Method for corrosion detection and infiltration assessment by measuring the value of a magnetic property

Description

DOMAINE TECHNIQUE DE L'INVENTION

[0001] 1. The technical field of the invention is that of detecting and / or preventing corrosion by measuring a value of a magnetic characteristic. This measured value is that of a magnetic field of a magnet at a given location.

[0002] Document DE 10 2017 129573 A1 discloses a method for detecting corrosion comprising the use of a pair of magnets as corrosion coupons.

[0003] 2. In particular, the technical field of the invention is that of detecting and monitoring the corrosion of a metal portion attached to a magnet.

[0004] 3. It relates in particular to the measurement of a magnetic characteristic value at a given location of the magnetic field of a magnet of a corrosion-sensitive indicator present in a wall of a civil engineering structure, for the prevention of corrosion of at least one metal reinforcement in a porous coating forming the wall. The coating may be made of concrete, for example. By "a coating" is meant a porous layer in the solid state coating the metal reinforcement. For example, in the case of a civil engineering structure made of reinforced concrete, the concrete is considered to be the coating. Thus, the invention can, using an indicator, prevent corrosion of a metal reinforcement of a wall of a civil engineering structure.

[0005] 4. In general, the present invention can be used to detect and monitor corrosion in all types of metal reinforcement that is difficult to access.

[0006] 5. The present invention therefore relates to the detection of corrosion location from magnetic data and in particular the detection of corrosion of a member from a reading or measurement of a magnetic characteristic value at a given location of a magnetic field of a permanent magnet. ARRIERE-PLAN TECHNOLOGIQUE DE L'INVENTION

[0007] 6. There are different ways to deduce corrosion of a metal portion, whether visual, chemical, electrical resistivity or by wave such as ultrasound.

[0008] 7. However, some of these methods require direct access to the metal portion, which is not always the case, for example for the metal reinforcement embedded in concrete in a reinforced concrete wall.

[0009] 8. It is advantageous to know the condition of the coating to prevent aging of the reinforcement.

[0010] 9. Indeed, if we exclude its natural ageing, the lifespan of a civil engineering structure, for example in 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 flaking of surfaces exposed to de-icing salts.

[0011] 10. A common factor in these pathologies is corrosion. Quantifying this corrosion in average or surface gradient is subsequently a crucial phase in the diagnosis of maritime structures.

[0012] 11. Core sampling of reinforced concrete 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.

[0013] 12. 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.

[0014] 13. Furthermore, various 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.

[0015] 14. 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.

[0016] 15. 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.

[0017] 16. 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.

[0018] 17. 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.

[0019] 18. Further, the system may include an interrogator outside the wall of the structure and a witness in the wall, the interrogator includes a magnetic field generator including an induction or electromagnetic wave radar and a magnetic field reader and the witness may include a non-magnetic, non-metallic holding member such as a polymer or ABS resin and a corrodible conductor. Corrosion may be measured by providing a primary magnetic field to the concrete from the generator, 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.

[0020] 19. 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 reflect 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.

[0021] 20. 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.)

[0022] 21. Document US2004100278 further describes a device and its method, 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 state of the corroded reinforcement.

[0023] 22. 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.

[0024] 23. In addition, these different solutions lead to targeted assessments, which makes the assessment process tedious. For example, in the case of using a measuring sensor, it must be connected to the metal portion or have its antenna opposite the witness to be measured and therefore do not allow corrosion information to be obtained from a wider area, for example, than the volume in front of the sensor antenna.

[0025] 24. Finally, one of the problems is that the structure can vary, for example the thickness can increase or decrease due to the humidity level, heat can distort measurement values. RESUME DE L'INVENTION

[0026] 25. The invention provides a solution to at least one of the problems mentioned above, by comparing a measurement of a magnetic characteristic value at a given location of a magnetic field of a magnet with acquired magnetic field data to deduce corrosion.

[0027] 26. One aspect of the invention relates to a method for detecting corrosion of a ferromagnetic corrodible layer comprising: a) a step of acquiring and storing data in a memory including at least data: o from different locations of at least one permanent magnet in a global three-dimensional reference frame X, Y, Z, o magnetic characteristic value (B%, B, M) for each magnet in a three-dimensional frame of reference specific to the magnet, b) a step of determining a location of a device for measuring a value of a magnetic characteristic (B), distant from the at least one magnet, in the overall three-dimensional frame of reference by a processor of a detection device, c) a step of measuring a value of a magnetic characteristic (B) by means of the measuring device at the location determined in the determination step, and recording, in a database, the value of the measurement, the location and the date of the measurement, d) a step of estimating by the processor at least the value of a magnetic characteristic (B%, B, M) of at least one magnet according to its own three-dimensional reference frame, depending on the value measured in the measurement step and the location of the magnet acquired in the acquisition step, e) a step of comparing the magnetic characteristic value (B%, B, M) estimated for at least one magnet and that previously estimated or acquired, f) a step of deducing corrosion of the ferromagnetic corrodible layer between the at least one location of the magnet and the location of the device depending on the comparison of the magnetic characteristic value carried out in the comparison step.

[0028] 27. Thanks to the invention, it is possible to deduce corrosion of a ferromagnetic corrodible layer by comparing a value of a magnetic characteristic of the magnet according to its reference. En In fact, if a ferromagnetic corrodible layer is in perfect condition, the magnetic characteristic value is very low due to the magnetic field short-circuited by the ferromagnetic corrodible layer, whereas if it is corroded, the magnetic characteristic value approaches that of the magnet without a ferromagnetic corrodible layer since once corroded, it no longer short-circuits the magnetic field or only slightly. En furthermore, estimating the value of a magnetic characteristic (B , M ) of the magnet according to its own reference point makes it possible to reduce errors in estimating corrosion, for example an error linked to the distance between the measuring location and the magnet which has changed due for example to an expansion of the wall (for example due to freezing / thawing) En In fact, if the measured value is directly compared while the location is wrong, the corrosion deduction may also be wrong. Estimating the value of a magnetic characteristic (B, M ) of the magnet according to its own reference also makes it possible to limit the risk that an agent disturbing the magnetic flux could cause a measurement error, by detecting this disturbing agent and estimating the value by taking into account the disturbance by this agent.

[0029] 28. In addition to the characteristics which have just been mentioned in the preceding paragraph, the corrosion detection method according to one aspect of the invention may have one or more additional characteristics among the following, considered individually or according to all technically possible combinations: 29. According to one embodiment, the value of a measured magnetic characteristic is an arbitrary unit of a magnetic flux density.

[0030] 30. According to one embodiment, each magnet is attached to a ferromagnetic metallic element forming the ferromagnetic corrodible layer and in that the data acquisition and storage step further comprises data of magnetic characteristic values ​​(B, M ) as a function of the thickness of the uncorroded ferromagnetic metal element with the components of the three-dimensional reference frame specific to the magnet x, y and z.

[0031] 31. According to an example of this embodiment the acquired magnetic characteristic value of the magnet is based with the ferromagnetic metallic element forming the ferromagnetic corrodible layer, and if the estimated one is higher by at least one predetermined value than the acquired one then the deduction step f) deduces that the ferromagnetic corrodible layer is corroded.

[0032] 32. According to another example of this embodiment the acquired magnetic characteristic value of the magnet is based without the ferromagnetic metallic element forming the ferromagnetic corrodible layer, and if that estimated is less than a predetermined value added to that acquired then the deduction step f) deduces that the ferromagnetic corrodible layer is corroded.

[0033] 33. According to another example of this embodiment, the acquired magnetic characteristic value of the magnet is based on a measurement at a first instant, if that estimated at an instant subsequent to the first instant is greater by at least one predetermined value than that acquired during a measurement at the first instant then the deduction step f) deduces that the ferromagnetic corrodible layer is corroded.

[0034] 34. According to an example of this embodiment, the memory comprises a calibration curve of the magnetic characteristic value of the magnet with respect to the corrosion of the ferromagnetic corrodible layer attached to the magnet and in that step f) of deducing corrosion of the ferromagnetic corrodible layer comprises the use of this curve by an inverse method to deduce therefrom a corrosion state value of the ferromagnetic corrodible layer.

[0035] 35. This example allows a state of corrosion to be given, for example a percentage of corrosion of the corrodible ferromagnetic layer or a thickness of the remaining uncorroded ferromagnetic layer.

[0036] 36. According to this example, the method uses an inverse method to determine from the calibration curve, a corrosion state and further a possibility of a pathogen infiltration state.Indeed, a calibration curve can be produced in the laboratory which can be modified by experiment, in which a plurality of characteristic magnetic values ​​of witnesses are measured in the laboratory, each comprising an identical magnet and a corrodible layer initially identical (same material and same thickness) at the same or different predetermined distances to calculate from the Biot Savart law a magnetic moment of each witness, in which the different measurements are carried out by having their corrodible layers according to different predetermined corrosion rates which can be determined as a function of a chloride content in a structure by experiment, by relating each measured magnetic moment value to the chloride content in a corresponding structure.Thus, by the inverse method, the magnetic moment of the sensor is measured or calculated from the device and by the inverse method, the corrosion rate and / or, as required, a chloride content is deduced from the measurement of the magnetic moment using the calibration curve produced in the laboratory.

[0037] 37. According to one embodiment, the method comprises a plurality of steps of determining the location of the device and steps of measurements at different locations in a limited area and in that the estimation of the characteristic value is refined at each measurement of magnetic flux density (B) of a location. This makes it possible to adapt to a change in the actual distance between the magnet and the measuring device or another magnet producing a magnetic field in an area close to the magnet for which a value is estimated.For example, in the case of two magnets close to each other against a ferromagnetic corrodible layer, as long as the ferromagnetic corrodible layer is not corroded, there will be no impact on the measurement because the ferromagnetic corrodible layer of the magnets will short-circuit their magnetic field but when the ferromagnetic corrodible layer of the magnets is corroded, the measurement by the measuring device at the determined location may pick up the magnetic field of one or both magnets and estimate a wrong value of a magnetic characteristic (B, . M) of the magnet and therefore a deduction of corrosion of the ferromagnetic corroded layer but not necessarily in the right place. Using several measurements in a predetermined area relative to the magnet makes it possible to estimate more precisely the value of a magnetic characteristic (B, M) of the actual magnet since one of the measurements by the device at the first, second, third, etc. locations will necessarily be less influenced by the other magnet.

[0038] 38. According to one embodiment, the model uses the Biot-Savart law to estimate the value of a magnetic characteristic (B, M) according to its own three-dimensional reference frame x, y and z. Knowing the location of the magnet and that of the device, this makes it possible to easily calculate and estimate the magnetic moment of the magnet in its reference frame.

[0039] 39. According to one embodiment, the data acquisition and storage step further comprises orientation data of the poles of the magnets relative to the point of origin of the global reference frame. This makes it possible to estimate a magnet oriented randomly relative to the measurement location. For example, in the case of a wall comprising a porous coating in which a witness comprising the magnet and the corrodible layer, the witness may have been either randomly positioned or disoriented during the procedure of pouring the porous coating, the acquisition step comprises a measurement of the orientation of the magnet relative to the monitoring surface to acquire the orientation and thus to take into account the orientation of the magnet relative to the surface against which the measuring device performs the measurement at the determined location. The orientation of the magnet may be carried out using the measuring device or a compass.So only in the case where the magnet is positioned with its face having the north and south poles in parallel with the face of the wall, the magnet will be unusable or difficult to use.

[0040] 40. For example, the device includes a magnetometer with a Hall probe to measure the directions and senses of the magnetic field and thus determine the orientation of the magnet. For example, a regular dome will represent a magnet positioned such that a face comprising north and south is perpendicular to the probe of the magnetometer, making it possible to indicate that this face of the magnet is parallel to the auscultation surface, and if the dome is irregular, the device may include a correction factor making it possible to indicate the orientation of the surface of the magnet relative to its probe.

[0041] 41. According to one embodiment, the method may further comprise a step of estimating the orientation of the magnet using the measured values ​​of a magnetic characteristic and the step of estimating by the processor at least the value of an estimated magnetic characteristic is a function of the determined orientation. This makes it possible to take into account the case of a different orientation of the magnet relative to the origin. For example, in the case of a witness comprising a magnet and a corrodible layer in a wall, in the event of a crack crossing the area of ​​the magnet, the magnet may be slightly inclined relative to its initial orientation and therefore modify the value of the magnetic characteristic (B, m) to be measured on the face of the wall relative to a value measured if the magnet had kept its initial orientation.

[0042] 42. According to one embodiment, in which a corrosion indicator is located in a porous coating of a wall, in particular reinforced concrete, the corrosion indicator comprising one of the magnets and at least one ferromagnetic corrodible layer integral with the magnet connecting the north pole to the south, and in which the step of deducing corrosion of the ferromagnetic corrodible layer and that of the ferromagnetic corrodible layer of the indicator.

[0043] 43. Such a witness thus allows, when it is placed between the reinforcement and the surface of the wall against which the measurement is carried out by the measuring device, in the event of deterioration of the corrodible ferromagnetic layer to estimate by the processor at least the value of a magnetic characteristic (B, M)of the magnet by means of the measurement or measurements by the measuring device, greater than that estimated when the ferromagnetic corrodible layer was in perfect condition and thus indicate to a maintenance operator that the porous wall includes corrosion pathogens at least up to the location of the witness comprising this ferromagnetic corrodible layer.

[0044] 44. According to one embodiment, the step of acquiring location data of the magnets in the global reference frame X, Y, Z comprises: a) a sub-step of searching for a signal of a magnetic flux density (B) by means of the measuring apparatus for a magnetic characteristic value of the magnetic field in a limited area and measuring and recording the most significant signal value of each limited area, b) a sub-step of recording a location in the global three-dimensional frame by a processor in a memory when the measuring apparatus has measured a value of a most significant magnetic characteristic, c) a sub-step of deducing the distance between the magnet and the location and recording the location of each permanent magnet in a global three-dimensional frame.

[0045] 45. Such an embodiment makes it possible, on the one hand, to obtain a starting reference of one or more values ​​of a magnetic characteristic of a magnetic field provided by the magnet located at a measurement location, and on the other hand to acquire data to map the location of the magnet, for example of the witness.

[0046] 46. ​​Another aspect of the invention also relates to a method for estimating the content of a pathogenic agent in a wall comprising a porous packaging, comprising: the corrosion detection method described above, a step of deducing infiltration of pathogenic agent into the wall based on a calibration curve, locations of the witness(es) comprising the magnet and corrosion assessments of the ferromagnetic corrodible layer.

[0047] 47. This allows maintenance agents to estimate the time and location of preventive maintenance by resolidifying the porous coating of the wall to prevent the reinforcement from being damaged.

[0048] 48. According to this example, the method uses an inverse method to determine from a calibration curve produced by laboratory or by experiment, the infiltration state of a pathogenic agent. Indeed, a calibration curve can be produced in the laboratory which can be modified by experiment, in which a plurality of values ​​of a magnetic characteristic of controls comprising the magnet and a corrodible layer initially identical (same material and same thickness) at the same or different predetermined distances are measured in the laboratory, and having their corrodible layers according to different predetermined corrosion rates as a function of a chloride content in a structure and from Biot Savart's law to determine a magnetic moment value to produce this calibration curve by relating each magnetic moment value to the chloride content in a corresponding structure.Thus, by the inverse method, we can measure or calculate the magnetic moment of the sensor from the device and use the calibration curve produced in the laboratory to determine the chloride content of the portion of the structure comprising the witness as a function of the magnetic moment of the sensor, and therefore by the inverse approach deduce a chloride content from the measurement of the magnetic moment.

[0049] 49. The invention is therefore likely to be used more particularly for the detection of infiltration of pathogenic agent which can cause corrosion of a reinforcement in the structures of civil engineering infrastructures (buildings, dams, bridges, tunnels, swimming pools, etc.), in particular the structures of maritime infrastructures and the foundations of coastal or offshore works (dykes, jetties, wind turbines, offshore platforms, tidal dams, hydro turbines, etc.). A structure of an infrastructure can be a part of the infrastructure, for example a wall, post or even a portion thereof.

[0050] 50. The invention can be used in a preventive maintenance context since by detecting an area infiltrated by a pathogenic agent, it is possible for the maintenance agent to limit the spread of a pathology, for example by sealing the detected area or by replacing (destruction of the porous wall (for example concrete) and masonry with a new porous wall) the detected area to extend the residual life of the infrastructure and therefore limit the costs related to the maintenance of the structure. En In fact, by carrying out preventive maintenance, the structure will not be corroded or will be corroded less quickly and therefore the building will have a longer lifespan without very major work which is very expensive.

[0051] 51. According to one embodiment, the step of deducing infiltration of pathogenic agent in the wall based on a calibration curve comprises a sub-step: estimating the location of infiltrated areas in the overall pathogen marker in the coating based on the magnet locations of each witness having a ferromagnetic corroded layer, determined corrosion status value information of the different ferromagnetic layers and the time between measurements inferring corrosion of the ferromagnetic corrodible layer of the witness, deducing pathogen infiltration rates in each estimated area in the wall based on a calibration curve, the magnet locations of each witness and corrosion assessments.

[0052] 52. According to an example, the sub-step of estimating the location of the infiltrated zones also estimates an intermediate zone between two neighboring non-juxtaposed infiltrated zones and the sub-step of deducing the pathogen infiltration rate deduces an infiltration rate.

[0053] 53. According to one embodiment, the step of deducing infiltration of pathogenic agent in the wall is a function of the different corrosion estimates of one or more witnesses over time.

[0054] 54. According to one embodiment, the step of deducing pathogen infiltration in the wall is a function of the different corrosion estimates of other neighboring areas over time. This makes it possible to identify the pathogen infiltration radiation.

[0055] 55. According to one embodiment, the method comprises a step of modeling the wall comprising a porous coating and a reinforcement cast in the coating and the location of the different magnets of each witness comprising the magnets in the wall and a step of modeling and displaying the estimation of the content of an agent in the wall by displaying the estimated localized areas and their deduced infiltration rates. This makes it possible to produce a map of the infiltration of the pathogenic agent.

[0056] 56. The invention also relates to an autonomous detection device for implementing the corrosion detection method described above with or without the different characteristics of the embodiments described above, comprising the processor, the memory and the magnetic flux density measuring device and in that it comprises a device for moving the measuring device along at least two axes of the global reference frame to move it facing the auscultation surface of a wall and in that the movements of the measuring device are controlled by a control unit comprising the processor as a function of the measured values ​​of a magnetic characteristic and / or location of different acquired magnets.

[0057] 57. According to one embodiment, the apparatus is further adapted to produce the method for estimating the content of a pathogen described above.

[0058] 58. The invention and its various applications will be better understood by reading the following description and examining the accompanying figures. BREVE DESCRIPTION DES FIGURES

[0059] 59. The figures are presented for information purposes only and do not in any way limit the invention. 60. [ Fig. 1 ] shows a schematic representation of a block diagram of the corrosion detection method according to the invention. 61. [ Fig. 2 ] shows a schematic representation of a first example of the implementation of the corrosion detection method according to the invention 62. [ Fig. 3 ] shows a schematic representation of a magnetic flux measurement curve of witnesses according to a state of corrosion of their ferromagnetic corrodible layer. 63. [ Fig. 4 ] shows a schematic representation of a curve representing the variation of magnetic flux as a function of time of witnesses according to different states of corrosion and measurement distance. 64. [ Fig. 5 ] shows a schematic representation of the example of the figure 2 in a situation of pathogen infiltration. 65. [ Fig. 6 ] shows a schematic temporal representation of the relative variation of magnetic flux B% of the witnesses of the example of the figure 5 . 66. [ Fig. 7 ] shows a schematic representation of pathogen infiltration deduction from the example of the figure 2 with the crack shown from the figure 5 after a period. 67. [ Fig. 8 ] shows a schematic representation of a second example of the implementation of the corrosion detection method according to the invention. 68. [ Fig. 9 ] shows a representation of calibration curves between a corrosion rate and a value of a magnetic characteristic of two corrodible layers of different thicknesses. DESCRIPTION DETAILLEE

[0060] 69. The figure 1 shows a schematic representation of a block diagram of the corrosion detection method according to the invention.

[0061] 70. The method for detecting corrosion of a ferromagnetic corrodible layer comprises a first step E1 of acquiring and storing data in a memory including at least data from different locations Emx, from at least one permanent magnet 10 in a global three-dimensional frame of reference X, Y, Z and values ​​of a magnetic characteristic among the flux density (B) or one or more magnetic moments (M), for each magnet (10) in a three-dimensional frame of reference specific to the magnet.

[0062] 71. The figure 2 shows a schematic representation of a first example of the corrosion detection method according to the invention.

[0063] 72. The figure 2 includes the global X, Y, Z reference frame. The figure 2 represents a portion of a reinforced concrete wall 4, hereinafter called wall 4, cut along the X axis in a reinforcement 41 of the wall 4 and witnesses 1 placed in a porous coating 42 of the wall 4.

[0064] 73. The witnesses 1 are located at different depths of an auscultation surface 421. In this case, in this example, the witnesses 1 are all located between the auscultation surface 421 and the frame 41.

[0065] 74. In an example of this embodiment, the witnesses 1 of the wall 4 are identical and each witness 1 comprises a magnet 10, 11, 12 having a magnetic flux density of 1 tesla. Of course, the magnetic flux density can be less than or greater than 1 tesla. In this example, each witness 1 also comprises a corrodible layer of ferromagnetic materials 100, 110, 120.

[0066] 75. The wall 4 comprises an auscultation surface 421, in this case in the X,Y plane. The X,Y plane further allows a detection apparatus 2 explained below to represent the position of the indicators by projection onto the osculation surface 421 and the Z direction represents the depth of the indicators relative to the auscultation surface 421. The indicators 1 are, in this exemplary embodiment, located at different depths relative to the auscultation surface 421, in this case, they are spaced from each other by 10 mm and at different locations, in this case located on the same value of the X axis (for example 1 meter) and at different values ​​along the Y axis, in this case spaced from each other by 40 mm.The first witness is located at a location Em10 such that the magnet 10 has its boundary of the two poles located at +35 mm on the Y axis, its surface facing its corrodible layer at +5 mm on the Z axis, and at +1 m for the X axis, the second witness is located at a location Em11 such that the magnet 11 has its boundary of the two poles located at -5 mm on the Y axis, its surface facing its corrodible layer at +15 mm on the Z axis and at +1 m for the X axis, the third witness is located at a location Em12 such that the magnet 12 has its boundary of the two poles located at -45 mm on the Y axis, its surface facing its corrodible layer at +25 mm on the Z axis and at +1 m for the X axis.

[0067] 76. Each witness 1 is oriented in the porous coating 42 such that the layer of ferromagnetic materials 100, 110, 120 is closer to the auscultation surface 421 than the magnet 10, 11, 12.

[0068] 77. The witnesses may have a different inclination from each other relative to the auscultation surface. The apparatus may comprise a means for determining the inclination of the magnet by measuring different values ​​of a magnetic characteristic. An example of inclination is that a face comprising the north and the south is inclined relative to the Y and / or X axis, i.e. not parallel to the auscultation surface. For example, the apparatus comprises a magnetometer with a Hall probe to enable the directions and senses of the magnetic field to be measured and thus the orientation of the magnet to be determined.For example, a regular dome will represent a magnet positioned such that the face comprising north and south is perpendicular to the probe of the magnetometer, making it possible to indicate that this face of the magnet is parallel to the auscultation surface and if the dome is irregular, the device can include a corrective factor making it possible to indicate the orientation of the surface of the magnet relative to its probe.

[0069] 78. In this example, the first step E1 of acquiring and storing data in a memory of the data of different locations Em10, Em11, Em12, of the three permanent magnets 10, 11, 12 in the global three-dimensional reference frame X, Y, Z is carried out using a detection device 2 comprising a measuring apparatus 20 for measuring magnetic characteristic values ​​which is in this example the magnetic flux density B.

[0070] 79. The first step E1 comprises a sub-step E10 of searching for a signal of a magnetic flux density B by moving the measuring device 20 against the auscultation surface 421 and when a signal of a magnetic flux density B is measured, moving the measuring device 20 in a limited area until obtaining the largest value of magnetic flux density in order to identify the location of the magnet 10, 11, 12.

[0071] 80. The detection device 2 comprises a processor 21 and a memory 22, configured to record in the memory 22 the magnetic flux density and location values ​​when its measuring device 20 has measured the value of the largest magnetic flux density B.

[0072] 81. The first step E1 therefore comprises a sub-step E11 of recording the value B10, B11, B12 of the most significant signal of the limited zone as well as the location Emx when the measuring device 20 has measured the value of the most significant magnetic flux density B for each magnet 10, 11, 12.

[0073] 82. The first step E1 further comprises a sub-step E12 of deducing the distance between the magnet and the location Ea210, Ea211, Ea212 of the detection device 2 and recording the location Em10, Em11, Em12 of each permanent magnet 10, 11, 12 in the global three-dimensional reference frame X, Y, Z as a function of the location of its measuring device 20 and the deduced distance. Furthermore, this deduction sub-step E12 deduces and records at least one magnetic characteristic value of the magnet in a three-dimensional reference frame x10, y10, z10, specific to the magnet as a function of the deduced distance and the value B10, B11, B12 of the most significant signal as well as the magnetic flux density, in this case a value of 1 tesla.

[0074] 83.

[0075] 84. B r = μ 0 4 π 3 r . m r − r ∧ 2 m / r ∧ 5

[0076] 85. For example, the magnetic characteristic value "magnetic flux density B" of magnet 10 in its frame X10, Y10, Z10 is 76.5 milli-teslas at the location X= +1000 mm, Y= +35 mm, Z= 0 mm. The magnetic characteristic value can be the magnetic flux density B or magnetic moments ( M ) or the intensity of the magnetic field H at a given point.

[0077] 86. The figure 3 shows a schematic representation of a measurement curve of the magnetic flux density B of the magnets 10, 11, 12 of each witness 1 according to a state of corrosion of their corrodible layer 100, 110, 120.

[0078] 87. This measurement curve may, for example, be a calibration curve produced in the laboratory which may be modified by experiment, in which a plurality of values ​​of a magnetic characteristic of witnesses are measured in the laboratory, each comprising an identical magnet and a corrodible layer which are initially identical (same material and same thickness) at the same or different predetermined distances to calculate, from the Biot Savart law explained below, a magnetic moment of each witness, in which the different measurements are carried out by having their corrodible layers according to different predetermined corrosion rates which may be determined as a function of a chloride content in a structure by experiment, by relating each estimated magnetic moment value to the chloride content in a corresponding structure.

[0079] 88. It can be seen in this figure that when the corrodible layer of the witnesses 1 is not corroded, the detection device 2 measures by its measuring device 20 a density of the magnetic flux B10 of approximately 24.38mT at the location Ea210 opposite for the first witness, of approximately 5mT at the location Ea211 opposite the second witness, of approximately 1.49mT at the location Ea212 opposite the third witness.

[0080] 89. The figure 9 represents two calibration curves of two fictitious witnesses produced in the laboratory, in a reference frame representing on the ordinate a value of a magnetic characteristic, here an arbitrary unit AU of which 1 AU equals 0.05mT and on the abscissa the relative mass loss of the corrodible layer of the witness corresponding here to the corrosion rate. In this example, the same cylindrical magnet of 8 mm diameter and 2 mm thickness was placed under two series of corrodible layers at different corrosion rates to carry out the magnetic measurements. Each corrodible layer of a series was identical to the initial state. In this case the corrodible layers in the initial state (not corroded) of a first series had a thickness of 0.5mm and those of the second series had a thickness of 1mm. In this example, each corrodible layer is a right square prism with square faces of dimension 15 x 15 mm.In particular, measurements with the magnet and the magnetometer device with the distance of 3 cm were carried out on each corrodible layer to ensure that each corrodible layer of a series gives an identical signal to the other corrodible layers of the same series within the measurement errors. The corrodible layers were then placed in a 30 gL-1 NaCl solution to accelerate their corrosion, for example by electrolysis in a solution of 30 g of NaCl per liter of water, weighed and measured their values ​​of a magnetic characteristic AU by a magnetometer sensor placed 3 cm from the corrodible surface of the ferromagnetic layer opposite the magnet.

[0081] 90. We can see on this figure 9 that the thicker the corrodible layer, the smaller the value of a measured magnetic characteristic for the same corrosion rate, but that they vary substantially identically in relation to their corrosion rate. Thus, according to an example having witnesses with different corrodible layer thicknesses, the witnesses with a thicker corrodible layer will be closer to the monitoring surface and those with a smaller thickness closer to the reinforcement, thus making it easier to use the same measurement range of a measuring device.

[0082] 91. In this example, the measurement of mass loss relative to the corrodible layer in the initial state in percentage corresponds to the different corrosion rates in percentage. The corrosion rate can be measured differently, for example by geometric dimensioning.

[0083] 92. The measuring distance between the measuring device 20 and the magnet therefore has a significant influence on the measured value. Indeed, it can be seen that for a 1 cm difference in depth between the first and second witnesses, the measurement is divided in this example by 5 for a non-corroded layer and that for a 1 cm difference in depth between the second and third witnesses, the measurement is divided in this example by 3.33 for a non-corroded layer.

[0084] 93. Furthermore, it can be seen that the greater the measurement distance between the measuring device 20, the less the corrosion of the corrodible layer 100, 110, 120 influences the measured magnetic flux density B. Indeed, for a witness having its layer corroded at 100% in this example, the third witness having a measurement distance of 25mm has a variation of 19% while the first witness also having its corrodible layer corroded at 100% has a variation of 214% or 2.14 times the measurement carried out at time T0.

[0085] 94. The measuring device 20 may be modified or may have different configurations or calibrations to adapt to the sensitivity of the magnetic fields to be measured.

[0086] 95. Thus, according to an example, thanks to the knowledge of a magnetic flux density of 1 tesla, and the Biot-Savart law (deduced from Maxwell's equations): B r = μ 0 4 π 3 r . m r − r 2 m r 5

[0087] 96. Where r = the predetermined distance and B(r) the measured flux density and µ 0 is the magnetic permeability of the vacuum and is equal to: − μ 0 ≃ 4 π ⋅ 10 − 7 SI

[0088] 97. The detection device 2 with its processor 21 is configured, by having a program, for example, to calculate and estimate or deduce therefrom the value of the moment m 11 z< ,m 12 z< , m 13 z< according to its own three-dimensional reference frame x, y and z at a location Ea210, Ea211, Ea212, which is recorded in a memory. The recorded location may be that acquired in step E1 by determining the location where the value of a measured magnetic characteristic best corresponds to the location opposite a witness. This estimation of the value of the magnetic moment m 11 z< ,m 12 z< , m 13 z< is therefore carried out from the Biot Savard law and the measured magnetic flux density value B and the distance r deduced during the distance deduction sub-step r E12 of step E1.

[0089] 98. The detection device 2 therefore stores in memory a value of a magnetic characteristic acquired in step E1 of each magnet 10, 11, 12 according to their reference at a predetermined location which may be the origin.

[0090] 99. The method further comprises a step of determining a location of the measuring device 20 in the global three-dimensional reference frame by the processor 21.

[0091] 100. For example, the determined location may be the predetermined location or another recorded location Ea210, Ea211, Ea212 if the auscultation surface has not changed, otherwise it may determine another one. In particular, the detection device 2 may comprise a human-machine interface to guide the user to move the device to the recorded location and furthermore, for example, a button, so that the user can give the information that the location Ea210 or Ea211 or Ea212 is no longer accessible (for example, swelling of the wall 4 due to freezing / thawing), in this case the device determines another location which may, for example, be the one closest to the location Ea210 or Ea211 or Ea212.

[0092] 101. The method further comprises a step E3 of measuring a value of a magnetic characteristic, here a magnetic flux density B, by means of the measuring device 20 at the determined location, in this case at each of the locations Ea210, Ea211, Ea212, and recording in a database the data concerning the measurement, the location and the date of the measurement.

[0093] 102. The method may comprise a plurality of location determination steps E2 and measurement steps E3 at different locations within a limited area and in that the estimation of the characteristic value is refined at each measurement of the magnetic flux density (B) of a location. This may be the case, for example, if the auscultation surface is modified or if a magnetic field disturbance agent requires different measures to reduce the risk of estimation error E4 described below. Furthermore, these repeated steps may also be carried out at the beginning during the data acquisition step E1 explained above.

[0094] 103. For example, the corrodible layer 100, 110, 120 of each witness 1 has undergone corrosion due to infiltration of a pathogenic agent, for example ions from salt water.

[0095] 104. The corrodible layer 100, 110, 120 of each witness 1 has a thickness of 1 mm and is made of iron, at time T0 when step E1 has been carried out. The corrodible layer of the witness may be made of materials other than iron, such as nickel and cobalt or other alloys.

[0096] 105. The figure 4 represents a curve representing the relative variation of magnetic flux B% as a function of time of the three witnesses according to different states of corrosion and distance, for example T1 is 2 years after T0, T2 is 4 years after T0 and T3 6 years after T0. In this example, the value of a magnetic characteristic is a percentage variation of a flux density (B) or of one or more magnetic moments (M) at a point in the x,y,z frame specific to the magnet 10, 11, 12. The frame specific to the magnet can also be the global frame.

[0097] 106. On the figure 3 , it can be seen that the more the corrodible layer 100, 110, 120 is corroded, the greater the magnetic field density B measured at the location Ea210, Ea211, Ea212, by the measuring device 20.

[0098] 107. For example, at T2, each corrodible layer is corroded by 2 / 3, that is to say that the corrodible layer 100, 110, 120 in the form of an iron plate with a thickness of 1 mm now comprises only 33% of its initial thickness, i.e. in this example 0.3 mm of non-corroded iron.

[0099] 108. The curves of the figures 3 And 4 may for example be calibration curves allowing the detection device 2, using a program in the memory 22 using the processor 21, to correlate the variation in corrosion of a type of indicator with its magnetic field as a function of the measurement distance by the inverse method.

[0100] 109. The method comprises an estimation step E4, carried out by the detection device 2 using a program in the memory 22 and the processor 21, of at least the value of a magnetic characteristic, of at least one magnet 10, 11, 12 according to its own three-dimensional reference frame, as a function of the magnetic flux density B measured in step E3 and the location Em10, Em11, Em, 12 of the magnet acquired in step E1 at the location Ea210, Ea211, Ea212 of the measuring device 20. Thus, by modifying the location of the measuring device 20 with respect to that memorized Ea210, Ea211, Ea212 there will be a significant influence, even for 1 mm. The method makes it possible from this new location to identify the value of a magnetic characteristic (B, M) of at least one magnet according to its own three-dimensional reference frame. The first location Ea210, Ea211, Ea212 of the measuring device 20 for the measurement in step 10 may be inaccessible for various reasons, for example an expansion of the wall linked to the infiltration of agent, or even heat, a layer added to the wall, for example linked to a repair, etc.

[0101] 110. The method therefore comprises a step E5 of comparing the value of a magnetic characteristic (B, M ) of at least one magnet, estimated at this instant in step E4 and that previously estimated at an instant prior to step E4 or acquired in step E10 of this same magnet. By "previously estimated magnetic characteristic" is meant here a comparison of the value of a magnetic characteristic (B, M ) of at least one magnet deduced with the measurement carried out at an instant T, for example at time T2 and value of a magnetic characteristic (B, M) of at least one magnet at an instant deduced with the measurement carried out at a previous instant for example T-1, in this example at instant T1.

[0102] 111. Finally, the method comprises a step E6 of deducing corrosion of the corrodible layer 100, 110, 120 between the at least one location of the magnet and the location of the measuring device 20 as a function of the comparison of the value of a magnetic characteristic in step e) and that acquired in step a).

[0103] 112. Indeed, if the value of a magnetic characteristic increases, corrosion can be deduced.

[0104] 113. The method may further comprise in the memory a calibration curve of the value of a characteristic of the magnet 10, 11, 12 with respect to the corrosion of the corrodible layer 100, 110, 120 of the witness 1 and in that the step E6 of deducing a corrosion of the corrodible layer 100, 110, 120 is produced by the processor which uses this curve to deduce therefrom a corrosion state value of the corrodible layer 100, 110, 120. For example, the corrosion calibration curve is such that that of the figure 3 , to estimate the percentage of corrosion of the corrodible layer 100, 110, 120. The method can therefore use an inverse method to determine from a calibration curve, produced by laboratory or by experiment, a corrosion rate of the corrodible layer linked to a magnetic moment value deduced from the measurement of the value of a magnetic characteristic.

[0105] 114. The figure 5 schematically represents an example of a situation of the wall in which a crack 43 has formed allowing infiltration of a pathogenic agent represented by arrows, such as sodium chloride has become embedded in the wall with a deformation 4210 of the auscultation surface 421.

[0106] 115. Measurements are taken at T1 2 years, T2, and T3 after the time T0 in which the data acquisition step 10 was carried out.

[0107] 116. Due to the deformation of the auscultation surface 421 during the crack 43, the method comprises in the determination step E2, the determination of the location Ea210', Ea211', (the location Ea212 being able to be exploitable) of the measuring device of a value of a magnetic characteristic may comprise a sub-step of indicating the location, for example by indicating by visible signals, the direction to take to position the measuring device 20 at the determined location Ea210', Ea211'. The origin of the global reference point, to locate the measuring device and the magnets in space, may be a physical point, for example visible on an outlet of the wall of one end of a stake of the reinforcement 41. The determination of the reference point may be manual at each instant and recorded in the memory by the processor.The determination of the location Ea210' can also be carried out manually by being imposed by the operator, and the processor determines the location Ea210', Ea211' according to the origin of the global reference frame. The determination of the location Ea210' can be carried out by the processor using information from a sensor of the detection device 2, of the radar type which can be infrared, or any other type of sensor to enable the identification of a precise location relative to an origin of a reference frame.

[0108] 117. In addition, the device may include software to indicate the angular orientation relative to the witness or correct the measurement relative to this angular orientation. Indeed, the auscultation surface may, due to its deformation, prevent it from having the same orientation as during the first acquisition or the old acquisition and thus give an incorrect measurement by having the device oriented differently relative to the magnet than during its old measurements.

[0109] 118. The figure 6 shows a schematic temporal representation of the relative variation of the flux density (B%) or one or more magnetic moments (M), witnesses of the example of the figure 5 at different periods T1, T2, T3 compared to T0.

[0110] 119. As can be seen, at time T1, the infiltration of pathogenic agent caused corrosion of the corrodible layer 110 of the second witness since there was a determination of a variation in magnetic flux of approximately 5% compared to time T0 which, for example, according to the calibration curve of the second witness, may correspond to a corrosion state of 40%. The other two deductions of values ​​of a magnetic characteristic of the other two witnesses remained stable at time T1. At this time T1, the device can deduce that an infiltration of pathogenic agent in the coating has passed in front of or behind the first witness but that there is little risk that it has reached the depth of the reinforcement 41.

[0111] 120. At time T2, the infiltration of pathogenic agent caused corrosion of the corrodible layer 100 of the first witness since a variation in magnetic flux of approximately 20% is determined, i.e. corrosion having a corrosion state of approximately 33% according to the reference curve of the figure 3 and the corrodible layer 110 of the second witness since a variation in magnetic flux of approximately 12% is determined, i.e. corrosion for example having a corrosion state of approximately 70%.

[0112] 121. From time T1, the detection device 2 can be configured to perform a method of estimating the content of a pathogen in the wall 4 in the porous packaging further comprising, from the corrosion detection method, a step of deducing infiltration of pathogen E7 in the wall as a function of a calibration curve, locations of the witness(es) and corrosion assessments, the time between measurements between measurements deducing corrosion of the corrodible layer of the witness. The step of deducing infiltration of pathogen E7 in the wall 4 is a function of the different corrosion estimates of one or more witnesses 1 over time.

[0113] 122. The method uses an inverse method to determine from a calibration curve not shown, produced by laboratory or by experiment, the state of infiltration of a pathogenic agent. Indeed, a calibration curve can be produced in the laboratory which can be modified by experiment, in which a plurality of values ​​of a magnetic characteristic of controls comprising the magnet and a corrodible layer initially identical (same material and same thickness) are measured in the laboratory at the same or different predetermined distances, and having their corrodible layers according to different predetermined corrosion rates as a function of a chloride content in a structure and from Biot Savart's law to determine a magnetic moment value to produce this calibration curve by relating each magnetic moment value to the chloride content in a corresponding structure or even use the calibration curve of the figure 3 (magnetic moment, distance and corrosion rate) or that of the figure 9 (different corrosion rates, value of a magnetic characteristic at a predetermined distance and different initial thicknesses), with another calibration curve including the corrosion rate and a chloride content not shown. Thus, by the inverse method, the magnetic moment of the sensor can be measured or calculated from the device and the calibration curve produced in the laboratory can be used to determine the chloride content of the portion of the structure including the witness as a function of the magnetic moment of the sensor, and therefore, by the inverse approach, a chloride content can be deduced from the measurement of the magnetic moment.

[0114] 123. The step of deducing pathogen infiltration E7 includes a first sub-step of estimating the location of the infiltrated zones E70 in the global reference by pathogens in the coating based on the determined corrosion state values ​​of the different layers and the time between measurements.

[0115] 124. The step of estimating the content of a pathogen E7 includes a second sub-step of deducing the infiltration rate of pathogen E71 in each area estimated in particular by the inverse method with a calibration curve as described previously.

[0116] 125. For example, at time T1, the detection device is configured to estimate the location of an area infiltrated by a pathogenic agent around the magnet 11 of the second witness due to its corrodible layer 110 having a corrosion state of 40% deduced in step E6 and a deduction of pathogenic agent infiltration rate linked to this state.

[0117] 126. The figure 7 represents a graphical representation of estimation of the content of a pathogenic agent in the wall 4 at time T2, in which the detection device 2 is configured to estimate a first zone 44 around the magnet 10 comprising a certain size and a second zone 46 around the magnet 11 comprising a size greater than that of the first zone 44 since at time T1 a first zone had already been located and the deduction of the corrosion state value of the corrodible layer 110 continued to increase.

[0118] 127. Furthermore, the detection device 2 is configured to further estimate an intermediate zone 45 between the two neighboring infiltrated zones 44, 46 which is not juxtaposed and deduces therefrom a pathogen infiltration rate having a value between the two rates of the two other zones 44, 46.

[0119] 128. The detection device 2 can further be configured to model the wall 4 with the porous coating 42, the reinforcement 41, the location of the different indicators or magnets 10, 11, 12 in the wall 4 and a step of modeling and displaying the estimation of the content of an agent in the wall by displaying the estimated localized zones and their deduced infiltration rate, as shown in the figure 4 .

[0120] 129. The figure 8 represents a schematic representation of a second example of implementation of the corrosion detection method according to the invention in which the magnet 10 can be moved over a surface of a corrodible layer 90 as well as the measuring device 20 can be moved over an opposite surface of the corrodible layer 90.

[0121] 130. In this example, the corrodible layer 90 is a part of a bodywork 9 in which the magnet 10 is inserted through an opening 91. Thus the corrosion detection method makes it possible to detect the corrosion of a corrodible layer 90 whose surface is difficult to access.

[0122] 131. According to another example not shown, it is the measuring device 20 which is inside the bodywork and the magnet 10 which is outside on the corrodible layer 9.

[0123] 132. In this example, the thickness 92 or the different thicknesses of the corrodible layer 90 is / are known and the step E1 of acquiring and storing data in a memory of which at least data from different locations can correspond to a plurality of locations Em20 of the magnet 10 on the corrodible layer of which the measurement distance can change for example according to different thicknesses 92 of the corrodible layer 90. The value of a measured magnetic characteristic of the magnet 10 can correspond to the flux (B) or one or more magnetic moments (M) of the magnet 10 in its reference frame. This step can be carried out on the bodywork not comprising a corroded layer.

[0124] 133. In this example, the corrodible layer 9 comprises a corroded area 900 and a dented portion 901 also comprising a corroded area 902.

[0125] 134. The detection device 2 is also configured to carry out the step E2 of determining a location Ea20, Ea21 of the measuring device 20 of a magnetic characteristic value in its overall three-dimensional reference frame X, Y, Z relative to the location of the magnet Em20, Em21 or may also be based on previous measurements having deduced corrosion or a suspicion of corrosion by the user, for example at the dented portion 901.

[0126] 135. The detection device 2 is configured to carry out, as in the first example, the step E3 of measuring a magnetic flux density B and recording in a database the measurement, the location Ea20, Em20 of the measuring device 20 and of the magnet 10' and the date of the measurement and the step E4 of estimating by the processor at least the value of a magnetic characteristic (B%, M), of the magnet 10' according to its own three-dimensional reference frame x, y and z and comparison E5 of its value of a magnetic characteristic (B, M ) estimated and that previously estimated or acquired in step E1.

[0127] 136. Finally, the detection device 2 is configured to carry out the step E6 of deducing the corrosion of the corrodible layer 9 between the at least one location of the magnet Em20, Em21 and the location Ea20, Ea21 of the measuring device 20 based on the comparison of the value of a magnetic characteristic acquired in step E1.

[0128] 137. For example, the detection device 2 is configured to infer corrosion between the location of the magnet Em20, and the location of the device Ea20.

[0129] 138. The method may also comprise a step of estimating the orientation of the magnet 10'. For example, in the dented portion 901, the magnet 10' or the measuring device 20 can hardly be placed opposite each other. For example, as in the figure 8 , at the location of the magnet Em21, the magnet 10' can be inclined relative to the measuring device 20 at the location Ea21.

[0130] 139. Then the detection device 2 can be configured to take this orientation into account in the location determination step E2 and thus further determine the inclination of its measuring device 20 to be opposite the two poles of the magnet 10' in a balanced manner. The detection device 2 can be configured to in the estimation step E4 by the processor of at least the value of a magnetic characteristic (B%, M ) that it is also a function of the orientation of the magnet 10' relative to the measuring device 20.

[0131] 140. Unless otherwise specified, the same element appearing in different figures has a single reference.

Claims

1. Method for detecting corrosion of a ferromagnetic corrodible layer (100, 110, 120, 90), comprising: a) A step (E1) of acquiring and storing data in a memory, including at least: i) Data on the different locations (Em10, Em11, Em12, Em20, Em21) of at least one permanent magnet (10, 11, 12, 10') in a global three-dimensional reference frame (X, Y, Z), ii) Data on the value of a magnetic characteristic for each magnet (10, 11, 12, 10') in a three-dimensional reference frame specific to the magnet (x, y, z), b) A step (E2) of determining the location (Ea210, Ea211, Ea212, Ea20, Ea21) of a measuring device (20) for a magnetic characteristic value, distant from the at least one magnet, in the global three-dimensional reference frame (X, Y, Z), by a processor (21) of a detection device (2), c) A step (E3) of measuring a magnetic characteristic value (B) using the measuring device (20) at the location (Ea210, Ea211, Ea212, Ea20, Ea21) determined in step (E2), and recording in a database the measured value, the location of the device (Ea210, Ea211, Ea212, Ea20, Ea21), and the date of the measurement, d) A step (E4) of estimating, by the processor (21), a magnetic characteristic value (B%, B, M) of at least one magnet (10, 11, 12, 10') according to its own three-dimensional reference frame (x, y, z), based on the value measured in step (E3) and the location (Em10, Em11, Em12, Em20, Em21) of the magnet (10, 11, 12, 10') acquired in step (E1), e) A step (E5) of comparing the estimated magnetic characteristic value (B%, B, Mof at least one magnet (10, 11, 12, 10') with a previously estimated or acquired value for this magnet, f) A step (E6) of deducing corrosion of the ferromagnetic corrodible layer (100, 110, 120, 90) between the location (Em10, Em11, Em12, Em20, Em21) of the magnet (10, 11, 12, 10') and the location of the device (Ea210, Ea211, Ea212, Ea20, Ea21), based on the comparison of magnetic characteristic values performed in step (E5).

2. Method for detecting corrosion of a ferromagnetic corrodible layer (100, 110, 120, 90) according to claim 1, wherein each magnet (10, 11, 12, 10') is attached to a ferromagnetic metallic element forming the corrodible layer (100, 110, 120, 90), and wherein the step (E1) of acquiring and storing data further includes data on magnetic characteristic values (B, M) as a function of the thickness of the non-corroded metallic element, with the components of the three-dimensional reference frame specific to the magnet (x, y, z).

3. Method for detecting corrosion of a ferromagnetic corrodible layer (100, 110, 120, 90) according to claim 2, wherein the memory includes a calibration curve of the magnetic characteristic value of the magnet (10, 11, 12, 10') relative to the corrosion of the metallic element attached to the magnet (10, 11, 12, 10'), and wherein the step (E6) of deducing corrosion of the ferromagnetic corrodible layer (100, 110, 120, 90) uses this curve through an inverse method to deduce a corrosion state value of the ferromagnetic corrodible layer (100, 110, 120, 90).

4. Method for detecting corrosion of a ferromagnetic corrodible layer (100, 110, 120, 90) according to any of the preceding claims, further comprising multiple steps of determining locations (E2) and measuring (E3) at different locations (Ea1, Ea2) within a limited area (Z), wherein the estimation of the characteristic value is refined with each measurement of magnetic flux density (B) at a location (Ea1, Ea2).

5. Method for detecting corrosion of a ferromagnetic corrodible layer (100, 110, 120, 90) according to any of the preceding claims, wherein the step (E1) of acquiring and storing data further includes data on the orientation of the poles of the magnets relative to the origin point of the global reference frame.

6. Method for detecting corrosion of a ferromagnetic corrodible layer (100, 110, 120) according to any of the preceding claims, wherein a corrosion indicator (1) is located in a porous coating (42) of a wall (4), particularly made of reinforced concrete, the corrosion indicator (1) comprising one of the magnets (10, 11, 12) and a ferromagnetic corrodible layer (100, 110, 120) secured to the magnet (10, 11, 12), and wherein the step (E6) of deducing corrosion of a ferromagnetic corrodible layer pertains to the indicator (1).

7. Method for detecting corrosion of a ferromagnetic corrodible layer (100, 110, 120, 90) according to any of the preceding claims, wherein the step of acquiring location data (Em10, Em11, Em12, Em20, Em21) and the magnets (10, 11, 12) in the global reference frame (X, Y, Z) includes: a) A sub-step (E10) of searching for a magnetic flux density signal (B) using the measuring device (2) and measuring the highest signal value (B10, B11, B12) within a limited area, b) A sub-step (E11) of recording the location (Ea2) of the measuring device (20) in the global three-dimensional reference frame (X, Y, Z) by a processor (21) in a memory (22) when the measuring device (20) has measured the highest signal value (B10, B11, B12), c) A sub-step (E12) of deducing the distance between the magnet (10, 11, 12) and the location (Ea) and recording the location (Em10, Em11, Em12) of each permanent magnet (10, 11, 12) in the global three-dimensional reference frame (X, Y, Z).

8. Method for estimating the content of a pathogenic agent in a wall (4) comprising a porous coating (42), including the corrosion detection method according to any of claims 1 to 7, and a step (E7) of deducing infiltration of a pathogenic agent in the wall based on a calibration curve, the locations (Em10, Em11, Em12, Em20, Em21) of the magnet (10, 11, 12) of each indicator (1) comprising the magnet (10, 11, 12) and corrosion evaluations of the ferromagnetic corrodible layer (100, 110, 120).

9. Method for evaluating pathogen infiltration in a wall, including the method for estimating the content of a pathogenic agent in a wall (4) according to claim 8, wherein the step (E7) of deducing pathogen infiltration in the wall (4) is based on the different corrosion estimates of one or more indicators over time.

10. Method for evaluating pathogen infiltration in a wall according to claim 9, wherein the step (E7) of deducing pathogen infiltration includes: a) A sub-step (E70) of estimating the location of infiltrated zones in the global pathogen agent reference frame within the coating, based on the locations (Em10, Em11, Em12, Em20, Em21) of the magnet (10, 11, 12) of each indicator (1) having a corroded ferromagnetic layer, determined corrosion state value information of the different ferromagnetic corrodible layers, and the time between measurements deducing corrosion of the ferromagnetic corrodible layer of the indicator (1), b) A sub-step (E71) of deducing the pathogen infiltration rate in each estimated area in the wall based on a calibration curve, the locations (Em10, Em11, Em12, Em20, Em21) of the magnet (10, 11, 12) of each indicator (1), and corrosion assessments.

11. Method for evaluating pathogen infiltration in a wall according to the preceding claim, including a step of modeling the wall (4) comprising a porous coating (42) and a reinforcement (41) cast in the coating (42), the location of the different indicators (1) comprising the magnets (10, 11, 12) in the wall (4), and a step of modeling an evaluation of pathogen infiltration in the wall (4) based on the step (E7) of deducing pathogen infiltration.

12. Detection apparatus (2) for implementing the corrosion detection method according to any of claims 1 to 7, comprising the processor (21), the memory (22), and the flux density measuring device (20).

13. Detection apparatus (2) according to the preceding claim, further comprising a device for moving the measuring apparatus along at least two axes of the global reference frame (X, Y, Z) to move it facing an auscultation surface (421) of a wall (4), wherein the movements of the measuring apparatus (20) are controlled by a control unit comprising the processor (21) based on the values of magnetic flux density measurements (B) and / or the location (Em) of different magnets acquired.