Dissertations / Theses on the topic 'Corrosion ; Magnesium ; Magnesium alloys'

Mestrado em Engenharia de Materiais
A biodegradação de biomateriais em ordem a conseguir uma dissolução completa de um determinado equipamento após a realização do seu propósito, tem sido visto como uma ideia atrativa pela comunidade cientifica, devido ao elevado potencial nas melhorias a qualidade de vida do paciente e devido aos custos pós operatorios que podem ser melhorados. O comportamento de biodegradação é consequência da elevada susceptibilidade à corrosão, inerente às ligas metálicas como o magnésio. Esta característica deve-se à instabilidade química causada pela inserção das ligas num ambiente agressivo às mesmas. Esta afirmação continua a ser verdadeira no caso em que ligas de mágnesio são introduzidas no corpo humano, em contacto com iões agressivos ao metal, presentes nos fluídos corporais. O trabalho de investigação proposto nesta tese, tem como temática o estudo de mágnesio puro, ligas de Mg-XGd e Mg-XGd-YMn, onde o rácio estequiométrico dos elementos é X=2,5 e Y=1. As ligas usadas não se encontram comercializadas, mas existe um forte interesse no seu uso como material biodegradável devido às boas propriedades mecânicas apresentadas pelas mesmas. No entanto as taxas de corrosão necessitam de ser modeladas de forma a viabilizar o seu uso como biomaterial, e uma melhor compreensão sobre os mecanismos de corrosão podem ajudar no design de futuras ligas. O foco deste trabalho consiste em desvendar a natureza da corrosão e devido a isso diversos fatores serão estudados, usando diferentes técnicas de caracterização i) Observar a microestrutura e os microconstituintes presentes, o seu tamanho, morfologia e composição elemental, usando para tais fins técnicas de MEV e EDS. A rugosidade e o potential Volta apresentada pelos diversos constituintes da microstrutura será levado a cabo por técnicas de MFA e SKFM. ii) Técnicas eletroquímicas, como a eletroquímca de impedância e polarização dinâmica, serão usadas de forma a perceber o comportamento do sistemas em diversos meios eletrolíticos. Tempos longos de imersão foram realizados durante medições de Impedância eletroquímica. iii) A composição quimica e o estudo de fases dos produtos de corrosão são realizados usando técnias de EDS e DRX, o que permite identificar os tipos de produtos preferencialmente formados durante o processo de corrosão. iv) Uma série de outras técnicas proporcionaram uma informação mais consistente sobre o comportamento de corrosão nas ligas de mágnesio, como a evolução do hidrogénio e a observação das secções de corte. A reproducibilidade foi estudada usando uma amostragem em diversas técnicas. Entretanto este trabalho é baseado numa comparação qualitativa que permite entender e desvendar o porquê, como e qual o tipo de corrosão que é apresentado pelos diversos sistemas em estudo. Os resultados obtidos pelas diversas técnias revelaram que os fenómenos de corrosão são dependentes do tipo de ambiente e das suas condições. A presença de níveis de impurezas superiores aos limites de tolerância, como o ferro, mostram que a taxa de corrosão é aumentada na presença dos mesmos, visto que aumenta a actividade catódica dos intermetálicos. O manganês como elemento de liga reduz esse efeito, diminuindo a respetiva taxa de corrosão. A formação de produtos de corrosão é dependente do pH do meio, e assim, a precipitação de compostos vai diferir com o eletrólito em uso. O sistema ternário e o magnésio puro demonstraram taxas de corrosão aproximadamente de 0,18 mm/a a 330h de imersão, imerso na solução de PBS. Estas taxas de corrosão podem ser adequadas para aplicações biomédicas.
Biomaterials bring valuable improvements to the biomedical field. The idea behind the biodegradation behaviour of a biomaterial which can be used as an implant in the human body has attracted the attention of the scientific community, due to various benefits which may improve quality of life of injured humans. The biodegradation behaviour of metals arises from the high susceptibility to corrosion of metallic alloys in the human body, which are in contact with aggressive ions presented by human body fluids. This especially concerns magnesium and its alloys. Magnesium alloys must comply with the requirements which are put on the biodegradable materials. Among such requirements one can name mechanical properties and controlled corrosion activity. Investigation in this work performed on several Mg samples, including a pure magnesium (HP Mg), Mg-XGd and Mg-XGd-YMn systems with variation in stoichiometry ratio of elements, X=2 and 5 and Y=1. These are non-commercial Mg alloys which may present interest due to their potential as biodegradable materials. A tailoring of the corrosion rate is required to reduce the corrosion rate of such alloys. For that, it is incredibly wise to understand the corrosion mechanisms in different electrolytes and conditions. To study the influence of factors which affects corrosion a series of characterization techniques were used. At first microstructure and microconstituents as intermetallics, their size, shape and elemental composition, were evaluated using SEM and EDS. Roughness and Volta potential of the different phases present in the microstructure were studied using AFM/SKFM technique, which allows to correlate the Volta potential with local corrosion of intermetallics and to observe dissolution and precipitation processes at the microscale. Also, electrochemical measurements, such as Electrochemical Impedance Spectroscopy (EIS), potential dynamic polarization, were conducted accessing corrosion behaviour of systems in different electrolytes during short immersion times. For electrochemical characterization, in the extended time of immersion EIS was used. To obtain the corrosion rate, it was used the hydrogen evolution method. Then corrosion products chemistry was studied using X-ray diffraction and energy dispersive spectroscopy techniques, which allow to identify the type of products formed in the different electrolytes and to correlate their formation with corrosion behaviour. Cross section analysis and identification of corrosion morphology were accessed on samples after EIS tests. Reproducibly of measurements were ensured by studying a set of replica samples. This work is based on qualitative/qualitative comparison of results which allowed a better understanding why, how and which corrosion is present in the different systems. The different techniques used revealed that corrosion is highly dependent on the environment and the conditions of measurements. The presence of high levels of impurities as iron induces high levels of corrosion by increasing the cathodic activity of intermetallic. Manganese as an alloying element reduces the effect of the impurities in corrosion. Corrosion products formation is pH dependent, and so, the precipitation of corrosion products compounds from different electrolytes may be beneficial or nonbeneficial to corrosion. The ternary system and the HP Magnesium demonstrate corrosion rates approximately 0.18 mm/year in PBS solution, which can be adequate for biomedical applications.

Magnesium has been identified as a promising biodegradable implant material because it does not cause systemic toxicity and can reduce stress shielding. However, it corrodes too quickly in the body. Titanium, which is already used ubiquitously for implants, was chosen as the alloying element because of its proven biocompatibility and corrosion resistance in physiological environments. Thus, alloying magnesium with titanium is expected to improve the corrosion resistance of magnesium. Mg-Ti alloys with a titanium content ranging from 5 to 35 at.-% were successfully synthesized by mechanical alloying. Spark plasma sintering was identified as a processing route to consolidate the alloy powders made by ball-milling into bulk material without destroying the alloy structure. This is an important finding as this metastable Mg-Ti alloy can only be heated up to max. 200C° for a limited time without reaching the stable state of separated magnesium and titanium. The superior corrosion behavior of Mg80-Ti20 alloy in a simulated physiological environment was shown through hydrogen evolution tests, where the corrosion rate was drastically reduced compared to pure magnesium and electrochemical measurements revealed an increased potential and resistance compared to pure magnesium. Cytotoxicity tests on murine pre-osteoblastic cells in vitro confirmed that supernatants made from Mg-Ti alloy were no more cytotoxic than supernatants prepared with pure magnesium. Mg and Mg-Ti alloys can also be used to make novel polymer-metal composites, e.g., with poly(lactic-co-glycolic acid) (PLGA) to avoid the polymer’s detrimental pH drop during degradation and alter its degradation pattern. Thus, Mg-Ti alloys can be fabricated and consolidated while achieving improved corrosion resistance and maintaining cytocompatibility. This work opens up the possibility of using Mg-Ti alloys for fracture fixation implants and other biomedical applications.

The influence of microstructure on the corrosion behaviour of magnesium alloys has been investigated using advanced microscopy approaches including optical microscopy, SEM, TEM and SKPFM with a focus on the effect of melt-conditioned twin roll casting (MCTRC) and friction stir welding (FSW) on the resultant microstructure of magnesium alloys.The microstructure characterization revealed that intense shearing, generated through the advanced shear technology, resulted in grain refinement and a uniform distribution of the β-phase and reduced micro-porosity in the MCTRC Mg-Al alloys, of which were attributed to the enhanced heterogeneous nucleation, which resulted in a highly refined grain structure. The TRC Mg-Al alloys displayed a coarse grained microstructure, with a random distribution of grain sizes. Deformation features like twinning, localized shear, microporosity and centre-line segregation were some of the commonly observed defects in the TRC alloys. The general microstructure of the AZ series Mg-Al alloys was composed of α-Mg grains, the β-phase, rosette-shaped Al8Mn5 intermetallic particles and β-precipitates.The MCTRC Mg-Al alloys showed improved corrosion resistance owing to the reduced grain size and the β-phase network acting as a corrosion barrier, thereby retarding the corrosion process. The TRC Mg-Al alloys exhibited higher susceptibility to galvanic corrosion due to the coarse and random distribution of grain sizes, and segregation. The corrosion testing results showed different corrosion morphologies, including filiform-like and spherical channel-like along with overall general corrosion. However, galvanic corrosion, initiating at localized sites due to Al8Mn5 intermetallic particles and the Si/Fe impurities accounted for a major deterioration in the performance of the Mg-Al alloys. The polarization curves revealed no evidence of passivation, suggesting that the alloy surface was continuously attacked. SKPFM results indicated that the micro-constituents, namely Al8Mn5 intermetallic particles and the β-phase exhibited higher nobility relative to the α-Mg matrix, suggesting formation of micro-galvanic couples at localized sites leading to the initiation of galvanic corrosion.The AM60 and AZ91 Mg-Al alloys, subjected to FSW, revealed that the traverse speed had a direct influence on the weld zone microstructure, where the size of the friction stir/weld nugget zone decreased with increase in the traverse speed and the increase in the rate of deformation, led to widening of the friction stir zone, below the shoulder. The weld microstructure displayed a prominent friction stir zone, with an ultrafine grain structure of an average grain size ranging from 2-10 μm. The localized increase in temperatures, in the TMAZ, due to the lower tool rotation rates and traverse speeds, which rise above the eutectic melting point (430°C), showed evidence of partial melting followed by re-solidification of the β-phase and evidence of liquation below the shoulder regions in the TMAZ. The morphology of the β-phase clearly revealed solute segregation, inconsistent with the β-phase observed in the parent alloy microstructure.The polarization curves obtained from the weld zones in the FSW AM60 alloy showed an improved corrosion resistance compared with the parent metal zone. SKPFM results revealed that the α-Mg matrix in the friction stir zone showed higher surface potential values compared with the parent alloy microstructure, due to the dissolution of the β-phase, suggesting higher nobility. However, the polarization behaviour of the AZ91 alloys did not show a significant difference in the corrosion resistance in the weld zones due to the higher volume fraction of the β-phase in the AZ91 alloys. The immersion testing results revealed higher susceptibility to corrosion in the transition zone due to the flash formation and the banded microstructure leading to failure of the weld zone.

The low density and high specific strength of magnesium alloys have created a great deal of interest in the use of these alloys in the automotive and aerospace industries and in portable electronics. All of these industries deal with applications in which weight is extremely important. However, an obstacle to overcome when using magnesium alloys in engineering applications are their unsatisfactory corrosion properties. This thesis is devoted to the atmospheric corrosion of the two magnesium alloys AZ91D and AM50, in particular the ways the microstructure and exposure parameters of these alloys influence their corrosion behaviour. The work includes both laboratory and field studies. The results obtained show that the microstructure is of vital importance for the corrosion behaviour under atmospheric conditions. The microstructure of magnesium-aluminium alloys contains different intermetallic phases, e.g. Al8Mn5 and β-Mg17Al12. The local nobility of these intermetallic phases was measured on a submicron level in an atmospheric environment. It was shown that particles of the Al-Mn type exhibit the highest Volta potential among the microstructure constituents of the AZ91D magnesium alloy. Further, it was shown that the Volta potential was highly dependent on the aluminium content of the magnesiumaluminium phases in the surface layer. When thin electrolyte layers are present, CO2 diffuses readily to the surface forming magnesium carbonate, hydromagnesite. The CO2 lowers the pH in areas on the surface that are alkaline due to the cathodic reaction. This stabilises the aluminium-containing surface film, the result being increased corrosion protection of phases rich in aluminium. Both in the laboratory and under field conditions the corrosion attack was initiated in large α-phase grains, which is explained by the lower aluminium content in these grains. The thin electrolyte film, which is formed under atmospheric conditions, decreases the possibility of galvanic coupling of alloy constituents located at larger distances from each other. Thus the cathodic process is in most cases located in the eutectic α-/β phase close to the α-phases, instead of in intermetallic Al-Mn particles, even though the driving force for the initiation of the corrosion attack in Al-Mn particles should be high, due to their high nobility.

QC 20100802

Étant donné leur capacité à se dégrader à l'intérieur du corps, les implants biodégradables ont fait l'objet de nombreuses recherches médicales. Parmi tous les matériaux, c'est le magnésium, un élément indispensable du corps humain, qui conduit aux résultats les plus favorables car son module d'Young est similaire à celui de l'os. De ce fait, les méthodes adoptées afin d'améliorer le comportement du magnésium pur vis-à-vis de la corrosion sont les suivantes: a)Ajout d'éléments d'alliage comme le zinc, le calcium et l'erbium (Mg-2Zn-2Er, Mg-2Zn-0.6Ca-1Er, etc.) pour contrôler le comportement de dégradation b) Procédés secondaires tels que l'extrusion pour modifier sa microstructure c)Revêtements de surface à base de fluorure pour mieux protéger la surface. La première partie de cette thèse porte sur la caractérisation microstructurale d'alliages. La caractérisation microstructurale révèle la présence de MgZn2, de phases W (Mg3Zn3Er2) et i (Mg3Zn6Er) dans différents alliages. L'évaluation des propriétés mécaniques a révélé une augmentation des propriétés de traction et de compression des alliages ternaires et quaternaires par rapport aux alliages de Mg et de Mg-2Zn. Ces propriétés mécaniques améliorées sont attribuées à une réduction de la taille des grains, à la présence d'atomes de soluté et à des phases secondaires. Mg-2Zn-2Er et Mg-3Zn-0.5Er présentaient une résistance à la corrosion améliorée en raison de la microstructure à granulométrie fine et d'une répartition uniforme des phases secondaires. La viabilité cellulaire a été améliorée avec l'épaisseur du temps de revêtement et ces alliages pourraient servir de candidats potentiels pour d'autres tests in vivo
With the ability to bio-degrade and thereby reducing the stress-shielding effect, biodegradable implants are of great importance in medical research. Among all the materials, magnesium is the one which shows promising results being bio-degradable and with the properties comparable with its young's modulus to that of bones. In the present study, the approaches adopted to improve the mechanical and corrosion behaviors of pure magnesium using carefully chosen: (a) Alloying elements like zinc, calcium and erbium (Mg-2Zn-2Er, Mg-2Zn-0.6Ca-1Er, etc.) to control the degradation behavior (b) Secondary processes like extrusion to alter and improve the microstructure (c) Surface treatments like fluoride coatings to further protect the surface to resist the rapid dissolution. The first part of this thesis focuses on the microstructural characterization of as-DMDed and as-extruded alloys. The microstructural characterization (XRD and TEM) reveals the presence of MgZn2, W-phase (Mg3Zn3Er2) and i-phases (Mg3Zn6Er) in different alloys. The mechanical property assessment revealed an increment in the tensile and compressive properties of ternary and quaternary alloys as compared to pure Mg and Mg-2Zn binary alloy. These values are attributed to a reduction in grain size, presence of solute atoms and secondary phases. Mg-2Zn-2Er and Mg-3Zn-0.5Er showed enhanced corrosion resistance due to the fine grain sized microstructure and a uniform distribution of secondary phases. The cell viability values were enhanced with increased coating time and it was found that these alloys could serve as potential candidates for further in-vivo tests to establish their applicability

Mg-V and Mg-Zr alloys with nominal compositions 1, 6, 17.5, 27 wt% V and 2, 8.6 and 10.6 wt% Zr respectively were produced by PVD. All deposits exhibited compositional inhomogeneity, columnar microstructures and a strong basal texture. The solid solubilities of V and Zr in Mg were extended approximately to 17 wt% V and 10 wt% respectively. Grain refinement occurred with increasing solute content. The solid solution break up temperature decreased as the V and Zr content in the alloys increased. Pure V precipitated when the extended solid solubility of was exceeded. Both c and a lattice parameters, as well as the c/a ratio decreased with increasing V content in the Mg-V alloys. The slight increase of the a-lattice parameter and the decrease of the c one led to a decrease of the c/a ratio with increasing Zr additions in the Mg-Zr alloys. The air-formed oxide on the surfaces of the Mg-V alloys consisted predominantly of hydromagnesite at the outermost surface with Mg(OH)2 in excess of MgO underneath. No evidence of V oxide in the surface film was found. Magnesium oxide was also found between the grains of the deposits. The air-formed oxide on the surfaces of the Mg-Zr alloys consisted of ZrO2, MgO and possibly Zr sub-oxide. The presence of the oxides beween the columnar grains gave rise to graded metal/oxide interfaces. The outermost surfaces of the Mg-Zr alloys were similar to the Mg-V ones. Analysis of changes of the Auger parameters of the Mg-V and Mg-Zr alloys was also undertaken in order to investigate the electronic changes that take place upon alloying Mg with V and Zr. Charge transfer between 0.09 and 0.11 electrons/atom from Mg to V as well as changes in the V d charge were calculated by measuring the Mg and V Auger parameters and using the charge transfer model of Thomas and Weightman. Electron transfer between 0.02 and 0.03 electrons/atom from Mg to Zr was also found to occur upon alloying Mg with Zr. The electron transfer has been related to changes in crystal structure. The Mg-V and Mg-Zr alloys were examined after immersion in 3 wt% NaCl solution for 5 and 15 minutes, 9 hours and 7 days. The dramatic increase in the corrosion rate of the Mg-V alloys was attributed to the precipitation of pure V. The unsatisfactory corrosion performance of the Mg-V alloys was attributed to the absence of compositional uniformity through the thickness of the Mg-V deposits and the low thermodynamic stability of the corrosion products in the saline environment. Hydromagnesite at the outermost surface and Mg(OH)2, MgO and V2O4 in the bulk of the corrosion layer were the corrosion products. MgH2 and areas enriched in metallic V within the bulk of the corrosion products were also detected. The low corrosion rates of the Mg-Zr alloys, the lowest ever reported for Mg alloys, were attributed to the nature of the corrosion products and particularly the Zr contribution. The corrosion products were enriched in Zr, and were non-porous and in many cases well adherent. X-ray and electron diffraction suggested the existence of only Mg(OH)2 and MgO in the corrosion products, indirectly implying the participation of zirconium oxide/hydroxide in an amorphous/nanocrystalline state. Surface analysis indicated that a Zr oxide coexisted with Mg(OH)2 and MgO below a magnesium carbonate overlayer and also suggested the existence of Zr hydrous oxide (hydroxide). The repetition of the substrate pattern, as well as the fact that Zr hydroxide was replaced with ZrO2 and Zr sub-oxide as the metal-oxide interface was approached, implied a corrosion mechanism involving inwards diffusion of the anionic species.

The high susceptibility to corrosion limits the broad application of magnesium alloys, and therefore, the corrosion protection of magnesium is of major concern in practical conditions. A great effort has been made in the last few decades to solve this problem. Various types of surface coatings have been developed to provide corrosion protection for magnesium alloys, among which plasma electrolytic oxidation (PEO) is one of the most promising techniques. The PEO treatment can produce a hard ceramic-like oxide coating on magnesium and its alloys, leading to significantly enhanced wear and corrosion resistance. However, the intrinsic porous morphology of the PEO coatings still limits their effect of corrosion protection. The objective of the present work is to overcome this microstructural drawback, and further improve the corrosion protection ability of PEO coatings on magnesium and its alloys. Different approaches have been adopted to reduce the degradation rate of PEO coatings, including optimisation of the PEO treatment itself, sequential processing combining PEO coating with various post-treatments and formation of smart self-healing PEO coatings inspired by biological systems. The PEO process was investigated by analysing the current/voltage transients, and the PEO coatings were systematically characterised by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). Fourier transform infrared spectroscopy (FTIR) was also used to study the chemical composition of the plasma enhanced chemical vapour deposition (PECVD) coatings on the PEO coated magnesium alloy. The corrosion resistance of the PEO coatings in 3.5 wt.% NaCl solution was investigated by the electrochemical methods, including open circuit potential (OCP) monitoring, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation scans (PDP). In addition, the mechanical behaviour of PEO coatings was also examined by scratch testing. It was found that both voltage and frequency have significant effect on the properties of PEO coatings, and a more compact coating was produced by pulsed bipolar voltage mode. The PECVD post-treatment was proven to be an effective method of improving the corrosion protection ability if appropriate precursors were used, as this method could cause both positive and negative effects. Coating degradation could also occur during immersion post-treatments, although the corrosion resistance of the PEO coating was also improved by the Ce deposition and benzotriazole (BTA) adsorption. In this case, a better corrosion protection was achieved by combining the PEO coating with Ce-based immersion post-treatment, as the insoluble Ce-containing compounds provided both sealing effect and the inhibition of cathodic reaction. Finally, the self-healing PEO coating incorporated with inhibitor loaded nanocontainers was developed and shown a good potential for providing a long-term corrosion protection for magnesium alloys, even though the corrosion resistance was not significantly increased compared with conventional PEO coating. However, none of the above approaches was perfect, indicating that there is still plenty of work to be done in the future.

As an alternative to the present toxic chromate-based coating system now in use, the Mg-rich primer technology has been designed to protect Al alloys (in particular Al 2024 T3) and developed in analogy to Zn-rich primers for steel substrate. As an expansion of this concept, metal-rich primer systems based on Mg alloy particles as pigments were studied. Five different Mg alloy pigments, AM60, AZ91B, LNR91, AM503 and AZG, were characterized by using the same epoxy-polyamide polymer as binder, a same dispersion additive and the same solvent. Different Mg alloy-rich primers were formulated by varying the Mg alloy particles and their pigment volume concentrations (PVC). The electrochemical performance of each Mg alloy-rich primer after the cyclic exposure in Prohesion chamber was investigated by electrochemical impedance Spectroscopy (EIS). The results indicated that all the Mg alloy-rich primers could provide cathodic protection for AA 2024 T3 substrates. However, the Mg alloys as pigments in metal-rich primers seemed to exhibit the different anti-corrosion protection performances, such as the barrier properties, due to the different properties of these pigments. In these investigations, multiple samples of each system were studied and statistical methods were used in analyzing the EIS data. From these results, the recommendation for improved EIS data analysis was made. CPVC studies were carried out on the Mg alloy-rich primers by using three Mg alloy pigments, AM60, AZ91B and LNR91. A modified model for predicting CPVC is proposed, and the results showed much better agreement between the CPVC values obtained from the experimental and mathematical methods. Using the data from the AM60 alloy pigment system, an estimate of experimental coarseness was done on a coating system, the first time such an estimate has been performed. By combining various surface analysis techniques, such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and confocal Raman microscopy, the oxidation products formed after exposure were identified. It was found that variation of Al content in Mg alloy could significantly affect the pH of the microenvironment in the primer films and result in the formation of various oxidation products.
Air Force Office of Scientific Research (Grant No. 49620-02-1-0398)

As an alternative to the present toxic chromate-based coating system now in use, the Mg-rich primer technology has been designed to protect A1 alloys (in particular A1 2024 T3) and developed in analogy to Zn-rich primers for steel substrate. As an expansion of this concept, metal-rich primer systems based on Mg alloy particles as pigments were studied. Five different Mg alloy pigments. AM60, A719B, LNR91, AM503 and AZG, were characterized by using the same epoxy-polyamide polymer as binder, a same dispersion additive and the same solvent. Different Mg alloy-rich primers were formulated by varying the Mg alloy particles and their pigment volume concentrations (PVC). The electrochemical performance of each Mg alloy-rich primer alter the cyclic exposure in Prohesion chamber was investigated by electrochemical impedance Spectroscopy (EIS). The results indicated that all the Mg alloy-rich primers could provide cathodic protection for AA 2024 T3 substrates. However, the Mg alloys as pigments in metal-rich primers seemed to exhibit the different anti-corrosion protection performances, such as the barrier properties, due to the different properties of these pigments. In these investigations, multiple samples of each system were studied and statistical methods were used in analyzing the EIS data. From these results. the recommendation for improved EIS data analysis was made. CPVC studies were carried out on the Mg alloy-rich primers by using three Mg alloy pigments, AM60, A2918 and LNR91. A modified model for predicting CPVC is proposed, and the results showed much better agreement between the CPVC values obtained from the experimental and mathematical methods. Using the data from the AM60 alloy pigment system, an estimate of experimental coarseness was done on a coating system, the first time such an estimate has been performed. By combining various surface analysis techniques, such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and confocal Raman microscopy, the oxidation products formed alter exposure were identified. It was found that variation of A1 content in Mg alloy could significantly affect the pH of the microenvironment in the primer films and result in the formation of various oxidation products.
Air Force Office of Scientific Research (AFOSR) (Grant No. 49620-02-1-0398)

Biomaterials have been used for more than a century in the human body to improve body functions and replace damaged tissues. Currently approved and commonly used metallic biomaterials such as, stainless steel, titanium, cobalt chromium and other alloys have been found to have adverse effects leading in some cases, to mechanical failure and rejection of the implant. The physical or chemical nature of the degradation products of some implants initiates an adverse foreign body reaction in the tissue. Some metallic implants remain as permanent fixtures, whereas others such as plates, screws and pins used to secure serious fractures are removed by a second surgical procedure after the tissue has healed sufficiently. However, repeat surgical procedures increase the cost of health care and the possibility of patient morbidity. This study focuses on the development of magnesium based biodegradable alloys/metal matrix composites (MMCs) for orthopedic and cardiovascular applications. The Mg alloys/MMCs possessed good mechanical properties and biocompatible properties. Nine different compositions of Mg alloys/MMCs were manufactured and surface treated. Their degradation behavior, ion leaching, wettability, morphology, cytotoxicity and mechanical properties were determined. Alloying with Zn, Ca, HA and Gd and surface treatment resulted in improved mechanical properties, corrosion resistance, reduced cytotoxicity, lower pH and hydrogen evolution. Anodization resulted in the formation of a distinct oxide layer (thickness 5-10 μm) as compared with that produced on mechanically polished samples (~20-50 nm) under ambient conditions. It is envisaged that the findings of this research will introduce a new class of Mg based biodegradable alloys/MMCs and the emergence of innovative cardiovascular and orthopedic implant devices.

Magnesium and its alloys are considered as the potential biomaterials due to their biocompatibility and biodegradable characteristics but suffer from poor corrosion performance. Various surface modification techniques are employed to improve their corrosion resistance. In present case, laser surface melting was carried out on AZ31B Mg alloy with various laser energy densities using a continuous wave ytterbium laser. Effect of laser treatment on phase and microstructure evolution was evaluated by X ray diffraction and scanning electron microscopy. Multi-physics thermal model predicted time temperature evolution along the depth of the laser treatment zone. Additionally, electrochemical method and bio-immersion test were employed to evaluate the corrosion behavior in simulated body fluid medium. Microstructure revealed grain refinement and even distribution of Mg17Al12 phase along the grain boundary for laser treated samples leading to substantial enhancement in the corrosion resistance of the laser treated samples compared to the untreated alloy. The laser processed samples also possessed a superior wettability in SBF solution than the untreated sample. This was further reflected in enhanced bio-integration behavior of laser processed samples. By changing the parameters of laser processing such as power, scanning speed, and fill spacing, a controllable corrosion resistance and bioactivity/biocompatibility of the implant material was achieved.

Les alliages de Magnésium (Mg) sont une nouvelle génération de matériaux biodégradables ayant une bonne ostéointégration et un module d'élasticité similaire à celle de l'os humain. Ces propriétés rendent ces matériaux attrayant pour produire des implants temporaires pour la réparation osseuse. Toutefois, les alliages Mg se dégradent rapidement in vivo, rendant nécessaire de contrôler leur vitesse de corrosion pour accompagner la régénération tissulaire. Parmi les approches proposées pour réduire la corrosion et la biocompatibilité des alliages Mg, les plus utilisées sont les couches de conversion et les revêtements de surface. Dans ce travail une approche synergique qui combine une réduction du taux de corrosion avec l'amélioration de la biocompatibilité des alliages Mg est proposée. De nouveaux revêtements bicouches ont été déposés sur la surface d'alliages AZ31 et ZE41 : (i) un revêtement de silane-TiO2 déposé par dip-coating et (ii) des couches supérieures de collagène de type I et/ou de chitosane. Le revêtement inférieur a été efficace pour réduire la corrosion des alliages dans un fluide corporel simulé et en milieu de culture. La culture cellulaire in vitro de fibroblastes et ostéoblastes, a révélé que le dépôt additionnel de biopolymères a amélioré la réponse biologique du revêtement de silane-TiO2. Ces résultats montrent qu'il existe un effet combiné des revêtements bicouches et de la composition des alliages sur la réponse à la corrosion et sur le comportement cellulaire. Ce travail apporte donc une nouvelle contribution à la compréhension de l'évolution de la corrosion des alliages Mg dans des environnements biologiques
Magnesium (Mg) alloys are a new generation of biodegradable materials with good osseointegration and elastic modulus similar to that of human bone. These properties make them attractive materials to produce biodegradable implants for bone repairing applications that require temporary support. However, Mg alloys degrade rapidly in the in vivo environment making necessary to control their corrosion rate to accompany the tissue healing processes. Several approaches have been proposed for reducing corrosion rate and improving biocompatibility of Mg alloys. The most used ones are conversion films and surface coatings. This project proposes a synergistic approach that combines both decreased corrosion rate and improved biocompatibility of Mg alloys: we developed novel bi-layered coatings to functionalize the surface of AZ31 and ZE41 Mg alloys for bone repair applications. First, a bottom silane-TiO2 coating was formulated and deposited on both alloys by the dip-coating technique. The silane-based coating was effective in slowing down the corrosion rate of the substrates in simulated body fluid (SBF) and in Dulbecco’s Modified Eagle’s Medium (DMEM). Secondly, top layers of type I collagen and/or chitosan were developed. Cell in vitro tests, with fibroblasts and osteoblasts, revealed that the biopolymers enhanced the biological response of the silane-TiO2 coating. Furthermore, the findings showed that there is a combined effect of the bi-layered coatings and the nature of the alloys on their final corrosion response and on the fate of the cells. In the same way, this work contributes to elucidating corrosion processes of Mg alloys in organic solutions in the long-term

The goals of this research are to develop deeper understanding of the corrosion protection mechanism of Mg-rich primer (MgRPs), improve corrosion protection performance of MgRPs, and extend the application of MgRPs. To address these research goals, the following studies were performed: 1. Early blistering problems encountered during constant immersion or ASTM B117 exposure of top-coated MgRPs over AA2024-T3 substrate were investigated. The results suggest that hydrogen entrapment by topcoat, instead of Al corrosion, contributes significantly to the formation of early blistering. Meanwhile, simultaneous real-time hydrogen collection and open circuit potential measurement was demonstrated as a new method for studying the corrosion protection mechanism of MgRPs. Moreover, the gas generated from MgRPs was unequivocally identified as hydrogen by cyclic voltammetry. 2. Degradation behaviors of MgRP in 1% NaCl solution and Dilute Harrison Solution (DHS) were compared through scanning electron microscopy, hydrogen volume collection and electrochemical tests. The effects of connection modes between Mg pigment and Al substrate, different ions on the formation and stability of Mg oxidation products, and cathodic reaction sites on the microstructure of MgRP were discussed. In addition, an in situ method for the estimation of remaining Mg pigment in MgRP was developed based on H2 volume collection. 3. The effects of adding sodium benzoate (SB), sodium dodecylbenzenesulfonate (SDBS), and 8-hydroxyquinoline (HQ) to MgRP on its corrosion protection of AA 2024-T3 were investigated. The results show that addition of SB, SDBS and HQ into MgRP improved the corrosion protection performance of MgRP by decelerating the oxidation rate of Mg, improving coating barrier properties and inhibiting the corrosion of Al alloy substrate. 4. The (MgRP-powder topcoat) coating system was developed and characterized in this research for the corrosion protection of Al alloys. The results show that powder topcoat can be applied on top of MgRP through both fluidized bed and electrostatic spray methods. Moreover, this (MgRP-powder topcoat) coating system provided much longer corrosion protection time to Al substrate than the powder coat by itself, without degrading other coating properties.
AkzoNobel, US DOD, OSD, Technical Corrosion Collaboration (TCC) and NDSU Center for Surface Protection

Magnesium alloys have been widely explored as potential biomaterials, but several limitations to using these materials have prevented their widespread use, such as uncontrollable degradation kinetics which alter their mechanical properties. In an attempt to further the applicability of magnesium and its alloys for biomedical purposes, two novel magnesium alloys Mg-Zn-Cu and Mg-Zn-Se were developed with the expectation of improving upon the unfavorable qualities shown by similar magnesium based materials that have previously been explored. The overall performance of these novel magnesium alloys has been assessesed in three distinct phases of research: 1) analysing the mechanical properties of the as-cast magnesium alloys, 2) evaluating the biocompatibility of the as-cast magnesium alloys through the use of in-vitro cellular studies, and 3) profiling the degradation kinetics of the as-cast magnesium alloys through the use of electrochemical potentiodynamic polarization techqnique as well as gravimetric weight-loss methods. As compared to currently available shape memory alloys and degradable as-cast alloys, these experimental alloys possess superior as-cast mechanical properties with elongation at failure values of 12% and 13% for the Mg-Zn-Se and Mg-Zn-Se alloys, respectively. This is substantially higher than other as-cast magnesium alloys that have elongation at failure values that range from 7-10%. Biocompatibility tests revealed that both the Mg-Zn-Se and Mg-Zn-Cu alloys exhibit low cytotoxicity levels which are suitable for biomaterial applications. Gravimetric and electrochemical testing was indicative of the weight loss and initial corrosion behavior of the alloys once immersed within a simulated body fluid. The development of these novel as-cast magnesium alloys provide an advancement to the field of degradable metallic materials, while experimental results indicate their potential as cost-effective medical devices.

Advances in biomaterials have enabled medical practitioners to replace diseased body parts or to assist in the healing process. In situations where a permanent biomaterial implant is used for a temporary application, additional surgeries are required to remove these implants once the healing process is complete, which increases medical costs and patient morbidity. Bio-absorbable materials dissolve and are metabolized by the body after the healing process is complete thereby negating additional surgeries for removal of implants. Magnesium alloys as novel bio-absorbable biomaterials, have attracted great attention recently because of their good mechanical properties, biocompatibility and corrosion rate in physiological environments. However, usage of Mg as biodegradable implant has been limited by its poor corrosion resistance in the physiological solutions. An optimal biodegradable implant must initially have slow degradation to ensure total mechanical integrity then degrade over time as the tissue heals. The current research focuses on surface modification of Mg alloy (MZC) by surface treatment and polymer coating in an effort to enhance the corrosion rate and biocompatibility. It is envisaged that the results obtained from this investigation would provide the academic community with insights for the utilization of bio-absorbable implants particularly for patients suffering from atherosclerosis. The alloying elements used in this study are zinc and calcium both of which are essential minerals in the human metabolic and healing processes. A hydrophobic biodegradable co-polymer, polyglycolic-co-caprolactone (PGCL), was used to coat the surface treated MZC to retard the initial degradation rate. Two surface treatments were selected: (a) acid etching and (b) anodization to produce different surface morphologies, roughness, surface energy, chemistry and hydrophobicity that are pivotal for PGCL adhesion onto the MZC. Additionally, analyses of biodegradation, biocompatibility, and mechanical integrity were performed in order to investigate the optimum surface modification process, suitable for biomaterial implants. The study concluded that anodization created better adhesion between the MZC and PGCL coating. Furthermore, PGCL coated anodized MZC exhibited lower corrosion rate, good mechanical integrity, and better biocompatibility as compared with acid etched.

Magnesium (Mg) and its alloys are known for their high chemical reactivity. This property often poses issues related to undesirable corrosion, or degradation of exposed surfaces. The chemical reactivity of Mg can be also exploited, and as a result Mg alloys often find use as anode materials for fuel cells. However, due to a long term immersion of the anodes in highly alkaline environments, the problem of corrosion remains and needs to be evaluated. Therefore, in this research, the corrosion behavior of a commercially available magnesium alloy AZ31 in 45 wt% potassium hydroxide (KOH), a common electrolyte for alkaline fuel cells, was studied. Immersion tests were performed for a total duration of 20 days to study the growth of corrosion products on the alloy’s surface. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) were carried out to characterize the structure and chemistry of the corrosion products. Also, electrochemical studies were carried out to study the kinetics of corrosion of the AZ31 alloy. Finally, the effect of adding 2 wt% sodium silicate (Na₂SiO₃) to the KOH electrolyte in order to manipulate the corrosion rate was also examined. Tafel analysis confirmed that the corrosion potential of the AZ31 sample immersed in the Na₂SiO₃ + KOH solution reduced by 16% with respect to that of sample immersed in pure KOH. Although the AZ31 alloy contains only a trace amount of nickel, SEM-EDS characterization of the corrosion products revealed that they contained high levels of nickel, with XRD analysis confirming the presence of a nickel hydroxide layer. In the case of the sample immersed in Na₂SiO₃ + KOH electrolyte, an additional layer rich in silicates developed, and likely acted as a barrier for diffusion of ions from surface of the AZ31 sample to the electrolyte. EIS results of modeling the surface corrosion phenomena revealed that a modified Randle’s circuit represented the electrochemical processes occurring on the surface of the alloy. Warburg impedance for the sample immersed in Na₂SiO₃ + KOH was relatively high, suggesting a dissolution of ions from the surface into the highly alkaline KOH electrolyte.
Graduate Studies, College of (Okanagan)
Graduate

@L'objet de ce travail était l'étude de l'oxydation d'un alliage aluminium-5% magnésium à l'état liquide sous oxygène, par analyse thermogravimétrique, en s'appuyant sur celle du magnésium à l'état solide. Dans la plupart des cas, la vitesse absolue d'une réaction est proportionnelle au produit de la réactivité de croissance et de la fonction d'espace caractérisant les dimensions des zones réactionnelles mises en jeu dans le mécanisme de croissance. Nous avons vérifié que c'est le cas de la réaction étudiée ici c'est-à-dire la formation de l'oxyde de magnésium. Les mécanismes de formation de la magnésie dépendent de la pression partielle d'oxygène fixée dans l'enceinte réactionnelle, l'allure des courbes cinétiques ainsi que les morphologies des oxydes formés étant très différentes selon la pression d'oxygène. Sous de faibles pressions partielles, la réaction a lieu en phase gaz après évaporation du magnésium. Dans le cas de l'alliage, un mécanisme de formation de l'oxyde en phase gaz a été proposé. Ce modèle permet de rendre compte de l'influence de la pression partielle sur la vitesse de formation par unité de surface d'évaporation de la magnésie. Dans le domaine de fortes pressions d'oxygène et pour le magnésium solide, une modélisation physico-chimique décrivant les mécanismes de croissance de la magnésie et prévoyant l'évolution au cours du temps de la fonction d'espace a été proposée et validée par confrontation aux résultats expérimentaux. Dans le cas de l'alliage, les courbes cinétiques obtenues dans des conditions isothermes et isobares se sont révélées non reproductibles. Une méthodologie d'étude, basée sur l'utilisation de la méthode des décrochements, a rendu possible la modélisation de la croissance de l'oxyde. En effet, cette méthode a permis de déterminer l'origine de la non reproductibilité des courbes cinétiques, à savoir l'évolution aléatoire de la fonction d'espace, et d'obtenir les variations de la réactivité de croissance avec la pression d'oxygène. En nous appuyant sur la modélisation proposée pour le magnésium, nous avons proposé un modèle physico-chimique décrivant les mécanismes de croissance de l'oxyde et rendant compte de l'influence de la pression d'oxygène.

The main aim of this research was to develop novel nanocomposite PVD coatings for magnesium alloys, to improve their wear and corrosion resistance – and thereby explore the potential to extend the use of such alloys to moving parts for light-weighting of tribological components, where the potential for cumulative weight savings is immense if key parts can be made from magnesium, but the alloys cannot currently be used successfully due to their poor wear and corrosion behaviour under dynamic loading. The work comprises two main stages. The first stage was to produce a base layer for subsequent PVD ceramic nitride (or nitrogen-doped hard metallic) coating deposition. The second stage was to deposit a nanocomposite coating with improved tribological performance, by introducing sequentially nitrogen reactive gas, subsequent to the base layer preparation step. In the first stage, sixteen AlCuMoMgZrB PVD coating layers were prepared by pulsed direct current closed-field unbalanced magnetron sputtering. Four deposition runs were carried out, with substrate negative bias voltages of 50 V, 60 V, 75 V and 100 V being applied. For each deposition run, four proprietary WE43 magnesium alloy substrates were placed at different positions (P1-P4) on the substrate holder, between AlMgB and ZrMoCu composite sputter targets mounted at 90° to each other. Investigations into composition, microstructure, mechanical and electrochemical properties were then carried out, to select the most suitable base layer. The P1-60 layer (i.e. deposited at P1 position, closest to the AlMgB composite target, with substrate negative bias of 60 V) was chosen as the most suitable candidate amongst the sixteen AlCuMoMgZrB coating layers due to its superior mechanical properties, electrochemical properties, and amorphous microstructure. In the second stage, four novel AlCuMoMgZrB(N) nanocomposite PVD coatings with different nitrogen reactive gas flow rates (i.e. 5 sccm, 10 sccm, 15 sccm and 20 sccm), introduced partway through the sputter deposition process, were produced sequentially, on top of the selected P1-60 base layer. Further detailed investigations into composition, microstructure, mechanical, tribological and electrochemical properties were performed to evaluate the improved wear and corrosion resistance. For practical applications, P1-60-15sccm (46.27 at.% Al, 8.71. at.% Mg, 5.35 at.% Cu, 3.63 at.% Mo, 1.30 at.% Zr, 2.65 at.% B and 32.08 at.% N) seems a likely candidate to provide an optimal combination of wear and corrosion resistance – in terms of the best and the second-best performance in micro-abrasion and in corrosion tests, respectively.

The present study concerns the application of LSM using an excimer laser to enhance the corrosion resistance of rare-earth Elektron 21 magnesium alloy. The alloy has been treated by an excimer laser to produce a highly homogeneous and refined microstructure for improvement of corrosion resistance. The laser surface treatment was applied on two different prepared surfaces of the alloy; i) a ground surface up to 1200 SiC grit; ii) a chemically cleaned surface using CrO3 +AgNO3 boiling solution. The intermetallic phases within the α-matrix that are believed to initiate corrosion have been dissolved by two methods. The first is by the excimer laser, where they were dissolved in the melted layers. The second is by a chemical dissolution prior LSM. Variation of the laser parameters such as changed laser influence (low, medium and high) and increased number of pulses, resulted in formation of thicker melted layers, but promoted the formation of porosity and micro-cracks particularly at overlap regions. The initial stage of this study was aimed at optimising the laser conditions for production of a uniform microstructure, with an increase in the corrosion resistance of the alloy being determined by potentiodynamic polarization measurements in sodium chloride solution. A laser fluence of 6 and 7 J/cm2 with 10, 20, 25, 40 and 50 pulses with a different overlap ratio of 7%, 20% and 50% were subsequently selected as the optimum condition to treat the surface of the alloy. After laser treatment, the top surfaces and the cross-sections of the alloy showed a relatively homogenous melted layer and a significant reduction in the number of large intergranular Mg-Zn-RE phase was achieved resulting in a significant improvement of the corrosion resistance of the alloy. This work also investigated the mechanism of corrosion and the interaction between the intergranular Mg-Zn-RE phase, the Zr-rich regions within the grains and the bulk Mg-rich matrix. The results obtained by scanning electron microscopy (SEM) / energy-dispersive X-ray (EDX) and scanning Kelvin prop forced microscopy (SKPFM) potential map measurements as well as transmission electron microscopy (TEM) / energy-dispersive X-ray (EDX) have shown the importance of the microstructure in the initiation of corrosion in 3.5 wt% NaCl solution, where the Zr-rich regions played a distinct role in the early stages of corrosion in this alloy. However, the obtained results have demonstrated that such laser melted layers improved the corrosion resistance of the alloy, but further work is still needed to obtain the fully understanding of such behaviour which can better the research results, particularly the selectively chemical dissolution of the second phases prior LSM.

AZ31 magnesium alloy sheet offers a promising candidate for the weight reduction of large-scale automotive components in order to save fuel and reduce the carbon dioxide emission. Despite its good corrosion performance in air, high humidity and the presence of aggressive ions such as chlorides significantly reduce the corrosion resistance leading to localised and fast dissolution. Consequently, the use of magnesium-based alloys is limited, in particular in external automotive applications. A comprehensive understanding of the corrosion behaviour of AZ31 sheet, taking into account the alloy's microstructure, is needed in order to improve its corrosion protection. This research programme elaborately investigated the commercial AZ31B-H24 magnesium alloy sheet with regards to its microstructure and corrosion performance in concentrated and dilute sodium chloride solutions. The grain structure and typical AlMn intermetallic phases were characterized in addition to grain boundary particles of γ-Mg17(Al,Zn)12. Possessing nobler electrochemical potentials with respect to the magnesium matrix, all the intermetallic phases provided cathodically active sites corroding at significantly slower rates. During corrosion, coarse AlMn intermetallic particles were found to dealloy primarily with selective dissolution of aluminium. High-resolution microscopy revealed that zinc and aluminium segregation on the grain boundaries led to intergranular corrosion. The corrosion in AZ31B-H24 alloy proceeded in three well-defined sequential stages upon immersion in sodium chloride solutions, namely corrosion initiation (stage I), shallow filiform-like propagation (stage II) and localised in-depth corrosion (stage III). It was concluded that the corrosion stages were mainly determined by the formation of resistive corrosion products and defects in the alloy. The comprehensive ex-situ microscopic study of the three corrosion stages in combination with real-time measurement of hydrogen evolution and conventional electrochemical investigations provided new insights such as the formation of crystallographically oriented pores (COP) indicating corrosion propagation mechanism. The COP preferentially developed parallel to the {10-10} and the {11-20} families of planes as identified by electron backscatter diffraction. Mechanistic corrosion aspects including the cathodic activation of the intact surface were discussed and explained. The intact surface was covered by a bilayer, consisting of an outer layer of crystalline Mg(OH)2 platelets and an inner layer of hydrated and amorphous Mg(OH)2, that grew with immersion time. Zinc enrichment formed under the bilayer and its thickness increased with increasing exposure reaching 7 ± 1 nm after 4 days immersion. Nano-sized particles in the enrichment were detected and identified as MgZn2. Enrichment in zinc and aluminium under the corrosion products seemed to have minor influences on the overall cathodic activity.

The low density and high relative strength of Mg alloys means they can offer engineering benefits over steels or Al alloys. However, the susceptibility of Mg alloys to corrosion has limited their exploitation and restricted their use to more benign environments. An Mg-Al intermetallic surface layer is a good candidate for a robust corrosion protection method. This work demonstrates their development by using a novel ionic liquid electroplating process to deposit Al on to Mg substrates that when heat treated diffuses to form discrete intermetallic layers. Examination of three Mg-Al-Zn alloys showed that the amount Mg-Al intermetallic phases in their microstructures was linked to the quantity of Al they contained. Subsequent self-corrosion measurements using electrochemical impedance spectroscopy demonstrated that their performance was connected to the amount of intermetallic present, and in particular the strength of the micro-galvanic couples generated between the anodic and cathodic phases. Measurements of the self-corrosion behaviour of manufactured samples of the Mg-Al intermetallics confirmed that they could provide significant improvements, but it was acknowledged that their noble nature compared to an Mg substrate would encourage galvanic corrosion if a surface layer was damaged. As such the galvanic activity of the Mg-Al-Zn alloys and Mg-Al intermetallics was compared against a pure Mg standard using zero resistance ammetry and the resistance box technique. Galvanic models of alloy self-corrosion and a damaged intermetallic surface layer were also used to assess the potential problem. These measurements demonstrated that the intermetallics could act as strong cathodes, but further discussion on the nature of the behaviour suggested means by which galvanic corrosion might self-limit or self-repair. The galvanic corrosion experiments also revealed how the combination of current flow and a solution saturated with Mg2+ ions could lead to the formation of a highly protective Mg(OH)2 film with promising characteristics.

[ES] En los últimos años, se han desarrollado numerosos tipos de recubrimientos superficiales frente a la corrosión basados especialmente en aleaciones de cinc-aluminio-magnesio (conocidas como aleaciones "ZM"), como alternativas a los recubrimientos tradicionales basados en cinc (conocidos como "Z"), con el fin de mejorar sus características técnicas y reducir su coste. Los fabricantes de estos nuevos tratamientos reivindican una mayor resistencia a la corrosión, basándose en ensayos de corrosión acelerada y ensayos de campo, estos últimos de muy pocos años de duración. La presente tesis, tiene como principal objetivo la estructuración y análisis de toda la información existente en el actual estado de la técnica, y en particular, el estudio de los ensayos de campo existentes para corroborar su resistencia a la corrosión en distintos tipos de ambientes y a partir de ello, proponer un modelo matemático que facilite su cálculo a largo plazo. Se presenta una revisión del estado de la técnica de recubrimientos metálicos basados en aleaciones ZM, que cubre su evolución en el tiempo, las diferentes calidades y designaciones existentes en el mercado, su estructura y composición, normas internacionales que los regulan y una detallada investigación sobre ensayos de campo en localizaciones de todo el mundo, habiéndose encontrado ensayos de una duración máxima de 6 años. A partir del análisis de estos ensayos de campo, se propone una Metodología para verificar el rendimiento y la evolución de la función corrosión-tiempo, en los diferentes ambientes de exposición, categorizados a través de la norma internacional ISO 9223 (ISO, 2012), que los denomina "categorías de corrosividad", y que abarcan desde C1 (muy bajo) hasta CX (extremo). Este análisis ha clasificado todos los resultados de los ensayos por material, categoría de corrosividad y evolución a lo largo del tiempo. De esta forma, cada categoría de corrosividad ha sido investigada en profundidad, mediante un análisis estadístico, poniendo especial énfasis en la corrosión anual, medida como pérdida de masa (µm / año), la función corrosión-tiempo y su ajuste a un determinado comportamiento. Se han analizado asimismo los recubrimientos Z con el fin de poder comparar ambas alternativas y corroborar la hipótesis de partida, cuyo supuesto principal es la mayor resistencia a la corrosión de las aleaciones ZM frente a los recubrimientos Z. Este análisis ha sido el punto de entrada, para establecer un modelo matemático que determine el rendimiento de la corrosión a largo plazo, con el fin de proporcionar a los profesionales de proyectos en la ingeniería, una herramienta que permita estimar la resistencia a la corrosión y la optimización del coste de una instalación cuando se utilizan diferentes tipos de materiales. El compendio de todo este análisis se ha reflejado en el apartado de Resultados y comentarios. La referida metodología, se ha aplicado a un caso de estudio para mostrar cómo seleccionar la calidad del recubrimiento y su espesor óptimo, así como un cálculo de costes, con el objetivo de garantizar los requisitos de un determinado proyecto, en términos de resistencia a la corrosión y coste. Las conclusiones finales ponen de manifiesto que existen algunas ventajas de las aleaciones ZM frente a recubrimientos Z, principalmente en lo que respecta a la resistencia a la corrosión, al haber encontrado relaciones que pueden duplicar y triplicar su rendimiento, en los períodos para los que hay datos disponibles. Del mismo modo, se han encontrado algunas desventajas, que deben investigarse más a fondo en futuros trabajos de investigación, para dar continuidad a esta tesis. Por ejemplo, la limitación de estos recubrimientos para lograr grandes espesores, la limitada duración de los ensayos de campo, el rendimiento en partes específicas de los componentes (cortes, embuticiones, doblados, soldaduras...), etc.
[CA] En els últims anys, s'han desenvolupat nombrosos tipus de recobriments superficials enfront de la corrosió basats especialment en aliatges de zinc-alumini-magnesi (conegudes com a aliatges "ZM"), com a alternatives als recobriments tradicionals basats en zinc (coneguts com a "Z"), amb la finalitat de millorar les seues característiques tècniques i reduir el seu cost. Els fabricants d'aquests nous tractaments reivindiquen una major resistència a la corrosió, basant-se en assajos de corrosió accelerada i assajos de camp, aquests últims de molt pocs anys de duració. La present tesi, té com a principal objectiu l'estructuració i anàlisi de tota la informació existent en l'actual estat de la tècnica, i en particular, l'estudi dels assajos de camp existents per a corroborar la seua resistència a la corrosió en diferents tipus d'ambients i a partir d'això, proposar un model matemàtic que facilite el seu càlcul a llarg termini. Es presenta una revisió de l'estat de la tècnica de recobriments metàl·lics basats en aliatges ZM, que cobreix la seua evolució en el temps, les diferents qualitats i designacions existents en el mercat, la seua estructura i composició, normes internacionals que els regulen i una detallada investigació sobre assajos de camp en localitzacions de tot el món, havent-se trobat assajos d'una duració màxima de 6 anys. A partir de l'anàlisi d'aquests assajos de camp, es proposa una metodologia per a verificar el rendiment i l'evolució de la funció corrosió-temps, en els diferents ambients d'exposició, categoritzats a través de la norma internacional ISO 9223 (ISO, 2012), que els denomina "categories de corrosivitat", i que abasten des de C1 (molt baix) fins a CX (extrem). Aquesta anàlisi ha classificat tots els resultats dels assajos per material, categoria de corrosivitat i evolució al llarg del temps. D'aquesta manera, cada categoria de corrosivitat ha sigut investigada en profunditat, mitjançant una anàlisi estadística, posant especial èmfasi en la corrosió anual, mesura com a pèrdua de massa (µm / any), la funció corrosió-temps i el seu ajust a un determinat comportament. S'han analitzat així mateix els recobriments Z amb la finalitat de poder comparar totes dues alternatives i corroborar la hipòtesi de partida, el supòsit principal de la qual és la major resistència a la corrosió dels aliatges ZM enfront dels recobriments Z. Aquesta anàlisi ha sigut el punt d'entrada, per a establir un model matemàtic que determine el rendiment de la corrosió a llarg termini, amb la finalitat de proporcionar als professionals de projectes en l'enginyeria, una eina que permeta estimar la resistència a la corrosió i l'optimització del cost d'una instal·lació quan s'utilitzen diferents tipus de materials. El compendi de tota aquesta anàlisi s'ha reflectit en l'apartat de Resultats i comentaris. La referida metodologia, s'ha aplicat a un cas d'estudi per a mostrar com seleccionar la qualitat del recobriment i la seua grossària òptima, així com un càlcul de costos, amb l'objectiu de garantir els requisits d'un determinat projecte, en termes de resistència a la corrosió i cost. Les conclusions finals posen de manifest que existeixen alguns avantatges dels aliatges ZM enfront de recobriments Z, principalment pel que fa a la resistència a la corrosió, en haver trobat relacions que poden duplicar i triplicar el seu rendiment, en els períodes per als quals hi ha dades disponibles. De la mateixa manera, s'han trobat alguns desavantatges, que han d'investigar-se més a fons en futurs treballs de recerca, per a donar continuïtat a aquesta tesi. Per exemple, la limitació d'aquests recobriments per a aconseguir grans grossàries, la limitada duració dels assajos de camp, el rendiment en parts específiques dels components (talls, embuticions, doblegats, soldadures...), etc.
[EN] In recent years, numerous types of surface corrosion coatings, based especially on zinc-aluminium-magnesium alloys (known as "ZM" alloys), have been developed as alternatives to traditional zinc-based coatings (known as "Z"), to improve its technical characteristics and reduce its cost. The manufacturers of these new treatments claim greater resistance to corrosion, based on accelerated corrosion tests and field tests, the latter lasting only a few years. The main objective of this thesis is the structuring and analysis of all the existing information in the current state of the art, and in particular, the study of the existing field tests to corroborate their resistance to corrosion in different types of environments and based on this, propose a mathematical model that facilitates its long-term calculation. A review of the state of the art of metal coatings based on ZM alloys is presented, which covers their evolution over time, the different qualities and designations existing in the market, their structure and composition, international standards that regulate them and a detailed research on field tests in different locations around the world, having found tests of a maximum duration of 6 years. From the analysis of these field tests, a methodology is proposed to verify the performance and evolution of the corrosion-time function in the different exposure environments, categorized through the international standard ISO 9223 (ISO, 2012), which calls them "corrosivity classes", and which range is from C1 (very low) to CX (extreme). This analysis has classified all the test results by material, corrosivity class and evolution over time. In this way, each corrosivity class has been investigated in depth, through statistical analysis, with special emphasis on annual corrosion, measured as mass loss (µm / year), the corrosion-time function and its adjustment to a certain behaviour. The Z coatings have also been analysed to be able to compare both alternatives and corroborate the main hypothesis, whose main assumption is the greater resistance to corrosion of ZM alloys compared to Z coatings. This analysis has been the entry point to establish a mathematical model that determines the long-term corrosion performance, to provide project engineering professionals, with a tool to estimate the corrosion resistance and optimize the cost of an installation when different types of materials are used. The summary of all this analysis has been reflected in the Results and discussion section. The referred methodology has been applied to a case study to show how to select the quality of the coating and its optimal thickness, as well as a cost calculation, in order to guarantee the requirements of a specific project, in terms of resistance to corrosion and cost. The final conclusions show that there are some advantages of ZM alloys over Z coatings, mainly with regard to corrosion resistance, having found relationships that can double and triple their performance, in the periods for which there are data available. In the same way, some disadvantages have been found, which must be investigated further in future research works, to give continuity to this thesis. For example, the limitation of these coatings to achieve large thicknesses, the limited duration of field tests, the performance of specific parts of the components (cuts, embossments, bends, welds ...), etc.
Chenoll Mora, E. (2021). Analysis of metallic coatings based in zinc-aluminium-magnesium alloys, in terms of performance and long-term corrosion. Case study: Electrical cable trays selection in project design [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/167418
TESIS

The hot forming-quenching (HFQ) process has introduced grains and subgrain growth, accompanied with modification of the intermetallic particle distribution in AZ31 magnesium alloys. Each region of the HFQ component represents significant grain structure variation and surface conditions that contributed to the corrosion susceptibility. The homogeneous grain structure significantly ruled the corrosion propagation features by filiform-like corrosion. Immersion of AZ31 alloys in 3.5 wt.% NaCl indicated higher corrosion rate of HFQ TRC (corrosion rate: 10.129 mm/year), a factor of 10 times, higher than the rolled alloy (corrosion rate: 0.853 mm/year) and a factor of 2 times, higher than the corrosion rate of MCTRC alloy (corrosion rate: 5.956 mm/year). Much lower corrosion rate was indicated in the as-cast TRC and MCTRC alloys, compared to the alloys after HFQ process that revealed the contribution of network or continuous distribution of β-Mg17Al12 phase particles to reduce the corrosion driven in chloride solution. In contrast, discontinuous distribution of cathodic β-Mg17Al12 phase particles increases the corrosion rate of HFQ TRC alloy by promoting the cathodic reaction and intense filament propagation resembling the coarse interdendritic and grain boundaries attack. The presence of high population densities of cathodic Al8Mn5 particles in HFQ rolled AZ31B-H24 alloy significantly reduced the corrosion driven for intense corrosion attack on the rolled alloy. The surface preparation by mechanical grinding process induced MgO and Zn-enrichment layer, accompanied with near surface deformed layer that consisted of nanograins in the range size of 40 to 250 nm. The grinding process refined the surface by removing the cutting damage and marks that formed during the thermomechanical process and led to stable potential of the HFQ AZ31 alloys, in the range of -1.59 to -1.57 V, during open circuit potential (OCP) measurement. The surface regularity with grinding path causing the filament to propagate following the grinding direction. The as-received surface contained many cutting damages and deep scratch marks from the rolling and casting processes that could introduce many corrosion initiation sites. The absence of the grinding direction on the as-received surface could control intense corrosion susceptibility, due to the non-linear filament propagation. The surface irregularity on chromic acid cleaned surface of HFQ rolled AZ31B-H24 alloy also contributed to low corrosion potential of the rolled alloy during OCP and potentiodynamic polarization measurement.

The high magnesium alloy Magnox Al80 is used as a clad for nuclear fuel employed in the UK gas-cooled, graphite moderated power station reactors of the same name. Following irradiation, spent fuel elements are stored in aqueous environments. Historical corrosion studies and plant experience have identified suitable chemistry regimes to ensure passivity of the material, by maintaining high pH and very low concentrations of aggressive anions, notably chloride. Despite this large body of work, certain aspects of the corrosion mechanism are not well characterised, notably, the growth kinetics of surfaces with thick passive films, the manner in which chloride initiates film breakdown and the underlying mechanism of localised corrosion in alkaline environments. A variety of corrosion investigation techniques have been applied here to address these knowledge gaps.Extensive electrochemical work has been supported by characterisation of the corrosion morphology with in situ observations including correlated videomicroscopy, quantitative image analysis and micro-tomography. These results have allowed targeting of quantum mechanical atomistic calculations on ionic adsorption and substitution at a simulated passive film lattice interface. Production of magnesium microelectrodes and application for corrosion study have been reported for the first time. Finite element modelling has been used for interpretation and reconciliation of these results, allowing comparison between techniques, previous observations and with plant experience.The findings of these studies have provided clarity on a number of aspects of Magnox, and magnesium, corrosion behaviour. An extension of the high field film growth model has been proposed, incorporating the effect of an outer hydroxide layer which stabilises the very thin, dense inner oxide at the metal surface. The outcomes of atomistic simulations relating to anions on the passive film lattice interface have been related semi-quantitatively to macroscopic results. Magnesium has been clearly shown to undergo salt film corrosion in conditions not substantially different from those in a fuel storage environment.Consideration of these findings in the context of previous mechanistic work has led to the proposal of a reaction scheme which reconciles the very different behaviours of these materials with a small number of underlying reactions. These describe corrosion according to reactions across a very thin surface film (whether oxide or salt film) with the rate, evolution and morphology being determined largely by the specific mass transport processes at work.

The design of new engineering materials resistant to both wear damage and corrosion degradation becomes increasingly demanding in complex service conditions. Unfortunately, there is typically a tradeoff between wear and corrosion resistance, even for important passive metals such as Al alloys. This is because the presence of precipitates hardens the material but at the same time lead to unfavorable galvanic coupling between the precipitates and the matrix, resulting in accelerated corrosion. This work showed that Al (or Mg) supersaturated solid solution formed using non-equilibrium methods exhibited enhanced corrosion resistance without compromising strength. For Al, alloying with Mn up to ~ 20.at.% simultaneously increased the wear resistance of Al as well as the protectiveness of the passive layer, thus improving the overall tribocorrosion resistance. For Mg, alloying with Y (4.67 wt.%), Zr (0.45 wt%), and Nd (1.79 wt%) in solid solution led to ~ 8 fold increment in corrosion resistance in physiological environment. Magnetron-sputtered aluminum (Al) and aluminum–manganese (Al-Mn) films with structures ranging from nanocrystalline to amorphous were obtained by tuning the Mn% up to 20.5 at.%. Corrosion behavior of the films was investigated in 0.6 M and 0.01 M NaCl aqueous solutions by potentiodynamic polarization (PD) and electrochemical impedance spectroscopy (EIS). Pitting corrosion was found to be strongly affected by alloy composition. The amorphous Al–20.5 at.% Mn exhibited the best pitting resistance during short term exposure. However, over longer immersion in 0.01 M NaCl up to 108 hrs, nanocrystalline Al–5.2 at.% Mn showed the highest corrosion resistance. The dual-phase Al-11.5 at % Mn alloy was found to have higher nominal corrosion rate compared to its nanocrystalline or amorphous counterparts. The effects of Mn alloying on the tribocorrosion behavior of magnetron-sputtered Al-Mn thin films with 5.2 at.% and 20.5 at.% Mn were investigated in 0.6 M NaCl aqueous solution. Tribocorrosion resistance of Al-Mn was found to be strongly affected by the alloying composition and applied potential. Higher Mn content increased H/E ratio and promoted the formation of denser and more compact passive film, hence improving tribocorrosion resistance of Al. In particular, alloying with 20.5 at.% Mn led to an increase of the corrosion resistance by ~ 10 times and the hardness ~ 8 times compared to pure Al. The total material loss during tribocorrosion was found to increase with applied potential. When the applied potential was increased from cathodic to anodic, simultaneous contribution of the mechanical and the electrochemical wear leads to accelerated material loss. A galvanic cell model was used to investigate the depassivation-repassivation kinetics during tribocorrosion. It was found that alloying with 5.2 at.% Mn led to more than 10-fold reduction in the current density required to re-passivate similar worn areas compared to pure Al. The origin of wear-corrosion synergy was discussed based on these observations. Magnesium alloys such as WE43 are considered for biomedical applications including cardiovascular stents and bone implants due to their biocompatibility, good cell adhesion, and mechanical properties close to that of bones. Unfortunately, their high degradation rate and subsequent loss of structural integrity in physiological environments hinders such applications. To improve the corrosion resistance of WE43 magnesium alloy, its microstructure was optimized to prevent micro-galvanic coupling between Mg matrix and precipitates. Chemically homogeneous WE43 with nanoscale surface roughness was obtained by magnetron sputtering with high effective quench rate. The effect of chemical heterogeneity on the corrosion resistance of biodegradable WE43 magnesium alloy was studied by performing corrosion tests in blood bank buffered saline using samples from two metallurgical states, cast and deposited. The microstructure of all samples was investigated by grazing incidence X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The deposited samples, prepared by magnetron sputtering using targets with the same global composition as cast WE43, exhibited chemically homogeneous microstructure without the formation of secondary phases typically observed in the cast alloy. The corrosion behavior was studied by PD and EIS tests. It was found that the deposited alloy showed enhanced corrosion resistance, ~8-fold reduction in corrosion rate compared to the cast alloy, owing to the elimination of micro-galvanic coupling between the Mg matrix and the precipitates. In-situ monitoring of hydrogen bubble evolution during corrosion indicated significantly reduced cathodic reaction kinetics in the deposited alloy. Post-corrosion surface and cross-sectional SEM studies showed that the high corrosion rate in the cast alloy was associated with the formation of severely cracked corrosion products preferably around Zr- and Y-containing precipitates.

The present work provides an insight in the development of silica films prepared by water-based sol-gel process. The weaknesses of the coating technology are identified, also solutions are discussed. The silica film is applied on aluminium alloy 6082-T6 and magnesium alloy AZ31. The development of the coating properties such as cost-efficiency, crack-free, self-healing and long-term corrosion protection is the main topic of this work. Cracking is the major drawback of silica films; the cracks are generated due to shrinkage of the film during the heat treatment, nanoparticles-doped silica film is successfully reduced the shrinkage which leads to crack-free silica films. The self-healing of the coated aluminium and magnesium samples is generated by corrosion inhibitors-doped silica film. When a defect appears in the film, the corrosion inhibitors leach out of the silica film to the defect area to heal the corroded surface. The long-term corrosion protection is realized by means of a mixture of corrosion inhibitors-doped silica film
Die vorliegende Arbeit liefert einen Einblick in die Entwicklung von Silikafilmen, die mittels Sol-Gel-Prozess auf Wasserbasis hergestellt wurden. Die Schwächen der Beschichtungstechnologie werden dargestellt und Lösungen diskutiert. Der Silikafilm wird auf Aluminiumlegierung 6082-T6 und Magnesium-legierung AZ31 aufgebracht. Schwerpunkt dieser Arbeit ist die Entwicklung der Schichteigenschaften, wie Kosteneffizienz, Rissfreiheit, Selbstheilung so wie langfristiger Korrosionsschutz. Rissbildung ist ein wesentlicher Nachteil von Silikafilmen; rissfreie Filme wurden mittels nanopartikeldotierter Silikafilme hergestellt. Die Selbstheilung von Aluminium-und Magnesiumsubstraten mit Silikafilm wird durch den Effekt der wasserlöslichen Korrosionsinhibitoren generiert. Die Experimente haben gezeigt, dass die Proben mit inhibitordotierter Beschichtung selbst gegen Korrosion geschützt sind. Ein langfristiger Korrosionsschutz wird durch eine Mischung aus Korrosionsinhibitor-dotierten Silika-Film realisiert

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Dissertação (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

The use of metallic biomaterials, such as titanium alloys, stainless steels and cobalt-chromium alloys in bone implant devices is indispensable to support the bone during the healing period. However, prolonged use of implantation devices made of these inert biomaterials may lead to stress shielding due to their high elastic moduli resulting in the loss of bone density during the remodeling process. Therefore, magnesium is being investigated as a possible biomaterial for temporary fixation devices as it has low elastic modulus close to that of the human cortical bone in addition to being biocompatible and biodegradable in the aqueous chloride environment of the human body. However, the high corrosion rates of magnesium due to its low standard electrode potential compromise the mechanical integrity of the implant during the bone healing process. This thesis involved the fabrication of four magnesium-zinc alloys with 1, 1.5, 2 and 5 weight percent Zn in Mg in an attempt to tailor the corrosion rate of the alloy. The hardness and densities of the alloys were determined. The microstructure of the alloys was characterized by chemical analysis both before and after corrosion experiments. Both gravimetric and electrochemical studies were used to understand the in vitro corrosion behaviour of the alloys in static Simulated Body Fluid (SBF). The weight loss measurements after immersion tests indicated that the as-cast Mg-2.0Zn alloy had the lowest corrosion rate owing to network-like second phase precipitations. The potentiodynamic polarization experiments yielded extremely low corrosion rates for all the alloys, and were correlated to the microstructures of the corroded surfaces. Electrochemical Impedance Spectroscopy (EIS) was conducted to study and model the interface between the sample and the SBF. The electrochemical test results indicated that the Mg-1.0Zn exhibited the highest polarization resistance leading to the decreased corrosion rate of the alloy. The corrosion products consisted of magnesium carbonates, magnesium hydroxides and calcium phosphates as indicated by Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD) and X-ray Energy Dispersive Spectroscopy (X-EDS). Results were suggestive that, Mg-Zn alloys with ≤2 wt. % Zn were promising as a suitable metallic biomaterial for orthopaedic applications and should be considered for further in vitro and in vivo studies.
Applied Science, Faculty of
Engineering, School of (Okanagan)
Graduate

Magnesium alloys exhibit desirable properties for use in transportation technology. In particular, the low density and high specific strength of these alloys is of interest to the aerospace community. However, the concerns of flammability and susceptibility to corrosion have limited the use of magnesium alloys within the aircraft cabin. This work studies a magnesium alloy containing rare earth elements designed to increase resistance to ignition while lowering rate of corrosion. The microstructure of the alloy was documented using scanning electron microscopy. Specimens underwent salt spray testing and the corrosion products were examined using energy dispersive spectroscopy.

Les alliages de magnésium représentent une alternative à l’utilisation d’alliages d’aluminium ou de matériaux composites, en particulier dans le secteur aéronautique dans l’objectif de réduire la masse des structures. Ces travaux de thèse ont pour but de participer au développement de nouvelles méthodes de protection des alliages de magnésium, plus efficaces et respectueuses de l’environnement. Pour mener à bien ces travaux, des techniques électrochimiques, en particulier la spectroscopie d’impédance électrochimique, ont été couplées à des mesures par microscopie à force atomique (AFM), à des analyses par spectroscopie d’émission atomique à plasma induit (ICP-AES) et par spectroscopie de masse d’ions secondaires à temps de vol (Tof-SIMS) ainsi que des essais normalisés industriels. Tout d’abord, la résistance à la corrosion en milieu Na2SO4 de trois alliages de magnésium contenant des terres rares (WE43, EV31 et ZE41) a été étudiée et comparée à celle de deux alliages riches en aluminium (AZ31 et AZ91) et à celle du magnésium pur. Pour tous les alliages, il a été montré que les particules intermétalliques agissent comme des cathodes locales. Cet effet de couplage galvanique est plus marqué pour les particules riches en terres rares, en particulier dans le cas de l’alliage EV31. Conjointement, la corrosion est contrôlée par la dissolution de la matrice riche en magnésium et par le recouvrement progressif de la surface métallique par un film d’oxydes/hydroxydes. Ce film est plus protecteur pour les alliages que pour le magnésium pur mais cet effet bénéfique n’est toutefois pas suffisant pour compenser le rôle néfaste joué par les particules intermétalliques. Au final, l’ajout de terres rares augmente la vitesse de corrosion des alliages de magnésium en milieu Na2SO4 par rapport à celle des alliages AZ ou celle du magnésium pur. Dans le cas de l’alliage WE43, qui a été retenu pour la suite de l’étude, il a été montré que le film protecteur d’oxydes est plus mince et plus stable que celui formé sur le Mg pur, en particulier en présence d’ions chlorure. Ces résultats ont été expliqués par l’incorporation des éléments d’alliages, comme l’yttrium, qui serait responsable de la formation d’un film d’oxydes plus compact. Puis, plusieurs méthodes de protection ont été envisagées dans le but d’obtenir une résistance à la corrosion compatible avec les exigences de l’industrie aéronautique. Un traitement d’anodisation, développé par la société Prodem et appelé CEP, en combinaison avec plusieurs primaires de peinture sans chromate, proposés par la société Mapaéro (hydrodiluable ou haut-extrait sec) ont été évalués et comparés aux solutions de référence chromatées. Il a été montré que les traitements de conversion actuels, même en présence de primaire chromaté, ne permettent pas une protection efficace des alliages de magnésium. Le traitement CEP, de par sa structure poreuse, permet une bonne adhésion avec les primaires. Les meilleures performances ont été obtenues pour le traitement CEP revêtu par le primaire haut-extrait sec. Des analyses supplémentaires ont montré que l’ajout d’un vernis permettait d’obtenir un système de protection prometteur pour le remplacement des systèmes de référence sur la base des exigences clés aéronautiques.

Surface integrity of manufactured components has a critical impact on their functional performance. Magnesium alloys are lightweight materials used in the transportation industry and are also emerging as a potential material for biodegradable medical implants. However, the unsatisfactory corrosion performance of Mg alloys limits their application to a great extent. Surface integrity factors, such as grain size, crystallographic orientation and residual stress, have been proved to remarkably influence the functional performance of magnesium alloys, including corrosion resistance, wear resistance and fatigue life. In this dissertation, the influence of machining conditions, including dry and cryogenic cooling (liquid nitrogen was sprayed to the machined surface during machining), cutting edge radius, cutting speed and feed rate, on the surface integrity of AZ31B Mg alloy was investigated. Cryogenic machining led to the formation of a "featureless layer" on the machined surface where significant grain refinement from 12 μm to 31 nm occurred due to dynamic recrystallization (DRX), as well as increased intensity of basal plane on the surface and more compressive residual stresses. Dry and cryogenic burnishing experiments of the same material were conducted using a fixed roller setup. The thickness of the processed-influenced layer, where remarkable microstructural changes occurred, was dramatically increased from the maximum value of 20 μm during machining to 3.4 mm during burnishing. The burnishing process also produced a stronger basal texture on the surface than the machining process. Preliminary corrosion tests were conducted to evaluate the corrosion performance of selected machined and burnished AZ31B Mg samples in 5% NaCl solution and simulated body fluid (SBF). Cryogenic cooling and large edge radius tools were found to significantly improve the corrosion performance of machined samples in both solutions. The largest improvement in the material's corrosion performance was achieved by burnishing. A finite element study was conducted for machining of AZ31B Mg alloy and calibrated using the experimental data. A user subroutine was developed and incorporated to predict the grain size changes induced by machining. Good agreements between the predicted and measured grain size as well as thickness of featureless layers were achieved. Numerical studies were extended to include the influence of rake angle, feed rate and cutting speed on the featureless layer formation.

Aim of this study is to improve corrosion resistance of magnesium alloy AZ91 by conversion coatings. Influence of alloy microstructure on conversion coating growth and corrosion resistance was evaluated. Properties of pure magnesium and magnesium alloy AZ91 as well as the influence of alloying elements on properties of this alloy are described in theoretical part. Recent results of corrosion protection by conversion coatings on AZ type magnesium alloys are summarised in recherché part. Practical part focuses on preparation of hexavalent chromium based conversion coating and phosphate-permanganate based conversion coating on as cast AZ91 magnesium alloy, these coatings were subsequently applied on annealed AZ91 magnesium alloy. Corrosion protection of the coatings prepared on as cast and annealed alloy was evaluated by potentiodynamic measurements and testing in neutral salt spray. Furthermore the influence of plasma activation on phosphate-permanganate coating surface was studied.