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3,393,138 United States Patent Office Paterated July 16, 1968 2 cathodic protection of iron or steel, since (at theoretically 3,393,138 perfect efficiency) one ampere hour is provided by the ALURANURM ALLY ANODE AND METHOD OF USING SAME IN CATH ODEC PROTECTION dissolution of 0.335 gram of aluminum as compared with Roy A. Hine, Banbury, England, assignor to Aliminitia 0.454 gram of magnesium, so that, other things being LaboraÉories Linied, Montreal, Quebec, Canada, a equal, it might be expected that aluminum would have a comparay of Canada wide application for the production of anodes for cathodic No Drawing. Original application May 1, 1962, Ser. No. protection. The difficulty however, lies in the fact that 19,430. Divided and this application Apr. 7, 1965, Ser. aluminum of commercial purity is not sufficiently or con No. 446,426 sistently electronegative to serve as a sacrificial anode for Claims priority, application Great Britain, May 11, 1961, cathodic protection against the effects of sea water. 17,277/61 O Magnesium undoubtedly gives very satisfactory protec 2 Clairas, (C. 204-148) tion and maintains the steel or iron at a high negative potential. Owing to the unduly-negative electrode poten tial of magnesium a strong galvanic current still flows ABSTRACT OF THE DISCLOSURE even when the steel has been polarised to a protective (1) A sacrificial aluminum anode for the cathodic pro level, with the result that anode material is wasteful and tection of a metal body in sea water, and the method of uneconomically consumed. protection using this anode, consisting essentially of tin, It will be appreciated that the greatest economy of 0.01-2%; gallium, at least 0.005%, certain specified pos anodic material will be achieved if the negative potential sible impurities; and the balance aluminum. 20 of the cathodically protected metal is kept down to a (2) A sacrificial aiuminum anode for the cathodic pro figure which is not unduly in excess of that required to tection of a metal body in sea water, and the method of protect it from corrosion. protection using this anode, consisting essentially of tin, Zinc and aluminum-zinc alloys have less negative elec 0.01-2%; zinc, 0.5-10%; certain possible impurities; and trode potentials than magnesium and they are therefore the balance aluminum. 2 5 more economic in use, especially when consideration is given to their electrical output in terms of ampere-hours in relation to cost of metal. However, in a number of ap plications it is essential that the polarisation of the steel The instant application is a division of application Se to a protective potential shall be brought about in as short rial No. 191430, filed May 1, 1962, now U.S. Patent No. 30 a time as possible; such situations arise, for example in 3,321,305. ships' ballast tanks (oil tankers, ore carriers, etc.) where The present invention relates to the protection of iron the bailast water may only be present for short and inter and steel structure against corrosion by contact with Sea mittent periods. During these periods of intermittent im water by the employment of sacrificial metal anodes for mersion the anode must rapidly bring the steel structure the cathodic protection of metal structures. 3. 5 to protective potential and moreover should cause the It has long been known that the corrosion of metal, for electrochemical deposition of calcium and magnesium example, iron and steel, my contact with an electrolyte can compounds on the Steel cathode as an adherent film be largely or completely prevented by making the metal which serves to protect the steel when not immersed in cathodic by electrically connecting a suitable anode metal sea-water. The less negative potential of zinc and alu to it. 40 minum-zinc alloys results in a rate of polarisation of the One of the principal applications of cathodic protec steel which is too low for intermittent immersion applica tion is in the case of iron and steel subjected to corrosion tions, unless masses of anode material having unduly by contact with sea water. Thus the hulls of ships, the large Surface areas are used or unless unduly large num storage tanks of tankers which may be filled with Sea bers of individual anodes are mounted at relatively small water as ballast and stationary steel structures, such as intervals over the steel surface. For this reason mag jetties, may all be protected in this way to prevent or re nesium anodes have been favoured for protection of duce the corrosion of exposed portions of the metal. ballast tanks etc., despite the disadvantage of wasteful For use as a sacrificial anode the anode metal must be consumption. given a suitable shape then electrically connected with In the selection of a material for use in a sacrificial and situated in close proximity to the iron or steel part anode, the initial negative potential provided by the anode to be protected. However, many shapes and constructions is not the only quality which has to be considered. The of sacrificial anodes are already known for this purpose. potential must also be maintained at a satisfactory level The anode metal is usually cast around an iron or Steel over a long period, so that the anode does not have to be core, by means of which it is secured to the iron or steel discarded prematurely. to be protected. The selection of a suitable shape presents 5 5 In experimental work we have found that aluminum, no problems to one skilled in the art and the shape and containing the usual commercial impurities within cer construction of sacrificial anodes forms no part of the tain ranges becomes markedly more electronegative by present invention. being alloyed with a small amount of tin, (in excess of It is generally accepted that steel or iron will not rust 0.01%). The electronegative character of this alloy would in sea water if its potential (expressed relative to a stand 60 appear to make it suitable for use in a sacrificial anode for aird copper-copper Sulphate electrode) is more negative cathodic protection of steel and iron against sea water than -0.85 volt. Steel immersed in Sea water without corrosion, but corrosion tests conducted with aluminum cathodic protection usually has a potential of about tin alloy containing 0.2% tin were disappointing, as it was -0.6 volt. found that the negative potential of steel sheets employed The maintenance of the steel or iron at a negative po 65 in the test fell in about two months to a voltage of about tential which will prevent corrosion may be achieved by -0.83 volt and this figure was not sufficiently electronega connecting it electrically to a sacrificial anode of a more tive for wholly effective cathodic protection of iron and electro-negative metal. Magnesium or a magnesium-base steel structures against sea water corrosion, as was evi alloy has been used to form an anode for this purpose, denced in the test by the formation of a thin black oxide as also has zinc and aluminum-zinc alloys. 70 film (though without red rust) on further immersion of Aluminum is theoretically a better metal than mag one year, during which the negative potential remained nesium to employ in the construction of an anode for substantially constant. 3,393,138 3. 4. In particular the potential developed was not negative um is so more than an inconsequential impurity. When enough to permit anodes of this material to be used in Zinc is present only as impurity, the preferred level of the ships ballast tanks. then essential element gallium on the other hand is in ex It has been found that there are several factors which cess of 0.015% and in fact this amount of gallium may be have to be taken into consideration before a commercially found in selected commercial available 99.8% purity acceptable anode can be prepared from aluminum tin aluminum. alloys. Additional impurities such as boron, which may be add Firstly the alloy must have a structure such that the tin ed in very small quantities for grain refinement purposes, is held as a fine precipitate dispersed throughout the alloy may be tolerated. matrix and not in the form of localised particles in the A binary alloy of aluminum and tin (without the added grain boundaries. O Zinc content) prepared in accordance with the present in The desired structure can be achieved by the direct chill vention has an electrode potential intermediate between casting process, in which the molten metal is rapidly that of zinc and that of magnesium; this potential is suffi cooled and solidified by passing the metal continuously ciently negative to enable an anode of the alloy to bring a through a chilled neck, so that during solidification there 5 steel structure rapidly and effectively to a protective po is insufficient time to permit the tin to migrate to the grain tential, but not so unduly negative that the anode material boundaries. An iron or steel core is preferably cast into is wastefully consumed. The following table shows the the metal at the same time. Alternatively the tin may be electrode potential of the aluminum-tin alloy with respect dispersed through the aluminium by a heat treatment after to other materials, the potentials being measured in sea the alloy has been cast in a permanent mould about an 2. Water against a copper/copper Sulphate auxiliary elec iron or steel core. The heat treatment may comprise a trode. solution heat treatment at 500-530 C.