Metal surface scale conditioning

Abstract

Methods and systems are provided for treating oxide scale on the surface of a metal object. In one embodiment, a system temperature control apparatus controls the temperature of metal object's surface to an application temperature below the Leidenfrost temperature point of an alkali metal hydroxide aqueous conditioning solution. An application apparatus wets the metal object's surface at the controlled temperature with a thin layer of the solution that engages the oxide scale, and a heating apparatus heats the wetted surface to a final conditioning temperature above a melting point of the alkali metal hydroxide by an additional value selected to effect conditioning of the oxide scale at a reasonable but not excessive rate by the melting alkali metal hydroxide reacting with the oxide scale. The system terminates additional conditioning to prevent creation of additional oxide scale beyond the conditioned depth.

1 . A system for treating oxide scale on the surface of a metal object, comprising:
 a temperature control apparatus that controls the temperature of metal object's surface to an application temperature below the Leidenfrost temperature point of an aqueous conditioning solution comprising an alkali metal hydroxide, the metal object's surface comprising an oxide scale having an initial depth from the metal object's surface;   an application apparatus that wets the metal object's surface at the controlled temperature with a thin layer of the aqueous conditioning solution, the aqueous conditioning solution thin layer engaging the oxide scale; and   a heating apparatus that heats the wetted metal object surface to a final conditioning temperature that is above a melting point of the alkali metal hydroxide in an anhydrous form by an additional value selected to effect conditioning of the oxide scale on the metal surface at a reasonable but not excessive rate, the wetted metal object surface heated to the final conditioning temperature evaporating water in the aqueous conditioning solution and melting the alkali metal hydroxide in the anhydrous form on the metal object's surface, the melting alkali metal hydroxide reacting with the engaged oxide scale and reducing the oxide scale to a conditioned depth from the metal object's surface that is less than the initial depth;   wherein the system terminates additional conditioning of the metal object's surface beyond the conditioned depth, the terminating preventing a creation of an additional oxide scale beyond the conditioned depth from the metal object's surface.   
     
     
         2 . The system of  claim 1 , wherein the system terminates the additional conditioning of the metal object's surface beyond the conditioned depth by the application apparatus applying the aqueous conditioning solution thin layer in an amount wherein the reacting of the melting alkali metal hydroxide with the engaged oxide scale consumes enough of the melting alkali metal hydroxide in the thin layer on the metal object's surface to prevent additional conditioning of the metal object's surface. 
     
     
         3 . The system of  claim 1 , further comprising:
 cooling apparatus that quenches the metal object's surface to a temperature below the final conditioning temperature at an end of a conditioning time period that is selected to terminate the additional conditioning as a function of a material parameter and a dimension parameter of the metal object.   
     
     
         4 . (canceled) 
     
     
         5 . The system of  claim 3 , wherein the final conditioning temperature and the conditioning time period are selected to produce a specified extent of conditioning of the scale on the metal object's surface. 
     
     
         6 . The system of  claim 5 , wherein the additional value to effect conditioning of the oxide scale is selected from a range of zero to about 200° F. (94° C.). 
     
     
         7 - 11 . (canceled) 
     
     
         12 . The system of  claim 6  wherein the aqueous conditioning solution comprises:
 a eutectic blend of sodium and potassium hydroxides at about 30% by weight; 
 about 3% by weight sodium nitrate; 
 about 67% by weight water; and 
 less than about 1% by weight of at least one nonionic surfactant. 
 
     
     
         13 . The system of  claim 12  wherein the eutectic blend comprises about 18% by weight potassium hydroxide and about 12% by weight sodium hydroxide. 
     
     
         14 - 16 . (canceled) 
     
     
         17 . The system of  claim 6 , wherein the final conditioning temperature is an increase over the application temperature ranging from about 150° F. (65° C.) to about 200° F. (94° C.)). 
     
     
         18 . A method of treating oxide scale on the surface of a metal object, comprising:
 controlling a temperature of a metal object's surface to an application temperature below the Leidenfrost temperature point of an aqueous conditioning solution comprising an alkali metal hydroxide, the metal object's surface comprising an oxide scale having an initial depth from the metal object's surface;   wetting the metal object's surface at the controlled temperature with a thin layer of the aqueous conditioning solution, the aqueous conditioning solution thin layer engaging the oxide scale;   heating the wetted metal object surface to a final conditioning temperature that is above a melting point of the alkali metal hydroxide in an anhydrous form by an additional value selected to effect conditioning of the oxide scale on the metal surface at a reasonable but not excessive rate, thereby evaporating water in the aqueous conditioning solution and melting the alkali metal hydroxide in the anhydrous form on the metal object's surface at the final conditioning temperature and causing the melting alkali metal hydroxide to react with the engaged oxide scale and reduce the oxide scale to a conditioned depth from the metal object's surface that is less than the initial depth; and   terminating additional conditioning of the metal object's surface beyond the conditioned depth, the terminating preventing a creation of an additional oxide scale beyond the conditioned depth from the metal object's surface.   
     
     
         19 . The method of  claim 18 , wherein the terminating the additional conditioning comprises the reacting of the melting alkali metal hydroxide with the engaged oxide scale consuming enough of the melting alkali metal hydroxide on the metal object's surface to prevent additional conditioning of the metal object's surface. 
     
     
         20 . The method of  claim 18 , further comprising:
 selecting a conditioning time period to terminate the additional conditioning as a function of a material parameter and a dimension parameter of the metal object; and   wherein the terminating the additional conditioning comprises quenching the metal object's surface to a temperature below the final conditioning temperature at an end of the conditioning time period.   
     
     
         21 . The method of  claim 20  wherein the quenching comprises rinsing the alkali metal hydroxide from the metal object's surface. 
     
     
         22 . The method of  claim 20 , further comprising selecting the final conditioning temperature and the conditioning time period to produce a specified extent of conditioning of the scale on the metal object's surface. 
     
     
         23 . The method of  claim 22 , further comprising:
 selecting the additional value to effect conditioning of the oxide scale from a range of zero to about 200° F. (94° C.).   
     
     
         24 . The method of  claim 22 , further comprising optimizing the conditioned depth by varying at least one of the final conditioning temperature, components of the aqueous conditioning solution, relative amounts of reactants utilized in the aqueous conditioning solution and the conditioning time period as a function of the material parameter and the dimension parameter. 
     
     
         25 . The method of  claim 22 , wherein the specified extent of conditioning is a least-oxide-to-pickle level of conditioning of scale on the metal object's surface and a minimal base metal effect, the method further comprising:
 selecting the final conditioning temperature and the conditioning time period as a function of an amount of scale on the metal object's surface.   
     
     
         26 . The method of  claim 22  wherein the conditioning time period is no more than about thirty seconds. 
     
     
         27 . The method of  claim 26  wherein the conditioning time period is about three seconds. 
     
     
         28 . The method of  claim 22  wherein the heating the wetted metal object surface comprises heating in an oxygen-containing atmosphere. 
     
     
         29 . The method of  claim 22  wherein the aqueous conditioning solution comprises:
 a eutectic blend of sodium and potassium hydroxides at about 30% by weight; 
 about 3% by weight sodium nitrate; 
 about 67% by weight water; and 
 less than about 1% by weight of at least one nonionic surfactant. 
 
     
     
         30 . The method of  claim 29  wherein the eutectic blend comprises about 18% by weight potassium hydroxide and about 12% by weight sodium hydroxide. 
     
     
         31 . The method of  claim 22 , further comprising determining the Leidenfrost temperature point as a function of a caustic concentration of the aqueous conditioning solution. 
     
     
         32 . The method of  claim 31 , further comprising:
 selecting the application temperature near the Leidenfrost temperature point and above a boiling point of the conditioning solution.   
     
     
         33 . The method of  claim 32 , further comprising selecting the application temperature above a salt fusion temperature point of the conditioning solution. 
     
     
         34 . The method of  claim 33 , further comprising:
 selecting the final conditioning temperature as an increase over the application temperature ranging from about 150° F. (65° C.) to about 200° F. (94° C.)).