Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
1995-06-23
2003-02-25
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S277000, C427S237000, C427S249170, C427S249190, C427S255150, C427S255180, C428S450000, C428S472000
Reexamination Certificate
active
06524402
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns a passivation method for metallic articles of nickel and iron-based superalloy. Passivation concerns the surface of these metallic articles. The invention also concerns the metallic product in which at least a portion of its surface has been treated using the passivation method of the present application. The method of the present invention produces metallic articles which have a better resistance to oxidizing atmospheres at high temperature, and greatly reduced carburation and coking in hydrocarbons, compared with untreated articles.
In particular, the present method can limit corrosion of the external surfaces of the articles by oxidation during contact of these surfaces with an oxidizing atmosphere and can also limit cementation and coking when in contact with hydrocarbons at high temperature. This method is particularly applicable to the treatment of reactor or furnace surfaces, in particular to pipe stills used in the industry. Within the context of the present description, the terms carburation and carburizing are deemed to be equivalent.
More particularly, this method can limit coking in thermal hydrocarbon cracking reactors and in pipe stills which heat hydrocarbons to high temperature, for example to a temperature of more than 350° C. The skilled person is well aware that coking reactions at the reactor and/or furnace tube walls appear above this temperature. The method of the present invention is more particularly applicable to apparatus for carrying out steam cracking processes or pyrolysis processes in the absence of a diluent or in the presence of a diluent other than steam, for example in the presence of hydrogen, hydrocarbons or hydrocarbon cuts.
The method of the present invention is also applicable to apparatus for carrying out catalytic dehydrogenation or steam reforming processes, and hydrocarbon or hydrocarbon cut visbreaking processes.
Many documents describe the coke formation reaction in a variety of reactions which bring hydrocarbons into contact with the walls at high temperature. The coking phenomenon is particularly well described and studied within the framework of thermal hydrocarbon cracking. The leading article is considered to be that of Professor Froment, published in 1990 in the periodical “Review of Chemical Engineering”, volume 6, number 4, pages 292 to 328, entitled “Coke formation in the thermal cracking of hydrocarbons”. A more recent article appeared in the Review of the Institut Frangais du Petrole of 1992, by Billaud, Broutin, Busson, Gueret and Weill, volume 47, number 4, pages 537 to 549, entitled “Coke formation during hydrocarbon pyrolysis”. The first section appeared in that review and the second section appeared in the same review in 1993, volume 48, number 2, pages 115 to 125, with the same title.
Summarising the observations described in the prior art, coke formation during thermal hydrocarbon cracking is a complex phenomenon which brings several mechanisms into play, at least one of which involving reactions which are catalysed by the presence of oxides of metallic elements such as nickel, iron or cobalt at the walls of the apparatus used to carry out these processes. These metallic elements are in general contained in large quantities in the refractory superalloys employed, primarily because of the thermal conditions encountered at the walls of these apparatus. The catalytic mechanism is very dominant: observations have shown that if this mechanism is inhibited, then in the case of steam cracking the period between successive shut-downs to decoke the furnaces is increased by a factor of at least about 3.
A few documents have described methods for inhibiting the catalytic formation of coke: a particular example is U.S. Pat. No. 5,208,069, which describes a passivation method for the metallic surface of reactor tubes which come into contact with hydrocarbons by in-situ decomposition (i.e., in the assembled apparatus) of an organometallic non oxygenated silicone derivative under conditions which form a fine layer of ceramic material on the surface of these tubes. This method, in which deposition is effected at atmospheric pressure or at a slight underpressure, generally does not produce relatively uniform deposition over the entire length of the tubes since the rate of growth of the deposit is not uniform and thus the thickness, and also the quality of the deposit, varies along the tube. These variations entrain a risk of producing zones with thick deposits and hence low adherence and/or zones where the silicon carbide deposit is of poor quality and thus also of low adherence. The pressure at which the vapour phase deposition is carried out (according to the examples in that patent) to deposit the organometallic silicon compound is much higher and a homogeneous deposit cannot be produced since the gas diffusion distances are much smaller than in a vacuum, for example at a pressure of less than 10
+4
Pascals. Further, the deposited silicon carbide is a compound with a low expansion coefficient, while the superalloy usually possesses a much higher expansion coefficient, which means that with time and with the cycles of heating and cooling, there is a substantial risk that, at least at some points, the integrity of the silicon carbide coating will fail and, as a consequence, contact will occur between the hydrocarbons and the superalloy, which will increase the coking rate of the apparatus.
U.S. Pat. No. 5,242,574 also describes a passivation method for the walls of a reactor formed from a nickel and iron-based superalloy and containing a high aluminium content (4% to 6% by weight). This passivation method consists of forming a metal oxide layer by preoxidation of the alloy surface, which involves the formation of oxides of the metals forming the alloy. Passivation in this case is explained by the beneficial effect of the aluminium in the matrix, as it makes possible the generation of a stable superficial alumina layer by simple selective partial oxidation (at the oxygen partial pressure and temperature conditions described in that patent). This method does not generally produce a uniform layer either from the aspect of composition or from other physical characteristics. Further, this passivation method is only readily applicable in the case of a superalloy with a high aluminium content.
In addition, when using alloys with high aluminium contents, very frequently an intermetallic phase with composition Ni
3
Al is formed. This intermetallic phase is formed during use at high temperature and renders the material brittle: the material ages and the creep strength reduces with time. This phenomenon means that the maximum temperature at which this type of material can be used must be 100° C. lower than those of superalloys based on nickel and iron which only contain small to very small quantities of aluminium, for example less than 0.5% by weight.
Further, the presence of aluminium in the matrix can lead to precipitation of the intermetallic phase on the grain boundaries during welding. This adversely affects mechanical behaviour (appearance of cracks and even breaks). Welding this type of material is difficult, as with all nickel-based superalloys with an aluminium content of more than about 3%. These materials are classified as difficult to weld (see E Bradley, “Welding of superalloys”, Superalloys—a technical guide, 1988, Chapter 13, pages 197 to 220).
The presence of aluminium in the matrix means that the coatings adhere to the material with difficulty. Coatings obtained by chemical vapour deposition (CVD) (for example those described in U.S. Pat. No. 5,242,574) on this type of material are less adhesive and weaker, in particular during thermal cycling.
The present invention provides a passivation method for the metallic surface of a superalloy containing little or no aluminium which at least partially overcomes the drawbacks of the prior art methods and produces a coating which is highly resistant to thermal cycling and has good adhesion and homogeneity. The quality o
Broutin Paul
Nisio Pascal
Ropital François
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
Oltmans Andrew L.
Sheehan John
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