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

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).

Effect of surface preparation treatments on copper enrichment on 2024 aluminium alloy surface

“…This phenomenon already observed for the 2024-T3 alloy after 15 min treatment in 2.8 M HNO 3 bath [27] is known as dealloying. This was also evidenced for Al(Cu,Mg) phase in the case of AA-2024-T3 after oxidising acid pickling in Turgo Smut Go NC by Alba-Galvin et al [17]. This type of morphology results from the preferential dissolution of aluminium, and magnesium in the case of S-phase.…”

“…Surface preparation is known to influence the properties of the coatings [4,[12][13][14][15][16][17]. The pretreatment should allow to remove lubricant to clean the surface, standardize and control the chemical composition of the alloy surface to improve conversion coating performances.…”

Application of Commercial Surface Pretreatments on the Formation of Cerium Conversion Coating (CeCC) over High-Strength Aluminum Alloys 2024-T3 and 7075-T6

“…EDX analysis included in Table 2 was performed in the areas marked in each image (Supplementary Materials contain original EDX spectra in Figure S1). Sample 2-Ce-1 showed the typical microstructure of the 2024 alloy described in previous works [3,29], where the most common coarse IMPs were Al(Cu,Mg) and Al-Cu-Mn-Fe-(Si). SEM-EDX did not capture the presence of the peaks corresponding to Ce.…”

“…Therefore, the deposition, if any, is not detectable with this technique. Samples 2-Ce-5, 2-Ce-10, and 2-Ce-15 presented three types of morphologies associated with cerium layers: (a) deposits or copper islands typically formed on the intermetallic phases [24,29,32], (b) characteristic areas with cracked coating, and (c) uniform areas without cracking. However, there were still some areas in sample 2-Ce-5 where IMPs were appreciated.…”

“…Otherwise, the investments required to adapt the corrosion protective treatment must include the pretreatments as well. Thus, 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 CeCl 3 and H 2 O 2 . The standard pretreatment comprises the immersion of the parts in three consecutive chemical solutions: (a) Turco 6849: a silicatecontaining alkaline degreaser, (b) Turco 4215 NC-LT: a free of chromate and silicate alkaline etching, and (c) Turco Smut Go NC: an oxidizing acid pickling.…”