JPS6239703B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for determining the corrosion rate of a metal sample under predetermined conditions by using the metal sample as a working electrode and measuring its polarization characteristics.

As a method for measuring or determining the corrosion rate of metal, an electrochemical method is known in which the corrosion rate is determined from the polarization resistance Rp of a sample metal piece. This method focuses on the fact that metal corrosion is a type of electrode reaction that elutes metal ions from the metal surface, and the rate of this electrode reaction depends on the magnitude of the current flowing at that time. That is, when a metal sample piece is immersed in a corrosive system, for example, a corrosive solution, the metal sample piece exhibits a certain corrosion potential, but a small constant anode current or cathode current I is applied to the sample metal piece through the counter electrode. When applied to , a potential change occurs. However, if the amount of potential change in this lever is η, then the polarization resistance Rp of the sample metal piece is given by Rp=η/I.Theoretically, the polarization resistance Rp is inversely proportional to the corrosion current Icorr as shown in the following equation. It is believed that.

Rp Icorr=K (K is a constant and a unique value for corrosion reactions)
However, in reality, the difference in K depending on the type of reaction is small compared to the change in the corrosion current Icorr, and the corrosion rate V is calculated using the following formula: V=(M/ZF)Icorr (M is the atomic weight of the sample metal, Z is the eluted metal The valence number of the ion, F is Faraday's constant), so if the corrosion current Icorr is known, the corrosion rate V can be calculated.

However, the method of determining Rp from the potential change when a constant current is applied and determining the corrosion rate of the sample metal piece from this method has the following disadvantages. First of all, it takes a relatively long time for the potential change η of the sample metal piece to reach a constant value. If a long time is required here, the change in the surface condition of the sample metal piece or the deposition of corrosion products on the surface is promoted, which tends to lead to inaccuracy in the measurement of the amount of potential change η. Second, it is difficult to accurately measure the potential of the sample metal piece due to the influence of ohmic drop caused by solution resistance between the sample metal piece and the reference electrode. This Ohmic drop is the amount of potential change η
This poses a major obstacle in tracking changes in the temperature, and also requires complicated operations to compensate for the effects of Ohmic drop.

As a method to deal with the above-mentioned drawbacks, the present applicant has previously proposed that a fixed amount of electric charge is instantaneously applied to a sample metal piece immersed in a corrosive system via a counter electrode, and the potential change of the sample metal piece due to the charge application is The electrode value (η) is measured through a reference electrode provided close to the sample metal piece.
A patent application was made for a metal corrosion rate determination method in which the relationship between time (t) is measured in an open circuit state and the relationship between polarization value (η) and time (t) is analyzed to determine the corrosion rate of a sample metal piece. No. 51-56076 (Japanese Unexamined Patent Publication No. 52-139491), etc. This method is called the coulostat method because it regulates the charge applied to the sample metal piece, and (i) it does not require compensation or correction for ohmic drop even in a high resistance liquid. (ii) Measurement time is short. (iii) Measurement can be performed under conditions close to natural without disturbing the measurement system. It has various advantages such as

By the way, the main method of measurement using the coulostat method is to store the electric charge to be given to the sample metal piece in advance in a capacitor, and then transfer a predetermined amount of electric charge stored in the capacitor to the sample metal piece. granted through the opposite pole,
The subsequent potential change of the sample metal piece is the polarization value (η) −
The polarization resistance of corrosion reaction is obtained by analyzing the polarization value (η)-time (t) relationship.
Rp, Terfel gradient β _a , β _c , corrosion current density
There is a method that uses a capacitor as a charge supply source to determine Icorr, differential capacitance Cd, etc. Polarization resistance Rp, terfer gradient β _a , β _c , corrosion current density Icorr, differential capacitance Cd are determined by the above Courostat method.
The principle by which such calculations can be obtained is as shown below.

That is, considering the following equation, which is often used to express corrosion reactions, I/Icorr=exp(2.3/β _a η)−exp(−2.3/
β _c η)……(1) When the polarization value η is small such as η≦10mV, (1)
The formula is expressed as I = 2.3 (β _a · β _c / β _a + β _c ) η · Icorr ... (2) Rp = β _a β _c / {2.3 ( _{β a} + β _c )
Icorr}, equation (2) can be expressed as I=η/Rp (3). Here, I is the Faraday current density flowing through the sample metal piece. Using formula (3), when η≦10mV, the polarization value η of the sample metal piece after applying a predetermined amount of charge is as follows: η=η ₀ exp (-t/Cd Rp) ......( 4) Derived as a function of time t. Here, η ₀ is the polarization value immediately after applying charge, and the obtained η
It can be obtained by extrapolating the -t curve to t=0 (while considering equation (4)). It is known that once η ₀ is determined, the applied charge density Δq can be expressed as shown in the following equation.

△q=η ₀ Cd ......(5) The differential capacitance Cd can be found from the relationship in equation (5). Once Cd is determined, the polarization resistance Rp can be determined from the time constant Cd Rp of the decay of the polarization value η, which is determined by analyzing the obtained η-t curve according to equation (4).

The above is the principle for finding Rp, but the principle for finding β _a and β _c is as follows. In other words, consider a case where a large predetermined amount of electric charge is applied to the sample metal piece as an absolute value such that η>50 mV or η<−50 mV. Especially when η>50mV (1)
The equation can be expressed in a simple form with the second term omitted, as shown in equation (6) below.

I = Icorr exp (2.3/β _a η) ......(6) Therefore, immediately after a predetermined amount of relatively large positive charge is applied, after a certain amount of time t has passed, η _i is reached. If time t = ₀ when /β _a +exp(−2.3/β _a η _i )……(7). Here, the differential capacitance Cd can be considered to be constant within a minute potential range.

Looking at this equation (7), from the measured η-t curve,
By reading the polarization values η ₁ , η ₂ , η ₃ at three different times t ₁ , t ₂ , t ₃ respectively, 3
equation exp(-2.3/β _a η ₁ )=Icorr/Cd×2.3/β _a t ₁ +exp(-2.3/β _a η _i )……(8) exp(-2 .3/β _a η ₂ )=Icorr/Cd×2.3/β _a t ₂ +exp(−2.3/β _a η _i )……(9) exp(−2.3/β _a η ₃ )=Icorr/Cd×2.3/β _a t ₃ +exp(−2.3/β _a η _i )……(10
) can be obtained.

Subtracting equation (9) from equation (8), we get exp(-2.3/β _a η ₁ )−exp(-2.3/β _a η ₂ )=Icorr/Cd×2.3/β _a (t ₁ − t ₂ )……
(11) Subtracting equation (10) from equation (9), exp(-2.3/β _a η ₂ )−exp(-2.3/β _a η ₃ )=Icorr/Cd×2.3 /β _a (t ₂ − t ₃ )……
(12) Interpreting equation (11) with equation (12) Dividing equation (12) by equation (13) From equation (14), it can be seen that β _a can be found if the polarization values η ₁ , η ₂ , η ₃ corresponding to three different times t ₁ , t ₂ , t ₃ are known.

By the way, now η ₁ > η ₂ > η ₃ (>50m
V), consider η ₁ , η ₂ , η ₃ such that η ₁ = η 2 +△η, η ₃ = η ₂ −△η _, and calculate the times t ₁ , t ₂ , corresponding to these polarization values. Suppose that t ₃ is sampled from each obtained η-t curve. (However, △η>0) That is, η ₁ - _{η 2} =
η _{1 ,} η ₂ , and η ₃ are determined to be η 2 −η ₃ , and the times corresponding to these values are sampled.

Using such η ₁ , η ₂ , η ₃ (14)
The left side of the equation is It is simplified to the form

Therefore, From equation (15), β _a is the measured η-t curve (η
>50mV), first find η ₂ at a certain time t ₂ , and then calculate η ₁ = η ₂ which is larger than η ₂ by △η.
If we read the times t 1 and t ₃ corresponding to η ₃ = η ₂ −△η, which are smaller by + _{△η and △η, we can easily use these t 1 ,} t ₂ , t ₃ , and △η. It turns out that it can be calculated. The above was an analysis method for β _a , but from the η-t curve after applying a charge so that η < -50 mV, η ₁ < η ₂ < η ₃ < -50 mV
, η ₁ = η ₂ −△η, η ₃ = η ₂ +△
η, η ₃ = η ₂ +△η η ₁ that satisfies △η>0,
If we read t _{1 ,} t 2 , and t ₃ corresponding to η ₂ and η ₃ , we can theoretically find β _c by using the following relationship, as in the case of β _a . I understand. By the way, the method using the above-mentioned capacitor as a means to store charge in advance is an effective method for tracking the corrosion rate of various sample pieces.
Various good measured data have also been obtained.

(References: Ken-ichi Kanno, Masayuki
Suzuki and Yuichi Sato, Boshoku Gijutsu,
26 , 697 (1977). ) However, as we proceeded with further investigation, we found that, as is often encountered in actual measurement systems, the surface area S of the sample metal piece is relatively small, and therefore the differential capacitance Cd×S over the total surface area is also relatively small. S
is not sufficiently large with respect to the capacitance C ₁ of the capacitor, for example, when Cd×S/C ₁ <100, and the natural potential Ecount opposite to the natural potential of the sample metal piece, that is, the corrosion potential Ecorr If the difference between . It was found that this phenomenon is observed along with the Ohmick descent. In particular, when the charge stored in the capacitor in advance is 0, the potential fluctuation Vd of the sample metal piece including the ohmic drop due to the charging of the capacitor _C1 based on the potential difference between the sample metal piece and the counter electrode.
Only (t) will be observed instead of the change in polarization value η(t), which should originally be measured as an output signal. Therefore, if a predetermined amount of charge is stored in the capacitor in advance, the potential fluctuation will affect the polarization value η(t) that should originally be measured.
Vd(t) will be added. to η(t)
Adding Vd(t) is undesirable because the measurement result will include an error.

The present invention has been proposed in order to deal with the above-mentioned drawbacks, and the differential capacitance Cd×S in the entire length area of the sample metal piece is not sufficiently large with respect to the capacitance C ₁ of the capacitor, and It is an object of the present invention to provide a method for determining the corrosion rate using the Crowstad method, which causes little measurement error even when the natural potential difference between the sample metal piece and the counter electrode is large.

Hereinafter, the present invention will be explained in detail.
A predetermined amount of electric charge stored in a capacitor in advance is applied instantaneously to a sample metal piece immersed in a corrosive system via a counter electrode, and the potential change of the sample metal due to the charge application is applied close to the sample metal piece. The relationship between polarization value (η) and time (t) is measured in an open circuit state through a reference electrode provided, and the corrosion rate of the sample metal piece is determined by analyzing the relationship between polarization value (η) and time (t). In the corrosion rate determination method, when applying the electric charge, a voltage of the same value and opposite polarity as the electromotive force generated between the sample metal piece and the counter electrode is applied between the sample metal piece and the counter electrode. This is a corrosion rate determination method based on the coulostat method, which makes it possible to accurately measure the time change in the polarization value (overvoltage) η(t) due only to the consumption of a predetermined amount of charge stored in the capacitor. be.

Next, examples of the present invention will be described. FIG. 1 shows an example of a corrosion rate determination apparatus using the method of the present invention. In the figure, 1 is a sample metal piece, 2 is a counter electrode, and 3
is a reference electrode, and 4 is a cell for measuring corrosion rate. 5-1, 5-2, 5-3, and 5-4 are capacitors for storing charge in advance, and each has a different capacity. A rotary switch 6 is provided so that one of the capacitors with an appropriate capacity can be selected. 7 is a battery, and 8 is a variable resistor, and these batteries and the variable resistor form a potentiometer for supplying electric charge to the capacitors 5-1 to 5-4. 9 is a switch for changing polarity. 10 is a relay, which is connected in advance to the capacitors 5-1 to 5-5.
-4 is for applying the charge stored in the cell 4 between the sample metal piece 1 and the counter electrode 2. The battery 11 and the switch 12 are connected to the relay 1.
This is for driving 0. 13 is a recording mechanism that records the change in polarization value (η) after charge is applied. 14 is an analysis mechanism for analyzing the changes in the recorded polarization value and calculating corrosion rate factors such as the polarization resistance Rp, turf gradient β _a , β _c , corrosion current density Icorr, and differential capacity of the target corrosion reaction. It is. The recording mechanism 13 is designed so that a synchroscope 15 for monitoring the obtained η-t curve can be connected. By the way, a portion 16 surrounded by a dotted line is used to apply a voltage of the same value and opposite polarity to the electromotive force generated between the sample metal piece 1 and the counter electrode 2 during charge application between the sample metal piece 1 and the counter electrode. It is a voltage application mechanism. 17 is an operational amplifier for generating the constant voltage of opposite polarity, and 18 is a resistor. This resistor 18 is inserted in series in a closed circuit composed of the sample metal piece 1, the counter electrode 2, and the capacitor 5, and is designed so that a voltage of the opposite polarity is applied to this resistor 18. battery 1
9. The variable resistor 20 constitutes a potentiometer and is used to determine the voltage of the opposite polarity applied to the resistor 18. 21 is a voltmeter for measuring the voltage between the sample metal piece 1 and the counter electrode 2 before being charged, and 22 is an operational amplifier used to convert the impedance of the voltage signal. Further, the switch 23 is connected to the operational amplifier 17,
This is for cutting off the connection between the voltage applying circuit and the resistor 18 when the voltage applying circuit composed of the battery 19 and the variable resistor 20 is not operated. In addition,
The switch 23 is linked with the power switch of the operational amplifier 17, so that when the switch 23 is in an open circuit state, the power of the operational amplifier 17 is also turned off. Note that 24 is for measuring the initial voltage _V1 of the capacitor.

Note that the voltage application mechanism 16 is operated in the following order.

(i) With the switch 23 open, the voltage (electromotive force) between the sample metal piece 1 and the counter electrode 2 is measured using the voltmeter 21.

(ii) Close the switch 23 and adjust the knob of the variable resistor 20 so that a voltage of opposite polarity to the voltage measured in (i) is applied to the resistor 18. At this time, you can check whether the prescribed voltage is applied to the resistor 18 by looking at the scale on the dial of the variable resistor 20, and also by checking whether the voltmeter 21 reads 0V. can.

The above operations are operations performed on the voltage application mechanism when applying a predetermined amount of charge.

Using the apparatus assembled as described above, a copper plate (TCu-P1) with a surface area of 2 cm ² was used as the sample metal piece 1,
Separate platinum electrodes coated with platinum were used as the counter electrode 2 and the reference electrode 3, and a 300-day immersion test was conducted with city water contained in the cell 4. As a result, for example, the measurement results at 140 days after immersion were as follows.

First, measurements were taken with the switch 23 kept open in the voltage applying mechanism indicated by 16 surrounded by a dotted line. When a charge of 0.168 μC was applied to the sample metal piece, a η-t curve as shown in FIG. 2 was obtained, and when the curve was analyzed by the analysis mechanism 14, Rp = 180 KΩcm ² and Cd = 39.9 μF/cm. ² was obtained. Furthermore, the switch 23 is kept open, and the charge ΔQ (=Δ
By analyzing the positive and negative η-t curves when the absolute value of q·S) was increased to |ΔQ|>15 μC, β _a =53 mV and β _c =78 mV were obtained. From these obtained values of Rp, β _a and β _c , Icorr=
0.0762 μA/cm ² was obtained.

Furthermore, when the potential difference between the sample metal piece 1 and the counter electrode 2 was measured with the voltmeter 21 while the switch 23 remained open, it was 0.321V (the sample metal piece 1 was the positive electrode, and the counter electrode was the platinum black electrode). Since the platinum electrode is the negative electrode, the resistor 18 is connected to - by the voltage applying circuit.
The variable resistor 20 was adjusted so that a voltage of the opposite sign of 0.321V was applied. Next, the switch 23 was closed, and the voltage indicated by the voltmeter 21 was confirmed to be 0V.

Therefore, as in the previous case, a charge of 0.168 μC was applied to sample metal piece 1. As a result, a η-t curve as shown in FIG. 3 was obtained, and when this curve was analyzed by the analysis mechanism 14, Rp=86.3KΩcm ²
Cd=107 μF/cm ² was obtained. Furthermore, when the switch 23 is kept closed and the absolute value of the charge △Q (=△q・S) applied to the sample metal piece is increased to |△Q|>15 μC, the positive and negative By analyzing the η-t curve, β _a =51 mV,
β _c =74 mV was obtained. These obtained Rp,
Icorr=0.152 μA/cm ² was obtained from the values of β _a and β _c . On the other hand, from the weight loss measured over 300 days for the same system, the average corrosion rate
A value of 0.450mdd was obtained. Converting this average corrosion rate value to corrosion flow density Icorr = 0.153μ
It was found that the value of Icorr was A/cm ² and was very close to the value of Icorr obtained by operating the voltage applying circuit. The above was a comparison between the corrosion rate obtained by the Coulostat method at 140 days after immersion and the average corrosion rate determined from weight loss.In addition, we also compared the change in Icorr over time using the Coulostat method over a period of 300 days. When the average value of the corrosion current density Icorr was determined from the trace, a value of 0.155 μA/cm ² was obtained.
On the other hand, the average value of the corrosion current density Icorr obtained without operating the voltage application circuit was 0.0632 μA/cm ² . Comparing these obtained Icorr values, it is clear that the voltage applying circuit is activated and the voltmeter 21
It can be seen that when the reading is set to 0V, a value that is much closer to the value obtained by the weight reduction method is obtained.
The above results demonstrate that the method of the present invention was effective in determining accurate corrosion rates when the sample metal piece was pure copper.

In the above-mentioned embodiment, only one example of a device using the method of the present invention has been shown, but it is also possible to use a circuit device as shown in FIG. 4 to realize the method of the present invention. 1 in the figure is a sample metal piece,
2 is a counter electrode, 3 is a reference electrode, and 4 is a cell for measuring corrosion rate. 5-1, 5-2, 5-3, 5-
4 are capacitors for storing charge in advance, and each has a different capacity. A rotary switch 6 is provided so that one of the capacitors with an appropriate capacity can be selected. 7 is a battery, 8 is a variable resistor, and these batteries and variable resistors are connected to the capacitors 5-1 to 5-4.
It forms a potentiometer for supplying electric charge to. 9 is a switch for changing polarity. Reference numeral 10 denotes a relay, which is used to apply electric charges previously stored in the capacitors 5-1 to 5-4 between the sample metal piece 1 and the counter electrode 2 in the cell 4. battery 11 and switch 12
is for driving the relay 10. 13 is a recording mechanism that records the change in polarization value (η) after charge is applied. Reference numeral 14 denotes an analysis mechanism for analyzing the changes in the recorded polarization value to calculate corrosion rate factors such as polarization resistance Rp, turf gradients β _a , β _c , corrosion current density Icorr, etc. of the target corrosion reaction. The recording mechanism 13 is designed so that a synchroscope 15 for monitoring the obtained η-t curve can be connected.
By the way, a portion 31 surrounded by a dotted line is a voltage having the same magnitude and opposite polarity as the electromotive force generated between the two electrodes of the sample metal piece and the counter electrode, and is applied to the capacitors 5-1 to 5-5.
The object of the present invention is achieved by subtracting in advance the voltage applied to the sample metal piece 1 and the counter electrode 2 from the charging voltage of the capacitor by applying the voltage applied to the sample metal piece 1 and the counter electrode 2 in a closed circuit consisting of -1 and a variable resistor. This is a voltage application circuit for The voltage applying circuit includes an operational amplifier 35, a battery 32, a polarity switch 33, a variable resistor 34, etc., and serves as a voltage source of a predetermined magnitude. The magnitude of the voltage can be adjusted by a variable resistor 34, and its polarity can be changed by a switch 33. Operational amplifier 36 and voltmeter 37
is for measuring the potential difference between the sample metal piece 1 and the counter electrode 2 in advance. The voltmeter 40 is used to determine the amount of charge stored in the capacitor in advance. Note that the voltage measured by the voltmeter is the voltage when the voltage application circuit is not operated.

Further, the specific apparatus using the method of the present invention can be appropriately modified other than those shown in FIGS. 1 and 4 without departing from the gist of the present invention.

[Brief explanation of the drawing]

Figures 1 and 4 are circuit diagrams showing an example of a corrosion rate determination apparatus using the method of the present invention, and Figure 2 is a circuit diagram showing an example of a corrosion rate determination apparatus using the method of the present invention. Figure 3 shows the η-t curve obtained by operating the voltage applying mechanism of the present invention using the same sample metal piece as in Figure 2. . 1... Sample metal piece, 2... Counter electrode, 5-1 to 5-
4... Capacitor.

Claims (1)

[Claims] 1. A predetermined amount of electric charge stored in a capacitor in advance is applied instantaneously to a sample metal piece immersed in a corrosive system via a counter electrode, and the potential change of the sample metal piece due to the charge application is applied in the vicinity of the sample metal piece. The polarization value (η) is measured in an open circuit state as the relationship between polarization value (η) and time (t) through a reference electrode provided.
- a corrosion rate determination method for determining the corrosion rate of the sample metal piece by analyzing the relationship between time (t),
Determination of corrosion rate, characterized in that when applying the charge, a voltage of the same value and opposite polarity as the electromotive force generated between the sample metal piece and the counter electrode is applied between the sample metal piece and the counter electrode. Method.