US2721172A - Consumable metal anodes - Google Patents

Description

Oct. 18, 1955 w. F. HIGGINS ET AL.

CONSUMABLE METAL ANODES Filed July 5, 1951 Inventor William Frederick Higgins & William Godfrey Waite Aitornev United States Patent 0 CONSUMABLE METAL ANODES William F. Higgins, Salford, and William Godfrey Waite, Piccadilly, London, England, assignors to F. A. A. Hughes & Co. Limited, London, England, a British company Application July 5, 1951, Serial No. 235,214

Claims priority, application Great Britain July 12, 1950 2 Claims. (Cl. 204197) This invention relates to an improved construction for consumable metal anodes used in the galvanic protection of corrodible metal structures.

In the galvanic protection of buried metallic structures or structures immersed in an aqueous solution, consumable electrodes of a metal anodic to the metal of the structure are buried in the earth or immersed in the water near the structure and are connected thereto by electrical conductors. The resulting flow of current maintains the structure cathodic with respect to the soil or water and greatly arrests the corrosion. The consumable anodes hitherto used have consisted of blocks of strongly basic metal such as zinc or magnesium or base alloys of these metals with a plain surface which is intended to be consumed at a fairly even rate. Such anodes whilst effective, maintain a fairly constant current output throughout most of their lives.

It is common experience that the current generated is in certain cases greater than that required for adequate protection, and the rate of usage of the anode is consequently higher than is desirable. This is especially the case in sea water where the electrolytic resistance may be very low. In such circumstances the anode has a shorter life than might otherwise be the case.

It is possible to limit the current actually applied to the structure to be protected by means of suitable electric resistances inserted in the metallic circuit, but this is not an ideal method since it tends to reduce the useful current without materially affecting the rate of chemical reaction on the anode surface which gives rise to the current. Thus the use of resistances is a wasteful process.

The current generated by the anode is a function, among other factors, of the surface area of the anode exposed to the electrolyte and taking part in the chemical process resulting from contact of the anode with the electrolyte.

One obvious way of limiting the current, is to use anodes of small superficial area, but this method largely defeats its object since such small anodes would necessarily have a short life and would require to be replaced at unduly short intervals.

According to the present invention the current is controlled at the place of origin, that is, the interface between anode and electrolyte, by providing a coating on its surface which partly shields the anode surface from the electrolyte thereby reducing the rate of reaction of the anode with the electrolyte, said coating being made of a material which is substantially unattacked by the electrolyte.

The shielding means may be such that only a portion of the anode surface is exposed to reaction with the electrolyte at any time; or, alternatively such that all the surface, or substantially all the surface, is exposed, but in such a way that the access of electrolyte is restricted and the rate of current generation consequently reduced; or by a combination of both these methods.

It is possible by this arrangement to use a large anode containing a volume of material suflicient for long conice tinued protection and to ensure long continued protection by exposing the material of the anode regularly and progressively to the electrolyte. It will be shown later that it is possible also by a slight variation in technique. at the time of installing the anode, to secure an initial high rate followed, after a due period, by a smaller regular rate, a method which is often of considerable value in the art of galvanic protection.

In carrying the invention into effect the anode, (which may be of any convenient metal or alloy known to be suitable for the purpose of supplying galvanic current) according to one example, may be cast or wrought as a solid cylinder or bar, having a galvanised steel or other metal connection provided at one end. This connection is for the purpose of coupling the anode to the structure to be protected. This connection however is preferably not located deeply within the length of the anode but is confined to one end. The anode body itself, including the end bearing the connection, is sheathed in impervious and electrically insulating material with the exception of one area, conveniently, but not necessarily, the end remote from the connector. Thus the extreme end only of the anode is, in this case, presented to the electrolyte, and as the area of this end can be made much smaller than the total area of the anode, and in any desired proportion thereto, the current generated will be smaller in like proportion. Thus the operation of the anode is restricted so that it is consumed from the end only, and as the surface is used up in the reaction fresh metal is exposed progressively throughout the length of the anode until it is entirely consumed.

In experiments we have made, a cylindrical anode was constructed on the above described principle, of such dimensions that the area of the exposed end was onetenth of the total area. On immersion in sea water the current generated was found to be approximately onetenth of that generated by an identical anode in which the whole of the body was exposed. Consumption of the totally exposed anode was uniform over the whole surface, while that of the shrouded anode took place only on the exposed end and progressed steadily up the containing tube formed by the shroud. Since the two identical steel sheets to which the two anodes were severally coupled remained quite bright and free from corrosion it is obvious that the smaller current delivered by the shrouded anode was sufficient for galvanic protection of the steel and that the ten times larger current delivered by the unshrouded anode was abnormally wasteful.

If desired the shrouding of the anodes may be carried out in such a Way that the impervious wrapping can be readily stripped back from the surface to any degree desired, thus enabling the user to commence protection with a greater surface (and therefore a proportionately higher current) than would be obtained from the exposed end surface alone; later, when the uncovered length has been consumed, the end surface only will remain exposed, generating current at a rate proportional to its area. Thus the advantage will be secured of high initial current where desired, coupled with long life at a lower subsequent rate.

In the form of the invention described above the sheathing material is designed to remain in situ as the anode is consumed, so that access of the electrolyte to the end surface becomes gradually more restricted as the surface retreats down the tube. This is of no practical consequence because the diameter of the anode is so chosen that the restriction of circulation of fresh electrolyte down the gradually increasing length of sheathing tube is negligible.

In another form of the invention the sheathing takes the .form .of .an impervious .but somewhat .friable .material, sufiiciently strong to withstand handling when containing the anode, but capable of being gradually broken away byithe'iincreasing volume of the corrosion product of tthel'anode and "theaaction -of .sea water, as ;the anode iszdissolved away. In this form 'of the-device the reacti-vezendzofithe anode is alwayssp'resented to the :elec trolyte-zcompletelyunrestricted. A suitable material for the ipurpose :intended can 'be produced from l'bitumen or bitumen-like :compounds incorporating :'a :proportion of fillers such as powdered slate-orstone.

'0-In2another2form :ofithe invention, instead of a more or less fcompletely impervious sheath, an elastic -membranenis used, designed ;to be of a semipermeable or microporousmature. In this case vitzmay .be appliediover .thCftWhQlC iSuIfa-Cel.Of?fl16 anode (and limits the access of electrolyte byavirtue .of, and sin :proportion to, its permeability- The elastic :nature ;of the sheath :provides tfor-the timzased volume .due to :the corrosion .product of :the anode iits. r.eaction with .the electrolyte. :For this gummh fathsofzmicroporous rubber may be used, .or the same. (effect may {be achieved by ithe 1 application if certaiulpaint According to;a;s till further form ;of the invention the anode igproyidedwith a sheathofa metal-or alloy more noble-thandhe metahoralloy of which the anodeis made. For example :the sheath ;may' be made of aluminium nor aluminiumqbasealloy which ;may be of foil =thiCkness-. or maymavea thickness up to say one quarter of'an inch. If desired an :aluminium tube may have a molten magnesiumfalloy poured into it to formithe sheathedanode. We have .found that .a magnesium anode sheathed with aluminium or aluminium alloy with a metal 'to metal contact between'the magnesium alloy and the aluminium or aluminium alloy gives satisfactory ,results. If desired however anelectn'cally insulating coating e. g. of bitumen may be disposedbetween the two metals.

"The invention isillustrated by wayof example in the accompanying diagrammatic drawings wherein: I

Figure 1 is a perspective view of an anode made in accordance .with the invention;

Figure .2,is an underneath plan view thereof;

consists .of .a .cylindrical .hLock .10 .of magnesium .base

alloy having a steel rod 11 cast into its top end, and having a tubular coating 12 of' aluminium having a top wall 13 which encloses the upper surface of the block 10, this wall 13 having a centralihdle through which the rod 11 passes. Ihistubecanbe invertedand the said central hole closed'by a 'sand mould whilethe molten magnesium -.is pouredlinto it. The finished anode thus ;has the lower circular I'CIId surface .of the magnesium :alloy uncovered and this is the only area subject to the :electrolytic action. As shown in Figure 3 the magnesium alloy mayextendbelowihe lower end ,of the aluminium coating so that a larger area is initially exposed to elec- I trolytic attack'which is sometimes desirable in orderto provide a larger .current during an initial period until polarisation is effected whereafter a smaller current is desired. After the lower projecting endof the magnesium alloy has been consumed, only the I circular lower surface is subject to attack.

We claim:

'1. A consumable metal anode 'for the galvanic protection of metal structures comprising an anode block made essentially 'of magnesium-ormagnesium base alloys,

' left' free of 'the s heath,'saidsheath'being made essentially" .Figure 3 is ,a perspective view-of a modified formzof A a ferrous-metal' rod having one end embedded in-one end-0f said block-and extending outwardly therefrom, a protective sheath surrounding and contacting the outer surface of said block and covering the end face from which' 'the rodextendg the other end'of the block'being of-more=-noble aluminum or aluminum base alloys to reduce-the rate of-reaction of theanode-with the electroylte. 2. A-consumablemetal anode for galvanic protection of metal -structures =as defined-in claim 1 and said anode being free of the sheath 'for a small part of the outer surface adjacent to' and immediately abovethe end face which is-already free of the sheath.

:References-Citedin theafile of-this patent UNITED STATES PATENTS 685,176 Ross Oct. 22, L901 .1,50,6,3.06 Kirkaldy Aug. 26, 1924 .,2,157.,1.8O Little, May 9 1939 2,459,123 Bateset al. Jan. 11, 1949 7 1,489,739 Bialosky Nov. 29, 1949 2,54l;062

.Hoxeng Feb. 13, 1951

Claims (1)

1. A CONSUMABLE METAL ANODE FOR THE GALVANIC PROTECTION OF METAL STRUCTURES COMPRISING AN ANODE BLOCK MAKE ESSENTIALLY OF MAGNESIUM OR MAGNESIUM BASE ALLOYS, A FERROUS METAL ROD HAVING ONE END EMBEDDED IN ONE END OF SAID BLOCK AND EXTENDING OUTWARDLY THEREFROM, A PROTECTIVE SHEATH SURROUNDING AND CONTACTING THE OUTER SURFACE OF SAID BLOCK AND COVERING THE END FACE FROM WHICH THE ROD EXTENDS, THE OTHER END OF THE BLOCK BEING LEFT FREE OF THE SHEATH, SAID SHEATH BEING MADE ESSENTIALLY OF MORE NOBLE ALUMINUM OR ALUMINUM BASE ALLOYS TO REDUCE THE RATE OF REACTION OF THE ANODE WITH THE ELECTROYLTE.

Images