1,721,069 research outputs found
Polymer bonding and adhesion on aluminium and magnesium alloys
No full textMaterials Science & EngineeringMechanical, Maritime and Materials Engineerin
Cr(VI) and Cr(III)-Based Conversion Coatings on Zinc
No full textWith the aims of understanding the protective mechanism of chromate conversion coatings and developing alternatives to chromate treatments, the physical natures and corrosion properties of Cr(VI) and Cr(III) treated zinc have been investigated in this work. The Cr(VI) treatments were carried out in dichromate and sulfuric acid solution with different dipping times. The Cr(III) treatments were carried out in two commercial solutions (A and B). The thickness of the coatings was measured using ellipsometry. The morphologies and the compositions of the treated zinc have been studied by means of SEM, AFM, AES, FTIR and XPS. The drying temperature influence on the corrosion performance of the Cr(VI)âtreated zinc has been investigated. The Volta potential in treated and untreated areas has been measured using scanning Kelvin probe (SKP) and SKPFM. The corrosion behavior of the Cr(VI) and Cr(III) treated zinc has been investigated using polarization, electrochemical impedance measurements (EIS), and salt spray tests. Both Cr(VI) and Cr(III) species were detected by XPS in the outermost layer of the Cr(VI) coatings, while no Cr(VI) species was found in the Cr(III) coatings. AES depth profile results show that chromium oxides are the main components in the Cr(VI) coatings. Zinc oxide is mainly located at the chromium oxides / zinc interface. The Cr(III) coating is a mixture of chromium oxides and zinc oxide. Both the Cr(VI) and the Cr(III) treatments can supply corrosion protection to zinc. The corrosion resistance of the Cr(III)-B coating is greater than that of the Cr(III)-A coating. However, the inhibition of the corrosion of zinc by Cr(VI) coating is more effective than by the Cr(III) coatings. The inhibition of the corrosion of zinc by the Cr(VI) and the Cr(III) treatments is discussed, and future research topics are suggested.Mechanical, Maritime and Materials Engineerin
The role of magnesium in the electrochemical behaviour of 5XXX aluminium-magnesium alloys
No full textAn investigation concerning the effects of magnesium on the intergranular corrosion susceptibility of AA5XXX aluminium alloys was carried out. In the present work, magnesium is found to be highly mobile in the bulk metal as well as in the aluminium oxide. This mobility is also found to be dependent on the temperature and bulk magnesium concentration. Interestingly, the corrosion susceptibility of the AA5XXX series appears to be directly related to the magnesium mobility as well. It was found that magnesium is the most favoured alloying element in the aluminium metal that actively contributes to the formation and composition of the oxide layer. Nevertheless, the extent of this contribution strongly depends on the magnesium content of the alloy and the applied temperature. Auger composition-depth profiles performed on alloys with different Mg contents indicate that when the bulk magnesium concentration is high, the Mg enrichment on the oxide layer becomes stronger. This situation brings as a consequence the formation of thicker oxide layers. At grain boundaries, magnesium enrichment was measured and quantify. A mechanism for intergranular corrosion is proposed. This mechanism is based on the combination of three main factors: magnesium-rich precipitates at grain boundaries, magnesium enrichments at grain boundaries as free-atoms, and hydrogen diffusion into the grain boundaries.Mechanical Maritime and Materials Engineerin
Enhanced corrosion protection by microstructural control of aluminium brazing sheet
No full textAluminium brazing sheet is a sandwich material made out of two aluminium alloys (AA4xxx/AA3xxx) and is widely used in automotive heat exchangers. One of the main performance criteria for heat exchanger units is the lifetime of the product. The lifetime of the heat exchanger units is determined by their corrosion resistance. The most common way to measure the corrosion resistance of heat exchanger materials is the so called Sea Water Acidified Accelerated Test (SWAAT). Unfortunately there is no agreement on how corrosion performance should be evaluated in SWAAT. The first aim of the current research work was to perform a statistical and in-depth analysis of the critical parameters of the SWAAT. By keeping the climatic parameters constant, the correlation between the variation of the test results and the actual fundamental differences in the material condition i.e. temper and brazing condition was elucidated. Parameters that potentially could influence the outcome of the SWAAT test were investigated. The results were used to narrow the operation window of the test and making it more reproducible and reliable. The possibility of the presence of some links between SWAAT and electrochemical characterization were also investigated. Corrosion mechanism and morphology of attack in SWAAT and potentiodynamic test were compared. The result confirmed that the SWAAT performance of the material can be reflected in potentiodynamic responses. Finally a new statistical reliability evaluation approach and a novel result interpretation method were developed. Nowadays cost reductions are key within automotive industry and the current corrosion resistant alloys become less competitive. There is a strong demand for low cost alloys but still with the excellent corrosion protection properties of the expensive alloys. In depth knowledge of how alloy composition, microstructure and thermo-mechanical processing alter the corrosion resistance of aluminium brazing sheet is crucial to allow production of cost effective alloys with sufficient properties in the near future. Therefore the second aim of this research was to develop a fundamental understanding over the correlation between microstructural features and the resulting electrochemical responses of aluminium brazing sheet materials. The detailed and fundamental mechanistic understanding of the effects of microstructural variations and the consequent electrochemical reactivities was aimed at providing solutions for enhanced corrosion protection by microstructural control of aluminium brazing sheet. A full microstructural characterization of the material was btained. A complete electrochemical characterization of the structure at macro, localized and micro scales was achieved and correlated to the microstructural features developed as consequences of brazing treatment. The results were used to explain the corrosion propagation mechanism through the structure of the brazed material and to investigate possibilities for improving the corrosion resistance of the brazed structure. Using the generated knowledge over the correlation between the microstructural features and the electrochemical responses, it was concluded that applying a proper post-braze heat treatment may improve the corrosion resistance of the aluminium brazing sheet. An innovative technique for improving corrosion resistance of a modified AA4xxx/AA3xxx aluminium brazing sheet has been developed. In this process the controlled atmosphere brazed (CAB) material is subjected to a thermal post-brazing treatment. The investigations of the corrosion resistance of the post-braze heat treated material confirmed the effectiveness of the developed thermal treatment for improving the corrosion resistance of the brazing sheet material. The relatively high tolerance to increased Si and Fe alloying element concentrations in this post-braze heat treated structure will provide the opportunity for further usage of recycled aluminium in its production process. Therefore this new material is referred to as a green corrosion resistant aluminium brazing sheet. This new generation of aluminium brazing sheet is able to tolerate a higher amount of previously considered detrimental alloying elements and thus a lower need for purification during primary and recycling processing saving energy, material and labor costs.Materials Science & EngineeringMechanical, Maritime and Materials Engineerin
Microstructuur-gecorreleerde eigenschappen van conversielagen op aluminium legeringen
No full textApplied Science
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Influence of the Microstructure of the Corrosion Performance of DP Steels
No full textWeight reduction of car bodies can be achieved by application of steel components with a lower thickness; however mechanical properties (for constructive and safety reasons) must be maintained, which can be achieved by using (U)HSS steels. These steels have been designed and optimized for improved mechanical behavior and therefore normally have a poorer or at least not well documented and understood corrosion resistance. One of these HSS steels are the so called Dual Phase steels, or DP steels. The aim of this work is to investigate the effect of the individual and combined ferrite and martensite phases in model DP steels on the corrosion performance and electrochemical behavior. For this, the first part of this work consisted in the development of a range of dual phase microstructures with different ferrite-martensite ratios ranging from virtually fully ferritic to fully martensitic. Controlled heat treatments were developed with the CASIM (Continuous Annealing SIMulator) and the dilatometer in order to obtain the desired microstructures. After the microstructures were developed, standard 3 electrode electrochemical setups were used to perform open circuit potential and potentiodynamic polarization measurements, approach which was then complemented by the use of Micro-Capillary cell. The use of the localized technique was focused on the possibility to perform single phase measurements which are not possible with the standard setups. The influence of the individual and relative amounts of ferrite and martensite on the electrochemical response of model DP steels was discussed. The first conclusions that were drawn from the electrochemical work was that due to the absence of a stable passive layer, performing measurements on standard setupsproved to be a challenge because crevice was a dominating factor in the system. Modifications to standard cell and Avesta cell were done in the attempt to avoid the presence of crevice. With the Micro-Capillary setup, even though crevice can also be an issue, reliable and reproducible polarization curves were obtained. A clear effect of sodium chloride concentration and the presence of a carbonate buffer on the electrochemical response of the model DP steels was observed. However, it was also found that the differences in the electrochemical behavior induced by the amount of martensite present are not significant enough to be distinguished from the reproducibility of the polarization curves. This was also the case for the base material sample, which even though composed of a highly deformed ferritic matrix with cementite-carbides islands, showed a similar response in the polarizations. The influence of the polarization system (setup, electrolyte, and microstructure) was discussed, as well as the importance of acknowledging the influence of crevice and reproducibility while performing electrochemical measurements.Materials Science & EngineeringMechanical, Maritime and Materials Engineerin
Local electrochemical behaviour of 7xxx aluminium alloys
No full textAluminium alloys of the 7xxx series (Al-Zn-Mg-Cu) are susceptible to localized types of corrosion like pitting, intergranular corrosion and exfoliation corrosion. This represents a limitation for the application of these alloys in the aerospace components because localized corrosion might have a negative effect on safety and costs. This PhD thesis investigates the relation between electrochemical behaviour and microstructure of a number of 7xxx aluminium alloys: AA7075, AA7349 and an experimental alloy (EA1) with composition in the range of AA7449. To this aim, the experimental approach of this PhD consisted in the characterization of the microstructure of the alloys combined with the study of their electrochemical behaviour. The investigation of the microstructure focused on intermetallics and strengthening particles because these second phase particles strongly affect the corrosion behaviour of 7xxx aluminium alloys. The study of the electrochemical behaviour was approached on two different levels: macro- and micro-electrochemical characterization of the alloys. The macro-electrochemical characterization employed open circuit potential and potentiodynamic polarization measurements to study the overall corrosion behaviour of the alloys, while the micro-electrochemical characterization applied local electrochemical techniques (scanning Kelvin probe force microscope and micro-capillary cell) to study the local behaviour of the intermetallics. There are three main types of intermetallics in the 7xxx alloys investigated: - Al7Cu2Fe - (Al,Cu)6(Fe,Cu) - Mg2Si In AA7349 and EA1 the intermetallics contain small amounts of Mn and Si. The strengthening particles are Mg2Zn phase for all alloys. The intermetallics are not affected by heat treatments (solution heat treatment, aging and overaging), while the strengthening particles undergo strong changes. The strengthening particles are dissolved by solution heat treatment resulting in supersaturation of solute elements (Zn and Mg) in the matrix and in segregation of these elements at the grain boundaries. They precipitate in the matrix and at the grain boundaries during aging and overaging progressively reducing the supersaturation of the matrix and the segregation at the grain boundaries. The intermetallics are the initiation sites for localized corrosion, which takes place as local dissolution of the matrix at the location of the intermetallics and results in pitting of the alloy surface. At a later stage, the localized attack propagates as intergranular corrosion and might turn into exfoliation corrosion for microstructures particularly susceptible to the intergranular attack. The types of intermetallics in the microstructure of the alloys exhibit different micro- electrochemical behaviour. The Al7Cu2Fe and (Al,Cu)6(Fe,Cu) intermetallics have cathodic behaviour relative to the matrix (positive Volta potential difference relative to the matrix), while the Mg2Si intermetallics have anodic behaviour (negative Volta potential difference relative to the matrix). Moreover, the Al7Cu2Fe intermetallics have stronger cathodic behaviour than the (Al,Cu)6(Fe,Cu) intermetallics. The microstructural changes taking place during solution heat treatment strongly affect the electrochemical behaviour of the alloys. Solution heat treatment strongly increases the Volta potential difference between the intermetallics and the matrix (for all types of intermetallics) increasing the susceptibility to pitting. Moreover, it increases the susceptibility to intergranular and exfoliation corrosion because of the segregation of Zn and Mg at the grain boundaries. Aging and overaging decrease the Volta potential difference between the intermetallics and the matrix improving the resistance to pitting. Aging improves the susceptibility to intergranular corrosion as compared to solution heat treatment. However, the alloys aged to the peak-strength tempers are still prone to severe intergranular corrosion and exfoliation corrosion because they undergo precipitation of small strengthening particles at the grain boundaries with very small interparticle spacing. Overaging strongly improves the resistance to intergranular corrosion and eliminates the susceptibility to exfoliation corrosion because it increases the size and the interparticle spacing of the particles at the grain boundaries. The characterization using the local electrochemical techniques (scanning Kelvin probe force microscope and micro-capillary cell) showed that the micro- electrochemical behaviour of the intermetallics is different for each intermetallic. This means that some areas of the alloy surface might be very susceptible to localized attack and might be preferential sites for the initiation of localized corrosion. The local electrochemical techniques used in this PhD enable the relation of the electrochemical behaviour to the microstructure of the alloys. In particular, it was possible to establish this relationship for the intermetallics due to the lateral resolution of these techniques (in the micrometer range). This thesis has proved that the complementary use of local electrochemical techniques is very useful for the characterization of metals that are susceptible to localized types of corrosion.Applied Science
Interfacial bonding mechanisms of carboxylic coatings on pretreated zinc surfaces
No full textThe aim of this Ph.D. work is to investigate the adhesion and de-adhesion of organic coatings on galvanized steel substrate. Chapter 1 introduces the project aim and research outline. Chapter 2 investigates Zn oxide semiconductor properties as the major compound formed on galvanized steel. The results showed a clear effect of surface treatment on Zn oxide doping concentration and flatband potential. Moreover, an obvious relation between Zn oxide composition and flatband potential was detected. The deposition mechanism of Zr-based conversion coatings, as potential replacement of the existing conversion coatings, on differently pretreated Zn surfaces has been studied in chapter 3. It was found that Zr deposition rate and the composition of the obtained conversion layer strongly depend on the initial hydroxyl fraction and surface roughness. Studying the metal-polymer interfacial bonding is extremely difficult due to the relatively high polymer thickness masking the interface region. Consequently, the metal-polymer interfaces are modeled in chapters 4-7 by adsorption of the representative molecules. Chapter 4 compares the adsorption properties of carboxylic based molecules on Zn and Fe substrates. It was shown that the obtained interfacial configurations are different on Zn and Fe substrates. In this case, one end adsorption of succinic acid molecules was promoted on Fe, while adsorption of Zn was conducted mainly with both ends. Consequently, Zn was selected for continuation of the research to explore more details of the interfacial properties. Chapter 5 examines the effects of an external potential on adsorption properties of the carboxylic model compounds. The results showed that the applied potential slightly increases the amount of adsorbates, which may contribute to a better adhesion. Moreover, the applied potential increases one-end adsorption of succinic acid molecules due to a C-C bonding cleavage. Desorption of the formed carboxylates was examined by means of cyclic voltammetry in chapter 6. It was shown that the molecular type and surface pretreatment affect the desorption process and consequently the interfacial bonding strength. It was found the metal-polymer interaction is accompanied by an electron exchange in the interfacial region. Consequently, scanning Kevin probe (SKP) measurements were performed to evaluate the Volta potential changes due to the adsorption. The results showed that the amount of electron transfer is correlated to the interfacial bonding density and molecular chain length. Additionally, an effect of adsorption configuration was detected on the obtained Volta potential shift. In the next step, the interfacial bonding properties of the real carboxylic coatings and Zn substrate were evaluated in chapters 8-10. Chapter 8 assesses the interfacial bonding of the residue polymer layers obtained after a dedicated polymer removal procedure. A comparable interfacial bonding mechanism was detected for the real coatings and model compounds applied on Zn surfaces. Chapter 9 investigates in-situ the interfacial bonding formation upon application of the polymers on Zn surfaces in a Kretschmann geometry. An equivalent interfacial bonding mechanism was detected compared to those obtained through the adsorption of the model compounds and residue layers. Additionally, it was shown that the metal-polymer interaction takes place in a relatively short time, while polymerization proceeds afterwards. The interfacial bonding degradation was examined by means of an integrated ATR-FTIR and EIS set-up in a Kretschmann geometry and described in chapter 10. The results showed a beneficial role of the polymerization process in the formation of lateral bondings blocking water penetration through the polymer structure. Moreover, it was found that the duration of the disbonding process depended on the Zn surface hydroxyl fraction.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin
- …
