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The tribocorrosion behaviour of Ti-6Al-4 V exposed to phosphate buffered saline was investigated under a range of normal forces in both cathodic and anodic regions to provide a basis for properly deriving the tribological constants of this alloy. To achieve this, a new customised tribotester was designed and manufactured to rub the Ti-6Al-4 V disks against zirconia balls. The tests were conducted at a sliding frequency of 1 Hz and a sliding distance of 4.8 mm under various normal forces and potentials as 17.5, 10.8, 6, 3.5 N, and − 1.2, − 0.6, 0, 0.4, 0.8 V/VAg/AgCl, respectively. The damaged surfaces were characterised by scanning electron microscopy and energy-dispersive X-ray spectroscopy, profilometer, and micro-hardness tester. The post analyses confirmed the appearance of some minor cracks together with third-body wear particles. No significant changes in the hardness were detected after the tribocorrosion tests. The results of profilometry and electrochemical current indicated that in the anodic region the chemical losses accounted for a significant proportion (up to 36%) of the total loss. The proportional chemical loss increased with the potential; however, neither direct nor reverse relationship was found with the normal force. Overall, in the anodic domain, the material loss increased with the potential level due to the formation of oxide layer which may induce more shear cutting. In the cathodic domain, hydrogen embrittlement changed the properties of the interface and thus, the amount of material loss. Both the mechanical and chemical wear were described by an existing tribocorrosion theory; thereby, the theory was equipped with its tribocorrosive constants for future analyses on the tribocorrosion of this alloy extensively used in various applications including biomedical implants.

reactors by continuous ones. The reactor system which is investigated here comprises several inputs and due to the near plug fl ow characteristics between the inputs and the outputs, reacts to the changes of the infl ows with large time delays. Model-based optimizing control is the obvious candidate to control such a multi-input plants with significant delays. While the standard implementations of model predictive control penalize the deviations of the states or outputs from reference values and the control moves in the cost function, in this work the controller is formulated to account directly for the economics of the reactor system while the product quality parameters are imposed as constraints. [ 1 ] Some applications of optimizing control can be found in refs. [ 27 -30 ] . Limited computation power and lack of effi cient optimization algorithms have caused thatIn this contribution, a nonlinear model-based optimizing control for continuous polymerization of acrylic acid in tubular reactors with multiple side injections of monomer is developed. The background of this work is to transfer the production of polymers from semibatch to continuous operations. The confi guration of the tubular reactor, which imposes long delays between the inputs and the measurements, and the lack of intermediate measurements as well as the nonlinear reaction kinetics and sharp moving fronts of concentrations when the infl ows are changed, make the application of the optimizing control very challenging. In order to simulate the sharp fronts of the reactor system faithfully and fast, the spatial domain of the partial differential equations model of the tubular reactor is discretized by applying the weighted essentially non-oscillatory scheme. We formulate the controller such that it optimizes the productivity of the reactor directly, while the product quality parameters are imposed as constraints. A particle fi lter is implemented to estimate the states of the reactor system and to initialize the process model. The simulation results show that the controller can increase the product throughput considerably compared to an initial operating point and has a robust performance against the measurement noise. Furthermore, the effect of formulating the quality constraints as hard or soft constraints as well as a changeover scenario between the different grades of a polymer are studied.