*Magnesium Alloys for Sustainable Weight-Saving Approach: A Brief Market Overview, New… DOI: http://dx.doi.org/10.5772/intechopen.102777*

magnesium when molten. In the air, molten magnesium is coated with an oxide layer, which, in contrast to the aluminum oxide layer, cannot protect highly reactive magnesium from oxygen. It is true that molten aluminum, too, is highly reactive with oxygen. However, the alumina layer that instantaneously forms on liquid metal in oxygen is dense and not permeable to further oxygen. The magnesium oxide layer formed during melting is characterized by low density. The Pilling–Bedworth number (PBR) explains the high-temperature oxidation behavior of different metals and their oxides [23] by the ratio between the molar volume of oxide and the molar volume of metal. This volume change is responsible for varying types of surface stress in the oxide layer.

For PBR <1, tensile stress on the oxide layer promotes the layer to crack: that's what happens in molten magnesium metal. When the PBR is equal to 1 (the better situation with PBR above 1), it represents a safe condition. Sound compressive stresses develop in the thin dense, and stable oxide layer, protecting the molten metal from the outer atmosphere. This happens for iron, aluminum, titanium, and other metals. Unfortunately, this does not apply to magnesium. Emley [24] found that up to 450°C, magnesium forms a protective magnesium oxide layer, but it becomes porous and non-protective over 450°C. The high reactivity with oxygen causes magnesium to easily ignite and endangers the workers and the production line.

The time to ignition depends on the magnesium alloy composition [23, 25]. The real big problem of flame ignition in magnesium is that the oxidation reaction is highly exothermic. Magnesium oxide, white powder, creates a net release of energy in the form of heat. Magnesium ignited burns with flame at more than 2000°C; thus, no crucible can resist if the flame is not extinguished. Furthermore, magnesium atoms are also capable of reducing water to the highly flammable hydrogen gas by the reaction Mg(s) + 2H2O → Mg (OH)2(s) + H2(g); meanwhile, hydrogen gas could be easily ignited by the excess heat given by the magnesium reduction reaction. Magnesium metal can also react with carbon dioxide when present in the atmosphere to promote and sustain magnesium oxide formation accordingly with the following: Mg(s) + CO2 → 2MgO(s) + C(s). For this reason, conventional carbon dioxide fire extinguishers cannot be used for extinguishing magnesium fires (required Class D dry chemical fire extinguisher or covering the fire with sand to remove air source).

This hazardous behavior of magnesium metal is therefore historically correlated to conditions that lead to flame ignition of molten magnesium or magnesium in the form of powder, ribbon, thin strips, and foils, namely those fine structure forms that can be quickly heated up just by relatively low heating source, for example by friction. The highly exothermic oxidation reaction could bring explosive hazards in the presence of moisture when flame ignition is not adequately managed by specific knowledge and expertise. For these reasons, particularly in the presence of molten magnesium (for example, during cast shaping), fluorine-based compounds, such as SF6, for protection of molten magnesium have been used since the 1930s [26]. Before introducing SF6, magnesium was protected with alkali metal halide fluxes, sulfur dioxide (SO2), or even elemental sulfur. The decomposition and following reaction between the fluorine and liquid magnesium keeps separate highly reactive molten magnesium from oxygen. On the one hand, these reactions are thought capable of creating on the molten metal surface an elastic, nonporous protective film containing MgO and MgF2 with a Pilling-Bedworth ratio larger than 1 [27]. On the other hand, significant impact is ascribable to the use of SF6 as a cover gas. The SF6 environmental impact has been calculated to be 22,800 kg CO2eq/kg of SF6 used (in other words, 22,800 times greater than 1 kg of CO2 emitted). Usually, 1 kg of SF6 is required as cover gas per ton of melting magnesium, resulting in a 22,800 kg CO2 equivalent per kilogram of melt

magnesium. In Europe, SF6 is banned, while in the United States, its use is optional for the industry.

To present date, banded SF6 has been substituted by less impacting hydrofluorocarbons such as HFC-134a, however, considered a greenhouse gas but much less impacting. A much lower impact is for sulfur dioxide, but it presents limits for its toxicity and its corrosive properties. Usually, a specific blend of them is used. Recently the Novec 612 fluid—registered by the 3 M Company— promises a meager global warming potential (GWP, expressed as kgCO2eq/kg product) of 1, equivalent to CO2. Furthermore, to limit the intense use of protective substances, an old approach recently proposed consists of adding unique alloying elements to improve the ignition resistance of magnesium alloys. In the past, Emley [24] claimed that additions of small amounts of Be, Al, and Ca enhanced the oxidation resistance of solid Mg alloy near the melting point. Such magnesium alloys could be melted in the air if the oxide skin on the ingot was not broken. Sakamoto et al. [28] verified the oxide film on the Mg-Ca consists of a CaO surface thin layer, and just below this layer, a mixture of MgO-CaO exists. To date, the main reason for this protective effect from Ca-O is not clarified. One prominent hypothesis embraces the PBR rule. The higher thermodynamic stability of Ca-O added oxide layer and the kinetics of the diffusion and reaction of Mg ion at and through the oxide layer formed by a mixture of MgO and CaO. When the oxide layer consists of a combination of MgO and CaO, the large volume of CaO might compensate for the shrinkage due to MgO formation. Phenomena involved in retarding flame ignition in Mg alloys systems when alloyed with Ca, Be, and Y has been studied for years but not wholly clarified today.

To summarize, reasonable and sustainable practices are available today in the marketplace to safely treat magnesium and significantly reduce the pollutant emissions in handling molten magnesium in foundries.

But the second source of pollution for the magnesium industry, much more relevant and challenging to control, depends on the vast amount of energy necessary for the magnesium extractive and refining phase, namely the primary magnesium fabrication. There are only a few processes available for the primary magnesium fabrication as they are based on sources of the raw materials by which magnesium can be extracted: raw materials ores (such as dolomite, magnesite, hydroxide mineral brucite, halide mineral carnallite) and brine, which is a mainly a highly concentrated water solution of common salts like hydrated magnesium chloride, magnesium sulfate and magnesium bromide, whose preferred reservoirs are the higher concentrated seawater such as the Great Salt Lake and the Dead Sea. By the way, magnesium raw material sources are considered practically inexhaustible, as magnesium is the 4th abundant metal in the Hearth crust, following iron, aluminum, and silicon. Moreover, inexhaustibility is properly true for seawater reservoirs of magnesium chloride salts. Depending on the type of magnesium source employed, we can distinguish two prominent process patterns to produce magnesium metal: (a) the electrolysis of fused anhydrous magnesium chloride obtained by various refining upstream processes (e.g., dehydration of magnesium chloride brines or chlorination of magnesium oxide) and (b) the thermal reduction of magnesium oxide by ferrosilicon derived from carbonate ores. Today's electrolytic processes are mainly based on the oldest and original Dow process employing seawater as a primary magnesium source.

The Dow process was developed in the first decade of the twentieth century, as the USA started an extensive magnesium production for military scopes. Electrolytic cells are vessels equipped with multiple steel cathodes and graphite anodes partially submerged in the dehydrated molten salt electrolyte. They generally operate to
