EP0951582A1 - Surface processing of steel or nickel alloy and processed steel or nickel alloy - Google Patents

Abstract

The invention relates to a method for surface processing of steel and/or nickel alloy. It also relates to the use of said method and steel or nickel alloy processed using such a method. Processing involves the use of peroxide or a water-peroxide mixture or a peroxide solution.

Description

description

Surface treatment of steel or nickel alloy and treated steel or nickel alloy

The invention relates to a method for the surface treatment of steel, in particular stainless steel, and / or of a nickel alloy. The invention also relates to uses for such a method and a steel or nickel alloy produced by such a method.

The steel or the nickel alloy are, for example, of the type Incoloy 800, Inconel 600 or the like. The method can be used in nuclear technology to reduce the later activity absorption (contamination) of components of the primary circuit of water-cooled nuclear power plants before new installation or after decontamination.

In water-cooled nuclear power plants, such as boiling water reactors (SWR) and pressurized water reactors (DWR), oxide layers form on the wetted surfaces, which largely consist of zirconium alloys and austenitic chromium-nickel steels (so-called stainless steel), by reaction with the hot water and / or steam. Some of these oxide layers get into the water cycles through solution or erosion and can be activated in the neutron field. If the activated corrosion products are stored outside of the reactor core on surfaces of components in oxide layers present there or as particles on them, these components are radioactively contaminated. Components in the pressurized water reactor that are at risk of contamination are mainly main coolant pumps and steam generators, in older boiling water reactors with external circulation, they are Recirculation lines and the reactor water purification system.

In order to avoid an inadmissible radiation exposure of the operating staff during operation, inspection, maintenance work and repairs, this contamination must be kept as low as possible. This can be done through a careful choice of materials and operating parameters, e.g. water chemistry. If the contamination nevertheless rises to inadmissible values, such systems must be decontaminated. This is done by a chemical treatment by which the oxide layer and thus the activated corrosion products contained therein are removed.

In the past, many subsystems of nuclear power plants have been routinely decontaminated, such as the main coolant pumps, steam generators or parts of them in pressurized water reactors, the circulation loops and the cleaning system in boiling water reactors. These decontamination procedures are today

State of the art and are offered commercially. As a rule, decontamination factors between 10 and well over 100 are achieved.

For economic and technical reasons, in most cases only the directly disruptive contaminations are removed, while the majority of the surfaces, such as the surfaces of the fuel elements, are not treated in order to keep the volume of radioactive waste to be disposed of during decontamination as low as possible . If the components cleaned in this way are put back into operation, their surfaces in contact with the coolant quickly become covered with an oxide layer. This oxide layer settles with the one present in the coolant, or with that from the non-decontaminated surfaces activation products entering the coolant into equilibrium. The consequence of this is a very rapid recontamination of the cleaned surfaces. Very rapid contamination of components is also observed when components are replaced. The recontamination of cleaned surfaces or the contamination of newly installed surfaces can quickly reach values that are higher than before decontamination. This was observed, for example, in the circulation loops in a nuclear power plant after the replacement of the circulation loops.

In the past, various attempts have been made to pretreat the decontaminated or newly used surfaces in such a way that the contamination only occurs to a reduced extent. The following ways are basically available

- Reduction of the available activation products. This can be done by a so-called full system decontamination including the fuel elements. A major disadvantage is that there are large volumes of radioactive waste.

- Manufacture of surfaces that are contaminated more slowly, e.g. by electropolishing. However, this is only practical when replacing systems and has not been successful in the case of a trial selected nuclear power plant.

- Coating the cleaned or new surface with a non-contaminated oxide layer. This can be done by various methods, such as with oxygen-containing steam or with water with high oxygen contents. This requires treatments for relatively long times and / or at high temperatures. These procedures have not been very successful so far, for example the treatment remained the new and electropolished recirculation line in a selected nuclear power plant with no discernible influence.

From the problems outlined above, the object on which the invention is based is that of developing a method which has the following properties:

- Easy handling, if possible in the power plant itself;

- short treatment time and low process temperatures;

- unproblematic aids, i.e. non-hazardous chemicals which, even if residues remain in the systems, cannot lead to consequential and long-term damage;

- The generated surfaces respectively. Protective layers must be effective and stable in the subsequent operation of the system for a very long time, in particular they must not come off;

- the components must not be damaged by the treatment, and

- Protective layers and layers obtained, which form during the subsequent normal operation of the nuclear power plants, must be removable again by means of decontamination processes introduced in practice today.

The invention is also based on the object of specifying suitable uses of the method and a steel or a nickel alloy to be obtained with such a method.

The stated object is achieved with regard to the method by the combination of features of claim 1.

Preferred embodiments are mentioned in the dependent claims 2 to 12.

It is proposed that the surface of the steel be used with peroxide or with a water-peroxide mixture or with treated with a peroxide solution. In the following, "peroxide" also stands for a water-peroxide mixture or a peroxide solution. "Peroxide" can always take the place of "hydrogen peroxide" and vice versa.

The treatment is carried out, for example, under water and, for example, by adding hydrogen peroxide to the water. For example, the process for the surface treatment of stainless steel and / or nickel alloys, for example of the type Incoloy 800, Inconel 600, and the like, which are used particularly in steam generators of pressurized water reactors, provides that the surface to be treated is initially treated using generally customary techniques, such as, for example through chemical treatment, is cleaned and degreased. The actual treatment can be carried out with a peroxide solution with a peroxide concentration of 0.1 to 200 mmol H ₂ 0 ₂ in, for example, pure or deionized water. This treatment can be done in an open or closed system.

As a rule, concentrations higher than 10 mmol H ₂ 0 _{2 are} not used, but can nevertheless be used, but no longer bring any significant advantages in the sense of the invention, since excess hydrogen peroxide is rapidly decomposed thermally and catalytically, ie remains ineffective for the desired layer formation .

The peroxide solution is e.g. a solution of peroxide in pure or deionized water.

The object of specifying a suitable use for the method is achieved according to the invention by the combination of features of claims 13 or 14. The method is used to treat surfaces wetted in water-cooled nuclear power plants, which are largely made of stainless steels and / or nickel alloys, for example of the type Incoloy 800, Inconel 600, or the like exist, which are used particularly in steam generators of pressurized water reactors and which in the past have led to the problems mentioned at the outset due to contamination.

The object of specifying a suitable steel and / or a suitable nickel alloy is achieved according to the invention by the combination of features of claim 15.

If a protective layer that is stable during operation is to be built up in a short time and at temperatures that are low in relation to the operating temperatures, then oxide layers such as are formed during normal operation are necessary. However, since the reaction rate of the oxidation by decomposition of the water or with oxygen is not sufficient at these temperatures, various oxidizing agents were investigated, such as ozone, chromic acid and its salts, permanganic acid and its salts, and hydrogen peroxide. Of all the additives examined in the preliminary tests, hydrogen peroxide proved to be extremely suitable because

- With hydrogen peroxide at relatively low temperatures and during short treatment times stable, long-term resistant protective layers can be achieved even in normal operation of boiling water and pressurized water reactors,

- Hydrogen peroxide in the water chemistry of nuclear power plants is not a foreign substance, since this is continuously formed in the coolant by radiolysis, and due to the high decomposition rate at operating temperature, no significant concentration is reached, and

- These protective layers have proven to be extremely stable during attempts to outsource them under operating conditions, ie they did not change qualitatively for a long time still quantitative and compared to untreated surfaces, they absorbed more than 80% less activated corrosion products, eg the cobalt isotopes Co-60 and Co-58, which are particularly disadvantageous for radiation protection reasons.

The illustration according to FIG. 1 serves as an example of this behavior, where the deposition behavior of Co-58 is shown under primary water conditions of a boiling water reactor on steel material number 1.4571 (X10CrNiMoTil810).

The composition and structure of the layer produced by means of hydrogen peroxide differs fundamentally from that formed atmospherically or only with hot water. While mainly chromium-rich oxide layers are formed during atmospheric oxidation and mainly nickel spinels are formed in hot water without hydrogen peroxide, almost pure iron III oxides are formed with hydrogen peroxide. Oxide layers without hydrogen peroxide form little or no protection against further oxidation in hot water / steam (the spinel layers continue to form), while layers formed under hydrogen peroxide are very stable and in particular when used later under the conditions of the primary circuit Nuclear power plant do not allow spinel formation.

An advantage of the present invention is that the

Parts that have surfaces to be treated can either be removed or treated separately. Systems, such as tanks, pipes, pumps, can also be treated when installed by coating them on their water-exposed inside.

Another advantage of the treatment of steels with hydrogen peroxide is that the surface treatment of the steel can be checked visually by changing the color. As a result of interference, the drive according to the invention treated surface from deep gold to dark blue-violet. Basically, all spectral colors result, depending on the layer thickness and the type of light reflection.

The coloration of stainless steel that occurs is not known from the prior art. Thus, the method according to the invention is also generally suitable for coloring stainless steel or nickel alloys, in that the cleaned surface e.g. is treated with peroxide under water. The coloring is not only interesting in connection with the specified uses of the process in nuclear technology, but also in non-nuclear areas.

Those from approx. 20 nm to approx. 300 nm or between approx. 0.02 to approx. 0.3 µm. Such a layer thickness is e.g. achieved with a treatment time between 10h and 300h.

Using some process conditions, the method according to the invention will be described, for example, and with reference to the following concrete examples.

The preoxidation with hydrogen peroxide takes place in the

Way that the coating can be carried out at below 100 ° C in an open or closed container, or at temperatures above 100 ° C in a closed container. Systems to be coated, e.g. in nuclear technology, can also be used directly as containers. The latter is the

Case when the systems, containers, pipelines, pumps, etc. are to be coated on their water-exposed inside, as is the case, for example, with cooling systems in nuclear power plants. The procedure is as follows: The container or the systems are filled with pure water or deionized water. The pure water is preferably circulated or metered in continuously. The water is brought to the desired treatment temperature and maintained by suitable devices. At the same time, a dilute solution of hydrogen peroxide is metered in, so that the desired concentration of hydrogen peroxide can be maintained at all times. When treating closed systems, e.g. at temperatures above 100 ° C and pressures greater than 1 bar, effective ventilation is expediently applied in order to remove the oxygen gas which is formed during the thermal and catalytic decomposition of excess hydrogen peroxide and to avoid a gas cushion. which could prevent the complete coating of the systems / containers.

Example 1: Open container (unpressurized) Temperature: <100 ° C Pressure: atmospheric

Concentration: 0.1 to 100 mmol H ₂ 0 ₂ *

Water speed 0 to <10 m / s Treatment time: 10 to <300 h

Example 2: Closed container or system

Temperature:> 100 ° C Pressure:> 1 bar, depending on the temperature, so that no steam cushion is created, respectively. the system remains completely filled with pressurized water.

Concentration: 0.1 to 100 mmol H ₂ 0 ₂ * Water speed 0 to> 10 m / s Treatment time: 10 to> 300 h

Example 3: 140 ° C pressure:> 5 bar

Concentration: 0.1 to 100 mmol H ₂ 0 ₂ * water velocity 0 to> 10 m / s treatment time: 10 to 200 h

Example 4: 170 ° C pressure:> 10 bar

Concentration: 0.1 to 100 mmol H ₂ 02 'Water speed: 0 to> 10 m / s Treatment time: 10 to 100 h

* Concentrations higher than 100 mmol can be used, but do not bring any advantages in the sense of the method according to the invention, since excess hydrogen peroxide is rapidly thermally and catalytically decomposed, ie it remains ineffective for the desired layer formation.

Of course, the present invention is not limited to the examples given. These are only for better understanding of the present invention.

Basically, the present invention proposes a method for the surface treatment of steel, such as in particular stainless steel, as well as nickel alloys, for example of the type Incoloy 800, Inconel 600, and the like, as are used particularly in steam generators of pressurized water reactors, the treatment with Peroxide or a water per oxide mixture or solution takes place. It is important that a protective layer of oxide is formed on the treated surface. The surface can be the surface of a container, pipeline, pump, etc. In the case of boiling water reactors, for example, the feed water preheaters could also be treated using the method mentioned here in order to reduce the introduction of corrosion products into the reactor.

With the coating according to the invention, the advantage is achieved in particular that surface contamination can occur later or only with difficulty when operating a system.

Claims

claims 1. Process for the surface treatment of steel, in particular stainless steel, and / or of a nickel alloy, so that the treatment is carried out with peroxide or with a water-peroxide mixture or with a peroxide solution. 2. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that the surface to be treated is first cleaned and / or degreased. 3. The method according to claim 1 or 2, d a d u r c h g e k e n n z e i c h n e t that the treatment is carried out under water. 4. The method according to claim 3, d a d u r c h g e k e n n z e i c h n e t that peroxide is added to the water. 5. The method according to any one of claims 1 to 4, characterized in that the surface is treated with a peroxide solution with a peroxide concentration of 0.1 to 200 mmol H2O2, preferably a concentration of 0.1 to 100 mmol H ₂ 0 ₂ . 6. The method according to any one of claims 1 to 5, so that the peroxide solution is a solution in pure or deionized water. 7. The method according to any one of claims 1 to 6, characterized in that the peroxide solution is circulated and / or is metered in continuously. 8. The method according to any one of claims 1 to 7, that the treatment time is 10 to 300 hours. 9. The method according to any one of claims 1 to 8, characterized in that the treatment takes place in an open container or system at a solution temperature of <100 ° C and atmospheric pressure and a dissolution speed along the surface of 0 to <10 m / s, and that a desired concentration of peroxide is constantly maintained by metering in peroxide. 10. The method according to any one of claims 1 to 8, characterized in that the treatment takes place in a closed salary or system at a temperature of> 100 ° C, preferably in a range of 100 to 200 ° C, and at a pressure of> 1 bar, and that a vent is provided for the oxygen gas which arises during the thermal and catalytic decomposition of the excess peroxide. 11. The method according to claim 10, that the treatment takes place in a temperature range from 120 to 180 ° C and in a pressure range from 5 to 15 bar, and at a dissolution speed along the surface of 0 to 10 m / s. 12. The method according to any one of claims 1 to 11, characterized in that the surface to be treated of a system is coated with oxide in the installed state on the water-exposed inside of the system. 13. Use of the method according to one of claims 1 to 12 for the surface treatment of a component wetted on its surface in a water-cooled nuclear power plant, for example the primary circuit, to reduce the activity absorption (contamination) of the component. 14. Use of the method according to any one of claims 1 to 12 for coloring steel, in particular stainless steel, and / or a nickel alloy. 15. Steel, in particular stainless steel, and / or nickel alloy containing an oxide layer at least at partial areas of the surface, obtainable by a method according to one of claims 1 to 12.