US2805198A - Cathodic protection system and anode therefor - Google Patents

Description

Sept- 3, 1957 H. A. ROBINSON ET A1. 2,805,198

CATHODIC PROTECTION SYSTEM ANDYANODE THEREFOR Filed Feb. 29, 1956 mmw N k0 T.

QQQN uw tou United States Patent CATHODIC PROTECTIN SYSTEM AND ANUDE THEREFOR 1 Harold A. Robinson, Midland, Mich., and .lohn J. Newport and Oliver Osborn, Lake Jackson, Tex., assigner-s to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application February 29, 1956, Serial No. 568,6i9

8 Claims. (Cl. 204-197) This invention relates to a cathodic protection anode formed of a galvanically active metal and to a cathodic protection system wherein such a protection anode is employed. Y

Cathodic protective anodes have been used for many purposes, such as preventing corrosion of structures in sea water, pipelines buried in the soil and internal areas of hot water heaters. For the most part, these anodes are made of readily expendable magnesium which has been alloyed with other metals to give the anode properties which particularly adapt it for its intended use. For example, cell magnesium has a satisfactory solution potential, but a poor current eciency. Certain magnesium alloys have improved current eticiencies, but lower solution potentials than cell magnesium. Consequently, more alloy anodes are required to protect a given structure, but under predetermined conditions of use these anodes will have a longer useful life. Offsetting to a certain extent some of the advantages of these anodes made from magnesium alloy is the cost of the additional process steps and alloying ingredients.

It is an object of this invention to provide an improved cathodic protection anode composed essentially of magnesium and containing minor amounts of manganese and aluminum present in a predetermined ratio to each other. Another object of this invention is to provide an improved system for cathodically protecting structures immersed in media in which they are subject to corrosion. Still another object of this invention is to provide a process for making an improved cathodic protection anode directly from the melt in cell in which the magnesium metal is rst prepared.

It has been discovered that an improved cathodic pro tection anode is obtained by forming the anode from a magnesium base alloy containing from 0.50 to 1.3 percent by weight manganese and not more than 0.010 percent by weight aluminum. A preferred method of making these anodes is to add a suitable manganese cornpound to the feed of a magnesium electrolytic cell, and the preferred composition comprises from 0.50 to .80 manganese and not over 0.005 aluminum (in weight percent), with the balance essentially magnesium. When such anodes are employed in a system for cathodically protecting structures composed of a metal less anodic than'magnesium, they have a greatly improved current eiciency combined with an unusually high solution potential. These systems are thus advantageous in that they'require fewer anodes and have a longer life than cell magnesium anodes, for example.

The present invention is useful in providing cathodic protection for structures immersed in sea water, and can be vused under some conditions as a protective anode in hot water heaters. Yet anodes and systems comprisly fall between 200 and 300 hours per pound. Anodes at a given voltage or potential. Consequently, .an anode. metal having relatively low solution potential is desir- L able to avoid useless current generation which is in turn accompanied by rapid consumption of the anode metal.

When the structure to be protected is buried in high resistance soil, this high resistance so Vreduces the current owingthrough the soil from the anode to the corrodible structure thatV a high voltage or anodesolution potential is needed to adequately protect the stiucture. The all-around adaptability of the anode and the cathodic protective system comprising the present invention ismanifested Vby the, fact that it retains its high current eiciency and solution potential in sea water, salinersoils and non-saline soils where the anode usually operates in a sulfate rich backfill.

Anodes comprising the present invention have the following composition in percent by weight:

Magnesium; At least l98.5. Manganese 0.50 to 1.3. Aluminum Not over 0.010. Iron .001 to 0.03. Tin Not over 0.01. Nickel Not over .002. Lead Notover 0.01. Other metals (each) Not over 0.05.

In the electrolytic production of magnesium, the molten magnesium in the electrolytic cell is referred to as cellmagnesium. Essentially, cell kmagnesium is pure magnesium, but certain adventitious elements are present in small amounts. In .thel production of anodes comprising the present invention, amounts of these adventitious elements must be controlled. Iron is present in at least 0.001 percent by weight and must be limited to a maximum of 0.03 percent if an anode of the desirable characteristics of the present invention is to be obtained. Nickel is restricted toy not over 0.002 .percent by weight and the tin and lead Vshould rnot exceed 0.01 percent. The other metals present do not exceed 0.05 percent each. The total of all such adventitious elements does not exceed 0.2 percent in the cell magnesium.

Aluminum is always present in small amounts in cell magnesium and will vary from 0.0001 up to 0.02 weight percent. Although it has previously been recognized that high percentages of iron were detrimental in anodes used for cathodic protection, little if any attention was directed to the amount of aluminum in the cell magnesium.

' amount of aluminum present, an alloy is produced which,

when made into anodes, combines an even higher solution potential than cell magnesium with a high currentY .efliciencv Measuring current eiciency in ampere hours per pound of anode metal, cell magnesium anodes usualcomprising the present invention, on the other hand, will It has now vbeen found that the current eiciency of anodes made from cell magnesium is closely related to the amount of aluminum present. With larger amounts of aluminum, i. e. over .O05 Weight percent,

have current eici'encies of at least 400 and as high as H565 hours` per pound: Although the exactv reason for this remarkableincrease in current eliciency is not known, it is believed that the manganese in some way ties up or counteracts the detrimental eiects of the ironaluminum phase which is known to adversely affect the current elliciency of the anodes.

The solution potential of anodes comprising the present invention is substantially uniform wheuthe manganese solutionpotentials does decrease. In-thisI samerange, thel amount of manganese is adjustedrelative' to the amount of aluminum` present toxobtainenhanced current efficiency and the highest solutionpotentials, Figure l illustratesthe. critical effectof the Mn/Al ratio on the current eiciency1ofthe anode by plot-tingxtest data from alloys comprising the present invention which* have similar aluminum content on a graph in whichfthe ordinates represent ampere hoursl per pound of'anode metal and theabscissas `represent the percentby weight of manganese. The details-of thel tests and data` upon which-Figure l is-basedV are set forth in Examples ll and 2 below.

It is apparent from Figure l that the low aluminum alloys (less than .001 percent) have consistently good current efficiencies when between 0.5 percent and 1.3 percent by weight of manganese is'present. As the amount ofv aluminum increases, the decrease in current eiliciency becomesmore pronounced in the lower ranges of manganese content. Higher percentages of manganese result in satisfactory efliciencies for all aluminum contents not over .01%. to aluminum is the critical factor. Since itris desirable to know how little manganese can be added and still overcome the detrimental eifect of the aluminum, an

lt is thus seen that the ratio of manganese of the' examples were obtained under conditions simulat- The-preferredi manner 'of producing such alloyed anodes,

however, comprises adding a manganese compound, such as manganese dioxide or manganese dichloride, to the feed of an electrolytic cell for making magnesium. The constituents of the cell are analyzed at frequent intervals and the manganese added in the feed is adjusted according. to the aluminum content of the cell magnesium. In this connection it is important to note that manganese is an, expensive alloying metal and minimum amounts useable areemploycd to' reduce the cost of these sacrificial anodes which maybe employed tothe extent of several tons foruses such as protecting pipelines.r The casting of anodes from this alloy follows. the conventional procedures althoughv it is preferred to employ Arelatively cool molds. Y

In order to enable those skilled in the art to more readily-practice the present invention, the following examples are set forth to illustrate the present invention:

. ing a composition as set forth above, exceptf'or the varia-Y Example 1 The melt in a magnesium cell was analyzed by a direct reading spectrophotometer and found to contain by weight, .0052 percent Al, .0013 percent Cu, .014 percent Fe, less than .0l percent Ca, less than .0005 percent Ni, less than .001 percent Pb, less than .001 percent Si, less than .0l percent Sn, less than .02 percent Zn, and the balance magnesium. To ther feed in this cell was added enough manganese dioxide to produce in the melt a minimum of 0.81 percent by weight of manganese (0.5-l-60(.0052) =0.5-{-0.3l). Final analysis showed 0.92 Weight percent of manganese.

From this melt, magnesium `anodes used for laboratory investigations were cast in iron molds just slightly over C. and immediately air cooled to room temperature. Y

These anodes were prepared bysectioning specimens roughly 7 inches long and 1/2` inch square cross section from l7 pound D-shaped production anodes. These specimens were machined togive'small rods,.5/16 inch in diameter by 6 inches long. A 2 inch section at one end of each rod was threaded and'a at was filed near the middle of the threaded section to allow for the stamping of identification numbers. After weighing, the threaded vsection was smeared lightly with petrolatum grease and the anode` was then screwed into atransparent sleeve threaded at the lower end to take the anode. The 4'inch length of anodeextending beyond the grease-sealed sleeve was then vapor-degreased with trichlorethylene.

Employing the. anodes so prepared, experimental cellsv were assembled using a 5.5 inch length of 3.5 inch. standardpipe for the cathode, and an` aqueous solution saturated with CaSOi and Mg(OH)2 interposed between the rodandthe cathode in the electrolyte. This cell was then: connected in series with other test cells and operated at an anode current densityv of 36.milliamperes per square footfor 14 days. The anodes were then removed from the cells andthe loose corrosion product was brushed or rubbed off in a stream of runningwater. This wasv followed by immersion in agitated, aqueous 20 percent chromic acid solution, containing` l percent AgNOs, to. remove residual corrosion products, after which the anodes were rinsed, dried and reweighed. Current e-4 ciencies were calculated from the weight loss andthe quantity of current passed. On vthis basis, this anode showed a current eciency of 521 ampere hours, per pound of anode-metal.

Example 2 A series of tests following the procedures above outlined weremade andthe results of these tests formed the basis for Figure 1.. The alloy in each case contained approximately .0005 percent Cu, less than .01 percent Ca, less'than .0005 percent Ni, less than .001 percent' Pb,` less than .001 percent Si, less than .0l percent Sn andV less than .02 percent Zn, with the balance magnesium 'Y except for aluminum, iron and manganese `as tabulated.

In addition to the current eflciency test above outlined' in4 Example l, these same test anodes were tested for solution potential by immersion, following the current efficiency test, in an aqueous solution saturated with CaSO4 and Mg(OH)2, and allowed to soak for one hour. Y

shows a number of alloys tested both for current eiiciency and solution potential, each of these alloys havtions in the amount of aluminum, iron and magnanese as noted. To parallel Figure l, the data, is arranged according to theY aluminum content, each set of data con,- stituting an average of three test specimens. Alloys falling outside the prescribed range of manganese, have Percent by Weight of metms Amp. Open Cir- Y Y Hours per cuit solug pound of tion poten-V Percent Percent Percent Anode tial ref: A1 Fe Mn Metal calomel 0005 008 70 557 1. 665 0005 013 84 520 1. 647 0005 012 86 516 1. 644 0005 0116 99 449 1. 622 0006 0107 l. 20 523 1. 664 0007 0091 80 543 1. 654 0008 0163 77 536 1. 646 0008 0050 84 548 1. 646 0008 0055 85 529 1. 624 0008 0118 87 479 1. 653 0008 005 90 563 1. 672 0008 0059 92 532 1. 649 0008 0107 97 544 1. 654 0009 .010 61 468 1. 629 0009 010 73 518 1. 650 0009 009 78 503 1. 672 0009 0145 80 527 l. 638 .0009 0086 84 520 `1. 655 0009 0062 86 540 1. 633 0010 0066 85 550 1. 656 0011 017 86 520 1. 634 0011 0086 86 486 1. 638 0012 0094 81 520 1. 639 0013 016 45 355 1. 628 0013 014 74 483 1. 635 0013 0074 76 498 1. 642 .0013 007 77 542 1. 675 0013 .0072 82 538 1. 655 0014 0084 97 498 l. 647 0015 015 .68 460 1. 666 0016 020 64 400 1. 637 0016 012 69 461 1. 628 0016 013 72 495 1. 661 0016 009 85 489 1. 635 0016 0074 91 528 1. 641 0017 0076 74 498 1. 633 0017 0046 1.01 560 1. 644 0019 018 72 448 1. 634 0019 017 74 507 1. 654 0019 0067 82 .486 1. 628 0019 0096 1. 32 548 1. 642 0020 014 c 81 527 1. 641 0022 015 65 443 1. 634 0022 0107 72 450 1. 644 0023 012 68 438 1. 627 0023 0082 91 508 l. 640 0024 0116 1.20 546 1. 651 0025 005 78 413 1.668 0025 0068 1. 29 541 1. 626 0027 019 77 451 1. 643 0028 0072 73 442 1. 625 0028 0113 76 402 1. 658 0028 009 89 567 1. 661 0034 0064 72 398 1. 629 0034 0040 1. 06 507 1. 624 0035 0096 84 466 1. 648 0035 0140 90 507 1. 629 0042 0091 96 509 1. 652 0045 015 51 202 1. 668 0045 0159 76 438 l. 644 0052 014 92 521 1. 665 0055 010 65 298 1. 627 0055 0098 70 347 1. 658 0067 0110 .63 223 l. 631 0097 0092 l. 13 567 1. 626 03 012 1. 71 508 1. 554 03 011 2. 06 515 l. 590 03 009 2. 48 450 1. 585 03 014 1.36 517 592 The data from this table relating to solution potential are plotted in Figure 2, which graphically `shows high solution potentials between 0.5 and 1.3 percent manganese with rather sharp decreases in solution potential above and below these points. It is thus seen that the alloys of the above table which fall within the scope of the present invention combine a high current efficiency with a high solution potential.

In order to describe the cathodic protection system comprising the present invention, reference is made to Figure 3, which is a schematic illustration of an anode field positioned about a bare ferrous metal pipeline, in this case, a bare metal 8 inch diameter pipe which is to be protected. The pipeline 1 is connected through an insulated electrical conductor 2 to an insulated collector wire 3, so-called because it interconnects through lead wires 4-4 with anodes, 5-5. These anodes, which are formed from the alloy.. above described, may be of various sizes, but are in `this'instance 4 inches by 20 inches and are spaced at least 10 feet from the pipeline 1 and from 50 to 75 feet from each'iother.' These anodes operate 'most eiciently and have Va longer life when surounded 'by a backll 6-6. For high' resistivity soils, the backll 6V may be composed of 20 parts 'bentonite (dry, powdered), 75 parts gypsum (dry, powdered) and 5 parts sodium sulfate (anhydrous). This backiill isolates the anode chemically and acts as an electrolytic bridge carrying electricity from anode to earth; V

For most installations, anodes need be used only on one side of the-,pipeline t`o be protected, lalthough exceptionally large pipes may require anodes on both sides. Thespacing of the anodes-in the station shown in Figure 3, as we ll as the total numbrof stations required, is a function of the solution potential of the anode, theresistivity of the soil and the amount of current required to protect the pipeline.` Other factors being equal, anodes of the present invention which have higher solution potentials than thoseofthe prior iart, may be spaced farther apart, thus reducing the total number of anodes needed. Those rskilled in tharfwillrecognize that the distances, size of anodes, etc.A shown in Figure 3 may be varied to suit the particular conditions encountered.

It is apparent vtr'or'n the above detailed description of the invention that al athodic protection anode has been produced which,lwh'en litiliz'ed Vina cathodic protection system, iS characterized by remarkable 'current efficiency combinedwitli high' solution potential. These results are obtained by restricting `theialloy composition of the anode -within verynarrow'and Icritical ranges and particularly by 'maintainingthe ratio of manganese `to aluminum at a prescribed minimum.' The invention is especially adapted forluse in'highresistivity soils, wherein a high solution potential is needed.

VvWhat is 'claimed is:

1.y A cathodicprotection'anode formed of galvanically active metal having the following composition in percent by weight:

the amount of manganese in weight percent being at least equal to 0.5+60 (percent by Weight of aluminum).

2. A cathodic protection anode formed of a cast body of galvanically active metal having the following cornposition in percent by weight:

Magnesium At least 99.16. Manganese 0.5 to' 0.8. Aluminum Not over 0.005. Iron Y 0.001 to 0.03.

Nickel Not over 0.002. Tin Not over 0.01. Lead Not over 0.01. Other metals (each) Not over 0.05.

the amount of manganese in weight percent being at least equal to 0.5-|-60(percent by weight of aluminum).

3. In combination with a structure composed of corrodible metal less anodic than magnesium immersed in a medium wherein it is corrodible, a protective system of sacrificial metal submerged in the corroding medium and electrically connected to said structure and comprising 7 an expendable quantityof inagnesinnimetal;hin/ingthe`v following composition in percent by Weight:

Magnesium At least 9825. Manganese 0.50. to 14.3. Aluminum f Not over 0.041. Tin Not over OO 1'. Lead Not over 0101, Ii'nriv f 0.001 `to` 0.03. NickeL Not over 0.002'. Other metals (each) Not over"0.05.

the percent by weightv omanganese' beingat leastequaly to 0.5.-1-60 (percent byweighti of aluminum)- 4. The combination dcned in claim: 3' wherein theA corroding medium is sea waten.

5. The combination; defined in. claimk 34 wherein: the co'rro'ding mediumisasoil..

6. In combination. withv a' structureY composedioi''feraY rous metal immersed inahighlresistii/ity-soil; aprote'ctive system ofr sacrii'cial metal.l submerge'dfin` the? high; re sistivity soil. andl electrically .connected to Vsaid structure. so asA to form al galvanic cell therewith; and comprising an expendable 'quantity of ama'gnesiimr alloy having the following! comp usitio'n` in. percent by weightc Magnesium At.leastl98.5. Manganese 0.50 to`1'.3. v Aluminum No't over'00'1`.. Tin- Notover 0:01'. Lead Not over" ;'01". Iron 0.0011602031 Nickel Notover': 0.1002; Other metals (each) Noti over 0L05".

the amount of manganese Yinweight percent-being' at: least equal to 0.5-1-60 (percent by Weightiof. aluminum).

7. A method of cathodically protecting a 'structure composed of a corrodible metalless anodic thanr mag.- nesium immersed in a Ymedium wherein it iscori'odible, which comprises placing a protective systerh^=of sacrificial4 metal in the corroding medium composed-0f anexpendable quantity `of magnesium.` alloy'haying; the.Y following composition in percent weight: f

Y seA .A the amountfof manganese in weightpercent being' at least equal to 0560 (percent byzweight of' aluminum), and

electrically-connecting said structure and the sacricial metal to produce a galvanic cell in which the sacrificial metal is the anode and thestructure is the cathode.

8. A cathodic protection anode formed of galvanieally active metal'having the following composition in'percent by weight: v

Magnesium At least 98.5. Manganese 0.50 to 1.3. Aluminum Not over 0.01. Tin Not over 0.01. Lead Not over 0.01. Iron 0.001 to 0.03. Nickel Not over 0.002. -Other metals i(each) No't over 0.05.

the amount of manganese in weight percent being at least equal to 02541- (percent by weight of aluminum) when the percent by weight of aluminum exceeds .007.

References Cited in the' le of this patent UNITED STATES PATENTS 1,960,700 G'ann' et a1. May 29, 1934 2,431,723? Yerkes Dec. 2, 1947 2,478,479 Grebe et al. Aug. 9, 1949 2,645,612 Taylorf j July 14, 1953 2,698,230 Boyle' Dec. 28, 1954 2,742,355 Emley Apr. 17, 1956 OTHER REFERENCES H. H. Robinson, Transactions of the Electrochemical Society; V01. (1946), pp. 496-499.

Claims (1)

1. A CATHODIC PROTECTION ANODE FORMED OF GALVANICALLY ACTIVE METAL HAVING THE FOLLOWING COMPOSITION IN PERCENT BY WEIGHT:

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