Passivating layer for metal-containing substrates

A method for sequential production of metal-containing substrates which have a conversion layer comprising chromium and zirconium on at least part of their surface, the method comprising:

• bringing into contact at least part of the surface of metal-containing substrates with an electrolyte solution comprising at least one trivalent chromium compound and at least one tetravalent zirconium compound to form the conversion layer;
• adding fresh trivalent chromium and tetravalent zirconium compounds to the electrolyte solution to compensate for chromium and zirconium deposited in the conversion layer;
• and
• controlling or regulating a Cr:Zr mass ratio in the electrolyte solution and/or the addition of trivalent chromium compounds and tetravalent zirconium compounds to the electrolyte solution such that the Cr:Zr mass ratio in the conversion layer, based upon a sum of the metals Zr and Cr, is >0.7,
• wherein a limit value for the Cr:Zr mass ratio in the conversion layer, based upon the sum of the metals Zr and Cr, is ≤0.7, and
• wherein a limit value for the Cr:Zr mass ratio in the electrolyte solution, based upon the sum of the metals Zr and Cr, is ≤0.55.