JP2000162167A - Method for measuring faraday resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily measure the Faraday resistance (polarization resistance) in the state where the electrochemical effect on a sample metal is minimized by sweeping the voltage to be applied to a sample metal electrode in the vicinity of open voltage within the range where the analysis by Tafel's plot is not applicable. SOLUTION: The DC voltage between a reference electrode and a sample (metal) electrode is detected, and a current source is regulated so that the applied voltage shows a specified change. The Faraday resistance is determined from the voltage-current characteristic at that time. In this circuit, a resistor R, a solution resistor Re, a Faraday resistor Rp, a capacitor component Cd, and an electromotive force (battery) factor E are provided between a counter electrode and the sample metal electrode. In this measurement, a measuring system is handled as an electronic circuit element, and the voltage-current characteristic in the vicinity of open voltage is examined to determine the Faraday resistance. As the vicinity of open voltage, up to about ±20 mV is set relative to the open voltage as a standard.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for measuring Faraday resistance (polarization resistance). The present invention relates to a method for evaluating metal corrosion.

[0002]

2. Description of the Related Art There is a close relationship between metal corrosion and Faraday resistance (polarization resistance). If the metal corrodes in the electrolyte, some form of battery is formed. At this time, current flows due to dissociation of metal atoms into metal ions and electrons. This current is the corrosion current, and the Faraday resistance (polarization resistance) reduces the corrosion current.

A method of measuring the Faraday resistance (polarization resistance) includes a method of performing a Tafel plot by sweeping an applied voltage in a wide range centering on an open voltage of a sample metal electrode. When performing Tafel plots, typically ± 100
Although the voltage is swept in the range of about mV, the portion near the open-circuit voltage (open-circuit voltage about ± 20 mV) cannot be used because it does not form a straight line in the Tafel plot.

[0004]

In order to sweep the applied voltage so that Tafel plotting can be performed by sweeping the applied voltage, a large current needs to flow. As a result, the electrochemical reaction at the sample metal electrode becomes intense. Examples of the reaction (phenomenon) include generation of hydrogen gas when the applied voltage is negative, permeation of the generated hydrogen into the electrode metal, and dissolution of the electrode when the applied voltage is positive. These greatly change the electrode surface and change the electrode characteristics. Therefore, the Faraday resistance (polarization resistance) measured under the conditions using the Tafel plot is likely to be different from the original Faraday resistance (polarization resistance), and is hard to use as a scale for actual corrosion evaluation.

Several methods for measuring Faraday resistance (polarization resistance) that are less likely to disturb the electrode surface have been proposed. Among them, there are a method in which a minute AC voltage is applied to measure, a method in which a charge is given by a capacitor, and a method in which the voltage is obtained from a time change of the generated voltage ("New edition electrochemical measurement method" edited by The Electrochemical Society of Japan). These have a dynamic component in the measurement, and there may be a slight difference in the measured Faraday resistance (polarization resistance). Further, the configuration of the device itself is relatively complicated.

[0006]

This measurement method focuses on the equivalent electronic circuit of the measurement system, and sweeps the voltage applied to the sample metal electrode in the vicinity of the open circuit voltage to the extent that analysis by Tafel plot cannot be applied, and the Faraday resistance ( (Polarization resistance). That is, the method is a method for measuring Faraday resistance (polarization resistance) or a method for evaluating metal corrosion, in which a voltage applied to a sample metal electrode is swept within a range of an open voltage ± 20 mV or less.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The measuring system used in the present invention is one in which an electrode made of a sample metal for evaluating corrosion, an electrode of a counter electrode, and a reference electrode are immersed in an electrolytic solution. The measurement system forms a kind of battery. When simplified in consideration of the connection with the present measurement method, the basic functions of the measurement system can be represented by the equivalent electronic circuit of FIG. In this measuring method, as shown in FIG. 1, a DC voltage (applied voltage) between a reference electrode and a sample (metal) electrode is detected, and a current source is adjusted so that the applied voltage shows a predetermined change. . The Faraday resistance is obtained from the voltage-current characteristics at this time. In this circuit, the resistance (R), solution resistance (Re), Faraday resistance (polarization resistance) (Rp), capacitor component (Cdl), electromotive force (battery) factor (E) are provided between the counter electrode and the sample metal electrode. ), And Rp and Cdl are derived from the sample metal electrode. In the present measurement method, the counter electrode only plays a role as a path of a current flowing along with the measurement, and thus the factor of the electromotive force caused by the counter electrode is omitted in FIG. R in FIG.
Shows the solution resistance between the reference electrode and the counter electrode and the Faraday resistance of the counter electrode collectively.

In FIG. 1, the Faraday resistance (polarization resistance) is Rp. In this measurement, the measurement system is treated as an electronic circuit element, and the Faraday resistance (also referred to as polarization resistance, abbreviated as Rp in the following description) is obtained by examining the voltage-current characteristics near the open circuit voltage. The open voltage in the present invention is the potential of the sample metal electrode with respect to the reference electrode, and means the voltage measured by the voltmeter of FIG. 1 when the counter electrode is not connected to the current source. Therefore, in the embodiment using the calomel electrode as the reference electrode and the iron electrode as the sample metal electrode, the open circuit voltage has a negative value. The voltmeter used here is of course a high input resistance (FET input type electronic voltmeter or the like). The term "close to the open circuit voltage" is used as a standard up to about ± 20 mV relative to the open circuit voltage. Since the electrochemical reaction is less likely to occur as the current flowing through the sample metal electrode is smaller, the applied voltage should be closer to the open voltage unless there is a problem such as noise. Therefore, it is preferable that the sweep voltage range for examining the voltage-current characteristics is narrower around the open circuit voltage.
In particular, when Rp is low, it is preferable to narrow the sweep voltage range in order to reduce the current flowing through the sample metal electrode.

When the voltage-current characteristics are measured, the current change (ΔI) with respect to the voltage change (ΔV) at the open voltage can be determined, so that Re + Rp = ΔV / ΔI is obtained. In this case, if the solution resistance (Re) is measured in advance, Rp = ΔV / ΔI-R
It can be easily calculated as e. Also, if Re is sufficiently smaller than (Re + Rp) and the measurement error range is (Re + Rp),
Rp = ΔV / ΔI.

If the solution resistance (Re) is unknown, it must be measured by a known method. Any of these solution resistance measurement methods may be used, but a method of instantaneously interrupting the applied voltage is one of the preferable choices in consideration of the configuration of the measurement device (electronic circuit for measurement). That is, when the counter electrode is opened for a very short time to stop the energization when the applied voltage is swept, the residual voltage Vres is observed at the sample metal electrode and the reference electrode. Vres
Is the sum of the voltage of the electromotive force (battery) factor E and the voltage remaining in the capacitor component Cdl. The energization stop time is the same as in the case of a known measurement, and is preferably 1 millisecond or less, and the lower limit is determined by response characteristics of an electronic circuit for measurement and the like. Assuming that the applied voltage immediately before the stop of energization is Vapp and the current at that time is Iapp, the voltage generated at the solution resistance Re is (Vapp-Vres). Therefore, using Ohm's law, Re = │ (Vapp− Vres)
/ Iapp |. Since the applied voltage in this measurement method is near the open-circuit voltage, the operation of measuring the solution resistance (Re) described here may be performed at the start of the voltage sweep or at the end of the voltage sweep. In this case, the instantaneous interruption of the voltage is advantageous because it does not disturb the voltage-current characteristics.

As a sample metal electrode constituting the measuring system, a metal sample whose corrosion is to be evaluated is used. As the counter electrode, any material can be used as long as it is electrochemically stable and has high conductivity in the electrolytic solution for measurement. For example, conductive materials such as graphite, platinum, and gold can be used. Further, as the reference electrode, those used in ordinary electrochemical measurements such as a calomel electrode, a silver-silver chloride electrode and a hydrogen electrode can be used.

The sample metal electrode and the electrolyte can be determined according to the purpose. For example, as an application of this measurement method, there is a metal corrosion evaluation in an aqueous solution containing sodium chloride as a main electrolyte. At this time, it is desirable that the concentration range of the main electrolyte such as sodium chloride is saturated up to 1 g / l (liter) or more, preferably up to 10 g / l (liter). In particular, there are many examples in which iron-based metals such as iron and stainless steel are used in a state where they are exposed to an aqueous solution containing sodium chloride as a main electrolyte. Examples include the transfer of hot springs and seawater, and oil well pipes. In such a case,
As a sample metal electrode, a small piece of metal used for transfer can be used, and as an electrolytic solution, hot spring water, seawater, oil field salt water, or a mixture thereof can be used. Here, the terms hot spring water, sea water, and oil field salt water also include hot spring water analogs, sea water analogs, and oil field salt water analogs prepared by mixing and dissolving inorganic salts.

[0013] The evaluation of metal corrosion is very important in the maintenance and management of industrial facilities, and it is considered that it is sometimes preferable that the corrosion evaluation be measured in the field. Various electronic circuits for measurement required for the measurement method of the present invention can be used depending on the measurement accuracy and the data processing method. The simplest measuring device (measuring electronic circuit) includes an applied voltage generating circuit and a current measuring circuit. It is easy to linearly change the change (dV / dt) of the applied voltage with respect to time. Therefore, the rate of change (dI / d
Re + R by adding a differentiation function to detect t)
Data for calculating p = (dV / dt) / (dI / dt) is easily obtained. Vr if measurement of solution resistance (Re) is required
Only a switch and a sample hold circuit for obtaining and outputting es are added. The circuit described here can be manufactured inexpensively and is easy to operate on batteries, so it can be used in the field. This measurement method is also useful in configuring a measurement device.

The relative evaluation of the corrosion of the sample metal electrode is expressed as (1 /
Rp). This is because it is generally recognized that the corrosion rate (V) is proportional to the corrosion current (I), and the corrosion current is inversely proportional to Rp. In order to easily evaluate the corrosion rate, assuming that the unit of I is mA, the unit of Rp is Ω, and the unit of corrosion rate is milli (chemical) equivalent / second, the corrosion current value is obtained by I = K / Rp, K = 26 (mV) for iron and iron-based metals, corrosion rate (V)
Is the metal ion generation rate {milli (chemical) equivalent / sec}, which is possible according to Faraday's law (V = I / 96500).

The corrosion inhibitory ability of the corrosion inhibitor is obtained from the corrosion rate (Vco) before the addition of the corrosion inhibitor and the corrosion rate (Vsa) after the addition of the corrosion inhibitor (corrosion inhibitory ability) = (Vco−Vs).
a) It is required as / Vco. Since the corrosion current is directly proportional to the corrosion rate, the relationship between the corrosion prevention ability and the corrosion current is (corrosion prevention ability) = (Ico−Isa) / Ico. Here, Ico is the corrosion current before the addition of the corrosion inhibitor, and Isa is the corrosion current after the addition of the corrosion inhibitor. Rp is Rpco before adding the corrosion inhibitor and Rpsa is Rp after adding the corrosion inhibitor.
Then, since the corrosion current and Rp are inversely proportional, Ico = K / R
pco and Isa = K / Rpsa hold. When the corrosion prevention ability is expressed using Rp based on these relationships, (corrosion prevention ability) = (Rpsa−Rpco) / Rpsa.

In the case of iron, the proportionality coefficient K is often empirically set to 25 mV or 26 mV. The inventor uses K = 26 mV as needed, but this does not pose a problem because the K value is canceled in the calculation of the corrosion prevention ability. In the examples of the present invention, the numerical value calculated by the above equation was multiplied by 100, and the corrosion inhibitory ability of the corrosion inhibitor was shown as a percentage.

[0017]

EXAMPLES Hereinafter, the present invention will be described specifically with reference to examples, but the present invention is not limited to these examples. Example 1 The electrode and the electrolyte sample metal electrode were cylindrical iron materials having a diameter of 9.5 mm and a length of 12.1 mm, and were almost polished to a mirror surface. This is called an iron electrode in the present invention. An iron electrode was attached to a round bottom flask, and two graphite electrodes as counter electrodes around the iron electrode and a saturated calomel electrode (SC) as a reference electrode.
E) was set. Table 1 shows the compositions of 900 ml of the electrolytic solution. As the electronic circuit, an electrochemical measurement device SI 1280B manufactured by Solartron was used.
An operation equivalent to that of the electronic circuit of FIG. 1 was performed together with the measurement cell.

[0018]

[Table 1]

In the case of corrosion measurement, it is common to saturate carbon dioxide at the same time as removing air by passing carbon dioxide through an electrolyte. Here, in order to increase reproducibility, sodium hydrogen carbonate and hydrochloric acid were added after passing nitrogen gas through the electrolyte solution B to obtain a predetermined electrolyte solution composition (electrolyte solution A).
Electrolyte A has a pH of 5.9, but has a high pH stability because of the buffer system of carbonic acid-bicarbonate. In addition, in order to prevent the volatilization of carbon dioxide, the electrolyte A was not ventilated.

After performing a control measurement of a sample (corrosion inhibitor) concentration of 0 using the electrolyte solution A, a corrosion inhibitor for measurement was added, and the effect was measured. The corrosion inhibitor for measurement was a 2% aqueous solution. The amount of corrosion inhibitor added was 0.135 ml, 2 at the measured concentration of 3 ppm.
Since it is 0.900 ml at 0 ppm, the dilution of the electrolytic solution A by the addition of the corrosion inhibitor hardly occurs. When a corrosion inhibitor or the like was added to the measurement system, the mixture was stirred to make the electrolyte homogeneous. During the measurement, the stirring was stopped to prevent the electrolyte from swinging. The measurement and the calculation of the corrosion prevention ability were performed by the open circuit ± 15 mV of the iron electrode (sample metal electrode) -SCE (reference electrode) in the equivalent circuit shown in FIG.
The test was performed by sweeping the applied voltage in a DC range within the range.

When the electrolytic solution B was used, the Rp value was around 1700 Ω. The Rp value when using electrolyte A was about 80 Ω. From this Rp value, V = (26 / Rp) / 96500
When the corrosion rate is estimated as follows, it becomes 0.0000034 milli (chemical) equivalent / second {3.4 nano (chemical) equivalent / second}.

For comparison, a known method of impedance measurement was also attempted. 1 kHz for impedance measurement
The Cole-Cole plot is performed by changing the frequency in the range of about -0.1 Hz to analyze the resistance component. The DC component of the voltage applied during the measurement was an open circuit voltage, and the AC component was 5 mV. Although a slightly distorted and almost semicircular figure was obtained in the Cole-Cole plot, the resistance components obtained from the low frequency side were about 80Ω, respectively, {estimated corrosion rate 3.4 nano (chemical) equivalents / sec}. It was a value equivalent to the measurement result.

An attempt was made to evaluate the ability of laurylamine, a corrosion inhibitor, to inhibit corrosion. When laurylamine was not added, that is, when the concentration was 0 ppm, the Rp value was 83.5 Ω ｛the estimated corrosion rate was 3.2.
The Rp value at nano (chemical) equivalent / sec｝ and 3 ppm concentration is 9
4.1 Ω {estimated corrosion rate 2.9 nano (chemical) equivalents / sec},
The Rp value at a concentration of 20 ppm is 177Ω ｛Estimated corrosion rate 1.
52 nano (chemical) equivalents / sec. As a result, the corrosion inhibiting ability of laurylamine was 11% (concentration 3 ppm), 53%
(Concentration: 20 ppm). A known impedance measurement method was also tried, but the agreement was usually within 3%.

[0024]

According to the measuring method of the present invention, since the current flowing during the measurement is small, the disturbance on the electrode surface is small, and the Faraday resistance (polarization resistance) can be measured in a static state in principle.

According to the measuring method of the present invention, the Faraday resistance (polarization resistance) can be easily measured in a state where the electrochemical influence on the sample metal is small, and the corrosion of the sample metal can be evaluated. Since the minimum configuration of the measuring electronic circuit required for the measurement is simple, it can be expected to be used in the field. In addition, the corrosion prevention ability of the corrosion inhibitor can be easily evaluated.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of an equivalent electronic circuit used to measure Faraday resistance according to the present invention.

Claims (6)

[Claims] 1. A method for measuring Faraday resistance (polarization resistance), wherein a voltage applied to a sample metal electrode is swept within an open voltage range of ± 20 mV or less. 2. The method for measuring Faraday resistance according to claim 1, wherein the sample metal electrode according to claim 1 is made of iron. 3. The Faraday resistance of the Faraday resistance according to claim 1 or 2, wherein an electrolyte solution containing sodium chloride up to saturation in a concentration range of 1 g / l or more as a main electrolyte is used. Measuring method. 4. The hot spring water, sea water, oil field salt water or a mixture of sea water and oil field salt water (where the term hot spring water, sea water, oil field salt water is a mixture of inorganic salts, A hot spring water analog, a seawater analog, and an oilfield saltwater analog prepared as described above) as an electrolytic solution. 5. A method for evaluating metal corrosion, comprising measuring a Faraday resistance by the measuring method according to claim 1 and obtaining a corrosion rate (V) by the following equation: K / (96500 × Rp) where K is a proportional constant, and Rp is a Faraday resistance. 6. A method for evaluating the corrosion inhibitory ability of a corrosion inhibitor, wherein the measurement according to claim 1 is performed in the presence of a corrosion inhibitor.