US 3817852 A
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June 18, 1974 J TQEDTMAN ETAL 3,817,852
ZINC-STEEL SACRIFICIAL ANODE GROUND ROD Original Filed Sept. 23, 1969 i J.A. Toedtman 5 I By G.W. Petri and G. Kayariun A TTO/P/VEY United States Patent Int. Cl. C23f 13/00 US. Cl. 204-197 4 Claims ABSTRACT OF THE DISCLOSURE A ground rod is made from a sacrificial anode core inside a hardened sheath or jacket. The sheath is split longitudinally, and then the longitudinal corners or edges are turned in to provide a locking key which fits into longitudinal grooves on the anode core, thus completing the ground rod. This makes a strong rod while exposing the anode core. This way, a steel jacket, for example, provides the mechanical strength required for a driven ground rod and an exposed anode (for example, zinc) provides a sacrificial anode. Other anode material such as magnesium and aluminum may also be used.
This is a continuation of application Ser. No. 860,214, filed Sept. 23, 1969, now abandoned.
This invention relates to electrical power distribution systems and more particularly, means for protecting underground equipment against electrolytic corrosion.
The basic theory behind corrosion of metals is that some of the metallic molecules, immersed in an electrolyte, disassociate themselves into ions when they free electrons. This theory of metal corrosion is rather straightforward. However, it is difficult to evaluate the relative importance of the many different factors which aifect undergorund corrosion. In fact, the conditions under which corrosion occur are so poorly defined that the theory is often more helpful in explaining why a corrosion has already taken place rather than predicting what is to be expected.
Usualy, the metallic state (which is a high energy form) is uncommon in nature where most metals are found as oxides or compounds of oxides (which are a low energy form). These oxides achieve the higher state of internal energy during a refining process which ultimately leads to a useful metal. Corrosion is the process by which the metal returns from this unnaturally higher level of energy to the original lower energy level of the oxide state.
For instance, when iron and copper are immersed in a common electrolyte, iron molecules have a greater tendency than copper molecules to lose their electrons, become positive ions, and disassociate themselves from the metal. When this occurs, the iron becomes negatively charged with respect to the electrolyte or liquid in which it is immersed. Therefore, a galvanic couple is formed between the iron and copper. The resulting imbalance of electrons causes a current to flow from the iron to the copper. As the electrons are deposed at the copper, a chemical reaction occurs in the iron to form an oxide. From this, it is clear that if iron and copper are immersed in a common electrolyte, the oxidization of the iron is greatly accelerated.
A typical underground electrical power distribution system, and the component parts thereof, have all of the ingredients for making a corrosion cell. Although the mild steel transformed tank is often protected with a surface coating, scratches, nicks, and chips expose the bare steel. The soil with its accompanying moisture or ground water is the electrolyte. Thus, a corrosion cell exists between the bare steel and any exposed copper.
To overcome this problem, another metal (sometimes called a sacrificial anode) is often immersed in the electrolyte to form another galvanic couple. This other metal is selected from a class of metals having anodic voltage potentials relative to steel. Then, the other metal gives up its electrons, is broken down, and sacrified to absorb corrosive potentials, thus diverting these potentials from the galvanic couple formed by copper and steel. Fundamentally, therefore, cathodic protection consists of impressing, on an underground structure, an electromotive force which makes the entire structure cathodic with respect to the adjacent soil.
Accordingly, an object of the invention is to provide new and improved anodic or cathodic protection of underground electrical equipment. In this connection, an object is to mechanically protect the sacrificial metal without affecting its function of making the equipment cathodic with respect to the ground.
In keeping with an aspect of the invention, these and other objects are accomplished by providing a zinc-steel ground rod for the dual purpose of completing a sacrificial anode in a galvanic couple between the zinc and other metals and of grounding the electrical equipment to preclude a heavy build-up of electrical potentials thereon. To protect the zinc against mechanical damage when it is driven into the ground and to insure continued good electrical connections, the zinc is surrounded by a split steel jacket. The steel is crimp connected into the zinc to make a rugged unitary structure. The zinc exposed by the split provides the anode.
The above mentioned and other features and objects of this invention and the manner of obtaining them will become more apparent, and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a cross-section view which shows the end of a solid sacrificial anode rod as it appears at the start of a manufacturing process;
FIG. 2 shows a cross-section of the end of the same rod after it has been initially formed, as by being extruded or drawn through a first die;
FIG. 3 shows the end of a cross-section of the same rod after it is drawn through a second die;
FIG. 4 is a cross-section end view which shows a galvanized steel sheath or jacket after an initial forming step;
FIG. 5 shows the combination of the steel sheath or jacket of FIG. 4 and the sacrificial anode rod of FIG. 3;
FIG. 6 is a perspective view which shows successive die reductions of the combination rod of FIG. 5; and
FIG. 7 is a perspective view which shows a completed drivable, bi-metal sacrificial anode, ground rod.
A below grade housing or vault for electrical equipment, for example, may be either coated mild steel or stainless steel. In keeping with the usual electrical installation practice, all of the parts of the housing and equipment therein are connected together by means of common ground wiring which is, in turn, connected to a ground rod. In the case of a mild steel housing in an electrolyte of ground water, the further addition of a zinc, alumi num, or magnesium sacrificial anode greatly reduces the corrosion rate of the steel.
In greater detail, the potential standing on mild steel, per se, is approximately 0.4 volts. The steel is protected if its potential can be driven to 0.85 volts. This is accomplished by electrically connecting, say a zinc anode to the steel because the zinc is more active than the mild steel, and it supplies an electron flow to the steel which drives it further negative. Assume, for example, that a transformer tank has a surface resistance of 3,000 ohms, assume also 15 ohms for the electrolyte ground waterand scarificial anode,.and .01 ohms for a concentric copper neutral. A zinc anode has a driving voltage of 0.25 volts, and a magnesium anode has 0.9
volts. The current can now be calculated. Using these magnesium. Thus, in a typical situation, a pound of zinc would last 2.4 years and a pound of magnesium would last 1.1 years.
7 To make the inventive ground rod, a bar of sacrificial anode material, such as zinc, aluminum, or magnesium, is drawn through a Vaughn draw block to have a generally circular cross-section 20 with a specific diameter. For example, as received, the bar stock might be a coil which is nominally circular in cross-section and approximately three-quarters of an inch in diameter, but its circumference may be irregular with any diameter varying by, perhaps, a sixteenth of an inch as compared with any other diameter. After the draw, the diameter could be approximately five-eighths of an inch, and the cross-section -is perfectly circularwithin acceptable tolerances.
The circular cross-section, drawn rod 20, FIG. 1, is then milled to reduce the circumference at one end to acc'ept a second forming, draw die. Then, the rod 20, of FIG. 1, is drawn through a second Vaughn draw block to form a circular cross-section 21 having two longitudinal slots or grooves 22, 23 extending along the length thereof. If the circular stock of FIG. 1 is initially coiled, the grooves 22, 23 are formed on the outside of the coil.
Then, the rod 21 is again drawn through a draw block to form a rod 24 having grooves 25, 26 of full depth. Under the foregoing assumptions that the initial circular cross-section 20 had a diameter of five-eighths of an inch, the completely formed rod 24 has a diameter of about .650 inches.
Next, the rod 24 is cut into seven or eight foot lengths and straightened.
The sacrificial anode rod, per se, cannot easily be used for making a ground rod. First, if zinc is used, it is too soft to drive, and second, zinc creeps or cold flows over time and responsive to any added heat. Therefore, if a connector is tightly attached to zinc, the zinc flows out or creeps away to loosen the point of connection. This looseness, in turn, makes a high resistance electrical joint which generates more heat and causes a still greater cold flow of anode material.
Therefore, to form the sacrificial anode rod 24 into a usable ground rod, a steel reinforcing sheath or jacket 30 (FIG. 4) is given contours complementary to the cross-section of the rod. Initially, the steel is purchased or made to any convenient shape, such as a coiled ribbon. Then, the ribbon is galvanized, preferably with a lock forming quality, hot dipped commercial coat. Next, the ribbon is bent or otherwise formed into a sheath or jacket having a broken-circular cross-section with hooks or keys 31, 32 turned in at the open ends of the break in the circle. The hooks 31, 32 provide interlocking edges, add strength and provide additional material in the area of the rod 24 which is otherwise weakened by the guideways 25, 26.
The overall inside dimensions of the steel sheath 30 are slightly larger than the outside dimensions of the anode rod 24. Thus, the rod 24 slips easily inside the loose steel sheath 30. This looseness is important since the softer anode metal does not always yield to the harder steel. Instead, it tends cold flow until it builds-up a lump which is large enough to bind against the steel and keep the anode rod from sliding into the sheath.
Next, the loose assembled sheath 30 and anode rod 24 are placed in draw dies 35, 36 and reduced in dimeter in successive steps, as indiated at 37, 38 in FIG. 6. If successive passes are made through the dies, the steel sheath is first reduced to make a perfectly mated sheet-.to-core combination. Thereafter, and throughout the reduction process, the volume per cross-section area is the important criteria. As the overall ground rod diameter continues to be reduced, the ratio of steel-to-anode core material antomatically continues to be maintained. Moreover, if a zinc core is used and if care is taken to protect the galvanized layer of zinc, all of the exposed metal' is zinc, and all junctions exposed to the electrolyte are zinc-tozinc junctions; therefore, no galvanic couples are formed between the steel jacket 30 and zinc core 24.
According to the invention, the completed product forms a drivable, bi-metal sacrificial (with respect to steel) anode ground rod. Briefly, the term drivable implies a metal which is hard enough so that the top Will not mushroom or otherwise deform when it is struck while being driven into the ground. The driving tip may encounter a normal number of stones without becoming unusable, and the rod will not bend an unacceptable amount. The term sacrificial with respect to steel means that the anode metal is higher in the galvanic series than steel. By way of example, one such series is as follows:
Approximate potentials with respect to saturated copper-copper sulphate electrode, volts* Material:
Commercially pure magnesium l.75.
Magnesium alloy (6 percent Al, 3 percent An, 0.15 percent Mn) 1.6. Zinc 1.1. Aluminum alloy (5 percent Zn) l.0. Commercially pure aluminum 0.8. Cadmium 0.8. Mild steel (clean and shiny) -0.5-O.8. Mild steel (rusted) -0.2-0.5. Cast iron (not graphitized) 0.5. Lead -O.5. Tin 0.5. Stainless steel, Type 304 (active state) -0.5. Copper, brass, bronze --0.2. Mild steel (in concrete) -0.2. Titanium -0.2. High silicon cast iron --0.2. Nickel +0.1 to 0.25. Monel 0.15. Silver solder (40 percent) 0.l. Stainless steel, Type 304 (passive state) +0.1. Carbon, graphite, coke 5+0.3.
*These values are representative of the potentials normally observed in soils and waters which are neither markedly acid nor markedly alkaline. Of the metals listed in this particular table, magnesium, zinc, aluminum, and cadmium are sacrificial, with respect to steel. Also, it might be well to point out that a sacrificial anode is a device which is intentionally allowed to disintegrate to protect the steel. Thus, the zinc in a galvanized steel rod is not sacrificial since any sacrifice of the zinc would leave the steel without its rust protective coating, and further since the total amount of zinc is not adequate to provide a sacrificial function.
The ground rod has the strength of steel to withstand the hammer blows which drives the rod into the ground and the stones which the rod strikes as it moves through the ground. More particular, after the zinc ground rod is reduced to its final diameter a steel drive point 42 is attached to one end and a drive cap 43 is attached to the other end. Again, these parts may be galvanized to preclude the formation of a galvanic couple within the ground rod itself. If it is assumed that the finished outside diameter of the ground rod is reduced to about fiveeighths of an inch in diameter, the anode core 24 is about one-half inch in diameter. The exposed section 40 of the core is about a quarter-inch at the circumference. This exposed section forms the sacrificial anode.
While the principles of the invention have been described above in connection with specific apparatus and applications, it is to be understood that this description is made only by way of example and not as a limitation on the scope of the invention.
1. A sacrificial anode adapted to be driven into the ground and comprising a sheet steel jacket having generally a C-shaped cross section enclosing most of the peripheral surface of an elongated substantially straight metal rod of sacrificial metal, the metal of said rod being higher on the galvanic chart than steel, inturned ends of said jacket cross section engaging said rod for structurally supporting said rod along the length thereof and for maintaining surface contact with said rod along the length thereof, and said inturned ends defining therebetween a longitudinal slit in said jacket permitting an elongated section of said rod to be exposed as a sacrificial anode through said slit.
2. An anode as claimed in claim 1, in which the jacket inturned ends comprise a locking key resting within a groove formed in the rod with the metal of said rod being me.
3. A combined ground rod and sacrificial anode adapted to be driven into the ground and comprising a steel tubular elongated sheath forming a jacket having an open slot along the full length of said jacket parallel to the length of the sheath, said sheath being crimp connected to an elongated substantially straight inner core of generally uniform cross section along the length of said core and enclosing most of the peripheral surface of said core said core being fabricated of zinc with a section of the core protruding through the open slot along the length thereof to form a sacrificial anode.
4. A combined ground rod and anode as claimed in claim 3, in which the core has at least one groove ex-- tending along the length of the core, and the sheath includes at the edge of the slot at least one looking key fitting into the groove to lock the core and the sheath together, with said jacket compressed against said rod for contact therewith.
References Cited UNITED STATES PATENTS 2,157,180 5/1939 Little 204-197 2,829,099 4/1958 Marsh 204-197 3,012,958 12/1961 Vixler 204-197 3,391,072 7/1968 Pearson 204-197 TA-HSUNG TUNG, Primary Examiner US. Cl. X.R.