Surface Studies on Industrial Aluminium Alloys

Aluminium alloys are used in a wide range of applications due to their high tensile strength concomitant with low density. Additionally, aluminium alloys form a naturally occurring and passivating oxide layer, which leads to high corrosion and weathering resistance. In industrial manufacturing, aluminium alloys acquire their desired properties through specially designed heating processes. A common method to join aluminium work pieces together is brazing. The work pieces designed for brazing applications are coated with an aluminium alloy, which has a lower melting point as the base material. During brazing the top alloy is molten and the covering oxide layer needs to be broken up to achieve a strong and durable connection between the work pieces. This thesis present how temperature treatment affectss the surface layer of two different aluminium alloys. The processes taking place during heat treatment were studied by a combination of microscopy and spectroscopy techniques. Both alloys were heated in an UHV chamber and characterized after subsequent heating by using different operational modes of the SPELEEM (Spectroscopic PhotoEmission and Low Energy Electron Microscope). The SPELEEM is situated at beamline I311 at the MAX II storage ring at the national Swedish synchrotron radiation facility, MAX IV Laboratory. The operation modes of the SPELEEM that were used for this study are MEM (Mirror Electron Microscopy), XPEEM (X-ray PhotoEmission Electron Microscopy) and XPS (X-ray Photoelectron Spectroscopy). In addition, the samples were examined by SEM (Scanning Electron Microscopy) before and after the heat treatment. Using the above described combination of different surface science techniques, the changes occurring upon heat treatment at the aluminium-magnesium-silicon alloy 6063 surface were studied at room temperature and after heating to 300°C and 400°C. The aluminium alloy used in brazing applications was studied at higher temperatures up to 500°C to follow the decomposition of the aluminium oxide layer. Using the previously mentioned techniques allows to follow the diffusion, reactions and sublimation of the different elements

Industrial Alloys Studied by Surface Sensitive Techniques

This thesis reports on surface studies of industrial materials whose importance for society manifests in the vast range of applications. In industrial materials alloying is performed in order to improve the parent material's physical and mechanical properties such as strength, corrosion and wear resistance, as well as high temperature performance in comparison to the pure metal.Even though metals like copper, tin, and zinc have been alloyed since approximately 2500 BCE, current research projects still try to unravel the complex interactions between the single alloying elements on an atomic scale. This thesis focuses on two groups of metallic alloys: aluminum alloys and steels. For both alloy groups, the properties of their protective surface oxides are of major importance as they determine the material's performance with respect to corrosion, erosion, wear, joining, and coating. Therefore, a surface science approach was employed to study the chemical composition, thickness and distribution of phases and particles at the oxide surfaces. Contemporary research in this area is characterized by attempts to bridge the gap between classical model systems in highly controlled environments and industrial complex alloys in experimental conditions mimicking their working environment.Here, the material gap is bridged by transitioning from single crystals to industrial alloy standards, see Papers I and IV. Custom made composite aluminum alloys made for brazing applications, Papers II and III, and multiphase steel, Paper V, are investigated in this thesis. Simultaneously to the material gap, the pressure gap is addressed by exposing the materials to more realistic conditions using ambient pressure X-ray photoelectron spectroscopy, see Papers III and IV. This technique allows for measurements while the sample is exposed to different gases up to the lower mbar regime. By comparing X-ray photoelectron spectroscopy with standard UHV measurements major differences in the resulting surface oxide composition are observed. The thickness of the native oxide films of the different samples is determined by X-ray reflectivity measurements performed at different experimental conditions ranging from UHV to air and water environments, Papers I and IV. X-ray photoelectron emission microscopy and low-energy electron microscopy was used to follow the surface development during heating of aluminum and steel samples on a sub-micron length scale, Papers II, IV, and V. The microscopic imaging allows for the identification of the chemical state of the alloying elements and their lateral distribution in the surface layer

Anodization of Al(100), Al(111) and Al Alloy 6063 studied in situ with X-ray reflectivity and electrochemical impedance spectroscopy

We present results from the anodization of single crystal Al(100) and Al(111) surfaces and the aluminum alloy AA 6063 studied in situ by X-ray reflectivity and electrochemical impedance spectroscopy. We observe that the anodic oxide layer grows linearly with the anodization potential and that the thicknesses are similar for all samples. However, the thicknesses obtained from X-ray reflectivity are higher than that obtained from electrochemical impedance spectroscopy. We attribute the higher thicknesses to an outer porous oxide layer, which is not detected by electrochemical impedance spectroscopy. Both, electrochemical impedance spectroscopy and X-ray reflectivity suggests that a more heterogeneous and rough oxide is formed on AA 6063 due to the influence of the alloying elements and intermetallic particles during the growth

Integration of electrochemical and synchrotron-based X-ray techniques for in-situ investigation of aluminum anodization

Anodization of aluminum alloys AA 6082 and AA 7075 was investigated in-situ with integrated electrochemical and synchrotron-based X-ray reflectivity (XRR) methods providing complementary information about the anodic processes taking place on the alloys. The stepwise potentiostatic polarization measurements reveal dynamic processes of the anodic oxide formation and dissolution, and the following electrochemical impedance spectroscopy measurements detect the break of the native oxide and the growth of typical two-layer anodic oxide film, while the XRR measurements show the growth of entire anodic oxide film whose thickness increases linearly with the increasing applied potential. The results indicate that while a stable anodic oxide can be formed on the both alloys with a similar growth factor, AA 7075 shows a thinner thickness of the barrier layer and a lower resistance of the oxide film. The electrochemical results suggest both localized and uniform anodic dissolution processes, which are more pronounced on AA 7075, demonstrating the effect of alloying elements on the growth of anodic oxides

Observation of Pore Growth and Self-Organization in Anodic Alumina by Time-Resolved X-ray Scattering

The anodic oxidation of metals such as aluminum and titanium can lead to the development of self-ordering pores. These pores make excellent templates for a range of nanoscale objects with many applications in nanoscience. Theoretical studies on pore formation have proposed several models for the establishment, growth, and ordering of these pores; however, experimental verification has mostly been limited to ex situ measurements. Here we show that the lateral and vertical pore structure can be probed in situ with high precision, using grazing transmission X-ray scattering. By making use of the high flux available at modern synchrotrons and fitting only the difference between scattering patterns we show the nearly real-time evolution of the pore’s arrangement. We observe no dependence on the substrate crystallographic orientation for domain size or pore separation. We do however observe an anisotropy in the oxide growth rate for the different substrate surfaces. This experimental approach can be applied to the study of a large variety of electrochemically produced materials such as magnetic nanowires, novel solar cell designs, and catalysts

Surface oxide development on aluminum alloy 6063 during heat treatment

We report on the influence of oxygen partial pressure for the development of surface oxides covering the industrial aluminum alloy standard 6063 at temperatures ranging from room temperature to 500° C. Using an array of synchrotron-based techniques, we followed the change in oxide thickness, chemical composition, and the lateral distribution of alloying elements. The impact of the oxygen chemical potential is most visible at high temperatures where the oxide composition changes from mostly Al based to mostly Mg based. This is in stark contrast to the ultra-high vacuum (UHV) conditions where only a partial compositional transition is observed. The microscopy data demonstrate that in the UHV case, Mg segregation onto the surface occurs firstly at grain boundaries at 300° C and secondly at sites over the entire surface at 400° C. Further, the initial oxide thickness is 45 Å, as determined by XPS and XRR, decreases in all observed cases after heating to 300° C. At higher temperatures, however, the oxygen partial pressure highly influences the resulting oxide thickness as evident from our X-ray reflectivity data

Influence of Surface Strain on Passive Film Formation of Duplex Stainless Steel and Its Degradation in Corrosive Environment

The effect of surface strain on the passive film evolution of SAF 2507 super duplex stainless steel exposed to ambient air and 0.1 M NaCl solution with varying anodic polarization at room temperature has been investigated using in-situ grazing incidence X-ray diffraction (GIXRD) in combination with electrochemical measurements. Surface strain affected the crystallinity of the passive film as such that the surface oxides/hydroxides were predominantly amorphous, with some minor crystalline CrOOH and FeOOH present in the film. Crystalline CrOOH was seen to diminish in volume upon immersion in the NaCl solution, well-possibly becoming amorphous during anodic polarization, whereas crystalline FeOOH was seen to increase in volume during polarization to the passive potential regime. Strain relaxation, associated with metal dissolution, occurred in both austenitic and ferritic grains during immersion in the electrolyte. Anodic polarization to the transpassive regime led to maximum strain relaxation, occurring more on the austenite than the ferrite. The selective transpassive dissolution nature of the ferrite was significantly reduced due to large strains in the austenite. Passive film breakdown was reflected by enhanced dissolution of Fe, Cr, Mo and Ni occurring simultaneously around 1300 mV vs. Ag/AgCl

Structure dependent effect of silicon on the oxidation of Al(111) and Al(100)

The effect of sub-monolayer silicon on the oxidation of Al(111) and Al(100) surfaces was investigated using X-ray Photoelectron Spectroscopy (XPS) and density functional theory (DFT) calculations. On both surfaces the adatom site is preferred over substituting Si into the Al-lattice; on Al(100) the four fold hollow site is vastly favored whereas on Al(111) bridge and hollow sites are almost equal in energy. Upon O 2 exposure, Si is not oxidized but buried at the metal/oxide interface under the growing aluminum oxide. On Al(111), Si has a catalytic effect on both the initial oxidation by aiding in creating a higher local oxygen coverage in the early stages of oxidation and, in particular, at higher oxide coverages by facilitating lifting Al from the metal into the oxide. The final oxide, as measured from the Al2p intensity, is 25–30% thicker with Si than without. This observation is valid for both 0.1 monolayer (ML) and 0.3 ML Si coverage. On Al(100), on the other hand, at 0.16 ML Si coverage, the initial oxidation is faster than for the bare surface due to Si island edges being active in the oxide growth. At 0.5 ML Si coverage the oxidation is slower, as the islands coalesce and he amount of edges reduces. Upon oxide formation the effect of Si vanishes as it is overgrown by Al 2 O 3 , and the oxide thickness is only 6% higher than on bare Al(100), for both Si coverages studied. Our findings indicate that, in addition to a vanishing oxygen adsorption energy and Mott potential, a detailed picture of atom exchange and transport at the metal/oxide interface has to be taken into account to explain the limiting oxide thickness

In-situ synchrotron GIXRD study of passive film evolution on duplex stainless steel in corrosive environment

This paper presents new findings about the passive film formed on super duplex stainless steel in ambient air and corrosive environments, studied by synchrotron grazing-incidence X-ray diffraction (GIXRD). The passive film, formed in air, was seen to be a nano-crystalline mixed-oxide. Electrochemical polarisation to the passive region in aqueous 1 M NaCl at room temperature resulted in an increase of the passive film thickness, preferential dissolution of Fe, and partial loss of crystallinity. After termination of polarization to the transpassive regime, reformation of the mixed-oxides was observed, showing a thicker, semi-crystalline, and more defective nature (more vacancies) with further new oxides/hydroxides

Redefining passivity breakdown of super duplex stainless steel by electrochemical operando synchrotron near surface X-ray analyses

Passivity determines corrosion resistance and stability of highly-alloyed stainless steels, and passivity breakdown is commonly believed to occur at a fixed potential due to formation and dissolution of Cr(VI) species. In this work, the study of a 25Cr–7Ni super duplex stainless steel in 1 M NaCl solution revealed that the passivity breakdown is a continuous degradation progress of the passive film over a potential range, associated with enhanced Fe dissolution before rapid Cr dissolution and removal of the oxide. The breakdown involves structural and compositional changes of the passive film and the underlying alloy surface layer, as well as selective metal dissolution depending on the anodic potential. The onset of passivity breakdown occurred at 1000 mV/Ag/AgCl, and Fe dissolved more on the ferrite than the austenite phase. With increasing potential, the passive film became thicker but less dense, while the underlying alloy surface layer became denser indicating Ni and Mo enrichment. Rapid Cr dissolution occurred at ≥1300 mV/Ag/AgCl