JP7108291B2 - Stainless steel surface treatment method - Google Patents

Description

TECHNICAL FIELD The present invention relates to surface processing of stainless steel, and more particularly to surface processing of stainless steel using pure steam cleaning.

There are various types of stainless steel products, such as electropolishing, passivation after electropolishing, and neither electropolishing nor passivation. When passivation is performed after electropolishing, the pitting potential is also high, and therefore high chemical durability and acid resistance are provided.

On the other hand, tanks used by pharmaceutical companies for manufacturing chemicals and tanks for food manufacturing by food companies are cleaned with high-temperature pure water steam (pure steam cleaning: hereinafter referred to as PS cleaning) after installation. Then, the inside of the tank is sterilized or sterilized repeatedly.

The above electrolytic polishing, passivation treatment and PS cleaning are originally performed without any mutual relationship, but in the stainless steel processing industry, the purpose is to speed up the start-up after delivering stainless steel containers to the chemical industry or food industry. After electrolytic polishing and passivation treatment, the PS cleaning is performed, and the corrosion products generated by the PS cleaning are wiped off with a cloth impregnated with ethanol before delivery.

In other words, the reason why PS cleaning is conventionally applied in the processing of stainless steel is to reduce the labor of sterilization and sterilization work at the delivery destination.

JP-A-2006-122699

When a stainless steel product is passivated after electropolishing, it has a high pitting potential and therefore high chemical durability.

On the other hand, if PS cleaning is performed before delivery as described above, corrosion products will be generated, and if this is wiped off before delivery, the oxide film formed by the passivation treatment will be destroyed during the wiping work and pitting corrosion will occur. May reduce potential. Moreover, at the factory to which the product is delivered, a substance that easily corrodes stainless steel is put in (touched by), so the stainless steel product will be corroded and the durability will be significantly impaired.

The present invention has been proposed in view of the above-mentioned conventional circumstances, and provides a surface processing method for stainless steel that does not impair the durability of stainless steel products even if they are subjected to PS cleaning in chemical factories, food factories, etc. It is intended to

The present invention is to improve the pitting corrosion potential by using PS cleaning at any stage of the stainless steel surface processing process.

In this mode, the stainless steel surface is subjected to electrolytic polishing, after the electrolytic polishing, PS cleaning is performed for a predetermined time with pure steam at a predetermined temperature, and after wiping off corrosion products generated by the PS cleaning, passivation It is a surface treatment method for stainless steel that performs a heat treatment.

By subjecting the electropolished stainless steel surface to PS cleaning, the pitting potential was improved, which means that the corrosion resistance was correspondingly enhanced. When passivating a stainless steel surface, if the PS cleaning is performed after passivation, the oxide film created by the passivation treatment is destroyed by wiping off the corrosion products produced by the PS cleaning. However, if PS cleaning is performed before the passivation treatment, the oxide film will be revived by the passivation treatment, and it will be able to withstand use at the delivery destination. It will be.

FIG. 3 shows the pitting potential (HCl) of PS-cleaned samples after passivation. The figure which shows the pitting potential (NaCl) at the time of PS washing|cleaning to the sample which is not processed. The figure which shows the pitting corrosion potential (NaCl) of the sample which PS washing|cleaned for different time after electropolishing. Pitting potential (NaCl) of samples subjected to PS cleaning for different times after passivation. The figure which shows the pitting corrosion potential (HCl) of the sample which carried out PS washing|cleaning with different temperature after electropolishing. A diagram showing the pitting potential (HCl) of samples subjected to PS cleaning at different temperatures after passivation. A photograph showing the results of wiping off corroded matter generated by repeated PS cleaning after passivation. Fig. 2 shows the pitting potential (HCl) of PS-cleaned samples in different processes. Photographs showing PS-cleaned samples in different processes.

Buffing, electropolishing, and passivation are common procedures for surface treatment of stainless steel products. Passivation treatment is a process to form an oxide film on the surface of stainless steel with nitric acid for the purpose of improving chemical resistance. There is also

In the following description, electropolishing is performed in a mixed solution of sulfuric acid and phosphoric acid with the object to be processed as the positive electrode. Passivation treatment is also 2 hours treatment in 30% HNO ₃ solution unless otherwise specified. Unless otherwise specified, all samples were washed with PS under conditions of 121° C., 2.1 atm, and treatment time of 2 hours, but when the temperature is changed, the time may be changed. SUS316L is used as a stainless steel sample.

First, FIG. 1 shows a stainless steel sample after electropolishing (hereinafter sometimes simply referred to as a sample), a sample subjected to PS cleaning after electropolishing, a sample subjected to passivation treatment after electropolishing, and electropolishing and passivation treatment. After that, the pitting potential in a 0.02M-HCl solution was measured for the sample that was further subjected to PS cleaning.

A pitting potential of about 900 mV is obtained by electropolishing + passivation treatment + PS cleaning (thick solid line). The value is somewhat higher than that of the hardening process (one-dot chain line). From this, it means that the effectiveness of PS cleaning for the stainless steel sample has been confirmed.

FIG. 2 shows the results of pitting potential measurements in a 3.5% NaCl solution when PS cleaning was performed for various times on stainless steel samples that were neither electropolished nor passivated. be. The treatment time for PS washing was varied from 0.1 hour to 6 hours.

In this case, the pitting potential of the sample not washed with PS was a little less than 600 mV, whereas the pitting potential increased to 800 mV or more after PS washing for 0.1 hour or more, and a value exceeding 1000 mV was obtained after treatment for 2 hours. was made.

Based on this result, the effect of improving the pitting corrosion potential can be expected by applying PS cleaning at any stage of the surface treatment of stainless steel.

FIG. 3 shows the results of measuring the pitting corrosion potential in a 3.5% NaCl solution after the electropolished stainless steel sample was washed with PS for different times (0.1 h to 6 h).

The pitting potential was a little less than 900 mV before PS cleaning, but increased to about 1100 mV after PS cleaning, which is a higher value than the sample passivated after electropolishing. In addition, at least in this experiment, the dependence of the PS washing itself on the time is not recognized so much.

FIG. 4 shows the pitting potential in a 3.5% NaCl solution of a stainless steel sample that was passivated after electropolishing and PS cleaned for different times (0.1 h to 6 h). It is. While the value is about 1050 mV without PS cleaning, the sample with PS cleaning exceeds 1100 mV, and the effectiveness of PS cleaning is expected even for passivated stainless steel samples. can.

However, as will be described later, wiping off corrosion products generated by PS cleaning after PS cleaning destroys the oxide film formed by passivation. (Touching) may reduce the durability of the product.

FIG. 5 shows the pitting potential measured in a 0.15M-HCl (3.5% NaCl in FIG. 3) solution after subjecting an electropolished stainless steel sample to PS cleaning treatment at different temperatures.

According to this, when the PS cleaning steam temperature is 80°C, only a little less than 600 mV is obtained, but when it is 100°C or higher, a value of around 700 mV is obtained. be.

FIG. 6 shows pitting corrosion measured in a 0.15M-HCl solution (3.5% NaCl solution in FIG. 4) after subjecting a stainless steel sample passivated after electropolishing to PS cleaning treatment at different temperatures. It indicates electric potential. A value of around 1000 mV is obtained at a steam temperature of 121° C. or higher, but only a little over 900 mV is obtained at 100° C. and a little less than 900 mV at 80° C. This value is not much different from the sample not washed with PS.

As described above, it can be understood from FIGS. 5 and 6 that the practical steam temperature is 100° C. or higher, preferably 120° C. or higher when PS cleaning is used for surface treatment of stainless steel.

FIG. 7 shows the state of contamination after wiping off the corrosion products on a sample subjected to passivation treatment after electropolishing and repeatedly exposed to steam at 121° C. for 2 hours (PS cleaning). Bemcott shown. A considerable amount of corrosion products can be visually observed in the wiping after the first PS cleaning. When it comes to wiping after PS cleaning, it is no longer observed.

By the way, as described in the prior art, stainless steel products used for drug manufacturing and food processing are hand-washed with PS for the purpose of sterilization and sterilization by the user after being delivered to the user. On the side of the surface processing company, assuming this, PS cleaning is performed after the surface processing, and the surface is wiped off before shipment. As a result, although the start-up time on the user side is shortened, there arises a problem of deterioration in corrosion resistance due to wiping.

FIG. 8 shows the pitting potential in 0.15M-HCl for different treatments. As shown by the thin solid line in FIG. 8, the pitting potential of the sample subjected to passivation treatment after electrolytic polishing shows a value of around 900 mV.

When the PS cleaning and wiping are further performed after the passivation treatment, the corrosion products generated by the PS cleaning are wiped off, and the oxide film generated by the passivation is also scraped off. The surface is scratched as shown. Therefore, in this case, the pitting potential is remarkably lowered as indicated by the dashed line in FIG. This pattern corresponds to the product delivered to the user from the former processor. When the user further cleans with PS for sterilization and sterilization, the sample surface (without wiping the surface) is thickly covered with corrosion products (Fig. 9(b)).

After PS cleaning after electropolishing, the surface is wiped off and then passivated. Even if PS cleaning is performed after that, the generation of corrosion products becomes extremely small, and the surface is corroded as shown in FIG. 9(c). It will be in a state where there is very little adhesion of matter. At this time, the pitting potential is around 1000 mV as indicated by the thick solid line in FIG. 8, and the sample exhibits high pitting corrosion resistance.

As described above, when PS cleaning, which has been conventionally used for the purpose of sterilization and sterilization, is used before passivation treatment in surface treatment of stainless steel, the corrosion resistance of the surface of stainless steel to chloride solutions can be significantly improved. is shown.

As described above, the present invention can obtain a high pitting corrosion potential for a stainless steel sample against a chloride environment, and the stainless steel product can be durable even if the substance handled by the user is a substance that easily corrodes the stainless steel. Since it can be maintained, it can be used for surface treatment of stainless steel.

Claims (3)

electropolishing a stainless steel surface;
a step of performing pure steam cleaning with pure steam at a predetermined temperature for a predetermined time after the electropolishing, and wiping off the generated corrosive matter;
a step of performing a passivation treatment after the pure steam cleaning;
A surface treatment method for stainless steel, comprising: 2. The method of surface treatment of stainless steel according to claim 1 , wherein the pure steam cleaning and wiping off the generated corrosive products are performed a plurality of times. 3. The method for surface treatment of stainless steel according to claim 1, wherein the pure steam cleaning is performed at a temperature of 100[deg.] C. or higher for 0.1 hour or longer.

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