Method for the production of a metal strip coated with a coating of chromium and chromium oxide using an electrolyte solution with a trivalent chromium compound

A method for producing a metal strip coated with a coating containing chromium metal and chromium oxide, the method comprising:

• connecting the metal strip as a cathode and passing the metal strip at a predefined strip speed through a plurality of electrolysis tanks successively arranged in a strip travel direction and filled with electrolyte solution containing a trivalent chromium compound, the plurality of electrolysis tanks including at least a first electrolysis tank or a front group of electrolysis tanks and a last electrolysis tank or a rear group of electrolysis tanks;
• wherein the electrolyte solution in the first electrolysis tank or in the front group of electrolysis tanks has a first temperature averaged across a volume of an electrolysis tank and the electrolyte solution in the last electrolysis tank or in the rear group of electrolysis tanks has a second temperature, averaged across a volume of an electrolysis tank, less than 40° C. and lower than the first temperature;
• and
• bringing the metal strip into contact with the electrolyte solution during an electrolysis time to electrolytically deposit the coating onto the metal strip, the coating having at least two layers with different compositions, the at least two layers including a lower layer facing the metal strip and having a first proportion by weight of chromium oxide, and an upper layer having a second proportion by weight of chromium oxide, the second proportion by weight of chromium oxide being higher than the first proportion by weight of chromium oxide;
• wherein the electrolysis time, during which the metal strip is in contact with the electrolyte solution in the last electrolysis tank or in the rear group of electrolysis tanks, is less than 2.0 seconds.