Abstract
The selection of appropriate surface pretreatments is one of the pending issues for the industrial application of cerium-based chemical conversion coatings (CeCC) as an alternative for toxic chromate conversion coating (CrCC). A two-step surface pretreatment based on commercial products has been successfully used here to obtain CeCC on AA2024-T3 and AA7075-T6. Specimens processed for 1 to 15 min in solutions containing CeCl3 and H2O2 have been studied by scanning electron microscopy coupled with energy-dispersive X-ray analysis (SEM-EDX), glow discharge optical emission spectroscopy (GDOES), potentiodynamic linear polarization (LP), electrochemical impedance spectroscopy (EIS), and neutral salt spray (NSS) tests. SEM-EDX showed that CeCC was firstly observed as deposits, followed by a general coverage of the surface with the formation of cracks where the coating was getting thicker. GDOES confirmed an increase of the CeCC thickness as the deposition proceed, the formation of CeCC over 7075 being faster than over 2024. There was a Ce-rich layer in both alloys and an aluminum oxide/hydroxide layer on 7075 between the upper Ce-rich layer and the aluminum matrix. According to LP and EIS, CeCC in all samples offered cathodic protection and comparable degradation in chloride-containing media. Finally, the NSS test corroborated the anti-corrosion properties of the CeCC obtained after the commercial pretreatments employed.
Highlights
New steps were taken in the implementation of chemical conversion coatings (CeCC) as alternatives to
chromate conversion coating (CrCC), showing that commercial surface pretreatments used in the aeronautical industry allow CeCC to be deposited in aluminum alloys 2024 and 7075
The CeCC proved to be protective in NaCl solutions and in the neutral salt spray (NSS) test
Summary
Chromates are used as inhibitors in closed systems and paints, in Cr-based chemical conversion coatings (CrCC), and in anodizing and sealing baths [9,10,11] Among these applications, CrCC is especially successful since chromium species are present in two oxidation states: Cr(III) and. In a previous paper [29], the authors investigated the effect of a standard surface pretreatment for aluminum alloys in the aeronautical industry on the 2024-T3 surface and its subsequent influence in the formation of CeCC in solutions of CeCl3 and H2 O2. Potentiodynamic linear polarization (LP), electrochemical impedance spectroscopy (EIS), and neutral salt spray (NSS) tests were employed to analyze the corrosion properties
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