Coatings for Metal Surfaces

Abstract

An iron-containing metal surface, such as a brushed stainless steel, comprises a heat-set layer of a metal phosphate reaction product formed between a surface metal and phosphate precursor wherein the reaction product layer has an average thickness less than 2 micrometers. A metal surface, that resists retention of environmental stains or marks as well as resisting tarnish at elevated temperatures, is formed by forming a reaction product with the metal surface by heat setting a surface liquid layer containing a phosphate precursor such as a phosphate ester-containing solution or phosphoric acid solution, and optionally containing a phospho alumina precursor, onto the metal surface. An article coated in accordance with this invention is similar in appearance to a similar article without such a coating for maintaining original aesthetic properties.

What is claimed is: 
     
         1 . A method of coating a surface of an iron-containing material, the process comprising:
 applying a first layer that comprises a phospho alumina precursor solution that has an aluminum to phosphorous atomic ratio of 0.1 to 0.7 to the surface;   forming an iron-phosphate precursor on the surface;   drying the first layer; and then   heat setting the first layer by heating to a temperature above 200° C.   
     
     
         2 . The method of  claim 1 , wherein the first layer comprises a phospho alumina precursor solution that has an aluminum to phosphorous atomic ratio of 0.1 to 0.5. 
     
     
         3 . The method of  claim 1 , wherein the surface has a heterogeneous surface microstructure that includes valleys and peaks; wherein the first layer fills valleys in the heterogeneous surface microstructure. 
     
     
         4 . The method of  claim 3 , wherein the surface has an average roughness (Ra) of 0.2 to 1.5 microns. 
     
     
         5 . The method of  claim 4 , wherein the heat set first layer has a thickness of about 0.05 to 2 microns. 
     
     
         6 . The method of  claim 5 , wherein the surface has an Ra of 0.4 to 0.6; and the heat set first layer has a thickness of about 0.1 to 0.3 microns. 
     
     
         7 . The method of  claim 5 , wherein, after heat setting, the valleys are filled with an iron phosphate. 
     
     
         8 . The method of  claim 3 , wherein the surface is a brushed surface. 
     
     
         9 . The method of  claim 1  further comprising applying a second layer that comprises a phosphor-alumina precursor solution which has an aluminum to phosphorous ratio of about 1 to 10 to the heat set first layer;
 drying the second layer; and then 
 heat setting the second layer at a temperature above 250° C. to 1000° C. 
 
     
     
         10 . The method of  claim 9 , wherein the second layer comprises a phosphor-alumina precursor solution that has an aluminum to phosphorous ratio of about 2 to 8. 
     
     
         11 . The method of  claim 9 , wherein the surface is a brushed surface; wherein the brushed surface has an average roughness (Ra) of 0.2 to 1.5 microns; and wherein the second layer provides a chemically uniform and substantially smooth surface. 
     
     
         12 . The method of  claim 11 , wherein the heat set first layer has a thickness of about 0.05 to 0.5 microns; wherein the heat set first layer fills valleys in the brushed surface; and wherein the heat set second layer has a thickness of about 0.05 to 0.5 microns. 
     
     
         13 . The method of  claim 12 , wherein the brushed surface has an Ra of 0.4 to 0.6; and the heat set first layer has a thickness of about 0.1 to 0.3 microns. 
     
     
         14 . The method of  claim 1 , wherein the iron-containing material is stainless steel.