Method for passivating tinplate

Abstract

The invention relates to a method for passivating the surface of a tinned steel strip which is moved through a coating system at a strip speed of at least 200 m/min. After the steel strip is tinned, the surface is anodically oxidized, and a liquid solution of a chromium-free aftertreatment agent is applied onto the oxide layer. The invention further relates to a tinned steel strip or sheet with a support layer made of a fine or ultrafine sheet of steel, a tin layer which is deposited thereon, and a surface layer of a chromium-free aftertreatment agent. An oxide layer is formed between the tin layer and the surface layer of the aftertreatment agent, said oxide layer substantially consisting of tetravalent tin oxide (SnO 2 ).

1 . Method for passivating the surface of a tinned steel strip, which is moving at a strip speed of at least 200 m/min through a coating system, wherein after tinning the steel strip the surface is first anodically oxidized in order to form an oxide layer, which substantially consists of tetravalent tin oxide (SnO 2 ), and then a liquid solution of a chromium-free post-treatment agent is applied to the oxide layer. 
     
     
         2 . Method of  claim 1 , wherein the anodic oxidation is conducted with a basic electrolyte and at a current density of 1.0 A/dm 2  to 3 A/dm 2  by passing the tinned steel strip through an electrolysis bath. 
     
     
         3 . Method of  claim 2 , wherein the basic electrolyte is chosen from an alkali metal or alkaline earth metal hydroxide or carbonate, a basic alkali metal phosphate, and a basic organic alkali metal or alkaline earth metal salt. 
     
     
         4 . Method of  claim 2 , where wherein the electrolyte contains sodium carbonate. 
     
     
         5 . Method of  claim 2 , wherein the electrolysis bath is adjusted so that a charge of at least 0.2 C is achieved. 
     
     
         6 . Method of  claim 1 , wherein the post-treatment agent is chosen from copolymers of acrylates, polymethylsiloxanes with polyether side chains, acid polyethers, polymers with heterocyclic groups, and acidic, aqueous, liquid compositions that contain complex metal fluoride anions with di- to tetravalent cations and polymer substances. 
     
     
         7 . Method of  claim 1 , wherein the post-treatment agent contains titanium and/or zirconium. 
     
     
         8 . Method  claim 1 , wherein after the anodic oxidation, an aqueous solution of the post-treatment agent or a ready to use solution of the post-treatment agent is applied by a passage through a bath containing the post-treatment agent. 
     
     
         9 . Method  claim 1 , wherein the post-treatment agent is sprayed through at least one tube, which is arranged at a distance from the coated metal strip surface and has at least one hole or nozzle through which the post-treatment agent is sprayed onto one or each coated surface of the steel strip. 
     
     
         10 . Method  claim 1 , wherein the solution of the post-treatment agent is applied to both sides of the steel strip. 
     
     
         11 . Method of  claim 9 , wherein the post-treatment agent is sprayed onto the tinned steel strip surface(s) in the form of liquid jets, where the liquid jets strike the surface in an angle range of between +45° and −45° to the normal to the surface. 
     
     
         12 . Method of  claim 1 , wherein the solution of the post-treatment agent, after being applied to the tinned and oxidized surface of the steel strip, is squeezed off by means of squeeze rollers. 
     
     
         13 . Method of  claim 12 , wherein the tinned steel strip is dried after squeezing off the post-treatment agent. 
     
     
         14 . Method as in  claim 13 , wherein after squeezing off the post-treatment agent and drying, a thin film of the post-treatment agent with a weight between 2 mg/m 2  and 30 mg/m 2  is present on the tinned steel strip. 
     
     
         15 . Method as in  claim 13 , wherein the post-treatment agent is titanium-containing and that after squeezing off the titanium-containing post-treatment agent and after drying, a thin film of the post-treatment agent with a titanium weight between 0.5 mg/m 2  and 2 mg/m 2  is present on the surface of the tinned steel strip. 
     
     
         16 . Method of  claim 1 , wherein the anodic oxidation of the tinned surface of the steel strip takes place within an anodization time of 0.1 to 1.0 second and preferably within an anodization time between 0.2 and 0.7 second. 
     
     
         17 . Tinned steel strip or sheet with a support layer of a fine or ultrafine sheet of steel, a tin layer deposited thereon, and a surface layer of a chromium-free post-treatment agent, characterized in that between the tin layer and the surface layer of the post-treatment agent, an oxide layer is formed, which substantially consists of tetravalent tin oxide (SnO 2 ).