Method for the selective removal of zinc ions from alkaline bath solutions in the serial surface treatment of metal components

A method for wet-chemical surface treatment of metal components, which have surfaces of zinc and aluminum and which are serially wet-chemically pretreated comprising steps of:

• A.) contacting metal components having surfaces of zinc and aluminum with an alkaline bath solution, which is stored in a system tank and contains:
• a) a complexing agent Y in the form of water-soluble condensed phosphates and/or in the form of water-soluble organic compounds, which have at least one functional group selected from —OPO3X2/n and/or —PO3X2/n, wherein X is either a hydrogen atom or an alkali metal and/or alkaline-earth metal atom having the particular valency n, and
• b) iron(III) ions,
• wherein the alkaline bath solution in the wet-chemical pretreatment has a pH value that is greater than 10 and a free alkalinity that is at least 0.5 points, but less than 50 points;
• wherein a maximum value “Znmax” for concentration of dissolved zinc in the alkaline bath solution of the system tank is not greater than Znmax according to Formula I:
• Znmax=0.0004×(pH−9)×[FA]+0.6×[Y] (I)
• pH is pH value;
• Znmax is the maximum value for concentration of dissolved zinc in mmol/l;
• [FA] is free alkalinity in mmol/l;
• [Y] is concentration in mmol/l of complexing agents Y in the form of water-soluble condensed phosphates, calculated as P2O6, and/or in the form of water-soluble organic compounds that have at least one functional group selected from —COOX1/n, —OPO3X2/n, and/or —PO3X2/n, wherein X is either a hydrogen atom or an alkali metal and/or alkaline-earth metal atom having the particular valency n;
• and
• B.) preventing the maximum value Znmax in the wet-chemical pretreatment from being exceeded by:
• 1) contacting at least part of the alkaline bath solution of the system tank with an ion exchange resin that bears functional groups containing —OPO3X2/n, and/or —PO3X2/n, wherein X is either a hydrogen atom or an alkali metal and/or alkaline-earth metal atom to be exchanged having the particular valency n, and
• 2) returning the part of the alkaline bath solution that was brought in contact with the ion exchange resin to the system tank.