Influence of Aerospace Standard Surface Pretreatment on the Intermetallic Phases and CeCC of 2024-T3 Al-Cu Alloy

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2019-03Department
Ciencia de los Materiales e Ingeniería Metalúrgica y Química InorgánicaSource
Metals 2019, 9, 320Abstract
A standard three-step surface pretreatment employed in the aerospace sector for Al alloys
have been investigated prior to the generation of cerium conversion coatings (CeCC) on aluminiumcopper
alloy 2024. Two pretreatments were analysed, one without final acid etching (Pretreatment
1) and another with this step (Pretreatment 2). Both pretreatments affect the alloy intermetallic
phases, playing a key role in the development of the CeCC, and also in the susceptibility to localised
corrosion in NaCl medium. Scanning electron microscopy coupled with energy-dispersive X-ray
analysis (SEM-EDX) revealed that after Pretreatment 2, Al(Cu,Mg) phases were partially or totally
removed through dealloying with their subsequent copper enrichment. Conversely, none of these
intermetallic phases were affected when the final acid step was not employed (Pretreatment 1).
Meanwhile, Al-Cu-Fe-Mn-(Si) phases, the other major Al–Cu alloys intermetallics, suffers minor
changes through the whole pretreatments chain. The protective efficiency of CeCC was evaluated
using electrochemical techniques based on linear polarisation (LP) and electrochemical impedance
spectroscopy (EIS). Samples with CeCC deposited after the Pretreatment 1 gave higher polarisation
resistance and impedance module than CeCC deposited after Pretreatment 2. SEM-EDX and X-ray
photoelectron spectroscopy analysis (XPS) indicate that the main factors explaining the corrosion
resistance of the coatings is the existence of Al(Cu,Mg) intermetallics in the surface of the alloy,
which promote the deposition of a cerium-based coating rich in Ce4+ compounds. These Al(Cu,Mg)
intermetallics were kept in the 2024 alloy when acid etching was not employed (Pretreatment 1).
Subjects
aluminium; alloy phases; corrosion; surface pretreatments; CeCCCollections
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