US3117070A - Bronze half-cell for cathodic protection system - Google Patents

Description

Jan. 7,

fironze 1964 E. P. ANDERSON 7,070

BRONZE HALF-CELL FOR CATHODIC PROTECTION SYSTEM Filed March '7, 1960 2\,14 5% -5\ f a i 20 k 24 3 26 Z i g 4% Ca/fioaz'c Profecfz'on System INVENTOR. EDWARD P. ANDERSON y km! yum JWMM yffldn d/dw- K lam.

ATTORNEYS 3,117,970 BRONZE HALF-QELL FOR CATHOBIC PROTECTHON SYSTEM Edward 1. Andersen, Livingston, N.J., assignnr, by mesne assignments, to Engelhard Industries, inc, Newark, NJ., a corporation of Delaware Filed Mar. 7, 1960, Ser. No. 13,631 1 Claim. (Cl. 204196) This invention relates to cathodic protection systems, and more particularly to sensing half-cells for such systems.

In the cathodic protection of surfaces against corrosion, two types of electrodes are normally used. These electrodes are the anodes from which the protective current flows and the sensing half-cell which indicates the level of protection which is present on the surface which is being protected. For large ships, calomel or silver-silver chloride cells are widely used, and they are generally satisfactory.

For the cathodic protection of the steel hulls or the submerged metal appendages of wooden hulls on small craft such as cabin cruisers and the like, calomel or silversilver chloride sensing halt-cells are too expensive. Instead of these expensive half-cells, it has been proposed to use an exposed steel disk as a half-cell. Steel half-cells have a number of disadvantages. First, they have an abrupt change in characteristics during the first few days of operation. In addition, they produce both positive and negative output readings. Accordingly, when a steel cell is used, a center-reading meter must be used instead of a less expensive meter which is sensitive to signals of but one polarity. Finally, after prolonged periods, the steel half-cell rusts.

Accordingly, a principal object of the present invention is to increase the stability and corrosion resistance of cathodic protection half-cells made of inexpensive materials.

Another object of the invention is to provide an inexpensive half-cell which will give voltage indications of a single polarity with both steel hull and wooden hull craft, under all normal cathodic protection conditions.

A further object of the invention is the development of a half-cell assembly for a cathodic protection system which may readily be used with both steel and wooden hulled craft.

In accordance with the present invention, the foregoing objects may be secured by the use of a bronze sensing half-cell in combination with a cathodic protection system. When a bronze half-cell is employed, all of the voltage readings between the half-cell and the steel hull or the protected fittings of a wooden hulled ship are of the same polarity. The readings are relatively constant as compared with a steel cell, for example. In addition, the high copper content of bronze tends to poison marine growth and prevent fouling. Furthermore, bronze-type alloys include little zinc, so that the physical and electrochemical properties of the cell are not adversely aifected with the passage of time.

In accordance with a broad feature of the invention, a cathodic protection system is provided with a sensing half-cell made of bronze-type material.

In accordance with another feature of the invention, a cathodic protection system includes a sensing half cell of bronze-type material, a meter Which is sensitive to signals of only one polarity, and circuitry for connecting the meter between the sensing half-cell and the surface to be cathodically protected.

Other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description and from the single FIGURE of the drawings which shows a sensing half-cell in detail and also shows schematically a cathodic protection system.

Patented Jan. 7, 1%54 With reference to the drawing, a sensing half-cell having an active surface 12 of bronze-type material is shown secured to the steel hull 14 of a boat. The structural details of the mounting arrangement will be considered but briefly in the present application as it is the subject matter of a patent application Serial No. 13,250, filed March 7, 1960, entitled Electrode Mounting Assembly. This copending application, which was filed concurrently with the present specification, is the invention of Joseph Zarra.

Other components of the sensing cell assembly include the plastic support 16, the lead-in wire 18, and the clamping nut 2i). An insulating mat 22 is provided between the reference electrode assembly and the hull 14. A suitable fiexible sealing washer 24, a regular washer 26, and a lock washer 22'; are provided between the bolt 20 and the steel hull 14. The connection from wire 18 to the insulated Wire 30 leading to the cathodic protection system 32 is protected against water and dampness by the closures 34 and as.

The cathodic protection system 32 may be of the type disclosed in my co-pending patent application Serial No. 766,147, filed October 8, 1958. The cathodic protection system may also take the form of the system disclosed in Paul Byrnes application Serial No. 11,593, entitled Cathodic Protection System, which was filed in the Patent Ofiice on February 29, 1960, now abandoned. The present cathodic protection system includes the usual ground connection 38 to the hull 14, and also includes an anode in to which cathodic protection current is supplied.

A meter 42 is also included in the cathodic protection system. The meter 42 is of the inexpensive type in which voltages of but one polarity may be sensed.

To facilitate a consideration of the electro-chemical voltages which are involved in the operation of the present cathodic protection system, a table giving the galvanic series of certain selected metals and alloys in sea Water will now be set forth. Table I shows the relative potential of different materials with reference to a saturated calomel half-cell in sea water having a velocity of 13 feet per second:

Table 1 Avg. Sea

Material Potential Water Volts Temp.

Zin 1. 03 22 Aluminum 633T. O. 24 Carbon Stecl 0. 61 24 Yellow Brass" 0.36 24 Red Brass 0. 33 24 CompositionG Bronze. 0.31 24 Admiralty Brass. 0. 29 24 9010 CuNi+.82 F 0.28 26 7030 Cu-Ni+.06 F 0. 27 '26 7030 CuNi+.45 Fe" 0.25 26 Inconel 0. 17 2t Monel 0.075 22 With regard to the composition of the various materials set forth in Table I, aluminum 635T is principally aluminum with about 0.4 of one percent of silicon and 0.7 of one percent of magnesium; yellow brass includes 67 percent copper and 33 percent zinc; red brass includes percent copper and 15 percent zinc; admiralty brass includes about 70 percent copper, 29 percent zinc and one percent tin; composition G bronze includes 88 percent copper, 10 percent tin, and 2 percent zinc; Inconel is an alloy containing 72 percent nickel, 610 percent iron, 14- 17 percent chromium, 1.0 percent manganese, .5 percent copper, .5 percent silicon and .15 percent carbon; and Monel includes 60-70 percent nickel, 2535 percent copper, 1-3 percent iron, .252 percent manganese, 02-15 percent silicon and .05.3 percent carbon.

To point up the present invention, the potential between a steel half-cell and the hull of a steel boat as employed heretofore will first be considered. Prior to the application of any cathodic protection current, the usual steel hull might .be expected to have a negative potential of about 0.61 volt, or 610 millivolts, by reference to Table I. In fact, however, because of the presence of certain bronze, Monel or other fittings, the average potential of the steel hull with fittings may actually be about 400 millivolts.

With a steel sensing cell having a negative potential of about 610 millivolts, a negative potential of about 210 millivolts is present at the half-cell with respect to the hull of the ship. As cathodic protection current flows between the anode and the hull, a thin film of hydrogen is formed which protects the steel hull of the vessel. As this film is formed, the potential on the hull builds up. Under optimum cathodic protection conditions, the potential on the hull should be about 250 millivolts above the potential of the hull as indicated in the galvanic series shown in Table I. Thus, for example, with a steel hull having a normal potential of about 0.61 volt, or 610 millivolts, the potential will be approximately 860 millivolts when optimum cathodic protection conditions are present.

When the hull potential is raised to 860 millvolts, its potential is now negative with respect to the sensing cell. It is clear, therefore, that when a steel half-cell is employed, a meter should be provided which reads potentials of either polarity.

When boats having wooden hulls are employed, cathodic protection techniques may be used to protect the fittings. These fittings are frequently of bronze materials having a potential comparable .to that of the bronze reference cell. Accordingly, the initial potential between a bronze reference cell and the fittings on a boat having a wooden hull, will be approximately 0. As a protective film builds up on the fittings, the effective negative potential on the fittings becomes higher, and may increase, for example, from about 300 millivolts to about 550 millivolts. When using a steel reference cell with wooden boats, however, the voltage between the steel half-cell and the fittings is always negative, and may decrease from a difference of about 310 millivolts to a difference of about 60 millivolts.

When a bronze half-cell is employed with a steel hull, the readings from the half-cell to the steel hull are always of the same polarity, as the half-cell voltage is about 300 or 320 millivolts, and the voltage on the steel increases from about 400 millivolts up to approximately 860 millivolts, as discussed above. tection units, it is convenient to have a single system which is applicable both to steel and wooden hulled boats. When a bronze sensing cell is employed, this is practical for the reasons discussed above. Thus, for either type of boat, the same polarity of sensing voltage is obtained. Furthermore, in either system the meter may be sensitive to a single polarity and need not be of the more expensive center-reading type. i

In the sale of cathodic pro' In addition to the adverse structural effects, it has been found that the presence of zinc in excess of about ten percent produces a significant shift in the sensing halfcell voltage, upon the leaching out of the zinc. Other satisfactory bronze-type alloys would include to percent copper and the remainder mostly nickel. Ac-

cordingly, when the term bronze-type alloy is employed in I the present specification and claims, reference is made to alloys containing at least fifty percent copper and less than ten percent zinc, and having a voltage near 0.3 volt in accordance with the galvanic series as set forth in Table I,

Several advantages of the present invention will now be restated. These advantages include simplification of the circuitry and stability of the half-cell output. In addition, the anti-fouling action of the copper present in the cathodic protection half-cell, is helpful in avoiding undesired fiuctuations in the half-cell output.

It is to be understood that the above described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

In a cathodic protection system for a boat having metal on at least a portion of the hull, an anode mounted on the hull, means connected between the anode and the hull to supply direct current to the anode for cathodically protecting said metal, a reference half-cell comprising a bronze electrode mounted on the hull and insulated therefrom, a voltmeter capable of reading signals of one polarity connected between the hull and the bronze electrode, and means for varying the current supplied to said anode in accordance with the signals provided by said reference half-cell.

References Cited in the file of this patent UNITED STATES PATENTS 1,900,011 Durham Mar. 7, 1933 2,221,997 Polin Nov. 18, 1935 2,338,713 Ewing Jan. 11, 1944 2,934,484 Anderson Apr. 26, 1960 2,982,714 Sabins May 2, 1961 OTHER REFERENCES Chemical and Metallurgical Egn, October 1940, page 691. i I

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