The effect of different carboxylic acids on the sulfuric acid anodizing of AA2024

Porous anodic oxides on AA2024-T3, grown in Cr(VI)-free mixtures of H2SO4 with carboxylic acids (malic, tartaric, oxalic, malonic, citric) under identical experimental conditions were comparatively studied. The corrosion resistance determined by pit initiation times in 0. 1 M NaCl is improved by the addition of malic, tartaric, and citric acid and impaired by malonic and oxalic acid. It is shown that the current efficiency of oxide formation can be obtained from the partial derivative E-cell/partial derivative t value verified during barrier layer formation. The ranking of the corrosion performance can be deduced from electrochemical parameters of anodic oxide formation, morphology, structure, and composition of the oxide films. Beneficial for the corrosion resistance are higher voltage slopes partial derivative E-cell/partial derivative t during barrier layer formation, which also indicates higher formation efficiency, as well as smaller flaws, denser oxides, lower OH- and S-content, as well as higher Cu content of the anodic oxide.