JPH06136600A - Method for roughening stainless steel surface - Google Patents

Abstract

PURPOSE:To roughen the surface of stainless steel with a sufficient anchor effect by anodizing the stainless steel in nitric acid or in an aq. soln. consisting essentially of nitric acid. CONSTITUTION:A stainless steel is anodized in nitric acid or in an aq. soln. consisting essentially of nitric acid. The stainless steel is anodized and cathodized in nitric acid or in an aq. soln. consisting essentially of nitric acid. Consequently, a coating film is formed or an adhesive is applied on the surface with greater working effect.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface roughening method for stainless steel, and more particularly to a surface roughening method for stainless steel which causes a large number of pitting corrosion or grain boundary corrosion.

[0002]

2. Description of the Related Art When coating or coating stainless steel, the surface of stainless steel is sometimes roughened (increased unevenness) in order to improve the adhesion of a coating film or a coating material. In particular, since PTFE (polytetrafluoroethylene) has a weak adhesive force, the unevenness of the stainless steel surface is increased to provide the "anchor effect", that is, the effect similar to the anchor of a ship being caught on the seabed when it is thrown. It is said that it is indispensable to improve the fit.

Also, when stainless steel is bonded with an adhesive, when it is plated, or when it is used as an electrode or a catalyst, it is advantageous that a large number of surface irregularities (large) and a large surface area work well. There are many.

For such a purpose, a common method for roughening stainless steel is to pickle it with nitric hydrofluoric acid, hydrochloric acid + ferric chloride solution, etc. No significant improvement effect for improving It is considered that this is partly due to the fact that the shape of the surface irregularities after pickling has less "scratching".

There is also a method of spraying a granular material onto the surface of the material to be treated such as blasting or honing to roughen the surface. However, when the material to be treated is a thin plate, the plate is warped or work-hardened. Inconvenience is likely to occur.

As a unique method, Japanese Patent Publication Sho 61-59399
The publication discloses a method of electrolyzing in an aqueous solution of ferric chloride and then immersing it to cause a lot of pitting corrosion on the surface of the stainless steel, thereby providing unevenness with which an anchor effect can be expected. However, it is difficult for this method to uniformly generate pitting corrosion over the entire surface, and there is a phenomenon in which deep pitting corrosion occurs locally depending on the surface condition of the material to be treated and pitting corrosion does not occur in other portions. Since it easily occurs, quality control in industrial production is difficult.

Also disclosed are a method of applying an acid-resistant ink or paint in a mist state and then applying a corrosive solution (see Japanese Patent Publication No. 62-12310) and a method of using a photoresist (see Japanese Patent Laid-Open No. 4-99288). However, all of them have drawbacks of long processing time and high processing cost, and cannot be said to be inexpensive methods suitable for mass production.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for surface roughening stainless steel, and more particularly, an inexpensive method for surface roughening stainless steel for increasing the adhesion of a coating film or coating material. Is to provide.

[0009]

The present inventors have found that the anchor
As a result of research on a method for uniformly forming a rough surface with a large number of "scratches" that can be expected to be effective, it was found that the method of anodic electrolysis in nitric acid is preferable. Here, the gist of the present invention is a method for roughening stainless steel, which comprises subjecting stainless steel to anodic electrolysis in nitric acid or an aqueous solution containing nitric acid as a main component.

From another aspect, the present invention is a method for roughening stainless steel, characterized in that anodic electrolysis and cathodic electrolysis of stainless steel are carried out in nitric acid or an aqueous solution containing nitric acid as a main component. Thus, according to the present invention, a large number of pitting corrosion or grain boundary erosion can be generated on the entire surface of the stainless steel.

[0011]

The pit obtained by the method according to the present invention,
That is, the generation density of pitting corrosion is 10 ⁴ pieces / cm ² or more, and these are evenly distributed over the entire surface to be treated.
In addition, the grain boundary erosion by the method of the present invention is not a form in which only a very narrow range near the grain boundary is dissolved, but a “form that tends to be deeply eroded as it is closer to the grain boundary”, and probably the Since the erosion depth differs depending on the orientation, the surface profile is irregular as a whole. Therefore, the surface shape is completely different from the groove-shaped intergranular erosion, which is caused by pickling the austenitic stainless steel after annealing, which is generally well known, with a deep erosion depth. By the way, the depth of the grain boundary groove by nitric hydrofluoric acid pickling is 1 to
On the other hand, the erosion depth by the method of the present invention tends to become deeper as the crystal grains become larger, while it is several μm.
reach m

FIG. 1 shows an example of the result of examining the surface profile of the rough surface by the method of the present invention with a stylus type roughness meter (stylus tip radius: 1.5 μm). Here, what is important is that the peaks and valleys of the profile are closely connected and the steeper the slope, the more preferable the rough surface. Further, according to the method of the present invention, the average inclination angle (θ) of the profile calculated by the following formula can be at least 10 ° or more, and in some cases, 30 ° or more.

[0013]

[Equation 1]

Where x: horizontal distance of the surface profile of the rough surface (μm) y: vertical distance of the surface profile of the rough surface (μm) x ₀ : scanning distance of the stylus of the surface roughness meter (mm) In the above measurement of θ, x ₀ is at least 2.5 mm or more, preferably 10 mm or less in order to reduce the measurement error.
It is better to set it to mm or more.

Further, the smaller the tip radius of the stylus of the roughness gauge, the finer the irregularities can be traced and the larger the average inclination angle tends to be. However, there is a problem in practical use. Therefore, the tip radius of 1.5 μm is used here. Based on the profile measured with the needle. Thus, according to the present invention, effective roughening is achieved by anodic electrolysis (or anodic electrolysis + cathodic electrolysis) in nitric acid.

That is, a desired roughened surface can be easily obtained by electrolyzing stainless steel as a material to be treated as an anode (or a cathode) in nitric acid having a concentration of several% to several +%. Appropriate conditions for electrolysis at this time vary depending on the type and surface condition of stainless steel, but are approximately as follows.

(1) Nitric acid concentration: If it is less than 1%, it is difficult to obtain a uniform rough surface because the oxidizing power is insufficient. If it is 50% or more, the oxidizing power is too strong and uneven pitting corrosion is likely to occur, and furthermore, fume is remarkably generated, which causes a trouble in work. Desirably 10-30% HN
If the O ₃ solution is used, a desired rough surface can be stably obtained.

(2) Electrolytic current: When the current is 1.0 mA / cm ² or less, so-called hyperpassive dissolution does not occur and the surface cannot be roughened because the stainless steel is not sufficiently oxidized. On the other hand, although the upper limit of the electrolysis current is theoretically not large, if the current is too large, a large amount of current is consumed not only for melting the stainless steel but also for generating O ₂ gas. Further, on the surface of the counter electrode (cathode), H ₂ gas is generated, nitric acid is decomposed, or NOx gas is generated, which causes economic and hygiene disadvantages. Therefore, the upper limit is set to 1000 mA / cm ² .

Although anodic electrolysis of the material to be treated is necessary for roughening the surface, cathode electrolysis may be supplementarily used. In this case, if the current density of the cathodic electrolysis is less than 10 mA / cm ² , there is no effect, and if it exceeds 10,000 mA / cm ² , H ₂ gas is often generated, which causes economic and safety problems. Desirably 50 ~
The best effect is obtained with 1000mA / cm ² .

(3) Surface potential of stainless steel at the time of electrolysis: If the potential is nobler than 1.0 V (vs.SCE) during anodic electrolysis, no passive passivation occurs and roughening cannot be achieved. Also,
At 2.2 V (vs. SCE), 0 ₂ is more noticeable in nobles. On the other hand, at the time of cathodic electrolysis, there is no effect unless the potential is lower than -0.2 V (vs.SCE), but H ₂ gas is remarkably generated when the potential is lower than -2.0 V (vs.SCE).

Whether or not to use the cathodic electrolysis may be determined depending on the type and surface condition of the stainless steel. For example, as shown in the examples below, the surface wet-polished with SUS430 steel or SUS410 steel with No. 600 abrasive paper is used for anodic electrolysis. While only roughening surface is obtained by pitting corrosion on the whole surface, only the oxide scale is removed by neutral salt electrolysis of the annealed material of SUS304 steel with sodium sulfate solution etc. In the case of the surface, by using anodic electrolysis in nitric acid and cathodic electrolysis together, it is easy to obtain a rough surface with uniform pitting corrosion on the entire surface.

As described above, the reason why a rough surface such as pitting corrosion can be obtained by the method of the present invention is not clear at present, but it is presumed as follows.
Generally, pitting corrosion is a kind of phenomenon in which a part of the passive film of stainless steel is destroyed and corrosion locally progresses, and it is presumed that passivation and active dissolution are mixed.

Even in the case of the treatment conditions of the method of the present invention, a mixture of passivation with nitric acid, hyperpassive dissolution by anodic electrolysis, and active dissolution by cathodic electrolysis coexist, resulting in pores of relatively uniform size and depth. It is speculated that localized dissolution of the edible food occurs. An advantage of the method of the present invention is that pitting corrosion can be uniformly generated over the entire surface,
It can be applied to various steel types and stainless steels with different surface conditions depending on the control of electrolysis conditions.

For example, the surface of a SUS304 steel annealed material that has been descaled by a neutral salt electrolysis method has a Si-enriched layer on the outermost surface, and anodic electrolysis alone in nitric acid results in uniform pitting corrosion. Is less likely to occur, but uniform pitting is likely to occur due to partial dissolution of the Si layer by cathodic electrolysis.

Here, the meaning of the term "rough surface" used in the present specification will be further described. The surface roughness is usually measured using a surface roughness meter and is displayed by the maximum roughness (Rmax) or the like. This is a way of thinking that roughness is evaluated by the vertical distance between the uneven valley and the mountain. However, this method does not know the horizontal distance between the valley and the mountain, so it is not known whether the steep mountain and the valley are continuous or gentle. Therefore, the term "rough surface" in the present invention means the steep unevenness of the former, in other words, the distance between the valley and the mountain or the distance between the mountain and the adjacent mountain (pitch) is short, and the distance between the mountain and the valley in the vertical direction is also to some extent. Is a surface state that exhibits a large, so-called anchor effect.

Various effects of such a rough surface can be considered as described above. First, it is expected that the adhesion of the coating film, the coating film, the plating film or the adhesive will be improved. However, as mentioned above, it is quite contingent to determine whether or not the rough surface of the cross-sectional shape with many "traps" that effectively obtains the anchor effect is apt to be controlled by chance. Think about the case.

2 (a) and 2 (b) schematically show the cross-sectional shape of the surface. Comparing these cross-sectional shapes, the height of the peaks in Fig. 2 (b) is half that in Fig. 2 (a), and the pitch is 1/4.
Even at the same horizontal distance, the total length of the sawtooth line is longer in Fig. 2 (b). Therefore, the rough surface having the cross-sectional shape shown in Fig. 2 (b) has a smaller maximum roughness than the rough surface shown in Fig. 2 (a), but since it has a substantial surface area, it contributes greatly to the adhesion of the coating film. It is expected that the "fan der walth power" that is said to be large. A large substantial surface area is advantageous when used as an electrode or a catalyst. In other words, even if electrodes and catalysts (or their supports) that are apparently the same size have a large surface area, it is expected that the electrolysis rate and reaction rate will increase accordingly.

Further, the rough surface realized by the method of the present invention has a peculiar effect on reflection and absorption of light, and is gray in appearance and has no gloss at all. Therefore, although it is still usable as a calm stainless steel surface, it may be possible to obtain a unique appearance by further adding treatments such as oxidative coloring and chemical coloring.

The effect described above does not depend on the cross-sectional shape of the rough surface alone. For example, the adhesive force is easily affected by the chemical composition of the finished surface, but chemical conversion treatment is performed as a post-treatment. And a strong adhesive force can be obtained stably. However, since such an effect is obtained based on the rough surface by the method of the present invention, it will be easily understood that such various improved techniques are also within the scope of the present invention.

Next, the function and effect of the present invention will be described in more detail with reference to examples.

[0031]

【Example】

(Example 1) 2 of SUS430 steel, SUS410S steel and SUS304 steel
The B finishing material (sheet thickness 0.5 mm) was wet-polished using No. 600 abrasive paper as the test material, and the electrolytic conditions shown in Table 1 were used.
Anodic electrolysis was performed in a 12% HNO ₃ aqueous solution at ℃ with platinum as the counter electrode.

As a method for evaluating the effect of the present invention, it is considered more objective and strict to express it by the shape of the cross section or the state of surface irregularities. Therefore, the roughness was measured using a surface roughness meter or an optical microscope, and at the same time, the surface was observed by an SEM (scanning electron microscope) to examine the fineness of the horizontal unevenness.

When the surface after electrolysis was observed by SEM, as shown in the electron micrograph of FIG.
It was found that the treated S430 steel and SUS410 steel were all covered with pits with a diameter of 0.5 to 6 μm, the pit-to-pit spacing was 1 to 5 μm, and there was no flat part. . As shown in the electron micrograph of FIG. 4, the SUS304 steel treated material is a “grain boundary erosion type” rough surface, the average inclination angle of the surface profile is 18 to 27 °, and the maximum roughness (Rmax) is
It was 25 to 37 μm.

On the other hand, for comparison, only cathodic electrolysis was carried out using the same aqueous nitric acid solution, and 1% HF-10% HNO ₃
The SUS304 steel immersed in the solution (50 ° C) for 3 minutes had no pits. The average tilt angle and maximum roughness of the surface profile measured by the surface roughness meter were small. From the above, it was found that the surface of SUS430 steel and SUS410S steel treated by the method of the present invention has a significantly large unevenness due to the formation of pits and a substantially large surface area.

[0035]

[Table 1]

(Example 2) Cold rolled steel sheet of SUS304 steel (sheet thickness 0.4
mm) was annealed at 1100 ° C for 60 seconds in a hydrocarbon gas combustion atmosphere, and then the current density was changed to 70 ° C in a 20% Na ₂ SO ₄ aqueous solution.
Alternating electrolysis at 80mA / cm ² (Anode electrolysis 2sec → Cathode electrolysis 1se
The sample that had been subjected to descaling for a total of 60 seconds by repeating c) was used as a test material, and alternating electrolysis was performed under the electrolytic conditions shown in Table 2 in a 20% HNO ₃ solution at 50 ° C. using a SUS304 plate as a counter electrode. When the surface after electrolysis was observed by SEM, it was found that pits having a diameter of 15 to 30 μm were formed on the entire surface of the treated material by the method of the present invention as shown in the electron micrograph of FIG.

On the other hand, for the purpose of comparison, the above-mentioned test materials were subjected to 0.7% HF-8% HNO ₃ solution (50 ° C.) as usual.
The grain boundaries of the specimens pickled by immersion for 60 seconds or 300 seconds were eroded, but the inside of the grains was almost smooth. Further, when the depth of the pits generated by the method of the present invention and the depth of groove-like erosion generated by the comparison method were measured using an optical microscope, it was found that the former was much deeper. As described above, since the surface of the SUS304 steel treated by the method of the present invention is covered with relatively uniform pits over the entire surface, the substantial surface area is large and the anchor effect for the coating film or the adhesive can be expected greatly.

[0038]

[Table 2]

(Example 3) 2B finish material of SUS304 steel (sheet thickness
0.4 mm) was wet-polished with No. 600 abrasive paper to 50
Current density using a SUS304 plate as a counter electrode in a 20% HNO ₃ solution at ℃
After anodic electrolysis for 30 minutes at 24mA / cm ² (potential +1.2 V vs. SCE) and cold rolled steel sheet of SUS304 (sheet thickness 0.4 mm) were annealed in a hydrocarbon gas combustion atmosphere at 1100 ° C for 60 seconds, then 80 ℃,
After removing the oxide scale from the surface by alternating electrolysis in a 20% Na ₂ SO ₄ aqueous solution at a current density of 80 mA / cm ² ,
Alternating electrolysis in% HNO ₃ (+200 mA / cm ² × 2 sec → −200 mA
/ cm ² × 1 sec for a total of 180 seconds) as a test material according to the method of the present invention. Separately, as a test material according to the comparative method, 2B material of the above SUS304 steel was used at 60 ° C, 10% F ₂ Cl ₃ After anodic electrolysis for 5 minutes at a current density of 100 mA / cm ^{2 in} an aqueous solution,
Soaked in a 15% FeCl ₃ aqueous solution at 20 ° C for 3 hours, 60
What was immersed in a 3% HF-15% HNO ₃ aqueous solution for 3 minutes at ℃ was prepared, and polytetrafluoroethylene (P
After coating a TFE) -based coating, the adhesion of the coating was examined by the peel test method.

The results are shown in Table 3 together with the processing conditions. According to it, the treated material according to the present invention has a large film adhesion of 5 to 7 kg / 25 mm width, whereas the comparative example
The width was 4.3 kgf / 25 mm, and the comparative example was as small as 1 kg or less / 25 mm width. In the example of the present invention, although the treatment time for surface roughening was relatively short as 3 to 30 minutes, the effect of surface roughening was great and sufficient adhesion of the PTFE film was obtained.

[0041]

[Table 3]

[0042]

As described above, according to the present invention, it is possible to roughen the surface having a great anchoring effect, and therefore, the action and effect as a surface for forming a coating film, applying an adhesive agent, etc. are great.

[Brief description of drawings]

1 is a profile of a stainless steel surface roughened by a method according to the present invention.

2 (a) and 2 (b) are schematic explanatory views of the cross-sectional shape of the roughened surface of the material to be treated.

FIG. 3 is an electron micrograph of a stainless steel surface roughened by the method according to the present invention.

FIG. 4 is an electron micrograph of a stainless steel surface roughened by the method according to the present invention.

FIG. 5 is an electron micrograph of a stainless steel surface roughened by the method according to the present invention.

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[Procedure amendment]

[Submission date] November 10, 1992

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Claims (2)

[Claims] 1. A method of roughening stainless steel, which comprises subjecting stainless steel to anodic electrolysis in nitric acid or an aqueous solution containing nitric acid as a main component. 2. A method for roughening stainless steel, which comprises subjecting stainless steel to anodic electrolysis and cathodic electrolysis in nitric acid or an aqueous solution containing nitric acid as a main component.