MAGNETIC CORROSION INDICATOR

Description

DOMAINE TECHNIQUE DE L'INVENTION

[0001] The technical field of the invention is that of detecting and monitoring the corrosion of metallic components present in a porous medium, in particular in civil engineering structures, comprising at least one metallic reinforcement and a concrete coating for example. By "a coating" is meant a porous layer in the solid state coating the metallic reinforcement. For example, in the case of a civil engineering structure made of reinforced concrete, the concrete is considered to be the coating.

[0002] It relates more particularly to a witness allowing the measurement of magnetic characteristics sensitive to corrosion of a metal reinforcement of a wall of a civil engineering structure comprising a porous coating such as concrete, to civil engineering structures, and binders comprising such a witness, to the method of detecting and monitoring the corrosion front in a porous coating comprising such a witness, and to a method of manufacturing a reinforced concrete wall comprising such a witness.

[0003] In general, the present invention can be used to detect and monitor corrosion in all types of civil engineering structures comprising a porous coating and a metal reinforcement coated in the coating which are likely to undergo corrosion (in particular structures having reinforced concrete walls).

[0004] The invention is therefore likely to be used more particularly for the detection of corrosion in civil engineering structures (buildings, dams, bridges, tunnels, swimming pools, etc.), in particular maritime structures and the foundations of coastal or offshore works (dykes, jetties, wind turbines, offshore platforms, tidal dams, hydro turbines, etc.). ARRIERE-PLAN TECHNOLOGIQUE DE L'INVENTION

[0005] If we exclude its natural aging, the lifespan of a civil engineering structure, for example reinforced concrete, can be limited by the penetration of aggressive agents such as chloride ions or by freeze-thaw cycles which cause swelling and cracking as well as spalling of surfaces exposed to de-icing salts.

[0006] A common factor in these pathologies is corrosion. Quantifying this corrosion in average or surface gradients is subsequently a crucial phase in the diagnosis of maritime structures.

[0007] Reinforced concrete core sampling is regularly carried out by project owners to determine the chloride ion concentration and thus estimate the progress of any deterioration using predictive models to optimize coating repairs. Unfortunately, these core sampling tests are destructive, which minimizes sampling via core sampling, and therefore limits the estimation of the progress of deterioration at the level of the entire structure.

[0008] Also, in operational terms, the method commonly used for monitoring and preventing such pathologies involves regular visual inspections to identify cracks emerging on the surface of the structure's walls, followed by repairs in the form of coating to prevent chlorinated water from reaching the steel.

[0009] Furthermore, different non-destructive testing (NDT) techniques exist and make it possible to estimate significant parameters (performance indicators) of the lifespan of a structure, such as the saturation rate of concrete and porosity.

[0010] However, difficulties arise in identifying cost-effective and reproducible methods that provide useful data over long periods of time, since the corrosion of structural reinforcement tends to occur slowly.

[0011] In some applications, integrated monitoring techniques are implemented using sensors of different technologies placed in the concrete coating in order to measure the concentration of chlorides.

[0012] For example, a corrosion detection system is known comprising a witness comprising an electrical resistance which increases when the witness is corroded, a means of measuring the resistance and an RFID chip allowing, during a check, the means of measuring the resistance to be powered and the information transmitted to an external reader.

[0013] However, such a witness has a high cost and in the event of deterioration of the RFID chip or the measuring means, or even of a connection between the different elements of the witness, the external reader will receive erroneous information.

[0014] In addition, the system may further comprise an interrogator outside the wall of the structure and a witness in the wall, the interrogator comprises a magnetic field generator comprising an induction or electromagnetic wave radar and a magnetic field reader and the witness may comprise a non-magnetic and non-metallic holding member such as a polymer or ABS resin and a corrodible conductor. Corrosion may be measured by supplying by the generator a primary magnetic field to the concrete, the corrodible conductor generates an induced current forming a second magnetic field and the interrogator reads the second magnetic field generated by the corrodible conductor.

[0015] However, such a witness does not provide information when the corrodible conductor has undergone sufficiently high corrosion to no longer have conductivity between these ends. In addition, the electromagnetic radar is an active tool that propagates a wave to the witness which depends on several factors including environmental factors such as the humidity level and the chloride level between the radar and the witness, that is to say in the structure and outside. Thus, the reading result will not be identical depending on the weather and are not precise since they are sensitive to different environmental factors. Finally, such a witness must be mapped during the construction of the structure, otherwise there is a risk that the radar transmits and the reader reads waves coming from other elements of the structure which can return the waves such as the reinforcement of the structure or metal debris in the concrete.Finally, such witnesses must be large enough to reflect the wave coming from the active device, for example electromagnetic radar; this volume can cause damage to the structure.

[0016] These different witnesses and systems therefore have extremely high costs in terms of deployment on the scale of a large structure, and still present major technical constraints (robustness, reliability, autonomy.)

[0017] Also known from DE102017129573 is a corrosion sensor and a corrosion monitoring method used, for example, to determine and monitor the progress of corrosion of reinforcements on reinforced concrete elements. The magnetic corrosion sensor consists of a corrosion rod and a reference rod made of reinforcing steel, the rods having permanent magnets at their ends. The corrosion sensor is installed in the concrete cover. The magnetic field is detected using a magnetic field scanner and the measurement result is sent to the display device.

[0018] US2004100278 further describes a device and method thereof, measuring ferromagnetic resonance properties by applying a magnetic field to the structure by arranging a magnet near the structure. The ferromagnetic resonance will be different depending on the corrosion condition of the corroded reinforcement.

[0019] This device does not prevent corrosion but measures the state of corrosion of the reinforcement. It is therefore not possible to carry out preventive maintenance to prevent corrosion of the reinforcement since corrosion of the reinforcement has already begun when a state of corrosion is detected. RESUME DE L'INVENTION

[0020] For many structures, particularly in the maritime environment, corrosion prevention for the reinforced concrete structures of the structure is essential to improve their lifespan. Many techniques exist that provide information on the presence of chloride ions in the coating at a specific location in the structure, but none provide precise information on its penetration front, let alone in a cost-effective and reliable manner.

[0021] The invention provides a solution to the problems mentioned above, by enabling an innovative technique for the non-destructive, reliable, autonomous, remote and low-cost evaluation of coating concrete undergoing degradation by the penetration of chloride ions.

[0022] This technique responds to a need to provide a simple, inexpensive means of detecting corrosion for reinforced concrete structures.

[0023] According to a first aspect of the invention, this need is tended to be satisfied by providing a liquid binder according to claim 1, comprising a corrosion indicator for insertion into a porous coating of a reinforced concrete structure, comprising at least one permanent magnet generating an induced magnetic field, and a layer corrodible by pathogenic agents, in particular chloride ions, the corrodible layer being integral with the magnet, the corrodible layer comprising a ferromagnetic material making it possible to modify the magnetic field generated by the magnet depending on the state of corrosion of the ferromagnetic material of the corrodible layer.

[0024] Thanks to the invention, such a corrosion indicator is inexpensive and simply allows, using a passive reader external to the structure (magnetic induction loop, magnetometer, teslameter), to know the state of corrosion of the corrosion layer located at a certain depth and therefore the penetration front of the pathogenic agent in the reinforced concrete structure.

[0025] Indeed, the introduction of a ferromagnetic element into a magnetic field increases the deformation of the field lines of the magnetic field, which translates, in terms of result, by the summation of the magnetic field generated by the magnet and the magnetic field generated by the magnetization of the ferromagnetic element. The coating has a relative permeability close to 1 while that of the corrodible layer between 200 and 10000. Thus when the corrodible layer is corroded, the non-corroded ferromagnetic mass decreases and modifies the amplitude of the magnetic field lines and therefore the intensity of the magnetic field measured at an auscultation surface of the structure.

[0026] Thus, the degree of corrosion of the ferromagnetic material leads to a variation in the ferromagnetic mass of the corrodible layer in contact with the magnet, which leads to a variation in the intensity of the magnetic field induced by the assembly.

[0027] The corrosion indicator of the invention has many advantages: beyond being non-destructive, it does not require any power source of its own, it allows remote data acquisition, wirelessly, with great reliability and at low cost. Furthermore, in the event of intense corrosion of the corrodible layer, the indicator remains reliable because the magnet of the indicator continues to generate a magnetic field, thus informing of a state of intense corrosion of the corrodible layer of the indicator.

[0028] Furthermore, the reader is a purely passive tool allowing the reading of the magnetic field wave coming from the witness, this wave is much more stable compared to the reading of a wave from an active tool according to different weather conditions, i.e. a humidity level or the chloride level. Thus the reading is much more reliable than an active tool such as in the prior art.

[0029] Operationally, after positioning corrosion indicators upstream in the porous coating of a reinforced concrete structure during its construction, the manager can produce an initial reference map of his structure by measuring the magnetic intensity at each point relative to a corrosion indicator position. The diagnostic operation will then consist of regularly carrying out new magnetic intensity measurement campaigns at these reference points and evaluating the differential with respect to the reference measurement in order to conclude on a state of corrosion of the coating.

[0030] Thus, for a sustainable management strategy for structures, particularly maritime structures, it is possible to carry out a method of non-destructive electromagnetic monitoring and measurement in an efficient manner and thus to diagnose the state of the structure at a given moment, to monitor the evolution of aging, and therefore to carry out preventive maintenance of the structure.

[0031] The invention also relates to a binder in a liquid state, in particular hydraulic concrete, comprising a mortar and corrosion indicators according to the first aspect of the invention, the binder in the liquid state becoming a porous coating in the solid state and being intended to become a coating of a reinforced concrete structure.

[0032] Thus, this binder has the advantage of not complicating the manufacturing stages of the reinforced concrete structure. Indeed, such a binder, which includes corrosion indicators, facilitates the operation of pouring the concrete into a formwork containing the metal reinforcement of the structure without adding a step of handling the corrosion indicators. Once the coating is in the solid state, the reinforced concrete structure includes indicators at different positions and depths relative to the monitoring surface. Thus, it is possible to carry out measurements at different depths and different positions of the structure to assess the corrosion state of the coating. Of course, such a binder requires having a sufficient number of corrosion indicators to statistically guarantee a good distribution and positioning of these corrosion indicators, allowing an effective diagnosis of the corrosion state of the entire structure.By binder in a liquid state we mean that it can flow for example between two supports, even if it includes solid particles, in particular corrosion indicators, but also for example gravel, fibers etc.

[0033] The invention also relates to a reinforced concrete wall of a structure, the wall comprising a metal frame, a porous coating, in particular made of concrete, covering the metal frame and forming at least one monitoring surface of the wall and corrosion indicators, according to the first aspect of the invention, located in the porous coating between the metal frame and the monitoring surface and in which the corrosion indicators are distributed at different positions between the monitoring surface and the metal frame.

[0034] Thus, the user can, by measuring the intensity of the magnetic field at different positions, conclude on the degree of infiltration of pathogenic agents into the coating at different positions of the structure in order to carry out preventive repairs if necessary.

[0035] The invention also relates to a maritime structure comprising a reinforced concrete wall such as that described above, in which the monitoring surface is external and the wall comprises a tidal zone, in which at least one of the corrosion indicators is located between the monitoring surface of the tidal zone and the reinforcement.

[0036] The invention thus makes it possible to provide a means of measuring corrosion in the areas of maritime structures most exposed to tidal cycles. Such a structure can include more witnesses in the tidal zone than in other areas in order to be able to increase the number of measurement samples and thus have a better mapping of the tidal zone in order to identify the parts of the tidal zone most infiltrated by chloride ions.

[0037] The invention also relates to a land structure comprising a reinforced concrete wall as described above, in which the external surface comprises at least one zone accessible to water and air, in which at least one of the magnetic corrosion indicators is located between the external surface and the metal frame.

[0038] The invention also relates to several methods of manufacturing the structure by implementing a different implementation for the installation of corrosion indicators in the coating.

[0039] A first type of manufacturing process of the structure is by the implementation of corrosion indicators is the installation of these corrosion indicators by predetermined placement on the metal reinforcement or between it and the external surface of the reinforced concrete wall, for example before coating.

[0040] The invention thus aims at a first method for manufacturing a reinforced concrete wall comprising a porous coating, comprising: a step of mounting and positioning a metal frame, a step of placing at least one corrosion indicator according to the first aspect of the invention, by means of a non-magnetic holding member, in particular PVC, to hold the indicator between the metal frame and the formwork, a step of formwork around the metal frame, after the three preceding steps, a step of pouring a binder, in particular comprising cement, into the formwork containing at least one corrosion indicator and the metal frame, a step of formwork removal when the binder is dry forming a porous coating surrounding the metal frame and the corrosion indicators.

[0041] The first manufacturing process has the advantage of perfectly controlling the positioning of the corrosion indicators in order to simplify and optimize a process for monitoring the state of corrosion of the coating described later and allow a reliable and robust diagnosis.

[0042] A second type of method for manufacturing the structure is by implementing corrosion indicators using the liquid binder as described previously, therefore comprising corrosion indicators, in order to achieve the coating through pouring or projection of the binder around a reinforcement.

[0043] The invention thus aims at a method for manufacturing a reinforced concrete wall comprising: a step of mounting a metal frame, a step of formwork around the metal frame and of pouring or spraying the liquid binder described above onto the metal frame coating the metal frame by placing the corrosion indicators in the liquid mortar randomly in the wall, and in that when the binder is dry, the liquid mortar forms a porous coating surrounding the metal frame and the corrosion indicators.

[0044] According to an example of this embodiment comprising the formwork and casting step, the method further comprises a formwork removal step.

[0045] The second type of corrosion indicator implementation process has the advantage of manufacturing the instrumented reinforced concrete wall without adding any preparation steps. Adding the corrosion indicators directly into the binder optimizes the manufacturing costs of such a wall. When the binder is in the solid state, it becomes the coating of the wall, coating both the indicators and the metal reinforcement of the wall.

[0046] The invention also relates to a method for deducing corrosion of a reinforced concrete wall described above according to one of the two manufacturing methods comprising: a step of acquiring data at a time t of a measurement of an intensity of the induced magnetic field for each magnetic corrosion witness at a reference point of each witness, a step of comparing the acquired data and an initial data previously acquired for this same reference point corresponding to an intensity of magnetic fields of the witness magnet at this reference point in a new state of the structure, a step of estimating the state of the corrosion according to the comparison according to an abacus or a database, in particular curves or reference table making it possible to identify the corrosion according to the difference between the measurement and the data.

[0047] The measurement of the magnetic field intensity by a reader at time t is more stable than with an active device such as a radar which will be sensitive to the water state, the physical state, the evolution of the porous state, and hydration rate (function of the concrete manufacture) and the concentration of chloride ions.

[0048] The initial data previously acquired can be an intensity of the induced magnetic field previously measured at this same reference point or previously estimated for this reference point or previously acquired during the manufacture of the wall for this reference point.

[0049] The step of estimating the state of corrosion can also depend on knowledge of the depth of the witness.

[0050] The abacus or database can also take into account the type of concrete to estimate its state of corrosion.

[0051] The invention also relates to a method for monitoring the corrosion front in the porous coating of a wall as described above, comprising the steps of the method for monitoring the corrosion front described above and: a step of acquiring initial data for each witness comprising a witness identification data item, the initial data item of the magnetic field intensity of the witness magnet at the reference point on the monitoring surface, a reference point location data item relative to the monitoring surface, a depth data item between the reference point on the monitoring surface and the witness, a step of measuring the intensity of the magnetic field induced by the witness at each reference point at a time t after the initial data acquisition, in which each measurement at time t allows the acquisition of the data item at time t of the acquisition step at time t of the corrosion deduction method described previously, and a step of differential evolution between the different estimations of the corrosion state estimated for each witness at each time t.

[0052] Such a monitoring method makes it possible to monitor, at each reference point, the evolution of the intensity of the magnetic field induced by each corresponding magnetic corrosion indicator and thus deduce and conclude on a state of corrosion of the coating surface examined at time t.

[0053] Such a process makes it possible to establish, on the scale of an external surface and without complex implementation, a reference through a 3D mapping of the magnetic intensity levels at each reference point located by the presence of a magnetic corrosion indicator.

[0054] According to a first example of this method of monitoring the corrosion front in the porous coating of a wall as described previously, the acquisition of the initial data for each reference point includes: a step of initial reference measurement of the intensity of the magnetic field induced by each detected magnetic corrosion witness at different reference locations relative to the external surface after hardening of the materials of the reinforced structures, a step of identification, localization and recording of the reference location relative to the external surface and acquisition of the initial reference measurement of the intensity of the magnetic field induced by each detected corrosion witness as initial data.

[0055] The initial measurement step allows for the consideration of the perpetual magnetic environment at a fixed point in the structure. For example, a nuclear power plant alternator may generate part of the reader's magnetic field.

[0056] According to a second example of this method of monitoring the corrosion front in the porous coating of a wall as described previously, the acquisition of the initial data for each reference point includes: a step of measuring the orientation of the magnetic field coming from the magnet of a witness, a step of determining the depth of the magnet of the witness as a function of its orientation and its intensity and of the inclination of the witness in relation to the occultation surface of the structure produced as a function of the measurement of the orientation and the intensity of the magnetic field of the magnet of the witness, a step of identifying, locating and recording the reference location in relation to the external surface and the initial reference measurement of the intensity of the magnetic field induced by each detected corrosion witness,

[0057] According to a third example of this method of monitoring the corrosion front in the porous coating of a wall as described previously, the acquisition of the initial data for each reference point includes: a step of estimating the initial data from predetermined knowledge or measuring the depth of the witness in the structure and the type of witness.

[0058] This step requires knowing the location of the witnesses and therefore can only be used according to the second type of reinforced concrete wall manufacturing process but only according to the first type of implementation process whereas the first and second examples can be used with the first or second wall manufacturing process described previously.

[0059] The corrosion front monitoring method may further include: steps of measuring and recording successive and delayed measurements in time at different times t of the intensity of the magnetic field induced by each corrosion indicator detected in the same locations identified in the identification and location step.

[0060] In addition, for each reference point, steps of measuring and recording successive and delayed measurements in time at different times t also include the measurement of the orientation of the magnetic field of the witness magnet.

[0061] The corrosion monitoring method may further comprise a step before or after the first measurement of field intensity, a step of measuring the orientation of the magnetic field coming from the witness magnet and a step of determining the depth of the witness magnet as a function of its orientation and its intensity and the inclination of the witness relative to the occulting surface of the structure produced as a function of the measurement of the orientation and the intensity of the magnetic field of the witness magnet.

[0062] The corrosion front monitoring method may further comprise at each time t a step of differential evolution between the different estimates of the corrosion state estimated for each witness at each previous time t to deduce a potential estimate of the evolution of the corrosion at a time t+1.

[0063] The initial and / or successive and delayed measurement steps of the intensity of the magnetic field induced by each detected corrosion indicator are carried out using a magnetic field reader, for example a magnetic induction loop, magnetometer or teslameter. The reader can thus be passive, i.e. it reads the magnetic fields and does not transmit waves to the indicator, such as a radar.

[0064] The steps of identifying, locating and recording the reference location relative to the external surface and of successive and time-delayed recording of the measurements of the intensity of the induced magnetic field can be carried out humanly or by computer and automatically by means of a recording device comprising a microcontroller, software, a memory and a location means, for example a GPS communicating with the reader.

[0065] The differential evaluation step between the initial reference measurement and the following ones for each recorded location can be carried out electronically by the recording device or in a program in which an operator has transmitted the data or manually by an operator for example using a reference table or an Abacus or a database, in particular curves or reference table allowing the corrosion to be identified according to the difference between the two measurements.

[0066] The abacus or database can also take into account the type of concrete to deduce the state of corrosion.

[0067] According to an implementation of one of these two methods, a drone comprises the reader allowing to carry out the initial and successive and delayed measurement steps in time of the intensity of the magnetic field induced by each detected corrosion indicator. The fact that the reader is passive makes it possible to be lighter compared to the active device, in particular by avoiding carrying the energy necessary for the formation of the magnetic wave or magnetic induction.

[0068] The drone may be piloted by an operator or be autonomous through an automatic navigation program. The drone may further include the recording device or communicate with the recording device.

[0069] The indicator or liquid binder or structure or building or corrosion monitoring method or one of the two types of manufacturing method of the reinforced concrete wall described above according to the invention may also have one or more of the characteristics below, considered individually or in all technically possible combinations: In one embodiment of the indicator, the indicator further comprises a protective layer such as plastic between the permanent magnet and the corrodible layer to protect the permanent magnet.

[0070] In one embodiment of the witness, the permanent magnet is entirely enclosed by the protective layer to protect it.

[0071] In one embodiment of the indicator, the ferromagnetic material of the corrodible layer is iron. Such a material is inexpensive and is highly corrosive, allowing it to deteriorate rapidly and thus alter the magnetic field of the indicator.

[0072] The indicator is the size of a concrete granule or smaller than a concrete granule. Therefore, it causes little or no change in the strength characteristics of the wall materials compared to larger sensors with electronics.

[0073] In one embodiment of the witness, the permanent magnet is a parallelepiped, for example a paving stone, and in that the corrodible layer is also a parallelepiped, for example a paving stone, the parallelepiped having a contact surface parallel to a magnetic field surface of the magnet. This shape makes it possible to reduce the modification of the resistance characteristics of the wall materials. According to an example of this embodiment, the contact surface is less than the propagation surface.

[0074] In an example of this embodiment, the magnet is a pad with permanent magnetization greater than or equal to 1 Tesla, with a thickness of 0.5 cm and the propagation surface of the pad comprises a dimension 1 cm * 1 cm.

[0075] In one example of this embodiment, the corrodible layer is an iron pad having a contact area of ​​0.8cm * 0.8cm and a thickness of 0.3cm.

[0076] This size allows it to be small enough to be close to the size of a concrete aggregate.

[0077] In one embodiment of the witness, the permanent magnet comprises at least 90% ferrite and has a magnetic field intensity in contact with the corrodible layer of approximately 0.01 to 0.045 Tesla. Such a magnet is inexpensive and allows for a magnetic field to be measured with a reader external to the structure (magnetic induction loop, magnetometer or teslameter) at several centimeters.

[0078] In another embodiment of the witness than the previous one, the permanent magnet comprises at least 90% of neodymium iron boron and has a magnetic field intensity in contact with the corrodible layer of approximately 1 to 1.3 Tesla. Such a magnet is powerful enough to allow to have a measurable magnetic field with a reader external to the structure (magnetic induction loop, Teslameter magnetometer) at several centimeters.

[0079] Indeed, as the induction of the magnetic field (expressed in Gauss or Tesla) of the magnet decreases as the distance increases, depending on the structure, it may be necessary to have witnesses having a material allowing to have a more powerful magnetic field intensity on its surface.

[0080] According to one embodiment of the witness, the corrodible layer covers a face of the permanent magnet having a South polarity and a North polarity.

[0081] The magnetic field is strongest directly on the surface at the edges of the poles (at the corners in the case of a parallelepiped magnet), and weakest in the center. However, near the edge, the magnetic field decreases much faster than the field on the central magnetic axis (border between the north and south poles) as the distance increases. Just a few millimeters from the surface of the pole, the field on the axis can be stronger than near the edge.

[0082] The corrodible layer only covers the face of the permanent magnet with a South polarity and a North polarity.

[0083] An implementation of the magnet in the concrete by placing the boundary opposite the monitoring surface therefore allows for a better measurement of the magnet of the witness of the monitoring surface.

[0084] In one embodiment of the corrosion indicator, the indicator comprises two layers of ferromagnetic material covering the two largest surfaces of the magnet comprising the two polarities.

[0085] This allows the corrosion indicator to be placed in two positions.

[0086] In another embodiment of the indicator, the indicator comprises a corrodible layer covering all surfaces of the magnet comprising two polarities.

[0087] This allows for multiple positions of the witness in the coating.

[0088] In another embodiment of the indicator, the indicator comprises a cylindrical magnet comprising a south polarity on one portion of the cylinder and a north polarity on another portion of the cylinder, the two cylinder portions being separated by a radial boundary relative to the axis of the cylinder of the magnet. In this example, the corrodible layer may surround the cylindrical surface of the magnet. Thus, this indicator comprises a plurality of positions.

[0089] In one embodiment of the liquid binder, the mortar comprises cement.

[0090] The liquid binder allows for indicators to be placed at different heights in the reinforced structure when the mortar becomes a solid coating. Indeed, for example, in the case of cement-based concrete, the very low viscosity of the concrete prevents the indicators from running or rising to the surface when the reinforced structure is being manufactured.

[0091] The advantage of cement is that it has excellent compressive strength. It also dries faster than most other solutions.

[0092] Such a coating is porous, the indicators thus make it possible to send information on the infiltration of pathogenic agents, such as chloride ions, by modifying the intensity of the magnetic field generated by the magnet according to the state of corrosion of the corrodible layer.

[0093] In one example, mortar is made to form shotcrete.

[0094] In a variation of this liquid binder example, the binder is made to be poured into a formwork.

[0095] According to one embodiment of the binder, the mortar is concrete, in particular cement-based.

[0096] In one embodiment of the reinforced concrete wall, the corrosion indicators are distributed at different depths between the monitoring surface and the reinforcement.

[0097] This makes it possible to carry out a three-dimensional mapping of the infiltration of pathogenic agents, for example chloride ions, in order to either carry out preventive maintenance to prevent the spread of these pathogenic agents in the coating towards the reinforcement, or to identify and budget for maintenance of a repair to be carried out.

[0098] According to one embodiment of the reinforced concrete wall, the reinforced concrete wall comprises holding members fixed to the reinforcement and to the witnesses to hold the witnesses.

[0099] These holding members allow the indicators to be held in position relative to the metal reinforcement during the liquid mortar coating stage. The liquid mortar becomes the coating after the coating stage on the metal structure and when the mortar is in the solid state.

[0100] According to one example, the holding members are rigid to maintain the inclination of the witnesses during a coating step.

[0101] According to one example, the holding members are wires, in particular nylon, each wire of which is fixed to a protective layer of the witness and to two fixed parts, for example to the formwork and to the reinforcement or to a bar above the formwork fixed for example to the formwork and a bar at the bottom of the formwork.

[0102] According to one embodiment of the reinforced concrete wall, the structure comprises several types of indicators, each indicator of a type of indicators is identical, in which the types of indicators are different from each other at least in that the magnets and / or the corrodible layers of the types of indicators comprise a different volume, the types of indicators comprising larger magnets and / or layers are further from the occultation surface than the types of indicators close to the monitoring surface. This allows for better precision in the measurements of the intensity of the magnetic field as a function of the depth of the indicators relative to the monitoring surface and thus a better evaluation of the infiltration of pathogenic agents into the reinforced concrete wall.

[0103] According to a variant of the previous embodiment of the reinforced concrete wall, the witnesses are all identical. In this example, the intensity of the magnetic field of the witnesses is sufficient so that the witness, closest to the reinforcement between the reinforcement and the monitoring surface, generates a magnetic field detectable at the monitoring surface.

[0104] According to one embodiment of the corrosion front monitoring method, the method comprises a step of mapping the structure before the step of the first reference measurement.

[0105] For example, the mapping step is performed by identifying the position of the reinforcement relative to a reference point.

[0106] According to another example, the mapping step consists of a three-dimensional realization of the metal frame closest to the auscultation surface and the positions of the witnesses in relation to this metal frame.

[0107] According to one example, the mapping step is performed by identifying the position of a plurality of witnesses relative to the auscultation surface.

[0108] The tracking method according to the last two examples is carried out when the support manufacturing method is carried out according to the first type of reinforced concrete wall manufacturing method. The mapping step may further include identifications of witness types in the case of the reinforced concrete wall comprising different types of witnesses described previously

[0109] According to a variant of these last two examples, the mapping step further comprises a step of determining the depth and orientation of the indicator calculated as a function of the first measurement. Such a step can in particular be carried out when the reinforced concrete wall comprises identical indicators and a method for manufacturing a reinforced concrete wall according to the second type of the invention. Indeed, when the reinforced concrete wall is manufactured according to the second type of manufacturing method, the indicators are at different depths and the first measurements can make it possible to calculate the depth of the magnet to integrate it into the mapping step.

[0110] The invention and its various applications will be better understood by reading the following description and examining the accompanying figures. BREVE DESCRIPTION DES FIGURES

[0111] The figures are presented for information purposes only and in no way limit the invention. THE figures 1a et 1b schematically represent a section of a witness according to a first embodiment respectively in a new state and a state of advanced corrosion. The figures 2a And 2b represents a simulation of the magnetic field of a witness of the example of the first embodiment respectively in a new state and a state of total corrosion, in a portion of a reinforced concrete wall. The figure 3 represents a schematic diagram of a section of a portion of a reinforced concrete wall including witnesses at different positions and different depths. The figure 4 is a graph including on the ordinate the intensity in Tesla of a magnetic field measured by a reader external to the structure at an occultation surface of a reinforced concrete wall and on the abscissa different thicknesses of the corrodible layer of a witness generating the measured magnetic field. The figure 5 represents a table showing the evolution of the induced magnetic field B on the surface of a concrete slab as a function of the thickness of the corrodible layer of a witness and as a function of different dielectric characteristics of the concrete coating. The figure 6 represents a graph representing the differential value of the magnetic field intensity in Tesla as a function of the quantity of chloride ions.

[0112] For clarity, identical or similar elements are identified by identical reference signs throughout the figures. DESCRIPTION DETAILLEE

[0113] The figures are presented for information purposes only and in no way limit the invention.

[0114] [ Fig. 1A ] represents a corrosion indicator 1, hereinafter referred to as indicator 1, according to an embodiment comprising a corrodible layer 12 and a magnet 10 secured to the corrodible layer 12.

[0115] In this embodiment, the indicator 1 further comprises a protective layer 11 surrounding the magnet 10 to protect it. The protective layer 11 may be, as in this example, a portion of plastic overmolding on the magnet 10, but may also be, for example, a closed box, by clipping for example, in which the magnet 10 is located and may be wedged there. The indicator 1 comprises, in this example, an adhesive 13 between the corrodible layer 12 and the protective layer 11. According to another example, the overmolding is also carried out on the corrodible layer 12. According to another example not shown, the indicator 1 may have the corrodible layer 12 fixed directly against a propagation surface of the magnet 10 and may either be devoid of a protective layer or have a protective layer 11 surrounding the rest of the surfaces of the magnet 10 (i.e. surrounding the magnet 10 except the portion of the propagation surface in contact with the ferromagnetic layer).The protective layer 11 thus protects the magnet against pathogens.

[0116] Witness 1 has the shape of a parallelepiped and in this case has the shape of a block in this embodiment but could also have another shape, for example in the shape of a cylinder.

[0117] Magnet 1 has the shape of a parallelepiped and in this case has the shape of a paving stone but could also have another shape such as for example a cylinder shape, particularly in the case of a cylindrical witness.

[0118] In this embodiment, the magnet 10 comprises a propagation surface 100 visible in section on the figure 1A . The magnet 10 comprises a positive polarity 102 and a negative polarity 104. The propagation surface 100 of the magnet 10 comprises a face of the positive polarity 102 and a face of the negative polarity 104. On the figures 1A et 1B , dotted line shows the boundary between the two polarities. The propagation surface 100 is covered by a contact surface 120 of the corrodible layer 12. The propagation surface 100 of the magnet 10 is in this case one of the two largest faces of the magnet having the two polarities of the magnet 10. It can be said that the propagation surface 100 supports the corrodible layer 12. The corrodible layer 12 is made of ferromagnetic material and is in this case made of iron. Thus, the fact that the corrodible layer 12 comprises a ferromagnetic material opposite this propagation surface 100 of the magnet 10 makes it possible to modify the magnetic field on its largest detectable face.

[0119] In this embodiment, the propagation surface 100 comprises in this case a value of 1 cm2 but may comprise an area of ​​a value between 0.5 cm2 and 5 cm2 for example. In this embodiment, the propagation surface 100 has a square shape but could be a rectangle or another shape, for example a disc or a portion of a cylinder in the case of a cylindrical magnet.

[0120] The thickness of the magnet, in this example, is 0.5 centimeters but can be smaller or larger, for example between 0.3 cm and 2 cm thick.

[0121] The greater the thickness of the magnet and / or the larger its propagation surface, the more easily its magnetic field and its variation will be detectable.

[0122] The corrodible layer is also of the same shape as that of the magnet, in this case a paving stone. In this example, a contact surface 120 is smaller than that of the propagation surface 100 of the magnet 10. In this case, the contact surface is 0.8cm*0.8cm, or, in this example, a square surface thirty-six percent smaller than the square surface of the propagation surface 100 of the magnet 10.

[0123] Such a witness volume reduces the changes in the characteristics of a coating in a wall of a structure compared to a witness of larger volume and is also less expensive.

[0124] [ Fig. 1B ] represents a section of witness 1 identical to that of the figure 1A except that the corrodible layer 12 has undergone corrosion.

[0125] The corroded corrodible layer reduces the amount of uncorroded ferromagnetic material, which causes a reduction in the magnetic field strength of the magnet 10 relative to the magnetic field strength generated by the magnet 10 of the witness 1 comprising a corrodible layer 12 in the new state.

[0126] [ Fig. 2A ] represents a simulation of the magnetic field of witness 1 in the new state of the figure1 whose magnet 10 has a residual magnetic flux density of 1 Tesla.

[0127] The magnetic field simulation is performed by making a box 2 simulating the permeability of a concrete coating to simulate the witness in a concrete wall. The simulation was performed with a relative permittivity of 3 εr and an electrical conductivity of 0.1 Siemens per meter. The simulation also takes into account the permittivity of the protective layer 13.

[0128] The propagation surface 100 of the magnet 10 of the witness 1 is simulated to be eight centimeters from an auscultation surface 21 of the box 2 simulating an auscultation surface in this example of a support.

[0129] The magnetic field, generated by the magnet 10 of the witness 1 in the new state, is represented by waves 4. As a reminder, the witness 1 comprises, in this example of this embodiment in the new state, a thickness of its corrodible layer 12 equal to 0.3 cm.

[0130] A passive external reader to the wall of the structure, in this case a magnetometer 3, is schematically represented by a point on the monitoring surface 21 of the concrete box 2. In this embodiment, the propagation surface 100 is parallel to the monitoring surface 21 and the magnetometer 3 is located at the intersection of the monitoring surface and a plane passing through the boundary of the magnet (i.e. perpendicular to the propagation surface 100 and to the center thereof). The magnetometer may be a teslameter.

[0131] The simulation indicates that the value of the magnetic field intensity measured at the point representing magnetometer 3 is 0.00026 Tesla or 2.6 Gauss.

[0132] [ Fig. 2B ] represents the same simulation of magnetic field generated by magnet 10 of witness 1 except that in this simulation, the corrodible layer is removed corresponding to a completely corroded layer of material.

[0133] The simulation indicates that the value of the magnetic field intensity measured at the point representing magnetometer 3 is 0.00022 Tesla or 2.2 Gauss.

[0134] Thus we can see that the thickness of the ferromagnetic layer modifies the intensity of the magnetic field measured by the magnetometer.

[0135] [ Fig. 3 ] represents a schematic diagram of a section of a portion of a reinforced concrete wall 4 of a civil engineering structure, comprising witnesses 1 at different positions in the reinforced concrete wall 4.

[0136] There figure 3 includes an X, Y, Z reference frame, the section of which was carried out in the X, Z plane.

[0137] The portion of the reinforced concrete wall, 4 hereinafter called wall 4, comprises a reinforcement 41 and a porous coating 42.

[0138] In an example of this embodiment, the witnesses 1 of the wall 4 are identical to that of the example of the figure 1A . In this case, each witness 1 includes a witness 10 having a magnetic flux density of 1 Tesla. On the figure 3 , lines, surrounding each witness of the portion of the wall 4, represent the magnetic field at different intensity values.

[0139] According to another example not shown, the witnesses 1 may be different in that witnesses 1 comprise a magnet having a different magnetic flux density value than those of other magnets, for example by being of a different size or of a different composition. For example, the wall 4 comprises three types of witnesses, each type of witness 1 is located at one of the three levels described below and the witnesses of each type are identical.

[0140] The wall 4 comprises an oscillating surface 421, in this case in the X, Y plane.

[0141] The X, Y plane represents the position of the witnesses and the Z direction represents the depth of the witnesses relative to an auscultation surface. The witnesses 1 are, in this exemplary embodiment, located at different depths relative to the wall 4. Each witness 1 is oriented in the porous coating 42 such that the layer of ferromagnetic material 12 is closer to the auscultation surface than the magnet 10. In this case, in this example, the witnesses 1 are all located between the auscultation surface 421 and the frame 41. The wall 4 can comprise hundreds of witnesses depending on the air of the auscultation surface.

[0142] The wall 4 could also include other indicators 1 in the porous coating 42 between a frame 41 and another auscultation surface not shown.

[0143] The wall 4 in this case comprises several reinforcements, only one of which is shown in the portion of the wall 4. The reinforcements 41 are in this case at different depths relative to the auscultation surface.

[0144] The method of manufacturing this wall 4 is according to a first type of method consisting of arranging the indicators 1, in a predetermined manner, before coating the reinforcement 41 with the coating 42.

[0145] For example, the method for manufacturing the wall 4 comprises a step of mounting and positioning a metal frame, a step of forming the formwork around the metal frame, a step of placing several indicators 1, for example using a nylon thread fixed to the protective layer of the indicator to hold the indicator between the metal frame and the formwork. The nylon thread can be fixed to the frame and / or to the formwork or to a plate above the formwork or the frame. The order of these three steps is not important. When these three steps are carried out, the method also comprises a step of pouring or spraying the concrete into the formwork, coating the indicators and the metal frame. When the coating is dry, the method comprises a formwork removal step.

[0146] In this embodiment, at least one indicator is located between the monitoring surface 421 and the reinforcement 41 closest to the monitoring surface 41. This thus makes it possible to warn the user of infiltration of a pathogenic agent into the coating 42 of the concrete.

[0147] The section of the portion of wall 4 represents five witnesses numbered 1A, 1B, 1C, 1D and 1E which extend one after the other in the direction X. In this example, wall 4 comprises three levels of witnesses 1 but wall 4 could comprise only one level or two or more than three levels.

[0148] In this portion of wall 4, the first witness 1A and the fourth witness 1D are located on the first floor, the second witness 1B and the fifth witness 1E are located on the second floor, and finally the third witness 1C is located on the top floor.

[0149] The first stage is closer to the auscultation surface 421 than to the armature 41 and the last stage is on the contrary closer to the armature 41 than to the auscultation surface 421. The second stage is located between the first and second stages and is in this case located in the middle between the auscultation surface 421 and the first armature 41.

[0150] In other words, the witnesses 1 are arranged in the XZ plane by successive groups of stairs in the V direction, comprising in this case three witnesses 1, in this case three floors descending in the X direction one after the other. The witnesses 1 could also be in the XZ plane by V-shaped group of five witnesses, that is to say a staircase which, in the X direction, descends from the first floor to the third floor and then rises from this third floor to the first floor. According to another embodiment not shown, the witnesses can be aligned in a group of 7 witnesses in a W in the XZ plane, in which in the X direction, the first witness and the last witness of the group are on the first floor, the second, fourth and sixth witnesses 1 are on the second floor and the third and fifth witnesses 1 are on the third floor.

[0151] According to yet another embodiment, the witnesses 1 are located randomly in depth and position.

[0152] Of course, wall 4 includes other witnesses, particularly in the Y direction. For example, in this example, witnesses 1 are aligned in the YZ plane in the same way as in the YX plane, that is to say in groups of three witnesses and floors descending in the Y direction but can also all be aligned at the same height or even in groups of three witnesses and floors in staircases ascending in the Y direction.

[0153] The wall 4 may have an impermeable layer on its inspection surface; not shown; to prevent the infiltration of pathogenic agents. However, this impermeable layer may have a crack due, for example, to a mechanical shock or poor workmanship or even to a vibration of the structure, for example a seismic vibration. The impermeable layer may then allow pathogenic agents to enter the porous coating 42 of the wall 4.

[0154] In the portion of the wall 4, a degraded portion 422 of the porous coating 42 is damaged by pathogenic agents infiltrated into the porous coating 42.

[0155] The pathogens are, in this case, sodium chloride ions and wall 4 is a reinforced concrete wall of a marine structure, for example a dike or a bridge.

[0156] The wall portion 4 is for example a portion of the tidal zone of the marine building. The auscultation surface 422 is therefore in this example vertical (the figure 3 is therefore rotated 90° and the X direction is vertical).

[0157] The fourth witness 1D is located in this degraded part 422, the chloride ions have corroded the corrodible layer 12 which is degrading. The degradation of the corrodible layer 12 leads to a reduction in the thickness of the non-corroded ferromagnetic material and therefore reduces the intensity of the magnetic field at the monitoring surface 421. The other witnesses 1 each have their non-degraded corrodible layer 12.

[0158] Thus, it is possible to carry out a method for monitoring the corrosion front by carrying out at a first moment, when the coating is dry and before a part of the wall 4 is degraded, to carry out using a magnetometer on the side of the monitoring surface 21 a step of detecting a magnetic field coming from the wall corresponding to a magnetic field of a witness 1 and for each detection of a magnetic field of a witness, a step of first reference measurement of the intensity of the induced magnetic field of the witness, a step of locating a reference point of the measurement and a step of recording the location and the intensity of the magnetic field measured at the reference point.

[0159] Thus, for each witness 1 detected, the location and the first measurement of the intensity of the magnetic field provided by witness 1 are recorded when the wall 4 is still in good condition. The reference point can be distant from the monitoring surface or located on the monitoring surface 421.

[0160] Then, the corrosion front monitoring process carries out at several successive and delayed moments in time, which can be several months or years between each moment, a step consisting of measuring, at each location of the recorded reference point, the intensity of the magnetic field and a step of differential evaluation between this new measurement and that of the first reference measurement recorded at each location of the recorded reference point.

[0161] There figure 3 represents the portion of wall 4 at the second moment, for example three years after the first moment of the first reference measurement of the identified witnesses. At this second moment, witnesses 1A, 1B, 1C and 1E have not undergone or have undergone very little corrosion compared to the first measurement but the corrodible layer of the fourth witness 1D is corroded compared to the first moment of the first measurement causing a strong variation of the magnetic field at the reference point.

[0162] Thus, during the differential evaluation step, the comparison of the two measurements is identical or very close (less than 0.01 milliteslas difference between the two measurements) for the controls 1A, 1B, 1C and 1E and shows a difference between the two measurements of the fourth control 1D of more than 0.01 milliteslas. It is therefore possible to deduce that the corrodible layer 12 is thinner than during the first measurement (thickness less than at the time of the first reference measurement) and therefore conclude that there is an infiltration of pathogenic agent responsible for the corrosion of the corrodible layer 12 of the fourth control 1D. Depending on the difference in measurement, the user or the machine can conclude, using a reference table or graph, the state of corrosion of the corrodible layer 12.

[0163] [ Fig. 4 ] represents a reference graph of witness 1 measured at the surface of the monitoring surface as a function of the corrosion of the corrodible layer 12. On the abscissa, the graph represents the thickness of the ferromagnetic layer 12. The more advanced the corrosion of the ferromagnetic layer, the more the thickness of the corrosion material layer is reduced. On the ordinate, the reference graph represents the intensity of the magnetic field measured at the surface of the monitoring surface and at the boundary of witness 1.

[0164] The reference graph, in this case, was produced with a witness 1 from the example of the figure 1 and at a depth of 8cm from the auscultation surface as in the simulation of figures 2a And 2b. We can see from this graph that the evolution of the magnetic field intensity at the reference point evolves in a staircase and that every millimeter of loss of uncorroded ferromagnetic material from the corrodible layer, the magnetic field intensity varies by at least 0.01 millitesla, in this example, 0.04 millitesla for a difference of 3mm in magnetic charge thickness.

[0165] So the user, in the example shown in the figure 3 , can conclude that the degraded part 422 has not yet reached, in this section of the portion of the wall, the depth of the reinforcement 41. Indeed this deduction can be made using the measurements of the intensity of the magnetic field, of the third and fifth witnesses 1C, 1E closer to the reinforcement 41 than the fourth witness 1D, which allows to deduce that their corrodible layer has not had a reduction of 1 mm of non-corroded ferromagnetic material.

[0166] As already mentioned, wall 4 also includes other indicators in the Y direction which also allow us to conclude whether or not the damaged part has reached the reinforcement in the Y direction.

[0167] The user can thus either carry out maintenance on this degraded part or wait for a corrodible layer 12 of a second-stage indicator to corrode.

[0168] [ Fig. 5 ] represents a table showing magnetic field intensity measurements at the surface of the auscultation surface of different coatings and thicknesses of a corrodible layer of a witness 1. The columns of the table represent different dielectric characteristics of concrete coating and the rows represent different thicknesses of the corrodible layer of witness. Thus each box represents a measurement of magnetic field intensity as a function of a thickness of the corrodible layer and as a function of a dielectric characteristic of a coating. It can be seen that the magnetic field intensity varies as a function of the thickness of the corrodible layer but not or less than 0.000001 mT as a function of its different types of coating.

[0169] The invention therefore makes it possible to operate with different coating walls in a simple manner since the types of coatings have little or no influence on the magnetic field intensity measurements.

[0170] [ Fig. 6 ] represents a graph representing the variations in measurement of the intensity of the magnetic field in vertical component Z as a function of the quantity of chloride ions to corrode during experimental tests the layers of ferromagnetic material 12 of a witness 1 of the figure 1 at a depth of 8cm in a porous coating having one of the dielectric characteristics of the table of the figure 5 .

[0171] The graph represents on the ordinate the differences in measured values ​​of magnetic field intensity with a magnetometer at a reference point and on the abscissa the quantity of chloride ions in g / l to corrode the layers of ferromagnetic material.

[0172] It can be seen that the amount of chloride ions influences the measured values ​​in the vertical component by 0.011 millitesla to 0.017 millitesla. Thus, the magnetic field intensity measurements made by the magnetometer can allow conclusions to be drawn about the state of corrosion in the wall.

Claims

1. A binder in a liquid state, especially hydraulic concrete, comprising a mortar and corrosion indicators (1A, 1B, 1C, 1D, 1E) comprising at least one permanent magnet (10) generating a magnetic field, and a layer (12) being corrodible by pathogenic agents, especially chloride ions, the corrodible layer (12) being integral with the magnet, the corrodible layer (12) comprising a ferromagnetic material for modifying the magnetic field generated by the magnet (10) as a function of the corrosion state of the ferromagnetic material of the corrodible layer (12), and in that the corrodible layer (12) covers a face of the permanent magnet (10) having a South polarity (102) and a North polarity (104), the binder becoming a porous coating (42) in the solid state and being intended to become a coating (42) of a reinforced concrete wall (4).

2. The binder in a liquid state according to claim 1, wherein the binder comprises a protective layer (11), such as plastic, between the permanent magnet (10) and the corrodible layer (12) for protecting the permanent magnet (10).

3. The binder in a liquid state according to the preceding claim, wherein the ferromagnetic material of the corrodible layer (12) is iron.

4. The binder in a liquid state according to any of the preceding claims, wherein the permanent magnet (10) comprises at least 90% ferrite and has a magnetic field intensity in contact with the corrodible layer (12) of about 0.01 to 0.045T.

5. A reinforced concrete wall (4) of a structure comprising: - a metal reinforcement (41), - a porous coating (42), especially of concrete, covering the metal reinforcement (41) and forming at least one inspection surface (421) of the wall (4), and - corrosion indicators (10) comprising at least one permanent magnet (10) generating a magnetic field, and a layer (12) being corrodible by pathogenic agents, especially chloride ions, the corrodible layer (12) being integral with the magnet, the corrodible layer (12) comprising a ferromagnetic material for modifying the magnetic field generated by the magnet (10) as a function of the corrosion state of the ferromagnetic material of the corrodible layer (12), and in that the corrodible layer (12) covers a face of the permanent magnet (10) having a South polarity (102) and a North polarity (104), located in the porous coating (42) between the metal reinforcement (41) and the inspection surface (421) and wherein the corrosion indicators (1) are distributed at different positions between the inspection surface (421) and the metal reinforcement (41).

6. The reinforced concrete wall (4) of a structure according to claim 5, wherein the corrosion indicator (1) comprises a protective layer (11), such as plastic, between the permanent magnet (10) and the corrodible layer (12) to protect the permanent magnet (10).

7. The reinforced concrete wall (4) of a structure according to the preceding claim, wherein the ferromagnetic material of the corrodible layer (12) is iron.

8. A reinforced concrete wall (4) of a structure according to any of claims 5 to 7, wherein the permanent magnet (10) comprises at least 90% ferrite and has a magnetic field intensity in contact with the corrodible layer (12) of about 0.01 to 0.045T.

9. A marine structure comprising a reinforced concrete wall (4) according to one of claims 5 to 8, wherein the inspection surface (421) is external and the wall comprises at least one tidal zone, wherein at least one of the corrosion indicators (1) is located between the external inspection surface of the tidal zone and the metal reinforcement (41).

10. A method for deducing corrosion of a reinforced concrete wall according to one of claims 5 to 8 comprising: - a step of acquiring a piece of data at a time t from a measurement of an intensity of the induced magnetic field for each magnetic corrosion indicator at a reference point of each indicator, - a step of comparing the piece of data acquired with an initial piece of data previously acquired for this same reference point corresponding to a magnetic field intensity of the indicator magnet at this reference point in a new state of the structure, - a step of estimating the corrosion state as a function of the comparison as a function of a chart or a database, especially curves or a reference table for identifying corrosion as a function of the difference between the measurement and the piece of data.

11. A method for monitoring corrosion front in a porous coating (42) of a wall (4) according to one of claims 5 to 8, comprising the steps of the deduction method of claim 10 and: - a step of acquiring initial data for each indicator comprising a piece of data for identifying the indicator, the initial piece of data for the magnetic field intensity of the magnet of the indicator at the reference point on the inspection surface, a piece of data for locating the reference point relative to the inspection surface, a piece of data for a depth between the reference point on the inspection surface and the indicator, - a step of measuring intensity of the magnetic field induced by the indicator at each reference point at a time t after acquiring the initial piece of data, wherein each measurement at time t enables acquisition of the piece of data at time t of the acquisition step at a time t of the corrosion deduction method, and - a step of differentially evaluating between the different estimates of the corrosion state estimated for each indicator at each time t.