Hexavalent chromium-free sealing method applicable after sulfuric anodization of aluminum alloys, a sealing solution used in said method, and an article treated using said method

Abstract

The invention concerns a sealing method for producing a film of oxide having good saline corrosion resistance properties on a metal substrate. Said method comprises steps consisting of: providing a buffered sealing solution comprising at least one simple cobalt II salt and at least one simple lithium III salt; and bringing said metal substrate having a previously oxidized surface into contact with said sealing solution for a period sufficient to form a mixed cobalt/lithium oxide film. More particularly, this method is applicable to a metal substrate produced from aluminum or from aluminum alloy anodized in a sulfuric medium.

1 . A sealing method for producing an oxide film having saline corrosion resistance properties on a metal substrate, comprising steps consisting of: 
 providing a buffered sealing solution comprising at least one simple cobalt II salt and at least one simple lithium III salt; and    bringing said metal substrate having a previously anodized surface into contact with said sealing solution for a period sufficient to form a mixed cobalt/lithium oxide film.    
     
     
         2 . A sealing method according to  claim 1 , wherein said simple cobalt II salt is from the group constituted by cobalt sulfate, cobalt nitrate, cobalt carbonate and cobalt acetate.  
     
     
         3 . A sealing method according to  claim 2 , wherein said simple cobalt II salt is cobalt acetate Co(CH 3 COO) 2 ,4H 2 O in a concentration in the range 3 g/liter to 6 g/liter, i.e. in the range 1.2×10 −2  mole/liter to 2.41×10 −2  mole/liter, preferably in the range 4 g/liter to 5 g/liter, i.e. in the range 1.61×10 −2  mole/liter to 2.01×10 −2  mole/liter.  
     
     
         4 . A sealing method according to  claim 1 , wherein said simple lithium III salt is from the group constituted by lithium sulfate, lithium nitrate, lithium carbonate and lithium acetate.  
     
     
         5 . A sealing method according to  claim 4 , wherein said simple lithium III salt is lithium carbonate LiCO 3  in a concentration in the range 0.5 g/liter to 1.5 g/liter, i.e. in the range 6.77×10 −3  mole/liter to 2.03×10 −2  mole/liter, preferably in the range 0.75 g/liter to 1 g/liter, i.e. in the range 1.02×10 −2  mole/liter to 1.35×10 −2  mole/liter.  
     
     
         6 . A sealing method according to  claim 1 , said sealing solution further comprising at least one weak acid from the group constituted by boric acid, acetic acid, citric acid and tartaric acid.  
     
     
         7 . A sealing method according to  claim 6 , wherein said weak acid is boric acid H 3 BO 3  in a concentration in the range 3 g/liter to 6 g/liter, i.e. in the range 4.85×10 −2  mole/liter to 9.7×10 −2  mole/liter, preferably in the range 4 g/liter to 5 g/liter, i.e. in the range 6.47×10 −2  mole/liter to 8.09×10 −2  mole/liter.  
     
     
         8 . A sealing method according to  claim 1 , wherein the sealing solution has a pH in the range 5 to 6.  
     
     
         9 . A sealing method according to  claim 1 , the sealing solution further comprising a surfactant comprising sodium lauryl sulfate and/or sodium dodecyl sulfate.  
     
     
         10 . A sealing method according to  claim 9 , wherein said surfactant is sodium lauryl sulfate present in a concentration in the range 1.5 mg/liter to 3.5 mg/liter, i.e. in the range 5.20×10 −6  mole/liter to 1.21×10 −5  mole/liter, preferably in the range 2 mg/liter to 3 mg/liter, i.e. in the range 6.94×10 −6  mole/liter to 1.04×10 −5  mole/liter.  
     
     
         11 . A sealing method according to  claim 1 , wherein the temperature of the sealing solution is more than 87° C.  
     
     
         12 . A sealing method according to  claim 1 , wherein said substrate is formed from aluminum or aluminum alloy.  
     
     
         13 . A sealing method according to  claim 1 , wherein the duration of said contact of the substrate with the sealing solution is more than 15 minutes.  
     
     
         14 . A sealing solution for producing a film of oxide on a metal substrate, comprising at least one simple cobalt II salt, at least one simple lithium III salt and being buffered, by means of which a mixed cobalt/lithium film is obtained.  
     
     
         15 . A sealing solution according to  claim 14 , wherein said simple cobalt II salt is from the group constituted by cobalt sulfate, cobalt nitrate, cobalt carbonate and cobalt acetate.  
     
     
         16 . A sealing solution according to  claim 15 , wherein simple cobalt II salt is cobalt acetate Co(CH 3 COO) 2 ,4H 2 O in a concentration in the range 3 g/liter to 6 g/liter, i.e. in the range 1.2×10 −2  mole/liter to 2.41×10 −2  mole/liter, preferably in the range 4 g/liter to 5 g/liter, i.e. in the range 1.61×10 −2  mole/liter to 2.01×10 −2  mole/liter.  
     
     
         17 . A sealing solution according to  claim 14 , wherein said simple lithium III salt is from the group constituted by lithium sulfate, lithium nitrate, lithium carbonate and lithium acetate.  
     
     
         18 . A sealing solution according to  claim 17 , wherein simple lithium II salt is lithium carbonate LiCO 3  in a concentration in the range 0.5 g/liter to 1.5 g/liter, i.e. in the range 6.77×10 −3  mole/liter to 2.03×10 −2  mole/liter, preferably in the range 0.75 g/liter to 1 g/liter, i.e. in the range 1.02×10 −2  mole/liter to 1.35×10 −2  mole/liter.  
     
     
         19 . A sealing solution according to  claim 14 , further comprising at least one weak acid from the group constituted by boric acid, acetic acid, citric acid and tartaric acid.  
     
     
         20 . A sealing solution according to  claim 19 , wherein said weak acid is boric acid in a concentration in the range 3 g/liter to 6 g/liter, i.e. in the range 4.85×10 −2  mole/liter to 9.7×10 −2  mole/liter, preferably in the range 4 g/liter to 5 g/liter, i.e. in the range 6.47×10 −2  mole/liter to 8.09×10 −2  mole/liter.  
     
     
         21 . A sealing solution according to  claim 14 , wherein its pH is in the range 5 to 6.  
     
     
         22 . A sealing solution according to  claim 14 , further comprising a surfactant comprising sodium lauryl sulfate and/or sodium dodecyl sulfate.  
     
     
         23 . A sealing solution according to  claim 22 , wherein said surfactant is sodium lauryl sulfate present in a concentration in the range 1.5 mg/liter to 3.5 mg/liter, i.e. in the range 5.20×10 −6  mole/liter to 1.21×10 −5  mole/liter, preferably in the range 2 mg/liter to 3 mg/liter, i.e. in the range 6.94×10 −6  mole/liter to 1.04×10 −5  mole/liter.  
     
     
         24 . A sealing solution according to  claim 14 , wherein its temperature is more than 87° C.  
     
     
         25 . A sealing solution according to  claim 14 , wherein said substrate is formed from aluminum or aluminum alloy.  
     
     
         26 . A coated article produced by the method according to  claim 1 , using a sealing solution comprising at least one simple cobalt II salt, at least one simple lithium III salt and being buffered, by means of which a mixed cobalt/lithium film is obtained.  
     
     
         27 . An article according to  claim 26 , comprising a film which is in the range 15 μm to 20 μm thick.  
     
     
         28 . An article according to  claim 26 , wherein it has corrosion resistance properties, in particular to saline corrosion, said article comprising: 
 a metal substrate formed from aluminum or an aluminum alloy;    a film comprising aluminum oxide, cobalt oxide and lithium oxide.