Method for increasing the corrosion resistance of a chrome-plated substrate

1. A method for increasing the corrosion resistance of a chrome-plated substrate, the method comprising:

a) dipping at least one part of a chrome-plated surface of a chrome-plated substrate into an electrolyte, the electrolyte comprising

trivalent chromium ions, wherein the concentration of the trivalent chromium ions in the electrolyte is in the range of 0.001 to 0.1 M,

at least one conducting salt, wherein the concentration of the at least one conducting salt in the electrolyte is in the range of 2 to 50 g/L, and

at least one reducing agent, wherein the concentration of the at least one reducing agent in the electrolyte is in the range of 0.1 to 50 g/L, and

b) forming a trivalent chromium oxide film on the at least one part of the chrome-plated surface by applying a pulse reverse current between the chrome-plated surface and a counter electrode electrically connected with the chrome-plated surface through the electrolyte;

wherein the pulse reverse current is applied for a time period from 30 to 300 seconds.

2. The method according to claim 1 , wherein the applied pulse reverse current has

a frequency in the range of 0.1 to 1000 Hz,

a current density in the range of 0.01 to 10 A/dm 2 , and/or

a duty cycle in the range of 40 to 95%.

3. The method according to claim 1 , wherein the chrome-plated surface of the substrate has been obtained by trivalent chromium electroplating.

4. The method according to claim 1 , wherein the substrate comprises a main part made of plastic and at least one under layer arranged on the main part, wherein the at least one under layer is composed of a deposit selected from a metal, a metal alloy or mixtures thereof.

5. The method according to claim 4 , wherein the deposit is selected from the group consisting of nickel, alloys of nickel, copper, alloys of copper, and mixtures thereof.

6. The method according to claim 1 , wherein the concentration of the trivalent chromium ions in the electrolyte is in the range of 0.002 to 0.08 M.

7. The method according to claim 1 , wherein the electrolyte comprises at least one trivalent chromium salt comprising the trivalent chromium ions.

8. The method according to claim 7 , wherein the at least one trivalent chromium salt comprising the trivalent chromium ions is selected from the group consisting of chromium sulfate, chromium potassium sulfate, chromium chloride, and mixtures thereof.

9. The method according to claim 1 , wherein the at least one conducting salt is selected from the group consisting of sulfates, nitrates, phosphates, carbonates, bicarbonates, acetates, chlorides, and mixtures thereof.

10. The method according to claim 1 , wherein the at least one reducing agent is selected from the group consisting of sulfites, metabisulfites, thiosulfates, hydrosulfites, hydrazine, hydroxylamine, hydroxylammonium salts, ascorbic acid and its Na and K salts, formic acid and its Na and K salts, glyoxylic acid and its Na and K salts, glyoxal, glucose, sorbitol, and mixtures thereof.

11. The method according to claim 1 , wherein the electrolyte comprises at least one Cr(III) complexing agent.

12. The method according to claim 1 , wherein the pH value of the electrolyte is in the range of 2 to 10.

13. The method according to claim 1 , wherein the counter electrode is made of stainless steel, graphite, or titanium.

14. The method according to claim 13 , wherein the counter electrode is covered by a mixed metal oxide or platinum.