Corrosion mechanism of high pressure die cast (HPDC) AM50 magnesium alloy from a planar microstructure context is discussed. The progress of corrosion is mapped by means of electrochemical impedance spectroscopy, microscopy, and by elemental composition distribution analysis. Corrosion resistance of the alloy surface increased on prolonged exposure to the electrolyte (1.6wt.% NaCl solution), which is attributed to the selective dissolution of Mg-rich alpha phase, and to the evolution of Al-rich surface comprising of Mg17Al12 (beta) and Al-containing eutectic phases. A comparative study between HPDC and sand cast AM50 alloys revealed superior corrosion performance of die cast alloy.

Squeeze casting of magnesium alloys potentially can be used in lightweight chassis components such as control arms and knuckles. This study documents the microstructural analysis and corrosion behavior of AM50 alloys squeeze cast at different pressures between 40 and 120 MPa and compares them with high-pressure die cast (HPDC) AM50 alloy castings and an AM50 squeeze cast prototype control arm. Although the corrosion rates of the squeeze cast samples are slightly higher than those observed for the HPDC AM50 alloy, the former does produce virtually porosity-free castings that are required for structural applications like control arms and wheels. This outcome is extremely encouraging as it provides an opportunity for additional alloy and process development by squeeze casting that has remained relatively unexplored for magnesium alloys compared with aluminum. Among the microstructural parameters analyzed, it seems that the β-phase interfacial area, indicating a greater degree of β network, leads to a lower corrosion rate. Weight loss was the better method for determining corrosion behavior in these alloys that contain a large fraction of second phase, which can cause perturbations to an overall uniform surface corrosion behavior.

The passivation and pitting corrosion behaviour of stoichiometric Laves phase magnetostrictive compound Tb0.3Dy0.7Fe1.92, was investigated in 0.02M NaOH solution at different chloride ion concentrations (0.001-0.1M) by cyclic potentiodynamic polarisation. A systematic decrease in breakdown potential and corresponding increase in anodic current densities was observed with increasing chloride concentration. Potentiostatic currents were measured at several applied potentials in the passive range. The incubation period of pitting decreased with increasing chloride concentration. Microscopic examination of corroded surfaces after cyclic polarisation scans revealed that the attack by chloride ions was localised along arrays of active rare earth rich phase. At higher chloride levels, these phases were undermined. The number of pits and pit size increased with increasing chloride concentration. Electrochemical impedance spectroscopy conducted at open circuit potential and 50 mV above breakdown potential confirmed increasing corrosion rate with increasing chloride concentration. It has been concluded that the significant mechanistic step in the pitting corrosion of Tb0.3Dy0.7Fe1.92 is adsorption of metal-chloride complex species.

A logical approach towards the development of protective metallic coatings for the newly developed pseudo-binary rare earth magnetostrictive material Tb0.3Dy0.7Fe1.92 has been put forth. Based on the proposal, experimental trials were conducted on nickel-based coatings, produced by electrodeposition and electroless plating techniques. Direct current electrodeposition of nickel was conducted using both aqueous and non-aqueous plating baths. Non-aqueous electrodeposition produced adherent nickel deposits unlike aqueous electrodeposition and electroless plating methods. The main cause of poor coating adherence in aqueous electrodeposition and electroless plating has been identified as excessive hydrogen evolution. The physical disintegration of the substrate due to hydrogen embrittlement has been briefly addressed.

The corrosion behaviour of pseudo-binary compound Tb0.3Dy0.7Fe1.92 (Terfenol-D) was studied in 3.5% NaCl and 0.01N Na2SO4 solutions using Tafel polarisation, linear polarisation, electrochemical impedance spectroscopy and weight loss techniques. Electrochemical testing revealed the adverse effect of chloride ions, as corrosion rates were higher in 3.5% NaCl solutions than in 0.01N Na2SO4 solutions. The effect of aeration was studied by conducting experiments in freely-aerated, deaerated and fully-aerated conditions. Absence of oxygen accelerated corrosion in chloride environment, due to increased hydrogen interaction in presence of destabilised surface films. The improved corrosion resistance in chloride-free environment was correlated with morphological features of the corroded surfaces. Micro-compositional analysis of the corrosion products revealed that the corrosion products were primarily rich in rare earth elements. A comparison of all experimental techniques showed that Tafel extrapolation and EIS techniques provided reliable estimates of corrosion rate.

The effect of hydrogen on the corrosion behavior of rare-earth-based magnetostrictive material Tb0.3Dy0.7Fe1.92 was studied after cathodic charging of hydrogen. The aqueous solutions used for understanding electrochemical behavior were 3.5% NaCl and 0.01 N Na2SO4, in freely aerated and deaerated conditions. The severity of hydrogen attack in the presence of chloride ions increased with increasing hydrogen charging duration. Terraced and cleavagelike faceted features were evident on the surfaces after hydrogen charging and they were related to hydrogen embrittlement of the material. The destabilizing effect of chloride ions on the surface films, resulting in increased hydrogen uptake and consequent lowering of corrosion resistance, was verified by testing in chloride-free 0.01 N Na2SO4 environment in both freely aerated and deaerated conditions. The material was more tolerant to charged hydrogen in the absence of chloride ions. Protective surface film formation in 0.01 N Na2SO4 solution resulted in improved corrosion resistance of Terfenol-D compared to that in 3.5% NaCl solution. (c) 2007 The Electrochemical Society.

Nanocrystalline nickel (nc-Ni) coatings were produced by pulse electrodeposition using Watts bath with sodium citrate and saccharin added as grain refining agents. The electrochemical nature of nc-Ni coatings, evaluated in 1M H2SO4 solution by electrochemical impedance spectroscopy. The corrosion rate of bulk nickel was lower than that of nc-Ni after stabilisation of free corrosion potential.

The effect of hydrogen on the corrosion behavior of rare-earth-based magnetostrictive material Tb 0.3 Dy 0.7 Fe 1.92 was studied after cathodic charging of hydrogen. The aqueous solutions used for understanding electrochemical behavior were 3.5% NaCl and 0.01 N Na 2 SO 4 , in freely aerated and deaerated conditions. The severity of hydrogen attack in the presence of chloride ions increased with increasing hydrogen charging duration. Terraced and cleavagelike faceted features were evident on the surfaces after hydrogen charging and they were related to hydrogen embrittlement of the material. The destabilizing effect of chloride ions on the surface films, resulting in increased hydrogen uptake and consequent lowering of corrosion resistance, was verified by testing in chloride-free 0.01 N Na 2 SO 4 environment in both freely aerated and deaerated conditions. The material was more tolerant to charged hydrogen in the absence of chloride ions. Protective surface film formation in 0.01 N Na 2 SO 4 solution resulted in improved corrosion resistance of Terfenol-D compared to that in 3.5% NaCl solution.

The effect of constituent elements Tb, Dy, and Fe on corrosion of Terfenol-D (Tb0.3Dy0.7Fe1.92) was studied by potentiodynamic polarization studies in freely aerated, deaerated, and fully aerated 3.5% NaCl and 0.01 N Na2SO4 solutions. The corrosion products obtained from immersion and cyclic spray tests were characterized by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The polarization behavior of Terfenol-D closely resembled that of pure iron in all environments. Characterization of spalled corrosion products after immersion testing revealed a higher rare earth (RE) content. An increase in iron content of the adherent corrosion products with increasing duration of exposure pointed out the significant role of iron in the general corrosion processes of Terfenol-D. The polarization and characterization results have been related to the dealloying corrosion mechanism, wherein selective dissolution of REs from the surface results in corrosion from an essentially iron-enriched surface. (c) 2008 The Electrochemical Society.