Oxidation Start Detection on Heating Parts of Thermogravimetry Curves for High Temperature Alloys Based on Nickel, Cobalt or Iron
The heating parts of thermogravimetry curves performed for cast alloys in air at high temperatures were analysed, after correction from the effects of air buoyancy variation, to characterize the transient oxidation before reaching the targeted temperature for the isothermal stage usually applied thereafter. This was done here for three binary M-30Cr alloys, six ternary M-30Cr-0.4 and 0.8%C alloys, and three M-30Cr-0.4C-6Ta quaternary alloys, heated in synthetic air at 20 K min-1 until 1000, 1100 or 1200°C. The cobalt alloys begin to be oxidized with a sufficient mass gain to be detected by thermogravimetry, earlier than the nickel or iron alloys (i.e. at lower temperatures). The temperature of oxidation start is lowered by the presence of tantalum for the three families of alloys. When the carbon content (or the carbides density) increases in the alloy, the temperature of oxidation start decreases for the cobalt alloys and remains almost constant for the nickel or iron alloys, while the total mass gain due to oxidation during heating increases for the nickel and cobalt alloys and decreases for the iron alloys.
Role of nickel in the oxidation of Fe–Cr–Ni alloys in air–water vapour atmospheres
Abstract A series of experimental austenitic alloys has been produced in which the nickel content ranges from 14 to 43%, with constant levels of 20%Cr, 1%Mn and 0.5%Si. A combination of isothermal, discontinuous and cyclic oxidation testing has been used to elucidate the performance in dry air and in air with 10%, 45% or 62% water vapour at 700°C and 1000°C. Evaluation was by means of thermogravimetry, surface analysis with glow discharge optical emission spectroscopy and scanning electron microscopy. Nickel is shown to have several roles: it accelerates the kinetics of chromia formation yet suppresses chromia spallation at 700°C. At 1000°C, it strongly decreases the breakaway oxidation and spalling associated with iron oxide formation. This effect is particularly marked in environments containing water vapour, where the material loss may be decreased 10-fold by an increase in the nickel content. Results correlate to thermodynamic and kinetic data which show nickel to increase the chromium activity and diffusivity in the alloy.
An anionic shell shields a cationic core allowing for uptake and release of polyelectrolytes within core-shell responsive microgels.
To realize carriers for drug delivery, cationic containers are required for anionic guests. Nevertheless, the toxicity of cationic carriers limits their practical use. In this study, we investigate a model system of polyampholyte N-isopropylacrylamide (NIPAM)-based microgels with a cationic core and an anionic shell to study whether the presence of a negative shell allows the cationic core to be shielded while still enabling the uptake and release of the anionic guest polyelectrolytes. These microgels are loaded with polystyrene sulfonate of different molecular weights to investigate the influence of their chain length on the uptake and release process. By means of small-angle neutron scattering, we evaluate the spatial distribution of polystyrene sulfonate within the microgels. The guest molecules are located in different parts of the core-shell microgels depending on their size. By combining these scattering results with UV-vis spectroscopy, electrophoretic mobility and potentiometric titrations we gain complementary results to investigate the uptake and release process of polyelectrolytes in polyampholyte core-shell microgels. Moreover, Brownian molecular dynamic simulations are performed to compare the experimental and theoretical results of this model. Our findings demonstrate that the presence of a shell still enables efficient uptake of guest molecules into the cationic core. These anionic guest molecules can be released through an anionic shell. Furthermore, the presence of a shell enhances the stability of the microgel-polyelectrolyte complexes with respect to the cationic precursor microgel alone.