Effect of chloride ions on passivity of Mg based materials

Abstract

The effect of chloride ions on passivation of pure Mg, Mg–0·6 vol.-%Mo composite, Mg–0·6 vol.-%Cu composite and Mg alloy AZ91D has been studied in 0·1M NaOH solution by cyclic polarisation and electrochemical impedance spectroscopy (EIS). An addition of even 0·05M of chloride ions was sufficient to cause breakdown of passivity. Cyclic polarisation curves revealed a positive hysteresis loop in the presence of chloride ions. Breakdown potentials decreased continuously, for all materials, with increasing addition of chloride. Electrochemical impedance spectroscopy studies revealed that the film resistance of all Mg based materials continuously decreased with the addition of chloride ions. The film resistance of Mg–0·6%Cu and Mg–0·6%Mo composites was lower than that of pure Mg and AZ91D. Mg–0·6%Mo composite showed the lowest film resistance in all the solutions. The increase in film capacitance, for pure Mg and Mg based composites, with the addition of chloride ions, was attributed to surface roughening. Mo reinforcements were more detrimental than copper reinforcements.     

Electrolyte and temperature effects on pitting corrosion of type 316LN stainless steels

Abstract

The influence of electrolyte composition and temperature on the pitting corrosion resistance of nitrogen bearing (0·015, 0·198, and 0·56%N) type 316L stainless steels has been investi gated. Anodic polarisation curves were determined in neutral chloride solution at room temperature and at elevated temperatures of 308, 318, and 333 K. Similar polarisation studies were also conducted at room temperature in 1N H₂ SO₄ and in an acidic chloride solution containing 1N H₂ SO₄ and 0·5M NaCl. The results show that the critical pitting potential E_pp in neutral chloride and acidic chloride media increases as the nitrogen content of the alloy increases, indicating that resistance to pitting increases with the addition of nitrogen. However, in 1N H₂ SO₄ solution, the transpassive potential was almost independent of the nitrogen content. As the temperature of the neutral chloride medium was increased, the E_pp values decreased, irrespective of nitrogen content. The decrease in E_pp is attributed not only to the temperature induced modification of the passive oxide film but also to chloride induced activity at the passive film/solution interface. Nevertheless, the alloy con taining 0·56% nitrogen showed better resistance to pitting at 333 K than did the alloy with 0·015% nitrogen at room temperature. SEM examination of the pitted specimens showed clear evidence of pitting for the alloy with 0·015% nitrogen, but insignificant pitting attack for the alloy with 0·56%N. However, the alloy with 0·56%N displayed some pitting attack when the experiments were conducted at 333 K.     

A globally stable state feedback controller for flexible joint robots

The paper addresses the problem of controlling the joints of a flexible joint robot with a state feedback controller and proposes a gradual way of extending such a controller towards the complete decoupling of the robot dynamics. The global asymptotic stability for the state feedback controller with gravity compensation is proven, followed by some theoretical remarks on its passivity properties. By proper parameterization, the proposed controller structure can implement a position, a stiffness or a torque controller. Experimental results on the DLR lightweight robots validate the method.