Solidification and spangle formation of hot-dip-galvanized zinc coatings

Solidification of hot dip coatings was studied regarding thermal conditions. Optical phenomena occurring at the liquid zinc surface were documented and the solid zinc surface was characterized in respect to optical and microscopic appearance, distribution of Pb and Al, crystal orientation, and topography. Resulting from these observations, a solidification model can be derived: zinc nucleation occurs at the steel/zinc interface. Due to thermal conditions in the slightly undercooled liquid zinc film, solidification occurs by rapid sideways dendritic expansion of the nucleated grains along the steel/zinc interface. Dendritic growth is controlled by interaction of crystal orientation of the nucleated zinc grain and thermal conditions in the undercooled layer. This leads to formation of different shaped grains with thicker and thinner sectors. The mechanism of sideways expansion continues until the entire interface is covered with dendritic zinc grains. Even though the zinc outer surface is still a liquid phase, final spangle size, as well as surface appearance and shape of the grains, is already determined at that early stage of solidification. Further growth only leads to a thickening of the solid layer; however, its relief remains almost unchanged. Thickening occurs relatively slowly due to the fact that marginal heat flow toward the surface now represents the limiting factor. Growth of the solid zinc layer results in continuous enrichment of Pb and Al in the residual liquid. Then, outer surface solidification starts as segments of single grains emerge. Distribution of the enriched residual melt in between the already solid areas depends on the relief of the solid layer. Finally, eutectic Zn-Pb reaction with precipitation of Pb particles takes place, which defines the dull appearance of these regions. Solidification for “lead-free” coatings is essentially the same, except that the final eutectic Zn-Pb reaction is missing. Additional investigations of dendritic secondary arm spacing indicate that Pb does not act by suppressing zinc nucleation. Pronounced dendritic growth is proposed to be favored by a change in interfacial energy. The new solidification model is applicable for a wide range of processing conditions and explains the origin of the typical spangle structure.     

Integration of an electrooptic polymer in an integrated opticscircuit on silicon

This paper presents the joining of active nonlinear polymer waveguides with passive silicon nitride waveguides (SiO₂-Si ₃N₄-SiO₂) to form an integrated Mach-Zehnder modulator with a lateral electrode configuration on a silicon substrate. Passive and active waveguides are based on a silicon-nitride-strip guiding structure. In the active waveguide a nonlinear polymer layer is used to obtain an index modulation via the electrooptic effect. Despite the silicon nitride strip based guiding structure, 63% of the energy of the fundamental mode is guided in the nonlinear polymer (provided by Flamel Technology, Venissieux, France). Poling with field strengths up to 75 V/μm applied to the lateral electrodes has been employed to orient the chromophores. A half wave voltage of 35 V has been measured for an electrooptic coefficient of 5.8 pm/V at a wavelength of 1.3 μm. Optical loss measurements have been done on polymer and passive waveguides. The best results have been 1.8 dB/cm for the active and 0.78 dB/cm for the passive waveguides leading to a total loss of 6 dB for a modulator with an interaction length of 2.5 cm. The coupling loss between a laser diode and the passive waveguide structure was measured to be at least 4.6 dB using a microscope objective and piezo-electric displacement elements. Stability tests under atmospheric conditions have shown a decrease of the electrooptic coefficient which might be due to the hygroscopic behavior of the active polymer. The bandwidth of the modulator has been determined to be 4 MHz     

Surface and Coating Analysis of Press‐Hardened Hot‐Dip Galvanized Steel Sheet

The use of continuous galvanized steel sheet as feedstock material for press hardening leads to components of very high strength levels with classical cathodic corrosion protection. The present work provides an insight into this technology with special focus on surface oxidation and intermetallic phase formation during the austenisation process. For that reason hot dipped galvanized 22MnB5 steel sheets, with a blank thickness of 1.5 mm and an average coating weight of 70 g m^?2, were annealed in a temperature range of 400–900°C in steps of 50°C without soaking before quenching in water. Surface and cross‐cuts were analyzed by SEM, EDX, and XRD to illustrate the phase formation and the surface changes during thermal treatment. Corrosion behavior was also studied based on electrochemical investigations and an accelerated, cyclic, automotive corrosion tests called VDA, which is a mixture of salt spray test and alternating climate test. It was found that austenisation of galvanized steel sheet causes a significant change of the coating. The generated coating still offers cathodic protection for the steel substrate and has higher corrosion resistance than standard galvanized steel sheet. Surface oxidation also occurs during the press hardening process leading to a surface covered with successively arranged layers of Al₂O₃ and ZnO, containing also further oxidized alloying elements.