JPH1048174A - Apparatus for measuring corrosion of outer face of metallic piping - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inspect the state of a metallic piping coated with an exterior material, a thermal insulating material, etc., industrially advantageously with good reliability, by setting a data-transmitting part for transmitting corrosion measuring signals taken out to a signal detector, and the like. SOLUTION: A sensor cable 20 for measuring an outer face corrosion is arranged in a longitudinal direction by a fixing band 13 below an outer face of a metallic piping 10 coated with a thermal insulating material 11 and an exterior material 12. An end of the cable is connected to a signal detector 30. The sensor cable 20 has a plurality of cables. Measuring signals of an outer face corrosion of the metallic piping 10 are detected by sensors, taken out to a signal detector 30, and transmitted to a data-processing part 50 via a data-transmitting part 40. At the data-processing part, a state of the outer face corrosion of the metallic piping 10 is analyzed according to an electrochemical measuring method on the basis of the transmitted corrosion measuring signals. In this manner, the state of the outer face corrosion of the metallic piping 10 can be easily, surely and reliably detected and grasped without removing the thermal insulating material 11. exterior material 12, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a corrosion measuring apparatus applied to the measurement of external corrosion of metal pipes, and more particularly, to the measurement of external corrosion of metal pipes by electrochemically measuring the external corrosion of metal pipes. It concerns the device.

[0002]

2. Description of the Related Art In a facility such as a chemical plant or a petrochemical complex, a large number of pipelines such as pipelines are used for transporting applied raw materials, products, utilities (steam, nitrogen gas, fuel gas, etc.). These pipes, especially metal pipes, are generally used by being exposed to the atmosphere.In general, in order to prevent external corrosion, the outer surfaces of the metal pipes are subjected to anticorrosion coating or the like. Above, measures are taken to prevent moisture, rainwater, and the like from entering the inside by performing coating with a heat insulating material, an exterior material, and the like.

[0003]

By the way, the external corrosion in this kind of metal pipe is caused by various assumed factors,
For example, structural factors (such as branch portions of branch pipes and boundary portions with exterior materials), operational factors (operating temperature and dew condensation, years of use, etc.), and design factors (such as piping materials and wall thickness) And implementation methods), exterior factors (deterioration and poor implementation of heat insulation and exterior materials, etc.), and environmental factors (temperature conditions, rainfall, etc.), and the like in many cases.

That is, for example, in the case of a metal pipe using carbon steel as a pipe material, external corrosion occurs due to the following factors. (A) The exterior material applied to the outer surface of the metal pipe becomes poor in sealing due to poor execution or deterioration over time, etc., and rainwater infiltrates inside and accumulates, causing corrosion on the corresponding outer surface portion. (B) The anticorrosive coating applied to the outer surface of the metal pipe is peeled off due to aging due to heat history, deterioration due to contact with moisture, etc., and the metal surface is exposed to the outside, causing outer surface corrosion. (C) When the working temperature of the metal pipe is always in the range of about 20 to 85 ° C. or when the working life exceeds about 15 years, the rate of occurrence of external corrosion becomes extremely high.

In the case of a metal pipe using stainless steel as a pipe material, stress corrosion cracking occurs due to infiltration of rainwater and concentration of chlorine ions from a heat insulating material.

On the other hand, with respect to the method of measuring or inspecting the external corrosion of a metal pipe having the above configuration, conventionally, an outer material or a heat insulating material which is previously coated on the outer surface of the pipe itself is removed, and then the metal is removed. Means for visually confirming and implementing the outer surface of a pipe are well known.

However, this visual inspection method is very difficult and troublesome in practice because the inspection target portion extends over the entire length of a relatively wide metal pipe. In addition, it is necessary to coat the exterior material and the heat insulating material again, which is troublesome, and inevitably increases the cost, which cannot be said to be an industrially advantageous means.

[0008] The present invention is intended to solve the problems of such a conventional method for inspecting the outer surface of a metal pipe.
It is an object of the present invention to provide a metal pipe outer surface corrosion measuring device which enables a reliable and industrially advantageous inspection of the state of outer surface corrosion in a metal pipe covered with an exterior material or a heat insulating material. It is.

[0009]

In order to achieve the above-mentioned object, an apparatus for measuring corrosion of an outer surface of a metal pipe according to the present invention is particularly applicable to an outer material, a heat insulating material, etc., which are previously coated on the outer surface of a metal pipe. Without removing it, the state of corrosion of the outer surface of the inspection target portion can be quantitatively measured using an electrochemical measurement method.

That is, the invention according to claim 1 of the present invention is an apparatus for measuring the corrosion state of an outer surface of a metal pipe having an outer surface coated with a heat insulating material and / or an exterior material, and the like. It is held in close contact with the outer surface in the longitudinal direction, and arranges individual corrosion measurement sensors for each predetermined section along the longitudinal direction,
A plurality of sensor cables sequentially and continuously provided at a slight interval in the longitudinal direction of the metal pipe; and the same number of signal detectors for sequentially extracting each corrosion measurement signal from each of the individual corrosion measurement sensors. A data transmission unit that transmits each corrosion measurement signal taken out to each of the signal detectors; and, based on each corrosion measurement signal transmitted by the data transmission unit, electrically determines an external corrosion state of a corresponding portion of the metal pipe. An external corrosion measurement apparatus for metal pipes, comprising at least a data processing unit for performing analysis by a chemical measurement method.

According to the first aspect of the present invention, the corrosion measurement signal of the corresponding outer surface portion of the metal pipe is detected by each of the corrosion measurement sensors corresponding to each of the sensor cables and taken out by each of the signal detectors. Each of the corrosion measurement signals taken out by each of the signal detectors is transmitted to a data processing unit via a data transmission unit. In the data processing unit, based on each of the transmitted corrosion measurement signals, a corresponding portion of a metal pipe is The external corrosion state of the is analyzed by electrochemical measurement.

According to a second aspect of the present invention, in the configuration of the apparatus for measuring corrosion of an outer surface of a metal pipe according to the first aspect, the sensor cable held in close contact with the outer surface in the longitudinal direction of the metal pipe comprises: It is characterized in that the metal pipe is disposed on an outer surface portion where rainwater or the like entering the outer surface easily accumulates.

According to the second aspect of the present invention, the corrosion state of the outer surface portion of the metal pipe, in which rainwater or the like easily enters and stagnates, and thus often corrodes, is completely detected.

According to a third aspect of the present invention, in the configuration of the apparatus for measuring corrosion of an external surface of a metal pipe according to any one of the first and second aspects, there is provided an apparatus for measuring the corrosion from each of the signal detectors to a data processing unit. The data transmission unit is based on a wired transmission system in which each of the signal detectors and the data processing unit is directly connected individually.

According to the third aspect of the present invention, each of the corrosion measurement signals taken out by each of the signal detectors is transmitted to the data processing section by a wired transmission method by direct connection.

According to a fourth aspect of the present invention, in the configuration of the metal pipe outer surface corrosion measuring apparatus according to any one of the first and second aspects, there is provided an apparatus for measuring the corrosion of each of the signal detectors to the data processing unit. It is characterized in that the data transmission unit is of a wired transmission system that enables connection of each of these signal detectors and the data processing unit by a telephone line.

According to the fourth aspect of the present invention, each of the corrosion measurement signals taken out by each of the signal detectors is transmitted to the data processing unit by a wire transmission method using a telephone line.

According to a fifth aspect of the present invention, in the configuration of the metal pipe outer surface corrosion measuring apparatus according to any one of the first and second aspects, a method is provided in which each signal detector is connected to a data processing unit. The data transmission unit is based on a wireless transmission system that connects each of the signal detectors and the data processing unit by wireless communication.

According to the fifth aspect of the present invention, each of the corrosion measurement signals taken out by each of the signal detectors is transmitted to the data processing unit by a wireless transmission method.

[0020]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a metal pipe outer surface corrosion measuring apparatus according to an embodiment of the present invention; FIG.

FIG. 1 is a perspective explanatory view schematically showing an outline of the overall configuration of an external surface corrosion measuring apparatus including a metal pipe to be measured to which the present embodiment is applied, and FIG. FIG. 3 is a vertical cross-sectional view schematically illustrating an example of an aspect in which the sensor cables are arranged in the direction, and FIG. 3 is an explanatory view schematically illustrating a sensor cable taken out therefrom. FIGS. 4 (a) and 4 (b) are a cross-sectional view and a vertical cross-sectional view schematically showing an example of an embodiment in which the metal pipes are arranged in a vertical direction.

In the apparatus configuration shown in FIGS. 1 to 4, in the case of the present embodiment, the metal pipe 10 is covered with a heat insulating material 11 and an exterior material 12 in contact with the outer surface over the entire length in order. I have.

The metal pipe 10, here, on the lower side of the outer surface of the metal pipe 10 arranged in the horizontal direction, that is, on the bottom portion corresponding to the lower side of the outer surface where rainwater or the like which causes corrosion may enter and accumulate. As is clear from FIGS. 1 and 2, a predetermined section along the longitudinal direction, for example, a range of about 10 to 100 m, preferably about 20 to 50 m, particularly about 20 to 50 m by appropriately using the fixing band 13 or the like, continuously plurality of surface corrosion measuring sensor cable 20 at a small distance for each about _{30m (20 1, 20 2,} ‥‥, 20 n) causes deployed with close contact to each signal input and output terminal , One of the cable ends is pulled out to the outside, and the corresponding signal detector 30 (30 ₁ , 30 ₂ ,.
30 _n ).

In this case, each of the individual sensor cables 20 (20 ₁ , 20 ₂ ,..., 20 _n ) is, for example, as shown in FIG. A plurality of sensor units 21 ₁ , 2 having a length range of about
1 ₂ , ‥‥, and 21 _n , respectively, and the sensor units 21 ₁ , 21 ₂ , ‥‥, and 21 _n are connected to the cable 20.
The coating is peeled off to expose the internal copper wire to form an electrode, and the corrosion measurement signal (electric signal) detected by each of the sensor units 21 ₁ , 21 ₂ , ‥‥, 21 _n is supplied to the corresponding sensor cable 20 ( 20 ₁ , 20 ₂ ,..., 20 _n ).
₁ , 30 ₂ , ‥‥, 30 _n ). here,
As each sensor cable 20, for example, a 2-core or 3-core household power cable or the like can be used, and a dissimilar metal such as a guaranteed copper wire for a thermocouple or chromel-allomel has an excellent signal characteristic and is effective. Can be used.

On the other hand, when the metal pipe 10 is arranged vertically, as shown in FIG. 4, the infiltrating rainwater flows from above to below in contact with the outer surface and stays in the flange portion 10a connecting the pipes. In this case, it is preferable in this case to dispose it along the outer side including the flange portion 10a.

The signal detectors 30 (30 ₁ , 30 ₁₎
30 ₂ , 取 り 出, 30 _n ), the individual corrosion measurement signals are output to the corresponding data transmission units 40 (40
₁ , 40 ₂ ,..., 40 _n )
The corrosion analysis is performed on the corresponding outer surface of the metal pipe 10 from the corrosion measurement signal.

Here, the detection method of this type of metal corrosion, that is, the method of measuring the outer surface corrosion of the metal pipe 10 according to the present embodiment is, in this case, an electrochemical measurement method, for example, an electric resistance measurement method, a polarization resistance measurement method. Each of the measuring methods (DC polarization resistance method, AC polarization resistance method (so-called impedance method)) and electrochemical noise method (so-called noise method) can be applied to each.

The polarization resistance measuring method is a means for determining the corrosion rate at the time of measurement from electrochemical polarization resistance. For example, a DC or AC minute potential difference (1) is measured between two electrodes.
(Approximately 0 mV), and a change in current caused by the applied voltage is measured to instantaneously determine the overall corrosion rate of the corresponding metal portion.

The above-mentioned electric resistance measuring method utilizes the fact that the cross-sectional area decreases with the corrosion of the metal element and the electric resistance increases, and the electric resistance of the metal element is measured at regular intervals. Then, it is a means for obtaining the average corrosion rate at the corresponding time from the measured value gradient.

The electrochemical noise method measures the coupling current between two electrodes and the electrochemical current noise, compares the two, and determines the degree of local corrosion of the corresponding metal part. Measuring the electrochemical potential noise occurring between the two electrodes, obtaining a resistance / impedance noise by comparing the potential noise with the current noise, and then obtaining the resistance / impedance noise output and the local corrosion. This is a means for judging the corrosion rate of local corrosion by comparing the degree of corrosion.

As described above, each of the signal detectors 3
Corrosion measurement signals (electric signals) detected at 0 (30 ₁ , 30 ₂ , ‥‥, 30 _n ) are transmitted to the corresponding data transmission units 40 (40 ₁ , 40 ₂ , ‥‥, 40 _n ). Through this, the data is sent to the data processing unit 50 and analyzed. In this case, the transmission means of each data transmission unit 40 is not particularly limited, and various transmission formats can be adopted.

Next, each of the signal detectors 30 (30 ₁ , 3)
0 ₂ , ‥‥, 30 _n ), each of the data transmission units 40 (40 ₁ , 40 ₂ , ‥‥, 40 _n ) for transmitting the individual corrosion measurement signals to the data processing unit 50. Will be described.

FIG. 5 shows an example of a case where a wired transmission system by direct connection is applied as the transmission means of the data transmission unit.

In the wired transmission system by direct connection shown in the configuration of FIG. 5, the data transmission section 40 is provided between each of the signal detectors 30 ₁ , 30 ₂ ,..., 30 _n and the data processing section 50. To each of the separate connection lines 41 ₁ , 41 ₂ , ‥‥,
And a state of being directly connected to each other by 41 _n, respective signal detector 30 _1, 30 _2, ‥‥, the corrosion measurement signal (electrical signal) a current signal (or voltage signal if necessary) has accepted the 30 _n , And directly transmit the data to the data processing unit 50. In the data processing unit 50, the signal detectors 30 ₁ , 30 ₂ ,.
_The corrosion measurement signal converted into a current signal (or a voltage signal as required) from _n is sequentially or selectively taken in and analyzed.

[0035] Therefore, when the data transmission unit 40 according to the wired transmission method according to this direct connection, each signal detector 30 _1, 30 _2, ‥‥, each of corrosion measurement signal extracted in 30 _n To the separate connection lines 41 ₁ , 41 ₂ ,
４１, 41 _n can be transmitted to the data processing unit 50, and the transmission configuration here can be extremely simplified. However, on the other hand, if the number of measurement points increases and the number of the respective signal detectors 30 ₁ , 30 ₂ , ‥‥, and 30 _n is large, cable construction for each connection becomes complicated. can not deny.

FIG. 6 shows an example of a mode in which a wire transmission system using a telephone line is applied as the transmission means of the data transmission unit.

[0037] In the wired transmission methods through the use of telephone lines shown in the configuration of this Figure 6, as the data transmission unit 40, each of the signal detectors 30 _1, 30 _2, ‥‥, corresponding individually to 30 _n Of each signal detection side modem 42 ₁ , 42
₂ ,..., 42 _n, and a signal processing modem 43 for the data processing unit 50, respectively, and these signal detection modems 42 ₁ , 42 ₂ ,.
‥, 42 _n and the modem 43 on the signal processing side can be connected via the telephone line 44, and by using the telephone line 44 in this manner, each signal can be detected in the same manner as in the method of FIG. The corrosion measurement signals from the instruments 30 ₁ , 30 ₂ ,..., 30 _n are analyzed and processed by the data processing unit 50.

[0038] Therefore, when the data transmission unit 40 according to the wired transmission method through the use of this telephone line, each signal detector 30 _1, 30 _2, ‥‥, each of the corrosion measurements to be taken 30 _n The signals are modulated by the corresponding signal detecting modems 42 ₁ , 42 ₂ ,..., 42 _n and transmitted to the signal processing modem 43 through the telephone line.
Further, after demodulation by the signal processing side modem 43, it can be taken into the data processing section 50 as expected. Therefore, as described above, even if the number of measurement points increases, the signal detectors 30 ₁ , 30 ₂ ,..., 30 _n are also large, unlike the case of the wired transmission method by the direct connection, there is no fear that the cable construction or the like becomes complicated, and the signal detectors 30 ₁ , 30 2,. 30 _2, ‥‥, it can be relatively easily transmit the corrosion measurement signals from 30 _n.

FIG. 7 shows an example of a case where a wireless transmission system is applied as the transmission means of the data transmission unit.

In the wireless transmission system shown in the configuration of FIG. 7, each of the signal detectors 3 is used as the data transmission section 40.
The corrosion measurement signals (electric signals) received respectively for the 0 ₁ , 30 ₂ ,..., 30 _n sides are converted into current signals (or voltage signals as required) and transmitted sequentially or selectively. A common wireless transmitter 45 is provided, and a wireless receiver 46 for receiving the transmission signal is provided on the data processing unit 50 side.
5 and a wireless receiver 46 on the signal processing side
Again, the signal detectors 30 ₁ , 30 ₂ ,.
The data processing unit 50 analyzes each corrosion measurement signal from 30 _n .

Therefore, in the case of the data transmission section 40 to which this radio transmission system is applied, the signal detectors 30 ₁ , 3
O ₂ , ‥‥, and 30 _n are sequentially or selectively modulated by the common radio transmitter 45 by the built-in modulator (not shown), and then the signal processing side radio signal is transmitted. By transmitting the signal to the receiver 46 and receiving each corrosion measurement signal transmitted by the wireless receiver 46 and demodulating the signal by the built-in demodulator (not shown) in the same manner, the signal is again transmitted as expected. Can be easily incorporated into the data processing unit 50.

Next, a processing mode in the data processing unit 50 will be described.

Each of the signal detectors 30 ₁ , 30 ₂ ,.
_30n from the data transmission unit 40 by the data transmission unit 50
Each of the corrosion measurement signals transmitted to the data processing unit 50 is processed by the electrochemical measurement method in the data processing unit 50 by using a measurement data analysis computer (not shown again) or the like. In this way, the corrosion data analysis of the corresponding pipe outer surface is performed as expected.

In this case, the analysis of the corrosion data on the outer surface of the pipe by the processing by the computer or the like is based on the types of the electrochemical measurement methods described above, for example, the electric resistance measurement method, the polarization resistance measurement method and the electrochemical noise method. Although it differs depending on each case, no matter which means is adopted, based on each corrosion measurement signal data, the state of the external surface corrosion in each metal pipe 10 and the analysis such as the transition and comparison of the trend value are performed, The analysis result is output and displayed on a CRT screen or a printer.

Here, as an example of the corrosion data analysis of the outer surface of the pipe, a case where an electrochemical noise method (a so-called noise method) is applied among electrochemical measurement methods will be described below.

That is, the electrochemical noise method uses the coupling current (I _mean ) as an external corrosion signal as described above.
The coupling current (I _mean ) and the electrochemical current noise (I _n ) and the electrochemical current noise (I _n ) and the electrochemical current noise (I _n ). By comparing with _n ), the degree of external corrosion, in other words, the degree of local corrosion can be easily grasped.

In this case, the type of corrosion can be classified into the following four _{types according to} the ratio of the coupling current I _n / electrochemical current noise I _mean . All surface _{corrosion: 0.001 <I n / I mean} <0.01 _{mixed-corrosion: 0.01 <I n / I mean} <0.1 station plant corrosion (partial): 0.1 <I _n / I _mean <1.0 Pitching (pitting corrosion): 1.0 <I _n / I _mean

[0048] Then, the I ratio of _n / I _mean that to be output and displayed on the CRT screen, a printer or the like, each signal detector 30 _1, 30 _2, ‥‥, Ya instantaneous value of corrosion measurement every 30 _n The transition of the form of corrosion can be extremely easily confirmed and grasped from the transition of the trend value in which the instantaneous values are displayed in time series.

Further, regarding the output display, in addition to the above-mentioned form of corrosion, the above-mentioned coupling current (I _mean ), electrochemical current noise (I _n ), electrochemical potential noise (V _n ), etc. By displaying the instantaneous value of each data, the transition of the trend value displayed in chronological order and its accumulated value, etc. separately or in association with each other, it is possible to more accurately judge the outer surface corrosion of the corresponding pipe, is there.

In the embodiment described above, the transmission mode of the data transmission section 40 is the
_Although only unidirectional transmission of each corrosion measurement signal from ₁ , 30 ₂ ,..., 30 _n to the data processing unit 50 has been described, bidirectional transmission is enabled, and for example, each signal detector 30 _1, 30 _2, ‥‥, it is possible to perform control such by specifying either 30 _n incorporate only corrosion measurement signal of the relevant portion.

[0051]

As described in detail in the above embodiment, according to the present invention, there is provided an apparatus for measuring the external corrosion state of a metal pipe having an outer surface coated with a heat insulating material and / or an exterior material. The corresponding corrosion measurement signal of the corresponding outer surface portion is detected by each corresponding corrosion measurement sensor arranged on each sensor cable and taken out to each signal detector, and the detected corrosion measurement signal is transmitted to the data transmission unit. By transmitting the data to the data processing unit and analyzing it by the electrochemical measurement method in the data processing unit, without removing the heat insulating material and / or the exterior material coated on the outer surface of the metal pipe at all, The corrosion state of the outer surface of the corresponding portion of the metal pipe can be detected and grasped easily and accurately with high reliability, and effective, practical, and industrially advantageous measurement can be performed. The one having the features.

[Brief description of the drawings]

FIG. 1 is a perspective explanatory view schematically showing an outline of an overall configuration of an external corrosion measuring apparatus including a metal pipe to be measured to which an embodiment of the present invention is applied.

FIG. 2 is a longitudinal sectional view schematically showing an example of an embodiment in which the metal pipe is arranged in a horizontal direction.

FIG. 3 is an explanatory diagram schematically showing a sensor cable of the metal pipe taken out in the same manner.

FIG. 4 is a cross-sectional view (FIG. 4A) and a longitudinal cross-sectional view (FIG. 4B) schematically showing an example of an embodiment in which the metal pipes are arranged in the vertical direction.

FIG. 5 is an explanatory diagram showing an example of a case where a wired transmission system by direct connection is applied as a transmission unit of the data transmission unit.

FIG. 6 is an explanatory diagram showing an example of a case where a wired transmission system using a telephone line is applied as a transmission unit of the data transmission unit.

FIG. 7 is an explanatory diagram showing an example of a case where a wireless transmission system is applied as a transmission unit of the data transmission unit.

[Explanation of symbols]

10 metal pipe 11 heat insulator 12 exterior material 13 securing band _{20 (20 1, 20 2,} ‥‥, 20 n) sensor cable _{21 1, 21 2, ‥‥,} 21 n corrosion measurement sensor unit 30 (30 _1, 30 _2, ‥‥, 30 _n) signal detector 40 the data transmission unit _{41 1, 41 2, ‥‥,} 41 n connection lines _{42 1, 42 2, ‥‥,} 42 n signal detection modem 43 signal processing modem 44 telephone Line 45 Wireless transmitter 46 Wireless receiver 47 Wireless communication 50 Data processing unit

 ──────────────────────────────────────────────────続 き Continued on front page (72) William M. Inventor. Cox 3-10-10 Ushidori, Kurashiki City, Okayama Prefecture Mitsubishi Chemical Mizushima Plant

Claims (5)

[Claims] An apparatus for measuring the corrosion state of an outer surface of a metal pipe having an outer surface coated with a heat insulating material and / or an exterior material, wherein the metal pipe is held in close contact with an outer surface in a longitudinal direction of the metal pipe, and is provided in the longitudinal direction. A plurality of sensor cables, each of which is provided with an individual corrosion measurement sensor for each predetermined section along the length of the metal pipe, and which is successively provided at a slight interval in the longitudinal direction of the metal pipe; The same number of signal detectors sequentially taking out each corrosion measurement signal from the, and a data transmission unit that transmits each corrosion measurement signal taken out to each signal detector,
Based on each corrosion measurement signal transmitted by the data transmission unit, an external surface of the metal pipe characterized by comprising at least a data processing unit for analyzing the external corrosion state of the corresponding portion of the metal pipe by an electrochemical measurement method Corrosion measuring device. 2. The sensor cable according to claim 1, wherein each of the sensor cables held in close contact with the outer surface of the metal pipe in the longitudinal direction is disposed on an outer surface portion where rainwater or the like entering the outer surface of the metal pipe easily accumulates. 2. The apparatus for measuring external corrosion of a metal pipe according to claim 1. 3. A data transmission unit from each of the signal detectors to the data processing unit is of a wired transmission system in which each of the signal detectors and the data processing unit is directly connected individually. An apparatus for measuring the external corrosion of metal pipes according to claim 1. 4. A data transmission unit from each of the signal detectors to the data processing unit is of a wired transmission system which enables connection of each of the signal detectors and the data processing unit by a telephone line. The apparatus for measuring external corrosion of metal pipes according to claim 1 or 2, wherein: 5. The data transmission unit from each signal detector to a data processing unit is based on a wireless transmission system that connects each of the signal detectors and the data processing unit by wireless communication. Item 3. An apparatus for measuring external corrosion of metal pipes according to items 1 and 2.