US2377792A - Preventing corrosion of ferrous metals by solutions of electrolytes - Google Patents

Description

June 1945. K. LAWRENCE ETAL 2,377,792

PREVENTING CORROSION OF FERROUS METALS BY SOLUTIONS OF ELECTROLYTES Filed Feb. 28, 1941 INVENTORS (WA/HE h. LAWRENCE am? ROBERTFE/VGLE ATTORNEY Patentcd June 5, 1945 orrice PREVENTING CORROSION OF FERROUS METALS BY "SOLUTIONS OF ELECTBO.

LYTES Charles K. Lawrence, Baldwinsvillc, and Robert F. Engle, Syracuse, N. Y., minors to The 801- vay Process Company, New York, N. Y., a corporation of New York Application February 28, 1941, Serial No. 380,995

2 Claims. (Cl. 20%-147) This invention relates to a process for preventing corrosion of a ferrous metal in contact with aqueous solutions of electrolytes which are corrosive towards the metal, and more specifically of electrolytes which liberate oxygen at the anode when electrolytically decomposed.

Numerous such electrolytes are known and, in general, their aqueous solutions are corrosive towards ferrous metals. For example, aqueous ammoniacal solutions of urea and carbon dioxide are on the market particularly as a material for use in the production of fertilizers. The tank cars commonly available for the shipment of these solutions as well as the equipment in which they are handled by the fertilizer manufacturers are made of various ferrous metals such as iron, steel and special alloy steels. These solutions are, however, corrosive toward the ferrous metals and numerous methods for reducing the rate of corrosion offerrous metals by these solutions have been proposed. For example, it has been proposed to add to the solution certain inhibitors. While the addition of inhibitor may make it practicable to handle the solutions in apparatus made of ferrous metals, even with an inhibitor present there is generally some residual small rate of corrosion of the metal by the solution. Furthermore, under some conditions of production, shipment or .use of these solutions, it would be advantageous if some means were available whereby corrosion offerrous metals by the solutions could be prevented without having to add a foreign material to the solution, with the attendant expense of providing the foreign material for addition to the solution and contamination of the solution therewith.

It has also been proposed to passify a ferrous metal by treatment with a strongly oxidizing agent such as chromic acid and then to contact the passified metal with an ammoniacal solution of ammonium nitrate (e. g-., a solution containing 60% ammonium nitrate, 20% ammonia and 20% water) in which a chromate has been dissolved to maintainthe passivity of the ferrous metal in contact with the solution. As in the methods described above, a disadvantage of this method is the necessity for dissolving in the ammonium nitrate-ammonia solution a foreign material. x l

In some cases it is not possible to employ in such solutions certain inhibitors. For-example, in producing ammonium nitrate by reaction of ammonia and nitric acid fed into a saturator vessel containing an aqueous ammonium nitrate solution. it is frequently desirable to maintain a low concentration of nitric acid in the solution. It is impracticable to dissolve in such a solution a thiocyanate, which may be used in an ammoniacal solution of ammonium nitrate, and yet it would be advantageous to have available some means for reducing the rate of attackby the acidammonium nitrate solution of the ferrous metal used as a material of construction for the saturator.

It is an object of this invention to provide a method for reducing the rate of corrosion of a ferrous metal by a solution which may be used without the need for contaminating the solution by addition thereto of foreign substances, or may be used in coniunction with the use of chemical agents, such as inhibitors, to further reduce a residual slow rate of corrosion of a ferrous metal by the inhibited solution.

We have discovered that by first passlvating the surfaces of a ferrous metal which is to be contacted by an aqueous solution of an electrolyte which liberates oxygen at the anode when electrolytically decomposed, and then making the passivated surfaces an anode in an electric circult which includes a cathode and said solution and a source of direct electric current which imposes upon the anode an electric potential within a particular range, corrosion of that ferrous surface by the solution in contact therewith is completely prevented or at least greatly reduced. The range of voltages within which the potential imposed on the anode must lie willvary somewhat, depending upon the speciflc metal of which the anode is constructed, composition of solution with which it is in contact, etc. In all cases the voltage is below the decomposition potential of the solution in contact with the inert electrode used as a cathode and the ferrous metal of the anode and above that at which the flow of current acts to promote corrosion of the ferrous metal surfaces of the anode. In the case of a tank car in which the interior surface of the container for an aqueous ammoniacal solution of tential applied to the tank and cathode therein should be at least 1.25 volts and not above 2.2

' volts. Somewhat below 1.25 volts the flow of 2 volts the current acts to electrolytically decompose the solution, liberating gaseous products which, accumulating in the free space in the container above the liquid surface, present a hazardous condition. Furthermore, decomposition of the material in the solution represents a loss of valuable material. With a voltage in the range 1.25 to 2.2 volts, practically complete protection of the ferrous metal surfaces is obtained without the dimculties, pointed out above, encountered at lower Or higher potentials.

Any method for passivating a ferrous metal by treatment with a chemical oxidizing agent may be used to effect the initial passivation of the ferrous metal surfaces. The metal surfaces may be treated with an oxidizing agent such as a solution of chromic acid, dichromate, potassium permanganate or nitric acid, as disclosed in U. s. P. 2,135,160. The metal surface may be washed with a solution of a strong oxidizing agent. e. g. KMBO4+5% chromic acid (CrOa) solution in water. Even relatively milder oxidation treatments are effective in coniimction with the electrolytic treatment of the containers with the solution of the electrolyte in contact with the metal surfaces as herein described. Thus, the oxidizing treatment to passivate the metal may be accomplished by steaming the metal surface at temperatures of at least 85 C. and then air-drying the surfaces.

We have discovered that by first passivating the metal surfaces by treating them with a chemical oxidizing agent and thereafter making the passive surfaces the anode in contact with the solution of the above described electrolytes, voltages below that causing electrolyte decomposition of the solution and above that at which the flow of current acts to promote corrosion of the ferrous metal surfaces of the anode will not only maintain the anode surfaces passive against attack by the solution but will even increase the effectiveness of the passive film on the metal surfaces. This has even been found to be true when the container for the solution is of large size, such a a tank car or storage tank of the size customarily used for the shipment and storage of these solutions, in which it is impracticable to employ cathodes having surface areas of the same order of magnitude as the surface areas of the container which are to be protected against corrosion by the solution.

In employing the above described means for preventing corrosion of ferrous metals by aqueous solutions of electrolytes, the cathode immersed in the solution may be of any electrically conducting material suitably resistant to attack by the solution. For example, aluminum is a suitably inert metal for the cathode in ammoniacal, acid or in neutral sodium nitrate solutions or aqueous solutions of nitric acid. Thus, a container of ferrous metal may be provided with an anode suitably inert towards an aqueous ammoniacal solution of sodium nitrate. The surfaces of the container which are to be contacted with the solution are treated with a chemical oxidizing agent, as described above, to passivate these surfaces. Thereafter the solution is filled into the container and the container connected to the positive and the cathode to the negative terminals of a source of direct electric current at a voltage below that at which the solution is electrolytically decomposed by the current and above that at which the flow of current acts to promote corrosion of the ferrous metal surfaces by the solution in contact therewith. Aluminum cathodes are also suitable for solutions of urea, which may also contain ammonia, carbon dioxide and/or salts such as ammonium nitrate.

For aqueous solutions of maleic acid, with or without other acids, cathodes of chrome-nickeliron alloys, such as the well known l8-8 group of these alloys, are suitably inert.

Despite the relatively low potential applied to the electrodes in contact with the solution, protection of the surfaces of large containers for the solution, such as tank cars or storage tanks or the commercial equipment of plants in which the solutions are produced or used, is obtained without having to unduly complicate the size and arrangement of the cathode in the container. Protection of the ferrous metal anode surfaces may be obtained with cathodes of very small surface area as compared with the anode surface area and unsymmetrically disposed with respect to the anode surfaces.

Prepassivating and making the container for the solution the anode under the conditions described above has been found to prevent or suitably reduce the rate of corrosion of the ferrous metal of which the container is constructed below the level of liquid in the tank. Corrosion may start, however, at surfaces above the liquid level and then extend downwardly to and below the liquid level. This is particularly liable to happen in the case of tank cars in which upper surfaces may be intermittently wetted with the solution when the car is in transit. Corrosion of the container surfaces above the liquid level which may be wetted by the liquid may be inhibited by having present in the solution an inhibitor.

To counteract the tendency of the surfaces at and above the liquid level to corrode, we prefer to paint these surfaces with a corrosionresistant paint after the metal surface has been passivated in addition to making the surfaces in contact with the solution an anode under the above described conditions. Any paint resistant to attack by the solution may be employed. A chlorinated rubber paint, marketed under the trade name Tornesit" has been found particularly good for use in a container for ammoniacal nitrate solutions. Other chlorinated rubber paints, such as that sold under the trade name "Densol or a coat of red lead primer under a coat of a chlorinated rubber paint sold under the trade name "Socony White" may be used for this purpose. The entire interior surface of the tank car may, of course, be painted in the above manner or the painted area may be confined to a strip immediately above and below the normal liquid level. This painting of the tank not only prevents corrosion immediately above the solution level but reduces the amount of current which must.be supplied in anodically protecting the metal surfaces by the process of our invention.

The simplicity of the apparatus required by the process of our invention for protecting ferrous metal surfaces against attack by solutions of electrolytes which liberate oxygen at the mode when electrolytically decomposed makes that process peculiarly adaptable for use in connection with large storage tanks or the tank cars in which such solutions are. shipped. The accompanving drawing illustrates diagrammatically an apparatus suitable for carrying out the process of this invention in such a tank car.

With reference to the drawing the numeral l indicates the tank for containing the solution. As is customary in tank cars for liquids, the tank is provided with a dome 2 through which an unloading pipe 3 passes to a sump in the bottom of the tank. This tank is carried by conventional running gear 4 which also carries a storage battery 5. This storage battery may comprise one or more 2-volt lead storage cells tank I to near the bottom of the tank. Preferably a second conductor rod I extends for some distance along the bottom of the tank and very close thereto. Rod 1 is in electrical contact with pipe and is supported on electrically insulatingsupports I. While pipe I is not a necessary feature of the apparatus, it isdesirable to insure protection of the tank when practically empty of solution. The positive terminal of battery 5 is electrically connected with tank I either directly, as shown in the drawing, or through the running gear of the car. The negative terminal of battery 8 is electrically connected with the top of rod 0. Thus, when solution of an electrolyte is introduced into tank I an electric circuit is completed through the solu' tion in contact with tank I as anode and rod 0, and rod 1 when this rod is included in the apparatus, as'cathode. A potential difference of 2 or 1.5 volts is thus applied to the anode and cathode, depending upon the type of battery used.

In employing the process of this invention for preventing'corrosion, the interior of the car is first passivated, for example by steaming it at temperatures above 85" C. and then admitting air to dry the car while it remains at these elevated temperatures. The dome may then be painted on the inside with a paint resistant to attack by the solution'to be shipped. The car is filled with an aqueous solution of an electrolyte such as described above, until the level of liquid rises into the dome of the car.

The following examples of methods for protecting ferrous metals against corrosion by aqueous solutions of electrolytes will further illustrate our invention:

An aqueous solution of urea, ammonia and carbon dioxide in the proportions by weight of 5.1 parts urea and 3.6 parts ammonia for every 1 part of carbon dioxide, corroded hot rolled, mild steel at the rate of 0.003 inch penetration per year. By passivating the steel and making it an anode in contact with the solution at a voltage of 1.2 between the steel and a cathode immersed in the solution, the metal was corroded at a rate of 0.0004 inch penetration per year; only about one-tenth the rate at which the metal was corroded when not protected by the process of our invention.

Hot rolled, mild steel is actively attacked by 40% maleic acid at room temperature. When the metal was passivated and made the anode in contactwith the aqueous maleic acid with an applied potential of 1.7 volts (i. e., a potential difference of 1.7 volts between the anode and a cathode immersed in the maleic acid solution).

the corrosion rate was reduced to 0.0009 inch potential of 0.9 volt, the rate of corrosion was reduced to 0.000! inch penetration per year.

It is known that iron contacted with 63% nitric acid becomes passive. reaction between the passive iron and acid does not take place, there is, nevertheless, a continued corrosion of the iron by the acid. After an initial period of one hour contact between 63% nitric acid and mild steel it was found that the steel was corroded by the acid at the rate of 0.01 inch penetration per year. By making the steel an anode in contact with the 63% nitric acid, with an aluminum cathode serving as the other electrode and a potential of 1 volt applied to the electrodes, it was found that the rate of corrosion of the metal by the acid following the initial one hour contact was reduced to 0.0061 inch penetration per year. Furthermore, when steel which has not been made an anode in the acid solution is removed from the solution, it spontaneously becomes active and subject to corrosion, whereas steel which has been made the anode remains passive upon removal from the acid. This ability of the process of our invention to make more stable the passivity of the iron in contact with the electrolyte is of particular importance when a situation is encountered in which the iron to be protected against corrosion is alternately in contact and out of contact with the corroding solution.

The use of an aluminum. cathode of small area as compared with the area of the ferrous metal anode is particularly important in connection with protecting ferrous metals against corrosion by concentrated nitric acid. Using an iron or platinum cathode in place of the aluminum, applied voltages above 0.6 for an iron cathode and over 0.4 for a smooth platinum cathode cause decomposition of the nitric acid at the cathode surface. Accordingly, when these cathodes are used the applied voltages which do not cause electrolytic decomposition of the solution and thus may be used for protecting the ferrous metal anode against corrosion, are low. An aluminum cathode in contact with the acid supplies, a relatively high back E. M. F. and potentials up to about 1.3 volts then may be applied to the cathode without decomposing the nitric acid. The use of these higher voltages with an aluminum cathode of small area as compared with the area of the anode is advantageous inasmuch as the higher the potential which is used the more eifectively is the ferrous metal anode protected against attack by the nitric acid.

The applied potentials which are efiective to prevent corrosion of the anode surfaces depend not-only upon the particular solution with which one is concerned, but also upon the type and size of the equipment in which surfaces are to be protected by the process of this invention, since the interfacial potential drop between anode and solution depends upon the size and arrangement of the anode and cathode. As indicated above for nitric acid solutions, the material used for the cathode may have an influence upon the range of suitable potentials applied to the electrodes. Generally, the larger theapparatus, the higher the applied potentials which will be used. although in all cases the applied potentials are low.

The process for protecting a ferrous metal against corrosion by ammoniacal solutions of ammonium nitrate, described but not specifically claimed herein, is described and specifi- Although rapid cally claimed in our copending application 8erial No. 380,994, filed February 28, 1941.

We claim:

1. The process for protecting a ferrous metal against corrosion by an aqueous solution of maleic acid, said metal being one subject to corrosion by said solution, which comprises passivating the surface of said metal which is to be contacted with said solution by treating said surface with a chemical oxidizing agent, thereafter placing said metal surface in contact with said solution and making said ferrous metal the anode in an electric circuit completed through said solution in contact with said metal, a cathode in said solution and a source of direct current at a voltage which is below that at which said solution is electrolytically decomposed in contact with said anode and cathode and is above those at which the flow of current increases the rate of corrosion of said surface by the solution in contact therewith as compared with the rate of corrosion of the same surfaces in contact with the solution without the application of the potential.

2. The process for protecting the ferrous metal surface of a container for an aqueous solution of maleic acid against corrosion thereby, said metal being one sublect to earoeion by said solution, which comprises polluting said eurface by treating it with a chemical oxidizing agent, thereafter contacting said surface with said solution and immereins in the solution an electrode of small size and uneymmetricaliy disposed with respect to said surface of ferrous metal, electrically connecting the negative and podtive terminals of a source of direct electric current to said electrode and to said ferrous metal, respectively, thereby making said ferrous metal the anode in the electric circuit completed through said solution, and by means of said source of electric current maintaining between said ferrous metal and said inert electrode a potential difference which is below that at which said solution is electrolytically decomposed in contact with said ferrous metal anode and said inert electrode and is above those at which the flow of current increases the rate of corrosion of the ferrous metal surface by the solution in contact therewith as compared with the rate of corrosion of the same surfaces in contact with the solution without the application of the potential.

- CHARLES K. LAWRENCE.

ROBERT F. ENGLEV

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