Note: Descriptions are shown in the official language in which they were submitted.
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METHOD QF APPLYING
COi\lDUCTlVE COATING
.~
The invention relates to a method for the formation of a corrosion resistant electrically
conductive coating on a substrate. The substrate may be an electrode, for say an
electrochemical cell.
It has been realised that titanium suboxides are useful as coatings. It is also known that
a layer of such a ,I,aLe,ial may be formed on a substrate by flame spraying or plasma
spraying, see e.g. GB-A-1438462; US-A-5225382; and G8-A-159506~. In these
disclosures mixtures which are precursors for titanium suboxide are plasma sprayed on to
a substrate under conditions selected to form the suboxide. Typically the suboxides are
of the formula TiOx, where x is between 1.95 and 1.99, see e.g. GB-A-1438462 and lJS
5225382. While such suboxides are corrosion resistant, their electrical conductivity is not
acceptable.
US-A-4252629 discloses a corrosion resistant carrier consisting of sintered TiOx where x
is 0.25 to 1.5 having a coating of manganese dioxide. The carrier can be made by
plasma spraying the titanium oxide under argon.
EP-A-00475g5 discloses that bulk titanium suboxide of the formula TiOx where x is 1.55
to 1.95 has corrosion resistance and electrical conductivity. The disclosure teaches that
the material may be formed into shapes by pressing or extrusion.
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It has now been discovered that if a m~Lerial of known stoichiometry is appiied to a
substrate one can obtain a coating of predetermined composition and which has a
particularly enhanced corrosion resistance and electrical conductivity.
According to the invention in one aspect there is provided a method of forming acorrosion resistant electrically conductive coating on a substrate, com~ ,ing forming a
predetermined titanium suboxide of the forrrlula TiOx where x is 1.5~ to 1.95 and then
applying that ~I,aLenal on to the substrate to form a coating of the same substantially
stoichiometry on the substrate.
The ~l~ lial may be applied by any suitable sFF'iG; ~ion technique which will not affect
the sloichio.,lel~y of the ,-,dlerial. Such techniques include plasma spraying and laser
coating. De-gun coating in which the material is melted and then projected by delondliu,~
and other thermal coating techniques may also be considered.
Because the starting Illd~ lial is of a known :,: hio~ L,y having desired properties one
can obtain directly a coating of the desired stoichiometry and hence the desiredcombination of properties, e.g. electrical conductivity, co., osion resistance etc. It is
possible to apply a mixture of the suboxides or just a single one. Most preferably the
plasma spraying of the defined titanium suboxide is carried out under conditions which
preclude the risk that the stoicl,;o~,lelly will be affected. To this end the applicalion for
example may be carried out under vacuum or under a shield of a suitable inert gas, e.g. v
argon.
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It is advantageous to pretreat the surface of the substrate to enhance the adhesion of the
applied coating. The pretreatment may be mec;hani~al, chemical or ele.,l,uchemical, e.g.
sand/grit blasting, deposition, laser ablation, etching, stove codlil 19, or the iike.
~)
The method may include the variation of adding minor properties of a dopant to the
,I,dLeridl to be applied. Examples include electrocatalysts and other stabilisers, e.g. Pt, Ir,
Ta, Nb, Ru, V; and the like, and oxides, e.g. tin oxide and mixtures thereof. These may
influence conductivity, adhesion, CGr. osion resisLa"ce properties. Typically the
composition will be in particulate form, e.g. a powder. Such additions may be made as a
subsequent treatment, e.g. a further coating of an electrocatalyst may be applied by
electroplating, thennal stove-coating, or the like to influence the electrochemical
properties of the end product.
The application equipment, e.g. plasma gun, may be of known type with appropriate
additional ~LIacllments to ensure an inert gas blanket, e.g. as described in GB-A-
2281488.
The laser will be adapted to melt the powder en route to the substrate, but is particularly
efficacious since it can be operated in a more co,..~ Iete vacuum or purer inert gas
atmosphere. In addition it is possible to clean the surface of the substrate to be coated
by a laser surface ablation technique wherein a second laser process can deliver a high
energy pulse (typically 35MW over 20 nS) to blast away surface oxides and other
conldllli, la-.L:, from the substrate immediately prior to the arrival of the molten powder.
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The coated item can be used as an eiectrode (either anode or cathode) in low current
density applications directly, or where necessary for high current density with the
~FP'i- on of a suitable electrocatalyst as noted above. The surface may also be
electroplated with, for instance, lead or zinc metals to assist electrical connectivity to or
adhesion of active materials in a battery. The electrode may be used in electrochemical
cells, metal winning, chlorine cells, effluent treatment; and the like.
The method allows coatings to be applied up to thicknesses well in excess of those
required for corrosion resistance alone, whilst ~"ai"Ldi~-ing good conductivities. The
coating thickness may range from about 50 to about 1000xS0~3mm. For very high
corrosion resistances a typical coating would be either of the order of 200-500x103mm
where x is about 1.75 to 1.8, or a thinner coating of about 100-200 x10 3 of a
stoichiometry where x is about 1.85 to 1.9. For less arduous duty, high conductivity
coatings of about 100-200x10~3mm and a stoichiometry of x = 1.75-1.8 are preferred. A
coating of the invention will have conductivity of the order of 10 to 100 S/cm. In even less
arduous corrosion duty, dopants can be added which further increase the conductivity.
In order that the invention may be well understood it will now be described with reference
to the following examples which are given by way of illu:,L,dlion only.
EXAMPLE I
Test electrodes for electrowinning of zinc were prepared in the following manner Two
sheets of commercial grade aluminium 2 mm thick and 230 x 80 mm width and breadth,
were lightly filed around all edges to remove sharp edges and burrs, and then grit
blasted. The items were then plasma sprayed using a Metco gun equipped with a
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Shroud as described in GB-A-2281488 which was controlled by a robot which ensured an
even coating over the entire surface and edges. The gun was fed with a pre-prepared
powder of whose ~i- h;a.~etry had been measured by X-ray diffraction as being as of
composition TiOx where x = 1.72. The conductivity was found to be ~0S/cm. The coating
was found to be 120 microns thick with a bulk density of 3.84 gm/cc. Subsequently
adiusl"~enb were made to the argon and hydrogen flowrates and pressures and a
stoichiometry of TiOx where x = 1.76 was obtained and a bulk density of 4.03 gm/cc
obtained. (It is generally accepted that if the bulk density is within 5% of the theoretical
full density then no interconnected porosity is present).
The coated anode was further treated with an electrocatalytic coating based on mixed
oxides of tantalum and iridium. During bhe stove coating one of the test electrodes lost a
high proportion of its coating due to the thermal expansion difference between the
ceramic and metallic components and was discarded. However the other survived
temperature cycling to about 400~C through 30 coats. This anode was used in a test cell
as an anode in the electrodeposition of zinc from a solution of 50 gmAitres Zn2+ and 180--
200 gm/litre H2SO4 with traces of fluoride (- 10 mgAitre) Mn2+ (5-8 mg/litre) and other
conld",il,ant ",ale,ia;s found in a commercial zinc electrowinning stream performing
sucGessfully at a current density of 450A/m2 for over 1000 hours of operation. It was
notable in this test that although the electrode potential did rise from an initially low (and
preferred) level in the long term the anode had a potenbal of about 250mV lower than
the Pb/Ag anode ,-,aterial more conventionally used and that any MnO2 which formed on
the anode fell off of its own accord from time to time or could be assisted by occasional
ullldsol1ic treatment.
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EXAMPLE 11
Token pieces of titanium metal were coated by the process of Example 1. Because of the
smaller difference between the expansion coefficients of the metai and the coating, they
were stove coated with electrocatalysts without any spalling of the coating layer. ",~
EXAMPLE 111
Solid pieces and expanded metai meshes of aluminium and of titanium were coated with
conductive ceramic by the method of Example I and then electrocoated at modest
temperatures with lead dioxide7 platinum metal, and antimony/tin oxide coatings.
EXAMPLE IV
Conductive ceramic coatings were applied using the method of Example I to test pieces
made of steel, brass, copper, lead and KEVL~R (which is not metallic).
