Corrosion behavior of rare earth metal (REM) conversion coatings on aluminum alloy LY12

The processes of the double layer rare earth metal (REM) conversion coating on aluminum alloy LY12 (2024) were introduced. The results of polarization tests showed that the corrosion resistance of the double layer REM conversion coating was superior to that of chromate conversion coating. The corrosion behavior of REM conversion coatings on LY12 alloy was studied with optical microscopy and energy dispersive spectroscopy (EDS). The results showed that in NaCl solution, pits initiated on the coatings formed on Cu-rich intermetalics. According to the equivalent circuit of pitting, a new method for analyzing the pit growth of REM conversion coatings was put forward by employing electrochemical impedance spectroscopy (EIS). Transmission line W in equivalent circuit could be neglected in short term exposure of REM conversion coatings to NaCl solution. This led to a formula for calculating the area fraction of pitting (F) on the surface of REM conversion coatings by simplifying the equivalent circuit.