPOSITION Temperature Phase Component Amount Duration 304 °C Gas CO 2 205 psi a 30 days H 2 S 16.6 psi a N 2 15.9 psi a Water vapor 776.4 psi a Liquid NaCl 4 molar (233.8 g/L) KCl 0.45 molar (33.5 g/L) CaCl 2 ·4H 2 O 20,000 ppm Ca 2+ (73.4 g/L) MnCl 2 ·4H 2 O 2,000 ppm Mn 2+ (7.2 g/L) FeCl 2 ·4H 2 O 2,000 ppm Fe 2+ (7.1 g/L) NaHCO 3 3.01 g/L Total Cl – 198,270 ppm Cl – pH 5.51 @ RT 4.41 at 580 °F predicted

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