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

Juan Alba-Galvín,Francisco Botana,José Sánchez-Amaya,Manuel Bethencourt,Leandro González-Rovira

doi:10.3390/met9030320

Abstract

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 aluminium-copper 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).

Highlights

Al–Cu alloys, commonly employed in the aerospace industry, offer excellent mechanical properties to weight ratios
The microstructure analysis performed by scanning electron microscopy (SEM)-energy-dispersive X-ray analysis (EDX) has revealed the presence of the main types of intermetallic particles of the alloy AA2024-T3, whose compositions are comparable with those described by Campestrini et al [28] for alloys of the same group
A standard surface pretreatment chain of aluminium alloys in the aerospace industry has been studied for the preparation of cerium conversion coatings on aluminium-copper alloy 2024-T3

Summary

Introduction

Al–Cu alloys, commonly employed in the aerospace industry, offer excellent mechanical properties to weight ratios. Despite their excellent mechanical properties, provided by the alloying elements, these alloys suffer from localised corrosion problems, especially in chloride-containing media. The intermetallic particles provide sites on the substrate surface where O2 reduction can take place, leading to corrosion such as pitting or filiform corrosion. The principal alloy phases in 2024 are. Al(Cu,Mg) and Al-Cu-Fe-Mn-Si [1,2,3,4,5,6,7], with different stoichiometry depending on the author consulted. Al–Cu alloys surfaces are often treated in order to reduce the corrosion extension.

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