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Localized impedance-anodization measurements to characterize corrosion films on irradiated zirconium alloys
226
Journal
of Nuclear
Materials
183 (1991) 226-228 North-Holland
Letter to the Editors
Localized corrosion
impedance-anodization measurements films on irradiated zirconium alloys
N. Ramasubramanian AECL
Research,
Received
and V.C. Ling
Chalk River Laboratories,
3 December
1990; accepted
Chalk River, Ontario, Canada KW IJO
20 February
1991
1. Introduction In CANDU (CANada Deuterium Uranium) reactors the pressure tubes, 0.1 m in diameter and - 6 m in length, are made of Zr-2.5 Nb alloy. Prior to installation the pressure tube is stress-relieved by autoclaving in steam at 1.05 MPa and 673 K for a day. This autoclaving procedure forms an oxide - 1 pm in thickness. In reactor the tube surfaces are exposed to two different environments. The inside of the tube is exposed to the pressurized lithiated heavy water coolant which enters the channel at - 510 K and exits at - 570 K. The outside of the tube forms an annulus with the calandria tube and is exposed to nitrogen or carbon dioxide. Low concentrations of deuterium and heavy water vapour are also present in the annulus. The oxide produced by the stress-relieving step initially provides a barrier for deuterium ingress from the annulus. However, the environment in the annulus is not strictly oxidizing and the protective quality of the oxide may become impaired. It is well established that when insufficient oxidant is present a breakdown of the oxide occurs and the effectiveness of the barrier oxide for hydrogen ingress is decreased [l]. The breakdown of the oxide is identified with oxide dissolution, preferentially occurring at the grain boundaries in the metal
111. As a part of an on-going surveillance program, pressure tubes removed from the reactors are monitored for corrosion and deuterium uptake. A high level of deuterium, recently found in one of the tubes, has raised concern about the possibility of deuterium ingress from the annulus. In order to evaluate the protective quality of the oxide on the outside surface of pressure tubes removed from reactors, a simple testing procedure, that can be easily adapted for use in a hot cell, was required. Elsevier
Science Publishers
to characterize
B.V. (North-Holland)
Long sections of the removed tubes had to be examined around their circumference and along their lengths. Two simple electrochemical techniques, applicable to corrosion films on zirconium alloys, are impedance and anodization measurements which are carried out using an aqueous electrolyte contact. The solution penetrates the porous layers in the corrosion film and the measurements are thus representative of the non-porous part of the corrosion film. We have developed a procedure to map the outer surface of removed pressure tubes by localizing the electrochemical measurements to small areas.
2. Experimental The pressure tube specimen to be examined was the working electrode; oxide was abraded off a small area at one end so that electrical contact could be made. Locations on the outside surface, free of scratches, were carefully selected. A piece of insulating tape with several punched holes, varying in diameter from 2 to 6 mm, was pressed onto the surface at a location. Several small test spots, exposed oxide film of known areas were thus made available at a given location. A few drops of saturated sodium nitrite solution were placed on one of the test spots. A platinum foil electrode, contacting the solution, formed the counter electrode. Measurements were mostly made with this two electrode arrangement. When needed, for example, during impedance measurements over a range of frequencies, a platinum wire inserted in the solution between the specimen and the counter electrode served as the reference electrode. The potential of the specimen was slightly negative to that of platinum; the potential measured at different locations varied in the range of 10 to 30 mV and was
N. Ramasubramanian,
V.C. Ling / Localized impedance-anodization
unaffected by the impedance measurements. Impedance measurements were made on a General Radio 1692 RLC digibridge at 1 kHz and changes in capacitance were followed with time for five minutes. Anodization was then carried out under galvanostatic conditions at 3 and 14 PA mm-’ and the potential of the specimen relative to the counter or reference electrode was followed with time. A final impedance measurement completed the testing. For comparison purposes similar measurements were made on off-cuts (rings cut and saved from the two ends of the stress-relieved pressure tube prior to installation in the reactor) and on specimens of pressure tube material in the pickled and oxidised conditions. Specimens from the off-cuts and pickled specimens oxidised in steam to form - 1 pm thick oxide were exposed to 1% deuterium in nitrogen and vacuum at 573 to 673 K for various times and changes in the impedance and anodization behaviour were followed.
The increases in the capacitance, measured soon after contacting the specimen with the electrolyte solution, were large and occurred within a couple of minutes. Small increases in the measured capacitance were then observed with increasing contact time with the solution; however, the capacitance measured after 5 min of contact was nearly steady and this was used in thickness calculations according to the relation: t, =0.1947/c,
(1)
where t, is the average thickness of the oxide (in pm) not penetrated by the solution and C is the capacitance in nF mmm2 [2]. The non-porous oxide, so called “barrier layer”, is considered to be non-uniform in thickness and the measured capacitance, used in eq. (1). is the sum of the capacitances of the fractional areas of varying thicknesses of the non-porous oxide. During anodization oxide growth by the applied field is expected to initially ocur at the thinnest regions of the non-porous oxide. The charging potential at which anodic oxide growth commences is mainly determined by the minimum thickness of the non-porous oxide which was calculated in nm, as
t M = 2.5 x v,
(2)
where I’ is the initial charging potential in volts. A film formation ratio of 2.5 nm V-i was obtained by comparing interference colours of films formed on Zr-2.5 Nb alloy in sodium nitrite solution with those formed on
227
Table 1 Average and minimum thickness op oxide, not-penetrated by the electrolyte solution, measured by capacitance and ancdization measurenents localized to spots 18 mm2 in area, on the outside surface of pressure tube P3LO9 removed ing Unit 3; anodization at 14 PA mm-* Distance from inlet
Capacitance (nF mm-2)
3.2 4.63 5.0 5.5 5.8
Charging potential
(V)
(m)
Off-cut
3. Results and discussion
measurements
0.80 1.41 0.69 3.89 1.78 4.60 2.64 9.36 2.34 6.72 0.55
Non-porous thickness
‘A
from Picker-
oxide
1M
average
minimum
(P)
(pm)
30
0.24
0.075
20
0.14
0.050
50
0.28
0.125
8
0.05
0.020
0.11 0.04 0.074 0.021 0.083 0.029 0.35
0.025
10 10 8 2 9 2 80
0.025 0.020 0.005 0.023 0.005 0.200
zirconium. Anodic films on zirconium were produced by fixed voltage anodization in 1 wt% KOH solution [3]. In table 1 are shown some typical results of capacitance and anodization measurements on pressure tube P3LO9 removed after 12 years of operation in Pickering Unit 3; data on the off-cut are included for comparison. In the case of the off-cut the average thickness of the non-porous portion of the oxide, estimated according to eq. (1) from measurements at four locations, is 0.35 pm. Infrared interferometry indicated a total oxide thickness of 0.8 pm, produced by the stress-relieving treatment in steam [4]. A little more than half of the steam-grown oxide is thus porous and easily penetrated by the sodium nitrite solution. The minimum thickness in the non-porous oxide is only 0.20 pm. In the case of specimens from the tube P3LO9 infrared interferometry indicated a slight increase in total oxide thickness on the outside surface; when compared to the initial thickness of 0.8 pm the thickness measured, after removal of the tube from the reactor, was - 2 pm [4]. However, despite this increase in total oxide thickness a general degradation of the oxide quality on the removed pressure tube is clearly indicated when the results for the tube P3LO9 are compared to those of the off-cut. Both the average and minimum thickness of the non-porous oxide have become greatly reduced. There were locations where the non-porous oxide was only 5 nm thick; a thickness same as that resulting from exposure of the bare alloy
228
N. Ramasubramanion.
2
3
4
5
V.C. Ling / Localized Impedance-anodization
6
Charge (millicoulomb) Fig. 1. Potential versus charge anodization behaviour of oxide pressure tube LO9 removed from mm2 and current density 14 HA off-cut,
curves typical of the localized on the outside surface of the Pickering Unit 3; spot area 18 mm-*; (a) pickled surface, (b)
and (c to f) four locations on tube LO9 at 4.5 to 5 m from the inlet end.
to air at room temperature. If the non-porous oxide is a barrier to deuterium ingress from the annulus then its effectiveness has been reduced and also seriously impaired at some locations on tube P3L09. In fig. 1 a set of typical potential versus charge curves obtained during galvanostatic anodization measurements on tube P3LO9 is compared with those obtained on the off-cut and a bare alloy specimen. The varying degree of degradation of the oxide on the outside of the tube, along the channel length, is clearly evident. The conclusion that the electrochemical measurements represent changes in the properties of the oxide is supported by the results obtained in laboratory experiments. The outside oxide on specimens from off-cuts and the oxide grown on pickled specimens were free of scratches. With increasing exposure to the non-oxidising environment, 1% deuterium in nitrogen or vacuum, at 573 to 673 K, the measured capacitance increased and the charging potential during anodization decreased. Anodization behaviour similar to curves d and f in fig. 1, indicating oxide breakdown, was observed after prolonged exposures. There were no visible changes in the appearance of the black oxide and infrared interferometry revealed no changes in the total oxide thickness. The environment in the gas annulus in reactor had been oxidising but the low concentration of oxidant and the presence of deuterium led to a degradation of the
measurements
existing oxide and growth of a non-protective oxide. Oxide dissolution had been simultaneously occurring with oxide growth. When calculating the average and minimum thickness of the non-porous oxide from eqs. (1) and (2) it is assumed that the oxide is Zrq. This is not strictly true in the case of Zr-2.5 Nb pressure tube material. It is a two phase alloy and undergoes microstructural changes during its life in the reactor. The effective dielectric constant, refractive index and the electrical conductivity for films grown on Zr-2.5 Nb pressure tube material could be different from those of zirconia films on zirconium. Nevertheless, comparisons, such as those shown in table 1 and fig. 1, can be made of the environmentally induced changes in the quality of oxide films on a material. The localized impedance and anodization measurements provide a quick means to evaluate the protectiveness of thin zirconia films especially, because penetration of the porous oxide by the contacting solution can be achieved in a relatively short time.
4. Summary Impedance and anodization measurements, using an aqueous electrolyte contact and localized to small areas, were used to grade the quality of the corrosion film on irradiated zirconium alloys. This method was developed for examining oxide films on the outside surface of pressure tubes removed from CANDU reactors.
Acknowledgement This project was supported CANDU Owners Group.
and
funded
by
the
References [I] B. Cox, Mechanism of Hydrogen Absorption by Zirconium Alloys, Atomic Energy of Canada Limited Report, AECL8702 (1985). [2] P.M. Rosecrans, ASTM-STP 824 (1984) 531. [3] N.J.M. Wilkins, Corrosion Sci. 5 (1965) 3. [4] N. Ramasubramanian and V.C. Ling, J. Nucl. (1990) 237.
Mater. 175
Journal
of Nuclear
Materials
183 (1991) 226-228 North-Holland
Letter to the Editors
Localized corrosion
impedance-anodization measurements films on irradiated zirconium alloys
N. Ramasubramanian AECL
Research,
Received
and V.C. Ling
Chalk River Laboratories,
3 December
1990; accepted
Chalk River, Ontario, Canada KW IJO
20 February
1991
1. Introduction In CANDU (CANada Deuterium Uranium) reactors the pressure tubes, 0.1 m in diameter and - 6 m in length, are made of Zr-2.5 Nb alloy. Prior to installation the pressure tube is stress-relieved by autoclaving in steam at 1.05 MPa and 673 K for a day. This autoclaving procedure forms an oxide - 1 pm in thickness. In reactor the tube surfaces are exposed to two different environments. The inside of the tube is exposed to the pressurized lithiated heavy water coolant which enters the channel at - 510 K and exits at - 570 K. The outside of the tube forms an annulus with the calandria tube and is exposed to nitrogen or carbon dioxide. Low concentrations of deuterium and heavy water vapour are also present in the annulus. The oxide produced by the stress-relieving step initially provides a barrier for deuterium ingress from the annulus. However, the environment in the annulus is not strictly oxidizing and the protective quality of the oxide may become impaired. It is well established that when insufficient oxidant is present a breakdown of the oxide occurs and the effectiveness of the barrier oxide for hydrogen ingress is decreased [l]. The breakdown of the oxide is identified with oxide dissolution, preferentially occurring at the grain boundaries in the metal
111. As a part of an on-going surveillance program, pressure tubes removed from the reactors are monitored for corrosion and deuterium uptake. A high level of deuterium, recently found in one of the tubes, has raised concern about the possibility of deuterium ingress from the annulus. In order to evaluate the protective quality of the oxide on the outside surface of pressure tubes removed from reactors, a simple testing procedure, that can be easily adapted for use in a hot cell, was required. Elsevier
Science Publishers
to characterize
B.V. (North-Holland)
Long sections of the removed tubes had to be examined around their circumference and along their lengths. Two simple electrochemical techniques, applicable to corrosion films on zirconium alloys, are impedance and anodization measurements which are carried out using an aqueous electrolyte contact. The solution penetrates the porous layers in the corrosion film and the measurements are thus representative of the non-porous part of the corrosion film. We have developed a procedure to map the outer surface of removed pressure tubes by localizing the electrochemical measurements to small areas.
2. Experimental The pressure tube specimen to be examined was the working electrode; oxide was abraded off a small area at one end so that electrical contact could be made. Locations on the outside surface, free of scratches, were carefully selected. A piece of insulating tape with several punched holes, varying in diameter from 2 to 6 mm, was pressed onto the surface at a location. Several small test spots, exposed oxide film of known areas were thus made available at a given location. A few drops of saturated sodium nitrite solution were placed on one of the test spots. A platinum foil electrode, contacting the solution, formed the counter electrode. Measurements were mostly made with this two electrode arrangement. When needed, for example, during impedance measurements over a range of frequencies, a platinum wire inserted in the solution between the specimen and the counter electrode served as the reference electrode. The potential of the specimen was slightly negative to that of platinum; the potential measured at different locations varied in the range of 10 to 30 mV and was
N. Ramasubramanian,
V.C. Ling / Localized impedance-anodization
unaffected by the impedance measurements. Impedance measurements were made on a General Radio 1692 RLC digibridge at 1 kHz and changes in capacitance were followed with time for five minutes. Anodization was then carried out under galvanostatic conditions at 3 and 14 PA mm-’ and the potential of the specimen relative to the counter or reference electrode was followed with time. A final impedance measurement completed the testing. For comparison purposes similar measurements were made on off-cuts (rings cut and saved from the two ends of the stress-relieved pressure tube prior to installation in the reactor) and on specimens of pressure tube material in the pickled and oxidised conditions. Specimens from the off-cuts and pickled specimens oxidised in steam to form - 1 pm thick oxide were exposed to 1% deuterium in nitrogen and vacuum at 573 to 673 K for various times and changes in the impedance and anodization behaviour were followed.
The increases in the capacitance, measured soon after contacting the specimen with the electrolyte solution, were large and occurred within a couple of minutes. Small increases in the measured capacitance were then observed with increasing contact time with the solution; however, the capacitance measured after 5 min of contact was nearly steady and this was used in thickness calculations according to the relation: t, =0.1947/c,
(1)
where t, is the average thickness of the oxide (in pm) not penetrated by the solution and C is the capacitance in nF mmm2 [2]. The non-porous oxide, so called “barrier layer”, is considered to be non-uniform in thickness and the measured capacitance, used in eq. (1). is the sum of the capacitances of the fractional areas of varying thicknesses of the non-porous oxide. During anodization oxide growth by the applied field is expected to initially ocur at the thinnest regions of the non-porous oxide. The charging potential at which anodic oxide growth commences is mainly determined by the minimum thickness of the non-porous oxide which was calculated in nm, as
t M = 2.5 x v,
(2)
where I’ is the initial charging potential in volts. A film formation ratio of 2.5 nm V-i was obtained by comparing interference colours of films formed on Zr-2.5 Nb alloy in sodium nitrite solution with those formed on
227
Table 1 Average and minimum thickness op oxide, not-penetrated by the electrolyte solution, measured by capacitance and ancdization measurenents localized to spots 18 mm2 in area, on the outside surface of pressure tube P3LO9 removed ing Unit 3; anodization at 14 PA mm-* Distance from inlet
Capacitance (nF mm-2)
3.2 4.63 5.0 5.5 5.8
Charging potential
(V)
(m)
Off-cut
3. Results and discussion
measurements
0.80 1.41 0.69 3.89 1.78 4.60 2.64 9.36 2.34 6.72 0.55
Non-porous thickness
‘A
from Picker-
oxide
1M
average
minimum
(P)
(pm)
30
0.24
0.075
20
0.14
0.050
50
0.28
0.125
8
0.05
0.020
0.11 0.04 0.074 0.021 0.083 0.029 0.35
0.025
10 10 8 2 9 2 80
0.025 0.020 0.005 0.023 0.005 0.200
zirconium. Anodic films on zirconium were produced by fixed voltage anodization in 1 wt% KOH solution [3]. In table 1 are shown some typical results of capacitance and anodization measurements on pressure tube P3LO9 removed after 12 years of operation in Pickering Unit 3; data on the off-cut are included for comparison. In the case of the off-cut the average thickness of the non-porous portion of the oxide, estimated according to eq. (1) from measurements at four locations, is 0.35 pm. Infrared interferometry indicated a total oxide thickness of 0.8 pm, produced by the stress-relieving treatment in steam [4]. A little more than half of the steam-grown oxide is thus porous and easily penetrated by the sodium nitrite solution. The minimum thickness in the non-porous oxide is only 0.20 pm. In the case of specimens from the tube P3LO9 infrared interferometry indicated a slight increase in total oxide thickness on the outside surface; when compared to the initial thickness of 0.8 pm the thickness measured, after removal of the tube from the reactor, was - 2 pm [4]. However, despite this increase in total oxide thickness a general degradation of the oxide quality on the removed pressure tube is clearly indicated when the results for the tube P3LO9 are compared to those of the off-cut. Both the average and minimum thickness of the non-porous oxide have become greatly reduced. There were locations where the non-porous oxide was only 5 nm thick; a thickness same as that resulting from exposure of the bare alloy
228
N. Ramasubramanion.
2
3
4
5
V.C. Ling / Localized Impedance-anodization
6
Charge (millicoulomb) Fig. 1. Potential versus charge anodization behaviour of oxide pressure tube LO9 removed from mm2 and current density 14 HA off-cut,
curves typical of the localized on the outside surface of the Pickering Unit 3; spot area 18 mm-*; (a) pickled surface, (b)
and (c to f) four locations on tube LO9 at 4.5 to 5 m from the inlet end.
to air at room temperature. If the non-porous oxide is a barrier to deuterium ingress from the annulus then its effectiveness has been reduced and also seriously impaired at some locations on tube P3L09. In fig. 1 a set of typical potential versus charge curves obtained during galvanostatic anodization measurements on tube P3LO9 is compared with those obtained on the off-cut and a bare alloy specimen. The varying degree of degradation of the oxide on the outside of the tube, along the channel length, is clearly evident. The conclusion that the electrochemical measurements represent changes in the properties of the oxide is supported by the results obtained in laboratory experiments. The outside oxide on specimens from off-cuts and the oxide grown on pickled specimens were free of scratches. With increasing exposure to the non-oxidising environment, 1% deuterium in nitrogen or vacuum, at 573 to 673 K, the measured capacitance increased and the charging potential during anodization decreased. Anodization behaviour similar to curves d and f in fig. 1, indicating oxide breakdown, was observed after prolonged exposures. There were no visible changes in the appearance of the black oxide and infrared interferometry revealed no changes in the total oxide thickness. The environment in the gas annulus in reactor had been oxidising but the low concentration of oxidant and the presence of deuterium led to a degradation of the
measurements
existing oxide and growth of a non-protective oxide. Oxide dissolution had been simultaneously occurring with oxide growth. When calculating the average and minimum thickness of the non-porous oxide from eqs. (1) and (2) it is assumed that the oxide is Zrq. This is not strictly true in the case of Zr-2.5 Nb pressure tube material. It is a two phase alloy and undergoes microstructural changes during its life in the reactor. The effective dielectric constant, refractive index and the electrical conductivity for films grown on Zr-2.5 Nb pressure tube material could be different from those of zirconia films on zirconium. Nevertheless, comparisons, such as those shown in table 1 and fig. 1, can be made of the environmentally induced changes in the quality of oxide films on a material. The localized impedance and anodization measurements provide a quick means to evaluate the protectiveness of thin zirconia films especially, because penetration of the porous oxide by the contacting solution can be achieved in a relatively short time.
4. Summary Impedance and anodization measurements, using an aqueous electrolyte contact and localized to small areas, were used to grade the quality of the corrosion film on irradiated zirconium alloys. This method was developed for examining oxide films on the outside surface of pressure tubes removed from CANDU reactors.
Acknowledgement This project was supported CANDU Owners Group.
and
funded
by
the
References [I] B. Cox, Mechanism of Hydrogen Absorption by Zirconium Alloys, Atomic Energy of Canada Limited Report, AECL8702 (1985). [2] P.M. Rosecrans, ASTM-STP 824 (1984) 531. [3] N.J.M. Wilkins, Corrosion Sci. 5 (1965) 3. [4] N. Ramasubramanian and V.C. Ling, J. Nucl. (1990) 237.
Mater. 175