Improving the properties of alumina films on carbon steel by optimizing the production of boehmite conversion coatings

In this work, multivariate analysis was applied to evaluate the influence of the deposition time (t), withdraw speed (v), and heating time (HT) used to produce boehmite conversion coatings and to optimize these conditions aiming the improvement of the final alumina-coated system's anti-corrosive properties. It was verified that v was the most influent factor on the studied variables, showing a positive effect in boehmite conversion coating thickness (BxSthickness) and a negative effect on the global corrosion resistance (Rg) and polarization resistance (Rp) of the whole coating systems. The highest anti-corrosive protection was obtained when conversion coating was produced using t = 284 s; v = 116 mm min−1 and HT = 221 min. The boehmite conversion coating prepared under this condition exhibited a homogeneous morphology, with sharp grains and no defects, high roughness (0.406 μm), and a complex mixture of aluminum and iron oxides in its composition, which may have contributed to the performance observed for the system in the saline medium. This combination also promoted the chemical stability of the whole system in the studied aggressive medium and increased the system's mechanical resistance.