Coated substrate and process for production thereof

Abstract

A process for electrolytically coating a substrate at a preselected second voltage. The process comprises the steps of: (i) immersing the substrate in an aqueous electrolytic bath comprising at least one soluble salt, the substrate acting as a first electrode; (ii) contacting a second electrode with the aqueous electrolytic bath; (iii) passing a current through the aqueous electrolytic bath at a first voltage which is less than the second voltage, the current comprising alternating current superimposed over direct current; (iv) increasing the first voltage to a second voltage over a period of time while continuing to pass the current through the aqueous electrolytic bath; (v) keeping the second voltage substantially constant thereby allowing the current to decrease; and (vi) thereby forming an oxide coating on the substrate.

What is claimed is:  
     
         1 . A process for electrolytically coating a substrate at a preselected second voltage, the process comprising the steps of: 
 (i) immersing the substrate in an aqueous electrolytic bath comprising at least one soluble salt, the substrate acting as a first electrode;    (ii) contacting a second electrode with the aqueous electrolytic bath;    (iii) passing a current through the aqueous electrolytic bath at a first voltage which is less than the second voltage, the current comprising alternating current superimposed over direct current;    (iv) increasing the first voltage to a second voltage over a period of time while continuing to pass the current through the aqueous electrolytic bath;    (v) keeping the second voltage substantially constant thereby allowing the current to decrease; and    (vi) thereby forming an oxide coating on the substrate.    
     
     
         2 . The process defined in  claim 1 , wherein the substrate is a metal or an alloy thereof.  
     
     
         3 . The process defined in  claim 1 , wherein the metal is selected from the group comprising aluminum, magnesium, lithium, beryllium, gallium, chromium, titanium, tantalum, zirconium, niobium, haffiium, antimony, tungsten, molybdenum, vanadium, bismuth and an alloy containing one or more of these metals.  
     
     
         4 . The process defined in  claim 2 , wherein the metal is magnesium or an alloy containing magnesium.  
     
     
         5 . The process defined in  claim 1 , wherein the substrate acts as an anode and the second electrode acts as an cathode.  
     
     
         6 . The process defined in  claim 1 , wherein the second voltage is at least about 300 V.  
     
     
         7 . The process defined in  claim 1 , wherein the second voltage is at least about 350 V.  
     
     
         8 . The process defined in  claim 1 , wherein the second voltage is at least about 400 V.  
     
     
         9 . The process defined in  claim 1 , wherein Step (iv) comprises increasing the first voltage to the second voltage in a substantially non-linear manner.  
     
     
         10 . The process defined in  claim 1 , wherein Step (iv) comprises increasing the first voltage to the second voltage in a substantially linear manner.  
     
     
         11 . The process defined in  claim 1 , wherein the electrolytic bath comprises a water soluble hydroxide, a water soluble fluoride compound and a water soluble silicate.  
     
     
         12 . The process defined in  claim 11 , wherein the water soluble hydroxide is present in an amount in the range of from about 2 to about 15 g per liter of electrolytic bath.  
     
     
         13 . The process defined in  claim 11 , wherein the water soluble hydroxide is present in an amount in the range of from about 4 to about IO g per liter of electrolytic bath.  
     
     
         14 . The process defined in  claim 11 , wherein the water soluble hydroxide is present in an amount in the range of from about 5 to about 8 g per liter of electrolytic bath.  
     
     
         15 . The process defined in  claim 1   1 , wherein the water soluble hydroxide is selected from the group comprising alkali metal hydroxides, ammonium hydroxide and mixtures thereof.  
     
     
         16 . The process defined in  claim 11 , wherein the water soluble hydroxide is potassium hydroxide.  
     
     
         17 . The process defined in  claim 11 , wherein the water soluble fluoride compound is present in an amount in the range of from about 2 to about 14 g per liter of electrolytic bath.  
     
     
         18 . The process defined in  claim 11 , wherein the water soluble fluoride compound is present in an amount in the range of from about 6 to about 12 g per liter of electrolytic bath.  
     
     
         19 . The process defined in  claim 11 , wherein the water soluble fluoride compound is present in an amount in the range of from about 8 to about 10 g per liter of electrolytic bath.  
     
     
         20 . The process defined in  claim 11 , wherein the water soluble fluoride is selected from the group consisting of alkali metal fluorides, ammonium fluoride, ammonium bifluoride, alkali metal fluorosilicates, hydrogen fluoride and mixtures thereof.  
     
     
         21 . The process defined in  claim 1   1 , wherein the water soluble fluoride is selected from the group consisting of alkali metal fluorides, alkali metal fluorosilicates, hydrogen fluoride and mixtures thereof.  
     
     
         22 . The process defined in  claim 11 , wherein the water soluble fluoride is potassium fluoride.  
     
     
         23 . The process defined in  claim 11 , wherein the water soluble silicate is present in an amount in the range of from about 5 to about 40 g per liter of electrolytic bath.  
     
     
         24 . The process defined in  claim 11 , wherein the water soluble silicate is present in an amount in the range of from about 10 to about 25 g per liter of electrolytic bath.  
     
     
         25 . The process defined in  claim 11 , wherein the water soluble silicate is present in an amount in the range of from about 15 to about 20 g per liter of electrolytic bath.  
     
     
         26 . The process defined in  claim 11 , wherein the water soluble silicate is selected from the group comprising alkali metal silicates, alkali metal fluorosilicates and mixtures thereof.  
     
     
         27 . The process defined in  claim 11 , wherein the water soluble silicate is sodium silicate.  
     
     
         28 . The process defined in  claim 1 , wherein the electrolytic bath has a pH of at least about 11.  
     
     
         29 . The process defined in  claim 1 , wherein the electrolytic bath has a pH in the range of from about 11.5 to about 13.  
     
     
         30 . The process defined in  claim 1 , wherein the electrolytic bath is maintained during the process at a temperature in the range of from about 5° to about 35° C.  
     
     
         31 . The process defined in  claim 1 , wherein the electrolytic bath is maintained during the process at a temperature in the range of from about 50 to about 25° C.  
     
     
         32 . The process defined in  claim 1 , wherein the electrolytic bath is maintained during the process at a temperature in the range of from about 5° to about 20° C.