Note: Descriptions are shown in the official language in which they were submitted.
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A Continuous-Action Reference Electrode for
the Cathodic Protection of Metallic Structure
Description
The invention concerns a continuous-action reference
electrode for the cathodic protection of metallic
structures.
The invention relates to the field of techniques for
the protection of underground and underwater plants from
electrochemical corrosion.
The reference electrode is predisposed for the
measurement of the polarised potential of the plant and
can be used for the measurement of the corrosion and also
for the automatic control of the cathodic protection
station.
Some prior art continuous-action reference electrodes
are composed of two metallic pivots of different metals
having high chemical purity, placed in mutual contact at a
point where the connection cable is also connected, which
cable leads to the cathodic protection station.
The said pivots are generally made of antimony and
bismuth and are covered with the oxides of the above
metals.
Another type of reference electrode in the prior art
is composed of two magnesium cylinders with an iron,
copper of nickel pivot inside.
The cylinder and the pivot are mutually insulated
with insulator compound and their point of mutual contact
is on a surface of the head, where they both also contact
the connection cable.
Common defects of the reference electrodes described
above are high metal consumption and low internal
conductivity which prevents use in dry environments with
high impedance.
In such cases, in order to increase the conductivity
of the electrode, it is necessary to increase its size,
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2 214477~ 14 MARCH 19~41
leading to a high consumption of metal as well as to high
costs.
A further type of prior art reference electrode
comprises- a ceramic box element, filled with high-
viscosity electrolyte, in which a copper pivot-shaped
electrode is inserted.
The connection cable is welded to the copper pivot.
On the external surface of the ceramic element an
electrochemical potentiometric end instrument is attached,
consisting of a steel plate with a surface of 625 mm, to
which plate another connection cable is welded.
This type of reference electrode also has several
drawbacks, such as a high specific consumption of metal
and the need for periodic maintenance for filling the
ceramic element with new electrolyte due to the inevitable
consumption of the said electrolyte.
Furthermore, the measurement precision of the
electrochemical potential by means of the relative steel
end instrument is not high due to the difference of the
chemical composition of the steel plate in the end
instrument and of the material of the underground metal
plant to be protected.
The reference electrode can be installed only in a
vertical position and thus its field of application is
limited particularly with regard to large-diameter piping.
Still further, the said reference electrode can be
used only when the ground temperature is above 0.
A principal aim of the present invention is to
perfect the precision of electrochemical potential
measurement and to reduce the maintenance costs of the
plant.
A further aim is to economise on the costs of the
reference electrode with regard to the specific
consumption of metal.
These and other aims are all at least partially
attained by a continuous-action reference electrode for
the cathodic protection of metallic structures, object of
the present invention, characterised in that the
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electrode: comprises a support (1) substantially plate-
shaped and made of ceramic material porous to electrolyte
having the function of a base for a copper plate (2)
having the function of an electrode, situated on both flat
faces of the said ceramic support 1 and of a titanium
plate (3) having the function of an electrode, isolated
from the said copper plate (2) on both flat faces; the
electric connection for both said plates (2), (3) being
realised by means of connection to a further copper plate
applied on a lateral face of the said support (1), the
arrangement of the said copper and titanium plate (2),
(3) on the first surface of the said support (1) in such a
way that a copper plate (2) on a flat surface corresponds
to a titanium plate (3) on an opposite surface, with a
metal membrane (6) constituting a electrochemical
potential transducer being situated on a lateral face of
the support (1) and being electrically isolated from the
other metal plates of the said support (1), said membrane
being made of the same material as the metallic
structure(s).
The total surface of the said copper plates (2) may
be more that the total surface of the titanium plates (3)
by 1.4 to 1.8 times
The plate shaped support (1) may be made of porous
ceramic material of the Sital type.
The said plates (2) and (3) may be flat-spiral
shaped, the said spirals being split into spiral couples,
one spiral of the said couples being of copper and one of
titanium, each spiral of the said couples being of
different length and width and being connected together
and to a coupling cable (5); said couples of spirals being
situated on two flat surfaces of the said support (1) in
such a way that one copper spiral on one flat surface of
the said support (1) corresponds to one titanium spiral
situated on a flat surface of the opposite face of said
support (1).
The reference electrode may further comprise a first
and second group of contacts, respectively (7) and (4),
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made in a conical hole situated between two adjacent
faces of the support (1) and below a corner of the said
support (1), for which reason the conical hole interior
part is covered with a layer of copper to which is welded
an electroductor wire for the first and second coupling
cables to a cathodic protection stationi the chamber
constituted by the conical hole being filled with an
isolating compound.
The said first and second group of contacts may be
made in a groove situated on a lateral face of the said
support (1), the groove internal surface being covered
with copper to which copper is soldered on
electroconductor wire of the first and the second coupling
cables (5), the said groove being filled with an isolating
compound.
One of the plates functioning as electrodes arranged
on the flat surface of the support (1) made of porous
ceramic material may instead be made of a nickel-based
alloy.
The said copper and titanium or nickel-based alloy
plates (2) and (3) may be applied to the ceramic support
(1) by means of a plasma process or by means of potting or
another type of coating aimed at creating a uniform
thickness over the surface.
The aforementioned aims and other besides will better
emerge from the detailed description which follows, of
some preferred embodiments illustrated in the form of non-
limiting examples in the accompanying drawings, in which:
Figure 1 shows a general view of a first
embodiment of the electrode
without the electro-chemical
potential transducer;
Figure 2 shows the electrode of Figure 1
with an electrochemical potential
transducer;
Figure 3 shows the arrangement diagram of
the spluttered plasma electrodes
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on the opposite ends of the
support;
Figure 4 shows a second embodiment of the
- reference electrode with the
electrochemical potential
transducer and electrodes
arranged on a spiral path;
Figs 5, 6 and 7 show a first embodiment of the
construction of the contact point
of the reference electrode;
Figs 8, 9 10 and 11 show a second embodiment of the
construction of the contact point
of the reference electrode;
Figs 12, 13 and 14 show the construction of the
electrochemical potential
transducer and of the contact
point of the reference electrode;
Figs 15 and 16 show the arrangement of the
contact point on the surface of
the support head between the two
faces of the reference electrode;
Figure 17 shows the arrangement of
potential measurement and
cathodal protection station
control electrode in the earth;
Figure 18 shows the arrangement diagram of
the reference electrode and the
measurement of the polarising
potential by the pulse
commutation method; and
Figure 19 shows the arrangement and wiring
diagram of the reference
electrode for the determination
of the efficiency of cathod~c ~
protection on an isolating defect
with the method of the output
electrode.
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With reference to Figures 1 to 3 and the first
embodiment of the electrode illustrated therein, 1 denotes
a rectangular body, substantially plate-shaped and made of
porous sital, that is of a ceramic material having a
porosity of between 2-50 percent volume.
On each face of the plate, metal plates 2 and 3 are
arranged, which plates constitute electrodes. The plates
2 are made of copper, while the plates 3 are made of
titanium.
Both metal plates 2 and 3 are plasma-coated either by
hot spraying or by vacuum deposition, or by means of
another constant-thickness coating system. On a lateral
surface of the plate-shaped support 1 a first contact
point 4 with the coupling cable 5 is installed.
The metal plates 2 and 3 are alternated on each
surface of the support 1.
Apart from this, the reciprocal arrangement of the
metal plates 2 and 3 on the opposite sides of the support
1 are alternated in such a way that if a copper plate 2 is
arranged on one side of a support 1, on the other surface
of the said support 1 a plate of titanium will be
arranged, and vice-versa.
The reference electrode with the electrochemical
potentiometer end instrument, apart from the copper plate
2 and the titanium plate 3 alternately situated on the
surfaces of the support 1, contains a metal membrane 6, a
contact group 7 for the electrochemical potential
transducer with the second coupling cable 8, as is
illustrated in Figure 2.
The arrangement of the copper plates 2 and the
titanium plates 3 on the opposite faces of the rectangular
base or support 1 is represented in Figure 3.
The metal membrane 6 of the electrochemical potential
transducer is made from the same material as the
underground plant to be protected, and is applied by means
of cathodic action.
Referring to Fig 4 there is illustrated a second
embodiment of the electrode. This reference electrode
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with the electrochemical potential transducer (see Figure
4) contains the continuous-belt electrodes made of copper
2 and titanium 3 plasma-coated on both surfaces of the
ceramic support 1.
The belt-electrodes 2 and 3 are flat-spiral shaped,
have different length and width and are situated on the
opposite faces of the support 1 in such a way that the
copper 2 belt, on the one side, corresponds to the
titanium 3 belt of the same width on the opposite surface
of the support 1.
Both belts are coupled between themselves as well as
with the second coupling cable 8.
The contact point 4 contains the first coupling cable
5 and is situated on one of the lateral faces of the
support 1.
All of the support 1 faces are copper-plated with
copper of a high degree of chemical purity.
The first metal membrane 6 of the electrochemical
potential transducer is on the non-copper-coated head with
the first contact group 7 and the second coupling cable 8.
A first embodiment of the construction of the contact
group of the reference electrode is represented in Figures
5, 6 and 7.
The coupling cable 5, see Figure 5, is inserted in a
conical hole situated between the two faces of the support
1 head below the support 1 corner.
The internal surface of the conical hole is copper-
coated, while the electrode 10 of the first coupling cable
5, by means of an alloy 11 inserted by brazing, is
electrically coupled with the copper layer 9 situated in
the conical hole.
The brazing point and the space inside the conical
hole is filled with an insulating compound 12.
To prevent the escape of the insulating compound 12
during the filling operation, the entrance section of the
conical hole is equal to the section of the coupling cable
5 including the insulation.
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In the second embodiment of the contact group
illustrated in Figures 8, 9, 10 and 11, on one of the
faces of the support 1 head a notch or groove is cut, see
Figure 9,- which notch or groove's internal surface,
together with the surface of the support 1 head, is
copper-coated, which copper 9 is plasma-applied on the
support 1.
The electrode 10 of the coupling cable 5 is brazed to
the copper layer 9 inside the notch or groove. The whole
notch is thus filled with insulating compound 12.
The electrochemical potential transducer, see Figures
12, 13 and 14 contains a metal membrane 13 situated on one
of the faces of the support 1 head.
The contact group 7 of the electrochemical potential
transducer is the same as the contact point group 4 of the
reference electrode (see Figure 5).
The metal membrane 13 of the electrochemical
potential transducer is directly applied on the ceramic
body, while the electro-conductor wire 14, by means of the
alloy 15, is welded by brazing to the metal membrane 13
inside the conical hole which is internally filled with
insulating compound 16.
Referring to the first embodiment of the electrode
the first contact point group 4 thereof, see Figures 15
and 16, is situated on the face of the support 1 head
between its edges.
The internal surface of the notch or groove is
copper-coated by brazing and contains the electrode 10
wire of the first coupling cable 5. The notch or groove
chamber is filled with insulating compound 12.
The second contact group 7 (not shown) for the
electrochemical potential transducer is made similarly to
the one represented in Figures 15 and 16.
As illustrated in Figure 17, the continuous-action
reference electrode 17 is installed close to the
underground piping 18 to be protected.
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21~4775
The continuous-action reference electrode 17 is
connected to the voltmeter 19 and to the input of the
cathodic protection station 20.
The second clamp of the voltmeter 19 is connected to
the underground piping 18 and to the input of the cathodic
protection station 20.
The outputs of the cathodic protection station 20 are
connected to the reference electrode 17 and to the
underground piping 18. Integer 21 of Figure 17 represents
an anode earth rod of the cathode protection station 20.
During the measuring of the polarising potential, as
illustrated in Figure 18, because of interrupted
polarisation, the continuous-action reference electrode 21
is placed close to the underground piping 18 to be
protected and parallel with the said underground piping 18
axis, while the metal membrane 22 of the electrochemical
potential transducer is arranged perpendicular to the axis
of the underground piping 18.
The coupling cable 5 of the reference electrode 21,
the second coupling cable 8 of the electrochemical
potential transducer and the clamp of the underground
piping 18 are respectively connected to first, second and
third clamps of a modulating device 23 which contains a
commutator 24, an integrator capacitor 25 and a measuring
device 26.
In Figure 19, which is a diagram of cathodic
protection efficiency measurement, the reference electrode
21 with the electrochemical potential transducer 22 are
installed in the earth above the insulation fault 27.
The coupling cable 5 of the reference electrode is
connected to the first clamp of the voltmeter 19 while the
second coupling cable 8 of the electrochemical potential
transducer is connected to the first clamp of the
milliammeter 28.
The second clamps of the voltmeter 19 and the
milliammeter 28 are connected to each other and also to
the underground piping 18.
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The voltmeter 19 is connected between the continuous-
action reference electrode 21 and the underground piping
18 and measures the polarising potential of the said
underground piping 18.
The tension measured by the voltmeter 19 reaches the
input of the cathodic protection station and increases or
reduces the polarising current between the reference
electrode 21 and the underground piping 18.
This phenomenon guarantees the continuous polarising
potential of the underground piping 18, which prevents its
corroslon .
The continuous-action reference electrode 21 can be
installed at any point on the surface of the underground
piping 18 and requires no maintenance. The whole surface
area of the copper plate 2 of Figures 1, 2 and 4 at the
copper-coated head surface of the support 1 is 1.4 - 1.8
times larger than the whole surface area of the titanium
plates.
The support 1 made of porous sital is impregnated
with phreatic electrolyte before being installed in the
earth, so no special impregnation is required.
The above-described reference electrodes are more
efficient in dry earth, sandy or perfect frozen
conditions.
The above-described reference electrodes can also
function in sea-water during the measurement of the
polarising potential with the interrupted polarisation
system illustrated in Figure 18.
In this case the commutator 24 periodically
commutates the metal membrane 22, alternatively at the
underground piping 18 and the measuring device 26.
In the meantime the integrator capacitor 25
attenuates the measured tension.
The metal membrane 6, illustrated in Figure 2, is
made of the same material as the underground piping 18: to
make it, first the metal dust of the underground piping 18
is produced and then the said underground piping 18 is
plasma-dusted on the base of porous ceramic material.
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14 MARGH 1994
214~77~
This operation is generally performed in an inert-gas
atmosphere.
The production of the plates and their coating on the
support 1-is performed by means of special dies.
s The metal plate is electrically insulated from the
other copper and titanium plates situated on the support
1.
The overall surface of the metal membrane 6 must be
not lower than 625 mm2 which is the standard set by
various countries.
In order to ensure that there is no contact between
the alloy 11 and the ground, the contact group is filled
with insulating compound 16, which, when it has become
solid, guarantees the mechanical resistance of the contact
group.
The contact group illustrated in Figures 8 to 11 is
simpler to produce with respect to the group illustrated
in Figures 5 and 12, but it requires special equipment for
the solidification of the insulating compound 16.
The cathodic protection efficiency degree definition
when no insulation is afforded by means of the external
electrode as illustrated in the diagram of Figure 19 is
performed in the following way.
First, the insulation fault 27 point is identified,
then, above the fault point, the reference electrode 21 is
installed in the earth with the electrochemical potential
transducer.
The input of the polarising potential is established
by means of the voltmeter 19.
If the value exceeds, for example 0.87 volts, the
insulating defect can be cathodically protectible. The
surface of the metal membrane 6 as illustrated in Figure 2
of the electrochemical potential transducer characterises
the m~;mllm surface of the insulation fault, protected
cathodically with the given measured polarising potential,
that is indicated by the maximum allowable value for the
surface of the insulation fault.
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12 21~77~) 14 MAR~ 1994
Plates of nickel alloy may be used in the reference
electrode instead of plates of titanium.
In such cases, however, the reference electrode needs
firstly to be set to prevent systematic errors.
Before plasma-coating the metal plates on the porous
ceramic base, the said base undergoes sand-blasting or
equivalent treatment to ensure that the said plasma-coated
plates will adhere perfectly.
In order to avoid errors in the measuring of the
polarised potential, the thickness of the metal membrane
6, illustrated in Figures 2 and 4, must be between 4-5 mm.
By bending the reference electrode a high degree of
saving in metal and maintenance is obtained.
Furthermore, having a large surface in contact with
the earth, as well as the possibility of great nearness to
the underground metal plant to be protected guarantees
high interior conductivity of the reference electrode,
which permits of increasing the measurement precision of
the polarising potential.
The present reference electrode can function in any
heat conditions, including constant turning in any ground
composition.
Before being installed in the earth the reference
electrode is potted with a watery solution of 3~ sodium
chloride.
If used in rocky ground the reference electrode would
be coated with bentonite.
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