Investigations were conducted to determine optimum bath compositions for the anodizing of aluminum, and the properties of the coatings obtained were investigated using organic (choline-fluoride base) alkaline baths containing ammonium fluoride and organic acid salts.
In the anodizing of aluminum in baths containing organic acid salts ((NH4
) uniform films were obtained, but in baths without these additives and with added ammonium acetate, non-uniform films were formed. The thickest films (about 15μm) were formed by anodizing for 30min at 20°C at a current density of 2A/dm2
in 0.2mol/L cholins solution containing 0.2mol/L ammonium fluoride and 0.1mol/L ammonium tartrate or in 0.2mol/L choline solution containing 0.1mol/L ammonium fluoride and 0.2mol/L ammonium citrate.
The film formed in 0.2mol/L choline solution containing 0.3mol/L ammonium fluoride and 0.1mol/L ammonium oxalate showed a high hardness (about 28 in Marten's scratch hardness test, load 50gf) and high corrosion resistance. It was found by SEM observation that pores of films prepared in baths containing ammonium citrate were about 50∼100nm in diameter, while those prepared in baths containing ammonium oxalate measured only about 40nm.