Determination of the corrosion rate of metallic structural

Abstract

Method for determining the corrosion rate of metallic structural materials, also in media where the ohmic potential drop is very high, by measuring the corrosion rate by electrochemical polarization methods at lower and higher polarizations. In the method, current and voltage are first measured at a fixed polarization potential, during which the system is set to a current and potential value during the stationary polarization, and then by turning off the current, a transient potential decay (depolarization) is induced. The invention is also an electrode which can be used for the implementation of the method, which is suitable for a short-term significant current load with a very small potential change. The invention also includes sensors for measuring the corrosion rate of the examined structural material (8) in water- and salt-containing liquid hydrocarbons and wet gases.

Description

[0001] DETERMINATION OF THE CORROSION RATE OF METALLIC STRUCTURAL MATERIALS

[0002] The invention relates to a method for determining the corrosion rate of metallic structural materials also in media where the ohmic potential drop is very high. The determination is made by measuring the polarization resistance from the stationary current value before switching off the polarization (which is the same as the Faraday current density value immediately after switching off) and from the first polarization value after switching the rate off, which no longer contains the ohmic potential drop, and from the value In doing so, first the current and voltage is measured at a fixed polarization potential, during which the system is set to an approximately constant current and potential value during the stationary polarization. After that, by turning off the current, a transient potential decay (depolarization) is induced. The corrosion rate is obtained from these data as explained below. The invention also relates to an electrode that is suitable for a short-term significant current load with a very small potential change. It is also suitable as a counter electrode and reference electrode. The electrode is attached to sensors to implement the method for determining the corrosion rate of metallic structural materials.

[0003] The invention also relates to a sensor for measuring the corrosion rate of the examined structural material in liquid hydrocarbons containing water and salt. The sensor consists of a sensor head and a threaded sensor connector. It is suitable for implementing the method for determining the corrosion rate of metallic structural materials.

[0004] The invention further relates to a sensor for measuring the corrosion rate of the examined structural material in wet gases. The sensor consists of a sensor head, a threaded sensor connector, and a sensor body. It is suitable for implementing the method for determining the corrosion rate of metallic structural materials.

[0005] One of the important tools for corrosion protection is the measurement of the corrosion rate, which in an industrial environment is called corrosion monitoring. There are two main methods of corrosion monitoring, the mass loss measurement and the electrochemical polarization methods. In principle, mass loss measurement can be used in any system, however, its response time is long (minimum weeks, but rather months), which significantly limits its practical applicability. The advantage of electrochemical processes is the short (typically minute or faster) response time, which enables rapid identification of corrosion factors and optimization of corrosion protection. At the same time, electrochemical polarization methods cannot, or can only be used with very high inaccuracy, in cases where the solution resistance is very high and / or the measurement geometry is very unfavorable and, as a result, the ohmic potential drop appearing in the measured potential data is significant, which makes the evaluation extremely difficult. Such cases include DCRM measurements, low-water, typically hydrocarbon bulk phase systems, low-salinity, high-purity (i.e., high-resistivity) aqueous systems, and wet gas phases.

[0006] The state of art of the field is analyzed on the basis of the documents C. Zausinger, K. Osterminski and C. Gehlen: Transient and gradient analyses of depolarization criteria, Valuable tools in chloride-induced rebar corrosion monitoring, Materials and Corrosion 2022; 73:932-939 (hereinafter: Announcement). The Announcement reflects the standard procedures currently in use, and based on them, it is easy to explain the essential innovations of the present invention.

[0007] The Announcement includes the method of measuring the polarization resistance, in which the current that can be measured at the moment before the circuit is interrupted and after the circuit is interrupted, and measured after the ohmic potential drop decay, i.e. the value which does not contain ohmic potential drop, are used to determine the polarization resistance (equation (5). At the same time, the correlation provided by the Authors (which, by the way, reflects the standard industrial unpretentiousness of the field) is extremely imprecise, and the calculation procedures which are the subject of the present invention serve to eliminate exactly these inaccuracies. The procedure stated in the Announcement is insufficiently precise for at least the following reasons:

[0008] 1 . The schematic transient decay diagram of the corrosion systems also presented by the Authors actually consists of a combination of many exponential decay curves. In the initial phase of the decay transient, where the change is very fast, therefore it is difficult to produce accurate data from a measuring technique the filters, generally applied on the potential measurement inputs used for measuring of the typically noisy corrosion systems, exert a distortion of the input signals. This filtering, especially if its time constant is larger, introduces an additional time-dependent transient into the measured potential data, which in turn reflects the characteristic of the measuring circuit and not the measured system. Therefore, the potential value existing at the moment of switch-off can only be determined with great inaccuracy (because the potential changes very quickly in this range), and in order to obtain more accurate data, extrapolation must be applied to the moment of switch-off based on the data measured later. The authors of the Announcement are apparently aware of this, because they propose a rudimentary and for several reasons objectionable, but at least in principle adequate method for determining the value of the initial potential (Figure 5 and the relevant explanation). The extrapolation can be linear, if we otherwise know that the process in the current potential range is accurately described by the Stern-Geary equation (claim 1), or the polynomial fitting described in the description (claim 2) is otherwise required. 2. At the end of the decay transient, the potential settles more or less to a more or less constant potential. The Authors identify this potential with the corrosion potential (cf. the comments after equations (6), (7) regarding Et30A, Et30C, but wrongly, because at the end of the decay transient, the transport processes change the concentration conditions on the surface and the longer, slower potential changes are already the consequence of these processes. Therefore, the value of the corrosion potential assigned to the charge transfer process must be determined from the points taken in the middle region of the decay transient, where the discharge of the capacity of the double layer actually occurs with more or less accuracy through a polarization resistance that can be considered constant. The relevant method (claim 1) enables the determination of the appropriate value of the corrosion potential.

[0009] 3. Equations (6), (7) of the Announcement treat as evidence that the current-potential relationship is linear, i.e., it follows the Stern-Geary equation, in an arbitrarily wide potential range. This is a very rough simplification which can lead to a significant error in the corrosion rate calculation. In corrosion practice, linearity, i.e. the validity of the Stern-Geary equation, is usually considered to be valid in a polarization range of 10-15 mV at most. An exact limit cannot be given, because it depends on the value of the cathodic and anodic Tafel slopes on the one hand, and on the other hand on the expected accuracy of the calculation. The informative data related to this, the examination of the quantity is a novelty of the present invention (claim 2). In those regions where the accuracy of the linear approximation is not adequate, a linearized form of the Butler-Volmer equation is used, the version of which can be used under the conditions of transient decay, also as a novelty, is included in claim 2.

[0010] It should be added to the points 1-3 above that the determination of the polarization resistance is not yet sufficient to determine the corrosion rate. This also requires the determination of the sum of the reciprocals of the anodic and cathodic Tafel slopes, which is also made possible by the calculation procedure described in claim 2. It is to be noted that in corrosion practice, due to the lack of the related theory, the sum of the reciprocals of the Tafel slopes was previously replaced by empirical values, which in itself can be a significant source of error. In this regard, only the calculation procedure described in claim 2 provides methods that can be used in p ra cti ce . (End of evaluation of the Announcement.)

[0011] It has been realized that the experimental methodology detailed in the description and the associated sensor types make electrochemical polarization methods applicable in many such systems. The most important element of this methodology is that the measurement is carried out after the polarization, by interrupting it, making use of the capacitive properties of the interfacial double layer.

[0012] The invention is a method for determining the corrosion rate of metallic structural materials also in media where the ohmic potential drop is very high. At small polarizations, by measuring the polarization resistance and using the Stern-Geary equation from the stationary current value before the polarization is switched off (which is the same as the Faraday current density value immediately after switching off) and from the first polarization value after switching off, which no longer contains the ohmic potential drop, and from the quantity the corrosion rate is determined by first measuring the current and voltage at fixed polarization potential. In doing so, during the stationary polarization, the system is set to an approximately constant current and potential value. After that, by turning off the current, a transient potential decay (depolarization) is induced. Then, the potential-time function of the system is measured on sections of suitably chosen length by sliding fitting the above equation. In order to eliminate disturbing factors arising as a result of transport processes, the most probable time constant t0of the transient depolarization and the most probable corrosion potential of the depolarizing system, as well as the polarization and the dimensionless reduced time of the system is determined by linear transformation of the differential equation describing the transient decay, i.e., by using the equation or equation (8) below. During this, the subrange of the potential¬ time function that results in the best linear fit of the measurement data series is selected. The fitted parameters determined in this way are used to determine the parameters and the transformed measurement data That is, the dimensionless differential equation of the system is defined using these quantities: 1, cf. the solution of the differential equation (4). If, based on the measurement results, this relationship is fulfilled with a good approximation even at the beginning of the transient decay (i.e. the Stern-Geary equation can also be considered valid at the beginning of the transient decay), then the value of the polarization resistance is calculated based on:RP =

[0013] The present invention also provides a method for determining the corrosion rate of metallic structural materials also in media where the ohmic potential drop is very high, at higher polarizations. Our method is applicable in cases where the Stern-Geary equation is no longer valid with the required accuracy. In the method, the polarization resistance is measured and the sum of the reciprocals of the cathodic and anodic Tafel slopes is determined. Calculation is carried out from the stationary current value before switching off the polarization (which is the same as the Faraday current density value immediately after switching off) and after switching off from the first polarization value which no longer contains an ohmic potential drop, and from the quantity First, current and voltage are measured at a fixed polarization potential. In doing so, during the stationary polarization, the system is set to an approximately constant current and potential value. After that, by turning off the current, a transient potential decay (depolarization) is induced. During this process, the potential-time function of the system is measured on sections of suitably chosen length by sliding fitting of the equation (8) or (9) below. In order to eliminate disturbing factors arising as a result of transport processes, the most probable time constant of the transient depolarization and the most probable corrosion potential Ecorrof the depolarizing system, as well as the polarization and the dimensionless reduced time of the system are defined by linear transformation of the differential equation describing the transient decay. That is, the equation or Equation (8) below is applied by choosing the subrange of the potential-time function that results in the best linear fit of the measurement data series. The fitted parameters determined in this way are used to determine the parameters and the transformed measurement data and In doing so, the dimensionless differential equation of the system is defined using these quantities: and its value is determined from the measurement data by polynomial extrapolation to the time From this, the value of the polarization resistance: is calculated. Furthermore, the variables of the equation In ’ are determined for I positive and negative polarization points, preferably closest to the In this way, I pairs of points are obtained, and by substituting them the following expression is obtained, for which the derivatives are obtained by numerically deriving the function and by using it the corrosion current: is calculated.

[0014] Further, the invention is an electrode that is suitable for a short-term significant current load with a very small potential change. It is also suitable as a counter electrode and reference electrode. The electrode is attached to the sensors to implement the methods. The electrode consists of a cylindrical, plastic or ceramic electrode housing with insulating material. A silver wire is routed through the electrode housing, which is connected to a silver disc electrode to which a silver foam electrode is soldered. Furthermore, the silver disk electrode and the silver foam electrode are surrounded by a sintered glass diaphragm.

[0015] Further, the invention is a sensor for measuring the corrosion rate of the examined structural material in liquid hydrocarbons containing water and salt, for the implementation of the method according to the present invention, which consists of a sensor head and a threaded sensor connector. The sensor is inserted, into the examined structural material by means of a threaded sensor connector under the sensor head, through a threaded pipe stub fixed preferably by welding to the examined structural material. A cylindrical steel working electrode is attached to the threaded sensor connector, which is grounded to the examined structural material. The first plastic intermediate piece is fixed inside the working electrode, and then the electrode according to the invention is also fixed to it as a counter electrode and a reference electrode.

[0016] Further, the invention is a sensor for measuring the corrosion rate of the examined structural material in wet gases, for the implementation of the method according to the invention, which consists of a sensor head, a threaded sensor connector, and a sensor body. The sensor is fitted to the examined structural material through a threaded pipe stub, preferably fixed by welding, into the examined structural material with the threaded sensor connector under the sensor head. The sensor body is grounded to the examined structural material as a counter electrode. Attached to the sensor body is the first plastic intermediate piece made of hydrophilic material, the working electrode made with a steel ring, the second plastic intermediate piece made of hydrophilic material and the reference electrode formed with a steel cap.

[0017] The invention will be described with reference to the attached drawings in which:

[0018] Figure 1 shows the equivalent circuit of the metal-electrolyte interfacial double layer in the simplest case. RPis the polarization resistance, Rsis the solution resistance, CDLis the interfacial double layer capacitance;

[0019] Fig. 2 is the electrochemical system of Fig. 1 , expanded with the circuit elements used for polarization (bottom) and potential measurement (top);

[0020] Figure 3 is a schematic drawing of the HLPC electrode in section;

[0021] Figure 4 is a schematic diagram of the DCRM-LC sensor in section;

[0022] Figure 5 is a schematic diagram of the DCRM-HC sensor in section;

[0023] Figure 6 is a schematic diagram of the HRCM sensor in section;

[0024] Figure 7 is a schematic diagram of the GCM sensor in section;

[0025] Fig. 8 is the general circuit diagram of the measurement arrangement. The experimental methodology used in the methods according to the present invention and the associated sensor types make the electrochemical polarization methods applicable in many such systems. The most important element of this methodology is that the measurement is carried out after the polarization, interrupting it, making use of the capacitive properties of the interfacial double layer.

[0026] The basic equation for calculating the corrosion rate is the Butler-Volmer equation: where JFis the so-called Faraday current density, which involves an electrochemical reaction, Jcorris the corrosion current density, which is proportional to the corrosion rate, EPis the polarized potential (which does not include the ohmic potential drop in the solution due to the current), is the corrosion potential (the potential value that the corrosion system reaches when the sum of the anodic and cathodic currents is zero) and bAand bcare the so-called anodic and cathodic Tafel-slope. Introducing the concept of polarization: is obtained from equation (1):

[0027] The experimental determination of the relation JFvs. (and consequently the determination of the corrosion current Jcovv) is often difficult, so in practice the Stern-Geary equation is often used to determine the corrosion rate:

[0028] Where RPis the polarization resistance. Its value can be determined from the polarization current density data at small polarizations. The Stern-Geary equation can be derived from the Butler-Volmer equation by performing its Taylor (McLaurin) expansion and leaving only the linear terms. Consequently, the method only can be used with relatively small 1 values where |?71 « bA, bc. Another flaw of the method is that the quantities bA, bcmust be determined from independent measurements or - as is often done in practice - must be replaced by empirical values, which can be a significant source of error in the calculated corrosion rate values.

[0029] Metal-electrolyte interfaces not only have electrochemical (Faraday) properties described by equations (1), (2), but also capacitive properties. From a practical point of view, the system can be described with sufficient accuracy with the model that includes a so-called interfacial double layer capacity (CDLsee Figure 1) connected parallel with the polarization resistance. In this system, parallel to the Faraday current, a capacitive current path is also opened through the double layer capacitance, and the total measurable current is the sum of the two quantities: where Jcis the value of the capacitive (i.e. flowing through the double layer capacitance) current density. Using the above relationships, the aim of the invention is to significantly reduce the inaccuracy of the electrochemical polarization methods for determining the corrosion rate and to provide a measurement method by which the methods described above can be extended to systems with high ohmic potential drop, such as DCRM (Direct Corrosion Rate Measurement) measurement procedures, to systems with low water content, typically hydrocarbon block phase, and low-salinity, high-purity (that is high-resistance) water systems and wet gas phases.

[0030] Based on equation (3), in the process of the transient potential decay after switching off, it is true that

[0031] Based on equation (4), however, the corrosion rate could only be determined with a large error, for at least the following three reasons:

[0032] 1. Determining polarization directly from experimentally measurable potential-time data is never accurate. Current meter inputs, with relatively long time constant (>100 ps) used for potential measurement of typically very noisy corrosion systems, at the moment of switching off the current and the beginning of the transient decay make the potential reading inaccurate in the first time-range after switching off, just when the potential changes very quickly and therefore there would be a great need for accuracy.

[0033] 2. At the end of the transient depolarization, when the potential would approach the corrosion potential, transport processes typically make the potential settling to the stationary value associated with the decay process uncertain.

[0034] 3. Lack of accurate experimental determination of the anodic and cathodic Tafel-slopes in equation (2).

[0035] With the present invention, among other things, the three problems listed above are eliminated, and since for determining the kinetic parameters the values of the transient decay of the potential that can be measured in the current-free state according to equation (4) are used, the determination is therefore not disturbed by the distorting effect of the ohmic potential drop resulting from the solution resistance.

[0036] To determine the exact value of the corrosion potential mentioned in point 2 above, the values of the potential transient decay are used in the following relationship: By introducing the transient time constant obtained,

[0037] And the equation with the appropriate transformations is linearized: from which the corrosion potential Ecorrcan be determined by calculation. For the calculation, we use the section of function that gives the best linear fit (highest linear correlation coefficient). The section providing the best linear fit is determined by sliding fitting, during which we fit a contiguous sub-range (at least 5 points) of the measurement points, and this is done over the entire range by adding another point to the sub-range from one end and leaving one at the other from the end.

[0038] In principle, the derivative can be determined from the measurement data, but the numerical determination of the derivative functions can be very imprecise if the noise of the measurement data is high, so it is advisable to use the integral (or possibly the double integral) of equation (8):

[0039] Equation (9) is essentially insensitive to zero-mean noise. With the method, we can produce the value of the polarization q with high accuracy from the measurable data EP= EP(t) .

[0040] The dimensionless reduced time is defined using the transient time constant t0:

[0041] With which (6) it takes the following form: with which the value of 7 at any point of the curve can be calculated from the value of the difference between another point and the time, from which it follows that the value of 7(0) for time can be calculated from any point in the time range where (11) is fulfilled with a good approximation:

[0042] The above relations (5)-(12) are true for relatively small 7 values with a good approximation (cf. the comments after equation 2), for larger polarization values the error of the r approximation can be significant, but this gives an option to calculate the quantit which can be used to refine the calculation of the corrosion current in equation (2). It is known (Zoltan Lukacs, Kristof Tamas: Determination of kinetic parameters from a new quadratic approximation of the Butler- Volmer equation, Journal of Electroanalytical Chemistry, 918 (2022), 16443.) that

[0043] The quadratic approximation (13) is very accurate (with an error below 1%) if Srj < 0.5. By determining the value of S, the determination of the corrosion rate can significantly be refined according to equation (2) (it can be shown that equation (14) is equivalent to the commonly used Stern-Geary equation (2):

[0044] (14)

[0045] For this purpose, it is used that n the current-free transient decay state according to (4), and by substituting (6) into (13): is obtained, from which the parameters A and S can be determined.

[0046] Note that the definition of the parameter A is not necessary according to (14), so it can be omitted from the equations, by which an unnecessary degree of freedom is eliminated. For this, we stipulate that during the measurement, we measure the potential at the same I number of time points, during the transient decay following the positive and negative polarization.

[0047] Equation (15) therefore becomes as follows: where Ti +and ri;_ are the reduced time of the i-th measurement point, are the polarization of the i-th measurement point, during the transient decay after the positive and negative polarization.

[0048] Let us subtract equation (16b) from (16a), and take into account that the signs of are always opposite, thereby the value of A is dropped from the equation:

[0049] In principle, the value of S can be determined from the equation, however, by double dividing with polarization the weight of the points with low polarization, but with a large uncertainty for several reasons is greatly increased, so it is compensated with an appropriate weight factor in the summing:

[0050] And it is obtained that the quantity — + — which is necessary to calculate the corrosion current density, can be calculated with the following relation:

[0051] The polarization resistance Rpcan be determined in the case of higher polarizations according to the following equation:

[0052] And (cf. equations (3) - (5))

[0053] Where the value 0) is obtained from the current value measured at the end of the stationary polarization before the transient depolarization, the values of should be extrapolated from the initial phase of the transient decay, where the distortion of the measuring system is already acceptably low.

[0054] With this, the values of all the parameters required for the calculation of the corrosion current according to equation (2) are given. The corrosion rate is directly derived from the corrosion current according to equation (19): where K is the corrosion rate g / cm2 / year in unit of measure, s is the number of seconds in a year, M is the molar mass in g / mol, z is the (molar) electron number change during the reaction and F is the Faraday constant, 96494 As / mol.

[0055] It should be emphasized that small and large polarization are not two measurement-evaluation procedures that can be separated from each other, but branches of a new, complex measurement-evaluation procedure based on the measurement results. The measurement results (i.e. decay potential vs. time transient) go through at least the following three rounds of quality control until the final result is obtained: 1. Determination of the most probable value of the corrosion potential and the decay time constant based on equations (8) and (9), using the sliding fitting method.

[0056] 2. The analysis of the point pairs based on equation (11). The subdomain that falls within the range a predetermined quality assurance parameter is accepted and the value of ln|?7(0)| can be calculated based on equation (11), and from it the polarization resistance can be calculated based on claim 1 . If there is no such subrange, the measurement result must be discarded.

[0057] 3. If it was possible to determine the value of In | (0) | and the polarization resistance based on the subdomain of the previous point (ii), then based on this subrange and between the points T = 0, the quantity must be determined, based on equation (15) or (17), and statistical significance tests (e.g. t-test) must be performed regarding the applicability of the obtained results. If the result is positive, the corrosion rate must be determined using the obtained parameters using equations (14) and (19).

[0058] In general, Fig. 1. shows substitution switching of the metal-electrolyte interfacial double layer in the simplest case. Where RPis the polarization resistance, Rsis the solution resistance, CDLis the interfacial double layer capacitance.

[0059] The individual potential values are measured against a reference electrode (ER) in a known manner.

[0060] The potential of the reference electrode drops out when determining the polarization

[0061] It should be noted that t] is the ohmically compensated polarization, i.e. it does not include the ohmic potential drop occurring during the reaction. The value of RPcan be directly determined experimentally at small polarizations, however, to determine the corrosion current, it is also necessary to know the Tafel slopes. In principle, these can be determined from equation (1a), however, in practice this is usually very imprecise, basically for two reasons:

[0062] 1) transport processes can distort the shape of the curve and;

[0063] 2) in medium-high resistance electrolytes, the ohmic potential drop occurring in the solution as a result of current conduction is added to the polarization value and only the sum of these two quantities can be measured, which also has the consequence that the corrosion current must be calculated from distorted values, often with very large errors.

[0064] In corrosion engineering practice, empirical values are often used for the Tafel slope to calculate the corrosion current according to equation (1a), typically 60 / ln10 and 120 / ln10 mV for the anodic and cathodic slopes, respectively.

[0065] Equations (1a), (2) can only be satisfied with a good approximation if the potential change is sufficiently slow, because the interfacial double layer has a capacitive term too (double layer capacity, see Figures 1 and 2). If the polarization is small enough to satisfy equation (2) with sufficient accuracy, then the transfer function of the corrosion system can be represented by the substitution circuit shown in Fig. 1.

[0066] If we exceed the relatively narrow potential range where equation (2) can be considered valid, the value of RPin Figure 1 is not constant and decreases rapidly with increasing the polarization (cf. equation (1). On the other hand, the value of the double-layer capacitance is almost constant in wide potential ranges, and possibly changes linearly with the potential.

[0067] The transfer function, i.e. the potential-current characteristic, is typically determined by setting the value of the potential or current according to a sort of program and re-measuring the current or potential. (Potentiodynamic and galvanodynamic polarization.)

[0068] The method works well if Rs« RP(with the notations in Figure 1). In many corrosion systems, however, the ohmic resistance of the solution and the resulting ohmic potential decay make it difficult, and in many cases practically impossible, to determine the transfer function of the interfacial double layer and, consequently, the corrosion current and the corrosion rate. Taking into account the ohmic and capacitive characteristics of the interfacial double layer it can be seen that there is an alternative possibility for measuring the transfer function. To outline this, Fig. 1 is expanded with the measurement system shown in Fig. 2, where the switch opens and closes the polarizing circuit very quickly (in a few ps). Jcis the capacitive current density, JFis the Faraday current density. Charge transfer, i.e. corrosion processes, are exclusively related to the Faraday current (by the Faraday current the current that causes changes in the material balance is meant), so their separation and determination is a fundamental task. In Figure 2, the circuit of the control unit, i.e. the path of the polarizing current, can be switched on and off. The measurement procedure has two phases: 1) stationary polarization and 2) transient depolarization.

[0069] 1) During stationary polarization, the switch, i.e. the lower polarizing circuit, is closed. In this state, according to any program (potentiostatic, galvanostatic or constant output voltage), the corrosion system is stationary polarized from the corrosion potential. If the polarization is carried out with an optimal duration (of the order of 1 s), then the potential and current values are set to approximately constant values. With the current and voltage measuring instrument (which is in practice a digital recorder) the establishment of the stationary state with a high sampling rate (with a resolution of at least 1 ms) is followed. During the process, the interfacial double layer capacity CDLis recharged.

[0070] 2) At the end of the process, the switch is opened, thereby eliminating the external potential effect that maintains the polarization, and the excess charge accumulated on the capacitance is discharged through the polarization resistance. Taking into account that no current flows in the external polarizing circuit, therefore the voltage across the solution resistance Rsis zero, and the potential measuring unit can directly measure the voltage and voltage changes in the interfacial double layer. (The physical system also includes the potential contribution of the interfacial double layer of the reference electrode, but this does not change over time and is not shown here for the sake of clarity.) The entire measurement procedure, which is aimed at compensating the ohmic potential drop, is based on this phenomenon.

[0071] Before the switch is opened, the sum of the current densities can be described by equation (3), after the switch is opened by equation (4). The current density value (J(T = O)required for the calculations is determined based on the current value at the end of the stationary polarization, and the potential values are determined from the potential data measured during the transient depolarization, which do not include the ohmic potential drop. This procedure is made possible by the fact that the voltage drop across the polarization resistance immediately before and after switching off is the same.

[0072] Based on the above, the corrosion rate calculated from potential data measured during transient depolarization, i.e. in a current-free state, can be given, which does not contain distortions due to the lack of knowledge of either the ohmic potential drop or the Tafel slopes. This procedure provides the real values until the solution resistance increases so much that it is comparable to the input resistance of the potential measuring system and consequently distorts the measured potential data.

[0073] The described measurement and evaluation procedure makes the extension of corrosion monitoring procedures possible to systems in which electrochemical monitoring procedures were not applicable until now. In addition to the above results, new types of electrodes and sensors are also required. Figure 8 schematically shows the measurement arrangement including these new developments. This is disclosed only to the extent that provides sufficient information for one skilled in the art.

[0074] The measuring instrument 1 contains the control unit 2, the potential measuring unit 3, the switch 4 and the current meter A and voltage meter V, although they are shown separately in the Figure for better identification. Minimum specifications for the measuring instrument 1 : The control unit 2 is capable of outputting a predefined signal form at the sufficiently noise-free power output in at least in a potential range of + / - 5V and current range of + / - 200 mA with a resolution of at least 1 ms. In addition, it can break the output circuit in a maximum of 0.1 ms, at a predetermined time (from the beginning of the measurement).

[0075] The potential measuring unit 3 can measure potential on at least three channels with a resolution of at least 1 ms and at least 1 mV, store the measurement data and send it to the client computer. Furthermore, it is able to re-measure the issued potential signal (between the 7 working electrodes and the 5 counter electrodes) on one of the channels for control purposes. It measures the potential difference between the working electrode 7 and the reference electrode 6 on another channel.

[0076] It measures the output current on an additional channel, either through a built-in current measuring circuit or an external shunt resistor.

[0077] By electrode, a structure made of metallic material is meant which, when in contact with an electrolyte, gives an electrochemical reaction and a potential can be measured on it against another electrode, and its potential changes when a current passes through it (polarization).

[0078] The sensor is an assembled structural unit of several electrodes, which can measure an electrochemical or corrosion property. The sensor is installed in the structure to be examined (e.g. pipeline, reactor) with a threaded connection, which also fulfills the role of grounding. The electrodes can be of three types: working electrode, reference electrode and counter electrode. The working or counter electrode may be grounded to the structure, but this is not absolutely necessary. The working electrode is the one whose potential changes are followed with the reference electrode. The only purpose of the counter electrode is to close the path of the current necessary to achieve polarization in the circuit.

[0079] During the execution of the procedure, several electrodes, depending on the measurement tasks are used, which will be presented below.

[0080] First, the so-called High Load Polarization Cell (HLPC) electrode shown in Figure 3 is described. Its structure is as follows: an insulating, plastic or ceramic electrode housing 16 surrounds the silver wire 17, which is connected to the silver disc electrode 18, to which a silver foam electrode 19 is soldered. The silver disk electrode 18 and the silver foam electrode 19 are surrounded by the sintered glass diaphragm 20.

[0081] The HLPC electrode is a special electrode that can withstand extremely high currents for a short time without suffering significant polarization. It consists of two main parts: a specially treated diaphragm and an also specially treated silver / silver chloride electrode. The HLPC electrode fulfills a dual role in sensors: it can be a counter electrode with a high load capacity, and if the load is smaller and as a result its potential changes only minimally depending on the passing current, it can also be used as a reference electrode.

[0082] The sintered glass diaphragm must provide ionic conduction with a low resistance, but at the same time form a strong barrier against other transport processes (diffusion). There is a significant concentration of chloride inside, but it should only be released into the environment to a very limited extent. In order to meet the contradictory requirements of the sintered glass diaphragm described above, the sintered glass housing is treated in the following way:

[0083] After the sinter glass housing is geometrically designed (cut to size), it is placed in a smaller beaker, which is placed into a larger beaker partially filled with water, then the whole thing is covered and the water is boiled. It is feasible also to use containers which can be placed under vacuum, so there is no need to boil, the operation can be done at a lower temperature. After a while, the water vapor completely fills the larger beaker, almost displaces the air, and slowly diffuses into the sintered glass diaphragm and displaces the air from there as well. After a longer (approx. 30 minutes) boiling, it is assured that only a minimal amount of air is present in the diaphragm. Then the boiling is stopped and a concentrated (at least 5 mol / l) calcium chloride or barium chloride solution is poured into the small beaker onto the diaphragm (a strontium chloride solution would also be suitable, but due to the relatively frequent radioactive isotope of strontium it should be avoided), and as the system cools, the external air pressure pushes the calcium chloride solution into the porous material of the diaphragm. After complete cooling, the calcium chloride solution is poured from the diaphragm and a concentrated (concentration of at least 5 mol / L) sodium sulfate or potassium sulfate solution is poured over it. The following reaction takes place in the system:

[0084] Na2SO4 + CaCI2 = 2NaCI + CaSO4

[0085] The formed calcium sulfate clogs the channels of the porous sintered glass, which significantly reduces the rate of the transport processes, but due to its water content, it retains its ionic conductive character and, together with the accumulated significant amount of soluble chloride salt, continues to ensure low-resistance conduction, thus the system fulfills the controversial set of requirements. In a few days, the sodium sulfate passes through the entire space of the diaphragm and the conversion indicated in the equation above can be considered complete.

[0086] The HLPC electrode can be used advantageously in liquid hydrocarbons for monitoring the water and salt content of hydrocarbons by electrochemical methods.

[0087] Knowledge of water and salinity content is extremely important in technological systems containing liquid hydrocarbons (e.g. distillation equipment). This goal is achieved by the HLPC electrode in a special circuit. In this process, two HLPC electrodes are connected opposite each other in the medium to be examined, where one is the working electrode and the other is the counter electrode. Furthermore, the counter electrode is also connected to the input of the reference electrode of the instrument (Figure 8). With this, the reference electrode and the counter electrode inputs are shorted to each other and the real and imaginary conduction of the system is determined by obtaining the current in the form of a voltage falling on a resistor of known value connected in series. The measurement can be performed using the transient polarization / depolarization method, or by re- measuring a sinusoidal alternating voltage signal. Since the resistance of the HLPC electrodes is negligible compared to the resistance of the medium, only the resistance and impedance of the medium are measured in the re-measured current and voltage signals. The system has a single time constant, which is determined by the electrical properties (conductivity and capacitance) of the solution between the two electrodes. Since the resistance of the medium (liquid hydrocarbons) is typically so great that its capacity already affects the impedance of the solution in the kilohertz frequency range, the real and imaginary part of the impedance of the solution, i.e. its ohmic resistance and capacity, can be measured. As a result, the value of the relative permittivity of the solution is obtained, from which, after proper calibration, the permittivity of the solution and its water content can be calculated in a known manner. The salt content of the solution can be calculated from the value of the water content and the ohmic (real) impedance, also after proper calibration. It should be noted that the method is fundamentally different from traditional capacitance and permittivity measurements in that, due to the use of HLPC electrodes, the real part of the impedance can also be evaluated and utilized.

[0088] Figure 4 shows the DCRM-LC (Low Conductivity, Low-Corrosiveness) corrosion monitoring sensor. Via a threaded pipe socket, the sensor is connected to the pipeline which corrosion rate is measured. The reference electrode is a steel ring, which is preferably placed as close as possible to the wall of the pipeline, but is not in galvanic contact with it. The counter electrode, also made of steel, is approximately in the geometric center of the tube, its special, crown-like design gives a large electrode surface. This reduces the electrode polarization and also the solution resistance. The working electrode is the sensor body or the pipeline itself, into which it is screwed through the pipe stub.

[0089] In Figure 4 the structure of the DCRM-LC sensor is shown, which consists of the following elements: sensor head 10, threaded pipe stub 11 , through which the sensor is inserted into the examined structural material 8 with the threaded sensor connector 12. Then, the sensor body 13 grounded to the examined structural material 8 extends into the interior of the examined structural material 8. The first plastic intermediate piece 14, the reference electrode 6 realized with a steel ring, the second plastic intermediate piece 15, and the large-surface steel counter electrode 5 are connected to the sensor body.

[0090] This type of electrode is used in clean or very clean water systems, where chloride pollution must be avoided, and the low corrosivity of the system and the relatively high polarization resistance of the pipe wall, i.e. its relatively low current consumption, ensure that the amount of current supplied by the counter electrode is sufficient.

[0091] The DCRM-HC (High Conductivity, High Corrosiveness) corrosion monitoring sensor shown in Figure 5 is basically suitable for the DCRM (Direct Corrosion Rate Measurement) measurement procedure. The structure of the DCRM-HC sensor is as follows: threaded pipe stub 11 advantageously fixed by welding to the examined structural material 8, through which the sensor is inserted into the examined structural material 8 with the threaded sensor connector 12 under the sensor head 10. Then, the sensor body 13, which are grounded to the examined structural material 8 and functions as working electrode 7, extend into the interior of the examined structural material 8. The first plastic intermediate piece 14, the reference electrode 6 realized with a steel ring, the second plastic intermediate piece 15, and the counter electrode 5 realized with the HLPC electrode are connected to the sensor body 13.

[0092] Like the DCRM-LC sensor, the DCRM-HC is connected via a threaded pipe socket to the pipeline whose corrosion rate is measured. The reference electrode here is also a steel ring, which is advantageously located as close as possible to the wall of the pipeline, but is not in galvanic contact with it. The difference compared to the DCRM-LC sensor is the counter electrode, where the counter electrode is an HLPC electrode, in order to allow the system to deliver higher currents.

[0093] Figure 6 shows the HRCM (High Resistance Corrosion Monitoring) sensor.

[0094] Its design is similar to that of the previously presented sensors: it has a threaded pipe stub 11 advantageously fixed by welding to the examined structural material 8, through which the sensor is inserted into the examined structural material 8 with the threaded sensor connector 12 under the sensor head 10. The threaded sensor connector 12 continues in a cylindrical steel working electrode 7, which is grounded to the examined structural material 8. The first plastic intermediate piece 14 is fixed to the threaded sensor connector 12, within the working electrode 7, and then the HLPC electrode, which functions as a counter electrode 5 and as a reference electrode 6, is also fixed to this.

[0095] This sensor is the most advantageous for electrochemical corrosion measurement procedures in extremely high resistance systems, such as wet diesel or gasoline. During this, an HLPC electrode is built into the metal cylinder, the corrosion rate of which is examined. The polarization is realized between the metal cylinder and the HLPC electrode. The metal cylinder electrode is grounded to the material of the structure through the thread. Since it is used in extremely high resistance systems, extremely small currents pass through the HLPC electrode, therefore it functions as both a counter electrode and a reference electrode, accordingly the counter and reference electrode inputs are short-circuited to each other in the measuring instrument. As a result of the high solution resistance, extremely large ohmic potential drops occur during polarization, but the depolarization process can be followed in the same way based on equations (7) and (7a). The GCM (Gas Corrosion Monitoring) gas corrosion sensor according to Figure 7 and the related process are suitable for electrochemically measuring the rapid corrosion rate in gases containing condensable water. The construction of the GCM sensor is as follows: the sensor is inserted, into the examined structural material 8 by means of a threaded sensor connector 12 under the sensor head 10, through a threaded pipe stub 11 fixed preferably by welding to the examined structural material 8. Then, the sensor body 13 which is grounded to the examined structural material 8 and acts as counter electrode 5, extends into the interior of the examined structural material 8. Attached to the sensor body 13 is the first plastic intermediate piece 14 made of hydrophilic material, the working electrode 7 made with a steel ring, the second plastic intermediate piece 15 made of hydrophilic material and the reference electrode 6 formed with a steel cap.

[0096] The GCM sensor consists of a grounded metal housing — in this case used as a counter electrode — an annular working electrode separated from the counter electrode by a hydrophilic plastic or ceramic insulating spacer, and a closing metal cap at the end of the electrode body separated by another hydrophilic plastic or ceramic insulating spacer and connected as a reference electrode. The course of the measurement during the procedure is as follows: it is assumed that the water content of the gas condenses on the hydrophilic plastic or ceramic insulating spacer, thereby creating ionic conduction between the working and counter electrodes, and between the working and reference electrodes. Since the geometry of the electrochemical system is unfavorable and the resistances — despite the ionic conduction — are relatively high, this system also can only be measured using the transient depolarization method. As a first step, a polarized state (between the working and counter electrodes) is created and it is monitored by measuring the potential between the working and reference electrodes. If the polarization settles into a stationary state, the polarizing circuit is interrupted and the polarization resistance is determined by the transient depolarization method using equations (7), (7a) and (8), and then the corrosion rate is determined. If the system can be sufficiently polarized, equations (10-18) are used, which give more accurate results.

Claims

Claims1 . Method for determining the corrosion rate of metallic structural materials also in media where the ohmic potential drop is very high, by measuring the polarization resistance from the stationary current value before the polarization is switched off (which is the same asthe Faraday current density value immediately after switching off) and from the first polarization value after switching off, which no longer contains the ohmic potential drop, andfrom the quantity the corrosion rate is determined by first measuring the current andvoltage af fixed polarization potential, in doing so, during the stationary polarization, the system is set to an approximately constant current and potential value, after that, by turning off the current, a transient potential decay (depolarization) is induced, then, the potential-time function of the system is measured on sections of suitably chosen length by sliding fitting the above equation, characterized in that at small polarizations the measuring is performed by using the Stern-Geary equation during which in order to eliminate disturbing factors arising as a result of transport processes, the most probable time constant t0of the transient depolarization and the most probable corrosion potential Ecorrof the depolarizing system, as well as the polarization and the dimensionless reduced time of the system isdetermined by linear transformation of the differential equation describing the transient decay,1.e. by using the equation in such a way that thesubrange of the potential-time function which results in the best linear fit of the measurement data series is selected, the fitted parameters determined in this way are used to determine the parameters Ecorrand t0and the transformed measurement data then the dimensionless differential equation of the system is defined using these quantities:and if based on the measurement results this relationship is fulfilled with a good approximation even at the beginning of the transient decay (i.e. the Stern-Geary equation can also be considered valid at the beginning of the transient decay), then the value of the polarization resistance is calculated based on:

2. Method for determining the corrosion rate of metallic structural materials also in media where the ohmic potential drop is very high, by measuring the polarization resistance from the stationary current valuebefore switching off the polarization (which is the same as the Faraday current density value immediately after switching off) and from the first polarization value (T](T = 0)), after switching off, which no longer contains the ohmic potential drop, as well as from the quantitydit is determined so that current and voltage are first measured ata fixed polarization potential, in doing so, during the stationary polarization, the system is set to an approximately constant current and potential value, then, by turning off the current, a transient potential decay (depolarization) is induced, during this process, the potential-time function of the system is measured on sections of suitably chosen length by sliding fitting of the above equation, characterized in that at higher polarizations the measurement is carried out by determining the cathodic and anodic Tafel slopes, during which in order to eliminate disturbing factors arising as a result of transport processes, the most probable time constant t0of the transient depolarization and the most probable corrosion potential Ecorrof the depolarizing system, as well as the polarization η = EP- Ecorrand the dimensionless reduced time T = t / t0of the system are defined by linear transformation of the differential equation describing the transient decay, that is, the equation ISapplied by choosing the subrange of the potential-time function that results in the best linear fit of the measurement data series, and the fitted parameters determined in this way are used to determine the parameters Ecorrand t0and the transformed measurement data and T, in doing so, the dimensionless differential equation of the system is defined using these quantities: f(T) =and its value is determined from the measurement data by polynomial extrapolation to the time T = 0, from this, the value of the polarization resistance: s calculated, furthermore, the variables of the equation Inare determined for I positive and negative polarization points, preferably closest tothe T = 0, in this way, I pairs of points are obtained, and bysubstituting them the following expressionis obtained, for which the derivatives are obtained by numerically deriving the functionand by using it the corrosion current is calculated.

3. Electrode that is suitable for a short-term significant current load with a very small potential change, and it is also suitable as a counter electrode and reference electrode, the electrode is attached to the sensors to implement the methods for determining the corrosion rate of metallic structural materials according to claims 1 and 2, characterized in that it consists of a cylindrical, plastic or ceramic electrode housing (16) with insulating material, a silver wire is routed through the electrode housing (16), which is connected to a silver disc electrode (18) to which a silverfoam electrode (19) is soldered, furthermore, the silver disk electrode (18) and the silver foam electrode (19) are surrounded by a sintered glass diaphragm (20).

4. Sensor for measuring the corrosion rate of the examined structural material (8) in liquid hydrocarbons containing water and salt, which consists of a sensor head (10) and a threaded sensor connector (12) to implement the methods for determining the corrosion rate of metallic structural materials according to claims 1 and 2, the sensor is inserted, into the examined structural material (8) by means of a threaded sensor connector (12) under the sensor head (10), through a threaded pipe stub (11) fixed preferably by welding to the examined structural material (8), characterized in that a cylindrical steel working electrode (7) is attached to the threaded sensor connector (12), which is grounded to the examined structural material (8), the first plastic intermediate piece(14) is fixed inside the working electrode (7), and then the electrode according to claim 3 is also fixed to it as a counter electrode (5) and a reference electrode (6).

5. Sensor for measuring the corrosion rate of the examined structural material (8) in wet gases consisting of a sensor head (10), a threaded sensor connector (12), and a sensor body (13) to implement the method for determining the corrosion rate of metallic structural materials according to claims 1 and 2, the sensor is inserted into the examined structural material (8) by means of a threaded sensor connector (12) under the sensor head (10), through a threaded pipe stub (11) fixed preferably by welding to the examined structural material (8), characterized in that the sensor body (13) is grounded to the examined structural material (8) as a counter electrode (5), the first plastic intermediate piece (14) made of hydrophilic material, the working electrode (7) made with a steel ring, the second plastic intermediate piece (15) made of hydrophilic material and the reference electrode (6) formed with a steel cap are attached to the sensor body (13).

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