JPS5937461B2 - Method for measuring rate factors of corrosion reactions - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses a metal sample piece as a working electrode, obtains its polarization characteristics, and further analyzes the rate factor, polarization resistance, and terf. This invention relates to a method for measuring the rate factor of a corrosion reaction, which allows obtaining gradients and the like.

It is known that one way to evaluate the corrosion reaction of metal is to determine it based on the corrosion rate.

As a method for measuring the corrosion rate, an electrochemical method is known in which the corrosion rate is determined from the polarization resistance of a sample metal piece. By the way, there is a relationship between the rate factor of corrosion reaction, polarization resistance, the Faraday current I flowing through the sample metal piece, and the polarization value η as Rp=η/I (1) It is known that there is

On the other hand, theoretically, the polarization resistance horse and the corrosion current meter corr
There is a relationship between them as shown in the following equation. βaβc Rplcorr: ゜゜'゜'゜゜゜゜゜(2)2.3(
βa+βc) In equation (2), βa and βc are the Terfel gradients.

On the other hand, there is a relationship between Icorr and corrosion rate V as shown in the following equation.

V=(M/ZF)・Icorr・・・・・・・・・(
3) In the formula (3), M is the atomic weight of the sample metal piece, Z is the valence number of the eluted metal ion, and F is the Faraday constant.

Here, the terfer gradients βa and βc are considered to be constants specific to the system, but they vary depending on the type of system and measurement conditions. Therefore, in order to accurately grasp the corrosion rate v, polarization resistance Rp The values of the terfel gradient ^ and βc are required.

Furthermore, the terfel gradient ^ and βc are important parameters that characterize the nature of corrosion reactions. From this perspective,
It is important to simultaneously and accurately determine the values of the polarization resistance equation and the terfel gradients βA and βc for evaluating the corrosivity of metal materials or for corrosion prevention. The polarization resistance method is conventionally known as a means for determining the polarization resistance formula. In this polarization resistance method (linear polarization method), the polarization resistance formula and the applied current 1 are (1
) is a method that utilizes the relationship shown in the equation, constant current 1
The polarization resistance Rp of the sample metal piece is determined by reading the polarization value η when flowing the sample metal. However, with this method, it is not possible to obtain the Terfel gradients βA and βc. Furthermore, this method has the disadvantage that when applied to a system with a slow corrosion rate, it takes a long time for the polarization value η to approach a constant value. Furthermore, in the case of a system with high solution resistance, such as in distilled water, the IR-DrOp generated due to the flow of a constant current is measured overlapping the η that should be determined, so the IR-DrOp must be corrected. There is also an inconvenience. in this way,? Only the value of βA. In contrast to the above polarization resistance method (linear polarization method) in which the value of βc cannot be evaluated, by passing a large current 111 that increases the value of lηI and using the formula representing the following corrosion reaction - Cυ5▲ Rvrv,
There are several methods for determining βA, βc, and.

A typical method is based on the measurement of a polarization curve (a schematic diagram is shown in FIG. 1) showing the relationship between 10g111 and η. This method utilizes the fact that when η>0, equation (4) is expressed as the following equation r-, and when η<0, it is expressed as the following equation.

That is, ^ and βc are determined from the slope of the straight line representing the relationship η-10g111. Furthermore, by interpolating the relationship η-10g111 to η→0, IO. rr is calculated, and Rp is calculated from this. Furthermore, by substituting the polarization values obtained when three constant currents of different magnitudes are passed through the sample metal piece into the relationship of equation (4), 3 is obtained.
R, . There is also a method of determining βA and βc at the same time. However, conventional Rp, Ba, βc
In all methods of determining , a constant current is passed through the test solution, so in systems with high solution resistance, correction or compensation of IR-DrOp becomes a problem, and in systems with low corrosion rate, the polarization value η There is a problem that it takes too much time to obtain a steady value.

Prolonging the measurement time not only increases the time it takes to obtain results, but also increases the risk of significantly changing the surface condition of the sample metal piece during measurement, which can lead to measurement errors. Therefore, the present inventors first applied for patent application No. 51-5.
6076 (Japanese Patent Publication No. 56-12812), etc., polarization resistance Rp, Terfel gradient βA. We have filed an application for a measurement method that can easily determine βc, etc.

In this method, a predetermined amount of charge is applied to a sample metal piece placed in a corrosive system, and the potential change of the sample metal piece at this time is determined as a relationship between polarization value η and time t, and the relationship between polarization value η and time t is The objective was to find rate factors in corrosion reactions such as polarization resistance and terfel gradients βA and βc. In addition, in the above measurement method, since the amount of charge to be applied necessary for measuring each rate factor of corrosion reaction (polarization resistance type, terfel gradient βA, βc) is limited, at least two of the rate factors of the same or different types are used. The above measurements required multiple measurements.

However, when performing multiple measurements as described above, a predetermined amount of charge is applied to the sample metal piece exhibiting the corrosion potential EcO in the first measurement, the potential is stepped, and the relationship between the polarization value η and time t is After finding the required speed factor from
Move on to the next measurement.

In other words, if the next measurement is not performed after the polarization value due to the charge applied in the first measurement has decayed to zero (the potential of the sample metal piece is the corrosion potential EcOrr), it is necessary to obtain the accurate rate factor of the corrosion reaction in the corroded state. I couldn't do it. In particular, in systems with slow corrosion rates, it takes a long time to measure one rate factor before the next measurement, and in systems with relatively fast corrosion rates, multiple rate factors are measured during the measurement of the sample metal piece. Since the corrosion state changes, continuous measurement in as short a time as possible is desired for accurate measurement. In view of the above points, the present invention provides IR-DrO using solution resistance.
It is an object of the present invention to provide a measurement method that can easily measure the rate factors of a plurality of corrosion reactions of a sample metal piece in an extremely short time and accurately without requiring correction or compensation of p.
The present invention deals with the corrosion potential E placed in a corrosive system.

A predetermined amount of charge is instantaneously applied to a sample metal piece exhibiting Orr, and a potential change of the sample metal piece is measured by a reference electrode placed close to the sample metal piece from the relationship of polarization value η and time t. In the method for measuring the rate factor of a corrosion reaction involved in a corrosion reaction, when measuring a plurality of rate factors of different or the same type of corrosion reactions, after measuring the rate factor of one corrosion reaction, the rate factor of the next corrosion reaction is measured. The potential of the sample metal piece before measuring the rate factor. Force the corrosion potential E. This is a method for measuring the rate factor of the corrosion reaction that returns to Orr, and when determining the polarization resistance as the rate factor of the corrosion reaction, an electric charge is applied so that the absolute value 1η1 of the polarization value in the sample metal piece is 10 mV or less. , Terfel slope βA. as a rate factor of corrosion reaction. When determining βc, more accurate measurement becomes possible by applying a charge so that the absolute value 1η1 of the polarization value in the sample metal piece is 60 mV or more. In addition, as the rate factor of the corrosion reaction in the method of the present invention, for example, the polarization resistance formula, the terfel gradient, etc. βC is mentioned, and these rate factors are determined as follows.

First, the polarization resistance equation is based on the following formula: When the polarization value η of the sample metal piece due to charge application is sufficiently small, the relationship between the Farade current 1 due to corrosion reaction, the polarization resistance, and the polarization value η is as shown in the following equation, so the measured polarization The value η-time t curve is theoretically derived from the general formula (4) expressing the corrosion reaction as follows, where η.

is the polarization value immediately after applying a charge to the sample metal piece, and CO is the differential capacitance of the electric double layer of the sample metal piece).

Therefore, if a straight line is obtained when the polarization value η is obtained and 11η is plotted against time t, η can be obtained by extrapolating the straight line to time t=o. can be found. Therefore, the polarization value η immediately after applying a charge to the metal sample piece.

The differential capacitance et al. is calculated from the following equation from the amount of change Δq in the charge density given to the sample metal piece, and then these η.

and C. By using the value of , the polarization resistance 4 can be determined from the slope of the above 1nη-t line. In particular, when the absolute value 1ηl of the polarization value is 10 mV or less, the above equations (7) to (9) hold, and an accurate polarization resistance can be determined.

On the other hand, the Terfel gradients βA and βc can be obtained as follows.

That is, when η is 60 mV or more, equation (4) is applied as follows.

Therefore, if t=o is the time when η1 is reached after a certain amount of time t has passed immediately after applying a relatively large positive charge of a predetermined amount, then η and t at a certain time t are The relationship is theoretically derived from the following equation.

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 (12 dogs), from the measured η-t curve, the polarization potentials η1, η2 for three different times Tl, t2, t3, respectively.
, η3, it can be seen that three equations can be obtained.

(au) Subtracting the α method from 2, subtracting Cl Chenmin from o, then subtracting equation (b) from equation (15), and from equation (b), three different times Tl ．． Polarization potentials η1, η2 .corresponding to t2, t3. It is understood that βa can be found if η3 is known.

By the way, now η1>η2>η3, η1=η2+Δη
, η3=η2−Δη, and Tl, t2, and t3 corresponding to these polarization potentials are sampled from the obtained η-t curves.

(However, Δη〉0), that is, 1η1−η2(η
η1 so that it becomes 2-η3! η29η3 are determined, and the times corresponding to these values are sampled. Using such η1, η2, and η3, the left side of α◆min can be simplified into the form.

Therefore, from equation (17), βa is the measured η-t curve (η》0)
Therefore, first find η2 at a certain time T2, and then η
The times corresponding to η1 = η2 + Δη, which is larger than Δη from 2, and η3 = η2 − Δη, which is smaller by Δη, are Tl and t, respectively.
3, their Tl, t2, t3 and Δη
It can be seen that it can be easily calculated by using .

By the way, the above analysis method is easy because terms including IcOrr and Cd are eliminated well, but Δη
If .eta.1 and .eta.2, .eta., and .eta.3 are set too large, the difference between .eta.1 and .eta.2 and .eta.

Therefore, Δη needs to be made sufficiently small, for example, 10 mV or less, so that the change in the potential of Cd can be ignored. The above was when η>60mV,
Conversely, when η<-60mV, equation (4) can be written as follows.

Therefore, if we let t=o be the time when η1 is reached after a certain amount of time has passed immediately after applying a certain amount of negative charge, the relationship between η and t at a certain time t is theoretical. is led to the form of the following equation.

Here, η1〈η2〈η3, η,=η2−Δη, η3=
If the polarization values are determined as η2+Δη, Δη〉O, and the times T, , t2, and t3 corresponding to these η1, η2, and η3 are read from the measured η-t curve, βc is obtained as in the case of βa. can be determined by using the following relationship 1Z1.

Although the rate factors of individual corrosion reactions are measured as described above, when a plurality of rate factors are to be measured continuously and accurately in a short period of time, it is carried out as follows.

For example, the case of determining the polarization resistance equation and the turf gradient βA, βc as rate factors of corrosion reaction will be explained using Fig. 1, which shows the principle of the apparatus using the method of the present invention, and Fig. 2, which shows the potential change in the sample metal piece. do. First, when determining the polarization resistance, a predetermined amount of charge is instantaneously applied from the charge supply mechanism B to the sample metal piece 1 as a working electrode disposed in the corrosion determination cell A.

Note that the polarization value η of the sample metal piece 1 at this time is the second
As shown in area a of the figure, the polarization value η decays after several MV steps and is recorded by the potentiometric measurement mechanism C as a polarization value η versus time t curve. Further, based on this polarization value η-time t curve, the polarization resistance can be determined by the above-mentioned analysis method. In addition, prior to determining the Terfel gradient βa, the corrosion potential E was determined using a potential meter D before measuring the polarization resistance Rp.

With Orr as a reference, the polarization value ηa is set to O by the constant voltage mechanism E, and the potential of the sample metal piece 1 is set to the corrosion potential E. Orr (polarization value η=0) (region d1 in FIG. 2). When determining the polarization gradient βa, the polarization value η is determined in the same manner as in the method for determining the polarization resistance R2, except that the step amount of the polarization value η due to charge application is set to 60 mV or more as shown in area b of FIG.
Obtain a one-hour t-curve.

Therefore, the terfel gradient βa can be determined by the above-mentioned analysis method. Prior to determining the Terfel gradient βc again, the corrosion potential E was determined using the potentiometer D before measuring the polarization resistance.

Orr as a reference, the polarization value ηb is set to O by the constant voltage mechanism E, and the potential of the sample metal piece 1 is set to EcOrr.O Next, when determining the tarfel gradient βc, the step amount of the polarization value η due to charge application is shown in Fig. 2. -60m as shown in area C
The terfel gradient βc can be obtained in the same manner as the method for determining the terfel gradient βa described above, except that it is less than or equal to V.

As described above, when the method of the present invention is used, polarization resistance and terfel gradients βA and βc are always obtained as rate factors of the corrosion reaction based on the corrosion potential EcOrr of the sample metal piece,
Corrosion determination can be performed extremely accurately.

Furthermore, area A in FIG. Since b, c, dl, and d2 are carried out in about several tens of seconds, it goes without saying that the measurement is speeded up.

Note that in the above, polarization resistance Rp, terfel gradient β
Although the case where the measurements are performed in the order of A and βc is shown, the same effect can be obtained even if the order is changed as appropriate, and it is also possible to repeatedly measure the same type of speed factor.

Next, an embodiment according to the present invention will be described with reference to FIG. 3 showing a specific example of an apparatus using the method of the present invention.

In FIG. 3, a corrosion determination cell A is comprised of a sample metal piece 1 placed in a corrosion system, a reference electrode 3 such as a saturated electrode that maintains a stable potential, and a counter electrode 2. A charge supply mechanism B for supplying a predetermined amount of charge to the sample metal piece 1
is battery 5, polarity switch 6, 7 and variable resistor 9
It is composed of a charge supply source section and a capacitor 16 that stores charges, and 18 and 19 indicate an operational amplifier and a voltmeter, respectively, for measuring the amount of supplied charge.

That is, the voltage V1 of the capacitor 16 before charging the sample metal piece 1 and the voltage V2 after supplying the charge are measured with the electrometer 19, and ΔQ-C1(V1-V2)...
The amount of charge ΔQ imparted to the sample metal piece can be determined as C1: capacitor capacity). Note that the charge supply mechanism B may be appropriately replaced with a constant voltage pulse power source or the like that can instantaneously supply a predetermined amount of charge. Further, F indicates a charge applying mechanism for instantaneously applying a predetermined amount of charge supplied from the charge supplying mechanism B to the sample metal piece 1, and a charge applying mechanism for activating the relay 11 resistor 12, switch 14 and relay 11. It is composed of a power source 13.

Further, G indicates a blocking mechanism that is effective when a high resistance liquid is used in a corrosive system during measurement or when a metal sample 1 with a large surface area is used.

In other words, as mentioned above, using a high resistance liquid in a corrosive system,
When using a sample metal piece with a large surface area, it takes a long time to charge the electric double layer of the sample metal piece when a predetermined amount of charge is supplied to the sample metal piece, so the IR-
DrOp could make it difficult to obtain accurate polarization values. Therefore, after the charge applying mechanism F is activated, the timer 20 is activated when a predetermined period of time has elapsed, and the transistor 2 is activated.
1, the source and drain of the FET switch 22 are made non-conductive, stopping the charge supply to the sample metal piece 1, and activating the relay 30, which will be described later, to start measuring the polarization value η and time t. That is, by stopping the charge supply after a predetermined period of time, the influence of IR-DrOp on the polarization value η can be ignored. Therefore, the potential change in the sample metal piece 1 is measured by the potential difference measuring mechanism C via the relay 30, and the polarization value η
- measured as a time t relationship.

Note that the potential difference measuring mechanism C includes an operational amplifier 31 for impedance conversion, a potential difference recording circuit, and an analysis display circuit 33.
It is composed of It becomes possible to determine the rate factor of each corrosion reaction using the above-mentioned corrosion determination cell A, charge supply mechanism B, charge application mechanism F, potential difference measurement mechanism C, and if necessary, cutoff mechanism G.

Furthermore, in the method of the present invention, switch 3 is
5. The corrosion potential E of the sample metal piece 1 is measured in advance by a potential measuring meter D composed of an operational amplifier 36 and a voltmeter 37.

Measure Orr in advance. Therefore, after the measurement of each rate factor is completed, the potential of the sample metal piece is forcibly set to the corrosion potential EcOrr by the constant voltage mechanism E. The switching mechanism H includes a timer 27 and a relay 28 in order to apply a reverse bias from the constant voltage mechanism E.

Note that the apparatus using the method of the present invention as described above operates in the following order.

First, before measuring the reaction factor of corrosion rate, switch 35
Test the potential measurement meter D with the sample metal piece 1 in the corrosion judgment cell A.
The potential of the sample metal piece 1, that is, the corrosion potential EcO
Measure rr.

Therefore, the variable resistor 40 in the constant voltage mechanism E is adjusted so that EcOrr is always maintained between the reference electrode 3 and the sample metal piece 1. Next, capacitors 16-1 to 16-5 in charge supply mechanism B corresponding to the speed factor to be measured first are selected, and the amount of charge to be supplied is set.

When the switch 14 in the charge applying mechanism F is closed, a predetermined amount of charge is instantaneously supplied from the charge supply mechanism B to the sample metal piece 1 via the relay 11, the FET switch 22, and the relay 28. After a predetermined minute time, the timer 20 is activated, the FET switch 22 stops the supply of electric charge, and at the same time, the relay 30 measures the potential change in the sample metal piece 1 as a relationship between polarization value η and time t. From this relationship between polarization value η and time t, an initial velocity factor is determined by a predetermined analysis method. Subsequently, the timer 27 connects the sample metal piece 1 and the constant voltage mechanism E, the potential of the sample metal piece 1 is forced to EcOrr, and the next rate factor can then be measured. Thereafter, multiple velocity factors can be measured accurately and quickly by similar operations.
In the second and subsequent measurements, it is not necessary to adjust the potential measuring meter D and the constant voltage mechanism E unless the corrosion potential EcOrr changes significantly due to long-term standing. Hereinafter, the corrosion status of a mild steel plate in city water was measured using an apparatus using the method of the present invention shown in FIG. 3 above.

First, a mild steel plate with a surface area of 5 cd was placed in corrosion determination cell A as a sample metal piece, and the corrosion potential E after being immersed in city water for 72 hours.
When cOrr was measured using potential meter D, it was -0.6.
31V vs. It was SCE.

Subsequently, the variable resistor 40 of the constant voltage mechanism E is adjusted, and when the constant voltage mechanism E and the corrosion determination cell A are connected, the potential of the sample metal piece is -0.631Vvs. with reference to the reference electrode 3. SCE
I set it up so that Therefore, in order to find the polarization resistance,
A charge of 3 μc was applied to the sample metal piece 1 using the charge supply mechanism B, the charge application mechanism F, and the cutoff mechanism G, and as a result of analysis by the potential difference measurement mechanism C, ra = 140ttF/Cd
, R, = 2.2 md was obtained.

Subsequently, the potential of the sample metal piece 1 was forcibly set to EcOrr, and then a charge of 300 ttC was applied by the same operation, resulting in a Terfel gradient βa of 62 mV. E again sample metal piece 1. After setting the voltage to OO, a charge of -300 μC was applied, and the Terfel slope βc was determined to be 66 mV. Furthermore, the above-determined Rp, βA. Calculating the corrosion current based on βc, IcOrr=6.3μA/Cr! It became l. Note that the time required for the measurement was about 30 seconds. In addition, the same measurements as above were obtained using the conventional immersion method.Results 1
6mdd, converted to IcOrr is 6AμA/C
rii, which was almost the same as that obtained by the method of the present invention.
As described above, in the method of the present invention, rate factors of corrosion reactions, such as polarization resistance and terfel gradients βA and βc, can be determined continuously, quickly and accurately, and this is particularly true when measuring systems with relatively slow corrosion rates. It becomes valid.

[Brief explanation of the drawing]

Figure 1 is a principle diagram of an apparatus using the method of the present invention, Figure 2 is a characteristic diagram showing the potential change of a sample metal piece when the method of the present invention is used, and Figure 3 is a diagram of a specific example of the apparatus using the method of the present invention. A circuit diagram showing an example of the device. DESCRIPTION OF SYMBOLS 1... Sample metal piece, 3... Reference electrode, A... Corrosion judgment cell, B... Charge supply mechanism, C... Potential difference measuring mechanism, D... Potential measuring meter, E... - Constant voltage mechanism.

Claims (1)

[Scope of Claims] 1 A predetermined amount of electric charge is instantaneously applied to a sample metal piece exhibiting a corrosion potential E_c_o_r_r placed in a corrosion system, and a potential change of the sample metal piece is measured by a reference placed close to the sample metal piece. A method of obtaining a polarization value η-time t relationship with an electrode in an open circuit state and measuring a rate factor of a corrosion reaction involved in the sample metal piece from this polarization value η-time t relationship is repeated at least twice. In continuously measuring the rate factor of one corrosion reaction, after measuring the rate factor of one corrosion reaction and before measuring the rate factor of the next corrosion reaction, the potential of the sample metal piece is forcibly changed to A method for measuring the rate factor of a corrosion reaction, characterized by returning the potential to E_c_o_r_r. 2 In claim 1, when determining the polarization resistance R_P as a rate factor of the corrosion reaction, an electric charge is instantaneously applied so that the absolute value |η| of the polarization value in the sample metal piece becomes 10 mV or less. A method for measuring the rate factor of corrosion reactions characterized by: 3 In claim 1, when determining the terfel gradients βa and βc as rate factors of corrosion reaction, an electric charge is instantaneously applied so that the absolute value |η| of the polarization value in the sample metal piece becomes 60 mV or more. A method for measuring the rate factor of corrosion reactions.