JPS5847479B2 - Stainless steel surface treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for surface treatment of stainless steel in which electrochemical treatment is performed using stainless steel as a cathode to form a corrosion-resistant film on the surface.

A conventional surface treatment method of this type is chrome plating.

This chromium plating method is an electroplating method called -fi, which forms a surface film of pure chromium by electrodepositing metallic chromium on the surface of stainless steel.

This surface film has the advantage of not only corrosion resistance but also excellent gloss, and is used to treat stainless steels that are inexpensive but do not have sufficient corrosion resistance, such as SUS430 according to the JIS standard.

However, this chromium plating method uses a mixed solution of sulfuric acid and highly concentrated chromic acid as the treatment solution, which makes it difficult to dispose of the large amount of chromic acid contained in the chromic acid during wastewater treatment, which increases the cost of wastewater treatment. As the amount of pollution increases, the problem of pollution becomes more serious.

In addition, the chromium plating method requires a large current of several amperes/di or more, and the processing time is long, resulting in high power consumption and processing costs.

For this reason, as an alternative to chrome plating, electrochemical treatment is performed using stainless steel as a cathode.
A method of forming a corrosion-resistant film on the surface has been proposed.

For example, a method has been proposed in which stainless steel is electrochemically treated using a mixed solution of chromic acid, dichromic acid, etc., and phosphoric acid, etc. as a treatment liquid.

This method has the advantage that the chromic acid concentration in the treatment liquid is low, waste liquid treatment is easy, and corrosion resistance is equivalent to that of the chromium plating method even if the current application time and current density are short.

However, this method has the disadvantage that the sulfite resistance required for automotive stainless steel is poor, the stainless steel surface has poor gloss, and its commercial value is lower than that of the chrome plating method.

Based on this, the present inventor has improved the method by:
We previously proposed a method for cathodically treating stainless steel in a treatment solution containing phosphoric acid and molybdate.

According to this method, by adding molybdate to the treatment liquid, it is possible to improve the sulfite resistance and also to improve the gloss.

However, when implementing this method, it has been found that there are the following problems.

It takes a lot of time to dissolve the molybdate in the processing solution.

For example, when 0.5 weight φ of sodium molybdate, which is a type of molybdate, is dissolved in an aqueous solution (room temperature) of 20 weight or more phosphoric acid with stirring, it takes about 15 to 20 minutes.

Furthermore, during treatment, the impurities contained in the molybdate concentrate and the liquid deteriorates in a short period of time, resulting in a short liquid life.

The present invention has been made to improve the above-mentioned problems, and its main feature is to improve workability by using metallic molybdenum as the anode electrode instead of adding molybdate. and prevents liquid deterioration.
The object of the present invention is to provide a method for surface treatment of stainless steel that can extend the life of the liquid.

The method of the present invention will be explained in detail below.

In the present invention, stainless steel having a bright annealing film (hereinafter abbreviated as BA film) or a passive film obtained by chemical treatment, etc. is immersed in an aqueous solution of phosphoric acid of 0.1 to 50% by weight, and the stainless steel is Steel is used as the cathode side electrode, and metal molybdenum is used as the anode side electrode, and the current density is 0.1 to 5A/
dm"1 is a method of cathodic treatment of stainless steel under treatment conditions of a liquid temperature of 10 to 90°C.

This method can be applied to various stainless steels such as JIS standard SUS430, SUS434, and SUS304, but this method can be used to improve the corrosion resistance of SUS430, SUS420, etc., which are inexpensive but have low corrosion resistance. suitable.

However, in the present invention, BA is applied to stainless steel in advance.
It is necessary to form a film or a passive film.

Both the BA film and the passive film are types of films whose surfaces are inactive, and the BA film is a C r 2 film obtained by bright annealing.
It is a corrosion-resistant film with a thickness of several hundred layers, mainly composed of 0s or FeCr204.

Note that this BA film is different from a passive film.

The passive film can be left in the air or soaked in an aqueous solution containing 20 volumes of nitric acid for 30 minutes at room temperature or at 65°C for 10 minutes.
Obtained by soaking for about a minute.

The reason why the BA film and the passive film are formed in advance in the present invention is that unless this film is formed, the effect of the present invention of improving corrosion resistance cannot be exhibited during the subsequent cathodic treatment.

This was confirmed by experiments (described in Example 1) described later, and has not been fully theoretically elucidated.

Next, in the present invention, the stainless steel on which the above film has been formed is immersed in an aqueous solution of 0.1 to 50 parts by weight of phosphoric acid, and the stainless steel is used as a cathode side electrode, and metal molybdenum is used as an anode side electrode. Cathodize the steel.

The phosphoric acid in this treatment solution is necessary for forming a corrosion-resistant film on the surface of stainless steel.

Here, the reason why the phosphoric acid was set at 0.1 weight φ or more is because corrosion prevention effect cannot be obtained if it is less than this, and the reason why the phosphoric acid is set at 50 weight or less is because if it exceeds this, the gloss of the stainless steel surface will disappear and there will be loss of waste liquid. This is because it is thick.

The reason why metal molybdenum is used as the anode side electrode in this method is to cause metal molybdenum to be electrolytically eluted during electrolysis.

This metal molybdenum increases the sulfuric acid resistance of the corrosion-resistant film on the surface of stainless steel and maintains good gloss. High purity metal molybdenum is preferred to prevent deterioration of the processing solution and maintain improved corrosion resistance. It is.

The treatment conditions for this cathode treatment are as follows: current density: 0.1 to 5
A/dm, liquid temperature 10-90°C.

The reason why the electric melon is set in the above range is because of o. This is because if it is less than IA/dm, the electrolytic elution of metal molybdenum will be small and no anticorrosion effect will be obtained, and if it exceeds 5A/dm, surface gloss will not be obtained.

The reason why the liquid temperature is set in the above range is that if it is less than 10°C, the processing time will be longer and it will be difficult to maintain the temperature of the processing liquid, and if it exceeds 90°C, the processing liquid will deteriorate.

The processing time here may be short if the liquid is warm and highly wet and the current density is high, and if vice versa, a long time is required, but with the processing liquid and processing conditions of the present invention, it is usually 10 seconds to 10
Corrosion-resistant coatings can be formed in a processing time of minutes.

However, according to the present invention, since metal molybdenum is eluted into the treatment solution, a corrosion-resistant film that does not deteriorate even after processing and has excellent sulfite resistance is formed on the stainless steel surface, and the film has a glossy appearance. Maintained.

Therefore, it is possible to improve the corrosion resistance of stainless steel, which does not have sufficient corrosion resistance, and to increase its commercial value.

Furthermore, since the present invention elutes metal molybdenum from the anode side electrode, there is no need to elute molybdate salt in advance, and the workability is good.

Furthermore, since metal molybdenum has few impurities, it has the effect of preventing deterioration of the liquid and extending the life of the liquid.

This was confirmed by the following examples.

Example 1 As a stainless steel material, a JIS standard SUS430 brightly annealed to form a BA film was used, and this was used as a cathode, a metal molybdenum plate with a weight ratio of 99. It was processed in a processing solution containing

In this case, the electrolyte density was 2 A/d 7n'', the liquid temperature was 25°C, and the treatment time was 3 minutes (processing 1), and the electrolyte density was IA/dm, the liquid temperature was 60'C, and the treatment time was 1 minute. (Processing 2).

The stainless steel surface-treated in this way was subjected to a corrosion resistance test using the test method shown in Table 1, and the test results were used in the second test method.
Shown in the table.

Furthermore, in order to compare with the method of the present invention, 0.5 weight percent of sodium molybdate was further added to the treatment liquid shown in the example and the surface was treated with this treatment liquid. A corrosion resistance test was conducted and the results are also listed in Table 2.

Corrosion resistance tests were also conducted on stainless steel materials with BA coating, and the results are also listed in Table 2.

Further, a stainless steel material without either a BA film or a passive film was subjected to cathode treatment under the same conditions as in the above example.

The corrosion resistance test results are also listed in Table 2.

Further, in the above example, a similar surface treatment was performed using lead instead of metal molybdenum as the anode, and the results of the corrosion resistance test of the stainless steel obtained by this treatment are also listed in Table 2.

From the above table, it was found that the present invention significantly improved corrosion resistance.

Note that solutions containing sodium molybdate also have good corrosion resistance.

However, as is clear from Example 3, which will be described later, the processing liquid deteriorated rapidly.

Example 2 Next, the stainless steel (SUS43
0) The presence or absence of gloss was determined by comparing the surface gloss with the gloss of the stainless steel material on which the BA film was formed.

The results are shown in Table 3. In addition, for comparison, we will discuss an anode in which lead is used instead of metal molybdenum, a phosphoric acid composition that is outside the scope of the present invention, and a current density that is outside the scope of the present invention. The presence or absence of gloss was determined in the same manner.

The results are also listed in Table 3.

From the above table, it was confirmed that the method of the present invention provided excellent gloss.

Example 3 Next, the treatment in Example 1 (treatment 1) was carried out, and the deterioration of the treatment liquid was investigated.

As a result, no deterioration was observed even after 75 hours.

On the other hand, 0.5 weight φ of sodium molybdate was further added to the treatment solution shown in Example 1, and the treatment was performed using ferrite as an anode, and the deterioration of the treatment solution was investigated.

As a result, deterioration was already observed after 25 hours.

In order to determine deterioration, a 100-width sample strip is continuously treated with 1 liter of electrolytic solution, and when the rusting rate in the sulfur dioxide test at that point becomes 10% or more, the solution is considered to have deteriorated.

As described above, the method of the present invention provides excellent corrosion resistance, particularly sulfite resistance, and maintains good gloss, making it extremely effective as a surface treatment for automotive stainless steel.

Moreover, this method has remarkable effects such as being able to easily prepare for processing, preventing deterioration of the processing solution, and allowing long-term use.

Claims (1)

[Claims] 1. A stainless steel having a bright annealing film or a passive film is immersed in an aqueous solution of phosphoric acid having a weight of 0.1 to 50 weight φ, and the stainless steel is used as a cathode electrode, and metal molybdenum is used as an anode electrode. Current density 0.1-5A/di
A method for surface treatment of stainless steel, characterized in that stainless steel is cathodically treated under treatment conditions of a liquid temperature of 10 to 90'C.