JP2001194333A - Method of electrochemical measurement - Google Patents

Abstract

PROBLEM TO BE SOLVED: To quantitatively evaluate the corrosion proof performance of an oxide film on a surface of a steel product for a boiler, etc. SOLUTION: Sample electrodes 45 each fitted with a heating resistance element 48, a counter electrode (tube) 43a fitted with a resistance heating element 43b, and reference electrodes 46 are disposed on a pressure resistant case 40 of a dummy boiler. A voltage is impressed on the sample electrodes 45 to measure a film breakdown potential. The corrosion proof performance of the film is evaluated based on a difference between the natural potential of the sample electrodes 45 and the film breakdown potential.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrochemical measurement method suitable for evaluating the corrosion resistance of an oxide film on a metal member that comes into contact with high-temperature and high-pressure water, such as in a boiler can.

[0002]

2. Description of the Related Art When a protective film of iron oxide in a boiler is partially destroyed due to poor water quality, thermal stress, or the like, FIG.
As shown in (1), a local battery is formed between the exposed steel surface, water and the protective film surface, and iron elutes from the anode.

The eluted iron ion (II) (Fe ²⁺ )
Even if the pH of the boiler water is moderately high, if dissolved oxygen is present, it is oxidized to iron (III) hydroxide, and the corrosion products are deposited as a precipitate (sabicob) on the anode surface as shown in FIG. I do. In such a state, a concentration difference occurs between the oxygen concentration of the water in the precipitate and the oxygen concentration of the water covering the cathode surface. Further, iron elutes, and corrosion proceeds deeper on the steel surface.

[0004] Bicarbonate, which is an acid consumption (pH 4.8) component [M alkalinity of the old standard] in feed water, is thermally decomposed in a boiler to generate carbon dioxide (CO ₂ ) and NaOH. .

2NaHCO ₃ → Na ₂ CO ₃ + CO ₂ ↑ + H ₂ O Na ₂ CO ₃ + H ₂ O → 2NaOH + CO ₂鋼 If the NaOH concentration increases, the steel becomes corroded.

Fe + 2NaOH → Na₂FeO₂+ H₂  Na generated₂FeO₂Decomposes and free alkali again
Generate Corrosion progresses by this repeated reaction
Go.

[0007] 3Na₂FeO₂+ 4H₂O → 6NaOH
+ Fe₃O₄+ H₂  3Na₂FeO₂+ 3H₂O + 1 / 2O₂→ 6NaOH
+ Fe₃O₄  The corrosion status of boiler steel is checked periodically (once / year).
Stop and release the steel to check the corrosion state of the steel surface (for example, Sabiko
Visual observation of oxides such as metal oxides)
I have.

In this case, the internal corrosion state and color tone are important judgment criteria for judging the water treatment effect of the boiler.
Visually, the expression method is sensuous, the individual differences are large, and it is not quantitative. Also, during the operation of the boiler, it was not possible to know the corrosion status of the steel surface, so it was not possible to recognize that the corrosion was progressing until the open inspection, and problems such as the occurrence of rust on the steel material occurred. .

Therefore, the applicant of the present invention can monitor (monitor or estimate) the corrosion state of the boiler steel without stopping the boiler, and perform the monitoring quantitatively without causing individual differences. A method of monitoring corrosion that can be performed has been proposed in Japanese Patent Application Laid-Open No. 9-318962.

In the simulated boiler adopted in the corrosion monitoring method of the same publication, a counter electrode, a reference electrode, and a sample electrode made of the same material as the actual boiler steel are arranged so as to come into contact with the boiler water of the simulated boiler. It measures the polarization resistance of the sample electrode and monitors the corrosion state in the boiler can from the measured value.

[0011]

Problems to be Solved by the Invention Japanese Patent Application Laid-Open No. 9-31896
In the method described in Japanese Patent Application Laid-Open No. 2 (1999) -1995, since the overall corrosion rate of the sample on the heat transfer surface was determined, it was not possible to monitor local corrosion, which is an actual form of corrosion.

An object of the present invention is to provide a method capable of estimating local corrosion by measuring the corrosion resistance of an oxide film on a sample surface.

[0013]

SUMMARY OF THE INVENTION An electrochemical measuring method according to the present invention comprises a high-temperature high-pressure water circulating system having a feed water inlet and a blow water outlet, and a counter electrode serving also as a heater for heating the circulating water. A sample electrode made of a metal, a means for heating the sample electrode, and an electrochemical measurement means comprising a reference electrode, and after circulating high-temperature and high-pressure water for a predetermined period, the natural potential and the film breakdown potential of the sample electrode. And evaluating the anticorrosion performance of the sample electrode from the difference between the two.

According to the method of the present invention, the anticorrosion performance of an oxide film formed on the surface of a metal member such as a steel material can be quantitatively evaluated by the difference between the film breakdown potential and the natural potential.
The corrosion resistance of the film is evaluated based on the difference between the film breaking potential and the natural potential because the natural potential changes depending on the pH of the water, DO (dissolved oxygen), or the time elapsed in contact with the water. This is because the more accurate the anticorrosion performance evaluation can be performed by subtracting the natural potential from the breakdown potential.

[0015]

Embodiments of the present invention will be described below. FIG. 1 is a side view of a simulated boiler for evaluating the anticorrosion performance of an oxide film formed on the surface of a boiler steel, and FIG. 2 is FIG.
FIG. 2 is a sectional view taken along line II-II of FIG.

In FIG. 1, a pressure-resistant case 30 for separating steam and water is shown.
, A liquid level gauge 31, a thermometer 32, a steam extraction pipe 33, a water supply pipe 3
4. The blow pipe 35 is connected.

A heating pressure-resistant case 40 made of SUS316L is provided on the side of the case 30. The lower part and the upper part of the case 30 and the case 40 are connected by circulation pipes 41 and 42.

As shown in FIG. 2, the resistance heating element 43b is
A heater 43 inserted into a US tube 43 a is inserted into the case 40. The tube 43a of the heater 43 is a counter electrode.

A sample electrode 45 and a reference electrode 46 are inserted into the case 40. A resistance heating element 48 is disposed on the rear surface of the sample electrode 45, and energization is performed so that the sample electrode 45 has the same temperature (for example, 230 ° C.) as the tube 43 a of the heater 43.

The sample electrode 45 is held at the tip of a cylindrical holder 49 via an insulating sealing material 50 made of fluororesin. The resistance heating element 48 is provided on the rear surface of the sample electrode 45.
Is held down. The holding body 51 is a cylindrical body having a closed end surface, and includes a thermometer 52 for measuring the temperature at the end.

A holder 49 holding the sample electrode 45 and the like
Is watertightly inserted into a cylindrical holder holding tube 53 extending laterally outward from the case 40, and the sample electrode 45 is
It is arranged so as to be in contact with water in 0.

The reference electrode 46 is a pressure-balanced external reference electrode, and Ag / AgCl is used as a salt bridge 54 made of zirconium oxide.
And the sample electrode 45 through the capillary 55 made of fluororesin.
Is disposed so as to be in contact with the water in the case 40 in the immediate vicinity of the case 40.

By grounding the case of the reference electrode 46 (reference numeral 49), noise is reduced, and the reproducibility of the measurement is improved.
The accuracy is improved.

In order to evaluate the anticorrosion performance of the sample electrode 45 by this simulated boiler, the resistance heating elements 43b, 4 are set so that the temperature of the tube 43a and the sample electrode 45 becomes the same predetermined temperature.
8 and maintain this state for a predetermined period. Then, the potential of the sample electrode 45 is obtained using the sample electrode 45 and the reference electrode 46,
The potential when this value is stabilized is defined as the natural potential.

Next, the sample electrode 45 and the counter electrode (tube 43
a), a predetermined voltage is applied, and this voltage is gradually increased every predetermined time. Then, the voltage at which the value of the flowing current sharply increases is determined as the film breakdown potential.

The difference ΔE between the film breaking potential and the spontaneous potential obtained in this way is calculated, and based on the ΔE, the sample electrode 4
The anti-corrosion property of the film of No. 5 is judged. The higher the difference ΔE between the film breaking potential and the spontaneous potential is, the more the film is judged to be anticorrosive.

[0027]

EXAMPLES Examples 1 and 2 The anticorrosion performance of the sample electrode 45 by the simulated boilers shown in FIGS. 1 and 2 was evaluated as follows.

The tube 43a has a length of 313 m.
m × 41.5 mmφ made of SUS, heater 4
The heat transfer amount of No. 3 was 98900 Kcal / h · m ² . A 13 mmφ × 2 mmt mild steel flat plate was used as the sample electrode 45.

(1) The same boiler water as that supplied to the actual boiler is introduced from the water supply pipe 34 so that the insides of the cases 30 and 40 are filled with boiler water. The blow water is caused to flow out from the blow pipe 35 so as to be at the upper part of the blow pipe 30. Total water volume is 4L
And the flow velocity in the case 40 is 8 to 12 cm / sec.
The circulation ratio was 40 to 60, all of which matched the values of the actual boiler.

(2) The heater 43 and the resistance heating elements 43b and 48 of the sample electrode 45 are energized so that the temperature of the tube 43a of the heater 43 and the temperature of the sample electrode 45 become 230 ° C.

The pressure in the cases 30 and 40 is 1.0 MPa
And After 3 days, 10 m from the natural potential of the sample electrode 45
The applied voltage was increased stepwise while maintaining the voltage at 60 V for 60 seconds. Then, the voltage at which the current rapidly increased was determined as the film breakdown potential.

Boiler water having a pH of 11.5 and DO of 6 μg / L (Example 1) or 300 μg / L (Example 2) was circulated, and the film breaking potential and the spontaneous potential of the sample electrode 45 were measured. The result is shown in FIG. In FIG. 3, the horizontal axis is E (applied voltage), and the vertical axis is the current value. As shown in FIG. 3, in Example 1, the current rapidly increased near E = −100 mV, and in Example 2, the current rapidly increased near E = −300 mV, and it was recognized that the film was broken at this voltage. Was done. As a result, it was recognized that the film was less likely to be broken in Example 1.

Embodiments 3 and 4 The simulated boiler used in the above embodiment has a pH of 10.0, 11.
0,11.5 and dissolved oxygen 8ppm and 0.02p
pm, and a film breaking potential and a spontaneous potential were measured in the same manner as described above except that a potential step method of 40 mV / min was employed. Some polarization curves are shown in FIG. 4, and E is shown in FIG. In FIG. 4, each curve is plotted at regular intervals and shifted in the horizontal axis direction.

The boiler JIS (JIS B8223) describes that the pH of the boiler water is controlled to 11.0 to 11.8 and the dissolved oxygen is controlled to be kept as low as possible.

The results obtained from the above figures of the example of the present invention are:
It can be seen that the JIS standard is well matched.

Example 4 Water was supplied to the simulated boiler used in the above example at pH 11.5 and with dissolved oxygen amounts of 8.3 ppm and 0.02 ppm, and the capacity of the reference electrode incorporated in the simulated boiler was grounded. . Tables 1 and 2 show the results obtained with and without grounding.

[0037]

[Table 1]

[0038]

[Table 2]

Tables 1 and 2 show that grounding improves the reproducibility of polarization measurement.

[0040]

According to the present invention, the anticorrosion performance of an oxide film formed on the surface of a steel material or the like can be evaluated with good reproducibility.

[Brief description of the drawings]

FIG. 1 is a side view of a simulation boiler.

FIG. 2 is a sectional view taken along the line II-II in FIG.

FIG. 3 is a graph showing measurement results in Examples 1 and 2.

FIG. 4 is a graph showing measurement results in Example 3.

FIG. 5 is a graph showing measurement results in Example 3.

FIG. 6 is a schematic view showing a state of corrosion of a steel material surface.

FIG. 7 is a schematic view showing a corrosion state of a steel material surface.

[Explanation of symbols]

 Reference Signs List 30 Pressure-resistant case for water / water separation 31 Liquid level gauge 32 Thermometer 33 Steam extraction pipe 34 Water supply pipe 35 Blow pipe 40 Heat-resistant pressure case 41, 42 Circulation pipe 43 Heater 43a Counter electrode (tube) 43b Resistance heating element 45 Sample electrode 46 Reference pole 48 Resistance heating element 49 Earth

Claims (1)

[Claims] 1. A high temperature and high pressure water circulation system having a water supply inlet and a blow water outlet, a counter electrode also serving as a heater for heating circulating water, a sample electrode made of metal, and heating the sample electrode. Means and an electrochemical measuring means consisting of a reference electrode. An electrochemical measurement method characterized by evaluating: