Method for stripping nitride coatings
Methods for stripping partially oxidized nitride wear or release coatings from metal workpieces comprise disrupting surface oxidation layers present on the coatings following use, and causing electrical current to flow from the workpiece and release coating to a counter electrode while the workpiece, release coating and counter electrode are immersed in an aqueous alkaline electrolyte solution.
1. A method for stripping a partially oxidized nitride release coating from a metal workpiece comprising the steps of:
disrupting a surface oxidation layer on the release coating to increase release coating electrical conductivity, and
flowing an electrical current from the workpiece and release coating to a counter electrode while the workpiece, release coating and counter electrode are immersed in an aqueous alkaline electrolyte solution.
2. The method in accordance with claim 1 wherein the nitride release coating consists essentially of one or more nitrides selected from the group consisting of TiN, TiAlN, CrN, TiAlCrN, TiAlSiN and AN.
3. The method in accordance with claim 1 wherein the release coating is a TiAlN coating deposited by evaporation or sputtering and wherein the workpiece is composed of a nickel-chromium alloy.
4. The method in accordance with claim 3 wherein the workpiece is a glass molding element.
5. The method in accordance with claim 1 wherein the counter electrode is composed of a metal resistant to corrosion in aqueous alkaline media.
6. The method in accordance with claim 5 wherein the counter electrode is composed of a metal selected from the group consisting of platinum, titanium, niobium, steel alloys, and nickel-chromium alloys.
7. The method in accordance with claim 1 wherein the aqueous alkaline electrolyte solution comprises at least one compound selected from the group consisting of potassium hydroxide and sodium hydroxide.
8. The method in accordance with claim 7 wherein the aqueous alkaline electrolyte solution is a 1M-12M aqueous solution of KOH or NaOH.
9. The method in accordance with claim 8 wherein an electrical potential not exceeding a range of about 1V to about 15V is applied between the workpiece and the counter electrode to generate the electrical current.
10. The method in accordance with claim 1 wherein the step of disrupting the surface oxidation layer comprises exposing the release coating to a concentrated aqueous alkali metal hydroxide solution for a time sufficient to at least partially dissolve the layer.
11. The method in accordance with claim 1 wherein the step of disrupting the surface oxidation layer comprises abrading the surface of the release coating to at least partially remove oxidized material therefrom.
12. The method in accordance with claim 1 wherein the step of disrupting the surface oxidation layer comprises applying a high voltage electrical pulse across the release coating.
13. The method according to claim 1 , wherein the workpiece further includes an interlayer disposed between the release coating and the workpiece, and wherein the method further comprises removing at least a portion of the interlayer.
14. The method in accordance with claim 13 , wherein the interlayer is a TiAl, Ti, or Al interlayer deposited by evaporation or sputtering.
15. The method in accordance with claim 14 , wherein the TiAl, Ti, or Al interlayer contains at least one diffused metal contaminant selected from the group consisting of iron, nickel and chromium.
16. The method of claim 13 , wherein the step of removing at least a portion of the interlayer comprises polishing the interlayer.
17. A method in accordance with claim 1 which comprises a further step of polishing a stripped surface of the workpiece.
18. A method for stripping a partially oxidized nitride release coating and at least a portion of an interlayer from a metal workpiece, the interlayer disposed between the release coating and the workpiece, the method comprising the steps of:
disrupting a surface oxidation layer on the release coating to increase release coating electrical conductivity;
flowing an electrical current from the workpiece and release coating to a counter electrode while the workpiece, release coating and counter electrode are immersed in an aqueous alkaline electrolyte solution; and
removing at least a portion of the interlayer by passing electrical current pulses of reversing polarity through the workpiece for a time sufficient to dissolve the interlayer.