JP3076186B2 - Method for measuring surface area of galvanic anode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the surface area of a galvanic anode which measures the amount of consumption of the galvanic anode in service up to the present and measures the remaining life in cathodic protection of the galvanic anode system. It is.

[0002]

2. Description of the Related Art The cathodic protection method is used for metal structures and buried pipes installed in seawater, river water or underground, reinforcing steel in concrete, chemical devices in contact with industrial water and organic and inorganic liquids, and the like. Used as a way to prevent corrosion,
It has a great effect on corrosion prevention in infrastructure such as harbors and bridges and industries in various fields.

In the galvanic anode system as an application of the cathodic protection method, a galvanic anode of a base potential alloy such as a magnesium alloy, an aluminum alloy, or a zinc alloy is connected to a body to be protected, and the galvanic anode is formed by melting the anode. In this way, an anticorrosion current is supplied to the anticorrosion body to prevent corrosion of the anticorrosion body. The service life of the galvanic anode is determined by design conditions such as volume or weight, generated current, and the like. Therefore, knowing the amount of wear and tear of the current-carrying anode during service and estimating the remaining service life is not only to confirm the appropriateness of the anticorrosion design, but also to grasp the time to replace the anode. is necessary.

At present, in order to measure the remaining life, a method is adopted in which the circumference of the anode is measured to determine the surface area, and the volume of the anode is calculated from the equivalent radius. Therefore, it is almost impossible to measure the volume of the anode in the backfill in the soil by measuring the circumference at an appropriate position.

[0005]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the prior art, and in a cathodic protection of a galvanic anode system, the remaining service life of the galvanic anode in service can be easily and electrochemically determined. An object of the present invention is to provide a method for measuring the surface area of a galvanic anode that can be measured quickly.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in view of the above-mentioned object, and as a result, have applied a polarization resistance method for electrochemically measuring the corrosion rate of a metallic material, thereby obtaining the surface area of a galvanic anode. A method for quickly and easily measuring the amount of dissolution consumed until the measurement of the galvanic anode in service and the remaining useful life by knowing the surface area was led.

That is, in the method for measuring the surface area of a galvanic anode according to the present invention, in cathodic protection of the galvanic anode system, the anodic current of the galvanic anode in operation is stopped, and the polarization resistance of the galvanic anode in a natural corrosion state is stopped. And 300 to 4 after the start of operation
The apparent polarization resistance measured after the lapse of 00 hours was divided by the polarization resistance measured at an arbitrary time thereafter, and the resultant value was multiplied by the surface area of the current-flow anode before operation to obtain the current-flow anode. It is characterized in that the surface area at any time is measured.

A self-corrosion reaction involving the generation of hydrogen gas always progresses on the surface of a current-carrying anode having a base potential of less than -1 V (based on a saturated calomel electrode) placed in an environment such as water, thereby generating an anticorrosion current. During operation, there is some anodic dissolution of the anodic metal due to the self-corrosion reaction, which overlaps with the Faraday dissolution reaction of the anodic metal corresponding to the current and during the de-energization alone. It is known that the rate of this reaction elapses in an almost constant state over a long period of time in a steady state except for an initial period after the use of the galvanic anode. Therefore, the present inventors have found a method for numerically measuring the change in the surface area of the anode by tracking the self-corrosion rate of the galvanic anode, which elapses at a substantially constant value for most of the operation, by the polarization resistance method.

In the polarization resistance method, a small current ΔI is applied to a metal having a surface area S (cm ² ) in a natural corrosion state from the outside.
(A), a slight change in the potential of the metal, ie, polarization
E (V). ΔE /
ΔI is referred to as apparent polarization resistance Rap (Ω), and polarization is divided by current density in order to allocate this to a unit area.
/ (△ I / S) is the polarization resistance Rp (Ω ·
cm ² ). The corrosion rate per unit area of metal is I
When expressed as corr (A / cm ² ), I corr and R
The equation of Stern is established by using a constant K (V) between p and p.

[0010]

(Equation 1) Since Rp = Rap · S according to the above definition, the expression (2) is satisfied.

[0011]

(Equation 2) Therefore, equation (3) is derived.

[0012]

(Equation 3)

[0013] Here, K is a constant for a metal material called the K value of Stern, and if Icorr is the self-corrosion rate of the galvanic anode, it can be considered to be constant during most of the service period (K / Icorr ) Is also a constant, (3)
It can be seen from the equation that the surface area of the galvanic anode can be determined from the apparent polarization resistance measurement.

In equation (3), it is derived that the surface area of the galvanic anode is inversely proportional to the measured value of the apparent polarization resistance. In the initial stage of energization, the surface slightly dissolves and reaches a steady state,
rr is in a state where it can be regarded as substantially constant, and the apparent polarization resistance at the time of reference in a state where the surface area of the anode is almost the same as the surface area So at the time of manufacture is Rapo, and the apparent polarization resistance Rapt at the time of arbitrary measurement. Comparing (4)
The equation holds.

[0015]

(Equation 4) Can be used to calculate the surface area St at any measurement.

By the way, the numerator of the formula (4) is the polarization resistance Rp per unit area according to the above definition.
Since the self-corrosion rate hardly changes in a steady state, it can be considered that the self-corrosion rate always shows a substantially constant value. Therefore, when the value of Rp of the same type of galvanostatic anode in the same type of environment is determined, Rap is measured at an arbitrary time without measuring Rapo for each anode (5).
St is immediately obtained from the equation.

[0017]

(Equation 5)

In this way, if the surface area at an arbitrary time is obtained, the volume of the anode can be calculated on the assumption that the galvanic anode is regarded as cylindrical or that the anode is uniformly dissolved in accordance with the shape. It is possible to evaluate the amount of dissolution and consumption up to the time of measurement and the remaining service life.

The method for measuring the remaining life of a galvanic anode according to the present invention by measuring the surface area will be described below with reference to the block diagram of FIG.

In FIG. 1, reference numeral 1 denotes a galvanic anode made of a magnesium alloy, an aluminum alloy, a zinc alloy, or the like; Backfill mainly using sodium sulfate, bentonite, etc., 3 is environment such as soil,
Reference numeral 4 denotes an anticorrosion body such as a buried pipe, and reference numeral 5 denotes a conductor connection portion for connecting the conductor of the galvanic anode 1 to the conductor of the anticorrosion body 4 or for temporarily disconnecting when measuring polarization resistance. Reference numeral 6 denotes a counter electrode made of an appropriate material placed in the environment 3 to allow a measurement current required for polarization resistance measurement to flow through the current-carrying anode 1. Reference numeral 7 denotes a current-carrying anode used to measure the polarization of the protection target 1. A reference electrode 8 near the reference numeral 1 is a measuring device for measuring the polarization resistance by a three-electrode system or a two-electrode system. Further, C in the polarization resistance measuring device 8 is a counter electrode connection terminal, R is a reference electrode connection terminal,
W indicates working electrode connection terminals.

The measurement is performed as follows. That is, when the galvanic anode 1 and the anticorrosion-protected body 4 are connected at the wire connecting portion 5 and the galvanic anode 1 is in operation, the wire connecting portion 5 is disconnected and the energization of the galvanic anode 1 is stopped. The polarization resistance is measured after the self-potential of the non-energized current carrying anode 1 is stabilized. In the measurement, an appropriate amount of measurement current ΔI is passed from the polarization resistance measuring device 8, the polarization ΔE generated between the reference electrode 7 and the galvanic anode 1 is measured, and ΔE / ΔI is calculated. Obtain the apparent polarization resistance Rap. The measurement current uses a current of either a DC or low-frequency rectangular wave or a sine wave, and the resulting polarization (DC or AC polarization according to the waveform of the current used) has a value of about 10 mV. It is preferable to select an appropriate current value. When the environment 3 has high conductivity such as seawater, the apparent polarization resistance Rap may be calculated using the polarization value as it is, but when the environment 3 has low conductivity such as fresh water or soil, the polarization measurement is performed. The values include the resistance of the environment and the voltage drop due to the measured current, which must be subtracted to determine the true apparent polarization resistance.

As a method of correcting an error due to environmental resistance, AC impedance measurement is often used. That is, the polarization of the galvanic anode 1 is measured with the reference electrode 7 using a high-frequency current of 1 kHz or more, and a value obtained by dividing the polarization by the measured current indicates the environmental resistance between the galvanic anode 1 and the collation electrode 7. By subtracting the value of the environmental resistance from the value of the polarization resistance described above, a corrected true apparent polarization resistance can be obtained.

In the measurement of the polarization resistance, the measured current is m
Since the amount of electricity measured is very small within a few tens of seconds and the amount of electricity measured is extremely small, the counter electrode 6 may be a soluble electrode such as an iron bar. The anticorrosion body 4 may be used as a temporary counter electrode. As the reference electrode 7, a full-scale reference electrode such as a saturated calomel electrode or a copper sulfate electrode is usually used. However, since it is a polarization measurement, the absolute value of the potential is not required to be accurate.
Since the measurement time is short, it is only necessary to ensure the stability of the potential during that time. Therefore, a metal electrode such as an iron bar or zinc placed near the galvanic anode 1 can sufficiently serve as a reference electrode.

For such a three-electrode measurement, a measuring current from the measuring device 8 is passed between two adjacent current-carrying anodes to measure a potential between the two current-carrying anodes. There is also a two-electrode method in which half of the resistance obtained by dividing by the measurement current is used as the polarization resistance of each anode, but the measurement accuracy is inferior to that of the three-electrode method.

When the measurement of the polarization resistance is completed, the connection of the conductor connecting portion 5 is restored, and the current carrying anode 1 is returned to the service state again.

The surface area of the galvanic anode 1 is determined as follows. In the initial stage after the start of the operation of the galvanic anode 1, the surface of the anode is slightly melted, and the self-corrosion rate is in a substantially constant steady state. Apparent polarization resistance Rap (correcting the effect of environmental resistance)
o, and at any subsequent measurement, the apparent polarization resistance Rapt is measured again by the same measurement method as the reference time,
The surface area St of the galvanic anode at the time of arbitrary measurement can be calculated by the equation (4).

Here, the reference time is a time when it is judged that the dissolution state of the galvanic anode becomes steady, the self-corrosion rate becomes constant, and this self-corrosion rate may be used as a reference for a long period of time. . In order to determine this reference time, as a result of measuring the change over time of the self-corrosion rate of the flowing anode of various alloys by a chemical measurement method that measures the amount of hydrogen gas generated on the surface of the flowing anode, hydrogen gas Is generally small at first but gradually increases,
The generation rate of hydrogen gas became almost constant in about 0 to 400 hours, and this steady state of the self-corrosion rate continued thereafter. Therefore, it was found that it is desirable to measure Rapo at this point after the start of operation of the galvanic anode.

As another method of determining the reference time, a method of estimating from the service life may be considered. Assuming that the galvanic anode melts from the side only in a cylindrical shape with insulated end faces, when melting reduces the diameter by 1%, the volume decreases by about 2%, while the surface area decreases by about 1%. . That is,
Even when the galvanic anode is used for 2% of the service life, the surface area is reduced by only about 1%, indicating that the surface area can be regarded as being almost the same as that at the time of manufacture. Therefore, for example, in the case of a galvanic anode having a service life of 5 years, 1% of the current anode sufficiently satisfies the above-mentioned steady arrival time in 1.2 months.
Even if p is considered as Rapo, the error is within 1%, and there is no problem if this point is determined as the reference time.

When the steady-state value of the polarization resistance Rp per unit area of the galvanic anode 1 is determined, St at that time is immediately calculated by the equation (5) using Rapt measured after the reference time. it can.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments.

EXAMPLE FIG. 2 shows the results for 80 cm ² , 800 cm ² and 160 cm ² , respectively.
The polarization resistance was measured intermittently for 1200 hours while flowing an anode current of a current density of 1 mA / cm ^{2 in} seawater on a test piece of an aluminum alloy galvanic anode having a surface area (So) of 0 cm ² , It shows the change. Apparent polarization resistance (Rap) was measured by a rectangular wave alternating current method having a period of 4 seconds at a point of time when 30 minutes had passed after the current was turned off. The Rap values of all the test pieces rapidly decreased at the beginning of the test, but became almost steady in 200 to 300 hours, and thereafter became almost constant. The smaller the surface area of the test piece, the larger the steady value of Rap.

Table 1 shows the results of Rapo obtained from 300 hours as a reference time, Rapt after 400 hours, and the evaluation area (St) calculated by the equation (4).

[0033]

[Table 1]

As shown in Table 1, each test piece was evaluated for a surface area that almost coincides with the set test area, and the result was that the error was within 3%. Further, the dotted line shown in FIG. 2 shows a change with time of the calculation result of So · Rapt (= Rp), and after 300 hours, all three test pieces have almost constant values, and the average value is 1244 Ω · cm. ^{In 2} , the fluctuation was about 7%.

When these results are combined, in order to evaluate the remaining service life of the current-carrying anode during operation, when the polarization resistance method is used as a method for measuring the surface area of the anode, at least 300 hours have passed since the start of operation. The apparent polarization resistance measured later is used as a reference value for the anode, and the surface area at that time can be evaluated with an error of about 3% by multiplying the ratio of the apparent polarization resistance at the subsequent time to the initial surface area. I understood. When the reference value is not determined for each anode and the value of Rp is determined for that type of anode, the value is divided by the apparent polarization resistance at the time of measurement and the error is about 7%. It was found that the surface area at that time could be evaluated.

[0036]

As described above, by cutting off the current flowing through the galvanic anode and measuring the polarization resistance of the galvanic anode in a non-energized state, the surface area of the galvanic anode at that time can be evaluated. The remaining life can be calculated, which is very useful in determining when to replace the anode. The measurement is simple and quick, and has the advantage that no work such as diving or excavation of the anode in the soil is required.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram for explaining an example of the measurement method of the present invention.

FIG. 2 is a graph showing an example of a temporal change in apparent polarization resistance of a galvanic anode.

[Explanation of symbols]

1: Current flowing anode, 2: Backfill, 3: Environment, 4: Corrosion-proof body, 5: Conductor connection, 6: Counter electrode, 7: Reference electrode, 8:
Polarization resistance measuring device, C: counter electrode connection terminal, R: reference electrode connection terminal, W: working electrode connection terminal.

Claims (1)

(57) [Claims] In a cathodic protection of a galvanic anode system,
The anode current of the galvanic anode during operation was stopped, and the polarization resistance of the galvanic anode was measured in a natural corrosion state.
The apparent polarization resistance measured after 0 to 400 hours has elapsed is divided by the polarization resistance measured at an arbitrary point in time thereafter, and the resultant value is multiplied by the surface area of the current-flowing anode before operation, thereby obtaining the current-flow resistance. A method for measuring the surface area of a galvanic anode, comprising measuring the surface area of the anode at an arbitrary time point.