NO20150829A1

NO20150829A1 - Field Kelvin Probe

Info

Publication number: NO20150829A1
Application number: NO20150829A
Authority: NO
Inventors: Eugen Florin Turcu; Michael Rohwerder
Original assignee: Tecom Analytical Systems
Priority date: 2015-06-24
Filing date: 2015-06-24
Publication date: 2016-12-26
Also published as: NO342324B1

Description

Background of the Invention

Technical Field

The invention relates to measurement systems in general and more specifically a Kelvin probe system and a method for operating same.

Background Art

From prior art one should refer to Kelvin probes. The Scanning and the non-scanning versions of the Kelvin Probe (SKP and KP respectively) are standard, noncontact, laboratory instruments routinely used for the measurement of surface work functions / Volta potentials. In recent years, it has been demonstrated that KP can directly measure corrosion potentials on coated or bare metallic surfaces. This pioneering work was started M. Stratmann and co-workers. From this prior work one should refer to WO1991019972 and M. Stratmann, H. Streckel, Corr. Sei. 6-7, 1990, 681-696.

Briefly, the KP's core is an electrical capacitor in which one vibrating plate is actuated at a certain height above a metallic substrate acting as the second plate. Volta potentials measured from a sinusoidal current are converted in real-time into corrosion potentials.

This technology comes with several problems. First of all there is a strong vibration associated with the probe vibrating to change distance to the object to be measured leading to noise and mechanical problems. Secondly the probe tends to be thin and long to obtain precise measurements and thus is also fragile. This extended geometry combined with vibrations lead to acoustic modes that in turn make the thin probe also moving laterally to the object to be measured and not just transversally over a single spot. Thirdly long term tests will require calibrations which is problematic in the field and also require interruptions of operation.

Disclosure of the Invention

Problems to be Solved by the Invention

Therefore, a main objective of the present invention is to provide a system and a method that overcomes the above shortcomings.

Means for Solving the Problems

The objective is achieved according to the invention by a rotating Kelvin probe as defined in the preamble of claim 1, håving the features of the characterising portion of claim 1, a calibration system as defined in the preamble of claim 6, håving the features of the characterising portion of claim 6, and a method for operation as defined in the preamble of claim 8, håving the features of the characterising portion of claim 8.

A number of non-exhaustive embodiments, variants or alternatives of the invention are defined by the dependent claims.

The present invention attains the above-described objective by a probe håving a longitudinal axis wherein the probe rotates around an axis not coinciding with the longitudinal axis.

Effects of the Invention

The technical differences over prior art is that the probe rotates rather than vibrates.

These effects provide in turn several further advantageous effects:

• it makes it possible to avoid noise

• it makes the probe less fragile, particularly if the probe is embedded in a body • it avoids acoustic modes with associated uncertainties regarding actual positioning

• it allows for seif calibration, also continuous seif calibration

Brief Description of the Drawings

The above and further features of the invention are set forth with particularity in the appended claims and together with advantages thereof will become clearerfrom consideration of the following detailed description of an exemplary embodiment of the invention given with reference to the accompanying drawings.

The invention will be further described below in connection with exemplary embodiments which are schematically shown in the drawings, wherein:

Fig. 1 shows a probe according to prior art

Fig. 2 shows a diagram of Volta potentials between a probe and a sample

Fig. 3 shows an embodiment of the invention

Fig. 4 shows diagrammatically an embodiment of an instrumentation of the embodiment according to Fig. 3 Fig. 5 shows an example of measurement curves using an instrumentation system according to Fig. 4

Description of the Reference Signs

The following reference numbers and signs refer to the drawings:

Detailed Description

Various aspects of the disclosure are described more fully hereinafter with reference to the accompanying drawings. This disclosure may, however, be embodied in many different forms and should not be construed as limited to any specific structure or

function presented throughout this disclosure. Rather, these aspects are provided so that this disclosure will be thorough and complete, and will fully convey the scope of

the disclosure to those skilled in the art. Based on the teachings herein one skilled in the art should appreciate that the scope of the disclosure is intended to cover any aspect of the disclosure disclosed herein, whether implemented independently of or combined with any other aspect of the disclosure. For example, an apparatus may be implemented or a method may be practiced using any number of the aspects set forth herein. In addition, the scope of the disclosure is intended to cover such an apparatus or method which is practiced using other structure, functionality, or structure and functionality in addition to or other than the various aspects of the disclosure set forth herein. It should be understood that any aspect of the disclosure disclosed herein may be embodied by one or more elements of a claim.

The invention will be further described in connection with exemplary embodiments which are schematically shown in the drawings, wherein Fig. 1 shows a Kelvin probe system 200 according to prior art. A vibrator 202 controlled and powered by a drive control/power 204 drives a shaft 206 connected to a Kelvin probe head 220 and a shaft position readout 209. The Kelvin probe head comprises a Kelvin probe 222, typically a metal body, håving at the end a Kelvin probe face 224. As the vibrator drives the shaft longitudinally the distance between the Kelvin probe face and a sample for testing 234 varies. This gives rise to a varying coupling and thus a varying electrical potential between the sample and the probe and is read out through an electrical pickup 208. The distance is monitored using a shaft position readout 209. An electronics unit 204 is connected to the electrical pickup 208 and shaft position readout 209 and to the sample 234 to close the circuit, and determines the contact potential, typically using a lock-in amplifier.

Principles forming the basis of the invention

The Kelvin probe can be used to identify a material (e.g. corrosion product) based on the potential between the surfaces of a known conductor and a sample when they are brought into electrical contact. This contact potential, also known as the Volta potential, depends on the work functions of the materials being used. The work function is the amount of energy needed to release electrons from a material's surface.

Fig. 2 shows a diagram of Volta potential ( AW) difference established between a Kelvin probe and a test sample

The basic principle behind the Kelvin Probe is to vary the capacitive coupling between the known material (probe) and the sample to generate an alternating current with an amplitude proportional to the contact potential. By adding a bias voltage to the known material, a change in amplitude can be observed. The amplitude will drop to zero if the bias voltage is equal to, but with reversed polarity of the contact potential. Hence, the bias voltage, Uappi, is a measure of the contact potential.

The capacitance C between the probe and the sample is given by:

Since the voltage given as the sum of contact potential and added bias is kept constant during the measurement, a current is generated when the capacitance is varied. The current is given by:

the current becomes

There are fundamentally three different ways to vary the capacitance: By area, a, and by distance, d and by changing the dielectric properties (e =8re0) of the space between the plates (either progressively or intermittently). In addition, any possible combinations of these three parameters could be deployed to achieve a variable capacitance. A change of the dielectric properties can practically be achieved for instance by cyclically moving to and from the plates a sheet made of various substances eachcharacterised bya different dielectric constant.

When varying the capacitance by distance, the distance between probe and sample is given by

Since the Kelvin probe face is small compared to the Kelvin probe head the effective area changes are negligible in the normal working conditions where the Kelvin probe face is close to the sample to be tested. Changes in the effective area is therefore not discussed.

When varying the capacitance by dielectric, the area of the dielectric present between the plates at a particular time must be varied with time. The actual equation describing the induced AC current depends on the construction (number of material, distribution of materials within the agglomerate,..) and it can be generalised as:

However, for one single probe, rotating at frequency (v = 0)/ 2Jl)tne shape of the AC signal would have the shape similar to that corresponding to a vibrating needle (Fig. 5). Two probes mounted at 180° apart, would still resemble the curves in Fig. 5; however, three probes mounted at 120° would differ from the previous two cases. To enhance the magnitude of the AC current, an aggregate of alternating various material with high difference in the dielectric constant (for example: trivial plastic such as polypropylene for low dielectric constant and mixed oxides such as PbMgNb03+PbTi03 for high constant).

The core of the invention is to replace the vibrator 202 with a rotating structure wherein a revolution of a rotating structure changes the capacitive coupling between a probe face and a sample.

The capacitive coupling is determined by both the effective area A of the Kelvin probe face with respect to the sample for testing as well as the distance d between the Kelvin probe face and the sample for testing. The capacitive coupling between the Kelvin probe other than the face and the sample for testing is considered negligible.

Best Modes of Carrying Out the Invention

The embodiment of the apparatus according to the invention shown in Fig. 3 shows a Kelvin probe system 100. A drive 102 controlled and powered by a drive control/power 103 rotates a shaft 106 connected to a Kelvin probe head 120 and a shaft encoder 109. The Kelvin probe head comprises a Kelvin probe 122, typically a metal body, håving at the end a Kelvin probe face 124. As the drive rotates the shaft rotates the distance between the Kelvin probe face and a sample for testing 134 varies. This gives rise to a varying coupling and thus a varying electrical potential between the sample and the probe and is read out through an electrical pickup or slip ring 108. The distance between the Kelvin probe face 124 and the sample 134 is determined using a shaft encoder 109. An electronics unit 104 is connected to the electrical pickup or slip ring 109 and shaft encoder 109 and to the sample 134 to close the circuit, and determines the contact potential, typically using a lock-in amplifier.

The drive 102 is an electrical motor operating at a steady angular speed. Variations in angular speed, typically stated as rotations per minute or RPM, are undesirable as these can cause problems with the phase locked loop in the lock in amplifier of the electronics unit. Similarly the drive should be essentially vibration free and the rotating parts should be well balanced. Considering the rather large size (in the cm<2>range) of the sensing probes that can be accommodated in the rotating Kelvin probe head, it is expected that the signal-to-noise ratio is much higher compared to the traditional vibrating needle Kelvin probe. Therefore, the analogous/digital lock-in amplifier can be replaced by appropriate software to filter out unwanted signals (noise).

Unlike the pick-up in prior art a pick-up for a rotating system would need to be adapted. Tests show that a slip ring 108 works well in connecting the rotating Kelvin probe face with the non-rotating electronics unit.

The Kelvin probe head 120 is preferably a cylindrical body 128 extending coaxially from the shaft 106 where the Kelvin probe 122 is embedded. Again the Kelvin probe head, being a rotating part, should be well balanced to avoid excessive vibrations. Several materials can be used, though metallic or otherwise electrically conducting materials are beneficial in screening the Kelvin probe from electromagnetic noise.

The Kelvin probe 122 is a slim metal cylinder embedded into the Kelvin probe head and positioned non-coaxially with respect to the shaft 106 and positioned such that the end face of the probe extends to and is flush with the side face of the Kelvin probe head. If the Kelvin probe head 120 is conducting the Kelvin probe is electrically isolated from the Kelvin probe head using an electrical isolator 126.

The end face of the Kelvin probe 122 flush with the side face of the Kelvin probe forms the Kelvin probe face 124 and is the only Kelvin probe part which is exposed to the sample 134.

In an aspect of this embodiment the Kelvin probe system is provided with a calibration sample 132 located at a position away from the sample for testing 134. This means that during a full revolution of the Kelvin probe head the Kelvin probe face is alternatively exposed to the sample for testing and to the calibration sample.

Since the calibration sample is a known material located at a known position and a known distance this knowledge can be used for continuous calibration of the system. A plurality of calibration samples can be provided at different positions for instance comprising a calibration sample similar to the materials expected to be found in the sample for testing, thus providing better identification of the materials in the sample for testing. Materials expected to be found can be both un-corroded material, corroded material, corrosion by-products and other materials.

With a calibration sample the system is able to perform a calibration between measurements of the sample for testing. Since the position and material properties of the calibration sample is known a calibration system can compensate for drift in the measurement of the sample for testing without interrupting the measurement process.

The Volta potential difference depends on the material used for the Kelvin probe face and for the sample for testing. It can be beneficial to test with different materials for the Kelvin probe face.

An electronics unit 140 is provided. Fig. 4 shows a preferred embodiment of an electronics unit 300 comprising individual parts. An actuator control 302 controls an actuator 304. The actuator in this case is any device that can deliver a rotational movement i.e. rotation, to the measuring probe. The device can for instance be any motor: electric, spring-powered or air-driven. The measuring probe can be attached to the motor via a gearbox. A Kelvin probe is connected to a signal processing unit 308 the AC current generated by the probe movement is first pre-amplified together with the noise. Later, the amplified signal and noise, is digitised and filtered to improving the signal-to-noise ratio. Once the signal is digitised, it can be processed in any imaginable way. This in turn is connected to a unit with amplifiers/filters 306, providing data for visualisation 312 and/or data logger 314 for recording said data.

Preferably a seif diagnosis unit 316 is provided, operatively connected to the actuator controller 304, the amplifier/filters 306 and, if present, a calibration unit.

A preferred use of a system as disclosed above is for hydrogen measurements. Since a Kelvin Probe measures work function differences established between two electroconductive materials when they are electrically connected with each other, any phenomena affecting the value of a work function would be detectable by the Kelvin probe. By way of examples: 1. Corrosion will change surface chargés and interface dielectric

constants at a metallic interface (metal-gas, metal-electrolyte, etc).

2. Diffusible matter able to be permeate through metals can reach a

metallic interface from the bulk of the metal; hence, the work function of that permeated material will change compared to work function of the pristine metal (free of diffusible mater). Such diffusible mater can be for instance

hydrogen (or protons as it is quite unlikely that a hydrogen atom can diffuse through the metal lattice as an atomic entity). To some extent, ions of alloying elements and impurities could diffuse too, especially at high temperatures.

3. Chemical or physical interaction between the environment and a

surface of an electroconductive material (metal, semiconductor, oxide covered metal, etc.) could cause significant changes of the work function.

Hydrogen is in many situations a side product of a corrosion process. When hydrogen is formed, upon reduction of hydrated protons (H30+) on the cathodic area near an active corrosion site, it will enter, to various degree, the metal. The hydrogen atoms diffuse (permeate) throughout the steel at rates depending on the concentration of hydrogen on the entry site, on the chemical composition of the steel (number of traps).

Eventually they will reach the other side (exit side) and will be oxidised to protons. The hydrogen leaving the sample at the exit side will influence the work function.

To enhance the detection limit, Palladium (Pd) layers deposited (thin films) on the exit site allow hydrogen to concentrate in the Pd. In such a case, the Kelvin probe measures changes of the work function of the Pd. This makes the detection very sensitive (below 10 ppb hydrogen in Pd) and quantitative. See "The hydrogen electrode in the "dry": A Kelvin probe approach to measuring hydrogen in metals"

S. Evers, M. Rohwerder, Electrochemistry Communications 24 (2012) 85-88.

In use a Kelvin probe detects internal corrosion in steel tanks, pipelines, from the outside. The measurement is accomplished with or without the additional Palladium layer.

The Kelvin probe can be deployed as internal corrosion detector as above, as a hydrogen leakages sensor (i.e. for hydrogen tanks, transportation or storage, etc), as Hydrogen Induced Stress Cracking (HISC) risk indicator as it can monitor hydrogen build up in steel.

It should be noted that besides corrosion as a source, hydrogen can permeate steel during the fabrication process (i.e. in the raw steel), during assembly (welding), and in operation (corrosion due to transported fluids).

Alternative Embodiments

A number of variations on the above can be envisaged. For instance determination of rotational position can be performed in several different ways. A single position indicator such as an impulse switch can be used with a phase locked loop to determine position from timing between consecutive impulses, preferably filtered by for instance a Kalman filter.

Such an impulse switch can be a mechanical readout with an electric switch, a magnetic readout using for instance a Hall-sensor, an optical readout using a photo diode and a photo cell or similar methods.

Also a calibration sample in a known position can be used as the impulse switch for instance by determining a threshold value indicating a known position.

Alternatively the sample 134 itself can be used though this is more demanding as the true position of the sample is not always known, especially when repositioning the measurement system.

In another embodiment the drive 102 is a stepper motor or a servo motor wherein the drive control 104 sets the position of the Kelvin probe face 124. In this case a shaft encoder is not necessarily required.

In one embodiment the Kelvin probe head is replaceable with other heads håving different materials.

In another embodiment the Kelvin probe head is provided with a plurality of Kelvin probe faces of different materials. These can be positioned at different angular positions along the surface of the Kelvin probe head. These can be sampled in turn or be connected to one of a plurality of slip rings for connection to the electronics unit.

In yet another embodiment the Kelvin probe faces can also be distributed across the width of the Kelvin probe head, the width of the Kelvin probe head being in a direction substantially parallel to the rotational axis. The technical effect of this is to measure the sample for testing at different positions which in turn can provide faster measurement across a wider area even without håving to move the probe.

Obviously the above can be combined such as using a plurality of Kelvin probe faces can be distributed at different angular positions and also across the width of the Kelvin probe head. Said Kelvin probe faces can be made of one material or a plurality of materials.

While the strangest signals will be achieved by performing the measurement when

the Kelvin probe face is closest to the sample to be measured it should be noted that the rotational motion of the Kelvin probe head means that there is a range of angular positions of the Kelvin probe head where the Kelvin probe face is sufficiently close to the sample to be measured to return usable readings. These measurements will be

located spatially before and after the point of closest position. This means that a spatially extended reading can be provided.

Similarly the above calibration processes can also be performed at a range of angular positions of the Kelvin probe head where the Kelvin probe face is sufficiently close to the calibration sample to return usable readings.

This extent of measurements will be substantially perpendicular to the width of the Kelvin probe head being in a direction substantially parallel to the rotational axis and thus also Kelvin probe faces distributed across the width of the Kelvin probe head.

This can be used to create 2D mapping of a sample at a rate higher than measuring a single point at every rotation. This in turn can be used to determine a gradient of change, and a portable Kelvin probe system can be provided with indicators to show the direction of the centre of a corroded area. Such an indicator can be 4 arrows indicating if the Kelvin probe system should be moved forward or backwards with respect to the rotation of the Kelvin probe head, or sideways along the axis of rotation. This will allow an operator to quickly locate corrosion. Alternatively the system can be used to control a traverse mechanism to move the Kelvin probe system across a sample to be tested håving an extended area.

The shaft 106 provides mechanical contact between the drive 102 and the Kelvin probe head 120. In an alternative embodiment the drive is directly connected to the Kelvin probe head without the use of a shaft.

For specialised use the slip ring 108 can be replaced by different forms for pick-ups such as a wire spool that is coiled and un-coiled. In some cases there is a limit to the number of revolutions that can be made before håving to reverse the process. Also a mercury drop can be used for providing electrical contact.

Industrial Applicability

The invention according to the application finds use in material analysis, particularly measurements of corrosion and/or hydrogen permeation through metallic materials, and can be used in most situations where traditional Kelvin probe systems have been employed.

Claims

1. A Kelvin probe system (100) for analyzing a sample for testing (134), the Kelvin probe system comprising: a drive (102) controlled and powered by a drive control/power (103) for rotating a shaft (106), a Kelvin probe head (120) connected to the shaft (106), comprising a Kelvin probe (122), håving at the end a Kelvin probe face (124),characterised in thatthe Kelvin probe face is provided on a side face of the Kelvin probe head.

2. The Kelvin probe system according to claim 1, wherein the Kelvin probe head comprises a plurality of Kelvin probe faces.

3. The Kelvin probe system according to claim 2, wherein the plurality of Kelvin probe surfaces are of more than one material.

4. The Kelvin probe system according to claim 2 or 3, wherein the plurality of Kelvin probe faces are distributed at different angular positions along the surface of the Kelvin probe head (120).

5. The Kelvin probe system according to claim 2 or 3, wherein the plurality of Kelvin probe faces are distributed at different positions along the width of the Kelvin probe head (120).

6. A calibration system for a Kelvin probe system (100) according to one of claims 1 - 5, wherein the Kelvin probe system (100) further comprising: a calibration sample (134) located away from a sample to be measured (132), wherein the calibration sample operates as build-in reference surface for the calibration of the rotating Kelvin Probe.

7. A calibration system for a Kelvin probe system (100) according to claim 6, comprising a plurality of calibration samples (134), wherein the calibration samples are made of different materials.

8. A method for measuring hydrogen using a Kelvin probe system (100) according to one of claims 1 - 5, wherein the Kelvin probe system measures at least one from the group of surface chargés and interface dielectric constants, at a metallic interface in a sample to be measured (132).

9. The method according to claim 8, wherein the Kelvin probe system (100) further comprises a calibration system for the Kelvin probe system (100) according to claims 10-14, wherein a first calibration sample is made of a material corresponding to a sample to be measured håving not been exposed to hydrogen, and a second calibration sample is made of a material corresponding to a sample to be measured that has been exposed to hydrogen.

10. The method according to claim 8, wherein the method comprises depositing a Pd layer over the sample to be measured (132) prior to operating the Kelvin probe system.