EVALUATION OF PERMANENT REFERENCE CELLS FOR DIRECT BURIAL APPLICATIONS

J. Peter Ault, P.E., John Repp and John Travaglini Corrpro Companies, Inc.

Tennessee Avenue & Beach Thorofare Ocean City, NJ 08226

ABSTRACT

Several new permanent reference cells have emerged in recent years with the advantages of smaller size and apparent longer life. The present test program was developed to assess the relative service life provided by various permanent reference cell designs. The natural tendency for the potential of a Cu- CuSO4 reference cell to vary must also be considered. Some causes of this variation may be due to the age of the cell and differences in manufacturing. Four tests (CuSO4 leaching, chloride leaching, dry-out tendencies and "real-world" testing) were performed on six permanent reference cells (PRC) samples. This paper discusses the testing results and how these results can be useful for the development of a standardized test method.

INTRODUCTION

Permanent reference cells have been used for many years to monitor buried structures susceptible to corrosive elements. These reference cells were typically bagged with a backfill material that made the cells bulky. Several new reference cells have emerged with the advantages of smaller size and apparent longer life. The present test program was developed to assess the relative service life provided by various permanent reference cell designs.

Environmental factors must be considered while performing durability testing. Previous authors have discussed possible factors resulting in erroneous potential readings from a half-cell and their relevance to permanent reference electrodes. 1'2 Variables which usually affect copper-copper sulfate (Cu-CuSO4) reference cells are temperature and light variations. Both authors indicated that potential

readings can vary by 0.9 mV/°C, and that exposure to direct sunlight can cause potential variations of up to +20 mV. However, since permanent reference cells are typically used in locations with no direct sunlight exposure and limited temperature variations, such effects should be minimal. J Another environmental factor that can cause a change in potential, although not as common as temperature and sunlight, is a change in chloride concentration. As the chloride contamination increases the potential shifts away from its reference potential (potential with no chlorides present) to approximately -123 mV in a 60,000 ppm solution. 2 All of these factors must be considered when performing reference cell durability tests.

The natural tendency for the potential of a C u - C u S O 4 reference cell to vary must also be considered. Some causes of this variation may be due to the age of the cell and differences in manufacturing. Because of this, cells have been cited as repeatable to + 0.006 V (6 mV). 3 When potentials between a known "good" and test Cu-CuSO4 reference cell are outside this range it should be considered unusable. While this may be an acceptable range in a laboratory setting, an electrode found to be a few milli-volts outside this range is likely to be used when acquiring field data. This study considered cells that were within + 10 mV as being "accurate."

EXPERIMENTAL APPROACH

Six permanent reference cell (PRC) models were obtained from five manufacturers. Table 1 summarizes the cells tested. Cells A and C were bagged with plaster/bentonite and no membrane separating the copper sulfate from the backfill. Cells F was supplied unbagged but had a ceramic membrane. Cells D and E were bagged with plaster/bentonite and also had a membranes isolating the copper sulfate from the backfill. Finally, Cell B was constructed with a "double membrane" configuration. This included a ceramic membrane containing the copper element and copper sulfate gel. That membrane was packaged within a geomembrane that also contained a copper sulfate gel.

Table 1 - Summary of Cells Tested

Cell A B C D E F

Supplier Cell Type Supplier A Bagged Only (No Inner Membrane) Supplier A Double Membrane Supplier B Bagged Only (No Inner Membrane) Supplier C Membrane/Bagged Supplier D Membrane/Bagged Supplier E Membrane/Unbagged

Four tests were performed on each PRC type. Each test was conducted on one of each PRC. A total of 24 PRC's (one PRC per model per test) were used for testing. These tests, discussed below, were used to measure the specific physical properties and relative durability of each PRC. All cells were tested in the as-received condition (i.e., bags were not removed).

C u S 0 4 Leach ing

Testing was conducted to determine the rate at which copper sulfate solution will leach out of a permanent reference cell. PRC's from each supplier were placed in individual 5-gallon containers and

immersed in deionized water maintained at ambient conditions. During testing the atmospheric temperature ranged from 20 to 27 °C (68 to 80 °F) and the relative humidity ranged from 80 to 98 %.

The potential of each PRC was measured against a piece of copper placed in a saturated copper- sulfate solution (cell). This cell was connected to each container via a salt bridge. Following one day of testing DC resistance measurements were made across the electrode membrane. These resistance measurements were used to show differences in barrier properties among the PRC models.

During testing the potential between each PRC and the copper strip in copper-sulfate solution was continually monitored using a portable data logger with an input resistance of 300 mega-ohms. The potential was monitored to detect changes in the copper-sulfate gel concentrations within each PRC. The conductivity of the water was periodically monitored to measure the increase in conductivity caused by copper sulfate leaching from the PRC gel. The deionized water was replaced when the conductivity exceeded 2,000 ~xmhos/cm (200 mS/m).

Chloride Leaching

Testing was conducted to determine the rate at which sodium chloride would leach into a permanent reference cell. PRC's from each supplier were immersed in a holding tank containing a saturated sodium chloride solution. This holding tank was connected to a cell containing a copper-sulfate solution and a piece of copper (cell) via a salt bridge. The potential of each PRC was monitored throughout this test using this Cu-CuSO4 cell.

Following one day of testing DC resistance measurements were made across the electrode membrane. These resistance measurements were used to show differences in barrier properties of the membranes and backfill material among the PRC models.

During testing the potential between each PRC and the standard copper-sulfate cell was continually monitored using a portable data logger. The potential was monitored to detect leaching of the sodium chloride solution into the PRC.

Dry-out Tendencies of Copper-Sulfate Electrode

Moisture is required to maintain the copper-sulfate gel inside each PRC. Most cells require an initial wetting process before permanent installation. However, the tendency of the cells to dry out and re-wet is an issue affecting their longevity. Testing was conducted to determine the PRC's that would dry-out quickest in arid environments. This testing was also used to show the PRC models that would regain functionality quickest when re-wet.

PRC's from each supplier were prepared by initial immersion in deionized water. This immersion was conducted until no air bubbles were observed from the PRC (indicating saturation). Following this initial wetting, the PRC's were buried in washed beach sand, which was not wet.

Prior to burial a commercially pure zinc plate was placed at an average depth of 2-inches from the bottom of each PRC. The sand was maintained at ambient conditions inside the laboratory. Due to the lack of moisture in the sand the water contained within these cells was pulled into the surrounding soil by osmotic forces. The potential of each cell to a zinc plate was monitored to view changes in potential

as the PRC's dried out. This test was used to show the ability of the PRC models to resist drying-out in dry installations.

Once all PRC models achieved potential readings that were outside of the acceptable range, tapwater was added to the sand until saturation was visually evident. This allowed for the re-wetting of the PRC's by similar means as would occur in the field. These PRC's and sand were allowed to dry for an additional 50 days. Any changes in potential were noted during this test. After 50 days, the sand was re-wet with tapwater (to the point of saturation) and the time for the potential of the PRC's to stabilize was noted. This test demonstrated the ability of the cells to recover after an initial dry-out period.

"Real-World" Testing of Reference Cells

PRC's from each supplier were prepared by initial immersion in deionized water. This immersion was conducted until no air bubbles were observed from the PRC (indicating saturation). Following this initial wetting, the PRC's were buffed in professional grade potting mixture, which was also saturated (observed visually).

Prior to filling the tank with soil, a commercially pure zinc plate was placed at the bottom of the tank. The potential between the PRC's and the plate was monitored over time using a portable data logger. The potential of the zinc plate was also periodically measured using a portable, refreshed Cu- CuSO4 cell as a control. This test was used to simulate the actual service conditions that might be expected of each PRC.

DC Resistance measurements were made across the membrane of each cell following two days of testing. These measurements indicate the initial membrane properties of the PRC's in this environment.

The test was performed in a movable tank, which allowed for climatic changes in the local environment by moving the tank to different locations. The PRC's in soil were exposed to temperatures ranging from 4.4 °C to 23.9 °C (40 °F to 75 °F). This temperature variation allowed for representative results from possible real-world service conditions that may be expected of each reference.

RESULTS AND DISCUSSION

Physical Characteristics

Figure 1 shows the membrane resistance for each PRC in each exposure environment. Figure 2 shows the potential of each cell, referenced to a Cu-CuSO4 standard, following 48 hours of testing. The 48-hour period minimized any "noise" that may be caused prior to stabilization in its environment and allowed for comparisons to be made. This allowed for comparison between the stabilized potential of each test. Even at this early point performance trends were observed.

C u S 0 4 Leach ing

Copper-sulfate leaching testing was conducted over a period of 125 days. Potentials between each reference cell and the Cu-CuSO4 standard were monitored throughout this period. The potential differences between the PRC's and the standard were observed to be as much as 50 mV as shown in Figure 3.

Figure 3 shows the slope of each curve following the initial 24-hour wetting period. This slope was measured in millivolts per hour and showed little overall change. Most of the slopes were negative, thus there was probably a difference in temperature between the copper-sulfate solution and deionized water. However, the values shown in figure 3 were typically small, further suggesting that the temperature variations were likely to also be small. The data also shows that cells F, A and D experienced the highest initial CuSO4 depletion relative to the other cells (assuming all have the same relative temperature variations) as shown by their large initial slope.

Figure 4 shows the plot of the potential trends for the CuSO4 leaching test. Gaps indicate periods where no data was collected due to power failure of data loggers. Although there were differences in mass between the test environment (DI water) and the copper-sulfate standard, the temperature change between them was considered to be small and has not been corrected for in this graph. Figure 4 also shows the area of acceptable variation from the Cu-CuSO4 reference. This figure shows that after 125 days only cell B was within the acceptable range (+10 mV vs. Cu-CuSO4 standard). The cells having final values the farthest outside this range were cell D (which was never within the acceptable range), cell F and cell A. These three cells were observed to have the highest initial copper-sulfate leaching as indicated by their initial slopes.

Chloride Leaching

The chloride-leaching test was conducted over a period of 110 days. The initial data from this test has shown some unique characteristics. Similar to the copper-sulfate leach test, temperature dependent variations were considered to be relatively small. However, significant trends associated with cell performance have been observed. Figure 5 shows the potential versus a Cu-CuSO4 standard. Gaps indicate periods where no data was collected due to power failure of data loggers. This figure also shows the area of acceptable variation from the Cu-CuSO4 reference. If the acceptable range of a cell is considered to be +10 mV from this standard, this graph shows that only PRC's B and E were within the acceptable potential variation range.

Dry-out Tendencies of Copper-Sulfate Electrode

Figure 6 shows a typical drying/re-wetting cycle with the PRC's in the sand test. Drying rates for the six PRC's ranged from 0.8 (cell E) to 3.7 (cell B) mV/day. The cells that dried out the most (i.e., shifted the most during drying) exhibited the largest shift in potential upon re-wetting. Cell E, the most stable of the PRC's, exhibited the smallest shift upon re-wetting. However, upon re-wetting, most PRC's, with the exception of cell F achieved potential readings within or near the acceptable range.

"Real World" Testing of Reference Cells

Figure 7 shows the potential trends for the soil test. These potentials have been corrected for temperature variations using a 0.9-mV/°C temperature dependence and 20 °C as the reference temperature. This correction was done since the difference in exposure temperatures was deliberately varied by exposure to various environments.

Initially all PRC's had a potential of approximately 1.1 volts to the zinc plate, except for cell D, which had an initial potential of 1.7 volts. However, this potential reached approximately 1.1 volts

following 2 hours of testing. This deviation was likely due to this PRC needing additional time to wet. The plots in Figure 7 also show that all of the cells have an initial increase in potential during the first 24 hours of testing. This increase was likely the time each required to stabilize.

Figure 7 also shows the area of acceptable variation from the C u - C u S O 4 reference. The acceptable range was based on the average potential of the zinc sheet measured to a refreshed, portable Cu-CuSO4 reference cell, approximately 1.13 volts. After 95 days of exposure, none of the PRC's were within the acceptable range, however, they all appear to be following the same trend away from the standard potential. Reasons for this were not thoroughly explored.

CONCLUSIONS

Based on the results of this testing the following conclusions were drawn:

. PRC's with a high membrane resistance in deionized water and saturated NaCI solution typically had a high membrane resistance in soil.

. Following 110 days of saturated sodium chloride testing two PRC's were within the acceptable range. PRC's with low membrane resistance typically exhibited the worst performance in the saturated NaC1 testing.

. PRC's with an initial stabilization potential greater than +30 mV from a Cu-CuSO4 standard experienced greater effects from the CuSO 4 and chloride leaching tests.

. PRC's with the highest initial chloride leaching (as observed by the slope of the potential change) were among the worst performers in the both the CuSO4 and chloride leaching tests.

5. Following 125 days of deionized water exposure only one PRC was within the acceptable range.

6. The cells fabricated with double membrane technology tended to outperform the other cells.

R E F E R E N C E S

. Ansuini, Frank J. and James R. Dimond, "Factors Affecting the Accuracy of Reference Electrodes", Material Performance, November 1994.

. Lopez, R.J., E. Ondak and S.J. Pawel, "Chemical and Environmental Influences on Copper/Copper Sulfate Reference Electrode Half Cell Potential", Material Performance, May 1998.

3. Uhlig, Herbert H., Corrosion Handbook, John Wiley & Sons, 1948, p. 931.

Membrane Resistances

100(0)00

100000

1O0OO

i ! logo

log

I0

Ol Water Sat. NaCl Soil

TeSt Environment

m cei ik Nee a

i C e r l C

i m Cell D

I • Cell E IDCell F L . . . .

Figure 1. Membrane Resistance for Each PRC after One Day of Exposure in Deionized Water and Saturated NaCI and after Two Days of Exposure in Soil.

Cell Potential v. Cu-CuSO 4 Standard Following 48 Hours of Testing

40.0 T

20.0300 ~ - ~ ~ ~ ~ i . . . . . . . . . . . . . . . . . . . . . . ~" lo.o i . . . . . . . . . . . . . . . . . . . . . . i Cell k "~ Cell B m

0.0 C mCe

-10.0 , i iCe l l O

i mCell E i -2o.o I [oc~,, F -30.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

-400 . . . .

-50.0 I i DI Water Sat. NaCI SOl

Ta=d Environmon

Figure 2. Potential Readings following 48 Hours of Testing.

0.0100 _

Change In PotenUal Following Initial Wetting Period (24 hours)

-o ooso 4

-0.01 O0 . ._

-0.0150 e - - - - -

g i ~ . . 0 . 0 2 0 0 ~ . _ _

"°'°25° i

-00350 Ceil A Cell B Ceil C Ceil D Cell E Cell F

Refenlltce Cell

Figure 3. Slope Following Initial Wetting Period (24 hours) in C u C u S O 4 Leaching Test.

Potential v, Sat.. CuSO 4 Ol Wate r Test

e K:::::: >o 10.0

~ 5.o

i -1Q° I -20.0 1

-30.0 -

-40.0 . . . . .

-50.0 ~ _ 0 500 I o00 1500 2000

Bapsed Time (hours)

(a) Cell A

~ [ 2 5 o 0 3 o o 0 I

. . . . . . . . J

Potent ia l v. SaL CuSO 4 DI Wa te r Test

50.0 ~ • . ~ l

40 0 BOX ShOw s Acceptable Potential Variations from Sat, CuSO 4 Reference (+ or - 10 mV) | - - - -

J r 30,0 ~,

! 20.0 i ~[~IjL ~ !_- ~ 0,0

-10.0

~. -20,0

_ _ [ -

-30.0 l

-40.0 -50.0

! ~" . . . . . . . . . . . . . . . . . . ] 4 - . - ~ . . . . I

. . . . . . . i . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . ~ r _ . 1 , ';

0 500 1000 1500 2000 2500 3000 Bapsed Ttrne (hours)

. . . . . . . . . . . J

(b) Cell B

Potent ial v. Sat. CuSO 4 DI Wa te r Test

50.0 , . . . . . . . . . . . . , --. _

~oo ~; ! l ttt --iL - i' , t . . . . . I ~ 20.0

~ 10.0

' ~ 0,0

-10.0

r o -20.0

,,.-.L. ,k ,. k

-300

I ,

-50.0 . . . . . . . . .

0 5()0 1000

i

1500 2000 2500 3O00, Rapssd Time (hours) !

(c) Cell C

Figure 4. Potential Trends for CuSO 4 Leaching Test.

50.0

40.0

30.0

20.0

Potent ia l v. S a L C u S O 4

DI W a t e r Test

BOX Show S A c t a b l e P°tent~a$ Varlat~ns fr°m Sat' CuSO" I~l f erence (+ ° r " J

lOmV)

o 10.0

~ 0.0

I ~ -10,0 i ~ = i ~. -20.0

-30.0

-40.0

-500

50.0

40.0

" l - - m = l ~ " v = ~ " ~

0 500 1000 1500 Bapsed TIms (hours)

(d) Cell D

2000 _ k

2500 3OOO!

Poten t ia l v. Sat. CuSO 4 DI Wa te r Test

] ~ I I L BOX Show S Acceptable Polen~al Variations from Sat, CuSO 4 Reference (+ or- 10 mV)

30.0

200

' ~ 100 ,.~ I ~ 0.0

~¢ -10.0 o ~. -20.0

-30.0

-40.0

-50.0 : 0

50.0

~ , - . - . ~ -,

? ? . . . . . . . . . .

k

1000 1500 2000 2500 3000] Bapssd Time (hours)

(e) Cell E

500

PotenlJa l v. Sat . CuSO 4 DI Wa te r Test

Box ShOW s Acceptable Potential Variations from Sal. CuSO, Reference (+ or - 10 rnV 40.0

3 0 0

I 20.0 ¢) i_; lo.o I ~ 0.0 . . . . . . . .

-100 a

: ~ -20.0

-30.0 . . . . .

-40.0 i

-S&O ~

0 500 100(I 1500 2000 2500 3000 I Bsp lsd Time (hour i )

(f) Cell F

Figure 4. Potential Trends for CuSO4 Leaching Test.

• ~ -50

-100

ID

-200

Potent ia l v, Sat, C u S 0 4 Sat. NaCl So lu t ion Test

100 I Box Show s Acceptable Potential Var~ati~s from Sat. CuSO 4 Reference

50

,L, I I

.250

i J

i

f i

i

i .

i /

[ 3ooo 25O0

J

500 1000 1500 2000

Bapssd Time (hours)

(a) Cell A

"5 >

m

¢L

lOO

5O

0

-50

-100

-150

-200

-250

Poten t ia l v , Sat . CuSO4 Sat. NaCl So lu t ion Tost

Box Show s Acceptable Potential Variations from Sat. OJSO 4 Reference

I J

-4- T ! L

I

500 1000 lsoo 2oo0 2500 3ooo I i

Bapssd Time (hours)

(b) Cell B

Potent ia l v. SaL C u S 0 4 Sat. NaCI So lu t ion Test

100 I Box Show s Acceptable Potential Variations f rom Sat, CuSO 4 Re[ers,'~ce

50 1 i ~ -

-150

-200

-250

4

j _ ~ ~ JL m, .

500 1 OOO 1500 2000 2500 3000 I i

Bapssd Time (hours)

(c) Cell C

Figure 5. Potential Trends for Chloride Leaching Test.

Potential v. Sat. CuSO4 SaL NaCl Solution Test

10o

50

o

,> -50

-100

-150

-200

-250 o

r ~ BOX Show S Acceptable Potential Variations from Sat. CuSO 4 Reference

i \

l

J I I

I

I 500 IOO0 1500 2000 2500

Bapsed Time (houri)

3000

. . . . . J

(d) Cel l D

"6 -50

.~ 400

-150

-200

.250

Potential v. Sat. CuSO4 Sat. NaCI Solution Test

100 ![ Box Shows Acceptable Potential Variations from Sat, CuSO,I Reference I I

. . . . . . .

l

l 4

i I i l o o o 1 ~ 200o 2500 5000 I

i Bapeed Time (hours) !

(e) Cel l E

Potential v. Sat. CuSO4 Sat. NaCI Solution Test

100 Box Show s Acceptable Potential Varlatiens from Sat. CuSO 4 Reference

50 Z I -

0

qoo J 1 / 3 ~ ,

#,- -150 ~ 4 r ' ~ l l ~ [ '1=" I . . . . i

-200 ~

r i -250 ~ 3000 500 1000 1500 2000 2500

Elapled Time (houri) I

(f) Cel l F

F igure 5. Potent ia l T r e n d s for C h l o r i d e L e a c h i n g Test.

l

['10,900

Potential vs. Zinc S a n d - Drying P e r i o d 1

0 80Q 150

' : ' : " ~ ' : " - " ' - . - . ; - . _ . _ . ,~c~,,_E . ~

~ " " " . , . /Cell F ' ' ! "" ,~ I " ~ ~ ' , ,u ,~ . , . , ~ . . ,~ . . . . . , _ . -~, K.Cell D E'"""

i e• ~ ~ ~ ' lmmle • ~ =m==nd- I ~°°Oeoe ~ ~ e~! ~ ttmtnutt , tt m e ue e n •

~ eoll oeoe ° "'""" ""... --Cell C

Cell B ~4P" _ ~,, ,~ ~ ! o • o 0 • 0 • o ' e •

°o •

k " ' * ' * t Re-Wetting "-,. "-.,, .,::

Figure 6.

160 170 180 190 200 210

Elapsed Time (days) Typical Drying/Rewetting Cycle in Dry-out Tendency (Sand) Test

Potential v. Zinc Soll Test

1.16 I . . . . . . . . . i T

1,15 il . . . . _[ Box Show s Acceptable Potential V a r ~ n s from Sa t OaSC

1 14 i

~. 1 1 3 . . . . . . . . . . . . . . . . . . . . . . I I

111 . . . . . . 1

110 . . . . . . . . . . . . . . . . . 1

1,09 i

1,08 = 500

Reference

1000 1500 Bapsed Time (hours)

(a) Cell A

I

i . . . . . . I

20o0 12~

1.16

1.15 i J

Potential v. Zinc Soil Test

Box Shows Acceptable Potential VariaUons from Sat. CuSO 4 Reference 1

114

'~" 1.13 g '~ 112

11o | = = ~

: I ...............

_JL

0 500 I O00 1500 2000 Bapled Time (houri)

" " 1

250(

(b) Cell B

Potential v. Zinc Soil Test

1.16 .... i J i- ..... f

1 Box Show S Acceptable Potential Variations from Sat. CuSO, Reference 1. 15 L

1.14

1.13

- - 1 . 1 2

~. 1.11

1.10

-4-

1,09

1.08

I

0 500 10%apIed Time (hours15o0 2000 250

(c) Cell C

Figure 7. Potential Trends for Soil Test.

Potent ia l v , Z inc Soi l Tes t

1 16 . . . . . . . .

1.15 [ 8ox Show s Acceptable Poten=Wara~ons from Sat. CaSO, Reference } . . . .

1,14

"~" 113

- - 1.12

1.1C ; . . . . . . . . . . . . . . . .

1.09 -

0 500 1000 1500 2000 2500] Elapsed Time (hour i ) i

. . . . . . . . i

(d) Cell D

1,16

115

Potent ia l v. Z inc Soi l Tes t

Y ~ Y I BOx Stlow s Acceptable Potential Variations from Sat. CuSO 4 Reference I

1.14

"~ 1.13

g -~ 1.~2 ¢ .= o 1.11

1.10

1.09

1 0 8

!

500 1000 1500 2000 IRapaed Time (hour i )

I

_ . j i

2500

(e) Cell E

116

1.15

1.14

Pot ant 1,13 tat (vo Its) 1,12

1,11

1 10

109

1.08

Potent ia l v . Z i n c Soi l T e s t

I , T f BOx Shows Acceptable Potentia~ Variations from Sat. CuSO 4 Reference

i 500 1000 1500

Elapsed Time (houri)

(f) Cell F

2OOO 2500

Figure 7. Potential Trends for Soil Test.