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United States Patent Office Patented July 2, 1963

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3,096,261 United States Patent Office Patented July 2, 1963 1. 2 alkali metals, as well as mixtures thereof. Because of 3,096,261 avaliability and lower cost, however, sodium hydroxide SALT BATH FOR ELECTROLYTIC CLEANING is preferred in this composition, but this is not to be taken OF METALS as limiting the scope of the invention in any way. The Matthew Mekjean, Niagara Falls, N.Y., assignor to 5 alkali metal hydroxide must be present in at least fifty Hooker Chemical Corporation, Niagara Falls, N.Y., a percent by weight of the total composition, with the pre corporation of New York ferred composition containing at least eighty percent by No Drawing. Filed May 25, 1959, Ser. No. 85,270 weight. When larger amounts of alkali metal hydroxides 11. Cairns. (C. 204-4) are employed (e.g., ninety-five percent or greater), the The present invention relates to a salt bath for the O only other constituent may be the additives of the present electrolytic cleaning of metals. More particularly, the invention. When smaller amounts of alkali metal hy present invention resides in a new and improved molten droxide are employed the remainder of the composition alkali metal hydroxide based salt bath for the electrolytic in addition to the additives of the present invention may cleaning of metals. be any of a number of special additives to accomplish The commercially available alkali metal hydroxide based 5 special results, said special additives being known to the salt baths for the electrolytic cleaning of metals operate art, and some of which being enumerated hereinafter. unsatisfactorily after extended idling periods. The metal Technical grade caustic soda is normally preferred since workpieces which result from the use of these baths after it is commercially available at a relatively low cost. such shutdowns appear a dark gray or brown color and Hence, when technical grade caustic soda is employed, have a dirty unattractive appearance. The bath must be 20 small amounts of impurities normally associated there "primed’ electrolytically before workpieces can be ac with are present in the bath, i.e., around three percent ceptably descaled and cleaned. This priming may be cumulatively by weight of sodium chloride, sodium car accomplished by using the bath electrolytically for at least bonate, etc. based on the total composition. It will be one hour, i.e., at least one hour of valuable production noted that in use the molten salt bath slowly absorbs time must be consumed during which time the workpieces 25 carbon dioxide from the atmosphere and converts Some are rejected due to a dark gray or brown color and a non of the alkali hydroxide to alkali carbonate. Another metallic, dirty unattractive appearance. Furthermore, source of carbon dioxide in the proximity of the bath heretofore, when the bath is in the primed condition, the would be the exhausts of gas or oil-fired heating tubes workpieces emerge from the bath relatively clean, i.e., for maintaining the salt in the molten state. This con light metallic or slivery gray, but with stains varying in 30 version of alkali hydroxide to alkali carbonate in no color from straw yellow to dark brown in a patternless way adversely affects the utility of the Salt bath, in fact, mottled effect. carbonate below thirty percent increases the fluidity of It is therefore, an object of the present invention to the salt due to a lowering of the melting point of the provide an alkali metal hydroxide based salt bath for the mixture. electrolytic cleaning of metals, wherein said bath need 35 The additives of the present invention should be pres not be primed electrolytically after periods of inactivity. ent in at least 0.05 percent by weight based on the total It is a further object of the present invention to provide composition. The upper limit is determined by the a chemical means for priming an alkali metal hydroxide solubility of the additives in the caustic bath. It will be based salt bath for the electrolyte cleaning of metals, noted that the additives should be added slowly to the said chemical means being safe, inexpensive and ex 40 caustic bath due to the fact that the additives of the pres peditious. ent invention tend to react with available oxygen in the It is a still further object of the present invention to bath. A preferred amount of additives has been found provide an alkali metal hydroxide based salt bath for the to be around 0.5 percent by weight based on the total electrolytic cleaning of metals, wherein when said bath is composition; however, larger amounts may be profitably primed, it is not subject to objectionable staining of 45 employed. workpieces. The additives of the present invention are gettering Further objects of the present invention will appear agents selected from the group consisting of elemental hereinafter. phosphorus, elemental iron, an inorganic phosphide, an In accordance with the present invention, I have found inorganic phosphite, or mixtures thereof. Typical metal a novel, substantially anhydrous, molten alkali metal hy 50 lic phosphides that may be employed include ferrophos droxide based salt for the electrolytic cleaning of metals, phorus (iron phosphide), aluminum phosphide, calcium preferably ferrous metals, cuprous metals, nickelous phosphide, arsenous phosphide, boron phosphide, cobalt metals, chromium metals and alloys thereof, comprising phosphide, cupric phosphide, cuprous phosphide, molyb at least fifty percent by weight of an alkali metal hy denum phosphide, stannic phosphide, etc. Typical metal droxide and at least 0.05 percent by weight of an addi 55 lis phosphites include sodium hypophosphite, sodium tive selected from the group consisting of elemental phosphite, calcium phosphite, calcium hypophosphite, phosphorus, elemental iron, a phosphide, a phosphite and cobaltous orthophosphite, lead phosphite, magnesium or mixtures thereof. thophosphite, potassium orthophosphite, potassium hypo By employing the salt bath composition in accordance phosphite, etc. The preferred additives of the present with the process of the present invention, I have obviated 60 invention are the phosphides, and in particular ferrophos the necessity for expensive and time consuming electro phorus baceuse of commercial availability and low cost. lytic priming of the salt bath after periods of inactivity. The necessity for priming will recure after an extended In addition the process of the present invention is safe, idling period. The length of time after which priming inexpensive and expeditious. Furthermore, after the is required will vary depending on the size of the bath, bath is primed, metals may be effectively cleaned without 65 temperature of the bath, the type of metals descaled prior objectionable straining of workpieces. Further advan to idling, composition of the bath, etc. tages of the composition of the present invention will be The special additives which may be included to achieve apparent to those skilled in the art upon reading the more desirable results for specific applications include present specification. 70 one or more of the following: phosphates, chlorides, car The alkali hydroxides which may be used include sodi bonates, fluorides, silicates, aluminates, stannates, borates, um, potassium, cesium, lithium, rubidium and the other sulfates, sulfites, molybdates, fluosilicates and fluoborates. 3,096,261 w 3 4. It is preferred to use the alkali metal additives since this metallic appearance. After approximately one hour, the corresponds to the bath; however, others can be used. castings emerged descaled, bright and clean, except for The use of these additives would be apparent to one a yellow, mottled stain. All samples Subsequently proc skilled in the art. It is to be understood that the bath essed continued to emerge descaled, bright and clean, of the present invention is predominately non-oxidizing except for a yellow, mottled stain, as long as there were and non-reducing in character. no extended idling periods. The temperatures at which the molten salt may be economically used can be varied over a range between Example 3-Descaling Cast Iron in a Ferrophosphorus about 650 degrees Fahrenheit to about 1250 degrees Prined Bath Fahrenheit, although the preferred operating tempera 0. A molten salt bath was idled in a manner after Exam ture would be in the range of from about 800 degrees ple 1. Thereafter, ferrophosphorus of the molecular Fahrenheit to about 1000 degrees Fahrenheit. The bath composition Fe2P, equivalent to one-tenth of one per may be kept in the molten state by the conventional cent by weight of the bath, was slowly added to the bath. methods known to those skilled in the art. For example, When the chemical reaction ceased, the bath is said to small furnaces might be economically controlled by elec 15 be chemically primed. A small, badly scaled, cast iron tric heating. Larger commercially operated baths, which plug was electrolytically descaled in a manner after Ex would have high heat dragout, might be more econom ample 1. The casting emerged completely descaled and ically heated by gas fired immersion tubes. Other heat silvery white, with on evidence of the mottled stain of ing methods which appear convenient may be utilized. Example 2. Subsequent samples continued to be de In all cases the specific volume of the metal being 20 scaled with similar results. treated and the thickness of the scale on the workpiece will control the time of the descaling cycle. Another Example 4.-Descaling Nickel-Steel in a Ferrophos variable which will effect the length of time of immersion phorus Primed Bath is the temperature at which the bath is maintained. The Example 3 was repeated electrolytically descaling a current density or amperes per unit area of the total Sur nickel-steel panel 3.5 by 6.0 inches of the proximate face of the workpiece will also determine the length of composition of 50 percent nickel and 50 percent iron, the processing cycle.
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