JPS60236058A - Detecting method of corrosion speed under paint coated film - Google Patents

Abstract

PURPOSE:To measure quickly a corrosive reaction speed in a wide frequency range by deriving a transfer function of a measured voltage in case a white noise voltage is impressed, and obtaining a frequency characteristic of the transfer function and a constant of a Tafel's gradient of a corrosive reaction. CONSTITUTION:A voltage from a white noise generator 6 is impressed through an opposed pole 5 to a painting metallic piece placed in a corrosion system. In this case, a current flowing in the system is converted to a voltage value V1 by an I-V converting circuit consisting of an operational amplifier 7 and a standard resistance 8. The converted voltage V1 in this case is shown as the right expression by a voltage V2 of the white noise generator 6, an impedance Z of the corrosion system, and a resistance value Rstd of the standard resistance 8. Accordingly, a frequency characteristic of the impedance Z of the corrosion system can be obtained from a frequency characteristic of V2/V1. Subsequently, the transfer function V2/V1 is processed by a Fourier converter 9, and the frequency analysis is executed. This result is transferred to a computer 10, and a corrosion current and an instantaneous corrosion speed are derived from a Tafel's constant of the corrosive reaction.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] This invention relates to a method for accurately, rapidly, and automatically detecting the reaction rate of a corrosion reaction occurring under a coating film.

[Prior Art] Conventionally, in a redox system such as corrosion, it is known that the response current i when a voltage η is externally applied to the system is expressed by the following equation.

1=icorr-(exp (77/βa)-ex
p (-η/βC)) (1) However, 1 corr is the corrosion current, and βa and βC are constants of the anode and cathode turf gradients, respectively. now,
If the polarization η is sufficiently small (10 to 20 mV or less),
Equation (1) can be expanded with Mac Laurin with respect to η, and the following equation is obtained.

i=2.3 ・1corr ・77・ ((βa+β
C)/βa ・βC) ・ ・ ・ ・ ・ ・ ・
・ ・ ・ ・ ・ ・ ・(2) From equation (2), i/77-2.3 ・ 1corr ・ ((βa+β
C)/βa ・βcl -1/Rp ・ ・ ・ ・
・ ・ ・ ・ ・(31βa, βC become constants once the type of reaction is determined, so this system exhibits resistance behavior when slight polarization is applied. This resistance is called polarization resistance Rp, and corrosion current, that is, corrosion Since the value is inversely proportional to the speed, this polarization resistance Rp can represent the corrosion rate of the system.
This is an overview of the polarization resistance method proposed by RN and Geary.

In this way, the electrical behavior of the corrosion reaction occurring at the interface of the corrosion system is equivalent to the electrical resistance when the polarization is small, but when it is made into a cell to measure this, the polarization resistance R
Electric double layer capacitance cp 2 is placed in parallel with p 1, and solution resistance Rs 3 is placed in series, and the equivalent circuit of the entire cell is as shown in FIG.

A method to separate and measure only Rp from this equivalent circuit is to apply a pulse and measure Rp from the transient phenomenon.

Courostat method to separate and measure Cp (for example,
8-22697, etc.), but when trying to apply it to the evaluation of corrosion under the paint film, it is not suitable for practical use due to the following drawbacks.

(2) Generally, in the case of a coating system, the polarization resistance Rp is very large (10 to 102 Ω·C♂), so the time constant of transient phenomena becomes large and measurement accuracy cannot be obtained.

(2) The applied voltage is theoretically limited to within 10 to 20 mV, resulting in a low S/N ratio.

■Since measurement takes time, it may not be possible to follow changes in the corrosion system over time.

■It takes time to calculate the corrosion rate from the measurement results, making real-time monitoring impossible.

Furthermore, as another method, an AC impedance method has been proposed. That is, in the equivalent circuit of FIG. 1, the complex impedance Z of this system is Z-(Rp/<1 +;
Cp”Rr)+Rsl −j (ωcpRI)2/ (
1+ω'cpRp)l =Zr-jZX...(4
). ω is the angular frequency. Here, the relationship between the real part Zr and the imaginary part Zi of this complex impedance is (4)
From the formula, it is expressed as the following formula.

[ZR-(R3+Rp/2)l +ZI-(Rp/2f
(5) Therefore, the Nyst diagram, which plots the frequency characteristics of this system on a complex plane with Zr and 21 as axes, has a semicircular shape as shown in Figure 2, and has two intercepts and a half on the ZR wheel axis. From the angular frequency ωmax of the apex of the circle, Rp.

Cp and Rs can be separated. Measurement methods such as the Lissajous method, the AC bridge method, and the lock-in amplifier method are proposed in 1, but if these methods are to be applied to the evaluation of corrosion under the paint film, the following It is not practical due to its drawbacks.

■The applied voltage is theoretically limited to within 10 to 20 mV A/
c) and the S/N ratio is low.

■It is necessary to measure one point at a time over a wide range of frequencies, which increases the measurement time. Therefore, it may not be possible to follow changes in the corrosion system over time.

[Summary of the invention]

5- This invention was made in order to eliminate the above-mentioned drawbacks of the conventional method. White noise is applied to a painted metal piece placed in a corrosive system via a counter electrode, and at this time, the current flowing in the system is After converting j into a voltage value by a current-voltage conversion circuit,
The transfer function between this and the applied white noise is determined by a Fourier transform device, and the rate of the corrosion reaction occurring under the paint film is calculated from the frequency characteristics of the obtained transfer function and the Terfel slope constant of the corrosion reaction. It is.

[Embodiments of the invention]

FIG. 3 shows an outline of the method for detecting the corrosion rate on a paint film according to the present invention. The output from the white noise generator 6 is applied to the painted metal piece 4 placed in the corrosive system via the platinum counter electrode 5 so that the absolute value 1η1 of the polarization voltage becomes 20 mvP−P. At this time, the current i flowing in the system is connected to the FET input type OP
■-■ (1M
, current voltage) is converted into a voltage value Vl by a conversion circuit. The converted voltage vI at this time is the Wahite noise generator 6
Output from V2. The impedance resistance of the corrosion system is expressed by the following formula using 6-standard resistance Rstd.

V]=Rstd/Z・■2...(6) What we have just found is the impedance λ of the corrosion system, so by transforming equation (6), we obtain Z=Rstd-V2/V1...171. It will be done. R
Since std is a scalar constant, Ω2/ in formula (7)
◇If the frequency characteristic 1 is obtained, the frequency characteristic 2, that is, the Nyquist diagram shown in FIG. 2 can be obtained. For this purpose, the transfer function Ω2/Ω1 of </2 and Vl is processed by the fast Fourier transform device 9, and frequency analysis is performed. This result is transferred to the computer 10, multiplied by the standard resistance Rs td, and the constant βa of the turf slope of the corrosion reaction.
, βC to calculate the corrosion current tcorr and the instantaneous corrosion rate. Furthermore, the results are displayed on a graph ink display, printer, etc., or recorded on a floppy disk. These peripheral devices are designated as 1 in Figure 3.
It is indicated by 1.

Further, this computer 10 is a GPIB (Genera
lPur-poseInterface Bus
), it is connected to the fast Fourier transform device 9, white noise generator 6, and peripheral equipment 11 (arrows in FIG. 3), making continuous unmanned operation possible through automatic control.

Further, a block diagram of the fast Fourier transform device is shown in FIG. ◇2. ◇The analog input of 1 goes into the signal conditioner 12. This signal conditioner 12 includes a preamplifier and attenuator for signal level adjustment, and a preamplifier and an attenuator for preventing the aliasing phenomenon in which signals outside the analysis frequency band return ghost signals within the analysis band due to the hetero gain effect with the sampling frequency. It includes an antialiasing filter and an analog multiplexer for switching and inputting two signal inputs. The signal that has passed through the signal conditioner 12 is converted into a digital quantity by the next A/D converter 13. This A/D conversion section includes a Zumble hold circuit, an A/D converter, and a buffer register.

The digitized signals are stored in the data memory 14 in the order of input. The digital processor 17 performs fast Fourier transform on this according to the processing algorithm stored in the program memory 16, and relocates the calculation result in the data memory 14. The calculation result is D-
D- consists of A converter, analog filter, and output amplifier.
The X-Y block is converted into an analog quantity through the A converter 15.

It can be outputted to a synchroscope or the like, or it can be outputted as a digital quantity to an external computer, magnetic disk, etc. via the digital processor 17 and GPIB port 18.

In FIG. 4, arrows indicate data flow, exchange, and program flow.

The features of this system are listed below.

■The r-v conversion circuit at the signal reception port from the corrosion cell is FE.
Since it is a T input type, the input impedance is high <(
(10'Ω or more), accurate wave 1 constant is possible even in a coating n-side system with high cell impedance.

■Since A-D conversion is performed, the range is wide (72 dB), and it is possible to handle small signals, large signals, and 9-signals at the same time.

(3) A method is used in which a signal with a nearly uniform power spectrum is applied within the analysis band, such as white noise, and the transfer function frequency characteristics between the response current and the applied signal are subjected to fast Fourier transform. Therefore, the time required for one measurement is extremely short (within 1 second).

(2) By taking advantage of high speed and performing averaging processing to average the results of multiple measurements, accurate and rapid measurements are possible even in coating systems with very low S/N ratios.

Figure 5 shows a Nyquist diagram showing impedance changes in a painted metal corrosion system measured using this system during continuous unmanned operation. Corrosion type cell has 5pcc of epoxy resin 2
A clear paint consisting of 8 parts by weight, 20 parts by weight of n-buthylated urea resin, 26 parts by weight of Cellosolve, and 26 parts by weight of xylene was applied by dip coating, and then baked and cured at 200"C] for 5 minutes to obtain a film thickness of 25 μm. Painted metal pieces with 3%-Na
c] It was formed by immersing it in a solution and providing a platinum counter electrode. In Figure 5, after lhr, cell formation, etc.
That is, it represents the time after the painted metal piece was immersed in a 3%-NaCl solution. The measurement time per measurement remained within 15 seconds after 10 aheraging cycles, demonstrating high speed. Also, the measurement result is 5cant 1ebury
Transfer Function A
The results obtained using the ASTM analysis method (ASTM 5
pec Tech Publ, 727.187~+97
(1981), ) and was also proven to be accurate.

(Effects of the Invention) As explained above, the present invention applies a white noise voltage to a painted metal piece placed in a corrosive system via a counter electrode 7, and converts the current i flowing in this time series into a current-voltage conversion circuit. After converting it into a voltage value by Since the speed of the corrosion reaction occurring under the paint film is increased, rapid and accurate detection is possible.

[Brief explanation of the drawing]

Figure 1 shows the equivalent circuit of the entire cell that was constructed to measure the electrical behavior of the corrosion reaction occurring at the interface of the corrosion system.
Figure 2 is a Nyquist diagram plotting the frequency characteristics on a complex plane centered on the real and imaginary parts of the complex impedance in the equivalent circuit diagram of Figure 1, and Figure 3 is a Nyquist diagram plotting the corrosion rate on the coating film of this invention. Fig. 4 is a block diagram showing the configuration of the fast Fourier transform device in the apparatus for detecting corrosion rate on a coating film shown in Fig. 3. , FIG. 5 is a Nyquist diagram showing the impedance change of a painted metal corrosion system measured by the method of detecting the corrosion rate on a paint film of the present invention. 4...Painted metal piece, 5...Platinum counter electrode, 6...
White noise generator, 7... FET input operational amplifier, 8... Standard resistor R3td, 9...
...Fast Fourier transform device, 10...Computer,
11...Peripheral equipment. 3-o! -〇 [^Uko・ηOIX] IZ 33 Procedural amendment (voluntary) 2 Name of invention Method for detecting corrosion rate on paint film 3 Name of person making the amendment (601) Mitsubishi Electric Corporation Representative Jinhachibe Katayama 4. Agent 5: Detailed explanation of the invention to be amended, 1st sentence of the drawings, 6. Contents of the amendment (11 Amend “Zwa” to “noha” on page 4, line 15 of the specification) (2) In the same book, page 4, line 18, replace rjZIJ with rj
Correct with Zij. (3) Same book, page 5, line 2 r I Z R-(Rs +
Rp/2) l” + Z l”=j [(
Correct as Zr-(R8+Rp/2)l +Z +-J. (4) In the same book, No. 5, line 4, 1-Zr and Zi] is corrected to rZr and Zi by -1. (5) Page 5, line 5 of the same book, ``Nyquist 1'' has been corrected to ``Nyquist''. (6) In the same book, page 5, line 6, replace r Z Rj with [Z
Correct with rJ. (7) Page 6, line 19 of the same book [Wahhoit 1]
"White" is corrected. (8) Page 7, line 2 of the same book, “vJJ” is “91”
and correct it. (9) Domon page 7, line 3, amend the phrase ``sought'' to read, ``I want to request.'' 00) In the same book, page 7, line 20, rPur-poseJ is corrected to rPurposeJ. (1j) In the 8th edition of the same book, line 18, "buffer" is corrected to "buffer." (/2) In the same book, page 9, line 8, "Prosensor" is corrected to "Prosensor". (/3) In the same book, page 9, line 16, replace “10” with “1”.
0”. (2Ll) Same book, page 10, line 20 rNa c I J
Correct it to rNaclJ. (/9) Figures 2 and 5 of the drawings will be corrected as shown in the attached sheet. Figure 2 Zr Figure 5 Zr [X 10'n-cm')

Claims (3)

[Claims] (1) A white noise voltage is applied to the painted metal piece placed in the corroding system via the counter electrode, and at this time, the current flowing in the system is converted to a voltage value by a current-voltage conversion circuit, and this value and the applied voltage are The transfer function between the white noise and the white noise is determined by a Fourier transform device, and the rate of the corrosion reaction occurring under the paint film is calculated based on the frequency characteristics of the obtained transfer function and the constant of the Terfel slope of the corrosion reaction. A method for detecting corrosion rate on a paint film, characterized in that: (2) The method for detecting the corrosion rate on a paint film according to claim 1, wherein the absolute value 1ηl of the polarization voltage of the applied white noise is set to be within 20 mvP-P. (3) The method for detecting corrosion rate on a coating film according to claim 1, wherein the current-voltage conversion circuit is comprised of an FET input type operational amplifier.