CA1037902A - Prevention of crevice corrosion - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A novel method of preventing damage to the base of diaphragm-type cells used for the production of chlorine by electrolysis of brine solutions by incorporating into the in-sulating blanket covering the base a mixture of nickel powder and nickelous oxide, the said mixture being at least in the areas where crevice corrosion is likely to occur.

Description

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STATE CF ~HE ARr ~ ' D~aphragm-type cells ~or the electrolysis o~ aqueous ;: ; alkali metal halide brines generally employ a ~oraminous or , per~ora~ed metallic cathode and a fluid-permeable diaphragm ;;,;
-. overlaylng ~he ca~hode thereby permi~ting hydraulic ~low o~
, electrolyte from the anode chamber thxough the dlaphra~ and ca~hode into the cathode chamber. Such cells flrst made their appearance in the early part o~ the twentieth:century. The~
~ fluid permeable diaphragm~ by separating the anode and~cat~
.' hode chambers, avoids the dist~rbing ef~ect~ Or conuectlon currents and gas evolut~on, and generall~J lnhibits migratlon ,~
of hydroxyl ions towards the anode. Tho~e diaphragm-type .
cells most wld~ly used today are o~ the circulati~g electro~
, ~ lyte type, wherein the dia,~hragms and cathodes may be arra~g~
ed horizontally or ~ertically, but in most instance~, at . lea~t in the United States, the arran~ement-ls vertlcal, Such cells are in wide-spread use ln ~he industry ~or ~he "
:., :. . :-. production o~ chlorlne and caustlc soda ~rom sodium chlvride ;~
, 20 brin~s and,,through the u~e o~ ~c~rious ~ophlsticated modi~
~icat-~ons~ con~i~erable ef~iciency ha3 been obtalned ~rom the ~.
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i 1~37 9 ~I cells which have been operated at current den~ltles approach-i~ ing one ampere per square lnch. Howev2r, de~plte their wlde-- li spread acceptance~ the3e cell~ ne~erthele~ have certaln ! dra~bac~ and dlsadvantages whlch limit the ~urther modifica--5 ' ~lon and lmprovement thereof.
~- ~I Mo~t of the~e limltat1ons may be attrlbuted to the ,~ ract tha~ the ma~ority of cells in operation to-date employ - i graphite as the anode material~ Generally these anodes ~ake the ~orm o~ flat, vertlcally-dlspo~ed graphite blades which haYe their lower ends embedded ln the cell botto~ or base.
!~ A typical means of implanting these anode blades ln the cell base is described ln U.S. Patent No. 2,9879463 and consists - ~ in inserting the anode blades in~o the slots formed 4y a plurality o~ conductl~e metal grids, usually copper grids.
In order to impro~e electrical contact, it is then the stand-~ . ..
ard procedure to apply a bonding layer o~ an electrically conducti~e material such as molten lead~ ~rhich laye~ s~rves both to increase electrical conductivity and rig1dly se~ the anode blades in the conducti~e grid. Cver this electrically-; conductive bonding ~ayer there is ~hen applied an electrica - ly insulating coating, ~or example asphaltJ which pre~ents- ; acces9 of the corrosi~e anoly~e to the metalO In turn9 a layer of concrete is applied over thls asphalt layer to com plete the base construction. Obviously, there ~re a number of disadYantages to such a cumber-some technique, which are ~-` mainly due to the use of graphite anode.
i In the last few years, dlmensionally stable metal ! anodes wlth an electrically conductlve, electro-~atalytic ~ coa~ing thereon ha~e been adapted ~or commercial use in d~aphragm-~ype cells which have permitted change~ in the . . .
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1~37~02 de~ign o~ the cell, particularly ~he deslgn and mounting of the anodes. In a commercial cell ~hich ha~ become widely ~' u~ed is that of U.S. Patent No. 3~591,483, the cell bas~ is ~! made Or a conductlng material and anodes are posltioned , thereon by rl~ers passing through the cell base and provided ~ith a ~lange, preferably madc! o~ ti~anium, on th~ lower portion thereo~ and a-non-conducti~e, pre~erably rubber, sheet on the cell base to act as a compres~lble seal between the anodes and the cell base and between the cell can and the ~
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; cell base~
; Experience with cell of this type, however, ha~
- ` ~hown that there is a tendency ~or crevice corro~ion to appear on the riser flange where it rest~ on the insulatlng sheet o~ rubber. Crevice corrosion i~ due to a speoial type of galvanic cell arislng from a dif~erence in the electrolyte C - composition in the crevice and the electrolyte ln con~act with the main body Or the metal part or ~lange. The crevice corrosion could cause leaks Or electrolyte ~rom thè cell and other problem~ leading to premature shut down o~ the cell for~ -repair~

OBJECTS CF THE INVENTION ~ ~ -,~
It ls an ob~ect of the ~nventlon to provide a novel method of preventing crevice corrosion in valve metal con- ~ ;
nections ln dlaphragm-type cells.
, . ~. -It is a further ob~ect o~ the invention to provide an -~
impro~ed diaphragm cell using as the non-conductive, insula~
tlng sheet a rubber sheet having ~n the area around the anode rlser a mixture of nickel ~nd nickelous oxide. - ~:
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These i~nd other ob~ects and advan~ages of the invent- ;
ion will become obvlous ~rom the ~ollowi~g detalled descrip-tlon.
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THE INVENTION ~'~
The invention reIates to ~n improvement in dia-phragm cells wherein the anode~ are supported by risers which are secured in the base and the base ls insulated from the interior of the cell by a protebtive sheet, the improvement comprising providing in the area of the cell ::~
base where crevice corrosion normally occurs a mixture of .
metallic powder nickel and nicke].ous oxide in the protective ~lanket or gasket sealing means. Usually, it is e~fected by ~.
incorporating into the protective sheet at least in the area ~ -~
. j thereof through which the risers pass a mixture of metallic .
nickel powder and nickelous oxide.
~ The said mixture may be incorporated throughout ~:
-- the entire protective sheet or only in the area where the . risers pass there~hrough, The protective sheets are usually -~
made of chlorinated rubber or neoprene type material and if the said mixture is to be incorporated throughout the sheet, :.
the mixture of metallic nickel powder and nick~lous oxide :
may be substituted for the zinc and magnesium oxide filling .
and curing agents. The nickel and nickelous oxide provide the needed curing effect during vulcaniæation of the neoprene blanket and the omission of the zinc and magnesium compounds from the protective blanket avoids their possible chlorination which would aggravate the crevice corrosion pro~lem.
I~ the protective sheet is not a neoprene type material but a metal sheet such as a titanium sheet, tha gaskets used to seal the cell and the area around the anode ~
risers are made of neoprene type material and the nickel- ~ :
. nickelous oxide mixture is incorporated in this material to . prevent crevice corrosion. It is also possible to merely ~:
insert into the neoprene type blanket, small gasket portions which can be inserted into the blanket in those areas wherein .
.; the anode rlsers pass through the protective bla~ket.
The nsopr~ne type material ussd for the protective B ~
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~. i anket or the ~a,sk~3ting m~teria.l ar~3 ~ell known but the mo~t commonly used types are the G and W types. Neoprene : is made by polymerizing 2-chlorobut:adiene in thQ presence of suitable catalysts, emulsifying agents, mod~fiers and protective agents. The G-type neoprene differ~ from the `~-~
W-types in that G contains a thiuram disulfide stabilizer .`.. :
- and are interpolymerized with sulfur.
The additives in various neoprenes are well known ; -and perform various functions. For example, plasticizer -~
and softeners such as naphthenic oils and other petroleum .. ~
derivatives are used to increase flexibility thereof. ~ :
Fillers may be carbon black, clays, calcium carbonate, silicon dioxide and other mineral fillers. Antioxidants are added to provide maximum protection from heat, ozone, and/or dis-coloration. Processing acids may also be incorporated therein; .
: .this class . includes lubricants tackifiers and agents to `;~
control viscosity and nerve. ..
After the neoprene has been vulcanized and has been formed into the desired shape, the area of the neoprene in the crevice corrosion problem area should be cut or scraped to remove the film inherently formed on the surface of the ;~ ~:
neoprene during the vulcani~ation process. ... ~ ;
The amount of metallic nickel and nickelou~ oxide to be incorporated into the neoprene may be 25 to 45~ preferably 30 to 40~ % by weight. The weight ratio of the nickel_ o `~
nickelous oxide may Yary from 3:1 to 1:1, preferably 1.5:1 to ..

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In accordan~e with the present invention there is ~.
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provided an i~provement in a diaphragm cell haYing a cell base with anodes mount~d thereon and covered with a protective .
blankst and a cell aan with cathodes mounted there~n. The . improvement compris~s providing in the area of the cQll ~a~e :~
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~here creYice corroqion normally occurs a mixture of metallic powder nickeI ana nickelous oxide in the protective blanket or gasket sealing means.
In accordance with another aspect of the present invention there is also provided an improvement in a dia- :
phragm cell having a cell base with dimensionally stable metal anodes mounted thereon, protective rubber blanket covering the cell base and a cell can with cathodes mounted therein resting on the cell base. The improvement comprises a rubber :~
:: 10 blanket having incorporated therein a mixture of metallic ~ ~:
powdered nickel and nickelous oxide, the blanket surface in the ~.
areas where crevice corrosion is likely to occur being abraded.

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7~302 Referring now to the drawlngs-- .

¦l Fig, 1 ls a simlplified end view of a typical i! diaphragm-type electrolytic cell of the invention with the t~ cell can and cathodes removed for clarlty.
5 ~ Fig. 2 is a s~mplifled side view of a portion of the diaphragm-type electrolytic cell o~ Fig. 1 with cell can and cathodes not shown.
' In Fig. 1, the cell base 1 is constructed o~ a material such as aluminum or copper and hence serves as both 10 the supporting means for the cell and the conductor. The ~ `

power supply is attached directly to this base,for example, by means of a nut 9 and bolt 11. The non-conductive sheet . :

3 covers essentially all of the-cell base 1 and is construc~
` ted of an elastic material such as neoprene containing metal-C 15 lic nickel powder and nickelous oxide. The protrusions 5 and 6 on this non-conductive sheet 3 perform separate func- `
tions. Protrusion 5 serves as a gasket on which the cell ; can rests. A small amount of putty 29 lines the inside of `
; the protrusion to insure that no leakage occurs. Protrusion ~
20 1 6 serves as a deflector to prevent brine or water from get- ~ I
ting between the non-conductive sheet 3 and the cell base 1. ~
e valve metal anode 19 with the electrocatalytic coating ;-thereon is connected, for example by T~elding, to the anode , riser 13, which riser extends through the non-conductive sheet and cell base and is fastened on the bottom of the cell base - ' by means of a nut 17. Examples of suitable electrocatalytic, electrically conductiv-e coatings are described in U.S. patents 3,632,498 and 3,711,385. The preferred coating for chlorine production is TiO2 + Ru02. ,This is one of the problem areas ;-, -6-: ...................... . , ............ . :
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! ln Whlch crevlce corroslon generally occur~. The neoprene blanket ls scraped ln this area to remove the rilm on the ::
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blanket ~ormed durlng vulcanization. The riser i5 also '? provided with a ~lange 15 whlch upon tlghtening the nut 17, 5 il forms a hydraullc seal with ~he non-conduc~ive sheet of material 3 thereby preventing leakage of anolyte through the ~
,I cell base. While it is indicated in Fig. 1 that two anodes ~ ;
- j extend across the width of the cell, this number is not ' critical and may be changed as conditions warrant. `~
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~lg. 2, is a partial side view along the length of a cell, again with the conventional cathodes and cell can , removed. This figure shows essentially the same ~eatures as in Fig. 1, however, there is also indicated on the anode 19 the electrically conductive surface 21, greatly exaggerated for illustration, in fact belng on the order of from 1 to 5 ` mlcrons in thickness.
In the following example, there are described -- ~arious embodiments to illustrate the invention. However, lt should be understood that the invention is not intended -, . -~0 to be limited to the specific embodiments.

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;; - EXAMPLE

160 g of neoprene W~ 16~ g of carbon black, 16 g of -~
t aromatic ~etroleum plasticizer, 15 g of naphthalenic petro~
leum plasticizer, o.8 g of ethylene thiourea as accelerator 25 i 139 g Or powdered metalllc nlckel, 76 g o~ green nickelous oxide, 6.4 g of magnesium oxide in oll as a scorch resistan~
~- agent and 11.5 g of monobenzone type antl oxidan~ were throughly admixed and vulcanized in the usual fashion. The ; resulting neoprene had a durometer hardness rating o~ 65-70.
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- :....... j~ . , , A 40 mil sheet was cut into circular gaskets 2 inches in l! dlameter, said gaskets being abraded to expose the nickel.
¦¦ ~ In order to determine the degree of protection Prom ¦ crevice corrosion, a sandwich of alterna~ing discs of !~ tltanium and neoprene prepared above were clamped in a press i Or a tltanium frame, ~uts and bolts. A second test was run with alternating layers of titan~um and conventional neoprene !~ without the metallic nickel-nickelous oxide mixture. The ll 1' ; 1 sandwiches were then submerged for 13 days into a refluxing aqueous solution of 25 g~l of sodium chloride with a pH of 1.3 after which the sandwiches were checked for corrosion.
The sandwich with conventional neoprene exhlbited heavy ', tltanium corrosion and build up of deposits while the sand~
, wich of the invention was free of corrosion.
C Varlous modifications of the invention may be made i without departing from the spirit or scope thereof, and it ~ -. . ~ , .
is to be understood that the invention is to be limited only -as defined in the appended claims.

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Claims (12)

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

1. In a diaphragm cell comprising a cell base with anodes mounted thereon and covered with a protective blanket and a cell can with cathodes mounted therein, the improvement comprising providing in the area of the cell base where crevice corrosion normally occurs a mixture of metallic powder nickel and nickelous oxide in the protective blanket or gasket sealing means.

2. In an improved diaphragm cell comprising a cell base with dimensionally stable metal anodes mounted thereon, a protective rubber blanket covering said cell base and a cell can with cathodes mounted therein resting on the cell base, the improvement comprising a rubber blanket having incorporated therein a mixture of metallic powdered nickel and nickelous oxide, the blanket surface in the areas where crevice corrosion is likely to occur being abraded.

3. The diaphragm cell of claim 2 wherein the rubber blanket contains 25 to 45% by weight of the mixture of nickel and nickelous oxide.

4. The diaphragm cell of claim 2 wherein the weight ratio of nickel to nickelous oxide is 3:1 to 1:1.

5. The diaphragm cell of claim 2 wherein the weight ratio of nickel to nickelous oxide is 1.5:1 to 2:1.

6. The diaphragm cell of claim 2 wherein the rubber blanket contains 30 to 40% by weight of the mixture of nickel and nickelous oxide.

7. The diaphragm cell of claim 1 wherein the nickel and nickelous oxide is in the gasket sealing means.

8. The diaphragm cell of claim 7 wherein the weight ratio of nickel to nickelous oxide is 3:1 to 1:1

9. The diaphragm cell of claim 7 wherein the gasket sealing means contains 25 to 45% by weight of the nickel-nickelous oxide mixture.

10. A method of preventing crevice corrosion in diaphragm cells comprised of a cell base with dimensionally stable valve metal anodes mounted thereon and covered with a protective rubber blanket and a cell can with cathodes mounted therein comprising using a rubber blanket or gasket containing a mixture of metallic powdered nickel and nickel-ous oxide in the areas where crevice corrosion is likely and whose surface is abraded in the said area.

11. The method of claim 10 wherein the weight ratio of the nickel-nickelous oxide mixture is 25 to 45% by weight of the rubber blanket.

12. The method of claim 11 wherein the weight ratio of nickel to nickelous oxide is 3:1 to 1:1.