JP2645558B2 - Method for detecting hydrogen in metal - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to the detection of hydrogen in metal, and more particularly to the detection of hydrogen concentration in steel. The present invention is used for detecting, for example, embrittlement of a metal due to dissolution of hydrogen in a metal such as steel.

[Prior Art] The phenomenon of dissolution of hydrogen in metals such as steel is well known. As the dissolution of hydrogen in the metal proceeds, erosion of the metal, interfacial separation of the clad alloy, embrittlement of the metal, and the like occur. Here, the erosion by hydrogen is a phenomenon in which hydrogen atoms dissolved in a metal react with carbon to form bubbles such as methane, and this pressure destroys a metal such as steel. In addition, interface delamination is a phenomenon in which hydrogen is accumulated at a boundary portion of a clad alloy, causing delamination of the clad alloy at the interface. Further, embrittlement due to hydrogen is a phenomenon in which a metal that has absorbed hydrogen upon contact with high-temperature and high-pressure hydrogen is cooled, particularly embrittled, and destroyed.

The problem of metal destruction and deterioration due to dissolution of hydrogen is particularly significant when high-temperature and high-pressure hydrogen is used in plant piping, reactors, and the like. That is, there is a risk that the metal such as stainless steel used for the pipe wall and the vessel wall is destroyed by hydrogen. In order to prevent these phenomena, it is necessary to detect hydrogen in the metal.

Detection of hydrogen in a metal is also necessary for purification and transport of hydrogen by Pd or the like, or control of hydrogen storage using the hydrogen storage capacity of a metal such as Zr or Nb.

Separately, ZrO ₂ stabilized with CaO or Y ₂ O ₃ or CeO
_2. Oxygen ion conductivity such as β-alumina is known. These oxygen ion conductors are used, for example, for measuring the oxygen concentration of exhaust gas from an automatic engine. Also Sr
CeO ₃ , or Ce element among them, about 1 to 10%, Y or Y
A hydrogen ion conductor (proton conductor) substituted by b or Sc is also known. For example, Iwahara et al., “High-temperature hydrogen ion conductor and its application to electromotive force gas sensor”,
See "The 1st International Conference on Chemical Sensors", Kodansha, 1983, p.227. Proton conductors such as SrCe _0.95- Yb _0.05 O ₃ exhibit proton conductivity at high temperatures and are used as hydrogen sensors.

[Problems of the Invention] An object of the present invention is to detect the hydrogen concentration in a metal and to make the detection output in that case substantially proportional to the hydrogen concentration.

[Constitution of the Invention] The method for detecting hydrogen in a metal according to the present invention comprises attaching an ion-conductive solid electrolyte to a metal under airtight conditions, diffusing and permeating the metal in the metal, and converting the hydrogen flow into an ionization current to the solid electrolyte It is characterized in that hydrogen in the metal is detected from the ionization current.

For example, suppose that one side of the metal is in contact with high temperature, high pressure hydrogen. Hydrogen diffuses into the metal mainly as atomic hydrogen. When a proton conductor is brought into contact with the other surface of the metal, hydrogen diffuses and permeates through the metal due to the difference in hydrogen concentration. Hydrogen reaching the other surface of the metal is converted into protons and moves through the conductor. Here, the other surface of the proton conductor is connected to a metal via a low impedance load or the like. The current flowing through the load is equal to the permeation rate of hydrogen in the metal if proton conduction in the conductor is dominant and other side reactions at the contact surface between the metal and the conductor can be neglected. And if the permeation rate of hydrogen is known, it is corrected by the diffusion coefficient,
It is possible to know the hydrogen concentration in the metal and the concentration distribution of the hydrogen in the metal. When a load current is output, this output is almost proportional to the hydrogen concentration in the metal.

Here, it has been shown that the hydrogen flow in the metal is converted into an ionization current to the proton conductor, and the hydrogen concentration in the metal is measured. Next, this is generalized. First, the proton conductor may be changed to a carrier conductor capable of reacting with hydrogen, such as an oxygen ion conductor. For example, an oxygen ion conductor such as ZrO ₂ or CeO ₂ is brought into contact with a metal, and the other surface of the conductor is brought into contact with a small amount of oxygen. At the contact surface between the metal and the conductor, a reaction between hydrogen and oxygen occurs, and oxygen ions move to the metal side in the conductor. Assuming that all the diffused hydrogen reacts with oxygen ions, the current accompanying the movement of oxygen ions is equal to the permeation rate of hydrogen. The main solid electrolytes currently available are proton conductors and oxygen ion conductors, but they can also be used if a conductor capable of transporting ions that react with hydrogen can be obtained.

At room temperature, an active electrode such as Pd or Pt is required to oxidize hydrogen permeated into the metal into protons. However, the destruction of metal by hydrogen is a phenomenon mainly caused by contact with high-temperature and high-pressure hydrogen. Therefore, the detection itself, for example, 50
It is often performed at a high temperature of about 0 ° C. Because the temperature of the system is high, oxidation of hydrogen to protons and reduction of protons to hydrogen molecules in a proton conductor, reduction of oxygen molecules to oxygen ions in an oxygen ion conductor, and conversion of oxygen ions to hydrogen atoms The reaction proceeds easily. Thus, electrode activity itself is only a secondary factor. More importantly about the electrodes,
The purpose is to seal the contact surface between the conductor and the metal by using a soft and highly extensible material such as Pd for the contact surface between the conductor and the metal.

Examine the causes of detection errors. When oxygen enters the contact surface between the conductor and the metal, hydrogen is consumed by a side reaction between oxygen and hydrogen. Also, as is well known, hydrogen is a substance having a very large diffusion coefficient, and is a dense substance such as glass and permeates again. Therefore, if the conductor is arranged at a position separated from the metal and the permeated hydrogen is sampled and guided, hydrogen is lost at the sampling home.

In order to solve these problems, a conductor is airtightly arranged on a metal surface. The conductor is preferably in direct contact with the metal surface, but may be spaced apart via a seal. Preferably, a soft and highly extensible detection electrode such as Pd, Au, or Ag is used for the contact surface between the conductor and the metal, and this electrode is used to prevent intrusion of oxygen from the surroundings.

Next, it is preferable to keep the potential of the conductor with respect to the metal at an appropriate value to prevent oxidation of a metal such as steel. When the contact potential of the conductor to the metal surface is 1 to 2 V or more, oxidation of a metal such as steel occurs. The resulting oxide suppresses the diffusion of hydrogen,
This may cause a detection error. In order to prevent this problem, it is preferable that the potential of the contact surface of the conductor with respect to the metal surface is kept at 0.5 V or less, more preferably at 0.2 V or less, to prevent oxidation of the metal.

Embodiment FIG. 1 shows a basic embodiment. In the figure, reference numeral 02 denotes a pipe made of stainless steel or the like used for high-temperature and high-pressure hydrogen in a chemical plant or the like, and measures the hydrogen concentration in the pipe. Although an example for predicting metal destruction by hydrogen is shown here, the hydrogen content of a hydrogen storage metal such as Pd, Zr, or Nb can be similarly detected.

Reference numeral 2 denotes a proton conductive solid electrolyte such as SrCe _0.95 Yb _0.05 O _3. Yb may be substituted with Y or Sc, or plain SrCeO ₃ may be used. These proton conductors exhibit high proton conductivity in the range of about 300 to 800 ° C., and are suitable for detection at high temperatures because of their high heat resistance. Proton conductor 2 also includes antimonic acid (Sb ₂
O ₅ · nH ₂ O: n is usually about 2) or the like. In the embodiment, description will be made mainly on the proton conductor.
Oxygen ion conductive solid electrolytes such as ZrO ₂ stabilized with Y ₂ O _{3 and the} like, CeO ₂ and β-alumina may be used. Reference numeral 4 denotes a detection electrode, in which a conductive material having high spreadability such as Pd, Au, Ag, RuO ₂ , or an alloy or a mixture thereof is used. Since these materials have high extensibility, they adhere to the surface of the pipe 02 and prevent the residual oxygen in the atmosphere from diffusing to the detection electrode 4 and reacting with hydrogen. Reference numeral 6 denotes a comparison electrode, which can use any electrode material such as Pt, Pd, SnO, LaNiO ₃ , and carbon. Reference numeral 8 denotes a counter electrode which can use an arbitrary electrode material. The conductor 2 and the three electrodes 4, 6, 8 constitute a hydrogen sensor 10. In the embodiment, since the sensor 10 is operated at a temperature of about 500 ° C., the electrode reaction can be caused by the activity of the proton conductor 2 itself. Therefore electrode 4,
6 and 8 need not be provided.

Reference numeral 12 denotes a reference atmosphere setting cover which shuts off the sensor 10 from the external atmosphere and further prevents diffusion of oxygen to the detection electrode 4. The cover 12 Ar and He, but flowing inert gas such as N _2,
It may contain oxygen enough to react and consume the protons that have reached the counter electrode 8, and the allowable oxygen concentration is determined by the airtightness of the detection electrode 4.

Reference numeral 14 denotes an operational amplifier, 16 denotes a power supply for setting the described potential, and the output may be 0 or a negative value. Reference numeral 18 denotes an ammeter for measuring a proton current. Then, for example, one terminal of the power supply 16 and the pipe 02
And the detection electrode 4 are connected so that the current flowing through the conductor 2 flows through the power supply 16 to the pipe 02 and the detection electrode 4.

Note that the proton transport number of the proton conductor 2 is not strictly 1 and carriers such as electrons can also be involved. When the pipe metal is oxidized on the surface of the detection electrode 4, a dark current is generated due to the oxidation. Circuits for compensating for these dark currents are shown after the chain line in the figure. In this case, for example, replace the ammeter 18 with a resistor 20
And the like, and the voltage is amplified by the differential amplifier 22. The output of the differential amplifier 22 is compensated for the dark current by a reference potential determined by the power supply 24 and the resistors 26 and 28, and the difference between the two outputs is extracted by the differential amplifier 30 and displayed on a meter 32 or the like.

The operation of the embodiment will be described. Hydrogen inside the pipe 02 diffuses into the pipe metal mainly as atomic hydrogen. Piping 02
Is low, and a linear hydrogen concentration distribution is formed inside the pipe metal according to Fick's law of diffusion. Hydrogen dissolved in the pipe metal permeates to the outside,
Is oxidized to protons. The generated protons move to the counter electrode because the hydrogen concentration inside the cover 12 is low. On the other hand, the electrons generated at the detection electrode 4 accompanying the oxidation of hydrogen to protons are
The current flows from the ammeter 18 to the counter electrode 8 via 16. At the counter electrode 8, the protons and electrons recombine and are discharged as hydrogen molecules and water vapor together with the Ar gas flow.

Here, the current flowing through the ammeter 18 is equal to the proton current inside the conductor 2, and is equal to the hydrogen permeation rate if there is no side reaction at the detection electrode 4. The permeation rate of hydrogen is proportional to the hydrogen concentration inside the pipe metal, and when divided by the hydrogen diffusion constant of the pipe metal, the internal hydrogen concentration is obtained.

In the embodiment, since the outside air is blocked by the cover 12 and Pd or the like, which is highly extensible and airtight, is used for the detection electrode 4, the side reaction between hydrogen and oxygen at the detection electrode is small. Next, the potential of the comparison electrode 6 is controlled to determine the potential of the detection electrode 4 with respect to the pipe metal. The potential of the comparison electrode 6 is equal to the potential of the power supply 16 because the operational amplifier 14 is used. The potential of the detection electrode 4 with respect to the comparison electrode 6 is determined by the sum of the difference in the interface potential between the two electrodes 4, 6 and the proton conductor 2 and the voltage drop in the proton conductor 2. It is preferable that the potential of the detection electrode 4 with respect to the pipe metal is maintained at 0.5 V or less, more preferably at 0.2 V or less, to prevent oxidation of the pipe metal. In order to compensate for dark current due to factors other than hydrogen diffusion, an output corresponding to dark current may be separately obtained and compensated. SrCe _0.95 Yb _0.05 O ₃ and ZrO ₂ used next are stable ceramic materials and do not corrode metals.
In addition, even when an acidic and slightly corrosive conductor such as ammonic acid is used, the danger of corrosion can be eliminated by cutting off the pipe metal at the detection electrode 4.

Secondly, a measurement result when SrCe _0.95 Yb _0.05 O ₃ is used for the proton conductor 2 is shown. Using carbon steel having a thickness of 2.0 mm for the pipe 02, H ₂ at 500 ° C. and 1 atm and Ar at 500 ° C. and 1 atm were alternately flowed at random intervals for 16 cycles. A hydrogen concentration meter inside the pipe metal is obtained from the current of the ammeter 18 and displayed. The hydrogen concentration indicates a value near the inner surface of the pipe 02, and the diffusion coefficient of hydrogen in the pipe metal was determined by the transient response method shown in FIG. Here, the output of the power supply 16 was set to 0V.

The average value of the hydrogen concentration is 0.137ppm, the standard deviation is 0.024ppm,
The value for dark current is converted to hydrogen concentration, and the average value is 0.017p
pm, standard deviation 0.006 ppm. When the dark current was compensated, the result was that the average value of the hydrogen concentration was 0.12 ppm and the standard deviation was 0.03 ppm.

Although the description has been made mainly of the proton conductor in the embodiment,
An oxygen ion conductor may be used. In this case, if oxygen is mixed in the Ar gas flow, the oxygen ionized at the counter electrode 8 moves inside the conductor and reacts with hydrogen at the detection electrode 4. The generated water vapor diffuses inside the conductor 2 and the interface between the conductor 2 and the detection electrode 4 and is discharged to the outside. And the detection pole 4
Assuming that the hydrogen and oxygen completely react with each other, the current caused by the oxygen ions is proportional to the permeation rate of hydrogen. However, in this case, since oxygen is present in the atmosphere, the problem of side reactions due to oxygen that has entered the detection electrode 4 is greater.

3 and 4 show another embodiment. In FIG.
An ammeter 18 and a power supply 16 were directly connected to the counter electrode 8 without providing the comparison electrode 6. In FIG. 4, a ring-shaped pedestal 40 is
And the sensor 10 was fixed to the base 40.

FIG. 5 shows an outline of the measurement of the diffusion constant of hydrogen by the transient response. When the hydrogen concentration inside the pipe 02 is rapidly changed, the output current of the sensor 10 shows a transient phenomenon corresponding to the diffusion of hydrogen into the pipe metal. The response waveform is determined by Fick's diffusion law, and the diffusion constant can be obtained from the time until the output rises after hydrogen is introduced. The diagram schematically shows three response waveforms I and I in order of decreasing diffusion constant.
I and III are shown. For example, the inventor uses this method to
The hydrogen diffusion constant of -1% Mo steel was determined. The obtained diffusion constant was 2.4 × 10 ⁻³ · Exp (−4200 / RT) cm ² / sec (R is a gas constant, T is an absolute temperature).

[Effect of the Invention] According to the present invention, the hydrogen concentration in the metal can be obtained, and the detection output is almost proportional to the hydrogen concentration.

[Brief description of the drawings]

FIG. 1 is a diagram showing a detection device used in the embodiment, and FIG. 2 is a characteristic diagram showing a detection result in the embodiment. FIG. 3 is a diagram showing a detection device used in another embodiment, and FIG. 4 is a diagram showing a detection device used in still another embodiment. FIG. 5 is a characteristic diagram showing a response waveform of the embodiment to a change in hydrogen pressure. In the figure, 2 ... an ion-conductive solid electrolyte, 4 ... a detection electrode, 6 ... a comparison electrode, 8 ... a counter electrode.

Claims (3)

(57) [Claims] 1. An ion-conductive solid electrolyte is attached to a metal under airtight conditions, a hydrogen flow diffused and permeated through the metal is converted into an ionization current to the solid electrolyte, and hydrogen in the metal is detected from the ionization current. A method for detecting hydrogen in a metal, comprising: 2. The method for detecting hydrogen in a metal according to claim 1, wherein said solid electrolyte is a proton-conductive solid electrolyte. 3. The method for detecting hydrogen in metal according to claim 1, wherein said solid electrolyte is an oxygen ion conductive solid electrolyte.