JPS5814981B2 - Method for measuring rate factors of corrosion reactions - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring the rate factor, Terfel slope, of a corrosion reaction involving a metal specimen under predetermined conditions by using a metal specimen as a working electrode and measuring its polarization characteristics. .

One means of evaluating the corrosion reaction of metals is to make a determination based on the corrosion rate.

As a method for measuring 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.

By the way, it is known that there is a relationship between the rate factor of the corrosion reaction, the polarization resistance R1, the Faraday current I flowing through the sample metal piece, and the polarization value η.

On the other hand, theoretically, polarization resistance Rp and corrosion current Icorr
There is a relationship expressed by equation (2).

In addition, β8. β.

is the Terfel gradient. On the other hand, there is a relationship between Icorr and corrosion rate V as shown in equation (3).

In formula (3), M is the atomic weight of the sample metal piece, 2 is the valence number of the eluted metal ion, and F is the Faraday constant.

Here, Terfel gradient β3. β.

is considered to be a constant specific to the system, but it changes depending on the type of system and measurement conditions, so in order to accurately understand the corrosion rate V, in addition to the polarization resistance and R9, the value of the terfel slope β8°β0 is required. It is.

Also, the terfel gradient β8°β.

is an important parameter characterizing the nature of corrosion reaction.

From this point of view, the terfel gradient βyu,β.

Accurately determining the value of is important for evaluating the corrosivity of metal materials or preventing corrosion.

By the way, the Terfel gradient β8. β.

Conventionally, a method based on a polarization curve has been adopted as a means for determining the .

This method provides a polarization curve that is the basis for calculating the temperature gradient.

The principle is based on the fact that the current density factor and the polarization value η based on the corrosion reaction are expressed by the following equation.

In other words, as can be seen from equation (4), the polarization curve showing the relationship between loglIl and η (the schematic diagram is shown in Figure 1) is as follows when η>0 (the second term on the right side can be ignored)
Since it is expressed by the following equation, η-1o when η>0
Find 8 from the slope of the straight line showing the glIj relationship, and η
〈When 0, (the first term on the right side of equation (4) can be ignored)
Since it is expressed by the following equation, η-10 when η<0
β from the slope of the straight line showing the g1■1 relationship.

I'm trying to find out.

However, when measuring the polarization curve, it is necessary to measure the current density while changing the polarization potential η, or to measure η when changing the current density (2).

Moreover, when η or I is set to a certain constant value, it takes time for the value to be determined or η to reach a steady value, or in some cases, a clear steady value cannot be obtained.

For these reasons, the method based on the measurement of the polarization curve has the major disadvantage that the measurement is very complicated and takes too much time.

Note that the so-called constant current polarization method, which is often used as a rapid evaluation method for metal corrosion, has the disadvantage that although R2 can be determined, the Terfel gradient cannot be determined.

The present invention has been made in consideration of the above circumstances, and its purpose is to provide a method for easily and quickly measuring the terfel gradient.

The present invention will be explained in detail below.

The present invention is applied to the coulostat method, in which a predetermined amount of electric charge is instantaneously applied to a sample metal piece placed in a corrosive system via an electrode placed close to the sample metal piece, and the potential change of the sample metal piece is reflected by the electrode. The relationship between polarization value η and waiting time is measured in an open circuit state through
The present invention is characterized in that the velocity factor and terfel slope involved in the sample metal piece are determined using the portion of ~60 LV.

Figure 2 is a circuit diagram illustrating the principle of determining the Terfel gradient of a corrosion reaction using the Coulostand method, in which 1 is a pulse generator that generates a constant amount of electricity pulse, and 2 is a sample metal piece as a working electrode. , 3 indicates a reference electrode, and 4 indicates a potentiometer.

Therefore, when determining the terfel gradient of a corrosion reaction using the circuit shown in FIG.
A short constant electric quantity pulse (charge) of, for example, several μS to several ms is applied to the sample metal piece 2 located at
to instantaneously charge the electric double layer of the sample metal piece 2.

In this case, the magnitude of the charge applied is such that the potential of the sample metal piece is the corrosion potential E.

It is preferable to select it so that it changes by several tens of millivolts or more than or.

A certain amount of electric charge instantaneously applied in this way is consumed in the vicinity of the sample metal piece 2 as a working electrode by a corrosion reaction, and the potential of the sample metal piece tends to return to its original state, the corrosion potential (natural potential) ECOrr. Therefore, the relationship between the amount of potential change, that is, the polarization value η, and the time t is recorded using the potentiometer 4.

Note that as long as a potentiometer 4 with a large input impedance is used, the current between the sample metal piece 2 and the reference electrode 3 can be ignored and measurement can be performed in an open circuit state.

By the way, when η is several tens of LV or more, it can be written as equation (4).

Based on this equation (7), the Terfel gradient β8 can be determined as follows.

That is, a sufficiently large charge that satisfies the above conditions is applied to the sample metal piece at the corrosion potential Eoorr so that the overvoltage becomes ηm.

Since the applied charge is gradually dissipated by the corrosion reaction, the overvoltage η decreases.

Now, select a certain overvoltage ηi such that 0<ηi minus ηm, set the moment when η becomes ηi as 1=0, and count the elapsed time from this moment.

Potential E. orrJuηi and E.

Assuming that the differential capacitance cd between orr+η is constant,
The charge Δ(10−t) consumed when time elapses from 1=0 to t can be expressed as follows.

On the other hand, the electric current flowing due to the corrosion reaction during this time is
Since the integral is equal to △q0−j, the following relationship is obtained.

(8) (From equation (9) The differential equation of is obtained.

Equation (10) is solved under the initial condition of 1 = 0 and η = ηi (and is derived to the form.

(However, when ηi〉η〉50 to 60 mV).

Here, Cd is the differential capacitance of the electric double layer of the sample metal, and can be considered to be constant within a minute potential range.

Looking at this equation 00, the measured ηm can be plotted from the curve at the polarization potential η1. η2. By reading η3, it can be seen that three equations are obtained.

Subtracting equation (L3) from equation (12), subtracting equation (14) from equation (13), dividing equation (15) by (16), and finding the difference from equation (17). If the polarization potentials η1, η2, and η3 corresponding to the three times t1, t2, and t3 are known, β8 can be found.

By the way, now η1>η2>η3, η1=η2+Δη
, η1 such that η3=η2−Δη. Considering η2゜η3, tl corresponding to these polarization potentials.

t2. It is assumed that η- obtained at t3 is sampled from the curve.

If such η1, η2, and η3 are used, the left side of equation (17) can be simplified into the form.

Therefore, from equation (18), β8 is the curve (η〉0
), first find η2 at a certain time t2, and then calculate the times corresponding to η1 = η2 + Δη, which is larger than Δη from η2, and η3 = η2 − Δη, which is smaller than Δη, by 11, respectively.
1. If you read them, their tl, t2゜t3 and Δ
It can be seen that the calculation can be easily done by using η.

This type of analysis method is easy because terms including IC8rr and Cd are effectively eliminated, but if Δη is set too large, the difference between η and η2, and between η2 and η3 becomes too large, and the potential of Cd becomes too large. It is conceivable that changes will occur at the district attorney's office.

Therefore, Δη needs to be made small enough, for example, to 10 mV or less, so that the change in the potential of CD can be ignored.

The above was when η〉50-60mV, but conversely η
When the voltage is −50 to −60 mV, equation (4) can be written as follows.

Therefore, if we assume that the time when ηi is reached after a certain amount of time has passed immediately after applying a certain amount of negative charge is 1=0, then the relationship between η and t at a certain time t is expressed by equation 00. In the same way, the following formula is obtained.

Therefore, η1 × η2 × η3, η1 = η2 − Δη, η
3=η2+Δη, Δη〉0, and the measured η− is calculated from the curve to calculate these η1. η2. By reading the time tl)t2Ft3 corresponding to η3, β3
As in the case of β.

can be determined using the following formula.

Next, an apparatus for implementing the present invention will be explained.

FIG. 3 shows an example.

This device includes a cell 11 for measuring a corrosion reaction Q rate factor and a terfel gradient, which includes a sample metal piece (working electrode) 12 and a low-resistance reference electrode 13 arranged close to the sample metal piece (working electrode) 12; and a system A that applies a known amount of charge via a reference electrode 13 placed close to this;
and a system B that tracks potential changes of the sample metal piece 12.

System A for supplying a known amount of charge includes a power supply 14 for supplying charge, capacitors 151 to 154 for storing the supplied charge in advance, a variable resistor 16 for regulating the amount of electricity stored in the capacitors 151 to 154, and the above. It consists of a relay 17 that instantaneously applies the charges stored in the capacitors 15A to 154 to the sample metal piece 12 via the reference electrode 13 of the cell 11.

On the other hand, system B that tracks the potential change of the sample metal piece 12 includes an operational amplifier 18 that converts the impedance of the signal from the reference electrode 13 of the measurement cell 11 and the sample metal piece 12, and a system that converts the signal passing through the operational amplifier 18. It is comprised of a calculation control device 22 that analyzes according to the above-mentioned formula and calculates the corrosion reaction rate factor and terfel gradient, and a display device 23 that displays the calculated terfel gradient.

In the system B for tracking potential changes, the power supply 20 and the variable resistor 21 constitute a potentiometer, and this potentiometer is for applying a constant bias to the output signal.

Further, a voltmeter 25 is connected to both ends of the capacitors 151 to 154 via a switch 26 so as to check the voltage of each capacitor 151 to 154, and a rotary switch is connected to the capacitors 151 to 154 so that an appropriate capacity can be selected. 24 is provided.

Furthermore, a terminal 19 is provided on the output side of the operational amplifier 18 so that the shape of the separation value η-time t to be measured can be monitored using a display device such as a synchroscope.

Note that the measurement cell 11 may be of an electrode type consisting of a counter electrode provided opposite to the working electrode and a reference electrode provided close to the working electrode.

In the above-mentioned (corrosion reaction rate factor, terfel gradient measuring device), a mild steel plate 5S41 is used as a sample metal piece, that is, the working electrode 12, and with city water contained in the cell 11, the mild steel plate is first measured. The corrosion potential (natural potential) was determined to be -0,653VVS-8CE.

Next, when a positive charge of 3 μC was instantaneously applied via the reference electrode 13, the Kufer gradient β8 was 76.2 mV.
The value of was obtained.

On the other hand, the polarization potential one-hour curve at this time was measured by connecting a synchroscope to the terminal 19, and when the time during which ηi=120 mV was applied was rewritten as 1=0, it became as shown in FIG.

Note that the time when ηi was given was after 205 m5 had passed after the double layer charging, and the time was counted with that point newly set as 1=0.

From the curve, η1=100mV1η2=90mV, 13=
The time t for applying 80 mV is t1 = 35.3.
ms.

t2=62-8m81t3"'100m5 was obtained.

These tl, t2. The value of t3 and Δη=10mV are (
By substituting into Equation 151, β = 76.0 mV was obtained.

This Terfel gradient β = 76.0 mV and β output to the display device 23 = 76.2 mV were both approximately equal to 78 mV obtained by measuring the polarization curve.

As described above, according to the method of the present invention, a single η can be measured within a very short time of several hundred m5, compared to the conventional method based on the measurement of polarization curves, which is time-consuming. - can be determined from the curve.

Furthermore, the method of the present invention has the great feature that the terfel gradient can be determined without considering the surface area of the sample metal piece.

Furthermore, since the method of the present invention allows measurement in an open circuit state, corrections taking into account the potential drop are ignored even in systems with large solution resistance.

[Brief explanation of drawings]

Fig. 1 is a diagram schematically showing a polarization curve, Fig. 2 is a diagram for explaining the principle of determining the Terfel gradient of a corrosion reaction involving a sample metal piece by the Courostat method, and Fig. 3 is a diagram showing the present invention. FIG. 4, which is a circuit diagram of an example of an apparatus for implementing the method, is a relationship diagram showing the relationship between the polarization value obtained using the above apparatus and time. 11... Measurement cell, 12... Sample metal piece, 13... Reference electrode, 14... Power supply, 151-154... Capacitor, 16...
...Variable resistance, 17...Relay, 18...
...Operation amplifier, 19...Terminal, 20...
...Power supply, 21...Variable resistor, 22...
...Calculation control device, 23...Display device.

Claims (1)

[Claims] 1. A predetermined amount of electric charge is instantaneously applied to a sample metal piece placed in a corrosion system via an electrode placed close to it, and the potential change of the sample metal piece is measured via the electrode as a polarization value η - waiting time. The relationship was measured in an open circuit state, and the 1η1>50 to 60 mV portion of the above polarization value η - waiting time relationship was used to determine the rate factor of the corrosion reaction involving the sample metal piece, the Terfel gradient. A method for measuring a rate factor of a corrosion reaction, characterized in that: