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Effect of free carbon dioxide on corrosion behavior of copper in simulated water
Surface and Coatings Technology 169 – 170 (2003) 662–665
Effect of free carbon dioxide on corrosion behavior of copper in simulated water Kazuharu Sobuea,*, Akifumi Sugaharaa,1, Takeshi Nakataa, Hachiro Imaia, Shin’ichi Magainob a Shibaura Institute of Technology, 3-9-14, Shibaura, Minato-ku, Tokyo 108-8548, Japan Kanagawa Industrial Technology Research Institute, 705-1 Shimoimaizumi, Ebina-shi, Kanagawa-ken 243-0435, Japan
b
Abstract The effect of free carbon dioxide on the copper corrosion-behavior has been investigated by the immersion tests with electrochemical impedance spectroscopy. The copper corrosion-rate increased with the concentration of the free carbon dioxide. The free carbon dioxide continuously provides water with Hq ions if the CO2 gas existed in the atmosphere. The production of Hq might assist the reduction of the dissolved oxygen to start the oxidation of copper. This might be the reason why the free carbon dioxide promotes the oxidation of copper. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Pitting corrosion; Copper; Immersion test; Impedance spectroscopy
1. Introduction
2. Experimental
Copper tubes are widely used in water supply systems. In spite of their good corrosion resistance, copper tubes sometimes suffer from type I pitting corrosion. The type I pitting corrosion is apt to occur in cold-water supply systems using hard waters. The pits are covered with greenish corrosion products which have hemispherical shaped mounds. The mound mainly consists of a basic copper carbonate and calcium carbonate. It was reported that the type I pitting corrosion is due to carbonaceous films formed from drawing-lubricant residues during the annealing process in manufacture and that the carbonaceous films provide a large and efficient cathodic surface w1 x . We had examined the influence of the chemical components dissolved in water on the copper corrosionrate by statistic analysis w2x. This analysis has shown that the copper corrosion-rate might increase with the concentration of free carbon dioxide. In this study, the effect of the free carbon dioxide on the corrosion behavior of copper was experimentally examined.
2.1. Test electrode and test solution
*Corresponding author. Tel.: q81-3-5476-2422; fax: q81-3-54763161. E-mail address: [email protected] (K. Sobue). 1 Present address: Fujisash Co., Ltd, 135 Nakamaruko Nakaharaku, Kawasaki-shi 211-0012, Japan.
Phosphorous deoxidized copper specimens (Cu)99.9 wt.%, Ps0.026 wt.%, 15=15=1 mm3) were used for the immersion tests with electrochemical impedance spectroscopy (EIS) measurements. Conductor wires in glass tubes and the copper specimens were embedded in epoxy resin to form the copper electrodes. The test surfaces of the copper electrodes were mechanically polished with 600 grid emery paper and rinsed with distilled water. The simulated tap-water (pH 7.7) was prepared from analytical grade reagents dissolved in distilled water w3x. 2.2. Immersion test with EIS measurement In order to investigate the effect of the free carbon dioxide on the copper corrosion-behavior, the immersion tests were carried out in atmospheres of 77% N2 –21% O2 –2% CO2, 78.8% N2 –21% O2 –0.2% CO2 and 79% N2 –21% O2 gases (artificial air). The concentration of the free carbon dioxide in water increased with the CO2 concentration in the gas. The effect of pH was also investigated because the pH of water decreased with
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 3 . 0 0 0 4 9 - 5
K. Sobue et al. / Surface and Coatings Technology 169 – 170 (2003) 662–665
663
Table 1 Atmospheres and conditions of pH adjustment for the simulated tap-waters Condition name
2% CO2 0.2% CO2 0%CO2 0% CO2 with H2SO4 0% CO2 with HCl 0% CO2 with HNO3
Atmosphere
77% N2 –21% O2 –2.0% CO2 78.8%N2 –21% O2 –0.2% CO2 79% N2 –21% O2 –0% CO2 79% N2 –21% O2 –0% CO2 79% N2 –21% O2 –0% CO2 79% N2 –2l% O2 –0% CO2
Addition of acid solution to 1 dm3 simulated tap-water Acid
Volumey cm3
No addition No addition No addition 0.05 mol dmy3 H2SO4
– – – 3.75 4.00 4.25
0.1 mol dmy3 HCl 0.1 mol dmy3 HNO3
increasing concentration of the free carbon dioxide in water. For this purpose, the pH of the simulated tapwaters were adjusted to 6.6, which was the same as that of the simulated tap-water saturated with 77% N2 – 21%O2 –2% CO2 gas, with the addition of H2SO4, HCl or HNO3 without CO2 gas. Table 1 shows the atmospheres and conditions of the pH adjustment for the simulated tap-waters. The effect of the free carbon dioxide was investigated by comparing the conditions of 2% CO2, 0.2% CO2 and 0% CO2 as shown in Table 1. The effect of pH investigated by comparing the conditions of 2% CO2, 0% CO2 with H2SO4, 0% CO2 with HCl and 0% CO2 with HNO3 (pH 6.6) is shown in Table 1. The atmosphere of the test cell was maintained by the artificial air supply, and the temperature of the water (1.0 dm3) was maintained at 303 K by a thermostatic water bath throughout the experiment. After a 48 h-immersion, the EIS measurements were carried out with a four-lead two-electrode setup in the frequency range of 0.01 Hz–10 kHz with the potential modulation of 10 mV. After the EIS measurement, the electrochemical parameters were determined by a curve-fitting pro-
Fig. 1. Equivalent circuit of the copper specimen in the simulated tapwater. Rel, Cdl, RctA, RctC and ZW are electrolyte resistance, double layer capacitance, charge-transfer resistance for anodic reaction, charge-transfer resistance for cathodic reaction and Warburg impedance, respectively.
pH before immersion test
Free carbon dioxidey mg dmy3 Before immersion test
After 48 h
6.6 7.5 8.9 6.6 6.6 6.6
20 2.2 0 0 0 0
20 2.8 0 0 0 0
gram (Scribner Associates, Inc. Zview Version 2.2) with an equivalent circuit shown in Fig. 1. 3. Results and discussion Fig. 2 shows photographs of the copper specimens after the 48 h-immersion tests and the change in pH
Fig. 2. Photographs of the copper specimens after the 48-h immersion tests and the change in pH throughout the immersion tests.
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K. Sobue et al. / Surface and Coatings Technology 169 – 170 (2003) 662–665
impedance (ZW) determined by the curve-fitting with the equivalent circuit shown in Fig. 1 and the concentration of the free carbon dioxide. The values of RctA, RctC and ZW decreased with increasing concentration of the free carbon dioxide. A clear difference in the values of RctA, RctC and ZW was not observed among the specimens immersed in the solutions containing H2SO4, HCl and HNO3. These values were higher than those of the specimens immersed in the solution saturated with 77% N2 –21% O2 –2% CO2 gas. These results suggest that the corrosion-rate increases with the concentration of the free carbon dioxide. The free carbon dioxide (CO2(aq.)) continuously provides water with Hq ions (Eqs. (1)–(3)).
Fig. 3. Bode plots for impedance of the copper specimens in the simulated tap-water.
throughout the immersion tests. As shown in Fig. 2(a– c), the area of color-change on the specimen surface increased with the concentration of the free carbon dioxide. The pH values did not change throughout the immersion tests. As shown in Fig. 2(d–f), a clear difference in the corrosion behavior was not observed among the specimens immersed in the solutions containing H2SO4, HCl and HNO3. The pHs of these solutions increased throughout the immersion tests. The Bode plots for the impedance of the copper specimens in the simulated tap-waters are shown in Fig. 3. For the impedance diagram of copper in a neutral solution, the high frequency region might correspond to the charge-transfer resistance and the low frequency region might correspond to diffusion. Fig. 4 shows charge-transfer resistances (RctA and RctC), Warburg
CO2(g)™CO2(aq.)
(1)
CO2(aq.)qH2O™H2CO3
(2)
q H2CO3™HCOy 3 qH
(3)
The production of Hq via Eq. (3) might assist the reduction of the dissolved oxygen (Eq. (4)) to start the oxidation of copper. O2q4Hqq4ey™2H2O
(4)
Water might have a buffer capacity for pH if the CO2 gas existed in the atmosphere. For the acid addition, the pH might shift to a higher value because Hq decreased as the reduction of dissolved oxygen (Eq. (4)) proceeded without supplying Hq. The surface state and the detailed corrosion-mechanism are under investigation, and will be reported elsewhere. 4. Conclusion The copper corrosion-rate increases with the concentration of the free carbon dioxide. The free carbon dioxide continuously provides water with Hq ions if the CO2 gas exists in the atmosphere. The production of Hq might assist the reduction of the dissolved oxygen to start the oxidation of copper. This might be the reason why the free carbon dioxide promotes the oxidation of copper. Acknowledgments
Fig. 4. RctA, RctC, ZW determined by the curve fitting with the equivalent circuit shown in Fig. 1 and the concentration of the free carbon dioxide.
The authors wish to thank Japan Copper Development Association.
K. Sobue et al. / Surface and Coatings Technology 169 – 170 (2003) 662–665
References w1x H.S. Campbell, J. Inst. Metals 77 (1950) 345. w2x K. Sobue, S. Magaino, A. Kawaguchi, A. Sugahara, H. Imai,
665
Proceedings of the JSCE Materials and Environments 2000, (2000) 327. w3x A. Sugahara, K. Sobue, S. Magaino, A. Kawaguchi, H. imai, Proceedings of the 47th Japan Conference on Materials and Environments, (2000) 323.
Effect of free carbon dioxide on corrosion behavior of copper in simulated water Kazuharu Sobuea,*, Akifumi Sugaharaa,1, Takeshi Nakataa, Hachiro Imaia, Shin’ichi Magainob a Shibaura Institute of Technology, 3-9-14, Shibaura, Minato-ku, Tokyo 108-8548, Japan Kanagawa Industrial Technology Research Institute, 705-1 Shimoimaizumi, Ebina-shi, Kanagawa-ken 243-0435, Japan
b
Abstract The effect of free carbon dioxide on the copper corrosion-behavior has been investigated by the immersion tests with electrochemical impedance spectroscopy. The copper corrosion-rate increased with the concentration of the free carbon dioxide. The free carbon dioxide continuously provides water with Hq ions if the CO2 gas existed in the atmosphere. The production of Hq might assist the reduction of the dissolved oxygen to start the oxidation of copper. This might be the reason why the free carbon dioxide promotes the oxidation of copper. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Pitting corrosion; Copper; Immersion test; Impedance spectroscopy
1. Introduction
2. Experimental
Copper tubes are widely used in water supply systems. In spite of their good corrosion resistance, copper tubes sometimes suffer from type I pitting corrosion. The type I pitting corrosion is apt to occur in cold-water supply systems using hard waters. The pits are covered with greenish corrosion products which have hemispherical shaped mounds. The mound mainly consists of a basic copper carbonate and calcium carbonate. It was reported that the type I pitting corrosion is due to carbonaceous films formed from drawing-lubricant residues during the annealing process in manufacture and that the carbonaceous films provide a large and efficient cathodic surface w1 x . We had examined the influence of the chemical components dissolved in water on the copper corrosionrate by statistic analysis w2x. This analysis has shown that the copper corrosion-rate might increase with the concentration of free carbon dioxide. In this study, the effect of the free carbon dioxide on the corrosion behavior of copper was experimentally examined.
2.1. Test electrode and test solution
*Corresponding author. Tel.: q81-3-5476-2422; fax: q81-3-54763161. E-mail address: [email protected] (K. Sobue). 1 Present address: Fujisash Co., Ltd, 135 Nakamaruko Nakaharaku, Kawasaki-shi 211-0012, Japan.
Phosphorous deoxidized copper specimens (Cu)99.9 wt.%, Ps0.026 wt.%, 15=15=1 mm3) were used for the immersion tests with electrochemical impedance spectroscopy (EIS) measurements. Conductor wires in glass tubes and the copper specimens were embedded in epoxy resin to form the copper electrodes. The test surfaces of the copper electrodes were mechanically polished with 600 grid emery paper and rinsed with distilled water. The simulated tap-water (pH 7.7) was prepared from analytical grade reagents dissolved in distilled water w3x. 2.2. Immersion test with EIS measurement In order to investigate the effect of the free carbon dioxide on the copper corrosion-behavior, the immersion tests were carried out in atmospheres of 77% N2 –21% O2 –2% CO2, 78.8% N2 –21% O2 –0.2% CO2 and 79% N2 –21% O2 gases (artificial air). The concentration of the free carbon dioxide in water increased with the CO2 concentration in the gas. The effect of pH was also investigated because the pH of water decreased with
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 3 . 0 0 0 4 9 - 5
K. Sobue et al. / Surface and Coatings Technology 169 – 170 (2003) 662–665
663
Table 1 Atmospheres and conditions of pH adjustment for the simulated tap-waters Condition name
2% CO2 0.2% CO2 0%CO2 0% CO2 with H2SO4 0% CO2 with HCl 0% CO2 with HNO3
Atmosphere
77% N2 –21% O2 –2.0% CO2 78.8%N2 –21% O2 –0.2% CO2 79% N2 –21% O2 –0% CO2 79% N2 –21% O2 –0% CO2 79% N2 –21% O2 –0% CO2 79% N2 –2l% O2 –0% CO2
Addition of acid solution to 1 dm3 simulated tap-water Acid
Volumey cm3
No addition No addition No addition 0.05 mol dmy3 H2SO4
– – – 3.75 4.00 4.25
0.1 mol dmy3 HCl 0.1 mol dmy3 HNO3
increasing concentration of the free carbon dioxide in water. For this purpose, the pH of the simulated tapwaters were adjusted to 6.6, which was the same as that of the simulated tap-water saturated with 77% N2 – 21%O2 –2% CO2 gas, with the addition of H2SO4, HCl or HNO3 without CO2 gas. Table 1 shows the atmospheres and conditions of the pH adjustment for the simulated tap-waters. The effect of the free carbon dioxide was investigated by comparing the conditions of 2% CO2, 0.2% CO2 and 0% CO2 as shown in Table 1. The effect of pH investigated by comparing the conditions of 2% CO2, 0% CO2 with H2SO4, 0% CO2 with HCl and 0% CO2 with HNO3 (pH 6.6) is shown in Table 1. The atmosphere of the test cell was maintained by the artificial air supply, and the temperature of the water (1.0 dm3) was maintained at 303 K by a thermostatic water bath throughout the experiment. After a 48 h-immersion, the EIS measurements were carried out with a four-lead two-electrode setup in the frequency range of 0.01 Hz–10 kHz with the potential modulation of 10 mV. After the EIS measurement, the electrochemical parameters were determined by a curve-fitting pro-
Fig. 1. Equivalent circuit of the copper specimen in the simulated tapwater. Rel, Cdl, RctA, RctC and ZW are electrolyte resistance, double layer capacitance, charge-transfer resistance for anodic reaction, charge-transfer resistance for cathodic reaction and Warburg impedance, respectively.
pH before immersion test
Free carbon dioxidey mg dmy3 Before immersion test
After 48 h
6.6 7.5 8.9 6.6 6.6 6.6
20 2.2 0 0 0 0
20 2.8 0 0 0 0
gram (Scribner Associates, Inc. Zview Version 2.2) with an equivalent circuit shown in Fig. 1. 3. Results and discussion Fig. 2 shows photographs of the copper specimens after the 48 h-immersion tests and the change in pH
Fig. 2. Photographs of the copper specimens after the 48-h immersion tests and the change in pH throughout the immersion tests.
664
K. Sobue et al. / Surface and Coatings Technology 169 – 170 (2003) 662–665
impedance (ZW) determined by the curve-fitting with the equivalent circuit shown in Fig. 1 and the concentration of the free carbon dioxide. The values of RctA, RctC and ZW decreased with increasing concentration of the free carbon dioxide. A clear difference in the values of RctA, RctC and ZW was not observed among the specimens immersed in the solutions containing H2SO4, HCl and HNO3. These values were higher than those of the specimens immersed in the solution saturated with 77% N2 –21% O2 –2% CO2 gas. These results suggest that the corrosion-rate increases with the concentration of the free carbon dioxide. The free carbon dioxide (CO2(aq.)) continuously provides water with Hq ions (Eqs. (1)–(3)).
Fig. 3. Bode plots for impedance of the copper specimens in the simulated tap-water.
throughout the immersion tests. As shown in Fig. 2(a– c), the area of color-change on the specimen surface increased with the concentration of the free carbon dioxide. The pH values did not change throughout the immersion tests. As shown in Fig. 2(d–f), a clear difference in the corrosion behavior was not observed among the specimens immersed in the solutions containing H2SO4, HCl and HNO3. The pHs of these solutions increased throughout the immersion tests. The Bode plots for the impedance of the copper specimens in the simulated tap-waters are shown in Fig. 3. For the impedance diagram of copper in a neutral solution, the high frequency region might correspond to the charge-transfer resistance and the low frequency region might correspond to diffusion. Fig. 4 shows charge-transfer resistances (RctA and RctC), Warburg
CO2(g)™CO2(aq.)
(1)
CO2(aq.)qH2O™H2CO3
(2)
q H2CO3™HCOy 3 qH
(3)
The production of Hq via Eq. (3) might assist the reduction of the dissolved oxygen (Eq. (4)) to start the oxidation of copper. O2q4Hqq4ey™2H2O
(4)
Water might have a buffer capacity for pH if the CO2 gas existed in the atmosphere. For the acid addition, the pH might shift to a higher value because Hq decreased as the reduction of dissolved oxygen (Eq. (4)) proceeded without supplying Hq. The surface state and the detailed corrosion-mechanism are under investigation, and will be reported elsewhere. 4. Conclusion The copper corrosion-rate increases with the concentration of the free carbon dioxide. The free carbon dioxide continuously provides water with Hq ions if the CO2 gas exists in the atmosphere. The production of Hq might assist the reduction of the dissolved oxygen to start the oxidation of copper. This might be the reason why the free carbon dioxide promotes the oxidation of copper. Acknowledgments
Fig. 4. RctA, RctC, ZW determined by the curve fitting with the equivalent circuit shown in Fig. 1 and the concentration of the free carbon dioxide.
The authors wish to thank Japan Copper Development Association.
K. Sobue et al. / Surface and Coatings Technology 169 – 170 (2003) 662–665
References w1x H.S. Campbell, J. Inst. Metals 77 (1950) 345. w2x K. Sobue, S. Magaino, A. Kawaguchi, A. Sugahara, H. Imai,
665
Proceedings of the JSCE Materials and Environments 2000, (2000) 327. w3x A. Sugahara, K. Sobue, S. Magaino, A. Kawaguchi, H. imai, Proceedings of the 47th Japan Conference on Materials and Environments, (2000) 323.