This work is a part of the NEPAL project (NEw Protections for ALuminum) that aims to replace hexavalent chromium treatments largely used in aeronautic industry by new trivalent chromium conversion layer. We studied here the influence of major alloying elements of the alloy on the growth on the conversion layer and therefore its corrosion properties.
The study was carried out on an AA 2024 cold rolled 3 mm thin sheet. Three microstructures were studied: i) a T3 metallurgical state ii) a T3 state followed by a tempering at 190°C for 12 h to induce Cu precipitation (aged sample) and iii) a T3 state followed by a prolonged solution heat treatment at 494°C for 40 minutes to induce the formation of a Mg-depleted zone on the near surface of the alloy (solution heat treated sample). The microstructures were characterized by transmission electron microscopy (TEM).
The corrosion behavior of both uncoated and coated samples was evaluated by corrosion potential (Ecorr) measurements, plotting of polarization curves and electrochemical impedance spectroscopy (EIS) measurements performed at Ecorr for various immersion times. For a better understanding of the conversion layer coated alloy corrosion properties, surface analyses of the AA 2024 samples at each step of the conversion layer process were performed by combining X-ray photoelectron spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and cyclic voltammetry measurements.
Results showed the growth of a thick Mg-rich naturally formed oxide for the solution heat treated sample leading to a widening of the passivity plateau observed on the polarization curve for the uncoated sample compared to T3 uncoated sample. This was associated with the formation of a Mg-depleted zone on the near surface of the alloy; however, preferential dissolution of Mg occurred during the pre-treatment (degreasing and deoxidation) for both the T3 and solution heat treated samples so that the kinetics and mechanisms of the conversion layer growth and its corrosion properties were similar for both samples. For the aged alloy, TEM observations showed a dense precipitation of intergranular and intragranular Al2CuMg. These precipitates were assumed to contribute significantly to the Cu enrichment observed on the sample surface after degreasing and deoxidation leading to a Cu-rich conversion layer with a lower corrosion resistance compared to that formed on the T3 sample.