Fluoride-free zirconium-based metal pre-treatment for passivation

A method for anti-corrosion pretreatment of a metal substrate, comprising steps of:

• 1) contacting at least one surface of a metal substrate with an aqueous anti-corrosion agent comprising:
• (A) at least one water-soluble compound of the element zirconium;
• and
• (B) at least one polycyclic hydrocarbon compound which comprises at least one anellated benzene ring, wherein the at least one anellated benzene ring has at least two hydroxyl functional groups on the ring in an ortho position relative to one another;
• wherein the aqueous anti-corrosion agent comprises a homogenous aqueous phase having a molar ratio of the element zirconium to total fluoride that is greater than 1.

A concentrate of an anti-corrosion agent comprising:

• at least 1 wt. % of a water-soluble compound of the element zirconium, based on the element zirconium;
• and at least 0.01 wt. % of at least one polycyclic hydrocarbon compound which comprises at least one anellated benzene ring, wherein the at least one anellated benzene ring has at least two hydroxyl functional groups on the ring in an ortho position relative to one another, wherein the at least one anellated benzene rings are bridged in each case by being anellated on an acyclic hydrocarbon system;
• wherein the aqueous anti-corrosion agent has a weight ratio of the water-soluble compounds (A) of the element zirconium, based on the element zirconium, to the polycyclic hydrocarbons (B) that is less than 0.2:1;
• and wherein the concentrate has a pH in a range of from 0.5 to 2.0.