CA1147699A - Electrodes and their preparation - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Novel dimensionally stable electrodes constituted by a film forming metallic material alloyed with at least one other metal member of the group consisting of metals belonging to Groups VIB, VIIB, VIII, IIB, IB, IVA, lanthanum and lanthanide series of the Periodic Table, such as chromium, molybdenum, manganese, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, germanium, tin, lead and lanthanum having an electroconductive and corrosion resistant surface pre-activated on the surface thereof, preparation of said electrodes, use of said electrodes as anodes for electrolysis in aqueous and organic solutions or in fused salts as well as for cathodic protection and electrolysis methods using said electrodes.

Description

7~
Recently dimensionally stable electrocles for anodic and cathodic reactions in electrolysis cells have been used, for example in the manufacture of chlorine and caustic by electrolysis of aqueous solutions of alkali metal chloride, for metal electrowinning in hydrochloric acid and sulfuric acid solutions~ and for other processes in which an electric current is passed through an electro-lyte for the purpose of decomposing the elec-trolyte, for carryiny out organic oxidations and reductions, or to impress a cathodic potential to a metallic structure which has to be protected Erom corrosion.
They have been particularly valuable in flowing mercury cathode cells and in : diaphragm cells for the produc-tion of chlorine and caustic~ in metal electrowinning cells in which pure metal is recovered from a chloride or sulfate solution as well as in the cathodic protection of ship hulls and structures.
Dimensionally stable electrodes have been prepared with valve metal bases, such as titanium, ~antalum, zirconLum, hafnium, vanadium, niobium and tungsten, or "film forming" alloys, which in service develop a corrosion resistant but non-electrically con-ductive oxide or barrier layer which prevents the further flow ofanodic current through the anode except at substantially higher s ~c J~ e/e~ frodes voltage and, therefore//cannot be used successfully as anodes. It .~
has, therefore~ been considered necessary to cover at least a por-tion o~ the valve metal such as a titanium or tantalum anode with a conductive layer of noble metal from ~he platinum group (i.e.j platinum, palladium~ iridium, osmium~ rhodium, ruthenium) or con ductive and catalytic noble metal oxides as such or mixed with valve metal oxides and other metal oxidesO
These conductive layers usually completely cover the active surface of the electrically conductive base except for in-evitable pores through the coating, which pores were, however, sealed by the development of the barrier layer above refexred to on the "film forming'; base. In ~he present context, we identify ~b/~'~

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` with the words "film forllling metal"~ '~valve metal" and "film formin~
i~\\ '''`1-;
m~" a conductive metallic material which has the capacity of passivating itself under anodic polarisation by forminy a corrosion-rasistant and electrically-insulating barrier layer of oxides over the portion of its surface which is exposed ~o the electrolyte.
Coating made of, or containing, a platinum group metal or of platinum group metal oxides are, however, expensive and are consumed or deactivated in the electrolysis process and, therefore, reactivation processes or recoating are necessary to replace de-activated anodes.
Up to now~ the commercial electrodes for chlorine and oxygen evolution have been prepared by coating a valve metal base with a noble metal from the platinum group or with either a sepa--rately applied coating containiny oxides or with separate:Ly applied coating compositions which under thermal treatment generate a layer containing oxides.

~ . ~ , , It is an object of the invention to provide novel long lasting electrodes which are mechanically and chemically resistant to the conditions found in electrolytic cells as well as in cat hodic protection, and which do not require separately applied conductive coatings.
It is another object o~ the invention to provide novel processes for the preparation of electrodes for electrolysis cells.
It is another object to provide methods for pre-activat-ing, whenever necessary, electrodes made with the metal of the electrode for use in electrolysis cells.
It is a further object of the invention to provide novel electrolysis methods using the electrodes of the invention.
It is another object of the invention to provide novel dimensionally stable electrodes which form their own active coat-ing when used as anodes and are not passivated by prolonged -operationO

- ~b/S~

Another objcct of tile invention is to provide electrodes to be used as anodes which are able to generate a layer of oxidcs on their surface from the alloy form:ing the electrode or by automatic self-regeneration in an electrolysis cell under anodic oxygen evolution.
It is a further object of the invention to provicle a novel method of producing electrodes by alloying a valve metal with at least one metal belonging to Groups VIB, VIIB, VIII, I~B, IB, IVA, lanthanum and lanthanide series of the Periodic Table, such as chromium, man~anese, molybden~m, rhenium, iron, ruthenium, osmium, cobal-t, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, tin, lead, germanium and lanthanum and pre-activating, whenever necessary, said electrodes.
It is another object of the invention to provide a novel method of producing corrosion resistant electrodes by sin-tering a mixture of metal powders comprising at least a valve metal powder and a metal powder of at least one metal belonging to Groups VIB, VIIB, VIII, IIB, IB, IVA of the Periodic Table, such as chromium, manganese, molybdenum, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, tin, lead, germanium and lanthanum and pre-activating, whenever necessary, said electrodes.
It is another object of the invention to provide a novel method of producing corrosion resistant electrodes by sintering a mixture of metal powder and metal oxides or intermetallic compounds, the latter prcviding conductive nuclei on the surface of the electrode which remains permanently active.
It is an additional object of the invention to provide methods to pre-activate the surfaces of the novel electrodes of the invention.
These and other objects and advantages of the invention will become obvious from the foll~wing detailed description.

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T.~IE INVENTION
It has now surprisingly been found that by alloying the film forming metals such as titani~, tantalum, niobium~ tungsten, zirconium, haf:hium, vanadium, molybdenum, or silicon~iron alloys or other corrosion resistant iron alloys with appropriate quantities of certain other metals, the alloys obtained develop, under anodic polarization, an electrlcally conductive film and we have been able to obtain alloys whose developed surface films, besides being electrically conductive/ show also high catalytic properties.
1~ Alloys prepared according to the invention when connected into an electrolysis circuit have been used as electrodes working at low and economically acceptable over-voltages with extremely high mechanical and chemical resistance~
The novel electrodes of the invention are constituted by a film forming and corrosion resistant metallic material alloyed with at least one member of the group consisting of metals belong-in~ to Groups VIB, VII~, VIII, IIB, IB, IVA, lanthanum and lanth-~nide series of the Periodic Table. ~ layer of oxide is generated d U Y ~ pr~ ~l L~ d ~ r operation or ~4~6#~ed on the alloy by methods which are hereunder describedO
In another embodiment of the invention powder of a valve metal or of film forming alloys such as high silicon content Si-Fe alloys is sintered with powder of either at least a metal be-longing to Groups VIB, VIIB~ VIII~ IIB,IB,IVA,lanthanum and lant~-anide series of the Periodic Table or oxides _ or inter-metallic compounds o the same metalsO
In-this case the additive elements or compounds constitute the electrocatalytically active and electroconductive nuclei on the surface of the sintered electrodesO
In the latter embodiment it is not necessary that the c~ncentration of the additive element or compound be uniform through the entire section of the sintered. electrode but, by appropriate - 4 - .

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pOW(le:r ITliX:i.ll~J LeChlli.(:~lle or cl.he]^ me;.lns, Ll~e (le~ired collcenl:l-at:i.on of the adc~it:k~n;l:l. meLcll or metal compound in i:he su~face onJy ean he aclli.e~ed lea~ing i.n the surface .layers the bul]i o.~ the sintere~d el.ectr.ode composed only of the~ mat:ci~ ma~eri.al.
I-t has been Eound tllcll: in most eases the amount o the metal or metal eompound added i.s suEfleiellt when as low as 0.1~ by wei(~ht ~nd carl bc as hic~ll as 50~ by WeiCJh~..
E~amples of film-:For~ lc) me-l:als are titanium, tarltalum. :.
~ireonium, hafnium, vanadium, niobium and tungsten.
~n example o a film forming metal alloy is a silicon-iron alloy, wherein the silicon eontent is 14.5% by wt. as metallic si.licon.
E~amples of metals belongin~ to ~roups VI}3, V:[:LI~"
VIII, IIB, IB AND IVA lanthanum and lan-thanicle series of the Periodie Table are ehromi.um, molybdenum, manganese, rhenium, iron, ruthen.ium, osmium, eobalt, rhoclium, iridium, niekel, palladium, platinum, eopper, silver, gold, zinc, eadmium, tin, lead, ger-manium and lanthanum. The amount of said metals in the alloys ean be as low as 0.1 and as high as 50%, ?referably 10 to 306 by weight of the alloy.
Amony preferred eleetrode embodiments of the invention are eleetrodes made of titanium or any of other film-forming metals with l to 50% by weight of niekel or eobalt or an alloy of iron-silieon eontaining up to ~0% of silieon, preferably 14.5%, and 0.5 to 10% by weight of molybdenum or chromium. By increasing the amount of molybdenum or chromium or by adding nickel or cobalt, the amount of silieon in the alloy ean be mueh lower.
The said eleetrodes may then be subjeeted to one of the following aetivation proeesses whieh forms a layer of oxides of the metals eonstituting the alloy on the outer surfaee of the eleetrode or mixed erystals of oxides of said metals. Other aeti~ation proeesses than those specifically described may be used. The anodes of the invention are able to withstand operating conditlons in con~erc.ial. electrolysis cells for chlorine production equally as well as valve metal anodes coated with an ac~ive la~er of a platinum yroup metal or an.oxide of platinum group metal of the prior art, and they operate for cathodic protectio~ as welL as titanium anodes coated with an active layer as described in the pri or artO
The anodes are preferably cleaned before being subjectecl to the activation processes described herein. This may be ef~ected by sandblasting~ by li.ght etching in hydrochloric acid for 5 to 45 minutes followed by washing with distilled water~or by othe~
cleaning processes.
The electrodes are also provided, before or after ac_i-vation, with means to connect the electrodes to a source of el~ctric current.
One means of activating ~he electrode comprises dipping thP electrode in a molten salt for Up tQ 10 hours at a tempera_ure slightly higher than the melting point of the specific molten salt.
~aid salts are preferably inorganic alkali metal oxidizing sal_s ~r mlxtures thereof such as sodium nitrate, potassium persul ate, potassium pyrophosphate, sodium perborate and the like.
Another method of activating the electrodes comprise, heating the electrodes in an oxidizing atmosphereto a tempPrat-_re of from 500 to 1200C for up IO 10 hours and optionally maint_in-ing the electrodes at such temperature in an inert atm~Sphere auch as nitrogen or argon for up to lO hours. Preferably, the elec_rodes are slowly cooled at a rate of 10 to 80C per hour, usually in an inert atmosphereO

~b/S'~

~ third mothod oE activat~ng the electrodes comprises anodic polarization of the electrode in an aqueous sulfuric acid solution or an aqueous alkaline solution with a current density preferably of 600 to 3000 A/m2 at 30 to 50C for up -to 1~ hours.
Other activation methods which will oxidize the alloy may.be used to form active coatings on the surface of the alloy ~etal of the electrode.
Stated limits for temperature, time of oxidizing treatment~ current density are only indicative in so far as, during experiments, it has been found that comparable performance results were obtained from tes-t coupons after a definitive pre-activation treatment while for another set of different test coupons such a limit would be somewhat different.
Therefore it is assumed that the optimum conditions for pre-treatment will be easily recogni~ed by one skilled in the ar-t when practicing the present invention.
The activation methods of the invention appear to promote the formation of a mixed crystal or a composite crystal layer of oxides of the metals forming the outer surface of the alloy electrode based, which layer covers the entire surface of the electrode base and, in the instances where measurements have been made, is approximately 1 to 30 microns thick.
The oxide layer may, however, cover only a portion of the electrode metal.
In a modification of the invention, the cleaned electrode base without any pre-activation treatment may be used as an anode for oxygen evolution by electrolysis of a suitable aqueous electrolyte as, for instance, an electrolyte as used in the electrowinning of metals.
A thin layer of peroxide type compounds appears to be formed as soon as the electrodes are operated as anodes in such an oxygen evolution electrolysis, either in sulfuric or in phosphoric acid solutions. These anodes are exceptionally valuable for use in electrowinning of metals where sulfuric acid solutions of the metal are electrolyzed with oxygen formed at the anode and the metal to be won, such as copper, being deposited on the cathode, bm:~

and have the advalltages of beincJ economically produccd and o~
the activation bcing self-regenerating during the electrolysis process.
The electrodes of this invention are particularly useful for electrowinning processes used in the production of various metals because they do not add impurities to the elec-trolytic bath which would deposit onto the cathode, together with the metals being won, as do anodes of, for example, lead con-taining antimony and bismuth, which give impure cathode refined metals.
Moreover, their resistance to acid solu-tions and to oxygen evolution and their low anode potential make them deslrable for this use.
By the words "alloy" or "alloyed" used freely throughout the presen-t disclosure, for sake of simplicity, we intend to identify, where relevant, the true solid solutions of one or more metals into the crys-tal lat-tice of another metal, or intermetallic compounds and oxides as well as "mixtures" of said metals, oxides and intermetallic compounds wherein the degree of solution is incomplete or even quite small, like in the case when the "alloy" is obtained by sinterization of a mixture of metals, metal oxides or intermetallic compounds containing the appropriate metals or compounds in the correct proportions.
However, it should be understood that the invention is not intended to be limited to the specific embodiments.

Six coupons of a titanium-nickel t98.5~ - 1.5%) alloy . having a projected area of 4cm2 were sandblasted and were then activated by anodic polari~ation in sodium hydroxide for 10 hours at the concentration and current densities repor'.?d in Table 1.

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T~B I.E: I

.

Sample NaOH Solution Current Density No. % ~y wt~ kA~m2

2 10 3

3 20 8a cc/

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The ssmple coupons were used successfully as dimen_ . . . slonally stable anodes for cathodic protection. They were .: . . also tested ~s anodes f'or the electrolysis of a saturated " sodi~n chlorlde aqueous solution ~t 60C ~.ith a current den-.: 5 slty of ~ m2 for two days. The initial and final an-- : : ode potentials and the ~mount o~ weight loss .~rom the anode .. . . wcre determined. The results are reported in Table II.
: . TABLE II
. ~ _ _ - . . . Anode Potential V tMHE) Sample ~ After 2 days Weight Lo~ss . . . No~ Initial Value of O~er~tion in m~/cm-., . , ~ _ _ . ... . _ . . ,, ~
- . ~ .1 2~10 lhigh :> 0.
.~ 2.06 high ~ 0.5 . 3 ~2 ~.20 l.S , ~ .6 j ~49 ~70 102 .- 6 - 1050 -~7 .... .
~, , _ .. ._ . _ _ . . . ~ _ . . . _ .
The results of Table II show that the anocLe sample No. 4 has a particularly low anode potential which remained ~0 unchanged after 2 days of operation~ Moreover, the ;netal ~eight loss at the same time was only 0.6 mg/cm2.
'.' .
, ., ~:xA~lIpLE ? ' .
Six titanium-nickel alloy coupons having a projec-. ted surface areaof 4 cm~ of the composition in Table III were ~5 sandblasted and then activated by anodic polarization in a . 1 ~ b~ weight sodium hydroxide solutîon at a current density o~ 3 kA/m2 for 10 hours. The said coupons were then used .: . - .,' ''"
... . .. . , .: ~" . . . . ' ~ .~
., ' .
~. . 173.,042 1~ - L1~7~9 ~s anodes to generate chlorlne as in Example 1 and the ini-. tial and final anode po~entials and final weight loss are reported in Table III.
TABIE III

Alloy Com- Anode Potential . position . ~nitial A~-~r Weight Sample ~ as Metal ~alue ~ Days Loss V (NHE) mg/cm2 1 9500~.0 1,~9 1.50 0.5 2 ~0,010.~ 0 ~.45 0 3 ~00020.0 1.39 ~.42 . 4 70,030~0 ~3~ 3 ~O7 60.040~0 ~,3~, 1036 1.9 6 50 D 050.0 1.~40 1.69 2.2 .... ~ _ . _ , . ........ . ~V~_r .

'Test coupons ;Jere also used satis~ac~orily as ~nodes ~or cathodic protection.

Four coupons having a projected surface ~rea o

4 ~m2 and consisting of 9~ r 5~0 ~itanium and 1. 5~ cobalt were ~0 sandblasted and then were activated by dipping into a molten salt ba~,h as described in Table IV for 5 hours. The result-~ng samples were then used as anodes in chlorine e~oiution as in Table II o~ Example 1 and t~e anode potent1als and I weight lo were deter~ineF~ ¦
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TABLE ~V
. Anode Potential V(NHE~ Weight Sample Initial After 2 D~LY~ ~et2 Nci~, Molterl Salt Value of' Operation mg/cm , _ ~
. 5 3 ~ ~(N~332 2~,0 high ~.5 2 NaN03 2 ,0 high ,~ O. 5 3 ~S208 ~!~0 20~0 ~0 K4P27 ~0O high 2 0~5 . ~

E~AMPLE 4 r -10 Four titanium-cobalt coupor~ o~ the composi~iQn o:f ~able V with a pro,~ected sur~ace area o~ 4 cm2 were ~an~bla~ted . and then were activated by dlpping in molten pota~ium per-~ulfate ~or 5 hoursO The res~lting samples were then used . ~or chlorine e~olution as in 'rable II o~ Example ~ and the anode potential and weight loss were determined.
I . ' - '' ~AB~R ~ !
i Alloy Com-l po8ition Anode Potential V(NHE) ~Teight ¦Sample ~ by ~eight Initial A~ter Ios~ 2 20 ~ No. T1 Co ~alue 2 day~ mg/cm . I -1 9~ 05 2,01 ~lg 3.6 2 9000 ~0.0 ~D 86 ~.93 0.8 - 3 70.0 3-~ 1~50 1.50 0~2 4 50.0 50~0 1,60 1,89 0.2 . , qhe re~ults Or Table V ~hows that the anode o~ sample No~ 3 has a particularly low anode potential which remained ~nchar~ed after 2 days operation~ Moreo~er, the me~al weight ;loss at the same time wa~ only 002 mg/cm2.

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._ . Il ... ~ . ........ . !

'7t;'~3 . ~MP~

~Four coupon~ con~isting Or 98.5,5~ tltaniwn an~ 1.5,~ !
lron and having a pro~ected sur~ace area o~ 4 cm2 were and~
bïa~ed and then were heated in an oxygen atmo~phere ~or ~our hours at khe temperature~ in Table VI and then ~or ~hree hour~ in a nitrogen atT~phere~ ~e coupon~ were cooled in ' a nitrogen atmGsphere at a rate o~ 50C per hour and were then used as anodes ~or chlorlne evolution a~ in Example 1, . ~he anode potentials and weight losses were then determined to be a~ follow~:
ABI~E 'iJI

. Ac~iva- Anode Potential .
~hermal t~on in ~ ~ (MHE) , .
. ~ygen~ Nitrogen .~ 4 ~
Sample Atmos- Atmos- Inltial A~ter Weight ~OS9 No. phere phere Value2 Day~ mg/cm' , . . _ _ i 500 500 2 ~20 hlgh 2 . 5 ;00 ~;00 ~ ~ '352 03a 0, 9 3 650 ~00 2.36 2.90 ~5 4 700 5~0 7 3.0 ~gh 20.5 . , ~ . ~
E~tAMPLE 6 ~our tltanium-iron coupons having a proJected area Or 4 cm2 and the composition of Table VII were sandblasted and then heated at 600C for ~our hours in an oxygen at~os-p~ere followed by heating for three hours a~ 500C ln a -nitrogen atmosphereO The samples were cooled in the nitro-, gen atmosphere at a rate o~ 50C per hour a~d were then u~ed ~or chlorine evolution as in Table II of Ecample 1. The . ~e~ults are reported in Table ~TIIo li '7~!~g ~ABL~
. --Alloy Com-po~ltion Anode Potential V(N~ elght . . Sample ,~ b~r Weight Inltiall~ter Lo~
No<. Ti Fe Value2 Day~ mg/cm~
. . _.. . ~ _ . . . _ . .
1. ~8.5 1~5 Og6 2039 1,1 2 - 9~,0 ~0~0 l~ ~;,g9 1.5 3 70~ 30~0 1~ 47 ~o6 ~4 5~0 50~0 1~5~ 9 . _. ,. _ _.
lC~ . 51~he re~ults of Table VII show that the anode o~ sa~ple . No~ 3 has a particularly low- anode potential whlch remain~d unchary~e~ a~er 2 day~ o~ opera~ionD

. EXA~PLE 7 Se~en coupons o~ d~r~erent ti~aniusll alloys havlng a pro~ected surface area o:f 4 cm were sandbla~t~d and then were activated by dipping lnto molten potassium persul ate ~or ~i~e hours. The resuiting coupons were then used ~or chlor~ne eYolu~ion as in Table II o~ ~:xample T and ~he results are repor~ed in ~able VIII~ . ¦

~0 ~lloy Composition Anode Pot ential ' % V ~
Sample ~I:nit~al After We~:ht ~s No. Ti Co Ni Pb Mn Sn Value 2 I~a~s~ngfcm.
. , _ .
1 1 5025 25 . - ~ 2 O 0~; high2 0.
¦ 2 70 - 30 ~ 2002 2~,06Negligi~le 3 50 ~ ~;0 ~ ~1. 81 1~. 81 16.3 4 7 ~ ~ 30 ~ 3006 high ~.5 50 ~ 50 ~ lo90 1~2 25~
6 50 ~ ~5 ~5 L~ 60 1~ o5 ! 7 ~025 ~ ~5 ~ l.36 1.37 0"4 . ~ . ~
. ~he results of Table V~ how that i~ ls pos~ible, b~J
-~arylng the compo~ition Or ~he alloys ~o obtairl alloy~ w~th ti ~l-ow an~de potentials and low weight los~e~0 ,. ~
13.

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El~AMPLE 8 . Six ~itaniwn nlckel cou~ons ha~lng a proJected sur~-face area of 4 cm were sandb:Lasted and then were u~ed with i out ~urthe~ treatrnent a~ anodes ~or oxygen evolutlon ln the electrolysis o~ an agueolLs 10,~ sul~ c acid solu~lon at ~C at curre~ densities o~ 1,,2 and 6 kA/m20 ~e anode potentials and the welght loss were dete~rninedO ~rhe res~lts as~e in Table IX" . .
~ABIE IX
.
~ r , 4~lloy Anode Potential V ~N~) -2 ~Ie~al ~omposition At 1.2 kA/rn~ At 6.0 kA/m Wei~ht Sample ~ a~ Me~al Initial 40 Initial 40 Loss2 ~o. ~ N1 Value l~ays Day3 mg/cm 1 70 30 .2 .12 2 ~, 8 neglig -~

1~ ~ a 70 30 2 . 50 ~gh ~ 0 ~ 5 2 60 4~ lo 95 ~ . 98 negll~-. ible 2a 60 40 2.07 2.30 -7 ~
3 50 50 1~;0 lo 86 negllg-3a 50 ~0 ~ . 88 2 .12 1 ~ 6 These anodes may ~,e used in metal electrowinning pro-ce~s es .
. __ ~ourteen titanium alloy coupons o~ ~rarlous composi tlo~ as giYen in Table X9 having a pro~ected sur~ace area ~!5 ~ 4 cm, were sandblasted and were therl used without fu~ther treatment as anodes for the evolution of oxygen by electroly-. ~is of an aqueous 10% sulflLric acid solution at 70C and cur-.ren:~ denslties o~ l.2 and 6~cA/m20 The anode po~en~izl and ~elE~ht lo~es are reported in Table X0 .' ., '`.
1! ~ `14 ~
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. . ~ S~ ~ ~0 C` ~1 ~ O , ~
. - . . ~.a 4-~ 0 ~1 3 C'~

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. ~ 3 D ~ ~ O O O ~ ~ U~ ~
. o~ ~. ~ -- .. _ ~sS , ~ 1 1 1 1 9 8' .~ ' o U~ U~L I ~ ~ ~ I I 0~ 1 ' 8 3 0 t~

. ~ O G O O O O O O O O O O O O
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. ~ ~: ~ ~ ~, . ~ ,.sC U~, '¢ `.0 '¢ ~ C-.,. ,.- ,' . ~. ' '-'''' """''''""' '''" ",' - ~.

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,,, . , , , ,, ,,, , , . . ~., . ,.., . ,_ Il 173~04~' - 15 ~ . I

In thi~ te~t, Sample3 No. 1 (and lA) appear to be the best for u~e in electrolysi3 processes in tYhich oxygen :~ evolved at the anodeJ such a~ ln met~l electro~inning proce~ses.

5 . E~AMPIE1o ~Four coupons o:E a silicon-iron ;alloy con~isting o~
¦ 81~% lron, 15.1% 3ilicon~ 0.9,~ mo~ybdenwn and traces o~ czr-bon ~Lnd nltrogen wi~h a ~ ace OI 1~ c~m2 pro~ected area were ¦ cleaned by sandblasting and were then heated in a furnace ln an oxygen atmosp~ere ror ~i~e hours at temperatu~e~ Qf' 6Go to 900C. ~Che samples were then slowly cooled in a.n oxy~,en !
. a~mo~ihere a~ a cooling ra~e o~ ~0C per hour. qhe resul~-amples ~lere then used as anodes ~or chlorille evolut~n . in a sa~urated sodium chloride aqueou~ solution a~ 60C ~ h a curr~nt densl~y o~ 2.5 kA/m ~or ~ve days. l'he initi21 ~nd ~lnal anode potential and ~he amount o~ weight loss 2re reported ln ~ab~e XI.
~ABL~ XI
__ .
Anode Potential V (NXE) 20 Sample ~ea~ing Inltial AfterWei.ght ~o~s . ¦ NoO Temp. C Value 5 DaysIn mg/c~f 600 1.~ 2~8 ~ 2 700 1~9 h~gh _2~.5 3 800 1.80 ~o5 2 3 4 9~)0 2~10 hi~ 5 .

IPL~

. ¦ ~o~Lr soupor~ o~ the sllicon-iron alloy a used in ~ample 10 were sandblasted and then were ~lrst hea.ted at the j ~ 16 -.

'7~
tempcratures given in Table XII, in a furnace with an oxygen atmosphere for five hours and secondly heated in a nitrogen atmosphere for five more hours. The coupons were then slowly cooled in anitrogen atmosphere at a rate of 50C per hour.
The temperature was the same in each heating step for the individual COUpOAS. The sample coupons were then u i as anodes as in Example 1 for the evolution of chlorine for ten days and the results are reported in Table XII.

TABLE XII

Anodic Potentials Weight Sample Heating InitialAfter 10 Days Loss No. Temp. C ValueOf Operation in mg/cm2 _ 1 600 1.60 1.68 negligible 2 700 1.70 1.75 ne.gligible 3 800 1.50 1.50 negligible 4 900 1.98 high ~ 0.5 _ Table XII shows that the best anodic potential for chlorine evolution was obtained with the test coupons heated to 800C. The coupons were also used satisfactorilv as stable anodes for cathodic protection.

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... . . . . . . . . . .

.
E~MPLE 1 2 . . . _ .

Sintered materials obtained ~ a mixture of metal powders vf mesh NoS. comprised between 60 and 320 and having composition-' as indi-cated hereinbelow in Table XIII have been used as anodes for the electrolysis of H~S04 10% solution at 6 0 C under a current densi-ty over projected area of 1. 2 KAirn The experimental results are summarized in Table XIII.

~ABLE XIII
.
.. . . _ . .
Composition of sintered Anode potential Weight loss material ~ by wt. V(NHE) mg/cm2 InitialAfte r Ti C,o Ni TiO2 RuO2 Value 10 days ~3 0 3 4 0 ~!.,39 2~40 105 93 0 2 4 1 ILo 60 1~ 61 neg:Ligib~:e:
93 1 1 4 1 l o 66 1. 58 D.egligible 3 3 3 1 1~ 54 1~ 56 negligible . _ . . . .. _ ~he following remarks can be made:
i) ~he presence of Ru02 sharply improves the catalytic activity for oxygen evolutionO
ii) the addition of cobalt slightly increases the catalytic activity for the oxygen evolutionO
iii) ~he addition of RuO~ or cobalt and Ru0;2 sha~rply decrease5 the rnetal wei ght lo s s .

The last three samples are very suitable fior~ use as anodes in electrolysis processes in which oxygen is evolved at the anode, such as in most metal electrowinning processes~

_~ . O

7~jq3~

I_:CA/~i~ ' L ' I 3 - f ro~n Sintered materials obtained ~ a mixture of metal powders of mesh Nos. comprised between 60 and 320 and having compQsitio~4 as in-~icated in Ta.ble XIV ha~e been used as anodes o:r the ele~lysis of H2SO4 10% solution at 60C under a current density over projected axea of 1.2 KA/m~ .
The experimental results are summarized in Table XIV.

TABLE XIV
.

- Composition of sintered Anode potential Me~al V (NHE ) Wei ght - material % by wt. Initial After L,oss 2 Ti Co Ni TiO2 Ir IrO2value 10 days mglcm _ q3 0 Z 4 0 0 2.30 2.~0 1.5 93 0 2 4 0 1 1!60 . 1.63 rlegligible 93 0 1 4 1 1 1~ 5~ 1. 54 neg:Ligibl~
93 1 1 3 1 1 1. 53 1. 53 negligible .. ... .

The three last samples are c~ racterized by a low anodic potential which ~emained substantially uncha~ged after 10 days of operation and by all ex-tremely low metal weight loss.

~ ~.
~c,.o.n Sintered materials obtained ~ a mixture of metal powders of mesh Nos. comprised between 60 and 320 and having compositior~as indica--i:ed in Table X~l have been used as anodes for the electrolysis of H2S04 10% solution at 60C under a current density over projected area of 1. i~ KA Im The experimental results are indicated in the following table~

. . , .. ~.......... .. . ... .

TABLE XY

composition of sintered Anode Potential :Metal naterial q~ by . wt . V ~NHE ) Wei ght Initial After Loss TiCo Ni Pt Ir Yalue 10 days mg/cm ~33 0 7 ~ 2. 7 / 8~
'93 0 5 2 0 2.~) 202 ~.5 93- 0 ~ 0 2 . 1. 70 1. 72 negligible 93 0 5 1 1 1. 68 1. 70 ~egligible 932. 5 2. 5 1 1 1. 67 1. 68 n~gligible ~ _ _.. .
The ~ree last samples show a low anodic potential and an extremely low metal weight loss which makes them ~ery useful as anodes for electrolysis processes wherein o.~{ygen is e~olved at the anode.
.
EX~MPLE 1 5 Prv~
Sintered materials obtained ~7- a mixture of metal powders of m~sh Nos. comprised bet~,veen 60 and 320 and ha~ing ~ompositionSas indica-ted in Table XVI have been used as anodes for the elect:rolysis of the 2 4 10% solution at 60C under a current density over projected area of 1. 2 KA/m The experimental results are indicated in the following TableO

l`ABLE ~ ~!~

_ C;omposition of sintered Anode Potential Metal material % by~t. ~(NHE) Weight Initial After Loss 2 TiCo304 Fe304 E~u02 ~alue 10 days mg/cm 9010 0 0 1.90 2.0 1.~
go o lo n 1.97 2~,10 2.5 go 0 n 0 . lo 80 io B0 negligible 90 5 5 a 1 o 83 l o 87 negligible 902~ 5 ~o 5 5 lo 77 l o 78 ~egligible ...... ... ' '' 1-- ~

~ 2~ . , ~ !

'7~

The following remarks can be made:

i) the addition of Ru02 sharply improves the catalytic activity for oxygen evolution.
ii) the addition of Co304 ~ Fe3O~ slightly increases the catalytic activity.
iii) the addition of Ru02 and/or Co3O~ -t Fe304 sharply lowers the me-tal weight loss.
The last three samples show a low anodic potential and a very good resistance to corrosion.

Sintered materials obtained from a mixture of metal powders with mesh Nos. comprised between 60 and 320 and having compositions as indicated in Table XVII have been tested as anodes for the electrolysis of H2S04 10~ solution at 60C and at a current density of 1.2 ~A/m2. The experimental results are detailed in Table XVII.

TABLE XVII

Composition of sintered Anode Potential Weight V(NHE) material % by wt. Initial After Loss Fe Mo Cr W Si Value 10 days mg/cm . .
60 20 5 15 0 1.9 1.9 20 60 Z0 5 10 5 2.1 2.1 negligible 60 10 5 15 10 2.0 2.1 negligible 60 10 10 5 15 2.0 2.3 negligible The addition of Silicon greatly improves the metal corrosion resistance while lowering slightly the catalytic activity for oxygen evolution.

b m ~ P
C~.L
.

'7~3~

Sintered materials obtained from a mixture of metal powders with mesh Nos. comprised between 60 and 320 and having compositions as indicated in Table XVIXI have been -tested as anodes for the electrolysis f H250~ 10~ solution.at 60C and at a current density of 1.2 KA/m2, The experlmental results are reported in the following Table.

TABLE XVIII
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Composition of sintered Anode Potential Weight V~NHE) material % by wt. Initial After Loss TiSnTa207IrTa207 value 10 days mg/cm2 .
0 1.7 1.7 negligible 0 10 1.5 1.5 negligible _ The presence of metallates in the valve metal matrix sharply increases the electrocatalytic activity for oxygen evolution while not affecting the very good corrosion resistance.

, . EXAMPLE 18 Sintered materials of similar composition as described in Example 12 have been pre-activated by dipping the test coupons in a molten potassium persulfate bath for 5 hours.
They were then tested as anodes for the electrolysis of a saturated sodium chloride aqueous solution at 60C with a current density of 5 KA/m . The experimental results are reported in the following Table.

bm:~ -:, ~3 ,, :

L

.
TABLE ~X~E ~
. _ .
Gomposition of sintered Anode potential Weight material ~Q by~t. Initial After Loss i CoNi T 0 _ ~ 10 days mg/cm 93 0 3 4 0 2 D 9 3 . 3 ï 0 ~3 t1 2 ~ ,70 1075 200 ~3 1 1 4 1 1.6~ 1.70 1.() 3 3 3 1 l.b5 1.69 1.0 The presence of Ru02 sharply improves the catalytic activity for chlorine evolution and the metal weight loss is sharply reduced.
Addition of Cobalt and Nickel urther improves the performance of the anode s .
EXAMPLE ~q Sintered materials of similar composition as described in E~ample 13 lhas been pre-activated by anodic polarization in a 10% b.w.t.
~odium hydro~;ide solution at a eurrent density of 3 E~ /m for 10 hours. The test coupons were then tested as anodes for the electro-lysis of a saturated sodium chloride aqueous solution at 60C with a current density of 5 KA,~m .
The experimental results are reported in the following Table.

-TA B LE XX~_ Gomposition of si~tered Anode Potential Weight material ~c ~y ~to V(NHE) Loss `Ti CoNi TiO2 Ir IrO2 ralue10 days mg/cm :
93 0 3 ~ O 1~ 20552~,60 10 g3 0 2 4 1~ 1. 8$ 2~ 5 93 0 1 4 ~ 1 1 0 73 1 . 74 1 . 6 93 1 1 3 1 g 1.601~60 1~ 5 ~ 23 . ~ .. _ . .. . . . . .

- ~ o Test sample No. 4 shows a low anode potential which remained unchanged a:fter 10 days o:~ aperation. The metal weight 108~1 for ~e same period was 1. 5 mg/cm.
.

Sintered materials of similar composition as described in Example 14 ~have been pre~activated by anodic po~irization in a 10% b.,wt, sodium hydroxide solution at a c~lrrent density o 3 KA/m for 10 hours.
The test coupons were then tested as anodes for the electrolysis of a ~aturated sodium chloride aqueous solution at 60C with a current density of 5 KA/m7 The experimental results~ are reported in the follo~,virg TableO

TABLE XX~E

Composition of sintered A~1ode Potential Weight material % by wt. ~J(NHE) Loss Ti Co Ni Pt Ir value l O days ~ mg/cm 93 ~ 7 0 0 2c3 3.0 Z~0 93 0 5 2 0 . . . 2.2 2~ 5 . 10 93 ~ 5 0 2 2.() 203 5 g3 0 5 1 ~ 1"651.67 2 93 2~, 5 Z~5 1 1 1060 1~60 .,, , . _ _ . . . .

The two last samples of the table show a low anode potentiaI or chlorine evoiution which remained pratically unchanged after ten days of operation The corresponding metal weight losses were also lowO

-~ ~4 --.

EXAMPLE 2 ~l , Sintered materials of similar composition a8 deucribed in E:xample 15 }lave heen pre-activated by anodic polarization in a 10% bow~t~ 90-dium hydroxide solution at a curxent density of 3 E~A /rn for 10 hours~.
The test coupons were then tested as anodes for the electrolysis of a saturated sodium chloride aqueous solution at 600C with a current density ~f ~ KA/m.
The experimental results are reported in the following Table.

TABLE XXg~T
, C;omposition of sintered Anode potential We;ght material 7~ by~ ~t. ~NHE) - Loss Ti Co304 Fe304RuO2 value 10 days mg/c~n ~ . . . . . _ __ 90 i0 O 0 2011) 2 20 20 90 0 10 0 1097 1.98 ~
9 û 0 0 1 0 1 . 9 0 1 ~ 9 3 ne gli gible 90 5 5 0 1. 57 1. 57 negligible . 902~ 52a ~ 5 1~ 45 1.~ 4$ negligible .. . . . . __ . ... _~ _ . . . .
The last test salnple in the table show;, a remarkably lo~,v anode potential for chlorine evolution associated with very good corrosion resistanca.

EXA MPLE 2~
Sintered materials similar con~position as described in Example 17 haveb~en pre-acti~rated by anodi- poldr;~ation in a 10% b. ~vto sodium hydro-xide solution at a current clensity of 3 KA/~n for 10 hours., The test coupons were then teste~ as anodes for the e~rolysis of a -~aturated sodium chloride aqueous solution at 60C with a current den-of 5 KA /~n~
~he experimental re sults are reported in the follo~fing Table.

.

TABLE XX~

Composition of sintered Anode potential Weight V (PIHE ) ma$erial ~c by-~tOInitial After Loss .Ti SnTa207 IrTaz07value 10 days mg/cm 80 20 01. 7 1. 75 . negligible 0 101. 5 1,. 55 negligible The addition of met;~llates to the valve metal matrix sharply increases .
: the catalytic activity.
The last test sample in the table shows a low anode potential for chlorine evolution and a very good corrosion resistance.

.
. ' , .
; .

~ 26 --.. - .
. ~

Anodes prepared according to the invcntion, and comprising other film forming metals such as the valve metals tantalum, zirconium, niobium, vanadium, hafnium, tungsten and film forming iron alloys alloyed or sinterized with other metals, metal oxides and intermetallic compounds ~hich provide, on the surface of the film forming matrix, active nuclei which interrupt the non conductive barrier layer and permit the formation of an electrically conductive and electrocatalytic film thereon, may also be prepared and used in electrolysis processes for chlorine evolution, oxygen evolution and other purposes such as fused salt electrolysis, electrowinniny, electrophoresis, organic and~aqueous solutions electrolysis, cathodic protection and the like.
The electrodes produced accordingly to Examples 1 to 22 may be connected into an electrolysis cell circuit in any desired manner and are provided with suitable means to make connection to a source of electrolysis ~current in diaphragm or mercury cathode chlorine cells, electrcwinning cells or any other type of electrolysis cells.
As will be seen from the various examples, -the electrodes of this invention may be used in chlorine and oxygen evolution and other electrolysis processes by merely pre-activating the alloy compositions (or a portion of the alloy composition) forming the surface of the electrode.
The ac-tivation layer is formed from the alloy at the surface of the electrodes, without the application of a separate coating layer, and is, therefore, cheaper to produce, more adherent to the surface of the electroae and more easily restored (re-activated) after use if necessary than the separately applied coatings of the prior art. Moreover in some uses (i.e., oxygen evolution), the activation layer is self-generating and regenerating in service -- thereby giving rise to long-life, inexpensive ~anodes for use particularly in me~al electrowinning, which do not add impurities to the metal being recovered.
Various modifications of the products and processes of the invention may be made without depàrting from the spirit or scope thereof and it should be understood that the invention is not limited by the illustrative examples bm: d~ - 27 -1~-~,.. .

given and i~ intended to be limited only as dcined in the appcnded cl~im~ .

. This application ls a division of copending Canadian application Serial NoO 190,929, filed January 25, 1974.

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, _ . .

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A pre-activated electrode comprising an electrically conductive base made of an alloy of iron containing up to 20% by weight of silicon and 0.5 to 10%
by weight of molybdenum or chromium, having an electrically conductive and electrocatalytic outer layer consisting of oxides of the said alloyed metals on the surface of said electrode base formed by a pre-activated treatment of the base.

2. A process for the pre-activation of an electrode comprising heating an electrically conductive base made of an alloy of (A) at least one film forming metal and (B) from about 0.1 to about 50% by weight of at least one member of the group consisting of metals from Groups VIB, VIIB, VIII, IIB, IB, IVA and lanthanum and lanthanide series of the Periodic Table in an oxygen-containing atmosphere at 500° to 1200°C for one to ten hours and then slowly cooling the said base.

3. The process of claim 2 wherein the base is cooled at a rate of 10 to 80°C per hour.

4. The process of claim 2 wherein, after heating in the oxygen-containing atmosphere, the base is heated to 500° to 1200°C in an inert atmosphere for one to ten hours.

5. A process for the pre-activation of an electrode comprising subjecting an electrically conductive base made of an alloy of (A) at least one film forming metal and (B) from about 0.1 to about 50% by weight of at least one member of the group consisting of metals from Groups VIB, VIIB, VIII, IIB, IB, IVA and lanthanum and lanthanide series of the Periodic Table to anodic polarization with an aqueous solution of 5 to 50% by weight of an alkali metal salt for one to twenty hours at a current density of 600 to 5000 A/m2.

6. A process for the pre-activation of an electrode comprising dipping an electrically conductive base made of an alloy of (A) at least one film forming metal and (B) from about 0.1 to about 50% by weight of at least one member of the group consisting of metals from Groups VIB, VIIB, VIII, IIB, IB, IVA and lanthanum and lanthanide series of the Periodic Table into a molten oxidizing salt for one to ten hours at a temperature above the melting point of the salt but below the boiling point thereof.

7. The process of claim 6 wherein the oxidizing salt is selected from the group consisting of alkali metal nitrates, persulfates, perborates and pyrophosphates.

8. A method of preparing a pre-activated electrode comprising subjecting a base comprised of an alloy of (A) at least one film forming metal (B) from about 0.1 to 50%
by weight of at least one member of the group consisting of Groups VIB, VIIB, VIII, IIB, IB, IVA and lanthanum and lanthanide series of the Periodic Table to a surface oxidizing pre-activation treatment to form on the base surface a corrosion-resistant, non-passivating layer of the oxides of the metals of the alloy.

9. As a product of manufacture, a pre-activated anode comprising an electrically conductive base made of a silicon-iron alloy containing from 0.1 to 20% by weight of silicon and iron about 0.1 to 50% by weight of at least one member of the group consisting of Groups VIB, VIIB, VIII, IIB, IB, IVA
and lanthanum and lathanide series of the Periodic Table, and having a coating of oxides of the metals constituting said alloy on the surface of said anode base formed by a pre-activation treatment of said base and means on said anode to connect the said anode to a source of electrolysis current.