Reduction of crevice corrosion in bipolar chlorine diaphragm cells by locating the cathode screen at the crevice and maintaining the titanium within the crevice anodic
United States Patent Raetzsch, Jr. et al.  
 [ REDUCTION OF CREVICE CORROSION IN BIPOLAR CHLORINE DIAPHRAGM CELLS BY LOCATING THE CATHODE SCREEN AT THE CREVICE AND MAINTAINING THE TITANIUM WITHIN THE CREVICE ANODIC [75] Inventors: Carl W. Raetzsch, Jr.; William Bruce Darlington; Hugh Cunningham; Donald W. DuBois, all of Corpus Christi, Tex.  
 [73] Assignee: PPG Industries, Inc., Pittsburgh, Pa.  
 [22] Filed: July 20, 1973 [2]] Appl. No.1 38I,ll4  
 [52] U.S. Cl. 204/I47; 204/98; 204/128; 204/196; 204/254; 204/256 [5!] Int. Cl. C23! 13/00 [58] Field of Search 204/98. 147, I96, 254. 204/255. 256, 268  
 [56] References Cited UNITED STATES PATENTS 3.lU2.()86 8/[963 Cotton 204/l47 1 Apr. 8, 1975 3.118.823 1/1904 Cotton et al. 204/290 F 3.755.108 11/1973 Raetzsch et al 204/255 x Primary E.\&#39;antiner.l0hn H. Mack Assistant Examiner-W. 1. Solomon Attorney, Agent, or Firm-Richard M. Goldman [57] ABSTRACT Disclosed is a bipolar electrolyzer having a plurality of individual diaphragm cells electrically and mechanically in series. Each of the individual diaphragm cells has an anolyte chamber fabricated of an anolyteresistant material and a catholyte chamber fabricated of the catholyte-resistant material. The anolyte chamber is separated from the catholyte chamber by a diaphragm suitably mounted on the cathodes of the cell. The cell body is joined together at a gasketed joint. At the crevice formed within the joint between the anolyte-resistant member of the cell body and the gasket crevice corrosion is reduced by maintaining the surface of the anolyte-resistant material within the crevice anodic.  
 I0 Claims, 5 Drawing Figures PATENIEBAPR 85975 7.876.517  
 seam 1 [1F 3 REDUCTION OF CREVICE CURII OSION IN BIPOLAR CHLORINE DIAPHRAGM CELLS BY LOCATING TIIE CATI&#39;IODE SCREEN AT THE CREVICE AND MAINTAINING THE TITANIUM WITHIN THE CREVICE ANODIC BACKGROUND OF THE INVENTION Diaphragm cells for the electrolysis of brine to yield caustic soda and chlorine from saturated brine are disclosed generally in the article by Morton S. Kircher entitled Elecrrulysis of Brines in Diaphragm Cells appearing at pages 81 to 126 in James S. Sconce. Editor. Chlorine: lls Manufacture. Properties. and Uses, American Chemical Society Monograph 154. Rinehold Publishing Corp., New York. N.Y. (1962).  
  In a typical diaphragm cell. the anolyte is acidic. having a pH of from about 3 to about 4.5. The principle anode reaction is:  
  The catholyte, also referred to as catholyte liquor and cell liquor, typically contains from about 110 to about l50 grams per liter of sodium hydroxide and from about ISO to about 200 grams per liter of sodium chloride. The catholyte is strongly basic. At the cathode. the principal reaction is a discharge of hydrogen ion from the basic solution:  
  A bipolar electrolyzer contains a plurality of single individual diaphragm cells in a common unit. e.g.. three. or five. or eight or more cells. and possibly as many as 75 or more cells in a single electrolyzer. The individual diaphragm cells are electrically in series through a common structural member, called a backplate or support plate. The cathodes of one cell are electrically and mechanically connected to one surface of the backplate. ie, the catholyte-resistant surface of the backplate. The anodes of the next adjacent cell in the electrolyzer are mechanically and electrically connected to the opposite surface of the backplate. i.e.. the anolyte-resistant surface of the backplate.  
  In a bipolar electrolytic cell. the electrical current typically flows from an external power source into an anodic end unit and through the anodes of the anodic end unit into the anolyte of the end cell. The current then flows through the diaphragm of the end cell into the cathode of the end cell. and from the cathode and through the backplate into the anode of the next adjacent cell. Thereafter. the electrical current flows from an anode of a cell to and through the anolyte to the cathodes of the cell and from the cathodes through the backplate to the anodes of the next adjacent cell.  
  Two types of construction of bipolar electrolytic cells have been used. In one form of construction, the cell body is a catholyte-resistant material. e.g.. steel. and the interior surfaces of the cell contacted by anolyte, e.g.. the anolyte chamber and anodic surface of the backplate. are lined with chlorine-resistant rubber. However. at the high current densities necessary for economical operation. the rubber lining used in the electrolytic cells of the prior art loses its resistance to chlorine.  
  In an alternative cell design. the cell body is fabricated of metal and clad with an anolyte-resistant metal. e.g.. a valve metal such as titanium or the like. In the assembly of such cells. a titanium clad steel flange is gasketed to the titanium flange surface of the anolyteresistant side of the backplate, providing a pair of titanium to gasketing material crevices seal. These titanium to gasketing material crevices are particularly susceptible to crevice corrosion.  
  Titanium crevice corrosion is a phenomena which occurs under conditions of oxygen depletion at crev ices such as joints, laps, fillets. seals and the like. While the exact mechanism of crevice corrosion is not fully understood, it is generally found only in thin crevices, characterized by a high ratio of metal surface area to electrolyte volume within the crevice. It is generally believed that crevice corrosion is caused by the diffusion or seepage of electrolyte through gasketing into the crevice. establishing a local cell within the crevice. It has been found by previous workers that the electrolyte within the crevice is highly acidic. having a pH of less than 2, for example. of a pH of L5 or even lower. Within a crevice. the concentration of corrosion product is high. Concentrations on the order of more than l0 grams per liter and even higher. e.g.. as high as 20 or 30 grams per liter of corrosion product have been reported. Additionally. iron present in the titanium ap pears to serve as a site for the crevice corrosion of the titanium.  
  A local cell is reported to arise within the crevice. The local cell may be adjacent areas of a single sheet of titanium. The cathodic side of the local cell within the crevice is reported to contain a titanium hydride or a subhydride phase. e.g.. TiH which is brittle and readily flakes away to be hydrolyzed within the local cell. The anodic side of the local cell within the crevice is reported to contain incompletely formed sub-oxides of titanium which also flake away to form corrosion products which may subsequently be hydrolyzed.  
  The electrolyte within the local cell of the crevice is further characterized in that it is oxygen deficient and may contain large amounts of halogen ion.  
  There have been various attempts to solve the problem of crevice corrosion. For example, the degree of crevice corrosion may be reduced when the titanium is an alloy with nickel, such as a 2 percent nickel titanium alloy described in US. Pat. No. 3.469.975 to Bertea et al. It has also been found that reducing the surface iron inclusion content of the titanium. as disclosed in the commonly assigned. copending application of Donald W. DuBois for Method of Treating Titanium Containing Structures,&#34; Ser. No. 239,991, filed Mar. 3 l. I972. serves to reduce the rate of crevice corrosion. Additionally. it has been found that where the gasketing material is a rubber compound substantially free of calcium, such as is disclosed in the commonly assigned. copending application of Donald W. DuBois and William B. Darlington for Suppression of Crevice Corrosion in Gasket of Titanium Crevices by the Use of Rubber Compound Gaskets Substantially Free of Calcium,&#34; Ser. No. 348,452. filed Apr. 5, I973. the rate of titanium crevice corrosion is reduced still further.  
 SUMMARY It has now surprisingly been found that crevice corrosion at titanium joints may be reduced even further if the titanium is kept anodic within all of the crevices.  
  According to an exemplification of this invention. an electrolytic cell is provided having a cell body containing an anolyte chamber fabricated of an anolyteresistant material. e.g.. titanium or the like. and a catholyte chamber fabricated of a catholyte-resistant material. having a gasketed joint between the anolyteresistant material of the cell body and the catholyteresistant material of the cell body. Within the gasket joint the surface of the anolyte-resistant material within the crevice is maintained anodic with respect to the surface of the catholyte-resistant material within the crevice.  
  According to a further exemplification. the surface of the anolyte-resistant material within the crevice is maintained at an electrical potential, with respect to the catholyteresistant material within the joint, between the passivation potential of the anolyte-resistant material and the anodic breakdown potential of the anoIyte-resistant material. In the case of titanium, this is found to be from about 2 to about 4 volts.  
  According to a preferred exemplification of this invention. the anolyte resistant material may be main&#39; tained anodic with respect to the catholyte-resistant material within the gasketed joint by extending the cathode to the crevice.  
 DETAILED DESCRIPTION The cell structure apparatus useful in carrying out the invention may be more clearly understood by refer ence to the appended FIGS.  
  FIG. 1 is a partially exploded perspective view of a bipolar electrolyzer having the crevice control structure of the invention.  
  FIG. 2 is a perspective partial cutaway view of an in dividual bipolar unit of the electrolytic cell shown in FIG. 1. viewed from the anodic side.  
  FIG. 3 is a perspective partial cutaway view of an individual bipolar unit of the electrolyzer shown in FIG. I viewed from the cathodic side.  
  FIG. 4 is a cutaway plan view of an individual cell from the top.  
  FIG. 5 is a cutaway elevation view of an individual cell.  
  FIG. 1 shows an electrolyzer 1 having individual cell units 11. Each individual cell unit has a cell body 21 with flanges 23 and 25 at each end thereof and a backplate 31 inside. The cathodes 51 extend from the cathodic side of the backplate 31 and include individ ual cathode fingers 53 and a cathode backscreen 55. Anodes 41 extend from the anodic side 33 of the backplate 31. Hydrogen is recovered from the catholyte chamber 57 through the hydrogen pipe 17 and chlorine is recovered from the anolyte chamber 43 through the chlorine pipe 19.  
  An individual cell unit is shown in partial perspective from the anodic side in FIG. 2 and from the cathodic side in FIG. 3. and in cutaway plan view in FIG. 4 and cutaway elevation view in FIG. 5. The individual cell unit 11 includes a cell body 21 having an anolyte cham ber 43 clad with an anolyte-resistant material 27 and a flange 33 extending therefrom clad with an anolyteresistant material 27. The cell body 11 also has a catholyte chamber 57 with a flange 23 of a catholyteresistant material.  
  The anolyte chamber 43 is separated from the catholyte chamber 51 by a backplate 31. The backplate 31 is fabricated of a catholyte-resistant material. and clad with an anolyte-resistant material to provide an ano lyteresistant surface 33. The anodes 41 extend from the anolyte-resistant surface 33 of the backplate 31 into the anolyte chamber 43. The cathodes 51, includ- 4 ing the cathodekfinge&#39;rs53and the cathode screen-55 extend from the catholytewesistant side 35 mfthe backplate 31. The area within the icathodcfingers 53 and between the cathode screen 55. andthe catholyteresistant surface 35 of the back-plate 3l-is the catholyte chamber 57.  
  The cell body is fabricated of a c&#39;atholyte&#39;resistant material. such as steel. Within the anolyte chamber 43. the cell body 21 is clad with an anolyte-resistant material 27 such as titanium. Within the catholyte chamber 57, cladding is normally not necessary.  
  The cell unit is joined to the next cell unit at joint 61 which includes the catholyte-resistant flange 23, the anolyte-resistant flange 27, and a gasket 71. According to an exemplification of this invention, the cathode backscreen 55 extends to the crevice 61 contacting the catholyte-resistant material of the flange 23 in the crevice 6] and the gasket 71.  
  The anolyte chamber 43 and those portions of the cell body in contact with anolyte are clad or lined with ananolyte-resistant metal. Normally the anolyteresistant metal is a film-forming metal. Film-forming metals are those metals which form a tough. adherent. protective oxide film when contacted with acidic media under anodic conditions. Film-forming metals include titanium. zirconium. hafnium, vanadium. columbium. tantalum. tungsten. and their alloys. Most commonly, titanium is used in the fabrication of electrochemical apparatus because of its lower cost.  
  In the design and assembly ofthe bipolar electrolyzer of this invention. replicate units of the bipolar electrolyzer are joined together at a joint or seal 61. This joint 61 is such as to provide a substantially electrolyte tight seal so as to avoid seepage of electrolyte out of the cell and onto the cell room floor. This seal 61 provides a crevice wherein the titanium within the crevice is subject to crevice corrosion.  
  In a preferred exemplification of this invention. there is no titanium to gasketing material to titanium crevice. The crevice 61 present in the cell body is a titanium to gasketing material to steel crevice. According to this invention. the titanium is maintained anodic while the steel is maintained cathodic. The anodic potential on the surface of the anolyte resistant material, i.e.. titanium, within the crevice is maintained between the passivation potential and anodic breakdown potential of the material. For titanium. this is normally from about 2.0 to about 4.0 volts and preferably from about 2.1 to about 3.8 volts.  
  As shown in FIGS. 4 and 5. the catholyte-resistant member 23 ofthe joint 61 is electrically in parallel with the cathode 51 of the cell and at the same potential as the cathode 51. In a preferred exemplification of this invention shown in the Figures. the cathode 51. and more particularly the cathode screen 55, extends to crevice 61 thereby providing both a cathodic potential on the catholyte-resistant member of the joint and an anodic potential on the anolyte-resistant member of the joint. The cathode 51 serves to provide throw of electrical current in the crevice. rendering the anolyteresistant material 27 on the flange 33 within the crevice 61 anodic. I  
  In still another exemplification of the intention. the cathode screen 55 extends to and into the crevice 61 thereby providing increased throw of electrical current onto the surface of the anolyte-resistant material 27 on the flange 33 within the crevice 6i. maintaining the anolyte-resistant material 27 within the crevice M anodic.  
  Particularly good results are obtained where the anolyte-resistant material is titanium alloyed with nickel. such as an alloy containing 2 percent nickel. as disclosed in US. Pat. No. 3,469.975 to Bertea et al.. the disclosure of which is hereby incorporated by reference. Additionally, for particularly satisfactory suppression of crevice corrosion. the titanium cladding 27 of the cell body 21 should be treated to reduce the surface iron inclusion content ofthe titanium. as disclosed in the commonly assigned copending application of Donald W. DuBois. Ser. No. 239,99l, filed Mar. 3l, 1972, the disclosure ofwhich is hereby incorporated by reference.  
  Still further reduction of the titanium corrosion within the crevice 61 may be obtained if the gasket 71 is characterized by the substantial absence of calcium such as is disclosed in the commonly assigned copending application of DuBois and Darlington for Suppression of Crevice Corrosion in Gasket of Titanium Crevices by the Use of Rubber Compound Gaskets Substantially Free of Calcium, Ser. No. 348,452, filed Apr. 5, l973. the disclosure of which is hereby incorporated by reference. Still further reduction of the crevice corrosion of the titanium in the crevice may be provided when the normal gasket pressure exerted by the gasket on the steel and titanium is maintained in excess of 300 pounds per square inch as disclosed in the commonly assigned copending application of Carl W. Raetzsch and Hugh Cunningham for &#34;Elimination of Anodes Stem Corrosion,&#34; Ser. No. 38l,l l3 filed July 20, I973 the disclosure which is hereby incorporated by reference.  
  Additionally, further reduction of crevice corrosion is provided when the anolyte-resistant material 27 within the joint 61 is titanium, and the surface of the titanium 27 within the joint 61 is protected from contact with electrolyte within the joint by the pres ence oftantalum on the surface of the titanium 27. The tantalum need be present only as a thin film or layer on the titanium, e.g., 1 micro inch or more. Greater thicknesses of tantalum, e.g., several one-thousandths of an inch or more, may be used, for example when the tantalum is applied to the titanium as a tantalum sheet or foil. The thickness of the tantalum depends on the method of applying the tantalum to the titanium 27. However, the tantalum layer or surface, when present, should be free of pinholes and surface imperfections.  
  The tantalum layer or coating may be applied by vacuum deposition, sputtering, vapor phase deposition, detonation cladding, forging or any other equivalent method of providing a tight seal between the titanium and the tantalum.  
  According to this further exemplification, the anolyte-resistant material within the crevice is a titanium clad 27 flange having an external surface, e.g., coating or layer of tantalum.  
  lt is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.  
 We claim:  
  1. In a bipolar electrolyzer having a plurality of bipolar units electrically and mechanically in series forming a plurality of individual electrolytic cells electrically and mechanically in series, at least one of said bipolar units comprising a backplate having an anolyteresistant surface on one side with a plurality of anodes extending therefrom and a catholyte resistant surface on the opposite side with a cathodic backscreen spaced therefrom and parallel thereto and a plurality of earth odes extending therefrom; and having flanged peripheral walls between adjacent cell units whereby an electrolytic cell is provided between the anolyte-resistantsurface of the backplate of a first bipolar unit and the catholyte resistant surface of the backplate of the next adjacent bipolar unit; the improvement wherein the said backplate is within the cell body formed by said peripheral walls whereby a single peripheral wall extends from a first flange at a first seal with a flange of the cell unit prior thereto, past the backplate to a second flange at a second seal with a flange of the bipolar unit subsequent thereto, said peripheral wall having a catholyte-resistant surface extending from said first flange within said first seal to said backplate, and an anolyte-resistant surface extending from said backplate to said second flange within said second seal. and wherein the said cathodic backscreen is substantially coplanar with the first flange of said peripheral wall whereby said cathodic backscreen extends to said first seal.  
  2. The bipolar electrolyzer ofclaim 1 wherein the cathodic backscreen is electrically in parallel with catholyte-resistant surface of the first flange within the seal.  
  3. The bipolar electrolyzer ofclaim 2 wherein the cathodic backscreen is in contact with the catholyteresistant surface of the first flange within the seal.  
  4. The bipolar electrolyzer of claim 3 wherein the cathodic backscreen extends into the seal.  
  5. In a method of operating a bipolar electrolyzer having a plurality of individual electrolytic diaphragm cells in series, wherein electrical current passes through a first backplate of an electrolytic cell to an anode of the cell, from the anode through an acidic anolyte to a cathode of the cell, and from the cathode to a second backplate ofthe said cell, wherein the interior surfaces of the cell exposed to catholyte are fabricated of a catholyte-resistant material and the interior surfaces of the cell exposed to anolyte are fabricated of an anolyteresistant material, and wherein a flange of the anolyteresistant material, a flange of the catholyte-resistant material and a gasket, form a crevice wherein the anolyte-resistant material is subject to crevice corrosion; the improvement wherein the flange of the anolyteresistant material extends from the said first backplate, the flange of the catholyte-resistant material extends from the second backplate, and the surface of the catholyte-resistant material within the crevice is maintained electrically in parallel with the cathode of the individual cell whereby the surface of the anolyte-resistant material within the crevice is maintained anodic,  
  6. The method of claim 5 comprising maintaining the surface of the anolyte-resistant material within the crevice at a potential between the passivation potential and the anodic breakdown potential thereof.  
  7. The method of claim 6 comprising maintaining the surface of the anolyte-resistant material within the crevice at an electrical potential of from about 2.0 to about 4.0 volts.  
 tends into the crevice.  
 10. The method of claim 5 wherein the anoiyteresistant material within the &#39;crevice is titanium.