JPS6159399B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing stainless steel having a rough surface with a large anchoring effect, more specifically, a rough surface with an opening narrower than the bottom and excellent coating adhesion. Recently, stainless steel coated with various types of coatings has come into widespread use in many applications such as building materials, electrical appliances, and industrial instruments. There has been a strong demand for provision. By the way, when coating stainless steel with various coatings, the adhesion between the two is currently only improved by the following method. (a) A method of roughening the stainless steel surface by subjecting it to mechanical polishing such as polishing, shot blasting, liquid honing, or grinding. (b) A method of chemically roughening the stainless steel surface by corroding the entire surface by applying an acid such as sulfuric acid or nitric fluoride or an inorganic salt solution such as ferric chloride to the stainless steel surface. 〓〓〓〓〓
(c) A method in which a type of chemical conversion film is formed on the surface of stainless steel by treating it with an oxalic acid solution. Therefore, in the case of methods (a) and (b) above, no matter how much you try to increase the surface roughness by changing the processing conditions,
As shown in Figs. 1 and 2, the resulting rough surface is limited to one in which the width a of the opening is wider than the width b of the bottom, and the ratio of the depth c to the width a of the opening is small. Even if the material was covered, a sufficient anchoring effect could not be obtained. In addition, in method (c), at present, a film (phosphate film) with excellent adhesion to the coating, such as that normally obtained when phosphate treatment is applied to galvanized iron sheets or cold-rolled ordinary steel sheets, can be applied. ) has not been developed, and there are still problems with its adhesion. Generally, in order to increase the projection effect of the coating and improve the adhesion of the coating over a wide surface, the width of the rough surface opening is made smaller than the width of the bottom, and the depth of the rough surface and the width of the opening are made smaller. It is ideal to increase the ratio of rough surfaces and to distribute these rough surfaces evenly and densely over the entire surface. The present invention aims to provide a method for manufacturing stainless steel having such an ideal rough surface. The present inventors focused on the fact that stainless steel undergoes pitting corrosion due to halogen ions, forming a rough surface where the width of the opening is wider than the width of the bottom, and by developing this, the ideal rough surface described above was achieved. They succeeded in developing a method for manufacturing stainless steel with In other words, when stainless steel is subjected to pitting corrosion with halogen ions, the width a of the opening is wider than the width b of the bottom, as schematically shown in Figure 1, and a rough surface shaped like an octopus pot is formed. . Moreover, the depth c of this octopus-shaped rough surface can be 50 to 100 times or more than the width a of the opening depending on the pitting corrosion conditions, and in some cases, the depth c of this octopus-shaped rough surface can be increased to 50 to 100 times or more than the width a of the opening. In some cases, the bottoms of adjacent pitting corrodes communicate with each other, so an excellent anchoring effect can be expected. FIG. 14 shows a specific example (planar photograph) in which pitting corrosion is caused by the action of halogen ions on stainless steel, and it can be seen that deep pitting corrosion occurs on the surface. However, in general, when industrial products are targeted, satisfactory coating adhesion cannot be obtained simply by causing pitting corrosion with excellent anchoring effects as described above. In other words, in the case of industrial products, they generally have a large area, so
It is extremely important to uniformly and densely apply the above-mentioned pitting corrosion, which has an excellent anchoring effect, over a wide surface. However, it is extremely difficult to perform such pitting corrosion using a conventional method using halogen ions. Even if pitting is performed using the conventional conventional method,
The density of pitting corrosion will vary locally, and the shape of the pitting corrosion will also vary in size locally, making the rough surface uneven. This is because a rough surface cannot be formed. Therefore, the inventors of the present invention conducted extensive research to develop a method for obtaining a rough surface with uniform pitting density and shape over a wide area, and as a result, they combined electrolysis using an aqueous ferric chloride solution and immersion. This made it successful. That is, the present inventors first electrolyzed stainless steel for a short time using an aqueous ferric chloride solution as an electrolyte to form pitting corrosion, which is the starting point, and then turned off the electricity and immersed it in an aqueous ferric chloride solution. It was discovered that the above-mentioned rough surface could be obtained by a two-step process in which the pitting corrosion was grown using the above method. The rough surface of stainless steel obtained by this two-step process has an octopus-like pitting shape, and the size of the pitting is almost constant, and it occurs densely and uniformly, so it has excellent coating adhesion. It can be applied to many applications. However, depending on the application, higher adhesion may be required, and in applications such as enameling, where oxide-based coatings are coated, compatibility with the coating may be required. As a result of various studies, the inventors of the present invention have found that in order to cope with such uses, it is sufficient to perform post-treatment on the surface pitting areas after electrolysis with an aqueous ferric chloride solution. It's ivy. In other words, after mechanical polishing, chemical corrosion, short-term heating in a high-temperature oxidizing atmosphere, chemical conversion treatment, etc.,
If the treatment is applied, the rough surface of the end-pitting corrosion area, which does not contribute much to adhesion with electrolysis alone, will become coarser, and an oxide film or chemical conversion film will be applied there, resulting in the end-pitting corrosion area becoming coarser. The adhesion of the pitting area is improved, and together with the adhesion of the pitting area, the adhesion of the entire surface is significantly improved. The present invention will be explained in detail below. The present invention can be broadly divided into:
The pitting corrosion process consists of a first stage of electrolysis for a short time using stainless steel as an anode, and a second stage of immersion in a ferric chloride aqueous solution. There are two types of cases: cases in which a post-treatment step is added to coarsen the grain, or to apply a thin oxide film or chemical conversion film thereon. First, the pitting corrosion process will be explained. 1 First stage This first stage is a process in which stainless steel is used as an anode in an aqueous ferric chloride solution, and an object with good electrical conductivity (such as various metals or carbon) is used as a cathode for a short period of time. This is the process of forming pitting corrosion, which becomes the starting point of the pitting corrosion formed through the second stage. Therefore, it is important that the pitting corrosion obtained in this step be dense and uniformly distributed over a wide surface, and generally the size and depth do not matter. Even if it is small and shallow, it will be sufficiently reinforced in the second stage. By the way, the properties of stainless steel differ depending on the steel type, so in order to apply pitting corrosion to each steel type as described above, it is necessary to set the concentration of ferric chloride aqueous solution, temperature, current density, and electrolysis time within the appropriate range according to each steel type. Selection is important. Therefore, the present inventors selected the representative SUS304 and higher grade SUS316 for austenitic steels, and selected SUS430, a typical steel type, and lower grade SUS316 for ferrite steels.
SUS410 was selected, these steels (thickness 1.0 mm) were used as anodes, and ordinary steel plates (thickness 1.0 mm) were used as cathodes to investigate the appropriate range of the above-mentioned factors to obtain ideal pitting corrosion. When investigating the appropriate range, we fixed some of the factors, varied the two factors that most influence each other during electrolysis, and determined the appropriate range by taking into account the pitting corrosion state. I decided to do so. Figures 2 to 5 show the relationship between the pitting corrosion state, the ferric chloride solution concentration, and the current density when electrolyzed at a temperature of 60°C and an electrolysis time of 5 minutes.
Figure 5 shows SUS304, SUS316, and SUS4, respectively.
30, the case of SUS410 is shown. Figures 6 to 9 show the relationship between pitting corrosion state, ferric chloride solution concentration, and temperature at a current density of 10 A/dm ² and an electrolysis time of 5 minutes.
The figures are SUS304, SUS316, and SUS43, respectively.
0, the case of SUS410 is shown. Furthermore, Figures 10 to 13 show the relationship between the pitting corrosion state, current density, and electrolysis time at a ferric chloride solution concentration of 10% by weight and a temperature of 50°C.
Figures 12 and 13 are SUS304, SUS316, and SUS4.
30, the case of SUS410 is shown. By the way, in each of the above figures, the pitting corrosion state is classified as follows, and the appropriate electrolytic range is the range surrounded by line a and line b. It is dense and uniformly distributed over the entire surface, and the pore size is usually small, and the second
Condition requiring staged corrosion treatment● (black circle) Condition in which good pitting corrosion is obtained with only the first stage electrolytic treatment, and the subsequent second stage immersion treatment is not required △ The occurrence density of pitting corrosion is low , Non-uniformly distributed state x Full-face corrosion including the end pitting corrosion area. Therefore, in the case of SUS304, if we examine Figures 2, 6 and 10, the concentration of ferric chloride solution is 2.5
~17.3% by weight, and the current density is 1.0~14.9A/ ^dm2 . Also, from Figure 6, the concentration of the second chloride solution is 3.0~
17.5% by weight, temperature 10-74°C. Furthermore, the first
From Figure 0, the current density is 0.8 to 15.6 A/dm ² and the electrolysis time is 2.5 to 30 minutes. To summarize the above case of SUS304, the ferric chloride liquid concentration is 2.5 to 17.5
Weight%, temperature is 10~74℃, current density is 0.8~
15.6A/dm ² and electrolysis time is 2.5 to 30 minutes. To summarize in the same way below, in the case of SUS316, the ferric chloride solution concentration is from Figures 3, 7, and 11.
The content is 3.8 to 23.5% by weight, the temperature is 15.0% to 85.5°C, the current density is 2.0 to 18.8A/dm ² , and the electrolysis time is 3.8 to 30 minutes. In addition, in the case of SUS430, from Figures 4, 8, and 12, the ferric chloride liquid concentration is 0.5 to 13.2% by weight, and the temperature is 8.5 to 8.5%.
66℃, current density 0.7-14.5A/ ^dm2 , electrolysis time
1.3 to 30 minutes. Furthermore, in the case of SUS410, from Figures 5, 9, and 13, the ferric chloride liquid concentration is 0.5 to 11.0% by weight, and the temperature is
The temperature is 7.5 to 56.5°C, the current density is 0.7 to 10.6 A/dm ² , and the electrolysis time is 1.0 to 30 minutes. Therefore, to summarize stainless steel as a whole:
The appropriate electrolytic conditions are ferric chloride solution concentration 0.5 to 23.5.
Weight%, temperature 7.5~85.5℃, current density 0.7~
〓〓〓〓〓
18.8A/dm ² and electrolysis time 1.0 to 30 minutes. By the way, in this stage, as mentioned earlier, the pitting corrosion is concentrated,
Furthermore, although the purpose is to apply the corrosion uniformly over a wide surface, it is important to note that as shown in Figures 10 to 13, the diameter of pitting corrosion may become large and the subsequent second stage of corrosion treatment may not be necessary. worth it. 2 Second stage This second stage is performed by immersing the stainless steel after passing through the first stage in a ferric chloride aqueous solution.
This is a process in which the pitting corrosion formed in this stage grows and develops into pitting corrosion with a good anchoring effect, but since the corrosion progress differs depending on the steel type at this stage,
It is necessary to select the appropriate range depending on the steel type. In other words, in this process, if the corrosion conditions are too weak, the development of pitting corrosion will be slow, resulting in ideal pitting corrosion (octopus pot shape).
On the other hand, if it is too strong, corrosion will occur on the entire surface, and if the material is, for example, a steel plate, the plate thickness will be insufficient. Figures 14 to 17 show the growth of pitting corrosion when stainless steel that has undergone pitting corrosion by electrolysis under appropriate conditions in the first stage is immersed in a ferric chloride aqueous solution for 4 hours in the second stage. Figures 14, 15, 16, and 17 show the relationship between the ferric chloride solution concentration and temperature, and show the cases of SUS304, SUS430, and SUS410, respectively. Also at this stage, the appropriate corrosion condition range is the range surrounded by line a and line b. Therefore, the appropriate corrosion conditions for each steel are ferric chloride liquid concentration 4.5 to 22.5% by weight and temperature 8.5 to 67.0°C for SUS304 as shown in Figure 14, and ferric chloride liquid concentration as shown in Figure 15 for SUS316. The concentration is 7.2 to 26.4% by weight, and the concentration is 16.0 to 67.0°C. In addition, in the case of SUS430, the ferric chloride liquid concentration is 2.1 to 18.8% by weight and the temperature is 7.0 to 54°C from Fig. 16, and in the case of SUS410, the ferric chloride liquid concentration is 1.9 to 14.6% by weight and the temperature is 8.0°C from Fig. 17. ~53℃. Therefore, the appropriate corrosion conditions for stainless steel as a whole are a ferric chloride liquid concentration of 1.9 to 26.4% by weight and a temperature of 7.0 to 26.4% by weight.
The temperature will be 67.0℃. Next, a post-treatment step for improving the adhesion of the end pitting corrosion portion which does not contribute much to the adhesion after the first and second stages of the pitting corrosion step will be described. Depending on the electrolytic conditions, this step may not be necessary in the pitting corrosion process (Figs. 10 to 13), but due to its purpose, the pitting corrosion process does not impart much adhesion to the terminally corroded portion. Therefore, in this process, the adhesion of the end-pitting corrosion area is improved to cope with applications that require a high degree of adhesion. In this step, methods for improving the adhesion of the end-pitting corrosion area include a method of increasing the surface roughness and a method of applying an oxide film. Chemical surface etching methods and mechanical surface polishing methods can be used to increase the surface roughness, but in the case of the present invention, hydrochloric acid, sulfuric acid, or It is sufficient to apply a corrosive liquid such as an acid to the stainless steel surface after pitting corrosion under conventional conditions, and the latter method can also be carried out by polishing with various abrasives under conventional treatment conditions. , shot blasting, liquid honing or grinding may be applied to the stainless steel surface. By these methods, the roughness of the end-pitting corrosion area becomes even coarser, and the adhesion of the coating inevitably improves accordingly. The method for applying the oxide film is to use a short-time heating method in a high-temperature oxidizing atmosphere. Specifically, stainless steel after pitting is heated to 0.5 to 2 It is heated for 1 minute. As a result, a thin oxide film is formed on the stainless steel surface, and since it is the same as the metal oxide that is the main component of glass or ceramic, when these are coated, they become compatible with each other, and in some cases, there may be a problem between the two. Chemical bonding can be expected, further improving adhesion with the coating. One example of a method for applying a chemical conversion coating is a method of treating stainless steel with a solution containing oxalic acid as a main component. That is, if stainless steel is immersed in this solution or sprayed with this solution, a silicide of iron, chromium, or nickel will be formed on the surface of the stainless steel, and the interaction between this silica and the coating will occur. Chemical bonding is expected, and the tightness of the coating is further improved.Next, the present invention will be explained with reference to Examples. Example 1 A SUS304 cold-rolled steel plate (1.0 mm x
1000mm×2000mm. BA finish) is used as an anode for 10A/
Electrolysis was carried out for 5 minutes at a current density of dm ² , and then further
As a two-stage process, the material was immersed for 3 hours in an aqueous solution of ferric chloride at a temperature of 20° C. and a concentration of 15% by weight, resulting in an octopus-shaped rough surface with a uniform and high pitting density and a depth of about 245 μm. FIG. 18 shows a cross section of the surface of the pitting stainless steel. Example 2 As a first step, a SUS316 cold rolled steel plate (1.0 mm×
1000mm×2000mm. BA finish) was electrolytically treated for 10 minutes at a current density of 15 A/ ^dm2 , and then as a second step, immersed in a 20 wt% ferric chloride aqueous solution at a temperature of 40°C for 4 hours to reduce pitting corrosion. An octopus pot-shaped rough surface with a uniform and high depth of approximately 240 μm was obtained. FIG. 19 shows a cross section of the surface layer of the pitting corrosion stainless steel. Example 3 As a first step, a SUS430 cold rolled steel plate (1.0 mm x
1000mm×2000mm. BA finish) is used as an anode for 10A/
Electrolysis was carried out for 5 minutes at a current density of dm ² , and then, as a second step, immersion was performed for 4 hours in an aqueous solution of ferric chloride at a temperature of 40°C and a concentration of 5% by weight, resulting in uniform and high pitting corrosion density and depth. An octopus pot-shaped rough surface of approximately 240μ was obtained. The surface layer cross section of the pitting corrosion stainless steel is No. 20.
As shown in the figure. Example 4 The surface of the stainless steel plate obtained in Example 1 was further polished using ground aluminum with #120 abrasive grains to coarsen the roughness of the end pitting area. A cross section of the surface layer of this material is shown in FIG. Example 5 A mixed aqueous solution containing 5% by volume of each of nitric acid and hydrofluoric acid was heated to 70°C, and the stainless steel plate obtained in Example 2 was immersed in the solution for 5 minutes to increase the roughness of the end pitting corrosion area.
A cross section of the surface layer of this product is shown in FIG. Example 6 The stainless steel plate obtained in Example 3 was further heated in the air at 800°C for 3 minutes to form an oxide film. A cross section of the surface layer of this material is shown in FIG. Example 7 Various commercially available synthetic resin paints were applied to the rough-surfaced stainless steel formed in Examples 1 to 6 and the satin-finished stainless steel sheets manufactured by conventional methods so that the dry coating thickness would be 30 to 40μ. After painting, JIS33
12 (Colored galvanized iron plate) and 8.5 (Cross grid test) A grid pattern was drawn, and then the area was peeled off with cellophane tape to conduct a paint film adhesion test. Table 1 shows the results and shows that the stainless steel plates produced by the method of the present invention exhibit superior coating film adhesion compared to those produced by the conventional method. Further, although not shown in Table 1, more severe tests were conducted, and it was found that those with coarser end-pitting corrosion areas exhibited particularly excellent coating film adhesion.

[Table] 〓〓〓〓〓

[Table] As described above, the rough surface of stainless steel formed according to the present invention has dense pitting corrosion, and there are pitting areas that are evenly formed, and areas where the roughness is coarsened, or where there is an oxide film or a chemical conversion coating. Since the coating is composed of a pitting corrosion part and a pitting part which has been subjected to the above-mentioned methods, it exhibits excellent adhesion to the coating due to the relative effects of the two. Furthermore, since the pitting corrosion process of the present invention is comprised of short-time electrolytic treatment and immersion treatment, it is possible to save electricity costs compared to the case where ideal pitting corrosion is performed only by electrolytic treatment. In addition, when implementing the method, if the electrolytic cell and the immersion tank are separately provided and used for exclusive use, electrolysis efficiency and work efficiency can be improved, and manufacturing costs including fixed costs can be reduced. Since the rough surface obtained by the present invention exhibits excellent coating adhesion as described above, it can be coated with a thickness of 100 μm or more, and in some cases, 1000 μm or more.

[Brief explanation of the drawing]

Figure 1 shows rough stainless steel surfaces formed by the conventional method and the method of the present invention, 1 and 2 are cross-sectional views schematically showing the rough surfaces formed by the conventional method, and 3 is a cross-sectional view schematically showing the rough surfaces formed by the method of the present invention. 4 is a cross-sectional view schematically showing a rough surface formed by the method of the present invention, and 4 is a plan view of the rough surface formed by the method of the present invention. Figures 2 to 5 show how the concentration of ferric chloride solution and current density affect pitting corrosion when stainless steel is electrolyzed at an electrolysis temperature of 60°C and an electrolysis time of 5 minutes in electrolytic treatment, which is the first stage of the pitting corrosion process. Figures 2, 3, 4, and 5 are diagrams showing the relationship between SUS304 and SUS304, respectively.
The cases of SUS316, SUS430, and SUS410 are shown. Figures 6 to 9 show the current density in the above electrolytic treatment.
6, 7, and 7 are diagrams showing the relationship between the concentration of ferric chloride solution and the electrolysis temperature on pitting corrosion when stainless steel is electrolyzed at 10 A/dm ² and for an electrolysis time of 5 minutes.
8, 9, the figures are SUS304, SUS316, respectively.
The case of SUS430 and SUS410 is shown. Figures 10 to 13 are diagrams showing the relationship between current density and electrolysis time on pitting corrosion when stainless steel is electrolyzed at a ferric chloride solution concentration of 10% by weight and an electrolysis temperature of 50°C in the above electrolytic treatment. Atte, No. 10, 1
Figures 1, 12, and 13 are SUS304 and SUS3, respectively.
16, SUS430, and SUS410 are shown. The symbols in FIGS. 2 to 13 indicate the following pitting corrosion occurrence states. 〇 (white circle) Pitting corrosion on the stainless steel surface is dense and uniformly distributed over the entire surface, and the pore diameter is usually small, requiring a second stage of corrosion treatment. 〓〓〓〓〓
● (black circle) Condition in which good pitting corrosion is obtained only by the electrolytic treatment in the first stage of the pitting corrosion process, and the subsequent second stage of immersion treatment is not required. △ Pitting corrosion occurs at low density and unevenly distributed. × A state where corrosion has occurred on the entire surface including the end pitting corrosion area. Figures 14 to 17 show that stainless steel is electrolyzed in the pitting corrosion process stage under the conditions of a ferric chloride solution concentration of 10% by weight, a current density of 10 A//dm ² , and an electrolytic temperature of 50°C. Figures 14, 15, 16, and 17 show the relationship between the concentration of ferric chloride solution and temperature on the growth of pitting corrosion when immersed in ferric chloride solution for 4 hours. SUS43
0, the case of SUS410 is shown. Further, symbols in the figure indicate the following states. 〇 The growth of pitting corrosion progresses uniformly over the entire surface, and the density of pitting corrosion is high. △ The growth of pitting corrosion has progressed partially, and the density of pitting corrosion is low. × General corrosion state Figures 18, 19, 20, 21, 22, and 23 are cross-sectional views of the stainless steel surface layers formed in Examples 1, 2, 3, 4, 5, and 6, respectively. 〓〓〓〓〓

Claims (1)

[Claims] 1. In a ferric chloride aqueous solution at a temperature of 7.5 to 85.5°C and a concentration of 0.5 to 23.5% by weight, stainless steel is used as an anode,
After performing electrolysis for 1 to 30 minutes with direct current at a current density of 0.7 to 18.8 A/ ^dm2 , the current was turned off, and then immersed in an aqueous ferric chloride solution at a temperature of 7 to 67°C and a concentration of 1.9 to 26.4% by weight. A method for producing stainless steel with excellent adhesion to coatings, which is characterized by applying pitting corrosion. 2 Electrolyze in a ferric chloride aqueous solution with a concentration of 0.5 to 23.5% by weight at a temperature of 7.5 to 85.5°C using stainless steel as an anode and direct current with a current density of 0.7 to 18.8 A/dm ² for 1 to 30 minutes, then turn off the electricity. , then temperature 7-67℃, concentration
Pitting corrosion is applied by immersion in a 1.9 to 26.4% by weight ferric chloride aqueous solution, and then the surface of the pitting area is roughened by chemical corrosion or mechanical polishing, or by short-term immersion in an oxidizing atmosphere. A method for producing stainless steel with excellent coating adhesion, which is characterized by applying an oxide film or a chemical conversion film to the end-pitting corrosion area using a heating method or a chemical conversion treatment method.