JPS63495A - Improvement of corrosion resistance of stainless steel - Google Patents

Abstract

PURPOSE:To stably improve the corrosion resistance of a stainless steel material having a lustrous surface without deteriorating the luster by immersing the steel material in a nonoxidative acid bath and holding it in the bath for a specified time. CONSTITUTION:A coil 1 of a stainless steel material having fine surface luster such as a 2B stainless steel material or a bright-annealed stainless steel material is continuously uncoiled, immersed in a nonoxidative acid bath 2 and held in the bath 2 for such a time that the natural potential suddenly changes to the lower side and the surface luster rate does not suddenly drop. A reference electrode 4 is placed in a treatment tank 3 contg. the acid bath 2, the potential difference between the electrode 4 and one end of the coil 1 is continuously measured and a sink roll 5 is moved up and down according to the measured difference to control the immersion time.

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention improves the corrosion resistance of stainless steel with a glossy surface, such as so-called "2B specification material" and "BA treated material (bright annealing treated material)". The present invention relates to a surface treatment method that can be carried out simply and at low cost on an industrial scale, in order to stably improve the surface gloss without impairing it.

<Background Art> In general, stainless steel plates for normal use are manufactured by cold-rolling plates to a desired thickness and annealing them in a heating furnace.
Since the annealing atmosphere contains a large amount of 02 and H and O produced by combustion of fuel, a thick oxide scale is formed on the surface of the stainless steel sheet after annealing. Therefore, it is essential to remove oxidized scale from stainless steel sheets after annealing by molten salt bath treatment and pickling, but such treatments have the inconvenience of impairing the gloss of the stainless steel sheet surface. there were.

Therefore, for applications where gloss is important, the stainless steel plate after pickling should be temper-rolled again to give it luster (2B specification), or the inside of the furnace during annealing should be heated with ammonia decomposition gas (Hz). Measures have been taken to maintain the gloss by annealing (bright annealing: BA) in an inert gas atmosphere such as: Nz = 3: 1).

However, when manufacturing 2B specification materials, if we try to perform sufficient pickling, the surface of the stainless steel plate becomes extremely rough, and after that the surface of the stainless steel plate becomes 8m long! Even with rolling, sufficient gloss cannot be obtained, and on the other hand, if you try to carry out weak pickling with emphasis on gloss,
There was a problem in that the so-called chromium-deficient layer (a layer with poor corrosion resistance caused by preferential oxidation of chromium during annealing) on the surface could not be sufficiently removed.

In addition, when bright annealing (BA) is used, pickling is not necessary because oxide scale does not form on the surface, and it is said that stainless steel Et& that retains excellent surface gloss and other properties can be obtained as is. If we examine it strictly, we will find that trace amounts of H, O, and 0□ are mixed into the inert atmosphere mentioned above, and therefore an oxide film will be formed on the surface of the stainless steel sheet even in this case, although it is extremely thin. Among the components of stainless steel, SlSMn
Since 5Cr and the like are elements that are relatively easily oxidized, it is thought that the oxide film formed during bright annealing normally contains a large amount of these elements. It is said that nitrides may be formed by reacting with N2 gas. In any case, the properties of the film formed on the stainless steel surface by bright annealing vary greatly depending on the temperature, time, atmospheric gas, etc. during annealing, and it may not have a protective property like the so-called "non-film". In some cases, a strong film was formed, and in other cases, a film was formed that had very weak protective properties and actually prevented passivation of the stainless steel surface.In this way, bright annealing conditions and the corrosion resistance of stainless steel The relationship between these two conditions is extremely complex and there are many varying factors (currently, it is common for the corrosion resistance of bright annealed stainless steel materials to vary greatly depending on the annealing conditions, and it is necessary to elucidate these mutual relationships and determine the bright annealing conditions from the perspective of corrosion resistance. It was still at a stage where it was industrially impossible to control it.

As mentioned above, the corrosion resistance of commercially available stainless steels with glossy surfaces such as F12B specification stainless steel materials and bright annealed stainless steel materials varies considerably due to subtle differences in the manufacturing process, even if the steel type and surface specifications are the same. Moreover, since it had more or less a layer of relatively poor corrosion resistance on its surface, it was not uncommon for it to speak during use. However, even if a stainless steel material has a layer with relatively poor corrosion resistance on its surface, the outermost surface has been made passivated by the action of 02, etc. in the atmosphere, so it still maintains the corrosion resistance of stainless steel. However, if the outermost surface is removed due to eaves or acid adhesion, localized corrosion such as pitting corrosion is likely to occur, leading to speech.

For this reason, as a method to improve the corrosion resistance of bright annealed stainless steel materials without deteriorating the gloss, there have been several methods to improve the corrosion resistance of bright annealed stainless steel materials without deteriorating the gloss. A method has been proposed in which the material is further immersed in an oxidizing acid such as nitric acid for passivation treatment if necessary (Japanese Unexamined Patent Publication No. 142140/1983, No. 142142/1983). , JP-A-59-6398, and JP-A-59-23882), each of these methods had the following problems. That is, i) Since both methods involve electrolytic treatment, more expensive electrolytic equipment is required for implementation.

Ii) All of these can be easily carried out in a laboratory beaker test, but when attempting to process continuous ME strips on an industrial scale, special measures are required to conduct electricity to the moving copper strips. However, it is difficult to avoid inconveniences such as the occurrence of spark defects and a significant decrease in current efficiency.

iii) Since the surface corrosion resistance of the treated materials is not uniform and differs from material to material, it may be possible to produce a glossy product even if treated under the same processing conditions (bath temperature, bath concentration, current density, processing time). This results in large variations in the quality of the resulting products, such as the production of products that lack luster.

<Means for Solving the Problems> From the above-mentioned viewpoint, the present inventors have developed materials with 2B specification and optical treatment, which inevitably have variations in corrosion resistance and which cannot ignore the existence of a layer with slightly inferior corrosion resistance. In order to find a way to easily and stably improve the corrosion resistance of "stainless steel materials with a glossy surface" such as annealed materials, without using expensive equipment such as electrolytic equipment, and without reducing the gloss of the material. As a result of the research, the following findings were obtained. That is, (a) As a means to improve the corrosion resistance of stainless steel without using electrolytic equipment, it is possible to strengthen the passive film (oxide film) by immersing stainless steel in oxidizing acid such as nitric acid. In this case, the chromium-deficient layer present on the surface of the above-mentioned 2B specification material or bright annealed material may not be easily removed, so that no significant effect on pitting corrosion resistance can be obtained.

(b) On the other hand, when stainless steel 2B specification material or bright annealed material is immersed in non-oxidizing acid such as oxalic acid or sulfuric acid, the removal of the layer with poor corrosion resistance on the surface of the stainless steel material is comparable. Although it is easy to do, the surface gloss is often lost, and if you try to maintain the surface gloss, the "layer with poor corrosion resistance" mentioned above cannot be removed sufficiently. It is extremely difficult and poses many problems in actual work.

(C) However, when the self-potential of stainless steel immersed in a non-oxidizing acid is measured, as shown in Figure 1, it usually shows a tendency to suddenly change in the direction of base after a few seconds to a few minutes. On the other hand, the surface gloss of stainless steel immersed in a non-oxidizing acid does not change for a while even after the point of sudden change in the self-potential. Showing a tendency for rapid deterioration after a certain period of time.

Table 1 shows the ``time at which the natural potential suddenly changes'' (T in Figure 1) when various stainless steel materials are immersed in non-oxidizing acid.
, and the time at which the surface gloss suddenly drops (represented by T2 in Figure 1), but even if the materials and acid baths are different, T and T2 can be clearly recognized. It will be done.

(d) This sudden change in spontaneous potential occurs because the passive film (oxide film) on the stainless steel surface is destroyed by the acid, and the subsequent stable potential is due to the "layer with poor corrosion resistance" on the surface and this. It is assumed that this indicates the subsequent smooth elution of the metal.

(e) By the way, in general, elution on the surface is not completely uniform, and the unevenness that occurs due to this inevitably has a negative effect on the surface gloss, but the curve showing the change in surface gloss in Figure 1. ``In the above-mentioned ``layer with poor corrosion resistance'' and the leaching process of the base metal, the ``layer with poor corrosion resistance'' has a thickness of several tens to hundreds of layers and is easily melted. When this layer is eluted, it has almost no effect on the surface gloss, and when the ``layer with poor corrosion resistance'' finishes eluting and the base metal starts eluting, the surface gloss suddenly decreases. What is assumed to be indicated.

(fl) Then, the 2B specification stainless steel material or the bright annealed stainless steel material is immersed in a non-oxidizing acid, and the natural potential of the stainless steel suddenly changes in the range shown by (T2-TI) in Figure 1 above. Even if the lifting time is at the beginning of the range indicated by T above, a fairly noticeable improvement in corrosion resistance can be obtained. To achieve this, a material can be created that can be used to improve surface gloss while maintaining excellent surface gloss.

(g) Therefore, if you understand the "self-potential change curve" and "gloss rate change curve" as exemplified in Figure 1 when stainless steel material with a glossy surface is immersed in non-oxidizing acid, , It is possible to stably improve the corrosion resistance of the stainless steel material with simple equipment without causing a decrease in gloss.

This invention was made based on the above findings, and involves immersing a stainless steel material with a glossy surface in a bath of non-oxidizing acid, and exceeding the point at which the natural potential of the stainless steel suddenly changes in the direction of base. ．． The stainless steel material is characterized in that the corrosion resistance of the stainless steel material is stably and reliably improved without impairing the surface gloss by holding the stainless steel material in the acid bath for a period of time that does not exceed the point at which the surface gloss ratio suddenly drops.

Here, the term "stainless steel material with a glossy surface" refers to stainless steel materials with excellent surface gloss, such as 2B specification stainless steel and bright annealed stainless steel materials, as described above, and the quality of the material is limited. It's not a thing. Furthermore, there are no particular restrictions on the "bath of non-oxidizing acid," but it is preferable to use an aqueous solution of oxalic acid, sulfuric acid, or the like.

The holding time in the non-oxidizing acid bath is calculated based on the ``point of time when the natural potential of stainless steel suddenly changes to the base'' and ``the point of time when the surface gloss rate suddenly drops'', which were determined experimentally in advance. However, the stainless steel material is immersed in a non-oxidizing acid bath and its potential is tracked and measured at the same time, and the ``point at which the surface gloss rate suddenly drops'' determined experimentally in advance is also taken into account. It is preferable to adjust the

Stainless steel materials whose "layers with poor corrosion resistance" on the surface have been removed by such "immersion treatment in a non-oxidizing acid bath"
Immediately after treatment, the desired corrosion resistance is not exhibited because the passive film on the surface is insufficient, but the passive film is regenerated by being kept in the atmosphere for several days, and at this point corrosion resistance exceeds the original state. .

In other words, the corrosion resistance improving effect of the surface treatment method according to the present invention is achieved by appropriately removing a surface film with a relatively low level of corrosion resistance, such as that seen on commercially available glossy stainless steel mouthpieces, etc.
It is created by creating a base that can smoothly form the high level of corrosion-resistant film that the metal should originally have, but after this treatment, in order to regenerate a strong passive film in a shorter time, it is necessary to It is preferable to immerse it in a heated aqueous nitric acid solution of about 30% by weight. In this case, several percent in nitric acid
By adding CrO3, dichromate, permanganate, etc., or blowing ozone, the time required to regenerate the passive film can be further shortened.

In addition, if skin pass rolling is performed after treatment with a non-oxidizing acid bath or after passivation treatment with an aqueous nitric acid solution, the surface gloss will further improve, which is an effective means for increasing product value. becomes.

Next, the present invention will be specifically explained using examples and comparing with comparative examples.

<Example> First, stainless steel mouthpieces having the component compositions and surface conditions as shown in Table 2 were prepared, and were treated under the conditions shown in Table 3.

Next, the gloss retention rate and corrosion resistance of the obtained treated materials were investigated, and the results are also shown in Table 3 in comparison with the untreated materials.

In addition, the "gloss retention rate" referred to here is a conflict expressed by the formula, and the corrosion resistance is determined by the "Cast test (JIS DO
201)” and “Atmospheric exposure test”, the Cath test time was 400 hours and the number of atmospheric exposure test days was 6.
It was for 5 days.

As is clear from the results shown in Table 3, the untreated material does not have sufficient corrosion resistance, while the surface-treated material of the present invention shows almost no decrease in gloss (after temper rolling). (On the contrary, the gloss increases when the coating is applied), and it can be seen that it has extremely good corrosion resistance.

By the way, FIG. 2 is a schematic diagram showing an example of the case where the stainless steel coil material 1 is continuously subjected to the treatment of the present invention. A comparison electrode 4 is inserted into the treatment tank 3, the potential difference between the comparison electrode 4 and one end of the coil 1 is continuously measured, and the immersion time is controlled by moving the zinc roll 5 up and down according to the measured value. It is a method of In FIG. 2, the reference numeral 6 is a potentiometer, and the reference numeral 7 is a deflation crawl.

As is clear from this example, the surface treatment method of the present invention can be carried out simply and easily without the use of particularly expensive and troublesome equipment, and is an extremely advantageous means industrially. I can say that there is.

Overall effect> As explained above, according to the present invention, the corrosion resistance of stainless steel with good surface gloss, which was considered to be somewhat difficult, has been improved.
It becomes possible to easily and stably improve the surface gloss without adversely affecting it, and it brings about extremely useful effects industrially, such as further expanding the field of application of stainless steel materials.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the ``soaking time'', the ``natural potential of the stainless steel field'', and the ``surface gloss retention rate'' when a stainless steel material with a glossy surface is immersed in a non-oxidizing acid. The figure is a schematic diagram showing how a stainless steel coil material is subjected to the surface treatment of the present invention. In the drawings, 1... Stainless steel coil material, 2... Non-oxidizing acid bath, 3... Treatment tank, 4... Comparative Ti electrode, 5... Zinc roll, 6... Potentiometer , 7... Deflation crawl.

Claims (1)

[Claims] A stainless steel material with a glossy surface is immersed in a bath of non-oxidizing acid, and the acid is immersed for a period of time that exceeds the point at which the self-potential of the stainless steel suddenly changes toward the base, but does not exceed the point at which the surface gloss rate suddenly drops. characterized by being kept in a bath;
A method for improving the corrosion resistance of stainless steel without impairing its surface gloss.