Method for forming conversion coating on outer surface of magnesium or magnesium alloy

Abstract

Disclosed is a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy by surface treatment with acidic and alkaline solutions. The method is carried out in an environmentally friendly manner to improve the corrosion resistance of the magnesium or magnesium alloy. The method minimizes the damage to the appearance of the magnesium or magnesium even after surface treatment and maximizes the attractive appearance of the magnesium or magnesium alloy.

1 . A method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy, the method comprising:
 degreasing the outer surface of magnesium or a magnesium alloy, followed by water washing;   etching the degreased magnesium or magnesium alloy to remove an oxide coating present on the outer surface thereof, followed by water washing;   removing smut present on the outer surface of the magnesium or magnesium alloy, followed by water washing;   forming a conversion coating on the desmutted outer surface of the magnesium or magnesium alloy, followed by water washing; and   forming a coating on the conversion coating.   
     
     
         2 . The method of  claim 1 , wherein, in the step of degreasing and water washing, the degreasing is performed using a first aqueous solution comprising 30-60 g/L of potassium hydroxide (KOH), 150-200 g/L of sodium hydroxide (NaOH), 20-30 g/L of an amine oxide, 5-10 g/L of sodium gluconate (C 6 H 11 NaO 7 ) and a wetting agent. 
     
     
         3 . The method of  claim 1 , wherein, in the step of etching and water washing, the etching is performed using a second aqueous solution comprising 600-800 g/L of nitric acid (HNO 3 ), 200-400 g/L of phosphoric acid (H 3 PO 4 ) and a wetting agent. 
     
     
         4 . The method of  claim 3 , wherein the second aqueous solution is diluted with distilled water to a pH of 1.0-2.0. 
     
     
         5 . The method of  claim 1 , wherein, in the step of removing smut and water washing, removing smut is performed by immersing the etched metal in a third aqueous solution comprising 50-200 g/L of potassium hydroxide (KOH) and 5-10 g/L of a wetting agent. 
     
     
         6 . The method of  claim 1 , wherein, in the step of forming a conversion coating and water washing, the conversion coating is formed using a fourth aqueous solution comprising 600-800 g/L of nitric acid (HNO 3 ), 200-400 g/L of phosphoric acid (H 3 PO 4 ) and 0.5-10 g/L of hydrofluoric acid (HF). 
     
     
         7 . The method of  claim 6 , wherein the fourth aqueous solution is diluted with distilled water to a pH of 2.0-4.0.