JPS6056797B2 - Method for removing surface oxide film of Zr and Hf and their alloys - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to Zr. ．． Zr-based alloy, Hf. ．． The present invention relates to a method for removing a surface oxide film of group ①a metals such as Hf-based alloys, or alloys thereof.

In general, most metals or alloys form an oxide film on their surface due to casting, forging, heat treatment, or use in corrosive environments. It was necessary to carry out removal treatment.

By the way, in recent years, with the remarkable development of the aviation industry, nuclear power industry, seawater utilization industry, etc., Zr. ．． Zr
Base alloy or Hf. ．． Demand for Hf-based alloys is rapidly increasing, and oxide film removal treatment has become important for product quality and overall efficiency of the product processing process, especially for these alloys that are prone to forming oxide films on their surfaces. I'm getting old. Conventionally, Zr. ．． Surface oxide films formed on materials made of Zr-based alloys, Hfl, and Hf-based alloys can be removed by mechanical surface grinding or LiF. ．． Generally, fluoride molten salt immersion treatment such as ZrF4 was used, but in the former case, it was difficult to remove the coating from narrow areas, small recesses, or corners, and the bare metal also required considerable cutting. In addition, the oxide film was extremely hard, causing significant wear and tear on the blades of the grinding machine.

On the other hand, in the latter case, highly corrosive molten salt is used, which poses safety and cost problems.
The melting rate of the metal is also extremely high, resulting in low yields.
Furthermore, after the film removal process, it is necessary to remove salts that adhere to the metal surface. Therefore,
In addition to such methods for removing the surface oxide film, attempts have been made to use 1 nitric hydrofluoric acid treatment, 2 blowing gas treatment, 3N1I1F alternating current electrolytic treatment, etc.; The dissolution rate of the base metal is significantly higher than that of the base metal, so the coating is extremely thin (a). It has the disadvantage that it can only be used to remove particles (approximately 1 μm or less) and that it also dissolves a large amount of metal.

In addition, the "humidus gas treatment method" means that the object to be treated is first heated to 500 to 600°C in a gas flow containing gas to form a layer of fluorine compound between the oxide film and the base metal, and then After that, it was immersed in a mixed solution (90 to 95°C) of 0.4 mol of ammonium oxalate, 0.16 mol of ammonium citrate, 0.1 mol of ammonium fluoride, and 0.3 mol of hydrogen peroxide. There is a method of removing the oxide film by eluting the fluorine compound, but this method uses HF at high temperatures, which increases the cost of safety measures and increases the processing cost.

The "AC electrolytic treatment method in NH4F" was developed as a pretreatment method for dissolving a zircaloy alloy in which an oxide film is formed. Perform AC electrolysis at density,
This method removes the oxide film by eluting the base metal to some extent, but since this method is originally a pretreatment for a process that aims to melt the entire metal, local corrosion is severe and only the oxide film is removed. When used for the purpose of removing metal, there was a problem in that the residual oxide film and the heavily corroded base metal were mixed together.

From the above-mentioned viewpoints, the inventors of the present invention have discovered that even products with complex shapes can be completely processed without causing problems of safety or yield reduction due to excessive metal loss. As a result of conducting research to find a low-cost method for removing the surface oxide film of group a metal-based alloys, we came to the knowledge shown in (a) to (e) below.

That is, (a) Zr. Zr-based alloy, Hf. Although the surface oxide film of Hf-based alloys has high electrical insulating properties, when an AC voltage of 200 V or less is applied in an electrolyte solution, dielectric breakdown occurs over the entire surface, and the surface oxide film is damaged by the base metal. Small pores with a diameter of 10 to 100 μm are generated at a density of about 1 pore/d. FIG. 1 is a photograph showing the state of the surface of Zircaloy 4 5 minutes after the dielectric breakdown reaction of the oxide film of Zircaloy 4 started due to the voltage application in the ammonium fluoride aqueous solution; Parts where the metal has fallen off and micropores (which look like black dots) caused by dielectric breakdown can be seen.

Moreover, this phenomenon occurs because even if bare metal is already present, its surface is immediately oxidized to a thin layer in the air.
This occurs in the same manner as in the case where the exposed portion does not exist.

Even if the thickness of the film is about 5 μm or more, the thickness of the protective part is only 5 μm at most, and the other parts have many cracks and pores, so it is difficult to apply an electric current to cause dielectric breakdown. The voltage does not increase beyond the above level even if the coating thickness increases, and unlike ordinary electrolysis, the main purpose is dielectric breakdown, so even if the object to be treated has a complex shape, it will not increase Pores can be formed uniformly on the surface. )) It is well known that ammonium fluoride solution dissolves Group A metal-based alloys, but it also contains citric acid, oxalic acid,
Alternatively, if an aqueous solution of ammonium salt, sodium salt, potassium salt of tartaric acid, or a mixture thereof is added to maintain the pH at 5 to 6, the dissolution rate of the metal itself is kept extremely low, and rather, the dissolution rate of the metal itself is reduced. The rate of dissolution of the boundary layer near the interface of the oxide film increases.

FIG. 2 is a photograph showing the progression of corrosion due to the treatment solution, with the upper left showing the bare metal and the rest showing the oxide film.

The white part of the oxide film is an area where the lower part has already been corroded and has lost its bond with the base metal. :
) Therefore, add an aqueous solution of ammonium salt, sodium salt, potassium salt of citric acid, oxalic acid, or tartaric acid, or a mixture thereof to an aqueous solution of ammonium fluoride 1 to maintain the pH at 5 to 6. ZrlZr groups and Hf. When an alternating current is passed through the surface oxide film of the Hf-based alloy at a specific current density, uniform pores reaching the base metal can be formed in the oxide film, and further,
By dissolving the boundary layer through these pores, the bond between the oxide film and the base metal is lost, and the oxide film is peeled off from the base metal.

]) The presence of a small amount of ammonium nitrate in the electrolyte solution further improves the oxide film stripping ability, especially in the case of Sn-containing alloys. ]) Because of the mechanism described above, when hydrogen gas is generated on the surface of the metal by applying electricity or when ultrasonic vibration is applied to the electrolyte solution, new reaction liquid is replenished into the boundary layer. , the effect of promoting the diffusion of eluted gold layer ions from the gap to the outside, or promoting the peeling of an oxide film with loose bonds, etc. can be obtained.

And the generation of hydrogen gas is further increased by the addition of hydrogen peroxide. This invention was made based on the above findings, and includes Zr. Zr-based alloy, or (F,. 112 concentration of one or more hydrogen peroxides, and citric acid,
P by adding one or more of ammonium salt, sodium salt, and potassium salt of oxalic acid or tartaric acid.
In an electrolytic aqueous solution maintaining H5-6, as it is,
Alternatively, by performing alternating current electrolysis at a current density of 0.2 to 0.5A1c#f while applying ultrasonic vibration to the electrolytic aqueous solution, the surface oxide film can be efficiently removed without excessively dissolving the base metal. It has characteristics. Next, the reason why the composition and pH of the treatment liquid and the current density are limited as described above in the method of the present invention will be explained.

Ammonium pentafluoride concentration Figure 3 is a diagram showing the influence of ammonium fluoride concentration in an aqueous solution on the applied voltage required to obtain the same current density when Zircaloy 4 is used as the material to be treated. , FIG. 4 is a diagram showing the influence of liquid concentration on the dissolution rate of the same treated material, and as is clear from these diagrams, when the ammonium fluoride concentration is less than 0.1 mol, the applied voltage Not only does this increase the rate of deterioration of the treatment liquid, but the rate of deterioration of the treatment liquid also increases, making treatment work difficult.

On the other hand, if the ammonium fluoride concentration exceeds 0.5 mol, the corrosion rate becomes excessive and local corrosion tends to occur, and the buffer agent added at the same time becomes supersaturated at a considerably high temperature and precipitates, which is not preferable. . For this reason, the ammonium fluoride concentration in the electrolyte should be set at 0.1 to 0.5.
It was determined as mole. (Xi) PH value Figure 5 is a diagram showing the influence of PH on the dissolution rate, and Figure 6 is a diagram showing the influence of PH on the applied voltage required to obtain the same current density. From the figure,
If the pH is less than 5, the corrosion rate becomes excessive and local corrosion occurs, and if the pH exceeds 6, the corrosion rate decreases but the power consumption increases, both of which are unfavorable. It was set as 5-6.

Note that when conducting the experiments shown in Figures 5 and 6, if the pH value is less than 5, the concentration of ammonium citrate and ammonium oxalate is decreased, and if it exceeds 5, the concentration of ammonia is decreased. By addition, the desired pH value was adjusted.

Concentration of buffering agents The concentration of buffering agents such as ammonium nitrate, hydrogen peroxide, and ammonium, sodium, and potassium salts of citric acid, oxalic acid, or tartaric acid should be determined when removing an oxide film using a processing solution. There is a required upper limit of 60 hours.
In order to maintain the pH value of the liquid between 5 and 6 during the period of 12 to 12 hours, it is sufficient that the concentration is 1.5 times or more the ammonium fluoride concentration, as shown in FIG.

In particular, the ammonium nitrate concentration was determined to be 11,100 to 115 of ammonium fluoride because, in the case of alloys containing Sn such as Zircaloy, the content is at most 1.
This is because it remains at ~2%.

In addition, the concentration of hydrogen peroxide was changed to 111% of ammonium fluoride.
The reason for setting the concentration to be between 0 and 112 is that if the concentration is less than 1% of the ammonium fluoride concentration, it will be difficult to maintain the foaming effect on the surface of the treated object for the necessary time, whereas if it exceeds 112, the foaming effect will be difficult to maintain for the necessary time. Foaming on the surface of the treated object becomes excessive, and the liquid level rises excessively due to the foaming.

Current Density If the current density is less than 0.02Alc71f, the amount of bubbles generated during treatment will be small and the effect of promoting oxide film peeling will be weak.On the other hand, if it exceeds 0.5A1d, foaming will be excessive and the liquid level will rise considerably. , the liquid temperature rises significantly due to energization, and 7
As the temperature reaches 0°C or higher, cooling becomes necessary, and the amount of metal eluted also increases, resulting in a decrease in yield.
It was set as 1.

Regarding the temperature of the treatment liquid, as is clear from the diagram showing the influence of liquid temperature on the dissolution rate in Figure 8, the corrosion rate increases as the temperature increases, but if the temperature exceeds 70°C, foaming will be excessive. If the temperature exceeds 80'C, this phenomenon becomes extremely severe, so it is preferable to keep the temperature below 70°C. Note that the data shown above is for Zircaloy 4U, but for other Zr, . Z
r base combination and Hf, . Similar results were obtained for the Hf-based alloy.

By the way, in carrying out this invention, it is preferable to apply thermal shock or mechanical shock to the material to be treated as a pretreatment for electrolytic treatment, since fine racks are formed in the oxide film and the oxide film is easily peeled off. In addition, if some oxide film remains for some reason after carrying out the method of this invention, the remaining film can be removed by immersion in a nitric-hydrofluoric acid solution as a post-treatment or by sanding with a metal brush or sanding. can be easily removed. It was also confirmed that when immersing in a nitric-hydrofluoric acid solution, better effects can be obtained by oscillating ultrasonic waves into the treatment solution. Next, the present invention will be explained with reference to examples. Example 1 First, the component composition was Sn:
1.50% by weight, Fe: 0.20% by weight, Cr: 0.1
0% by weight, Ni: 0.002% by weight, Zr: remainder, 4 tubes of Zircaloy (outer diameter: 10.7Wrft1 wall thickness:
0.62Wfm1 Length: 100cm, Oxide film thickness: 3~5μ7TL, although it varies depending on the part. ) was prepared, and AC electrolysis was carried out using an AC electrolysis surface decontamination experimental apparatus as shown in FIG.

In FIG. 9, 1 is a fluororesin beaker, 2 is a material to be treated, 3 is a fluororesin-coated heater, 4 is a fluororesin-coated thermometer, 5 is an ultrasonic cleaner, 6 is a voltmeter, 7 is an ammeter,
8 is a slider. The electrolysis conditions at this time were as follows.

Treatment liquid composition: ammonium fluoride...0.5 mol, ammonium nitrate...0.05 mol, ammonium oxalate...0.1 mol, ammonium citrate... ...1 mole, hydrogen peroxide...
・0.1 mol, liquid volume: 700 ml, liquid PH: 5, current density: 0.1 AIcI, liquid temperature: 70°C1, processing time: 4 gin.

After the electrolytic treatment, the surface of the Zircaloy 4 tube as the material to be treated was observed, and it was confirmed that the oxide film had been completely peeled off and removed.

Example 2 Electrolysis conditions were as follows: Treatment liquid composition: ammonium fluoride...0.1 mol, ammonium citrate ◆●...0.5 mol, liquid volume: 700 ml, liquid PH: 5, Current density: 0.03AIcIt1 Liquid temperature: 70°C Treatment time: 9 powders When four tubes of Silcaloy were treated in the same manner as in Example 1, it was found that the oxide film was completely peeled off and removed. confirmed.

Example 3 Hf plate material (width: 20mm, thickness: 2T!Rln, length:
100'' order, oxide film thickness: 6 μm) was prepared, and AC electrolysis was performed using an AC electrolysis surface decontamination experimental apparatus as shown in FIG.

The electrolysis conditions at this time were as follows.

Treatment liquid composition: ammonium fluoride...0.5 mol, ammonium citrate...1.5 mol, liquid volume: 700 ml, liquid PH: 5, current density: 0.5 AId1 liquid Temperature: 70℃, Processing time: 6 Gin.

When the material to be treated after the electrolytic treatment was observed, it was confirmed that the oxide film had been completely peeled off and removed.

Example 4 Using 4 Zircaloy tubes with an oxide film thickness of 6 μm, the electrolytic conditions were maintained at liquid temperature: 65°C and current density: 0.1 AIcI (however, the applied voltage was
The electrolytic treatment was carried out under the same conditions as in Example 1, except that the initial voltage was 130 V and the final stage was 60 V), and ultrasonic waves were oscillated during the electrolytic treatment.

After the treatment, it was observed that matte gray-brown scale (mainly composed of zirconium oxide) was attached to the surface of the Zircaloy 4 tube.

Unlike the initial black oxide film, this scale can be easily removed by scrubbing with a wire brush, etc.
It was a scale that was loosely connected to the base metal. Therefore, as a post-treatment, this tube was immersed for 3 minutes in a solution of 8 mol of nitric acid and 0.5 mol of hydrofluoric acid under ultrasonic vibration oscillation, and the oxide film was completely removed from both the inner and outer surfaces of the tube. removed.

In addition, the corrosion rate calculated including the peeling of the oxide film and the elution of the base metal is: 1st stage treatment: 61.4 m 91 d Imi
nl post-treatment: 45.4m91ddImin1.

Example 5 Zircaloy 4 tubes were treated under the same conditions as in Example 4, except that no ultrasonic vibration was applied during the treatment.

As a result, the corrosion rate decreased to 1st stage treatment: 53.7m91drTI, Im1n1 post-treatment: 43.8mg1dIminl, and spots of black oxide film remained.

Example 6 Example 4 except that hydrogen peroxide is not added to the treatment liquid composition
Four tubes of Zircaloy were treated under the same conditions.

As a result, the corrosion rate decreased to 45.8 m91d Inno Minl in the first stage treatment,
A decrease in bubble generation was observed.

Example 7 Zircaloy 4 tubes similar to those in Example 4 were
The procedure of inserting the sample into an electric furnace maintained at 600°C and, after reaching that temperature, placing it into water at room temperature was repeated three times.

As a result, fine cracks were generated on the surface of the oxide film as shown in the photograph shown in Figure 10, and even under the same treatment conditions as in Example 4, the applied voltage: 80 V (initial) -
A current density of 1A1c could be given even at 40cm (final stage). The oxide film could be removed in a treatment time of 4 powders, and a clean surface with metallic luster could be obtained without the need for post-treatment. As mentioned above, according to the invention, Zr. ．． Zr-based alloy, or Hf. When removing an oxide film from an Hf-based alloy, (1) the oxide film in narrow parts and recesses can be easily removed, and (Ii) the amount of loss of metal is small and the yield is improved.

That is, the elution of the base metal falls within the range of 40 to 80 μm in terms of thickness; (iii) it can be processed with inexpensive equipment and materials; (iv) an easily available 50Hz or 60Hz AC power source can be used as is; v) High operational safety; (Vi) Excellent effects such as the ability to process two objects at the same time, as shown in FIG. 9, can be obtained; 1Zr. ．． Zr-based alloy, or Hf. Removal of oxide film formed on the ingot surface during Hf-based alloy casting, 5.
Removal of an oxide film formed on the surface of a forged product during forging, [Phase] Similarly, removal of an oxide film formed on the surface of a forged product during heat treatment, [Co.] Similarly, during use in a corrosive environment. Removal of generated surface oxide film, Zr.9 on which surface oxide film is formed. ．． Zr-based alloy, or Hf.
．． It can be applied to the removal of oxide films as a pre-treatment for recovering scrap materials from Hf-based alloys, producing excellent results and producing industrially useful effects.
Ru.

[Brief explanation of the drawing]

Figure 1 is an electron micrograph showing how the dielectric breakdown reaction of the metal oxide film begins due to voltage application in the electrolyte;
Figure 2 is an electron micrograph showing how corrosion progresses due to the electrolytic solution, Figure 3 is a diagram showing the influence of solution concentration on the applied voltage required to obtain the same current density, and Figure 4 is a diagram showing the effect of solution concentration on the dissolution rate. A diagram showing the influence of liquid concentration, FIG. 5 a diagram showing the influence of PH on dissolution rate, and FIG. 6 a diagram showing the influence of PH on the applied voltage required to obtain the same current density. Figure 7 is a diagram showing the influence of liquid composition on changes in pH value, Figure 8 is a diagram showing the influence of liquid temperature on dissolution rate, Figure 9 is a schematic diagram of the AC electrolytic surface decontamination equipment, FIG. 10 is an electron micrograph showing an oxide film in which cracks were generated due to the pretreatment. In the drawings, 1... fluororesin beaker,
2... Treated material, 3... Fluorine resin coated heater, 4... Fluorine resin coated thermometer, 5...
...Ultrasonic cleaning machine, 6...Voltmeter, 7...
... Ammeter, 8 ... Slider.

Claims (1)

[Scope of Claims] 1. Zr and Hf having an oxide film, and their alloys, at an ammonium fluoride concentration of 0.1 to 0.5 mol, and an ammonium salt of citric acid, oxalic acid, or tartaric acid. In an electrolytic aqueous solution whose pH is maintained at 5 to 6 by adding one or more of sodium salt, and potassium salt, current density: 0.02 to 0.5 A/cm^2
A method for removing a surface oxide film of Zr, Hf, and alloys thereof, the method comprising performing alternating current electrolysis. 2 Zr and Hf having an oxide film, and their alloys, ammonium fluoride concentration: 0.1 to 0.5 mol, ammonium nitrate concentration: 1/1 of ammonium fluoride
00 to 1/5 concentration, and citric acid, oxalic acid,
Or, in an electrolytic aqueous solution whose pH is maintained at 5 to 6 by adding one or more of ammonium salt, sodium salt, and potassium salt of tartaric acid, current density: 0.02 to
Characterized by AC electrolysis at 0.5A/cm^2,
A method for removing a surface oxide film of Zr, Hf, and alloys thereof. 3 Zr and Hf having an oxide film, and their alloys, ammonium fluoride concentration: 0.1 to 0.5 mol, hydrogen peroxide concentration: 1/10 to 1 of ammonium fluoride.
/2 concentration, and in an electrolytic aqueous solution whose pH was maintained at 5 to 6 by adding one or more of ammonium salt, sodium salt, and potassium salt of citric acid, oxalic acid, or tartaric acid, Current density: 0.02-0.5A
A method for removing a surface oxide film of Zr, Hf, and alloys thereof, characterized by carrying out alternating current electrolysis at /cm^2. 4 Zr and Hf having an oxide film, and their alloys, ammonium fluoride concentration: 0.1 to 0.5 mol,
Ammonium nitrate concentration: 1/10 of ammonium fluoride
0 to 1/5 concentration, hydrogen peroxide concentration: 1/10 to 1/2 concentration of ammonium fluoride, and ammonium salt, sodium salt, and potassium salt of citric acid, oxalic acid, or tartaric acid. pH by adding one or more
Current density: 0 in an electrolytic aqueous solution maintained at 5 to 6.
．． A method for removing a surface oxide film of Zr, Hf, and alloys thereof, characterized by carrying out alternating current electrolysis at 02 to 0.5 A/cm^2. 5 Zr and Hf having an oxide film, and their alloys, with ammonium fluoride concentration: 0.1 to 0.5 mol, and ammonium salt, sodium salt, and potassium of citric acid, oxalic acid, or tartaric acid. AC electrolysis at a current density of 0.02 to 0.5 A/cm^2 in an electrolytic aqueous solution to which pH 5 to 6 is maintained by adding one or more types of salts and ultrasonic vibrations are applied. A method for removing a surface oxide film of Zr, Hf, and alloys thereof, characterized by: 6 Zr and Hf having an oxide film, and their alloys, ammonium fluoride concentration: 0.1 to 0.5 mol, ammonium nitrate concentration: 1/1 of ammonium fluoride
00 to 1/5 concentration, and citric acid, oxalic acid,
Or, by adding one or more of ammonium salt, sodium salt, and potassium salt of tartaric acid, the pH is maintained at 5 to 6, and in an electrolyte solution to which ultrasonic vibration is applied, the current density is 0.02 to 0. A method for removing a surface oxide film of Zr and Hf and their alloys, the method comprising performing alternating current electrolysis at .5 A/cm^2. 7 Zr and Hf having an oxide film, and their alloys, ammonium fluoride concentration: 0.1 to 0.5 mol, hydrogen peroxide concentration: 1/10 to 1 of ammonium fluoride
/2 concentration, and by adding one or more of ammonium salt, sodium salt, and potassium salt of citric acid, oxalic acid, or tartaric acid, the pH is maintained at 5 to 6, and ultrasonic vibration is applied. A method for removing surface oxide films of Zr, Hf, and alloys thereof, characterized by carrying out alternating current electrolysis in an electrolytic aqueous solution at a current density of 0.02 to 0.5 A/cm^2. 8 Zr and Hf having an oxide film, and their alloys, ammonium fluoride concentration: 0.1 to 0.5 mol, ammonium nitrate concentration: 1/1 of ammonium fluoride
00 to 1/5 concentration, hydrogen peroxide concentration: 1/10 to 1/2 concentration of ammonium fluoride, and citric acid,
P by adding one or more of ammonium salt, sodium salt, and potassium salt of oxalic acid or tartaric acid.
Current density: 0.02 to 0.5 A/
A method for removing a surface oxide film of Zr, Hf, and alloys thereof, characterized by performing alternating current electrolysis at cm^2.