- Email: [email protected]
On-line corrosion monitoring in geothermal district heating systems. I. General corrosion rates
Corrosion Science 48 (2006) 1770–1778 www.elsevier.com/locate/corsci
On-line corrosion monitoring in geothermal district heating systems. I. General corrosion rates S. Richter a
a,*
, L.R. Hilbert b, R.I. Thorarinsdottir
c
The Icelandic Building Research Institute, Keldnaholti, 112 Reykjavik, Iceland b IPL, Technical University of Denmark, 2800 Lyngby, Denmark c The National Energy Authority, Grensasvegi 9, 108 Reykjavik, Iceland Received 14 December 2004; accepted 9 June 2005 Available online 8 September 2005
Abstract General corrosion rates in the geothermal district heating systems in Iceland are generally low, of the magnitude 1 lm/y. The reason is high pH (9.5), low-conductivity (200 lm/y) and negligible dissolved oxygen. The geothermal hot water is either used directly from source or to heat up cold ground water. The fluid naturally contains sulphide, which helps keeping the fluid oxygen-free but complicates the electrochemical environment. In this research on-line techniques for corrosion monitoring were tested and evaluated in this medium. Electrochemical methods worked well as long as frequency was kept low but ER worked better if oxygen was present. 2005 Elsevier Ltd. All rights reserved. Keywords: Geothermal; On-line; Carbon steel; Sulphide
*
Corresponding author. Tel.: +354 570 7300; fax: +354 570 7311. E-mail address: [email protected] (S. Richter).
0010-938X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2005.06.019
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
1771
1. Introduction Majority (87%) of Icelandic households are heated with district heating using geothermal energy [1]. Low-temperature (<150 C at 1000 m depth) sources can be used directly for district heating but high-temperature (>200 C at 1000 m depth) geothermal water is too rich in minerals and is instead used to heat up cold ground water, which is distributed to consumers. From Table 1 it can be seen that the geothermal water is soft and instead of strong oxide or carbonate protective layer, weaker ferrous sulphide layer is formed. The type of ferrous sulphide layer formed depends on the pH and concentration of sulphide in the medium. For pH > 8.8 [2] and relatively low concentration of sulphide as in the geothermal system, pyrite or pyrrhotite is formed rather then the weaker mackinawite. Pyrite/pyrrhotite are two-dimensional and therefore more protective than the three-dimensional mackinawite. However, pyrite and pyrrhotite break down in the presence of oxygen, facilitating localized corrosion. Corrosion rate has been measured in geothermal district heating systems with offline methods, such as weight loss coupons. The aim of this work is to employ on-line methods, such as electrochemical methods. Linear polarization resistance is relatively simple and easy to use. However, the theory behind it, developed by Stern and Geary [3], has certain limitations [4,5]. It does, for instance, not work well in low-conductivity medium or where more than one anodic and one cathodic reaction take place. The geothermal hot water has low conductivity, about 200 lS/cm and rather complex sulphur chemistry, where number of reactions are possible for the cathodic reaction. Earlier research [6] on the use of electrochemical corrosion monitoring in geothermal water showed strong non-linearity in the form of hysteresis. The aim of this work is to accommodate electrochemical methods to the low-conductivity environment of geothermal water.
2. Experimental The research was carried out in three district heating pumping stations, from where the water is pumped out to consumers. Corrosion probes and weight loss coupons were inserted into a by-pass unit. The differences between the stations are outlined in Table 2. In Keflavik the district heating water is ground water, heated with geothermal steam in heat Table 1 Characteristics of geothermal district heating water used in the experiment Station
T (C)
pH
O2
H2S
Na
K
Ca
Mg
Cl
CO2
Si
Keflavik Bolholt Oskjuhlid
105 80 80
8.9 9.5 9.5
<0.003 <0.003 <0.1
0.08 0.5 <0.6
34 59 43
1.5 2 0.9
8.1 3.4 2.5
6.9 0.01 0.01
75 56 20
6 17 20
13 150 88
The unit is (ppm) unless stated otherwise.
1772
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
Table 2 Characteristics of the pumping stations, where the research was conducted Station
Source of water
Oxygen
Hydrogen sulphide
Keflavik Bolholt Oskjuhlid
Heated Boreholes Reservoir tanks
Not detected Not detected Present
Changing Constant Changing
exchangers. Hot springs in and around Reykjavik supply hot water, which is pumped out from Bolholt and Oskjuhlid. In Bolholt, it is pumped directly from source but in Oskjuhlid it is pumped from a reservoir tank. Oxygen ingress in the reservoir tanks in Oskjuhlid can occur so the level of dissolved oxygen is ca. 0–100 ppb but the other two stations are oxygen free (<3 ppb). Despite using heated ground water, some sulphide is occasionally found in the Keflavik water (0–150 ppb). The Bolholt water has a steady concentration of 500 ppb and the Oskjuhlid water has about 0–600 ppb concentration of sulphide. Weight loss coupons were made of carbon steel, same as the piping. The coupons were degreased with acetone and weighed to the nearest 0.1 mg. The size of the coupons was 50 mm · 25 mm · 1 mm. After the exposure the coupons were cleaned according to standard ISO 8407 [7]. The solution was 6 M hydrochloric acid, 0.025 M hexamethylenetetramine. The coupons were submerged in the cleaning solution for 10 min, after which they were rinsed with cold tap water, distilled water and finally with acetone before they were weighed. The procedure was repeated four times. The electrochemical measurements were linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS). They were performed with three identical carbon steel electrodes, which were polished to 1000 grit. The area of each of the electrode was 9.2 cm2. Polarization of 10 mV was used with scan rates 0.01 mV/s, 0.005 mV/s, 0.001 mV/s and 0.0005 mV/s for LPR measurements and the frequency range of 105–105 Hz for EIS measurements. Additional measurements were carried out in Oskjuhlid, these include dissolved oxygen and electrical resistance (ER). The dissolved oxygen was measured with colorimetric ampoules. The ER monitoring was performed with a newly developed differential technique from MetriCorr [8]. This technique measures not only the change in the resistance with time but also the rate of the change. This enables more sensitive monitoring, which is independent of electrochemical processes. The equipment was exposed to the systems for 6 months during the year 2004.
3. Results Because of low solution conductivity and to avoid capacitive effects, it is necessary to maintain low scan rates [9]. However, measurements with low scan rates take long time, which is not only inconvenient but also increases errors caused by the transient
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
1773
Table 3 Average corrosion rate (lm/y) results from the electrochemical monitoring Scan rate (mV/s)
Keflavik Bolholt Oskjuhlid
0.01
0.005
0.001
0.0005
3.8 5.6 59
3.1 3.9 43
1.7 1.4 23
1.2 0.76 N/A
EIS
Weight loss
Zero point calculation
1.3 0.4 N/A
2.3 1.8 16
1.3 0.91 N/A
Comparison is given by 12 months weight loss coupons and zero point calculation. N/A—Not available.
behaviour of the system. For example scan rate of 0.001 mV/s and 0.0005 mV/s take 5.5 and 11 h, respectively, for polarization of ±10 mV. According to the measured corrosion rate in Table 3, it is not possible to use higher scan rates without influencing the results. Scan rate as low as 0.005 mV/s produces results that are about twice the result from weight loss. Two methods were used to evaluate the results according to electrochemical measurements. One was to use weight loss measurements and the other was to use the electrochemical data and extrapolate to zero scan rate [10]. The advantages of the latter method are that present measurements can be used and there is no need for external measurements. Thus it is quicker more readily available. Eqs. (1) and (2) are obtained from linear regression of the results of LPR measurements for Keflavik and Bolholt, respectively and are used to calculate corrosion rate vcorr at zero scan rate, v. The result is displayed in Table 3 as well as the results of a 12 month weight loss measurement. Keflavik vcorr ¼ 268v þ 1:3 Bolholt vcorr ¼ 495v þ 0:91
ð1Þ ð2Þ
The lowest scan rate produces the most accurate corrosion rate [11] since it has the minimal capacitive effect. The zero point calculation corresponds well to the slowest scan rate, 0.0005 mV/s, so it can be concluded that the polarization resistance results are very accurate at that scan rate. The weight loss coupons correspond better to the scan rate of 0.001 mV/s. The weight loss coupons seem to overestimate the general corrosion rate but perhaps they display the general behaviour of the system, which can include underdeposit pitting [12]. Electrochemical measurements in Oskjuhlid are somewhat difficult due to oxygen ingress in the presence of sulphur compounds. The dissolved oxygen concentration varies typically between 0 and 100 ppb. Only few measurement points were valid and not enough to model the results to produce a linear correlation between the scan rate and the corrosion rate. The measurements in Oskjuhlid took place over one month (December 2003) and during that time the dissolved oxygen was measured between 30 and 80 ppb by colorimetric ampoules. The oxygen concentration was at maximum when the valid measurements were made and yield to a corrosion rate of 23 lm/y for the second lowest scan rate (0.001 mV/s). No valid results were gathered for the lowest scan rate.
1774
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
Earlier research [13], conducted in July 2003 gave corrosion rate in the vicinity of 5 lm/y whilst dissolved oxygen measured 6–15 ppb. The concentration of dissolved oxygen has a direct influence on the corrosion rate and is the most influential factor of corrosion in this system. Fig. 1 shows the LPR response from each of the stations. The hysteresis is plainly seen in the figure, especially for Oskjuhlid station. Linearity is observed around zero current density and the polarization resistance is calculated from the slope of that line. A close-up is shown in Fig. 1(b), where the response can be more easily distinguished. Figs. 2–4 show example of a Nyquist plots from each of the stations. The figures only show partial semi-circles so in order to determine the corrosion rate, the measurements need to be extrapolated until they cross the x-axis. In Oskjuhlid the measurements are faulty due to the oxygen ingress. The extrapolation was performed with equivalent circuit program [14] and the average results are shown in Table 3. The outcome is very close to the LPR result at 0.0005 mV/s. At the high frequency end of the Nyquist plots small semi-circles can be seen. In Keflavik, the semi-circle is followed by a diffusion line before the big semi-circle begins. In Bolholt two semi-circles occur without diffusion line and the first circle is very small. Despite going to very low frequencies (104 Hz), the semi-circle is rather incomplete. This is due to the high capacitive effect. The capacitance is in the mF range (1– 8 mF), which is rather high. This effect can be seen on the Bode plot as well (Fig. 5) where the modulus is around zero for most of the frequency range and only starts to rise at around 0.1–0.01 Hz (Keflavik/Bolholt) and does not reach the break point where Rs + Rp starts. The phase is incomplete for all stations, reaching maximum of 74 in Bolholt. The maximum phase is reached close to the minimum frequency.
Fig. 1. Linear polarization curves from each of the stations. A hysteresis is apparent in all cases but linearity prevails around zero current density. The difference between Oskjuhlid and the other stations is great enough that the polarization curves from Keflavik and Bolholt cannot be easily distinguished: (a) the entire polarization (±10 mV) and (b) close-up around zero current density, where Keflavik and Bolholt can be seen separately.
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
1775
Fig. 2. An example of Nyquist graph displaying EIS results from Keflavik: (a) all frequencies and (b) close-up on high frequencies.
Fig. 3. An example of Nyquist graph displaying EIS results from Bolholt: (a) all frequencies and (b) close-up on high frequencies.
Fig. 6 shows results from on-line ER monitoring and dissolved oxygen measurements. It can be seen that the ER monitoring responds well to the changes in dissolved oxygen, which is the main corrosive factor in the system. When no oxygen is measured (July 2004) the ER data drops to zero although it oscillates indicating that there is an on–off effect where trace amounts of oxygen causes corrosive effects but reacts with the sulphide and is eliminated.
1776
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
Fig. 4. An example of Nyquist graph displaying EIS results from Oskjuhlid.
Fig. 5. Modulus and phase of EIS results displayed on Bode graphs from: (a) Keflavik, (b) Bolholt and (c) Oskjuhlid.
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
1777
Fig. 6. Corrosion monitoring with differential ER compared to point measurements of dissolved oxygen. The dissolved oxygen is displayed as a thick line corresponding to an axis to the right.
A peak occurs May 4th and coincides with an action that was performed at the reservoir, i.e. emptying of one of the tanks. The system recuperates quickly and is back to normal (<25 lm/y) after one day. 4. Conclusions Polarization resistance methods, such as LPR and EIS, worked well in geothermal water, in the absence of dissolved oxygen. However, they are not preferred for monitoring in this medium because of the low conductivity. The scan rate for LPR has to be as low as 0.001 mV/s and EIS measurements are not completed even for frequencies around 105 and show diffusional effects. These methods are time consuming and allows for errors due to the transient behaviour of the system. Problems occurred in the Oskjuhlid station, where oxygen can enter the water via reservoir tanks. The polarization resistance methods were hindered by deposits, which precipitate on the electrodes and will distort the monitoring of general corrosion rates. The low lifetime of the probe along with the potentially distorted measurements makes these methods unfeasible in this case. Non-electrochemical methods, such as differential ER, proved to be more reliable in the Oskjuhlid station, where oxygen is present in the water. This method is exempted from the limitations of the linear polarization method and has a response time far superior to both of the polarization methods tested. The research is on going and differential ER measurements will be performed in other points in the district heating network. References [1] National Energy Authority, Energy in Iceland. Historical Perspective, Present Status, Future Outlook, February 2004.
1778
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
[2] J.S. Smith, J.D.A. Miller, Nature of sulphides and their corrosive effect on Ferrous metals: a review, Br. Corros. J. 10 (1975) 136–143. [3] M. Stern, A.L. Geary, Electrochemical polarization, I. A theoretical analysis of the shape of polarization curves, J. Electrochem. Soc. 104 (1957) 56–63. [4] L.M. Callow, J.A. Richardson, J.L. Dawson, Corrosion monitoring using polarisation resistance measurements, Br. Corros. J. 11 (1976) 132–139. [5] K.B. Oldham, F. Mansfeld, On the so-called linear polarization method for measurement of corrosion rates, Corrosion 27 (1971) 434–435. [6] R.I. Thorarinsdottir, Corrosion of Copper and Copper Alloys in Geothermal District Heating Water Containing Sulphide, Ph.D. Thesis, The Technical University of Denmark, 2000. [7] ISO 8407, Corrosion of metals and alloys—Removal of corrosion products from corrosion test specimens, 1991. [8] L.V. Nielsen, K.V. Nielsen, Differential ER-technology for measuring degree of accumulated corrosion as well as instant corrosion rateProceedings Corrosion 2003, NACE International, Houston, TX, USA, 2003. [9] J.R. Scully, Polarization resistance method for determination of instantaneous corrosion rates, Corrosion 56 (2000) 199–218. [10] D.D. MacDonald, An impedance interpretation of small amplitude cyclic voltammetry. I. Theoretical analysis for a resistive–capacitive system, J. Electrochem. Soc. 125 (1978) 1443–1449. [11] R.G. Kelly et al., Electrochemical Techniques in Corrosion Science and Engineering, Marcel Dekker, 2003 (Chapter 4). [12] D.A. Jones, Principles and Prevention of Corrosion, second ed., Pearson Education, 1996 (Chapter 7). [13] S. Richter, R.I. Thorarinsdottir, On-line monitoring of corrosion in low-corrosive environment, in: Proceedings 13, Scandinavian Corrosion Congress, Reykjavik, Iceland, 2004. [14] B.A. Boukamp, Equivalent circuit (EQUIVCRT.PAS), Users manual, University of Twente, 1989.
On-line corrosion monitoring in geothermal district heating systems. I. General corrosion rates S. Richter a
a,*
, L.R. Hilbert b, R.I. Thorarinsdottir
c
The Icelandic Building Research Institute, Keldnaholti, 112 Reykjavik, Iceland b IPL, Technical University of Denmark, 2800 Lyngby, Denmark c The National Energy Authority, Grensasvegi 9, 108 Reykjavik, Iceland Received 14 December 2004; accepted 9 June 2005 Available online 8 September 2005
Abstract General corrosion rates in the geothermal district heating systems in Iceland are generally low, of the magnitude 1 lm/y. The reason is high pH (9.5), low-conductivity (200 lm/y) and negligible dissolved oxygen. The geothermal hot water is either used directly from source or to heat up cold ground water. The fluid naturally contains sulphide, which helps keeping the fluid oxygen-free but complicates the electrochemical environment. In this research on-line techniques for corrosion monitoring were tested and evaluated in this medium. Electrochemical methods worked well as long as frequency was kept low but ER worked better if oxygen was present. 2005 Elsevier Ltd. All rights reserved. Keywords: Geothermal; On-line; Carbon steel; Sulphide
*
Corresponding author. Tel.: +354 570 7300; fax: +354 570 7311. E-mail address: [email protected] (S. Richter).
0010-938X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2005.06.019
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
1771
1. Introduction Majority (87%) of Icelandic households are heated with district heating using geothermal energy [1]. Low-temperature (<150 C at 1000 m depth) sources can be used directly for district heating but high-temperature (>200 C at 1000 m depth) geothermal water is too rich in minerals and is instead used to heat up cold ground water, which is distributed to consumers. From Table 1 it can be seen that the geothermal water is soft and instead of strong oxide or carbonate protective layer, weaker ferrous sulphide layer is formed. The type of ferrous sulphide layer formed depends on the pH and concentration of sulphide in the medium. For pH > 8.8 [2] and relatively low concentration of sulphide as in the geothermal system, pyrite or pyrrhotite is formed rather then the weaker mackinawite. Pyrite/pyrrhotite are two-dimensional and therefore more protective than the three-dimensional mackinawite. However, pyrite and pyrrhotite break down in the presence of oxygen, facilitating localized corrosion. Corrosion rate has been measured in geothermal district heating systems with offline methods, such as weight loss coupons. The aim of this work is to employ on-line methods, such as electrochemical methods. Linear polarization resistance is relatively simple and easy to use. However, the theory behind it, developed by Stern and Geary [3], has certain limitations [4,5]. It does, for instance, not work well in low-conductivity medium or where more than one anodic and one cathodic reaction take place. The geothermal hot water has low conductivity, about 200 lS/cm and rather complex sulphur chemistry, where number of reactions are possible for the cathodic reaction. Earlier research [6] on the use of electrochemical corrosion monitoring in geothermal water showed strong non-linearity in the form of hysteresis. The aim of this work is to accommodate electrochemical methods to the low-conductivity environment of geothermal water.
2. Experimental The research was carried out in three district heating pumping stations, from where the water is pumped out to consumers. Corrosion probes and weight loss coupons were inserted into a by-pass unit. The differences between the stations are outlined in Table 2. In Keflavik the district heating water is ground water, heated with geothermal steam in heat Table 1 Characteristics of geothermal district heating water used in the experiment Station
T (C)
pH
O2
H2S
Na
K
Ca
Mg
Cl
CO2
Si
Keflavik Bolholt Oskjuhlid
105 80 80
8.9 9.5 9.5
<0.003 <0.003 <0.1
0.08 0.5 <0.6
34 59 43
1.5 2 0.9
8.1 3.4 2.5
6.9 0.01 0.01
75 56 20
6 17 20
13 150 88
The unit is (ppm) unless stated otherwise.
1772
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
Table 2 Characteristics of the pumping stations, where the research was conducted Station
Source of water
Oxygen
Hydrogen sulphide
Keflavik Bolholt Oskjuhlid
Heated Boreholes Reservoir tanks
Not detected Not detected Present
Changing Constant Changing
exchangers. Hot springs in and around Reykjavik supply hot water, which is pumped out from Bolholt and Oskjuhlid. In Bolholt, it is pumped directly from source but in Oskjuhlid it is pumped from a reservoir tank. Oxygen ingress in the reservoir tanks in Oskjuhlid can occur so the level of dissolved oxygen is ca. 0–100 ppb but the other two stations are oxygen free (<3 ppb). Despite using heated ground water, some sulphide is occasionally found in the Keflavik water (0–150 ppb). The Bolholt water has a steady concentration of 500 ppb and the Oskjuhlid water has about 0–600 ppb concentration of sulphide. Weight loss coupons were made of carbon steel, same as the piping. The coupons were degreased with acetone and weighed to the nearest 0.1 mg. The size of the coupons was 50 mm · 25 mm · 1 mm. After the exposure the coupons were cleaned according to standard ISO 8407 [7]. The solution was 6 M hydrochloric acid, 0.025 M hexamethylenetetramine. The coupons were submerged in the cleaning solution for 10 min, after which they were rinsed with cold tap water, distilled water and finally with acetone before they were weighed. The procedure was repeated four times. The electrochemical measurements were linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS). They were performed with three identical carbon steel electrodes, which were polished to 1000 grit. The area of each of the electrode was 9.2 cm2. Polarization of 10 mV was used with scan rates 0.01 mV/s, 0.005 mV/s, 0.001 mV/s and 0.0005 mV/s for LPR measurements and the frequency range of 105–105 Hz for EIS measurements. Additional measurements were carried out in Oskjuhlid, these include dissolved oxygen and electrical resistance (ER). The dissolved oxygen was measured with colorimetric ampoules. The ER monitoring was performed with a newly developed differential technique from MetriCorr [8]. This technique measures not only the change in the resistance with time but also the rate of the change. This enables more sensitive monitoring, which is independent of electrochemical processes. The equipment was exposed to the systems for 6 months during the year 2004.
3. Results Because of low solution conductivity and to avoid capacitive effects, it is necessary to maintain low scan rates [9]. However, measurements with low scan rates take long time, which is not only inconvenient but also increases errors caused by the transient
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
1773
Table 3 Average corrosion rate (lm/y) results from the electrochemical monitoring Scan rate (mV/s)
Keflavik Bolholt Oskjuhlid
0.01
0.005
0.001
0.0005
3.8 5.6 59
3.1 3.9 43
1.7 1.4 23
1.2 0.76 N/A
EIS
Weight loss
Zero point calculation
1.3 0.4 N/A
2.3 1.8 16
1.3 0.91 N/A
Comparison is given by 12 months weight loss coupons and zero point calculation. N/A—Not available.
behaviour of the system. For example scan rate of 0.001 mV/s and 0.0005 mV/s take 5.5 and 11 h, respectively, for polarization of ±10 mV. According to the measured corrosion rate in Table 3, it is not possible to use higher scan rates without influencing the results. Scan rate as low as 0.005 mV/s produces results that are about twice the result from weight loss. Two methods were used to evaluate the results according to electrochemical measurements. One was to use weight loss measurements and the other was to use the electrochemical data and extrapolate to zero scan rate [10]. The advantages of the latter method are that present measurements can be used and there is no need for external measurements. Thus it is quicker more readily available. Eqs. (1) and (2) are obtained from linear regression of the results of LPR measurements for Keflavik and Bolholt, respectively and are used to calculate corrosion rate vcorr at zero scan rate, v. The result is displayed in Table 3 as well as the results of a 12 month weight loss measurement. Keflavik vcorr ¼ 268v þ 1:3 Bolholt vcorr ¼ 495v þ 0:91
ð1Þ ð2Þ
The lowest scan rate produces the most accurate corrosion rate [11] since it has the minimal capacitive effect. The zero point calculation corresponds well to the slowest scan rate, 0.0005 mV/s, so it can be concluded that the polarization resistance results are very accurate at that scan rate. The weight loss coupons correspond better to the scan rate of 0.001 mV/s. The weight loss coupons seem to overestimate the general corrosion rate but perhaps they display the general behaviour of the system, which can include underdeposit pitting [12]. Electrochemical measurements in Oskjuhlid are somewhat difficult due to oxygen ingress in the presence of sulphur compounds. The dissolved oxygen concentration varies typically between 0 and 100 ppb. Only few measurement points were valid and not enough to model the results to produce a linear correlation between the scan rate and the corrosion rate. The measurements in Oskjuhlid took place over one month (December 2003) and during that time the dissolved oxygen was measured between 30 and 80 ppb by colorimetric ampoules. The oxygen concentration was at maximum when the valid measurements were made and yield to a corrosion rate of 23 lm/y for the second lowest scan rate (0.001 mV/s). No valid results were gathered for the lowest scan rate.
1774
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
Earlier research [13], conducted in July 2003 gave corrosion rate in the vicinity of 5 lm/y whilst dissolved oxygen measured 6–15 ppb. The concentration of dissolved oxygen has a direct influence on the corrosion rate and is the most influential factor of corrosion in this system. Fig. 1 shows the LPR response from each of the stations. The hysteresis is plainly seen in the figure, especially for Oskjuhlid station. Linearity is observed around zero current density and the polarization resistance is calculated from the slope of that line. A close-up is shown in Fig. 1(b), where the response can be more easily distinguished. Figs. 2–4 show example of a Nyquist plots from each of the stations. The figures only show partial semi-circles so in order to determine the corrosion rate, the measurements need to be extrapolated until they cross the x-axis. In Oskjuhlid the measurements are faulty due to the oxygen ingress. The extrapolation was performed with equivalent circuit program [14] and the average results are shown in Table 3. The outcome is very close to the LPR result at 0.0005 mV/s. At the high frequency end of the Nyquist plots small semi-circles can be seen. In Keflavik, the semi-circle is followed by a diffusion line before the big semi-circle begins. In Bolholt two semi-circles occur without diffusion line and the first circle is very small. Despite going to very low frequencies (104 Hz), the semi-circle is rather incomplete. This is due to the high capacitive effect. The capacitance is in the mF range (1– 8 mF), which is rather high. This effect can be seen on the Bode plot as well (Fig. 5) where the modulus is around zero for most of the frequency range and only starts to rise at around 0.1–0.01 Hz (Keflavik/Bolholt) and does not reach the break point where Rs + Rp starts. The phase is incomplete for all stations, reaching maximum of 74 in Bolholt. The maximum phase is reached close to the minimum frequency.
Fig. 1. Linear polarization curves from each of the stations. A hysteresis is apparent in all cases but linearity prevails around zero current density. The difference between Oskjuhlid and the other stations is great enough that the polarization curves from Keflavik and Bolholt cannot be easily distinguished: (a) the entire polarization (±10 mV) and (b) close-up around zero current density, where Keflavik and Bolholt can be seen separately.
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
1775
Fig. 2. An example of Nyquist graph displaying EIS results from Keflavik: (a) all frequencies and (b) close-up on high frequencies.
Fig. 3. An example of Nyquist graph displaying EIS results from Bolholt: (a) all frequencies and (b) close-up on high frequencies.
Fig. 6 shows results from on-line ER monitoring and dissolved oxygen measurements. It can be seen that the ER monitoring responds well to the changes in dissolved oxygen, which is the main corrosive factor in the system. When no oxygen is measured (July 2004) the ER data drops to zero although it oscillates indicating that there is an on–off effect where trace amounts of oxygen causes corrosive effects but reacts with the sulphide and is eliminated.
1776
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
Fig. 4. An example of Nyquist graph displaying EIS results from Oskjuhlid.
Fig. 5. Modulus and phase of EIS results displayed on Bode graphs from: (a) Keflavik, (b) Bolholt and (c) Oskjuhlid.
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
1777
Fig. 6. Corrosion monitoring with differential ER compared to point measurements of dissolved oxygen. The dissolved oxygen is displayed as a thick line corresponding to an axis to the right.
A peak occurs May 4th and coincides with an action that was performed at the reservoir, i.e. emptying of one of the tanks. The system recuperates quickly and is back to normal (<25 lm/y) after one day. 4. Conclusions Polarization resistance methods, such as LPR and EIS, worked well in geothermal water, in the absence of dissolved oxygen. However, they are not preferred for monitoring in this medium because of the low conductivity. The scan rate for LPR has to be as low as 0.001 mV/s and EIS measurements are not completed even for frequencies around 105 and show diffusional effects. These methods are time consuming and allows for errors due to the transient behaviour of the system. Problems occurred in the Oskjuhlid station, where oxygen can enter the water via reservoir tanks. The polarization resistance methods were hindered by deposits, which precipitate on the electrodes and will distort the monitoring of general corrosion rates. The low lifetime of the probe along with the potentially distorted measurements makes these methods unfeasible in this case. Non-electrochemical methods, such as differential ER, proved to be more reliable in the Oskjuhlid station, where oxygen is present in the water. This method is exempted from the limitations of the linear polarization method and has a response time far superior to both of the polarization methods tested. The research is on going and differential ER measurements will be performed in other points in the district heating network. References [1] National Energy Authority, Energy in Iceland. Historical Perspective, Present Status, Future Outlook, February 2004.
1778
S. Richter et al. / Corrosion Science 48 (2006) 1770–1778
[2] J.S. Smith, J.D.A. Miller, Nature of sulphides and their corrosive effect on Ferrous metals: a review, Br. Corros. J. 10 (1975) 136–143. [3] M. Stern, A.L. Geary, Electrochemical polarization, I. A theoretical analysis of the shape of polarization curves, J. Electrochem. Soc. 104 (1957) 56–63. [4] L.M. Callow, J.A. Richardson, J.L. Dawson, Corrosion monitoring using polarisation resistance measurements, Br. Corros. J. 11 (1976) 132–139. [5] K.B. Oldham, F. Mansfeld, On the so-called linear polarization method for measurement of corrosion rates, Corrosion 27 (1971) 434–435. [6] R.I. Thorarinsdottir, Corrosion of Copper and Copper Alloys in Geothermal District Heating Water Containing Sulphide, Ph.D. Thesis, The Technical University of Denmark, 2000. [7] ISO 8407, Corrosion of metals and alloys—Removal of corrosion products from corrosion test specimens, 1991. [8] L.V. Nielsen, K.V. Nielsen, Differential ER-technology for measuring degree of accumulated corrosion as well as instant corrosion rateProceedings Corrosion 2003, NACE International, Houston, TX, USA, 2003. [9] J.R. Scully, Polarization resistance method for determination of instantaneous corrosion rates, Corrosion 56 (2000) 199–218. [10] D.D. MacDonald, An impedance interpretation of small amplitude cyclic voltammetry. I. Theoretical analysis for a resistive–capacitive system, J. Electrochem. Soc. 125 (1978) 1443–1449. [11] R.G. Kelly et al., Electrochemical Techniques in Corrosion Science and Engineering, Marcel Dekker, 2003 (Chapter 4). [12] D.A. Jones, Principles and Prevention of Corrosion, second ed., Pearson Education, 1996 (Chapter 7). [13] S. Richter, R.I. Thorarinsdottir, On-line monitoring of corrosion in low-corrosive environment, in: Proceedings 13, Scandinavian Corrosion Congress, Reykjavik, Iceland, 2004. [14] B.A. Boukamp, Equivalent circuit (EQUIVCRT.PAS), Users manual, University of Twente, 1989.