US3137642A - Method and means for protecting structures, machinery containers, etc. made of steel, copper, brass, bronze or similar materials against corrosion - Google Patents

Description

United States Patent METHOD AND MEANS FOR PRGTECTING STRUC- TURES, lWACHINERY CONTAINERS, ETC. MADE OF STEEL, CGEPER, BRASS, BRQNZE 0R SIMI- LAR MATERIALS AGAE'JST C(BRRQSIGN Winthrop A. Johns, 766 Hyslip Ave., Westfield, NJ. No Drawing. Filed Apr. 13, 1960, Ser. No. 21,875

2 Claims. ((31. 204-148) Structures, machinery, containers, piping, etc., when made of steel, iron, brass, some bronzes and even some stainless steels are subject to corrosion when they are in contact with water, water mixtures or other ionizing liquids containing negative ions such as Cl: 50 80 CO Brand the like. Examples of such equipment are found in chemical processing where the corrosive medium may be the product itself; an intermediate, or one of the media for heating or cooling, also in underground pipelines where the path leads through salt marshes or even along sections of the ocean bottom. Sea water is an excellent example of such a corrosive liquid, and indeed we find some of the most serious corrosion problems in the control of sea water corrosion. Thus the entire maritime industry is in a continuous struggle against this prevalent, costly problem.

Examples of these structures are all dockside facilities other than wood, barges, tugs, metal sea-walls, sheet piling, cables, chains, etc., and every boat that has any exposed metalright from the smallest to the very largest. Plus bilges, ballast tanks, fuel tanks and even the cargo tanks themselves of the petroleum transporting tankers. Due to the use of ballast sea-water in these cargo tanks, the chain of sea-water has extended considerably inland from coastal regions, and we now find salt water in the bottoms of small domestic heating oil tanks and in the oil burners as well.

1. Selection of Materials Thus structures exposed to this type of attack can be made of wood, stone, tile or corrosion resistant metals.

II. Protective Coatings In this category fall paints, mastics, bitumens, rubber and rubber-like plastics, chemical surface treatments, metal coatings sprayed or electrolytically deposited, vitreous enamels and in enclosed containers, the use of inhibitors.

III. Cathodic Protection This method involves the use of an impressed voltage on the metal to be protected. Ideally this voltage is applied at such a value that the metal to be protected becomes cathodic with respect to the hydrogen ion and at such points that a relatively even negative voltage can be maintained over the entire area without excessive current flow in localized areas.

It is in the scope of this latter method that the present invention falls. The protective current can be supplied from any source of direct current such as a battery, a rectifier with suitable controls on an alternating current line or by the use of suitable metals connected to the structure. These metals have higher values of negative voltages or anode potentials than the metals being protected. The result is that these anodes corrode instead of the structural metals. In effect the system becomes a short circuited battery with electrons flowing from the anode, through the metal-to-metal or conductive connection to the cathode and thence into the liquid to discharge hydrogen ions. The electro-chemistry of this reaction is treated more fully in text books on corrosion.

This method is quite old as modern technology goes. Sir Humphry Davy in a paper read before the Royal Society of Philosophy in London in 1823 described the use "ice of zinc blocks attached to the copper sheathing of wooden boats. World-wide technical literature is abundant on the problems and proposed solution. Commercial anodic materials are essentially of the three metals, zinc, magnesium and aluminum, either separately as in the case of Zn and Mg or in combinations of any two or all three.

Theoretically all three of these metals should work as anodes. They are sufficiently negative in ion solutions. A measure of this voltage is made by what is known as a half-cell. This consists of a metal such as silver, mercury or copper in contact with a saturated solution of chloride or sulfate and separated from the corroding medium by a porous membrane or a porous Wooden plug. Thus the solution potentials of iron and these three metals with reference to a CuCuSO (copper-copper sulfate) halfcell are:

Volts Al, aluminum l.0l6 Zn, zinc -1.l06 Mg, magnesium l.750 Fe, iron l 0.68

In actual practice however we find that aluminum is nearly useless in its pure state as well as in most of its alloys because of an impervious surface coating that forms which prevents access to the surface of the ions that is necessary to complete the electrolytic circuit. This is called polarization of the anode and is obviously undesirable since it prevents the anode from generating its current and developing the voltage required.

Experience has shown that in order to safely prevent corrosion of an iron structure, it should be maintained at a voltage of less than 0.85 volt. There is some disagreement on this value; some authorities hold that -0.82 volt is a sufiicient value, but the figure 0.85 volt has been proven safe except for certain conditions where sulfate reducing bacteria are a factor. In these cases the even lower voltage of O.9O volt is recommended.

It is obvious that with so low a voltage difference, an active surface on the anode and a very low electrical resistance between the anode and the cathode must be maintained in order to obtain these negative values.

These are two of the basic requirements of a good anode system. If these are met We generally find good results at the cathode or the structure being protected. However there are other factors that may enter into the picture:

The first four of these items are quite closely related. The ideal anode installation should use inexpensive materials, should be easy to install, should have a reasonably long life and should be easily replaced.

Zinc, the historical material, has proven quite eifective. It is inexpensive. However, it has had a disadvantage in fouling or as pointed out above, in forming an impervious coating over itself that limits or prevents its action, or that the metal disintegrated rapidly without pro ducing any protective current. Recently it has been found that these actions are caused by impurities in the metal.

The reasons are thought to be related to the formation of anode and cathode areas on the surface of the anode itself with the result that spontaneous wastage occurs, or to the impurities causing the formation of non-reactive, insoluble surface coatings. When the metal is prepared and processed to ensure a purity of about 99.999% these effects are minimized. The gassing of the anode is not a problem, and the reaction-precipitation products are fairly low in volume. No sparking danger has been found. The theoretical electrical current energy available is about 350 ampere-hours per lb. The pure anode operates at about 95% efiiciency; in other words, its spontaneous wastage in normal installations is about This gives a cost factor based on $0.127 per lb. fabricated of about 1300 ampere-hours per dollar.

Aluminum should be effective, but as pointed out it tends to polarize and become inactive. However, researchers have recently found that if it is alloyed with about 5% of zinc. The anode potential is raised slightly and this polarizing tendency disappears. But in use this alloy operates at only about 53% efiioiency, and fairly large volumes of gases and precipitation products are formed. The volume and nature of these deposits could be objectionable in enclosed containers. No sparking danger has been found. The theoretical energy available is about 1370 ampere-hours per pound, but there are other factors that affect the rate of consumption. At lower current outputs the aluminum anode has a lower efliciency and hence a lesser amount of the current going for protection and a larger amount being consumed in self destruction. At maximum current delivery, with the alloy operating at 53% efficiency, the cost factor based on a fabricated cost of $0.47 per pound is 1540 ampere-hours per dollar.

Magnesium has had some success as an anode material. It has a relatively high anode potential Which in operation causes it to distintegrate quite rapidly compared to zinc or the aluminum alloy for two reasons. First, impurities in the metal form cathode areas of such a driving potential (voltage difference) that a high rate of self destruction occurs. At maximum current output the magnesium alloys operate at about 50%, and at lower output values the efiiciency approaches zero. The second reason is that the voltage is so great that the cathode cannot accumulate a protective calcareous deposit. The current continues to flow causing a continuous increase in the deposits on the surface and causes them to spall or flake off in chips. At maximum current output and at the 50% rated efiiciency at this output, the cost factor for the magnesium anodes at a fabricated cost of $0.45 per pound is 1111 ampere-hours per dollar.

It was apparent that a slightly higher voltage than that delivered by either the zinc or the aluminum would be desirable, also greater stability and less deposit formation. Some combination of the three metals suggested itself as a possibility, but trials of these alloys or mixtures were disappointing since the deposits that formed fouled or polarized the anode. Then I discovered that if small percentages of manganese and silicon were added to the mixture, this polarizing effect was reduced and the effects of other impurities in reducing efiiciencies were nearly eliminated.

During this experimental work there were two unexpected findings. The first concerned the mutual solubility of zinc and aluminum. At the solidification temperature, zinc is soluble in aluminum to a limit of about 7%; the solubility of aluminum in zinc is even lower at 2 or 3%, therefore, I had expected that difficulty would be encountered in going to higher percentages of either metal. However I also wanted a higher anode potential so I added some magnesiumranging from 2% to as high as 20% in some mixtures. At between 4% ad 8% it appeared to act as a mixing agent and allowed me to use about equal proportions of the remainder in zinc and aluminum. Several different combinations were prepared and tested both in the laboratory and in actual use. The second unexpected finding was in the effect of small additions of manganese and silicon. The presence of either manganese up to about 0.8% or silicon up to about 0.5% or better yet both of these elements, seemed to stabilize the alloy. With these small additions, the tolerance for other metal impurities increased sharply. I found that unlike other anode materials of like efficiencies, where iron and nickel had to be kept below about 0.00159 0, this alloy could tolerate 0.05% without sacrificing efiiciency. Higher percentages may be tolerable. Instead of surface polarization or self wastage, the alloy remains inactive in the corroding medium until it is connected to a cathode. At that time it delivers a current to the cathode lowering its (the cathodes) solution potential to below 0.82 volt. As polarization of the cathode takes place, its current output decreases gradually as the polarizing coating increases and its anode potential approaches its open circuit value of 1.186 volts.

The corrosion rate of the alloy by itself is unusually low. Laboratory testing indicates a rate of about 0.18 mdd. (milligrams per square decimeter per day). But some experimental data have shown an even lesser value. One sample of about 250 gm. and an estimated surface area of about one square decimeter has been exposed to sea water but not attached to a cathode for 120 days. It shows no visible evidence of corrosion.

An experimental laboratory anode of the composition shown has been on a cycling test to simulate use in ballast tanks of petroleum tankers. In these tanks, sea water ballast is carried for about 13 days followed by empty under humid conditions for two days, then filled with cargo for about 13 days, empty for two days, then repeat. The table below shows the data that have been collected to date:

Gm. Weight of anode at start of test, December 31,

1958 58.42 Weight of anode after 247 days 57.89 Weight loss 0.53 Weight of anode after 371 days 57.85 Weight loss since start 0.57 Weight loss in last 124 days (accuracy $0.010

The cathode shows no corrosion whatsoever. When it is dried it has a light gray color. Originally it would dry white, but the oil cycles have darkened the coating. The anode is free from any fouling deposits, the cathode has a hard adherent coating estimated at about 0.08 to 0.13 mm. thick. It has been experimentally ruptured in small areas to determine its thickness and characteristics. The coating was observed to reform within 6 or 7 days of the sea water cycle. Considering the total weight loss, the anode life projects to over years and when the final 124 days are taken by themselves, even longer. Thus it is possible with this material to install anodes when the ship is built and have them last with complete protection for its entire useful life. By contrast, magnesium type anodes now in use require almost twice as much weight but are depleted to the replacement point within 2 or 3 years.

This low self corrosion rate is very important from the standpoint of installed and renewal cost. The cost factors given above are true only at maximum current output. The self corrosion continues at the same rate regardless of the current demand; thus at lower outputs, which is the usual situation, the self corrosion rate becomes extremely important. The higher efficiency anode materials may show lower ultimate cost in spite of a higher initial cost per pound.

This anode material with its nearly 50% of aluminum and its exceptionally high efficiency has more amperehours capacity and a lower cost factor. Its energy potential at 98% efliciency is about 885 ampere-hours, and

As appears from the foregoing Fe, Ni, Cu and other impurities generally are present in small amounts, the total of such impurities not exceeding about 0.50%. These impurities are regarded as being non-essential. The essential ingredients are Al, Mn, Mg, Si and Zn in the amounts stated.

These alloys can be cast in most shapes. The castings are relatively hard although the constituent metals are soft. Hardness readings have been found between 70 and 86 Rockwell B. I have experienced surface hardness in some castings much harder than these figures would indicate as indicated by its feeling and workability with metal cutting tools such as files and saws.

This alloy composition approximates the ideal anode composition. Its voltage or anode potential is ample to polarize the cathode, but not great enough to blast paint, millscale or calcareous deposits from the surfaces. Its self corrosion rate is very low, and thus its efficiency high. It is inexpensive, has a high energy producing potential and is thus inexpensive to install since minimum weight is required. It has long service life. Its gassing is minimal since there is little or no wasted current. The calcareous deposit formed in sea water types of media is unusual in that it is thin, hard and quite adherent, and that its polarizing effect on the cathode is so nearly complete. The reaction products of the anode itself are soft and porous and do not interfere with its anode functions. The precipitation products evolved in sea Water types of media are low in volume and extremely fluffy." They form as fine fiocculent flake-like agglomerations that gradually settle toward the bottom, if there is no movement to keep them dispersed. It is easily handled by all kinds of pumps; it does not tend to cake or cause any trouble in the bottoms of tanks.

For comparison purposes some of the presently available data are given below in tabular form:

High Alumi- Mag- Subject Purity um 95%, nesium Material Zinc Zine Alloy Density:

Pounds per cu. in 0.258 0.100 0.063 0.132 Grams per cc 7. 14 2. 77 1. 74 3.65 PotentialVolts (Ref; Cu-

CuSOr in sea water 1. 106 1.050 1. 750 -1. 186 Etficiency in sea water-percent at maximum output 95 53 50 98 Ampere-hours per-pound at the above eflieiency 350 725 500 885 Cost Factor-ampere-hours per dollar at present prices. 1, 300 1, 540 1,111 1, 770 Volume of precipitation products Low Medium High Low Gas evolution Low Medium High Low In general these anode materials will prevent corrosion on other metals that have a less negative potential. Their radius of action depends upon their ability to supply enough current to polarize adjacent surfaces. This ability depends on:

(1) The surface area of the anode material exposed to the electrolyte.

(2) The surface area of the cathode exposed to the electrolyte.

(3) The ability of the surface of the anode material to sustain a continuous current without fouling or polarizing itself.

(4) The electrical resistance through the metal-to-metal path between the anode material surface and the cathode surface.

(5) The electrolytic resistance of the corroding medium.

Of these, the ability of the anode to sustain a reasonably large current density concerns the material itself. The other factors concern the installation conditions. Normal anode current density of other materials range up to 10 milliamperes per square inch or 1.55 amperes per square cm. The current density of this new anode material has been measured at over 40 milliamperes per square inch or 6.2 amperes per square cm.

Thus in operation the current generated by the anode polarizes adjacent surfaces first and ranges outward spherically (generally the metallic resistance of the circuit is insignificant when compared with the electrolytic resistance of the corroding medium). As the nearby surfaces become more and more coated, the current flows farther and farther until either all of the cathode surface is coated or until the total flow of current through the coating just balances the ability of the anode to produce. It is known that the corroded condition of the cathode surface affects this balance. Thus if the surface has been corroded prior to the installation of anodes and if there are adherent corrosion products remaining, more polarizing current is required.

The superior nature of the thin, hard, adherent coating deposited by this metal alloy in sea water applications makes it a highly superior anode metal. This combined with its high current capacity and its resistance to self corrosion makes the material almost completely selfregulating, producing only enough current to protect the cathode without waste.

I claim:

1. Method of protecting a body of iron, steel or other metals against corrosion in the presence of a water solution containing at least one ion selected from the group consisting of Cl, SO SO CO and Br which comprises attaching thereto in metal-to-metal contact a body of an alloy having a composition within the ranges Ingredients: Percent by weight Al 44.0-52.5. Mn 0.050.80. Mg 2.0-l0.0. Si nos-0.50. Fe Not more than 0.05. Ni Not more than 0.05. Cu Not more than 0.05 Impurities Not more than 0.30. Zn Remainder.

2. A device for protecting iron, steel or other metals against corrosion when in contact with a. water solution containing at least one ion selected from the group consisting of Cl, SO CO and Brconsisting of a body of an alloy having a composition within the ranges Ingredients: Percent by weight Al 44.0-52.5. Mn ODS-0.80. Mg 2.0-10.0. Si ODS-0.50. Fe Not more than 0.05. Ni Not more than 0.05 Cu Not more than 0.05 Impurities Not more than 0.30. Zn Remainder.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Brown Apr. 9, 1935 Christen July 11, 1939 Christen Dec. 31, 1940 Bonsack Mar. 3, 1942 Neu Jan. 20, 1959 Staley Nov. 17, 1959 Battis et a1. Sept. 26, 1961 Colwell Jan. 30, 1962 FOREIGN PATENTS Great Britain Mar. 6, 1957 OTHER REFERENCES

Claims (1)

1. METHOD OF PROTECTING A BODY OF IRON, STEEL OR OTHER METALS AGAINST COROSION IN THE PRESENCE OF A WATER SOLUTION CONTAINING AT LEAST ONE ION SELECTED FROM THE GROUP CONSISTING OF CL-, SO3--, SO4--, CO3-- AND BRWHICH COMPRISES ATTACHING THERETO IN METAL-TO-METAL CONTACT A BODY OF AN ALLOY HAVING A COMPOSITION WITHIN THE RANGES-