Abstract
In the present investigation, an anodic oxide layer was developed on AA2024 specimens by sulfuric acid anodization process and subsequently sealed with chromate free Mn–Mo oxyanions (PMMO) by simple dip method. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy studies showed the presence of multiple oxidation states of Mn and Mo and the polymolybdate in the sealed anodic oxide layer respectively. Various electrochemical techniques were used to evaluate the corrosion performance of the developed coatings. Cyclic polarization test results showed a wide trans-passive potential region (> 2 V) for PMMO. Electrochemical noise measurement (ENM) results revealed pit growth of PMMO is highly controlled due to the presence of inhibitors in the sealed oxide layer. Scanning vibrating electrode technique (SVET) results revealed a minimal localized distribution of anodic and cathodic corrosion sites on PMMO compared to plain oxide layer (PO) and HTO after 168 h of immersion in 0.6 M NaCl solution. The results obtained from electrochemical impedance spectroscopy (EIS) showed one to two orders magnitude higher impedance (│Z│0.1 Hz) value for PMMO compared to PO and HTO. Neutral salt spray test result of PMMO after 1000 h showed comparable corrosion performance with conventional chromic acid anodized (CAA) specimen.
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