Methods of making protective coating for metal surfaces

Abstract

A coating for a metal surface that provides excellent resistance to both electrochemical corrosion and mechanical insult is provided. The coating involves at least an inner coating that is a sacrificial anodic layer and an outer coating that is a protective dielectric material made of inorganic metal oxide. Some versions of the coating include an intermediate layer as well that serves to improve adhesion between the coatings and may provide additional galvanic protection. Although the coating can be made by a variety of methods, advanced methods of spray application are provided for making high-quality lightweight versions the coating.

We claim: 
     
         1 . A process for coating a ductile iron surface, comprising:
 (a) thermally spraying a first spray onto the ductile iron surface, the first spray comprising at least 95% w/w metallic zinc, to form an inner galvanizing coating that is anodic relative to the ductile iron surface and in direct contact with the ductile iron surface; and   (b) thermally spraying a second spray onto the inner coating, the second spray comprising at least 90% w/w Al 3 O 2 , to form an outer dielectric coating in direct contact with the inner coating; wherein the sum of the thicknesses of the inner and outer coatings is at most 10 mils (254 μm).   
     
     
         2 . A process for coating a metal surface, comprising:
 (a) thermally spraying a first spray to form a galvanizing inner coating substantially electrically connected to the metal surface, wherein
 (i) the first spray comprises a first metal that is anodic to the metal surface, and 
 (ii) the inner coating is anodic to the metal surface; and 
   (b) thermally spraying a second spray to form a dielectric outer coating on the side of the inner coating opposite the metal surface, wherein the second spray comprises a dielectric oxide of a second metal.   
     
     
         3 . The process of  claim 2 , wherein the first spray is thermally sprayed directly onto the metal surface. 
     
     
         4 . The process of  claim 2 , wherein the second spray is thermally sprayed directly onto the inner coating. 
     
     
         5 . The process of  claim 2 , wherein the first metal is selected from the group consisting of: zinc, aluminum, magnesium, indium, gallium, and tellurium. 
     
     
         6 . The process of  claim 2 , wherein the first spray comprises at least 95% w/w of the first metal, which is selected from the group consisting of: zinc, aluminum, magnesium, indium, gallium, tellurium, or a combination of any of the foregoing. 
     
     
         7 . The process of  claim 2 , wherein the first spray comprises at least 85% w/w of the first metal, which is zinc. 
     
     
         8 . The process of  claim 2 , wherein the first spray comprises at least 95% w/w of the first metal, which is zinc. 
     
     
         9 . The process of  claim 2 , wherein the first metal is zinc and the oxide of the second metal is an oxide of aluminum. 
     
     
         10 . The process of  claim 2 , wherein the inner coating is spayed to a thickness of about 2-3 mils (50.8-76.2 μm). 
     
     
         11 . The process of  claim 2 , wherein the outer coating is sprayed to a thickness of about 2-6 mils (50.8-152.4 μm). 
     
     
         12 . The process of  claim 2 , wherein the sum of the thicknesses of the inner coating and the outer coating does not exceed about 10 mils (254 μm). 
     
     
         13 . The process of  claim 2 , wherein the oxide of the second metal is selected from the group consisting of: an oxide of aluminum, an oxide of magnesium, and an oxide of titanium. 
     
     
         14 . The process of  claim 2 , wherein the second spray comprises at least 95% w/w of the oxide of the second metal. 
     
     
         15 . The process of  claim 2 , wherein the second spray comprises at least 95% w/w of the oxide of a second metal, which is selected from the group consisting of: zinc, aluminum, magnesium, titanium, or a combination of any of the foregoing. 
     
     
         16 . The process of  claim 2 , wherein the second spray comprises at least 95% w/w Al 2 O 3 . 
     
     
         17 . The process of  claim 2 , comprising applying an organic topcoat outside of the outer coating. 
     
     
         18 . The process of  claim 2 , comprising applying an organic topcoat outside of the outer coating, and the topcoat is selected from the group consisting of: asphalt, paint, and wax. 
     
     
         19 . The process of  claim 2 , comprising: thermally spraying a third spray prior to forming the outer coating, to form an intermediate galvanizing coating between the inner coating and the outer coating, wherein the second spray comprises a third metal that is anodic to the metal surface. 
     
     
         20 . The process of  claim 19 , wherein the intermediate coating is anodic to the metal surface. 
     
     
         21 . The process of  claim 19 , wherein the intermediate coating is anodic to the metal surface and the inner coating. 
     
     
         22 . The process of  claim 19 , wherein:
 (a) the first spray comprises at least 80% w/w of the first metal which is zinc;   (b) the third spray comprises at least 90% w/w of the third metal which is selected from aluminum, titanium, or a mixture of both; and   (c) the second spray comprises at least 90% w/w Al 2 O 3 .   
     
     
         23 . The process of  claim 19 , wherein the third metal is aluminum. 
     
     
         24 . A coated metal surface that is the product of the process of  claim 2 . 
     
     
         25 . A fluid conduit comprising the coated metal surface that is the product of the process of  claim 2 .