Generally, a barrier-type film of anodized aluminum is formed in baths of neutral salts (ammonium carbonate or ammonium borate), but a porous-type anodized film was formed in ammonium carbonate or ammonium borate baths through the addition of ammonium fluoride.
In this paper the effect of carbonic acid derivatives and acid amides on aluminum anodizing were investigated in ammonium carbonate (carbonate-fluoride) baths and ammonium borate (borate-fluoride) baths containing ammonium fluoride.
During the anodizing of aluminum in baths containing carbonic acid derivatives or acid amides, bath voltage decreased and uniform films were formed but nonuniform films were obtained in the baths without these additives.
Anodizing in a borate-fluoride bath containing ammonium carbamate ((NH4
F 0.2mol/L, NH2
0.1mol/L) for 30min at 20°C with a current density of 2A/dm2
yielded the thickest (about 16μm) and most uniform films. These films also had high grade hardness and corrosion resistance.
SEM observation of the film surface and cross-sections proved that the film formed in carbonate-fluoride baths and borate-fluoride baths containing carbonic acid derivatives and acid amides had larger pores (500∼700Å) and cells than films formed in sulfuric acid baths.
Cracks formed by partial dissolution of cells were observed on the upper parts of the films formed in carbonatefluoride baths containing formamide, acetamide, or urea.