JPH0350983B2 - - Google Patents

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention relates to an electrochemical measurement method for non-destructively evaluating the corrosion resistance etc. of a metal structure using an electrolytic bath, and more specifically, to This invention relates to an electrochemical measurement method for non-destructively evaluating the corrosion status of metal structures coated with paint.

[Technical background of the invention and its problems]

In order to evaluate the corrosion status of metal structures coated with coating layers such as paint, visual observation of the coating film surface and tan δ measurement of the coating film have been conventionally performed. but,
Visual observation has the problem that the corrosion criteria tend to be subjective and that changes do not appear on the coating surface unless corrosion under the coating has progressed considerably.
In addition, in the method of measuring tan δ of a paint film, the obtained value often does not correspond to the actual corrosion situation, and it cannot be said to be a practical evaluation technique.

Therefore, in order to solve these problems, attempts have been made to apply electrochemical measurement methods such as the coulostat method, current interrupter method, and AC impedance method. , it is necessary to take out the lead wire from the underlying metal. However, in ordinary structures, the entire surface of the structure is often coated with anti-corrosion paint, and it is necessary to remove a portion of the coated surface in order to pull out the lead wire. Therefore, it is necessary to destroy and repair a part of the paint film each time a measurement is made, which is inefficient and, if the repair is incorrect, there is a high possibility that the corrosion resistance of that part will deteriorate and corrosion will occur. In addition, in the case of structures where aesthetics are important, the deterioration of the appearance of the repaired area could cause major problems. Furthermore, it has been difficult to obtain sufficient measurement accuracy.

[Purpose of the invention]

The present invention has been made in view of the above-mentioned circumstances, and is a method for measuring electrochemical conditions such as corrosion on the surface of a metal structure coated with a coating layer such as paint, non-destructively and with high precision from above the coating layer. The purpose is to provide a possible electrochemical measurement method.

[Summary of the invention]

The present invention provides an electrochemical measurement method for measuring the corrosion state of a metal structure through a coating layer on the surface of the metal structure, in which a pair of electrolytic cells having the same shape and having a reference electrode and an insoluble electrode are attached to the metal structure. An electric signal is applied between each insoluble electrode to polarize one electrolytic cell to cathode polarization and the other to anode polarization, and the impedance between the reference electrodes is measured at this time. This is an electrochemical measurement method characterized by:

Polarization here refers to an operation in which the electrode potential is moved from its natural state toward the anode or cathode by passing a current through the electrode from the outside. Therefore, in the case of measurement using alternating current, polarization states appear alternately at the alternating current frequency. That is, an anode polarization state in which the potential of the object to be measured is polarized in the noble direction and a cathode polarization state in which the potential of the object to be measured is polarized in the negative direction alternately appear. Since the power source used for the measurement according to the method of the present invention does not matter whether it is AC or DC, the impedance can also be AC or DC.

Note that the coating layer in the present invention is a coating layer made of an organic substance, an inorganic substance, etc., and includes a coating layer formed by painting or the like.

Next, in order to explain the present invention, the electrolytic cell conventionally used and the electrolytic cell used in the present invention will be replaced with an equivalent circuit model and analyzed.

FIG. 1 shows an embodiment of the present invention, and FIG. 2 shows a cross-sectional view of an example of an electrolytic cell conventionally used.

In FIG. 1 and FIG. 2, the electrolytic cell body 1,
1' or 1 is a surface to be measured 4 of a metal structure 3 covered with a coating film 2 as a coating layer,
4' or 4' in a watertight manner, and the electrolytic cell body 1,
Inside each of 1' or 1'' there is a counter electrode 5,
5' or 5, a reference electrode 6, 6' or 6'' and an electrolyte solution 7 are held.A counter electrode 5, 5' or 5'' is attached to the surface to be measured 4, 4' or 4'' for electrochemical measurements. The reference electrode 6, 6' or 6'' is used to provide electrical stimulation, and an insoluble electrode made of an insoluble metal such as platinum is usually used. Usually, a saturated sweetened electrode (SCE), a silver-silver chloride electrode, or platinum with platinum black is used.
If the polarization of the counter electrodes 5, 5', 5" is negligibly small compared to the polarization of the electrode 4", the counter electrodes 5, 5', 5" can also function as reference electrodes. Usually, saline is used, but any liquid that has ionic conductivity and does not excessively deteriorate the coating film 2 can be used.

Figures 3 and 4 are equivalent circuit models that replace Figures 1 and 2, respectively, and Rc, Cc
are the polarization resistance and double layer capacitance of the counter electrodes 5, 5', and 5'', respectively, and Rs is the solution resistance of the electrolyte 7. Rf, Cf
are the insulation resistance and electric capacitance at the surfaces to be measured 4, 4, and 4'' of the coating film 2, respectively, and Rp and Cd are the surface to be measured 4 and
The reaction resistance and double layer capacity at 4,4″ are
Rm indicates the electrical resistance of the metal structure 3, respectively. Terminals A, B, A', B', C, D, and E have the same content as shown in FIGS. 1 and 2.

Among these various capacitances C and resistances R, the solution resistance
Rs is usually very small compared to coating film resistances Rf, Rf' and reaction resistances Rp, Rp', and since polarization measurement is performed between reference electrode terminals B, B', counter electrodes 5, 5',
5" polarization resistance Rc and double layer capacitance Cc are not involved in polarization measurement, so their contribution can be excluded in polarization analysis. Also, in normal metals, Rm is almost zero and can be ignored. Therefore, when electrical stimulation is applied between terminals A and A' in Figure 1 and between terminals C and E in Figure 2, the first
Extract the equivalent circuit that is involved in the polarization measured between terminals B and B' in the figure or between terminals D and E in Figure 2.
It can be simplified as shown in FIG.

The impedance between terminals B and B' in Figure 5 is Z ₁
Then, Z ₁ is expressed by equation (1).

Z ₁ = Rf/1+jωRf・Cf+Rp/1+jωRp・Cd+Rp
'/1+jωRp'·Cd+Rf'/1+jωRf'·Cf (1) Let _Z2 be the impedance between terminals DE and E in FIG. 6, and _Z2 is expressed by equation (2).

Z ₂ = Rf/1+jωRf・Cf+Rp/1+jωRpCd…(2) Next, by making the two sets of electrolytic cells used in Fig. 1 have the same shape, Rf=Rf′, Cf=
Cf′, Rp=Rp′, and Cd=Cd′, so equation (1) is simplified as shown in equation (1).

Z ₁ =2・Rf/1+jωRf・Cf+2・Rp/1+jωRp・Cd
...(1)' As is clear from the above equation, the impedance Z ₁ obtained by the method of the present invention is twice the impedance (Z ₂ ) obtained by the conventional method using a single electrode. Therefore, by simply halving the impedance obtained by the method of the present invention, the same impedance as in the conventional method can be obtained.

As explained above, according to the electrochemical measurement method of the present invention, it is possible to eliminate the need to draw out lead wires from the underlying metal, which was indispensable in the past.

In the case of direct current measurement, conventional measurement methods can only measure either anode polarization or cathode polarization. Therefore, polarity-dependent corrosion measurements cannot provide accurate measurements. Furthermore, even in AC measurements, the measurement time is not necessarily an integral multiple of the period of the measurement AC power supply, so the times of the cathode polarization state and the anode polarization state are not the same, so the influence of one of the polarization states will appear strongly. Therefore, there were problems with measurement accuracy and reproducibility, but according to the method of the present invention, when one electrode is polarized as an anode, the other is polarized as a cathode, and the sum of their impedances, that is, the impedance between both electrodes, can be measured. Therefore, the average value of the anode polarization state and the cathode polarization state can be determined. Therefore, even if the measurement frequency becomes low, the influence of one polarization state does not appear strongly, and the measurement accuracy and reproducibility are improved compared to the conventional method. The same thing can also be said in the case of direct current measurement.

[Embodiments of the invention]

In order to simplify the electrochemical measuring method of the present invention, a prototype electrolytic cell with fixed magnets, shown partially in cross section in FIG. 7, was manufactured and measured. The test piece was coated steel with a pigment-free epoxy coating 8 on the mild steel plate 16, and 3% saline was used as the electrolyte 12. AC impedance was measured using a Solartron 1250 model. . Note that 9 in Figure 7,
9' is an electrolytic cell container, 10 and 10' are saturated sweet electrodes, 11 is a platinum plate as an insoluble electrode, and 1
3 is a magnetic yoke, 14 is a magnet, 15, 15'
indicate a watertight seal, respectively. Figure 8 shows a Cole-Cole plot (a plot of impedance measured in response to a stimulus on a complex plane) that is commonly used to analyze the response when electrical stimulation is applied to a corrosion reaction system. Shown in a. Next, for comparison, a lead wire was drawn out from the mild steel plate 16, and AC measurement was performed using the same Solartron 1250 model, and a call-call plot obtained is shown in FIG. 8b. In FIG. 8, a and b have the same shape, and the values of a and b on the real and imaginary axes are respectively twice as large as those of b.

This result confirmed that the above calculation result was correct.

Furthermore, in the present invention, even if the measurement time is appropriately selected, when one electrode is in the cathodic polarization state, the other is always in the anode polarization state, and the fact that the impedance between the electrodes is measured means that the cathodic polarization is This means that we are measuring the average of anode polarization. When one electrolytic cell is used as in the conventional method, the measurement time is not necessarily an integral multiple of the period of the AC power source for measurement, so the influence of either anode polarization or cathode polarization will appear. In electrochemical measurements, measurements are usually performed at frequencies as low as mHz, and this effect is very likely to occur. It is very difficult to accurately control the measurement time, resulting in poor reproducibility and difficulty in highly accurate measurement. On the other hand, the present invention, which always measures the average value, does not have this problem and can perform highly accurate measurements with good reproducibility.

〔Effect of the invention〕

As mentioned above, by using the electrochemical measurement method of the present invention to evaluate coated steel, it becomes unnecessary to draw out lead wires from the base metal, which was indispensable in the past. In addition to making it possible to significantly improve the efficiency of evaluation work by eliminating the need for film destruction and repair work, on-site measurements have become significantly easier, and there is no need to damage the durability of the paint film. Most of my fears have disappeared.

Further, in the present invention, by using a pair of electrolytic cells, each electrolytic cell has anode polarization and cathode polarization, making it possible to perform measurements with high precision and excellent reproducibility. In other words, since corrosion measurement generally has polarity dependence, accurate measurement is not always possible when measuring only with one polarity. On the other hand, in the present invention, the impedance between the two electrodes is measured when one is cathodically polarized and the other is cathodically polarized, so the results are not biased towards one polarity and are highly reproducible. Accuracy can be measured.

In addition, in normal metal structures, the base metal is grounded, so if there is noise input in conventional electrochemical measurement equipment even if a differential amplifier is used, this noise input itself will be amplified and accurate measurements will not be possible. However, in the case of the present invention, by using a pair of electrolytic cells, it is possible to cancel out the noise input with a differential amplifier, which greatly improves measurement accuracy.
It can be said that it is extremely effective industrially.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram explaining the basic configuration of an electrolytic cell according to the present invention, FIG. 2 is an explanatory diagram explaining the basic configuration of a conventional electrolytic cell, FIG. 3 is an equivalent circuit model of FIG. The figure shows the equivalent circuit model of Figure 2, Figure 5 shows the equivalent circuit model that extracts the parts involved in potential response from Figure 3, and Figure 6 shows the equivalent circuit model that extracts the parts involved in potential response from Figure 4. Figure 7 is a configuration diagram of an electrolytic cell used in an example according to the present invention, Figure 8a is a Cole-Cole plot measured in an example, and Figure 8b is a measurement using a conventional electrolytic cell. The following shows the call plot. 1, 1', 1''... Electrolytic cell body, 2... Coating layer (coating film), 3... Metal structure (base metal), 4,
4', 4"...Measurement surface, 5, 5', 5"...Insoluble electrode (counter electrode), 6, 6', 6"...Reference electrode, 7...
Electrolyte solution, 8...Epoxy paint, 9,9'...
Electrolytic cell container, 10, 10'...saturated sweet electrode,
11... Insoluble electrode (platinum plate), 12... 3% saline, 13... Magnetic yoke, 14... Magnet, 1
5, 15'... Watertight seal, 16... Mild steel plate.

Claims (1)

[Claims] 1. In an electrochemical measurement method for measuring the corrosion state of the metal structure through the coating layer on the surface of the metal structure, a pair of electrolytic cells of the same shape equipped with a reference electrode and an insoluble electrode are connected to the coating layer of the metal structure. By applying an electric signal between the insoluble electrodes, one electrolytic cell is polarized as a cathode and the other as an anode, and the impedance between the reference electrodes is measured. Characteristic electrochemical measurement method.