Electroceramic Coating for Magnesium Alloys

Abstract

This invention relates to articles having magnesium-containing metal surfaces with an electroceramic coating chemically bonded to the metal surfaces and to articles having a composite coating comprising first sectors of electroceramic coating and second sectors comprising organic and/or inorganic components different from the electroceramic coating. The invention further relates to processes of making and using the articles.

What is claimed is: 
     
         1 . A method of improving corrosion resistance of magnesium containing metal substrates comprising:
 A) providing an alkaline electrolyte comprised of water, a source of hydroxide ion, and one or more additional components selected from the group consisting of: water-soluble inorganic fluorides, water-soluble organic fluorides, water-dispersible inorganic fluorides, and water-dispersible organic fluorides and mixtures thereof;   B) providing a cathode in contact with the electrolyte;   C) placing a magnesium containing article having at least one bare metallic magnesium or magnesium alloy surface in contact with the electrolyte and electrically connected thereto such that said surface acts as an anode;   D) passing a current between the anode and cathode through the electrolyte solution for a time effective to generate a first layer of an inorganic-based coating chemically bonded directly to said surface;   E) removing the article having the first layer of an inorganic-based coating from the electrolyte and optionally drying it;   F) optionally post-treating the article having the first layer of an inorganic-based coating by:
 i. infusing the first layer of an inorganic-based coating with a second component that is different from the inorganic-based coating thereby distributing the second component throughout at least a portion of the inorganic-based coating and/or 
 ii. contacting the first layer of an inorganic-based coating with a polymeric composition thereby forming a second layer comprising organic polymer chains and/or inorganic polymer chains; and 
   G) optionally applying a layer of paint after the post-treating step.   
     
     
         2 . The method of  claim 1  wherein said method is performed in the absence of any step prior to step D) that deposits silicate and/or fluoride on the magnesium surface. 
     
     
         3 . The method of  claim 2  further comprising performing at least one step selected from cleaning, etching, deoxidizing, desmutting, and combinations thereof prior to placing the magnesium containing article in contact with the electrolyte such that prior to generating the first layer, from 0.5 to 50 g/m 2  of metal is removed from the bare metallic magnesium or magnesium alloy surface. 
     
     
         4 . The method of  claim 1  comprising masking portions of the magnesium containing article prior to placing the magnesium containing article in contact with the electrolyte. 
     
     
         5 . The method of  claim 1  comprising controlling temperature and concentration of the electrolyte and time and waveform of the current in step D) to thereby produce the inorganic-based coating at a thickness of 1-20 microns and comprises carbon, oxygen, fluoride, magnesium and aluminum. 
     
     
         6 . The method of  claim 5  wherein forming the first layer in step D) utilizes less than 10 kWh per square meter of the magnesium containing surface coated. 
     
     
         7 . The method of  claim 1  wherein after step E), no more than 10 mg/m2 of the inorganic-based coating is removed. 
     
     
         8 . The method of  claim 1  wherein said current is pulsed direct current having an average voltage in a range of 50 to 600 volts 
     
     
         9 . The method of  claim 5  wherein the oxygen has a ratio to the fluorine in the inorganic-based coating that exhibits a concentration gradient wherein amount of oxygen relative to amount of fluorine increases as a function of distance from the magnesium-containing article's metal surface. 
     
     
         10 . The method of  claim 5  wherein the inorganic-based coating deposited in step D) has a bilayer structure, comprising:
 a. a first sub-layer directly bonded to the bare metallic magnesium or magnesium alloy surface at a first interface, said first sub-layer comprising at least 70 wt. % of a combined mass of fluorine and magnesium, and a positive amount of oxygen present in an amount of less than about 25 wt. %; 
 b. a second sub-layer integrally connected to the first sub-layer, said second sub-layer comprising external surfaces at the outer boundary of the inorganic-based coating, and internal surfaces defined by pores in the second sub-layer lying interior to the outer boundary of the inorganic-based coating and in communication therewith, said second sub-layer having a composition wherein:
 first sub-layer Mg wt. % >second sub-layer Mg wt. % 
 first sub-layer F wt. % >second sub-layer F wt. % 
 first sub-layer O wt. % <second sub-layer O wt. %. 
 
 
     
     
         11 . The method of  claim 1  wherein the post-treating step F) is present as a step of contacting a matrix of the first layer of inorganic-based coating with a second component different from the inorganic-based coating; distributing the second component throughout at least a portion of the matrix; and depositing a second layer that is different from the inorganic-based coating and is adhered to at least external surfaces of the inorganic-based coating, 
     
     
         12 . The method of  claim 10  wherein step F) i) is present and comprises a step of introducing at least one vanadium containing composition as the second component to the second sub-layer of inorganic-based coating, contacting at least the external surfaces and desirably at least some of the internal surfaces of the second sub-layer, whereby said second component forms a thin film in contact with the external surfaces of the inorganic-based coating and lining at least a portion of the pores in the inorganic-based coating. 
     
     
         13 . The method of  claim 12  wherein the infusing step comprises reacting the vanadium containing composition and elements of the inorganic-based coating to thereby form a portion of the second component, which is different from the inorganic-based coating and the vanadium containing composition. 
     
     
         14 . The method of  claim 1  wherein step F) ii) is present and comprises contacting the first layer of an inorganic-based coating with a polymeric composition thereby forming a second layer comprising organic polymer chains and/or inorganic polymer chains; and optionally applying a layer of paint after the post-treating step. 
     
     
         15 . A magnesium-containing article comprising at least one metallic magnesium or magnesium alloy surface coated according to  claim 1 . 
     
     
         16 . A magnesium-containing article comprising at least one metallic magnesium or magnesium alloy surface coated with a first layer of an inorganic-based coating chemically bonded directly to said surface wherein the inorganic-based coating has a bilayer structure, comprising:
 a. a first sub-layer directly bonded to the bare metallic magnesium or magnesium alloy surface at a first interface, said first sub-layer comprising at least 70 wt. % of a combined mass of fluorine and magnesium, and a positive amount of oxygen present in an average amount of less than about 20 wt. %;   b. a second sub-layer integrally connected to the first sub-layer, said second sub-layer comprising external surfaces at the outer boundary of the inorganic-based coating, and internal surfaces defined by pores in the second sub-layer lying interior to the outer boundary of the inorganic-based coating and in communication therewith, said second sub-layer comprising carbon, oxygen, fluoride, magnesium and aluminum, said oxygen present in the inorganic-based coating second sub-layer in an average amount of greater than about 25 wt. %   
     
     
         17 . A magnesium-containing article having a composite coating comprising:
 a. a matrix formed by a first layer of an inorganic-based coating chemically bound directly to at least one metallic magnesium or magnesium alloy surface, said matrix having pores and internal surfaces defined by pores, at least some of said pores being in communication with an external surface of the first layer and forming openings therein; and   b. a second component, different from the inorganic-based coating, distributed throughout at least a portion of the matrix comprising the pores, said second component being in contact with at least some of the internal surfaces and external surfaces.   
     
     
         18 . The magnesium-containing article of  claim 17  further comprising a second layer that is different from the inorganic-based coating and is adhered to at least external surfaces of the inorganic-based coating.