Anodization method for corrosion protection of aluminium alloy elements used in an aircraft structure

Abstract

An anodization method for corrosion protection of an aluminium or aluminium alloy element used in an aircraft structure, comprising the following steps: a) subjecting the element to a degreasing step by means of an alkaline bath for removing contaminating elements; b) subjecting the element to a subsequent first washing in water; c) subjecting the element to an acid pickling step by dipping the element in an acid solution and then extracting the element from the acid solution and subjecting the element to a subsequent washing in water; subjecting the washed element to a subsequent electrochemical treatment step in a tank by dipping the element in a solution of tartaric acid (C 4 H 6 O 6 ) and sulphuric acid (H 2 SO 4 ); e) subjecting the element to a subsequent washing in water; f) dipping the element in a bath in which a solution of chromium, with an oxidation number of +3, and zirconium ions and fluorides is present, in order to carry out a first post-anodization sealing step; g) extracting the element from the bath of step f) and subjecting it to a subsequent final washing and a subsequent dipping in a tank of boiling water (second sealing step), and then drying the element.

1 . An anodization method for corrosion protection of an aluminium or aluminium alloy element used in an aircraft structure, 
       comprising the following steps:
 a) subjecting the element to a degreasing step by means of an alkaline bath (block  100 ) for removing contaminating elements; 
 b) subjecting the element to a subsequent first washing in water (block  110 ); 
 c) subjecting the element to an acid pickling step ( 120 ) by dipping the element in an acid solution and then extracting the element from the acid solution and subjecting the element to a subsequent washing in water; 
 d) subjecting the washed element to a subsequent electrochemical treatment step in a tank ( 140 ) by dipping the element in a solution of tartaric acid (C 4 H 6 O 6 ) and sulphuric acid (H 2 SO 4 ) and applying an electric potential to said element; 
 e) subjecting the element to a subsequent second washing in water ( 150 ); 
 f) dipping (block  170 ) the element in a bath in which a solution of chromium, with an oxidation number of +3, and zirconium ions and fluorides is present, in order to carry out a post-anodization sealing step; 
 g) extracting the element from the bath of step f) and subjecting it to a third final washing in water and a subsequent dipping in a tank of boiling water, which provides a second sealing step, and then drying the element (block  180 ). 
 
     
     
         2 . The anodization method as defined in  claim 1 , wherein step c) is carried out by dipping the element in an acid bath for a time interval of 5 to 10 minutes. 
     
     
         3 . The anodization method as defined in  claim 1 , wherein step c) is carried out by dipping the element in an acid bath having a temperature of 20° C. to 40° C. 
     
     
         4 . The anodization method as defined in  claim 1 , wherein step d) is configured to perform the following chemical reactions: 
       Electrochemical reaction at the anode:
   2Al+3H 2 O═Al 2 O 3 +6H + +6 e   − 
 
 
       Electrochemical reaction at the cathode:
   6H + +6 e   − =3H 2    
 
       Resulting anodization reaction:
   2Al+3H 2 O═>Al 2 O 3 +3H 2  
 
 
     
     
         5 . The anodization method as defined in  claim 1 , wherein step d) is carried out using the following parameters:
 applying the voltage within one minute from the dipping of the element in the solution;   applying an increasing voltage with a ramp not exceeding 3 volts per minute;   applying a constant voltage for approximately 20 minutes, and thereafter;   gradually reducing the voltage to a null value;   removing the element from the solution within 3 minutes from the switching off of the voltage.   
     
     
         6 . The anodization method as defined in  claim 1 , wherein step d) is carried out using a solution having a temperature ranging between 36° C. and 39° C. 
     
     
         7 . The anodization method as defined in  claim 1 , wherein step g) is carried out in a tank of boiling water with a temperature higher than 95° C. and a pH ranging between 4.5 and 7 for approximately 30 minutes. 
     
     
         8 . The anodization method as defined in  claim 1 , wherein in step d) the concentration of tartaric acid is 72-88 g/l and the concentration of sulphuric acid is 36-44 g/l. 
     
     
         9 . The anodization method as defined in  claim 1 , wherein step f) performs the following chemical reactions:
   4Al2O3+24F − +3Zr +4 +4Cr +3 →8AlF3+3ZrO2+2Cr2O3
   
     
     
         10 . The anodization method as defined in  claim 1 , wherein between step b) and step d) the following further steps are also carried out:
 alkaline chemical etching ( 200 ) in order to prepare/activate the aluminium/aluminium alloy surfaces for the electrochemical treatment in step d); and   aluminium/aluminium alloy desmutting and rinsing ( 210 ).   
     
     
         11 . The anodization method as defined in  claim 1 , wherein said boiling water has a temperature higher than 95° C. 
     
     
         12 . The anodization method as defined in  claim 1 , wherein said boiling water has a pH ranging between 4.5 and 7. 
     
     
         13 . The anodization method as defined in  claim 1 , wherein said fluorides result from salts and fluorozirconates/silicates.