OA18800A - A reference electrode for electrochemical measurements at high temperatures. - Google Patents
A reference electrode for electrochemical measurements at high temperatures. Download PDFInfo
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- OA18800A OA18800A OA1201800203 OA18800A OA 18800 A OA18800 A OA 18800A OA 1201800203 OA1201800203 OA 1201800203 OA 18800 A OA18800 A OA 18800A
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- reference electrode
- sait
- electrolyte
- electrode
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- 238000002848 electrochemical method Methods 0.000 title description 5
- 239000010453 quartz Substances 0.000 claims abstract description 29
- 229910052904 quartz Inorganic materials 0.000 claims abstract description 29
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N silicon dioxide Inorganic materials O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims abstract description 29
- WCUXLLCKKVVCTQ-UHFFFAOYSA-M potassium chloride Chemical compound [Cl-].[K+] WCUXLLCKKVVCTQ-UHFFFAOYSA-M 0.000 claims abstract description 22
- 239000000919 ceramic Substances 0.000 claims abstract description 12
- HKZLPVFGJNLROG-UHFFFAOYSA-M Silver chloride Chemical compound [Cl-].[Ag+] HKZLPVFGJNLROG-UHFFFAOYSA-M 0.000 claims abstract description 11
- 229910052709 silver Inorganic materials 0.000 claims abstract description 9
- 239000004332 silver Substances 0.000 claims abstract description 9
- 239000003792 electrolyte Substances 0.000 claims abstract 10
- 238000007789 sealing Methods 0.000 claims abstract 7
- MCMNRKCIXSYSNV-UHFFFAOYSA-N ZrO2 Chemical compound O=[Zr]=O MCMNRKCIXSYSNV-UHFFFAOYSA-N 0.000 claims description 14
- PNEYBMLMFCGWSK-UHFFFAOYSA-N AI2O3 Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 claims description 9
- 239000001103 potassium chloride Substances 0.000 claims description 9
- 235000011164 potassium chloride Nutrition 0.000 claims description 9
- VEXZGXHMUGYJMC-UHFFFAOYSA-M chloride anion Chemical compound [Cl-] VEXZGXHMUGYJMC-UHFFFAOYSA-M 0.000 claims description 2
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 claims 1
- 229910052721 tungsten Inorganic materials 0.000 claims 1
- 239000010937 tungsten Substances 0.000 claims 1
- BQCADISMDOOEFD-UHFFFAOYSA-N silver Chemical compound [Ag] BQCADISMDOOEFD-UHFFFAOYSA-N 0.000 abstract description 8
- 239000000203 mixture Substances 0.000 abstract description 6
- 150000002500 ions Chemical class 0.000 abstract description 4
- 150000003839 salts Chemical class 0.000 abstract description 4
- 239000011780 sodium chloride Substances 0.000 abstract description 4
- 239000000126 substance Substances 0.000 abstract description 3
- 229910052751 metal Inorganic materials 0.000 abstract description 2
- 239000002184 metal Substances 0.000 abstract description 2
- -1 AgCl and KCl) Chemical class 0.000 abstract 1
- 238000005260 corrosion Methods 0.000 description 47
- XKRFYHLGVUSROY-UHFFFAOYSA-N argon Chemical compound [Ar] XKRFYHLGVUSROY-UHFFFAOYSA-N 0.000 description 16
- 239000007789 gas Substances 0.000 description 13
- 239000001301 oxygen Substances 0.000 description 12
- 229910052760 oxygen Inorganic materials 0.000 description 12
- MYMOFIZGZYHOMD-UHFFFAOYSA-N oxygen Chemical compound O=O MYMOFIZGZYHOMD-UHFFFAOYSA-N 0.000 description 12
- 229910000856 hastalloy Inorganic materials 0.000 description 10
- 238000000034 method Methods 0.000 description 9
- 229910052786 argon Inorganic materials 0.000 description 8
- 238000006056 electrooxidation reaction Methods 0.000 description 7
- 229910045601 alloy Inorganic materials 0.000 description 6
- 239000000956 alloy Substances 0.000 description 6
- 239000000243 solution Substances 0.000 description 6
- 230000004913 activation Effects 0.000 description 5
- REDXJYDRNCIFBQ-UHFFFAOYSA-N aluminium(3+) Chemical class [Al+3] REDXJYDRNCIFBQ-UHFFFAOYSA-N 0.000 description 5
- OXBLHERUFWYNTN-UHFFFAOYSA-M Copper(I) chloride Chemical compound [Cu]Cl OXBLHERUFWYNTN-UHFFFAOYSA-M 0.000 description 4
- 238000007654 immersion Methods 0.000 description 4
- 239000000843 powder Substances 0.000 description 4
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 4
- 229910020586 KCl—ZnCl2 Inorganic materials 0.000 description 3
- 230000005587 bubbling Effects 0.000 description 3
- 239000010949 copper Substances 0.000 description 3
- 238000000840 electrochemical analysis Methods 0.000 description 3
- 239000001257 hydrogen Substances 0.000 description 3
- 229910052739 hydrogen Inorganic materials 0.000 description 3
- 238000003780 insertion Methods 0.000 description 3
- 238000005259 measurement Methods 0.000 description 3
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 3
- ORTQZVOHEJQUHG-UHFFFAOYSA-L Copper(II) chloride Chemical compound Cl[Cu]Cl ORTQZVOHEJQUHG-UHFFFAOYSA-L 0.000 description 2
- ZOMNIUBKTOKEHS-UHFFFAOYSA-L Mercury(I) chloride Chemical class Cl[Hg][Hg]Cl ZOMNIUBKTOKEHS-UHFFFAOYSA-L 0.000 description 2
- CSCPPACGZOOCGX-UHFFFAOYSA-N acetone Chemical compound CC(C)=O CSCPPACGZOOCGX-UHFFFAOYSA-N 0.000 description 2
- 239000007864 aqueous solution Substances 0.000 description 2
- 229940045803 cuprous chloride Drugs 0.000 description 2
- 230000003247 decreasing Effects 0.000 description 2
- 150000002431 hydrogen Chemical class 0.000 description 2
- 239000000463 material Substances 0.000 description 2
- 230000035699 permeability Effects 0.000 description 2
- 230000003334 potential Effects 0.000 description 2
- 239000008346 aqueous phase Substances 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 238000010349 cathodic reaction Methods 0.000 description 1
- 125000002091 cationic group Chemical group 0.000 description 1
- RYGMFSIKBFXOCR-UHFFFAOYSA-N copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 230000005496 eutectics Effects 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 238000005755 formation reaction Methods 0.000 description 1
- UFHFLCQGNIYNRP-UHFFFAOYSA-N hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 1
- 230000003993 interaction Effects 0.000 description 1
- 238000005342 ion exchange Methods 0.000 description 1
- 150000002739 metals Chemical class 0.000 description 1
- 239000007800 oxidant agent Substances 0.000 description 1
- 230000001590 oxidative Effects 0.000 description 1
- 229910052697 platinum Inorganic materials 0.000 description 1
- 150000003378 silver Chemical class 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- HCHKCACWOHOZIP-UHFFFAOYSA-N zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 1
- 229910052725 zinc Inorganic materials 0.000 description 1
- 239000011701 zinc Substances 0.000 description 1
Abstract
A reference electrode which is stable over a wide range of temperatures, pressures and chemical conditions is provided. The subject reference electrode according to the present invention comprises a tubular enclosure composed of quartz having a distal, closed end and a proximal, open end. An insulating ceramic rod is seemingly connected to the opening in the closed distal end of the enclosure to form micro-cracks between the ceramic rod and the quartz enclosure (called a cracked junction, CJ). The CJ gives a very tortuous path for ion conduction from inside the reference electrode (RE) to a working electrode (WE). Inside the tubular enclosure is an electrical lead (e.g., a silver wire) disposed in an electrolyte comprising a mixture of alkaline metal salts (e.g., AgCl and KCl), extending from the electrolyte upward through a sealing means at the proximal end of the quartz enclosure.
Description
A REFERENCE ELECTRODE FOR ELECTROCHEMICAL MEASUREMENTS AT HIGH TEMPERATURES
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
This invention was made with govemment support under Grant No. DE-EE0005942, awarded by DOE. The govemment has certain rights in the invention.
BACK.GROUND OF THE INVENTION
Field of the Invention
The present invention relates to a reference electrode for use at high températures up to 1000° C.
Description of Related Art
A reference electrode (RE) is an electrode in an ionic conducting solution, called a halfcell, with a constant electrode potential. The reference electrode is connected by a sait bridge to a second half-cell with another electrode, called a working electrode (WE), and voltage (potential différence) is measured between the RE and WE to find the potential at the working electrode versus the reference-electrode potential.
The RE is an essential component in an electrochemical cell to quantitatively observe behavior of the working electrode. A steady current can be passed between the working electrode and another electrode called a counter electrode (CE) while the WE potential is measured versus the RE. This can be repeated for a number of currents between the WE and CE. In this way, a plot of WE current versus WE potential (called the polarization of the working electrode) can be made, and the corrosion rate of a working métal electrode can be determined by this plot of WE current as a function of WE potential.
PA513282/O/V4167761.1
SUMMARY OF THE INVENTION
A stable and robust reference electrode according to the present invention has been made from a métal wire (like silver wire, Ag-wire) in contact with its ionic métal sait (like silver chloride, Ag+CF) and an alkaline métal sait (like potassium chloride, KCl) inside a quartz tube with an insulating ceramic rod (like alumina or zirconia rod) melted into one end of the quartz tube so that micro-cracks form between the ceramic rod and quartz (called a cracked junction, CJ). The CJ gives a very tortuous path for ion conduction from inside the quartz tube to outside the tube.
This reference electrode of the present invention has been calibrated and used for quantitatively estimating the electrochemical corrosion of Hastelloy C-276 in a zinc eutectic molten sait (l8.6NaCl-2l.9KCl-59.5ZnCl2 mol %, MP=2l3°C) equilibrated with air at températures up to 900°C. In the electrochemical polarization experiment, the métal is immersed in molten sait equilibrated with air (or Argon for anaérobie tests) along with counter and reference électrodes for about 10 minutes to détermine the open circuit potential (OCP) of the alloy versus the reference electrode. Then, the test alloy is polarized from -30 mV from the OCP to + 30 mV above OCP. The reference electrode must hâve a stable potential (be ideally nonpolarizable), must be stable over a wide range of températures up to 900 °C, even as high as 1300 °C, for corrosion studies of alloys in molten salts and should not perturb the alloy sample or molten sait under test.
In one embodiment, the housing is made of quartz so that the reference electrode could be used at températures up to 900°C. The quartz tube was terminated with a “cracked junction’’ (CJ) for ionic connection between the reference electrode and the working electrode (test alloy) of the electrochemical cell. This quartz tube was filled with proper amounts of l part Ag métal powder,
ΡΑ513282ΌΑ'4167761.1 l part AgCl powder and l part KCI powder which were mixed well by grinding and then poured into the quartz tube. A silver wire was inserted almost completely down the tube for electrical connection as shown in Fig. I. The CJ was made by fusing the quartz tube over an alumina rod so that the rod was firmly held in place as if sealed into the quartz. However, due to différences in expansion coefficients of the alumina and quartz, micro cracks form at the quartz and alumina interface resulting in a very tortuous path for ion diffusion between the inside of the quartz tube containing the reference electrode and the outside which gives ionic contact to the electrochemical cell. This reference electrode, shown in Fig. I, is referred to as the Alumina CJ.
BRIEF DESCRIPTION OFTHE DRAWINGS
Fig. I is a depiction of a high-temperature alumina cracked junction reference electrode according to the present invention;
Fig. 2 is a graph depicting différences in potential in time for the cracked junction (CJ) electrode, the saturated calomel electrode (SCE) and the saturated silver/ silver chloride electrode (Ag/AgCl or SSE). Electrode to left is “high” and electrode on right is “low” on a meter (saturated means the aqueous phase is equilibrated and in contact with solid KCI);
Fig. 3 is a depiction of a newly-prepared silver/silver chloride electrode (SSE) in quartz housing with a zirconia rod formîng a cracked junction;
Fig. 4 is a depiction of a newly prepared copper/cuprous chloride electrode (CCE) in quartz housing;
Fig. 5 is a graph showing the change in potential différences (E) as a function of time of SSE and CCE versus SCE and CCE in sat KCI solution at room température;
Fig. 6 is a depiction of a réversible hydrogen (RHE) and réversible oxygen electrode (ROE) used to measure the relative electrode potential of the SSE and CCE;
PA5l3282/OA'4l6776l.l
Fig. 7 is a depiction of a quartz housing for a electrochemical réversible gas half-cell;
Fig. 8 is a graph showing a linear polarization curve showing how to calculate the corrosion current density (/);
Fig. 9 is a graph showing a comparison of polarization curves of C-276 Hastelloy samples in 150 gm of NaCl-KCl-ZnCI2 sait at different températures in air;
Fig. 10 is a graph showing a comparison of polarization curves of C-276 Hastelloy samples in NaCl-KCl-ZnCl2 sait at 800 C when the cell is open to air or under flowing of dry air;
Fig. 11 is a graph showing a comparison of polarization curves of C-276 Hastelloy samples in NaCl-KCl-ZnCl2 sait at different températures under argon atmosphère; and
Fig. 12 is a graph showing log Icorr as a function of (l/T) of C-276 Hastelloy samples in NaCl-KCl-ZnCl2 molten sait.
DESCRIPTION OF THE INVENTION
A reference electrode according to the present invention is used in order to measure the potential of a métal sample in molten sait at high températures (up to 900° C or more). A métal in contact with its cationic sait has constant potential and is the basis for making a reference electrode. The new reference electrode used in molten sait was developed to simulate the traditional silver/sîlver chloride (Ag/AgCI) reference electrode (SSE) used in aqueous solutions. The new RE has a silver wire inserted into a mixture of Chemicals (Ag métal powder + AgCl + KCl) housed in a quartz tube with a ceramic rod (Zirconia) sealed at the bottom making a cracked junction for ion conduction needed to complété the electric circuit for measuring and controlling potential of a métal sample in molten sait at high températures (up to 900° C or more). The main improvement in this reference electrode is that a zirconia rod was melted into
PA5 J3282/OA/4167761.1 one end of heavy-walled quartz tubing was used to form the cracked junction. This is much more stable than thin walled quartz and alumina.
In another embodiment, a combination of métal and metal-cationic sait was used to make another reference electrode, a copper/cuprous chloride reference electrode (CCE). In the CCE, a copper wire is inserted into a mixture of Chemicals (Cu + CuCl + KCl) housed tn a quartz tube terminating with a sealed ceramic rod (Zirconia) at the bottom of the tube. The zirconia sealed in quartz has a tortuous crack for ionic exchange between the reference chamber and main chamber of sait holding the electrode under test. This ion exchange is needed in order to complété the electrical connection between the reference electrode (RE) and the working electrode (WE) under test in the molten sait, so the potentiel of the working electrode under test can be measured and controlled during the electrochemical polarization measurements of the WE under test.
Example l: Testing the potential of the new reference électrodes in saturated aqueous KCl. To verify that these new combinations (Fig. 3 and 4) can serve as reference électrodes, their potential was measured against the well-known standard saturated calomel reference electrode (SCE) in aqueous saturated potassium chloride solution.
Fig. 5 shows a plot of the time dependence of the potential différence ( ΔΕ ) measured in aqueous saturated KCl solution at room température for SSE versus SCE, CCE versus SCE and the CCE versus SSE. As shown in figure 5, the proposed reference électrodes (SSE and CCE) showed the expected ΔΕ values versus the SCE (based on thennodynamic calculations) after Ih and 24h of immersion in sat. KCl solution. Moreover, the ΔΕ values are essentially constant throughout the immersion time. The ΔΕ values are summarized in Table I.
PA5I3282/OA/4I67761.1
Table 1. Potential différences (JE) between different électrodes in sat. KO solution.
| ΔΕ | lh immersion | 24h immersion | Standard value |
| SSE - SCE | -59 mV | -57 mV | -45 mV |
| CCE-SCE | -151 mV | -156 mV | -145 mV |
| CCE- SSE | -91 mV | -99 mV | -101 mV |
This data confirms that these proposed électrodes (SSE and CCE) can serve as reference électrodes in aqueous solutions as the SCE does.
Example 2: Testing the potential of the proposed électrodes în NaCl-KCl-ZnCU (M.P.:
204q C) at high températures. The potentials of the new électrodes (SSE and CCE) were measured against réversible gas électrodes. These gas électrodes are the réversible hydrogen electrode (RHE) and réversible oxygen electrode (ROE). A platinum wire was welded to platinum-mesh in molten sait, which was housed in quartz, and hydrogen (or oxygen) gas was 10 sent in the quartz housing at a flow rate of 90 SCCM as shown in Fig. 6. The gas was sent to the Pt wire in a quartz housing as dry gas or was pre-saturated with DI water by passing the gas through a gas wash bottle at room température.
Table 2. Potential différences (AE) between SSE, CCE, RHE and ROE under different conditions and températures
| ΔΕ (mV) __---- | Standard 4Eat 25°C (mV) | ||||||
| T(’C) | 250°C | 300°C | 350°C | 400°C | 500°C | 800°C | |
| SSE vs. RHE (dry HJ | +294 | +370“ | +396 | +463 | +2?5 | ||
| SSE vs. RHE (Hz/HjO) | +230 | +238 | +241 | +237 | +225 | ||
| CCE vs. RHE (dry HJ | +160 | +206 | +269 | +280 | +124 | ||
| CCE vs. RHE (Hi/EhO) | +94 | +102 | +96 | . +62. | +124 | ||
| SSE vs. ROE (dry OJ | -403 | -190 | -165 | -1005 | |||
| SSE vs. ROE (Oï/HîO) | -775 | -814 | -806 | -1005 | |||
| CCE vs. ROE (dry O2) | -560 | -370 | -348 | -1106 | |||
| CCE vs. ROE (OjfH2O) | -935 | -968 | 1-987 | -1106 | |||
| CCE vs. SSE | -135 | -142 | -150 | -163 | -182 | -225 | -100 |
Measuring the voltage (ΔΛ) of a first reference electrode with a known potential (E RE1 kno'-'·') against the potential of a second electrode (E RE2 unknown) is done to see if the potential 6
P A513282/0A4167761.1 différence (AP) is constant, which establishes the suitabilîty of this electrode (RE2) as a reference electrode [see Electroanalytical Chemistry, James J. Lingane, 2nd édition. Interscience Publishers ( 1958)] and to establish the potential of the second electrode (RE2).
Following this method, the potential différences shown in Table 2 were found to be constant in time and the measured potentials are in fairly good agreement with the thermodynamically expected potential différences (ΔΕ) calculated from the calorimetric data for the free energy (AG) of formation of the various materials. The small observed déviations are quite reasonable since the tabulated thermodynamic data do not take into account interactions between the various materials (Ag, AgCl, KCl, Cu, CuCl, etc.) and the molten sait. So the various électrodes in Table 2 are found to be suitable as reference électrodes.
The silver/silver chloride electrode used for this work can certaînly be used as a reference electrode to détermine and control the potential of a métal under test during an electrochemical détermination of the corrosion rate of a métal in a molten sait at températures up to 800°C.
Example 3: Electrochemical détermination of corrosion rates. The métal alloy used in ail electrochemical corrosion rate déterminations was Hastelloy C-276. Mass of the molten sait used during ail electrochemical experiments was 150 gni. The métal sample was abraded on wet or dry 600 grit SÎC paper, rinsed with deionized (DI) water and then rinsed with acetone. The electrochemical corrosion cell was made of quartz with spécifie dimensions to fit into an electrical fumace used to isothermally control température during ail tests. Fig. 7 shows the electrochemical corrosion cell used.
The electrochemical test of métal corrosion was carried out by using the linear polarization technique. In this technique, the métal sample was polarized ±30 mV versus the open circuit potential (OCP) at a scan rate of 0.2 mV/s. The potential of the métal under test 7
ΡΛ513282'0A.-4167761.1 started from the most cathodic value and was scanned to the most positive value, giving a linear polarization (I/V) curve. The measured linear polarization (I/V) curve was transformed to a loglO of the absolute value of the current plotted versus the potential, and this gave a plot that was used to calculate the corrosion current density (icorr) as shown in Fig. 8.
The corrosion rates were determined from the corrosion current density by using the formula derived from Faraday’s Law, which is given by ASTM Standards G59 and GI02 (ASTM International, 2003):
CR (pm/y) = ^1 * ^corr *
P where Αη = 3.27 in pm g pA'1 cm’1 yr'1, icorr is the corrosion current density in pA cm'2 (determined from the polarization curve, Fig. 22), EW and p are the équivalent weight (27.01 g/eq) and density (8.89 g cm’3) of the C-276 Hastelloy, respectively.
Example 4: Electrochemical corrosion rate measurements in NaCl-KCI-ZnCb (molar composition, 13.4-33.7-52.9, M.P.= 204° C). In these corrosion tests in aérobic molten sait, the electrochemical corrosion cell was kept open to the atmospheric air ail the time. The sait was melted at 300° C for 30 min, then a métal sample was inserted at this température (300° C). After reaching a stable OCP (about 5 min after samples insertion), the polarization (I-V) curve was measured. After measuring the I-V curve at 300° C, the température was raised to 500° C and after reaching a stable OCP, an I/V curve was measured at this température (500° C). The same procedure was used to measure the I/V curve at 800° C. Two different sîzes of samples in the same mass of sait (150 gm) were used to investigate the effect of sample size on the corrosion rate.
ΡΑ513282.ΌΑ/4167761.1
As shown in Figure 9, as expected from the Arrhenius équation with no change of mechanism, the polarization currents increase with an increase in température. In addition there is a clear positive shift in the OCP with increase in température due to higher oxygen concentration on the métal surface due to better transport of oxygen from air due to lower viscosity of the molten sait and higher permeability of oxygen in the molten sait.
Table 3. Corrosion parameters obtained from polarization curves in Fig. 23.
| Température (’C) / Atmosphère | Surface area for WE and CE | Corrosion potential, Et0„ (V) | Corrosion current density, lcorT (μΑ/cm2) | Corrosion rate, (pm/y) | |
| 300 (Small) .......... | . Air | WE=5.6 cm2 CE=10.5 cm2 | -0.065 | 3.98 | 39.52 |
| 300 (Large) .......... | . Air | WE=17.5 cm2 CE=27.3 cm2 | -0.115 | 5 | 49.65 |
| 500 (Small) .......... | . Air | WE=5.6 cm2 CE=10.5 cm2 | 0.125 | 39.8 | 395.21 |
| 500 (Large) .......... | . Air | WE=17.5 cm2 CE=27.3 cm2 | 0.08 | 43.6 | 432.94 |
| 800 (Small) .......... | . Air | WE=5.6 cm2 CE=10.5 cm2 | 0.284 | 251 | 2492.43 |
| 800 (Large) .......... | . Air | WE=17.5cm2 CE=27.3 cm2 | 0.291 | 239.88 | 2382 |
As shown in the table 3, the corrosion rates of the small size sample are very similar to those of the large size sample which suggests that in the short terni there is no dependency of corrosion rate on the métal coupon size when holding the mass ofthe molten sali constant under these conditions.
Example 5: Aérobic electrochemical tests at 800° C with flowing dry air in the molten sait. The sait was heated to melt at 500° C, then the dry air was sent into the sait at 175 SCCM for ih, then the température was raised to 800° C while dry air was still bubbling in the sait.
Then the samples (CE and WE) were inserted (the température was kept at 800° C) and the dry air bubbling stopped în the sait and started above the sait, and after the OCP became stable (about 5 min after sample insertion) then the I-V curve was measured for métal coupon in the molten sait equilibrated with dry air.
ΡΛ5Ι3282ΌΑ4Ι6776Ι.1
Table 4. Comparison of corrosion parameters of C-276 Hastelloy samples in NaCI-KClZnCh sait at 800°C when the cell is open to air or under flowing of dry air.
| Température (°C) / Atmosphère | Surface area for WE and CE | Corrosion potential, Ecorr (V) | Corrosion current density, icorr (pA/cm2) | Corrosion rate, (pm/y) |
| 800 / Cell open to air | WE=17.5 cm2 CE=27.3 cm2 | 0.291 | 239.88 | 2382 |
| 800 / Dry air | WE=11.2cm2 CE=21.7 cm2 | 0.23 | 223.87 | 2223 |
As shown from Fig. 10 and Table 4, the corrosion rate under flowing of dry air is very close to the corrosion rate when the cell was kept open to air. This indicates that the corrosion process was mainly driven by oxygen réduction (as the main cathodic reaction) in both cases.
Example 6: Anaérobie electrochemical tests. For anaérobie electrochemical corrosion testing, the sait was heated to melt at 300° C, then Argon gas was flowed into the sait at 175 SCCM for 30 min before inserting the counter and working electrode Hastelloy métal samples. When the CE and WE samples were inserted, the gas bubbling into the molten sait was stopped, and instead gas was flowed above the sait, and after the OCP became stable (about 5 min after sample insertion), then the I-V curve was measured. After the first I/V curve was acquired at 300° C, the argon gas was again flowed into the sait until the température reached 500° C. Then the argon flow was switched again to over the sait. After the OCP was stable, the l-V curve was measured at 500° C. The same procedure was used for acquiring the I/V curve at 800° C. The métal samples remained in the molten sait sînee they were first inserted at 300° C and until tests were fmished at 800° C.
As shown in figure H, the corrosion currents significantly decreased under anaérobie condition when compared with those measured under aérobic conditions (Figs. 9 and 10). Moreover, these polarization currents measured under anaérobie condition slightly increased and the OCP shifted to more positive values as the sait température increased. It is practically
PA51328WA/4167761.1 impossible to completely remove ail oxygen from the sait, so the positive shift in OCP is probably due the higher permeability of residual oxygen in the sait as the viscosity of the sait decreased with increasing température. Although there is positive shift in OCP values with încreasing température under anaérobie conditions, ali other things being equal, the OCP values measured under anaérobie conditions were still seen to be about 100 mV more négative than those OCP values measured under aérobic condition, particularly noticeable is the différence in OCPs for métal in aérobic and anaérobie molten sait at 800°C.
Table 5. Corrosion rates obtained from métal polarization in anaérobie sait (Fig. 25).
| Température (C) / Atmosphère | Surface area for WE and CE | Corrosion potential, E£O„ (V) | Corrosion current density, Icorr (μΑ/cm2) | Corrosion rate, (pm/y) |
| 300 (Small) ........... Argon | WE=3.5 cm2 CE=8.4 cm2 | -0.02 | 0.501 | 4.97 |
| 300 (Large) ........... Argon | WE=14 cm2 CE=24.5 cm2 | -0.08 | 0.795 | 7.89 |
| 500 (Small) ........... Argon | WE=3.5 cm2 CE=8.4 cm 2 | 0.004 | 1.58 | 15.68 |
| 500 (Large) ........... Argon | WE=14 cm2 CE=24.5 cm2 | -0.057 | 1.86 | 18.46 |
| 800 (Small) ........... Argon | WE=3.5 cm2 CE=8.4 cm2 | 0.15 | 3.98 | 39.52 |
| 800 (Large) ........... Argon | WE=14 cm2 CE=24.5 cm2 | 0.166 | 3.16 | 31.37 |
As shown in table 5, the corrosion rates under anaérobie condition at 800°C are about 50 times lower than the corrosion rates measured under aérobic conditions (Table 4) ail other things being equal. It is also noted that corrosion rates of the small-sized sample are again very similar 15 to those of the large-sized samples in anaérobie molten sait, which suggests that in these short term tests there is no dependency of corrosion rate on the métal size immersed in same sait mass ( 150 gm ) as previously found on testing under aérobic conditions (Table 3).
PA513282/0A'4l 6776 l.I
It is important to note that the corrosion rates calculated by the electrochemical method (linear polarization technique) are in good agreement with the corrosion rates previousiy calculated by the gravimétrie method. This strongly suggests that the corrosion rates are accurate as they give very similar values when they are determined by 2 different methods. This gravimétrie methods is considered inconvénient because it take a long time to do but is considered accurate. The agreement of the electrochemical method to the gravimétrie method indicates the electrochemical method is accurate and vérifiés two things i) the use of the linear polarization technique is reliable for estimating the corrosion rates of metals in molten salts at high températures up to 800°C and n) the newly developed silver / silver chloride reference electrode (SSE) is reliable for correctiy estimating and controlling the potential in molten salts at high températures up to 800°C.
Example 7: Activation energy of corrosion under aérobic and anaérobie conditions in NaCl-KCl-ZnCb molten sait as predicted by Arrhenius plots. The activation energy (Ea) of the corrosion process can be calculated from corrosion current densities (lCOTT) at different températures according to the Arrhenius équation, which is —Ea lûglcorr = 2 3RT + l°gA where R. is the gas constant (8.3 14 J/mol . K), A is Arrhenius constant and T is the absolute température. Activation energy was calculated from the slope of log Icorr as a function of (l/T) plots.
ΡΛ513282 OA.4167761.1
Table 6. Arrhenîus activation energy of the corrosion process of C-276 Hastelloy in NaCIKCl-ZnCh molten sait under aérobic and anaérobie conditions.
| Sample | Ea (kJ/mol) | |
| Small area ..... | ......Air | 4237 |
| Large area...... | ......Air | 39.59 |
| Small area ..... | ......Argon | 21.19 |
| Large area...... | ......Argon | 14.17 |
As shown in Fig. 12 and Table 6, the activation energy for corrosion under anaérobie conditions are almost half of values under aérobic conditions. This indicates that the corrosion rate has different dependency on température for aérobic and anaérobie conditions. It is clear that the corrosion rate is more dépendent on température under aérobic condition than anaérobie conditions. This reflects the greater irreversibility of oxygen réduction compared to proton 10 réduction on water (residual water is the thermodynamically weak oxidant in the sait equilibrated with Argon). Another considération is that oxygen transport is strongly température dépendent which agréés with viscosity measurement which show the viscosity of the molten sait is higher at low températures (300° C) and is lower at higher températures (800° C). Accordingly oxygen penneability is highest at 800° C giving rise to highest corrosion rate and is very low at 300° C 15 giving rise to corrosion rate approaching that in anaérobie sait at 300° C, as is seen in Figure 12.
Claims (10)
- THE INVENTION CLAIMED IS1. A reference electrode comprising:a tubular enclosure having a proximal end and a distal end, wherein said distal end comprises a junction for ionic conduction between the reference electrode and a working electrode;a non-porous insulating ceramic rod sealingly connected to said distal end of said enclosure to form micro-cracks between said ceramic rod and said enclosure at said junction;an electrolyte disposed inside of said enclosure, said electrolyte comprising an alkaline métal sait;a sealing means for sealing said enclosure at said proximal end; and an electrical lead disposed in said electrolyte in said enclosure and extending through said sealing means at the proximal end of said enclosure.
- 2. The reference electrode of claim l, wherein the enclosure is composed of heavywalled quartz.
- 3. The reference electrode of claim l, wherein the insulating ceramic rod is composed of alumina.
- 4. The reference electrode of claim l, wherein the electrolyte comprises sîlver chloride (AgCl) and potassium chloride (K.CI).
- 5. The reference electrode of claim l, wherein the electrical lead is composed of silver.
- 6. A reference electrode for use at high températures comprising:a tubular enclosure inert to températures above 900° C and having an open proximal end and a closed distal end, wherein said closed distal end comprises a junction for ionic conduction between the reference electrode and a working electrode;5 a non-porous insulating ceramic rod fused to said opening at said closed distal end to form micro-cracks between said ceramic rod and said enclosure at said junction;an electrolyte disposed inside of said enclosure, said electrolyte comprising an alkaline métal sait;a sealing means for sealing said enclosure at said proximal end; and10 an electrical lead disposed in said electrolyte in said enclosure and extending through said sealing means at the proximal end of said enclosure.
- 7. The reference electrode of claim 6, wherein the enclosure is composed of alumina.
- 8. The reference electrode of claim 6, wherein the insulating ceramic rod is15 composed of zirconia.
- 9. The reference electrode of claim 6, wherein the electrolyte comprises potassium chloride (KC1).
- 10. The reference electrode of claim 6, wherein the electrical lead is composed of tungsten.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US62/258,853 | 2015-11-23 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| OA18800A true OA18800A (en) | 2019-06-28 |
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