Chromating method and component obtained by this method

Abstract

A method of chromating a part including an aluminum-based alloy, the method including the steps of providing the part including a layer based on aluminum oxide on at least one of its surfaces, of immersing the part in a chromating bath including chromium III, and of performing chemical conversion of the part in the chromating bath. The invention also provides a part obtained by the method.

1 . A chromating method for chromating a part comprising an aluminum-based alloy, the method comprising the following steps:
 providing the part including a layer based on aluminum oxide on at least one surface of the part, which layer is of thickness that is greater than or equal to 5 nm;   immersing the part in a chromating bath including chromium III; and   performing chemical conversion of the part in the chromating bath.   
     
     
         2 . The chromating method according to  claim 1 , wherein the chromating bath comprises zirconium. 
     
     
         3 . The chromating method according to  claim 1 , wherein the layer based on aluminum oxide is a native oxide layer and wherein the part and the layer based on aluminum alloy are degreased before being immersed in the chromating bath. 
     
     
         4 . The chromating method according to  claim 1 , wherein the layer based on aluminum oxide is formed by a mechanical method. 
     
     
         5 . The chromating method according to  claim 4 , wherein the mechanical method comprises a step of projecting solid particles against the surface of the part. 
     
     
         6 . The chromating method according to  claim 4 , wherein the mechanical method comprises a step of abrading the surface of the part with abrasive paper or with a liquid containing abrasive particles. 
     
     
         7 . The chromating method according to  claim 1 , wherein the layer based on aluminum oxide is formed by a chemical method comprising a first step of chemical cleaning of the surface of the part followed by a step of chemical or electrochemical oxidation of the surface of the part. 
     
     
         8 . The chromating method according to  claim 1 , wherein the layer based on aluminum oxide is obtained by placing the part in an atmosphere having humidity in the range 30% to 100% and/or wherein the temperature lies in the range 30° C. to 200° C. 
     
     
         9 . A part comprising an aluminum-based alloy presenting, on at least one surface of the part, a coating comprising chromium III, the coating having a free surface and having a given thickness, wherein, in a layer of the coating lying in the range 15% to 30% of the given thickness of the coating as measured from the free surface, the amount of aluminum oxide measured in arbitrary units by TOF-SIMS varies at most by plus or minus 50%, preferably by plus or minus 40%, still more preferably by plus or minus 30%. 
     
     
         10 . The part according to  claim 9 , wherein the coating that comprises chromium III also includes zirconium.