JPS6131948A - Instrument for measuring impedance of coated film - Google Patents

Abstract

PURPOSE:To meassure exactly the impedance of a coated film without the influence of surface leak current on a measured value by disposing guard electrodes around measuring electrodes. CONSTITUTION:The measuring electrodes 4a, 4b are brought into contact directly or via a conductive material 3 with the surface of the paint-coated film 2 on a blank metal 1 and the guard electrodes 8a, 8b are provided around the same. A voltage is impressed to the electrodes 4a, 4b and the electrodes 8a, 8b by an AC power source 6 and the current is measured by ammeters 5a, 5b connected to the measuring electrodes 4a, 4b. The resistance of the ammeters 5a, 5b is small as compared to the surface leak resistance between the electrodes 4a, 4b and the guard electrodes 8a, 8b and therefore the impedance measurement is made possible without the influence of the surface leak current on the measured value.

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention relates to a coating used for detecting and evaluating deterioration of a coating film applied to a metal surface used for purposes such as rust prevention and aesthetics. The present invention relates to a membrane impedance measuring device.

[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 placed on a coating film 2 to be evaluated coated on a base metal 1 either directly or via a conductive liquid or a conductive gel 3.
contact. An ammeter 5 is connected between the base metal 1 and the measurement electrode 4.
An AC voltage is applied using an AC power supply 6 via the AC power source 6. 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 after coating, the paint film 2 has a very large electrical resistance (10"Ω・F or more DC resistance. The electrical equivalent circuit of the paint film 2 is that the paint film 2 is In a normal case, it is represented by a parallel circuit of a resistor Rf and a capacitor Cf as shown in Fig. 6.As the coating film 2 deteriorates, the resistance Rf decreases a little [as shown in Fig. 6]. A simple equinumerical circuit, as shown in Figure 1, begins to express complex impedances with multiple time constants.

However, at the initial stage of deterioration, it is possible to treat the resistor Rf and capacitor Cf as a parallel circuit.

The impedance Z (js) of the coating film is obtained from equation (1).

Z(jω) = e(jω)/i(jω) ・・・・・・
...(1) Here, ω is the angular frequency.

e(jω) is voltage.

i(jω) represents current.

Further, tan δ, which is another index of deterioration, is obtained by equation (2).

tanδ= IZmI/ IZeI −・−・
−・−(2) Here, zm is the imaginary part of impedance,
Z8 is the real part of impedance (resistance component whose impedance is in the same phase as the pure resistance).

As the coating film 2 deteriorates, the resistance R shown in FIG.

decreases, and when the same AC voltage e(jω) is applied, the current 1ts) increases and the impedance Z(jω) decreases. Similarly, when the resistance R, decreases, the real part 1Zel of the impedance decreases and janδ increases.

In this way, the impedance of the coating film 2 or t
Deterioration of the coating film 2 can be detected by measuring an δ.

Changes in impedance before and after the paint film deteriorates are shown in FIGS. 7 and 8.

Figure 7 is a plot of the absolute value of impedance versus frequency (logarithm), and is generally called a Bode diagram. Impedance song of undegraded paint film #
With respect to ia, as the paint film deteriorates, the impedance decreases markedly on the lower frequency side as shown in the curve. 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 terms of a real part and an imaginary part, which is a complex representation generally called a Nyquist diagram. When the paint film 2 deteriorates.

In the impedance locus d, the semicircle becomes smaller and the circle becomes deformed compared to the normal curve C. From this shape, the degree of deterioration of the paint film 2 can be estimated.O Although the method for detecting the deterioration of the paint film from the impedance measurement results is the same as above, when actually measuring the impedance of the paint film 2, There are cases where it is not possible to take out electrical lead wires directly from the base metal 1. As a countermeasure to this problem, the applicant previously proposed a double cell method (two electrodes). For example, Japanese Patent Application No. 58-198315, Japanese Patent Application No. 59-59
5818 etc. are such examples.

In this method, two measurement electrodes 4a are used as shown in FIG.
, 4b is applied, and the impedance is measured from the voltage and current 9 frequencies at that time in the same manner as the method described above, and deterioration of the coating film 2 is detected. In this case, the impedance measured is the sum of the impedances of the electrode 4a and the electrode 4b.

However, when measuring impedance using this method, if the impedance of the paint film 2 is very high, such as a thick paint film such as an offshore structure, leakage on the paint 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 for measurement causes electrical continuity between the electrodes, , errors may occur in the impedance 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 measured by the voltmeter 7.
Voltage e(jω) read by and current i read by ammeter 5
(jω), it can be found by equation (1), but the current i
(jω) is the paint film penetration current j that passed through the paint film 2 (in addition to the surface leakage current i), as a result, the impedance is a smaller value than the impedance of the paint film 2, p, in the case of deterioration. A similar judgment will be made.As a way to reduce surface leakage, increasing the distance between the measurement electrodes 4a and 4.
It is often limited by the size of the measuring device, the size of the object to be measured, and the measuring range, and it is not preferable to move the electrodes away from the viewpoint of noise.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a coating film impedance measuring device that overcomes the drawbacks of conventional devices, provides a guard electrode around the measuring electrode, and is capable of accurate measurement in which surface leakage current does not affect the measured value. is its purpose.

[Summary of the invention]

The present invention is a coating film impedance measuring device using two electrodes (double cell), so that leakage current between the electrodes does not affect the measured value.
This is a coating film impedance measuring device in which a guard electrode is arranged around the measuring electrode, and furthermore, it is a coating film impedance measuring device in which a card electrode is driven using an amplifier so that it has the same potential as the measuring electrode.

[Embodiments 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.

A measuring electrode 4 and 4. contact. The measuring electrodes 4a and 4. are ammeter 5a and 5. It is connected to the AC power supply 6 via.

Further, measurement electrodes 4a and 4. guard electrode 8 around
a and 8b are provided, and these guard electrodes 8a and 8b
is also brought into contact with the coating film 2 directly or via a conductive substance 3. These guard electrodes 8a and 8b are directly connected to the AC power source 6.

Therefore, the measurement electrodes 4a and 4b or the guard electrodes 8a and 8. The shape of is shown in Fig. 2(a).

As shown in the plan view (upper figure) and side sectional view C (lower figure) in (b), the guard electrode 8 may have any shape depending on the object to be measured, such as circular or square, but the guard electrode 8 is the measuring electrode. Preferably, it completely surrounds 4a and 4b.

With this configuration, the measurement electrode 4a and the guard electrode 8
However, since the resistance of the ammeter 5a is generally sufficiently small compared to the surface leak resistance between the measurement electrode 4a and the guard electrode 8, this potential difference causes a voltage drop between the guard electrode and the measurement electrode 4a. 8 to measuring electrode 4□
8 has a sufficiently small value compared to the measurement current ia.

Moreover, since the surface leakage current i flows only from the guard electrode 8a, the current ia flowing to the measurement electrode 4a is
Only those involved in impedance measurement.

Measuring electrode 4b, guard electrode 86. The same effect applies to the ammeter 5b.

In this way, accurate impedance measurement of the coating film 2 is possible.

In addition to sine waves, impulses or random noise containing many frequency components can be used as the AC power source 6 for impedance measurement. Rf, coating volume Of or impedance loci as shown in FIGS. 7 and 8 are obtained.

Table 1 shows the impedance measurement results of the coating film according to the present invention.

Table 1 Results of coating film impedance measurement of epoxy thick film paint In the two-electrode method without a guard electrode, when the surface is wet,
Or if a conductive gel is attached to the surface,
Due to surface leakage, the impedance of the coating film is lower than that of the one-electrode method. This tendency is particularly noticeable when the distance between the electrodes is small.

The measurement results of the present invention provided with a guard electrode showed values close to those of the one-electrode method, confirming the effectiveness of the present invention.

FIG. 3 is a block diagram showing the circuit configuration of another embodiment of the present invention.

This is an example showing the under connection required in actual measurement, and also the guard electrode 8 and the measurement electrode 4a or 4. Buffer amplifiers 108 and 10b are added to reduce the potential difference between the two. The current flowing through the measurement electrode 4a is obtained by extracting the voltage across the I (current→voltage) conversion resistor R by a high input impedance amplifier 118. Similarly, measurement electrode 4. The current is output as a voltage to the output of the high input impedance amplifier 11b. The voltage is taken from the output of the high input impedance under 2.

Furthermore, by driving the guard electrode 8 with the amplifiers 10a and 105, even if one surface leakage current is large, there is little effect on the measured current value. and 4. This allows the coating film impedance measuring device to be made smaller. Furthermore, being able to shorten the distance between the measurement electrodes improves the noise resistance performance, making it possible to reduce the distance between measurement electrodes.
It is highly effective for on-site measurements of grounding.

FIG. 4(a) shows a side sectional view and a plan sectional view of another embodiment of the present invention.
), shown in (b).

This is an example where the electrodes are arranged on the same circle, and by arranging the electrodes in this way, the measurement electrode 4 acts as a shield, so that the noise performance is improved.

Since the guard electrode 8 is inserted between the measurement electrodes 4a and 4b, the leakage current between the measurement electrodes 4a and 4b is not included in the measured value, so even with this configuration, the coating film impedance measurement device can be made smaller. can do. Miniaturization further improves noise performance and facilitates measurement.

The buffer amplifier 10 shown in FIG. 4 may be omitted, but in that case, the card electrode 8 must be connected to the output terminal of the AC power supply 6 either directly or via a suitable resistor. An appropriate resistance is one having a value such that the potentials of the measurement electrode 4a and the guard electrode 8 are equal during measurement.

〔Effect of the invention〕

Thus, according to the present invention, by providing a guard electrode to the impedance measurement device used for detecting paint film deterioration using the two-electrode method (double cell method), it is possible to It is possible to accurately measure the impedance of a high-resistance coating film, and the measurement electrodes can be moved closer together, making it possible to build a shed, and improving noise resistance, making it possible to measure accurately on-site. Ta.

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 an 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 a block diagram showing the circuit configuration of an embodiment of the present invention. FIG. 4(a) is a block diagram showing the circuit configuration of another embodiment. (b) is a side sectional view and a plan sectional view of another embodiment of the present invention, FIG. 5 is an explanatory diagram showing a conventional electrochemical evaluation method for paint film deterioration, and FIG. 6 is a deterioration of paint film. Figure 7 is an explanatory diagram showing the change in frequency dependence of impedance due to deterioration of the paint film, and Figure 8 shows Figure 7 on a complex plane. Explanatory diagram,
FIG. 9 is an explanatory diagram showing a method for electrochemically evaluating paint film deterioration using the two-electrode method. 1... Base metal 2... Paint film 3... Conductive substance 4 + 4a, 4b... Measuring electrodes 5, 5
a.5. ... Ammeter 6 ... AC power supply 7 ... Voltmeter 818a, 8. ... Guard electrode 9 ...
Insulators 10, 10a, IOb... Buffer amplifiers 11, 11a, 11b, 12... High input impedance amplifiers a, c... Normal paint film impedance, its trajectory , d... Impedance of deteriorated paint film. Its locus R,... Coating film resistance Cf... Coating film capacitance ia... Measuring current a i,... Measuring current b 15...・Surface leakage current R, Ra, R,...IV conversion resistance. Applicant's Representative Kiyoshi Inomata Figure 1 Figure 2 Figure 3 Figure 4 Figure 6 Figure 6 Scale f Figure 7 Frequency 'I? l bowl (f) real part

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. A guard electrode is placed around the base metal surface coating either directly or via a conductive material and connected to both ends of the AC power source, and a voltmeter is used to measure the voltage between the electrodes for both measurements. A coating film impedance measuring device comprising: an ammeter that measures a current flowing through an electrode; 2. The coating film impedance measuring device according to claim 1, wherein the guard electrode surrounds the measuring electrode. 3. Claim 1 in which the guard electrode is driven to have the same potential as the measurement electrode using an amplifier.
The coating film impedance measuring device described in . 4. Install a high input impedance amplifier in front of the voltmeter to detect the voltage, connect an IV (current → voltage) conversion resistor instead of the ammeter, and have a high input impedance amplifier at both ends of the resistor to derive the current 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: