Process for corrosion protection of iron containing materials

A process for corrosion protection of an iron-containing substrate comprising, in this order, the steps of

• (i) providing a substrate made of an iron-containing material,
• (ii) electroplating onto said substrate a first zinc-nickel alloy layer having a nickel concentration in the range of 6 to 15 wt.-%,
• (iii) thereon, electroplating a second zinc-nickel alloy layer having a nickel concentration in the range of 12 to 30 wt.-% onto the first zinc-nickel alloy layer with the proviso that the concentration of nickel in the second zinc-nickel alloy layer is higher than the nickel concentration in the first zinc-nickel alloy layer, and
• (iv) depositing a black passivation layer onto the second zinc-nickel alloy layer,
• wherein an acidic zinc-nickel electrolyte used for electroplating the first zinc-nickel alloy layer and an acidic zinc-nickel electrolyte used for electroplating the second zinc-nickel alloy layer are the same acidic zinc-nickel electrolyte and
• wherein the nickel concentrations in both zinc-nickel alloy layers are adjusted by changing pH value, temperature, or both the pH value and the temperature of one or both of the acidic zinc-nickel electrolyte used for electroplating the first zinc-nickel alloy layer and the acidic zinc-nickel electrolyte used for electroplating the second zinc-nickel alloy layer during the electroplating steps.