WO2009157773A1

WO2009157773A1 - Method and system for determination of coating performance

Info

Publication number: WO2009157773A1
Application number: PCT/NO2009/000145
Authority: WO
Inventors: Kjell Hatteland
Original assignee: Emt Research As
Priority date: 2008-06-24
Filing date: 2009-04-16
Publication date: 2009-12-30
Also published as: GB2461272A; GB2461272B; GB0811569D0

Abstract

The present invention relates to a method and system for determination of coating performance of a nonconductive coating applied to a metal structure connected to a common terminal. The system comprises a first electrode at a first location, a second electrode at a second location, where the second location is axially displaced in relation to the first location. A low-frequent voltage is applied to the first electrode by means of a voltage source and the resulting potential at the second electrode is measured by means of the voltage meter. According to the method, the mean leakage resistance Rle can be computed.

Description

METHOD AND SYSTEM FOR DETERMINATION OF COATING PERFORMANCE

FIELD OF THE INVENTION

The present invention relates to a method and system for determination of coating performance.

BACKGROUND OF THE INVENTION

In many applications, metallic surfaces are coated with a nonconductive coating, basically with the purpose of protecting the metal surface against corrosion. A second purpose is to reduce friction between the metallic surface and flowing fluid, for example fluid flowing in a metal pipe or tube.

Coating is applied in many applications to stop any electrolytic barrier between the (fluid) electrolyte and the metal as this will result in metal ions transportation and thus corrosion. Moreover, the purpose of any coating system is to reduce micropores to a minimum, hence contributing to reduction of corrosion. One parameter for evaluating the protection quality of a coating is the leakage resistance or the leakage current between the fluid and the metal surface. According to data sheets for modern coating paintings, as e.g. based on two component epoxy, the value of the volumetric leakage resistance is in range of 10^Λ10 to 10^Λ15 Ohm*cm. This is outside the range of previously known technology for measuring leakage resistance, especially in closed systems as e.g. inside polluted or fouled pipe systems.

One frequently used method for measuring the leakage current in a pipe is to apply a voltage between the metal surface and to provide an electrode with a foam dipped in a proper electrolyte in the fluid in the flow and hence to measure any leakage currents radial to the surface. Any breakage in the insulation provided by the nonconducting coating will lead to a measurable current. This direct method can detect equivalent leakage resistance typical up 10**9 0hms/m2, and maybe more with application of dedicated expensive laboratory equipment as electronmeters.

The object of the present invention is to provide an improved system and method for determination of coating performance. In particular it is an object of the invention to measure coating performance over substantial length of a pipe. SUMMARY OF THE INVENTION

The invention relates to a method for determination of coating performance of a nonconductive coating applied to a metal structure connected to a common terminal, where the method comprises the following steps: - providing a conductive fluid in contact with the coating, where the specific conductivity of the fluid is known; applying a first electrode at a first location, where the first electrode is in contact with the conductive fluid, where the first electrode is connected to a common terminal via a voltage source; - applying a second electrode at a second location, where the second location is axially displaced in relation to the first location, where the second electrode is in contact with the conductive fluid, and where the second electrode is connected to the common terminal via a voltage meter; measuring a length L between the first and second electrode; - applying a low-frequent voltage to the first electrode by means of the voltage source; measuring a resulting potential at the second electrode by means of the voltage meter; computing a mean leakage resistance RJe based on the applied low-frequent voltage, the resulting potential at the second electrode, the specific conductivity of the fluid and the length L.

In an aspect of the invention, the method further comprises the following steps: applying an AC voltage to the first electrode by means of the voltage source; measuring the resulting potential and the phase difference at the second electrode by means of the voltage meter; computing the mean capacitance C based on the applied AC voltage, the resulting potential and the phase difference at the second electrode, the specific conductivity of the fluid, the length L and the mean leakage resistance RIe.

In an aspect of the invention, the low-frequent voltage is a step voltage with duration of approximately 0,5 - 10 seconds.

In an aspect of the invention, the low-frequent voltage is a voltage with frequency 0 - and some few kHz. In an aspect of the invention, the computation of the mean leakage resistance is performed by means of an equivalent simulation model in which the leakage resistance RIe is adjusted until the voltage at the second electrode of the simulation model is equal to the potential at the second electrode measured by means of the voltage meter.

In an aspect of the invention, the computation of the mean capacitance C is performed by means of an equivalent simulation model in which the capacitance C is adjusted until the voltage and phase difference at the second electrode of the simulation model is equal to the potential and phase difference at the second electrode measured by means of the voltage meter.

In an aspect of the invention, the common terminal is electrical ground.

In an aspect of the invention, the method comprises computing the mean coating thickness by means of the mean capacitance C and the dielectric constant for the coating. In an aspect of the invention, the fluid is water or a specific electrolyte.

In an aspect of the invention, the method comprises applying several electrodes, alternatingly connected to the voltage source and voltage meter respectively, to determine the mean leakage reactance and/or mean capacitance of the coating located between the respective electrodes. The present invention also relates to a system for determination of coating performance of a nonconductive coating applied to a metal structure connected to a common terminal, comprising: a container containing a conductive fluid in contact with the coating, where the specific conductivity of the fluid is known; - a first electrode provided at a first location, where the first electrode is in contact with the conductive fluid, a second electrode provided at a second location, where the second location is axially displaced in relation to the first location with a length L, where the second electrode is in contact with the conductive fluid a voltage source having a first terminal connected to the first electrode and a second terminal connected to a common terminal, where the voltage source is supplying a low-frequent voltage to the first electrode; a voltage meter having a first terminal connected to the second electrode and a second terminal connected to the common terminal, where the voltage meter is provided for measuring a resulting potential at the second electrode; and - computation means for computing a mean leakage resistance RIe. In an aspect of the invention, the voltage source further is provided for supplying an AC voltage to the first electrode, where the voltage meter further is provided for measuring both the resulting voltage potential and the phase difference at the second electrode, and where the computation means is provided for computing a mean capacitance C.

In an aspect of the invention, the low- frequent voltage is a voltage with frequency 0 - and some few kHz. In an aspect of the invention, the computation means comprises an equivalent simulation model for computing the mean leakage resistance, wherein the leakage resistance RIe is adjusted until the voltage at the second electrode of the simulation model is equal to the potential at the second electrode measured by means of the voltage meter. In an aspect of the invention, the computation means comprises an equivalent simulation model for computing the mean capacitance C, wherein the capacitance C is adjusted until the voltage and phase difference at the second electrode of the simulation model is equal to the potential and phase difference at the second electrode measured by means of the voltage meter. In an aspect of the invention, the container is the metal structure itself, or a separate container.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described in detail with reference to the enclosed drawings, where: Fig. 1 illustrates the system according to a first embodiment, where a simulation model is indicated with dashed lines;

Fig. 2 illustrates an alternative configuration of the system according to the first embodiment;

Fig. 3 shows details of the attenuation impedance of the simulation model; Fig. 4 illustrates the system according to a second embodiment, as a test bench setup for testing coating properties. First, it should be noted that determination of coating performance in this context is mainly represented by determination of the leakage resistance. The leakage resistance of the coating, which will cause a leakage current when exposed to a certain voltage, including the natural corrosion potentials, is the vital parameter for preventing corrosion attacks of the metal surface.

In an aspect of the invention, also the coating capacitance is used as an additional measure for the coating performance. The coating capacitance is corresponding to a mean coating thickness over a predefined length. The thickness of the coating is also important as it makes the coating surface more durable and resistance to microcracks, micropores etc., which again will lead to corrosion.

First embodiment

In the first embodiment, the method and system is used to measure the coating performance of a pipe, for example a water conduct pipe for an electricity power plant, water supply pipes, sewage purification plants, a process pipe in oil and gas industry, chemical industry etc.

Fig. 1 shows the overall system 1. A metal structure 2, for example a pipe, is coated with a coating 3. The pipe is filled with an electrically conductive fluid 4, hence the pipe serves as a fluid container. It should be noted that the fluid in normal situations only is in contact with the coating 3, and not the pipe 2, i.e. the pipe is coated to avoid corrosion of the pipe, to avoid scaling and biological fouling and to reduce the friction losses between the fluid and the pipe wall.

A first electrode 5 is located at a first location in the system 1 , and a second electrode 6 is located at a second location in the system 1. The first and second electrodes are spaced apart and are in contact with the fluid 4, but not in contact with the pipe 2.

In. fig. 1 , the electrodes 5, 6 close the respective ends of the pipe, while in fig. 2, the electrodes 5, 6 are provided in contact with the fluid 4 through manhole openings 7 of the pipe. The electrodes 5, 6 can be provided with electrical insulation at some areas, to avoid electrical contact with the pipe or manhole openings. It should be noted that in fig. 2, the electrodes 5 and 6, as in fig. 1 , are connected to the voltage source and voltage meter respectively even though these are not shown. Moreover, in fig. 2, an insulated plug 8 can be inserted in the pipe near the second electrode 6, to avoid that the coating and fluid in the pipe to the right (in fig. 2) influences the measurements. Alternatively, the simulation model described below can take the extra coated pipe/fluid into consideration in the simulation model. The first electrode 5 is connected to a voltage source Vin. A first terminal of the voltage source is connected to a common terminal such as electrical ground. The second terminal of the voltage source is connected to the first electrode 5. The voltage source can supply an alternating voltage with amplitude Ain and frequency Fin to the first electrode 5.

The second electrode 6 is connected to a voltage meter Vout. A first terminal of the voltage meter is connected to the common terminal, while the second terminal of the voltage meter is connected to the second electrode 6. The voltage meter can measure the voltage amplitude and phase difference between the input voltage Vin and the measured voltage Vout.

Also the pipe is connected to the common terminal.

The method according to the invention will now be described.

The metal structure is filled with a conductive fluid. Here water is used, but any conductive fluid can be used. The fluid is in contact with the coating and the specific conductivity of the fluid is known.

As described above, the first electrode 5 is applied at a first location of the metal structure. The second electrode 6 is applied at a second location of the metal structure, where the second location is axially displaced in relation to the first location. The first and second electrodes are in contact with the conductive fluid. In the present embodiment, a simulation model indicated by arrow 10 in fig. 1 is used to compute the mean leakage resistance. The simulation model is shown with dashed lines inside the pipe of fig. 1, and comprises a number of attenuation impedance networks connected in series between the first electrode 5 and the second electrode 6. The attenuation impedance network simulates the physical behaviour of the fluid and coating inside the pipe in fig. 1.

One of the attenuation impedance networks is illustrated in fig. 3. One such network could for example represent the total attenuation impedance of a pipe with length 1 meter. Hence, the attenuation impedance can be referred to as an incremental attenuation network, since it increases with the length of the metal structure. The attenuation network comprises an axial resistance Ra having a first terminal connected to the first electrode 5 (or a neighbour network) and a second terminal connected to the second electrode 6 (or a neighbour network), a radial resistance Rr connected between the second terminal of the axial resistance Ra and a node A, a coating capacitance C connected between the node A and ground and a leakage resistance RIe connected between the node A and ground. If any leakage current through the coating is present, an additional attenuation of the voltage is seen at the voltage meter. This is due to the attenuation formed by the incremental longitudinal impedance of the fluid and the same incremental impedance to the ground that is due to the coating pores and microfractures. By combining a set of measurements taken with different frequencies and comparing the measured attenuation with a simulated mathematical model of these cascaded incremental attenuation networks, an estimate for both the mean leakage resistance and coating capacitance can be found. In addition the longitudinal voltage distribution along the pipe coating can be assessed through the simulation model of cascaded networks.

In the present embodiment, the length L of the pipe is 10 meter and the pipe has an inner diameter of 10 cm. The pipe is internally coated with coating in the form of ordinary painting, which could be of vegetable oils or tar. The coating is exhibiting lots of micropores due to the solvent agents. The simulation model is built up consisting of 10 sections of equal attenuation networks according to Fig. 3, connected in series, where each section represents parameters of one meter length of the steel pipe.

The conductivity of the water has been measured to be 2.0 mS/meter. The specific conductivity of the fluid per meter is equal to l/(the axial resistance Ra). The specific conductivity is 2.0 mS/m * pi * r*r = 2.0* 3.14 *0.05*0.05 (mS) = 0.0157 mS, which equals an axial resistance Ra in the range of 65 kθhms/m.

In the simulation model, the radial resistance Rr can be set to zero, since its value will be a fraction of the leakage reactance (combination of leakage resistance and coating capacitance). In the simulation model, potentiometers of 10 MOhms are used to represent the leakage resistance RIe. It should be noted that for high quality coating products this value will be much higher.

In the simulation model, a set of capacitors are used for the coating capacitance C. An expected capacitance C is used as a start value. The expected capacitance is calculated in the following way: The nominal specific capacitance of such a coating is typically 10 pF/cm2 at a coating thickness of 100 um (typical parameter according to the coating specifications). The area of the steel pipe per meter length will be 2*pi*r* 100cm = 3125 cm2, giving an expected capacitance of approximately 30 nF/m (10 pF/cm2 * 3125 cm2). In the physical system 1 a low-frequent voltage is applied to the first electrode by means of the voltage source. The low-frequent voltage can for example be a step voltage with duration of approximately 0,5 - 10 seconds. Alternatively, the low- frequent voltage can be a voltage with frequency 0 to some few Hz. The low- frequent voltage will rule out the influence of the capacitor because the reactance of the capacitor at these frequencies will be at least a factor often higher than the expected leakage resistance. A typical value of the voltage could be up to 1 VDC or 1 Vrms to avoid to limit electrolysis of the fluid.

A good indicator for that the reactance of the coating capacitor can be ruled out is that the phase between Vin and Vout is nearly zero.

In the physical system 1 , the resulting potential at the second electrode 6 is measured by means of the voltage meter. In the simulation model 10, the potentiometers representing the leakage resistance RIe are adjusted, keeping all at the same resistance value until the measured output voltage read from the voltage meter is the same as the output voltage at the second electrode 6 in the model The mean leakage resistance per meter is thus determined as the resistance presently adjusted on the potentiometer. Consequently the mean leakage resistance RIe is computed based on the applied low-frequent voltage, the resulting potential at the second electrode, the specific conductivity of the fluid and the length L.

For this specific example, the leakage resistance of the coating could be between 1 MOhms/m and 10 MOhms/m, which indicates that the coating does not have the utmost performance.

In an aspect of the invention the capacitor C of the simulation model is found. Here, an AC voltage is applied to the first electrode 5 by means of the voltage source, with a frequency chosen so that the capacitor reactance becomes a fraction of the leakage resistance RIe found in the above example. In the present example a frequency of some few kilohertz is chosen.

An indicator for that the capacitance is dominating the reactance to ground is that a clear phase shift between Vin and Vout can be seen. The phase shift could be more than 90 degrees, especially for long pipes.

The resulting potential and phase difference are measured by means of the voltage meter at the second electrode 6.

In the simulation model, the values for all the capacitors C are adjusted (for example by replacing the capacitors with other capacitors). All capacitors values should be held equal to each other.

When the capacitances in the simulation model gives the same voltage and phase difference at the second electrode 6 in the model as the measured voltage and phase difference, a value for the capacitance C has been found. Hence, the mean coating capacitance per meter pipe is thus determined.

Consequently the mean capacitance C has been calculated based on the applied AC voltage, the resulting potential and the phase difference at the second electrode, the specific conductivity of the fluid, the length L and the mean leakage resistance RJe.

Knowing that the coating capacitance is inverse proportional to the thickness of the coating, the real thickness of the coating can thus easily be calculated. If e.g. the capacitance was measured to 15 nF/m, the equivalent thickness is 200 μm.

Now it is further possible to measure the voltage level at specific positions in the simulation model, which again represent the voltage distribution internally of pipe.

It should be noted that for long pipes of e.g. several hundred meters, it will be unpractical to use a physical simulation model as the computation means as described in the example. Instead, computation means using mathematical or numerical models based on network simulation are thus preferred. These mathematical or numerical models could for example be implemented in computation means such as a computer program for being run on a computer.

Another use of the new method will be to simulate the electric field distribution inside a coated pipe with the purpose of fouling reduction as described in patent WO 2004/094319 Al , titled 'Method for flow improvement and reduction of fouling in process equipment'. Here it is vital to obtain a voltage over the full length of the pipe that is above a certain value. By utilizing two electrodes, properly separated along the pipe, and applying the new method it is possible to predict or calculated e.g. the feeding voltage needed at both ends of the pipe in order to get sufficient voltage at the middle of the pipe to compensate for coating leakage currents. The voltage can thereby be adjusted to compensate for natural degradation of the coating resistance over time. By using a set of electrodes along a pipe or a pipe system, which again could be part of a processing system or a hydroelectric power station, it will also be possible to predict or control the voltage distribution over the actual part. The application of the new method in not restricted to determining coating performance in pipes or using water as the fluid. Other, indirect applications of the first embodiment will be within process and storage equipments or constructions where friction losses, reduction of scaling and biological fouling is attempted reduced by applying a specific voltage or a voltage range between the fluid and the coated metal surface. Second embodiment

In the second embodiment, the method and system is used as a test bench setup to measure the coating performance and hence the key coating properties of a coating material. Fig. 4 shows an alternative embodiment of the invention, where it is used as a test bench facility for testing coating performance.

In fig. 4 it is shown that the system 20 comprises a metal structure 22 in the form of a metal rod. The metal structure 22 is coated with a coating 23. The coated metal structure is located in a nonconductive pipe 27 (e.g. of PCV material). A packer element 28, such as an O-ring, is located in one end opening of the pipe 27, for easy insertion of a coated metal rod into the pipe 27 while at the same time no fluid is allowed to leak out past the coated metal rod when inserted.

A first electrode 25 is provided in one end of the pipe 27, while a second electrode 26 is provided axially displaced in relation to the first electrode. An inlet 30 and an outlet 31 are provided in the pipe 27 to fill it with fluid 24, hence, the pipe 27 serves as a fluid container.

One end of the coated metal rod is connected to the common terminal or ground. The first electrode 25 is connected to a voltage source Vin, and the second electrode is connected to a voltage meter Vout. As can bee seen, the system 20 of fig. 4 is corresponding to the system 1 in fig.l, however, the system 20 is adapted for easy replacement of one coated metal rod with another rod with a different coating for testing purposes. Consequently, the same simulation model as in the first embodiment above can be used to determine the leakage resistance RJe and also the capacitance C. It is preferred that the coated rod occupies a major part of the pipe, as this will increase the ratio between the coated area of the rod and the remaining cross- sectional area of the conductive fluid around, thus increasing the sensitivity of the measurement.

A vertical arrangement of the test bench could of course also be considered, where one end of the nonconductive pipe is permanently plugged and the electrolyte is filled from the top. Third embodiment

In the third embodiment, the method and system is based on applying two transmitting electrodes, connected to the voltage source, and one or more measurement electrodes in between, connected to a voltmeter, in order to measure the coating performance and hence the key coating properties of a coating material on a limited distance of e.g. a long pipe.

By applying two transmitting electrodes, the effect of the conducing fluid and its possible connection to grounding devices or isolating defects in the coating outside the measurement setup is strongly limited or inhibited. Thus any measurements of voltage drops in between these two electrodes will solely be depending on the coating performance in between the two transmitting electrodes.

If the coating has uniform properties between the two probes, the simulating network will consist of two symmetrical networks connected together halfway between the two probes. For the last lump of the network the impedance (capacitance reactance and leakage resistance) to ground should be doubled.

Above, three embodiments of the invention have been described. The system and methods has been shown to measure the mean leakage resistance over a predetermined section of a coated metal structure. The method is more sensitive, the longer the metal structure is. For long pipes, tests have been showing that the invention was capable of determining a mean coating leakage resistance beyond 10** 10 0hms/cm2.

Further modifications and variations will be obvious for a skilled man when reading the description above. The scope of the invention will appear from the following claims and their equivalents.

Claims

1. A method for determination of coating performance of a nonconductive coating applied to a metal structure connected to a common terminal, where the method comprises the following steps: - providing a conductive fluid in contact with the coating, where the specific conductivity of the fluid is known; applying a first electrode at a first location, where the first electrode is in contact with the conductive fluid, where the first electrode is connected to a common terminal via a voltage source; - applying a second electrode at a second location, where the second location is axially displaced in relation to the first location, where the second electrode is in contact with the conductive fluid, and where the second electrode is connected to the common terminal via a voltage meter; measuring a length L between the first and second electrode; - applying a low-frequent voltage to the first electrode by means of the voltage source; measuring a resulting potential at the second electrode by means of the voltage meter; computing a mean leakage resistance RIe based on the applied low-frequent voltage, the resulting potential at the second electrode, the specific conductivity of the fluid and the length L.

2. Method according to claim 1, where the method further comprises the following steps: applying an AC voltage to the first electrode by means of the voltage source; - measuring the resulting potential and the phase difference at the second electrode by means of the voltage meter; computing the mean capacitance C based on the applied AC voltage, the resulting potential and the phase difference at the second electrode, the specific conductivity of the fluid, the length L and the mean leakage resistance RIe.

3. Method according to claim 1, where the low-frequent voltage is a step voltage with duration of approximately 0,5 - 10 seconds.

4. Method according to claim 1, where the low-frequent voltage is a voltage with frequency 0 - some few kHz.

5. Method according to claim 1, where the computation of the mean leakage resistance is performed by means of an equivalent simulation model in which the leakage resistance RIe is adjusted until the voltage at the second electrode of the simulation model is equal to the potential at the second electrode measured by means of the voltage meter.

6. Method according to claim 2, where the computation of the mean capacitance C is performed by means of an equivalent simulation model in which the capacitance C is adjusted until the voltage and phase difference at the second electrode of the simulation model is equal to the potential and phase difference at the second electrode measured by means of the voltage meter.

7. Method according to claim 1, where the common terminal is electrical ground.

8. Method according to claim 2, where the method comprises computing the mean coating thickness by means of the mean capacitance C and the dielectric constant for the coating.

9. Method according to any of the claims above, where the fluid is water or a specific electrolyte.

10. Method according to any of the claims 2 - 19, where the method comprises applying several electrodes, alternatingly connected to the voltage source and voltage meter respectively, to determine the mean leakage reactance and/or mean capacitance of the coating located between the respective electrodes.

1 1. System for determination of coating performance of a nonconductive coating applied to a metal structure connected to a common terminal, comprising: a container containing a conductive fluid in contact with the coating, where the specific conductivity of the fluid is known; - a first electrode provided at a first location, where the first electrode is in contact with the conductive fluid, a second electrode provided at a second location, where the second location is axially displaced in relation to the first location with a length L, where the second electrode is in contact with the conductive fluid a voltage source having a first terminal connected to the first electrode and a second terminal connected to a common terminal, where the voltage source is supplying a low-frequent voltage to the first electrode; a voltage meter having a first terminal connected to the second electrode and a second terminal connected to the common terminal, where the voltage meter is provided for measuring a resulting potential at the second electrode; and - computation means for computing a mean leakage resistance RIe.

12. System according to the claim 1 1 , where the voltage source further is provided for supplying an AC voltage to the first electrode, where the voltage meter further is provided for measuring both the resulting voltage potential and the phase difference at the second electrode, and where the computation means is provided for computing a mean capacitance C.

13. System according to claim 1 1, where the low- frequent voltage is a step voltage with duration of approximately 0,5 - 10 seconds.

14. System according to claim 1 1, where the low-frequent voltage is a voltage with frequency 0 to some few kHz.

15. System according to claim 1 1, where the computation means comprises an equivalent simulation model for computing the mean leakage resistance, wherein the leakage resistance RIe is adjusted until the voltage at the second electrode of the simulation model is equal to the potential at the second electrode measured by means of the voltage meter.

16. System according to claim 1 1, where the computation means comprises an equivalent simulation model for computing the mean capacitance C, wherein the capacitance C is adjusted until the voltage and phase difference at the second electrode of the simulation model is equal to the potential and phase difference at the second electrode measured by means of the voltage meter.

17. System according to claim 1 1 , where the container is the metal structure itself, or a separate container.