US2366796A - Preventing corrosion of ferrous metals by ammoniacal solutions of ammonium nitrate - Google Patents

Description

1945- c. K. LAWRENCE ET AL 2,366,796

PREVENTING CORROSION OFFERROUS METALS BY AMMONIACAL SOLUTIONS OF AMMONIUM NITRATE Filed Feb. 28, 1941 7 7 F l ll INVENTOR5 ("HA/H55 ATTO R N EY Patented Jan. 9, 1945 PREVENTING CORROSION OF FERROUS METALS BY AMIMONIACAL SOLUTIONS OF AMMONIUM NITRATE Charles K. Lawrence, Baldwinsville, and Robert F. Engle, Syracuse, N. Y., assignors to The Solvay ProcessCompany,NewYork, N. Y., a corporation of New York Application February 28, 1941, Serial No. 380,994

11 Claims.

This'invention relates to a process for preventing corrosion of a ferrousmetal in contact with aqueous solutions of ammonium nitrate and particularly of aqueous ammoniacal solutions of ammonium nitrate. contact with the solution. As in the methods For many years past aqueous ammoniacal soludescribed above, a disadvantage of this method tions of ammonium nitrate havebeen on the maris the necessity for dissolving in th ammonium ket particularly as a material for use in the pronitrate-ammonia solution a, foreign material. duction of fertilizers. The tank cars commonly While non-ammoniacal aqueous solutions of available for the shipment of these solutions as ammonium nitrate do not have the high corrowell as the equipment in which they are handled siveness towards ferrous metals that the amby the fertilizer manufacturers are made of varmbniacal solutions do, nevertheless such solutions ious ferrous metals such as iron, steel and special are corrosive towards these metals. For example, a l y e e e s t s fir, 001- a saturated aqueous ammonium nitrate solution rosive towards the ferrous metals. For example, was found to corrode a hot rolled, mild steel at a hot rolled, mild steel in contact with a solution the rate of 0.006 inch penetration per year and containing ammonium nitrate, am-' the presence in such a solution of as little as 0.1% monia and 20% water was found to be corroded nitric acid increased the rate of corrosion to 0.047 at the rate of 0.55 inch penetration per year. A inch penetration per year. Such non-ammoniacal saturated aqueous solution of ammonium nitrate 20 ammonium nitrate solutions are frequently encontaining as little as 1% ammonia was found to countered, in commercial practice and. in some corrode this steel at the rate of 0.57 inch penetracases it is not possible to employ in such solution P Y tions certain inhibitors which can be successfully In view of the highly corrosive nature of these used in the ammoniacal solutions. For example,

solutions, extensive research has .been carried out in producing ammonium nitrate by reaction of to find methods for sufllciently reducing the rate ammonia and nitric acid fed into a saturator of corrosion of ferrous metals by these solutions vessel containing an aqueous ammonium nitrate $0 t WOuld be p a e t S re, ship and solution, it is frequently desirable to maintain handle them in iron and steel equipment. One a low concentration of nitric acid in the solution. type of method developed involved the addition It is impracticable to dissolve in such a solution to. the solution of certain inhibitors. For exa thiocyanate as an inhibitor and yet it would ample, as disclosed in U. S. P. 2,215,077, which. be advantageous to have available some means issued September 17, 1940, to Beekhuis and Mafor reducing the rate of attack by the acid-amcomber, the addition of a small amount of cermonium nitrate solution of the ferrous metal used tain sulfur compounds, e. g.,' ammonium thioas a material of construction for the saturator. cyanate, greatly reduced the rate of attack of It is an object of this invention to provide a iron and steel by the solution. While the addimethod for reducing the rate of corrosion of a tion to the solution of such inhibitors has made ferrous metal by an aqueous am nitrate it practicable to handle the'solutions in 'apparasolution which may be used without the need for tus made of ferrous metals, even with these incontaminating the solution by addition thereto of hibitors present there is generally some residual foreign substances, or may be used in conjunction small rate of corrosion of the metal by the soluwith the use of chemical agents, such as inhibition. Furthermore, under some conditions of tors, to further reduce the residual slow rate of production, shipment or use of these solutions, corrosion of a ferrous metalby the inhibited it would be'advantageous if some means were solution.

available whereby corrosion of ferrous metals by thesolutions 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 solut on and contamination of the solution therewith.

It has also been proposed, in U. S. P. 2,135,160, 7-. which issued November 1,' 1938, toHerman A. Beekhuis, Jr., 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 in which a chromate has been dissolved to maintain the passivity of the ferrous metal in We have discovered that by making the surfaces of a ferrous metal in contact with an aqueous solution of ammonium nitrate the anode in an electric circuit which includes a cathode in contact with the solution and a source of'direct electric current which imposes upon the anode an electric potentialwithina particular range, corrosion of that ferrous surface is completely prevented or at least greatly reduced. The range of voltages w thin which the potential imposed on the anode must lie will vary somewhat, depending uponthe specific metal of which the anode is constructed, composition of solution with which it is in contact, etc. 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 case the container for the ammoniacal ammonium nitrate solution is of large size, such as 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, the application to the electrodes of voltages below the decomposition potential of the solution will not of itself be suificient'to prevent corrosion of an initially active metal. In such cases, however,we have discovered that by first passivating the metal surfaces and thereafter making the passive surfaces the anode in contact with the ammoniacal ammonium nitrate solution, applied voltages below the decomposition potential 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.

Any method for passivating a ferrous meta I may be used to effect the initial passivation of the ferrous metal surfaces. Preferably, 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 oxidiz- In all cases the voltage is ing agent. e. g., 10% KMnO4 5% chromic acid (CrOg) solution in water. Even relatively milder oxidation treatments are effective in conjunction with the electrolytic treatment of the containers with the ammoniacal ammonium nitrate solution 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.

In the case of a tank car in which the interior surface of the container for an aqueous ammoniacal solution of ammonium nitrate has been passified and then made an anode with a cathode dipping into the solution in the central part of,the tank, the potential 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 electrical current between the electrodes in the solution acts to promote corrosion of the ferrous metal anode by the solution. Above 2.2 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 difflculties. pointed out above, encountered at lower or higher potentials.

asearee In employing the above described means for preventing corrosion of ferrous metals by aqueous ammonium nitrate solutions the cathode immersed in the solution may be of any electricallyconducting 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 ammonium nitrate solutions.

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.

Making the container for the ammonium nitrate 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 ammonium nitrate 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 corrosion-resistant paint after giving the metal surface an oxidizing passivating treatment in addition to making the surfaces in contact with the solution an anode under the above described conditions. Any paint resistant to attack by an aqueous ammonium nitrate 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 ammonium 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. 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 currentwhich must be supplied in anodically protecting themetal surfaces by the process of our invention.

The simplicity of the apparatus required for by ammonium nitrate solutions in accordance with the process of our invention makes that process peculiarly adaptable for use in connec-' tion with the tank cars in which aqueous ammonium nitrate solutions are shipped. The accompanying drawing illustrates diagrammatically an apparatus suitable for carrying out the process of this invention in such a tank car. 7

With reference to the drawing the numeral I aaeavoe 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 a conventional running gear 4 which also carries a storage battery 5. This storage battery may comprise one or more'2-volt lead storage cells or about 1.5 volt Edison alkaline batteries for delivery of a direct electric current at a poten-- tial of 2 or 1.5 volts, respectively. An aluminum rod 6 extends through a fitting 9 in dome 2 electrically insulating the pipe from the dome and reaches downwardly through the interior of tank I to near the bottom of the tank. Prefer ably a second aluminum rod 1 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 insulat- -ing supports 8. While pipe. I is not a necessary feature of the apparatus, it is desirable 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 5 is electrically connected with the top of rod 6. Thus, when an aqueous ammoniacal solution of ammonium nitrate is introduced into tank I an electric circuit 'is completed through the solution in contact with tank I as anode and rod 6, and rod.! 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, an ordinary tank car supplied for shipping an ammonium nitrate solution was provided with an aluminum cathode consisting of a rod passing through an insulator in the dome in the car with the lower end of the rod extending within six inches of the bottom of the car. This rod corresponded to rod 6 in the apparatus of the drawing. As a source of longer standin however, the solution started then painted with Densol.

direct electric current two lead storage batteries connected in parallel were used. The positive terminals of the batteries were connected with the dome of the car and the negativev terminals with rod 5.

The interior of the car was first steamed at temperatures above 85 C. and then air was admitted to dry the car while it remained at these elevated temperatures. The dome was then painted on the inside 'with two coats of "Tornesit. The car was filled with solution containing ammonium nitrate, 20% ammonia and 20% water to which had been added 0.1% ammonium thiocyanate nd 0.5%. AS203 as an arsenite. The car was led until the level of liquid rose into the dome of the car.

While the car was being filled, the .current passing between the cartank and aluminum cathode rose to a maximum of about I! amperes. Twenty-four hours after loading the current had decreased to aboutfi'amperes and it continued to drop thereafter'so that the daily average current had dropped to 0.7 ampere on the 12th day. With a surface area of the tank in contact with the solution. of about 850 square feet, this was .equal to about 0.0008 ampere of current per square foot of anode area. At the endof twelve days, the battery was disconnected from the car and for a'period of ten days the tank remained passive and unattacked. by the solution. On

' ammonium nitrate, 20%

year.

' to corrode the interior surfaces of the tank.

As shown by this example, in utilizing the process of our invention it is not necessary, although in some cases it may be. convenient and desirable, to continuously maintain an electric potential on the container as an anode to prevent corrosion of the container surfaces. ,Contact of the ferrous metal surfaces as anode with the ammonium nitrate solution under the conditions described above forcarrying out a specific procedure embodying our invention develops in the passive surfaces of the ferrous metal a passivity of a nature such that it will persist for a period of time after the source of electric current is disconnected from the container and prevent substantial corrosion of the container for some time after the electric current has ceased to flow. Accordingly, our invention contemplates a procedure in which a container which is to be filled and later emptied of the ammonium nitrate solution, as in shipping the solution in a tank car, is made the anode in contact with the passivity which will persist and will itself prevent attack of the metal by the solution. The electric current is then cut off from the container, although the solution is still left in contact with the surfaces thereof. Or a container for the ammonium nitrate solution may be connectedas anode to a source of electric current to develop this resistance to attack by the solution and thereafter the electric potential is applied to thecontainer only intermittently, as needed to maintain the passivity of the container surface against attack by the solution.

The procedure of the above example may be varied in numerous ways without, departing from the scope of our invention. For example, the interior of the tank car may'be dried out in a current of air, without first steaming it, and

Under these conditions, after filling the car with the ammoniacal solution of the above example, the average daily,

current passing at 2 volts potential applied to the tank was 7 amperes for the first day. By the tenth day the current had fallen to a daily average of about 3 amperes. When the car was un-, loaded on the 16th day inspection of the surfaces of the tank showed no corrosion.

Hot rolled, mild steel which, as pointed out above, was corroded by a solution containing 60% ammonia and 20% water at the rate of 0.5 inch penetration per year showed no corrosion when first passivated and then made the anode in contact with the solution with a potential of 1 volt applied'to the anode and a cathode in contactwith'the solution. The average current passing was only 0.00035 ampere per square foot of anode surface.

In contact with the same solution containing 0.1% ammonium thiocyanate, this steel was corroded at the rate of 0.011 inch penetration per 0.00011 ampere per square foot of anode surface.

A saturated aqueous solution of ammonium nitrate containing 0.1% nitric acid corroded] this steel at the rate of 0.047 inch penetration per When the steel was made the anode in contact with the solution with a potential of 1.2 volts applied to the electrodes the rate of corrosion dropped to 0.004 inch penetration per year with an average currentpassing of 0.0039 ampere per square foot of anode surface. These applied potentials of 1 to 1.2 volts were effective to substantially prevent corrosion of previously passivated anodes with areas much smaller than the interior of tank cars used for shipping ammoniacal solutions of ammonium nitrate, The applied potentials which are effective to prevent corrosion of the anode surfaces depend 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. The material used for the cathode may have an influence upon the range of suitable potentials applied to the electrodes. Generally, the larger the apparatus, the higher the applied potentials which will be used, although in all cases the applied potentials are low, e. g., the 1.25 to 2.2 volts which are suitable for tank cars.

The process of protecting a ferrous metal against corrosion by an aqueous ammonium nitrate solution disclosed and claimed in this application is disclosed and claimed in our copending application Serial No. 380,995, filed February 28, 1941, for protecting ferrous 'metals against corrosion by aqueous solutions of electrolytes which liberate oxygen at the anode when electrolytically decomposed.

We claim:

1. The process for protecting passivated surfaces of a ferrous metal against corrosion by an aqueous ammonium nitrate solution contacted therewith, said ferrous metal being one subject to corrosion by said solution, which comprises making said ferrous metal the anode in an electric circuit completed through said solution in contact with said passivated surfaces of the metal, an inert cathod in said solution and a source of direct current at a voltage which is below that at which said solution is electrolytically decomposedin contact with said anode and cathode and is above those at which the flow of current increases the rate of corrosion of said surfaces 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 thereto of an electrical potential.

2. The process for protecting a ferrous metal against corrosion by an aqueous ammonium nitrate solution contacted therewith, said metal being one subject to corrosion by said solution, which comprises passiv'ating the surface of said metal which is to be contacted with said solution by treating said surfaces with an oxidizin agent, thereafter placing said surfaces in contact with said solution and making said surfaces the anode in an electric circuit completed through said solution in contact with said metal, a cath-' aseavae corrosion by the solution, which comprises immersing an inert electrode in the solution in said container, electrically connecting the negative and positive terminals of a source of direct electric current to said electrode and to said ferrous metal, respectively, thereby making said passivated ferrous metal surfaces the anode in an 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 surfaces and said inert electrode and is above those at which the flow of current increases the rate of corrosion of the ferrous metal surfaces by the solution in contact therewith as compared with the rate of corrosion of the same ode 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'surfaces by the solution in contact therewith as compared with the rate of corrosion of the same surfaces in contact with of a passivated ferrous metal which is subject to 1 surfaces in contact with the solution without the application thereto of an electrical potential.

4. The process for protecting the ferrous metal surfaces of a container for an aqueous solution of ammonium nitrate against corrosion thereby,

said ferrous metal being onewsubject to corrosion by said solution, which comprises passivating said surfaces by treating it with an oxidizing agent, thereafter contacting said surfaces with said solution and immersing in the solution an aluminum electrode of small size and unsymmetrically disposed with respect to said surfaces of ferrous metal, electrically connecting the negative and positive terminals ofa source of direct electric current to said electrode and to said ferrous metal, respectively, thereby making said ferrous metal surfaces 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 aluminum electrode a potential difference below that at which said solution is electrically decomposed in contact with said ferrousmetal and aluminum electrode and above those at which the flow of current increases the rate of corrosion of the ferrous metal surfaces by the solution as compared with the rate of corrosion of the same surfaces in contact with the solution without the application of the potential.

5. The process for protecting a ferrous metal against corrosion by an aqueous ammonium nitrate solution contacted therewith, said metal being one subject to corrosion by said solution, which comprises subjecting the surfaces of said metal which are to be contacted by said solution to an oxidizing passivating treatment, coating thus treated surfaces with a paint resistant to attack by said solution, and making the thus treated'and coated ferrous metal the anode in an electric circuit completed through said solution in contact with said surfaces, 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 ferrous metal surfaces and said cathode and is above those at which the flow of current increases the rate of corrosion of said'metal by the solution in contact therewith as compared with the rate of corrosion ofthe same surfaces in contact with the solution without the appli- I against corrosion thereby, said container having interior surfaces of a ferrous metal subject to corrosion by said solution which comprises subjecting said interior surfaces. of said container to an oxidizing passivating .treatment, coating thus treated surfaces adjacent to and above the level of liquid to be placed in said container with a paint resistant to attack by said solution, introducing said solution into the thus treated container, immersing an electrode in the. solu-- tion in said container, electrically connecting the negative and positive terminals of a source of direct electric current to said electrode and to said ferrous metal, respectively, thereby making the ferrous metal of the container which is in contact with said solution the anode in the electric cirat a potential of 1.25 to 2.2 volts, and completcuit completed through said solution, and by means of said source of electric current maintaining between said ferrous metal and said electrade a potential difference below that at which said solution is electrically decomposed in contact with said ferrous metal'and said electrode and above those at which the flow of current increases the rate of corrosion of the ferrous metal surfaces 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.

7. The process for preventing corrosion of a tank car during shipment therein of an aqueous ammoniacal solution of ammonium nitrate, the surfaces of said tank car exposed to contact with said solution being composed of a ferrous metal subject to corrosion by the solution, which com prises passivating said ferrous metal surfaces by treatment with an oxidizing agent, introducing said solution into said tank-car and into contact with the thus passified surfaces, electrically connecting said ferrous metal surfaces to the positive terminal, and electrically connecting an aluminum cathode immersed in the solution in the tank car to the negative terminal of asource of direct current at a voltage of 1.25 to. 2.2 volts thereby making said ferrous metal surfaces of said tank car the anode in a circuit completed through said solution and the cathode and substantially preventing corrosion of the ferrous metal surfaces by said solution.

8. The process for preventing corrosion of a tank car during shipment therein of an aqueous ammoniacal solution of ammonium nitrate, said tank car having surfaces exposed to contact with said solution which are composed of a ferrous metal subject to corrosion by the solution, which comprises passivating said surfaces by treatment with an oxidizing agent, introducing said solution into said tank car the surfaces of which have been thus passified, immersing in the solution insaid tank car an aluminum electrode of small size and unsymmetrically disposed with ing an electric circuit including said ferrous metal as an anode in contact with said solution and an aluminum cathode immersed in the solution and electrically connected with the negative terminal of said source of direct current and preventing corrosion of the ferrous metal incontact with said solution.

' 10. The process for loading and preparing a tank car for shipment therein of an aqueous ammoniacal solution of ammonium nitrate, said tank car having surfaces of ferrous metal to be exposed to contact with said solution, which metal is subject to corrosion by said solution, and said tank car being provided with an aluminum electrode adapted to be immersed in the solution, which comprises passivating said surfaces by treating them with an oxidizing agent, electrically connecting said electrode and said ferrous metal of the tank car to the negative and positive terminals, respectively, of a source of direct electric current at a potential of 1.25 to 2.2 volts, introducing said solution into said tank car to complete an electric circuit through the solution between said electrode and the ferrous metal surfaces of the tank car and after introduction of said solution into the tank car maintaining said electrode and surfaces of the tank car electrically connected to said source of current for a substantial period of time long enough to impart to said ferrous metal surfaces'a passivity against corrosion by said solution which persists when the metal surfaces of the tank car are electrically disconnected from said source of electric current.

"11. Thie process for protecting passivated surfaces of a ferrous metal against corrosion by anaqueous ammonium nitrate solution contacted therewith, said ferrous metal being one subject to corrosion by said solution, which comprises making said ferrous metal the anode in an electric circuit completed through said solution in respect to said surfaces of ferrous metal, and

contact with said passivated surfaces of the metal, an inert 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 surfaces of ferrous metal 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 thereto of an electrical potential, and incorporating in said solution an inhibitor of corrosion of said ferrous.

metal by the solution.

CHARLES K. LAWRENCE. ROBERT F. ENGLE.

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