- Email: [email protected]
In-situ surface potential characterization of a cathodically polarized coating
DESALINATION ELSEVIER
Desalination 158 (2003) 29-34 www.elsevier.comhcate/desal
In-situ surface potential characterization of a cathodically polarized coating A. Husain”*, A. Fakhraldeenb “Building and Ener.gy Technologies Department, ‘Department of Advanced Systems, Kuwait Institute,for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait Tel. + 96.5 4845763; Fax + 965 4815223; email: [email protected]
Received I6 January 2003; accepted 22 January 2003
Abstract The relevance of this study pertains to coating material performance in desalination and power plants and particularly during cathodic protection (CP) of coated steel piping network. Organic coatings find various uses in many industries and such coatings are used extensively in conjunction with CP by the petroleum and water industry for corrosion protection. One of the most common technical problems encountered by the inspector or engineers is that of micro-defects (pinholes) in coating and optimization of material selection. The nature of the surface electrochemical reactions associated with defects in these protective coatings and the ability ofthe Scanning Potential Mapping (SCM) to detect them is the concern of this investigation. The results indicate that this method can monitor the qualitative effectiveness of the cathodic depolarization CP. A demonstration problem is elaborated which gives information on the surface potential distribution on the exterior surface of the coal tar epoxy coated steel substrate with predefined defects. The results demonstrate the evolution with time of the SCM for the coated specimen, which shows initial increase of the surface potential. As the polarization potentials are concerned, the cathodic potential affected mainly their cathodic regions. Kqiwords:
Coal tar epoxy; Surface potential mapping (SCM); Coating defect; Cathodic depolarization
-_____ 1. Introduction Offshore pipelines and structures for desalination plants are commonly protected against
corrosion by cathodic protection using sacrificial anodes and/or impressed current (CP). In many instances additional corrosion protection is obtained by combining CP and coating system.
*Corresponding author. Presented nt the European Conference on Desalination and the Environment: Europea, Desalination Society, International Water Association. 00 I I -9164/03/$-
See front
PII: SO0 I l-9 164(03)00429-6
Fresh Walterfor All, Malta, 4--8 May 2003.
matter 0 2003 Elsevier Science B.V. All rights reserved
30
.-1.Ifmain, A. Fakhraldeen
Llesalination 1.58 (2003) 29.~34
Corrosion of metals exposed to seawater environments, frequently in the presence of deposits, has long been a major problem in industrial process plants and desalination units as well as piping piers. The anodic reaction of interest in this particular case is the oxidation of iron, indicated by a straight line in the schematic potential-log current density plot in Fig. 1. The cathodic reactions may be reduction of oxygen and water. The oxygen reduction reaction is expressed by 1120, + H,O + 2e- -+ 20H
(1)
This is shown as being Fig. 1, while the reduction controlled (or polarization be under the pH condition be expressed by
diffusion controlled in of water is activation controlled), and may prevalent in seawater
2H20 + 2e- -+ H2 + 20H-
(2)
By supplying a cathodic current density it,,the anodic current density is reduced to i,, and the potential is lowered from EC to E,,.
112% + H20 -------> 20H -
Fe----->Fe
2+
I_I2 O---;--> $ Hz +OH-i I
I ip
ic
log (current density)----------> Fig. I. Schematic diagram showing relationship between potential and log current density for the anodic and cathodic reactions on steel in seawater.
For potential E,, lower than -800 mV vs. reference electrode with reference to a silver/ silver chloride electrode (Ag/AgCI), the anodic reaction has virtually stopped. At much lower potentials, water reduction becomes the predominant cathodic reaction, and the hydrogen atom, which is produced, involves a danger of hydrogen embrittlement. For these reasons offshore steel structures under aerobic environments are in practice need to be polarized to a potential between -800 mV and -1500 mV vs. AglAgCl. Both reaction ( 1) and (2) normally produce hydroxide ions, and the pH at the steelkeawatel interface will increase. As a consequence, slightly soluble sea salts, primarily CaCO, and Mg(OH),, will precipitate on the steel surface and form a layer of “calcareous deposits” which will reduce the amount of oxygen reaching the steel surface. This means that the limiting current density for oxygen reduction is reduced; the effect of this is that the current necessary to polarize the structure becomes less. At the same time the relative importance ofthe water reduction reaction increases at potentials more positive than -1500 mV vs. Ag/AgCl reference electrode. This study describes laboratory experiments that confirm the above scientific facts using Surface Corrosion Potential Mapping Technique (SCM) OIJa coated steel substrate. Surface Corrosion Mapping (SCM) has been employed in this study in conjunction with an applied polarization potential to detect a predefined paint defects. The main purpose of this experiment is to demonstrate the following: (i) Whether the SCM technique is capable to perform its detection ability while scanning a cathodically polarized painted panel. (ii) Whetherth e sizes, morphology, and locations of the deliberate defects or pinholes can be correlated on the SCM map during the application of polarization potential, which will activate the localized corrosion current densities above the normal free corrosion level of the sample.
A. Husain, A. Fakhraldeen / Desalination
(iii) What is the effect of applying cathodic protection potential on the surface of the painted sample.
2. Specimen conditions
preparation
and experimental
The principal steps of SCM operation and specimen preparation for surface scanning have been described previously [l-5]. The technique is a modified version of the scanning reference electrode technique (SRET) developed earlier by Isaacs and others [6,7]. The present SCM experiment incorporated a potentiostatigalvanostat instrument to control the electrode potential of the painted sample during surface potential scanning with the SCM. The SCM measuring technique in this work used the three electrode method (i.e. auxiliary, working, and reference electrodes) employed extensively in potentiostatic electrochemical studies. An auxiliary platinum electrode acted as a current source and did not affect the measurement between the other two scanning electrodes. Another reference electrode in the form of silver-silver chloride (Agl AgCl) in a salt bridge was incorporated in the SCM test set up. The reference electrode sets the DC polarization potential of the sample to a preselected value prior to scanning. A test surface composed of coal tar epoxy (MPl) painted on contaminated (3%NaCl fine mist) steel substrate (10 cmx I.5 cm). A special electrically shielded coax wire was connected to the specimen prior to solution immersion, and the contact point was bolted to the bare steel substrate after removal of a small portion of the paint, and then covered with glass tube filled with chemical resistant epoxy. The purpose ofthe electrical wire connection is to obtain a stable electrical contact for polarizing the painted sample. The painted samples was configured as the working electrode to the specified potential using the potentiostat, as well as to adjust its potential with the aid of a high impedance voltmeter connected between the specimen and Ag/AgCI electrode.
1.58 (2003) 29-34
31
Four micro-defects (pinholes) of different sizes were introduced into the painted layer using a twist drill. The diameters of the pinholes were in the following order (1.5 mm, 1 mm, 0.5 mm and 0.25 mm), and the pinholes were drilled at a fixed depth of penetration equal to 500 microns. The immersion solution was 5% sodium chloride dissolved in distilled water and the pH was kept at 7-8. The sample was then mounted into the SCM scanning tray. Three SCM tests were then performed, while cathodically polarizing the sample, over the same area (10 mmx 10 mm), which consisted of the four different sizes of defects. The potentiostatic polarization potential test was in the following order of magnitude -400 mV vs. AglAgCl, -800 mV vs. AgJAgCl, and -1200 mV vs. AglAgCl. The painted panel was immersed in the test solution for a period of 24 h before the start of SCM scanning with cathodic polarization. The following scanning parameters have been employed on the SCM main program: Samples Coal tar epoxy (MPI) Area per SCM scan, mm2 100 Total number of SCM tests 4 Potential reading per scanning area 2000 readings Scanning duration, h 1 No. of readings along X axes 50 readings No. of readings along Y axes 40 readings
3. Results and discussion When the painted panel was slowly polarized in the cathodic direction to (-400 mV vs. Ag/AgCl saturated electrode), simultaneously, the microtip reference electrode was scanned over the face of the defects. The result of this SCM scan has indicated that the surface potential region exhibited different types of activities as shown in the SCM map in Fig. 2. The reason for such behavior could be attributed to the presence of corrosion reaction processes undermining of the iron oxide corrosion products film formed inside each defect, existed during the f?ee natural corrosion
Hzaain, ‘4. Fakhrzrldeen f Desalination 1.58 (2003) 29-34
32
AntXiiC Above -229 -232 -234 -23Y -247 -249 -333 -34s -365 -it10 RdW
-226 -226 -229 -232 -234 -239 -247 -24Y -333 -345 -365 -110
Cathodic
X distance ( 10 mm)
potential. However, the reproducibility of the defects was poor and may be due to the insufficient polarization potential. At more negative potentials, where the sample is cathodic at -800 mV the painted panel may have exhibited larger cathodic current densities than the defective area. The SCM 2D map in Fig. 3 shows clearly four defects that are delineated and correlated well with the actual size and morphology of the defects introduced on the painted sample and being marked successfully on the
Fig. 2. SCM 2-D map for coal tar epcw paint with the introduced defects dul?ng cathodic polarization to 400 mV VS. AgiAgC1.
SCM map. Fig. 4 shows a 3D display ofthe same map, which clearly indicates the presence of four active anodic peaks that correspond well with each defect size. Therefore, the reproducibility of the defects was excellent and matches properly the actual defect location on the real sample surface. When further cathodically polarizing to -I .2 mV vs. Ag/AgCI, the painted sample surface responded to the effect of polarization potential by showing complete disappearance ofthe anodic active peaks. as indicated in Fig. 5. This pheno-
Fig. 3. SCM 2D map forthe same painted area as above during cathodic polarization to -800 mV. It shows the presence of 4 pinholes delineated during mapping.
A. Husain, A. Fakhraldeen
i Desalination
158 (2003) 29-34
33
Fig. 4. SCM 3-D display map of Fig. 3.
Fig. 5SCM 3-D map for the same area during cathodic polarization at -1.2 V Vs. Ag/AgCl, where the sample is being cathodically protected.
menon is known as the protection effect caused by cathodic protection. Under this condition the painted panel is said to be immune from corrosion attack or being polarized into the immunity region with respect to the potential pH diagram of Pourbaix and Evans diagram previously shown in Fig. 1. Therefore, it can be said that cathodic polarization of the intentionally damaged painted sample required an optimum protection potential of - 1.2 V for it to survive from degradation caused by seawater chloride attack.
4. Conclusions The most interesting results from this study is the unique information obtained about cathodic protection processes of the introduced defects, whether it is partial or total and what factors control the electrochemical stability of the defects. The reproducibility of the active potential peaks as the defect sites when the samples were tested in seawater solution was excellent and depended upon the applied polarization potential. The SCM has been successfully used in this
34
‘4. Husain, A. Fakhratdeen
respect and can give clearer indication of the defective sites and degree of cathodic protection process for various sizes of paint defects. References [I] A. Husain, Precise determination of surface micro[2] [3]
galvanic behavior, Desalination, 139(2001) 333-340. A. Husain, 2nd UAE Symposium on Materials Science, Nov. 2427, 1998. A. Husain, Proc. 13th International Corrosion Congress, Melbourne, Australia, Nov.25529, 1996, pp. 215/l-215/6.
Desulination 158 (2003) 29-34 [41 A. Husain and F Al-Sabti, Proc. IV international Symposium on Electrochemical Impedance Spectroscopy, Rio-De Janeiro, Brazil, Aug.2-7. 1998, pp. I86 188. 151 A. Husain and A. AI-Hazza, Proc. 10th APCCC Conference, Bali, Indonesia, Oct. 27.~3I ~ 1997, pp. F2.1/6--F2.616. [61 H.S. lsaacs and N. Vyas, Electrochemical Corrosion Testing, Mansfield and U. Bertocii, Eds., ASTM STP727, 1981, p.3. [71 H.S. Isaacs andY. Ishikawa, Proc. NACE Corrosion Conference, Anaheim, CA, 1983, p.25.
Desalination 158 (2003) 29-34 www.elsevier.comhcate/desal
In-situ surface potential characterization of a cathodically polarized coating A. Husain”*, A. Fakhraldeenb “Building and Ener.gy Technologies Department, ‘Department of Advanced Systems, Kuwait Institute,for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait Tel. + 96.5 4845763; Fax + 965 4815223; email: [email protected]
Received I6 January 2003; accepted 22 January 2003
Abstract The relevance of this study pertains to coating material performance in desalination and power plants and particularly during cathodic protection (CP) of coated steel piping network. Organic coatings find various uses in many industries and such coatings are used extensively in conjunction with CP by the petroleum and water industry for corrosion protection. One of the most common technical problems encountered by the inspector or engineers is that of micro-defects (pinholes) in coating and optimization of material selection. The nature of the surface electrochemical reactions associated with defects in these protective coatings and the ability ofthe Scanning Potential Mapping (SCM) to detect them is the concern of this investigation. The results indicate that this method can monitor the qualitative effectiveness of the cathodic depolarization CP. A demonstration problem is elaborated which gives information on the surface potential distribution on the exterior surface of the coal tar epoxy coated steel substrate with predefined defects. The results demonstrate the evolution with time of the SCM for the coated specimen, which shows initial increase of the surface potential. As the polarization potentials are concerned, the cathodic potential affected mainly their cathodic regions. Kqiwords:
Coal tar epoxy; Surface potential mapping (SCM); Coating defect; Cathodic depolarization
-_____ 1. Introduction Offshore pipelines and structures for desalination plants are commonly protected against
corrosion by cathodic protection using sacrificial anodes and/or impressed current (CP). In many instances additional corrosion protection is obtained by combining CP and coating system.
*Corresponding author. Presented nt the European Conference on Desalination and the Environment: Europea, Desalination Society, International Water Association. 00 I I -9164/03/$-
See front
PII: SO0 I l-9 164(03)00429-6
Fresh Walterfor All, Malta, 4--8 May 2003.
matter 0 2003 Elsevier Science B.V. All rights reserved
30
.-1.Ifmain, A. Fakhraldeen
Llesalination 1.58 (2003) 29.~34
Corrosion of metals exposed to seawater environments, frequently in the presence of deposits, has long been a major problem in industrial process plants and desalination units as well as piping piers. The anodic reaction of interest in this particular case is the oxidation of iron, indicated by a straight line in the schematic potential-log current density plot in Fig. 1. The cathodic reactions may be reduction of oxygen and water. The oxygen reduction reaction is expressed by 1120, + H,O + 2e- -+ 20H
(1)
This is shown as being Fig. 1, while the reduction controlled (or polarization be under the pH condition be expressed by
diffusion controlled in of water is activation controlled), and may prevalent in seawater
2H20 + 2e- -+ H2 + 20H-
(2)
By supplying a cathodic current density it,,the anodic current density is reduced to i,, and the potential is lowered from EC to E,,.
112% + H20 -------> 20H -
Fe----->Fe
2+
I_I2 O---;--> $ Hz +OH-i I
I ip
ic
log (current density)----------> Fig. I. Schematic diagram showing relationship between potential and log current density for the anodic and cathodic reactions on steel in seawater.
For potential E,, lower than -800 mV vs. reference electrode with reference to a silver/ silver chloride electrode (Ag/AgCI), the anodic reaction has virtually stopped. At much lower potentials, water reduction becomes the predominant cathodic reaction, and the hydrogen atom, which is produced, involves a danger of hydrogen embrittlement. For these reasons offshore steel structures under aerobic environments are in practice need to be polarized to a potential between -800 mV and -1500 mV vs. AglAgCl. Both reaction ( 1) and (2) normally produce hydroxide ions, and the pH at the steelkeawatel interface will increase. As a consequence, slightly soluble sea salts, primarily CaCO, and Mg(OH),, will precipitate on the steel surface and form a layer of “calcareous deposits” which will reduce the amount of oxygen reaching the steel surface. This means that the limiting current density for oxygen reduction is reduced; the effect of this is that the current necessary to polarize the structure becomes less. At the same time the relative importance ofthe water reduction reaction increases at potentials more positive than -1500 mV vs. Ag/AgCl reference electrode. This study describes laboratory experiments that confirm the above scientific facts using Surface Corrosion Potential Mapping Technique (SCM) OIJa coated steel substrate. Surface Corrosion Mapping (SCM) has been employed in this study in conjunction with an applied polarization potential to detect a predefined paint defects. The main purpose of this experiment is to demonstrate the following: (i) Whether the SCM technique is capable to perform its detection ability while scanning a cathodically polarized painted panel. (ii) Whetherth e sizes, morphology, and locations of the deliberate defects or pinholes can be correlated on the SCM map during the application of polarization potential, which will activate the localized corrosion current densities above the normal free corrosion level of the sample.
A. Husain, A. Fakhraldeen / Desalination
(iii) What is the effect of applying cathodic protection potential on the surface of the painted sample.
2. Specimen conditions
preparation
and experimental
The principal steps of SCM operation and specimen preparation for surface scanning have been described previously [l-5]. The technique is a modified version of the scanning reference electrode technique (SRET) developed earlier by Isaacs and others [6,7]. The present SCM experiment incorporated a potentiostatigalvanostat instrument to control the electrode potential of the painted sample during surface potential scanning with the SCM. The SCM measuring technique in this work used the three electrode method (i.e. auxiliary, working, and reference electrodes) employed extensively in potentiostatic electrochemical studies. An auxiliary platinum electrode acted as a current source and did not affect the measurement between the other two scanning electrodes. Another reference electrode in the form of silver-silver chloride (Agl AgCl) in a salt bridge was incorporated in the SCM test set up. The reference electrode sets the DC polarization potential of the sample to a preselected value prior to scanning. A test surface composed of coal tar epoxy (MPl) painted on contaminated (3%NaCl fine mist) steel substrate (10 cmx I.5 cm). A special electrically shielded coax wire was connected to the specimen prior to solution immersion, and the contact point was bolted to the bare steel substrate after removal of a small portion of the paint, and then covered with glass tube filled with chemical resistant epoxy. The purpose ofthe electrical wire connection is to obtain a stable electrical contact for polarizing the painted sample. The painted samples was configured as the working electrode to the specified potential using the potentiostat, as well as to adjust its potential with the aid of a high impedance voltmeter connected between the specimen and Ag/AgCI electrode.
1.58 (2003) 29-34
31
Four micro-defects (pinholes) of different sizes were introduced into the painted layer using a twist drill. The diameters of the pinholes were in the following order (1.5 mm, 1 mm, 0.5 mm and 0.25 mm), and the pinholes were drilled at a fixed depth of penetration equal to 500 microns. The immersion solution was 5% sodium chloride dissolved in distilled water and the pH was kept at 7-8. The sample was then mounted into the SCM scanning tray. Three SCM tests were then performed, while cathodically polarizing the sample, over the same area (10 mmx 10 mm), which consisted of the four different sizes of defects. The potentiostatic polarization potential test was in the following order of magnitude -400 mV vs. AglAgCl, -800 mV vs. AgJAgCl, and -1200 mV vs. AglAgCl. The painted panel was immersed in the test solution for a period of 24 h before the start of SCM scanning with cathodic polarization. The following scanning parameters have been employed on the SCM main program: Samples Coal tar epoxy (MPI) Area per SCM scan, mm2 100 Total number of SCM tests 4 Potential reading per scanning area 2000 readings Scanning duration, h 1 No. of readings along X axes 50 readings No. of readings along Y axes 40 readings
3. Results and discussion When the painted panel was slowly polarized in the cathodic direction to (-400 mV vs. Ag/AgCl saturated electrode), simultaneously, the microtip reference electrode was scanned over the face of the defects. The result of this SCM scan has indicated that the surface potential region exhibited different types of activities as shown in the SCM map in Fig. 2. The reason for such behavior could be attributed to the presence of corrosion reaction processes undermining of the iron oxide corrosion products film formed inside each defect, existed during the f?ee natural corrosion
Hzaain, ‘4. Fakhrzrldeen f Desalination 1.58 (2003) 29-34
32
AntXiiC Above -229 -232 -234 -23Y -247 -249 -333 -34s -365 -it10 RdW
-226 -226 -229 -232 -234 -239 -247 -24Y -333 -345 -365 -110
Cathodic
X distance ( 10 mm)
potential. However, the reproducibility of the defects was poor and may be due to the insufficient polarization potential. At more negative potentials, where the sample is cathodic at -800 mV the painted panel may have exhibited larger cathodic current densities than the defective area. The SCM 2D map in Fig. 3 shows clearly four defects that are delineated and correlated well with the actual size and morphology of the defects introduced on the painted sample and being marked successfully on the
Fig. 2. SCM 2-D map for coal tar epcw paint with the introduced defects dul?ng cathodic polarization to 400 mV VS. AgiAgC1.
SCM map. Fig. 4 shows a 3D display ofthe same map, which clearly indicates the presence of four active anodic peaks that correspond well with each defect size. Therefore, the reproducibility of the defects was excellent and matches properly the actual defect location on the real sample surface. When further cathodically polarizing to -I .2 mV vs. Ag/AgCI, the painted sample surface responded to the effect of polarization potential by showing complete disappearance ofthe anodic active peaks. as indicated in Fig. 5. This pheno-
Fig. 3. SCM 2D map forthe same painted area as above during cathodic polarization to -800 mV. It shows the presence of 4 pinholes delineated during mapping.
A. Husain, A. Fakhraldeen
i Desalination
158 (2003) 29-34
33
Fig. 4. SCM 3-D display map of Fig. 3.
Fig. 5SCM 3-D map for the same area during cathodic polarization at -1.2 V Vs. Ag/AgCl, where the sample is being cathodically protected.
menon is known as the protection effect caused by cathodic protection. Under this condition the painted panel is said to be immune from corrosion attack or being polarized into the immunity region with respect to the potential pH diagram of Pourbaix and Evans diagram previously shown in Fig. 1. Therefore, it can be said that cathodic polarization of the intentionally damaged painted sample required an optimum protection potential of - 1.2 V for it to survive from degradation caused by seawater chloride attack.
4. Conclusions The most interesting results from this study is the unique information obtained about cathodic protection processes of the introduced defects, whether it is partial or total and what factors control the electrochemical stability of the defects. The reproducibility of the active potential peaks as the defect sites when the samples were tested in seawater solution was excellent and depended upon the applied polarization potential. The SCM has been successfully used in this
34
‘4. Husain, A. Fakhratdeen
respect and can give clearer indication of the defective sites and degree of cathodic protection process for various sizes of paint defects. References [I] A. Husain, Precise determination of surface micro[2] [3]
galvanic behavior, Desalination, 139(2001) 333-340. A. Husain, 2nd UAE Symposium on Materials Science, Nov. 2427, 1998. A. Husain, Proc. 13th International Corrosion Congress, Melbourne, Australia, Nov.25529, 1996, pp. 215/l-215/6.
Desulination 158 (2003) 29-34 [41 A. Husain and F Al-Sabti, Proc. IV international Symposium on Electrochemical Impedance Spectroscopy, Rio-De Janeiro, Brazil, Aug.2-7. 1998, pp. I86 188. 151 A. Husain and A. AI-Hazza, Proc. 10th APCCC Conference, Bali, Indonesia, Oct. 27.~3I ~ 1997, pp. F2.1/6--F2.616. [61 H.S. lsaacs and N. Vyas, Electrochemical Corrosion Testing, Mansfield and U. Bertocii, Eds., ASTM STP727, 1981, p.3. [71 H.S. Isaacs andY. Ishikawa, Proc. NACE Corrosion Conference, Anaheim, CA, 1983, p.25.