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47 NACE INTERNATIONAL: VOL. 56, NO. 5 MATERIALS PERFORMANCE MAY 2017 Coating Thickness Material Analysis Microhardness Material Testing cation brine solution such as concentrated seawater, and the change in pH are consid- ered, a test intended to evaluate the effects of 10 MPa of H 2 S based upon ideal chemis- try with Henry 's law can be misleading. A test of dissolved H 2 S content of 0.29 m H 2 S solution in 3% NaCl brine would not accu- rately simulate the conditions for an H 2 S fugacity of <0.17 m H 2 S solution and a brine of triple-strength seawater (Table 1). If the total pressure is increased from 10 to 17 MPa, these values become 0.44 and 0.21, respectively. Finally, if the pressure is increased to move the phase equilibrium into the LLE-like range w here a dense phase forms from the gas (Figure 1), the molality of H 2 S dissolved in the triple- strength brine decreases to 0.28 m instead of th e valu e based on ideal ch emi str y assumptions that produced 0.93 m , an error by a factor of 3.3. An assumption of ideal chemistry could result in a test where the presence of H 2 S in the aqueous phase was less than expected by a factor of 3.3 compared to a test de- signed with the assumption of nonideal chemistry. Conclusions The concepts of nonideal chemistr y, including fugacity and activity coefficients, complex ion chemistry, and the effects of high pressure upon multiphase equilib- rium, are significant for oil and gas produc- tion conditions that involve high pressures, high temperatures, heavy brines, and the presence of supercritical species such as CO 2 , H 2 S, and heavier hydrocarbons in the vapor phase. The design of corrosion tests and corro- sion rate simulations that do not incorpo- rate the concepts of nonideal chemistry can introduce very significant errors. These errors can become expensive when the test results are over- or under-conservative to the point that they lead the corrosion engi- neer to select a material that is not optimal for the production process. References 1 B.E. Poling, J.M. Prausnitz, J.P. O'Connell, The Properties of Gases and Liquids, 5th ed. (New York, NY: McGraw-Hill, 2001). 2 J.F. Zemaitis, Jr., et al., Handbook of Aqueous Electrolyte Thermodynamics (New York, NY: American Institute of Chemical Engineers, 1986). 3 M. Rafal, et al., Models for Electrolyte Solu- tions, in Models for Thermodynamic and Phase Equilibria Calculations, S.I. Sandler, ed. (New York, NY: Marcel Dekker, 1995). 4 A. Anderko, P. Wang, M. Rafal, Fluid Phase Equilibria 194 (2002): pp. 123-142. 5 R.D. Springer, P. Wang, A. Anderko, SPE Jour- nal 20, 5 (2015): pp. 1,120-1,134. 6 D. Koschel, J.-Y. Coxam, V. Majer, Industrial and Engineering Chemistry Research 52, 40 (2013): pp. 14,483-14,491. 7 W. Henry, "Experiments on the Quantity of Gases Absorbed by Water, at Different Tem- peratures, and Under Different Pressures," Phil. Trans. 93 (1803): pp. 29-274. 8 M.S. Gruszkiewicz, et al., J. Solution Chemis- try 36, 6 (2007): pp. 723-765. 9 P. Wang, et al ., Industrial & Engineering Chemistr y Research 52 (2013): pp. 15,968- 15,987. 10 J.L. Nelson, R.V. Reddy, "Selecting Represen- tative Laboratory Test Conditions for Fit- For-Purpose OCTG Material Evaluations," SPE High-Pressure/High-Temperature Sour Well Design Applied Technology Workshop, Society of Petroleum Engineers, 2005. 11 Engineering Data Book, Section 23, "Physical Properties" (Tulsa, OK: Gas Processors Sup- pliers Association, 1994). STEPHEN N. SMITH, FNACE, is a consul- tant based in Pueblo, Colorado, USA, email: stephen.smith.PE@gmail.com. He retired from ExxonMobil after 26 years in research, production, and projects. He has written 40 papers on topics including H 2 S corrosion and cathodic protection of offshore structures and pipelines. He has an M.S. degree in metallurgical engineer- ing, is a NACE International Fellow and Corrosion Specialist, and received a NACE Technical Achievement Award. He has been a member of NACE for 43 years. ANDRE ANDERKO, FNACE, is the manag- ing director of OLI Systems, Inc., Cedar Knolls, New Jersey, USA, email: aanderko@ olisystems.com. His activities focus on thermophysical properties of electrolyte systems, modeling of aqueous corrosion of metals, and management of simulation technology development. He has a Ph.D. and has authored more than 120 papers. A member of NACE since 1996, he was elected NACE Fellow in 2016. PEIMING WANG is a senior scientist at OLI Systems, Inc., email: pwang@olisystems. com. Her research focuses on modeling thermodynamic and transport properties for aqueous and mixed-solvent electrolyte systems. She has authored more than 50 papers in peer-reviewed scientific journals and conference proceedings. She has a Ph.D. in physical chemistry and has been an editorial board member of the Journal of Solution Chemistry since 2007. Effects of Nonideal Chemistry on Corrosion Testing