Synergistic Corrosion Inhibition Effect Of Molybdate And Phosphate Ions For Anodic Oxidation Film Formed On 2024 Aluminum Alloy

Owing to the properties of high strength and low density,the 2024 aluminum alloy is widely used in the fields of aerospace industries and other high-tech fields.However,aluminum metal is a relatively active metal,the natural oxide film formed on aluminum can not provide adequate protection against aggressive ions.Therefore,the surface treatment of aluminum and its alloys is extremely necessary to improve the corrosion resistance before being uesed in a variety of industries.Among them,the anodic oxidation film formed on the aluminum and its alloys by anodic oxidation method can provide a good protection.Many studies have pointed out that the corrosion resistance could be greatly improved by adding corrosion inhibitors to the acidic solution.In this paper,2024 aluminum alloy is used as the research object,and oxalic acid is used as electrolyte solution.The influence of two inorganic corrosion inhibitors(sodium molybdate and ammonium dihydrogen phosphate)on the corrosion resistance of anodic oxidation films on 2024 aluminum alloy,and the synergistic effect of two corrosion inhibitors are studied.The corrosion resistance of anodic oxidation film in 3.5 wt.% NaCl solution is evaluated by electrochemical impedance spectroscopy(EIS)and potentiodynamic polarization curves.Results point out that adding the single sodium molybdate or ammonium dihydrogen phosphate of 0.01mol/L to oxalic acid electrolyte,inhibition efficiencies of the anodized samples are 10% and 47%,respectively.However,the concentrations of two inhibitors are both 0.01mol/L,inhibition efficiency can be as high as 92%.Thus,these results indicated that the significantly synergistic corrosion inhibition effect of molybdate and phosphate ions for anodic oxidation film formed on 2024 aluminum alloy.The mechanism may be as follows: Phosphate and molybdate ions can form the phosphomolybdate heteropoly acid,this strong oxidizing acid could modulate the formation rate of the oxide film and improve the structure of film.Therefore,the formation of phosphomolybdate heteropoly acid plays an important role in improving the corrosion resistance of 2024 aluminum alloy.