Corrosion mitigation of magnesium and magnesium alloys

Abstract

A method is provided for reducing the corrosion rate of surfaces of formed magnesium or magnesium alloy articles in which the formed surface contains small embedded particles of iron. By exposing the iron particle-containing formed surface to an acidic, aqueous solution comprising alkali metal fluoride ions at a temperature of between 20° C. and 30° C., an adherent passivating layer of MgF2 is formed. Further, such exposure to the acidified, aqueous, fluoride ion-containing solution reduces or eliminates the concentration of cathodic, corrosion-promoting, iron-containing particles on the article surface as the magnesium fluoride layer is being formed. The development of the passivating layer reduces corrosion in a water-containing environment, and even if the passivating MgF2 layer is breached, the reduction in surface iron-containing particles reduces the inherent corrosion rate of the article.

1 . A method of reducing the corrosion rate of a formed magnesium or magnesium-based alloy article, the formed article having a formed surface which may be exposed to an aqueous environment in use, the formed surface having embedded iron-containing particles; the method comprising reacting the formed surface of the article with an acidified aqueous solution of one or more of an alkali metal fluoride compound and ammonium fluoride so as to form a coextensive layer of MgF 2  on the formed surface of the article, the embedded iron-containing particles being removed from the formed surface during the reaction, the acidified fluoride ion-containing solution having a pH of less than 2. 
     
     
         2 . The method of reducing the corrosion rate of a formed magnesium or magnesium-based article as stated in  claim 1  in which an acidified solution of one or more alkali metal fluoride compounds is reacted with the surface of the formed article. 
     
     
         3 . The method of reducing the corrosion rate of a formed magnesium or magnesium-based article as stated in  claim 1  in which the reaction is conducted with the formed part and the aqueous solution being at a temperature in the range of 20° C. to 30° C. 
     
     
         4 . The method of reducing the corrosion rate of a formed magnesium or magnesium-based article as stated in  claim 1  in which the thickness of the formed MgF 2  layer is in the range of 0.1 micrometer to 1000 micrometers 
     
     
         5 . The method of reducing the corrosion rate of a formed magnesium or magnesium-based article as stated in  claim 1  in which the fluoride ion-containing solution comprises hydrogen fluoride (HF). 
     
     
         6 . The method of reducing the corrosion rate of a formed magnesium or magnesium-based article as stated in  claim 1  in which the pH value of the aqueous solution is obtained with the addition of one of the group consisting of H 2 SO 4  and HNO 3 . 
     
     
         7 . The method of reducing the corrosion rate of a formed magnesium or magnesium-based article as stated in  claim 1  in which the alkali metal earth compound is at least one of sodium fluoride or potassium fluoride. 
     
     
         8 . The method of reducing the corrosion rate of a formed magnesium or magnesium-based article as stated in  claim 1  in which the fluoride ion-containing solution comprises between 0.1 mole and 28.9 moles of fluoride ion per liter. 
     
     
         9 . The method of reducing the corrosion rate of a formed magnesium or magnesium-based article as stated in  claim 1  in which the formed surface of the magnesium article is exposed to the fluoride ion-containing solution for between 30 and 300 seconds. 
     
     
         10 . The method of reducing the corrosion rate of a formed magnesium or magnesium-based article as stated in  claim 1  in which the formed magnesium article is exposed to the fluoride ion-containing solution by dipping the magnesium article into a bath of the fluoride-containing solution. 
     
     
         11 . The method of reducing the corrosion rate of a formed magnesium or magnesium-based article as stated in  claim 8  in which the bath of the fluoride-containing solution is agitated. 
     
     
         12 . The method of reducing the corrosion rate of a formed magnesium or magnesium-based article as stated in  claim 8  further comprising agitating the article during exposure of the article to the fluoride ion-containing solution. 
     
     
         13 . The method of reducing the corrosion rate of a formed magnesium or magnesium-based article as stated in  claim 1  in which the magnesium article is exposed to the fluoride ion-containing solution by spraying the fluoride-containing solution on the magnesium article. 
     
     
         14 . The method of reducing the corrosion rate of a formed magnesium or magnesium-based article as stated in  claim 1  further comprising rinsing, drying and optionally baking the article, after terminating exposure of the magnesium article to the fluoride ion-containing solution. 
     
     
         15 . A formed magnesium article, the formed article having a formed surface which may be exposed to an aqueous environment in use, the formed magnesium article having on its surface, a co-extensive layer of MgF 2  with a thickness ranging from 0.1 micrometer to 1000 micrometers, the article having a surface concentration of iron-containing particles which is less than 50% of the volume concentration of iron-containing particles in the body of the article. 
     
     
         16 . A formed magnesium article as stated in  claim 15  in which the layer of magnesium fluoride was formed by reacting the formed surface of the article with an acidified aqueous solution of one or more of an alkali metal fluoride compound and ammonium fluoride. 
     
     
         17 . A formed magnesium article as stated in  claim 15  in which the layer of magnesium fluoride was formed by reacting the formed surface of the article with an acidified aqueous solution of an alkali metal fluoride compound. 
     
     
         18 . A formed magnesium article as stated in  claim 15  in which the layer of magnesium fluoride was formed by reacting the formed surface of the article with an acidified aqueous solution of at least one of sodium fluoride and potassium fluoride.