Permanent zinc reference electrode for soil applications
Paolo Marcassoli, Maurizio Mori, Monica Ginocchio, Bruno Bazzoni,
Cescor srl, Milan, Italy
Summary
High purity zinc is a pseudo reference electrode extensively used in seawater where
it provides adequate stability because of the depassivating effect of chloride ions;
when used in soil, also if installed in activating backfill containing bentonite and calcium sulphate, its stability is poor and polarization occurs due to passivity effects. In
this paper, a new zinc reference electrode is illustrated which employs a stable electrolyte containing chlorides. Two prototype versions have been developed and tested. Results are reported and the specific advantages with respect to traditional electrodes are discussed.
Key Words: Monitoring. Cathodic Protection. Reference Electrode. Zinc. Telecontrol.
1
Introduction
The criterion universally used to verify the cathodic protection conditions of a structure is based on the measurement of the structure potential with respect to a reference electrode placed in contact with the environment where the structure is immersed in. The measurement is performed with a high impedance voltmeter, whose
positive pole is connected to the structure, and the negative one is connected to the
reference electrode. Both permanent or portable reference electrodes are used: the
former are installed in close proximity to the structure to be monitored, while the latter
are handled by CP engineers during periodic measurement activities. Permanent reference electrodes are also a component of the so called potential probes, that are
probes containing a steel coupon, simulating a coating defect of a buried structure,
and a permanent reference electrode embedded close to coupon. Potential probes
allow to completely eliminate spurious ohmic drop contributions in the potential readings, in particular in the presence of stray currents. Potential probes and permanent
reference electrodes shall meet the requirements of long term stability of their potential and of durability of the device in the exposure environment – typically soil or water.
In recent years potential probes designed precisely to ensure high durability and stability of measurement have been developed: this applies in particular to the so called
StrayProbe [13]: one type uses a reference electrode obtained with a special activation (MMO) of titanium in alkaline mortar, and another one with a non-polarizable zinc
electrode in neutral backfill. The equivalence between MMO and the copper-sulphate
electrode (CSE) is –0.1 V, whilst the equivalence between Zn and CSE is –1.1 V;
accordingly the conversion to CSE is immediately obtained by adding respectively –
0.1 V and –1.1 V to the values detected with the probes.
1
Test Post
Shunt
Posizione preferita per la sonda
(faccia in su)
Backfill
Disco di acciaio
Condotta
SMMO o Elettrodo di zinco
a) Installed Potential Probe
b) StrayProbe
Figure 1: Potential probes
The permanent reference electrode most widely used for applications in soil is copper – saturated copper sulphate. The version for permanent installation is distinguished compared to the portable one for the use of copper sulfate crystals, contained in a terracotta case, which capture the humidity of the soil, thus acting as electrolyte. The main limitation of such electrode is the poor durability. Due to leakage,
but also to contamination by chloride ions and calcium, the average life of the copper
- sulphate electrode is in fact in the order of a few years. The leakage of the electrode also causes some negative effects:
 The dispersion of metal ions in the environment,
 The accumulation near the structure of cupric ions, which constitute a chemical
species potentially corrosive to steel and alloys in general.
Another permanent reference electrode is high purity zinc. This one is used today as
typical application as a permanent electrode for cathodic protection in seawater,
where it is characterized by high strength, ease of realizations of different shapes,
and prolonged stability over time. The excellent behavior of zinc as a reference electrode in seawater is due to the activity conditions established on the surface, favored
by the high concentration of chloride ions in seawater. Vice versa, in fresh water or in
soil, zinc is not applicable for its tendency to passivation. Passivation causes a raising of free corrosion potential which makes zinc in direct contact with soil unusable
as a reference electrode. To maintain zinc active, it is normally laid in contact with a
backfill, normally made by gypsum and bentonite. However, the durability is, as for
the copper sulphate electrode, very limited because of the leakage affecting the activating species of the backfill, and passivation takes place in a period varying from a
few months to a maximum of a few years. This kind of limitation is particularly critical,
for example, in cathodic protection applications for aboveground tanks, where the
reference electrodes, installed below the tank bottom, cannot be replaced in any way.
2
2
Potential of zinc in soil
The zinc reference electrode is a so-called pseudo-reference electrode, being its potential determined not by a specific electrode semi-reaction, but rather by a corrosion
reaction in the exposure environment, characterized by a mixed potential as balance
of anodic and cathodic semi-reactions. In neutral and acid environments, the mixed
potential of zinc is determined by an anodic semi-reaction: Zn = Zn2+ + 2e‒, whose
equilibrium potential, E°, is equal to -0.762 V vs. SHE (equal to -1.006 vs. SCE). In
neutral-alkaline environments instead, with a pH higher than 8÷9, zinc gives rise to
the formation of hydroxide, Zn(OH)2, whilst in alkaline environments, soluble complexes are formed, zincates HZnO2‒ and ZnO22‒ (see Pourbaix diagram for zinc, Figure 2). In a neutral environment, zinc mixed potential can be calculated considering
the exchange current of the anodic semi-reaction of zinc oxidation, the concentration
of dissolved oxygen in water present in the environment in contact with metallic zinc,
the pH, the concentration of Zn2+ ions, fixed to 10-6 eq/l. Applying the mixed potentials theory [2], the cathodic characteristic can be obtained by algebraically adding
the current density of the cathodic processes, the reduction of oxygen and the hydrogen evolution by dissociation of water:
Since the equilibrium potential of oxygen is considerably higher than the equilibrium
potentials of the other reactions involved, the only portion of the cathodic characteristic curve of reduction of oxygen is the vertical one determined by the oxygen limit current density, ilimO2. The equilibrium potential for hydrogen evolution by dissociation of
water is calculated with Nernst equation [3-5]:
The equations relating overpotantial and current density are here expressed by Tafel
equation:
where the exchange current density is:
where i0 ref is 3·10-5 A/m2, the reaction enthalpy
is 30 kJ/mol, the reference temperature is 20 °C, the slope of Tafel line bc is of 0.120 V/decade [6]. For zinc the following anodic reactions and their respective equilibrium potentials [7] are valid, expressed in V vs SHE:
3
2,2
2,0
ZnO2
1,8
1,6
0
-2
1,4
-4
-6
-6
1,2
-4
1,0
-2
0
0,8
0,6
Zn2+
0,4
0,2
Zn(OH)2
0
-0,2
-0,4
HZnO2‒
-0,6
ZnO2‒‒
-0,8
0
-2
-1,0
-4
-6
-1,2
Zn
-1,4
-1,6
-1,8
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
Figure 2: Pourbaix diagram of zinc
Assuming a concentration of Zn2+ ions equal to 10-6, and a value of pH equal to 7, as
the one of the backfill used for the above described zinc electrode, the zinc equilibrium potential is constant and independent from pH itself, and results therefore equal
to -1.184 V vs SCE.
The potential-current density relation for zinc dissolution is described by Tafel equation:
Where the exchange current density is 2·10-5 A/m2 [7], whilst the slope of Tafel’s line
ba, is equal to 0.030 V/decade [8].
Considering that oxygen contained inside the electrode is rapidly consumed and
there shall be no appreciable exchange from the outside, and considering a pH of the
backfill equal to 7, a potential value of -1.070 V vs SCE is obtained in correspondence with the intersection between the anodic and cathodic characteristic. A variation
of +24 mV is estimated per each order of magnitude increase of zinc ions concentration, due to corrosion of zinc bar, however an acceptable variation of electrode potential is expected after decades of years of exposure.
4
Figure 3: Zinc potential as an intersection of anodic and cathodic characteristics for oxygen lower
than 1 ppm, pH equal to 7 and concentration of Zn2+ equal to 10-6
3
NEW ZINC ELECTRODES FOR SOIL APPLICATIONS
A. Brenna et al. [12] have developed and described a potential probe which uses zinc
as a permanent reference electrode. The electrode element of zinc is in contact with
a special solid backfill made of gypsum and cellulose, rich in sulphate ions as activating species for zinc. Starting from this version, specific solutions for the realization of
zinc reference electrodes for soil use have been developed. The requirements to
which an important feedback has been given have been the following:

Non-polarizability

Durability

Negligible release of chemical species

Strength

Simplicity of realization.
During the development phase, two prototype versions have been created [1]: the
first, henceforward named Type A, is composed by a cylindrical high-purity zinc element in contact with a backfill made of gypsum, bentonite and cellulose fiber similar
to the one described in [12], but combined with chloride ions. The backfill is then surrounded by a cylindrical element made with cement mortar rich in chloride ions,
thought as a reservoir of chloride ions, which serves as a porous membrane through
the environment. The presence of chloride ions, in the backfill and in cement mortar,
has the role of zinc activating species together with the sulfate ions. In a second version, type B, the cylindrical element in cement mortar has been eliminated. This second version, in addition to having a more simple manufacturing, prevents the risk of a
side effect of cement mortar alkalinity on the potential (see below). Two batches of
electrodes have been consequently produced: 48 specimens identical to type A and
30 specimens for the Type B (Figure 2).
5
c) Type A Electrodes
d) Type B Electrodes
Figure 3: Prototypes of permanent zinc electrodes for soil application
4
EXPERIMENTAL
4.1 Introduction
The aim of the experimental testing phase has been to verify the open circuit potential of the prototypes, especially from the point of view of stability and reproducibility
over time. The measurements were conducted on the two different versions of reference electrode, Type A and Type B.
Type A electrodes have been subjected to periodical measurements of potential for a
period of about two months, whilst the testing of Type B electrodes has been continued for about a year. The reference electrodes were maintained for the entire duration of the testing within a case filled with silica sand, kept moist with periodic addition
of fresh water. The electrodes were stored under these conditions, and during this
period they were periodically subjected to the measuring of the potential. The potential measures have been carried out starting from one hour after the application of the
electrodes, with a high-impedance voltmeter (>10 Mohm) compared with a saturated
calomel electrode in contact with the same environment. Contextually to each measurement of potential, the values of temperature and conductivity were also acquired.
4.2 Data Analysis
For each series of periodic measurements of potential (N. 48 measurements for Type
A electrode and N. 30 measurements for Type B) the significant statistical parameters have been calculated, as represented in Table 1. The statistical parameters were
then used to create the box plots. The box plot allows to examine synthetically the
spread of data and the main distribution of values. It is represented (horizontally or
vertically oriented) through a rectangle divided into two parts, from which two
whyskers are issued. In the box plot the central rectangle, named interquartile range
or IQR, spans the first quartile (Q1) to the third quartile (Q3). The height of the IQR
indicates the spread and sleekness of data, while the center horizontal line corresponds to the sample median and the central plus sign corresponds to the sample
mean.
6
The whiskers at the edges of the box are the maximum and minimum values. Eventual values beyond the set limit of 1.5 times the interquartile range, in both directions,
are represented with an asterisk and represent potential "outliers".
Table 1: Statistical parameters for the analysis of potential measurement data.
PARAMETER
Minimum
Maximum
Range
Average value
Variance
Standard deviation
Variation coefficient
1st quartile
Median (2o quartile)
3rd quartile
Range interquartile
5
SYMBOL
MIN
MAX
Range
µ
σ2
Σ
CV
Q1
Q2
Q3
IQR
FORMULA/DETERMINATION
MAX-MIN
∑xi/n
∑(xi-µ)2/(n-1)
√(σ2)
σ/µ
nearest value to position 1/4
central value mean of the two central values
nearest value to position 3/4
Q3 – Q1
RESULTS
On type A Electrodes, during two months of experimentation 24 series of potential
measurement have been carried out for each of the N. 48 electrodes. For each series
statistical parameters and related box-plot have been calculated. The results are reported in Figure 5.
For type B electrodes, during a year, 18 series of potential measurement have been
carried out, on which the same elaborations represented above have been made.
Figure 6 shows the graphic representation of results through the box-plot.
Figure 5: Results for the potentials of type A electrodes
7
Figure 6: Results for the potentials of type B electrodes
6
DISCUSSION AND CONCLUSIONS
New type zinc electrodes, developed for soil applications, have given a positive feedback according to the previously fixed targets. In particular, the addition of chloride
ions in the backfill in contact with the element of zinc proved to be very effective in
maintaining the desired conditions of activity, and none of the electrodes has shown
significant effects of anodic polarization due to passivation of zinc.
Some Type A electrode specimens have shown a shift of potential towards negative
values; they have been therefore sectioned and inspected, revealing a contact – or
proximity – between zinc and the cement mortar, with a possible shift to elevated pH
values, to which the formation of zinc hydroxides and the consequent decrease of the
zinc potential correspond (see Figure 2). Among the Types A and B, as also evidenced by the results of measurements of potential, the second is preferable, that is
without the surrounding mortar.
The results of a year of testing have confirmed the remarkable stability of the potential, which lies within a range in the order of ±25 mV (the average range of all the carried out measurements, is exactly equal to 50 mV). The average potential of the electrodes, in a year, has resulted equal to -1.108 V vs. SCE, corresponding to -1.180 V
vs. copper-sulphate electrode. The value is close to that estimated on the basis of
the electrochemical theory, equal to -1.070 V vs. SCE. The potential of the electrodes
tends to approach the expected value after a few days from the manufacturing.
It is also necessary to consider that the semi-reaction Zn = Zn2+ +2e‒, is characterized by a very high exchange current density i0: the values reported in literature depend on the chemical solution used in contact with zinc, but they generally result of a
higher order of magnitude, more elevated than the ones, for instance, with copper.
This data enables to predict a further resistance to the polarizability of this type of
electrode, making it particularly attractive in telemetry systems and tele-control,
where the reference electrode is fundamental and the measurement of potential is
carried out with a high frequency [14].
8
The developed electrode, is configured as a device of simple and robust construction, not subject to leakage and the release of chemical species, intrinsically very durable, and thus representing an interesting alternative to traditional copper - saturated
copper sulphate electrodes. In soil tests are in progress, aimed to the comparison
between Type B zinc electrodes and traditional copper saturated copper sulphate
electrodes. (Figure 7): the measurements carried out after six months confirm the
laboratory results and indicate a good stability of potential over time.
Figure 7: Prototype of zinc electrode for soil installed near pipeline
7
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9