Method for producing a hard coating with high corrosion resistance on articles made of anodizable metals or alloys

Abstract

A method for coating, a composition suitable for coating and a coating generated with the method of coating on anodizable metallic surfaces, especially on magnesium rich and aluminum rich surfaces. is disclosed. The composition is an aqueous solution including alkali metal or ammonium cations, phosphorus containing anions and silicon containing anions as well as optionally a peroxide or a compound of Al, Ti, Zr or any mixture of them. Preferably, the anodizing is carried out with a micro-arc oxidation process.

1 . A composition of an aqueous electrolyte solution useful for the oxidation of a surface of at least one anodizable metallic material with a pH greater than 6 comprising:
 i. at least two different phosphorus containing compounds showing different anions which are at least partially soluble in the aqueous solution used, at least a first being called component a) and at least a second being called component b);   ii. at least one silicon containing compound which is at least partially soluble in the aqueous solution used; and   iii. an amount of at least one type of cations selected from alkali metal cations and ammonium cations;   iv. whereby the electrolyte solution shows a total concentration of at least one hydroxide of Na, K, Li, NH 4  or any mixture of these intentionally added to the electrolyte solution below 0.8 g/L or whereby the electrolyte solution is free of any hydroxide of Na, K, Li, NH 4  or any mixture of these added intentionally.   
   
   
       2 . A composition of an aqueous electrolyte solution useful for the oxidation of a surface of at least one anodizable metallic material with a pH greater than 6 comprising:
 at least two different phosphorus containing compounds showing different anions which are at least partially soluble in the aqueous solution used, at least two of them being called component a) and component b), wherein there is contained a moiety of at least one phosphorus containing compound showing oxyanions;   an amount of at least one compound selected from organic silicates, inorganic silicates, silicon containing oxides, silanes, silanols, siloxanes and polysiloxanes, their derivatives or any mixture of them that are sufficiently stable in the electrolyte solution, essentially non-toxic and water-soluble or at least partially water-soluble;   a moiety of at least one type of cations of Na, K, Li, NH 4  or any mixture of these;   whereby the electrolyte solution shows a total concentration of at least one hydroxide of Na, K, Li, NH 4  or any mixture of these intentionally added to the electrolyte solution below 0.8 g/L or whereby the electrolyte solution is free of any hydroxide of Na, K, Li, NH 4  or any mixture of these added intentionally.   
   
   
       3 . The composition of  claim 1  wherein the electrolyte solution contains a moiety of at least one primary phosphate, of at least one secondary phosphate, of at least one orthophosphate, of at least one condensed phosphate, of at least one pyrophosphate, of at least one phosphonate, of at least one phosphonite, of at least one phosphite, of at least one derivative of them or of any mixture of them. 
   
   
       4 . The composition of  claim 1  wherein the electrolyte solution contains:
 as component a) a moiety of at least one primary, secondary or tertiary phosphate or of at least one derivative of them or of any mixture of them and   as component b) a moiety of at least one pyrophosphate or of at least one derivative of it or of any mixture of them.   
   
   
       5 . The composition of  claim 1  wherein at least one of said phosphorus containing compounds is chosen from the group consisting of K 3 PO 4 , Na 3 PO 4 , (NH 4 ) 3 PO 4 , K 2 HPO 4 , Na 2 HPO 4 , (NH 4 ) 2 HPO 4 , KH 2 PO 4 , NaH 2 PO 4 , NH 4 H 2 PO 4 , K 4 P 2 O 7 , Na 4 P 2 O 7  and (NH 4 ) 4 P 2 O 7 . 
   
   
       6 . The composition of  claim 1  wherein the electrolyte solution contains the at least two phosphorus containing compounds in a total concentration in the range from 0.2 to 250 g/L. 
   
   
       7 . The composition of  claim 1  wherein the concentration of said component a) in said electrolyte solution is in the range from 0.1 to 220 g/L and wherein said component b) in said electrolyte solution is in the range from 0.1 to 220 g/L. 
   
   
       8 . The composition of  claim 1  wherein the electrolyte solution contains a moiety of at least one alkali metal silicate or at least one of their derivatives or any mixture of them. 
   
   
       9 . The composition of  claim 1  wherein the total concentration of the at least one silicon containing compound in said electrolyte solution is in the range from 0.5 g/L to 70 g/L. 
   
   
       10 . The composition of  claim 1  wherein there is a total concentration of at least one hydroxide of Na, K, Li or NH 4  or of any mixture of them of no more than 0.8 g/L in the electrolyte solution. 
   
   
       11 . The composition of  claim 1  wherein the electrolyte solution contains additionally at least one peroxide. 
   
   
       12 . The composition of  claim 1  wherein the concentration of the at least one peroxide additionally contained in the electrolyte solution is in the range from 0.01 g/L to 20 g/L—calculated as 100% of H 2 O 2 . 
   
   
       13 . The composition of  claim 1  wherein the electrolyte solution contains additionally at least one compound containing atoms of Al, Ti, Zr or any mixture of these atoms or any mixture of these compounds. 
   
   
       14 . The composition of  claim 1  wherein at least one water-insoluble compound containing atoms of Al, Ti, Zr or any mixture of these atoms or any mixture of these compounds additionally contained in the electrolyte solution is contained in the form of particles showing a particle size distribution for all these particles essentially in the range from 0.01 to 20 microns. 
   
   
       15 . The composition of  claim 1  wherein the concentration of the at least one compound containing atoms of Al, Ti, Zr or of any mixture of these atoms or of any mixture of these compounds additionally contained in the electrolyte solution is in the range from 0.01 g/L to 50 g/L. 
   
   
       16 . The composition of  claim 1  wherein the electrolyte solution contains as solvent water or water and at least one alcohol. 
   
   
       17 . The composition of  claim 1  wherein the electrolyte solution contains a total concentration of at least one solvent besides of water in the range from 0.01 to 500 g/L. 
   
   
       18 . A method of treating a metallic workpiece comprising:
 providing a metallic surface chosen from metallic surfaces of at least one metallic material that may be anodized;   immersing said surface in an electrolyte solution whereby the solution may really be a solution, a sol, a gel, a suspension or any mixture of them;   providing at least one electrode in said electrolyte solution; and   passing a current between said surface and said electrode through said electrolyte solution wherein said electrolyte solution is an aqueous solution with a pH greater than 6 that has a composition as claimed in  claim 1 .   
   
   
       19 . The method of  claim 18  wherein a pulsed direct current or an alternating current is applied as the current between said metallic surface and said electrode. 
   
   
       20 . The method of  claim 18  wherein the current applied is an alternating current showing a frequency of the pulses in the range from 1 to 100 Hz.