A chemical mapping with high lateral resolution using an atomic force microscope in the pulsed force mode with chemically modified tips, introduced as "dynamic chemical force microscopy" (dCFM), was carried out to investigate the chemical properties of the cuticle of human hair and its changes following an oxidative treatment. Chemically modified atomic force microscopy (AFM) tips, CH3- and NH2-terminated, were applied to achieve a defined chemical contrast (hydrophobic and ionic) in aqueous medium. A comparative Fourier transform infrared spectroscopy-attenuated total reflection identified the dominant chemical groups of the surface vicinity of the hair fiber resulting from the bleaching process. The combined experimental results lead to the conclusion that the hydrophobic top layer is partially removed after bleaching, resulting mostly in hydrophilic SO3(-) end groups at the top of the surface of the hair, with a mean surface density "δ(mean)" of negatively charged groups of approximately 2.2 molecules/nm(2), corresponding to ∼600 μg/m(2) cysteic acid. This indicates that thioester bonds are disrupted and fatty acids are removed as a result of cysteine oxidation. At the molecular level, our results indicate a clustered "self-assembled monolayer" alignment of cysteic acid with a crystal-like structuring, reminiscent of the "fluid mosaic model of cell membranes", with a surface energy of approximately 0.04 N/m. Despite previous extensive works of AFM on human hair, this is, to our knowledge, the first time that the hydrophobic and ionic sites at the top surface of hair have been imaged at the nanoscale with dCFM.
A laser welding-brazing (LWB) process to join zinc coated steel and aluminium sheets in two different flange geometries is reported. The deep drawing steel sheets are covered by a zinc layer of maximum thickness 10 mm, and a zinc based filler wire was used in the welding experiments with a Nd-YAG laser. Because of the differences in melting temperatures between iron (1808 K), aluminium (933 K), and zinc (693 K), it is possible to weld the aluminium alloy only. Owing to the zinc coating on the steel side, a Zn-Al alloy can be brazed onto the steel without any flux agent. The inevitable formation of a Fe-Al intermetallic phase at the bondline of the weld seam and the steel can be limited to a thickness of less than 5 mm and to a proportion of the contact area only. Mechanical as well as dynamic tests show results comparable to those obtained via other joining techniques. Salt chamber corrosion tests of varnished specimens display minor damage and no decline in tensile strength.
Joining of iron with aluminium in the liquid phase is complicated due to the formation of brittle intermetallics within Fe-Al melts. In this work we present the technology of a laser weldingbrazing (LWB) process to join zinc-coated steel and aluminium sheets in an overlap geometry. The process is divided into welding of the aluminium sheet and brazing of filler material and molten aluminium sheet onto the zinc-covered steel sheet to avoid the liquid stage of iron. It is possible to join those materials with a single laser beam. However, with a second laser beam, the melt flow can be controlled more effectively and the wetting behaviour can be improved by preheating the zinc-covered steel sheet surface. This is beneficial since the process works without any flux agent. Wetting lengths increase by using this technique and generate a joint that exceeds the tensile strength of the base material. Due to the zinc cover on the steel sheet there are two possible filler materials that have miscibility with zinc as well as aluminium. Zinc based filler materials have the advantage of a lower melting point and thus lower process temperatures. And aluminium based filler wires result in more ductile brazed seams, making it possible to deform such joints in a later application as tailored blanks.
To lower the freezing point of deicing salt, calcium chloride (CaCl 2 ) is commonly used. The influence of the presence and absence of the deicing salt ingredient Ca 2+ on the corrosion behavior of die-cast AM50 magnesium alloy was investigated using an electrolyte close to the deicing salt composition with and without Ca 2+ addition. The goal of the work was to point out the differences in the corrosion mechanism with and without Ca 2+ addition by electrochemical investigations, H 2 evolution, and weight-loss characterization. Under polarizing conditions the resulting current densities are significantly lower in the presence of Ca 2+ . Electrochemical impedance spectroscopy (EIS) measurements up to 4.5 h showed an increasing polarization resistance, R P , with time, and after 4.5 h, a breakdown of R P values in the presence of Ca 2+ combined with the appearance of an inductive loop. Mass-gain, mass-loss, and hydrogen evolution measurements confirmed the inhibiting behavior in the presence of Ca 2+ up to 1 day. During immersion, the pH does not exceed a value of 10. Therefore, under these experimental conditions it is not possible for the system to reach the passivating pH of 12. The influence of the pH value on the corrosion rate was found to be limited in the pH region of 8 to 11.
Zur Senkung des Fahrzeuggewichtes sollen zukünftig warmgewalzte Magnesium‐Blech‐Legierungen wie auch CFK‐Bauteile in die Karosseriestruktur integriert werden. Beide Leichtbaumaterialien sind mit verzinkten Stählen sowie mit Aluminium ohne aufwendige Korrosionsschutzmaßnahmen nicht kombinierbar. Aus elektrochemischen Untersuchungen wird bezüglich der Vorbehandlung von Magnesium eine oxidkeramische Oberflächenausbildung durch plasmachemische Oxydation als aussichtsreich eingeschätzt. Nach dem mechanischen Fügen mit Aluminium können derartige porenbehaftete Substrate mittels KTL in hoher Qualität lackiert werden, bevor der weitere Lackaufbau erfolgt. CFK‐Bauteile indessen haben – im Gegensatz zu Magnesium – elektrochemisch ein sehr edles Verhalten und verursachen starke Kontaktkorrosion an metallischen Anbauteilen, wenn die C‐Fasern durch den Bauteil‐Beschnitt frei liegen. Für das mechanische Fügen von CFK mittels Schraub‐ und Niettechnik eignen sich daher nur Verbindungselemente aus austenitischen Stählen oder aus Titan‐Legierungen (z.B. TiAl6V4). Aus Sicht der Materialverträglichkeit von CFK mit anderen Blech‐Materialien, vorzugsweise mit Aluminium, ist die Verklebung mit zusätzlicher Abdichtung entsprechender Flansche zwingend erforderlich.
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