Method to Produce Golden Bronze by Diffusion of Tin Into Copper Under Controlled Conditions

Abstract

The present invention relates to the production of a bronze plated substrate having a golden appearance thanks to a multiple-layer plating method. The method notably comprises plating a substrate having at least one layer of copper with a tin layer which thickness represents 3.5% to 12% of the copper layer thickness. The method further comprises annealing the plated substrate in an annealing furnace comprising a plurality of heating zones, the last heating zone of the furnace having an annealing temperature ranging from 600° C. to 815° C. The annealing step of the method is performed under controlled operating parameters which comprise an annealing residence time, the annealing temperature and the relative thickness of the tin layer in view of the copper layer. The operating parameters are controlled in accordance to each other to ensure obtaining the gold-like appearance of the plated bronze.

1 . A method for producing a golden bronze plated substrate, the method comprising
 providing a copper plated substrate comprising a core plated with at least one copper layer having a copper layer thickness;   plating the copper plated substrate with a tin layer having a tin layer thickness representing 3.5 to 12% of the copper layer thickness, thereby obtaining a multiple-layer substrate;   annealing the multiple-layer substrate at a gradually increasing annealing temperature for a predetermined annealing residence time, the annealing temperature and annealing residence time being controlled in accordance to each other for allowing complete diffusion of the tin layer into the at least one copper layer and producing an inter-diffused outer bronze layer having a golden appearance; and   burnishing the inter-diffused outer bronze layer to remove undesired oxide formations.   
     
     
         2 . The method according to  claim 1 , wherein the annealing is performed in an annealing furnace comprising a plurality of heating zones in which the gradually increasing annealing temperature ranges from 425° C. to 815° C. 
     
     
         3 . The method according to  claim 2 , wherein the plurality of heating zones comprises at least five heating zones including a last heating zone in which the gradually increasing annealing temperature ranges from 600° C. to 815° C. 
     
     
         4 . The method according to  claim 3 , wherein the at least five heating zones comprises:
 a first heating zone having a first annealing temperature ranging from 425° C. to 500° C.;   a second heating zone having a second annealing temperature ranging from 500° C. to 650° C.;   a third heating zone having a third annealing temperature ranging from 600° C. to 700° C.;   a fourth heating zone having a fourth annealing temperature ranging from 700° C. to 775° C.; and   a fifth heating zone having a fifth annealing temperature ranging from 775° C. to 815° C.   
     
     
         5 . The method according to  claim 1 , wherein the annealing residence time ranges from 10 to 90 minutes. 
     
     
         6 . The method according to  claim 1 , wherein the annealing residence time ranges from 20 to 30 minutes. 
     
     
         7 . The method according to  claim 1 , wherein the annealing of the multiple-layer substrate is performed under controlled annealing atmosphere. 
     
     
         8 . The method according to  claim 7 , wherein the annealing atmosphere is a reducing atmosphere comprising nitrogen, hydrogen or any combination thereof. 
     
     
         9 . The method according to  claim 1 , comprising etching the copper plated substrate with an acidic solution to produce an etched copper layer surface, the adhesion of the tin layer being optimized on the etched copper layer surface. 
     
     
         10 . The method according to  claim 1 , comprising plating a metallic substrate with an inner layer of nickel and an outer layer of copper, to produce the copper plated substrate. 
     
     
         11 . The method according to  claim 10 , wherein the plating of the respective nickel layer, copper layer and tin layer is performed by electroplating with an electroplating solution comprising acidic, cyanide, non-cyanide, neutral, slightly basic solution or any combination thereof. 
     
     
         12 . The method according to  claim 11 , wherein the electroplating solution is a non-cyanide solution. 
     
     
         13 . The method according to  claim 1 , wherein the copper plated substrate is a coinage blank. 
     
     
         14 . The method according to  claim 1 , wherein the annealing furnace comprises a belt conveyor of a screw conveyor to guide the multiple-layer substrate through the plurality of heating zones. 
     
     
         15 . The method according to  claim 1 , wherein the annealing furnace comprises a forced convection system to ensure even conduction and distribution of the heat within the furnace. 
     
     
         16 . The method according to  claim 1 , comprising quenching the multiple-layer substrate comprising the inter-diffused outer bronze layer after annealing in order to stop instantaneously the interdiffusion. 
     
     
         17 . The method according to  claim 1 , further comprising cleaning and drying the golden bronze plated substrate after burnishing to reveal the golden appearance of the bronze.