JPH0372156B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a passivation treatment method for bright annealed (hereinafter referred to as BA) stainless steel, which is characterized by improving the surface properties of the stainless steel and imparting effective corrosion resistance. BA-treated stainless steel has a beautiful surface and is widely used in automobiles, bicycle parts, household appliances, kitchen appliances, building materials, etc. However, in the case of ferritic stainless steel in particular, the surface film formed by BA is Corrosion resistance cannot be said to be sufficient, and BA-finished materials tend to be inferior to those polished with emery abrasive paper or the like. Therefore, depending on the usage environment, it is necessary to improve the surface properties of BA-finished stainless steel to impart effective corrosion resistance. Passivation treatment of stainless steel by immersion in nitric acid is often carried out to improve corrosion resistance.
BA-treated stainless steel is no exception, and immersion in nitric acid tends to improve its corrosion resistance. However, the BA film itself is an oxide film with a thickness of about 50 Å, and simply immersing it in nitric acid does not sufficiently improve the properties of the entire film. In view of the above circumstances, the present inventors have conducted various studies and found that the best method is to first remove the BA film, which cannot be called a corrosion-resistant film, by alternating electrolysis in nitric acid, and then immerse it in nitric acid to passivate it. I came to the conclusion that there is. To dissolve and remove the BA film, electrolysis using the sample as an anode or pure chemical dissolution using acid can be considered, but in either case, only the film on specific crystal grains or the film on grain boundaries is selectively removed. A phenomenon occurs in which the film on certain crystal grains hardly dissolves, and the beautiful appearance characteristic of BA finish is lost.
This results in a whitish, dull appearance and causes variations in corrosion resistance even after passivation. Even if it takes a long time to dissolve and remove the film, once the film starts dissolving in a specific place, it will not move on to dissolving the film in other places even after the film has finished dissolving.
In other words, if the base material in certain areas is simply melted down, the appearance becomes worse and the variation in corrosion resistance only increases. Regarding the dissolution behavior during alternating electrolysis, during the first anodic electrolysis, only the coating on specific crystal grains or the coating on crystal grain boundaries begins to selectively melt. However, when a pause occurs and cathodic electrolysis occurs, the pause plays the role of passivating the dissolved area, keeping it at a higher potential than the previously undissolved area, so that it will not dissolve during the next anodic electrolysis, and this By repeating the process, the entire surface will be uniformly dissolved. Also, during cathode electrolysis, the BA film, which is an oxide film, is partially reduced. During alternating electrolysis, the polarity of the plates may initially be either, but it is necessary to arrange the plates so that the final polarity is the same, and to energize the plates alternately so that the polarity is at least twice. The current application time is 0.5 to 10 seconds at each pole, and the total is within 120 seconds, but the shorter the current application time at each pole, the more effective it is. The current density is 5 to 300 mA/cm ² and the liquid temperature is from room temperature to 80°C. If the nitric acid concentration is less than 5 wt%, the passivation ability will be insufficient, and if it exceeds 30 wt%, the workability will deteriorate and the manufacturing cost will become unnecessarily high, so it is set at 5 to 30 wt%. The liquid temperature and concentration of the nitric acid immersion treatment are in the same range as in the case of the alternating electrolytic treatment, and the treatment time is generally 30 seconds to 60 minutes. Hereinafter, the present invention will be explained based on examples. BA material of SUS430 was prepared under the conditions shown in Table 1.
Electrolytic immersion treatment in nitric acid was performed. The test number is the same as , but it is a single-bath process, in which alternating electrolytic treatment was performed in the first part of the treatment tank of the line, and immersion treatment was performed in the remaining part of the same treatment tank. This is a highly productive method when assuming an actual production line.

【table】

[Table] Table 1 shows the results of the rust resistance test and exposure test. From the results in Table 1, it can be seen that the rust resistance of the stainless steel treated with nitric acid electrolysis and immersion is extremely superior to that of BA stainless steel. When the surface film composition of Test No.
0.92 to 0.93, whereas the material treated with nitric acid electrolysis and immersion showed a remarkable enrichment of chromium of 1.74 to 1.95, and a passive film enriched with chromium was formed on the surface of the material treated with nitric acid electrolysis and immersion. It was confirmed that the material exhibited excellent rust resistance.

Claims (1)

[Claims] 1. Bright annealed stainless steel is subjected to intermittent alternating electrolysis in nitric acid with a concentration of 5 to 30 wt% so that the stainless steel is on the anode side at least twice, including the final electrolysis, and then immersed in nitric acid to make it inert. A method for passivating bright annealed stainless steel, which is characterized by converting it into a passive state.