CH647265A5 - METHOD FOR PRODUCING PROTECTIVE OXIDE LAYERS. - Google Patents

Description

The invention relates to a method for producing protective oxide layers on a metallic article, in which the article is subjected to an oxidation process at elevated temperature after pretreatment.

It plays a role in the processes of nuclear coal gasification

Permeation plays an important role. For safety reasons, it is necessary to provide the components that come into contact with the working gases containing hydrogen or tritium with protective layers which are intended to prevent these elements from penetrating.

A known method of protecting materials or objects against the entry of foreign elements is to provide the surface of the corresponding object with an oxide layer.

In the known method, the oxide layer on the object is obtained simply by exposing the object to the atmosphere of the gasification process in question under the physical conditions on which the process is based.

However, such a method is not suitable for use in cases where the object is exposed to extreme conditions, in particular hydrogen at high temperatures, such as is the case with nuclear coal gasification.

The layers produced by the known method do not have sufficient tightness and, moreover, do not have sufficient mechanical stability. Under relatively low loads, cracks already form in the protective layer or the layer even flakes off.

The object of the invention is to improve the known method in such a way that the oxide layer offers effective protection against the permeation of foreign elements, in particular hydrogen or tritium, even at high temperatures.

The object is achieved in that the object is subjected to mechanical and / or chemical pretreatment using an object made of high-temperature alloys and that the subsequent oxidation process is carried out using a low oxidation potential and a temperature between approximately 900 and 1000 ° C.

Due to the low oxidation potential, selective oxidation is possible, with which, with a suitable choice of the partial pressure of the oxidizing agent, it can be achieved that only individual elements, preferably only one element from the material to be treated, are included in the oxidation process.

In the case of high-alloy steels and nickel-based alloys, it has been observed by using the process according to the invention that the component which forms the oxide with the lowest decomposition pressure, namely chromium, is oxidized. Due to the slow growth of the chromium oxide, a uniform formation of the oxide layer is achieved. This layer formation was also favored by the fact that these alloys have relatively good chromium mobility. This mobility of chrome results in a certain replenishment of chrome from the inner area to the surface, which contributes to the formation of a far more compact CnOs protective layer.

Investigations have shown that this Cn03 layer gives a uniformly dense coating which satisfactorily inhibits the permeation of hydrogen or tritium and other elements in the material, even at very high temperatures. It was also found that this oxide layer simultaneously provides good protection against high-temperature oxidation, against carburization and against hydrogen sulfide, sulfur oxide and halogen corrosion. The Cn03 layer also exhibits improved mechanical stability compared to the prior art.

The quality of the protective layer can be further improved by subjecting it to mechanical pretreatment2

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such as cold working, and then subjected to an annealing treatment under hydrogen.

The mechanical treatment, which can be grinding, honing, turning or shot peening, in conjunction with the subsequent temperature treatment, results in a refinement of the grain sizes on the object surface and thus an increase in the mobility of chromium. This is used in the subsequent chemical pretreatment in such a way that the Cr segregation of the alloy caused by the hydrogen in the annealing process causes a remarkable accumulation of chromium in the surface area. An oxidation which is approximately homogeneously distributed over the surface and which leads to a very dense and well-adhering and therefore mechanically very stable barrier layer takes place on such a pretreated, chromium-enriched surface which is made directly accessible for the oxidation process.

The annealing process is preferably carried out at a temperature which is approximately the same for the subsequent oxidation process. This has the advantage that the two temperature-dependent process steps can be carried out quickly one after the other.

COz can be used as an oxidizing agent for the oxidation process. This enables the auxiliary balance 2CO: = 2CO + O2 to be used to reduce the oxygen partial pressure.

A preferred oxidizing agent is water vapor. With water vapor, an even lower oxidation potential can be achieved under the auxiliary equilibrium 2H: 0 = 2H2 + O2 than in the case of CO2. The use of this oxidizing agent in connection with the hydrogen reduction as pretreatment has the further advantage that no rinsing process has to be used between the chemical pretreatment and the oxidation process. The excess hydrogen present during the oxidation even has a positive effect on the process in that this hydrogen causes a further reduction in the oxygen partial pressure.

In order to avoid carrying out the oxidation process under reduced pressure and thus the use of vacuum apparatuses, it is proposed to pass the oxidizing agent over the object to be coated in an inert carrier gas, preferably an inert gas, in particular helium or argon. The oxidizing agent can preferably be carried out in a closed circuit, but also in a partially closed or open mode of operation.

When using CO2 as the oxidizing agent, an oxidation potential of less than 50 mbar, preferably about 10 mbar, is used, while the water vapor partial pressure is less than 100 mbar, these values being based on normal conditions. The use of the oxidation process with steam under a partial pressure of approximately 20 mbar is particularly advantageous. These conditions

647 265

can be achieved directly at atmospheric pressure and room temperature.

It is advantageous if the oxide layer thickness is less than 4 μm, preferably in the range of 2 μm. Such a layer is resistant to stresses and other stresses and is consequently stable.

Examples:

example 1

The following were used to coat a nickel-based alloy known under the trademark “Hasteloy X” or “Inconel 625” with the composition: 22% Cr, 9% Mo, 19% Fe, Si, Mn, balance nickel or with the designation NiCr22Mo9Nb Process steps carried out:

a) First, the surface was mechanically pretreated by grinding (320 grit), honing or shot peening.

b) The object was then reduced with H2 at 1000 ° C. for 5 hours and then flushed with argon.

c) The oxidation process was then initiated at the same temperature, that is 1000 ° C., with 20 mbar hydrogen in argon.

d) After a 4-hour oxidation process, a dense Cn03 layer of 1 to 2 jo.ni was obtained.

Example 2

An article made of high-alloy steel with 32% Ni, 20% Cr, 0.1% C, Al, Ti, rest Fe was subjected to a pretreatment as in Example 1 [process steps a) and b)].

c) Then the surface was oxidized at 900 to 950 ° C with 10 to 20 mbar water vapor in argon.

d) In this case too, a compact chromium oxide layer of 1 to 2 (in.

Example 3

An article made of high-alloy steel as in Example 2 was used with the same pretreatment, but the hydrogen was not removed after the annealing treatment, but was retained for the oxidation process. The oxidation was initiated at 1000 ° C by adding steam in argon.

The water vapor partial pressure was between 10 to 20 mbar and that of H2 0.1 to 0.8 bar. The layer thickness after 4 hours of oxidation was also 1 to 2 μm.

In this process, a higher adhesive strength of the oxide layer was found compared to Example 2. In contrast, example 2 can be carried out at lower temperatures.

In all cases it was found that the oxide layer had a high stability and caused a remarkable protection against hydrogen or tritium permeation.

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Claims (10)

647265 PATENT CLAIMS 1. A process for the production of protective oxide layers on articles, in which the article is subjected to an oxidation process at elevated temperature after pretreatment, characterized in that the article is subjected to a mechanical and / or chemical pretreatment in order to use the process for articles made of high-temperature alloys the subsequent oxidation process is carried out using a low oxidation potential and a temperature between 900 and 1000 ° C. 2. The method according to claim 1, characterized in that the pretreatment consists in a mechanical and a subsequent annealing treatment under hydrogen. 3. The method according to claim 2, characterized in that the chemical surface treatment is carried out at a temperature which is approximately the same for the subsequent oxidation process. 4. The method according to any one of the preceding claims, characterized in that the oxidizing agent is CO2 and the COa partial pressure, based on normal conditions, is less than 50 mbar, preferably about 10 mbar. 5. The method according to any one of claims 1 to 4, characterized in that the oxidizing agent is water vapor and the water vapor partial pressure, based on normal conditions, lower than 100 mbar, preferably about Is 20 mbar. 6. The method according to claim 4 or 5, characterized in that the oxidizing agent is passed over the object to be coated in an inert carrier gas, preferably noble gas, such as argon or helium. 7. The method according to any one of claims 1 to 6, characterized in that the oxidation duration is between 2 and 8 hours depending on the desired layer thickness and the oxide layer thickness is less than 4 Jim, preferably 3 (im). 8. The method according to any one of the preceding claims, using an object made of nickel-based alloy, characterized in that the object is reduced after a mechanical pretreatment with H2 at 1000 ° C for about 3 hours and then a 4 to 8 hour oxidation process at 1000 ° C with about 20 mbar water vapor in noble gas is subjected. 9. The method according to any one of claims 1 to 7 and using an article made of high-alloy steel, characterized in that the article after a mechanical pretreatment with about 3 hours with H2 Is reduced to 1000 ° C and then subjected to a 4 to 8 hour oxidation treatment at about 1000 ° C and that the oxidation atmosphere consists of water vapor, hydrogen and argon. 10. The method according to any one of claims 1 to 7 and using an article made of high-alloy steel, characterized in that the article is reduced after a mechanical pretreatment with H2 at 1000 ° C for about 3 hours and then a 4 to 8 hour oxidation process is subjected to steam in noble gas at 900 to 950 ° C.