JP2000144442A - Passivating treatment of stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To strengthen a passivating film and to improve corrosion resistance by immersing a stainless steel in a sulfate solution containing Ce(IV) ion of specific concentration, washing it in water, and then drying it. SOLUTION: An aqueous solution of ceric ammonium sulfate dihydrate [Ce(NH4)4(SO4)4.2H2O] is prepared so that 0.2-2.5% of Ce(IV) ions are contained. This sulfate solution is held at about 40 to 60 deg.C, and a stainless steel is immersed in this solution or this solution is applied to the stainless steel into liquid film state to carry out passivating treatment for about >=10 min. Then, the stainless steel is washed in water and dried. In this case, application into liquid film state is recommended for increasing the amount of oxygen supplied from the air and densifying the resultant passivating film (oxide film). As the means of application, application by the use of a brush, etc., or atomization by the use of an atomizer is adopted. By this method, passivating treatment can be performed with minimized harmfulness while simplifying the aftertreatment stage after passivating treatment using a strongly oxidizing acid.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a passivation treatment method for strengthening a passivation film and improving corrosion resistance when stainless steel is used in a chloride environment such as air, fresh water, and a chemical plant environment. is there.

[0002]

2. Description of the Related Art Passivation treatment methods for stainless steel are roughly classified into the following three methods. (1) A method of immersing in a solution containing nitric acid and other strong oxidizing agents. (2) A method using oxygen or low-temperature heating in clean air. (3) A method using anodic polarization in a solution containing an oxidizing agent.

[0003] Among them, the method according to (1) is general,
Nitric hydrofluoric acid (about 1% HF, 30% HNO _Three, Remaining H_TwoO),
Immerse in a highly oxidizing acid solution such as 10-30% nitric acid.
Or electrolytically treated ("Stainless Steel Handbook" 19
August 30, 1973, published by Nikkan Kogyo Shimbun, p. 84
6). Alternatively, JP-A-52-106333 discloses that
Contains 5-40% nitric acid and 0.5-2.0% hydrochloric acid
50 consisting of water and 0.1 g to 10 g / l of inhibitor
Immerse stainless steel in liquid at ~ 70 ° C for 30 ~ 90 seconds
And a passivation treatment method for stainless steel surface characterized by
It has been disclosed.

[0004] The structure of the passivation film on the surface of stainless steel has not yet been completely elucidated, but is essentially 10 mm thick, as represented by Cr ₂ O ₃ .NH ₂ O. ~
It is an oxide film of 30 ° (angstrom), which is considered to be a uniform amorphous thin chemically stable film such as glass. It is considered that the passivation treatment in nitric hydrofluoric acid or nitric acid forms a stable passivation film by concentrating the Cr element in the passivation film ("Stainless Steel Handbook, 3rd Edition", January 1995). 24, published by Nikkan Kogyo Shimbun, p. 427).

[0005] Among these passivation treatments, hydrofluoric acid (HF) is specified as a second-class substance such as a specified chemical substance in the Ordinance for Enforcement of the Industrial Safety and Health Act, and nitrogen in a mixture with nitric acid Since oxides are generated, it has been desired to develop a process that does not use these.

[0006]

SUMMARY OF THE INVENTION The present invention simplifies the post-treatment process of a passivation treatment using a strong oxidizing acid such as nitric acid according to the prior art, is less harmful, and uses a conventional passivation process using nitric acid. It is an object of the present invention to provide a passivation treatment method that provides an effect equal to or greater than that of passivation treatment.

[0007]

In order to achieve the above object, the present invention has the following constitution. That is,
(1) A method for passivating stainless steel, characterized by being immersed in a sulfate solution containing 0.2% to 2.5% of Ce (IV) ions, washed with water, and dried, and (2) A) passivation method for stainless steel, which comprises applying a sulfate solution containing 0.2% to 2.5% of Ce (IV) ions in a liquid film state, followed by washing with water and drying.
And (3) after the final atmosphere annealing or the final annealing pickling of the stainless steel strip, Ce (IV) ions are added in an amount of 0.2% to 2.5%.
% Passivation treatment of a stainless steel strip, characterized by repeating spraying or immersion drying of a sulfate solution containing not more than 10% by weight, followed by washing and drying.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. First, the reason why Ce (IV) ion was selected is that the oxidation-reduction potential represented by the following chemical reaction formula is high. Ce ⁴ ++ e ⁻ ＣCe ³⁺ (1.74 V vs NHE) Furthermore, relatively high safety and no harmful effects such as Cr (IV) having a high redox potential have not been reported. ).

Next, Ce (IV) ions are added in an amount of 0.2% or more.
The reason specified below 5% will be explained based on experiments. An aqueous solution in which the amount of added ammonium cerium sulfate dihydrate was changed was prepared, and SUS304 was added in each solution at 50 ° C.
(No. 600 emery polished paper wet finish) for 10 minutes, then JIS G05 in 3.5% NaCl solution at 30 ° C.
The pitting potential was measured by the method of No. 77. The results are shown in FIG.

FIG. 1 shows that the higher the pitting potential, the better the passivation and the higher the corrosion resistance. As a result of the measurement, the pitting potential is dependent on the Ce (IV) concentration. And 0.4 V or more at 0.2% or more.
2% or more. In addition, as is clear from the figure, a more desirable value is 1% or more. Also, add weight 2.5%
The upper limit is set to 2.5% because if it exceeds this, the oxidizing action is too strong and the surface tends to corrode.

The reason for washing with water and drying after treatment with a Ce (IV) solution is that substances such as Ce (IV) ions and sulfates remaining on the surface accelerate the corrosion of the stainless steel matrix in the use environment. This is to prevent adverse effects.

Although the temperature of the immobilization treatment is not particularly specified,
40-60 ° C is desirable. If it is too high, the oxidizing power of the solution becomes strong and the passivation film is easily broken, so that the surface of the stainless steel may be dissolved and the skin may be roughened. On the other hand, if it is too low, it takes a long time to complete the passivation treatment, which is not industrially preferable.

Although the time for the passivation treatment is not specified,
It is appropriately selected according to the temperature and the concentration. For example, if the temperature is 40 to 60 ° C., it is desirable that the heating time be 10 minutes or more.

The stainless steel is immersed in a solution under the above conditions or applied in a liquid film state. In this case, when applied in a liquid film state, the supply amount of oxygen from the atmosphere can be increased, so that the passivation film (oxide film) can be made denser. As a method of applying the aqueous solution in a liquid film state, any of a method of applying with a brush or the like, a method of spraying with a sprayer, and a method of repeating exposure in the air while immersing (repetition of wet and dry) may be used.

By performing the passivation treatment on the stainless steel strip as described above after the final atmosphere annealing step or the final annealing pickling step, a passivated stainless steel strip can be obtained.

[0016]

(Example 1) Table 1 shows the pitting potential (JIS G 0577) of the passivated stainless steel sheet in 3.5% NaCl according to the present invention and the passivation treatment. The pitting corrosion potential of the stainless steel sheet not subjected to the passivation is shown in comparison.
It is clear that the pitting corrosion resistance is improved by the passivation treatment method of the present invention. Also,
The pitting potential after passivation treatment using nitric hydrofluoric acid or nitric acid is 60 mV to 500 mV higher than the pitting potential of a stainless steel sheet not subjected to passivation treatment. It is clear that it has the same or better effect as hydrofluoric acid and nitric acid.

[0017]

[Table 1]

(Example 2) Table 2 shows the artificial seawater spray (35 ° C, 4 hours) of the passivated stainless steel strip and the non-passivated stainless steel sheet according to the present invention. Dry (60 ° C., 2 hours) -wet (95% RH, 5
0 ° C, 2 hours), the results of a combined cycle corrosion test (3 cycles) are shown. The passivated stainless steel sheet has a rating of 1 according to the Stainless Steel Association rating number.
It is apparent that the rust-casting resistance is improved by the passivation treatment method of the present invention.

[0019]

[Table 2]

[0020]

According to the passivation treatment method of the present invention, the passivation film can be strengthened more than the conventional method using nitric hydrofluoric acid or nitric acid. This is a method using a solution, and the present invention can be widely applied to a stainless steel strip manufacturing process and an outdoor stainless steel device assembly and installation site.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the amount of Ce (IV) ions in a passivation solution and the pitting potential.

Claims (3)

[Claims] 1. The method according to claim 1, wherein the content of Ce (IV) ions is 0.2% or more and 2.5% or more.
A passivation treatment method for stainless steel, comprising immersing in a sulfate solution containing the following, washing with water, and drying. 2. The method according to claim 1, wherein the content of Ce (IV) ions is 0.2% or more and 2.5% or more.
A passivation treatment method for stainless steel, comprising applying a sulfate solution contained below in a liquid film state, washing with water, and drying. 3. A passivation treatment method for a stainless steel strip, which is carried out by the method according to claim 1 after the final atmosphere annealing or the final annealing and pickling in the stainless steel strip manufacturing process.