Coating processes – Applying superposed diverse coating or coating a coated base – Metallic compound-containing coating
Reexamination Certificate
1996-04-30
2002-09-24
Valentine, Donald R. (Department: 1741)
Coating processes
Applying superposed diverse coating or coating a coated base
Metallic compound-containing coating
C427S113000, C427S126100, C427S419700, C501S031000, C501S053000, C501S102000, C501S133000
Reexamination Certificate
active
06455107
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to bodies of materials such as, for example, carbonaceous materials, for use in corrosive environments such as oxidising media or gaseous or liquid. corrosive agents at elevated temperatures, coated with a protective surface coating which improves the resistance of the bodies to oxidation or corrosion and which may also enhance the electrical conductivity and/or electrochemical activity of the body.
BACKGROUND OF THE INVENTION
Carbonaceous materials are important engineering materials used in diverse applications such as aircraft bodies, electrodes, heating elements, structural materials, rocket nozzles, metallurgical crucibles, pump shafts, furnace fixtures, sintering trays, induction furnace susceptors, continuous casting dies, ingot molds, extrusion canisters and dies, heat exchangers, anodes, high temperature insulation (porous graphite), gas diffusers, aerospace structural materials, bearings, substrates in electronics industry, brazing and joining fixtures, diamond wheel molds, nozzles, glass molds etc. Although carbonaceous materials have properties which make them useful for the applications mentioned above, the resistance to oxidation is one property which has limited the use of these materials. Much effort is therefore underway to improve the resistance to oxidation of such materials.
Traditional methods of preventing oxidation of carbonaceous materials have involved the deposition of adherent and highly continuous layers of materials such as silicon carbide or metals such as aluminum. The deposit of such materials has normally been carried out by techniques such as vapor deposition (both PVD and CVD) or by electrochemical methods. Vapor deposition is an extremely slow and costly process and additionally may not be carried out for large parts such as electrodes. It is also known to plasma spray alumina/aluminium onto the sides of carbon anodes used as anodes for aluminium electrowinning, but this coating method is expensive. Other techniques such as electrochemical methods are limited in the type of materials that may be applied as coatings and size limitations again may be present. Sol-gel techniques are known for the application of coatings. However, it is well known that these techniques are not adequate for oxidation protection, because they produce extremely thin films, usually of the order of 1 micrometer thick, that are most often porous and have a tendency to delaminate especially under conditions of thermal expansion mismatch with the substrate.
Therefore, there is a need for developing a cost effective versatile method for coating carbonaceous materials with an adherent coating that will effectively prevent oxidation and the loss of the carbonaceous substrate because of rapid or slow burning.
SUMMARY OF THE INVENTION
According to the invention, a protective coating on a body of carbonaceous or other material which improves the resistance of the body to oxidation, and which may also enhance the bodies electrical conductivity and/or its electrochemical activity is applied from a colloidal slurry containing particulate reactant or non-reactant substances, or a mixture of particulate reactant and non-reactant substances, which when the body is heated to a sufficient elevated temperature form the protective coating by reaction sintering and/or by sintering without reaction.
The coatings of the invention are “thick” coatings, of the order of tens of micrometers thick, and contain refractory particulate materials which adjust to the thermal expansion mismatch and, in most embodiments, after sintering or oxidation during use, are able to provide a continuous thick silica layer for oxidation prevention.
The invention is particularly advantageous when the body is made of carbonaceous material, for instance petroleum coke, metallurgical coke, anthracite, graphite, amorphous carbon, fulerene such as fulerene C
60
or C
70
or of a related family, low density carbon or mixtures thereof. The coatings are particularly adherent on carbon substrates because the high surface activity bonds the particles to the carbon.
It is advantageous for bodies of low-density carbon to be protected by the coating of the invention, for example if the component is exposed to oxidising gas released in operation of an electrolytic cell, or also when the substrate is part of a cell bottom. Low density carbon embraces various types of relatively inexpensive forms of carbon which are relatively porous and very conductive, but hitherto could not be used successfully in the environment of aluminium production cells on account of the fact that they were subject to excessive corrosion or oxidation. Now it is possible by coating these low density carbons according to the invention, to make use of them in these cells instead of the more expensive high density anthracite and graphite, taking advantage of their excellent conductivity and low cost.
The invention also concerns coated bodies with substrates of a metal, alloy, intermetallic compound or refractory material, to which the protective coating is applied.
Two types of coatings have been developed and are described in this application. One will be called the micropyretic type and the other the non-micropyretic type. Micropyretic coatings contain combustible materials which provide heat during combustion and also add desired constituents to the coating after combustion of the coating. The non-micropyretic type does not contain any combustible. Mixtures of micropyretic and non-micropyretic coatings are also possible. Both coatings involve the application of a colloidal slurry which is applied to the substrate by painting, spraying, dipping or pouring onto the substrate. When several layers of such coatings are applied, it is possible that some may contain micropyretic constituents and some may not.
Thus, the applied colloidal slurry may contain micropyretic particulate reactant substances which undergo a sustained micropyretic reaction to produce for example refractory borides, silicides, nitrides, carbides, phosphides, oxides., aluminides, metal alloys, intermetallics, and mixtures thereof, of titanium, zirconium, hafnium, vanadium, silicon, niobium, tantalum, nickel, molybdenum and iron, the micropyretic reactant substances being finely divided particulates including elements making up the refractory material produced.
Such micropyretic reactant substances may for instance compriseparticles, fibers or foils of Ni, Al, Ti, B, Si, Nb, C, Cr
2
O
3
, Zr, Ta, TiO
2
, B
2
O
3
, Fe, Mo or combinations thereof.
It is essential to use colloids and mixtures of colloids for application of the coatings. Three types of colloidal processing are possible. The first involves the gelation of certain polysaccharide solutions. This, however, is relatively unimportant to this invention. The other two which involve colloids and metal organic compounds are relevant to this invention. These two involve the mixing of materials in a very fine scale. Colloids are defined as comprising a dispersed phase with at least one dimension between 0.5 nm (nanometer) and about micrometers in a dispersion medium which in our case is a liquid. The magnitude of this dimension distinguishes colloids from bulk systems in the following way: (a) an extremely large surface area and (b) a significant percentage of molecules reside in the surface of colloidal systems. Up to 40% of molecules may reside on the surface. The colloidal systems which are important to this invention are both the thermodynamically stable lyophylic type (which include macromolecular systems such as polymers) and the kinetically stable lyophobic type (those that contain particles).
Insoluble oxides in aqueous suspension develop surface electric charges by surface hydroxylation followed by dissociation of surface hydroxyl groups. Typical equations could be:
M
(
OH
) surface +H
2
O⇄
MO
−
surface +H
3
O
+
M
(
OH
) surface +H
2
O⇄
M(OH
2
)
+
surface +OH
−
where M represents a metal or a complex cation.
Such surface charges a
de Nora Vittorio
Sekhar Jainagesh A.
Deshmukh Jayadeep R.
Moltech Invent S.A.
Valentine Donald R.
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