JP2018205124A - Device and method for measuring corrosion rate - Google Patents

Abstract

To achieve a more accurate measurement of the corrosion rate of a metal material buried in the ground.SOLUTION: An AC measuring unit 220 measures the AC resistance Rby an AC impedance method, and the DC measuring unit 221 measures the DC resistance Rby a DC polarization resistance method. A polarization resistance calculation unit 40 calculates a polarization resistance Rby subtracting the AC resistance Rfrom the DC resistance R. The calculation recording unit 50 derives the corrosion rate r, using the polarization resistance R.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a technique for deriving the corrosion rate of a metal material embedded in the ground.

  Among the infrastructure structures that support our lives, metal materials such as water and gas pipelines, power cable pipelines, underground tanks, steel pipe columns, branch anchors, and other steel materials that are buried and used underground There are many. These structures buried in the ground deteriorate due to soil corrosion and cause a failure or the like by shortening the life of the equipment (Non-patent Document 1). Therefore, appropriate renewal is indispensable to ensure high reliability and safety of these infrastructure structures.

  As a method of judging the renewal timing, there is mainly a method by inspection. Although direct inspection by humans or machines is possible in the ground part of the equipment, direct inspection is often difficult in the ground part of the equipment. Therefore, maintenance / inspection of the current equipment is carried out only on the ground and on the ground, and the degree of deterioration in the underground is hardly confirmed.

  In order to minimize the cost while ensuring the safety and security of the underground part of the facility where it is difficult to evaluate the deterioration due to soil corrosion, predict and estimate the corrosion state of the underground part of the facility and give priority to renewal. Proactive management to deal with is effective. In order to predict and estimate the corrosion state of metal materials buried underground, it is important to know the corrosion rate of metal materials due to soil corrosion, and the corrosion rate depends on environmental factors. It is necessary to determine the relationship between corrosion rate and corrosion rate.

Morio Kadoi, Shoaki Takahashi, Kotaro Yano, “Studies on Soil Corrosion of Metallic Materials (1st Report)”, Corrosion Protection Technology, 1967, Vol. 16, No. 6, pp. 10-18 Atsushi Nishikata, “Impedance Characteristics and Corrosion Monitoring of Corrosion Systems”, Materials and Environment, 1999, Vol. 48, No. 11, pp. 686-692 Atsushi Nishikata, Takehiko Takahashi, Hoei Kiyo, Toru Mizurata, “Monitoring and Corrosion Mechanism of Carbon Steel Corrosion Rate in Repeated Wet and Wet Environments”, Materials and Environment, 1994, Vol. 43, No. 4, pp. 188-193 Satoru Yamamoto, Kenjiro Takeko, Satoshi Takaya, “Development of corrosion rate measurement method (CIPE method) for steel in concrete”, Rust, Nippon Corrosion Industry Co., Ltd., January 2015, No. 148, pp. 2-8 Yoshikazu Miyata, Keiji Asakura, “Measurement of Soil Corrosion Focusing on Electrochemical Method (Part 1)”, Materials and Environment, 1997, Vol. 46, No. 9, pp. 541-551

  The corrosion rate of the embedded metal material is derived from a value obtained by measuring the polarization resistance by using an electrochemical method (Non-patent Document 2). A correlation between the polarization resistance and the corrosion current density proportional to the corrosion rate has been found both theoretically and experimentally. Among the electrochemical methods, there are a DC polarization resistance method and an AC impedance method as a method for measuring the polarization resistance. Since the resistance value of the entire system is calculated in the measurement by the direct current polarization resistance method, it is difficult to separate and evaluate only the polarization resistance for evaluating the corrosion reaction in the soil environment where the soil resistance is high. . The AC impedance method is capable of frequency-separating many phenomena on the surface of the corrosion reaction, and is an excellent method for extracting only the polarization resistance derived from the corrosion reaction in the soil environment containing various phenomena. . The polarization resistance can be calculated from measurement data in the low frequency region of the AC impedance method (Non-patent Document 3).

  FIG. 10 shows an example of a Nyquist diagram obtained by measuring the metallic material embedded in ideal soil by the AC impedance method. The measurement was performed by the three-electrode method. In the Nyquist diagram of FIG. 10, arcs were confirmed in each of the high frequency region and the low frequency region. It is considered that the polarization resistance is derived from a low frequency arc. Accordingly, the polarization resistance is calculated from the value on the horizontal axis (impedance real part) from the start point to the end point of the arc in the low frequency region.

  However, since the time required for the measurement is long in the low frequency region of the AC impedance method, there is a problem that the amount of the environmental factor changes greatly before and after each measurement in the actual soil where the environmental factor fluctuates over time. The measurement time of data constituting the arc of the low frequency region derived from the polarization resistance was 7 minutes.

  In the actual soil environment, environmental factors fluctuate with time of weather. A prominent example is the change in soil moisture content due to rainfall. The fluctuation of the soil moisture content in the actual soil environment constantly repeats the cycle of “wet”, “drainage”, and “dry” in conjunction with rainfall. Therefore, considering that the progress of corrosion depends on the amount of environmental factors, it is considered that the corrosion rate is different even within one day.

  FIG. 11 shows an example of the change over time in the soil moisture content, which is one of the environmental factors. The soil moisture content is linked to the rainfall and repeats the cycle of wetting, draining and drying. If one cycle is defined as the period from rainfall to the next rainfall, the soil moisture content changes by 19.4% in one cycle in the example of FIG. In the example of FIG. 11, when the measurement in the low frequency region by the AC impedance method is performed in the region where the change is greatest, the soil moisture content changes by 6.39% before and after the measurement. This corresponds to 32.9% of the total change in soil moisture content in one cycle.

  In order to accurately predict and estimate the amount of corrosion of a metal material embedded in a real soil environment, it is necessary to accurately monitor temporal changes in the corrosion rate. However, when the corrosion rate is derived from the conventional electrochemical measurement method, the amount of environmental factors greatly changes before and after the measurement due to the length of the measurement time. As a result, in monitoring the corrosion rate, there is a problem that an accurate corrosion rate at each measurement time cannot be derived.

  The present invention has been made in view of the above, and an object thereof is to more accurately measure the corrosion rate of a metal material embedded in the ground.

  The corrosion rate measuring apparatus according to the first aspect of the present invention includes an electrode unit including a metal material for which a corrosion rate is desired to be obtained, an AC measurement unit that performs measurement by an AC impedance method on the electrode unit embedded in soil, DC measurement means for performing measurement by the DC polarization resistance method on the electrode part, and polarization resistance by subtracting the AC resistance obtained by the measurement by the AC measurement means from the DC resistance obtained by the measurement by the DC measurement means. And a corrosion rate deriving unit for deriving a corrosion rate from the polarization resistance.

  The corrosion rate measuring method according to the second aspect of the present invention includes a step of performing measurement by an AC impedance method on an electrode part including a metal material for which a corrosion rate is to be obtained embedded in soil, and a direct current is applied to the electrode part. Performing a measurement by a polarization resistance method, calculating a polarization resistance by subtracting an AC resistance obtained by the measurement by the DC polarization resistance method from a DC resistance obtained by the measurement by the AC impedance method, and the polarization Deriving the corrosion rate from the resistance.

  ADVANTAGE OF THE INVENTION According to this invention, the corrosion rate of the metal material embed | buried under the ground can be measured more correctly.

It is a functional block diagram which shows the structure of the corrosion rate measuring apparatus of this embodiment. It is a flowchart which shows the flow of a process of the corrosion rate measuring apparatus of this embodiment. It is a figure which shows an example of the measurement result by an alternating current measurement part. It is a figure which shows an example of the measurement result by a direct-current measuring part. It is a figure which shows an example of the Nyquist diagram obtained by the measurement by an alternating current impedance method. It is a figure which shows an example of the electric current-potential characteristic obtained by the direct current | flow polarization resistance method. It is a functional block diagram which shows the structure of the corrosion rate measuring apparatus which has a some electrode part. It is a functional block diagram which shows the structure of another corrosion rate measuring apparatus which has a some electrode part. It is a functional block diagram which shows the structure of another corrosion rate measuring apparatus which has a some electrode part. It is a figure which shows an example of the Nyquist diagram obtained by the measurement by an alternating current impedance method. It is a figure which shows the example of a time-dependent change of the soil moisture content which is one of the environmental factors.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a functional block diagram showing the configuration of the corrosion rate measuring apparatus 1 of the present embodiment. The corrosion rate measuring apparatus 1 shown in FIG. 1 includes an electrode unit 10, a measuring unit 20, an instruction determining unit 30, a polarization resistance calculating unit 40, and a calculation recording unit 50.

  The electrode unit 10 includes three electrodes using a counter electrode 11, a working electrode (metal sample whose polarization resistance is desired to be obtained) 12, and a reference electrode 13. These electrodes are embedded in the soil 100. The working electrode 12 needs to know the electrode area in advance.

  In deriving the corrosion rate with the present corrosion rate measuring apparatus 1, electrochemical measurement may be performed on the site where the metal material for which the corrosion rate is desired to be buried is embedded, or a simulated environment constructed by collecting local soil. You may implement below. If it is actually difficult to reach the site, it is preferable to use similar soil that contains similar components, exhibits similar physical quantities, and exhibits similar soil classification (soil particle size distribution). If you want to know the corrosion rate of a metal material buried in a certain area without being limited to a specific place, use the representative soil in that area or the soil described in the soil map issued by the Geospatial Information Authority of Japan. For example, Kanto Loam may be adopted in the Kanto region.

  The measurement unit 20 includes a switch unit 21 and an electrochemical measurement unit 22. The electrochemical measurement unit 22 includes an AC measurement unit 220 and a DC measurement unit 221.

  The switch unit 21 switches the connection destination of the electrode unit 10 between the AC measurement unit 220 and the DC measurement unit 221, the AC measurement unit 220 performs measurement by the AC impedance method, and the DC measurement unit 221 performs measurement by the DC polarization resistance method. . Switching control of the switch unit 21 depends on the determination of the instruction determination unit 30.

  The instruction determination unit 30 detects that the AC resistance or DC resistance is calculated by the polarization resistance calculation unit 40 and instructs the measurement unit 20 to switch the measurement method. When the measurement of both the AC measurement unit 220 and the DC measurement unit 221 is completed, the number of measurements n is incremented by 1, and the (n + 1) th measurement is started.

  The polarization resistance calculation unit 40 calculates AC resistance from the measurement result of the AC measurement unit 220, and calculates DC resistance from the measurement result of the DC measurement unit 221.

  The calculation recording unit 50 calculates the polarization resistance by subtracting the AC resistance from the DC resistance, and derives the corrosion rate using the polarization resistance.

  Next, the process flow of the corrosion rate measuring apparatus 1 of the present embodiment will be described.

  FIG. 2 is a flowchart showing a processing flow of the corrosion rate measuring apparatus 1 of the present embodiment.

First, it is determined whether to perform AC measurement by the AC measurement unit 220 or DC measurement by the DC measurement unit 221 (step S11). In this embodiment, since alternating current measurement and direct current measurement are performed alternately, it is determined that a measurement method different from the previously performed measurement method is performed. In the present embodiment, when the instruction determination unit 30 detects that the AC resistance R _Cn is calculated in the polarization resistance calculation unit 40, it is considered that the AC measurement has ended, and the instruction determination unit 30 performs DC measurement on the switch unit 21. The unit 221 is instructed to switch measurement. When the instruction determination unit 30 detects that the DC resistance R _Dn is calculated in the polarization resistance calculation unit 40, it is considered that the DC measurement is finished, and the instruction determination unit 30 measures the AC measurement unit 220 with respect to the switch unit 21. Give instructions to switch. Thereby, the measurement by the alternating current measuring unit 220 and the direct current measuring unit 221 is alternately performed. The first measurement may be performed from the AC measurement unit 220 or from the DC measurement unit 221.

  When it is determined that the AC measurement is to be performed, the switch unit 21 switches the connection destination of the electrode unit 10 to the AC measurement unit 220, and the AC measurement unit 220 performs the measurement by the AC impedance method on the electrode unit 10 (step S12). ).

  In FIG. 3, an example of the measurement result by the alternating current measurement part 220 is shown. “Wet” indicates a state in which the soil is immersed, and “Dry” indicates a state after the soil is immersed for one day by draining.

  The AC measurement unit 220 performs measurement such that the value of the vertical axis (impedance imaginary part) of the Nyquist diagram at the measurement point reaches a reference value or less from the high frequency toward the low frequency. When the measurement start is a high frequency of about 50 kHz, for example, the value on the vertical axis of the Nyquist diagram at the measurement start point may be less than the reference value. Therefore, in the measurement from high frequency to low frequency, the measurement end condition is that the value on the vertical axis of the Nyquist diagram at the measurement point continues to decrease and reaches the reference value or less.

  On the other hand, when it is determined that the direct current measurement is to be performed, the switch unit 21 switches the connection destination of the electrode unit 10 to the direct current measurement unit 221, and the direct current measurement unit 221 performs the measurement by the direct current polarization resistance method on the electrode unit 10. (Step S13).

  FIG. 4 shows an example of a measurement result obtained by the direct current measurement unit 221. “Wet” and “dry” are the same conditions as in the measurement by the AC impedance method.

  The direct current measuring unit 221 performs sweeping in a range in which the steel surface is not roughened based on the natural potential and in a potential range in which the resistance value can be calculated from the obtained current-potential characteristics. For example, you may implement by +/- 5 [mV] which is the applied voltage in the alternating current impedance method considered that the influence on the steel surface is small in an electrochemical measurement. The measurement result of FIG. 4 is an example obtained by sweeping at ± 5 [mV] with reference to the natural potential.

  The sweep potential in the DC measurement unit 221 can be set by the instruction determination unit 30. When the instruction determination unit 30 detects that the sweep has been completed within the set potential range, the measurement ends.

The polarization resistance calculation unit 40 acquires the measurement data obtained by the AC measurement unit 220 and the DC measurement unit 221 and calculates the AC resistance _RCn or the DC resistance _RDn (Steps S14 and S15).

The AC resistance R _Cn is a value on the horizontal axis (impedance real part) of the Nyquist diagram of the final measurement point in the measurement result of the AC measurement unit 220. Therefore, the reference value on the vertical axis of the Nyquist diagram for detecting the end of AC measurement is set within a range in which the value of the AC resistance _RCn is equal to or approximately approximate to the value at which the arc intersects the horizontal axis of the Nyquist diagram. There is a need to.

The direct current resistance R _Dn is calculated from the slope of the current-potential characteristic obtained by the measurement of the direct current measuring unit 221. As a calculation method of the slope, for example, a least square method may be used, or an extrapolation method may be used.

If AC resistance _RCn or DC resistance _RDn is obtained, it will return to step S11 and the next measurement by the measurement part 20 will be performed. At this time, when both the n-th AC resistance _RCn and the DC resistance _RDn are obtained, the number of measurements n is increased by 1, and the measurement unit 20 starts the (n + 1) th measurement (step S16).

When both DC resistance _{R Dn} and n-th AC resistance _{R Cn} has been obtained, the polarization resistance calculation unit 40 calculates the polarization resistance _{R pn} by subtracting the AC resistance _{R Cn} from the DC resistance _{R Dn} (step S17).

Calculating the recording unit 50 derives the corrosion rate _{r n} by using the polarization resistance _{R pn} (step S18). Specifically, to derive the corrosion rate r _n as follows.

The corrosion current density i _corrn is calculated from the polarization resistance R _pn based on the following equation (1).

Here, i _corrn is a corrosion current density [A / cm ² ], K is a conversion coefficient [V], and R _pn is a polarization resistance [Ω · cm ² ]. The conversion coefficient K is obtained in advance. The conversion coefficient K is calculated based on the following equation (2) by deriving a Tafel gradient from the anode and cathode polarization curves (see Non-Patent Document 4).

  Here, βa is an anode gradient [V / decade], and βc is a cathode gradient [V / decade].

  Alternatively, the conversion coefficient K may be calculated on the assumption that βa = βc = 0.1 [V / decade] without measuring the Tafel gradient (see Non-Patent Document 5).

Then, to derive the corrosion rate r _n based on the following equation (3).

Here, _{r n} corrosion rate [cm / sec], z is an ion valence, [rho is the density [g / cm ^2], F is the Faraday constant [C], M is the atomic weight [g / mol].

The calculation recording unit 50 records values of the corrosion rate r _n , the AC resistance R _Cn , and the DC resistance R _Dn as the nth measurement result.

The corrosion rate measurement apparatus 1 repeats the above processing, to derive the corrosion rate r _n at each measurement time point. For the end of measurement, a measurement time may be set in advance, or the end of measurement may be determined by an external factor.

  Next, it will be described that the polarization resistance can be calculated by subtracting the AC resistance from the DC resistance.

FIG. 5 shows an example of a Nyquist diagram obtained by measurement by the AC impedance method. As shown in the figure, circular arcs are confirmed in each of the high frequency region and the low frequency region. The AC resistance _RC is calculated from the arc in the high frequency region, and the polarization resistance _Rp is calculated from the arc in the low frequency region.

FIG. 6 shows an example of current-potential characteristics obtained by the DC polarization resistance method. FIG. 6 shows the results obtained by sweeping ± 15 [mV] based on the natural potential. The DC resistance _RD is calculated from the slope of the current-potential characteristics of FIG.

Table 1 shows the AC resistance R _C obtained in Figure 5 the sum of the polarization resistance R _p, and the value of the DC resistance R _D obtained in FIG.

It can be seen from Table 1 that both values are approximated even under different conditions of “wet” and “dry”. That is, the polarization resistance R _p by subtracting the AC resistance R _C of the DC resistance R _D can be calculated.

In the present embodiment, the DC resistance R _D may be treated as equivalent to the AC resistance R _C + polarization resistance R _p , or the previously determined DC resistance R _D and AC resistance R _C + polarization resistance R _p The correction term α may be obtained from the value, and a value obtained by multiplying the DC resistance _RD by the correction term α may be treated as equivalent to AC resistance R _C + polarization resistance R _p .

  In this embodiment, the measurement is finished within 2 minutes in total, within 1 minute for measurement in the high frequency region by the AC impedance method, within 1 minute for measurement by the DC polarization resistance method.

  Here, in a case where the soil moisture content has changed most in 7 minutes required for the conventional method in one cycle of the soil moisture content in FIG. 11, the change is compared with the change in the soil moisture content when the present embodiment is applied.

  Table 2 shows changes in soil moisture content when the conventional method and this embodiment are used for one cycle of soil moisture content in FIG. Before and after the measurement by the conventional method (7 minutes), the soil moisture content changed by 6.39%, and the rate of change in the soil moisture content was 32.9% with respect to the entire change in soil moisture content in one cycle. . On the other hand, when this embodiment (measurement time is 1 minute 40 seconds) is applied 4 times in 7 minutes of the conventional method, the rate of the same change in one cycle is 14.2% at the maximum and 3.1 at the minimum. % Can be suppressed.

  Next, a configuration example of a corrosion rate measuring device having a plurality of electrode portions will be described.

  FIG. 7 is a functional block diagram showing a configuration of the corrosion rate measuring apparatus 1 having a plurality of electrode portions.

  The corrosion rate measuring apparatus 1 shown in FIG. 7 includes a plurality of electrode portions 10A and 10B, and the measurement portions 20A and 20B are connected to the electrode portions 10A and 10B, respectively. Each of the measurement units 20A and 20B includes switch units 21A and 21B and electrochemical measurement units 22A and 22B, similarly to the corrosion rate measurement device 1 of FIG.

  The polarization resistance calculation unit 40 acquires measurement data obtained by the electrode units 10A and 10B and the measurement units 20A and 20B, and the polarization resistance calculation unit 40 and the calculation recording unit 50 calculate the polarization resistance and the Derivation of corrosion rate. In addition, the measurement units 20A and 20B are controlled by one instruction determination unit 30.

  In addition, you may embed electrode part 10A, 10B in different soil 100A, 100B. When embedded in different soils 100A and 100B, the corrosion rates of metallic materials with different embedded environments can be derived in parallel.

  FIG. 8 is a functional block diagram showing a configuration of another corrosion rate measuring apparatus 1 having a plurality of electrode portions.

  The corrosion rate measuring apparatus 1 shown in FIG. 8 has a configuration in which a plurality of electrode portions 10A and 10B are connected to one measuring portion 20.

  Each of the AC measurement unit 220 and the DC measurement unit 221 uses separate electrode units 10A and 10B. For example, when the AC impedance measurement of the AC measurement unit 220 is performed in the electrode unit 10A, the electrode unit 10B performs the DC polarization resistance measurement of the DC measurement unit 221. On the contrary, when the direct current polarization resistance measurement of the direct current measurement unit 221 is performed in the electrode unit 10A, the electrode unit 10B performs the alternating current impedance measurement of the alternating current measurement unit 220.

  7, the electrode portions 10A and 10B may be embedded in different soils 100A and 100B in the configuration of FIG.

  FIG. 9 is a functional block diagram showing a configuration of still another corrosion rate measuring apparatus 1 having a plurality of electrode portions.

  The corrosion rate measuring apparatus 1 shown in FIG. 9 has a configuration in which the measurement unit 20 does not include the switch unit 21 and each of the plurality of electrode units 10A and 10B is directly connected to the AC measurement unit 220 and the DC measurement unit 221. .

Each of electrode parts 10A and 10B is embedded in the same soil 100A and 100B, and performs only AC measurement or DC measurement continuously. The instruction determination unit 30 instructs the measurement unit 20 to start the next measurement when detecting that the AC resistance R _Cn and the DC resistance R _Dn are calculated in the polarization resistance calculation unit 40.

  Any corrosion rate measuring device 1 may include three or more electrode portions.

As described above, according to the present embodiment, the AC measuring unit 220 measures the AC resistance R _Cn by the AC impedance method, the DC measuring unit 221 measures the DC resistance R _Dn by the DC polarization resistance method, and the polarization by resistance calculating unit 40 calculates the polarization resistance _{R pn} pulling the AC resistance _{R Cn} from the DC resistance _{R Dn,} calculated recording unit 50 derives the corrosion rate _{r n} by using the polarization resistance _{R pn,} AC impedance method it is sufficient measure only high frequency region in the conventional technique can be derived corrosion rate r _n in a sufficiently short time with respect to variations in environmental factors compared to environmental factors in the conditions vary with time, when the measurement it is possible to more accurately derive the corrosion rate r _n in.

  According to the present embodiment, since the plurality of electrode portions 10A and 10B are provided, each measurement can be performed in parallel, so that the corrosion rate can be derived in a shorter time.

DESCRIPTION OF SYMBOLS 1 ... Corrosion rate measuring apparatus 10, 10A, 10B ... Electrode part 11, 11A, 11B ... Counter electrode 12, 12A, 12B ... Working electrode 13, 13A, 13B ... Reference electrode 20, 20A, 20B ... Measuring part 21, 21A, 21B Switch unit 22, 22A, 22B Electrochemical measurement unit 220, 220A, 220B AC measurement unit 221, 221A, 221B DC measurement unit 30 Instruction determination unit 40 Polarization resistance calculation unit 50 Calculation recording unit

Claims (5)

An electrode part containing a metal material whose corrosion rate is to be determined;
AC measuring means for carrying out measurement by the AC impedance method on the electrode part embedded in the soil,
DC measuring means for performing measurement by the DC polarization resistance method on the electrode part;
Polarization resistance calculation means for calculating polarization resistance by subtracting the AC resistance obtained by measurement by the AC measurement means from the DC resistance obtained by measurement by the DC measurement means;
Corrosion rate deriving means for deriving the corrosion rate from the polarization resistance;
A corrosion rate measuring device characterized by comprising:   The AC measuring means performs measurement from a high frequency to a low frequency, and determines the value of the impedance real part when the value of the imaginary impedance part in the Nyquist diagram of the measurement point reaches a predetermined reference value or less. The corrosion rate measuring device according to claim 1, wherein the corrosion rate measuring device is a resistance.   The corrosion rate measuring device according to claim 1, comprising a plurality of the electrode portions. A step of performing measurement by an alternating current impedance method on an electrode portion including a metal material for which a corrosion rate is to be obtained embedded in soil;
Performing a measurement by a direct current polarization resistance method on the electrode part;
Subtracting the AC resistance obtained by the measurement by the DC polarization resistance method from the DC resistance obtained by the measurement by the AC impedance method, and calculating the polarization resistance;
Deriving a corrosion rate from the polarization resistance;
Corrosion rate measuring method characterized by having.   In the measurement by the AC impedance method, measurement is performed from high frequency to low frequency, and the value of the real part of the impedance when the value of the imaginary part of the impedance in the Nyquist diagram of the measurement point reaches a predetermined reference value or less is obtained. The corrosion rate measuring method according to claim 4, wherein the AC resistance is used.

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