U.S. patent number 3,878,083 [Application Number 05/361,022] was granted by the patent office on 1975-04-15 for anode for oxygen evolution.
This patent grant is currently assigned to Electronor Corporation. Invention is credited to Giuseppe Bianchi, Oronzio De Nora, Antonio Nidola, Giovanni Trisoglio.
United States Patent |
3,878,083 |
De Nora , et al. |
April 15, 1975 |
Anode for oxygen evolution
Abstract
Novel electrode for oxygen evolution comprising an
electroconductive base provided with an outer coating containing a
mixed material of tantalum oxide and iridium oxide and preparation
and use thereof.
Inventors: |
De Nora; Oronzio (Milan,
IT), Bianchi; Giuseppe (Milan, IT), Nidola;
Antonio (Milan, IT), Trisoglio; Giovanni (Milan,
IT) |
Assignee: |
Electronor Corporation (Panama
City, PA)
|
Family
ID: |
11213858 |
Appl.
No.: |
05/361,022 |
Filed: |
May 17, 1973 |
Foreign Application Priority Data
|
|
|
|
|
May 18, 1972 [IT] |
|
|
24526/72 |
|
Current U.S.
Class: |
204/290.13 |
Current CPC
Class: |
C25C
7/02 (20130101); C25B 11/061 (20210101); C25B
11/093 (20210101) |
Current International
Class: |
C25C
7/00 (20060101); C25B 11/00 (20060101); C25B
11/04 (20060101); C25C 7/02 (20060101); B01k
003/06 (); C22d 001/00 () |
Field of
Search: |
;204/29F,29R
;117/201,230 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Mack; John H.
Assistant Examiner: Weisstuch; Aaron
Attorney, Agent or Firm: Hammond & Littell
Claims
We claim:
1. An electrode comprising an electroconductive base provided with
a coating over at least a portion of its outer surface of a mixed
material of tantalum oxide and iridium oxide, in which the ratio of
tantalum to iridium calculated as metal is 1:1 to 0.34 to 1.
2. The electrode of claim 1 wherein the said base is a valve
metal.
3. The electrode of claim 1 wherein the said base is an alloy of a
valve metal and at least one of the platinum group metals.
4. The electrode of claim 2 wherein the valve metal is
titanium.
5. The electrode of claim 1 wherein the said base is an alloy
containing at least two valve metals.
6. The electrode of claim 1 wherein the said base is an alloy
containing at least two valve metals and at least one of the
platinum group metals.
7. The electrode of claim 1 wherein the base is an alloy of
titanium containing up to 0.2% by weight of palladium.
8. An electrode comprising a base of titanium alloyed with up to
0.2% by weight of palladium provided with a coating over 50 to 100%
of the base of a mixed material of tantalum oxide and iridium
oxide, the ratio of tantalum to iridium calculated as metal being
1:1 to 0.34 to 1.
9. An electrode comprising an electroconductive base provided with
a coating over at least a portion of its outer surface of a mixed
material of tantalum oxide and iridium oxide, in which the ratio of
the tantalum to the iridium calculated as metal is 1:1 to 0.34 to
1, said coating further contains 0.1 to 5.0% by weight of an oxide
of a metal selected from the group consisting of alkaline earth
metals, cobalt, iron, nickel, chromium, molybdenum and
manganese.
10. An electrode of claim 9 wherein the metal is selected from the
group consisting of cobalt and an alkaline earth metal.
Description
STATE OF THE ART
In various electrochemical process such as, for example, in the
production of chlorine and other halogens, the production of
chlorates, the electrolysis of other salts which undergo
decomposition under electrolysis conditions and other electrolysis
processes, it has recently become commercially possible to use
dimensionally stable electrodes in place of graphite. These
dimensionally stable electrodes usually have a film forming valve
metal base such as titanium, tantalum, zirconium, aluminum, niobium
and tungsten, which has the capacity to conduct current in the
cathodic direction and to resist the passage of current in the
anodic direction and are sufficiently resistant to the electrolyte
and conditions used within an electrolytic cell, for example, in
the production of chlorine and caustic soda, to be used as
electrodes in electrolytic processes. In the anodic direction,
however, the resistance of the valve metals to the passage of
current goes up rapidly, due to the formation of an oxide layer
thereon, so that it is no longer possible to conduct current to the
electrolyte in any substantial amount without substantial increase
in voltage which makes continued use of uncoated valve metal
electrodes in an electrolytic process uneconomical.
It is, therefore, customary to apply electrically conductive
electrocatalytic coatings to these dimensionally stable valve metal
electrode bases. The electrode coatings must have the capacity to
continue to conduct current to the electrolyte over long periods of
time without becoming passivated, and in chlorine production, must
have the capacity to catalyze the formation of chlorine molecules
from the chloride ions at an anode. They must be electroconductive
and electrocatalytic and must adhere firmly to the valve metal base
over long periods of time under cell operating conditions.
The commercially available coatings contain a catalytic metal or
oxide from the platinum group metals, i.e., platinum, palladium,
iridium, ruthenium, rhodium, osmium and a binding or protective
agent such as titanium, dioxide, tantalum pentoxide and other valve
metal oxides in sufficient amount to protect the platinum group
metal or oxide from being removed from the electrode in the
electrolysis process and to bind the platinum group metal or oxide
to the electrode base. The binding and protective metal oxide is
usually in excess of the platinum group metal or oxide. Anodes of
this nature have been described in British Pat. No. 1,231,280.
In anodes for the recovering of metals by electrowinning, a
continual source of difficulty has been the selection of a suitable
material for the anode. The requirements are insolubility,
resistance to the mechanical and chemical effects of oxygen
liberated on its surface, low oxygen overvoltage, and resistance to
breakage in handling. Lead anodes containing 6 to 15 percent
antimony have been used in most plants. Such anodes are attacked by
chloride if present in the electrolyte. This is the case at the
huge plant at Chuquicamata, Chile, where it is necessary to remove
cupric chloride dissolved from the ore by passing the solution over
cement copper, reducing the cupric to insoluble cuprous chloride.
At this plant there was also developed an anode of a copper-silicon
alloy, called the Chilex anode, used in a portion of the tank-room.
It has a longer life but raises the power consumption because of
greater resistance and greater oxygen overvoltage.
Attempts to use mixed oxide coatings such as RuO.sub.2 -TiO.sub.2
for oxygen evolution have not been satisfactory in commercial use
because passivation takes place after 200 to 1,000 hours of
operation at a current density of 1.2 KA per m.sup.2. The use of a
Ta.sub.2 O.sub.5 - RuO.sub.2 mixed oxide coating improves the
electrocatalytic activity and the life of the anode somewhat but
not enough for commercial use. The use of a TiO.sub.2 -IrO.sub.2
coating has lower electrocatalytic activity.
OBJECTS OF THE INVENTION
It is an object of the invention to provide a novel anode for
oxygen evolution having an outer coating containing a mixed
material containing tantalum oxide and iridium oxide.
It is an additional object of the invention to provide a novel
electrode with an outer coating of tantalum oxide and iridium oxide
doped to improve the catalytic activity for oxygen evolution.
It is another object of the invention to provide novel electrodes
having a coating of a mixed material of Ta.sub.2 O.sub.5 -IrO.sub.2
on a valve metal alloy base having improved mechanical
stability.
It is a further object of the invention to provide a novel process
for the electrowinning of metals.
These and other objects and advantages of the invention will become
obvious from the following detail description.
THE INVENTION
The novel electrodes of the invention are comprised of an
electroconductive base provided with a coating over at least a
portion of its outer surface of a mixed material of tantalum oxide
and iridium oxide. The coating may be as little as 5% of the outer
surface of the electrode but preferably covers 50 to 100% of the
active face of the electrode. The preferred ratio of tantalum to
iridium calculated in percent of metal is 1:1 to 0.34:1.
The electrode base may be made of any electroconductive material
such as iron, nickel, lead, copper, etc. or alloys thereof but is
preferably a valve metal such as tungsten, titanium, tantalum,
niobium, aluminum or zirconium or alloys of two or more of said
metals. The valve metals' bases may be provided with an
intermediate layer such as an oxide of the valve metal or a coating
of another metal such as platinum group metals. The base may be a
valve metal and at least either one metal having a low hydrogen
overvoltage such as alloy of titanium with iron, cobalt, nickel,
palladium, vandadium or molybdenum, or mixtures of two or more of
said metals; or one metal suitable to form with titanium a
protective oxide film even in acid solution such as an alloy of
titanium with niobium, tantalum, zirconium or mixtures of two or
more of said metals.
In a preferred embodiment of the invention, the electroconductive
base is an alloy of a valve metal with a platinum group metal which
has an improved corrosion resistance to acid electrolytes
encountered in the use of the electrodes such as 5 to 15% sulfuric
acid or 1 to 5% hydrochloric acid. A particularly useful alloy is
titanium containing 0.1 to 0.20% by weight of palladium. This
corrosion resistance of the support of the coating prevents
chipping off of the coating even if the anode is immersed for a few
hours in an acid electrolyte without anodic polarization.
In a modification of the invention, the coating containing tantalum
oxide and iridium oxide can be doped with an oxide of a metal with
a valence of less than +4 to increase the catalytic activity for
oxygen evolution without adversely effecting the mechanical
properties of the coatings.
Without wishing to be limited to the following theoretical
discussion, it is believed that the semi-conductivity of the
Ta.sub.2 O.sub.5 -IrO.sub.2 system is of the n type and that the
addition of the doping metal oxide reverses the type of
conductivity from n-type to p-type which improves the anodic
process by producing electronic holes.
The doping metal oxide may be present in the coating in amounts
ranging from 0.5 to 5.0% preferably 1.5 to 3.0% by weight of the
said system calculated as metal. Examples of suitable doping metal
oxides are alkaline earth metals such as calcium, magnesium, barium
and members of Groups VIII, VI B and VII B of the periodic Table
such as cobalt, iron and nickel, chromium, molybdenum, manganese,
etc.
The increase in the catalytic activity of the doped coatings is
shown by the lower anode potential of doped anodes as compared to
undoped anodes after 8,000 hours of operation of the anodes under
identical working conditions. The doping seems to have no adverse
effect on the mechanical properties of the coatings as there is no
coating loss in either instance even after 8,000 hours
operation.
The electrodes of the invention are particularly useful for
electrolytic processes such as cathodic protection,
electroflotation, organic electrosynthesis such as
hydrodimerization of acrylonitrile and most particularly the
electrowinning of metals. The said electrodes have a high
electrocatalytic activity and a very low passivation rate of a few
millivolts per month at a current density of 1.2 to 2.0 KA per
m.sup.2 and a negligible weight loss if kept under anodic
polarization.
The novel method of the invention for the preparation of the
electrodes of the invention comprises applying to an
electroconductive electrode base a solution of a thermally
decomposible compound of tantalum and a thermally decomposible
compound of iridium, drying the coated electrode base by
evaporation of the solvent and then heating the dried electrode
base in the presence of an oxygen containing gas such as to form
the desired electrode.
The heating step is preferably effected at temperatures of
350.degree. to 600.degree.C, the optinum temperature being
500.degree. to 550.degree.C. At temperatures below 350.degree.C,
the oxidation is not completed or requires too long heating time
and at temperatures above 600.degree.C, the electrode base is
likely to be subjected to distortions and/or destruction by the
high temperatures.
The preliminary drying step is preferably effected by gentle
heating in air to evaporate the solvent and codeposit the metal
compounds. However, any convenient procedure may be used to remove
the solvent such as standing under reduced pressure.
In a preferred embodiment of the process, the coating is applied in
multiple coats with short periods of intermediate heating such as
500.degree. to 550.degree.C for 5 to 15 minutes with a longer final
heating after the last coat such as 500 to 550.degree.C for 45
minutes to 1 1/2 hours. The coating obtained thereby is very
adherent and quite uniform.
The electrodes of this invention are particularly useful for
electrowinning process used in the production of various metals
because they do not add impurities to the bath which deposit on the
cathode, with the metals being won, and thereby contaminate the
refined metal, as do anodes of for example lead containing antimony
and bismuth which give impure cathode refined metals. Moreover,
their resistance to the acid solutions and oxygen evolution and
their excellent anode potential makes them desirable for this
use.
In the following examples there are described several preferred
embodiments to illustrate the invention. However, it should be
understood that the invention is not intended to be limited to the
specific embodiments.
EXAMPLE I
24 Titanium plates 10 mm by 10 mm were etched in boiling 20%
hydrochoric acid for 60 minutes and were then thoroughly washed
with water. The plates were then coated with an aqueous solution of
the compositions of Table I in 12 to 15 coats. After the
application of each coat, the plates were dried and then heated for
10 minutes at 450.degree.C to 600.degree.C in an oven with forced
air circulation and then allowed to air cool. After the last coat,
the plates were heated in the oven at the same temperature for 1
hour and were then air cooled. The values of Table I are calculated
as weight of free metal. The tantalum chloride was used as a
solution in 20% hydrochloric acid.
TABLE I ______________________________________ Coating composition
heating temp Sample No. in mg (metal) in .degree.C
______________________________________ A.sub.1 TaCi.sub.5 - 16 450
A.sub.2 IrCl.sub.3 - 16 500 A.sub.3 550 A.sub.4 600 B.sub.1
TaCl.sub.5 - 13.10 450 B.sub.2 IrCl.sub.3 - 16.0 500 B.sub.3 550
B.sub.4 600 C.sub.1 TaCl.sub.5 - 10.70 450 C.sub.2 IrCl.sub.3 -
16.0 500 C.sub.3 550 C.sub.4 600 D.sub.1 TaCl.sub.5 - 8.6 450
D.sub.2 IrCl.sub.3 - 16.0 500 D.sub.3 550 D.sub.4 600 E.sub.1
TaCl.sub.5 - 6.85 450 E.sub.2 IrCl.sub.3 - 16.0 500 E.sub.3 550
E.sub.4 600 F.sub.1 TaCl.sub.5 - 5.46 450 F.sub.2 IrCl.sub.3 - 16.0
500 F.sub.3 550 F.sub.4 600
______________________________________
The anode potential for each anode was then determined by
electrolysis of 10% by weight sulfuric acid at 60.degree.C and a
current density of 1.2 KA/m.sup.2. The initial anode potential
(against NHE) and the anode potential after 3,000 and 6,000 hours
was determined and the coating loss was then determined. The values
are reported in Table II.
TABLE II ______________________________________ Coating weight
Anode Potential V(NHE) loss Sample initial after after No. value
3000 hs. 6000 hs. mg./cm.sup.2
______________________________________ A.sub.1 1.50 1.68 1.77 0.2
A.sub.2 1.52 1.62 1.70 0.0 A.sub.3 1.52 1.62 1.70 0.0 A.sub.4 1.52
1.62 1.77 0.0 B.sub.1 1.51 1.63 1.73 0.6 B.sub.2 1.51 1.62 1.68 0.0
B.sub.3 1.52 1.62 1.68 0.0 B.sub.4 1.51 1.63 1.79 0.0 C.sub.1 1.50
1.65 1.70 0.3 C.sub.2 1.51 1.63 1.64 0.0 C.sub.3 1.52 1.58 1.63 0.0
C.sub.4 1.52 1.63 1.73 0.0 D.sub.1 1.49 1.61 1.68 0.5 D.sub.2 1.50
1.60 1.62 0.0 D.sub.3 1.52 1.58 1.62 0.0 D.sub.4 1.52 1.60 1.67 0.0
E.sub.1 1.52 1.62 1.66 0.2 E.sub.2 1.50 1.60 1.61 0.0 E.sub.3 1.52
1.56 1.61 0.0 E.sub.4 1.52 1.63 1.67 0.5 F.sub.1 1.48 1.56 1.73 0.9
F.sub.2 1.47 1.53 1.74 0.6 F.sub.3 1.47 1.52 1.77 1.2 F.sub.4 1.49
1.60 1.80 1.3 ______________________________________
The results of Table II show that the electrodes of the invention
have high electrocatalytic activity and a very low passivation rate
and that the weight loss of the coating is negligible when within
the limits of the invention. It should be noted that the ratio of
Ta to Ir for samples F.sub.1 to F.sub.4 is about 0.34. Optimum
values are obtained in the heating range of
500.degree.-550.degree.C.
EXAMPLE II
For comparative purposes, electrodes were prepared as follows.
Titanium plates 10 mm by 10 mm were etched in boiling 20%
hydrochloric acid for 60 minutes and were then thoroughly washed
with water. The plates were then coated with an aqueous solution of
the compositions of Table III in 12 to 15 coats. After the
application of each coat, the plates were dried and then heated for
10 minutes at 450.degree. to 550.degree.C in an oven with forced
air circulation and then allowed to air cool. After the last coat,
the plates were heated in the oven at the same temperature for 1
hour and where then air cooled. The values of Table III are
calculated as weight of free metal.
TABLE III ______________________________________ Coating
composition heating temp Sample No. in mg (metal) in .degree.C
______________________________________ 1 TiCl.sub.3 - 19.5 450
RuCl.sub.3 ' 3H.sub.2 O - 16.0 2 TiCl.sub.3 - 10.7 450 RuCl.sub.3 '
3H.sub.2 O - 16.0 3 TaCl.sub.5 - 19.5 450 RuCl.sub.3 '3H.sub.2 O -
16.0 4 TaCl.sub.5 - 10.7 450 RuCl.sub.3 ' 3H.sub.2 O - 16.0 5
TiCl.sub.3 - 19.5 500 IrCl.sub.3 - 16.0 6 TiCl.sub.3 - 19.5 550
IrCl.sub.3 - 16.0 7 TiCl.sub.3 - 10.7 500 IrCl.sub.3 - 16.0 8
TiCl.sub.3 - 10.7 550 IrCl.sub.3 - 16.0
______________________________________
The anode potential for each anode was then determined by
electrolysis of 10% by weight sulfuric acid at 60.degree.C at a
current density of 1.2 KA/m.sup.2. The initial anode potential and
the anode potential after 600, 1,000 or 1,200 hours are reported in
Table IV. The final loss of the coating was determined at the end
of the test.
TABLE IV ______________________________________ Sample Anode
potential (NHE) in Volts after coating loss No. initial 600 h 1000
h 1200 h in mg/cm.sup.2 ______________________________________ 1
1.48 -- 2.00 > 2.5 0 2 1.47 -- 1.95 > 2.2 0 3 1.46 -- 1.85
2.09 0 4 1.45 -- 1.79 2.00 0 5 1.52 1.82 1.86 -- 0 6 1.52 1.89 1.93
-- 0 7 1.51 1.81 1.85 -- 0 8 1.52 1.85 1.90 -- 0
______________________________________
The results of Table IV show that RuO.sub.2 -TiO.sub.2 coated
electrodes become passivated after only 1,000 hours and the
Ta.sub.2 O.sub.5 -RuO.sub.2 coated electrodes are only slightly
improved and the TiO.sub.2 -IrO.sub.2 coated electrodes are no
better.
EXAMPLE III
10 plates of titanium containing 0.15% of palladium (10 .times. 10
mm) were sandblasted and then etched in refluxing 20% hydrochloric
acid for 60 minutes. The plates were then coated with the
compositions of Table V. The compositions were applied in 15 to 20
coats with intermediate heating at 450.degree.C for 10 minutes in
an oven with forced air circulation and cooling in air. The final
heating was effected at the temperatures in Table V for 1 hour
followed by air cooling.
TABLE V ______________________________________ Coating compositions
in mg of Final heating Sample No. free metal in .degree.C
______________________________________ AA TaCl.sub.5 - 16 + 500 BB
IrCl.sub.3 - 16 550 CC TaCl.sub.5 - 13.10 + 500 DD IrCl.sub.3 -
16.0 550 EE TaCl.sub.5 - 10.70 + 500 FF IrCl.sub.3 - 16.0 550 GG
TaCl.sub.5 - 8.60 + 500 HH IrCl.sub.3 - 16 550 II TaCl.sub.5 - 6.85
+ 500 JJ IrCl.sub.3 - 16 550
______________________________________
The anode potentials and coating weight loss were determined as in
Example I and the results are reported in Table VI.
TABLE VI ______________________________________ Anode Potential in
V(NHE) Coating weight initial after loss in mg/cm.sup.2 Sample No.
value 3000 hs. 6000 hs. after 2000 hs.
______________________________________ AA 1.52 1.62 1.69 0.0 BB
1.51 1.63 1.70 0.0 CC 1.51 1.62 1.68 0.0 DD 1.52 1.62 1.68 0.0 EE
1.51 1.63 1.65 0.0 FF 1.52 1.58 1.64 0.0 GG 1.50 1.59 1.63 0.0 HH
1.52 1.58 1.62 0.0 II 1.50 1.57 1.60 0.0 JJ 1.52 1.58 1.60 0.0
______________________________________
The results of Table VI show that the electrodes of the invention
with a titanium - palladium alloy base have excellent
electrocatalytic activity and low passivation rates.
EXAMPLE IV
To demonstrate the improved corrosion resistance of a titanium -
palladium alloy, 10 plates made of titanium containing 0.15% by
weight of palladium (10 .times. 10 mm) were sand-blasted and then
etched in refluxing 20% hydrochloric acid for 60 minutes. The
plates were then coated with the compositions of Table V using the
procedure of Example III. The anode potential was determined for
each electrode by electrolysis of 10% sulfuric acid at 60.degree.C
and a current density of 1.2 KA/m.sup.2. The initial anode
potential and the value after 1,000 and 2,000 hours and the coating
weight loss after 2,000 hours was determined. Moreover, the current
was halted for 15 minutes in each 24 hour period without removing
the electrode from the acid bath. The results are reported in Table
VII.
TABLE VII ______________________________________ Anode Potential
V(NHE) Coating weight Sample initial loss in No. value 1000 hs.
2000 hs. mg/cm.sup.2 ______________________________________ AA 1.52
1.66 1.68 0.2 BB 1.51 1.66 1.67 0.3 CC 1.51 1.57 1.62 0.3 DD 1.52
1.58 1.60 0.3 EE 1.50 1.56 1.58 0.3 FF 1.52 1.56 1.58 0.3 GG 1.50
1.55 1.56 0.3 HH 1.52 1.54 1.56 0.3 II 1.52 1.54 1.55 0.3 JJ 1.51
1.55 1.55 0.3 ______________________________________
The results of Table VII show that the electrodes of the invention
having a titanium - palladium alloy base have excellent
electrocatalytic activity and low passivation rates and the coating
does not chip off even without anodic polarization.
EXAMPLE V
10 titanium plates (20 .times. 20 mm) were etched in refluxing 20%
hydrochloric acid for 60 minutes and after being thoroughly washed
with water, the plates were coated with an aqueous solution
containing 2.01 mg (as free metal) of TaCl.sub.5, 3.2 mg (as free
metal) of IrCl.sub.3 and 0.0394 ml of hydrochloric acid. The
solution was applied in 12 coats with intermediate heating and
cooling and a final heating as described in Example I.
The coated titanium plates were used as anodes in cells for the
recovery of zinc from an aqueous electrolyte containing 100 g/liter
of Zn SO.sub.4 (as free metal), 10% sulfuric acid and 10 to 50 ppm
of glue. The cathode was a pure aluminum sheet with a smooth
surface and the electrolyte gap was 10 mm. The current density was
500 A/m.sup.2 and the electrolyte temperature was 35.degree.C. The
anode potential, loss of coating, zinc thickness on the cathode and
the morphology of the zinc deposit are reported in Table VIII
TABLE VIII ______________________________________ Test Anode
potential Coating Zn deposit Zn deposit No. V (NHE) weight loss
thickness morphology in mm ______________________________________ 1
1.47 0 3.2 smooth 2 1.48 0 3.5 do. 3 1.49 0 4.1 do. 4 1.47 0 3.5
do. 5 1.48 0 3.1 do. 6 1.50 0 3.0 do. 7 1.49 0 3.0 do. 8 1.50 0 4.1
do. 9 1.48 0 4.1 do. 10 1.47 0 4.0 do.
______________________________________
The cathodic current efficiency was found to be 92-95% in all cases
and the purity of the zinc deposit was 99.9999%.
EXAMPLE VI
Using the procedure of Example V, five titanium plates (20 .times.
20 cm) were coated with the composition of Example V. The coated
plates were used as anodes in a cell for recovery of copper from an
aqueous electrolyte containing 100 g/liter (as free metal) of
CuSO.sub.4 and 10 g/liter of sulfuric acid and the cathode was a
smooth steel plate. The electrolyte gap was 15 mm and the bath
temperature was 60.degree.C. The current density was 500 A/m.sup.2.
The anode potential, loss of coating and copper thickness and
morphology of the copper deposit are reported in Table IX.
TABLE IX ______________________________________ Test Anode
potential Coating Cu deposit Cu deposit No. V (NHE) weight loss
thickness morphology in mm ______________________________________ 1
1.47 0 4.5 smooth 2 1.50 0 4.5 do. 3 1.48 0 5.8 do. 4 1.47 0 4.1
do. 5 1.49 0 5.8 do. ______________________________________
The cathodic current efficiency was found to be 100% in all cases
and the purity of the copper was 99.9999%.
EXAMPLE VII
16 titanium coupons (20 .times. 20 mm) were etched in boilng
azeotropic 20% hydrochloric acid for 40 minutes and were thoroughly
washed. The coupons were then coated with the composition of Table
X in 20 coats. After the first 19 coats, the coupons were heated in
a forced air circulation oven at 500.degree.C and then were air
cooled. The last heating was at 500.degree. or 550.degree.C for 1
hour followed by air cooling.
TABLE X
__________________________________________________________________________
Specimen Liquid Coating per gm of noble metal/m.sup.2 No. each
titanium sheet coupon
__________________________________________________________________________
1, 1A, 1B, 1C TaCl.sub.5 2.077 mg. as Ta IrCl.sub.3 3.2 do. do. Ir
16 CaCl.sub.2.6H.sub.2 O 0.053 do. do. Ca HCl 0.0413 mls.
2,2A,2B,2C TaCl.sub.5 2.000 mg. as Ta IrCl.sub.3 3.2 do. do. Ir 16
CaCl.sub.2.6H.sub.2 O 0.13 do. do. Ca HCl 0.0394 mls. 3,3A,3B,3C
TaCl.sub.5 1.92 mg. as Ta IrCl.sub.3 3.2 do. do. Ir 16
CaCl.sub.2.6H.sub.2 O 0.21 do. do. Ca HCl 0.0374 mls. 4,4A,4B,4C
TaCl.sub.5 1.87 mg. as Ta IrCl.sub.3 3.2 do. do. Ir 16
CaCl.sub.2.6H.sub.2 O 0.26 do. do. Ca HCl 0.0362 mls.
__________________________________________________________________________
The samples were then tested in 10% sulfuric acid at 60.degree.C
with an anodic current density of 1.2 KA/m.sup.2 to determine the
anode potential and coating loss after 2,500 hours. The results are
shown in Table XI.
TABLE XI ______________________________________ Specimen Temp. Ca
Anode Potential Weight No. final heat content initial after loss
treatment % b.w.t. value 2500 hs. mg/cm.sup.2 V(NHE)
______________________________________ 1 500.degree.C 1 1.51 1.55 0
1A do. 1.51 1.56 do. 1B 550.degree.C 1 1.51 1.56 do. 1C do. 1.51
1.56 do. 2 500.degree.C 2.5 1.50 1.51 do. 2A do. 1.51 1.51 do. 2B
550.degree.C 2.5 1.50 1.52 do. 2C do. 1.50 1.52 do. 3 500.degree.C
4.0 1.51 1.58 do. 3A do. 1.52 1.58 do. 3B 550.degree.C 4.0 1.51
1.55 do. 3C do. 1.51 1.58 do. 4 500.degree.C 5.0 1.51 1.60 do. 4A
do. 1.52 1.60 do. 4B 550.degree.C 5.0 1.52 1.65 do. 4C do. 1.52
1.65 do. ______________________________________
EXAMPLE VIII
Using the procedure of Example VII, 20 .times. 20mm titanium
coupons were coated with the composition of Table XII with the same
heatings.
TABLE XII ______________________________________ Specimen Liquid
coating per N.M. No. each titanium sheet coupon amount gr/m.sup.2
______________________________________ 1,1A,1B,1C TaCl.sub.5 2.077
mg. as Ta IrCl.sub.3 3.200 do. do. Ir 16 CoCl.sub.2.6H.sub.2 O
0.053 do. do. Co HCl 0.0412 mls. 2,2A,2B,2C TaCl.sub.5 2.000 mg. as
Ta IrCl.sub.3 3.200 do. do. Ir 16 CoCl.sub.2.6H.sub.2 O 0.13 do.
do. Co HCl 0.0394 mls. 3,3A,3B,3C TaCl.sub.5 1.92 mg. as Ta
IrCl.sub.3 3.200 do. do. Ir 16 CoCl.sub.2.6H.sub.2 O 0.21 do. do.
Co HCl 0.0374 mls. 4,4A,4B,4C TaCl.sub.5 1.87 mg. as Ta IrCl.sub.3
3.200 do. do. Ir 16 CoCl.sub.2.6H.sub.2 0.26 do. do. Co HCl 0.0362
mls. ______________________________________
The anode potentials and the coating losses after 8,000 hours in
10% sulfuric acid at 60.degree. C with an anodic current density of
1.2 KA/m.sup.2 was determined as in Example VII and the results are
reported in Table XIII.
TABLE XIII ______________________________________ Speci- Temp. Co
content Anode Potential Coating men final heat % b.w.t. initial
after weight treatment value 8000 hs. loss V(NHE) mg/cm.sup.2
______________________________________ 1 500.degree.C 1 1.52 1.56 0
1A do. do. 1.52 1.56 do. 1B 550.degree.C do. 1.52 1.57 do. 1C do.
do. 1.52 1.57 do. 2 500.degree.C 2.5 1.52 1.53 do. 2A do. do. 1.52
1.53 do. 2B 550.degree.C do. 1.52 1.54 do. 2C do. do. 1.52 1.54 do.
3 500.degree.C 4 1.52 1.56 do. 3A do. do. 1.52 1.56 do. 3B
550.degree.C do. 1.52 1.56 do. 3C do. do. 1.52 1.56 do. 4
500.degree.C 5 1.52 1.56 do. 4A do. do. 1.52 1.57 do. 4B
550.degree.C do. 1.52 1.56 do. 4C do. do. 1.52 1.57 do.
______________________________________
Various modifications of the electrodes are processes of the
invention may be made without departing from the spirit or scope
thereof and it should be understood that the invention is to be
limited only as defined in the appended claims.
* * * * *