JPH0546495B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a coating film impedance measuring device used for detecting and evaluating the deterioration of a coating film applied to a metal surface used for purposes such as rust prevention and aesthetics. [Technical background of the invention and its problems] First, a conventional electrochemical evaluation method for paint film deterioration will be explained. As shown in FIG. 5, a measuring electrode 4 is brought into contact with the coating film 2 to be evaluated coated on the base metal 1 either directly or via a conductive liquid or conductive gel, that is, a conductive fluid 3. . An alternating current voltage is applied between the base metal 1 and the measurement electrode 4 using an alternating current power source 6 via an ammeter 5. The voltage applied at that time is read by a voltmeter 7 connected to the base metal 1 and the measuring electrode 4. In the normal state immediately after coating, the coating film 2 has a very high electrical resistance, and has a direct current resistance of 10 ⁸ Ω·cm or more. When the paint film 2 is normal, the electrical equivalent circuit of the paint film 2 is represented by a parallel circuit of a resistor R _f and a capacitor C _f as shown in FIG. As the paint film 2 deteriorates, this resistance R _f decreases, and the simple equivalent circuit shown in Figure 6 begins to represent a complex impedance with multiple time constants. It's coming.
However, at the initial stage of deterioration, it is possible to treat the resistor R _f and the capacitor C _f as a parallel circuit. The impedance Z (jω) of the coating film is obtained from equation (1). Z(jω)=e(jω)/i(jω)...(1) Here, ω represents the angular frequency, e(jω) represents the voltage, and i(jω) represents the current. Further, tan δ, which is another index of deterioration, is obtained by equation (2). tanδ=|Z _n |/|Z _e | ...(2) Here, Z _n is the imaginary part of the impedance, and Z _e is the real part of the impedance (resistance component whose impedance is in the same phase as the pure resistance). As the coating film 2 deteriorates, the resistance R _f in Fig. 6
decreases, and when the same AC voltage e(jω) is applied, the current i(jω) increases, and the impedance Z
(jω) decreases. Similarly, as resistance R _f decreases, the real part of impedance |Z _e | decreases.
tanδ increases. In this way, deterioration of the coating film 2 can be detected by measuring the impedance or tan δ of the coating film 2 by the AC impedance method. Changes in impedance before and after the paint film deteriorates are shown in FIGS. 7 and 8. FIG. 7 shows the absolute value of impedance blotted against frequency (logarithm), and is generally referred to as a Bode diagram. As opposed to the impedance curve a of an undegraded paint film, as the paint film deteriorates, the impedance decreases more markedly on the lower frequency side as shown by curve b. Therefore, when detecting deterioration only from the absolute value of impedance, it is more effective to measure at a lower frequency. FIG. 8 shows the impedance in real and imaginary parts, which is a multiple representation generally called a Nyquist diagram. As the paint film 2 deteriorates, the impedance locus d changes to the normal curve C.
As the semicircle becomes smaller compared to , the circle becomes deformed. From this shape, the degree of deterioration of the coating film 2 can be estimated. The method for detecting the deterioration of the paint film from the impedance measurement results is as described above, but when actually measuring the impedance of the paint film 2, the base metal 1
It may not be possible to take out the electrical lead wires directly from the As a countermeasure to this problem, the applicant previously proposed a double cell method (two-electrode method). For example, Japanese Patent Application No. 58-198315 (Japanese Unexamined Patent Publication No. 60-91250) and Japanese Patent Application No. 59-55818 (Japanese Unexamined Patent Publication No. 60-20015) are examples. In this method, as shown in Figure 9, an AC voltage is applied between two measuring electrodes _4a and _4b , and the impedance is measured from the voltage, current, and frequency in the same manner as the method described above. This is a method for detecting deterioration of the membrane 2. The impedance measured in this case is the sum of the impedances of the electrode 4 _a and the electrode 4 _b . However, when measuring impedance using this method, when measuring a coating system in which the impedance of the coating film 2 is very high, for example, when measuring a thick coating film such as a marine structure, leakage on the coating film surface may occur. The current can no longer be ignored. If the surface of the paint film is dirty or wet, or if a conductive liquid or gel inserted between the electrode and the paint film creates continuity between the electrodes, the impedance Errors may occur in the measurement results, and the original impedance of the paint film may not be measured. In FIG. 9, the impedance of the coating film 2 is determined by equation (1) from the voltage e(jω) read by the voltmeter 7 and the current i(jω) read by the ammeter 5. ) includes the surface leakage current _{i s in} addition to the paint film penetration current i f passing through the paint film 2, resulting in an impedance smaller than the impedance of the paint film 2, similar to the case of deterioration. A judgment will be made. One possible way to reduce surface leakage is to increase the distance between the measurement electrodes _4a and _4b , but this may increase the size of the measurement device or be limited by the size of the object to be measured or the measurement range. In many cases, it is not preferable to move the electrode away from the noise standpoint. [Object of the Invention] The present invention overcomes the drawbacks of conventional devices, provides a guard electrode around the measurement electrode, and provides coating film impedance that enables accurate measurement without surface leakage current affecting the measured value. Its purpose is to provide a measuring device. [Summary of the Invention] The present invention provides a coating film impedance measurement device using two electrodes (double cell), in which a guard electrode is arranged around the measurement electrode so that leakage current between the electrodes does not affect the measured value. It is a membrane impedance measurement device, and furthermore, it is a coating film impedance measurement device that uses an amplifier to drive the guard electrode to have the same potential as the measurement electrode. [Embodiment of the Invention] FIG. 1 is a block diagram showing a circuit configuration in an embodiment of the present invention. The same reference numerals indicate the same or corresponding parts in all drawings. Measuring electrodes 4 a and 4 are applied directly or via a conductive substance 3 such as a conductive liquid or gel or a porous material containing them onto a coating film 2 applied on a base _metal 1 . Bring _b into contact. The measurement electrodes 4 _a and 4 _b are connected to an AC power source 6 via ammeters 5 _a and 5 _b . Furthermore, guard electrodes 8 _a and 8 _b are provided around the measurement electrodes 4 _a and 4 _b , and these guard electrodes 8 _a
and _8b are also brought into contact with the coating film 2 directly or via the conductive substance 3. These guard electrodes 8 _a and 8 _b are directly connected to the AC power source 6 . Therefore, the shape of the measurement electrodes 4 _a and 4 _b or the guard electrodes 8 _a and 8 _b is as shown in the plan view (upper figure) and side sectional view (lower figure) in FIGS. 2 a and b, respectively. Although the guard electrode 8 may have any shape depending on the object to be measured, such as circular or square, it is preferable that the guard electrode 8 completely surround the measurement electrodes 4 _a and 4 _b . With this configuration, a potential difference equal to the voltage drop due to the resistance of the ammeter _5a is generated between the measurement electrode 4a _and the guard electrode _8a , but the surface leak resistance between the measurement electrode _4a and the guard electrode _8a is In comparison, the resistance of the ammeter _5a is generally sufficiently small, so that the flow from the guard electrode _8a to the measurement electrode _4a due to this potential difference has a sufficiently small value compared to the measurement electrode _ia . Further, since the surface leakage current i _s flows only from the guard electrode 8 _a , the current i _a flowing to the measurement electrode 4 _a is only that which is involved in measuring the impedance of the coating film 2. The same effect applies to the measurement electrode _4b , the guard electrode _8b , and the ammeter _5b . In this way, accurate impedance measurement of the coating film 2 becomes possible. As the AC power supply 6 for impedance measurement,
In addition to sine waves, impulses or random noise containing many frequency components simultaneously can be used, and by connecting the current and voltage to a transfer function meter, the coating resistance R _f , coating capacitance C _f or the seventh
Impedance trajectories as shown in FIG. 8 and FIG. 8 are obtained. Table 1 shows the impedance measurement results of the coating film according to the present invention.

〔Effect of the invention〕

Thus, according to the present invention, by providing a guard electrode in an impedance measurement device used for detecting paint film deterioration by the two-electrode method (double cell method),
It is possible to accurately measure the impedance of high-resistance coatings such as those used on marine structures, etc., and it also enables measurement electrodes to be brought closer together, making it more compact and having improved noise resistance. It has become possible to measure with high accuracy. As a result, deterioration of the paint film can be detected early, and life estimation, repainting timing determination, paint film evaluation, etc. can be performed in a short time and with high precision, which is of great industrial benefit.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the circuit configuration of one embodiment of the present invention, FIGS. 2 a and b are a plan sectional view and a side sectional view showing the shape of the electrode, and FIG. 3 is another embodiment of the present invention. A block diagram showing the circuit configuration of FIG. 4a,
b is a side sectional view and a plan sectional view of another embodiment of the present invention;
Figure 5 is an explanatory diagram showing a conventional method for electrochemically evaluating paint film deterioration, Figure 6 is an electrical equivalent circuit of a paint film used to evaluate paint film deterioration, and Figure 7 is a painting An explanatory diagram showing changes in the frequency dependence of impedance due to film deterioration. Figure 8 is an explanatory diagram showing Figure 7 on a complex plane. Figure 9 is an electrochemical evaluation of paint film deterioration using the two-electrode method. It is an explanatory diagram showing a method. DESCRIPTION OF SYMBOLS 1... Base metal, 2... Paint film, 3... Conductive substance, 4, 4 _a , 4 _b ... Measuring electrode, 5, 5 _a ,
5 _b ... Ammeter, 6... AC power supply, 7... Voltmeter,
8, 8 _a , 8 _b ...Guard electrode, 9...Insulator, 1
0,10 _a ,10 _b ……Buffer amplifier, 11,1
1 _a , 11 _b , 12... High input impedance amplifier, a, c... Impedance of normal paint film, its trajectory, b, d... Impedance of deteriorated paint film, its trajectory, R _f ... Paint film resistance , C _f ... Coating film capacity, i _a ... Measurement current a, i _b ... Measurement current b, i _s ...
...Surface leakage current, R, R _a , R _b ...IV conversion resistance.

Claims (1)

[Scope of Claims] 1. Measuring electrodes provided directly or via a conductive substance at two locations on the base metal surface coating, an AC power source connected between both measuring electrodes, and both measuring electrodes. Guard electrodes are placed around the electrodes either directly or via a conductive material on the painted base metal surface and are connected to both ends of the AC power source, a voltmeter that measures the voltage between the electrodes for both measurements, and electrodes for both measurements. A coating film impedance measurement device comprising: an ammeter that measures a current flowing through the impedance. 2. The coating film impedance measuring device according to claim 1, wherein the guard electrode surrounds the measurement electrode. 3. The coating film impedance measuring device according to claim 1, wherein the guard electrode is driven to have the same potential as the measurement electrode using an amplifier. 4 A high input impedance amplifier is installed in front of the voltmeter to detect the voltage, and instead of an ammeter, an IV (current → voltage) conversion resistor is connected, and a high input impedance amplifier is installed at both ends of the resistor, and the current is derived from the output of this amplifier. A coating film impedance measuring device according to claim 1, wherein the coating film impedance measuring device is configured to: