Patent Publication Number: US-2005121115-A1

Title: 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

Description:
The invention relates to a sealing method for producing a layer or film of oxide which is resistant to saline corrosion on a metal substrate, to the sealing solution used in said method, and to an article treated by said method.  
      In particular, the present invention relates to the application of said sealing method-or said sealing solution to a metal substrate formed from an aluminum or aluminum alloy substrate, said method being carried out after a prior sulfuric anodization step.  
     BACKGROUND OF THE INVENTION  
      Aluminum enjoys natural protection against atmospheric corrosion by oxidation of its surface, resulting in the formation of amorphous Al 2 O 3  type alumina. That alumina forms naturally in air and can also result from sulfuric acid anodization, which is preferable when a layer is to be produced rapidly and, moreover, when it has to be thicker. This is the case in conventional anodization of the sulfuric anodization (SA) type (sulfuric anodic oxydation), which produces a thickness of 10 micrometers (μm) to 20 μm and up to 100 μm in the case of “hard” anodization.  
      However, a thin layer of alumina on the surface of aluminum alloys is not sufficiently resistant to the corrosion to which the part is subjected.  
      In contrast, with anodization, the alumina which is formed has a columnar structure with a geometry which is close to a hexagonal symmetry, with pores rendering it relatively sensitive to its environment, in particular permeable to chemical attack.  
      For this reason, such pores have to be plugged by sealing. The sealing which is to be carried out is aimed at fixing (metallic) elements in said pores to reduce their permeability in order to enhance the anti-corrosive properties of the alumina layer without, however, degrading the mechanical properties of the alumina.  
      In particular, after sulfuric anodization, the alumina layer is sealed in known manner by placing the part in a chemical bath. In accordance with a current sealing method, that bath is a solution based on chromic acid, which allows the pores to be plugged with hexavalent chromium or chromium VI.  
      Sealing methods of that type, used to plug the pores of the alumina layer present on the surface of an aluminum or aluminum alloy part to form a film which is resistant to saline corrosion, have conventionally used acidic solutions comprising chromium VI, as described in U.S. Pat. Nos. 2,796,370 and 2,796,371.  
      However, it should be noted that using solutions based on chromium VI is now prohibited because of the toxicity of that heavy metal and new legal constraints based on environmental considerations.  
      Alternatively, other sealing solutions have been proposed in U.S. Pat. Nos. 5,411,606 and 5,472,524 and in European patent EP-A-0 488 430, which use sealing solutions comprising complex salts based on cobalt III.  
      However, that type of sealing solution is relatively expensive and difficult to use because the pH and the complex salts based on cobalt III are unstable—they have a tendency to precipitate out, depending on the age of the bath. For that reason, that type of solution is unstable, rendering operations not strictly reproducible, deleteriously affecting the quality of the layers obtained.  
     OBJECTS AND SUMMARY OF THE INVENTION  
      The present invention aims to overcome the problems with prior art sealing solutions, in particular problems with chromium toxicity, by proposing a novel sealing method using a sealing solution that contains no chromium, that produces good results and that is easy to carry out, in particular in an industrial context.  
      To this end, the sealing method of the present invention comprises steps consisting in: 
          providing a buffered sealing solution, preferably based on an aqueous reaction 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 that is sufficient to form a mixed cobalt/lithium oxide film obtained by chemical conversion.        

      Thus, said “mixed cobalt/lithium chemical sealing” can protect a previously anodized substrate, forming an extremely adhesive sealing layer which has anti-corrosion properties and good paint keying properties.  
      In the case of an aluminum substrate or an aluminum alloy substrate, this means that the film is formed when the pores of the alumina are plugged by metallic cobalt and lithium salts.  
      In this manner, it is understood that the presence of a simple cobalt II salt and a simple lithium II salt, which are products in routine use, renders said method extremely easy to carry out.  
      Surprisingly, a combination of at least one simple cobalt II salt and at least one simple lithium III salt provides very good results which are relatively superior to those obtained by using only at least one simple cobalt II salt or by using only at least one simple lithium III salt.  
      Preferably, said simple cobalt II salt is from the group constituted by cobalt sulfate, cobalt nitrate, cobalt carbonate and cobalt acetate. In particular, said simple cobalt II salt is cobalt acetate Co(CH 3 COO) 2 ,4H 2 O in a concentration in the range 3 grams/liter (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.  
      Preferably, said simple lithium III salt is from the group constituted by lithium sulfate, lithium nitrate, lithium carbonate and lithium acetate. In particular, 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.  
      In a preferred implementation, said sealing solution also comprises at least one weak acid from the group constituted by boric acid, acetic acid, citric acid and tartaric acid. In particular, 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.  
      Said solution has the additional advantage of being easy to reproduce, of giving a homogeneous result, and of allowing the sealing solution to be re-used because of the buffer effect of the weak acid which stabilizes the pH of the sealing solution.  
      Said weak acid can buffer the sealing solution which then has a pH in the range 5 to 6, advantageously in the range 5.1 to 5.9, preferably 5.5±0.1.  
      In a preferred but optional implementation, the sealing solution in question also comprises a surfactant such as sodium lauryl sulfate and/or sodium dodecyl sulfate C 12 H 25 NaO 4 S.  
      In particular, 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.  
      Adding such a compound can enhance the results (a more even layer and better distribution of simple cobalt and lithium salts in the pores of the alumina). In fact, it contributes to reducing the surface tension between the metal substrate and the sealing solution, and it also improves the pH stability of the solution by capturing hydrogen ions H +  liberated by the weak acid.  
      In a further preferred implementation, the temperature of the sealing solution is over 87° C., preferably over 90° C., advantageously over 95° C., and more preferably in the range 95° C. to 98° C.  
      Preferably, the duration of the step for bringing the substrate into contact with the sealing solution is more than 15 minutes (min.), advantageously more than 20 min., and preferably in the range 20 min. to 25 min.  
      The present invention also pertains to 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.  
      The present invention also pertains to a treated article resulting from carrying out the method of the type defined above, using the sealing solution of the type defined above.  
      In a preferred implementation, said article comprises a sealed anodization film having a thickness in the range 15 μm to 20 μm.  
      In general, the proposal of the present invention renders it possible to produce, in a simple and certain manner and without recourse to chromium, mixed cobalt/lithium sealing resulting in a film having corrosion resistance properties on a metal substrate.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other advantages and characteristics of the invention will become apparent from the following description made with reference to the accompanying drawings in which:  
       FIG. 1  shows a section through a specimen (AUZGN blade) treated using the method of the present invention, after 790 hours exposure to a saline mist;  
       FIG. 2  shows a top view of the surface appearance of a specimen treated using the method of the present invention. 
    
    
     MORE DETAILED DESCRIPTION  
      Highly conclusive tests were carried out using aluminum specimens which had undergone sulfuric anodization for 40 minutes (thickness of alumina layer obtained: 16 micronmeters (um) and which had been immersed in a sealing solution for more than 20 minutes at a temperature of 97° C.±2° C.  
      The sealing solution used was an aqueous solution which had the following characteristics: 
          simple cobalt II salt: 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;     simple lithium II salt: 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;     weak acid: 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;     as the surfactant: 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; and     temperature: 90° C.±3° C.        

      A sealing solution was obtained the pH of which was kept at 5.5±0.1 as the solution was buffered by the boric acid.  
      These anodized and sealed specimens in a cobalt-lithium medium had an anodic potential of −650 millivolts (mV) (measured with respect to a saturated calomel electrode, SCE). This high plate (anodic) protection value was thus of the same order of magnitude as that obtained for a prior art dichromate solution (−655 mV SCE).  
      Further, said aluminum alloy (2024) specimens withstood more than 700 hours of the saline mist resistance test in accordance with French standard AFNOR NFX 41002 or International standard ISO 9227.  
       FIG. 1  is a photograph showing a section of the specimen after exposure to a saline mist for 790 hours: 
          the substrate 10 produced from aluminum alloy 2024 is surmounted by the alumina layer  12  sealed by sulfuric anodization in which three different zones with different compositions can be distinguished:     the lower layer of sealed alumina  12   a  surmounting the substrate  10  and which is characterized by a composition having an absence of cobalt and carbon;     the upper layer of sealed alumina  12   b  surmounting the lower layer of sealed alumina  12   a  and which is characterized by a composition having a very low concentration of cobalt and carbon; and     the surface of the sealed alumina layer  12   c  surmounting the lower layer of sealed alumina  12   a  and which is characterized by a composition comprising cobalt and carbon relating to the presence of cobalt salts in the sealing solution and carbon from the lithium carbonate.        
      It will be understood that thanks to this sealing method and to the sealing solution of the invention, an article is obtained which has good corrosion resistance properties, in particular as regards saline corrosion.  
      In a preferred implementation, said article comprises: 
          a metal substrate based on aluminum or an aluminum alloy;     a sealed film comprising aluminum oxide, cobalt oxide and lithium oxide.        

      It can also be noted that the article treated using the method of the invention has a film formed by a layer of sealed alumina, which is non porous and has a conventional cracked structure at its surface. This can be seen in the accompanying photograph shown in  FIG. 2 .