JPS5926668B2 - Stainless steel surface processing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a strong golden to brown colored film on the surface of stainless steel that is less likely to cause color unevenness due to differences in dry and wet conditions.

Conventional methods for producing a colored film on the surface of stainless steel include dichromic acid oxidation method, sulfurization method, alkali oxidation method,
Although alkaline electrolysis methods and the like have been proposed, the method that has been put into practical use is mainly a method in which stainless steel is immersed in an aqueous sulfuric acid solution containing hexavalent chromium ions.

However, with this method that includes chromium ions, the color tone changes rapidly depending on the immersion time, from gray to brown to reddish-purple to yellow. It is very difficult to obtain it stably. On the other hand, a method of immersing stainless steel in an aqueous solution containing vanadate has also been proposed. For example, "Metal Surface Technology Handbook" (edited by the Metal Surface Technology Association, 1962 edition, 82
6) describes the acidic black oxidation method using 25 parts of sulfuric acid and 14 parts of vanadate (approximately 138 to 18 parts as pentavalent vanadium).
29/l) and 20 parts of water.
A method of processing at 0 to 100°C is shown. However, according to this method, the liquid may become slurry or solidify, making it difficult to handle. Furthermore, the colored film obtained by this method is thinner and more prone to fading and discoloration than when using a chromium-based coloring solution.
It also has the disadvantage of being prone to peeling and ulceration, and lacking in durability. Furthermore, a heated solution of sulfuric acid or a mixed acid of sulfuric acid and an inorganic acid other than hydrochloric acid in which vanadate, an oxidizing agent having a nitrate group, an oxidation aid having a chromate group, and a chelating agent consisting of a chain polymeric phosphate are dissolved. Method of immersing in
No. 18241) is also known.

In this liquid composition, vanadate is 2 to 40 fl/l (equivalent to about 0.9 to 17.49 fl/l as pentavalent vanadium)
It is. However, since this method also contains chromium ions, the color tone changes greatly depending on the immersion time, making it difficult to set the conditions to obtain the desired color tone, and the color tone of the obtained colored film changes between when it is dry and when it is wet. It had the disadvantage that it looked like The present invention creates a stable brown coating on the surface of stainless steel with a uniform color that is robust and does not change due to external conditions, by applying a transparent resin coating after a short period of treatment in an acid bath containing vanadium ions. The purpose was to provide a method for generating

That is, the first invention of the present invention is 20 to 65 f as pentavalent vanadium! /2 and a mixed acid of sulfuric acid or an inorganic acid selected from sulfuric acid and phosphoric acid, nitric acid or hydrochloric acid 450-12
009/l of stainless steel in a heating liquid containing 3 to 2
After immersing for 0 minutes to generate a colored film on the surface,
This is a stainless steel surface treatment method characterized by applying a transparent resin coating.

As mentioned above, with the conventional chromium-based coloring method, the change in color tone is large and the speed is fast, making it difficult to set the conditions and easily causing color unevenness.However, the method of the present invention produces a highly marketable golden yellow or brown color. Conditions are easy to set, and can be obtained in a short time, with little color unevenness.

The reason why color unevenness occurs less in the colored film of the present invention is thought to be due to the difference in the oxidizing power of the oxidizing agent in addition to the above. In other words, hexavalent chromium has a strong oxidizing power and therefore reacts rapidly, causing localized unevenness of reaction, whereas pentavalent vanadium has a weaker oxidizing power than hexavalent chromium, so it reacts more rapidly than chromium. It is thought that the process progresses slowly and evenly, producing a uniform film with little color unevenness. The pentavalent vanadium used in the present invention is metavanadate (Claw 03) or KVO3,N
aO3, Na3VO4, NH4VO3, (NH4)4V
It is practical to add it as an alkali or ammonium vanadate such as 40.2.

The concentration of pentavalent vanadium ranges from 20 to 659/l.

The lower limit of 209/l was determined from the following experimental results.

That is, Table 1 shows the color tone results of treatments with sulfuric acid concentration of 4509/2 and vanadium concentrations of 17.5 and 20.09/l at 80°C, and Figure 1 shows the color tone results for sulfuric acid concentration of 8009/1.
, vanadium concentration 17.5 and 20.09/l at 100℃
This figure shows the results obtained using a photoelectric colorimeter to determine the color difference with respect to the target color tone when treated with the composition liquid. From these results, it can be seen that a vanadium concentration of about 20 g/l is the critical point in terms of the degree of coloring and variation (stability) in the color tone of the colored film.

Furthermore, when the coloring operation according to the present invention is carried out in a continuous manner in which a fixed amount of liquid is replenished rather than in a batch manner in which aged liquid is replaced over a certain period of time, the vanadium concentration must be 259/l or more. is desirable.

On the other hand, the higher the vanadium concentration, the more effective it is in coloring, but there is a limit from the viewpoints of handling and economy, and the upper limit is about 659/l. In sulfuric acid with a concentration of 8009/2, even if this sulfuric acid is heated to the boiling point (124°C), it is impossible to dissolve more than 709/l of vanadium. Although the amount increases, the viscosity of the liquid increases, making it difficult to handle, and it is not economical because the amount of liquid that adheres to colored stainless steel equipment and the reduction reaction of vanadium increases.

Furthermore, as the change in color tone becomes more severe, undesirable phenomena such as this occur. The concentration of sulfuric acid or a mixed acid of sulfuric acid and an inorganic acid can be from 450 to 12009/j; if it is less than 4509/j, coloring is difficult, and if it is more than 12009/l, it is unfavorable for handling.

The inorganic acids used in the mixed acid are phosphoric acid, nitric acid, and hydrochloric acid, but the mixing ratio with respect to sulfuric acid is preferably 5% or less. Depending on the stainless steel material used, a mixture of these inorganic acids can give good coloring results. The higher the processing temperature, the easier it is to color. (If the temperature exceeds 100°C, evaporation will occur, concentration of the liquid and reduction reaction of vanadium will increase, and more mist will be generated, making the working environment worse. Below, the temperature is particularly preferably about 90°C.

If it is below 70°C, the processing time will be long and it is not practical.

In addition, the treatment time may be selected as appropriate depending on the liquid composition, treatment temperature, and desired color tone of the colored film, but the treatment time is shorter than the conventional method using vanadate, and generally Achieved in 20 minutes.

The color of the colored film can vary from golden yellow to brown by adjusting the liquid composition, treatment temperature, and treatment time.

Moreover, as described above, unlike chromium-based coloring liquids, there is little change in color tone and the reaction is slow, so it is possible to stably obtain the desired color. By further coating the colored film with a transparent resin, the present invention not only physically protects the disadvantages associated with the thickness of the film inherent to vanadium-based coloring liquids, but also improves the color of the colored film. It was found that the effect of eliminating unevenness, especially the change in color tone during drying and wetness, was obtained.

Moreover, this color unevenness erasing effect is a feature that is observed only in the vanadium-based colored film according to the present invention, and is not observed even when a chromium-based colored film is coated with a transparent resin.
Furthermore, the durability of the film after coating with a transparent resin was also superior to the vanadium-based colored film according to the present invention than to the conventional chromium-based colored film, indicating good adhesion to the resin coating. Therefore, according to the coloring method of the present invention,
Compared to conventional chromium-based coloring methods, a stable color tone can be obtained even at the stage of forming a colored film, but the above-mentioned color unevenness erasing effect is added by subsequently applying a transparent resin coating. , the color tone of the final finished product is unprecedented.

This improves the yield of the product and is particularly advantageous when coloring is carried out on a large scale, where strict control is difficult. Figure 2 shows the results of an experiment to see the effect of removing color unevenness by transparent resin coating.

FIG. 2 is a graph showing the color tone of the colored film according to the present invention and the film obtained by applying clear coating to this film as a color difference with respect to the target color tone. As is clear from this graph, the color difference after clear coating is smaller than that before coating, and therefore, it can be seen that color unevenness is reduced. The transparent resin that can be used can be selected from commercially available acrylic resins, amino resins, epoxy resins, etc. that are suitable for the purpose of the stainless steel colored product.

Furthermore, even if it is not completely transparent, it is also possible to use a material that does not lose the original purpose of coloring, such as a matte clear coating resin. Furthermore, a second invention of the present invention is a method using a heating liquid in which less than half of the vanadium content of the first invention is replaced with hexavalent molybdenum.

According to the second invention, a highly glittering colored film with less color unevenness can be stably obtained than when vanadium is used alone. The reason for the reduction in color unevenness and the appearance of brightness when molybdenum is mixed is not well understood, but hexavalent molybdenum has an oxidizing power, although it is weaker than pentavalent vanadium, so it has a buffering effect on oxidizing power. It is thought that it plays a role. The hexavalent molybdenum used in this second invention is Na2MOO4
- Molybdates such as 2H20, (NH4)6M070244H20.

The concentration of hexavalent molybdenum is less than half of the amount of vanadium in the case of vanadium alone, preferably less than 1/3, but if more than half of the amount of vanadium is replaced, coloring becomes extremely poor.

In both the first and second inventions described above, a cyclic condensed phosphate is used as a chelating agent at 109
By adding less than /l, good results are obtained in stabilizing the color tone and extending the life of the colored liquid.

As is clear from the above description, according to the present invention, by immersing stainless steel in an acid bath containing vanadium ions for a relatively short time, the surface of the stainless steel is coated with a solid and uniform brown color. A colored film can be stably produced, and by further applying a transparent resin coating to the produced colored film, it not only has the protective effect of the colored film, but also
It is possible to prevent color tone changes during drying and wetting, and to further improve the effect of erasing color unevenness. Examples of the present invention will be shown below regarding the case of coloring a SUS27 stainless steel plate.

Example 1 A stainless steel plate was immersed for 7 minutes in an aqueous solution of metavanadate 1009/11 sulfuric acid 4509/l and ultraphosphate 59/l heated to 85°C to form a colored film, and then thoroughly washed with water. , dried, and finished by spraying with a commercially available organic solvent type acrylic resin paint.

A stainless steel plate with a uniform golden color was obtained, and there were no defective products with uneven color due to differences in dry and wet conditions. This product was subjected to a salt spray test (5% saline solution, 35°C for 480 hours) and an accelerated weathering test (480 hours), and no abnormalities such as discoloration or fading were observed in either case. [Comparative example] A stainless steel plate was heated to 85°C in a solution containing 1259% of chromic anhydride, 2459% of sulfuric acid, and 3459% of water.
After being immersed for a minute to form a colored film, it was washed with water in the same manner as in Example 1, dried, and then coated with resin, and subjected to a salt spray test and an accelerated weathering test.

Color unevenness often occurred due to differences in dry and wet conditions after coloring, and this color unevenness did not disappear even with resin coating, and 8 out of 10 sheets were rejected based on the colored products of the present invention. In addition, slight fading of color was observed in the salt spray test results. Example 2 Stainless steel was immersed in a solution containing 759/11 sodium metavanadate, 509/11 sodium molybdate, and 10,009/l sulfuric acid heated to 80°C to form a colored film, and after thorough washing and drying, it was commercially available. Finished by dip coating with water-soluble acrylic resin.

In Example 1, a uniform golden-yellow stainless steel plate with brightness was stably obtained, and there were no defective products due to uneven coloring. This product was subjected to a salt spray test and an accelerated weathering test in the same manner as in Example 1, and no abnormality was observed. Example 3 In the test shown in Example 2, in which the immersion time in the colored liquid was extended to 20 minutes, uniform brown stainless steel was stably obtained, and the same defects due to color unevenness as in Example 1 were avoided. There were no good products.

Example 4 Unlike Examples 1 to 3, SUS24 stainless steel plate was
Potassium metavanadate 75f1/11 Sodium molybdate 509/11 Ultraphosphate 39/l, sulfuric acid 8009/l, nitric acid 509/2 concentration was immersed in a solution heated to 85°C for 10 minutes to color it, and the water-soluble Finished with acrylic resin paint.

Stainless steel with a uniform golden color was obtained, and there were no defective products due to color unevenness.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between vanadium concentration and color difference with respect to the target color tone, and FIG. 2 is a graph showing the change in color difference with respect to the target color tone before and after clear coating.

Claims (1)

[Claims] 1. Stainless steel is added to a heating liquid containing 20 to 65 g/l of pentavalent vanadium and 450 to 1200 g/l of sulfuric acid or a mixed acid of sulfuric acid and an inorganic acid selected from phosphoric acid, nitric acid, or hydrochloric acid. A method for surface processing stainless steel, which comprises soaking the steel for 3 to 20 minutes to form a colored film on its surface, and then applying a transparent resin coating. 2 20 to 65 as pentavalent vanadium and hexavalent molybdenum (however, molybdenum is less than half of vanadium)
Stainless steel is immersed for 3 to 20 minutes in a heating solution containing 450 to 1200 g/l of sulfuric acid or a mixed acid of sulfuric acid and an inorganic acid selected from phosphoric acid, nitric acid, or hydrochloric acid to form a colored film on its surface. A method of surface processing stainless steel that is characterized by applying a transparent resin coating after forming a stainless steel.