Method for depositing a corrosion-protection coating from a suspension

Abstract

Process for depositing a coating on a substrate having at least one cavity, comprising a step of mixing ground powders of an activating agent and a powder of the metal or of the alloy to be deposited on the substrate that were ground in step a) and a liquid so as to form a suspension, and a step of applying the suspension to the portion of the substrate to be coated.

Claims

1. A process for depositing a coating on a substrate having at least one cavity of minimum equivalent diameter e.sub.cm, comprising the following successive steps: a) grinding a powder of an activating agent and a powder of a metal or of an alloy to be deposited on the substrate, said grinding being carried out so as to obtain only particles having an equivalent diameter d such that de.sub.cm/10; b) mixing the ground powders and a liquid so as to form a suspension; c) pickling the portion of the substrate to be coated; d) applying the suspension to the pickled portion to provide a substrate-suspension assembly; e) heating the substrate-suspension assembly to a temperature of between 150 C. and 400 C. and for a duration of between 20 and 60 minutes so as to at least partly eliminate the liquid; f) further heating the substrate-suspension assembly resulting from step e) to a temperature of between 500 C. and a melting point of a least refractory element of the metal or of the alloy to be deposited and for a duration of between 1 and 10 hours so as to react the activating agent and the metal or the alloy to be deposited, completion of step f) resulting in production of a substrate-coating assembly and pulverulent residues; g) eliminating the pulverulent residues from the substrate-coating assembly; and a last step of heating the substrate-coating assembly at a temperature of between 900 C. and 1200 C. and for a duration of between 1 and 5 hours.

2. The process of claim 1, wherein application of the suspension to the pickled portion is carried out by immersing the substrate in the suspension, by injecting the suspension into the at least one cavity of the substrate, or by brush application of the suspension to the at least one cavity of the substrate.

3. The process of claim 1, wherein step g) of eliminating the residues is carried out by vibration or washing with aqueous solutions.

4. The process of claim 1, wherein the substrate is selected from metallic substrates, substrates made of alloy(s) or made of superalloy(s), composite substrates comprising one or more metals and/or alloy(s) and/or superalloy(s) containing Ni in order to react with Al deposited and form NiAl.

5. The process of claim 1, wherein the substrate is a metallic heat exchanger.

6. The process of claim 1, wherein step e) is carried out under an atmosphere that is renewed every x minutes with 1 minx20 min.

7. The process of claim 1, wherein step f) is carried out under a static inert gas atmosphere.

8. The process of claim 7, wherein the inert gas is argon.

9. The process of claim 8, wherein in step b) at least one organic compound is added to the mixture.

10. The process of claim 9, wherein the at least one organic compound is selected from the group consisting of binders, lubricants and dispersants.

11. The process of claim 10, wherein said process further comprises, between step e) and step f), a step of heating the substrate-suspension assembly resulting from step e) to a temperature of between 400 C. and a melting point of a least refractory element of the metal or of the alloy to be deposited and for a duration of between 1 and 10 hours so as to eliminate or degrade the at least one organic compound.

12. The process of claim 11, wherein said step of heating the substrate-suspension assembly resulting from step e) is carried out under an atmosphere that is renewed every x minutes with 1 minx20 min.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a flow chart showing steps of a first process according to the invention.

(2) FIG. 2 is a flow chart showing steps of a second process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(3) FIG. 1 schematically shows the various steps of the process according to the invention.

(4) The first step (step a) is a step of grinding a powder of an activating agent and a powder of the metal or of the alloy to be deposited on the substrate. This grinding step makes it possible to obtain a particle size suitable for the part to be coated. Preferably, the substrate has cavities of minimum equivalent diameter e.sub.cm and the grinding step a) is carried out so as to obtain only particles having an equivalent diameter d such that de.sub.cm/10. These powders must be able to be dispersed homogeneously in the liquid phase. The presence of agglomerates may lead to blocking of the entrance of the substrate cavities. The main difficulty consists in dispersing powders of different natures within a same suspension so as to obtain a homogeneous distribution of the elements in the suspension and a homogeneous deposition.

(5) The second step (step b) is a step of mixing the powders ground in step a) and a liquid so as to form a suspension. Organic compounds of binder or lubricant type may be added to the suspension, in particular to promote wetting on the surface to be coated and to control the thickness deposited. The characteristics of the suspension must also be adapted to the chosen application technique.

(6) The third step (step c) is a step of pickling the portion of the substrate to be coated. Without pickling of the surface of the part to be coated, no deposition is possible. Pickling enables a better adhesion of the coating on the substrate and a better uniformity of the deposition since the precursor is decomposed at the surface of the part with a catalytic effect of the surface. If there are impurities, the deposition may be degraded.

(7) The fourth step (step d) is a step of applying the suspension to the portion of the substrate to be coated. The application and in particular the application to the inside of the cavities of the substrate may be carried out according to several processes: immersion of the substrate in the suspension; injection of the suspension into the cavity of the substrate; or brush application of the suspension to the cavity of the substrate. Zones that do not have to be covered may be protected by a mask that will be removed before or during the heating steps. After the deposition of the suspension at the surface of the substrate, the excess may be eliminated if necessary.

(8) The fifth step (step e) is a first step of heating the substrate-suspension assembly at a temperature of between 50 C. and 300 C. and for a duration of between 30 min and 5 h so as to at least partly eliminate the liquid. The temperature is set as a function of the nature of the liquid (examples: ethanol, water). In order to facilitate the elimination of the liquid, gas flushing may be introduced inside the part in order to renew the atmosphere. During this step, the critical point is not to degrade the deposition.

(9) The following step (step i) is an intermediate step since it depends on the presence or absence of organic compounds in the suspension. This step is a step of heating the substrate-suspension assembly resulting from step e) at a temperature of between 400 C. and the melting point of the least refractory element of the metal or of the alloy to be deposited and for a duration of between 1 and 10 hours so as to eliminate or degrade the organic compounds. As for the elimination of the liquid, gas flushing may be carried out in order to renew the atmosphere and eliminate the products of the degradation of the organic compounds.

(10) The sixth step (step f) is a second step of heating the substrate-suspension assembly resulting from step e) at a temperature of between 500 C. and the melting point of the least refractory element of the metal or of the alloy to be deposited (for example 660 C. for aluminum) and for a duration of between 1 and 10 hours so as to react the activating agent and the metal or the alloy to be deposited. This treatment will preferably be carried out in a static atmosphere in order to avoid a rapid depletion of the element that forms the coating (aluminum) from the atmosphere. For this, the cavity or cavities may be blocked. In the latter case, it will be necessary to pay particular attention to the increase in pressure in the part. It will also be possible to place the open cavity in a bed of cement powder so that the latter saturates the atmosphere that is the precursor of the element(s) to be deposited.

(11) Steps e) and i) could be carried out simultaneously or successively in the case where the temperature difference between the temperature of the degradation of the organics (step i) and the temperature of the reaction between the activating agent and the metal or the alloy to be deposited (step f) is large enough (temperature difference of between 50-100 C.). In the opposite case, it will be preferred to carry out step e) independently of steps i) and f) which could be carried out simultaneously.

(12) The seventh step (step g) is a step of eliminating pulverulent residues. Several techniques may be used such as washing with aqueous solutions, chemical etching, erosion, etc.

(13) The eighth step (step h) is a step of recovering a substrate-coating assembly.

(14) The process according to the invention may comprise a last step of heating the substrate-coating assembly at a temperature of between 900 C. and 1200 C. and for a duration of between 1 and 5 hours.

(15) FIG. 2 schematically shows the various steps of the alternative process.

(16) Since the alternative process is very similar to the process described above, only the steps which are modified will be described in detail.

(17) The first step (step a) is a step of grinding a powder of a pickling flux and a powder of the metal or of the alloy to be deposited on the substrate. Preferably, the pickling flux has a low melting point, i.e. . . . An example of pickling flux is K.sub.3AlF.sub.6-KAlF.sub.4.

(18) The sixth step (step f) is a second step of heating the substrate-suspension assembly resulting from step e) at a temperature above the melting point of the pickling flux for a duration of between 20 min and 6 h so as to react the pickling flux and the metal or the alloy to be deposited.

(19) This alternative process has the advantage of enabling a heat treatment at higher temperature than the first process which is limited by the lowest melting point of the elements to be deposited. This degree of freedom may be advantageous for the elimination of the organic compounds used for producing the suspension. Specifically, a temperature of 600 to 800 C. may be necessary in certain cases for completely degrading these organic compounds.

(20) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

(21) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(22) Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing i.e. anything else may be additionally included and remain within the scope of comprising. Comprising is defined herein as necessarily encompassing the more limited transitional terms consisting essentially of and consisting of; comprising may therefore be replaced by consisting essentially of or consisting of and remain within the expressly defined scope of comprising.

(23) Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

(24) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(25) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

(26) All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.