Process for corrosion protection of iron containing materials
View Patent ↗The present invention relates to a process for corrosion protection of an iron-containing substrate wherein a first zinc-nickel alloy layer, a second zinc-nickel alloy layer and a black passivate layer are deposited onto the substrate. The nickel concentration in the second zinc-nickel alloy layer is higher than the nickel concentration in the first zinc-nickel alloy layer. The substrate surface obtained is homogenously black with an appealing decorative appearance and both resistance against white rust and red rust are improved.
1. 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.
2. The process for corrosion protection of a substrate according to claim 1 wherein the substrate is made of cast iron.
3. The process for corrosion protection of a substrate according to claim 1 wherein the concentration of nickel in the first zinc-nickel alloy layer ranges from 10 to 15 wt.-%.
4. The process for corrosion protection of a substrate according to claim 1 wherein the concentration of nickel in the first zinc-nickel alloy layer ranges from 12 to 15 wt.-%.
5. The process for corrosion protection of a substrate according to claim 1 wherein the concentration of nickel in the second zinc-nickel alloy layer ranges from 13 to 20 wt.-%.
6. The process for corrosion protection of a substrate according to claim 1 wherein the concentration of nickel in the second zinc-nickel alloy layer ranges from 15 to 18 wt.-%.
7. The process for corrosion protection of a substrate according to claim 1 wherein the concentration represented in weight-% of nickel in the first zinc-nickel alloy layer is 50 to 99% of the concentration represented in weight-% of nickel in the second zinc-nickel alloy layer.
8. The process for corrosion protection of a substrate according to claim 1 wherein the concentration represented in weight-% of nickel in the first zinc-nickel alloy layer is 60 to 95% of the concentration represented in weight-% of nickel in the second zinc-nickel alloy layer.
9. The process for corrosion protection of a substrate according to claim 1 wherein the concentration represented in weight-% of nickel in the first zinc-nickel alloy layer is 70 to 90% of the concentration represented in weight-% of nickel in the second zinc-nickel alloy layer.
10. The process for corrosion protection of a substrate according to claim 1 wherein the black passivation layer is deposited from an acidic aqueous solution comprising chromium(III) ions, a complexing agent and an oxidizing agent.
11. The process for corrosion protection of a substrate according to claim 1 wherein at least one further layer selected from sealing layer and non pigmented chromium(III) containing passivation layer is deposited onto the black passivate layer obtained in step (iv).