US3278404A - Method and apparatus for cathodic protection - Google Patents

Description

Oct. 11,1966 J. B. COTTON ETAL 3,

METHOD AND APPARATUS FOR CATHODIC vPROTECTION Filed July 16, 1958 //V VENTORS N4 m a: 75 NM f 5 0 m 031, Ma fl Nu i B, M m JR fl United States Patent M 3,278,404 METHOD AND APPARATUS FOR CATHODIC PROTECTTON Joseph Bernard Cotton, Sutton Coldfield, and Arthur Harold Barber, Walsall, England, assignors to Imperial Chemical Industries Limited, London, England, a corporation of Great Britain Filed July 16, 1958, Ser. No. 748,938 Claims priority, application Great Britain, July 17, 1957, 22,619/57 2 Claims. (Cl. 204-147) This invention relates to electrode structures suitable for use in a variety of electrochemical processes including electrolytic processes and the cathodic protection of metal structures.

According to the invention an electrode structure comprises a support of which the surface consists of titanium or a titanium base alloy having anodic polarisation properties comparable to those of pure titanium and having in contact with part or all of said surface a platinum group metal or alloy constituted by platinum group metals. It Will be understood that the platinum group of metals consists of those metals occurring in Group 8 and Periods 5 and 6 of the Periodic Table. As examples of the support material commercially pure titanium and a titanium-zirconium alloy containing 5% by weight of the latter may be mentioned. The platinum group metal may be for example pure platinum, alloys of platinum and palladium containing 10% or 20% of the latter, pure rhodium, platinum-rhodium and platinum-iridium alloys or osmiumiridium alloys.

The platinum group metal need only make good electrioal contact with the titanium for successful operation but in practice it will be preferred to attach the platinum group metal in secure manner to the titanium in order to resist the stresses of the environment in which the electrode is to be used.

A variety of methods of attachment of the components together are available, the particular methods being selected in accordance with cost requirements, general convenience, and also the purpose for which the electrode is to be used. The optimum method for any set of circumstances may readily be chosen after simple experiments. Usually the required key or bond between the titanium and the platinum group metal will be achieved by electroplating and platinum-plated titanium electrodes will give very good performance under various conditions. A satisfactory titanium surface for electroplating is produced by pretreatment with concentrated hydrochloric acid.

As an alternative to an electroplated layer, the platinum group metal may be in the form of bright sheet, wire, foil or pieces pressed, spot-welded, bolted or riveted on to the titanium. The platinum group metal may be used in powder form and attached to the titanium by rolling into the titanium surface. For certain applications a platinum black coating will be preferable because of its low overvoltage properties.

Other methods of attachment of the platinum group metal are by metal spraying, Electroless plating, thermal decomposition of an applied paint film, spark transfer, sputtering or evaporating. The surface character of the platinum group metal will be chosen in accordance with the overvoltage requirements of the gaseous species being discharged at the electrode.

Typical electrodes of the invention are prepared as follows. The titanium or alloy thereof in whatever wrought form required is etched for 4 hours at 60 C. in pure concent-rated hydrochloric acid as described in British patent specification No. 758,013, and rinsed, cleaned and then electroplated. Alternatively the titanium may be im- 3,278,404 Patented Oct. 11, 1966 ICC mer-sed for 3 days at room temperature in pure hydrochloric acid to prepare its surface. The electroplating bath will contain sodium hexahydroxy-platinate or rhodium sulphate solution in the case of platinum or rhodium respectively and plating will be conducted under conventional conditions to give a coating of the required thickness. For practical purposes a coating of 0.000 1 inch will be suitable to combine current density requirements with a useful working life of the electrode but in many cases thinner coatings, e.g. 0.00001 or 0000005 inch may be satisfactory. If only part of the surface is to he plated then stopping-off materials will be employed.

An important application of electrode structures according to this invention is as anodes in an eletcrical system. The electrodes may be used as anodes in a variety of electrolytes notably those in which the reaction at the anode involves the liberation of oxygen or chlorine. The electrodes make use of the strength and low weight of titanium and its high corrosion resistance in a wide range of media.

The electrodes permit the use of very high current densities, for example, at 1000 amperes per square foot of true surface area of the platinum group metal. There will of course be an upper limit to the current density which may be employed in any particular case and this is due to the fact that at extremely high current densities appreciable polarisation of the platinum group metal may occur; this in turn may render any exposed titanium more anodic than it would be at lower current densities and at a sufiiciently high voltage it may cause anodic dissolution of the exposed titanium. A further limitation in the use of electrodes of this invention in electrolytic cells is that it is necessary to avoid the use of alternating current or direct current carrying an appreciable proportion of superimposed alternating current or having any appreciable ripple effect. Experience indicates that direct current with up to 5% alternating current ripple is acceptable.

It will be understood from the foregoing that the support constitutes the main bulk of the electrode upon which the active component, i.e. the platinum group metal is disposed. In effect, therefore, the electrodes offer the advantages of solid platinum without incurring the high coat of solid paltinum. It is not necessary that the bulk of the electrode consists entirely of titanium or its alloy and, if desired, an inner core of another metal may be used.

The electrodes can be of a flexible form adapted to be conformed to the requirements of cell design by bending, e.g. flexible meshes or grilles of the form required. Such structures can be produced with expanded titanium mesh.

The invention will be further described with reference to particular applications of the electrodes.

(i) The cathodic protection of a steel jetty standing in estuarine water and presenting an area of approximately 15,000 square feet of metal may be effected by means of four anodes constructed in titanium and carrying electroplated coatings of platinum. The arrangement of anodes is shown in the accompanying drawing. The anodes 1 are in the form of rods 2 feet in length and /8 inch in diameter and each carries a layer of platinum 0.0001 inch thick. The anodes are mounted in a wooden frame 2 and are each tapped at one end for the admission of a screw terminal 3 to secure the current lead 4 to the electrode. The connection between the current lead 4 and the electrode is held in bitumen and the four leads are combined and connected to the positive pole of a source of direct current. No special electrical apparatus is required and conventional types of rectifier unit may be employed. These anodes operate at .anodic current densities varying between 70 amperes per square foot at high tide and 48 amperes at low tide. These current density ratings compare very favourably with those obtained from conventional anode materials such as graphite and lead which operating under optimum conditions normally function at only 12 amperes per square foot and -15 amperes per square foot respectively.

Smaller scale experiments show that rhodium may be used in place of platinum to give comparable results in the above example.

Cathodic protection may also be effected according to the invention in other waters of suitable conductivity. The cathodic protection of iron or steel pipelines carrying corrosive media is very conveniently effected by means of anodes in wire form, e.g. mounted c-oaxially inside the pipe. In general, for cathodic protection installations, an electrode having a low oxygen overvoltage is desirable in order to reduce the electrical energy consumed. For this application, the surface of the noble material should be as active as possible and this is achieved with matt or finely divided deposits.

(ii) For the electrolytic oxidation of sulphuric acid to form persulphuric acid, the anodes are conveniently in the form of titanium rod having wire of the platinum group metal spot-welded to its surface. A high oxygen overvoltage is desirable in certain electrolytic oxidation processes thereby achieving maximum oxidation efiiciency. Accordingly, in this application the surface of the noble metal should be bright.

(iii) In the electrolytic pickling of stainless steel sheet in 10% aqueous nitric acid at a current density of 200 amperes per square foot, platinum-plated titanium electrodes are markedly superior to silicon iron and stainless steel electrodes both from the point of view of rate of loss of metal and general performance.

(iv) In the electrodialysis of brackish waters platinised titanium electrodes may be used with advantage in place of both magnetite anodes and stainless steel cathodes. In contrast to magnetite anodes and stainless steel cathodes, no cell contamination problems are encountered where platinised titanium is employed for both anodes and cathodes whilst overall electrical efficiency is substantially the same.

(v) Platinum-plated titanium anodes may be used to replace graphite in the production of sodium hypochlorite by the electrolysis of solutions of sodium chloride. The current efficiency of the electrodes is greater than graphite electrodes when the process is worked to produw 2.5% available chlorine in the electrolysed solution. The working voltage of the anode more nearly approaches the theoretical decomposition voltage than graphite and the energy efficiency of the process is greater than that with graphite anodes. In contrast with pure platinum anodes, replacement costs of the platinised titanium anodes are small and there is no need for frequent adjustment oi voltage control to compensate for wear as with graphite anodes.

In addition to the above examples of uses of electrodes according to the invention, they may be used for electrolytic oxidation processes involving the production of nascent oxygen, for example, to reduce the oxygen demand of various industrial efiluents.

We claim:

1. A process for cathodically protecting a metal structure in contact with corrosive aqueous media which comprises passing current through an electrical system which includes a current source having a negative terminal connected to said structure and a positive terminal connected to an anode positioned in said media and insulated from said negative terminal, said anode comprising a support selected from the group consisting of titanium and titanium base alloys having anodic polarization properties essentially the same as those of titanium in said media, said support having an operative surface consisting essentially of at least one member of the group consisting of platinum, rhodium and platinum and rhodium base alloys in which all of the constituents are platinum group metals.

2. A cathodic protection system for protecting a metal structure in contact with corrosive aqueous media which comprises a current source having a negative terminal connected to said structure and a positive terminal connected to an anode positioned in said media and insulated from said negative terminal, said anode comprising a support selected from the group consisting of titanium and titanium base alloys having anodic polarization properties essentially the same as those of titanium in said media, said support having an operative surface consisting essentially of at least one member of the group consisting of latinum, rhodium and platinum and rhodium base alloys in which all of the constituents are platinum group metals.

References Cited by the Examiner UNITED STATES PATENTS 1,477,099 12/ 1923 Baum 204290 2,631,115 3/1953 Fox 204290 2,636,856 4/ 1953 Suggs et a1. 204-290 2,719,797 10/ 1955 Rosenblatt 204290 2,734,837 2/1956 Hands 204-32 2,795,541 6/1957 Muller 204-290 2,863,819 12/1958 Preiser 204196 2,873,233 2/1959 Schnable 20432 FOREIGN PATENTS 904,490 2/ 1954 Germany. 236,579 6/ 1945 Switzerland.

JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, Examiner.

T. TUNG, Assistant Examiner.

Claims (1)

1. A PROCESS FOR CATHODICALLY PROTECTING A METAL STRUCTURE IN CONTACT WITH CORROSIVE AQUEOUS MEDIA WHICH COMPRISES PASSING CURRENT THROUGH AN ELECTRICAL SYSTEM WHICH INCLUDES A CURRENT SOURCE HAVING A NEGATIVE TERMINAL CONNECTED TO SAID STRUCTURE AND A POSITIVE TERMINAL CONNECTED TO AN ANODE POSITIONED IN SAID MEDIA AND INSULATED FROM SAID NEGATIVE TERMINAL, SAID ANODE COMPRISING A SUPPORT SELECTED FROM THE GROUP CONSISTING OF TITANIUM AND TITANIUM BASE ALLOYS HAVING ANODIC POLARIZATION PROPERTIES ESSENTIALLY THE SAME AS THOSE OF TITANIUM IN SAID MEDIA, SAID SUPPORT HAVING AN OPERATIVE SURFACE CONSISTING ESSENTIALLY OF AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF PLATINUM, RHODIUM AND PLATINUM AND RHODIUM BASE ALLOYS IN WHICH ALL OF THE CONSTITUENTS ARE PLATINUM GROUP METALS.

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