Abstract
Corrosion resistance of aluminum and its alloys is closely related with properties of their anodized oxide film. This paper reports the corrosion resistance of the film determined by measuring electrode potentials in anodizing, for Al and Al-Si alloys.
After anodizing, the specimen was dipped in HCl-NaOH solution and its electrode potential was measured by arranging the specimen in negative pole and saturated calomel electrode in positive pole.
(1) When anodizing time was constant, holding times of the 1st and 2nd electrode potentials were longer with the increase of current density. The same results were obtained for the constant values of the product of current density and anodizing time.
(2) The 1st and 2nd electrode potentials gave constant values, in spite of variations in current density and anodizing time, as follows.
Pure aluminum: 1st electrode potential about -0.55V
2nd electrode potential-1.45--1.55V
Al-Si alloys 1st electrode potential about-0.45--0.55V
2nd electrode potential -1.35V
(3) When the product of current density and anodizing time was a constant value of 30A. min./dm2, holding times of the 1st and 2nd electrode potentials abruptly decreased with the increase of silicon content. However, they gave constant values for the specimens containing Si of 2% (which is over solubility).
(4) Under the same conditions as above (30A, min./dm2), the two electrode potentials gradually showed noble values for pure aluminum and its alloys containing up to 2% of silicon. When silicon content was above 2%, they always showed constant values for any contents of silicon.
