JP5527860B2 - How to passivate stainless steel - Google Patents

Description

  The present invention relates to the passivation of stainless steel.

  It has been conventionally known that an oxide film is formed on a stainless steel surface and passivated to provide a high pitting corrosion potential and improve corrosion resistance and corrosion resistance. In addition, it is a conventionally known technique that the inclusion of molybdenum as an alloy in stainless steel suppresses pitting corrosion due to chloride ions and improves the ability of the film to regenerate against scratches.

  There are stainless steels in which molybdenum is initially contained as a raw material and melted and alloyed with molybdenum, but there is a drawback that the price is high. In order to include molybdenum in stainless steel that does not include molybdenum, a method of performing electrolytic treatment using a stainless steel as a cathode in a solution containing a molybdenum salt is employed. Examples of the cathode treatment are disclosed in, for example, JP-A-005-298939, JP-A-07-188976, JP-A-64-4494, and JP-A-57-26196.

  However, the methods disclosed in these publications employ a technique of forming a metal molybdenum compound on the surface of the stainless steel by electrolytic treatment, and the passivating film is formed in a separate step.

  Among them, JP-A-64-4494 discloses a method for obtaining a high pitting corrosion potential by combining three-stage electrolytic treatment and electroless treatment. JP-A-57-26196 discloses electrolysis or electroless treatment in a solution containing phosphoric acid and molybdic acid.

JP 2005-298939 A JP 07-188976 A Japanese Unexamined Patent Publication No. 64-4494 JP-A-57-26196

  In Japanese Patent Laid-Open No. 2005-298939, electrolytic treatment using a stainless steel as a cathode in a solution of nitric acid and sodium molybdate is performed to form a metal molybdenum compound on the surface of the stainless steel, and then an oxide film is formed by constant potential treatment. It comes to form. Although the meaning of the constant potential treatment is not clear here, in order to form an oxide film, the object to be treated needs to receive electrons from the solution. It is understood that it means. That is, here, different types of electrolytic treatment are used for the electrodeposition of molybdenum on the workpiece and the oxidation treatment (passivation treatment).

  In Japanese Patent Laid-Open No. 07-188976, the object to be processed is subjected to anodic electropolishing in the first step to remove dirt, and in the second step, the object to be processed is subjected to cathodic electrolytic treatment to electrodeposit molybdenum. The pitting potential is 1000 mV or less, which is not sufficient.

  Further, JP-A-64-4494 discloses that chromic acid or molybdic acid is added to chromic acid after electroless treatment of stainless steel in a solution in which at least one of sulfuric acid, phosphoric acid, nitric acid and molybdate is added to chromic acid. An invention for cathodic electrolysis with an added aqueous solution is disclosed. In this invention, although the electroless treatment is performed in the third step, spray spraying is used as a treatment method. With this method, it is difficult to control the temperature during treatment, and the treatment becomes uneven. There are disadvantages such as the acid being easily scattered. In addition, since the cathodic electrolysis treatment in the second step is an essential requirement, this also includes a double step in which molybdenum is laminated on the workpiece surface by electrodeposition and then the laminate is oxidized. It will be.

  Further, JP-A-57-26196 discloses electrolysis or electroless treatment in a solution containing phosphoric acid and molybdic acid. Although electroless treatment (see Table 3 and Example 3) is disclosed here, it is not intended to obtain a high pitting corrosion potential, and the effect of pitting corrosion potential is not disclosed. Is unknown.

  Furthermore, in any of the above methods, molybdenum is electrodeposited on the surface of the object to be processed by cathodic electrolysis to form a passive film.

  Cathodic electrolytic treatment has a basic problem that the uniformity of characteristics cannot be ensured unless the temperature, concentration, and current density of the solution are strictly controlled, and the characteristic varies depending on the part of the workpiece with unevenness. is there. Accordingly, it is not preferable to use electrolytic treatment for a workpiece having a large area or a workpiece having a complicated shape from the viewpoint of the quality and uniformity of each product or portion.

  The present invention has been proposed in view of the above-mentioned conventional circumstances, and without using a cathodic electrolytic treatment, a passive film is formed on the surface of stainless steel in a state containing molybdenum that enhances the ability to suppress pitting corrosion by chlorine ions against scratches. It is intended to form.

  In order to achieve the above object, the present invention employs the following steps.

  Stainless steel is electropolished in a mixed solution of phosphoric acid and sulfuric acid (anodic electrolysis) to clean the surface of the stainless steel and to elute iron to form a chromium-rich passivated film.

  Next, a passive film containing a metal molybdenum compound on the stainless steel surface is formed by electroless treatment in a nitric acid solution of sodium molybdate at a predetermined temperature and a predetermined concentration for a predetermined time.

  A chromium-rich passivated film is formed on the stainless steel surface by the electropolishing treatment, and a passive film containing a metal molybdenum compound is formed on the stainless steel surface by a nitric acid solution of sodium molybdate used for the electroless treatment. be able to. As a result, a passive film containing molybdenum can be formed on the stainless steel surface without using cathodic electrolysis, and a high pitting corrosion potential can be obtained, and stainless steel with high regenerating ability can be realized with an inexpensive raw material. .

FIG. 1 is a distribution diagram of the pitting corrosion potential of one embodiment of the present invention. FIG. 2 is a distribution diagram of the pitting corrosion potential of the comparative example. FIG. 3 is a distribution diagram of the pitting potential of another embodiment of the present invention. FIG. 4 is an XPS analysis diagram of one embodiment of the present invention. FIG. 5 is an XPS analysis diagram of another embodiment of the present invention. FIG. 6 is an XPS analysis diagram of still another embodiment of the present invention. FIG. 7 is a diagram showing the composition obtained by the treatment of the comparative example of the present invention. FIG. 8 is a diagram showing the composition by the treatment of the present invention. FIG. 9 is a diagram showing the results of a corrosion resistance test in steam. FIG. 10 is a diagram showing the results of a corrosion resistance test in salt water.

  In the present invention, stainless steel is electropolished in a mixed solution of phosphoric acid and sulfuric acid (anodic electrolysis) to remove and clean non-metallic inclusions on the surface of the stainless steel, and iron is eluted, and chromium is applied to the surface. Forms a rich passivation film. Next, the stainless steel turned into chromium chromium is subjected to electroless treatment in a nitric acid solution of sodium molybdate at a predetermined temperature and a predetermined concentration for a predetermined time, thereby forming a passive film containing a metal molybdenum compound.

In the electrolytic treatment, when 85 wt% phosphoric acid and 98 wt % sulfuric acid are used, the ratio of phosphoric acid to sulfuric acid is 50% to 90%: 10% to 50% (volume%). When the phosphoric acid concentration is lower than this, the glossiness is lowered, and when it is higher than this, the cost is increased.

The electropolishing treatment is performed at a temperature of 30 ° C. to 80 ° C. and a current of 1 A to 20 A / dm ² for 30 seconds to 20 minutes. If the temperature is lower than 30 ° C., the dissolution rate of iron is not sufficient, and if it is higher than 80 ° C., the odor of sulfuric acid becomes strong, causing problems in workability. If the current density is less than 1 A / dm ² , the dissolution rate of iron is not sufficient, and if it is greater than 20 A / dm ² , the uniformity of the current density is lowered, resulting in a reduction in quality. Furthermore, the temperature, current density, and time are matters that should be determined relatively, but to the extent that the elution of iron does not become too large and is too small, it does not hinder the subsequent electroless treatment. The range of the degree.

  The nitric acid solution used for the electroless treatment is 10 to 60% by weight. If the concentration is lower than 10% by weight, an oxidizing action cannot be obtained at a sufficient rate. On the other hand, if it is higher than 60% by weight, the oxidizing power is too strong and a good passivation film cannot be obtained. The concentration of sodium molybdate to be added to nitric acid is determined by a relative relationship with the solution temperature and processing time, but is in the range of 0.05 M to 0.5 M / L. When sodium molybdate is 0.05 M / L or less, sufficient resistance to chloride ions cannot be obtained, and at the same time, the variation in pitting potential increases. On the other hand, when the sodium molybdate is 0.5 M / L or more, the resistance to chloride ions is sufficiently high, but the improvement of the pitting potential corresponding to the added amount cannot be obtained. Furthermore, the electroless treatment is performed at a temperature in the range of 20 to 80 ° C. When the temperature is 20 ° C. or lower, a sufficient reaction rate cannot be obtained, and a passivation film containing metal molybdenum is not formed. When the temperature is 80 ° C. or higher, a nitric acid odor is generated in the atmosphere, resulting in workability problems.

As the material itself, stainless steel SUS304 not containing molybdenum was used. This material was mixed in a mixed solution of 85% by weight phosphoric acid and 98% sulfuric acid (phosphoric acid: sulfuric acid = 50%: 50% (volume%)) at a temperature of 30 ° C. for 10 minutes at a current of 10 A / dm ^2. Electropolishing was performed to prepare samples for the following experiments.

  Next, as shown in Table 1, 10%, 30%, and 50% concentration nitric acid (solution group I) was prepared, and a solution in which 0.1 mol of sodium molybdate was added to 30% concentration nitric acid (solution group II) A 0.1 M aqueous solution of sodium molybdate (solution group III) is prepared.

  The electropolished samples of these solution groups are subjected to electroless treatment at a temperature of 25 ° C., 2 hours, 7 hours, a temperature of 60 ° C., 2 hours, and 7 hours, respectively, and the pitting corrosion potential is measured.

  The results are shown in Table 1. In Table 1 (unit: mV), Average is the average value of the pitting corrosion potential of a plurality of experiments, Lower is the lower limit value, and Upper is the upper limit value. In the following, the term “range” means the difference between Lower and Upper in a plurality of experiments. The number of samples is 5 for each test.

  After electropolishing, when electroless treatment is performed with a solution of 0.1 M sodium molybdate in 30% concentration of nitric acid, the pitting corrosion potential is in a wide range (60 ° C., 2 hours, 25 It was possible to obtain a stable pitting potential at a high temperature (about 1000 mV) and a small variation range (see Experiment Nos. (14) and (15)).

  On the other hand, as shown in FIGS. 2 (a), (b), and (c), when a pitting corrosion potential of about 1000 mV is obtained by oxidation treatment with nitric acid, each concentration is 10%, 30%, 50%. In both cases, only when the temperature is 25 ° C. for 7 hours (experiment number (3), (7), (11)), a result with a small range can be obtained, and the concentration is 10% (experiment number (3)). Slightly, the range was widened.

  Furthermore, when trying to obtain a pitting corrosion potential of around 1000 mV at a temperature of 60 ° C. by adding sodium molybdate to the 30% nitric acid solution according to the present invention (treatment time: 2 hours), as shown in FIG. The concentration of sodium acid is 0.05M / L to 0.2M / L (0.05M / L, 0.2M / L is not in Table 1, 0.1M / L is up to the experiment number (16) to obtain a stable pitting potential. On the other hand, in the treatment with nitric acid alone, the pitting corrosion potential varied with each treatment (see Experiment Nos. (6) and (8)).

  FIG. 4 shows the analysis results of the elemental analysis XPS of the sample according to Experiment 16 according to the present invention, and Table 2 shows specific numerical values (units A · C% (atomic percentage)). In Table 2, the sputtering time is shown in the left column of the table. According to Table 2, molybdenum is observed near the surface (see the Mo3d column), and correspondingly, a molybdenum peak is observed in FIG. 4 (see the Mo1 column). In addition, ESCA5600Ci made by ULVAC-PHI Co., Ltd. was used as an elemental analysis device. The vertical axis in FIG. 4 is the number of sputtering times, about 0.1 minute, and accordingly, a value corresponding to a numerical value of 1 min or less in Table 2 appears as a peak below the Mo1 column.

  Figure 5 shows the addition of 0.1M sodium molybdate to 30% nitric acid and 2 hours at 25 ° C (experiment number (13)). Figure 6 shows the addition of 0.1M sodium molybdate to 30% nitric acid. This is XPS data when electroless treatment (not shown in Table 1) for 7 hours at ℃. In the case of 40 ° C. for 7 hours, a molybdenum peak is observed, and in the case of 25 ° C. for 2 hours, a slight peak is also recognized, and it can be understood that molybdenum is incorporated into the stainless steel surface simultaneously with the oxidation treatment.

  It should be noted that the raw material (state not subjected to electrolytic polishing treatment) is directly passivated with nitric acid (for example, 30% nitric acid solution), or the raw material not subjected to electrolytic polishing treatment is added to nitric acid and molybdic acid (for example, 30% nitric acid solution). An experiment was conducted to measure the pitting potential of the sample passivated by adding 0.1 M / L molybdic acid. However, only a pitting potential of several hundred mV or less was obtained, and the electropolishing treatment of the present application was important. I can understand.

  Furthermore, when the electropolished sample was electrolessly treated with a 0.1 molybdic acid aqueous solution (solution group III: no nitric acid) and tested in the same manner as the above samples, the pitting potential was low and satisfactory. It can be understood that a passive film is not formed (experiment numbers (17) to (20)).

図７は、ステンレス（SUS304）を電解研磨（8Ｖ、10分、10A/dm²）＋30%硝酸での無電解処理（60℃、2時間）（実験番号(6)の試料）をしたときのXPSの分析結果、図８（本願発明）は電解研磨（8Ｖ、10分、10A/dm²）＋30%硝酸＋0.1Mモリブデン酸への無電解処理（60℃、2時間）（実験番号(14)の試料）の分析結果である。

Fig. 7 shows the results when stainless steel (SUS304) was electropolished (8V, 10 minutes, 10A / dm ² ) + 30% nitric acid for electroless treatment (60 ° C, 2 hours) (sample of experiment number (6)). As a result of XPS analysis, FIG. 8 (the present invention) shows electroless polishing (8 V, 10 minutes, 10 A / dm ² ) + 30% nitric acid + 0.1 M molybdic acid electroless treatment (60 ° C., 2 hours) (experiment number (14 ) Sample)).

  In both figures, A · C% on the vertical axis is atomic% and S. on the horizontal axis. T.A. (Min) is the sputtering time and corresponds to the distance in the depth direction from the surface. The relationship between the sputtering time and the depth is about 1.5 nm / min.

  As can be understood from the behavior of oxygen (black square), the oxide film (the position of ST time, which is about half of the rise of oxygen (position of ST 2 min.)) Is the present invention (FIG. 8). ), The position is 1.8 min. However, in the comparative example (FIG. 7), it is 1 min or less, and the oxide film of the present invention is about twice as deep as the comparative example.

  Next, as can be understood from the behavior of chromium (black triangle), in the present invention (FIG. 8), the chromium concentration is higher to a distance deeper than the surface, as compared with the comparative example (FIG. 7).

  Molybdenum has a very small proportion, so it cannot be confirmed from FIG. 8, but it is contained to some extent from Table 2 (sample of experiment number (16)) or from FIGS. 4 to 6. I understand that.

The sample was electropolished (8 V, 10 minutes, 10 A / dm ² ) + 30% nitric acid (40 ° C, 7 hours: sample corresponding to Fig. 6). Sputtering time was 1 min and oxygen was 33.45%. The results of 32.36% for chromium, 3.77% for oxygen with a sputtering time of 2 min, and 10.8% for chromium were obtained, and considering the results shown in FIG. 8, the treatment of the present invention has stability. I can understand that.

  The above difference in composition leads to the high pitting corrosion potential and stability shown in Table 1, and the stability when exposed to steam (121 ° C) described below or immersed in salt water. It is thought that it leads to sex.

  By the way, in the pharmaceutical and food industries, pure water vapor cleaning of stainless steel containers is sometimes performed for sterilization and sterilization, and at that time discoloration (corrosive organisms of iron derived from high-temperature oxide film and iron) occurs. May occur.

Therefore, a corrosion resistance evaluation test by steam treatment was performed, and the results are shown in FIG. Sample: Electropolishing (8V, 10 minutes, current 10A / dm ² ) + Electroless treatment with 30% nitric acid (60 ° C, 2 hours) (Comparative example, upper part of Fig. 9: Experiment number (6)), Electropolishing Treatment (8 V, 10 minutes, current 10 A / dm ² ) + electroless treatment (60 ° C., 2 hours) with 30% nitric acid + 0.1 M molybdic acid (the present invention, FIG. 9 lower row: experiment number (14)). It is a test result about a plurality of samples (5 samples) about each test. In FIG. 9, the voltage (horizontal axis) at which the current (vertical axis) suddenly rises is the pitting corrosion potential.

  In the steam treatment at 121 ° C. for up to 2 hours, a sample with a current rising at several hundred mV or less is seen in the comparative product, but it can be understood that the present invention stably shows a pitting potential of 1000 mV or more.

  Although we tried a corrosion resistance test in steam for the sample as it was and the sample that was only electropolished, it was not clear if there was a pitting potential in the sample as it was. In the sample, the average was several hundred mV, and the result was far inferior to the present invention or the nitric acid-treated product.

  Corrosion resistance tests were also performed on samples similar to those used for steam treatment and immersed in salt water (concentration 3.5%) (FIG. 10). In the treatment with nitric acid alone, a sample showing a pitting potential lower than 1000 mV is already observed after 10 days of immersion, but the product of the present invention shows 1000 mV or more in any sample when immersed for 10 days. In addition, it can be understood that the stability can be maintained for a considerably long time (100 days in this experiment).

  In FIG. 9 and FIG. 10, “no treatment” means that the product is not exposed to a steam atmosphere or is not immersed in salt water, and as shown in Table 1, here, the product of the present invention is more stable than the nitric acid-treated product. It can be understood that the nature is high.

  As described above, according to the present invention, a chromium-rich passivated film can be formed on the stainless steel surface by electropolishing, and a passivated film can be formed on the stainless steel surface by nitric acid used for electroless treatment. In addition to being able to penetrate molybdenum. As a result, it is possible to form a passive film containing a metal molybdenum compound on the stainless steel surface without using cathodic electrolysis, to obtain a high pitting corrosion potential, and to realize a highly recyclable stainless steel with an inexpensive raw material. Can do.

Industrial applicability

  As described above, the present invention has great industrial applicability because the formation of the passive film on the stainless steel surface and the molybdenum content can be simultaneously processed without electrolysis.

Claims (1)

Stainless steel is 1 Adm in a mixture ratio of phosphoric acid and sulfuric acid of 85 wt% phosphoric acid: 98 wt% sulfuric acid = 50 to 90:10 to 50 vol%, at a temperature of 30 to 80 ° C. ² ~ 20A dm ² Electrolytic polishing with current of 30 seconds to 20 minutes.
After the electropolishing, a step of performing an electroless treatment at a temperature of 20 ° C. to 80 ° C. for a predetermined time in a nitric acid solution of 10 wt% to 60 wt% of 0.05 M / L to 0.5 M / L sodium molybdate
A method for passivating stainless steel characterized by comprising:

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