In Situ Electrochemical Atomic Force Microscopy and Auger Electro Spectroscopy Study on the Passive Film Structure of 2024-T3 Aluminum Alloy Combined with a Density Functional Theory Calculation

The initial growth of the passive films on the Al2CuMg particle and Al matrix in AA2024-T3 are investigated by in situ electrochemical atomic force microscopy (ECAFM) and ex situ characterization techniques in citric acid. The passive films are mainly composed of Al2O3 (34%), Al(OH)(3) (8.4%), AlOOH (27.2%), CuO (19.4%), and Cu2O (11%). ECAFM images display that the film on the Al2CuMg particle is flawed and loose. Auger electro spectroscopy (AES) results show that the thickness of the passive film on the Al2CuMg particle is 1.4 nm, which is thinner than that on the Al matrix (2.4 nm). The combined ECAFM and AES results demonstrate that the corrosion resistance of the passive film on the Al2CuMg is weaker than that on the Al matrix. According to the density functional theory (DFT) calculations, the average charge transfer and adsorption energy of the adsorbates (O2-/OH-/H2O) on the Al surfaces are larger than that on the Al2CuMg surfaces, meaning that O-2 and H2O preferably dissociate and adsorb on the Al surfaces. Moreover, the electronic interactions between the adsorbates and Al surfaces are stronger. This is the reason that the growth rate of the passive film on the Al2CuMg particle is slower at initial stages.