Method for Electrolytic Stripping of Spray Metal Coated Substrate

Abstract

A method for stripping a fused thermal spray metal coating from the surface of a soft metallic substrate. The steps for the method include: immersing the coated metallic substrate in an aqueous solution of chromic acid, peroxide, and a second acid; immersing a metal cathode in the aqueous solution; applying a positive potential to the fused spray metal coated substrate and a negative potential to the metal cathode to generate a direct current between the substrate and the cathode; where the current is applied for a sufficient time to remove the coating. This method permits the electrochemical removal of fused and impermeable thermal spray metal coatings.

1 . A method for stripping a fused spray metal coating from the surface of an iron based substrate, comprising the steps of:
 immersing the fused spray metal coated iron based substrate in an aqueous solution comprising chromic acid, peroxide, and a second acid, the second acid being sulfuric acid, phosphoric acid or a mixture thereof;   immersing a metal cathode in the aqueous solution; and   applying a positive electrical potential to the substrate and a negative electrical potential to the cathode for creating a direct electrical current between the cathode and the anode through the fused spray metal coating for removal of the fused spray metal coating from the steel substrate.   
   
   
       2 . The method according to  claim 1 , wherein the positive and negative electrical potentials are maintained until complete removal of the fused spray metal coating. 
   
   
       3 . The method according to  claim 1  or  2 , wherein the spray metal coating is a NiCr, a WC, or a MoS 2  alloy. 
   
   
       4 . The method according to  claim 1 ,  2  or  3 , wherein the substrate is steel, mild steel, common steel, or iron. 
   
   
       5 . The method according to any one of  claims 1  to  4 , wherein the spray metal coating is an impermeable coating. 
   
   
       6 . The method according to any one of  claims 1  to  5 , the aqueous solution comprising:
 6 to 38 parts by volume of a 20 to 25 wt % aqueous chromic acid solution;   2 to 8 parts by volume of about a 35 wt % aqueous peroxide solution; and   100 parts by volume is 55°-35° Baume sulfuric acid.   
   
   
       7 . The method according to any one of  claims 1  to  6 , wherein the peroxide is hydrogen peroxide or barium peroxide. 
   
   
       8 . The method according to any one of  claims 1  to  7 , wherein the chromic acid is H 2 CrO 4 . 
   
   
       9 . The method according to any one of  claims 1  to  8 , wherein the current is 20 to 80 mA/cm 2  (20 to 80 A/ft 2 ). 
   
   
       10 . The method according to any one of claims I to  9 , the aqueous solution further comprising a catalyst. 
   
   
       11 . The method of  claim 10 , wherein the catalyst is hydrogen peroxide. 
   
   
       12 . The method according to any one of  claims 1  to  11 , wherein the aqueous solution is maintained at a temperature of 30 to 80° C. (90° F. to 180° F.). 
   
   
       13 . The method of  claim 12 , wherein the solution is maintained at 50 to 60° C. (120° F. to 140° F.). 
   
   
       14 . The method according to any one of  claims 1  to  12 , the cathode comprising an anodically deposited coating of lead, carbon or titanium or oxides thereof, covering a conductive material having a backing of non-conductive material.