Method for passivating a tinplate strip and apparatus for producing said passivated tinplate strip

A method for passivating a tinplate strip in a continuous process, comprising:

• after electrodepositing a tin layer or layers on a steel strip to make the tinplate strip or after an optional flow-melting of the electrodeposited tin layer or layers, passing the tinplate strip into a basic aqueous solution in an electrochemical treatment tank and then moving the tinplate strip in an entry-pass comprising moving the tinplate strip between anodes within the basic aqueous solution in the electrochemical treatment tank, and
• after the entry pass moving the tinplate strip in an exit pass comprising moving the tinplate strip between cathodes within the basic aqueous solution in the electrochemical treatment tank, and after the exit pass discharging the tinplate strip from the basic aqueous solution and the electrochemical treatment tank, wherein the tinplate strip moves from the entry pass to the exit pass without leaving the basic aqueous solution in the electrochemical treatment tank,
• wherein any pre-existing tin-oxide layer on the tinplate surface is cathodically removed from the tinplate surface during the entry-pass to produce a bare tin surface, and
• wherein the bare tin surface is subsequently immediately anodically re-oxidised during the exit pass to form a new tin-oxide layer,
• wherein the charge for cathodically removing the pre-existing tin-oxide layer from the tinplate surface is Q1, and wherein the charge for anodically re-oxidising the tinplate is Q2 and wherein Q1<Q2,
• wherein the imposed charge density for the anodic re-oxidation and the cathodic removal of the pre-existing tin-oxide layer is identical and equal to Q2 and is at least 15 C/m2 and
• wherein the anodically re-oxidised tinplate is rinsed and dried after exiting the basic aqueous solution.