Development of Corrosion Probe based on Solid State Reference Electrodes 1 Introduction

Transcription

1 Development of Corrosion Probe based on Solid State Reference Electrodes BYUNG GI PARK*, IN HYOUNG RHEE**, DAECHUL CHO** *FNC Technology Co. Ltd., SNU Research Park Innovation Center #421, San 4-1 Bongchun-dong, Gwanak-gu, Seoul , Republic of Korea **Department of Environmental Engineering, Soonchunhyang University, 646 Eupnae-ri Shinchang-myun, Asan-si, Chungnam , Republic of Korea Abstract: - The corrosion probe that is composed of two reference electrodes and one working electrode was designed and fabricated in order to provide early warning for preventing corrosion-related problem such as plugging and clogging in generator water-cooling circuit. A solid state reference electrode (SSRE), as an alternative to standard reference electrodes, was introduced for the fabrication of the corrosion probe. An Ag/AgCl SSRE plays a role of reference electrode for an electrochemical corrosion potential (ECP) of copper and ph-sensing electrode that is Ir 2 O 3 /IrO 2 SSRE. A stable potential was obtained during the stability test of SSREs. A potential of ph-sensing electrode was linearly proportional to ph of water. The fabricated corrosion probe was tested under a simulated environment of generator water-cooling circuit. The results exhibited that the ECP of copper was changed in response of a change of dissolved oxygen concentration. Key-Words: - Corrosion, Corrosion probe, Solid state reference electrode, Electrochemical corrosion potential, Copper corrosion, ph-sensing electrode, Generator water-cooling circuit 1 Introduction Copper corrosion-related problem such as plugging and/or clogging has been frequently reported in the generator stator water-cooling circuits. Plugging and clogging of generator stator water-cooling circuit are caused by a particle release from surface of hollow copper strands. Particulate release as corrosion products is dependent on an electrochemical corrosion potential (ECP) of copper. The ECP change across a critical value triggered a massive release of the particulate corrosion products followed by the plugging of hollow copper strands. Experimental result shows that about 250 mv SHE is a critical potential.[1] A stability of oxide is dependent on both of ph and equilibrium electrode potential according to thermodynamic theory. If the ECP can be approximated to equilibrium electrochemical potential at the stator water-cooling circuits, the potential-ph diagram is useful tool to indicate stability of oxide on copper surface in generator stator water-cooling circuits. Therefore, ECP and ph monitoring can provide early warning for preventing corrosion-related problem such as plugging and clogging in generator water-cooling circuits. Early warning can be achieved with a corrosion probe that has functions of monitoring the ECP of copper and ph of water. The corrosion probe was composed of a reference electrode, a ph-sensing electrode, and a copper electrode. A solid state reference electrode (SSRE), as an alternative of standard reference electrodes, was introduced for the fabrication of the corrosion probe. It has an advantage that SSRE itself does not introduce any significant amount of electrolyte into the monitored system. Ag/AgCl electrode reaction was used to fabricate SSRE. The ph-sensing electrode utilized a mixed oxide electrode reaction that is an electrode reaction between Ir2O3 and IrO2. For corrosion probe, direct electrochemical deposition of iridium oxide that was first introduced by Yamanaka [2] has been applied to fabricate ph-sensing electrode. The SSREs was finally coated with Nafion and thermally cured at 120 C. The Nafion that is cation exchange membrane would be played a role of minimization of the interfering effect of anionic redox species in iridium electrodeposited electrode and of prevention of a leakage of chlorine ion in Ag/AgCl electrode. The corrosion probe was assembled with the fabricated SSREs and copper electrode. It was tested under simulated condition of generator water-cooling circuit.

2 4 2 Experimental Corrosion Probe: The corrosion probe is composed of 4-electrode that are Ag/AgCl reference electrode, Ir 2 O 3 /IrO 4 reference electrode, Pt electrode, and copper. The schematic diagram of the corrosion probe is illustrated at Figure 1. As shown in Figure 1, an integral design of the corrosion probe was adopted for an easy installation into the monitored system. The functions of the corrosion probe were achieved as follows: The ECP of copper is measured against Ag/AgCl reference electrode. A potential difference between Ir 2 O 3 /IrO 2 reference electrode and Ag/AgCl reference electrode indicates ph of water. Pt electrode is a pseudo reference electrode and is used to monitor a health condition of Ag/AgCl reference and Ir 2 O 3 /IrO 2 reference electrode. Each electrode is a type of metal wire wound on a Teflon bobbin. A flow cell was fabricated with stainless steel. The assembled corrosion probe was shown in Figure 2. Ag/AgCl Reference Electrode: Pure silver wire (>99.9%) was used for the fabrication of SSRE. Silver wire was wound on a Teflon bobbin. Silver wire of 5cm from the bobbin was encapsulated with Teflon heat-shrinkable tubing. The silver wire wound on the bobbin was ultrasonically cleaned with acetone and rinsed with deionized water. A layer of silver chloride was formed on the silver wire by applying 2 ma/cm 2 of direct current for 30 min in 1M HCl solution. The appearance of Ag/AgCl electrode was brown. The silver wire wound on the bobbin was thoroughly washed with deionized water to remove chloride ion and stored in a saturated AgCl solution. An immobilized electrolyte was introduced by Nolan.[3] For the purpose of the fabrication of the corrosion probe, the immobilized electrolyte was adopted to Ag/AgCl SSRE. The immobilized electrolyte was freshly prepared by saturating 50 ml of THF (Tetrahydrofuran) with KCl at room temperature and then adding 1.7g of PVC. The solution was stored for 4 day before coating Ag/AgCl wire. The Ag/AgCl wire that is wound on the bobbin was dip-coated in the immobilized electrolyte solution. After several dipping, the Ag/AgCl electrode wound on the bobbin was dried in a desiccator for 2 days to evaporate solvent. After electrode was dried, the Ag/AgCl wire was dip-coated in Nafion 117 solution and cured at 120 C for 1 hour. After curing, the electrode was stored in a desiccator for 24 h. Before potential measurement with the stored Ag/AgCl SSRE, it has to be activated by dipping in deionized water for a 24 hour. Swagelok fitting Sampling water out 50 mm LEMO connector Pt electrode Ag/AgCl electrode Ir2O3/IrO4 electrode Sampling water in Flow chamber Figure 1. Schematics of the corrosion probe. Figure 2. Photo of the corrosion probe. ph-sensing electrode: Marzouk introduced the iridium coating on titanium substrate.[4] It was easier procedure than others and greatly enhanced the extent of adhesion to the metal surface. For the purpose of the fabrication of the corrosion probe, iridium oxide was coated on the titanium wire. Titanium wire was polished with fine polishing paper and then rinsed thoroughly with deionized water. Titanium wire was

3 wound on a Teflon bobbin. The titanium wire wound on the bobbin was ultrasonically cleaned with acetone and rinsed with deionized water. The surface of titanium wire was roughened by etching in 70% sulfuric acid at 80 C for 2 min. After etching, the bobbin with titanium wire was thoroughly rinsed with deionized water. Next, iridium oxide layer was electrodeposited by applying 2 ma/cm 2 of direct current for 3 min in the deposition solution and then rinsed with deionized water. The deposition solution was produced according to Marzouk procedures.[4] The solution was prepared by dissolving 75 mg of IrCl 4 H 2 O in 50 ml of deionized water, 0.5 ml of 30% H 2 O 2 was added, and then 365 mg of potassium oxalate hydrate was added. The solution was stirred for ~10 min between each step. The ph of solution was raised slowly to ph 10.5 by addition of anhydrous potassium carbonate and then was stored at refrigerator.[4] The appearance of the electrodeposited iridium oxide film was dark greenish-blue. After rinsing, the bobbin was dried in a desiccator for 2 days. After electrode was dried, titanium wire was dip-coated in Nafion 117 solution, cured at 120 C for 1 hour, and stored in a desiccator. For ph measurement with the stored iridium oxide electrode, it has to be activated by dipping in deionized water for a 24 hour. 3 Results and Discussion The stability of the potential of SSREs was investigated for 180 hours in deionized water. The potential was measured against conventional reference electrode (ORION Ag/AgCl reference electrode) under room temperature. Figure 3 exhibits the stability of potential for the fabricated SSREs that are two Ag/AgCl SSREs, an iridium oxide electrode, and platinum electrode. Two Ag/AgCl SSREs and an iridium oxide electrode were tested. Ag/AgCl SSREs exhibit potentials of about 105 mv and about 145 mv, respectively. The potential difference between Ag/AgCl SSREs was about 35 mv. A cause of potential difference may be resulted from the difference of an activity of chlorine ion in the immobilized electrolyte. It may be developed by the dipping process for coating the immobilized electrolytes on the AgCl-coated Ag wire wound the bobbin. Therefore, it is necessary to calibrate the fabricated Ag/AgCl SSREs, respectively. Iridium oxide electrode that is the ph-sensing electrode maintained at about 75 mv for 180 hours as shown in Figure 3. A potential perturbation during test was observed as shown in Figure 3. The stability of electrode potential was tested under an exposed condition at room temperature. The behavior of the perturbed potential may be resulted from the daily temperature excursion. A temperature dependence of the fabricated Ag/AgCl SSRE was determined with a cell of 1M KCl solution. The potential of the Ag/AgCl SSRE was measured against a bare AgCl-coated Ag rod in 1M KCl solution under temperature range of 20~80 C. The temperature dependence of the potential is shown in Figure 4. The measured potential was converted to the scale of standard hydrogen electrode. The temperature dependence of the fabricated Ag/AgCl SSRE was obtained by fitting the experimental data as follows; E ( mv SHE) = T T + 10 T where T is temperature (degree Celsius). This correlation can be only applied for this electrode under temperature range of 20~80 C. There is a small discrepancy between the fabricated Ag/AgCl SSREs. For accurate measurement with the fabricated SSRE, it has to be calibrated with calibration cell. A ph-sensing characteristic of iridium oxide coated electrode was tested by addition of HCl and NaOH to universal buffer solution. The potential of ph-sensing electrode was measured against Ag/AgCl SSRE. As shown in Figure 5, the potential of the electrode was linearly proportional to ph change. The super-nernstian slope of mv/ph was observed. This behavior of the electrode was reported for hydrous iridium oxide films prepared by anodic electrodeposition.[5] Figure 3. Stability of the fabricated SSREs.

4 surface of Cu electrode might be altered to satisfy an oxidizing condition in test environment. The change of surface condition of Cu electrode results in the decrease of the ECP of copper. Therefore, the corrosion probe should be conditioned for water chemistry environment during about 2~3 days before monitoring the ECP of copper and ph of water. Figure 4. Temperature dependence of the fabricated Ag/AgCl SSRE Figure 6. Influence of dissolved oxygen and dissolved hydrogen on the ECP of copper measured with the corrosion probe test under a simulated condition of generator water-cooling circuit. Water temperature at probe position was measured at about 50 C. Figure 5. Potential response of the fabricated ph-sensing electrode (Ir 2 O 3 /IrO 2 ) The assembled corrosion probe was tested under a simulation condition of the generator water-cooling circuit. Water temperature at the position of the corrosion probe was measured about 50 C. Figure 6 exhibits the behavior of the ECP of copper. A dissolved oxygen (DO) and dissolved hydrogen (DH) was changed for test. It was observed that the ECP of copper was related to a change of DO and DH.[1] In the test of corrosion probe, it was confirmed that the ECP of copper was changed in response to the change of DO and DH as shown in Figure 6. However, at the early stage of test, the ECP of copper was decreased even if the DO was maintained at about 4 ppm, as shown in Figure 6. During this stage, an oxide on the 4 Conclusion The corrosion probe based on the SSREs was developed and tested under the simulated condition of generator water-cooling circuit. Adopting the SSREs as an alternative standard reference electrode, the corrosion probe was integrally designed and could be easily installed into the monitored system. The ECP of copper measured with the corrosion probe was changed in response to the change of DO and DH. Therefore the corrosion probe with SSREs is useful to provide early warning for preventing corrosion-related problem such as plugging and clogging in generator water-cooling circuit. References: [1] B.G. Park, et al., Effect of Electrochemical Corrosion Potential of Copper on Plugging of Generator Water-cooling Circuit, NACE Corrosion, 61, 6, [2] K. Yamanaka, Anodically electrodeposited iridium oxide films from alkaline solutions for electrochromic display devices, Jpn. J. Appl. Phys., 28, , 1989.

5 [3] M.A. Nolan, et al., Fabrication and Characterization of a solid State Reference Electrode for Electroanalysis of Natural Waters with Ultramicroelectrodes, Anal. Chem. 69, pp , [4] S.A.M. Marzouk, Improved Electrodeposited Iridium Oxide ph Sensor Fabricated on Etched Titanium Substrates, Anal. Chem., 75, pp , [5] L.D. Burke, et al., Preparation of an oxidized iridium electrode and the variation of its potential with ph, J. Electroanal. Chem., 163, pp , 1984.