Back to EveryPatent.com
United States Patent |
5,514,411
|
Sekhar
,   et al.
|
May 7, 1996
|
Prevention of oxidation of carbonaceous and other materials at high
temperatures
Abstract
A body of carbonaceous or other material for use in corrosive environments
such as oxidising media or gaseous or liquid corrosive agents at elevated
temperatures, in particular in molten salts such as cryolite, is coated
with a protective surface coating which improves the resistance of the
body to oxidation or corrosion and which may also enhance the bodies
electrical conductivity and/or its electrochemical activity. The
protective coating is applied in one or more layers from a colloidal
slurry containing reactant or non-reactant substances, or a mixture of
reactant and non-reactant substances, in particular mixtures containing
silicon carbide and molybdenum silicide or silicon carbide and silicon
nitride, which when the body is heated to a sufficient elevated
temperature reaction sinter as a result of micropyretic reaction and/or
sinter without reaction to form the protective coating.
Inventors:
|
Sekhar; Jainagesh A. (Cincinnati, OH);
de Nora; Vittorio (Nassau, BS)
|
Assignee:
|
Moltech Invent S.A. (Luxembourg, LU)
|
Appl. No.:
|
320960 |
Filed:
|
October 12, 1994 |
Intern'l Class: |
B05D 005/12; B05D 003/02; B05D 001/02; B05D 001/18 |
Field of Search: |
204/290 R,294,243 R-247,279
;397.7;421;427;430.1;443.2;58;115;123-126.6
427/77,113,126.2,126.3,129.4,190-192,201,202,203,204,205,376.1-376.8,383.1
501/120,127
106/14.05
|
References Cited
U.S. Patent Documents
2859138 | Nov., 1958 | Blanchard | 117/169.
|
2866724 | Dec., 1958 | Alexander | 427/113.
|
3249460 | May., 1966 | Gerry | 427/376.
|
3348929 | Oct., 1967 | Valtschen et al. | 427/113.
|
3404031 | Oct., 1968 | Clayton et al. | 252/507.
|
3852107 | Dec., 1974 | Lorkin et al. | 427/113.
|
3859198 | Jan., 1975 | Emblem et al. | 204/294.
|
3939028 | Feb., 1976 | Schiffarth et al. | 427/376.
|
3964924 | Jun., 1976 | Kurzeja | 252/506.
|
4418097 | Nov., 1983 | Misra | 427/113.
|
4487804 | Dec., 1984 | Reven | 428/408.
|
4535035 | Aug., 1985 | Smialek et al. | 428/698.
|
4559270 | Dec., 1985 | Sara | 428/408.
|
4567103 | Jan., 1986 | Sara | 428/408.
|
4585675 | Apr., 1986 | Shuford | 427/376.
|
4650552 | Mar., 1987 | de Nora et al. | 501/127.
|
4711666 | Dec., 1987 | Chapman et al. | 252/508.
|
4726995 | Feb., 1988 | Chiu | 427/113.
|
4769074 | Sep., 1988 | Holcombe, Jr. et al. | 252/508.
|
4921731 | May., 1990 | Clark et al. | 427/314.
|
4983423 | Jan., 1991 | Goldsmith | 427/376.
|
5026422 | Jun., 1991 | Osborne | 106/14.
|
5112654 | May., 1992 | Claar | 427/376.
|
5164233 | Nov., 1992 | Sonuparlak et al. | 501/120.
|
5194330 | Mar., 1993 | Van den bulcke et al. | 428/698.
|
5250324 | Oct., 1993 | Claar | 427/376.
|
Primary Examiner: Valentine; Donald R.
Attorney, Agent or Firm: Dinsmore & Shohl
Parent Case Text
This is a divisional application of application Ser. No. 07/898,052, filed
Jun. 12, 1992, now U.S. Pat. No. 5,364,513.
Claims
We claim:
1. A method of improving the resistance to oxidation or corrosion of a body
of carbonaceous material for use in corrosive environments such as
oxidizing media or gaseous or liquid agents at elevated temperatures, the
body being, in particular, a component of an electrochemical cell for
molten salt electrolysis, which component in use is exposed to a corrosive
atmosphere, or to a molten salt electrolyte and/or to a product of
electrolysis in the cell, the method comprising:
applying to the body a non-glassy protective coating from a colloidal
slurry containing particular reactant or non-reactant substances, or a
mixture of particulate reactant and non-reactant substances;
heating the body prior to or during use to a sufficiently elevated
temperature; and
causing the reactant and/or non-reactant substances to reaction sinter
and/or to sinter without reaction to form said non-glassy coating, said
coating being adherent and protective.
2. A method according to claim 1, in which the body is made of carbonaceous
material.
3. A method according to claim 2, in which the carbonaceous material is
selected from petroleum coke, metallurgical coke, anthracite, graphite,
amorphous carbon, fulrene, low density carbon, or mixtures thereof.
4. A method according to claim 1 in which the applied colloidal slurry
contains micropyretic reactant substances which undergo a sustained
micropyretic reaction.
5. A method according to claim 4, in which the micropyretic reactant
substances react to produce 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 comprising finely divided particulates comprising elements
making up the refractory material produced.
6. A method according to claim 5, in which the micropyretic reactant
substances comprise particles, fibers or foils of Ni, Al, Ti, B, Si, Nb,
C, Cr.sub.2 O.sub.3,Zr, Ta, TiO.sub.2, B.sub.2 O.sub.3, Fe, Mo or
combinations thereof.
7. A method according to claim 1, in which the applied colloidal slurry
contains non-reactant substances which sinter above a given temperature.
8. A method according to claim 1, in which the reactant and/or non-reactant
substances reaction sinter and/or sinter without reaction above
900.degree. C.
9. A method according to claim 1, in which the reactant and/or non-reactant
substances are reaction sintered and/or sintered without reaction to
provide an adherent coating on the body prior to use.
10. A method according to claim 1, in which the non-reactant substances
comprise antioxidant or oxidation prevention materials such as boric acid
and its salts, and fluorides; bonding enhancing materials such as
methyl-cellulose, particulate alumina or urea; metallic, intermetallic,
semi-metallic, polymeric, refractory and/or ceramic materials such as
borides, carbides, nitrides, silicides, oxides, oxynitrides and mixtures
thereof; pyrolizable chlorosilanes, polycarbosilanes, polysilanes and
other organometal polymers which pyrolize to useful products for oxidation
prevention or enhancing bonding, or their pyrolized products;
thermosetting resins; thermoplastic resins; and mixtures thereof.
11. A method according to claim 10, containing metallic particles of Ni,
Pt, Al, Cr or intermetallic particles selected from NiAl, NiAl.sub.3,
CrSi, CrB, or combinations thereof.
12. A method according to claim 1, in which the non-reactant substances are
particulates with a particle size below 100 microns.
13. A method according to claim 12, in which different non-reactant
particulate substances have different particle sizes to optimize packing
of the particles, with particle size ratios in the range from 2:1 to 5:1,
preferably about 3:1.
14. A method according to claim 1, in which the particulate reactant and/or
non-reactant substances contain at least one silicon-containing compound.
15. A method according to claim 14, in which the particulate non-reactant
substances contain at least one carbide, nitride, boride or oxide of
silicon or combinations thereof, in combination with at least one silicide
of titanium, zirconium, hafnium, vanadium, niobium, tantalum, nickel,
molybdenum, chromium and iron, or a combination of at least two carbides,
nitrides, borides or oxides of silicon.
16. A method according to claim 15, in which the particulate non-reactant
substances contain silicon carbide and molybdenum silicide or silicon
carbide and silicon nitride.
17. A method according to claim 1, in which the colloidal slurry comprises
at least one of colloidal silica, alumina, yttria, ceria, thoria,
zirconia, magnesia, lithia and hydroxides, acetates and formates thereof
as well as oxides and hydroxides of other metals, cationic species and
mixtures thereof.
18. A method according to claim 17, in which the colloidal slurry is
derived from colloid precursors and reagents which are solutions of at
least one salt such as chlorides, sulfates, nitrates, chlorates,
perchlorates or metal organic compounds such as alkoxides, formates,
acetates of silicon, aluminium, yttrium, cerium, thorium zirconium,
magnesium, lithium and other metals and mixtures thereof.
19. A method according to claim 18, in which the colloid precursor or
colloid reagent contains a chelating agent such as acetyl acetone or
ethylacetoacetate.
20. A method according to claim 18, in which the solutions of metal organic
compounds, principally metal alkoxides, are of the general formula
M(OR).sub.z where M is a metal or complex cation, R is an alkyl chain and
z is a number.
21. A method according to claim 1, in which the substrate is a metal,
alloy, intermetallic compound or refractory material, to which the
protective coating is applied.
22. A method according to claim 1, in which the non-reactant substances
contain a binder to form a paste in which the binder is a suspension
containing one or more colloids, gels, colloid reagents, colloid
precursors, chelating agents, methyl cellulose, clays like kaolinite,
polyvinyl butyral, fused silica and activators.
23. A method according to claim 22, in which the reagents and precursors,
gels and colloids are modified in pH prior to or after mixing by the
addition of an acid or a base.
24. A method according to claim 1, in which the coating is applied by
dipping the body in a solution, painting, spraying or combinations of such
application techniques, in single or multi-layer coatings.
25. A method according to claim 1, in which the body is further painted,
sprayed, dipped or infiltrated with reagents and precursors, gels and/or
colloids.
26. A method of protecting a carbonaceous component of an electrochemical
cell for molten salt electrolysis which component in use is exposed to a
corrosive atmosphere, or to a molten salt electrolyte and/or to a product
of electrolysis in the cell, comprising:
applying to the component non-glassy protective coating from a colloidal
slurry containing particulate reactant or non-reactant substances, or a
mixture of particulate reactant and non-reactant substances;
heating the component prior to or during use to a sufficiently elevated
temperature; and
causing the reactant and/or non-reactant substances to reaction sinter
and/or to sinter without reaction to form said non-glassy coating, said
coating being adherent and protective.
Description
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.sub.60
or C.sub.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 comprise particles,
fibers or foils of Ni, Al, Ti, B, Si, Nb, C, Cr.sub.2 O.sub.3, Zr, Ta,
TiO.sub.2, B.sub.2 O.sub.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 10 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.sub.2 O.revreaction.MO.sup.- surface+H.sub.3 O.sup.-
M(OH)surface+H.sub.2 O.revreaction.M(OH.sub.2).sup.+ surface+OH.sup.-
where M represents a metal or a complex cation.
Such surface charges and the London and Van der Waals forces keep the
particles from agglomerating. An adsorbed layer of material, polymer or
surface active agent, modifies the interaction of particles in several
ways. In the mixing process described below we introduce new materials and
other agents into the colloids.
Colloids may form through cation hydrolysis. Many metal ions are subject to
hydrolysis because of high electronic charge or charge density. Initial
products of hydrolysis can condense and polymerize to form polyvalent
metal or polynuclear ions, which are themselves colloidal. Charge and pH
determine the ligands for central cations and the anion/cation ratio
controls the degree of polymerization and stability of the suspension. The
pH could vary from 0-14. A wide range of polynuclear cationic hydrolysis
products may exist with charge from 2+ to 6+. Polynuclear anionic
hydrolysis products could also have a wide range of charges.
The formation of colloids involves a starting material for example a
reagent grade metal salt which is converted in a chemical process to a
dispersible oxide which forms the colloidal solution on addition of dilute
acid or water. Removal of water (drying) and or removal of the anions from
the colloidal solution produces a gel like product. In the method of the
invention for oxidation resistant coatings, the colloid thus acts as a
binder to the other additives and also densifies the product formed. The
calcination process in air yields an oxide product after decomposition of
salts whereas carbon, silicon, boron etc. may be added to the colloid to
yield a non oxide ceramic in the coating. The colloidal solutions may also
be blended.
The colloidal carrier--usually colloidal alumina, colloidal ceria,
colloidal silica, colloidal yttria or colloidal monoaluminium phosphate
and usually in an aqueous medium--has been found to assist in moderating
the micropyretic reaction and to considerably improve the properties of
the coating whether produced by reaction sintering or non-reactive
sintering. It is however not necessary for all of the applied layers of
the slurry to have a colloidal carrier. Excellent results have been
obtained using some slurries with a colloidal carrier and others with an
organic solvent. Combinations of a colloidal carrier in aqueous medium and
an organic solvent have also worked well.
In the case of micropyretic coatings an additional step after the drying of
the applied slurry on the coating will be the firing (combustion) of the
constituents of the slurry by direct flame, concentrated sunlight, plasma,
laser, electron beam or by traditional methods such as passing a current
through the conductive substrate or placing the coated article inside a
furnace at a predetermined temperature or by heating the coating by an
induction method or by radiant heating.
The applied colloidal slurry contains particulate substances which sinter
above a given temperature, in particular reactant and/or non-reactant
substances like silicon carbide that reaction sinter and/or sinter without
reaction above 900.degree. C. The coating may be pre-formed prior to use,
in which case the reactant and/or non-reactant substances have been
reaction sintered and/or sintered without reaction to provide an adherent
coating on the body prior to use. Alternatively, the micropyretic reaction
sintering or the non-reactive sintering may take place only when the body
coated with the coating components is used at high temperature.
When use of a silicon-carbide-containing coating is contemplated at
temperatures below 900.degree. C. then normally micropyretic coatings are
preferred; when use is contemplated at above 900.degree. C. then
non-micropyretic coatings are also acceptable. This is because the
coatings become effective after they sinter. Above 900.degree. C.,
sintering may occur during exposure to the service conditions at the high
temperature. Below 900.degree. C. the micropyretic reaction and the
combustion initiation process will provide the required heat for the
sintering operation. Nevertheless, it remains possible to sinter
non-micropyretic coatings above 900.degree. C. and then use them below
900.degree. C.
In-situ repair of coatings during use is also contemplated by both types of
coatings.
The constituents of the slurries are:
(a) A carrier, chosen from colloidal liquids which could be colloidal
alumina, colloidal ceria, colloidal yttria, colloidal silica, colloidal
zirconia or mono-aluminum phosphate or colloidal cerium acetate or
mixtures thereof.
(b) A powder additive containing carbides, silicides, borides, nitrides,
oxides, nitrides, carbonitrides, oxynitrides, boric acid and its salts,
and combinations of these. When choosing combinations of powder additives
the particle size selection is of importance. It is preferable to choose
particle size below 100 microns and, when employing combinations of powder
additives, to choose particle sizes which are varied such that the packing
of particles is optimized. For example when choosing a composition
containing mostly SiC and some MoSi.sub.2 it is preferable to choose the
particle size of the MoSi.sub.2 much smaller (at least three times
smaller) than the SiC. Generally, the ratio of the particle sizes will be
in the range from 2:1 to 5:1, preferably about 3:1, for instance with
large particles in the range 15 to 30 micrometers and small particles in
the range 5 to 10 micrometers.
(c) Metallic particles such as for example Ni, Pt, Al, Cr or intermetallic
particles such as for example NiAl, NiAl.sub.3, CrSi, CrB etc. or
combinations thereof, in which case the particle sizes will be varied to
achieve optimum packing, as for powder additives.
(d) Micropyretic agents. These agents are particles, fibers or foils of
materials such as Ni, Al, Ti, B, Si, Nb, C, Cr.sub.2 O.sub.3, Zr, Ta,
TiO.sub.2, B.sub.2 O.sub.3, Fe, Mo or combinations which may react to
yield heat as well as yielding clean and nascent products from the
combustion. Typical reactions could be for example Cr.sub.2 O.sub.3
+2Al+2B which reacts spontaneously to give CrB.sub.2 and Al.sub.2 O.sub.3
with a large release of heat. The adiabatic temperature of such a
micropyretic reaction is 6500.degree. K. Tables I, II and III give a
partial listing of examples of micropyretic reactions and products and the
amount of heat released in each reaction. .DELTA.H(KJ/mole) is the
enthalpy release for the reaction and T.sub.ad K is the adiabatic
temperature (.degree.K.) which is expected to be reached in such
reactions.
TABLE I
______________________________________
FORMATION OF REFRACTORY COMPOUNDS
REACTION .DELTA.H(KJ/mole)
Tad K
______________________________________
Ti + 2B = TiB.sub.2
-293.00 3190
Zr + 2B = ZrB.sub.2
-263.75 3310
Nb + 2B = NbB.sub.2
-207.74 2400
Ti + B = TiB -158.84 3350
Hf + 2B = HfB.sub.2
-310.15 3520
Ta + 2B = TaB.sub.2
-193.53 3370
Ti + C = TiC -232.00 3210
______________________________________
TABLE II
______________________________________
FORMATION OF INTERMETALLICS
REACTION .DELTA.H(KJ/mole)
Tad K
______________________________________
Ti + Ni = TiNi -66.5 1773
Ti + Pd = TiPd -103.4 1873
Ni + Al = NiAl -118.4 1923
Ti + Al = TiAl -72.8 1734
Ti + Fe = TiFe -40.6 1423
______________________________________
TABLE III
______________________________________
FORMATION OF COMPOSITES
REACTION .DELTA.H(KJ/mole)
Tad K
______________________________________
Fe.sub.2 O.sub.3 + 2Al = Al.sub.2 O.sub.3 + 2Fe
-836.00 3753
Cr.sub.2 O.sub.3 + 2Al = Al.sub.2 O.sub.3 + 2Cr
-530.00 2460
2Cr.sub.2 O.sub.3 + 6Al + 6C = 6500
2Cr.sub.2 C.sub.3 + 3Al.sub.2 O.sub.3
0.86Ti + 1.72B + 1.48Al =
-293.00 1450
0.86TiB.sub.2 + 1.48Al
Ti + C + 0.68Ni = TiC + 0.68Ni
-232.00 1370
Zr + 2B + Cu = ZrB.sub.2 + Cu
-263.75 1100
______________________________________
(e) Metal organic compounds principally metal alkoxides of the general
formula M(OR).sub.z where M is a metal or complex cation made up of two or
more elements, R is an alkyl chain and z is a number usually in the range
1 to 12, or alternatively described as solutions in which molecules in
which organic groups are bound to a metal atom through oxygen. Examples
are silicon tetraisomyloxide, aluminum butoxide, aluminum isopropoxide,
tetraethyl orthosilicates, etc. Formates, acetates and acetylacetonates
are also considered in this category.
(f) Pyrolizable chlorosilanes, polycarbosilanes, polysilazanes and other
organosilicon polymers as binders which pyrolize to useful products for
oxidation prevention. Such compounds are expected to participate in the
micropyretic reaction in a beneficial but complex manner to increase the
yield of useful product with a morphology and size which assists in making
the coating more adherent and tight.
(g) Buffer solutions to modify the pH of the slurry. These are standard
laboratory grade alkalines or acids.
(h) Binding agents such as methyl cellulose, clays like kaolinite,
polyvinyl butyral, fused silica and activators, etc.
Considering the above defined constituent groups (a) to (h), the slurries
used in the invention are made up of at least one of the additives from
groups (b), (c) and/or (d) in a colloidal carrier from group (a),
optionally together with one or more components from groups (e) to (h).
Some materials may be present under more than one heading. For instance
silica or alumina in colloidal form can be included in the carrier, and in
powder form as additive. Particulate nickel and aluminium can be present
as a micropyretic reactant, but in excess of the stoichiometric amount,
whereby the excess forms a particulate additive. It is also possible for
the powder additive to be the same as the reaction product of the
micropyretic reaction.
The non-reactant substances may comprise antioxidant or oxidation
prevention materials such as boric acid and its salts, and fluorides;
bonding enhancing materials such as methyl-cellulose, particulate alumina
or urea; metallic, intermetallic, semi-metallic, polymeric, refractory
and/or ceramic materials such as borides, carbides, nitrides, silicides,
oxides, oxynitrides and mixtures thereof; pyrolizable chlorosilanes,
polycarbosilanes, polysilanes and other organometal polymers which
pyrolize to useful products for oxidation prevention or enhancing bonding,
or their pyrolized products: thermosetting resins; thermoplastic resins;
and mixtures thereof.
Examples of thermosetting resins are epoxides, phenolic resins and
polyimides. Examples of thermoplastic resins are polycarbonates, e.g.
Lexan.TM., polyphenylene sulfides, polyether ether ketones, polysulfones,
e.g. Udel.TM., polyetherimides and polyethersulfones.
The coating advantageously contains at least one silicon-containing
compound, which may be included as a reactant and/or as a non-reactant,
advantageously in a substantial amount, usually accounting for 30 wt % or
more of the coating, advantageously 50 wt % or more. Silicon compounds
when reacted or sintered form on the body a relatively impervious silica
skin, providing excellent resistance against oxidation and corrosion.
Formation of such a silicous skin can be enhanced by including colloidal
silica in the carrier.
The applied coating for instance contains at least one carbide, nitride,
boride or oxide of silicon or combinations thereof, in combination with at
least one silicide of titanium, zirconium, hafnium, vanadium, niobium,,
tantalum, nickel, molybdenum, chromium and iron, or a combination of at
least two carbides, nitrides, borides or oxides of silicon. One
particularly advantageous combination includes silicon carbide with
molybdenum silicide. Another includes silicon carbide with silicon
nitride. These silicon-based combinations can be used alone or in
combination with other silicon or non-silicon non-reactants or with
micropyretic reactants, and particularly with colloidal silica in the
carrier. When such coatings are sintered before use in an oxidising
atmosphere, or when such coatings are used in an oxidsing atmosphere, the
coatings are converted to produce a relatively impervious silica skin.
The invention is useful for protecting the various engineering materials
made of carbon listed at the outset. A main application of the invention
is however for the protection of components of electrochemical cells for
molten salt electrolysis which components in use are exposed to a
corrosive atmosphere, or to a molten salt electrolyte, such as cryolite,
and/or to a product of electrolyis in the cell. Such components are thus
coated with a protective surface coating which improves the resistance of
the components to oxidation or corrosion and which may also enhance the
electrical conductivity and/or electrochemical activity. The protective
coating 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 component is heated to a
sufficient elevated temperature, prior to or upon insertion in the cell,
form the protective coating by reaction sintering and/or by sintering
without reaction.
Such components may have a carbonaceous substrate, or a substrate of a
metal, alloy, intermetallic compound or refractory material, to which the
protective coating is applied. The component may be a cathode or a cathode
current feeder, an anode or an anode current feeder, e.g. for a
Hall-Heroult cell, or a bipolar electrode for new cell designs.
The invention is particularly applicable to components which are exposed to
corrosive or oxidising gas released in operation of the cell or present in
the cell operating conditions, the component comprising a substrate of
carbonaceous material (particularly low-density carbon), refractory
material or metal alloy that is subject to attack by the corrosive or
oxidising gas and is protected from corrosion or oxidation by the
protective surface coating.
The invention also concerns a method of improving the resistance to
oxidation or corrosion of a body of material for use in corrosive
environments such as oxidising media or gaseous or liquid agents at
elevated temperatures, the body being in particular a component of an
electrochemical cell for molten salt electrolysis which component in use
is exposed to a corrosive atmosphere, or to a molten salt electrolyte
and/or to a product of electrolysis in the cell. This method comprises
applying to the body a protective coating from a colloidal slurry
containing reactant or non-reactant substances, or a mixture of reactant
and non-reactant substances, followed by heating the body prior to or
during use to a sufficient temperature to cause the reactant and/or
non-reactant substances to reaction sinter and/or to sinter without
reaction to form an adherent protective coating.
The method of application of the slurry involves painting (by brush or
roller), dipping, spraying, or pouring the liquid onto the substrate and
allowing for drying before another layer is added. The coating need not
entirely dry before the application of the next layer. However if one or
more layers with micropyretic constituents are present, then it is
preferable to dry completely prior to firing. Layers may be added to
already fired coatings either for repair or for additional build up. Even
when micropyretic constituents are absent, it is preferred to heat the
coating with a suitable heat source such as a torch (butane or
oxyacetylene), a laser, a furnace, etc., so as to improve densification of
the coating. Heating takes place preferably in air but could be in other
oxidising atmospheres or in inert or reducing atmospheres.
The substrate may be treated by sand blasting or pickled with acids or
fluxes such as cryolite or other combinations of fluorides and chlorides
prior to the application of the coating. Similarly the substrate may be
cleaned with an organic solvent such as acetone to remove oily products
and other debris prior to the application of the coating. These treatments
will enhance the bonding of the coatings to the substrate.
After coating the substrate applied by dipping, painting, spraying or
combinations of such techniques in single or multi-layer coatings and
drying, a final coat of one or more of the liquids listed in (a)-(e) may
be applied lightly prior to use.
More generally, after fabrication and before use, the body can be painted,
sprayed, dipped or infiltrated with reagents and precursors, gels and/or
colloids.
Examples of some non-micropyretic slurries and some micropyretic slurries
are given in Table IV and Table V respectively.
TABLE IV
__________________________________________________________________________
EXAMPLES OF NON-MICROPYRETIC SLURRIES
Powder/
Composition Powder (w %)/Particle Size Carrier
Sample
-200 mesh
<10 microns
-325 mesh
-325 mesh
-325 mesh
Carrier, vol %
g/ml
__________________________________________________________________________
1 SiC 97.5%
Si.sub.3 N.sub.4 2.5%
-- -- -- Coll-Silica 50%
10/6
Coll-Alumina 50%
2 SiC 90%
Si.sub.3 N.sub.4 10%
-- -- -- Coll-Alumina 100%
10/6
3 SiC 90 %
Si.sub.3 N.sub.4 10%
-- -- -- Coll-Silica 100%
10/6
4 SiC 90%
-- Y.sub.2 O.sub.3 10%
-- -- Coll-Alumina 100%
10/6
5 SiC 90%
-- Y.sub.2 O.sub.3 10%
-- -- Coll-Silica 100%
10/6
6 SiC 92.5%
Si.sub.3 N.sub.4 2.5%
Y.sub.2 O.sub.3 5%
-- -- Coll-Silica 50%
10/6
-- Coll-Alumina 50%
7 SiC 90%
Si.sub.3 N.sub.4 10%
-- -- -- Coll-Yttria 100%
10/5
8 SiC 90%
Si.sub.3 N.sub.4 10%
-- -- -- Coll-Ceria 100%
10/5
9 SiC 90%
Si.sub.3 N.sub.4 5%
Y.sub.2 O.sub.3 2.5%
Al.sub.2 O.sub.3 2.5%
-- Coll-Silica 100%
10/5
10 SiC 85%
Si.sub.3 N.sub.4 5%
Y.sub.2 O.sub.3 2.5%
Al.sub.2 O.sub.3 2.5%
TiB.sub.2 5%
Coll-Silica 100%
10/5
11 SiC 85%
Si.sub.3 N.sub.4 5%
Y.sub.2 O.sub.3 2.5%
SiO.sub.2 2.5%
TiB.sub.2 5%
Coll-Alumina 100%
10/5
12 SiC 90%
MoSi.sub.2 5%
Y.sub.2 O.sub.3 2.5%
Al.sub.2 O.sub.3 2.5%
-- Coll-Silica 100%
10/5
13 SiC 85%
MoSi.sub.2 5%
Y.sub.2 O.sub.3 2.5%
Al.sub.2 O.sub.3 2.5%
TiB.sub.2 5%
Coll-Silica 100%
10/5
14 SiC 85%
MoSi.sub.2 5%
Y.sub.2 O.sub.3 2.5%
SiO.sub.2 2.5%
TiB.sub.2 5%
Coll-Alumina 100%
10/5
15 SiC 80%
PbSi.sub.2 10%
Y.sub.2 O.sub.3 10%
-- -- Coll-Silica 100%
10/5
16 SiC 70%
MoSi.sub.2 20%
Y.sub.2 O.sub.3 2.5%
Al.sub.2 O.sub.3 2.5%
TiB.sub.2 5%
Coll-Silica 100%
10/5
__________________________________________________________________________
TABLE V
__________________________________________________________________________
EXAMPLES OF MICROPYRETIC SLURRIES
Powder/
Composition Powder (w %)/Particle Size Carrier
Sample
-200 mesh
<10 microns
-325 mesh
-325 mesh
-325 mesh
Carrier, vol %
g/ml
__________________________________________________________________________
1 SiC 60%
Si.sub.3 N.sub.4 10%
Ti 17%
B 8% TiB.sub.2 5%
Coll-Silica 50%
10/6
Coll-Alumina 50%
2 SiC 50%
Si.sub.3 N.sub.4 8%
Ti 25%
B 10% TiB.sub.2 7%
Coll-Alumina 100%
10/6
3 SiC 50%
Si.sub.3 N.sub.4 7%
TiO.sub.2 25%
B.sub.2 O.sub.3 15%
Al 3% Coll-Silica 100%
10/6
4 SiC 50%
TiB.sub.2 10%
Y.sub.2 O.sub.3 8%
Ni 22%
Al 10%
Coll-Ceria Acetate 100%
10/6
5 SiC 50%
Zr.sub.2 5%
Y.sub.2 O.sub.3 2%
Ti 20%
Ni 23%
Coll-Silica 100%
10/6
Coll-Silica 50%/
10/6
6 SiC 32.5%
Si.sub.3 N.sub.4 2.5%
Y.sub.2 O.sub.3 5%
Ti 15%
Si 5% Coll-Alumina 50%
7 SiC 80%
Si.sub.3 N.sub.4 5%
Cr.sub.2 O.sub.3 10%
Al 3% C 2% Coll-Yttria 100%
10/5
8 SiC 50%
Si.sub.3 N.sub.4 10%
Ti 28%
C 7% BaO 5%
Coll-Ceria Acetate 100%
10/5
9 SiC 50%
Si.sub.3 N.sub.4 5%
Ti 26%
Al.sub.2 O.sub.3 3%
Al 16%
Polycarbosilane 20%
10/5
Coll-Silica 80%
10 SiC 40%
Si.sub.3 N.sub.4 5%
Y.sub.2 O.sub.3 5%
Ti 37%
Si 13%
Coll-Silica 100%
10/5
11 SiC 45%
Si.sub.3 N.sub.4 7.5%
Ti 30%
SiO.sub.2 2.5%
B 15% Coll-Alumina 100%
10/5
12 SiC 90%
Zr 4% Y.sub.2 O.sub.3 2.5%
Al.sub.2 O.sub.3 2.5%
B 1% Mono-Al-Phosphate 100%
10/5
13 SiC 85%
MoSi.sub.2 5%
Y.sub.2 O.sub.3 2.5%
Al.sub.2 O.sub.3 2.5%
TiB.sub.2 5%
Coll-Silica 100%
10/5
14 SiC 85%
MoSi.sub.2 5%
Y.sub.2 O.sub.3 2.5%
SiO.sub.2 2.5%
TiB.sub.2 5%
Coll-Alumina 100%
10/5
15 SiC 80%
MoSi.sub.2 5%
Y.sub.2 O.sub.3 10%
Ti 8% C 2% Tetraisomyloxide 20%
10/5
Coll-Silica 80%
16 SiC 68%
MoSi.sub.2 20%
Cr.sub.2 O.sub.3 2.5%
Al 7% C 2.5%
Coll-Silica 100%
10/5
__________________________________________________________________________
EXAMPLE I
A non-micropyretic slurry corresponding to sample 9 of Table IV was applied
to a graphite block 4 cm.times.4 cm.times.6 cm by painting to build up a
layer of approximately 500 micrometers. This block along with an uncoated
block were placed in a furnace under air at 1000.degree. C. The uncoated
block completely burnt in 24 hours. The coated block showed a weight loss
of 5% after an exposure of 27 hours. The coating was noted to remain very
stable and adherent. Scanning electron microscope pictures of the coating
before and after the exposure show that the coating self densified to
protect the carbonaceous substrate.
EXAMPLE II
The test of Example I was repeated but with a micropyretic slurry
corresponding to sample 10 of Table V applied as the coating. The coated
sample when placed in the furnace was noted to ignite. The weight loss
after a 27 hour test was 8%.
EXAMPLE III
A combination of a micropyretic slurry and a non-micropyretic slurry was
applied to a graphite substrate in the following manner. A slurry
corresponding to sample 13 of Table IV was applied by painting and allowed
to build up to 600 microns by applying several coats. Next a micropyretic
slurry was applied. This slurry corresponded to sample 11 of Table V
except that the SiC powder size was chosen to be 1 micron. This slurry was
built up to approximately 500 microns in thickness, thus making the total
thickness of the coating to be 1.1 mm. The micropyretic slurry-deposited
layer was then successfully fired by an oxyacetylene torch.
EXAMPLE IV
A slurry was made containing 40% by volume of sample 1 of the
non-micropyretic slurries in Table IV and 40% by volume of sample 1 of the
micropyretic slurries in Table V. To this was added 5% by volume of cerium
acetate (alkoxide), 5% by volume of polysilazanes, 2% buffer solution of
pH 10, 3% methyl cellulose and 5% by volume colloidal zirconia. This
coating was applied to an anthracite substrate. After drying, the
micropyretic slurry-deposited layer was successfully fired by an
oxyacetylene torch.
EXAMPLE V
A slurry was made containing 25 gm of TiB.sub.2 (particle size 10 microns)
and 15 ml colloidal alumina, 10 g titanium (particle size 44 microns) and
5 g boron (particle size 44 microns). This was then coated on a graphite
block as in Example I and subjected to the same test conditions. After 27
hours the loss in weight was 5%. The coating had changed in color from
black to yellow after 27 hours but remained adherent and protective of the
graphite.
EXAMPLE VI
Example I was repeated but a small crack was noted to be present at the
corner of the rectangular graphite piece. After a 27 hour test it was
noted that a substantial part of the graphite had now burnt by the passage
of air through the crack. Another similar test was repeated with a crack,
but the sample was removed from the furnace after 1 hour and the crack was
healed by the application of the same coating. After 27 hours the sample
was noted again to have been protected, thus showing that cracks may be
healed if discovered early.
EXAMPLE VII
The slurry of Example IV was mixed with carbon black in the ratio 6 ml of
slurry to 1 g of carbon black. This was applied to a graphite block.
Further, after drying, the sample was dipped in a mono-aluminum phosphate
(MAP) solution and allowed to dry again. The entire assembly was then
placed in a furnace at 1000.degree. C. under air. After 27 hours the
coating was noted to remain protective of the substrate.
Top