Corrosion protection of the model Al-Cu alloy by ALD alumina thin film coatings

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Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)

Time-of-Flight secondary ion mass spectrometry (ToF-SIMS) is a very sensitive surface analytical technique, well established for many industrial and research applications. ToF-SIMS is an acronym for the combination of the analytical technique SIMS (Secondary Ion Mass Spectrometry) with Time-of-Flight mass analysis (ToF). The technique provides detailed elemental and molecular information about the surface, thin layers, interfaces of the sample, and gives a full three-dimensional analysis. The use is widespread, including semiconductors, polymers, paint, coatings, glass, paper, metals, ceramics, biomaterials, pharmaceuticals and organic tissue. The average depth of analysis for a ToF-SIMS measurement is approximately 1 nm11,12.

Scanning Electron Microscopy (SEM) and Energy-dispersive X-Ray

spectroscopy (EDX) Scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM/EDX), the best known and most widely-used of the surface analytical techniques, was used as additional techniques to visualize the surface of some of the coated samples. SEM uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. The signals that derive from electron-sample interactions reveal information about the sample including external morphology (texture), chemical composition, and crystalline structure and orientation of materials making up the sample. In most applications, data are collected over a selected area of the surface of the sample, and a 2-dimensional image is generated that displays spatial variations in these properties. Areas ranging from approximately 1 cm to 5 microns in width can be imaged in a scanning mode using conventional SEM techniques (magnification ranging from 20X to approximately 30,000X, spatial resolution of 50 to 100 nm). The SEM is also capable of performing analyses of selected point locations on the sample. The EDX technique detects x-rays emitted from the sample during bombardment by an electron beam to characterize the elemental composition of the analyzed volume. Features or phases as small as 1 μm or less can be analyzed20-22.
Secondary electron images were taken at LISE (Laboratoire Interfaces et Systèmes Electrochimiques) in UPMC with a digital SEM S440 LEICA with a tungsten filament (FEG-SEM) as shown in the Fig. 2-15.

The annealing and aging thermal treatments

The Al-Cu polycrystalline alloy samples were thermally treated in two different ways:
1) the first thermal treatment was to oxidize the as prepared Al-Cu alloy samples in situ XPS at low pressure by exposure to O2 at 1.0×10-7 mbar at different temperatures (300 °C, 350 °C, and 400 °C) for 54 h totally, which was described detailed in the chapter 3.
2) the second thermal treatment was to anneal and age the as prepared Al-Cu alloy samples in the system shown in Fig. 2-17. After cleaning, the sample was placed inside a quartz tube, and pumped to vacuum (of around 1.0×10-5 mbar). Prior to the formal thermal treatment, the optimization of the annealing temperature among 520 °C, 540 °C, and 560 °C was performed on the Al-Cu samples, respectively. Afterwards, during the formal annealing treatment, the quartz tube with the sample inside was placed in the oven and the pressure of O2 was kept below 1.0×10-5 mbar at the optimized temperature for 1 hour. When the annealing was finished, the oven was moved away and the tube was quenched by ice water to room temperature rapidly. After a quick polishing (#1200→#2400→#4000→3μm→1μm, to obtain a mirror-like surface without modifying the surface significantly), the sample was placed inside the quartz tube again for aging treatment in air at 300 °C for 10 h followed by 450 °C for 5 minutes. When the aging treatment was finished, the sample was cooled down in the quartz tube and then transported directly to the XPS preparation and analysis chambers for the following measurements.

Chemical surface modifications induced by thermal treatments

The very low sputtering applied to the Al-Cu model surface, as specified in the experimental part, removed principally the carbonaceous contaminations decreasing by about 79.7 at.% on the sputtered surface compared with the polished surface.
Fig.3-2 shows the changes in the O1s core-level peak as a function of thermal treatment of the Al-Cu surface, starting from 0 Langmuir corresponding to the sputtered surface. For the sputtered surface before thermal treatment (0 L), the main component at lower binding energy of 530.4 ± 0.1 eV (BE) is attributed to Cu oxide, at higher BE of 531.4 ± 0.1 eV (BE) is attributed to Al oxide, and the peak at highest BE of 532.3 ± 0.1 eV to the hydroxyl groups (like AlOOH or Al(OH)3 species) and the contaminants (carboxyls and/or carbonates). After the low-pressure O2 exposure at 300°C for 6 hours (1620 L) the contributions of hydroxyl decreased significantly, what can be concluded from the more symmetrical shape of O1s. After 24-hour exposure to O2 (6481 L), the contributions from the hydroxyls groups is not observed anymore in the O1s region.
The change in the total area of O1s peak as a function of the exposure to LP O2 is presented in Fig. 3-3. The data (the binding energies (BE), the full-widths at half maximum (FWHM)) of the high resolution main O1s peak core level spectra are compiled in Tab.3-2. The significant increase of O1s peak area can be observed when the sample was exposed to O2 in the initial stage (about the first 3200 L) at 300 °C, which can be attributed to the growth of the Al oxide. However, the area of O1s peak decreased as the exposure to O2 at 300 °C went on (after about 3200 L), probably related to the surface dehydration and/or dehydroxylation, which has been reported by Papee et al.60 that heating of Al substrate covered by oxide/hydroxide layer (under vacuum at T ≥170 °C) leads to dehydration.

Corrosion performance of the polished and thermally treated Al-Cu alloys – immersion tests

Prior to this study, series of immersion tests (with different times of immersion) were carried out for measurement optimization in the neutral and alkaline electrolytes. The goal of these tests was to be able to observe the first signs of corrosion initiation and to control the surface state by surface sensitive techniques such as XPS and ToF- SIMS. Finally, the 7-minute-immersion test in near neutral, chloride electrolyte resulting in moderate corrosion signs favorable to analysis by surface sensitive techniques was chosen. In the case of alkaline electrolyte (pH=11.5), due to general studies by means of XPS and ToF-SIMS corrosion of the Al matrix and lack of visible corrosion spots (pitting corrosion), the immersion time for thermally treated sample was 7 minutes and for polished sample was prolonged from 7 minutes to (7+30) minutes.
The OCP measured for the thermally treated sample is only 30 mV higher than for the pristine, polished Al-Cu alloy sample in near neutral 0.01 M NaCl+0.3% vol H2O2 (pH≈6.2) electrolyte (Fig. 4-5). This small difference in the OCP between both samples indicates that the thermal treatments resulting in enlargement of the size of Cu-rich intermetallic particles has no marked influence on the electrochemical performances of the Al-Cu alloys. It should be noted that the alloy surface area exposed to the electrolyte is exactly the same and there is no significant difference in the thickness of the oxide layers present on both types of samples and thus changes in the OCP values are not expected. A small OCP decrease during the 7-minute- immersion test for both samples indicates minor surface modifications. The surface chemical and morphological modifications induced by immersion tests are discussed in the following parts.

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Surface chemical modifications of the polished and thermally treated Al-Cu alloys after immersion tests

The first series of figures (Fig. 4-6(a) and (b)) present the Al2s, Cu2p O1s and Cu Auger XPS spectra obtained after the immersions of the polished (non-treated) and thermally treated samples in neutral electrolytes. The XPS data are also compiled in Tab.4-2. Due to the strong interference of Al2p and Cu3p (already discussed in studies by means of XPS and ToF-SIMS chapter 3), the Al2s core level was chosen instead of the Al2p for analysis of the Al-species. The Al2s core level for the polished sample can be fitted with two peaks: Al2sA (at 118.8 ± 0.1 eV, assigned to Al(OH)3) and Al2sB (at 120.0 ± 0.1 eV, assigned to Al2O3). The Al2O3 film on aluminum alloy surface is prone to attract the hydrophilic groups in the solutions, which may promote the surface passivation by the formation of Al(OH)3 or AlOOH76,77. Furthermore, it should be noted that an extra peak can be observed at around 117.0 ± 0.1 eV. This peak is not a metallic aluminium and it can be attributed to the strong charging effect related to formation of thick layer of corrosion products characterized by insulating properties. The charging effect can be also easily verified in the area of C1s core level (not shown here) where characteristic multiple carbon peak was observed.
The Cu2p3/2 core level region is fitted with two peaks: Cu2p3/2A at 932.1 ± 0.1 eV, assigned to Cu2O78 and Cu2p3/2B at 934.0 ± 0.1 eV, assigned to CuO79,80. In addition, the presence of CuO can be confirmed by the characteristic shake-up satellite at 943.0 ± 0.1 eV81,82. The XPS analysis indicates that after corrosion the copper exists in a mixed oxidized form (CuO and Cu2O), with a ratio of 43:57, respectively. The area of satellite peak corresponding to CuO was included into the calculation of the different copper oxides ratio. The Cu Auger signal is very low and too broad owing to the charging effect, hence, does not allow for a qualitative characterization. The higher intensity of Cu2p peak compared with that before immersion tests can be explained by the preferential dissolution of Al through the pitting corrosion mechanisms occurring around Cu-rich particles, resulting from the well-known electrochemical coupling of the cathodic particles and the anodic Al substrate83-85.
In the O1s core level region, the peaks at 530.0, 531.2, 532.4 and 533.5 eV could be respectively ascribed to Cu oxides, Al oxides, hydroxyl groups or contaminants, and water molecules absorbed on the surface, respectively86-89. These XPS results indicate the formation of corrosion products mainly consisting of Al oxide and Al hydroxide rich in Cu oxides on the surface of non-treated sample after immersion in the neutral electrolyte.

Table of contents :

Abstract
Chapter 1 State-of-the-art and objectives
1.1 Aluminium and aluminium alloys
1.2 Thermal treatments of Al-Cu series alloys
1.3 Corrosion of Al-Cu alloys
1.3.1 Introduction to corrosion of aluminium alloys
1.3.2 Corrosion behavior of aluminium alloys and influence of intermetallic particles
1.4 Corrosion protection of aluminium alloys
1.4.1 Generalities about corrosion protection of aluminium alloys
1.4.2 Conversion coatings
1.4.3 Anodizing
1.4.4 Organic coatings
1.4.5 Coatings prepared by Atomic Layer Deposition
1.5 Objectives of this thesis
1.6 Contents of the thesis
Chapter 2 Techniques and sample preparations
2.1 X-ray Photoelectron Spectroscopy (XPS)
2.1.1 Principle
2.1.2 Instrument
2.1.3 Characterization
2.2 Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)
2.2.1 Principle
2.2.2 Instrument
2.2.3 Characterization
2.3 Scanning Electron Microscopy (SEM) and Energy-dispersive X-Ray Spectroscopy (EDX)
2.4 Sample preparations
2.4.1 Mechanical polishing
2.4.2 The annealing and aging thermal treatments
Chapter 3 Influence of thermal aging treatments on surface chemical modifications of model Al-Cu alloy studied by in situ XPS and ToF-SIMS
3.1 Introduction
3.2 Experimental
3.3 XPS results
3.4 ToF-SIMS results
3.5 Conclusions
Chapter 4 Corrosion performances of model Al-Cu alloy after thermal treatments (annealing and aging) – surface studies by means of XPS and ToF- SIMS
4.1 Introduction
4.2 Experimental
4.3 Results and discussions
4.3.1 Surface characterization of the polished and aged Al-Cu alloy before corrosion
4.3.2 Corrosion performance of the polished and thermally treated Al-Cu alloys – immersion tests
4.3.3 Surface chemical modifications of the polished and thermally treated Al-Cu alloys after immersion tests
4.4 Conclusions
Chapter 5 Corrosion protection of the model Al-Cu alloy by ALD alumina thin film coatings
5.1 Introduction
5.2 Experimental
5.3 Results and discussions
5.3.1 Surface characterization of the pristine and alumina-coated Al-Cu alloy samples
5.3.2 Corrosion resistance of the alumina ALD coated Al-Cu alloys – immersion tests
5.3.3 Surface characterization of non-coated and coated Al-Cu alloys after immersion tests
5.4 Conclusions
Chapter 6 Conclusions and perspectives
Abbreviations and acronyms
Acknowledgements

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