Pretreatment of magnesium substrates for electroplating
View Patent ↗The present invention relates to a method for depositing an adherent zinc coating onto a zinc-containing magnesium alloy substrate in order to render the surface suitable for electroplating. The coatings are applied from a pyrophosphate-based zinc electrolyte solution containing a small quantity of fluoride ions. Depending on the zinc and aluminum content of the magnesium alloy, the zinc electrolyte solution is applied by immersion deposition or electrolytically.
1. A method of depositing an adherent plated deposit onto a magnesium alloy article, the method comprising the steps of:
(a) cleaning the magnesium alloy article in an alkaline cleaning solution which has a pH above 10;
(b) applying a zinc layer on the cleaned magnesium alloy article by electrodeposition in a zinc coating solution through the application of electrical current; and
(c) applying a metal coating from an electrolyte solution that is compatible with the zinc coated magnesium surface,
wherein the magnesium alloy contains about 0-9% aluminum and about 6-20% zinc, and wherein the article is not etched prior to the applying of the zinc layer.
2. The method according to claim 1 , wherein the zinc coating solution is an aqueous solution comprising:
an alkali metal pyrophosphate;
a zinc salt; and
a water soluble fluoride salt or hydrofluoric acid.
3. The method according to claim 2 , wherein the alkali metal pyrophosphate is present in the solution in an amount sufficient to provide about 6 to 270 g/l or pyrophosphate ion.
4. The method according to claim 2 , wherein the zinc salt is present in the solution in an amount sufficient to provide about 1 to 40 g/l zinc ions.
5. The method according to claim 2 , wherein the fluoride salt or hydrofluoric acid is present in the solution in an amount sufficient to provide about 2-80 g/l fluoride ions.
6. The method according to claim 1 , wherein the zinc layer is applied electrolytically using a cathodic current density of about 0.5 to 5.0 A/dm 2 .
7. The method according to claim 6 , wherein the cathodic current density is between about 0.5-2.0 A/dm 2 .
8. The method according to claim 2 , wherein the zinc coating solution has a pH between about 8 and 11.
9. The method according to claim 2 wherein the temperature of the zinc coating solution is between about 10-100° C.
10. The method according to claim 9 , wherein the temperature of the zinc coating solution is between about 40-65° C.
11. The method according to claim 1 , wherein the magnesium alloy has an aluminum content of less than about 6%.