Effect of pH and Temperature on Corrosion of Nickel-Base Alloys in High Temperature and Pressure Aqueous Solutions

Abstract

The properties of water change significantly across the critical point of water and these changes affect the degradation behavior of constructional alloys. The corrosion behavior of nickel-base alloys is investigated across the critical temperature of water at a constant pressure in aqueous solutions. At subcritical temperatures, the corrosion behavior of alloys of 625 and C-276 is influenced by the pH values of the aqueous solutions. In neutral pH, a thin surface oxide film develops on the substrate. In pH 2, Ni and Fe are selectively dissolved, and Cr and Mo form stable oxides. In pH 1, Cr, Ni, and Fe are dissolved from the substrate and Mo forms a stable oxide. At supercritical temperatures, no dealloying is observed independent of pH and a thin surface oxide film develops. Changes in water properties and thermodynamics explain the dealloying behavior of elements and the highest corrosion rate at high subcritical temperature. Supercritical water is a single-phase fluid when both its pressure and temperature are beyond its critical point P: 22.1 MPa, T: 373.9°C. Supercritical water has different properties from ambient water at 25°C, 1 atm, in terms of density, dielectric constant, and ionic dissociation constant. 1-3 Utilizing the unique features of super-critical water, supercritical water oxidation SCWO technology has been developed to destroy aqueous toxic organic wastes with effi-ciencies up to 99.9999%. The by-products are CO 2 and H 2 O without generating either NO x or SO 2. Even though SCWO is a very effective technology in destroying organic wastes, corrosion of construc-tional materials is a major limiting factor in the further development of this technology. 4-6 In this study, Alloy 625 N06625 and Alloy C-276 N10276 have been tested as the candidate constructional alloys in aqueous solutions of molal pH 1, 2, and 7. Their chemical composition is represented in Table I. Nickel-base alloys are chosen because of their ability to withstand a wide variety of severe operating conditions involving corrosive environments, high temperatures, high stresses, and combinations of these factors. 7 Alloy 625 and C-276 are selected due to their commercial availability and wide application in industries. Alloys are tested with a tubular shape so that they can be exposed to both the chemical environment and the mechanical stress from the system pressure. The molal pH scale is chosen for convenience because molality does not depend on the density of the solution and, hence, does not vary with temperature. Materials and Methods A test facility was developed to test Alloy 625 tubes at a constant pressure at the supercritical and subcritical temperatures in ambient molal pH 2 and 7 aqueous solutions. The system shown in Fig. 1 draws from three feed-carboys, two of which contain deionized DI water, and the third, dilute aqueous hydrochloric acid pH 1.65 under ambient conditions. System pressure was developed by a continuously running two-head piston-type high pressure liquid chromatography feed pump at the system's upstream end and a back-pressure regulator BPR at the downstream end. The system pressure was measured by transducers, and was maintained at approximately 24.1 MPa throughout the system, after a thermal steady state has been achieved. The feed to one pump head was switchable between the HClaq and DI water containers so that a thermal steady state can be achieved without the aggressive species from HClaq solution. The system temperature was developed by means of a preheater coil inside a ceramic heater, and the air temperature inside the ceramic heater, insulated by a ceramic fiber blanket, was set to 660°C. The pressurized DI water stream flowed through the preheater coil to bring the fluid to a supercritical temperature, after which it proceeded to an insulated mixing tee, where it was mixed with the room-temperature acid or with DI water stream from the other pump head. To test the sample in pH 2 HCl solution, the volumetric flow ratio was set to 4.6:1.6 water:acid so that the fluid exits the mixing tee as pH 2 HCl solution, with a volumetric flow rate of 6.2 mL/min. To test the alloy in neutral pH solution, only DI water flowed throughout the system, with a flow rate of 2.3 mL/min, to obtain the highest temperature reading from the first thermocouple. From the mixing tee, the flow entered the Alloy 625 tube, which was instru-mented with five type-K thermocouples starting at the hot end. Each thermocouple was wrapped around and cemented to the tube using a high temperature metallic adhesive for thermal conductivity. These five thermocouples were used to monitor the temperatures of the test samples. After passing through the reaction vessel, the flow proceeded to a shell-and-tube heat exchanger where it was cooled to room temperature. The cooled stream then passed through the BPR and then to the effluent storage. The Alloy 625 tube with dimensions of 9.53 mm o.d., 4.76 mm i.d., and 61.4 cm length was tested in pH 2 HCl solutions. Because this tube served as a reaction vessel to test different nickel-base alloys, it experienced 11 operational chemical, thermal, and mechanical cycles for sample replacement. 8 The temperature of Alloy 625 in Table II is a time-weighted average of the 11 cyclic tests of nickel-base alloys. In each cycle test, only the temperature measured during the acid exposure was considered for the estimation of the average temperature. The Alloy 625 tube with the dimensions 3.18 mm o.d., 1.02 mm i.d., and 58.0 cm length was tested in DI water neutral pH. Alloy C-276 tubes with dimensions of 6.35 mm in o.d. and 4.57 mm in i.d. were prepared to expose the material in different pH solutions at the Center for Advanced Materials in Pennsylvania State University to measure in situ pH and electrochemical potential ECP at 350°C with the constant system pressure of 27.3 MPa. The detailed system configuration is well published in the literature. 9-11 After the ECP measurement, the samples were analyzed to compare the corrosion behavior of nickel-base alloys in different pH solutions. One tube was tested in a neutral pH solution of 0.1 M NaCl for 70 h, and another in an acidic solution of 0.1 M NaCl and 0.1 M HCl for 53 h. After the experiments, the test tubes were disassembled and filled with epoxy resin and hardener to protect the oxide scales before metallographic sample preparation. The samples were cut, cold-mounted using epoxy resin and hardener, and polished. When necessary , the samples were etched to identify the penetration path. The etchant was freshly prepared and consisted of 15 mL of concentrated C194) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75 Downloaded on 2016-03-06 to IP