Effect of Oxalic Acid Additives on Aluminum Anodizing in Formic Acid Containing Ammonium Heptamolybdate

Abstract

This paper reports a systematic study of the role of oxalic acid additives in aluminum anodizing in formic acid containing ammonium heptamolybdate. Adding oxalic acid in a concentration range of 5–20 mM to the 0.4 M formic acid solution containing 0.03 M ammonium heptamolybdate improves anodic film growth, increasing the film thickness and smoothing strongly wavy interface between the film and aluminum, and adding 100 mM of oxalic acid results in an almost complete block of the regular anodic film formation. In the case of aluminum anodizing in formic acid, the ammonium heptamolybdate additive prevents aluminum dissolution more effectively than only oxalic acid. The role of oxalic acid in this process is only to improve film growth and morphology. However, ammonium heptamolybdate improves film growth by increasing its thickness. Linear sweep voltammetry studies combined with SEM investigations of alumina growth show that in heptamolybdate-containing electrolytes, a thin porous alumina film is formed at the beginning of the process. Then, when the electrolyte oxidation potential is reached, the thin film on the surface breaks, resulting in a significant increase in the anodizing surface, and anodic oxide begins to grow rapidly.