Chromium-chromium oxide coatings applied to steel substrates for packaging applications and a method for producing said coatings

Abstract

A coated steel substrate for packaging applications, the substrate containing (i) a conventional non-passivated electrolytic, optionally flowmelted, tinplate, or (ii) a cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate, wherein one or both sides of the substrate is coated with a chromium metal—chromium oxide coating layer produced in a single process step by using a trivalent chromium electroplating process. A process for obtaining the coated steel substrate.

1 . A process for producing a coated steel substrate for packaging applications by depositing a chromium metal—chromium oxide coating on the substrate for packaging applications containing
 a) a conventional non-passivated electrolytic, optionally flowmelted, tinplate, Or 
 b) a cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate, 
 
       comprising electrolytically depositing on said substrate said chromium metal—chromium oxide coating in a single process step from a plating solution comprising a mixture of a trivalent chromium compound, a chelating agent, an optional conductivity enhancing salt, an optional depolarizer, an optional surfactant, to which an acid or base is optionally added to adjust pH, wherein the plating solution does not contain a buffering agent, and wherein a sufficiently high cathodic current density is being applied to deposit chromium metal. 
     
     
         2 . The process according to  claim 1 , wherein the chelating agent comprises a formic acid anion, the conductivity enhancing salt contains an alkali metal cation and the depolarizer comprises a bromide containing salt. 
     
     
         3 . The process according to  claim 1 , wherein the chelating agent, the conductivity enhancing salt and the depolarizer is potassium contains a cationic species and the cationic species in the chelating agent, the conductivity enhancing salt and the depolarizer is potassium. 
     
     
         4 . The process according to  claim 1 , wherein the coated substrate is further provided on one or both sides with an organic coating, consisting of a thermosetting organic coating by a lacquering step, or a thermoplastic single layer, or a thermoplastic multi-layer polymer by a film lamination step or a direct extrusion step. 
     
     
         5 . The process according to  claim 1 , wherein an anode is chosen that reduces or eliminates oxidation of Cr(III) ions to Cr(VI) ions during the plating step. 
     
     
         6 . The process according to  claim 1 , wherein the substrate is a tin coated substrate for packaging applications and is subjected to an electrolytic pre-treatment to minimise tin oxide layer thickness before coating one or both sides with the chromium metal—chromium oxide coating layer. 
     
     
         7 . The process according to  claim 6 , wherein the electrolytic pre-treatment consists of dipping the tin coated substrate into a sodium carbonate solution and applying a cathodic current density. 
     
     
         8 . The process according to  claim 7 , wherein the sodium carbonate solution has 2 to 5 g/l of Na 2 CO 3  at a temperature of between 35 and 65° C., and wherein the cathodic current density of between 0.5 and 2 A/dm 2  is applied for a period of between 0.5 and 5 seconds. 
     
     
         9 . The process according to  claim 1 , wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the non-passivated tinplate containing a total chromium content of at least 20 mg/m 2 . 
     
     
         10 . The process according to  claim 1 , wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the non-passivated tinplate containing a total chromium content of at most 140 mg/m 2 . 
     
     
         11 . The process according to  claim 1 , wherein the electrolytic deposition depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate containing a total chromium content of at least 20 mg/m 2 . 
     
     
         12 . The process according to  claim 1 , wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate containing a total chromium content of at most 140 mg/m 2 . 
     
     
         13 . The process according to  claim 1 , wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium meta—chromium oxide layer on the non-passivated tinplate containing a total chromium content of at least 40 mg/m 2 . 
     
     
         14 . The process according to  claim 1 , wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the non-passivated tinplate containing a total chromium content of at most 90 mg/m 2 . 
     
     
         15 . The process according to  claim 1 , wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate containing a total chromium content of at least 40 mg/m 2 . 
     
     
         16 . The process according to  claim 1 , wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate containing a total chromium content of at least 60 mg/m 2 . 
     
     
         17 . The process according to  claim 1 , wherein the electrolytic deposition deposits a chromium metal—chromium oxide layer on the cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate containing a total chromium content of at most 90 mg/m 2 . 
     
     
         18 . The process according to  claim 1 , wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate containing a total chromium content of at most 80 mg/m 2 . 
     
     
         19 . The process according to  claim 4 , wherein the thermoplastic single layer, or a thermoplastic multi-layer polymer is a polymer coating system comprising one or more layers comprising a thermoplastic resin selected from the group consisting of polyesters or polyolefins, acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, polycarbonates, styrene resins, ABS resins, chlorinated polyethers, ionomers, urethane resins and functionalised polymers; and/or copolymers thereof; and/or blends thereof. 
     
     
         20 . The process according to  claim 1 , wherein an anode is a gas diffusion anode that reduces or eliminates the oxidation of Cr(III) ions to Cr(VI) ions during the plating step.