JP2010271200A - Method for measuring resistance of coating film by fall-of-potential method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method which enables the accurate determination of the resistance of a coating film by excluding the effect of a stray current. <P>SOLUTION: In the method for measuring the resistance of the coating film, which is covered with a cover material, by a fall-of-potential method, an interruptor turned ON/OFF by an M system code pattern is installed on a current-carrying cable and the obtained potential difference is subjected to cross-correlation processing. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for measuring coating film resistance by a potential drop method.

  The coating film resistance is used for evaluation of the degree of coating film deterioration, and is measured by a potential drop method, a method using a ground resistance meter, or the like (Non-Patent Document 1).

  In the potential drop method, the anticorrosion measures such as the galvanic anode in the measurement object are removed, temporary energization is performed, and the amount of change in the tube-to-ground potential is measured. Temporary energization is a method of repeatedly turning on / off the galvanic anode at an intermediate position in the measurement target section at regular intervals. If the change in the tube-to-ground potential cannot be measured with the voltage of the galvanic anode, a battery or the like is used. Then, change the potential. As shown in FIG. 6, the tube-to-ground potential is measured at the temporary energization position and the terminals at both ends of the measurement target section, and the average value of the three potential change amounts is used to calculate the coating film resistance value as follows: It can be obtained by (1).

Ｒ・・・・・・塗膜抵抗（Ω・ｍ^２）
ΔＰ／Ｓ・・・管対地電位の変化量の平均値（Ｖ）
Ｉ・・・・・・流電陽極発生電流又は通電電流（Ａ）
Ｌ・・・・・・測定対象管延長（ｍ）
Ｄ・・・・・・管の実外径（ｍ）

R ··· coating resistance (Ω · m ² )
ΔP / S: Average value of change in tube-to-ground potential (V)
I ···· Electric current generation current or conduction current (A)
L · ·· Measurement tube extension (m)
D ··· Actual outer diameter of pipe (m)

Japan Gas Association, “High-Pressure Conduit Guidelines” JGA Finger-204-06, P256

In the measurement of coating film resistance by the conventional potential drop method, it is difficult to measure the amount of change in the tube-to-ground potential in an embedded environment with a large amount of stray current, and it is difficult to accurately measure the coating film resistance.
An object of the present invention is to provide a method capable of accurately measuring the coating film resistance while eliminating the influence of stray current.

  As a result of intensive studies to solve the above-mentioned problems, the inventor can improve the S / N ratio by using a pseudo-random signal for measurement and cross-correlating the detection signal, thereby achieving the object. It was.

  That is, according to the present invention, in a method of measuring the resistance of a coating film coated on a coating material by a potential drop method, an interrupter that is turned on / off by an M-sequence code pattern is installed in an energized cable, and the obtained potential difference is subjected to a cross correlation process. It is related with the measuring method of the coating-film resistance characterized by doing.

When cross-correlation processing is performed between the waveform of the applied voltage and the waveform of the detection signal, during the cross-correlation processing, for example, in the process of performing the multiplication by shifting the detection signal, the applied voltage and the shifted detection signal have the same pattern. Thus, a cross-correlation peak appears at the synchronized position. Noise having a different pattern from the applied voltage is canceled by performing the cross-correlation process, and therefore has little effect on the peak value. When a pseudo-random signal is used for the applied voltage, noise that does not depend on the applied voltage does not become the same pattern as the applied voltage, so that only the detection signal that results from the applied voltage is detected as a representative value of the potential difference. Thus, the S / N ratio is greatly improved. Using this representative value, the potential distribution is obtained, and the coating film resistance value can be obtained with high accuracy from the amount of change in the potential distribution by the same method as the conventional method described above.

  According to the present invention, the influence of stray current can be eliminated and the coating film resistance can be accurately measured.

It is a figure explaining the coating-film resistance measuring method which is one Example of this invention. It is a figure which shows the example of the shift register for M series signal generation. It is a figure which shows the example of an M series signal waveform and its autocorrelation signal waveform. It is a figure which shows the time series chart and M series correlation function of the tube ground potential and energization current which were obtained by this invention. It is a figure which shows the time series chart of the tube ground potential in a conventional method, and an energization current. It is a figure explaining the coating-film resistance measuring method by the conventional potential drop method.

  The coating material measured by the method of the present invention is obtained by coating a metal with a plastic or the like. The metal is not particularly limited as long as it is a metal such as steel such as iron, carbon steel, low alloy steel, and stainless steel, copper, copper alloy, aluminum, aluminum alloy, and the like. The shape may be any shape depending on the application, but typical shapes are tubes and plates. Although a coating film will not be restrict | limited especially if resistance can be measured, A typical thing is polyethylene, a polyurethane, etc.

  The covering material includes not only those buried in the soil but also those installed in water such as fresh water and seawater.

  In the measurement, first, the anode of the electrometer is connected to the terminal at both ends of the measurement target section of the covering material, and a saturated copper sulfate reference electrode is connected to the cathode. At this time, both ends of the measurement target section of the covering material are point 1 and point 3 from the left side, respectively, and the middle is point 2. Then, as shown in FIG. 1, the galvanic anode is connected to the anode of the electrometer connected in the middle. At this time, in the present invention, an M series interrupter is installed between them and energized by the M series code pattern. Is turned on / off, and the obtained potential difference is subjected to cross-correlation processing by a correlation calculator.

  The M-sequence code pattern is a code pattern based on a pseudo-random signal having a specific pattern and period generated by an M-sequence signal capable of highly sensitive signal detection. A pseudo-random signal is a signal that has a repetitive period in a long period and loses randomness, but maintains randomness in the period.

The M-sequence signal can be easily generated by a shift register having a feedback circuit as shown in FIG. The code length obtained by the six-stage shift register shown in FIG. 2 is 2 ⁶ −1 = 63.

  FIG. 3 shows an example of a signal waveform (a) of an M-sequence signal and its autocorrelation signal waveform (b). In FIG. 3, the horizontal axis represents time, the vertical axis represents the signal magnitude, and τa represents the period of the clock applied to the shift register that generates the M-sequence signal.

  Since the M-sequence signal is a pseudo-random signal with periodicity and has a period corresponding to the number of bits of the shift register (here, periodicity with a code length of 63), when autocorrelation is taken, FIG. Periodically have peak values as shown in FIG. From this, it is understood that when cross-correlation processing with other signals is performed, only signals having a pattern that matches the M-sequence signal have a cross-correlation value having a high peak value. By utilizing this property, it is possible to reduce the noise signal.

  When the cross-correlation process between the waveform of the applied voltage and the waveform of the detection signal is performed, during the calculation of the cross-correlation process, for example, in the process of performing the multiplication by shifting the detection signal, the applied voltage is the same as the shifted detection signal. A cross-correlation peak appears at a synchronized position in a pattern. Noise having a different pattern from the applied voltage is canceled out by performing the cross-correlation process, and hardly affects the peak value. When an M-sequence signal is used as the applied voltage, noise that is not based on the applied voltage does not become the same pattern as the applied voltage pattern. Therefore, only the detection signal resulting from the applied voltage is detected as a representative value of the potential difference, and the potential distribution is obtained using this representative value, and the change amount (ΔP / S) of the potential distribution is obtained. Furthermore, since the energizing current (I), the measurement target tube extension (L), and the actual outer diameter (D) of the tube are known, the coating film resistance value (R) can be obtained using the above-described equation (1). it can.

  In the present invention, since pseudo-random signals are used, measurement at a plurality of points and measurement at a point distant from the energization point are easy. The measurement of the coating film resistance is usually carried out at the time of embedding construction. If the resistance value is low, the construction state of the coating is judged to be poor, and there is a possibility that a coating defect exists.

The tube-to-ground potential and the energization current were measured by the arrangement shown in FIG. 4A to 4D show time-series charts obtained when energization is turned on / off by the M-sequence code pattern. An energization current chart at the point 2 is shown in FIG. 4A, and tube-to-ground potential charts at the respective points are shown in FIGS. 4B to 4D. When the measured values of FIGS. 4A to 4D are subjected to cross-correlation processing, the results are as shown in FIGS. 4E to 4H. FIG. 4E shows the M series correlation function of the energization current at the point 2, and FIGS. 4F to 4H show the M series correlation functions of the tube-to-ground potential at each point.

Each peak value in FIGS. 4E to 4H was the same value as the amount of change that can be read from FIGS. 4A to 4D.
On the other hand, FIGS. 5A to 5D show time series charts obtained when conventional energization on / off is repeatedly performed at regular intervals. The energization on time was 7 seconds, and the energization off time was 3 seconds. An energization current chart at point 2. FIG. 5A shows a tube-to-ground potential chart at each point in FIGS. In the measurement charts of FIGS. 5 (a) to 5 (d), it is difficult to read the amount of change in the pipe-to-ground potential at points 1 (FIG. 5 (b)) and 3 (FIG. 5 (d)) far from the energization point. It is.

  Thus, by applying a signal change with the M-sequence code pattern and performing cross-correlation processing, the noise signal component can be removed and the S / N ratio can be improved. As a result, the conduction current and the tube-to-ground potential can be reduced. It is possible to measure the change amount, that is, the coating film resistance value with high accuracy.

  According to the present invention, it is possible to accurately diagnose the degree of deterioration of a coating film such as an anticorrosion-coated steel pipe in the soil, and thus measures such as repair can be taken.

Claims (1)

  In a method of measuring the resistance of a coating film covered with a coating material by a potential drop method, an interrupter that is turned on / off by an M-sequence code pattern is installed in an energized cable, and the obtained potential difference is subjected to a cross-correlation process. Measuring method of coating film resistance.

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