US3383297A - Zinc-rare earth alloy anode for cathodic protection - Google Patents

Description

y 4, 1968 E. EBEBIUS 3,383,297

ZINC-RARE EARTH ALLOY ANODE FOR CATHODIC PROTECTIpN Filed March 5, 1965 Sfeel Corrosive Z Inc/Rare Medium Earfh Alloy Ernsf Eberius INVENTOR.

Attorney United States Patent Ofice A 4 15 Claims. (Cl. 204-148 The present invention relates to cathodic protection against corrosion, and more particularly to the provision of cathodic protection for minimizing corrosion of metals such as steel and like ferrous materials.

It is Well known that steel structures are subject to attack by corrosion, particularly when placed in an aggressive environment such as in ground soil, seawater or the like.

One of the methods used to prevent such corrosion is to provide means affording cathodic protection whereby the metal to be protected is made the cathode in the corrosive electrolyte with which it is in contact and an active anode is provided. These active anodes are generally made of magnesium, aluminum or zinc and are in electrical contact with the metal to be protected. These may be anodes buried in the ground for protec tion of underground pipelines or applied as attachments to surfaces of equipment such as condenser water boxes or on ships hulls. The anti-corrosion electrical current required is generated by electrochemical attack upon the anode material. These anodes, taking into consideration the common surface areas, are placed in short-circuit contact with the iron or other ferrous structures to be protected.

It is known that during the generation of current a concentration polarization of the anode surface occurs, which by transition into a covering polarization results in a more or less considerable loss of current from the anode. In order to minimize this inactivation, certain components have been alloyed with a zinc anode, including aluminum, cadmium and mercury. Despite, or to some extent because of, the activity of the anode, reaction products form on the surface thereof by reaction with constituents of the corrosive environment, for example seawater; these recation products mechanically coat the anode surface in the form of solid crusts. Such cementlike layers consist, when zinc anodes are used, for example, in sea water, of compounds of zinc with complex-forming substances naturally present among the salts of the seawater.

It is accordingly a primary object of the present invention to provide anodes for cathodic protection against corrosion which are characterized by minimum polarization effects and which also preclude the formation of cement-like scale on the anode surfaces.

It is another object of the present invention to provide an improved electrolytic-cell arrangement whereby metal used for construction purposes can be protected from corrosion.

With these and other objects in view, the present invention involves the use, as an anode to provide cathodic protection of metals subject to corrosion, of a zinc-alloy anode consisting essentially of metallic zinc and a rareearth alloying component which constitutes at least about 0.02% by weight of the body. The alloying component 3,383,297 Patented May 14, 1968 should consist of at least 35% by weight lanthanum and can be selected from the group consisting of lanthanum a combination of lanthanum with up to 50% of cerium and mischmetal.

Since quantities of the rare-earth alloying component above about 5.0% weight of the body do not provide additional improvement in the alloy for the purposes of the invention, and since the alloying substance is more expensive than zinc, as a practical matter the amount of alloying substance should not exceed about 5.0% by weight. However, it is to be understood that this is a practical maximum and that higher amounts are operable in accordance with the invention.

It has been found that the alloying of zinc with lanthanum alone, lanthanum and cerium combinations or with mischmetal, which is an alloy of lanthanum with cerium and other rare earths such as neodyium and praseodymium (e.g. 35% by weight lanthanum, 15% by weight of a mixture of neodymium and praseodymium and 50% by weight cerium) affords considerable activation of the anode, as compared to the use of zinc and conventional zinc alloys. The use of anodes made of such materials provide a significant reduction in the usual polarization inhibition of corrosion as well as in the covering effect, and simultaneously provide a high activity constant for the anode surface.

Comparative tests and measurements of high-grade zinc (99.999% purity) and smelter zinc (98.5% purity) alloyed, on the one hand, with mischmetal, and nonalloyed zinc anodes, on the other hand, show considerable advantages for the mischmetal-alloyed zinc anodes in comparison to the non-alloyed zinc anodes. It was found that the mischmetal-alloyed anodes, both during continuous operation and during interrupted operation, over a long period of time in synthetic seawater exhibited no marked reduction in potential, and in addition, the driving potential remained approximately constant over the electrochemical capacity of the anode.

The capacity and potential remain constant according to the present invention because the surfaces of the anodes of the invention do not become coated with solid, cementlike layers, but instead there is formed on the anodes a thin, brittle film which falls off itself, or a leafy film which is easily washed oft with water, so that the cf.- ficiency-determining interfacial contact between the clear anode metal and the surrounding electrolyte always remains constant.

The above and other objects, features and advantages of the present invention will become more readily apparent from the following examples and the accompanying drawing, the sole figure of which diagrammatically shows how a structural element is protected from corrosion by means of the anode of the present invention.

In the drawing there is shown a metal structural element 10 made, for example, of steel which is embedded in the ground 11. The earth includes salt-containing water 13 which serves as an electrolyte. There is connected to be structural element 10 an anode 12 which is made of pure zinc alloyed with 0.5% by weight of mischmetal consisting of 35% lanthanum, 10% neodymium, 5% praseodymium and 50% cerium. The body 12 serves as an anode while the body 10 serves as the cathode, the liquid 13 in the ground 11 acting as electrolyte; the corrosion on the meal body 10 is thus minimized.

3 Example I An anode for the cathodic protection of a steel pipe embedded in the ground was prepared by adding to 98.5% purity zinc in a molten state about 0.02% lanthanum. The body, after casting, was applied to the pipe and in surface contact therewith. It was found that the anode gave the cathodic protection of a conventional zinc anode but was substantially free from encrustation and scaling and functioned etlectively for a considerably longer period than did the ordinary zinc anode.

Example II The test of Example I was carried out on the steel hull of a ship with cerium substituted for half of the lanthanum of Example I. Again no cement-like scale was found.

Example III The test was repeated with substantially 0.02% by weight of mischmetal serving as the rare-earth alloying component. Again scaling was found to be of the thinlayer type readily sloughed off by the anode. The mischmetal consisted of 35% by weight lanthanum, 10% by weight neodymium, 5% by weight praseodymium and 50% by weight cerium.

Example IV The rare-earth components of Example I through III were alloyed with high-grade zinc (purity of 99.999% purity) in three separate tests with buried iron pipe. In each case, efiec ive cathodic protection was observed without the development of hard and inpenetrable scales upon the anode surface.

Example V The alloys of Examples I through IV were formed into anodes applied to the steel hull of a ship as a cathodic protection against corrosion in seawater. In each case the effective protection was obtained with a noticeable decrease in polarization phenomena and in the absence of significant scaling of the anode.

Example VI The tests of Examples 1 through V were carried out except that the rare-earth component in each alloy constituted 5% by weight thereof. Similar results were obtained. With alloys containing more than 5% by weight of the rare-earth component, no further increase in the antipolarization and antiscaling phenomena were observed.

What is claimed is:

1. A zinc alloy consisting essentially of metallic zinc and from 0.02% to substantially 5% by weight of a rareearth component consisting of at least one rare-earth element.

2. A zinc alloy consisting essentially of metallic zinc and from 0.02% to substantially 5% by weight of a rareearth component, said component being constituted by at least 35% by weight of lanthanum and at least one additional rare-earth element.

3. A zinc alloy consisting essentially of metallic zinc and from 0.02% to substantially 5% by weight of a rareearth component, said component being constituted by at least 35% by weight of lanthanum and at least one additional rare-earth element selected from the group consisting of neodymium, praseodymium and cerium.

4. A zinc alloy consisting essentially of metallic zinc of at least 98.5% purity and from 0.02% to substantially 5% by weight of a rare-earth component, said component being constituted by at least 35% by weight of lanthanum and at least one additional rare-earth element.

5. A zinc alloy consisting essentially of metallic zinc of at least 98.5% purity and from 0.02% to substantially 5% by weight of a rare-earth component, said component being constituted by at least 35% by weight of lanthanum and at least one additional rare-earth element selected from the group consisting of neodymium, praseodymium and cerium.

6. An alloy as defined in claim 5 wherein said component includes up to substantially 50% by weight thereof of cerium.

7. An alloy as defined in claim 6 wherein said component consists essentially of 50% by weight lanthanum and 50% by weight cerium.

8. An alloy as defined in claim 5 wherein said component is mischmetal.

9. An anode body for cathodic inhibition of corrosion composed of a Zinc alloy consisting es entially of metallic zinc and from 0.02% to substantially 5% by Weight of a rare-earth component, said component being constituted by at least 35% by weight of lanthanum and at least one additional rare-earth element.

10. An anode body for cathodic inhibition of corrosion composed of a zinc alloy consisting essentially of metallic Zinc of at least 98.5% purity and from 0.02% to substantially 5% by weight of a rare-earth component, said component being constituted by at least 35% by weight of lanthanum and at least one additional rareearth element selected from the group consisting of neodymium, praseodymium and cerium.

11. A corrosion-resistant assembly comprising a metal cathode in contact with an electrolyte and corrodable thereby; and an anode electrically connected wi h said cathode and in contact with said electrolyte, said anode being composed of a zinc alloy consisting essentially of metallic zinc and from 0.02% to substantially 5% by weight of a rare-earth component, said component being constiiuted by at least 35% by weight of lanthanum and at least one additional rare-earth element.

12. A corrosion-resistant assembly comprising a ferrous-metal cathode in contact with an e'ectrolyte and corrodable thereby; and an anode electrically connected with said cathode and in contact with said electrolyte, said anode being composed of a zinc alloy consisting essentially of metallic zinc and from 0.02% to substantially 5% by weight of a rare-earth component, said component being constituted by at least 35% by weight of lanthanum and at least one additional rare-earth element.

13. A corrosion-resistant assembly comprising a ferrous-metal cathode in contact with an electrolyte and corrodable thereby; and an anode electrically connected with said cathode and in contact with said electrolyte, said anode being composed of a zinc alloy consis'ing essentially of metallic zinc of at least 98.5% purity and from 0.02% to substantially 5% by weight of a rare-earth component, said component being constituted by at least 35% by weight of lanthanum and at least one additional rare-earth element selected from the group consisting of neodymium, praseodymium and cerium.

14. In the cathodic corrosion inhibition of a ferrousmetal body in contact with an electrolyte, the improvement which comprises the step of disposing a zinc-alloy body in electrically conductive relationship with said ferrous-metal body and in contact with said electrolyte, said body being composed of a zinc alloy consisting essentially of metallic Zinc and from 0.02% to substantially 5% by weight of a rare-earth component, said component being constituted by at least 35% by weight of lanthanum and at least one additional rare-earth element.

15. In the cathodic corrosion inhibition of a ferrousmetal body in contact with an electrolyte, the improvement which comprises the step of disposing a zinc-alloy body in electrically conductive relationship with said ferrous-metal body and in contact with said electrolyte, said body being composed of a zinc alloy consisting essentially of metallic Zinc of at least 98.5% purity and from 0.02% to substantially 5% by weight of a rare-earth component, said component being constituted by at least 35% by weight of lanthanum and at least one additional rare-earth element selected from the group consisting of neodymium, praseodymium and cerium.

(References on following page) 5 6 References Cited OTHER REFERENCES UNITED STATES PATENTS Chiotti et 21]., Trans. of the Metallurgical Soc. 0 Z 829 973 4/1958 J l 75 178 1 AIME, vol. 227, August 1963, pp. 910916. essup et a.

Sakano et 1 I 5 S. Przmary EXGIHZHGI'.

3,254,993 6 r an 8t 1- 5-1 -1 T. TUNG, Assistant Examiner.

Claims (1)

1. A ZINC ALLOY CONSISTING ESSENTIALLY OF METALLIC ZINC AND FROM 0.02% TO SUBSTANTIALLY 5% BY WEIGHT OF A RAREEARTH COMPONENT CONSISTING OF AT LEAST ONE RARE-EARTH ELEMENT.

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