Method for alkaline cleaning of zinc-magnesium-alloyed strip steel

An alkaline aqueous composition having a free alkalinity of greater than 3.0 points and a pH value of greater than 11.0, containing

• a) at least 1.0 mmol/kg of magnesium ions dissolved in water, and
• b) a system of complexing agents consisting of more than 2.0 mmol/kg, but less than 50.0 mmol/kg, of complexing agents having a complex formation constant pKL for magnesium of less than 2.0, preferably selected from glycolic acid, lactic acid, tartaric acid, malic acid, aldaric acids, aldonic acids and/or glucoheptonic acid, especially preferably from aldonic acids and/or glucoheptonic acid, particularly preferably from gluconic acid, and
• c1) more than 1.0 mmol/kg, but less than 10.0 mmol/kg of at least one di- and/or triphosphonic acid, and/or
• c2) more than 1.0 mmol/kg, but less than 10.0 mmol/kg of at least one organic compound having at least three carboxyl groups and at least one secondary and/or tertiary amino group.