US3293159A

US3293159A - Process for producing a fused reducing bath for descaling

Info

Publication number: US3293159A
Application number: US134967A
Authority: US
Inventors: Mekjean Matthew; Charles A Strack
Original assignee: Hooker Chemical Corp
Current assignee: Henkel Corp
Priority date: 1961-08-30
Filing date: 1961-08-30
Publication date: 1966-12-20
Anticipated expiration: 1983-12-20
Also published as: GB995263A; NL282584A; DE1196048B; BE621887A

Description

Dec. 20, 1966 M. MEKJEAN ETAL PROCESS FOR PRODUCING A FUSED REDUCING BATH FOR DESCALING Filed Aug. 50. 1961 3 Sheets-Shed; 1

Dec. 20, 1966 MEKJEAN ET AL.

PROCESS FOR PRODUCING A FUSE'D REDUCING BATH FOR DESCALING I Filed Aug. 50. 1961 3 Sheets-Sheet 2 m Q w l I t -|m\:I\\1 |1 ll i 1 1| ill! I I W 1111111 I11! :mrlll 0? WM I I IHW HI I I1 I I M l h WMMHHHH .H l l l z W 1% M m l N Dec. 20, 1966 MEKJEAN ET AL PROCESS FOR PRODUCING A FUSED REDUCING BATH FOR DESCALING Filed Aug. 30. 1961 3 Sheets-Sheet 3 United States Patent PROCESS FDR PRODUCING A F-USED REDUCING BATH FOR DESCALING Matthew Mekjean, Niagara Falls, and Charles A. Strack,

Lewiston, N.Y., assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York Filed Aug. 30, 1951, Ser. No. 134,967 6 Claims. (Cl. 20461) This invention relates to the art of removing oxygencontaining heat scale from metals. More specifically, the inventive concept herein defined resides in a unique type of reducing descaling fused salt bath.

Some descaling difliculties have been encountered because of the chemically resistant nature of the heat scale developed on certain metal species in the usual annealing and 'hot rolling procedures used in production. vHigh nickel steels, for example, have presented various descaling problems by presently existing molten salt systems. The present invention provides an economical alkalimetal hydroxide based descaling system particularly adapted for use in the descaling of nickel steels, high nickel alloys, stainless steels and many other metals and alloys. It has been found that presently commercially used reducing descaling systems, while somewhat eifective and desirable, present corrosion problems both on the bath containing pot, gas-fired heating tubes, immersion rolls and other component elements of the equipment. A further apparent drawback of the presently used reducing descaling baths is the ever present danger of violent reactions caused by the constant addition of various chemicals to the system. To be commercially desirable, a system of this type must not be effective as such, but must be economical to operate, while maintaining a high degree of safety. The system of this invention electrolytically creates and maintains a substantially constant source of reducing chemicals in the bath while providing the bonus effect of a highly desirable degree of safety.

The present invention is adaptable both to batch descaling units and to continuous descaling of strip. The trend in the primary metal industry is more and more toward automation, away from batch handling, and wherever possible and feasible, to produce the bulk of the products of the rolling mills in the form of strip. The rolling process, both on sheet products and strip, Workha-rdens the metal, which is then usually annealed or softened in a heat-treating step which leaves the surface of the metal covered with a heat scale which is difiicultly soluble in acid pickling baths. It is at this point, after annealing, that the fused salt reducing descaling bath of the present invention is most effectively utilized. In strip lines, the descaling bath is located just beyond the exit of the annealing furnace. The hot, annealed strip continuously passes directly from the annealing furnace into the descaling bath, where it is held submersed by special immersion rolls. The strip passes over exit rolls, then through a water quench tank, and special rolls lead it through one or more acid pickling tanks, spray rinses, air dryers, and past the inspection stations and the descaled metal is finally rolled into large coils.

The present invention provides a very versatile electrochemical process, where the proper control of direct current in suitable equipment produces and maintains an equilibrium composition of the reducing chemicals that effectively descale the metals and alloys of interest. While the electric current provided in this system does not directly participate in the descaling ope-ration, it is vital and necessary to insure the production of the reducing agents which are responsible for the deoxidatio-n of heat scales on metals. The typical system of this in- Patented Dec. 20, 1966 vention initially comprises a Na SO -NaOH melt maintained at a predetermined temperature in a melt pot. Immersed in the pot is a cathode means, an anode means, an anolyte and cath'olyte compartment determining structures. The initial composition of the bath is usually a molten Na SO -NaOH mixture. The preferred composition comprises from six (6.0) percent Na SO to eighteen (18.0) percent N-a SO the remainder substantially anhydrous NaOH. It would not however be considered without this invention to alter the amounts of either NaOH or the Na SO in the system. It has been found for example that any amount vof Na SO greater than three (3.0) percent may be utilized in this invention, although as mentioned above the preferred composition comprises from six (6.0) to eighteen (18.0) percent Na SO When the current is introduced into the system, the Na SO is progressively reduced to a complexity of ions which will be more clearly defined in the ensuing discussion. The Na SO itself is neither reducing nor oxidizing, thus is not the descaling medium as such. However, the reducing ions that are electrochemically formed when the current is introduced into the Na SO NaOH molten bath are highly desirable reducing-descaling agents.

Although the complexity of the chemical reactions that take place in this system have not been completely verified, it is believed that the below defined reactions occur. Electrolyte reduction of sodium sulfate yields a number of reducing agents in the melt, which, in turn, are also subjected to electrolytic reduction to be them-selves further reduced as follows:

Apparent Valence Na SO Sulfate +6 +0 l-o Disproportionation M12503 Sulfite +1;

Pyrosulfite Na O+Na S O5 na s o Dithionite +3 M25203 Thiesulfate N825 Sulfide -2 elec. reduction NaaSzOs *S Nil-23204 IAOQT At the cathode the S 0 takes on two electrons to create S O -f-O' The oxygen ion migrates to the anode where it loses two electrons to become 0 or molecular xygen gas which escapes from the system. The general verall reaction thus would be electrolytic reduction NazS2O5 azS204 %02 incompassed in this broad concept of the reduction of he Na SO -NaoH mixture to a number of reducing agents It compounds, is also the further teaching of a possible lath to be used in commercial manufacturing of these relucing agents or compounds or their acid forms.

The Na SO in the initial molten composition is in lectrochemical combination with and is electrolytically educed to the reducing sulfites, dithionites, thiosulfates, iyrosulfites, and sulfides among others. These reducing igents combine with the oxides of the scale on work- :ieces to be re-oxidized to the higher oxygen level, which hen again is electrolytically reduced; thus, a continuous )XldflllOl'l-l'fidllCllOll cycle takes place without any subtantial need for re-supplying the system externally with tdditional chemicals. A further effect is accomplished by hese ions in the transitory state; for example, Na SO vhile being an effective reducing agent, is also highly cor- 'osive in, the system. For example, the Na SO that is 'ormed, while supplying (together with the other ions Formed) a highly desirable descaling bath, is converted to he Na SO u-pon descaling since it will have a tendency o rob or absorb oxygen from the oxide scale. The cor- .OSlV6 effect of the Na SO thus is kept at a minimum vhile the descaling efforts of the Na SO are utilized together with the combined descaling effects of the other ons formed. It is believed that the descaling operation of his invention functions in the following manner: Using 5121 8 Na SO Na S O- as typical examples of the relucing agents in this system that are re-oxidized by the scale, the following are considered illustrative of the :hemistry involved in the descaling operation; any metal :0 be descaled will be indicated below by the symbol Me. Anhydrous alkali-metal hydroxide is the solvent:

The descaling reactions re-create the higher oxidation states (higher sulfur valences) of the alkali-metal sulfuroxygen salts, which in turn, are electrolytically reduced to repeat the cycle. The overall effect is to remove combined oxygen from the scale on workpieces of metal by the chemical reducing agents, and to release this as molecular oxygen gas by the electrolytic reduction of the oxidized reducing agents. Thus, the cycle is repeated ad infinitum; regeneration of the reducing agents electrolytically, and descaling of workpieces by chemical reductionof their oxide scales.

The elements of the reducing bath system of this invention include a fused, substantially anhydrous molten salt or electrolyte, a direct current electrical source, a cell converter (Recyclotron) comprising in combination a cathode means, an anode means, and a diaphragm means, a container pot, a passivating circuit, and a floating cover, which may be optionally used.

Utilization of the passivating circuit in the present system is a specially-devised and unique application of direct electrical current. Laboratory tests have demonstrated that otherwise soluble metals in the fused salt can be so passivated 'by this technique that the useful life of equipment can be extended ten (10), twenty (20) and even thirty (30) times normal. In its simplest concept, the passivating technique involves an anomaly: to use positive electricity in the presence of strong reducing agents, yet passivating the elements maintained positive without destroying or minimizing the reducing nature of the fused salt. Another unique factor is that this passivation technique is particularly effective in the substantially anhydrous molten salt mixture, and is less effective when water is present in the melt. In practice, those elements of the present system requiring passivation (such as the pot mean-s, diaphragm means, housing means, thermocouple probes, sludge pans, bafiles, or other specialized equipment such as rolls, agitators, pumps, etc.) are connected in parallel to the positive pole, for example of a rectifier. The negative pole is connected to either the recyclotron cathode or to a special electrode contacting the fused salt. The current in this separate, superimposed passivating circuit is adjusted within the range from about one-hundredth to a maximum of about one-tenth of the recyclotron current, and is somewhat dependent on the total area of equipment being passivated.

A floating cover of light particles may be optionally used on the surface of the catholyte. The purpose of such a cover would be to increase the efiiciency of the reducing bath by preventing unnecessary oxidation of the active re ducing agents with the oxygen of the atmosphere. A secondary advantage would be realized in conserving a portion of the radiant heat normally lost from the open surface of the bath. Such a floating cover on the fused salt should consist of a composite of large and small particles ranging approximately from one (1) to thirty-two (32) mesh. Such particles, whose density is less than that of the molten salt, could conveniently consist of porous materials substantially stable in the system.

Initially, the molten salt as above discussed preferably comprises NaOH and Na SO in the ratio of about from six (6.0) percent to eighteen (18.0) percent Na SO to ninety-four (94.0) percent to eighty-two (82.0) percent NaOH. This molten mixture is substantially anhydrous and is maintained preferably at temperatures from six hundred (600) degrees Fahrenheit to one thousand (1000) degrees Fahrenheit. Of course, any elevated tem perature below the boiling point of the molten mixture may be employed depending on the desired result. Any suitable type of heating may be used depending on the conditions and techniques involved. The NaOH used is preferably technical grade caustic soda (essentially anhydrous) having mixed therewith from six (6) to eighteen (18.0) percent by weight of the total dry components of Na SO When this medium is electrolyzed, first the elements of water are decomposed to oxygen at the anode and hydrogen at the cathode, leaving a substantially anhydrous salt in the melt (at temperatures ranging from about six hundred (600) to about one thousand (1000) degrees Fahrenheit, the preferred temperature being from about seven hundred (700) to about nine hundred (900) degrees Fahrenheit). When completely anhydrous, the continued electrolysis causes oxygen gas to be liberated at the anode, and reduction of the sulfate occurs at the cathode to yield reducing agents as above discussed (such as sodium sulfite, sodium thiosulfate, etc.).

It would not be considered without the scope of this invention to provide (at greater expense) an initial mixture of components such as NaOHNa SO Na SO Na S O Na S O -Na S O Na S in any combination of NaOH with one, two, three, four, five, or all six. The use of NaHS and even elemental sulfur, initially, will immediately react with NaOH to create some of the alkali metal sulfur-oxygen compounds listed, and without the reducing electric current, lW-Ollld eventually convert completely to Na SO the stable form.

Technical grade caustic soda is normally preferred since it is commercially available at a relatively low cost. Hence, when technical grade caustic soda is employed, small amounts of impurities normally associated therewith are present in the bath, i.e., around three (3) percent cumulatively by weight of sodium chloride, sodium carbonate, etc. based on the total composition. It will be noted. that in use the alkali hydroxide-based molten salt bath slowly absorbs carbon dioxide from the atmosphere and converts some of the hydroxide to alkali carbonate. Another source of carbon dioxide in the proximity of the bath would be the exhausts of gas or oil-fired. heating tubes for maintaining the salt in the molten state. This conversion of alkali metal hydroxide to alkali metal carbonate in no way adversely affects the utility of the salt bath; in fact, carbonate below thirty (30) percent increases the fluidity of the salt due to a lowering of the melting point of the mixture.

The melt also has a remarkable capacity for holding a number of contaminants, such as most metallic cations, for example, Cu, Ni, Cr, Sn+ Pb+ Zn, Ti+ Ag+, Fe+ Co+ Mo+ W+ B a++ Ca+ and V+ These would, as contaminants, be present usually in low concentrations and obtained, in most cases from the metallic scales being reduced by the catholyte. The NaOH will act as a solvent for these contaminants.

The sources of direct electric current in this system are preferably obtained by metallic rectifiers such as selenium, germanium or silicon. However, motor generators and batteries with their limitations, may be utilized especially in smaller systems. The direct current requirements of this system are in the approximate maximum range of about five hundred (500) amperes per ton of salt during start-ups, or during periods of increased demand, and approximately two hundred and fifty (250) amperes per ton minimum for normal operation. The actual operating level demand would determine the operating current requirements. The actual current demand in each particular installation would probably vary, however, with the efficiency of the equipment and type of scale being reduced, in conjunction with the particular analysis of the salt. It is most desirable for the system to operate at a maximum of about six (6) volts for personnel safety, as well as for economical power costs.

It is therefore an object of the present invention to provide a metal descaling system which will electrolytically maintain a substantially constant source of reducing chemicals within the reducing bath while providing the further advantage of a highly desirable degree of safety to operating personnel.

Another object of this invention is to provide a metal descaling composition which initially comprises (alkali) sodium sulfate in sodium hydroxide, which is electrolytically reduced to produce and maintain a number of reducing agents such as sulfite, pyrosulfite, dithionite, thiosulfate, sulfide and the like.

A further object of this invention is to provide a novel structure which will maintain the above-mentioned descaling compositions.

A still further object of this invention is to provide a novel cathode means which is particularly adapted for use in the present system.

A still further object of this invention is to provide an electrochemical process in which the proper control of direct current yields the reducing chemicals which effectively descale the metals and alloys of interest.

Other objects of this invention will become apparent from a further reading of this disclosure.

The invention will be further defined in relation to the accompanying drawings and following examples. It is to be understood however, that the drawings and examples in the following discussion are given merely to illustrate the invention, and are not by any means to be taken as limiting the invention to the particulars herein defined.

FIGURE I discloses a modification of the reducing salt bath system of this invention. FIGURE 11 illustrates a preferred embodiment of the reducing bath of this invention. FIGURE III illustrates a preferred cell structure included in the Recyclotron system of this invention.

Referring first to FIGURE I, the reducing system 1 comprises a metal pot 2. The salt or a molten descaling composition 17 is restricted or contained in pot 2. Most of the commercially used pots are comprised of steel. The material of constmction of the pot is of utmost importance in the present invention, since it should have a long life and be essentially inert chemically. Immersed in salt 17 is an anode means 3, a cathode means 4, and

a diaphragm means 7. The diaphragm means bisects the internal area of pot 2, thereby forming an anode compartment 5, and a cathode compartment 6, although the compartments do not have to be the same size. When anode means 3 and cathode means 4 are connected to an electrical source of direct current, the ions in the fused salt carry the current between the inert electrodes, forming a number of reducing ions at the cathode. The oxygen ion formed is attracted to the anode, passes through diaphragm means 17 to the anode where it loses its electrons, and is released as oxygen gas. When a direct current flows between the electrodes, ranging from 0.5 to 3.5 amperes per square inch of cathode surface at an impressed ranging from one (1.0) to 7.5 volts, a reducing bath is created in the catholyte. The metal to be descaled is generally simply immersed in the molten catholyte or cathode compartment for the descaling openation. However, eventually, on continuous operation, both the anode and cathode compartments 5 and 6 become effective for the descaling of steels or other metals and analysis of the anolyte and catholyte reveals the presence of the reducing agents in both. Although it is conceivable that a cell may be so constructed that might obviate the need for a diaphragm, it would probably be more complicated than to actually use one to assist in separating the anode product from the products of the cathode. The use of the diaphragm between anode 3 and cathode 4 permits miniaturization of the reducing bath generating system, which then allows the bulk of the pot volume to be utilized as an active working area for the descaling of metals. The more perfect the separation of the products of the electrodes from each other, the higher the electrical efiiciency of the system. Because of the dissolution of most diaphragm materials tested, only passivated iron, nickel or zirconium have proven satisfactory as materials of construction for the diaphragm. Especially designed and fabricated screens and porous or perforated plates have been successfully used; however, any specifically constructed diaphragm particularly suitable for the purpose for which it is designed needs to be protected by the passivating circuit.

The actual reduction of the sulfate essentially occurs at the inert cathode, whereby the reducing chemicals are liberated. The design, size, current density, voltage, current flow, and materials of construction are all important for the greatest efficiency of this system. Naturally, the composition of the salt, basically, has determined the former variables. The best cathode material of construction is copper and some of its alloys, due to its electrical efficiency, fairly low cost and relatively long life. It must be remembered that all cathode materials eventually dissolve. Other metals exhibiting long life as cathode materials are nickel, silver, zirconium and molybdenum. Economic considerations probably would restrict the choice of the cathode material to copper, nickel, or their alloys. Special precautions are necessary in the present system to insure maximum cathode life. This will be defined in greater detail in the ensuing discussion.

At the anode means 3, oxygen in the melt, obtained from the metal oxide scales, by absorption by the reducing agents of oxygen from the atmosphere, or from anions containing oxygen, is liberated as a gas, removing it from the system. This leaves the melt somewhat oxygen-shy, or hungry for more oxygen. The object is to supply the oxygen by the chemical reduction of scale on metallic workpieces. Iron, nickel and cobalt and their alloys with each other are the anode materials that have demonstrated the longest life, and economic considerations would indicate a preferance for iron or nickel, or an iron-nickel alloy for the composition most suitable for the anode. For fine laboratory work and scientific measurements, platinum may be utilized as an anode, but as a cathode, platinum dissolves.

Pot 2 is generally constructed of steel; however, laboratory use would indicate a preference for nickel. The

naterial of construction for the pot is of utmost imporance, since it should have long life and be essentially inert :hemically. Since the commercial preference is steel, the )16561112 system was designed for particular use with the netal steel-type pot. The cell structure therefore of FIG- JR'E I produces reducing compounds in the melt to create he reducing bath. The simplest cell is composed as illus- ;rated of the pot itself, separated into two compartments )y a dividing diaphragm, one the anode compartment con- ;aining the anode, and the other a cathode compartment :ontaining the cathode. The electrolyte, or molten (fused) salt in this case, completes a working cell ready for application of the electric current between the electrodes. When the current flows, the salt in the anode :ompartment becomes the anolyte. The salt in the cathode compartment therefore becomes the catholyte. In operation, the catholyte is chemically reducing in nature, and becomes the working area of the bath. It is Within the catholyte that workpieces of metal are exposed for the reduction of their oxide scales, or where the scale is conditioned or converted. In commercial baths of this type, it would be obviously advantageous to make the anode compartment relatively small and the cathode compartment relatively lange. In order to insure a reasonable life of the pot and the diaphragm in this system, they should be connected to the passivating circuit.

FIGURE II. A preferred embodiment of this invention is illustrated. This embodiment provides a metal pot 9, having therein a reductive housing means. This reductive, which we will identify in the ensuing discussion as a Recyclotron structure, comprises a steel housing 10 containing the anode 12 and a diaphragm means 11. This housing then would determine the anode compartment in the anolyte, and a cathode compartment in the catholyte. The cathode is usually separately situated in its position opposite the diaphragm of the housing and is electrically insulated from everything in the system. The main area of the pot, therefore, in which the cathode is located, becomes the cathode compartment, the salt within it becomes the catholyte, and therefore becomes the working area of this reducing bath. For this system to function safely, therefore, and to insure the safety of equipment, the housing, the diaphragm, the pot, the sludge pan, and other necessary accessories, are connected to the passivating circuit. The cell construction illustrated in FIGURE II is preferred over that of FIGURE 1, in that the structure of FIGURE I has certain limitations. The cell structure of FIGURE 1 would probably require modification of presently-used commercially existing systems. It would economically be more desirable to use the system illustrated in FIGURE II, in that the recyclotron 10 can be merely lowered into a commercially-used pot or bath. It would be self-contained, and when connected to the proper electrical source, would automatically create a reducing, descaling bath in situ. The size or number of these recyclotrons would depend upon the size of the bath and the application of the bath. In most cases, it is assumed that a special design would be utilized; in other words, the standard design may be modified to suit the particular needs.

FIGURE III illustrates an enlarged view of a recyclotron of this system. The recyclotron of this system comprises in combination a steel housing means 10, which encloses the anode means 12. On the lower portion of the combination bathe-diaphragm means 11 is positioned perforated diaphragm means 2-1. Outside of housing 1i? and immediately adjacent the diaphragm means 21 and parallel thereto, is positioned a cathode means 16. There has been a considerable amount of difficulty encountered in preventing the corrosion of the electrical cathode lead 18, to the submerged cathode means 16. One main difficulty in the corrosion of the electrical lead 18 occurs at the air-liquid interface where corrosion most readily occurs. Once the electrical lead 18 to the cathode fails because of corrosion, the entire cathode means 16 itself then becomes useless to the system. -It has been found that the life of the electrical lead '18 which connects the cathode 16 to the source of current has been greatly increased by the incorporation of an outer zirconium sheath 17 substantially completely enclosing the electrical lead 18. The zirconium is substantially inert in the system and does not hinder or harm the system of the operation of the present reducing bath. The zirconium sheath 17 concentrically encloses electrical lead 1-8, thereby substantially completely preventing any corrosion of the cathode electrical lead 18. By using a protective system of this nature, not only is the life of cathode 16 extended, but also the life of the entire system is extended. The reducing system (recyclotron), there-by comprises in combination a cathode .16, a cathode lead 18, a cathode lead sheath 17, an anode 12, anode lead 20, a housing 10, and a combination bathe-diaphragm means 111. The cathode bus bar 19 supports cathode lead .18, which is concentrically enclosed by zirconium sheath 17. Vertically suspended on cathode lead 18 is cathode means 16. Anode means .12 is suspended within the internal portion of housing I10.

A further preferred embodiment (not illustrated herein) of the recyclotron of this invention comprises a structure having an anode means, a diaphragm means, a cathode means, an anode electrical lead, a cathode electrical lead, and a source of current, said diaphragm means concentrically substantially enclosing said anode means, said cathode means concentrically substantially enclosing said diaphragm means, a top closure means positioned in a substantially gas tight manner between said anode means and said diaphragm means, and the upper area intermediate said diaphragm means and said cathode means defining a gas passage means extending from the internal portion of said recyclotnon to the atmosphere, said electrode electrical leads supplying electrical current to said electrodes from said source of current. This embodiment is preferred since it also may be immersed in presently existing pots, while having the bonus effects of occupying a minimum amount of space at high electrical efiiciency,

The following examples further define the specifics of this invention. The metals descaled in the following examples were primarily high nickel steel and typical stainless steels. However, other types of metals tested under proper descaling conditions yielded excellent results. Some of the other metals which may be descaied in the present system are straight-chrome stainless steels, chrome-nickel stainless steels, silicon steels, titanium and its alloys, mild steels (low carbon steels), nickel steels, copper and its alloys such as bronze, brass and so forth, molybdenum and its alloys, and such heat resistant and corrosion resistant nickel-based alloys with molybdenum and iron, with silicon; and with nickel-chromium alloys with molybdenum and tungsten, or with molybdenum and iron, or with cobalt and iron. We have undoubtedly not exhausted the number of diiferent alloys and metals that can be effectively descaled in the present system, but this list would be restricted essentially to those metals and alloys which are substantially unreactive in the salt, and to those Whose meltingpoints are above the operating temperature of the descaling medium.

Examples 1 through 14 inclusive have to do exclusively with a series of descaling tests performed on a nickelsteel alloy under successively-varied conditions of the descaling system in order to establish the ideal operating conditions. The particular nickel-steel used in these examples will be identified as 908, which has a typical or average alloy composition of the major elements of 77.2 percent nickel, 4.8 percent copper, 1.5 percent chromium and 14.9 percent iron. A large sheet of scaled 908 nickel steel was cut into standard panels of two (2) inches Wide by three (3) inches long, and each was given a number identification.

Examples 15 through 20 illustrate the effectiveness of the electrolyzed salt to descale various stainless steels in catholyte or anolyte in the temperature range from EXAMPLE 1 A salt composition A, comprising 91.5 percent technical grade sodium hydroxide and 8.5 percent anhydrous sodium sulfate, based on the total dry weight of the components, was charged into an experimental steel pot fitted with slots for positioning and holding the diaphragm screen. The diaphragm screen bisected the pot into two compartments of equal size, the one comprising the anode compartment containing a nickel anode, and the other comprising the cathode compartment containing a copper cathode. The total charge to the pot was one hundred and thirty (130) pounds of molten salt-electrolyte. The electrodes were so positioned that they Were opposite each other, with the diaphragm substantially centrally located between them. The initial distance between parallel faces of the electrodes was eight and one-half (8 /2) inches. The temperature of the melt was held in this instance at seven hundred and twenty-one (721) degrees Fahrenheit. The passivating circuit was connected to the pot and to the diaphragm screen, and was held at two (2) amperes at two (2) volts throughout these tests. The reducing current in the Recyclotron was set for forty (40) amperes at 4.1 volts. The salt was open to atmosphere without a cover. A sample panel #2 of 908 nickelsteel was suspended from a wire through a hole punched in its upper section, and immersed in the catholyte for fifteen minutes, then quenched in cold water to remove adhering salt and to cool the panel, and finally immersed in a fifteen (15 percent nitric acid (by weight) pickling bath at one hundred and sixty (160) degrees Fahrenheit for three (3) minutes. The sample of 908 was then rinsed in cold, followed by hot water, and inspected for evaluating the results. It was rated to be descaled ninety (90) percent on the front face and one hundred (100) percent on the reverse surface. A further test on panel #1 under these conditions yielded similar results.

EXAMPLE 2 To the conditions existing for Example 1 was added a floating cover on the catholyte to minimize atmospheric oxidation. A sample panel #3 of 908 nickel-steel was immersed for fifteen 15) minutes in the melt, water quenched, pickled three (3) minutes in nitric acid of Example 1, rinsed and dried. By inspection, the front face was ninety (90) percent and reverse face one hundred (100) percent descaled. Other panels tested yielded similar results.

EXAMPLE 3 To the conditions existing for Example 2, the Recyclotron current wa increased from forty (40) to eighty (80) amperes at 5.6 volts. Panel #8 of 908 immersed for fifteen (15 minutes in the catholyte, quenched and treated in nitric acid as in Example 2, yielded results of sixty (60) to one hundred (100) percent descaled on front and reverse faces repsectively.

EXAMPLE 4 To the conditions existing for Example 3, the Recyclotron current was raised from eighty (80) amperes to one hundred and twenty (120') amperes at 6.9 volts. Initial sample Panels #13 and #15 of 908 processed as described in Example 3, yielded better-than-average results ranging between ninety-five (95) and one hundred (100) percent descaling. As the electrolyte was maintained under these operating conditions, further tests on panels of 908 nickel-steel, panels #16, #17 and #18, yielded consistently perfect results, with descaling rated at one hundred (100) percent on both faces.

10 EXAMPLE 5 With the conditions existing under Example 4, the Recyclotron current was reduced from one hundred and twenty 120) to eighty amperes, and tests were conducted as in Example 3 on panels #19, #20 and #21 of 908. All the results were consistently excellent, all panels rating at one hundred percent descaled on both faces.

EXAMPLE 6 With the conditions existing under Example 5, the Recyclotron current was reduced from eighty (80) amperes to zero (0) amperes by shutting the system down. The only current still remaining was that flowing in the passivating circuit. Panels #22 through #26 of 908 were processed as before in Example 3, and examined. Results were consistently bad, descaling ranging between two (2) percent to fifteen (15 percent.

EXAMPLE 7 With the conditions existing under Example 6, the Recyclotron current was turned on, raising it from zero (0) to eighty (80) amperes, and the temperature of the melt was increased from seven hundred and twenty-one (721) degrees Fahrenheit to nine hundred and twenty-one (921) degrees Fahrenheit. Under these new conditions, panels of 908 nickel-steel were processed as before in Example 3. Initially, as the reducing chemicals werestill being created in the catholyte, results on panels #27 through #30 were eighty (80) to ninety-eight (98) percent descaled. Within two (2) hours, under the same conditions, however, one hundred (100) percent descaling was achieved on both faces of 908 panels #31 and #32.

EXAMPLE 8 With the conditions existing under Example 7, the Recyclotron current was reduced from eighty (80) to forty (40) amperes, and the temperature was allowed to drop slightly from nine hundred and twenty-one (921) to nine hundred (900) degrees Fahrenheit. Under these new conditions, panels #33 through #35 of 908 processed as in Example 3 were all perfectly descaled and rated at one hundred-one hundred (100 100) percent descaled on both front and reverse faces, respectively.

EXAMPLE 9 With the conditions existing as in Example 8, the electrodes were moved from a distance eight and one-half (8 /2) inches apart, to a new position where they were four and three-quarter inches apart. Under these new conditions, panels #36 through #40 of 908 nickel-steel were processed as in Example 3, with perfect results; all panels rated were one hundred 100-) percent descaled on both sides.

EXAMPLE 10 With identical conditions existing as in Example 9, panel #41 of 908 nickel-steel was processed in the catholyte for ten (10) minutes, then pickled for five (5) minutes in a ten (10) percent nitric acid at one hundred and sixty degrees Fahrenheit, rinsed and dried. Examination of panel #41 was near-perfect at ninety-nine (99) percent descaled on the front face and one hundred (100) percent on the reverse face.

EXAMPLE 11 Identical salt conditions as existing in Example 9, a panel #42 of 908 nickel-steel was processed in the catholyte for only five (5) minutes, then pickled for three (3) minutes in the ten (10) percent nitric acid at one hundred and sixty (160) degrees Fahrenheit, rinsed and dried. Panel #42 was rated ninety-nine (99) percent descaled on the front face and one hundred (100) percent on the reverse face.

1 1 EXAMPLE 12 Under identical salt conditions as existing in Example 9, a panel #43 of 908 nickel-steel was processed in the anolyte for fifteen (15) minutes, which after quenching, was pickled for three (3) minutes in ten percent nitric acid at one hundred and sixty (160) degrees Fahrenheit, rinsed and dried. Panel #43 was rated at ninetynine-ninety-nine (9999) percent descaled, front and rear faces.

EXAMPLE 13 With conditions existing under Example 9, only the temperature was lowered from nine hundred (900) degrees Fahrenheit to seven hundred and ninety-eight (798) degrees Fahrenheit. Under this condition, a panel #45 of 908 nickel-steel was processed in the catholyte for fifteen minutes, water-quenched, then pickled for three (3) minutes in the acid of Example 12. Panel #45 was rated at one hundred-one hundred (100-100) percent descaled on both front and reverse faces.

EXAMPLE 14 With conditions existing under Example 13, the temperature was again lowered from seven hundred and ninety-eight (798) to six hundred and ninety-five (695) degrees Fahrenheit. Under this condition, a panel #46 of 908 nickel-steel was processed exactly as in Example 13. Panel #46 was rated at one hundred-one hundred (100-100) percent descaled on both front and reverse faces.

Just a few words on the interpretation of these fourteen (14) examples. Initially, conditions existing in the melt were unsatisfactory to descale the tough 908 nickel-steel; this was the period of electrolytic dehydration of the traces of moisture present in the salt, undoubtedly existing during the periods of Examples 1, 2 and 3. During the operating period of Example 4, early tests (panels #13 and #15) were somewhat imperfect, due primarily to the fact that the dehydrated bath was now being supplied with suificient reducing chemicals by the Recyclotron current. As the concentration of reducing agents built up in the catholyte, however, later tests (panels #16, #17 and #18) yielded perfect descaling of 908 nickel-steel. Even reducing the Recyclotron current for Example 5 continued to yield perfect results on descaling 908 nickel-steel. Shutting the Recyclotron current off, however, as in Example 6, allowed all the reducing agents to quickly revert to their higher oxidation states, and panels of 908 processed during this period were definitely not descaled. When the Recyclotron current was restored, however, only enough time transpired to re-create the reducing agents, as the melt had already been previously dehydrated. The conditions of Example 7 depict this condition as well as the effect of raising the temperature. Once the reducing chemicals are electrolytically created and maintained, the amount of power input will depend on the amount of descaling work the bath is expected to perform. In Example 8, the Recyclotron current was again reduced simultaneously with lowering the temperature, and it continued to descale 908 panels perfectly. Moving the electrodes closely together, as in Example 9, did not adversely affect the functioning of the descaling bath, and in fact cutting processing times gradually from fifteen (15) to five (5) minutes in the salt and pickling the panels in weaker nitric acid as in Example 10 and Example 11 still continued to achieve remarkable descaling effectiveness. Even enough of the reducing chemicals migrated through the diaphragm screen into the anolyte to effectively descale 908 nickel-steel, as in Example 12. Continued lowerings of the temperature as in Example 13 and Example 14 did not appreciably reduce the ability of the catholyte to descale 908.

Further examples will illustrate the remarkable ability of the system of this invention to descale a variety of stainless steels. These steels are common enough to be standardized in composition ranges and are listed by both 12 SAE and A151 (Society of Automotive Engineers and American Iron and Steel Institute) standards steels composition lists.

The suffixes HRN, ERA and BA below noted designate Hot-Rolled Natural Sheet, Hot-Rolled and Annealed Sheet, and Box-Annealed Sheet respectively.

Stainless steels tested were the following:

Type 302 HRN Type 302 HRA Type 309 HRN Type 309 HRA Type 321 HRN Type 321 HRA Type 410 HRN Type 410 BA Type 430 HRN Type 446 HRN Type 446 HRA EXAMPLE 15 A salt composition A as in Example 1 supra, existing under the conditions of Example 9 supra. All eleven (11) types and conditions of stainless steel scaled panels were immersed in the catholyte for fifteen (15 minutes, then water-quenched. According to the alloy, each panel was then individually subjected to its own special pickling acid sequence. The following is a tabulation of the pickling acids and sequence, and the descaling results:

10% 10%-2% Percent Descaled Stainless Steel 10% H01 HNO; BNO;-

Type F), F HF mm. min. (120? F.) Front Reverse min.

5 100 100 3 100 100 5 100 100 4 100 99 2 100 100 5 100 100 2 100 98 5 100 100 4 100 100 2 100 100 446HRA 4 100 09. 0

EXAMPLE 16 Percent Desealed Stainless Steel Type Front Reverse 302 H RN 446 HRA EXAMPLE 17 With salt and conditions existing under Example 13 supra, the eleven (11) types and conditions of stainless steel scaled panelswere processed in the catholyte for fifteen (15 minutes. After water quenching, they were pickled, according to alloy and scale condition, as in Example 15 The front faces of all panels were one hundred (100) perwnt descaled; the reverse faces of all panels were one hundred (100) percent descaled with two (2) exceptions: 410 HRN was ninety-eight (98) percent and 309 HRA was ninety-nine (99) percent.

EXAMPLE 18 With salt and conditions existing under Example 13 and Example 17 supra, the eleven (11) types of stainless steel scaled panels were subjected to the action of the anolyte for fifteen 15) minutes and then water quenched. They were acid pickled as in Example 15. The front and reverse faces of all panels were one hundred-one hundred 100-100) percent descaled with the following exceptions: 410 HRN was rated one hundred-ninety-eight (100-98) and 446 HRA was rated ninety-nine-ninetyeight (99-98) percent descaled on front and reverse faces respectively.

EXAMPLE 19 With the molten electrolyte and conditions existing as under Example 14 supra, the eleven (11) types and conditions of stainless steel scaled panels were subjected to the reducing action of the catholyte for fifteen (15) minutes. After water quenching, the individual panels were then pickled according to the time cycles and sequences of Example 15. The results were as follows: all panels were one hundred-one hundred (ll00) percent descaled on both front and reverse faces with the exception of Type 446 HRA, which was rated ninetynine-ninety-eight (99-98) percent descaled on front and reverse faces respectively.

EXAMPLE 20 Under the same identical conditions as exist in Example 19 supra, the eleven (11) types and conditions of stainless steel scaled panels were subjected to the action of the anolyte for fifteen (15) minutes. After water quenching and acid pickling according to that of Example 15, all but three (3) panels were perfectly descaled. The three (3) were as follows: Type 410 HRN was one hundred-ninety-nine (100-99); Type 446 was ninety-fiveninety (95-90); and Type 309 HRA was ninety-fifty (90-50) percent descaled on front and reverse faces.

The first twenty (20) examples cited supra were descaling tests performed using a design of a Recyclotron as indicated in FIGURE I, which is the simplest of the possible designs. From a commercial point of view, the fundamental design of FIGURE II is preferred because of its greater adaptability to any presently-existing fused salt furnaces. Without any physical changes whatsoever of existing equipment, the installation of a Recyclotron incorporating the basic features of FIGURE II automatically converts such equipment to the reducing descaling system of the present disclosure (this assumes, of course, that the pot is charged with the proper salt composition as disclosed above). Example 21 through Example 28 inclusive make use of equipment adapted according to the basic design features outlined in FIGURE II.

EXAMPLE 21 A steel pot incorporated in an electrically-heated and automatically-controlled furnace, approximately three (3) feet long by three 3) feet wide by two and one-half (2%) feet deep was charged with two thousand, one hundred and sixty (2,160) pounds of technical grade caustic soda based on its dry weight, and the temperature raised to the fusion point. When the NaOH was in the fused, molten condition at about six hundred and fifty (650) degrees Fahrenheit, two hundred and forty (240) pounds of anhydrous sodium sulfate was added, which then dissolved to form a composition B comprising two thousand, four hundred (2,400) pounds of total salt in the percentage ratio of ninety (90) percent NaAH and ten percent sodium sulfate. When the temperature of this melt reached seven hundred (700) degrees Fahrenheit, a Recyclotron unit of the general design features of FIGURE II was lowered into the pot. This Recyclotron unit consisted of a steel housing approximately twenty (20) inches by twenty (20) inches by seven (7) inches, incorporating a steel combination baffle-diaphragm of twenty (20) inches by twenty (20) inches, in the lower half of which is positioned a twelve (12) inch by fourteen (14) inch diaphragm screen. Within the steel housing was positioned a nickel anode means comprising four hundred and eighty (480) square inches of immersed surface area. (The anode comprises a plurality of vertically-suspended slats to achieve a relatively high surface area.) A copper cathode means, having a submerged sheet portion twelve (12) inches by fourteen (14) inches, was positioned in the pot, externally of the housing, and immediately adjacent to and parallel with the outer face of the diaphragm screen.

A passivating circuit was connected to the pot, the housing means of the Recyclotron, the combination bafllediaphragm means, and was held at seventeen (17) amperes at 3.3 volts. The function of the passivating circuit, as indicated above, is to protect these listed elements in the system from the detrimental corrosion that would otherwise exist.

The Recyclotron current was initially established at approximately two hundred (200) amperes at 4.7 volts for a period of three (3) days for the purpose of electrolytic dehydration of the salt bath. The current was then increased to two hundred and seventy (270) amperes at six (6.0) volts for establishing the operating conditions in the bath.

The salt temperature was maintained at eight hundred (800) degrees Fahrenheit. A floating cover was added to the surface of the catholyte.

Three 3) panels of 908 nickel-steel, each eighteen 18) inches by eighteen (18) inches, were simultaneously immersed in the catholyte of this system for a period of twenty (20) minutes, then water-quenched. This was subsequently followed by pickling for three (3) minutes in ten (10) percent H 50 (at one hundred and seventy-five (175) degrees Fahrenheit), two (2) minutes in ten (10) percent HNO (one hundred and sixty (160) degrees Fahrenheit), and two (2) minutes in a mixed acid comprising ten (10) percent HNO with two (2) percent HF (at one hundred and twenty (120) degrees Fahrenheit). After rinsing and drying, examination of the 908 nickelsteel panels revealed that they were one hundred-one hundred (100-100) percent descaled on both faces.

EXAMPLE 22 Under the conditions of Example 21, a scaled panel of fifty (50) percent nickel-fifty (50) percent ir-on alloy, was immersed in the catholyte for thirty (30) minutes, water-quenched and pickled as follows: two (2) minutes in the previously mentioned (Example 21) H followed by one and one-half (1 /2) minutes in the previously mentioned (Example 21) HNOg-HF acid mixture. The panel was one hundred percent descaled on both faces.

EXAMPLE 23 Under the conditions of Example 21, immersed in the catholyte for thirty (30) minutes was a scaled panel of high nickel alloy of the following approximate composition: fifty-one (51) percent Ni, nineteen (19) percent Cr, eleven (11) percent Co, ten (10) percent M0, 3.2 percent Ti, four (4.0) percent Fe, 1.6 percent A1, 0.3 percent Mn. 0.1 percent C, and 0.005 percent B. The ten (10) inch by six and one-half (6 /2) inch panel was water-quenched, acid-treated under the same conditions and in the same manner as the panel treated in Example 22. This panel upon inspection, was indicated to be one hundred (100) percent descaled on both sides.

EXAMPLE 24 Under the conditions of Example 21, a scaled panel of Type 304 stainless steel, six and one-half (6 /2) inches by sixteen (16) inches, was immersed in the catholyte for a total of five (5) minutes and then quenched in water. The

panel was then acid-treated for three (3) minutes in ten (10) percent H 80 (at one hundred and fifty-eight (158) degrees Fahrenheit), followed by three (3) minutes. in ten (10) to two (2) percent HNO -HF (at one hundred and twenty (120) degrees Fahrenheit). The panel, upon rinsing and drying, was completely (one hundred (100) percent) descaled on both sides.

EXAMPLE 25 Under the conditions of Example 21, a hot-rolled and annealed scaled panel of Type 316 stainless steel, approximately nine and one-half (9 /2) inches by fifteen (15) inches was immersed in the catholyte for a period of twenty (20) minutes and then water-quenched. The panel was then pickled for five minutes in ten percent H 50 (at one hundred and fifty-eight (158) degrees Fahrenheit) followed by three (3) minutes in the HNO HF mixed acid of Example 21. After rinsing and drying, the Type 316 panel processed as above was examined and rated to be one hundred (100) percent descaled on both surfaces.

EXAMPLE 26 Under the conditions of Example 21, a low-nickel alloy steel paneL termed A181 4335, approximately twelve (12) inches by fifteen inches, was immersed in the catholyte for a period of five (5) minutes followed by a water quench. After pickling for ninety (90) seconds in the ten (10) percent H SOL, of Example 25, the panel was rinsed in cold water, and then dried. Although covered with a light smut, the panel was rated as being fully one hundred (100) percent descaled on both sides.

EXAMPLE 27 Under the conditions of Example 21, a box-annealed sheet of Type-410 stainless. steel, approximately twelve (12) inches by sixteen and one-half (116 /2) inches, was immersed in and subjected to the reducing action of the catholyte for a period of five (5) minutes. The sheet was quenched in cold water and followed by pickling for thirty (30) seconds in ten 10) percent H 50 under the conditions and manner of Example 25, followed by one (1) minute in ten (10) percent HNO at one hundred and sixty (160) degrees Fahrenheit. After rinsing and drying, the sheet of Type 410 was obviously clean and one hundred (100) percent scale-free on both faces.

EXAMPLE 28 EXAMPLE 29 A steel pot sixteen (16) inches by sixteen (16) inches by twelve (12) inches deep, fitted with slots for holding and positioning the nickel diaphragm screen twelve (12) inches by fifteen and one-half (15 /2) inches, was charged with a salt composition C which comprised sixteen and one-half (16 /2) percent of anhydrous sodium sulfate and eighty-three and one-half (83 /2) percent technical grade NaOH (based on one hundred and twenty-five (125) pounds total dry weight of the components). This mixture is closely equivalent to the eutectic composition of these two (2) components as determined by a series of freezing point determinations carried out over a large range of sulfate percentages in fused sodium hydroxide. The furnace Was controlled to maintain the fused salt mixture at eight hundred (800) degrees Fahrenheit. A

four (4) inch by five (5) inch submerged copper cathode means and a four (4) inch by five (5) inch submerged nickel anode means was positioned on either side of the centrally-positioned diaphragm means. A passivating circuit was employed and connected to the steel pot and to the diaphragm, with the current set and maintained at two (2) amperes at two (2) volts. The Recyclotron current flowing through the main electrodes was established at sixty ('60) amperes at 4.2 volts.

Salt composition C was electrolyzed for forty-eight (48) hours under these conditions for dehydration and to create the reducing agents in the catholyte. At this point, a scaled panel of 908 nickel steel, three (3) inches by six (6) inches, was immersed in the catholyte for a total of fifteen (15 minutes. After quenching in water, the panel was processed for three (3) minutes in fifteen (15) percent HNO (at one hundred and sixty (160) degrees Fahrenheit) rinsed and dried. The three 3) inches by five (5) inches nickel steel panel was completely stripped of its tough oxide scale, leaving one hundred percent descaled on both sides.

EXAMPLE 30 EXAMPLE 31 Under the conditions of Example 29, a six (6) inch by six (6) inch panel of scaled titanium of commercial purity was immersed in the catholyte for thirty (30) seconds, then water-quenched. The panel was then subjected to a five (5) second flash dip in ten (10) to two (2) percent HNO -HF mixture at one hundred and twenty degrees Fahrenheit, rinsed and dried. The panel of titanium was very bright, being one hundred (100) percent descaled on both surfaces.

EXAMPLE 32 Under the conditions of Example 29, a panel of a titanium alloy scaled at one thousand eight hundred (1,800)

.degrees Fahrenheit was immersed in the catholyte for a period of three and one-half (3%) minutes. The approximate composition of the alloy was 72.5 percent Ti, thirteen (13) percent V, eleven (11) percent Cr and three (3) percent Al. After water quenching, the panel was pickled for twenty (20) seconds in the HNO -HF acid mixture referred to in Example 31. After rinsing and drying, the 13-11-13 titanium alloy was rated to be one hundred (100) percent descaled on both sides.

EXAMPLE 33 Under the conditions of Example 29, a scaled panel of Type 302 stainless steel was immersed in the catholyte for ten (10) minutes, then water-quenched. This was followed by acid pickling in fifteen (15) percent HNO (at one hundred and sixty degrees Fahrenheit) for four (4) minutes, then in ten (10) percent to two (2) percent HNO -HF (at one hundred and twenty (120) degrees Fahrenheit) for thirty (30) seconds. After rinsing and drying, the panel of Type 30-2 was rated to be one hundred (100) percent descaled and bright on both faces.

EXAMPLE 34 Under the conditions of Example 29, a scaled panel of a typical stainless steel was immersed in the catholyte for a period of two (2) minutes. This alloy had the proxi mate composition of twenty-seven (27) percent Ni, twenty (20) percent Cr, two and one-half (2 /2) percent Mo, three (3) percent Cu and the remainder Fe. After water quenching, the panel was pickled for one (1) minute in ten (10) percent HCl (one hundred and fifty (150) degrees Fahrenheit) followed by one and one-half (1 /2) minutes in ten (10) percent to two (2) percent HNO -HF (one hundred and twenty (120) degrees Fahrenheit). The panel was descaled one hundred (100) percent on both sides.

EXAMPLE 35 Under the conditions of Example 29, a six (6) inch by six (6) inch by one-quarter inch plate of scaled Type 430 stainless steel was immersed in the catholyte for a total time of one (1) minute. After quenching in cold water, the plate was pickled for one (1) minute in fifteen (15) percent HNO (one hundred and sixty (160) degrees Fahrenheit) followed by a ten (10) second bright dip in ten (10) percent to two (2) percent HNO -HF (one hundred and twenty (120) degrees Fahrenheit), rinsed and dried. The type 430 plate was one hundred (100) percent descaled on both surfaces.

The above-defined composition can be easily adapted also for use as a home or other structure heating system. The sodium sulfate-sodium hydroxide composition can be located in a furnace or other containing means having positioned therein a composition heating means; such as coils, burners, etc. When required the bath would be heated thereby storing heat or energy which may be drawn upon over a period of time. The passivating circuit described in this disclosure is ideally adapted for use in this heating system, thereby providing a minimum of maintenance.

Although this invention has been illustrated and defined herein in terms of the above examples and accompanying drawings, it is to be understood that these are by no means all inclusive. Various modifications to the invention herein set out *will suggest themselves to those skilled in the art. These are intended to be comprehended within the spirit of this invention.

We claim:

1. A process for producing a reducing bath for the descaling of met-a1 oxides on the surface of a metal article wherein the reducing bath is formed in a metal container having therein a source of positive and negative current separated from each other by a diaphragm thereby forming an anode zone and a cathode zone comprising im-- posing a decomposition voltage through a molten salt mixture within said container, said molten salt mixture being comprised of a substantially anhydrous mixture of at least about three percent alkali-metal sulfate in a major proportion of alkali-metal hydroxide, reducing said alkali-metal sulfate, maintaining the reduced alkalimetal sulfate in said cathode zone, thereby producing a reducing bath of alkali-metal hydroxide and reduced alkali-metal sulfate selected from the group consisting of alkali-metal sulfites, alkali-metal dithionites, alkalimetal thiosulfates, alkali-metal pyrosulfites, alkali-metal sulfides and mixtures thereof.

2. The process of claim 1 wherein said alkali-metal sulfate is sodium sulfate and said alkali-metal hydroxide is sodium hydroxide.

3. A process for the descaling of metal oxides on the surface of a metal article in a reducing bath wherein the in a source of positive and negative current separated from each other by a diaphragm thereby forming an anode Zone and a cathode zone comprising imposing a decomposition voltage through a molten salt mixture within said container, said molten salt mixture being comprised of a substantially anhydrous mixture of at least three percent alkali-metal sulfate in a major proportion of alkali-metal hydroxide, reducing said alkali-metal sulfate thereby producing a reducing bath of alkali-metal hydroxide and reduced alkali-metal sulfate selected from the group consisting of alkali-metal sulfites, alkali-metal dithionites, alkali-metal thiosulfates, alkali-metal pyrosulfites, alkali-metal sulfides and mixtures thereof in said cathode zone, passing into said cathode zone of said molten salt mixture containing said reduced alkali-metal sulfate, a metal article having on its surface metal oxides, reacting said metal oxides with said reduced alkali-metal sulfates, thereby reducing said oxides and subsequently withdrawing said metal article from said molten salt mixture.

4. The process of claim 3 wherein said molten salt mixture is maintained at a temperature of about 700 to 900 degrees Fahrenheit and wherein the current density is maintained in the range of about 0.5 to 3.5 amperes per square inch of cathode area.

5. The process of claim 3 wherein on removing said metal article from said cathode zone, said metal article is quenched with cold water, immersed in an acid pickling solution and subsequently rinsed with water.

6. A process adapted to protect the component elements of a substantially anhydrous electrochemical descaling system from corrosion, said descaling system being a molten bath comprising a major proportion of an alkali- .metal hydroxide in a metal container having therein anode and cathode means comprising electrically connecting said anode and cathode means of an electrochemical descaling system to a primary source of direct electrical current, electrically connecting the elements to be protected to a positive pole of a secondary source of direct electrical current, electrically connecting the negative pole of said secondary source of electrical current to said cathode means, passing a primary and secondary electrical current through said cathode to said anodes, and maintaining said secondary current to said elements to be protected substantially less than the primary current to said anode means of said electrochemical descaling system.

References Cited by the Examiner UNITED STATES PATENTS 472,691 4/1892 Benjamin 204145 2,311,099 2/1943 Tainton 204 145 2,349,662 5/1944 Keating 204 252 2,448,262 8/1948 Gilbert 204 145 2,678,289 5/1954 Noble ell a1. 204 145 2,834,728 5/1958 G allone 204 147 2,847,374 8/1958 Webster et al. 204 -145 2,848,411 8/1958 Hartzell 204 290 2,860,100 11/1958 Krzyszkowski 204 252 2,890,157 6/1959 Raetzsch 204 147 2,909,471 10/1959 Nies 204 147 2,929,769 3/1960 Newell 204 290 2,936,278 5/1960 Shoemaker of al. 204 145 JOHN H. MACK, Primary Examiner. reducing bath is formed in a metal container having there- 60 P. SULLIVAN, R. L. GOOCH, R. MIHALEK,

Assistant Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,293,159 December 20, 1966 Matthew Mekjean et al.

rror appears in the above numbered pat- It is hereby certified that e etters Patent should read as ent requiring correction and that the said L corrected below.

Column 1, line 34, after "not" insert only column 2,

" read Electrolytic line 60,

line 28, for "Electrolyte for "+S O read +5 0 column 9, line 61, for

"repsectively" read respectively column 13, line 70, for "NaAH" read NaOH column 14, line 64, for "Mn." read Mn, column 15, line 49, for "(16)" read (l5) Signed and sealed this 3rd day of December 1968.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

Claims

1. A PROCESS FOR PRODUCING A REDUCING BATH FOR THE DESCALING OF METAL OXIDES ON THE SURFACE OF A METAL ARTICLE WHEREIN THE REDUCING BATH IS FORMED IN A METAL CONTAINER HAVING THEREIN A SOURCE OF POSITIVE AND NEGATIVE CURRENT SEPARATED FROM EACH OTHER BY A DIAPHRAGM THEREBY FORMING AN ANODE ZONE AND A CATHODE ZONE COMPRISING IMPOSING A DECOMPOSITION VOLTAGE THROUGH A MOLTEN SALT MIXTURE WITHIN SAID CONTAINER, SAID MOLTEN SALT MIXTURE BEING COMPRISES OF A SUBSTANTIALLY ANHYDROUS MIXTURE OF AT LEAST ABOUT THREE PERCENT ALKALI-METAL SULFATE IN A MAJOR PROPORTIONOF ALKALI-METAL HYDROXIDE, REDUCING SAID ALKALI-METAL SULFATE, MAINTAINING THE REDUCED ALKALIMETAL SULFATE IN SAID CATHODE ZONE, THEREBY PRODUCING A REDUCING BATH OF ALKALI-METAL HYDROXIDE AND REDUCED ALKALI--METAL SULFATE SELECTED FROM THE GROUP CONSISTING OF ALKALI-METAL SULFITES, ALKALI-METAL DITHIONITES, ALKALIMETAL THIOSULFATES, ALKALI-METAL PYROSULFITES, ALKALI-METAL SULFIDES AND MIXTURES THEREOF.

3. A PROCESS FOR THE DESCALING OF METAL OXIDES ON THE SURFACE OF A METAL ARTICLE IN A REDUCING BATH WHEREIN THE REDUCING BATH IS FORMED IN A METAL CONTAINER HAVING THEREIN A SOURCE OF POSITIVE AND NEGATIVE CURRENT SEPARATED FROM EACH OTHER BY A DIAPHRAGM THEREBY FORMING AN ANODE ZONE AND A CATHODE ZONE COMPRISING IMPOSING A DECOMPOSITION VOLTAGE THROUGH A MOLTEN SALT MIXTURE WITHIN SAID CONTAINER, SAID MOLTEN SALT MIXTURE BEING COMPRISED OF A SUBSTANTIALLY ANHYDROUS MIXTURE OF AT LEAST THREE PERCENT ALKALI-METAL SULFATE IN A MAJOR PROPORTION OF ALKALI-METAL HYDROXIDE, REDUCING SAID ALKALI-METAL SULFATE THEREBY PRODUCING A REDUCING BATH OF ALKALI-METAL HYDROXIDE AND REDUCED ALKALI-METAL SULFATE SELECTED FROM THE GROUP CONSISTING OF ALKALI-METAL SULFITES, ALKALI-METAL DITHIONITES, ALKAI-METAL THIOSULFATES, ALKALI-METAL PYROSULFITES, ALKALI-METAL SULFIDES AND MIXTURES THEREOF IN SAID CATHODE ZONE, PASSING INTO SAID CATHODE ZONE OF SAID MOLTEN SALT MIXTURE CONTAINING SAID REDUCED ALKALI-METAL SULFATE, A METAL ARTICLE HAVING ON ITS SUFACE METAL OXIDES, REACTING SAID METAL OXIDES WITH SAID REDUCED ALKALI-METAL SULFATES, THEREBY REDUCING SAID OXIDES AND SUBSEQUENTLY WITHDRAWING SAID METAL ARTICLE FROM SAID MOLTEN SALT MIXTURE.

6. A PROCESS ADAPTED TO PROTECT THE COMPONENT ELEMENTS OF A SUBSTANTIALLY ANHYDROUS ELECTROHEMICAL DESCALING SYSTEM FROM CORROSION, SAID DESCALING SYSTEM BEING A MOLTEN BATH COMPRISING A MAJOR PROPORTION OF AN ALKALIMETAL HYROXIDE IN A METAL CONTAINER HAVING THEREIN ANODE AND CATHODE MEANS COMPRISING ELECTRICALLY CONNECTING SAID ANODE AND CATHODE MEANS OF AN ELECTROCHEMICAL DESCALING SYSTEM TO A PRIMARY SOURCE OF DIRECT ELECTRICAL CURRENT, ELECTRICALLY CONNECTING THE ELEMENTS TO BE PROTECTED TO A POSITIVE POLE OF A SECONDARY SOURCE OF DIRECT ELECTRICAL CURRENT, ELECTRICALLY CONNECTING THE NEGATIVE POLE OF SAID SECONDARY SOURCE OF ELECTRICAL CURRENT TO SAID CATHODE MEANS, PASSING A PRIMARY AND SECONDARY ELECTRICAL CURRENT THROUGH SAID CATHODE TO SAID ANODES, AND MAINTAINING SAID SECONDARY CURRENT TO SAID ELEMENTS TO BE PROTECTED SUBSTANTIALY LESS THAN THE PRIMARY CURRENT TO SAID ANODE MEANS OF SAID ELECTROCHMICA DESCALING SYSTEM.