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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25 MATERIALS PERFORMANCE: VOL. 57, NO. 11 NOVEMBER 2018 Testing Corrosion-Resistant Alloys for Use in Geothermal Power Plants FIGURE 2 The UNS N06625 nickel alloy sample's U-bend apex after additional stressing, shown at 10X, 20X, and 40X magnification. FIGURE 3 The UNS R53400 titanium alloy sample's U-bend apex after additional stressing, shown at 10X, 20X, and 40X magnification. to electrically isolate the bolts from the specimens. MacDonald and Grauman pro- vided the desired gas and liquid composi- tions via stream reports, and this informa- tion was used to develop the test brine's desired gas and liquid compositions (see Table 1). A thermodynamic modeling soft- ware package was used to predict the con- centrations of H 2 S and CO 2 required to achieve the target partial pressures and pH at the test temperature (304 °C). The brine was mixed at ambient condi- tions and then bubbled with CO 2 . MacDon- ald and Grauman note that the solution turned brown as it was mixed, which was most likely caused by oxidation of iron ions from the ferrous chloride (FeCl 2 ). The deaer- ated solution was added to a deaerated autoclave containing the stressed speci- mens (duplicate specimens of each alloy material were tested in the same vessel), and the H 2 S and CO 2 were added separately by liquid loading. The vessel was then sealed and the temperature increased. The 30-day exposure period began when the temperature reached 304 °C. After testing, the specimens were cleaned in an ultrasonic bath with a warm phosphate detergent solution followed by a warmed and inhib- ited hydrochloric acid (HCl) solution and light scrubbing with a pig-bristle brush. Adherent black crystals remained on the UNS R53400 titanium alloy. Preliminary examination of all samples at 20X magnification (per ASTM G30) showed no obvious cracking or pitting on the apex of the U-bend (Figure 1). A thor- ough examination of the inside and outside diameter of each sample was completed at 20X to 40X magnification. Small shiny areas noted on the outside diameter of Sample 1 of the UNS N06625 nickel alloy were selected for further analysis using a high- magnification, three-dimensional micro- scope. The analysis confirmed these shiny areas were shallow corrosion pits on the surface of the sample. Shiny spots also were present on the inside diameter of the sam- ple, but could not be further examined or photographed without sectioning the sample. A relatively larger and deeper shiny area on Sample 2 of the UNS N06625 nickel alloy also was a major area of interest. This spot was located in a crevice covered by the ceramic washer. No grind marks were visi- ble in the affected area when it was ana- lyzed with the high-magnification micro- scope, which confirmed that the spot was crevice corrosion. While no shiny corrosion spots were observed on the surfaces of the UNS R53400 titanium alloy U-bend samples, several small, darkened areas were seen beneath the ceramic washer. Clearly visible, uninter- rupted grind marks were seen in this area, and light scratching removed the dark sur- face film to reveal unaffected metal under- neath. The film appeared to be a surface deposit, but was not attributed to any type of corrosion. Titanium corrosion deposits, which are known to be extremely tenacious, would not be as easily removed as this deposit. As a final check for crack susceptibility, U-bend samples of the UNS N06625 and UNS R53400 alloys were slowly stressed to increase the bend tension by 50% (i.e., the distance between the ends of the samples was reduced from 36 mm to 18 mm) to determine if any evidence of crack initiation could be observed on the apex of the sample surfaces. Analysis of the surface of both the UNS N06625 and UNS R53400 samples after the 50% bend increase showed no cracking. Figures 2 and 3 show the apex of the UNS N06625 and R53400 at different magnifica- tions, respectively. From the experiment, MacDonald and Grauman summarized that the initial sur- face examination showed no cracks on the UNS N06625 nickel alloy and UNS R53400 titanium alloy samples after exposure to the high-temperature brine, but did reveal small surface features for each. Applying further stress on U-bend samples did not lead to any type of crack on any of the alloy samples. According to MacDonald and Grau- man, titanium's resistance to hot reducing chloride environments makes it an appro- priate material for use in geothermal ser- vice, and they foresee the continued use of titanium as the Salton Sea KGRA is further utilized as a geothermal energy source. Future use of titanium also could possibly be seen in low-chloride geothermal steam and water service where corrosion occurs from condensed acid or non-condensable acid gases. Titanium is also being considered and tested for use in more typical steam gener- ating wells where HCl condensation has proven to be a corrosion issue, as well as enhanced geothermal systems where recir- culated water from surface-injected fluid becomes increasingly more corrosive over time. Bibliography Office of Energy Efficiency & Renewable Energy. "Geothermal Basics." https://www.energy. gov/eere/geothermal/geothermal-basics. October 12, 2018. References 1 W.D. MacDonald, J.S. Grauman, "Exposure Testing of UNS R53400, R56404 and N06625 in Simulated Salton Sea Geothermal Brine," CORROSION 2018 paper no. 11547 (Houston, TX: NACE International, 2018). 2 Office of Energy Efficiency & Renewable Energy, "Salton Sea Power Plant Recognized as Most Innovative Geothermal Project," https://www.energy.gov/eere/geothermal/ articles/salton-sea-power-plant-recognized- most-innovative-geothermal-project (Octo- ber 12, 2018). 3 Office of Energy Efficiency & Renewable Energy, "Imperial Valley Geothermal Area," https://www.energy.gov/eere/geothermal/ imperial-valley-geothermal-area (October 12, 2018). 4 ASTM G30-97(2016), "Standard Practice for Making and Using U-Bend Stress-Corrosion Test Specimens" (West Conshohocken, PA: ASTM, 2016).

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