US2738293A - Salt bath system and method for treating metals - Google Patents

Description

United States Patent ice SALT BATH SYSTEM AND METHOD FOR TREATING METALS Harry R. Spence, Huntly, Va., assignor to Diamond Alkali Company, Cleveland, Ohio, a corporation of Delaware No Drawing. Application July 25, 1952, Serial No. 300,985

10 Claims. (Cl. 1343) This invention relates to a new alkaline salt bath composition and to processes for conditioning and removing oxides on the surfaces of ferrous and non-ferrous metals and alloys.

It is known to follow the alkaline bath treatment of a metal to be descaled or cleaned with a light pickling treatment in a dilute aqueous solution of an inorganic acid to complete the removal of oxide which has been loosened and rendered easily removable by previous alkaline bath treatment. The process of this invention is advantageous in that it reduces the time of treatment in the alkaline bath heretofore required, does not result in attack on the metals themselves as distinguished from the oxide thereon, reduces the rigorousness of the subsequent acid treatment and thus saves acid, and, moreover, is useful when applied to metals known as stainless steel, such as chromium-iron alloys, nickel-iron alloys, and other chromium-nickel-iron alloys which re sist descaling treatment by prior art alkaline salts in a fused state.

It has heretofore been proposed to employ alkaline salt baths in the removal of oxides from steel surfaces. One prior practice of the general nature involved employs a bath comprising one or more fused caustic alkalies containing a minor proportion of an oxidizing agent, such as alkali nitrate. Experience has taught, however, that in employing such a composition, excessive removal of metal, as well as oxides, is encountered, and uneconomical corrosive destruction of containers used to hold the bath is encountered. Additional disabilities which accompany the practice of such prior art processes, especially those where the nitrate is present in an amount of the order of twenty odd percent, and particularly those wherein the remainder of the bath comprises an equal mixture of sodium and potassium hydroxides, is that in the treatment of stainless and high chromiumcontent steel, there is commonly a preferential action upon the oxides in favor of the chromium oxide, tending to leave undue quantities of the remaining oxide unaffected and thus not subject to complete removal by subsequent acid pickling treatment.

In accordance with the present invention, I have discovered that the oxides of all such metals can be removed or at least so conditioned by their immersion in my new fused salt bath that the various difliculties of the foregoing and other prior practices are overcome. In my new composition, I preferably employ an anhydrous alkaline salt which is preponderantly 5651515 hydroxide. Suc lisodiiinihyd roxide may be purchased in the form of commercial caustic soda which generally may have as much as about llblyleizhtnisodiumchlorige therein as a normal impurity. My new salt includes between about 0.25% and not more than 10% by weight, and preferably about 5%, of potassium hydroxide (KOH) which may be of commercial purity. The remainder of the salt comprises a compound adapted to function as an oxidizing agent in the environment of the bath. Suit- 2,738,293 Patented Mar. 13, 1956 able agents include alkali oga lkaline earth dichromates, nitrates, peroxides, manganates, permanganates or chlo ra'te'sj in general, sodium salts are more suitable and sodium nitrate is preferred. The oxidizing agent may also be of commercial purity. Further, it will be noted that the potassium hydroxide ingredient is a minor percentage of the new composition. The oxygen-containing ingredient, on the other hand, may vary in weight from a weight somewhat greater than the weight of the potassium hydroxide to a weight about equal to the weight of the sodium hydroxide.

In the application of my process, the salt is employed in a non-aqueous fused condition in an operative range of between about 650 F. and about 1250 F., optimum results being obtained between about 800 F. and about 950 F.

In the process of this invention as applied to stainless steels, nickel-chrome steels and straight chrome steels, oxides generally in the form of a scale which may be quite adherent before treatment are normally removed. In the cases where complete removal is not effected in the alkaline bath, the scale is changed so that a fine powderytype of surface is initially produced. No attack upon the metals themselves is observed in either case. Another material advantage is that the time of treatment is sub stantially reduced to as much as one-half the time required by the nearest comparable prior art procedures. Such an advance is of enormous moment in the industry.

In those cases in which the oxides have been conditioned by the new process rather than completely removed, an acid dip of short duration in a dilute aqueous solution of an inorganic non-oxidizing acid, such as sulfuric acid (H2504) or hydrochloric acid (HCl), will remove all trace of the conditioned oxides, leaving a clean and bright surface on the metals treated. In addition to providing for the removal of adherent scale, the present invention also incidentally frees the metal surfaces of oils and greases and removes all rust and impurities that may be thereon. Prior to the light acid pickling dip, it is desirable to rinse or wash the metals with water to avoid bringing the alkaline material into the acid-dip tank. The shorter period of time such metals need be in contact with the dilute inorganic acid is a highly advantageous aspect of this invention as any tendency of the dilute acid to attack the metals themselves is correspondingly diminished. In some cases it may be desirable to prolong the period of the acid dip to as much as five minutes which, however, will not change the benefits achievable by my new process. As will be well-understood by those in the art, the composition of the dilute acid may contain from 3% to 15% of concentrated acid by volume.

By way of example and not by way of limitation of the advantages of the new process, the following specific examples are offered:

Example I A piece of hot rolled, annealed stainless steel having about 18% nickel content, about 8% chromium content, with the balance substantially iron is immersed for about five minutes in a bath containing about 70% sodium hydroxide, about 28% sodium nitrate, and about 2% potassium hydroxide, allsuch percentages being by weight. The temperature of the bath is about 900 F. Upon removal of the piece from the bath, it is washed with water and then dipped for about one minute in a tank of 3% sulphuric acid. Upon removal of the piece of stainless steel from the acid, it is found to have a clean, bright metal surface without any trace thereon of the oxides that were present at the time it is immersed in the salt bath. In general, the times of immersion both in the salt bath and in any subsequent dilute acid bath are not critical, except as otherwise taught herein, and may be varied to suit operational requirements.

Example II A piece of low nickel-content steel, the scale on which is generally very difiicult to remove, is treated in accordance with Example I. Scale and other surface impurities are completely removed.

Example III A piece of severely surface-oxidized stainless steel is treated in a bath of composition 92% by weight of sodium hydroxide, 5% of sodium nitrate, and 3% of potassium hydroxide. Water wash, followed by one minute 5% sulfuric acid treatment, removes all oxides.

Example IV A sample of steel plate, as in Example I, is immersed in a molten bath consisting essentially on a Weight basis of about 75% sodium hydroxide, 22% sodium nitrate, and 3% potassium hydroxide. After one minute treatment at 900 F., the sample is removed, water washed, and acid dipped in HQ for 3 minutes. The sample emerges bright and completely cleaned. A procedure similar in all respects but omitting the 3% potassium hydroxide results in the removal of oxide to an extent of about only 50%.

Example V A. A hot rolled annealed plate of 18-8 stainless steel having a heavy layer of oxide is immersed for five minutes in a fused bath containing 50% caustic soda, 45% sodium nitrate, and 5% by weight of caustic potash at 1000 F. When removed and quenchced in water, the oxide is found to be changed to a sponge-like material and is removed completely in a one minute dip in 5% sulfuric acid, leaving a clean, bright surface as though polished, without attack on the base metal.

B. A piece of the same plate as that in Example V A. is treated in a bath of 50% caustic soda and 50% sodium nitrate for 5 minutes at 1000" F., all other conditions being duplicated. Only about 50% of the oxide is found to be changed. This plate requires 7 minutes in sulfuric acid to remove the sponge-like material. Upon inspection, there was still evidence of the original oxide that was not afiected by the fused bath or the acid.

Example VI A piece of the same plate as that in Example V is treated in the same way as in the first section of Example V, at temperatures of 940 F. and 775 F. The results are substantially the same except at 775 E, where the piece is noticeably less bright and shiny after acid treatment.

Example VII A piece of the same plate as in Example V is treated for 10 minutes in a fused bath of 50% caustic soda, 45% of sodium nitrate, and 5% of caustic potash by weight at 875 F. The result is comparable to that of the first section of Example V. Washing in water and one minute 5% sulfuric acid dip leaves the surface clean and bright.

Example VIII A piece of type 430 straight chrome hot rolled and annealed plate (14-48% Cr, 1% Si, 1% Mn, 0.12% C, remainder substantially Fe) with a heavy thick oxide coating is treated in a fused bath of 50% caustic soda, 45% sodium nitrate, and 5% caustic potash for 5 minutes at 850 F. When removed, the surface oxides appear to be changed to a soft spongy material having a reddish' brown color. When washed in water and given a one minute dip in warm weak sulfuric acid, the spongy material is removed, leaving a uniform clean white metal surface.

Example 1X A piece of the same plate as in Example VIII is treated in a fused bath of caustic soda and 50% sodium nitrate by weight without any caustic potash for 8 minutes at 850 F. When removed, the surface is a dull, glazed, black color and does not have the spongy material. A water wash and dip of one minute in weak warm acid remove only a small part of the black glaze.

Example X A piece of nickel steel with a heavy thick layer of black oxide is treated as in Example V with equally satisfactory results.

Similarly satisfactory results are obtained with many other alloys including pure titanium and titanium-nickel chrome stainless steel and heat resisting alloys.

The theoretical basis for this invention is not completely apparent. It appears that the combination of sodium hydroxide with 10% or less of potassium hydroxide has an action which is different from the action of sodium hydroxide alone and less injuriously corrosive both to work and containers of the bath than baths comprising a major amount of potassium hydroxide with caustic soda and an oxidizing agent.

While there have been described in detail certain forms of the invention and embodiments of its practice, the invention is not to be understood as being limited to the detailed disclosure as it is realized that changes within the scope of the invention are possible, and it is further intended that each step in the following claims shall refer to all equivalent steps for accomplishing the same result in substantially the same or equivalent manner, it being intended to cover this invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A composite salt for a metal cleaning salt bath process, consisting essentially of, on a weight basis, sodium hydroxide and sodium nitrate in approximately equal parts and potassium hydroxide in an amount not in excess of about 10% of the total weight of said salt.

2. A composite salt for a salt bath process, consisting essentially of, on a weight basis, potassium hydroxide from about 0.25% to about 5%, sodium nitrate in a quantity at least one and one-half times the weight of said potassium hydroxide but not in excess of the weight of sodium hydroxide, and the balance substantially sodium hydroxide.

3. In a process for removing oxides from metals, the steps comprising immersing such metals in a bath of fused salt consisting essentially of sodium hydroxide as the preponderant ingredient, potassium hydroxide in a quantity within the range from about 0.25% to about 10% by weight of the total dry weight of said salt, and an oxidizing agent selected from the group consisting of peroxide and oxygen yielding salts of the alkali and alkaline earth metals, said agent being stable in the presence of strong alkali at a temperature between from about 650 F. to 1050 F. for a period of time sufiicient to change the oxides on said metals into a porous readily removable condition, said oxidizing agent being present in a quantity of at least one and one-half times the weight of said potassium hydroxide but not in excess of the weight of sodium hydroxide.

4. In a process for removing oxides from metals, the steps comprising immersing such metals in a bath of fused salt consisting essentially of approximately equal parts of sodium hydroxide and sodium nitrate, and potassium hydroxide in a quantity between about 0.25% and about 5% by weight of the total weight of said salt for a period of time sufficient to change the oxides on said metals into a porous and at least loosely adherent readily removable condition.

5. In a process for removing oxides from metals, the steps comprising immersing such metals in a bath of fused salt consisting essentially of sodium hydroxide as the preponderant ingredient, potassium hydroxide in an amount within the range from about 0.25% to about 5% by weight of the total weight of said salt and an oxidizing agent selected from the group consisting of peroxide and oxygen yielding salts of the alkali and alkaline earth metals, said agent being stable in the presence of molten caustic alkali in a quantity at least one and one-half times the weight of said potassium hydroxide but not in excess of the weight of said sodium hydroxide for a period of time sufficient to change the oxides on said metal into a porous readily removable condition.

6. In a process for removing oxides from metals, the steps comprising immersing such metals in a bath of fused salt consisting essentially of sodium hydroxide as the preponderant ingredient, potassium hydroxide in an amount within the range from about 0.25% to about 5% by weight of the total weight of said salt and the balance sodium nitrate in an amount greater than the amount of potassium hydroxide but not greater than the weight of sodium hydroxide at a temperature between from about 650 F. to 1050 F. for a period of time sufficient to change the oxides on said metals into a porous readily removable condition.

7. In a process for removing oxides from metals, the steps comprising immersing such metals in a bath of fused salt consisting essentially of approximately equal parts of sodium hydroxide and sodium nitrate, and potassium hydroxide in a quantity between about 0.25% and about 5% by weight of the total weight of said salt, at a temperature between from about 800 F. to 950 F., for a period of time sufficient to change the oxides on said metals into a porous and at least loosely adherent readily removable condition.

8. In a process for removing oxides from metals, the steps comprising immersing such metals in a bath of fused salt consisting essentially of sodium hydroxide as the preponderant ingredient, potassium hydroxide in an amount within the range from about 0.25% to about 5% by weight of the total weight of said salt and the balance sodium nitrate in an amount greater than the amount of potassium hydroxide but not greater than the weight of sodium hydroxide for a period of time sufficient to change the oxides on said metals into a porous readily removable condition, and thereafter immersing said metals in a dilute inorganic acid for a period of time sufficient to complete the removal of said oxides.

9. In a process for removing oxides from metals, the steps comprising immersing such metals in a bath of fused salt consisting essentially of approximately equal parts of sodium hydroxide and sodium nitrate, potassium hydroxide in a quantity between about 0.25% and about 5% by weight of the total weight of said salt for a period of time sufficient to change the oxides on said metals into a porous and at least loosely adherent readily removable condition, and thereafter immersing said metals in a dilute inorganic acid for a period of time not in excess of about five minutes sufiicient to complete the removal of said oxides.

10. In a process for removing oxides from metals, the steps comprising immersing such metals in a bath of fused salt consisting essentially of sodium hydroxide as the preponderant ingredient, potassium hydroxide in an amount within the range from about 0.25 to about 5% by weight of the total weight of said salt and an oxidizing agent selected from the group consisting of peroxide and oxygen yielding salts of the alkali and alkaline earth metals, said agent being stable in the presence of molten inorganic alkali in a quantity at least one and one-half times the quantity of said potassium hydroxide but not in excess of the weight of sodium hydroxide for a period of time sufficient to change the oxides on said metal at least into a porous readily removable condition, and thereafter immersing said metals in a dilute inorganic acid for a period of time sufficient to complete the removal of said oxides.

References Cited in the file of this patent UNITED STATES PATENTS Re. 22,887 Spence et a1. June 3, 1947 2,271,375 McKay Ian. 27, 1942 2,567,456 Webster Sept. 11, 1951

Claims (2)

1. A COMPOSITE SALT FOR A METAL CLEANING SALT BATH PROCESS, CONSISTING ESSENTIALLY OF, ON A WEIGHT BASIS, SODIUM HYDROXIDE AND SODIUM NITRATE IN APPROXIMATELY EQUAL PARTS AND POTASSIUM HYDROXIDE IN AN AMOUNT NOT IN EXCESS OF ABOUT 10% OF THE TOTAL WEIGHT OF SAID SALT.

3. IN A PROCESS FOR REMOVING OXIDES FROM METALS, THE STEPS COMPRISING IMMERSING SUCH METALS IN A BATH OF FUSED SALT CONSISTING ESSENTIALLY OF SODIUM HYDROXIDE AS THE PREPONDERANT INGREDIENT, POTASSIUM HYDROXIDE IN A QUANTITY WITHIN THE RANGE FROM ABOUT 0.25% TO ABOUT 10% BY WEIGHT OF THE TOTAL DRY WEIGHT OF SAID SALT, AND AN OXIDIZING AGENT SELECTED FROM THE GROUP CONSISTING OF PEROXIDE AND OXYGEN YIELDING SALTS OF THE ALKALI AND ALKALINE EARTH METALS, SAID AGENT BEING STABLE IN THE PRESENCE OF STRONG ALKALI AT A TEMPERATURE BETWEEN FROM ABOUT 650* F. TO 1050* F. FOR A PERIOD OF TIME SUFFICIENT TO CHANGE THE OXIDES ON SAID METALS INTO A POROUS READILY REMOVABLE CONDITION, SAID OXIDIZING AGENT BEING PRESENT IN A QUANTITY OF AT LEAST ONE AND ONE-HALF TIMES THE WIEGHT OF SAID POTASSIUM HYDROXIDE BUT NOT IN EXCESS OF THE WEIGHT OF SODIUM HYDROXIDE.