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Application of the polarization resistance technique to corrosion monitoring
Corrosion Science, 1964, Vol. 4, pp. 245 to 251. Pergamon Press Ltd. Printed in Great Britain
APPLICATION OF THE POLARIZATION RESISTANCE TECHNIQUE TO CORROSION MONITORING* P. NEUFELD Imperial Chemical Industries Ltd., Billingham Division, Billingham, Co. Durham, England Abstract--The polarization resistance technique exists as a laboratory tool for the measurement of corrosion rates. A simple method of making measurements using this technique has been developed, making it usable for plant corrosion monitoring. Its effectiveness has been demonstrated by comparing the results with potential/time curves. The processes of film breakdown and repair are shown to be accompanied by corresponding variations in corrosion rate. R6sum6--La technique de r6sistance ft. la polarisation est un outil de laboratoire pour mesurer la vitesse de corrosion. Une m6thode simple pour effectuer les mesures en employant cette technique a 6t6 mise au point la rendant utilisable par les moniteurs d'usine. Son exactitude a ~tg dimontr~e en comparant ses rgsultants avec les courbes potentiel/temps. Les processus de fracture et de rd:paration du film se montrent accompagn6s de variations correspondantes de la vitesse de corrosion.
Zusammenfassung--Die Bestimmung des Polarisationswiderstandes ist eine Labormethode tiir die Messung der Korrosionsgeschwindigkeit. Eine einfache Messmethode wurde entwickelt, die dieses Verfahren ftir die I~berwachung der Korrosion in Anlagen brauchbar macht. Seine Brauchbarkeit wurde durch Vergleich der Ergebnisse mit Potential-Zeit-Kurven belegt. Die Vorg/inge des Filmdurchbruches und die der Filmausheilung sind, wie gezeigt werden kazan, mit ent'sprechenden .~,nderungen der Korrosionsgeschwindigkeit verbunden.
INTRODUCTION THE p o l a r i z a t i o n resistance technique is an electrical m e t h o d o f measuring corrosion rates. It originated with empirical observations by S i m m o n s 1 and Skold and L a r s o n 2 t h a t the slope, A V/AI, o f the c a t h o d i c p o l a r i z a t i o n curve near the corrosion potential bears a relationship to the c o r r o s i o n rate o f metal. L a t e r Stern and G e a r y a considered the theoretical basis o f this observation, showing t h a t A V/AI is inversely p r o p o r t i o n a l to the c o r r o s o i n rate as long as the p o t e n t i a l remains within a b o u t l0 mV o f the c o r r o sion potential. The term A VIAl has the dimensions o f resistance and was called the " p o l a r i z a t i o n resistance" by Stern. 4 The technique has been developed as a l a b o r a t o r y tool for m e a s u r i n g corrosion rates 5. e and the p r i m a r y p u r p o s e o f the present investigation was to simplify the m e a s u r i n g e q u i p m e n t and a d a p t the technique for p l a n t c o r r o s i o n monitoring. Previously the p o l a r i z a t i o n resistance has been measured from p o l a r i z a t i o n curves o b t a i n e d by p o t e n t i o s t a t i c methods, s a t u r a t e d calomel electrodes being used as reference electrodes. Since the c o r r o d i n g p o t e n t i a l o f m a n y metals is often a b o u t 500 mV from the reference p o t e n t i a l , it is not an easy m a t t e r to measure accurately the shift o f a b o u t l0 mV which encompasses the linear p o r t i o n o f the c a t h o d i c *Manuscript received 3 December 1963. 245
P.NEUI~LD
246
polarization curve. The use of delicate reference electrodes and complicated electronic equipment is also a drawback if the technique is to be applied under plant conditions. One solution that suggests itself is the use of a reference electrode made of metal similar to the test surface. This should normally attain a potential not greatly removed from that of the test metal, and is robust, although its stability is questionable. It is also important to know the sensi~tivity and speed of response of the technique to rapid changes in the corrosion rate. An existing method of detecting such changes is the use of potential[time curves. 7 Variations in potential have been interpreted
~M,O,
fOK,O,1 m~
$2 Current
;--
Potenlial
-i-
Test
Reference
Anode
Connector
Anode Ierminol ond support rod
Polythene Sleeving
@
End View
Anode
Araldite
Reference
Test
FIG. 1, Test probe and circuit,
Application of the polarization resistance technique to corrosion monitoring
247
as signifying changes in passivity or other corrosion processes. It has been difficult to confirm these interpretations by an independent method, so a comparison of the polarization resistance technique with simultaneous potential measurements has a double usefulness. Previous deductions from potential measurements can be checked, and the sensitivity of the polarization resistance technique can be investigated.
APPARATUS The forms of test probe and control circuit used are shown in Fig. 1. The control system is of the constant current type, switch $1 being used to turn the applied current on and off. The operation of $2 enables the valve voltmeter to be used to measure either current or potential. The anode ring was made of 18/8/Ti stainless steel and was also used as a former for the insulating resin. An Araldite resin was used in the experiments described here, but other resins have been used for special purposes. To obtain as even a current distribution on the test surface as possible the probe was constructed with a thickness of insulating material equal to the radius of the test electrode between the test electrode and anode ring. The test and reference electrodes were made of metal from the same bar and the potential between them in most corrodents was 50 mV or less, enabling the 100 mV full scale range of the valve voltmeter to be used. The reference electrode had sufficient short-term stability to permit accurate measurements of potential shifts. Long term stability is not required since measurement of absolute potentials is irrelevatat. When readings were to be made, the current was set to a convenient value so that the potential shift associated with its application was of the order of 10 mV. Accurate measurements were then made of the current, and of the potential with the current on and off. A continuous polarization curve was not attempted, it being assumed that it was a straight line in the range considered. A complete measurement of polarization resistance could then be made in less than one minute. It is thought that the application of only a small current has less effect on the condition of the corroding surface, and that this compensates for the uncertainty introduced by not obtaining a complete polarization curve.
E X P E R I M E N T A L WORK For convenience, the parameter used was the applied current in microamps per square centimeter divided by the potential shift in millivolts, A~A/AmV. This should be directly proportional to corrosion rate, being proportional to the inverse of the polarization resistance. Two series of tests were made: I. Corrosion tests
Polarization resistance measurements were compared with weight-loss measurements on specimens in the same solutions as the test probes. A series of solutions of organic acids in water were tested, as were some other common eorrodents. Only mild steel specimens were used in these tests. None of the experiments lasted longer than
248
P. NEUFELD
24 hours, as otherwise appreciable variation in the polarization resistance occurred as the corrosion rate changed with time. This would have introduced further complications into the experiment. TABLE 1. POLARIZATION RESISTANCE AND WEIGHT LOSS
Corroding medium
AIzA/AmV
mg cm-2h -1
Inv. pol. resist./wt, loss
3.1 1.9 0"13 6"3 2-5 3.1 1"0 0-65
0.28 0.25 0.016 0"62 0.24 0.35 0"10 0.025
11 8 8 10 10 9 10 26
0.025 1.5
0.0023 0.20
11 8
1 ~ propionic acid, air agitated 0.1 ~ propionic acid, air agitated 1 Yo propionic acid, nitrogen agitated 1 ~ formic acid, air agitated 0.05 ~ formic acid, air agitated I y. acetic acid, air agitated Tees-valley water, air agitated Tees-valley water (duplicate), air agitated Tees-valley water + 0.1% nitrite, air agitated 1 ~. hydrochloric acid, air agitated
Results are shown in Table 1. Except for one result the ratio of the in.verse of polarization resistance to the weight loss varies only between 8 and I 1. This confirms the conclusions of previous workers that the polarization resistance technique can give an accurate and consistent measure of corrosion rates. The claim of Stern and Weisert 6 that corrosion in an unknown corrodent can be estimated to within a factor of two seems to be justified.
2:C 1-8 1"6
~-o-.o--o~o
v
v
°°
Potential o o.
600
--
-
--
- 700
- -
-800
b4
-o
1"2
o
---900
::L E 1.0
--
- iooo ':'-
--
--I100
3
0.8
<
(3) 0'6 --
-1200
--
- 1300
0"4
~x
0"2
I I0
20
30
40
~__
50
A
60
70
Time,
SO
AmY
90
I00
- 1400
I10
r
120
rain
FIG. 2. 99"5 ~ A! in 3 ~ NaC1 solution.
L
I 130
I 140
-1500 f50
C)
rn
Application of the polarizationresistancetechniqueto corrosion monitoring 3c--
249
-- -I00
--
2c
-200
---300
"u o ~"
----400
a
--
-500
B
<
~L E
<]<3
-- - 600 H Cl
odded
IC
--
-700
--
- 800
(~ r'n
Potential
u
A m y
-- -900
240
L
250
[
260
i
270
^1
280
I
Z90
I^
300
I
I
I
310 3 2 0 330 Time, rain
340
I
350
I
360
[
370
I
380
-,ooo
390
FIG. 3. 99"5~, AI in 3~, NaC1 solution; HCI added after 340 rain to give a 2 ~ solution.
2. Potential/time curves Figures 2 and 3 show simultaneous potential and inverse polarization resistance measurements plotted against time. Aluminium (99.5 ~o) was tested in a 3 ~ sodium chloride solution after etching in 10~o HF and 5 min exposure to air. After 340 min, hydrochloric acid was added to give a 2 per cent solution. The process of initial film breakdown followed by film repair is shown to be accompanied by an initial increase in AtzA/AmV, followed by a steady decrease. The addition of hydrochloric acid evidently produced a major change in the corrosion reaction, so that the significance of the potential/time curve becomes difficult to interpret. The polarization resistance measurements however continue to give reasonable results, consistent with the behaviour before acid was added. Figure 4 shows the behaviour of air-filmed 18/8/Ti stainless steel in I 0 ~ sulphuric acid. Potassium chromate was added to give a 5 per cent solution after 220 min. The potential/time curve indicates that when the specimens were first immersed there was a weakening of passivity followed by the formation of a strongly passive surface. This is eordirmed by the polarization resistance measurements which show a peak corrosion rate at the point of lowest potential. When chromate was added to the 10 % acid, the potential of the stainless steel rose from 120 mV to 600 mV (SCE), rapidly at first and then more slowly. This rise in potential would be accompanied by a transient increase in corrosion current as charge builds up on the passive surface, and this is shown by the polarization resistance measurements. Figure 5 shows the current/time curve obtained using a Wenking Standard potentiostat when the potential of an 18/8/Ti specimen in I 0 ~ sulphuric acid was raised from 120 mV to 600 mV ($CE). The current is shown on an arbitrary scale. Here again there is a transient increase in anodie current as the surface becomes more strongly oxidized. The similarity with the results shown in Fig. 5 is apparent. The more rapid growth and decay of current in the potentiostat experiment can be accounted for by the more rapid and complete potential change involved.
250
P. NEUFELU
4oo
G 0"20
t -° 400
0"18 (
300
"lJ o
Oq4
200
z~
O.C,
I00
0"16
--
0 (n
I00
0"08
-
0'06
--200
0.04
--300 h
(>02
Chromate
I
I
20
)0
I
]
30
40
I
E
50
60
:~
70
8_1-2_0
Time,
F~
-400
added
I
2,0
I
l
220 230
I
(
240
[
I
250 260
270
I ~0 0
280
rain
FIG. 4. Stainless steel in 10~o H2SO,; potassium chromate added after 200 min to give a 5 ~ solution. 4--
,0.2 u c
I
I
I
I
2
I
3 Time,
I
4
1
5
I
6
I
7
min
FIo. 5. Current transient, stainless steel in 10~. H2SO,; potential shifted, 120-600 mV (SCE) at 30 s. In the study of reactions involving rapid and large changes in surface potential, such as those described above or active-passive transformations, it was found that often the large test electrode would change its condition at a different time from the small reference electrode. This gave rise to large potentials (of the order of 600 mV) between the reference and test electrodes, making accurate measurements difficult. It was also found that the electrode potentials tended to fluctuate rapidly at certain stages in the process of film formation. This effect is much less marked when normal reference electrodes are used, as the fluctuations are of the order of 20-30 mV, which is small compared with the large potential between reference and test electrodes.
Application of the polarization resistance technique to corrosion monitoring
251
CONCLUSIONS The polarization resistance technique offers a simple and sensitive measure of instantaneous corrosion rates. This makes it particularly useful in detecting the effects of changes in conditions on corrosion rates, even where these changes are rapid. A method of applying the technique under plant conditions for corro~sion monitoring has been developed. The difficulty of obtaining satisfactory measurements may be increased when reactions are involved that are accompanied by large shifts in the metal surface potential. The significance of potential/time curves in cases of passivity has been demonstrated. Low potentials accompanied by loss of passivity are shown to be associated with an increase in corrosion rate. High potentials associated with strengthening of passivity accompany a low corrosion rate. REFERENCES 1. E. J. SIMMONS,Corrosion 11,225 (1955). 2. R. V. SKOLDand T. E. LARSON,Corrosion 13, 139 (1957). 3. M. STERNand A. L. GEARY,J. Electrochem. Soc. 104, 56 0957). 4. M. STERN,Corrosion 14, 440 (1958). 5. S. EVANSand E. L. KOEHLER,J. Electrochem. Soc. 108, 509 (1961). 6. M. STERNand E. D. WEISERT,Proc. Amer. Soc. Test. Mater. 59, 1280 (1959). 7. U. R. EVANS,The Corrosion and Oxidation o f Metals. Chaps XIX, XXI. Arnold, London (1960).
APPLICATION OF THE POLARIZATION RESISTANCE TECHNIQUE TO CORROSION MONITORING* P. NEUFELD Imperial Chemical Industries Ltd., Billingham Division, Billingham, Co. Durham, England Abstract--The polarization resistance technique exists as a laboratory tool for the measurement of corrosion rates. A simple method of making measurements using this technique has been developed, making it usable for plant corrosion monitoring. Its effectiveness has been demonstrated by comparing the results with potential/time curves. The processes of film breakdown and repair are shown to be accompanied by corresponding variations in corrosion rate. R6sum6--La technique de r6sistance ft. la polarisation est un outil de laboratoire pour mesurer la vitesse de corrosion. Une m6thode simple pour effectuer les mesures en employant cette technique a 6t6 mise au point la rendant utilisable par les moniteurs d'usine. Son exactitude a ~tg dimontr~e en comparant ses rgsultants avec les courbes potentiel/temps. Les processus de fracture et de rd:paration du film se montrent accompagn6s de variations correspondantes de la vitesse de corrosion.
Zusammenfassung--Die Bestimmung des Polarisationswiderstandes ist eine Labormethode tiir die Messung der Korrosionsgeschwindigkeit. Eine einfache Messmethode wurde entwickelt, die dieses Verfahren ftir die I~berwachung der Korrosion in Anlagen brauchbar macht. Seine Brauchbarkeit wurde durch Vergleich der Ergebnisse mit Potential-Zeit-Kurven belegt. Die Vorg/inge des Filmdurchbruches und die der Filmausheilung sind, wie gezeigt werden kazan, mit ent'sprechenden .~,nderungen der Korrosionsgeschwindigkeit verbunden.
INTRODUCTION THE p o l a r i z a t i o n resistance technique is an electrical m e t h o d o f measuring corrosion rates. It originated with empirical observations by S i m m o n s 1 and Skold and L a r s o n 2 t h a t the slope, A V/AI, o f the c a t h o d i c p o l a r i z a t i o n curve near the corrosion potential bears a relationship to the c o r r o s i o n rate o f metal. L a t e r Stern and G e a r y a considered the theoretical basis o f this observation, showing t h a t A V/AI is inversely p r o p o r t i o n a l to the c o r r o s o i n rate as long as the p o t e n t i a l remains within a b o u t l0 mV o f the c o r r o sion potential. The term A VIAl has the dimensions o f resistance and was called the " p o l a r i z a t i o n resistance" by Stern. 4 The technique has been developed as a l a b o r a t o r y tool for m e a s u r i n g corrosion rates 5. e and the p r i m a r y p u r p o s e o f the present investigation was to simplify the m e a s u r i n g e q u i p m e n t and a d a p t the technique for p l a n t c o r r o s i o n monitoring. Previously the p o l a r i z a t i o n resistance has been measured from p o l a r i z a t i o n curves o b t a i n e d by p o t e n t i o s t a t i c methods, s a t u r a t e d calomel electrodes being used as reference electrodes. Since the c o r r o d i n g p o t e n t i a l o f m a n y metals is often a b o u t 500 mV from the reference p o t e n t i a l , it is not an easy m a t t e r to measure accurately the shift o f a b o u t l0 mV which encompasses the linear p o r t i o n o f the c a t h o d i c *Manuscript received 3 December 1963. 245
P.NEUI~LD
246
polarization curve. The use of delicate reference electrodes and complicated electronic equipment is also a drawback if the technique is to be applied under plant conditions. One solution that suggests itself is the use of a reference electrode made of metal similar to the test surface. This should normally attain a potential not greatly removed from that of the test metal, and is robust, although its stability is questionable. It is also important to know the sensi~tivity and speed of response of the technique to rapid changes in the corrosion rate. An existing method of detecting such changes is the use of potential[time curves. 7 Variations in potential have been interpreted
~M,O,
fOK,O,1 m~
$2 Current
;--
Potenlial
-i-
Test
Reference
Anode
Connector
Anode Ierminol ond support rod
Polythene Sleeving
@
End View
Anode
Araldite
Reference
Test
FIG. 1, Test probe and circuit,
Application of the polarization resistance technique to corrosion monitoring
247
as signifying changes in passivity or other corrosion processes. It has been difficult to confirm these interpretations by an independent method, so a comparison of the polarization resistance technique with simultaneous potential measurements has a double usefulness. Previous deductions from potential measurements can be checked, and the sensitivity of the polarization resistance technique can be investigated.
APPARATUS The forms of test probe and control circuit used are shown in Fig. 1. The control system is of the constant current type, switch $1 being used to turn the applied current on and off. The operation of $2 enables the valve voltmeter to be used to measure either current or potential. The anode ring was made of 18/8/Ti stainless steel and was also used as a former for the insulating resin. An Araldite resin was used in the experiments described here, but other resins have been used for special purposes. To obtain as even a current distribution on the test surface as possible the probe was constructed with a thickness of insulating material equal to the radius of the test electrode between the test electrode and anode ring. The test and reference electrodes were made of metal from the same bar and the potential between them in most corrodents was 50 mV or less, enabling the 100 mV full scale range of the valve voltmeter to be used. The reference electrode had sufficient short-term stability to permit accurate measurements of potential shifts. Long term stability is not required since measurement of absolute potentials is irrelevatat. When readings were to be made, the current was set to a convenient value so that the potential shift associated with its application was of the order of 10 mV. Accurate measurements were then made of the current, and of the potential with the current on and off. A continuous polarization curve was not attempted, it being assumed that it was a straight line in the range considered. A complete measurement of polarization resistance could then be made in less than one minute. It is thought that the application of only a small current has less effect on the condition of the corroding surface, and that this compensates for the uncertainty introduced by not obtaining a complete polarization curve.
E X P E R I M E N T A L WORK For convenience, the parameter used was the applied current in microamps per square centimeter divided by the potential shift in millivolts, A~A/AmV. This should be directly proportional to corrosion rate, being proportional to the inverse of the polarization resistance. Two series of tests were made: I. Corrosion tests
Polarization resistance measurements were compared with weight-loss measurements on specimens in the same solutions as the test probes. A series of solutions of organic acids in water were tested, as were some other common eorrodents. Only mild steel specimens were used in these tests. None of the experiments lasted longer than
248
P. NEUFELD
24 hours, as otherwise appreciable variation in the polarization resistance occurred as the corrosion rate changed with time. This would have introduced further complications into the experiment. TABLE 1. POLARIZATION RESISTANCE AND WEIGHT LOSS
Corroding medium
AIzA/AmV
mg cm-2h -1
Inv. pol. resist./wt, loss
3.1 1.9 0"13 6"3 2-5 3.1 1"0 0-65
0.28 0.25 0.016 0"62 0.24 0.35 0"10 0.025
11 8 8 10 10 9 10 26
0.025 1.5
0.0023 0.20
11 8
1 ~ propionic acid, air agitated 0.1 ~ propionic acid, air agitated 1 Yo propionic acid, nitrogen agitated 1 ~ formic acid, air agitated 0.05 ~ formic acid, air agitated I y. acetic acid, air agitated Tees-valley water, air agitated Tees-valley water (duplicate), air agitated Tees-valley water + 0.1% nitrite, air agitated 1 ~. hydrochloric acid, air agitated
Results are shown in Table 1. Except for one result the ratio of the in.verse of polarization resistance to the weight loss varies only between 8 and I 1. This confirms the conclusions of previous workers that the polarization resistance technique can give an accurate and consistent measure of corrosion rates. The claim of Stern and Weisert 6 that corrosion in an unknown corrodent can be estimated to within a factor of two seems to be justified.
2:C 1-8 1"6
~-o-.o--o~o
v
v
°°
Potential o o.
600
--
-
--
- 700
- -
-800
b4
-o
1"2
o
---900
::L E 1.0
--
- iooo ':'-
--
--I100
3
0.8
<
(3) 0'6 --
-1200
--
- 1300
0"4
~x
0"2
I I0
20
30
40
~__
50
A
60
70
Time,
SO
AmY
90
I00
- 1400
I10
r
120
rain
FIG. 2. 99"5 ~ A! in 3 ~ NaC1 solution.
L
I 130
I 140
-1500 f50
C)
rn
Application of the polarizationresistancetechniqueto corrosion monitoring 3c--
249
-- -I00
--
2c
-200
---300
"u o ~"
----400
a
--
-500
B
<
~L E
<]<3
-- - 600 H Cl
odded
IC
--
-700
--
- 800
(~ r'n
Potential
u
A m y
-- -900
240
L
250
[
260
i
270
^1
280
I
Z90
I^
300
I
I
I
310 3 2 0 330 Time, rain
340
I
350
I
360
[
370
I
380
-,ooo
390
FIG. 3. 99"5~, AI in 3~, NaC1 solution; HCI added after 340 rain to give a 2 ~ solution.
2. Potential/time curves Figures 2 and 3 show simultaneous potential and inverse polarization resistance measurements plotted against time. Aluminium (99.5 ~o) was tested in a 3 ~ sodium chloride solution after etching in 10~o HF and 5 min exposure to air. After 340 min, hydrochloric acid was added to give a 2 per cent solution. The process of initial film breakdown followed by film repair is shown to be accompanied by an initial increase in AtzA/AmV, followed by a steady decrease. The addition of hydrochloric acid evidently produced a major change in the corrosion reaction, so that the significance of the potential/time curve becomes difficult to interpret. The polarization resistance measurements however continue to give reasonable results, consistent with the behaviour before acid was added. Figure 4 shows the behaviour of air-filmed 18/8/Ti stainless steel in I 0 ~ sulphuric acid. Potassium chromate was added to give a 5 per cent solution after 220 min. The potential/time curve indicates that when the specimens were first immersed there was a weakening of passivity followed by the formation of a strongly passive surface. This is eordirmed by the polarization resistance measurements which show a peak corrosion rate at the point of lowest potential. When chromate was added to the 10 % acid, the potential of the stainless steel rose from 120 mV to 600 mV (SCE), rapidly at first and then more slowly. This rise in potential would be accompanied by a transient increase in corrosion current as charge builds up on the passive surface, and this is shown by the polarization resistance measurements. Figure 5 shows the current/time curve obtained using a Wenking Standard potentiostat when the potential of an 18/8/Ti specimen in I 0 ~ sulphuric acid was raised from 120 mV to 600 mV ($CE). The current is shown on an arbitrary scale. Here again there is a transient increase in anodie current as the surface becomes more strongly oxidized. The similarity with the results shown in Fig. 5 is apparent. The more rapid growth and decay of current in the potentiostat experiment can be accounted for by the more rapid and complete potential change involved.
250
P. NEUFELU
4oo
G 0"20
t -° 400
0"18 (
300
"lJ o
Oq4
200
z~
O.C,
I00
0"16
--
0 (n
I00
0"08
-
0'06
--200
0.04
--300 h
(>02
Chromate
I
I
20
)0
I
]
30
40
I
E
50
60
:~
70
8_1-2_0
Time,
F~
-400
added
I
2,0
I
l
220 230
I
(
240
[
I
250 260
270
I ~0 0
280
rain
FIG. 4. Stainless steel in 10~o H2SO,; potassium chromate added after 200 min to give a 5 ~ solution. 4--
,0.2 u c
I
I
I
I
2
I
3 Time,
I
4
1
5
I
6
I
7
min
FIo. 5. Current transient, stainless steel in 10~. H2SO,; potential shifted, 120-600 mV (SCE) at 30 s. In the study of reactions involving rapid and large changes in surface potential, such as those described above or active-passive transformations, it was found that often the large test electrode would change its condition at a different time from the small reference electrode. This gave rise to large potentials (of the order of 600 mV) between the reference and test electrodes, making accurate measurements difficult. It was also found that the electrode potentials tended to fluctuate rapidly at certain stages in the process of film formation. This effect is much less marked when normal reference electrodes are used, as the fluctuations are of the order of 20-30 mV, which is small compared with the large potential between reference and test electrodes.
Application of the polarization resistance technique to corrosion monitoring
251
CONCLUSIONS The polarization resistance technique offers a simple and sensitive measure of instantaneous corrosion rates. This makes it particularly useful in detecting the effects of changes in conditions on corrosion rates, even where these changes are rapid. A method of applying the technique under plant conditions for corro~sion monitoring has been developed. The difficulty of obtaining satisfactory measurements may be increased when reactions are involved that are accompanied by large shifts in the metal surface potential. The significance of potential/time curves in cases of passivity has been demonstrated. Low potentials accompanied by loss of passivity are shown to be associated with an increase in corrosion rate. High potentials associated with strengthening of passivity accompany a low corrosion rate. REFERENCES 1. E. J. SIMMONS,Corrosion 11,225 (1955). 2. R. V. SKOLDand T. E. LARSON,Corrosion 13, 139 (1957). 3. M. STERNand A. L. GEARY,J. Electrochem. Soc. 104, 56 0957). 4. M. STERN,Corrosion 14, 440 (1958). 5. S. EVANSand E. L. KOEHLER,J. Electrochem. Soc. 108, 509 (1961). 6. M. STERNand E. D. WEISERT,Proc. Amer. Soc. Test. Mater. 59, 1280 (1959). 7. U. R. EVANS,The Corrosion and Oxidation o f Metals. Chaps XIX, XXI. Arnold, London (1960).