JPS58618B2 - Method for determining corrosion rate of metals - Google Patents

Description

[Detailed description of the invention] The present invention uses two metal sample pieces of the same material as working electrodes,
This invention relates to a method for determining the corrosion rate of a metal specimen under predetermined conditions by 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 constant corrosion potential, but when a small constant current is applied to the sample metal piece through the counter electrode, the potential changes. cause.

However, if the amount of potential change is η, then the polarization resistance RP of the sample metal piece is given by RP=η/■ (1).

On the other hand, theoretically, it is believed that the polarization resistance Rp is inversely proportional to the corrosion current Icorr as shown in the following equation.

RpIcorr=K...(2) Here, K is a constant and a value unique to the corrosion reaction,
Actually, the difference in K depending on the type of reaction is small compared to the change in corrosion current Icorr.

Further, the corrosion rate V is expressed by the following formula % (3) (M is the atomic weight of the sample metal, Z is the valence number of the eluted metal ion, and F is the Faraday constant).

Therefore, find the polarization resistance RP from equation (1) and convert it to (2)
If the corrosion current Icorr is known by substituting it into the equation, the corrosion rate ■ 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 accumulation of corrosion products on the surface is promoted, which tends to lead to inaccurate measurement of the potential change η.

Second, it is difficult to accurately measure the potential of the sample metal piece due to the potential drop caused by the solution resistance between the sample metal piece and the reference electrode.

This potential drop becomes a major hindrance in tracking changes in the amount of potential change η, and there is also the inconvenience that compensating for the influence of the potential drop requires complicated operations.

Therefore, the present invention can measure the corrosion potential E within a relatively short measurement time.
The purpose of the present invention is to provide a method that is hardly affected by corr even in an unstable system to the extent that corr exists, and that can easily determine (measure) the corrosion rate of metal without requiring correction or compensation for voltage drop. .

To explain the present invention in detail below, the present invention instantaneously applies a certain amount of electric charge to two sample metal pieces of the same material placed in a corrosive system, one as an anode and the other as a cathode. The method is characterized in that the change in the potential difference δ between the sample metal pieces is measured in an open circuit state as a relationship between the potential difference δ and time t, and the corrosion rate of the sample metal piece is determined by analyzing the relationship between the potential difference δ and time t. This is a metal corrosion rate determination method that applies the coulostat method.

Figure 1 is a circuit diagram explaining the principle of measuring the polarization resistance Rp of two metal samples made of the same material using the coulostat method with one as an anode and the other as a cathode. 2 indicates a pulse generator for generating pulses, 2 indicates a sample metal piece as an anode, 3 indicates a sample metal piece as a cathode, and 4 indicates a potentiometer.

Using the circuit shown in Figure 1, we can calculate the polarization resistance R of the sample metal piece.
To obtain P, apply a constant electrical pulse (charge) of, for example, several microseconds to several micrometers between two sample metal pieces 2 and 3 that are at a predetermined corrosion potential Ecorr (natural potential Ecorr). The electric double layer of two sample metal pieces 2 and 3 is charged instantaneously.

In this case, the magnitude of the applied charge is preferably selected so that the potential of the sample metal piece changes by several mV from the corrosion potential Ecorr.

Further, it is desirable that both sample metal pieces be short-circuited until immediately before double layer charging in order to eliminate an asymmetric potential between the two sample metal pieces.

In this way, a certain amount of electric charge instantaneously applied is applied to both the sample metal piece 2 as an anode and a cathode by a corrosion reaction.
, 3, and the potential of the sample metal piece shows a tendency to return to its original corrosion potential (natural potential) Ecorr. Therefore, the change in potential difference δ with respect to time t can be expressed as the potential difference δ - time t relationship. Record with recorder 4.

As long as the potentiometer 4 has a large input impedance, the current between the two sample metal pieces 2 and 3 can be ignored and measurement can be performed in an open circuit state.

However, if we ignore the change in the differential capacitance CD of the electric double layer of the sample metal piece within a potential range of about several mV, when the potential difference δ is sufficiently small, the polarization value η is also sufficiently small and the faraday due to the anodic or cathodic reaction Since the relationship RP=η/■ approximately holds true between the current ■ and the polarization value η of the sample metal piece as an anode or the sample metal piece as a cathode, the curve for one hour is generally calculated theoretically. It is derived as follows.

η=η0exp(t/CDRP) (In the formula, η0 is the polarization value immediately after applying an electric charge to the sample metal piece.

) By the way, if a charge of ΔQ was given to the sample metal piece used as an anode during the initial double layer charging, a charge of 1 ΔQ was inevitably given to the sample metal piece used as a cathode. However, the surface area of both is Sa,
If Sc, the polarization values η0a and η0C of the sample metal piece used as an anode and the sample metal piece used as a cathode immediately after double layer charging are as follows, respectively.

η0a=ΔQ/Sa・CD・η0c=−ΔQ/SC・C
D Note that if the potential difference δ is sufficiently small, CD can be considered constant, and CD takes the same value as the natural potential.

Therefore, the polarization value ηa at time t of the sample metal piece used as an anode and the sample metal piece used as a cathode
, ηc are respectively η3-η0aexp(-t/CDRP) = (ΔQ/Sa・CD)exp(-1/CDRP)ηc
It is derived as follows: =η0cexp(-t/CDRp) =-(ΔQ/SC·CD)exp(-t/CDRp).

Therefore, the potential difference δ between the sample metal pieces at time t is further derived as a function of time.

Therefore, if a straight line is obtained when the potential difference δ is calculated and lnδ is plotted against time t, by removing the straight line at time t=0, ΔQ/CD(1/Sa+1/Sc
) can be obtained.

Here, ΔQ/CD (1/Sa+1/SC) corresponds to the potential difference δ0 between the anode and cathode at time t=0 immediately after double layer charging.

Therefore, the polarization value δ0 immediately after applying a charge to the sample metal piece is
Calculate the differential capacitance CD from the following formula and the amount of charge ΔQ given to the sample metal piece.Furthermore, using these values of δ0 and CD, the polarization resistance RP can be calculated from the slope of the curve using the above lnδ−. It can be sought.

Next, examples of the present invention will be described.

FIG. 2 shows an apparatus for measuring changes in potential difference between two sample metal pieces.

This is a system A that applies a known amount of charge between two sample metal pieces 12 and 13 made of the same material installed in the corrosion measurement cell 11, and a change in potential difference between the two sample metal pieces 12 and 13. It consists of a system B that tracks .

A system A that provides a known amount of charge includes a power source 14 for supplying the charge, and a capacitor 151 to store the above-mentioned charge in advance.
154, consisting of a variable resistor VR16 that regulates the amount of electricity stored in the capacitors 151 to 154, and a relay 17 that instantaneously provides the charge stored in the capacitors 151 to 154 between both sample metal pieces 12 and 13 of the cell 11. has been done.

Note that the relay 17 has two circuits and two contacts, so in order to be able to ignore the asymmetric potential at first, the sample metal pieces are short-circuited together, and after the short-circuit between the sample metal pieces is removed, a known amount of charge is immediately applied to the relay 17. It is used in a way that gives

On the other hand, the system B that tracks the potential difference change between the two sample metal pieces 12 and 13 is connected to the two sample metal pieces 12 in the corrosion rate measurement cell 11.
, 13, an operational amplifier 18, a potentiometer 19, and the like.

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.

Also, the capacitor 15 of system A that provides the known amount of charge.
The capacitors 151 to 154 are connected to both ends of 1 to 154.
A voltmeter 23 is connected via the switch 22 to check the voltage of the capacitors 151 to 154.
As for
A switch 24 is provided.

In the potential difference measurement device configured as described above, the sample metal pieces 12 and 13 are each 1 cm2.1.
Using a pure copper plate with a surface area of 5 cm2 and with city water contained in the cell 11, the corrosion potential (natural potential) of the copper plate was first determined using a 5CE (saturated corrosion electrode) as a reference electrode. , 230V vs. SCE.

Next, +0.14 was applied to both sample metal pieces 12 and 13, respectively.
When a charge of 3 .mu.C2-0.143 .mu.C was applied instantaneously, the relationship between the potential difference .delta. and the time t was as shown in FIG.

The logarithm logδ of the potential difference δ is calculated using the curve δ− shown in Figure 3.
When expressed as , there is a linear relationship with time t as shown in Figure 4, and if log δ is extrapolated to t = 0, a potential difference δ0 = 6.8 mV immediately after applying a charge to the sample metal piece is obtained, and From the equation, CD=35 μF/cm2, and from the slope of the straight line shown in FIG. 4, polarization resistance RP=213Ω was obtained.

The value of Ω for this polarization resistance RP=213 was approximately equal to the value 215 Ω determined by the constant current method.

When each of the above values was converted into a corrosion rate (■) using equations (2) and (3), the results were 0.27 mdd and 0.26 mdd, which were almost in agreement with the value measured by the immersion method, which was 0.266 mdd.

In addition, even if city water with high resistance to corrosive systems is used in the above,
The fact that values almost identical to those obtained by the immersion method can be obtained means that corrections taking into account the potential drop can be ignored.

As described above, according to the method of the present invention, there is no need for correction in consideration of potential drop, which not only eliminates the complexity of measurement but also improves measurement accuracy.

Furthermore, the corrosion rate can be determined or measured in a shorter time than the constant current method.

For example, when determining the corrosion rate of stainless steel using the constant current method, it takes a long time to charge the double layer, and it takes a considerable amount of time to reach a certain polarization value, resulting in a measurement time of several tens of thousands of hours.
The method of the present invention can be carried out in an extremely short time, on the order of several seconds, whereas the method of the present invention requires several minutes to several hours.

Therefore, according to the method of the present invention, determination can be made even when the natural potential of the sample metal piece (working electrode) tends to show instability due to changes in the surface condition of the sample metal piece (working electrode) or the corrosion system.

Furthermore, since the present invention utilizes a two-electrode coulostat method in which a sample metal piece of the same material is used as an anode and a cathode, the operation is simple and the erosion rate can be determined quickly.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the principle of determining the polarization resistance of a sample metal piece by the coulostat method, Fig. 2 is a circuit diagram of a potential difference change measuring device used in an embodiment of the present invention, and Figs. 3 and 4 are FIG. 6 is a relationship diagram showing the relationship between the potential difference and time determined in the above example. 11... Corrosion rate measurement cell, 12, 13...
...Sample metal piece, 14...Power supply, 151~
154... Capacitor, 16... Variable resistor, 17... Relay, 18... Operational amplifier, 19... Potentiometer, 20・・・・・・
...Power supply, 21...Variable resistance.

Claims (1)

[Claims] 1 Two sample metal pieces of the same material placed in a corrosive system,
A predetermined amount of charge is instantaneously applied to one of the metal pieces as an anode and the other as a cathode, and the change in the potential difference δ between the two sample metal pieces is measured in an open circuit state as a relationship between the potential difference δ and time t. A metal corrosion rate determination method characterized by determining the corrosion rate of a sample metal piece.