JPH04180559A - Production of high corrosion resistant stainless steel - Google Patents

Abstract

PURPOSE:To produce high corrosion resistant stainless steel stable in a range over a long period by introducing oxygen ions into the surface layer of stainless steel while stepwise changing acceleration voltage between high voltage and low voltage, and specifying the introduction amount of oxygen ions. CONSTITUTION:Introducing treatment of oxygen ions is performed on the surface layer of stainless steel. In this case, the acceleration voltage in introduction of oxygen ions is stepwise changed to a low voltage from not higher than 30keV till a high voltage not lower than 100keV or to a high voltage from not lower than 100keV till a low voltage not higher than 30keV. Furthermore, the introduction amount of oxygen ions due to acceleration voltage in the respective steps is regulated to >=1X10<15> ions/cm<2>. Moreover, the whole introduction amount of oxygen ions is regulated to >=1X10<16> ions/cm<2>. Thereby oxide is formed to the deep position of the surface layer. Accordingly, high corrosion resistant stainless steel is obtained which is extremely low in the eluted amount of metal in a passive electric potential region and capable of maintaining corrosion resistance in a range over a long period.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a method for manufacturing highly corrosion-resistant stainless steel, particularly in the food industry, semiconductor manufacturing industry, where trace amounts of metal elution are a problem,
The present invention also relates to a method for manufacturing highly corrosion-resistant stainless steel with extremely low metal elution, which is used in fields such as nuclear power generation.

(Prior art) Stainless steel is widely used as a corrosion-resistant material in various fields, but in fields such as the food industry and semiconductor manufacturing industry, even if a small amount of metal is leached due to corrosion, the quality of the product will deteriorate. This is a problem because it causes In addition, in nuclear power plants, trace amounts of metal elements eluted into the cooling water become radioactive in the reactor and become incorporated into the surface oxide film, which is a corrosion product formed as the steel materials used, such as stainless steel, corrode. This poses a health problem in that workers are exposed to radiation when inspecting equipment.

Stainless steel is originally used in the passive potential range where it exhibits corrosion resistance, but in the industrial field where metal elution is a problem, even in minute quantities, as mentioned above, stainless steel is used as a material with better corrosion resistance and extremely less metal elution. Development of materials is desired.

Conventionally, methods for suppressing metal elution from such electrode devices include a method using electrolytic polishing and a method of forming an oxide film by high temperature heat treatment in an oxidizing atmosphere (Japanese Patent Publication No. 2-191).
6), nitric acid treatment Pl! A method of forming an oxide film from this (Japanese Patent Publication No. 156719) has been proposed.

Further, general methods for improving corrosion resistance include a method of coating the metal surface with highly corrosion-resistant ceramic fuchs and a method of press-fitting metal ions such as Cr into the surface of the base metal.

(Problems to be Solved by the Invention) However, electrolytic polishing, high-temperature heat treatment in an oxidizing atmosphere, or nitric acid treatment do not have a sufficient suppressing effect on metal elution. In other words, all of these methods form a passive film (oxide) through a corrosion reaction on the steel surface or an equilibrium reaction with a gas. Elements that are formed and have a high affinity for oxygen in a gas are easily oxidized, and in the case of stainless steel, Cr oxides are first produced. However, since the formation reaction of the passive film is an equilibrium reaction in a solution or gas, it is difficult to form a film composition with sufficient corrosion resistance. In addition, an oxide film is likely to have defective parts, and may become cracked, cracked, or porous, and there is also a concern that it may peel off.

In addition, the method of coating with highly corrosion-resistant ceramics has a problem with adhesion, and if it peels off during use, the product defect rate will significantly increase due to metal elution. In the method of implanting metal ions such as Cr, the amount of ion current cannot be increased, so the processing time must be increased in order to obtain a predetermined performance, which increases the processing cost.

An object of the present invention is to provide a method for manufacturing highly corrosion-resistant stainless steel that has an extremely small amount of metal leached in the passive potential range and that can maintain its corrosion resistance over a long period of time.

(Means for Solving the Problems) Generally, metal elution is caused by an electrochemical reaction on the surface of the metal. Therefore, in order to suppress metal elution, it is necessary to treat the surface to suppress this electrochemical reaction. When the surface of a metal is covered with an oxide,
Because oxides have high electrical resistance, electrochemical reactions are not possible.
＜, corrosion is thought to be suppressed.

Based on this idea, the present inventor used oxygen ion implantation method as a method to form oxidized vIJ (Cr oxide) on the surface of stainless steel, and introduced oxygen ions into the steel in an amount exceeding the amount determined by thermal equilibrium. proposed a method for manufacturing highly corrosion-resistant stainless steel in which oxides are formed not only on the surface but also inside (Japanese Patent Application No. 127449/1999).

According to this method, highly corrosion-resistant stainless steel with extremely small amount of metal elution can be produced.

However, in a severe corrosive environment, the oxide film layer on the surface dissolves and the corrosion resistance gradually deteriorates.

Therefore, as a result of further detailed studies on the oxygen ion implantation method, the inventors of the present invention found that corrosion resistance could be extended by changing the oxygen ion acceleration voltage stepwise from high voltage to low voltage, or from low voltage to high voltage. It was found that it can be maintained for a long period of time.

The present invention was made based on such knowledge, and the gist thereof is ``a method for manufacturing highly corrosion-resistant stainless steel in which the surface layer is subjected to oxygen ion implantation treatment, the acceleration voltage of oxygen ion implantation treatment being set to 100 keV or higher. High voltage to 30keV
The oxygen and ion implantation amount is changed stepwise from the following low voltage, or from the low voltage of 30 keV or less to the high voltage of 100 keV or more, and the amount of oxygen and ion implantation is adjusted according to the acceleration voltage of each step.
ions/c+m2 or more, and the total amount of oxygen ions implanted is IXl0I6ions/cm2 or more.

The method of the present invention is applied to stainless steels with a Cr content of 13% or more (hereinafter, "%" means weight%), such as 5115409, SO3493, etc.
) 7 x Light stainless steel, SO5304, S
Austenitic stainless steel such as O3316 and SOS 310. In addition, A11oy 600, A11
This method is also effective for Nj-based alloys containing 13% or more of Cr, such as OY 690 (these are also included in the scope of "stainless steel" in this specification).

The oxygen ion implantation process can be performed, for example, by ionizing oxygen gas with a Freeman ion source, passing it through a mass separator, accelerating it at a predetermined acceleration voltage, and press-injecting it.

(Function) Generally, to maintain corrosion resistance and form the necessary Cr passive film, a C content of 13% or more is required.
If the r content is low, a sufficient passive film of Cr, which is necessary to maintain corrosion resistance, will not be formed.

Figure 1 schematically shows the formation of an oxide (Cr oxide) film by the method of the present invention in comparison with that by conventional methods (for example, acid treatment or high-temperature heat treatment in an oxidizing atmosphere). It's a country. In the figure, in the case of acid treatment (Figure (a)), the film 2 formed on the surface of the base stainless steel 1 is extremely thin. In addition, in the case of heat treatment (Figure (b)), the oxide film 3 is formed by an equilibrium reaction on the surface of the base stainless steel 1.
The film mainly grows on the outside of the surface of the rope 1. However, as mentioned above, it is difficult to form a uniform film. On the other hand, in the case of the method of the present invention (Figure (C)), a large amount of oxygen ions are forced out by applying an accelerating voltage to the oxygen ions, regardless of the surface reaction. ! In addition to increasing the oxygen tension not only on the surface of the stainless steel 1 but also inside it,
A uniform oxide film layer 4 can be formed.

The oxygen ion implantation method used in the present invention is a method characterized by forcibly introducing oxygen ions into steel, that is, in an amount exceeding the amount determined by thermal equilibrium, as described above, to form an oxide. This oxide (film) is not formed by a surface reaction, but is a layer formed by a reaction between the base steel and the injected oxygen ions, and is free from defects and is not susceptible to cracking or peeling. Not at all.

The total amount of oxygen ions implanted is as described in the above-mentioned Japanese Patent Application No. 2-12744.
As described in the specification of No. 9, lXl0Ihion
If it is less than "s/cm", there is almost no corrosion inhibition effect.
I Xl0I6ions/c! It is necessary to set it to 1" or more.

Although it is not necessary to set an upper limit for the total amount of oxygen ions to be implanted, the higher the amount of implanted oxygen ions, the longer the process will take, so the upper limit is usually about IXIO''1 ons/cm2.

The implantation depth of oxygen ions is, for example, an acceleration voltage of 120 keV.
, implanted ions 11 x 10"1ons/cm" approximately 2
There are 500 people.

The oxygen ion species to be implanted are 0゛ and 0□.

Either of these is fine.

The surface condition of the sample before implantation does not need to be determined in particular, but the flatter the surface, the more uniformly oxygen ions can be implanted.6 The surface roughness is 0.1 μ- in center line average roughness (Ra).
It is desirable to do the following.

Changing the acceleration voltage of oxygen ions from high voltage to low voltage or from low voltage to high voltage in stages changes the implantation depth of oxygen ions in stages, from shallow to deep on the surface. By making the amount of oxygen ions implanted in the depth direction of the surface layer portion uniform, the corrosion resistance in the depth direction is made uniform, and by forming a layer with a high amount of implanted oxygen ions on the outermost surface, it is possible to This is because it provides high corrosion resistance over a long period of time, thereby making it possible to maintain corrosion resistance for a long time.

The acceleration voltage is 100 keV or more on the high voltage side and 3 on the low voltage side.
Setting it to 0 keV or less: Well, if this condition is not met, there will be little effect in maintaining corrosion resistance over a long period of time. Note that the lower limit of the accelerating voltage is ↓ or 1okeV, and below this, the sputtering effect becomes more dominant than the injection effect, and effective implantation cannot be performed.

The oxygen ion implantation amount at each stage of acceleration voltage is I
0I6ions/c! 12 or more is because if the amount of oxygen ions implanted is less than this, the amount of implanted oxygen ions will not be uniform in the depth direction even with the implantation process in which the accelerating voltage is changed stepwise, and the corrosion resistance will not be improved. Because it's not ridiculous.

(Example) Stainless steel and Ni-based alloy shown in Table 1 were used as test materials, and the surface roughness Ra = 0. After polishing to 01μ, degrease with acetone and increase the acceleration voltage to 20 to 400k.
Oxygen ion (0°) implantation treatment was performed while changing the voltage stepwise from high voltage to low voltage or from low voltage to high voltage within the eV range. For comparison, a process was also performed in which a specific acceleration voltage was fixed.

Table 1 (4th place: 11L, %) Table 2 shows the stepwise changes in acceleration voltage, the amount of oxygen ions implanted at each acceleration voltage, and the total amount of implantation.

After oxygen ion implantation, a constant potential corrosion test was conducted in 5% sulfuric acid, and the corrosion current flowing through the test piece was measured. Melting of metals is caused by electrochemical reactions (e.g. Fe-*Fe"+'le
-, Cr-+Cr"°+3e-, Ni-+Ni"+2e
−), and the corrosion current corresponds to the amount of metal eluted, so
By comparing these current values, it is possible to determine the difference in the amount of metal eluted. The set potential in the constant potential corrosion test is 1.
lV (based on saturated calomel electrode). This potential is outside the passive potential range of stainless steel, and as shown in Figure 2, when oxygen ion implantation is not performed (failure! When oxygen ion injection treatment is performed (solid line), it takes about 20 minutes to show the same current value as the untreated material.This time is the time required until the corrosion resistance imparted by oxygen ion injection treatment is lost ( (corrosion resistance duration), corrosion resistance can be evaluated in a relatively short period of time without conducting long-term corrosion tests.

The measurement results are also shown in Table 2. From the same table, the minimum accelerating voltage is 30 keV or less, the maximum accelerating voltage is 100 keν or more, and the injection amount at each voltage is lXIO"1ons/c+
m” or more, and the total amount of oxygen ions implanted is lXl0
Examples of the present invention (No. 1 to 8) in which the value was ``tons/cm'' or more
), when implantation is performed using a single accelerating voltage (
It can be seen that the corrosion resistance duration was significantly extended compared to Comparative Examples Nos. 10.12 to 17 and 19).

Even if the acceleration voltage of oxygen ions is changed stepwise, the conditions deviate from the conditions specified in the method of the present invention (comparative example floors 20 to 2).
3) had a short corrosion resistance duration.

(Hereinafter, blank space) Table 2 (Effects of the invention) According to the method of the present invention, Fudo L! It is possible to produce highly corrosion-resistant stainless steel in which the amount of metal eluted in the i potential range is extremely small and whose corrosion resistance lasts for a long period of time. This steel is highly practical as a highly corrosion-resistant material in industrial fields where trace amounts of metal elution are a problem, such as the food industry and semiconductor manufacturing industry.

[Brief explanation of drawings]

Fig. 1 is a schematic cross-sectional view showing the formation of oxides on the steel surface, (Fig. 81 is when acid treatment is performed, 0))) is when high temperature heat treatment is applied, and (C) [m is when the method of the present invention is applied. It is. FIG. 2 is an explanatory diagram of the corrosion resistance determination test used in the examples.

Claims (1)

[Claims] A method for producing highly corrosion-resistant stainless steel in which the surface layer is subjected to oxygen ion implantation treatment, the acceleration voltage of oxygen ion implantation treatment being reduced from a high voltage of 100 keV or more to a low voltage of 30 keV or less, or from a low voltage of 30 keV or less to 100 kV.
The voltage was changed stepwise to a high voltage of eV or higher, and the amount of oxygen ions implanted was 1×10^1^5 ions depending on the acceleration voltage at each stage.
/cm^2 or more, and the total amount of oxygen ions implanted is 1×
A method for producing highly corrosion resistant stainless steel, characterized in that the corrosion resistance is 10^1^6 ions/cm^2 or more.