Slurry chromizing compositions

Abstract

Slurry coating composition for selectively enriching surface regions of a metal-based substrate, for example, the under-platform regions of a turbine blade, with chromium. The slurry coating composition contains metallic chromium, optionally metallic aluminum in a lesser amount by weight than chromium, and optionally other constituents. The composition further includes colloidal silica, and may also include one or more additional constituents, though in any event the composition is substantially free of hexavalent chromium and sources thereof. The coating composition can be used in a process that entails applying the coating composition to a surface region to form a slurry coating, and then heating the coating to remove any volatile components of the coating composition and thereafter cause diffusion of chromium from the coating into the surface region to form a chromium-rich diffusion coating.

1 . A slurry coating composition for enriching a surface region of a metal-based substrate with chromium, the slurry coating composition comprising a metallic powder, colloidal silica, and optionally one or more additional constituents though substantially free of hexavalent chromium and sources thereof, the metallic powder having a bulk composition comprising metallic chromium and optionally metallic aluminum in a lesser amount by weight than the metallic chromium. 
     
     
         2 . The slurry coating composition according to  claim 1 , wherein the metallic chromium constitutes at least 15 weight percent of the bulk composition of the metallic powder. 
     
     
         3 . The slurry coating composition according to  claim 1 , wherein the bulk composition of the metallic powder is predominantly the metallic chromium. 
     
     
         4 . The slurry coating composition according to  claim 1 , wherein the metallic powder consists of metallic chromium and incidental impurities. 
     
     
         5 . The slurry coating composition according to  claim 1 , wherein the metallic powder further comprises the metallic aluminum. 
     
     
         6 . The slurry coating composition according to  claim 5 , wherein the metallic aluminum constitutes about 2 to about 18 weight percent of the bulk composition of the metallic powder. 
     
     
         7 . The slurry coating composition according to  claim 5 , wherein the metallic aluminum constitutes about 5 to about 49 weight percent of the bulk composition of the metallic powder, the balance being the metallic chromium and incidental impurities. 
     
     
         8 . The slurry coating composition according to  claim 1 , wherein the colloidal silica comprises a liquid carrier selected from the group consisting of water, alcohols, halogenated hydrocarbon solvents, and compatible mixtures thereof. 
     
     
         9 . The slurry coating composition according to  claim 8 , wherein the liquid carrier is water. 
     
     
         10 . The slurry coating composition according to  claim 1 , wherein the slurry coating composition contains the one or more additional constituents selected from the group consisting of thickening agents, dispersants, deflocculants, anti-settling agents, anti-foaming agents, binders, plasticizers, emollients, surfactants, and lubricants. 
     
     
         11 . The slurry coating composition according to  claim 1 , wherein the metallic powder is present in the slurry coating composition at a level in the range of about 25% by weight to about 80% by weight of the slurry coating composition. 
     
     
         12 . The slurry coating composition according to  claim 1 , wherein the colloidal silica is present in the slurry coating composition at a level in the range of about 1% by weight to about 25% by weight, based on silica solids as a percentage of the slurry coating composition. 
     
     
         13 . The slurry coating composition according to  claim 1 , wherein the metallic powder further comprises at least one metal selected from the group consisting of platinum group metals, rare earth metals, scandium, yttrium, iron, and cobalt. 
     
     
         14 . The slurry coating composition according to  claim 1 , wherein the silica in the colloidal silica has an average particle size in the range of about 10 nanometers to about 100 nanometers. 
     
     
         15 . The slurry coating composition according to  claim 1 , further comprising at least one organic compound that contains at least two hydroxyl groups. 
     
     
         16 . The slurry coating composition according to  claim 15 , wherein the organic compound contains at least three hydroxyl groups. 
     
     
         17 . The slurry coating composition according to  claim 15 , wherein the organic compound is selected from the group consisting of alkane diols, glycerol, pentaerythritol, fats, and carbohydrates. 
     
     
         18 . The slurry coating composition according to  claim 15 , wherein the organic compound is present in an amount sufficient to chemically stabilize the metallic powder during contact with any aqueous component present in the slurry coating composition. 
     
     
         19 . The slurry coating composition according to  claim 18 , wherein the organic compound is present at a level in the range of about 0.1% by weight to about 20% by weight, based on the total weight of the slurry coating composition. 
     
     
         20 . The slurry coating composition according to  claim 1 , wherein the metallic powder has a particle size of −250 mesh. 
     
     
         21 . The slurry coating composition according to  claim 1 , wherein the slurry coating composition is in the form of a coating on the surface region of the substrate, and the substrate is formed of a nickel-based superalloy. 
     
     
         22 . The slurry coating composition according to  claim 21 , wherein the substrate is an under-platform region of a turbine blade of a gas turbine engine.