US3501261A - Corrosion control in sealed heat storage modules - Google Patents

Description

March 17, 1970 I R ETAL CORROSION CONTROL IN SEALED HEAT STORAGE MODULES Filed Sept. 26, 1967 u E nwl'll"! United States Patent 3,501,261 CORROSION CONTROL IN SEALED HEAT STORAGE MODULES Richard E. Rice and Willis Thompson Lawrence, Arlington, Mass., assignors, by mesne assignments, to Hookerv Chemical Corporation, Niagara Falls, N.Y., a corporation of New York Filed Sept. 26, 1967, Ser. No. 670,544

Int. Cl. C23f 11/06, 15/00; F28d 13/00 US. Cl. 212.5 9 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION In heat storage systems in which sensible and latent beats are stored at high temperatures in inorganic salt mixtures containing appreciable quantities of sodium hydroxide or other alkali metal hydroxides, the heat storing medium is most economically held in containers formed from ordinary mild steel, Such mixtures have thermal expansion coefficients substantially greater than that of steel. Consequently, it is necessary to provide an expansion space in the container above the surface of the heat storage medium.

Heretofore, this clearance space has been in communication with the ambient air through a breather system through which air could enter and leave as the heat storage medium is cooled and heated, respectively. This has insured that no excessive pressures develop inside the container, and that the heat storing salt mixture has access periodically to fresh air to maintain a substantially non-corrosive condition in the container, according to a chemical process which is described below. In order to prevent excessive heat loss to the surroundings, the tubes connecting the clearance space to the outside air must be small in diameter, and in order to prevent plugging by deposits of sodium carbonate formed by reaction of the carbon dioxide in air with the sodium hydroxide, it is necessary to use chemical absorbents or special configurations of the breathing device. Copending application Ser.

No. 556,230 filed by Richard E. Rice on Apr. 9, 1965, and US. 3,320,724, issued May 23, 1967, to Richard E. Rice, illustrate these embodiments.

In some applications of heat storage, it is highly desirable to avoid the complications of such protective devices, by hermetically sealing the container, thus preventing air from circulating through the clearance space. However, the alkali metal hydroxide compositions used for heat storing purposes cannot be operated in a sealed container throughout the desired temperature range which 'may extend to 900 degrees Fahrenheit or higher, because of excessive gas pressures which develop in the container, and because highly corrosive conditions develop if such compositions are deprived of air.

Materials which have been most successfully used for heat storing purposes in containers having breather systems comprise caustic soda, oxidizing agents such as sodium nitrate and/or sodium chromate, and catalytic agents such as manganese dioxide. When heated above 3,501,261 Patented Mar. 17, 1970 their fusion temperatures in steel containers, films of iron oxide form on the inner walls of the container. These films protect the steel from the excessively high rates of corrosion by the caustic soda, which would otherwise occur. Concurrently with the film formation, the oxidizing components of the scale mixture are reduced, and to maintain a permanently non-corrosive condition such a system requires continuous or periodic replenishment of the oxygen which is consumed. If such a mixture is sealed in a steel container and heated for protracted periods of time above its melting point, the sodium nitrate component, which in a typical case may be 8 percent of the total weight of the heat storing material, may be reduced first to sodium nitriate and subsequently to gaseous products which may include ammonia, water vapor, and hydrogen. This reaction can produce high pressures inside the container and an excessively high rate of corrosion.

Prior art salt compositions containing even minor amounts of alkali metal hydroxides have been noted for their tendency to corrode iron or steel receptacles and equipment. For example, Beck in US. 2,211,047, issued Aug. 13, 1940, used alkali metal chlorides containing minor amounts such as 5-15 percent alkali metal hydroxides as heat transfer media. He noted the excessive corrison of iron or steel equipment contacting his fused salt mixture and overcame the problem by addition of finely divided carbon into the heat transfer media. No mention is made in this patent of the problems attending the prevention of corrosion in sealed heat storage units or the actual cause of the corrosion problem.

Procedures have been recently developed for treating alkali metal hydroxides after which they can be hermetically sealed in ametal container and operated safely without excessive corrosion of the inner surfaces of the container. In essence, if completely free of other salts or impurities containing oxygen, and especially if completely free of water, caustic soda can under certain conditions be sealed in a steel container without deleterious results.

Caustic soda has an extremely high afiinity for water and even when heated considerably beyond its melting point of 605 degrees Fahrenheit will retain quantities of water which are deleterious in sealed heated containers. This water is very dilficult to eliminate, but it has been removed successfully by one or a combination of the following procedures.

One method consists in holding fused caustic soda for a period of time at a temperature of 900 degrees Fahrenheit or above while in contact with finely powdered metallic iron. This results in the elimination of moisture, and the saturation of the caustic soda with the by-products of the corrosion reaction. The principal chemical reactions involved are believed to be as follows:

Another method involves holding the caustic soda at a temperature of approximately 900 degrees Fahrenheit or higher and concurrently drying it by maintaining an atmosphere of dry air in contact with the caustic soda.

A third method involves contacting the caustic soda under the conditions presented in the preceding paragraph, with oxygen-free hydrogen rather than air.

As indicated in Equation I, one mole of iron powder reacts with one mole of water contained in the caustic soda. Taking into account molecular weights, one weight unit of water requires 3.1 weight units of iron for complete reaction. Therefore, a preferred ratio of iron powder to water contained in the caustic soda is at least 3.1 to 1, respectively, by weight. In many practical cases, iron in excess of this ratio will be used to insure that water is comletely eliminated.

The caustic soda, having been treated as described above, is charged into a steel container leaving a clearance space at the top. The clearance space may either be filled with hydrogen at atmospheric or sub-atmospheric pressure, or the air may be removed and the clearance space left substantially evacuated. The container is then hermetically sealed. The container is then heated relatively rapidly to a temperature of at least 700 degrees Fahrenheit, at which temperature hydrogen initially present in the clearance space, plus any additional hydrogen which is generated by reaction of the steel with caustic according to Equations I or II is rapidly eliminated by diffusion through the walls of the container.

During subsequent operation as a heat storing element, the container is thermally cycled according to a schedule which maintains it for an appreciable fraction of time at a temperature above approximately 700 degrees Fahrenheit, which insures that any additional hydrogen generated internally is eliminated by diffusion through the steel walls of the container. Under these conditions the pressure in the clearance space will normally assume a value in the vicinity of 50 torr (millimeters of mercury absolute).

It is an object of this invention to provide a method for reducing the internal corrosion of hermetically sealed, steel containers for alkali metal hydroxide heat storage compositions.

Also, it is an object of this invention to develop a steel heat storing apparatus which is adapted to house alkali metal hydroxide compositions without substantial corrosion internally.

Furthermore, it is an object of this invention to retard the corrosive chemical reaction occurring on the inside surfaces of a steel caustic soda storage container, by performing a reaction at the outside surface of the container which will retard the internal reaction by mass action.

Other objects and embodiments of this invention will become apparent from the following more detailed disclosure, examples and drawing.

BRIEF DESCRIPTION OF THE INVENTION We have now discovered that the internal corrosion reaction in a hermetically sealed metal container, holding a heat storing medium containing an alkali metal hydroxide, may be retarded by chemical reaction-s occurring on the external surfaces of the container. We have discovered that the rate of attack on the internal surfaces of the sealed steel container, by the caustic soda, can be controlled by the composition of the gaseous atmosphere in contact with the outside surfaces of the container while at elevated temperature. For example, if the surrounding atmosphere contains an appreciable partial pressure of water vapor, the rate of corrosion on the internal surfaces is reduced to or even the rate which occurs if the surrounding atmosphere is completely chemically inert with respect to the steel, or if the container is located in an evacuated space. Thus, it is within the scope of this invention to use the moisture content of the surrounding atmosphere, or the chemical composition of the atmosphere, or coatings which may be applied to the outside surfaces of the container, as means of controlling the internal corrosion rate during the use of the container as a heat storing element.

A suitable coating material is one which has the ability to absorb moisture from the air at temperatures near the lower end of the temperature range through which the heat storage container is cycled in normal operation. At temperatures above approximately 700 degrees Fahrenheit, where the internal corrosion rate would othenwise be high, this moisture is available to react with the steel according to the equation:

thus releasing atomic hydrogen to diffuse inward through the container wall to oppose the production of hydrogen by the internal corrosion reaction and reduce the corrosion rate.

The temperature range through which a container of an alkali metal hydroxide heat storage composition may be cycled repeatedly for long periods of time, in practice, varies with the thickness of the container wall, the type of metal or alloy from which the container is constructed, and with the specific heat storage material being used. For example, with thin steel containers housing an alkali metal hydroxide composition, the practical upper temperature limitation in operation is about 900 degrees Fahrenheit. For other container alloys and other heat storage materials the practical temperature limit is 1200 degrees Fahrenheit and above.

Actually, by generating hydrogen on the external surface of the container, the corrosion problem is not merely transferred to the outer surface, but the total corrosion of the container wall is reduced. The cumulative corrosion of internal and external surfaces of a container housing an alkali metal hydroxide composition is reduced by a factor of about ten when a chemical reaction is performed at the outer surface of the container to produce hydrogen.

DETAILED DESCRIPTION OF THE INVENTION The complete heat storage module contemplated by this invention can best be exemplified and understood by reference to the accompanying drawing. It is understood that an external coating of material which is hygroscopic is not a critical element of this invention although it is exemplified in the drawing. Actually, any method of bringing water into reactive contact with the outside surface of the steel heat storage tank may be used to produce atomic hydrogen in proximity to the surface of the storage vessel.

The drawing illustrates a partial central section view of a heat storage module, comprising a steel shell 10 and two end closures 14, sealed at 15, as by welding. The heat storage medium is an alkali metal hydroxide composition free of water which has been heated to 900 degrees Farhenheit in contact with iron powder, in a preferred embodiment of the invention. Gas 13 is principally hydrogen, which under operating conditions including high temperature operation, will pass through steel wall 10. A coating of a hygroscopic material 11 is placed upon the outside surface of the steel tank wall 10 to remove moisture from the atmosphere during periods of low operation temperature.

Example 1 A small steel container (hereinafter called a capsule) was partially filled with caustic soda and hermetically sealed. This capsule was placed in a glass and fused quartz vacuum system which contained a means for heating and maintaining the capsule at the desired temperature. The apparatus was also equipped with vacuum pumps, a valve by which the portion of the apparatus containing the capsule can be isolated from the vacuum pump, and a pressure measuring device for measuring the pressure in the isolated portion of the system. The only gaseous product produced by the drying and corrosion reactions was hydrogen. By maintaining the capsule at a temperature in the 700 800 degrees Fahrenheit range, the hydrogen diffused through the steel walls of the capsule. When a steady state was reached, the rate of dilfusion was the same as the rate at which hydrogen was produced. This rate was determined by closing the vacuum valve to isolate the portion of the system holding the capsule and measuring the rate of rise of pressure which resulted from eflusion of hydrogen from the capsule. The rate of the corrosion reactions were then determined from the constants of the system and the rate of pressure rise. Under the conditions of this experiment, the outside surface of the capsule was contacted by hydrogen only, at a pressure in the range of about 5 10- torr. When the test was finally terminated, the corrosion rates were confirmed by mechanical measurements made on the walls of the container itself.

The corrosion rates determined by this method were found to be between 0.012 and 0.040 inch per year. This corrosion rate is approximately 50 times as great as the corrosion rates occurring when the walls of the vessel are exposed to an atmosphere containing water vapor.

Example 2 A sealed cylindrical container having a capacity of approximately 700 grams of caustic soda was prepared. Inside the container two sheet steel coupons approximately 1" x 2" were positioned so that one was totally submerged in the molten material and the other so that most of its surface was exposed in the clearance space above the molten caustic soda. The container was heated at 900 degrees Fahrenheit with its exterior surfaces exposed to air containing water vapor at a partial pressure of 3-15 torr. The corrosion rate occurring on the inner surfaces of the container and on the samples was determined at intervals by noting the changes in weight which occurred and by measuring the change in the thickness of the metal sheets. The corrosion rate determined by this method was between 0.0002 to 0.0007 inch per year.

The diiference in corrosion rates between the preceding comparative examples was attributed to the different atmospheres or external environments of the two types of containers. The container used for the weighed coupon measurements was surrounded by the ambient air which normally contains an appreciable partial pressure of water vapor, whereas the capsules used in the hydrogen effusion test were completely blanketed by the emitted hydrogen which contained no appreciable water vapor.

It is clear from re-examination of Equations I and II that an increase in the pressure of hydrogen in such a system will tend to shift the equilibrium toward the left in both cases, by mass action. An increase of hydrogen pressure will decrease the corrosion rate on the internal surface of a sealed heat storage vessel containing an alkali metal hydroxide composition. In the range 700-900 degrees Fahrenheit, atomic hydrogen diffuses very readily through the steel walls of the container, and the effective hydrogen pressure at the corrodiug surfaces, i.e., the internal surface of the container, can therefore be influenced by a condition which produces atomic hydrogen at the external surfaces.

Atomic hydrogen is in equilibrium with molecular hydrogen. The solubility of atomic hydrogen in steel and the possible formation of molecular hydrogen within pockets or the interstices of the steel wall of a container may directly influence the diifusion of hydrogen through the steel.

It is believed that only atomic hydrogen diffuses through steel under these conditions, whereas molecular hydrogen does not pass through the steel. The hydrogen gas in the atmosphere surrounding the container is almost entirely molecular, and at practical pressures it would have an inappreciable influence on the internal corrosion rate.

Water vapor in the surrounding gas, however, can react with the iron at the outside surfaces of the container as is illustrated by Equation III, supra. The atomic hydrogen thus produced at the external surface can exert a pressure opposing that of the hydrogen produced on the internal surface by the corrosion reaction and thus influence the rate of corrosion. Likewise, the pressure of hydrogen resulting from the corrosion reaction tends to reduce the rate of the oxidation reaction, thus each reaction opposes the other.

are suflicient to have a profound influence on the corrosion rate of the internal surfaces of a steel heat storage container. This method of corrosion control can also be applied through the use of coatings applied to the outside surface of the container. For example, a coating having the ability to absorb moisture from the air at a temperature below approximately 700 degrees Fahrenheit may be used to increase the moisture concentration at the surface of the container in the temperature range above 700 degrees Fahrenheit. Examples of suitable coating materials include such things as silica gel, alumina and zeolites.

The extension of this idea to other metals is obvious. And in its broadest aspect, this invention embraces the concept that any combination of chemical and metal whose reaction produces atomic hydrogen can be controlled from the other side of the metal by another reaction which also produces atomic hydrogen.

Example 3 A heat storage container such as that described in the accompanying drawing, was filled with caustic soda which had been prepared for use in a sealed container for heat storage purposes by the following procedure.

A commercial grade of flaked caustic soda manu factured by the diaphragm cell process was heated to a temperature of 900 degrees Fahrenheit at which the material was molten in the steel melting pot.

Water was removed by holding the fused material at 900 degrees Fahrenheit for 24 hours while in contact with dry air. One percent of minus 325 mesh electrolytic iron powder was then added to remove any remaining water. The prepared caustic soda was then charged into a cylindrical container like that described in FIGURE 1. The top closure was welded into place, and the air in the clearance space removed with a vacuum pump and the container was completely sealed by welding.

The heat storing container was then tested by thermal cycling at the rate of four cycles per day between a top temperature of approximately 800 degree Fahrenheit and a bottom temperature of 200 degrees Fahrenheit, for eight months, with the outside surface of the container exposed to the ambient air which contained appreciable Water vapor. This caused the caustic soda to melt and solidify during each cycle. During this cycling, no excessive pressure developed in the clearance space, no deleterious corrosion occurred on the internal surfaces, and no distortion of the container resulted from the forces exerted by contraction and swelling of the solidified caustic soda.

Examples 45 To further investigate the function of water on the outside surface of a sealed module containing an alkali metal hydroxide composition, the following experiments were conducted.

An apparatus similar to that disclosed by Bloom et al., Journal of the Electrochemical Society, vol. 104, No. 5, pp. 264-269, was constructed. In essence, a vacuum system was constructed with a manometer, cold finger containing water and a capsule compartment. Valves were provided which would isolate the cold finger containing Water from the capsule and to isolate the system from the vacuum pump.

The capsule was heated to a temperature of 900 degrees Fahrenheit with the valve to the cold finger closed and valves to the manometer and vacuum pump open. The rate of hydrogen generation at any instant was measured by closing the valves to the vacuum pump and manometer and measuring the pressure change per unit time on the manometer. Other equivalent pressure measuring devices may be substituted for a manometer. With the valve to the cold finger closed, only a small amount of hydrogen is present in the system. Thus, the rate of pressure change is indicative of the corrosion rate in a vacuum and is directly related to the corrosion rate when the volume of the system, the internal surface area of the capsule and the equation governing the corrosion process are known.

To determine the corrosion rate with water vapor on the outside of the sealed capsule containing an alkali metal hydroxide composition, the cold finger is used. The water in the cold finger is brought to a constant temperature while the valve isolating the water from the rest of the apparatus is closed and while the valve to the vacuum pump and manometer are open. Then the valves to the vacuum pump and manometer are closed and the valve to the water in the cold finger is opened. This introduces water vapor into the system with a partial pressure equal to the saturation pressure of water at the constant temperature of the water in the cold finger as regulated by a constant temperature bath. The hydrogen generated by corrosion on the inside and outside of the capsule wall passes into the system causing the total pressure to rise above the partial pressure of the water vapor. The difference between the partial pressure of water vapor and the total pressure at any instant is the pressure of hydrogen in the system. The rate of change of hydrogen pressure may be used to calculate the corrosion rate. In this case, however, it is not known what portion of hydrogen is generated by the two dilferent reactions. Therefore, only an overall indication of corrosion rate may be obtained. It is apparent, however, that the presence of Water vapor on the outside of the capsule of alkali metal hydroxide composition significantly reduces the total corrosion rate. The reduction of the corrosion rate is proportional to some positive power of the water vapor pressure. The following data presents the results obtained in three experiments. These experiments are representative of the reduced corrosion rate observed when water vapor contacted the outside wall of a capsule containing an alkali metal hydroxide composition.

Having disclosed this invention, it will become apparent to those skilled in the art that obvious modifications and variations may be made which do not depart from the true spirit of this contribution. The preceding examples are presented herein for the purpose of illustrating specific embodiments of this invention rather than to limit its true scope. It is to be understood that although reference is made throughout this specification to atomic hydrogen and its diffusion through steel, this is a theoretical consideration only, while the inventive concept involved is not so restricted. When reference is made to hydrogen or atomic hydrogen, it is intended that the form of hydrogen which diffuses through steel or other metal alloys is embraced.

What is claimed is:

1. A process for retarding the corrosion of the metal of a hermetically sealed container at from about atmospheric to sub-atmospheric pressure housing an alkali metal hydroxide composition that reacts with said metal to generate hydrogen which comprises producing hydrogen at the external surface of the'container by chemi cal reaction at a temperature sufficient to effect diffusion of hydrogen through the container wall.

2. The process of claim 1 in which the metal of said sealed container is iron.

3. The process of claim. 1 in which the temperature at which hydrogen is produced at the outside surface of the container is above the melting point of said alkali metal hydroxide composition.

4. A process for reducing internal corrosion within a hermetically sealed steel container at from about atmospheric to sub-atmospheric pressure, said container being substantially filled with an alkali metal hydroxide composition, which comprises producing hydrogen at the outside surface of the container at a temperature above the melting point of the alkali metal hydroxide composition.

5. The process of claim 4 in which hydrogen and atomic hydrogen are generated at the outside surface of the container by the reaction of atmospheric water vapor with the iron of the steel container.

6. An apparatus comprising a hermetically sealed steel container substantially filled with an alkali metal hydroxide composition, said container being coated .with a hygroscopic material upon the exterior surface corresponding to the interior surface which contacts said alkali metal hydroxide composition.

7. The apparatus of claim 6 in which said coating material is a member selected from the group consisting of silica gel, alumina and a zeolite.

8. A process for reducing internal corrosion within a hermetically sealed steel container at from about atmospheric to sub-atmospheric pressure, said container being substantially filled with an alkali metal hydroxide composition, which comprises producing hydrogen at the outside surface of the container at a temperature above the melting point of the alkali metal hydroxide composition by the reaction of atmospheric water vapor with the iron of the steel container, said atmospheric Water vapor being held in close proximity to the outside surface of the steel container by a hygroscopic coated on said outside surface.

9. The process of claim 8 in which the hygroscopic coating material is a member selected from the group consisting of silica gel, alumina and a zeolite.

References Cited UNITED STATES PATENTS 2,211,047 8/1940 Beck 25271 2,453,471 11/1948 SWitZeI' et a1. 20684 XR 2,791,074 5/ 1957 Woodman 206-84 XR 2,924,274 2/ 1960 Richardson.

3,028,955 4/1962 Shorkey et al. 206-84 3,081,241 3/1963 Smith 2l-2.7 XR 3,299,945 1/1967 Rice et al l04 XR 3,320,724 5/1967 Rice 55269 3,382,919 5/1968 Rice 165-105 MORRIS O. WOLK, Primary Examiner B. S. RICHMAN, Assistant Examiner US. Cl. X.R.

"( 53 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 350 261 Dated March U. 1970 lnventofls) Richgrd E, Rice and Hi llis Thaw It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 33, delete "steel," and insert steel. Column 2, line l l, delete "nitriate" and insert nitrite Column 6, line #0, detete "degree" and insert degrees Column 8, Claim 8, line H, after "hygroscopic" insert material SIGNED AN SEALED JUL 2 81970 Anew Edward M. Fletcher, 11-. :gfiiff- 55mm,

A g officer can or Paton"

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