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Electrochemical behavior of thin masking coatings
Corrosion Science, Vol. 33, No. 7, pp. 1141-1145. 1992
0{110-938X/92 $5.(X) + 0.00 Pergamon Press Ltd
Printed in Great Britain.
ELECTROCHEMICAL
H. S.
ISAACS,*
A. J.
BEHAVIOR COATINGS
DAVENPORT,*
J.
OF THIN
HAWKINSJ"
MASKING
and G. E. THOMPSON+
* Brookhaven National Laboratory, Upton, NY 11973, U.S.A. +Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, Manchester, M60 IQD, U.K.
A b s t r a c t - - A s s e s s m e n t of a vinyl acetate/vinyl chloride co-polymer as a coating or masking material in electrochemical studies has been made. With a double dip coating on aluminum, copper, iron or platinum in a sodium chloride electrolyte, an open circuit potential was recorded close to that of the uncoated metals. Polarization m e a s u r e m e n t s were made at low current densities of both coated and uncoated metals. The rates of the interfaciat reactions were reduced by the coating, but not dramatically. Similar potentials were recorded in the absence of aqueous electrolyte for metal electrodes separated only by the coating. As a consequence, a masked copper connecting wire markedly influences the electrochemical behavior of uncoated aluminum undcr natural immersion conditions. The study indicated that the coating itself acts as an electrolyte, with electrochemical reactions taking place at the metal/coating interface.
INTRODUCTION
THIN PROTECTIVEinsulating layers or coatings are frequently employed to mask samples and to limit the area of the surface exposed to the electrolytes for electrochemical measurements. The effectiveness of the masking depends on the conduction processes taking place or the presence of pores in the coating material. Mayne and his coworkers ~'2 studied the ionic conduction of detached coatings and concluded that polymer films are heterogeneous and contain fine areas of locally high conductivities. The high conduction was considered in the case of cross-linked polymer films to be due to the degree of cross linking. Leidheiser and Kendig 3 attributed impedance changes in coatings to the penetration of electrolyte. These considerations led to the conclusion that the properties of the coating determine the sites of corrosion initiation. In a series of experiments in which the open circuit potential of aluminum was measured, two different types of masking material were used: beeswax 4 and vinyl acetate/vinyl chloride copolymer. The latter was chosen for experiments in which it was necessary to limit the coating thickness. Surprisingly, significant differences were found in the open circuit potentials of specimens prepared using the two different coatings. The experiments described in this paper were carried out to determine the cause of this effect. Although very thin well-characterized insulating coatings with controllable thickness are commercially available for electronic applications, these were not used since thermal treatments above 100°C are required. Such treatments can have a significant influence on the corrosion behavior of the substrate.5
Manuscript received 21 May 1991. 1141
1142
H . S . ISAACS et al.
. / GLASS SLIOE Fic. 1.
METALWIRES
Metal/lacquer/metal cell to test the electrochemical behavior of the lacquer. EXPERIMENTAL
METHOD
High purity aluminum electrodes (99.999%) were electropolished in a perchloric acid/ethanol mixture. 4 Iron, platinum and copper (>99.9%) wire electrodes were mechanically polished with 600 grade silicon carbide paper. A vinyl acetate/vinyl chloride co-polymer coating was applied by dipping metal electrodes into the lacquer, leaving a small region uncoated for electrical contact (the uncoated area was not exposed to the solution). The coating was allowed to dry for 15 rain at ambient temperature prior to the application of a second coating in a similar manner. The second coating was allowed to dry for 60 min. The final coating thickness was about 0.12 m m . The double dipping procedure was used to minimize the risk of continuous pores across the thickness of the coating. M e a s u r e m e n t s were made with wire electrodes or flag-shaped electrodes and no second metal was used for electrical connections for both coated and uncoated specimens. Electrometers were used to measure the open circuit potentials relative to a saturated calomel electrode. It was expected that very low currents would lead to the polarization of coated metal surfaces. In order to measure these low currents, two electrometers were used. O n e electrometer, a Keithley 616, was employed to introduce a known resistor between the coupled electrodes. The potential across the resistor was recorded and hence the current flowing was obtained. The resistors could be varied by orders of magnitude from 103 to 1011 , depending on the current range required. The second electrometer measured the potential of one of the electrodes relative to a calomel electrode and as the potential difference between it and the second electrode was obtained from the Keithley electrometer the polarization characteristics of both electrodes were generated. These m e a s u r e m e n t s were carried out in 1 M NaCI prepared from analytical grade reagent and distilled water exposed to air at room temperature. Metal/metal cells which used only the coating as a possible electrolyte were made by taping one end of two different metal wires or strips to hold them about 0.5 m m apart on a glass slide. The other parallel ends of the adjacent wires were then painted with the coating. The surfaces of the metals were prepared by abrading with 600 grade silicon carbide paper. Figure 1 illustrates this cell. EXPERIMENTAL
RESULTS
AND
DISCUSSION
The open circuit potentials of the various metals after immersion for 3 h in 1 M NaCI are given in Table 1. The electrodes were either uncoated or had been dipT A B L E 1.
THE
MAXIMUM
AND
MINIMUM
POTENTIALS
COATED AND UNCOATED METALS AFTER IMMERSION IN 1
OF
M
NaC1 Potential, V(SCE) Metal Platinum Copper Iron Aluminum
Uncoated -0.23, -0.24, -0.55, -1.12,
-0.14 -0.07 -0.48 -0.96
Coated 0.27, - 0 . 1 2 - 0 . 2 3 , 0.06 -0.50, -0.56 -0.80, -0.98
Electrochemical behavior of thin masking coatings 0,2 L
l
r
i •
I •
1 I43
I - coaled Cu
0 •
*
,m.
"-'* bare Cu
-0.2
>
-0.4 _•
v
-I0
Fic. 2.
k[
W
bore
-
-g
-8
-7
CURRENT
OENSITY
( k cm"~ )
-6
-5
The polarization behavior of coated and bare copper, iron and aluminum in air exposed 1 M NaCI.
coated in lacquer. It is evident that the open circuit potential of the uncoated and coated specimens are close although the coated samples generally have slightly more positive values. It is clear that the coating is not effectively masking the specimen surface and that electrochemical reactions are proceeding at the coating/metal interface or at the base of pores in the coating. No pores were observed in the coating by optical microscopy and it seems likely that the double-dipping procedure limits porosity. Furthermore a definite potential was established within a few seconds of immersion, suggesting that the diffusion of ions or water was unnecessary to establish the reaction. It is unlikely that the diffusion coefficient in the coating was greater than 10 -5 cm 2 s -1 , i.e. an order of magnitude too small to account for the initial behavior observable within a second. The penetration of pores would be more rapid and could account for the rapid response. As expected, the potentials vary with time due to changes in ionic composition or changes in the concentration of interfacial reactants. The polarization characteristics of the coated and uncoated metals observed on increasing the currents above 10 l0 A cm -2 are shown in Fig. 2. The coated aluminum shows distinct polarization even at the very low currents while the uncoated aluminum shows a similar polarization but at currents two orders of magnitude larger. Both the specimens showed current fluctuations, evidence that passivity breakdown was occurring when the potential increased above - 0 . 7 5 V(SCE) in the case of uncoated aluminum and - 0 . 6 V(SCE) for the coated surface. On decreasing the currents, corresponding potentials were within 100 mV of the values obtained with increasing currents, except for copper. With anodic currents in the case of iron (coupled to platinum) only the coated iron clearly showed polarization at 10 -5 A cm -2. From the results of Fig. 2, both the coated and the uncoated copper surfaces showed little effect attributable to external cathodic currents up to 10 -6 A c m -2. Coated copper, however, showed erratic results with major fluctuations in potential (<200 mV) but essentially no additional effects due to measured currents up to 10 -~' A cm -2. Comparison of the polarization result for each metal indicated that the behavior of the coated surfaces reflect the characteristics of the underlying metal when exposed directly to the solution. These results are possibly attributable to the characteristics of the metal/coating interface behaving electrochemically. Effects due to the presence of pores could not be ruled out even though the coating procedure was repeated specifically in an attempt to eliminate them.
1144
H.S. ISAACSet al. TABLE 2.
P O T E N T I A L D E V E L O P E D B E T W E E N METAL C O U P L E S IN AIR SEPARATED BY T H E COATING
Potential, V Approximate time after applying coating Couple
1.5h
4h
Pt-Cu Cu-A1 Fe-AI Pt-A1
o. 17 0.58 0.71 0.78
0.22 0.54 0.65 0.81
It is evident from the behavior of uncoated aluminum and coated copper (or coated iron) that the aluminum can be markedly polarized when galvanically coupled to coated copper. In Fig. 2, it can be deduced that an area of coated copper (or iron) of less than 0.5% of that of aluminum is sufficient to polarize the aluminum by about 50 inV. This demonstrates that the vinyl acetate-vinyl chloride coating (and possibly other coatings) is unsuitable for masking contacting dissimilar metals for corrosion studies. The effect is particularly marked when the natural immersion potential of a readily polarizable metal is monitored. It is interesting that a similar galvanic behavior may operate in the case of aluminum when only partially coated. The data in Table 1 and Fig. 2 show that the aluminum was at a more positive potential when dip-coated than when directly exposed to the electrolyte. In order to establish that pores in the coating were not a requirement for the electrochemical behavior of coated surfaces, measurements were made in air on the metal/metal couples with only the coating between them. Potentials which varied slowly with time were established for each of the cells (Table 2). Except for iron, the potentials of the metals show the potential classification expected from their position in the electrochemical series. However, iron may be in the passive state and thereby shows a potential more noble than copper. There is reasonable consistency in the results for copper and platinum as the potential difference between the P t - A I and Cu-A1 cells show approximately the same trend although the value of the P t - C u cell is about 0.05 V lower than the difference between the aluminum couples. The strong dependence of the potential on the nature of the underlying metal indicates that the properties of the metal determines the potential. Susceptible anodic areas present on the metal surface will remain anodic under the coating and dominate current flow to areas displaying cathodic behavior. Variations in the surface properties of metals with vinyl acetate/vinyl chloride coatings can, in much the same way as uncoated samples, be expected to play and important role in determining the locations where corrosion initiates. U n d e r these conditions the development of corrosion sites is determined by the behavior of the metal and not through variations in the properties of the coating which acts as electrolyte. Other coatings may also show these electrochemical characteristics. Acknowledgements--This research was performed under the auspicesof the U.S. Department of Energy,
Division of Materials Sciences, Office of Basic Energy Sciences under Contract No. DE-AC0276CH00016. The authors also wish to thank the Science and Engineering Research Council and the Royal Aircraft Establishment for the provisionof financial support to J. K. Hawkins and NATO for assistanceto H. S. Isaacs and G. E. Thompson, Research Grant No. 914(83).
Electrochemical behavior of thin masking coatings
1. 2. 3. 4. 5.
REFERENCES J. E. 0 . MAYNE and D. J. MII,LS, J.O.C.C.A. 58, 155 (1975). J. E. O. MAVNE and J. D. SCANTLESURY,J. Polym. Br. 1, 172 (1969). H. LIEDItElSEa, JR and M. W. KENDI6, Corrosion 32, 69 (1967). J. HAWKINS, Ph.D. Thcsis, Victoria University of Manchester (1988). H. S. ISAACS and G. KISSEL, J. electrochem. Soc. 119, 162 (I972).
1145
0{110-938X/92 $5.(X) + 0.00 Pergamon Press Ltd
Printed in Great Britain.
ELECTROCHEMICAL
H. S.
ISAACS,*
A. J.
BEHAVIOR COATINGS
DAVENPORT,*
J.
OF THIN
HAWKINSJ"
MASKING
and G. E. THOMPSON+
* Brookhaven National Laboratory, Upton, NY 11973, U.S.A. +Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, Manchester, M60 IQD, U.K.
A b s t r a c t - - A s s e s s m e n t of a vinyl acetate/vinyl chloride co-polymer as a coating or masking material in electrochemical studies has been made. With a double dip coating on aluminum, copper, iron or platinum in a sodium chloride electrolyte, an open circuit potential was recorded close to that of the uncoated metals. Polarization m e a s u r e m e n t s were made at low current densities of both coated and uncoated metals. The rates of the interfaciat reactions were reduced by the coating, but not dramatically. Similar potentials were recorded in the absence of aqueous electrolyte for metal electrodes separated only by the coating. As a consequence, a masked copper connecting wire markedly influences the electrochemical behavior of uncoated aluminum undcr natural immersion conditions. The study indicated that the coating itself acts as an electrolyte, with electrochemical reactions taking place at the metal/coating interface.
INTRODUCTION
THIN PROTECTIVEinsulating layers or coatings are frequently employed to mask samples and to limit the area of the surface exposed to the electrolytes for electrochemical measurements. The effectiveness of the masking depends on the conduction processes taking place or the presence of pores in the coating material. Mayne and his coworkers ~'2 studied the ionic conduction of detached coatings and concluded that polymer films are heterogeneous and contain fine areas of locally high conductivities. The high conduction was considered in the case of cross-linked polymer films to be due to the degree of cross linking. Leidheiser and Kendig 3 attributed impedance changes in coatings to the penetration of electrolyte. These considerations led to the conclusion that the properties of the coating determine the sites of corrosion initiation. In a series of experiments in which the open circuit potential of aluminum was measured, two different types of masking material were used: beeswax 4 and vinyl acetate/vinyl chloride copolymer. The latter was chosen for experiments in which it was necessary to limit the coating thickness. Surprisingly, significant differences were found in the open circuit potentials of specimens prepared using the two different coatings. The experiments described in this paper were carried out to determine the cause of this effect. Although very thin well-characterized insulating coatings with controllable thickness are commercially available for electronic applications, these were not used since thermal treatments above 100°C are required. Such treatments can have a significant influence on the corrosion behavior of the substrate.5
Manuscript received 21 May 1991. 1141
1142
H . S . ISAACS et al.
. / GLASS SLIOE Fic. 1.
METALWIRES
Metal/lacquer/metal cell to test the electrochemical behavior of the lacquer. EXPERIMENTAL
METHOD
High purity aluminum electrodes (99.999%) were electropolished in a perchloric acid/ethanol mixture. 4 Iron, platinum and copper (>99.9%) wire electrodes were mechanically polished with 600 grade silicon carbide paper. A vinyl acetate/vinyl chloride co-polymer coating was applied by dipping metal electrodes into the lacquer, leaving a small region uncoated for electrical contact (the uncoated area was not exposed to the solution). The coating was allowed to dry for 15 rain at ambient temperature prior to the application of a second coating in a similar manner. The second coating was allowed to dry for 60 min. The final coating thickness was about 0.12 m m . The double dipping procedure was used to minimize the risk of continuous pores across the thickness of the coating. M e a s u r e m e n t s were made with wire electrodes or flag-shaped electrodes and no second metal was used for electrical connections for both coated and uncoated specimens. Electrometers were used to measure the open circuit potentials relative to a saturated calomel electrode. It was expected that very low currents would lead to the polarization of coated metal surfaces. In order to measure these low currents, two electrometers were used. O n e electrometer, a Keithley 616, was employed to introduce a known resistor between the coupled electrodes. The potential across the resistor was recorded and hence the current flowing was obtained. The resistors could be varied by orders of magnitude from 103 to 1011 , depending on the current range required. The second electrometer measured the potential of one of the electrodes relative to a calomel electrode and as the potential difference between it and the second electrode was obtained from the Keithley electrometer the polarization characteristics of both electrodes were generated. These m e a s u r e m e n t s were carried out in 1 M NaCI prepared from analytical grade reagent and distilled water exposed to air at room temperature. Metal/metal cells which used only the coating as a possible electrolyte were made by taping one end of two different metal wires or strips to hold them about 0.5 m m apart on a glass slide. The other parallel ends of the adjacent wires were then painted with the coating. The surfaces of the metals were prepared by abrading with 600 grade silicon carbide paper. Figure 1 illustrates this cell. EXPERIMENTAL
RESULTS
AND
DISCUSSION
The open circuit potentials of the various metals after immersion for 3 h in 1 M NaCI are given in Table 1. The electrodes were either uncoated or had been dipT A B L E 1.
THE
MAXIMUM
AND
MINIMUM
POTENTIALS
COATED AND UNCOATED METALS AFTER IMMERSION IN 1
OF
M
NaC1 Potential, V(SCE) Metal Platinum Copper Iron Aluminum
Uncoated -0.23, -0.24, -0.55, -1.12,
-0.14 -0.07 -0.48 -0.96
Coated 0.27, - 0 . 1 2 - 0 . 2 3 , 0.06 -0.50, -0.56 -0.80, -0.98
Electrochemical behavior of thin masking coatings 0,2 L
l
r
i •
I •
1 I43
I - coaled Cu
0 •
*
,m.
"-'* bare Cu
-0.2
>
-0.4 _•
v
-I0
Fic. 2.
k[
W
bore
-
-g
-8
-7
CURRENT
OENSITY
( k cm"~ )
-6
-5
The polarization behavior of coated and bare copper, iron and aluminum in air exposed 1 M NaCI.
coated in lacquer. It is evident that the open circuit potential of the uncoated and coated specimens are close although the coated samples generally have slightly more positive values. It is clear that the coating is not effectively masking the specimen surface and that electrochemical reactions are proceeding at the coating/metal interface or at the base of pores in the coating. No pores were observed in the coating by optical microscopy and it seems likely that the double-dipping procedure limits porosity. Furthermore a definite potential was established within a few seconds of immersion, suggesting that the diffusion of ions or water was unnecessary to establish the reaction. It is unlikely that the diffusion coefficient in the coating was greater than 10 -5 cm 2 s -1 , i.e. an order of magnitude too small to account for the initial behavior observable within a second. The penetration of pores would be more rapid and could account for the rapid response. As expected, the potentials vary with time due to changes in ionic composition or changes in the concentration of interfacial reactants. The polarization characteristics of the coated and uncoated metals observed on increasing the currents above 10 l0 A cm -2 are shown in Fig. 2. The coated aluminum shows distinct polarization even at the very low currents while the uncoated aluminum shows a similar polarization but at currents two orders of magnitude larger. Both the specimens showed current fluctuations, evidence that passivity breakdown was occurring when the potential increased above - 0 . 7 5 V(SCE) in the case of uncoated aluminum and - 0 . 6 V(SCE) for the coated surface. On decreasing the currents, corresponding potentials were within 100 mV of the values obtained with increasing currents, except for copper. With anodic currents in the case of iron (coupled to platinum) only the coated iron clearly showed polarization at 10 -5 A cm -2. From the results of Fig. 2, both the coated and the uncoated copper surfaces showed little effect attributable to external cathodic currents up to 10 -6 A c m -2. Coated copper, however, showed erratic results with major fluctuations in potential (<200 mV) but essentially no additional effects due to measured currents up to 10 -~' A cm -2. Comparison of the polarization result for each metal indicated that the behavior of the coated surfaces reflect the characteristics of the underlying metal when exposed directly to the solution. These results are possibly attributable to the characteristics of the metal/coating interface behaving electrochemically. Effects due to the presence of pores could not be ruled out even though the coating procedure was repeated specifically in an attempt to eliminate them.
1144
H.S. ISAACSet al. TABLE 2.
P O T E N T I A L D E V E L O P E D B E T W E E N METAL C O U P L E S IN AIR SEPARATED BY T H E COATING
Potential, V Approximate time after applying coating Couple
1.5h
4h
Pt-Cu Cu-A1 Fe-AI Pt-A1
o. 17 0.58 0.71 0.78
0.22 0.54 0.65 0.81
It is evident from the behavior of uncoated aluminum and coated copper (or coated iron) that the aluminum can be markedly polarized when galvanically coupled to coated copper. In Fig. 2, it can be deduced that an area of coated copper (or iron) of less than 0.5% of that of aluminum is sufficient to polarize the aluminum by about 50 inV. This demonstrates that the vinyl acetate-vinyl chloride coating (and possibly other coatings) is unsuitable for masking contacting dissimilar metals for corrosion studies. The effect is particularly marked when the natural immersion potential of a readily polarizable metal is monitored. It is interesting that a similar galvanic behavior may operate in the case of aluminum when only partially coated. The data in Table 1 and Fig. 2 show that the aluminum was at a more positive potential when dip-coated than when directly exposed to the electrolyte. In order to establish that pores in the coating were not a requirement for the electrochemical behavior of coated surfaces, measurements were made in air on the metal/metal couples with only the coating between them. Potentials which varied slowly with time were established for each of the cells (Table 2). Except for iron, the potentials of the metals show the potential classification expected from their position in the electrochemical series. However, iron may be in the passive state and thereby shows a potential more noble than copper. There is reasonable consistency in the results for copper and platinum as the potential difference between the P t - A I and Cu-A1 cells show approximately the same trend although the value of the P t - C u cell is about 0.05 V lower than the difference between the aluminum couples. The strong dependence of the potential on the nature of the underlying metal indicates that the properties of the metal determines the potential. Susceptible anodic areas present on the metal surface will remain anodic under the coating and dominate current flow to areas displaying cathodic behavior. Variations in the surface properties of metals with vinyl acetate/vinyl chloride coatings can, in much the same way as uncoated samples, be expected to play and important role in determining the locations where corrosion initiates. U n d e r these conditions the development of corrosion sites is determined by the behavior of the metal and not through variations in the properties of the coating which acts as electrolyte. Other coatings may also show these electrochemical characteristics. Acknowledgements--This research was performed under the auspicesof the U.S. Department of Energy,
Division of Materials Sciences, Office of Basic Energy Sciences under Contract No. DE-AC0276CH00016. The authors also wish to thank the Science and Engineering Research Council and the Royal Aircraft Establishment for the provisionof financial support to J. K. Hawkins and NATO for assistanceto H. S. Isaacs and G. E. Thompson, Research Grant No. 914(83).
Electrochemical behavior of thin masking coatings
1. 2. 3. 4. 5.
REFERENCES J. E. 0 . MAYNE and D. J. MII,LS, J.O.C.C.A. 58, 155 (1975). J. E. O. MAVNE and J. D. SCANTLESURY,J. Polym. Br. 1, 172 (1969). H. LIEDItElSEa, JR and M. W. KENDI6, Corrosion 32, 69 (1967). J. HAWKINS, Ph.D. Thcsis, Victoria University of Manchester (1988). H. S. ISAACS and G. KISSEL, J. electrochem. Soc. 119, 162 (I972).
1145