FR2499593A1 - PROCESS FOR MAKING PROTECTIVE OXIDE LAYERS - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS FOR ACHIEVING PROTECTIVE OXIDE LAYERS ON OBJECTS, IN WHICH THE OBJECT IS SUBJECTED, AFTER A PRELIMINARY TREATMENT, TO A PROCESS OF OXIDIZATION UNDER HIGH TEMPERATURE. THE PROBLEM SOLVED CONSISTS OF PROVIDING FULLY EFFECTIVE PROTECTION AGAINST THE ACTION OF FOREIGN ELEMENTS. THIS PROCESS IS CHARACTERIZED IN THAT, FOR THE APPLICATION OF THE PROCESS TO STEEL OBJECTS ALLIED WITH CHROME AND OR NICKEL, THE OBJECT IS SUBJECT TO A MECHANICAL AND CHEMICAL PRIOR TREATMENT, AND IN THAT THE OXIDATION PROCESS IS PERFORMED WHICH FOLLOWS USING LOWER OXIDIZING POTENTIAL AND A TEMPERATURE BETWEEN APPROXIMATELY 480 AND 800C. THE INVENTION IS APPLICABLE IN PARTICULAR TO THERMAL PROCESSES IN THE CHEMICAL INDUSTRY.

Description

Process for producing layers of protective oxides The invention

  object is a method for producing protective oxide layers on a metal object, wherein the object is subjected, after preliminary treatment, to a high temperature oxidation process.

  In the technique of chemical processes

  during thermal processes,

  components at elevated or elevated temperatures

  subject to aggressive atmospheres which may

  damage to these materials. It should be mentioned before

  all the oxidation or catastrophic corrosion that occurs

  under the action of products that give off sulfur,

bone or halogens.

  A known method to protect

  materials or articles against the penetration of foreign elements

  is to provide the surface of the corresponding object with a

oxide layer.

  In the known process, the oxide layer is simply formed on the object in that the object is exposed to the atmosphere of the corresponding thermal process.

  under the basic physical conditions of this process.

  Such a method is however not suitable for the application cases in which the object is subject

  under extreme conditions, particularly if it is exposed to

  highly corrosive at high temperature, as is the case

  for example for thermo-chemical processes.

  The layers made with the known method do not have a sufficient seal and do not have

  no longer sufficient mechanical strength. Under requests

  With relatively low temperatures, cracks are already forming in the protective layer or this layer may even burst. She

  may also be constantly subject to local destruction under the

  aggressive components of the process atmosphere.

extra. -2-

  The object of the invention is to improve

  the known method so that the oxide layer provides a fully effective protection against the action of foreign elements, including oxygen, sulfur, halogens and carbon, even at high temperatures. The invention relates, for this purpose,

  a process of the above type, characterized in that for the purpose of

  application to an object made of chromium and / or nickel-alloy steels, the object is subjected to a prior mechanical and / or chemical treatment, and in that the following oxidation process is carried out using an oxidation potential weaker and a

  temperature between 480 and 8000 C approximately.

  By means of the lower oxidation potential, it is possible to carry out a selective oxidation by which, with a suitable choice of the partial pressure of the oxidation product, only certain

  elements, preferably a single element, of the material to be processed.

  ter participate in the oxidation process.

  For chromium-alloy steels and /

  or nickel, it has been observed, through the implementation of

  according to the invention, an oxidation of the constituent element which forms the oxide having the pressure of

  decomposition the weak folds, namely chromium or iron.

  In addition, the low oxidation potential leads to a control

  kinetics of the formation of the oxide, that is to say to a growth

  slow growth of the oxide layer and consequently to a

  uniform. In the case of chromium-alloy steels, this layer formation is also favored in that there is

  a relatively good mobility of chromium in these alloys.

  By this mobility of chromium, there is a certain shift in

  chromium from the inner zone to the surface, this

  placement involved in the formation of a protective layer

compact in Cr2 O3.

  On nickel-alloy steels, Fe3 04 * sealed layers are formed.

  Research has shown that these

  oxides gave uniform and watertight coatings satisfactorily arresting the arrival of oxygen, sulfur, halogens or hydrogen and other elements on the material, thereby preventing the progression of corrosion. even at high temperatures. Consequently,

  layers provide good protection against the progress

  oxidation, against carburization as well as against corrosion by hydrogen sulphide, sulfur dioxide and halogens. The layers have, in addition, improved mechanical and chemical resistance compared to the state of the art. The quality of the protective layer

  can be further improved by subjecting the object to

  mechanical preliminary treatment, eg cold forming

  followed in some cases by an annealing treatment in the hy-

  drogen. Mechanical treatment can be a

  grinding, honing, rolling or shot blasting. This treatment

  In conjunction with the subsequent heat treatment, this causes a greater fineness of the grains on the surface of the object and, consequently, an increase in the mobility of the

the alloy to be oxidized.

  For chromium steels, this model

  is used in the subsequent chemical pretreatment in that the Cr segregation of the alloy provoked

  hydrogen during the annealing operation leads

  richness in chromium in the superficial zone. On an over-

  face treated beforehand, enriched with chromium and rendered

  directly accessible for the oxidation operation, it is

  produces oxidation distributed substantially uniformly over the surface. This oxidation leads to a very damming layer

  compact and well adherent, this layer is therefore mechanical-

very resistant.

  -4- In chromium-free nickel steels,

  the transformation related to the oxidation process of the network

  space-centric network in a plan-centric network, driving

  does not adapt the network of the metal to the network of the oxide centered in plan. This results in improved adhesion. The transformation of the network, limited to the alliaga zone immediately below the oxide layer, depends on an iron depletion due to oxidation and corresponding to nickel enrichment. The annealing operation is carried out

  preferably at a temperature substantially equal to the temperature

  re planned for the following oxidation process. This has for

  advantage that the two stages of the process depend on the temperature

  ture, can be carried out directly one after the other.

  For the oxidation process, Co2 can be used as the oxidizing agent. It is thus possible to use the auxiliary balance 2CO2 = 2 CO + 02 to reduce

the partial pressure of oxygen.

  A preferred oxidizing agent is

  water vapour. With the water vapor can, using the equi-

  free auxiliary 2 H 20 = 2 H2 + 02 'to obtain an oxidation potential

  even lower than in the case of CO2. The use of this oxidizing agent provides, in connection with the reduction by hydrogen of the preliminary treatment, another advantage in that it is not necessary to interpose a washing operation between the preliminary chemical treatment and the oxidation process. The excess of hydrogen existing during the oxidation still acts positively on the process because this hydrogen causes a further reduction of the pressure

partial oxygen.

  To avoid having to carry out the oxidation process under reduced pressure and, consequently, to have to use vacuum devices, it is proposed to conduct the oxidation agent in an inert carrier gas, preferably a rare gas, especially helium or argon, on the object to be coated. The oxidation agent can then preferably be conducted in a closed circuit, but also in a

  partially closed or open operation.

  When Co 2 is used as the oxidizing agent, an oxidation potential of less than mbar, preferably substantially equal to 10 mbar, is used, while the partial pressure of water vapor is less than 100 mbar, these values being reported under normal conditions. It is

  particularly advantageous to implement the

  oxidation with water vapor under pressure

  substantially equal to 20 mbar. These conditions can be obtained directly at atmospheric pressure and at

ambient temperature.

  It is advantageous that the thickness of

  the oxide layer is less than 4 d and is

  in the range of 2 g. Such a layer is resistant to

  constraints and other solicitations; it is stable.

Examples of realization

Example 1

  To coat a nickel-chromium steel (18% Cr, 11% Ni), the following process steps were carried out:

  a) - the surface was first treated by means of

  grinding (grit 320), honing or blasting, b) - the object was then reduced by H2 to 8000 C for two hours and then flushed with argon, c) - then route the oxidation process at the same temperature, that is to say at 800 C, with water vapor under 20 mbar in argon,

  d) - after a four-hour oxidation process, we obtained

  held a tight layer of Cr2 O3 from 1 to 2 y, containing

little iron.

Example 2

  A 7% chromium steel article was subjected to mechanical pretreatment as in Example 1 (process step a)): c) - the surface was then oxidized at 6800 C with steam of water under 20 mbar in argon, d) - it was possible to achieve in this case, in six hours, a layer

  Fe Cr2 04 compact and adherent from 1 to 2 y.

  The oxidation temperature to be achieved depends on the concentration of chromium. She is so much more

  high that the chromium content is higher.

Example 3

  An 18% nickel steel article was used with the same pretreatment as in Example 2. The oxidation was carried out at 5000 C. The partial pressure of the water vapor was 20 mbar. The thickness of the waterproof layer

  of Fe3 041, was also 1 to 2 A after eight hours of oxy-

dation.

  In all cases it was found that the oxide layer had a high strength and

  outstanding protection against the progression of oxidation

  tion, sulfur and halogen corrosion,

  ration and fragility due to hydrogen.

-7-

Claims (6)

  1) - Process for producing   protective oxides on objects, in which   after preliminary treatment, the object undergoes an oxidation process under high temperature, characterized in that, for the application of the process to steel objects alloyed with   chromium and / or nickel, the object is subjected to a prior   mechanical and / or chemical, and in that the process is   subsequent oxidation using a lower oxidation potential and a temperature between about 480 and 800 C. 2) - Process according to claim 1, characterized in that, in the case of steels containing   chromium, the pretreatment consists of a   and a subsequent hydrogen annealing treatment.   Process according to Claim 2, characterized in that the chemical surface treatment is carried out at a temperature which is approximately equal to   the temperature of the oxidation process that follows.   4) - Process according to one of the   1 to 3, characterized in that the oxidizing agent is CO2.    ) - Process according to claim   4, characterized in that the partial pressure of CO2,   under normal conditions, is less than 50 mbar, and   preferably equal to about 10 mbar.   6) - Process according to any one   Claims 1 to 4, characterized in that the oxy-   used is water vapor.   7) - Process according to claim 6, characterized in that the partial pressure of water vapor, referred to normal conditions, is less than 100 mbar and   preferably equal to about 20 mbar.   8) - Process according to any one   Claims 4 to 7, characterized in that   The oxidizing agent on the object to be coated, in a carrier gas   inert, preferably a rare gas such as argon or helium.   9) - Process according to any one   Claims 1 to 8, characterized in that the duration of oxy-   between two and eight hours after the desired layer.   ) - Process according to any one   Claims 1 to 9, characterized in that the thickness of   the layer is less than 4 y, preferably less than 3. 11) Method according to any one   Claims 1 to 10, implemented on an object in   nickel-chromium steel, characterized in that after a   mechanical advance, we reduce the object with H2 to 8000C   about two hours and then submit it for   about four hours at an oxidation process at 8000 C with   water vapor at about 20 mbar in a rare gas.   ) - Process according to any one   Claims 1 to 10, implemented on an object in   chromium steel, characterized in that after a pre-treatment   mechanical lable, subject the object to oxidation treatment   for four to eight hours with hydrogen at -20 mbar   Viron in argon, at a temperature of between about 6000 ° C and 8000 ° C .; 13) - Process according to any one   Claims 1 to 10, implemented on an object in   nickel steel, characterized in that after a pre-treatment   mechanical lable, subject the object to an oxidation process for about eight hours at 500 C with steam   about 20 mbar in a rare gas.