Ingenieurwissenschaften
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- ItemDevelopment of Photocatalytically Active Anodized Layers by a Modified Phosphoric Acid Anodizing Process for Air Purification(Basel : MDPI, 2023) Lederer, Stephan; Benfer, Sigrid; Bloh, Jonathan; Javed, Rezan; Pashkova, Aneta; Fuerbeth, WolframOne of the key urban air quality issues is pollution by nitrogen oxides (NOx). To reduce NOx, facade cladding could be provided with photocatalytic properties by incorporating titanium dioxide nanoparticles. For this purpose, a modified phosphoric acid anodizing process (MPAA) was developed for the facade alloy EN AW-5005, in which highly ordered anodized structures with a low degree of arborization and tortuosity were produced. Pore widths between 70 nm and 150 nm and layer thicknesses of about 2–3 μm were obtained. The subsequent impregnation was carried out by dip coating from water-based systems. Depending on the dip-coating parameters and the suspension used, the pores can be filled up to 60% with the TiO2 nanoparticles. Photocatalytic tests according to ISO 22197-1 certify a high photocatalytic activity was obtained with rPCE values > 8 and with rPCE > 2, achieving “photocatalytically active for air purification”. Tests on the corrosion resistance of the anodized coatings with a commercially available aluminum and facade cleaner confirm a protective effect of the anodized coatings when compared with nonanodized aluminum material, as well as with compacted anodized layers.
- ItemPlasma Electrolytic Oxidation on Magnesium AZ31 with Sepiolite as Inhibitor Carrier for Improved Corrosion Protection(Basel : MDPI, 2023) Sottor, Robert; Gruen, Ricarda; Kremmer, Kerstin; Lederer, Stephan; Schneider, Michael; Fuerbeth, WolframPlasma electrolytic oxidation (PEO) in an alkaline silicate electrolyte containing nanosized sepiolite fibers was carried out on magnesium alloy AZ31. The mineral fibers were loaded with different corrosion inhibitors and incorporated in situ during the PEO treatment. The composition and microstructure of the PEO coatings were investigated by SEM. It was shown that the fibers are located on the surface as well as inside the “weak spots” of the coating, i.e., pores and discharge channels. The fixation of the particles is caused by sintering due to the heat developed during the PEO treatment. Investigations using electrochemical impedance spectroscopy and linear sweep voltammetry in 0.01 M NaCl solution confirmed an improvement of the corrosion protection. The use of the inhibitors shifts the critical pitting potential in the anodic direction. Regarding efficiency, cerium-loaded sepiolite showed the best behavior by shifting the pitting potential by +0.9 V.
- ItemMechanisms of Secondary Spreading and Micro Droplet Formation on Steel(Bristol : IOP Publishing, 2024-12) Seeger, Lea; Lobnig, Renate; Schuch, Elmar; Guo, Surong; Fürbeth, WolframA new theory for secondary spreading based on the wetting theory of thin films is presented. It explains how micro droplets within the spreading zone and the primary droplet retain their shape, although connected by a thin electrolyte film and how humidity and salt concentration affect the growth rate of micro droplets. The trigger for secondary spreading, polarization or alkalization, is identified by using droplets of sodium hydroxide solution. Secondary spreading thus occurs on steel from pH 13.5 without corrosion or external polarization. The limiting pH value found explains why secondary spreading on steel only occurs when certain salts are used. The effect of the substrate is investigated by changing the microstructure of the steel. By comparing the sizes of micro droplets and micro structural phases and by scanning electron microscopy/energy-dispersive X-ray analysis measurements of the spreading zone, the existence of an electrolyte film connecting the micro droplets is supported. Ecorr potential profiles of secondary spreading droplets of sodium chloride solution on steel acquired by means of SKP are used to assess the contribution of secondary spreading to the total corrosion current, which is estimated to be low compared to that of the cathodic zone at the edge of the droplet.
- ItemThe Influence of Mo Content and Annealing on the Oxidation Behavior of Arc-Melted Cr–xMo–8Si Alloys(Weinheim : Wiley-VCH Verl., 2024-03-21) Koliotassis, Lisa; White, Emma M. H.; Galetz, Mathias C.The influence of Mo (10-40at.%) additions to Cr-8at.%Si was systematically studied by producing arc melted alloys and analyzing the as-cast and annealed microstructures as well as the oxidation resistances. The majority of the alloys contained a solid solution A2 matrix with the intermetallic A15 = (Cr,Mo)3Si phase. The oxidation resistance was tested by thermogravimetric analysis (TGA) at 1200 °C in air for 50 and 100 h. Samples were characterized by XRD, SEM and EPMA (WDS). The studied Cr-xMo-8Si alloy series showed a beneficial effect of Mo on the corrosion behavior of the annealed arc melted alloys exposed at 1200 °C in air. An increased amount of A15 led to the formation of a continuous SiO2 scale beneath the Cr2O3 layer during high temperature exposure. The internal SiO2 layer inhibited further internal oxidation of the A15 phase. Prior annealing also influenced the oxidation behavior of the alloys. Higher annealing temperatures, resulted in an overall reduced fraction of the A15 phase, as well as coarsening, but also showed a more uniform size and distribution of the precipitates, which seems to benefit the formation of a protective duplex oxide scale. Cr-25Mo-8at.%Si annealed at 1350 °C for 100 h, showed the most promising oxidation behavior after exposure in air at 1200 °C for 100 h. For all of the investigated Mo contents, no nitridation was observed when compared to the Cr-8Si reference composition. The oxide scale adhesion also was improved. For Mo >25 at.% no spallation was observed. Evaluation of the TGA data showed it follows the paralinear law, with both the growth rate, kp, and volatilization rate, kv, of Cr2O3 being reduced by adding 10-25 at.% Mo. A binary Cr/Mo-Si phase diagram was generated for the studied alloy series in order to determine accurate annealing conditions and estimate the stability range of the metastable σ phase.
- ItemAdditively manufactured Ti2Mo for biomedical applications: surface engineering and corrosion properties(London : F1000 Research Ltd, 2024) Zander, Julian P.; Lederer, Stephan; Fürbeth, WolframAdditive manufacturing allows the fast production of small and complex parts with minimum waste for biomedical applications made of titanium alloys. However, the cytotoxicity of certain elements being widely used in these alloys has become a concern, initiating the development of new alloys avoiding their use as alloying elements. In this publication, surface engineering techniques like blasting and plasma electrolytic polishing have been applied to a new Ti2Mo alloy obtained by Selective Laser Melting (SLM) in order to enhance its surface properties. First aim of such surface engineering processes is to develop standardized surfaces for additively manufactured parts, comparable to conventionally produced parts. Residual stresses that arise due to the additive manufacturing process are examined as well as their potential impact on the mechanical and corrosion properties of the alloys. Via suitable heat treatment residual stresses can be lowered and adjusted. Additionally, the corrosion properties are determined using potentiodynamic polarization and tribocorrosion testing.