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 b-phase interfacial area, indicating a greater degree of b 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 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.
The effect of constituent elements Tb, Dy, and Fe on corrosion of Terfenol-D ͑Tb 0.3 Dy 0.7 Fe 1.92 ͒ was studied by potentiodynamic polarization studies in freely aerated, deaerated, and fully aerated 3.5% NaCl and 0.01 N Na 2 SO 4 solutions. The corrosion products obtained from immersion and cyclic spray tests were characterized by Fourier transform infrared ͑FTIR͒ spectroscopy and scanning electron microscopy ͑SEM͒. 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 ͑RE͒ 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.The corrosion of rare earth ͑RE͒ magnetic materials has been an important subject of study. The extensively studied materials in this category are materials based on Nd-Fe-B and Sm-Co alloys. A relatively new RE magnetic material is based on the Tb-Dy-Fe alloy. Detailed studies on several aspects of corrosion of commercial-grade Tb 0.3 Dy 0.7 Fe 1.92 ͑commercially called Terfenol-D͒ have recently been concluded. 1-5 Corrosion of these ͑Tb,Dy͒Fe 2 -based magnetostrictive materials is of scientific, commercial, and strategic concerns, as this material is the nextgeneration material for active sonar applications. Apart from this, Terfenol-D finds varied applications, such as in tools for well cleaning in the petroleum, oil, and gas industries, in fuel injection, and in medical appliances.The known aspects of corrosion of Terfenol-D are reviewed briefly. Terfenol-D is particularly corrosion prone in the presence of chloride ions. 3,4 Moreover, corrosion rates are higher in the absence of oxygen and with increased duration of hydrogen charging. 2 Hydrogen embrittlement accompanies the corrosion process of Terfenol-D, 2 which undermines the structural integrity of the material.The objective of the present communication is to understand the effect of constituent elements ͑Tb, Dy, and Fe͒ on the corrosion behavior of Terfenol-D. Understanding the corrosion mechanism of an alloy may be very complex when the components of the alloy differ widely in their corrosion behavior. Therefore, it may be convenient to understand the corrosion mechanisms of individual alloy components and then relate them to the alloy system under study. Characterization of corrosion products formed in different environments has been used extensively for understanding the corrosion mechanisms. The most common metallic corrosion products in atmospheric and corrosive environments include oxides ͑hydrated͒ and ͑oxy-͒ hydroxides.This study also characterizes the corrosion product on Terfenol-D and its constituents under similar t...
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