Method for passivation of the surface of blackplate or tinplate and electrolysis system for performance of the method

Abstract

A method for passivation of the surface of blackplate or tinplate by electrolytic deposition of a chromium oxide-containing passivation layer on the surface. The electrolytic deposition of the chromium-containing passivation layer occurs from an electrolyte solution that contains a trivalent chromium compound as well as at least one salt to increase the conductivity and at least one acid or a base to adjust a desired pH value. The electrolyte solution contains no additional components apart from the trivalent chromium compound as well as the at least one salt and the at least one acid or base and is especially free of organic complexing agents and free of buffering agents.

1 . A method for passivation of the surface of blackplate or tinplate, the method comprising:
 electrolytic deposition of a chromium oxide-containing passivation layer on the surface,   wherein the electrolytic deposition of the chromium-containing passivation layer occurs from an electrolyte solution that contains a trivalent chromium compound as well as at least one salt to increase conductivity and at least one acid or base to adjust a desired pH value,   wherein the electrolyte solution contains no additional components apart from the trivalent chromium compound and the at least one salt and the at least one acid or base and is free of organic sequestering agents and free of complexing agents.   
     
     
         2 . The method according to  claim 1 , wherein the passivation layer consists at least essentially of chromium oxide and/or chromium hydroxide. 
     
     
         3 . The method according to  claim 1 , wherein the passivation layer has a weight fraction of chromium oxide and/or chromium hydroxide of more than 90%. 
     
     
         4 . The method according to  claim 1 , wherein the blackplate or tinplate is connected as a cathode and is brought into contact with the electrolyte solution during an electrolysis time, wherein the electrolysis time lies in the range of 0.1 to 2.0 seconds. 
     
     
         5 . The method according to  claim 1 , wherein the blackplate or tinplate is passed through at least one electrolysis tank or several electrolysis tanks arranged one behind the other in a sheet travel direction at a prescribed sheet speed, during which the sheet speed is at least 100 m/min. 
     
     
         6 . The method according to  claim 5 , wherein a temperature of the electrolyte solution has a temperature averaged over a volume of the corresponding electrolysis tanks in the range of 20° C. to 65° C. 
     
     
         7 . The method according to  claim 5 , wherein an electrolysis time, in which the blackplate or tinplate is in effective electrolytic contact with the electrolyte solution is less than 1.0 seconds in each of the tanks, wherein a total electrolysis time in which the blackplate or tinplate is in effective electrolytic contact with the electrolyte solution in all the tanks is between 0.5 seconds and 2.0 seconds. 
     
     
         8 . The method according to  claim 1 , wherein the trivalent chromium compound is chosen from the group of basic Cr(III) sulfate (Cr 2 (SO 4 ) 3 ), Cr(III) nitrate (Cr(NO 3 ) 3 ), Cr(III) oxalate (CrC 2 O 4 ), Cr(III) acetate (C 12 H 36 ClCr 3 O 22 ), Cr(III) formate (Cr(OOCH) 3 ) or a mixture thereof, and wherein the salt of the electrolyte solution contains at least one alkali metal sulfate. 
     
     
         9 . The method according to  claim 1 , wherein the electrolyte solution has a pH value (measured at a temperature of 20° C.) in the range of 2.3 to 5.0, wherein the pH value is adjusted by adding at least one acid to the electrolyte solution. 
     
     
         10 . The method according to  claim 1 , wherein the concentration of trivalent chromium compound in the electrolyte solution is at least 10 g/L. 
     
     
         11 . The method according to  claim 1 , wherein the passivation layer applied from the electrolyte solution has a total coating weight of chromium oxide and/or chromium hydroxide of at least 3 mg/m 2 . 
     
     
         12 . The method according to  claim 1 , wherein by using an appropriate anode during electrolytic deposition of the passivation layer, oxidation of chromium(III) from the trivalent chromium compound of the electrolyte solution (E) to chromium(VI) is prevented. 
     
     
         13 . The method according to  claim 12 , wherein the anode contains no stainless steel and no platinum. 
     
     
         14 . The method according to  claim 11 , wherein the anode contains an outer surface or coating of a metal oxide, or wherein the anode consists of one of these materials. 
     
     
         15 . The method according to  claim 1 , wherein to produce the electrolyte solution the trivalent chromium compound that has been freed of organic residues except for unavoidable contaminants, the at least one salt as well as the at least one acid or base to adjust the desired pH value is dissolved in water and the solution thereby obtained is allowed to stand for complexation for at least 5 days, and then fine adjustment of the pH value occurs by addition of an acid or base. 
     
     
         16 . An electrolysis system for electrolytic passivation of the surface of a blackplate or tinplate by deposition of a chromium oxide-containing passivation layer on the surface, the electrolysis system comprising:
 at least one electrolysis tank filled with an electrolyte solution, or several electrolysis tanks arranged one behind the other, each of which is filled with the same electrolyte solution,   wherein the electrolyte solution contains no other components apart from the trivalent chromium compound, as well as at the least one salt to increase conductivity and the at least one acid or base to adjust the desired pH value and is especially free of organic components and free of complexing agents,   and the blackplate or tinplate for electrolytic deposition of the passivation layer is passed through the at least one electrolysis tank or through the several electrolysis tanks in succession with a stipulated sheet speed in a sheet travel direction so that the surface is electrolytically in effective contact with the electrolyte solution and a passivation layer consisting at least essentially of just chromium oxide and/or chromium hydroxide is deposited on the surface.   
     
     
         17 . The electrolysis system according to  claim 15 , wherein an anode with an outer surface or a coating of a metal oxide or of a mixed metal oxide is arranged in the or each electrolysis tank. 
     
     
         18 . A blackplate or tinplate with a surface passivated by electrolytic deposition of a chromium oxide containing passivation layer, wherein the passivation layer consists at least essentially of just chromium oxide and/or chromium hydroxide and has a weight fraction of chromium oxide and/or chromium hydroxide of more than 90%. 
     
     
         19 . The blackplate or tinplate according to  claim 17 , wherein the passivation layer includes at least of a first layer facing the surface of the blackplate or tinplate and a second layer forming the surface of the passivated blackplate or tinplate, the first layer containing metallic chromium and a second layer consisting of pure chromium oxide and/or chromium hydroxide. 
     
     
         20 . The blackplate or tinplate according to  claim 17 , wherein the passivation layer has a total coating weight of chromium oxide and/or chromium hydroxide of at least 3 mg/m 2 .