JPS63317680A - Formation of chromium oxide film having excellent resistance to heat and corrosion - Google Patents

Abstract

PURPOSE:To form the title tightly adhered chromium oxide film having excellent resistance to heat and corrosion by coating the ceramics slurry regulated by an aq. soln. of chromic acid on a base material, heating the material, and further impregnating the material with an aq. chrome oxide soln. contg. ammonium chromate. CONSTITUTION:The powder of SiO2 and Al2O3 is added to the chrome oxide soln. to prepare a slurry, and the slurry is coated on the surface of a metal and then heated to 500-600 deg.C to form a porous film. The film is impregnated with the aq. chrome oxide soln. added with <=10wt.% ammonium chromate or ammonium bichromate, and heated to 500-600 deg.C. The impregnation with chromic acid and heating are repeated. The particle diameter of the SiO2 and Al2O3 is preferably controlled to 1.0-2.0mum. The chromium oxide film obtained by this method is dense, and has a high bonding strength and excellent resistance to heat and corrosion.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention provides heat resistance and corrosion resistance suitable for surface modification of mechanical structural members or rolls used in high temperature environments and corrosive atmospheres. The present invention relates to a method for forming a chromium oxide-rich film.

(Prior art) Conventionally, chromic anhydride aqueous solution and 5i02. Al2O3
The first method for forming a fired film using powder is
The process shown in the figure is known. The aqueous solution of chromic acid anhydride refers to a solution in which crystals of chromic anhydride are dissolved in water; that is, after degreasing the metal material to be treated, a blasting process is performed to make the surface suitable for this treatment. This was mixed with an aqueous solution of chromic acid anhydride (hereinafter simply chromic acid) and SiO□・Al2O.
A slurry layer composed of powder is made by dipping or coating, and after drying to evaporate water,
Heat at 0-600°C for about 1 hour. Through this firing process, the fired product of chromic acid is made into a matrix on the surface of the metal material, and a 5in2. A film with Al2O3 as aggregate is formed. However, since this film is porous, it has weak adhesion with the total bending material, weak bonding strength of the film itself, and poor corrosion resistance. After impregnating with acid, this is again
The operation of heating to 00 to 600'C is repeated. By repeating this impregnation and heating, the pores are filled with a fired product of chromic acid (mainly (:r20:1)), and
A dense fired layer is also formed on the surface. However, even if this impregnation and heating are repeated, it is difficult to fill the pores and form a dense film on the surface, so it is necessary to repeat the impregnation and heating operations many times.

In addition, the conventional technology regarding impregnation liquid (Special Publication Publication No. 49-28
011. 51-18445, 55-14833. 56-:16215゜41-18708 and JP-A-5
9-47:182. 59-31867, 59-20
5480. 57-196779. 58-19729
3 If you look at >, you will find chromic acid, chromium chloride, and chromium nitrate.

Knowledge about the effects of various chromates such as chromium acetate and chromium sulfate is known, but even when these are used, the effects are insufficient. Furthermore, the use of ammonium chromate (Japanese Patent Publications No. 55-14833 and No. 41-18708) has also been tested, but its effect is to fill the pores in the fired film, but it does not produce a dense and hard film on the surface. It is said that it is impossible to do so, and current technology has not solved it.

(Problems to be Solved by the Invention) The conventional processing method in which a slurry of SiO□.Al2O3 powder is added to an aqueous chromic acid solution and the aqueous chromic acid solution is used for impregnation and heating has the following problems.

■When the particle size of SiO3/Al2O3 is large (usually 2
, 5 to 5 μm) Since the pores formed in the film after firing become larger, the contact area between the material to be treated (heat-resistant stainless steel) and the film becomes smaller, which is the cause of low adhesion between the two.

■5in2・Al2O3 distributed in the film after firing
When the particle size is large, the contact area between the particles and between the particles and the fired chromic acid product decreases, which is the cause of weak mutual bonding force.

■On the other hand, 5i02. When the Al2O3 particle size is reduced, the pores in the fired film become smaller, and the pore distribution becomes more even than that of coarse particles. There were drawbacks such as insufficient filling and inability to form a dense film on the surface of the fired film. For this reason, the impregnation #heating operation is performed from 10 to
The reality is that it is repeated 20 times.

■Repeated impregnation #heating causes expensive energy and labor costs.

(Means for solving the problem) In the present invention, the above-mentioned problem was solved by adopting the following method.

(1) Ammonium chromate [
(NH<)*Cr0n] or ammonium dichromate [(N
II4) 2Cr207] is added in a predetermined amount.

(2) SiO□ and Al2O, reducing the particle size,
Increase the number of particles per unit weight.

(Function) (1) When the impregnating liquid is an aqueous chromic acid solution to which ammonium chromate or ammonium dichromate is added, the following phenomenon occurs. In other words, ammonium chromate and ammonium dichromate begin to decompose around 190'C.
Ammonia is released and chromium oxide (Cr20
3) Generate fine particles. These fine particles become dispersed and filled in the few pores in the fired film, and when the temperature is further increased, from around 200℃, 1 weight of ammonium chromate and chromic acid coexisting with ammonium romate become the main component. Since the chromic acid itself melts, it acts like an adhesive and binds Sin and He1203 particles and Cr2.
0r become fixed to each other. Cr, O produced by decomposition of ammonium chromate and ammonium dichromate
Since the particles are fine particles of Igm or lower, even if they exist in the pores in the coating, they absorb the thermal and mechanical impact of the coating n9c and exhibit the effect of preventing the coating from being destroyed. Furthermore, when corrosive components enter from the outside, the effect of reducing the pore volume suppresses the amount of infiltration and improves corrosion resistance.

Furthermore, as is clear from the mechanism of action, this method using ammonium chromate and an aqueous chromic acid solution to which ammonium chromate has been added is effective against SiO□.

Even if the Al2O3 particles are somewhat large, they can penetrate into the pores formed in the fired film and fill them with fine Cr2O3 particles, which has the effect of expanding the range of use of SiO□, Al2O, and particle size.

When the pores in the fired film are filled with fine powder of Cr2O3,
The chromic acid aqueous solution supplied in the subsequent impregnation/heating step is used to form a dense fired layer on the surface of the film. In this case as well, if a chromic acid aqueous solution containing ammonium chromate and ammonium dichromate is used, each ammonium salt decomposes quickly during the temperature rising process to produce fine particles of chromium oxide (Cr203), which form the aggregate. As a result, the film formation rate is fast, and a dense and hard film is produced. In our investigation, we found that the object of the present invention can be achieved when the amount of ammonium chromate and ammonium dichromate added is within 1 oz by weight relative to the aqueous chromic acid solution. If it exceeds 10%, the film becomes brittle.

As described above, in the present invention, SiO□
, AI, 03 Even if the particle size was reduced, the filling of pores was insufficient, or it was difficult to form a dense film on the film surface. By using this, this problem was solved.

(2) By reducing the particle size of 5iO7 and Al2O3, the number of particles per unit weight increases. For example, if these particles are assumed to be spherical and the particle diameter, surface area ratio, and particle number ratio are calculated, the approximate relationship is as follows. In other words, if the surface area and number of particles of a particle with a particle diameter of 4.4 μm are each 1, then the particle diameter (μm) and the surface area ratio (4πr 2) 4.4
12.0 2
．． 2 1.0 4.40.2
22 Particle number ratio (4πr 3/3) 10.600 The particle diameter of 2.04 m is 2.2 and 11, lJ
The particle diameters of Lm are 4.4 and 85. In this way, even if the particles have the same weight, when they are made into fine particles, the surface area ratio and the particle number ratio become extremely large. In particular, the increase in the number of particles in the latter is remarkable;
It is uniformly dispersed in the fired film and increases the contact area with chromic acid, which has a great effect on strengthening the bonding strength.

Furthermore, even in the case of fine particles, it is difficult to fill the pores and form a dense film on the surface using the conventional method using chromic acid. By using a solution containing ammonium acid or ammonium dichromate, dense Cr, 0. In addition to forming a film, it is also possible to obtain a dense film on the inside. The effect of densification of the film due to this grain refinement is that SiO
□ and the average particle size of Al2O3 (cumulative value of particle size distribution is 5
(value when it becomes 0%) is significant at 2.0 gm or less.

This effect can be obtained even when the particle size is less than 1.0 gm, but it requires long grinding and kneading to make the particle size less than 1.071 m, which is not economical.
．． It is preferable to set it as 0-2.0ILm.

(Example) Example 1 The effects of the present invention were confirmed by the following experiments.

(1) Shape and dimensions of test piece (object to be treated) 100mm long
An SO8304 test piece measuring 50 cm in width and 5 cm in thickness was used as an object to be treated, and the fired film of the present invention was formed on its surface.

(2) Types and particle sizes of slurry particles Commercially available SiO□ and Al2O3 were mixed in a weight ratio of 9=1 as slurry particles.
After adding it to the aqueous chromic acid solution, it was kneaded with a magnetic ball mill to make the particles finer. The particle size was adjusted by the operating time of the ball mill, but in this experiment, the particles were reduced after 16 hours of operation. The average diameter distribution is 2.5
~3ILm, 48 hours average 1.5-2.0gm
There was. (Both values are when the cumulative value of particle size distribution is 50%) Hereinafter, the former will be referred to as coarse particles, and the latter will be referred to as fine particles.

(3) Film forming process and type of chromic acid aqueous solution The film forming process followed the usual process shown in FIG. 1 above. That is, after degreasing the S[JS:104 object to be treated with an organic solvent (xylene), it was blasted with alumina grid 524 to adjust the surface.

After that, it was immersed in the coarse grain and fine grain slurry produced by the method (2) above, and then gently pulled to coat the surface of the object to be treated.
A film of ~60 gm thickness was formed. Then 100-2
After evaporating moisture with heated air at 00℃, sso
The mixture was heated for 1 hour in an electric furnace maintained at . Through this treatment, a film with a porosity of 30 to SOS was formed on the surface of the object to be treated.

Next, the porous film was immersed in an aqueous solution of chromic acid to impregnate the pores of the film with chromic acid, and in this step, the conventional method and the method of the present invention were carried out separately. That is, <conventional method> 60 kg chromic acid aqueous solution (weight 2) vs. present invention>
■Chromic acid j1 contained in a 60-ton chromic acid aqueous solution
Added 3z of chromium and ammonium resistant to ¥ (weight 2
) ■3% of dichromic acid γ is added to the chromic acid μ contained in the 60z chromic acid aqueous solution (weight: 8%) After impregnation, each object to be treated is placed in an electric furnace at 550°C. After firing for 1 hour, it was taken out from the electric furnace and immersed again in an aqueous chromic acid solution. By repeating this operation 10 to 20 times, the voids formed in the fired film containing the 5i02 and A1□03 slurries were filled with chromium oxide, and a dense chromium oxide film was formed on the surface.

FIG. 2 shows a cross-sectional photograph of a fired film obtained by repeating impregnation and heating 10 times using the impregnating solution of the present invention. As is clear from this photograph, the pores in the film are filled, and a dense fired film is formed on the surface layer. SiO□, A
The film formed on the surface of fine particles (FIG. 2(b)) is very uniform, although it is not affected much by the size of the I□03 particles. Moreover, FIG. 3 shows a cross-sectional photograph of a product obtained by repeating impregnation and heating 10 times by the conventional method using coarse particles of SiO□ and Al2O3. In the conventional method, almost no film is observed on the surface of the fired film.

(4) Evaluation method The performance of the fired film of the present invention was evaluated by conducting the following tests.

(℃ Thermal Shock Test After leaving the SO3:104 test piece with a fired film formed in an electric furnace maintained at 770℃, 800℃, 900℃, tooo℃ for 30 minutes, pour it into tap water (20℃ ). Repeat this process of heating → pouring into water 10 times. 1
Thermal shock resistance of the film was evaluated based on the presence or absence of peeling of the fired film and its degree.

(2) Corrosion Resistance Test 5Z A cast test was conducted using a sulfuric acid aqueous solution (35°C) and JIS 8502, and the corrosion resistance was evaluated based on the time until the fired film peeled off.

(5) Test results - Thermal shock test results The test results are shown in Table 1. The following conclusions can be drawn from this result.

a. The conventional method is susceptible to thermal shock and easily peels off. This tendency is particularly noticeable when using coarse slurry.

b. The fine-grained slurry exhibited better performance than the coarse-grained slurry, and the thermal shock resistance improved as the number of impregnations with the chromic acid aqueous solution increased.

As shown in C0 and above, even in the conventional method, the heat resistance #N impact performance is improved as the fine particle slurry is used and the number of impregnations with the chromic acid aqueous solution is increased.

d. On the other hand, the method of the present invention, that is, the coating obtained by impregnating and baking a chromic acid aqueous solution containing ammonium chromate and ammonium dichromate, works effectively not only on fine-grained slurries but also on coarse-grained slurries. # Improved thermal shock performance. Particularly effective on coatings of fine slurry. No superiority difference was observed between ammonium chromate and ammonium dichromate, indicating that both are effective.

■Corrosion test results: The test results are shown in Table 2. As is clear from these results, the film fired by the method of the present invention can sufficiently withstand the corrosive conditions of sulfuric acid and Cath, forming an excellent corrosion-resistant film.

Table 2 Example 2 Impregnation: The influence of the amounts of ammonium chromate and ammonium dichromate added to the chromic acid aqueous solution for heating was investigated. The material and dimensions of the object to be treated and the film treatment process were the same as in Example 1, the slurry particles were limited to fine particles, and the composition of the chromic acid aqueous solution for impregnation and repeated heating was as follows. changed to.

Chromic acid aqueous solution for impregnation ■ 60% chromic acid aqueous solution (comparative solution) ■ Chromic acid ammonium l relative to the chromic acid content in the above solution
t 1, 3, 5, 10.15.20.30% addition (Tun-product-2) ■ Ammonium dichromate was added at 1, 3.5% relative to the chromic acid content in the above solution. Addition of +0.15.20.30% (same as above) (1) Evaluation method Using the aqueous chromic acid solution for impregnation, the slurry fired film was impregnated with 154; heated repeatedly, and the test piece was 8.
A water-cooled thermal shock test was conducted at 00° C. for 30 minutes to investigate the effects of adding ammonium chromate and ammonium dichromate.

(2) Thermal shock test results Table 3 shows the test results. This result is! It is expressed as the number of times when the total distance of 'A' of the film due to impact reaches 3z.

Thermal shock resistance improves as the concentration of ammonium salt increases, but if too much is added, the film formed on the surface of the fired film will become brittle, resulting in severe damage due to thermal shock! Lu1l becomes easier, and the performance deteriorates compared to an additive-free chromic acid aqueous solution. The amount of ammonium salt added is 1oz.
The following shows that the peeling phenomenon of the film is less likely to occur and that the thermal shock resistance is improved.

From the above results, it can be seen that ammonium salts should be added in a concentration range of 10z or less for both chromic acid and dichromic acid.

(Effects of the Invention) As described above, the method according to the present invention makes it possible to form a ceramic film that is dense and has strong bonding strength. This film has excellent heat resistance and corrosion resistance, and can be applied to a wide range of applications.

[Brief explanation of the drawings] Figure 1 shows a chromic anhydride aqueous solution and Sin, Al2O3
1 is a flowchart of a manufacturing process for forming a film using powder. Figure 2 shows the metal base material (SO3304) produced by the method of the present invention.
This is a photograph of the cross-sectional structure when a fired film is formed on the film, and (a) shows 5102. Al2O3 with conventional particle size, (
b) is 5i02. AIJi?2. Shows fine grains below OILm. Figure 3 shows the metal base material (SO
3:104) is a photograph of the cross-sectional structure when a film is formed.

Claims (2)

[Claims] (1) SiO_2 and Al_2O_ in chromic anhydride aqueous solution
3. After applying the slurry prepared by adding the mixed powder to the metal surface, it was heated to 500 to 600°C to form a porous film, which was then impregnated with an aqueous chromic acid anhydride solution and heated to 500 to 600°C. In the method for forming a chromium oxide film obtained by repeatedly performing the heating step to
A method for forming a chromium oxide film with high heat resistance and corrosion resistance, characterized in that the amount of chromium oxide added is 0% or less. (2) The average particle size of the SiO_2 and Al_2O_3 mixed powder (value when the cumulative value of particle size distribution is 50%) is 1.0
The method for forming a chromium oxide film according to claim 1, wherein the thickness is 2.0 μm.