EP2250481A1

EP2250481A1 - Corrosion detection device and method

Info

Publication number: EP2250481A1
Application number: EP08716317A
Authority: EP
Inventors: Christoph Paulitsch
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2008-03-06
Filing date: 2008-03-06
Publication date: 2010-11-17
Also published as: WO2009109203A1

Abstract

The invention relates to a corrosion detection device and method. The device according to the invention comprises an oscillating body (21, 22, 23) the mechanical oscillation properties of which are variable depending on corrosion. An actuator (21...23, 34) sets the oscillating body oscillating, the oscillations being detected by a sensor (21...23, 34) and being converted to a corresponding oscillation signal. An evaluation unit (37) detects corrosion depending on the oscillation signal. The device is characterized by the compactness of the sensor element (20). The method according to the invention is not susceptible to frictional electricity, temperature variations and interference potentials.

Description

description

Apparatus and method for corrosion detection

The invention relates to a device for corrosion detection and a method for operating such a device.

Such a device may, for example, be used to detect corrosion on a pipe or vessel wall which is in contact with an aggressive medium and therefore is to be monitored for corrosion.

Such a device for corrosion detection (corrosion sensor) is z. B. by products such as those offered by the company Pepperl & Fuchs under the brand CorrTran known. Such corrosion sensors or shortly sensors are used in contact with the process medium and comprise three similar potential-measuring electrodes. Corrosion on these electrodes is typically determined by the methods of resistance method, electrochemical noise analysis (EN) or linear polarization resistance measurement (LPR).

The resistance method is simple, robust and a single electrode is sufficient. The resistance of a loop-shaped electrode is measured, which changes in the event of corrosion due to the changed geometry of the electrode. Because of the necessary change in geometry, corrosion detection is only possible with strong corrosion of the electrodes. Thus, only larger amounts of corrosion can be detected. A resistance change due to temperature change is compensated by using a second similar electrode which is not exposed to the medium.

The electrochemical noise analysis (EN) is sensitive to electromagnetic interference and requires a complicated evaluation. Even low occurrence of corrosion can be detected. The electrodes must have not been greatly altered by corrosion, since already the electron exchange between the electrode surface and the process medium is detected.

In the LPR method, the electrodes are polarized and the current flowing is registered. After compensation of the electrode resistance, the corrosion current density is measured from the polarization resistance. Due to low corrosion current densities, this process is also sensitive to electromagnetic interference.

Accordingly, it is an object of the present invention to specify an apparatus and a method for corrosion detection in which the abovementioned disadvantages are avoided or at least their effects are reduced.

To solve this problem, the new device for corrosion detection of the type mentioned in the features specified in claim 1. In the further claims, advantageous embodiments of the device and a method for operating such a device are described.

The invention has the advantage that with such a device, the change in resistance of an electrode is not detected as a measure of any corrosion but a change in the mechanical vibration properties of a vibrating body, which occurs due to corrosion and therefore can be advantageously evaluated for their detection. Such corrosion detection is significantly more sensitive than methods and devices based on a pure resistance change. Thus, the dynamic mechanical behavior of a vibrating body immersed in an aggressive medium is used to monitor another component exposed to the aggressive medium for corrosion. By monitoring the position of the natural frequency of the oscillating body, a corrosion sensor can be constructed which outputs a signal proportional to the corrosion. Compared to using the resistance method, only low contact Lengths of the corroding sensor element with the aggressive medium necessary. In comparison with other known methods already described above, the size of the sensor can be chosen to be much smaller. In addition, the new device has the advantage that it is insensitive to static charge, temperature changes and potential interference, since only dynamic signals are evaluated. Electrical disturbance variables, such as alternating fields or electromagnetic fields, also do not interfere in an advantageous manner, since not the smallest electrical currents are used for the evaluation, but mechanical variables which are hardly influenced by such fields.

In a preferred embodiment, the actuator can be controlled by the evaluation device with a signal, and the evaluation device detects corrosion as a function of the drive signal and the oscillation signal. Thus, the natural frequency of the oscillating body can be determined by monitoring the phase position of the oscillation signal in comparison to the drive signal or by monitoring the amplitude of the oscillation signal in comparison to the drive signal in certain frequency ranges.

Preferably, the oscillating body or a part thereof consists of the same material as the component to be monitored for corrosion, for example a wall of a pipeline or a container. As a result, a better reliability of the diagnostic statement is achieved, since both the sensor element and the element to be monitored have identical corrosion properties.

An embodiment of the device in which the oscillating body is arranged substantially at a distance from a wall which corresponds to the width of a crevice corrosion to be detected has the advantage that, in addition to the detection of surface corrosion, it additionally permits monitoring for crevice corrosion. This form of corrosion occurs, for example, on metal parts in the presence of a corrosive sive medium in unfinished bearing gaps such as overlaps, fitted webs and welds that are not welded through. Although theoretically it could be prevented by constructively dispensing with gaps or by making them so large that crevice corrosion does not occur. In practice, however, this is not always possible, so that monitoring for crevice corrosion is useful in many cases.

An embodiment of the oscillating body with an electrode which is part of an electrostatic actuator with a second actuator electrode arranged at a short distance from the oscillating body has the advantage that the actuator and the sensor can be combined in one component. Separate sensors for receiving a vibration signal can thus be omitted. Alternatively, of course, additional electrostatic transducers can be provided as sensors. As a further alternative, it is possible, for example, to use a piezoelectric actuator for exciting the oscillating body, which likewise can be operated in a corresponding manner as a so-called sense-sensing actuator.

In an advantageous embodiment, the oscillating body is designed in the manner of a cantilever clamped on both sides. This ensures that the change in the natural frequency of the vibrating body is directly proportional to the corrosion and thus that a corrosion sensor with a linear characteristic curve by monitoring the position of the natural frequency is available.

In addition to the detection and evaluation of a mechanical vibration behavior means for measuring the electrical resistance of the bending beam can be provided. Advantageously, the product of the mechanical resonance frequency of the bending beam and the square of the electrical resistance can be used as a parameter for the detection of corrosion, since such a parameter is independent of the length of the bending beam and caused by this influence no errors.

If the device is designed as a transmitter and provided with a communication interface, via which a signal for corrosion message to a control station in an automation system can be output, the device is advantageously in the manner of a transmitter for process instrumentation for automated monitoring of the process plant ,

Reference to the drawings, in which a Ausfϋhrungsbeispiel of the invention is shown, the invention and refinements and advantages are explained in more detail below.

Show it:

1 shows a pipeline with a transmitter for corrosion detection,

Figure 2 is a corrosion-sensitive sensor element and

Figure 3 shows a basic structure of a device for corrosion detection.

In a process-technical plant, not further shown in FIG. 1, a pipeline 1 flows through a corrosive medium 2. A measuring transducer 3 for detecting corrosion of the pipeline 1 is attached to it in such a way that a corrosion-sensitive sensor element is in contact with the medium 2. A communication interface 4 serves to output ascertained values of the corrosion to a superordinate control station 5 in an automation system which is equipped, for example, with a field bus 6 as part of an automation network. When a limit value for a permissible corrosion is exceeded, an alarm message is additionally output via the fieldbus 6. On fieldbus 6 There are further transducers and actuators, for example a temperature transmitter 7.

A corrosion-sensitive sensor element 20 according to FIG. 2 can be embodied, for example, as a microelectromechanical sensor in a MEMS implementation. In principle, any shapes of vibrating bodies are conceivable, for example a simple plate which consists of the same material as the tube wall and is excited to oscillate perpendicular to the plate plane.

In the exemplary embodiment of a sensor element 20 shown in FIG. 2, three beams 21, 22 and 23, which are clamped on both sides, serve as vibrating bodies and are excited to bending oscillations perpendicular to the plane of the drawing. To produce the bending beams 21 to 23, four slots 24, 25, 26 and 27 are introduced, for example by an etching process, into a silicon carrier of the sensor element 20. On their upper sides visible in the illustration, a thick layer of the material, for example by sputtering, is applied to the bending beams 21, 22 and 23, from which the wall of the pipe 1 (FIG. 1) is made. Alternatively, the entire beam can be etched out of a previously applied layer of the material so that the bending beam as a whole consists of the material. With the aid of electrical lines 211 and 212 of the bending beam 21 can be connected to a later explained in more detail evaluation. Correspondingly, electrical lines 221 and 222 for the bending beam 22 and electrical lines 231 and 232 for the bending beam 23 are provided. The electrical lines can be produced on the sensor element 20, for example by doping the silicon carrier material.

As shown in Figure 3, the sensor element 20 is inserted like a lid in a cup-shaped housing 31, in the interior 32 in the operation of the corrosion sensor corrosive medium 2 (Figure 1) can flow. On a dome-shaped elevation 33 in the central region of the housing 31 is a Electrode 34 is arranged, which is protected by a protective coating 35 from corrosion. The electrode 34 forms part of an electrostatic actuator, the other part of which represent the bending beams 21, 22 and 23 (FIG. 2), which are located substantially above the dome-shaped elevation 33. Alternatively, the electrostatic actuator can be accommodated in a chamber separated from the corrosive medium. The distance between sensor element 20 and protective coating 35 is dimensioned such that they do not touch during vibrations. The distance between sensor element 20 and an inner surface 36 of housing 31 corresponds to the gap width of the crevice corrosion to be detected. If a crevice corrosion with different gap widths is to be detected, this can be realized as an alternative to the exemplary embodiment shown with a housing shape which has a stepped cup bottom. An evaluation device 37 is electrically connected by lines 38 to the sensor element 20 and by a line 39 to the electrode 34. The evaluation device 37 supplies control signals for excitation of oscillations in the sensor element 20 and evaluates the self-adjusting electrical signals as vibration signals for determining the vibration behavior of the sensor element 20 at the same time or during subsequent decay processes. Since the vibration behavior of the sensor element 20 changes as a function of the corrosion of the sputtered material, a characteristic value can be determined which characterizes the progress of the corrosion. If this exceeds a predetermined threshold value, an alarm message is output via a fieldbus 40.

The excitation of the sensor element 20 to vibrations can be carried out continuously or at regular time intervals. Since corrosion is slow to change compared to other monitoring quantities, monitoring at longer intervals is recommended to avoid unnecessary power consumption. In the exemplary embodiment shown, an electrostatic actuator is operated as a self-sensing actuator. Alternatively, of course, sensors for detecting vibrations can be formed separately from the actuator.

The evaluation of the vibration signal (s) is based on the change of the dynamic, mechanical behavior of the vibration body (s). For example, the height of the natural frequency of the vibrating body changes with the size of the vibrating body, which is reduced by corrosion. In the case of a cantilever beam, the natural frequency of the fundamental is given by the formula

where E is the modulus of elasticity, P _{1 is} the density of the

Beam material, I designate the area moment of inertia, r the radius and 1 the length of the beam.

The beam length is not changed by the corrosion, while the radius and the natural frequency are reduced by increasing corrosion. Thus, the change of the natural frequency is directly proportional to the corrosion and the monitoring of the position of the natural frequency allows a linear corrosion sensor. The natural frequency can be monitored by the

Phase position of the drive signal in comparison to the vibration signal or via the monitoring of the amplitude of the vibration signal can be determined in certain frequency ranges.

The evaluation of the mechanical vibration behavior can be combined with an evaluation according to the known resistance method. For this purpose, a current through the oscillating body, for example by the bending beam 21 by means of the lines 211 and 212 in Figure 2, is introduced, which leads to different voltage changes with different corro- degree leads. In the case of the bar, the resistance is through

R = P ₂ πr

where p denotes the electrical resistivity of the beam material. A detection independent of the actual length of the bending beam is achieved by the multiplication of the two characteristic values ω and R. In a characteristic value K = ωR ² , the influence of the length is just canceled, so that a length-independent characteristic value can be generated.

Claims

claims

1. A device for corrosion detection with a vibrating body (21, 22, 23) whose mechanical vibration properties are variable depending on corrosion, with an actuator (21 ... 23, 34), by which the vibrating body can be excited to mechanical vibrations with a sensor (21 ... 23, 34) for detecting mechanical vibrations of the vibration body and for generating a corresponding vibration signal and with an evaluation device (37) for detecting corrosion as a function of the vibration signal.

2. Apparatus according to claim 1, characterized in that the actuator (21 ... 23, 34) by the drive means (37) is controllable with a drive signal and that the evaluation device is additionally designed for the detection of corrosion in response to the drive signal.

3. Device according to claim 1 or 2, characterized in that the oscillating body (21 ... 23) consists of the same material as a wall, for example a pipe (1) or a container, the corrosion of which is to be detected.

4. Device according to one of the preceding claims, characterized in that the oscillating body (21 ... 23) is arranged substantially at a distance from a wall (36) which corresponds to the width of a crevice corrosion to be detected.

5. Device according to one of the preceding claims, characterized in that the oscillating body (21 ... 23) is provided with an electrode which is part of an electrostatic actuator with a small distance to the oscillating body arranged second actuator electrode (34).

6. Device according to one of the preceding claims, characterized in that the oscillating body (21 ... 23) is designed in the manner of a clamped bending beam.

7. The device according to claim 6, characterized in that means (37) for measuring the electrical resistance of the bending beam (21 ... 23) are provided and that as a parameter for detecting corrosion, the product of the mechanical resonance frequency of the bending beam and the square of the electrical resistance is determined.

8. Device according to one of the preceding claims, characterized in that the device is designed as a transmitter (3) and is provided with a communication interface (4) for outputting a signal to the corrosion message to a control station (5) in an automation system.

9. A method for operating a device for corrosion detection with a vibrating body (21 ... 23), the mechanical

Vibrational properties are variable depending on corrosion, wherein by an actuator (21 ... 23, 34) of the vibrating body is excited to mechanical vibrations detected with a sensor (21 ... 23, 34) mechanical vibrations of the vibrating body and a corresponding vibration signal is generated and wherein with an evaluation device (37) in response to the vibration signal, a corrosion is detected.