D. H. St John

THE ANODIC DISSOLUTION OF MAGNESIUM IN CHLORIDE AND SULPHATE SOLUTIONS

The electrochemical behaviour of magnesium was studied in representative chloride and sulphate solutions including NaCl, Na2SO4, NaOH and their mixed solutions, HCl, and H2SO4: (1) by measuring electrochemical polarisation curves, (2) by using electrochemical impedance spectroscopy (EIS), and (3) by simultaneous measurement of hydrogen gas evolution and measurement of magnesium dissolution rates using inductively coupled plasma atomic emission spectrophotometry (ICPEAS). These experiments showed that a partially protective surface film played an important role in the dissolution of magnesium in chloride and sulphate solutions. Furthermore, the experimental data were consistent with the involvement of the intermediate species Mg+ in magnesium dissolution at film imperfections or on a film-free surface. At such sites, magnesium first oxidised electrochemically to the intermediate species Mg+, and then the intermediate species chemically reacted with water to produce hydrogen and Mg2+. The presence of Cl- ions increased the film free area, and accelerated the electrochemical reaction rate from magnesium metal to Mg+

Formation of defect bands in high pressure die cast magnesium alloys

Die cast magnesium components are being increasingly used worldwide because of the excellent castability and properties that magnesium alloys offer. High pressure die casting of thin-walled components is particularly suitable because of the excellent flow characteristics of molten magnesium alloys. Typical automotive applications for thin-walled castings include components such as instrument panels, steering wheels, door frames and seat frames. These applications require optimisation of the quality and performance of the castings. It has been found that bands of porosity or segregation which follow contours parallel to the surface of the casting are formed under certain casting conditions in thin-walled magnesium high pressure die castings. The presence of this type of defect can have a significant effect on the mechanical properties. This paper investigates the effect of varied casting conditions on casting integrity and the appearance of the bands. A rationale for understanding the origin of these defects is related to the solidification behaviour, the mushy zone rheological properties and the filling pattern of the casting with associated shearing of the mushy zone. Methods to optimise the process parameters to control the occurrence of the banded defects, and thereby optimise the quality of high pressure die cast magnesium components, are outlined.

Grain Refinement of Magnesium Alloys: A Review of Recent Research, Theoretical Developments, and Their Application

This paper builds on the “Grain Refinement of Mg Alloys” published in 2005 and reviews the grain refinement research on Mg alloys that has been undertaken since then with an emphasis on the theoretical and analytical methods that have been developed. Consideration of recent research results and current theoretical knowledge has highlighted two important factors that affect an alloy’s as-cast grain size. The first factor applies to commercial Mg-Al alloys where it is concluded that impurity and minor elements such as Fe and Mn have a substantially negative impact on grain size because, in combination with Al, intermetallic phases can be formed that tend to poison the more potent native or deliberately added nucleant particles present in the melt. This factor appears to explain the contradictory experimental outcomes reported in the literature and suggests that the search for a more potent and reliable grain refining technology may need to take a different approach. The second factor applies to all alloys and is related to the role of constitutional supercooling which, on the one hand, promotes grain nucleation and, on the other hand, forms a nucleation-free zone preventing further nucleation within this zone, consequently limiting the grain refinement achievable, particularly in low solute-containing alloys. Strategies to reduce the negative impact of these two factors are discussed. Further, the Interdependence model has been shown to apply to a broad range of casting methods from slow cooling gravity die casting to fast cooling high pressure die casting and dynamic methods such as ultrasonic treatment.

A simple prediction of the rate of the peritectic transformation

The rate of a peritectic transformation is controlled by diffusion through the peritectic envelope. The rate constant is determined by the diffusion coefficient in the peritectic product phase and by the form of the phase diagram features. Peritectic systems can be classified on the basis of phase diagram form to permit easy prediction of relative rate constants.

The peritectic transformation

Some experimental observations on the Al-Al3Ti and Cd-Cd3Ag peritectic systems are discussed in relation to transformation rates. In steady-stage unidirectional solidification equilibrium between the solid and liquid phases is closely approached at all points of the solid-liquid interface. An approximate diffusion analysis permits prediction of the rate of growth of the peritectic envelope by diffusion through the envelope. In the Al-Al3Ti system the peritectic product is formed principally by direct transformation from the liquid phase, and attack on the properitectic phase is minimal. In the Cd-Cd3Ag system the peritectic transformation goes almost to completion, and direct transfor- mation from the liquid plays a minor part.

Microstructural features produced by the reduction of wustite in H2/H2O gas mixtures

The systematic study of the reduction of pure wustites (FeO) between 600 and 1100°C in H2/H2O gas mixtures has revealed a number of important morphological changes. It has been shown that dense wustite can decompose to form a highly porous wustite before iron nucleation takes place. The product morphologies of iron formed on the wustite on reduction have been classified into three types, (a) porous iron, (b) porous wustite covered by dense iron, and (c) dense wustite covered by dense iron.

The Breakdown of dense iron layers on wustite in CO/Co2 and H2/H2O systems

Examination of wustite single crystals reduced in CO/CO2 and H2/H2O gas mixtures has shown that in all cases a dense iron layer is formed initially on the oxide surface and that the porous growth of iron which is obtained under certain experimental conditions occurs as a direct result of the breakdown of this initial dense iron layer. Possible mechanisms of the iron layer breakdown are examined and compared with the experimental observations. A qualitative model of the breakdown process involving the nucleation of gas bubbles and the expansion of these bubbles in the iron layer is presented.

Establishment of product morphology during the initial stages of wustite reduction

The product morphologies obtained on the reduction of wustite in CO/CO2 gas mixtures between 1073 and 1373 K are reported. Three types of product morphology are identified, namely, type A (porous iron), type B (porous wustite covered with dense iron), and type C (dense wustite covered with dense iron). The reactions which occur during the reduction of wustite in CO/CO2 and H2/H2O systems both before and after iron nucleation are examined. The product morphologies obtained on reduction are explained qualitatively in terms of the relative rates of the chemical reaction with the gas and the mass transport processes both in and on the solid.

Thermal analysis of peritectic alloys

Predictions that increasing cooling rate will result in increasing elevation of the apparent peritectic arrest temperature have been confirmed for Cd-Ag and Zn-Cu alloys in line with observations of Al-Ti alloys. The shape of the peritectic cooling curve is related to the competition between the peritectic transformation and direct crystallization from the melt in the formation of the secondary α phase. The effect on solidification of the relative depressions of the α and β liquidus during cooling is considered.

The effects of impurity elements on the reduction of wustite and magnetite to iron in CO/CO2 and H2/H2O gas mixtures

The reduction of dense wustite and magnetite samples in CO/CO2 and H2/H2O gas mixtures has shown that impurity elements in solid solution in the oxides can significantly affect the reaction mechanisms operative during reduction and the conditions for porous iron growth. In general, the presence of P, Mg, Ti, Si, Ca, K, and Na in wustite favors, in order of increasing strength, the formation of the porous iron product morphology. Aluminum, on the other hand, significantly reduces the range of gas conditions over which the porous iron microstructure may be obtained.