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Age hardening and thermal treatment.
The age hardening takes a fundamental place in the alloy processing as it defines the microstructure and influence the mechanical and corrosion properties of the alloy. Of all the wrought alloys presented above, only the 2XXX, 6XXX and 7XXX series are heat treatable [5]. These alloys are designed by a letter T and a number which corresponds to specific heat treatment conditions. This designation follows the 4 number series attributed to each alloy. On the other hand, 1XXX, 3XXX, 4XXX and 5XXX are non-heat treatable and their mechanical properties are associated with strain hardening. They are usually alloyed with zinc, iron, chromium or magnesium whereas heat treatable alloys contain higher concentration of copper. Their mechanical properties increase with phase precipitation during the heat treatment. Generally, the age hardening is defined by three steps [6]:
– The solution treatment, generally at 460 – 565°C, where the soluble alloying elements are dissolved in the Aluminum solution.
– The quenching, where the solution is rapidly cooled -usually at room temperature- to obtain a supersaturated solid solution (SSSS) of the alloying element in the Aluminum matrix.
– An age hardening to form from the SSSS the fine precipitates in the Aluminum matrix. The aging parameters (time and temperature) will have an impact on the precipitates size and distribution. Usually the aging temperature is between 115-195°C.
The mechanical properties of the 2XXX series Al alloys are determined by the thermomechanical treatment. Usually, the alloying elements form clusters coherent with the matrix, called Guinier-Preston (GP) zones. These zones are ordered, and they are only one or two atoms planes in thickness. As they grow with temperature and time in the (Al) solid solution phase, they become incoherent with the lattice. In the case of the Al-Cu phase diagram, the formation of the phase (Al2Cu) follows this precipitation sequence:
→→ ′′ → ′ → ( 2 ) (3).
Aluminum 2024-T3: microstructure
In the past few years, alloy development has been continuously progressing to meet the requirements of the aircraft industry. Al-Cu alloys also referred as the 2000 series aluminum alloys are widely used as they provide an excellent strength to weight ratio, high damage tolerance and fatigue resistance. In the case of the AA2024-T3, an aluminum-copper-magnesium alloy, the addition of Cu and Mg considerably increases the mechanical properties. The enhanced mechanical properties are the consequence of the alloying elements addition with the effect of a heat treatment which allows the control of the alloy features without affecting its weight. Table 2 gives the general composition of the AA2024-T3 used for this study.
Alloy processing and effect of rolling on the surface microstructure
During alloy processing, the final step consists in a mechanical shaping of the alloy to achieve a semi-fabricated form [6]. The ingot passes through two sets of steel or copper rolls that rotate in opposite directions. As a consequence, the slab undergoes a severe deformation reducing its thickness to usually 16-20 mm and up to 1800 mm in width [6,11]. This process, causes a considerable modification to surface and generates a new surface layer, induced by the breakdown of the intermetallic particles, their coverage by an aluminum oxide layer and their redistribution on the surface. This phenomenon has been mentioned in the 1980s and is increasingly studied in the context of corrosion susceptibility and surface treatment of Aluminum alloys. The rolling process will create a new surface layer approximately 3 to 5 µm thick, characterized by a high porosity, fine grain structure [12] and incorporated oxides including -Al2O3 and MgO [13] (Fig. 3). The mechanism of the surface modification reported by Fishkis et al. [13] involves three steps:
– Formations of surface depressions by plowing, adhesive wear, delamination wear or transverse surface cracking.
– Filling of the cavities with wear debris which include oxide, metal and lubricants.
– Coverage of the cavities with a thin metal layer during the rolling process creating a “shingled” surface appearance.
The corrosion behavior of AA2024-T3
The electrochemical reactivity of AA2024-T3 is controlled by the highly heterogeneous microstructure, and more precisely by the second phase particles as they exhibit a different electrochemical behavior regarding the aluminum matrix [18–24]. It has been shown that Cu as well as the impurities are the main cause of corrosion failure as they create local sites where a micro-galvanic coupling between the particle and the matrix occurs [8,9,25–28]. Over the years, the reactivity of AA2024-T3 has been extensively studied to understand the mechanisms and determine the factors involved during the corrosion process. In this chapter, only the most common corrosion mechanisms along with the contribution of different intermetallic particles will be presented.
The pitting corrosion of Aluminum
Pitting is considered to be the most common mechanism of corrosion propagation for pure aluminum and high strength Al-alloys. The different steps involved during the pitting process were extensively studied and reported in the literature [29–34] (Fig. 4.): a) the film breakdown b) metastable pitting c) pit growth and d) pit stifling or death. The pit initiation – propagation is also facilitated by the presence of halides such as chloride ions in solution. These ions are very-well known to be aggressive to aluminum oxide as they will adsorb at the surface of the oxide film. Then adsorption of Cl- on the oxide occurs, preferentially localized on the irregularities of the film.
Secondly, the Cl- ions migrate through the film, creating a defect which exposes the bare metal to the solution. This will induce a pit propagation through the metal, nevertheless, a majority of the pits will not propagate and rapidly passivate. Those pits which initiate and grow only for a limited period before being passivated are called metastable pits. Indeed, the growth and propagation of pit requires very specific conditions such as local acidification and a high chloride concentration which is not always the case. At the bottom, the pit initiation is determined by the Aluminum oxidation and water hydrolysis:
→ 3+ + 3 − (5).
3+ + → ( )2+ + + (6).
followed by chloride hydrolysis:
( )2+ + − → ( ) + (7).
( ) + +→ ( ) + + (8).
The effect of the S-phase particle (Al2CuMg)
The S phase is a particle specific to Al-Cu-Mg containing alloy and they represent approximately 60 % of the intermetallic surface area [46]. This particle exhibits a significantly less noble potential regarding the aluminum matrix and undergoes active dissolution [23,24,46]. Most of the time, pits initiate at the periphery and on the particle. Many tried to understand the corrosion mechanisms involved with the S-phase and its effect on the corrosion of surface finishing. Several theories were discussed in the literature, for example, Buchheit suggested a corrosion process through dealloying (Fig. 6). First, Buchheit, Guillaumin and Mankowski detailed that the magnesium contained in the particle will dissolve, leaving a typical sponge-like shape on the surface [8,47]. The Cu left on the surface will lead to an increase of the particle potential. This Cu rich sponge will then act as a cathode and generate the dissolution of the surrounding matrix showed by characteristic trenches around the particles. Contrastingly, other studies observed the presence of Copper deposits at the periphery of the particles and presented a different mechanism. Buchheit et al. suggested that during the dealloying of the S-phase particle, some Cu-rich clusters will be transported in the solution through a non-faradaic process [47– 50]. The metallic copper, which is no longer in electrical contact with the matrix, will be oxidized by the oxygen present in solution and precipitate as copper oxide. In summary, the major difference between these two theories is that one dissolution mechanism is only driven by a galvanic coupling switch between the particle and the matrix, and the second however, involves a particle detachment/redeposition process. The role and the importance of the S-phase in the Cu enrichment and redistribution process has been also highlighted by the work of Vukmirovic et al. [51], who demonstrated a significant increase of Cu by comparing a commercial AA2024 and a synthetic AA2024 after 0.5 M NaCl exposure.
The effect of the phase (Al2Cu)
The corrosion process involved with the Al2Cu has also been extensively studied, mainly in chloride containing environment, and is determined to be driven by a dealloying process. It has been found that this particle acts as a local cathode as its corrosion potential is more noble than the aluminum matrix (corrosion potential). Lebouil et al. demonstrated by AESEC the Cu build-up on the surface of the particle during anodic polarization [53]. Nevertheless, the current densities supported by the Al-Cu intermetallic particles were shown by Lacroix et al. to be 10 times lower than the Al-Cu-Mg intermetallic particles [28]. Moreover, Buchheit et al. also noted the generation of copper ions when the particle is cathodically polarized demonstrating its major role during the corrosion process of Al alloys [47].
The effect of Al7Cu2Fe and Al-Cu-Fe-Mn particles
The Al-Cu-Fe-Mn containing phases represent roughly 40 % of the constituent particles found in the AA2024-T3 alloy. The effect of Fe-containing intermetallic particles such as Al7Cu2Fe on the corrosion behavior of Al alloys was for a long time not well documented. Fe usually comes from impurities and form the coarse precipitates known also as constituent particles which usually act as cathodic site [23,24,54]. The particles are irregularly shaped, sometimes break into pieces during the rolling process and the particles parts align along the rolling direction. Although the presence of this particle has been noted in AA2024, the electrochemical behavior and the role of Al7Cu2Fe have only been investigated recently by Ilebevare et al. [55] and further by Birbilis et al. [56]. For example, Birbilis showed that Al7Cu2Fe supports ORR (oxygen reduction reaction) at high current densities over a different range of Cl- concentration and pH (between 20 µA cm-2 and 2 mA cm-2) and the rates were sometimes about 3
Table of contents :
ACKNOWLEDGMENTS
LIST OF ACRONYMS & SYMBOLS
LIST OF FIGURES & TABLES
CHAPTER I: INTRODUCTION & STATE OF THE ART
1. INTRODUCTION
2. STATE OF THE ART
2.1. Generalities on Aluminum
2.2. Age hardening and thermal treatment.
2.3. Aluminum 2024-T3: microstructure
2.4. Alloy processing and effect of rolling on the surface microstructure
2.5. The corrosion behavior of AA2024-T3
2.5.1. The pitting corrosion of Aluminum
2.5.2. Intergranular corrosion
2.5.3. The effect of the S-phase particle (Al2CuMg)
2.5.4. The effect of the 𝜽 phase (Al2Cu)
2.5.5. The effect of Al7Cu2Fe and Al-Cu-Fe-Mn particles
2.6. Aluminum-Lithium alloys: microstructure
2.7. The corrosion behavior of AA2050-T3
2.7.1. Intergranular corrosion (IGC)
2.7.2. Stress corrosion cracking (SCC)
2.7.3. The effect of age hardening on the corrosion properties
3. THE SURFACE TREATMENT OF AL-ALLOYS
3.1. Solvent cleaning
3.2. Alkaline cleaning
3.3. Acid deoxidizer (acid pickling)
4. MOTIVATION AND OBJECTIVES OF THE THESIS
CHAPTER II: MATERIALS & METHODS
1. INTRODUCTION
2. MATERIALS & METHODS
2.1. Part A: The flow cell and electrolyte transportation.
2.1.1. The electrochemical flow cell
2.1.2. Flow injection valve system
2.2. Part B: The inductively coupled plasma atomic emission spectrometer (ICP-AES) .
2.2.1. Electrolyte introduction system
2.2.2. Internal standard and second peristaltic pump
2.2.3. Plasma: excitation source of the ICP-AES
2.2.4. Dispersive system
2.3. Part C: Element quantification and AESEC data treatment
2.3.1. Concentration, flow rate and convolution.
2.3.2. Hydrodynamics
2.4. Sample preparation
2.5. Electrochemical characterization
2.5.1. Potentiodynamic polarization curves
2.6. Surface ex-situ characterization techniques
2.6.1. Scanning electron microscopy (SEM)
2.6.2. Focused Ion Beam (FIB)
2.6.3. Glow discharge optical emission spectrometer (GDOES)
2.6.4. Profilometry
2.6.5. X-ray photoelectron spectroscopy (XPS)
2.6.6. Vibrational spectroscopy
2.6.7. X-ray diffraction (XRD)
CHAPTER III: IN SITU MONITORING OF ALLOY DISSOLUTION AND RESIDUAL FILM FORMATION DURING THE PRETREATMENT OF AL-ALLOY 2024-T3.
1. INTRODUCTION
2. EXPERIMENTAL
3. RESULTS AND DISCUSSION
3.1. In situ measurement of AA2024 pretreatment kinetics
3.2. Microstructural analysis of pretreated surfaces
3.3. Kinetics of Cu rich Particle Release in NaOH
3.4. Dissolution and passivation in HNO3
3.5. Polarization Behavior prior to, and following, pretreatment
4. CONCLUSIONS
5. APPENDIX
5.1. Hydrodynamics
5.2. Surface topography and etching rate
CHAPTER IV: ON-LINE REACTIVITY MEASUREMENT OF AL-LI ALLOY AA2050-T3 DURING A SURFACE PRETREATMENT SEQUENCE USING AESEC
1. INTRODUCTION
2. EXPERIMENTAL
3. RESULTS
3.1. Dissolution profile AA2050-T3 under pretreatment sequence
3.2. Reactivity of AA2050-T3 under HNO3 exposure
3.3. Microstructural analysis of AA2050-T3 before and after pretreatment
3.4. Particle detection under NaOH exposure
3.5. AESEC polarization curves prior & after pretreatment
3.6. GDOES profiles of the surface after pretreatment and polarization curves
3.7. Potentiodynamic polarization curve of AA2024-T3 in 0.5 M NaCl with the addition of 1 ppm of Li
4. DISCUSSION
5. CONCLUSIONS
CHAPTER V: CHARACTERIZATION OF AN AL-BASED CORROSION PRODUCT AFTER THE ANODIC POLARIZATION OF AN AL-LI ALLOY
1. INTRODUCTION
2. EXPERIMENTAL
3. RESULTS
3.1. GDOES analysis of the corroded surface
3.2. X-ray diffraction analysis of AA2050 prior and after corrosion testing
3.3. Identification of amorphous corrosion products by Raman spectroscopy
3.4. Complementary analysis of the corrosion product by Infrared spectroscopy
4. DISCUSSION
5. CONCLUSIONS
PRELIMINARY STUDIES & CONCLUSIONS
1. PRELIMINARY STUDIES
1.1. The pretreatment of intermetallic particles: the reactivity of S phase.
1.2. The statistical analysis of particle detachment: establishment of relationships between elements, signal intensities and particle nature.
2. CONCLUSIONS
2.1. General conclusions
2.2. Perspectives
REFERENCES
