Method of removing a case layer from a metal alloy

Abstract

A method of surface treating a metallic article includes the step of chemically removing a surface layer having titanium alloy alpha-phase precipitation to expose a core having titanium alloy beta-phase. In one example, the chemical removal includes using a first solution having nitric acid and hydrofluoric acid, and a second solution having nitric acid.

1 . A method of surface treating a metallic article, comprising the step:
 (a) chemically removing a surface layer having titanium alloy alpha-phase precipitation to expose a core having titanium alloy beta-phase.   
     
     
         2 . The method as recited in  claim 1 , including chemically removing the surface layer using a first solution having nitric acid and hydrofluoric acid, and a second solution having nitric acid. 
     
     
         3 . The method as recited in  claim 1 , including chemically removing the surface layer using a solution having unequal concentrations of nitric acid and hydrofluoric acid. 
     
     
         4 . The method as recited in  claim 1 , including wetting the surface layer with a first solution in a first step to remove at least a portion of the surface layer and expose the core, then wetting any remaining surface layer and the core with a second solution in a second step, and maintaining the surface layer and the core in a wet state between the first step and the second step. 
     
     
         5 . The method as recited in  claim 4 , including performing the first step at a temperature of about 75° F. and performing the second step at a temperature of about 55° F.-120° F. 
     
     
         6 . The method as recited in  claim 4 , wherein the first solution includes about 45 vol %-50 vol % nitric acid that is at least 42° Baume, about 5.6 vol %-12 vol % hydrofluoric acid that is 70% grade or 49% grade, and a remainder of water, and the second solution includes about 50 vol %-60 vol % nitric acid that is at least 42° Baume, and a remainder of water. 
     
     
         7 . The method as recited in  claim 1 , including wetting the surface layer with a first solution having about 45 vol %-50 vol % nitric acid that is at least 42° Baume, about 8 vol %-12 vol % hydrofluoric acid that is 49% grade, and a remainder of water. 
     
     
         8 . The method as recited in  claim 1 , including wetting the surface layer with a first solution having about 45 vol %-50 vol % nitric acid that is at least 42° Baume, about 5.6 vol %-8.4 vol % hydrofluoric acid that is about 70% grade, and a remainder of water. 
     
     
         9 . The method as recited in  claim 1 , including forming the surface layer and the core from a titanium alloy having a nominal composition that includes about 14 wt %-16 wt % molybdenum, about 2.5 wt %-3.5 wt % aluminum, about 2.4 wt %-3.2 wt % niobium, about 0.15 wt %-0.25 wt % silicon, and a remainder titanium. 
     
     
         10 . The method as recited in  claim 2 , including uniformly removing the surface layer without hydrogen embrittlement and intergranular attack of the titanium alloy beta-phase. 
     
     
         11 . A solution. system for chemically removing a surface layer having titanium alloy alpha-phase precipitation to expose a core having titanium alloy beta-phase of a metallic article, comprising:
 a first solution that includes nitric acid and hydrofluoric acid; and   a second solution that includes nitric acid, where the first solution and the second solution cooperate to remove the surface layer without smut formation to expose the core.   
     
     
         12 . The solution system as recited in  claim 11 , wherein the first solution includes about 45 vol %-50 vol % nitric acid that is at least 42° Baume, about 8 vol %-12 vol % hydrofluoric acid that is about 49% grade, and a remainder of water. 
     
     
         13 . The solution system as recited in  claim 11 , wherein the first solution includes about 45 vol %-50 vol % nitric acid that is at least 42° Baume, about 5.6 vol %-8.4 vol % hydrofluoric acid that is about 70% grade, and a remainder of water. 
     
     
         14 . The solution system as recited in  claim 11 , wherein the second solution includes about 50 vol %-60 vol % nitric acid that is at least 42° Baume and a remainder of water.