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Anodic oxidation of reactive metals such as Al and Ti produces oxide films with self-organized arrangements of nanoscale pores. Stress-driven mass transport of oxide is considered to play an important role in pore formation and self-ordering. Using in situ stress monitoring during both anodizing and subsequent open-circuit oxide dissolution, distributions of in-plane residual stress were measured in anodic alumina films formed by galvanostatic anodizing in phosphoric acid. Anodizing produced significant stress both in the oxide and at the metal-oxide interface. For oxides grown to 20nm thickness, the oxide stress was tensile below 3mA/cm2 and compressive above this current density, while the interface stress exhibited the opposite dependence. Stress generation correlated with interfacial volume change due to reactions and transport processes: oxide or interface stress was compressive when interfacial volume was created, and vice versa. Compressive stress buildup in the oxide is apparently required for self-ordered pore formation by flow-assisted mechanisms. From the present results, a simple criterion was derived specifying the conditions for compressive stress and pore formation in terms of parameters governing film composition, ionic transport and interfacial reaction kinetics.

The present study investigates the effect of fluorine species during anodizing of aluminium and AA2024-T3 alloy in sulphuric acid and tartaric-sulphuric acid (TSA) electrolytes. The investigation comprises four main parts; (i) Effects of fluoride on barrier film formation on aluminium. (ii) Effects of fluoride and fluorozirconic acid (FZ) on porous film growth on aluminium in sulphuric acid. (iii) Effects of FZ on porous film growth on aluminium and AA 2024-T3 alloy in sulphuric acid and TSA. (iv) Effects on anodizing of other fluoroacids (fluoroboric (FB), fluorosilicic (FS) and fluorotitanic acid (FT)). The anodic films were examined by analytical scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, Rutherford backscattering spectroscopy, nuclear reaction analysis and glow discharge optical emission spectroscopy. The behaviour of fluoride ions during the growth of barrier-type films on aluminium was investigated in ammonium pentaborate solution with added sodium fluoride. Additions of up to 3.5 x 10-3 M sodium fluoride had a negligible influence on the film growth. In contrast, 3.5 x 10-2 M sodium fluoride reduced the efficiency to 60% as fluoride ions promoted field-assisted ejection of Al3+ ions from the film. Incorporated fluoride ions migrated inwards at a rate about twice that of O2- ions, forming a fluoride-rich layer at the film base. The study of the influence of FZ on formation of porous anodic films in sulphuric acid and TSA employed a range of anodizing voltages, electrolyte temperatures and anodizing times. Fluoroacid increased the growth rate, with a reducing influence as the temperature increased. The films contained fluoride and sulphate ions, zirconium was not detected. The fluoride concentration decreased with increasing temperature, whereas the sulphate concentration was unaffected. Anodizing aluminium and AA 2024-T3 alloy in other fluoroacids resulted in similar influences on the anodizing behaviour as FZ. The differences in growth rate, film composition and film morphology were comparatively small and did not show a systematic dependence on the type of fluoroacid employed. Boron, silicon and titanium were not detected in the films.

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Trivalent chromium conversion coatings formed on Al and Al-Cu alloys has been investigated using high-resolution, analytical electron microscopy, atomic force microscopy, ion beam analysis, glow discharge optical emission spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, potentiodynamic polarization and electrochemical noise analysis. These coatings on the electropolished Al and sputtering-deposed Al consist of a chromium- and zirconium-rich outer layer and a thinner, aluminium-rich inner layer. Zirconium and chromium are presented in chemical states consistent with ZrO2, Cr(OH)3, Cr2(SO4)3, CrF3 and CrO3 or CrO42-. However, negligible amounts of hexavalent chromium species occurred in both coatings formed in de-aerated solution. On AA2024-T351 alloys, the coating above the second phase particles was thicker than that on the matrix due to the increased localized alkalinity. Moreover, the localized corrosion and copper enrichment of the matrix occurred at the coating base. The presence of copper resulted in a thinner coating at the matrix compared with superpure aluminium. An Fe(III)-containing D30 desmutter can effectively remove the protruded particles generated by alkaline etching. Moreover, a thinner oxide film on the D30-treated surface was evidenced to promote the coating initiation on the matrix. Further, the coating comprised the concentrated zirconium oxyfluorides and decreased contents of aluminium and copper. Coated alloys displayed a significantly enhanced corrosion protection, especially the cathoidc inhibition. In terms of coating post-treatments, the water immersion (40 ºC, pH 5) soon after conversion treatment revealed a simple but effective process to improve the fluoride enrichment in coatings and to enhance corrosion protection of freshly-developed coatings. Increasing concentration of solid-solution copper in sputtering-deposited alloys, in a range of 2, 24, 40 and 64 at.%Cu, significantly promotes the coating growth kinetics and an evident and a thicker corrosion layer by substrate dissolution was observed on Al-40 at.%Cu alloys. In addition, the relationship of copper-rich deposits with Cr(VI) transformation was demonstrated by Raman spectroscopy. Conversion coating process is a surface pre-treatment for aluminium and aluminium-copper alloys applied in the aerospace and automotive industries to provide corrosion resistance and appreciable durability for the coating system. However, the eco-friendly concern of the current chromate conversion coating motivates our research of the next-generation of promising replacement whist maintaining corrosion protection. Trivalent chromium conversion coating is of great importance and significance in the candidate processing.The formation, composition, chemistry and corrosion protection of the trivalent chromium conversion coatings on aluminium and aluminium-copper alloys have been investigated using the high-resolution electron microscopy, ion beam analyses, Raman spectroscopy and XPS. The fundamental studies worked out the mechanism of Cr III / Cr VI reaction in the coating formation on Al, and the possible eliminating methodologies were under discussion. Furthermore, the optimal trivalent chromium process includes the appropriate pre-treatment to produce a cleaner surface, conversion treatment for 5 min, water immersion post-treatment and air ageing for 24 hours.

The University of Manchester Dawod M H Elabar Doctor of philosophy Effect of sulphate impurity in chromic acid anodizing of aluminium and aluminium alloy 2016In this work, the nucleation and growth of pores in anodic films formed on aluminium in chromic acid and the effect of low levels of sulphate impurity in the anodizing bath on the formation of the films on aluminium and AA 2024 alloy are investigated. The sulphate concentrations considered include levels within specified limits for industrial processing. The anodizing is carried out either potentiostatically or by stepping the voltage. The films are examined by scanning electron microscopy, transmission electron microscopy and atomic force microscopy to determine the pore spacing, pore population densities, pore diameters and film thicknesses. Film compositions were determined using energy-dispersive X-ray spectroscopy, Rutherford backscattered microscopy and nuclear reaction analysis.In order to investigate the mechanism of pore formation, two tracer methods are employed. In one method, anodic films are formed first in an arsenate electrolyte in the second method, a tungsten tracer band deposited by magnetron sputtering. The behaviours of arsenic and the tungsten are investigated during the subsequent anodizing in chromic acid. The results suggest that the initiation and growth of pores in occurred as a result of electric field assisted chemical dissolution.The effect of sulphate impurity in the chromic acid is investigated using electrolytes with different sulphate content. In the initial stages of anodizing aluminium at 100 V, sulphate impurity at a level of 38 ppm in the chromic acid is shown to lead to significant incorporation of sulphate ions into the anodic film, a lower current density, a smaller cell size and less feathering of the pore walls. In addition, the efficiency of film formation is increased. In later stages of anodizing, the growth of larger pores and cells, leads to a duplex film morphology, with finer pores in the outer region. The change in pore size correlates with a reduction in the incorporation of sulphate into the film. From the results of sequential anodizing experiments, it is suggested that incorporated sulphate ions generate a space charge layer, which has an important role in determining the current density. The effects of higher sulphate concentrations up to 3000 ppm are investigated, which are shown to significantly affect the current density and the pore diameter.Anodizing of aluminium and AA 2024 alloy was also carried out according to industrial practice. The results show that there is significant effect of sulphur impurity on the film thickness. Corrosion tests in 3.5 % NaCl solution for the alloy after anodizing in low (≤ 1.5 ppm) and high (~38 ppm) sulphate-containing chromic acid electrolytes demonstrate a better corrosion resistance with films formed in the latter electrolyte.

Plasma electrolytic oxidation is used to prepare corrosion- and wear-resistant coatings on light metals. The extensive literature reports on coatings formed under a wide range of different electrical regimes and in diverse electrolyte compositions. However, little work is available that investigates systematically PEO of titanium under a range of electrical variables in a particular electrolyte. In the present work, coatings are formed in a silicate electrolyte under a range of current densities, duty cycles and rates of positive to negative current density. The coatings were found to contain anatase, rutile and amorphous silicate-rich material, with comparatively minor influences of the PEO parameters. Further, coatings were limited in thickness to 40 μm due to a decrease in voltage and intensity of sparks at longer treatment times. The coatings were relatively soft with poor wear- and corrosion- resistances, and a high coefficient of friction although the last could be reduced by incorporation of PTFE particles into the coating.The study also investigates coatings formed in aluminate-phosphate based electrolytes, which generated wear-resistant and corrosion-resistant coatings of increased hardness. A focus was on the use of high-resolution electron microscopy, which has not been reported previously, to determine the details of the coating composition and structure. The findings revealed the distribution of coating species, showing an aluminum-rich outer layer and a titanium-rich inner layer, with phosphorus enriched in a band near the base of the coating. However, the coatings also revealed highly localized variations in composition within their noanocrystalline structures, due to the melting and rapid solidification of the coating material. The study also examined the role of electrolyte purity on the formation and properties of the coatings, which has not been examined elsewhere. Importantly chlorine species from the lower purity electrolyte were shown to enrich near the substrate, resulting in a cracked interfacial layer and reduced adhesion of the coating. Such observations may account for reports of poor coating adhesion in the literature. Further, a reduced purity of the electrolyte results in an erratic voltage response, due to cycles of mechanical breakdown and healing of the coating, with high levels of chloride resulting in a highly porous coating. The distributions of phosphorus and chlorine species within the coatings suggest that these species migrate inwards, with chlorine species migrating faster than phosphorus species.

The present work concerns the plasma electrolytic oxidation (PEO) ofmagnesium alloys. The first part focuses on the optimisation of the PEO processparameters on Elektron 21 magnesium alloy in order to produce a protective coating onthe metal surface. Two electrolytes were employed, the first one contains 11 g/LNa2SiO4, 10 g/L Na4P2O7*10H2O and 2.5 g/L KOH, while the second one additionallyconsists of 8 g/L KF. The influence of the solution composition, current density andtime during the PEO process on the coating morphology and thickness are discussed. An addition of KF in the electrolyte, an increase of a current density and prolongationof the time of the process resulted in an increase of the thickness of the PEO coatings.Longer times of the process led to a transition to the so-called “soft-spark” regime.However, their occurrence caused the generation of a non-uniform coating in thesolution without KF. On the other hand, the prolongation of the time in the KFcontaining electrolyte led to a thicker and more uniform coating. XRD analysis revealedthe presence of amorphous and crystalline phases, with the latter including MgO,Mg3(PO4)2 and MgSiO4 for treatment in the solution without KF, and MgO and MgSiO4for the PEO process in the electrolyte with the addition of KF. The PEO coatings haveprotective properties and reduce the corrosion current density by two orders ofmagnitude. Moreover, an increase of noise resistance and impedance at low frequencyby up to two orders of magnitude were recorded for the PEO coated material. Thesecond part of the work was focused on application of a post-treatment to fill theporosity and defects present in the PEO coatings. A sol-gel technique was chosen. Itwas found that a sol can easily penetrate through the PEO layer and fill the pores. Thesol-gel coating was found to act as a barrier layer for the penetration of the corrosivesolution and decreases the corroson current density by one order of magnitude inpotentiodynamic polarization tests. In the last part of the work the PEO process wascarried out on AZ31 magnesium alloy. The PEO coating produced at a current densityof 420 mA/cm2 in the solution without addition of KF has a uniform and relativelycompact structure. The potentiodynamic polarization test showed that the coating hasprotective properties and results in a reduction of the corrosion current density by twoorders of magnitude. The present work concerns the plasma electrolytic oxidation (PEO) ofmagnesium alloys. The first part focuses on the optimisation of the PEO processparameters on Elektron 21 magnesium alloy in order to produce a protective coating onthe metal surface. Two electrolytes were employed, the first one contains 11 g/LNa2SiO4, 10 g/L Na4P2O7*10H2O and 2.5 g/L KOH, while the second one additionallyconsists of 8 g/L KF. The influence of the solution composition, current density andtime during the PEO process on the coating morphology and thickness are discussed. An addition of KF in the electrolyte, an increase of a current density and prolongationof the time of the process resulted in an increase of the thickness of the PEO coatings.Longer times of the process led to a transition to the so-called “soft-spark” regime.However, their occurrence caused the generation of a non-uniform coating in thesolution without KF. On the other hand, the prolongation of the time in the KFcontaining electrolyte led to a thicker and more uniform coating. XRD analysis revealedthe presence of amorphous and crystalline phases, with the latter including MgO,Mg3(PO4)2 and MgSiO4 for treatment in the solution without KF, and MgO and MgSiO4for the PEO process in the electrolyte with the addition of KF. The PEO coatings haveprotective properties and reduce the corrosion current density by two orders ofmagnitude. Moreover, an increase of noise resistance and impedance at low frequencyby up to two orders of magnitude were recorded for the PEO coated material. Thesecond part of the work was focused on application of a post-treatment to fill theporosity and defects present in the PEO coatings. A sol-gel technique was chosen. Itwas found that a sol can easily penetrate through the PEO layer and fill the pores. Thesol-gel coating was found to act as a barrier layer for the penetration of the corrosivesolution and decreases the corroson current density by one order of magnitude inpotentiodynamic polarization tests. In the last part of the work the PEO process wascarried out on AZ31 magnesium alloy. The PEO coating produced at a current densityof 420 mA/cm2 in the solution without addition of KF has a uniform and relativelycompact structure. The potentiodynamic polarization test showed that the coating hasprotective properties and results in a reduction of the corrosion current density by twoorders of magnitude.

High strength 7xxx aluminium alloys are used extensively in the aerospace industry because the alloys offer excellent mechanical properties. Unfortunately, the alloys can suffer localised corrosion due to the presence of large intermetallic particles at the alloy surface that are aligned in the rolling direction. Laser surface melting (LSM) techniques offer the potential to reduce and/or to eliminate the intermetallic phases from the surface of the alloy without affecting the alloy matrix.The present study concerns the application of LSM using an excimer laser to enhance the corrosion resistance of AA 7075-T6 aluminium alloy. The initial stage of the project was aimed at optimising the laser conditions for production of a uniform microstructure, with the increase in the corrosion resistance of the alloy being determined by potentiodynamic polarization measurements in sodium chloride solution. Low and high laser energy densities were used with a different number of pulses per unit area to treat the alloy surface, which were achieved by changing both the laser fluence and the pulse repetition frequency. A laser fluence of 3.3 J/cm2 with 80 pulses was subsequently selected as the optimum condition to treat the surface of the alloy. The composition and microstructure of the alloy before and after LSM treatment, and following corrosion tests, were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD).After the laser treatment, the surface and the cross-sections of the alloy showed a significant reduction in the number of large intermetallic particles and a relatively homogenous melted layer was generated that provided significant improvement in the resistance of the alloy against corrosion, as assessed by several corrosion test methods, including exfoliation corrosion (EXCO) tests. However, delamination of the melted layer was observed after extended testing in the EXCO solution which is possibly related to the formation of bands of fine magnesium and zinc-rich precipitates within the melted layer. Therefore, anodising in sulphuric acid was applied to the LSM alloy, in order to further increase the corrosion resistance and to protect the laser treated layer from delamination by generating a thin oxide film over the LSM layer. The results revealed that the anodic treatment increased the resistance of the alloy to exfoliation attack.

Friction stir welding (FSW) is a well recognised method for joining aluminium alloys and other engineering materials at a temperature below their melting point. However, the microstructure of the alloys may be modified during the welding process due to frictional heat and severe plastic deformation.In this study, the microstructures of friction stir welded AA5754-H111 and AA5083-O aluminium alloys have been investigated using optical microscopy, transmission and scanning electron microscopy equipped with electron backscatter diffraction (EBSD) and energy dispersive x-ray (EDX) facilities. Typical weld zones introduced by FSW were observed. Further, a joint line remnant flaw (JLR) within the thermomechanical affected zone (TMAZ) of the welds was also revealed. The formation of the JLR is attributed to dispersion of the magnesium rich oxides within the joining line.The effect of the modified alloy microstructure on the corrosion behaviour of the welds has been investigated by corrosion susceptibility testing and ex-situ SEM examination. Both parent alloys and welds showed good exfoliation and intergranular corrosion resistance (IGC). However, severe localized corrosion was observed at joint line remnant and the weld root.Reduced hardness was recorded in the heat affected zone (HAZ) of AA5754-H111 aluminium alloy weldment. This is attributed to the heat generated during welding that led to grain coarsening. In contrast, slightly increased hardness was recorded within the TMAZ. This was related to the grain refinement as a result of the recrystallization process that took place due to the effect of the thermal cycle and the plastic deformation. Little hardness change was recorded within AA5083-O aluminium alloy weldment. This was attributed to the effect of the alloy temper condition.Thermal simulation of the service environment of the friction stir welded alloys was conducted to assess the resistance to sensitization of welds. After exposure of the welded AA5754 and AA5083 alloys at 50, 70 and 170°C for prolonged time, the resistance of the AA5083 alloy weld to the IGC drastically decreased owing to the precipitation of magnesium rich particles known as β-phase at the grain boundaries. On the contrary, the resistance of the AA5754 alloy weld to IGC remained after the thermal exposure. Thus, the level of Mg content in Al-Mg alloys plays an important role in determining the corrosion characteristics of the alloys. The precipitation of Mg rich particles (β-phase) on the grain boundary is the determining factor for the resistance of the AA5xxx alloys to IGC owing to the difference in the electrode potentials between the β-phase and the grain interior, which leads to the generation of microgalvanic cells and selective dissolution of the grain boundary.

The contamination of 316L stainless steel surfaces within an electrospray ionisation source of a mass spectrometer is investigated. An accelerated method of contamination is used. Following initial test method development and investigation of the contamination resulting on the ion transfer components (sample cone, outer cone and extraction cone), flat samples are employed within the ionisation source. This enables characterisation of the contamination composition, morphology and build-up with time. Blood plasma is introduced into the mass spectrometer as it is a widely analysed substance that is known to result in contamination. The contamination from a mixture of human blood plasma, diluted in methanol, and a water/acetonitrile mobile phase is found to contain inorganic NaCl crystals embedded in a matrix of organic residues. The morphology shows self-organising features as the contamination builds. A model is proposed to explain the morphology, involving rapid evaporation of the droplets that impinge on the stainless steel surface.Two types of surface modification are considered for the stainless steel: electrochemically grown films and coatings deposited by vapour deposition. A method for electrochemical film growth is developed, enabling nanoporous films to be formed on the stainless steel in 5 M sulphuric acid at 60°C by square wave pulse polarisation between active or transpassive and passive potentials. The films are characterised using glow discharge optical emission spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, Rutherford backscattering spectroscopy and nuclear reaction analysis. The films are shown to be chromium- and molybdenum-rich relative to the substrate, and to consist mainly of sulphates, oxides and hydroxides. The morphology and composition of the films are discussed in relation to the polarisation conditions and mechanism of film formation.A range of vapour deposited coatings are considered: TiN, TiC, TiB2, Graphit-iC, and diamond-like carbon coatings with Si and N2 dopants and with varying sp2:sp3 ratios. In addition, a hydrophobic coating is deposited on the stainless steel by immersion, in order to provide a significant variation in surface energy. Surface analysis of the coatings is carried out, considering their sp2:sp3 ratios, their electrical conductivities, their water contact angle, and the various components of the surface energy.The contamination build-up on the surface of uncoated 316L stainless steel is compared with that on stainless steel with the various surface modifications. A method for quantification of the build-up of contamination on flat samples is developed using white light interferometry. The surface modifications which result in the slowest contamination build-up with time are then applied to the ion transfer components of the mass spectrometer. The robustness of the mass spectrometric response for the selected coated surfaces is compared with that of the uncoated stainless steel. The electrochemically grown films and two of the doped diamond-like carbon coatings are found to be successful in reducing the build-up of contamination.