GB2093073A - A method of producing protective oxide layers - Google Patents

Abstract

A method of producing oxide layers on high-tempeature alloys. The workpiece is pretreated mechanically and/or chemically, with hydrogen. Following pretreatment, an oxidation process using a low oxidation potential and a temperature running between 900 DEG and 1000 DEG C is effected. When the oxidizing agent, is water vapour at a partial pressure of about 20 mbar, the oxide surface layer produced provides a dense barrier inhibiting permeation by hydrogen and tritium. <IMAGE>

Description

SPECIFICATION A method of producing protective oxide layers This invention relates to a method of producing a protective oxide layer on metal.

In nuclear coal gasification processes, permeation is an important consideration. For reasons of safety, it is necessary to provide components wetted by hydrogen or tritium laden process with protective surface layers to prevent the ingress of these elements.

It is known that metal can be protected against the ingress of foreign elements by providing on the surface of the metal an oxide layer. This is produced by exposing the object to the atmosphere of the respective gasification process at the physical conditions underlying the process.

The above method, however, is not suitable for applications in which the metal is exposed to extreme conditions, more particularly to hydrogen at elevated temperatures, as it is the case, e.g., in the nuclear gasification of coal, since the surface layers produced afford insufficient sealing and mechanical stability. Also cracks tend to develop even under relatively moderate loads.

One object of the present invention is to enable the production of an oxide layer which affords effective protection against permeation by foreign elements, especially hydrogen or tritium, also at elevated temperatures.

According to this invention we propose a method of producing a protective oxide layer on metal, particuiarly a high temperature resistant alloy, in which, after mechanical and/or chemical pretreatment the metal is subjected to an oxidation process at a low oxidation potential and a temperature between about 9000 and 10000 C.

The low oxidation potential permits selective oxidation such that with the partial pressure of the oxidation agent being suitably selected, only single elements, preferably only one element, will enter in the oxidation process.

With high-alloy steels, as well as with nickelbase alloys, it was noted that oxidation will occur of that constituent which forms the oxide of minimum decomposition pressure, which is chromium. The slow growth of the chrome oxide results in uniform formation of the oxide layer.

Formation of this layer is promoted also by the fact that these alloys afford relatively great chromium mobility, which ensures continuity in the supply (i.e. migration) of chromium from internal regions to the surface and so contributes to the formation of a largely compact Cr203 surface layer.

This Cr203 layer provides a uniformiy dense coating which adequately inhibits permeation by hydrogen, tritium or other elements even at extremely high temperatures. Also, this layer is highly resistant to high-temperature oxidation, to carburizing and to corrosion by hydrogen sulphide, sulphur oxide and halogen. In addition, there is an improvement in mechanical stability as compared with conventionally produced oxide layers.

The integrity of the layer is further improved if the object is subjected to mechanical pretreatment, such as cold forming, and subsequently to heat treatment in hydrogen.

Mechanical treatment in the form of grinding, honing, turning or shot peening serves, together with the subsequent heat treatment, to produce a finer grain on the surface of the metal and, thus, to improve the chromium mobility. The latter is exploited in subsequent chemical pretreatment such that the chromium segregation caused in the alloy by the hydrogen during the heat treatment significantly enriches the chromium in the region of the surface. On a chromium enriched surface pretreated in said manner and made directly accessible for the oxidation process, oxidation will be approximately homogeneous over the surface area and produce a very dense, well adhering and, therefore, mechanically rather stable barrier layer.

Heat treatment is preferably carried out at a temperature approximating to the temperature of the subsequent oxidation stage. This has the advantage that the two thermal operations can be performed consecutively with minimum delay.

For the oxidation process, use can be made of CO2 as an oxidation agent, which makes it possible to utilize the 2CO2 = 2CO + 02 equilibrium for reducing the oxygen partial pressure.

Water vapour is a preferred oxidizing agent since at a 2H20 = 2H2 + 02 equilibrium, the oxidation potential can still be lower than that achieved with CO2. The use of water vapour as an oxidizing agent, in conjunction with hydrogen reduction as a pretreat, provides a further advantage in that the need for a flushing operation between chemical pretreatment and oxidation process is obviated. The hydrogen excess then prevailing during oxidation will even benefit the process in that the hydrogen still further reduces the oxygen partial pressure.

In order to avoid the need for performing the oxidation process at a reduced pressure and, thus the need to use vacuum apparatus, the oxidation agent is conveyed over the object to be coated in an inert carrier gas, which preferably is a rare gas, such as helium or argon. The oxidizing agent can then be transported preferably in a closed-loop circuit or alternatively in a partially closed-loop or open mode.

When CO2 is used as the oxidizing agent, use is made of an oxidation potential under 50 mbar, preferably about 10 mbar, while the water vapour partial pressure is under 100 mbar, these values being referred to standard conditions. Special advantage aecrues if the oxidation process is carried out using water vapour under a partial pressure of about 20 mbar. These conditions can be achieved directly at atmospheric pressure and room temperature.

An oxide layer less than 4 ym thick, and preferably in the 2-ym range, resists stresses and other loads and is thus stable.

Embodiments of the invention will now be described with reference to the following examples.

EXAMPLE 1 The following operations were performed to deposit a surface layer on a nickel-base alloy known commercially as Hasteloy X or Inconel 625 and designated NiCr22Mo9Nb, with the following analysis: 22% Cr, 9% M, 19% Fe, Si, Mn, the remainder being nickel: a) The surface was pretreated mechanically by grinding (320 mesh), honing or shot peening.

b) The metal was then reduced using H2 at 10000C for a duration of 5 hours c) The oxidation process was initiated at the same temperature, i.e. 10000C, using 20 mbar hydrogen in argon.

d) After 4 hours of oxidation a dense Cr203 layer 1 ,um to 2 jum thick had been obtained.

EXAMPLE 2 An object made of high-alloy steel containing 32% Ni, 20% Cr, 0.1% C, Al, Ti, the remainder Fe, was pretreated as in Example 1 (process operations a) and b)).

c) Thereafter the surface was oxidized in argon at 9000 to 9500C using 10 to 20 mbar water vapour.

d) After 4 hours of oxidation a compact chromium oxide layer having a thickness of 1 ,um to 2 ,um was produced.

EXAMPLE 3 An object of high-alloy steel as in Example 2 was subject to the same pretreatment, except that after heat treatment the hydrogen was not eliminated but instead retained for the oxidation process. Oxidation was initiated at 10000C in argon by adding water vapour.

The water vapour partial pressure ran between 10 and 20 mbar, and that of the H2 was 0.1 to 0.8 bar. The thickness of surface layer achieved after 4 hours of oxidation against was 1 ,um to 2 jut.

This process, when compared with that of Example 2, produced better bonding of the oxide layer, whereas the Example 2 is suitable for use at lower temperatures.

It was noted in all cases that the oxide layer possessed notable stability and afforded remarkable protection against permeation by hydrogen or tritium.

Claims (14)

1. Method of producing a protective oxide layer on metal in which, after mechanical and/or chemical pretreatment the metal is subjected to an oxidation process at a low oxidation potential and a temperature between about 9000 and 100000.

2. A method according to Claim 1, wherein the pretreatment comprises mechanical treatment followed by heat treatment in hydrogen.

3. A method according to Claim 2, wherein the chemical surface treatment is effected at a temperature approximately equal to the oxidation temperature.

4. A method according to any one of the preceding claims, wherein CO2 is used as an oxidising agent.

5. A method according to Claim 4, wherein the partial pressure of the CO2 (referred to standard conditions) is less than 50 mbar and preferably about 10 mbar.

6. A method according to any one of Claims 1 to 4, wherein water vapour is used as an oxidising agent.

7. A method according to Claim 6, wherein the partial pressure of water vapour (referred to standard conditions) is less than 100 mbar and preferably approximately 20 mbar.

8. A method according to any one of Claims 4 to 7, wherein the oxidising agent is passed over the metal to be coated in an inert carrier gas, preferably a rare gas, such as argon or helium.

9. A method according to any one of the preceding claims, wherein oxidation is carried out for a duration of 2 hours to 8 hours, depending on the intended thickness of surface layer.

10. A method according to any one of the preceding claims, wherein the thickness of oxide layer is less than 4 ,um and preferably less than 3 ,um.

11. A method according to any one of the preceding claims when applied to an object made of a nickel-base alloy, wherein after mechanical pretreatment, the object is reduced using H2 at 10000C for about 3 hours and is then subjected to an oxidation process at 10000C using about 20 mbar water vapour in rare gas for a duration of 4 hours to 8 hours.

12. A method according to any one of the Claims 1 to 10 when applied to an object made of high-alloy steel, wherein after mechanical pretreatment the metal is reduced using H2 at 10000C for about 3 hours and is then subjected to oxidation treatment at about 10000C for a duration of 4 to 8 hours, in an atmosphere of water vapour, hydrogen and argon.

13. A method according to any one of Claims 1 to 10 when applied to high-alloy steel, wherein after mechanical pretreatment, the metal is reduced using H2 at 10000C for about 3 hours and is then subjected to an oxidation process at 9000 to 9500C using water vapour in rare gas for a duration of 4 to 8 hours.

14. A method of producing a protective oxide layer on metal, substantially as hereinbefore described with reference to the accompanying examples.