Method and a device for detecting changes in a surface state and for monitoring the surface state

A method of detecting changes in a surface state and of monitoring said surface state, in particular for a body whose surface is at a given temperature and is in contact with a flowing liquid at a given temperature which may optionally be different from the temperature of said surface, wherein a reference metal body is placed in said liquid, said body having a surface at a temperature which is adjustable independently of the temperature of said liquid, and said body acting as an electrode which is optionally raised to an adjustable potential, and the surface state of said body is detected by measuring the intensity of light radiation reflected from at least one mirror-forming portion of the surface of the reference body.

The invention relates to a method and a device for detecting changes in a 
surface state and for monitoring the surface state, in particular for a 
surface which is in contact with a liquid flowing in pipework. 
BACKGROUND OF THE INVENTION 
Deposits occur in pipework in all fields, regardless of whether or not the 
liquids are water-based. When the liquid is water, specific mention may be 
made of problems which occur in piping potable water, industrial water, 
boiler water, cooling water, etc. 
Deposits may be formed by calcium carbonate (scale), by metal oxides, by 
atmospheric dirt in circuits open to the air, by microbes, or by corrosion 
products. With mixtures or solutions of various substances, other types of 
deposit may also be encountered (e.g. due to substances precipitating from 
the solution). 
The liquid may also attack the surface of the pipework, and one particular 
form of corrosion is due to metal, generally iron, being attacked by water 
since, thermo-dynamically speaking, there is no known domain over which 
water and iron can remain stably in contact under natural conditions. 
These phenomena give rise to considerable drawbacks. Deposits progressively 
block up pipes, thereby reducing flow rates or increasing head losses, and 
they also reduce heat exchange capacity, whereas corrosion damages 
pipework and may lead to breakage. 
With natural water, the practical equilibrium between calcium bicarbonate 
and carbon dioxide is governed by rather complex laws and a shift in the 
equilibrium position can give rise to chemical reactions in which calcium 
carbonate is dissolved (aggressiveness) or deposited (scaling), and these 
reactions may be superposed on the straight-forward electro-chemical 
corrosion reactions which are specific to metals. 
Thermodynamic calculation methods have been developed for attempting to 
estimate the scaling or corrosive nature of a given water. However, the 
large number of such methods (Tillmans' method, Langeliers's method, 
Hoover'diagram, Hallopeau's method, Franquin and Marceaux's diagram, . . . 
) is witness to the difficulty of this approach. These methods are based 
on studying pure solutions under determined conditions of pH, temperature, 
and concentration, and they are not capable of taking account of the 
complexity of practical situations. In addition, the results of such 
calculations are often of the YES/NO type as to the possibility of 
precipitation taking place, without giving any possibility of 
investigating the kinetics of the phenomena. 
In order to mitigate these drawbacks, methods and apparatuses have been 
developed for using the water of the circuit concerned to obtain a 
representation (which may be accelerated) of these phenomena so as to be 
able to correct them and possibly prevent them from taking place. 
A first method consists in placing thermocouples in a special circuit off 
the main circuit and in measuring variations in the heat exchange 
coefficient. This method gives an indication of the state of the apparatus 
without requiring direct inspection, e.g. in the cooling circuits of 
electricity power supply stations. This method thus does not make it 
possible to forecast scaling but only to observe it, and then only 
providing that the same conditions are maintained in the special circuit 
as in the main circuit, in particular with reference to temperature. This 
method is lengthy in application since the phenomenon takes place under 
real operating conditions and since cleaning the special circuit after it 
has been scaled turns out to be difficult. 
Another method makes use of measuring variation in current flow obtained by 
applying a constant potential (of about -1 V relative to a saturated 
calomel reference electrode). Recording current variation provides 
information on the scaling of the electrode constituted by the metal under 
investigation. The apparatus containing the metal sample, the reference 
electrode, and the auxiliary electrode in water taken from the main 
circuit is itself placed in a thermostatically controlled bath. This 
method has the advantage over the preceding method of making it possible 
to forecast scaling, e.g. over a period of three hours at 40.degree. C. 
However, it suffers from the drawback that sensitivity cannot be changed 
without changing either the temperature or the imposed potential, since 
the same means are being used both for giving rise to scaling and for 
measuring it. 
Further, since it is the bath itself which is heated or otherwise, rather 
than the metal sample, conditions on the surface of the sample are, by 
virtue of this very fact, very different from reality, in particular when 
considering heat exchangers. This means that the deposit is generally 
constituted by the calcite form of calcium carbonate, whereas in reality 
the aragonite form is obtained or else an association of both forms, 
depending on the temperature of the surface on which the deposit takes 
place. 
Similar problems occur with phenomena of deposition or corrosion in the 
presence of liquids other than natural water. 
In order to mitigate these drawbacks, the present invention seeks to 
provide a device enabling the conditions of the phenomenon to be created 
using parameters which are adjustable so as to reproduce the operating 
characteristics of the real circuit, or to create characteristics which 
accelerate the phenomenon, by using means for detecting the phenomenon and 
measuring variations therein, which means are separate from the means used 
for setting up experimental conditions. 
SUMMARY OF THE INVENTION 
The present invention thus provides a method of detecting changes in a 
surface state and of monitoring said surface state, in particular for a 
body whose surface is at a given temperature and is in contact with a 
flowing liquid at a given temperature which may optionally be different 
from the temperature of said surface, wherein a reference metal body is 
placed in said liquid, said body having a surface at a temperature which 
is adjustable independently of the temperature of said liquid, and said 
body acting as an electrode which is optionally raised to an adjustable 
potential, and the surface state of said body is detected by measuring the 
intensity of light radiation reflected from at least one mirror-forming 
portion of the surface of the reference body. 
By putting the surface of the reference body at the desired temperature, 
either merely by allowing the body to take up the temperature of its 
environment or else by heating the body to obtain a desired temperature at 
its surface, it is possible to track the phenomenon under normal 
conditions, and if the surface of the reference body is subjected to 
additional heating and/or to the application of a potential, then the 
phenomenon is accelerated, thereby making forecasting possible. 
Detection by measuring the intensity of radiation, e.g. infrared radiation, 
as reflected from at least one mirror-forming portion of the surface is 
completely independent from the means for setting up the phenomenon to be 
detected, and as a result the conditions under which the phenomenon 
appears can be changed without interferring with the conditions under 
which it is observed. 
The invention also provides a device for implementing the method, the 
device comprising a cell having a liquid inlet, a liquid outlet, two 
electrodes connected to a potentiostat, with one of said two electrodes 
being constituted by the reference body, and also a reference electrode, 
with at least a portion of the surface of the reference body constituting 
a mirror, with the reference body including heating means, and with the 
device further including an emitter-receiver of light radiation disposed 
in such a manner as to emit a light beam towards the mirror and receive 
the light beam reflected by the mirror. 
In a particular embodiment of the cell, the reference body is constituted 
by a hollow tube including a flat which serves as the mirror, and the 
heating means are constituted by a heating plug placed inside the tube. 
The method may be applied to monitoring the surface state of pipework by 
placing the device either directly in a main network or else by placing it 
in a secondary network which reproduces the conditions of a main network, 
with the phenomenon being accelerated or otherwise. The method and the 
device may also be used for performing studies on natural liquids or on 
synthetic liquids. 
The observed surface of the reference body is a surface on which a 
phenomenon occurs which is quantitatively and qualitatively similar to the 
phenomenon which actually takes place in the pipework. It may be 
constituted by a surface whose composition is the same as that of the real 
surface, or which is slightly different so long as it behaves in the same 
way. However, account must be taken of the fact that since the observed 
surface is initially polished so as to be reflective, the phenomenon may 
be initiated differently. 
In another embodiment of the cell, the portion serving as the mirror is a 
flat removable portion.

MORE DETAILED DESCRIPTION 
The detection and monitoring cell 1 as shown diagrammatically in FIG. 1 
includes a liquid inlet orifice 2 and a liquid outlet orifice 3. A 
reference electrode 4 is placed inside the cell. A main electrode or body 
5 fitted with heating means 6 is mounted inside the cell 1 together with 
an auxiliary electrode 7. The electrodes 5 and 7 are connected to 
corresponding terminals of a potentiostat 8, with the electrode 5 being a 
cathode or an anode. An emitter 9 of light radiation R is positioned at a 
certain angle (e.g. 45.degree.) relative to the electrode 5, and a 
receiver 10 is placed in such a manner as to receive the radiation 
reflected by the electrode 5. This receiver (a photodiode) is coupled to a 
resistance, and the voltage across the terminals of the resistance is 
measured, with variations in the voltage being displayed or recorded at 11 
and being representative of changes in the surface state of the electrode 
5. 
The cell operates as follows: the liquid is caused to flow through the cell 
1 at a determined flow rate; the surface of the electrode is brought to 
the desired temperature either by merely allowing thermal equilibrium to 
be established between the main electrode 5 and the liquid, or else by 
heating the electrode 5 using the heater means 6 until it reaches a 
temperature corresponding either to the temperature of the inside skin of 
the pipework (independently of the temperature of the liquid itself, e.g. 
in a heat exchanger), or else to a higher temperature in order to 
accelerate the phenomenon. 
The surface state of the main electrode then changes because of the 
deposition phenomenon and/or the corrosion phenomenon which is to be 
monitored. 
A light beam or ray R, e.g. of infrared light, is emitted by the emitter 9 
and travels towards the portion of the electrode 5 which constitutes a 
mirror at which it is reflected towards the receiver or photodiode 10. The 
formation of a deposit or of corrosion on the mirror-forming surface 
reduces the intensity of the reflected light beam and as a result the 
voltage provided by the readout device 11 is observed to diminish as a 
function of time. 
In this case, although the method serves to monitor the appearance of the 
phenomenon, it also serves, unlike the prior art, to reproduce the real 
phenomenon. This is particularly advantageous for scaling where the 
relative proportions of the calcite and aragonite forms of calcium 
carbonate vary, inter alia, as a function of the temperature of formation. 
In a particularly advantageous application of the method, after the 
electrode 5 has achieved the desired temperature, it is set back to a 
desired potential. To do this, a reference electrode 4 is included in the 
cell and a constant potential relative to the reference electrode 4 is 
applied to the electrodes (the cathode 5 and the auxiliary electrode 7, or 
vice versa) by means of the potentiostat. This application of a constant 
potential accelerates the phenomenon being investigated and thus makes it 
possible to determine its reaction kinetics and to predict how the 
phenomenon will evolve as a function of time. 
FIGS. 2 and 3a to 3d show a particular embodiment of a device for 
implementing the theoretical diagram of FIG. 1. 
This device is specifically designed for investigating scaling, but it 
could be used for studying phenomena other than scaling, if necessary with 
the aid of simple modifications, in particular, to take account, of the 
physico-chemical characteristics of the liquids being investigated. 
However, the description of this device is, for reasons of simplicity and 
clarity, restricted to a device for use with scaling. 
FIG. 2 is an overall view of a scaling cell 20. It is constituted by a 
generally cylindrical glass body 21 (where glass is inert relative to 
water and to the electrolysis reaction), which is disposed vertically and 
closed at its two ends by respective plugs. The body 21 includes a water 
inlet orifice 22 and a water outlet orifice 23 with the flow through the 
body being driven, for example, by a peristaltic pump (not shown). A 
reference electrode 24 is fitted to the duct leading to the orifice 22. It 
is preferable to use a simple and stable saturated calomel electrode, but 
it would also be possible to make use of any other type of reference 
electrode (e.g. a platinum wire). The main electrode 25 is constituted by 
a hollow tube, e.g. of stainless steel, which is fixed inside the cell 20 
or which passes lengthwise through the cell and through the plugs in order 
to make disassembly possible. For scaling it acts as a cathode, whereas 
for investigating corrosion it would act as an anode. In the description 
of a scaling cell, the main electrode is referred to as the cathode and 
the auxiliary electrode as the anode. A heating plug 26 is received in the 
hollow cathode and it is connected to a suitable adjustable source of 
heating energy (not shown). The anode 27 is a length of hollow tube having 
a greater diameter than the cathode and placed around the cathode. The 
plug 26 and the anode are placed in the middle region of the cell 20 in 
order to facilitate positioning the emitter-receiver of light radiation. 
The anode 27 is supported from above by a support device 30 which may be 
frustoconical as shown in the drawing, but which may naturally be of any 
other appropriate shape or structure. The anode and the cathode are 
connected in conventional manner to a potentiostat (not shown) which 
serves to apply a determined constant potential relative to the reference 
electrode 24. In a scaling cell, it is advantageous to make use of a 
potential of -1.060 V relative to the saturated calomel electrode. 
Naturally, the selected value will depend on the liquid passing through 
the cell and on the extent to which it is desired to accelerate the 
phenomenon under investigation. 
A conventional emitter-receiver (not shown) is positioned outside the cell 
20 such that the light radiation R (see FIG. 3c) emitted therefrom strikes 
the cathode 25. In order to enable the incident beam or ray to be 
reflected, at least a portion of the cathode 25 constitutes a mirror. This 
may be a flat 28 formed along the entire length of the 
cathode-constituting tube, or merely on the middle portion thereof, or on 
any other appropriate section. It may also be a reflecting pellet placed 
removably in a tube which is metal or otherwise. The utility of such a 
removable pellet is described below. 
By way of example, the emitter may be an infrared emitter powered at 7 V 
and placed at 45.degree. relative to the reflecting surface, and the 
receiver of the reflected light beam may be a photodiode driven at 15 V 
and also positioned at 45.degree. relative to the mirror-forming surface 
28 of the cathode 25. 
In order to enable the light beam to travel to the mirror, it is necessary 
to provide openings through the anode which are appropriately disposed 
relative to the positioning of the emitter-receiver and which allow the 
light rays to pass through the anode. For example, two openings may be 
provided which are spaced apart by 90.degree., either in a horizontal 
plane if the light beam travels in the horizontal plane, or else in a 
vertical plane if the light beam travels in a vertical plane. It would 
also be possible to provide a single elongate opening extending over more 
than 90.degree. (and more generally over twice the angle of incidence of 
the light beam). FIG. 3b shows, by way of example, a vertically elongate 
opening 29, whereas FIGS. 3a and 3c show two openings spaced apart at 
90.degree. in a horizontal plane. 
In another possible embodiment of the device, the light beam may be 
transmitted to the vicinity of the mirror-forming surface by means of 
optical fibers. The fibers pass through the body 21 in sealed manner, and 
also though the anode, if necessary. The angle of incidence is selected as 
desired, and may even by 90.degree. if the optical fibers are coaxial with 
each other. 
It is particularly advantageous to use optical fibers for conveying the 
light radiation when the water under investigation, or more generally the 
liquid under investigation, is highly colored or very turbid which has the 
effect of absorbing a portion of the emitted light prior to its reflection 
on the mirror and of spoiling the measurement results. However, this 
drawback may be mitigated by using an emitter-receiver system having two 
beams (a measuring beam and a reference beam) if optical fiber apparatus 
is not available. 
The following example is given to illustrate operation of the device. 
EXAMPLE 
Apparatus similar to that described above and shown in FIG. 2 was used 
under the following operating conditions to perform tests on five 
different water mixtures obtained by mixing natural water with deionized 
water in proportions given in the following table. 
OPERATING CONDITIONS 
Reference electrode 24: saturated calomel electrode; 
electrode potential: 1.080 V relative to the electrode 24; 
flow rate (adjustable between 9 and 20 ml/min): 9 ml/min; 
temperature obtained using a 100 W/220 V plug 26: 70.degree. C.; 
IR emitter: feed voltage 7 V--adjusted to 940 nm; 
receiver: photodiode driven at 15 V; and 
angle of incidence: 45.degree.. 
______________________________________ 
Concentration 
Natural Deionized of Ca.sup.++ and 
Mixture No. 
Water Water HCO.sub.3.sup.- in meq/1 
______________________________________ 
1 100% 0% 5 
2 75% 25% 4 
3 50% 50% 2.5 
4 25% 75% 1.25 
5 100% 0% 10 
______________________________________ 
.sup.+ additional CaCO.sub.3 
The curves shown on the graph of FIG. 4 were obtained, where each curve is 
referenced by the number of the corresponding water mixture in the table. 
It can be seen for low concentrations of calcium carbonate (mixtures No. 3 
and 4), that after a short period during which deposition takes place, the 
curves tend towards horizontal straight lines. There is no further change 
in the surface state, and scaling is thus no longer taking place. In 
contrast, at higher concentrations, the transmitted light intensity 
continues to diminish. However, it may be observed that after a certain 
length of time, on the order of 30 minutes, the curves have portions which 
are substantially linear. This characteristic makes it possible to define 
a time interval, e.g. of 10 minutes duration situated between 30 minutes 
and 40 minutes, over which the slopes of the curves may be measured in 
order to obtain an indication of the scaling power of the water by 
calculating a suitable index. 
When the apparatus is used for monitoring purposes (without applying a 
potential thereto) this index makes it possible, in particular, to detect 
any abnormal variation in the scaling power and thus to trigger an alarm 
or act on appropriate compensating devices either automatically or 
manually. To this end, the reading and/or recording apparatus 11 may be 
connected to a calculation unit which, if it determines that the slope of 
the voltage curve is greater than a predetermined value, provides an 
output signal which may trigger an alarm, for example. 
When the device is used for studying water prior to its being used in a 
real installation, results can be obtained rapidly by applying a potential 
to the device, for example making it possible to predict behavior, e.g. 
after a period of 3 hours at 40.degree. C., whereas in conventional 
experimental models, studies require up to fifteen days. 
Further, the electrode can easily be cleaned and at the same time the 
thickness of the deposit can be calculated, thereby making it possible to 
discover its effect on heat exchange, in particular. 
To this end, after the device has operated for a desired length of time, 
e.g. 40 minutes, for ensuring that deposition takes place and for 
measuring the slope of the voltage curve, heating is turned off and the 
electrolysis current is reversed and fixed at a value of 10 mA. 
This causes the following anode reaction to take place: 
EQU 2H.sub.2 O.fwdarw.O.sub.2 +4H.sup.+ +4e.sup.- 
The pH at the electrode drops and the protons formed redissolve the 
previously-deposited fur in accordance with the following reaction: 
EQU CaCO.sub.3 +H.sup.+ .fwdarw.HCO.sub.3.sup.- +Ca.sup.++ 
The reflecting surface is thus cleaned progressively, and the cleaning is 
completed when the measured voltage returns to its initial value. 
Since electrolysis has taken place at constant current, Faraday's law may 
be applied and the mass of calcium carbonate dissolved can consequently be 
calculated from the time required for redissolving it. 
Given the density of CaCO.sub.3, it is possible to calculate the thickness 
of the fur that had been formed. 
Further, since it is possible to vary the temperature of the reflecting 
surface independently both of the other formation parameters and of the 
detection system, it is possible to study the crystal forms of the deposit 
formed as a function of temperature and to verify that the calcite form, 
the aragonite form or a combination of those forms is obtained depending 
on the applied temperature. 
To this end, it may be desirable to provide a removable reflecting surface 
which can be removed from the apparatus prior to cleaning in order to 
examine the deposit formed thereon by other techniques such as electron 
microscopy, infrared spectrometry, etc. If the removable reflecting 
surface is a pellet, the deposit formed thereon will also be in the form 
of a pellet and may be directly analyzed in an infrared spectrometer. 
The above description of a particular embodiment of the device for a 
particular application (scaling) has naturally been given purely by way of 
example, and it is obvious that the invention has numerous applications 
both for monitoring purposes and for forecasting and investigation in all 
fields where phenomena may occur which change the state of a surface which 
is in contact with a liquid.