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Review

Are Magnesium Alloys Applied in Cars Sustainable and Environmentally Friendly? A Critical Review

by
Lucas Reijnders
IBED, Faculty of Science, University of Amsterdam, Science Park 904, 1090 GE Amsterdam, The Netherlands
Sustainability 2024, 16(17), 7799; https://doi.org/10.3390/su16177799
Submission received: 29 July 2024 / Revised: 25 August 2024 / Accepted: 5 September 2024 / Published: 6 September 2024
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Abstract

:
In the scientific literature, the terms sustainable, green, ecofriendly and environment(ally) friendly are used regarding magnesium alloys applied in cars. When sustainability is defined as remaining within safe planetary boundaries for mankind or as conserving natural capital for transfer to future generations, current alloys based on primary magnesium applied in cars are not sustainable. Current alloys based on primary magnesium are not green, ecofriendly or environmentally friendly when these terms mean that there is no burden to the environment or a minimal burden to the environment. Available environmental data do not support claims that current alloys based on magnesium originating from the Pidgeon process, which replace primary mild conventional steel in automotive applications, can be characterized as green, ecofriendly or environmentally friendly. There are options for substantially reducing contributions to the life cycle environmental burden of magnesium alloys. Minimizing the life cycle environmental burden of magnesium alloys may enable them to be characterized as environmentally friendly, ecofriendly or green in the sense of a minimal burden to the environment.

Graphical Abstract

1. Introduction

In the scientific literature, the terms sustainable, green, environment(ally) friendly and ecofriendly are used regarding magnesium alloys. For instance, D’Errico et al. [1] characterized the use of Mg alloys for weight reduction of cars as (eco-)sustainable and Emadi et al. [2] described this as sustainable. Singh et al. [3] characterized a novel Mg-Ca-Sc alloy as sustainable. There are currently hundreds of definitions and operationalizations (in terms of standars) of sustainability and most of them refer to environmental, economic and social matters [4,5]. In view of the latter, characterizing Mg alloys as sustainable might, for instance, have been supported by the finding that mining practices to generate the raw materials for magnesium alloy production meet environmental, social and economic mining sustainability standards [6]. D’Errico et al. and Emadi et al. [1,2], however, focused exclusively on environmental matters in their papers, without stating the sustainability definition that they used or stating the environmental sustainability standards that in their view should be met. Singh et al. [3] did not define sustainability when characterizing a Mg-Ca-Sc alloy as sustainable. Li et al. [7] and Shi et al. [8] characterized Mg alloys as ecofriendly in view of their role in lightweighting. Guo at al. [9] stated that lightweighting by magnesium has a role in green development. Lee et al. [10] characterized a recently developed alloy of Mg with Al, Ca and Y as environmentally friendly, referring to lightweighting of vehicles. Lightweighting by Mg alloys combined with their recyclability has been described as environmentally friendly by Kumar et al. [11] or green by Hu [12]. The recyclability of the Mg alloy AZ31 was referred to by Sabzehmeidani and Kazemzad [13] who presented this alloy as green. Liu et al. [14] characterized magnesium from the Qinghai electrolysis plant (China) as ‘generally a green product’, referring to ‘a very low carbon emission level’. Li et al. [15] presented magnesium alloys as ecofriendly structural materials without presenting reasons for this characterization. Common usage suggests that environmentally friendly, ecofriendly and green can be interchangeable and can have several meanings. These meanings are as follows: no burden to the environment, a minimal burden to the environment and, in comparisons, a reduced burden to the environment. The latter meaning often represents the least environmental version of environmentally friendly, ecofriendly or green.
The car industry is currently the most prominent user of magnesium used in alloys and accounts for about 55% of worldwide primary Mg metal use [14,16]. AM 50, AM60, AZ 31 and AZ 91 magnesium alloys are often used in cars [14,17]. Aluminum (A) and zinc (Z) present in Mg alloys improve strength and manganese (M) confers improved corrosion resistance [18,19]. In high-performance cars, magnesium alloys with rare earth metals (belonging to the WE series) are applied to engine blocks [20]. Examples of car parts to which magnesium alloys can be applied are given in Box 1.
Box 1. Examples of car parts to which magnesium alloys can be applied [1,14].
Box 1. Examples of car parts to which magnesium alloys can be applied [1,14].
BeamsOil pan
ConsolePedal bracket
Control boxRoof
DoorsSeals
EngineSeats
FramesSteering column and armature
HoodTransmission case
LiftgateWheels
When additional protection against corrosion is needed, a variety of surface treatments are available: conversion coatings, anodizing, plasma electrolytic oxidation, layered double hydroxides (that may include Zn), sol–gel coatings (that may include Ce, V or Zr), cold spraying, electrochemical plating and organic coatings [21,22]. Cr-based conversion coatings have been commonly used but are increasingly replaced by conversion coatings with other elements such as Ce, La, V and Zn [21,22,23]. The elements B, Cu, F, Ni and Sn may also be used in conferring coating-based corrosion resistance [13,21,24].
There are currently two major ways to produce Mg metal: a silicothermic reduction process, usually the Pidgeon process, and electrolysis of anhydrous MgCl2 at elevated temperatures [14,16,25,26,27]. Initially, electrolysis of MgCl2 dominated Mg production [28], but more recently Chinese producers using the Pidgeon process have had the largest share in the worldwide production of magnesium metal, currently estimated at about 87% [1,16,27,29]. Other countries where silicothermic processes are reportedly used to produce Mg metal include Brazil, Turkey and South Korea [16,26,30]. Electrolytic production of Mg has been reported as operational in China, Israel, Kazakhstan, Russia, the USA and the Ukraine [14,16,26,31].
The Pidgeon process commonly uses CaMg(CO3)2 (dolomite), FeSi and 2–3% CaF2 as input materials and fossil carbon compounds such as coal, coal-derived substances (e.g., coke, coke oven gas) and natural gas for heating [16,26,27,32]. The process flow for producing Mg metal with the Pidgeon process is in Figure 1.
The Pidgeon process has a production cycle of about 10 h in which there are switches between atmospheric pressure and vacuum [27,29]. In the Pidgeon process, fluorinated cover gas, in China this is commonly the potent greenhouse gas SF6, is applied to protect melted magnesium metal against oxidation and fire risk [33,34,35]. Current commercial electrolysis generating magnesium regards anhydrous MgCl2, which can be derived from MgCl2–brine or carnallite (KMgCl3.6H2O) or generated by chlorination from processed ores containing Mg, such as magnesite and serpentine [16,25,36]. The Mg output from electrolysis is protected by fluorinated cover gas [16,36]. Fluorinated cover gas is also often used when Mg is melted during processing, as in casting [16,37].
In what follows, the focus will be on the characterization of Mg alloys based on primary magnesium that are currently applied in cars as sustainable, ecofriendly, environmentally friendly or green. In Section 2, the method used will be briefly summarized. In Section 3, available evidence about the environmental impacts of the magnesium alloys applied in cars will be confronted with two definitions of sustainability that exclusively regard environmental matters. In Section 4, the available evidence will be reviewed to answer the question of whether current magnesium alloys in cars are ecofriendly, environmentally friendly or green in the comparative sense or in the sense of no burden to the environment. Section 5 will consider technical options for reducing and minimizing the life cycle environmental burden of magnesium alloys (and steel) used in cars. Section 6 presents the conclusions of this paper.

2. Methods

Databases of major publishers of scientific literature (ACS, Elsevier, Frontiers, IOP Publishing, MDPI, Sage, Springer-Nature, RSC, Taylor and Francis, Wiley), the Web of Science core collection and Google Scholar have been searched for use of the terms sustainable, green, ecofriendly and environmentally friendly to characterize magnesium alloys applicable in cars. The same databases were searched for publications regarding the environmental impacts of magnesium alloys applied in cars during their life cycles and for technological options to reduce these impacts. Available data about the environmental impacts of magnesium alloys turned out to be far from complete. In the case of missing data regarding the electrolytic production of magnesium metal, three companies active in this field were approached with questions, of which one answered. The data that were obtained were confronted with two definitions of (environmental) sustainability and three different definitions of the terms green, ecofriendly and environmentally friendly.

3. Are Current Mg Alloys Applied in Cars Sustainable?

Definitions of sustainability that focus on the relation between humanity and the environment have been used for a long time [4]. Current definitions regarding a sustainable relation between mankind and the environment published in the scientific literature include two major varieties [38]. The first one defines an operation space within which there is a sustainable relation. The second variety focuses on the conservation of natural capital for transfer to future generations. Natural capital can be defined as the stock of environmentally provided assets from which products and services that are useful to mankind can be derived. It comprises the physical environment providing services such as solar radiation, natural resources (e.g., ores) and ecosystems, providing services that benefit humans [38].
An influential example of the first variety of definitions is the safe operation space for mankind within planetary boundaries. The latter are defined as global environmental limits to avoid risks of abrupt non-linear environmentally induced changes in ecosystems [39]. Table 1 presents environmental matters for which planetary boundaries have been proposed and the current relation between planetary environmental burden and the proposed planetary boundary.
The emission of NOx linked to the combustion of fossil fuels during the (cradle-to-grave) life cycles of alloys based on primary magnesium in cars with internal combustion engines [17,33] currently contributes to transgressing the safe planetary boundary for N compounds (see Table 1). In view of current fossil energy sources for electricity production, the same would apply to the life cycles of Mg alloys applied in battery electric vehicles [42]. The emission of greenhouse gasses linked to Mg alloy life cycles [26,33] contributes to exceeding the planetary boundary for radiative forcing at the top-of-atmosphere (see Table 1). Mining of dolomite, magnesite, serpentine and carnallite has been associated with negative impacts on ecosystems, including the net primary production of biomass [43,44,45,46,47], and dumped slags originating from primary Mg production will also have negative impacts on the net primary production of biomass [29,48]. Thus, magnesium alloy life cycles contribute to transgressing the proposed planetary boundary for biosphere integrity (see Table 1). It can be concluded that current magnesium alloy life cycles are not within the safe operating space for humanity.
As to the conservation of natural capital for transfer to future generations, the focus is on the environmental burdens for future generations due to the interventions linked with the life cycles of magnesium alloys applied in cars, as shown in Box 2.
Box 2. Interventions to be considered in comprehensive environmental life cycle assessment (LCA) of magnesium alloys applied in cars, as used in state-of-the-art LCA methodologies [49,50].
Box 2. Interventions to be considered in comprehensive environmental life cycle assessment (LCA) of magnesium alloys applied in cars, as used in state-of-the-art LCA methodologies [49,50].
Interventions to be considered in comprehensive environmental life cycle assessment of magnesium alloys applied in cars
-
Consumption of mineral resources (e.g., ores).
-
Cumulative primary (non-renewable, fossil) energy consumption.
-
Fresh water consumption.
-
Land use and land use change affecting ecosystems and ecosystem services.
-
Greenhouse gas emissions affecting climate.
-
Emissions of fine particles (PM10) to air.
-
Emissions of acidifying substances, such as nitrogen compounds and sulfur oxides.
-
Emissions of nutrients, such as phosphate and nitrogen compounds.
-
Emissions of chlorinated and brominated compounds that can lead to ozone layer depletion.
-
Emissions of nitrogen oxides and hydrocarbons that may give rise to photo-oxidant smog.
-
Emission of substances that are hazardous to human health (human toxics).
-
Emission of substances that may lead to ecotoxicity.
The current input in the Mg alloy life cycles of fossil fuels, which are generated by slow geological processes [33,42], and the current emission of greenhouse gasses linked to magnesium alloy life cycles [26,33] violate the conservation of natural capital for transfer to future generations [38]. The same applies [38] to long-lasting negative impacts on ecosystem functions caused by mining [47,51,52] and the generation of persistent organochlorines in the primary electrolytic and silicothermic production of magnesium [53,54,55,56]. The generation of persistent polycyclic aromatic carbons by the Pidgeon process [57] and the substantial concentrations of leachable persistent hazardous elements, such as Cd, Cr, Hg, Mo and Pb, present in slags from the Pidgeon process [29,48] are also burdens to future generations, negatively affecting natural capital [38]. The same holds for long-lasting groundwater pollution linked to carnallite, magnesite and dolomite production [58,59,60,61]. Fresh water consumption for the primary production of magnesium may negatively affect fresh water resources for transfer to future generations in areas with severe water stress [16]. Current extraction of minerals for Mg production can lead to local or regional exhaustion of natural magnesium resource stocks [62]. However, worldwide stocks of natural Mg resources are very large, as the abundance of Mg in the Earth’s crust is estimated at about 2% and Mg is the third most plentiful dissolved element in sea water [16,63]. When Homo sapiens are assumed to have existed for about 106 years, it might be argued that, at the present consumption rate of Mg resources for the production of metal, worldwide natural Mg resources may be considered inexhaustible during this period. A similar argumentation may be used to characterize as sustainable the use of the (abundant) elements Fe and Si (in the Pidgeon process) and the abundant alloying elements Al and Ca. Such argumentation is, however, not justified for rare elements [63] that are currently applied in the production and processing of magnesium alloys and that are currently not functionally recycled at the end-of-life stage [16,64,65]. These include Zn and rare earths used in automotive AZ and WE Mg alloys and the rare elements B, Ce, Cu, Cr, La, Ni, Sn, V, Zn and Zr that can be used in coatings for Mg alloys used in cars [20,21,22,23,24]. Sc and Y used in novel Mg alloys proposed by Singh et al. [3] and Lee et al. [10], respectively, are also rare elements [63] that would currently not be functionally recycled at the end-of-life stage [16,64,65]. Current use of these rare elements in Mg alloy life cycles corresponds with a loss of natural capital [38]. It can be concluded that current life cycles of alloys based on primary magnesium do not allow for the conservation of natural capital for transfer to future generations.
Table 2 summarizes the interventions linked to magnesium alloy life cycles that contribute to the transgression of planetary boundaries and negatively affect natural capital for transfer to future generations.
All in all, when sustainability is defined as remaining within safe planetary boundaries for mankind or as conserving natural capital for transfer to future generations, the life cycles of current alloys based on primary Mg applied in cars are not sustainable, irrespective of whether they originate in the Pidgeon process or in electrolytic production.

4. Are Current Magnesium Alloys in Cars Environment Friendly, Ecofriendly or Green?

Kumar et al. [11], Hu [12] and Sabzehmeidani and Kazemzad [13] stressed the recyclability of magnesium alloys as an argument for the characterization of magnesium alloys as environmentally friendly or green. The terms environmentally friendly and green are commonly used regarding the real world. For this reason, not recyclability but actual recycling should be considered. It has been reported that actual recycling may matter substantially for the life cycle emissions of greenhouse gasses linked to the application of magnesium alloys in cars [66]. Metals, all being recyclable, vary widely in actual recycling [67]. Graedel and Miatto [67] estimated the overall worldwide end-of-use recycling rate of magnesium metal (including its alloys) at 39%, whereas the worldwide overall end-of-use recycling rate of steel is estimated by these authors at 78%. Guo et al. [68] reported that in China, which processes more than 50% of domestic Mg production [28], the recycling of magnesium alloys is at a relatively low level. Casting of Mg alloys in foundries is used in nearly 98% of the production processes generating structural components for cars [11]. In such casting, the amount of scrap generated is frequently 60–70% of the final casting mass [69]). Of this scrap, reportedly about one-third is recycled [69]. Recycling of end-of-use Mg alloys derived from end-of-life cars is currently mostly open-loop recycling or downcycling, with the Mg alloys becoming inputs in Al alloy production or additives in iron and steel production [16,64,65]. In view of the available evidence about actual recycling, there is no reason to qualify Mg alloys as environmentally friendly or green, if compared with steel.
If compared with conventional mild steel car parts, Mg alloys can reduce the weight of car parts by 55–60% [16,26,33,70,71]. Ceteris paribus (all other things being equal), weight reduction by magnesium alloys allows for reduced energy consumption in the use stage of cars. Though lower weight ceteris paribus reduces the environmental burden in the use stage of cars, other stages in the magnesium alloy life cycle may be linked to heavier environmental burdens than those of primary mild steel that is replaced. In view thereof, there is a case to evaluate the environmental burden of lightweighting cars by applying comprehensive (environmental) life cycle assessment (also: [35,72]). Interventions that are to be considered in comprehensive life cycle assessments are shown in Box 2. A lower environmental burden of Mg alloys, if compared with primary conventional mild steel, established by comprehensive life cycle assessment would be a firm basis for characterizing magnesium alloys in cars as green, ecofriendly or environmentally friendly in the comparative sense.
The peer reviewed (cradle-to-grave) life cycle assessment (LCA) with the widest coverage of interventions presented in Box 2, that analyses the replacement of primary conventional mild steel by magnesium, has been authored by Raugei et al. [33]. They studied the following interventions: cumulative non-renewable energy demand, the emission of greenhouse gasses, the emission of the acidifying substances NOx and SOx and the emission of human toxics. The LCA authored by Raugei et al. [33] concerns compact cars with internal combustion engines (ICEVs) and a service life of 150,000 km. The replacement studied regards the body of the car and selected chassis parts. As to this replacement, a ceteris paribus assumption has been made. The assessment by Raugei et al. [33] assumed a 75% ‘standard’ Pidgeon process and 25% ‘new/green’ Pidgeon process with coal as the main primary energy source. As to the cover gas SF6, Raugei et al. [33] considered a range of life cycle inputs. Measurements suggest that the average emission of SF6 from Chinese primary Mg production plants using the Pidgeon process has not increased over the period of 2011–2021 [73], while primary Mg production by these plants has increased [16]. This suggests that in China, the SF6 emission per average kg Mg originating from the Pidgeon process has been reduced. In view thereof, the lower end of the range for SF6 use given by Raugei et al. [33], 1.65 kg SF6/1000 kg Mg, will be used here. Raugei et al. [33] (‘pessimistically’) assumed 75% Mg recycling. This is at variance with the relatively low level of Mg recycling in China [68]) and the value of 39% reported for worldwide Mg metal recycling [67].
Raugei et al. [33] did not directly address the most important magnesium alloys applied in cars (AM 50, AM60, AZ 31 and AZ 91). These have Al as main alloying element, with the Al content varying between about 3 and 9%. Raugei et al. [33] included Al in their life cycle assessment, assuming 75% recycling, as in the case of Mg. As to cumulative non-renewable energy demand, the emission of greenhouse gasses and the emission of the acidifying substances NOx and SOx, Al did somewhat better than Mg, but the human toxicity potential was worse. In view of the limited Al content, the effect of the Al in the magnesium alloys AM 50, AM60, AZ 31 and AZ 91 on the quantitative results obtained by Raugei et al. [33] regarding magnesium would be small. In Mg alloys applied in cars, manganese may be present in concentrations up to 0.6% and zinc at concentrations up to 1.4%. Nuss and Eckelman [74] have collected cradle-to-gate environmental impact data about these metals. Cradle-to-gate regards the life cycle stages up to the metal leaving the production plant. In view of the data presented by Nuss and Eckelman [74], no substantial effect on the quantitative results obtained by Raugei et al. [33] would be expected. All in all, the quantitative outcomes obtained by Raugei et al. [33] for Mg would be close to the outcomes that would have been obtained when Mg alloys applied in cars would have been directly addressed.
The LCA provided by Raugei et al. [33] covers a part of the interventions presented in Box 2 that are relevant to magnesium alloy life cycles. Though magnesium alloy life cycles are linked to the consumption of ores [16]), Raugei et al. [33] did not study this intervention. Life cycle environmental burdens linked to ecotoxicity, photo-oxidant smog, the emission of fine particulates, groundwater pollution, land use and land use change affecting ecosystems, and fresh water consumption were also not quantified by Raugei et al. [33]. Available data regarding such interventions, and the impacts thereof, linked to magnesium alloy life cycles show that not including these neglects part of what occurs in the real world. Mining of the mineral dolomite is linked to land use and land use change that may negatively impact ecosystems [43,45]. Mining and associated water use for ore processing and dust suppression may negatively impact fresh water availability in the vicinity of mines [16,75]. The use of explosives in dolomite mining can give rise to emissions of fine particles in the air [76] and to nitrate concentrations in groundwater above the drinking water limit [61]. The Pidgeon process is associated with the emission to air of compounds leading to photo-oxidant smog [77], of fine particles [78], of ecotoxic HF [78,79], and of ecotoxic persistent organochlorines and polycyclic aromatics [55,57]. The Pidgeon process furthermore generates slag (at a rate of about 5–9 times the product output) that is often dumped [29,48]. This slag is highly alkaline, which can cause ecotoxicity, and contains ecotoxic substances such as Be, Cd, Cr, F, Hg and Pb [29,48]. Leachate from magnesium slag was found to exceed the Chinese groundwater quality limits for Cr, Cu, Hg and F [80]. Also, the Pidgeon process consumes substantial amounts of fresh water for cooling [16]. The partial coverage of environmental burdens by Raugei et al. [33] means that a firm conclusion as to the relative life cycle environmental burdens of Mg alloys, originating from the Pidgeon process, and conventional mild steel cannot be drawn from this study.
For substituting primary conventional mild steel by magnesium originating from the Pidgeon process, Raugei et al. [33] found no reduction as to life cycle cumulative non-renewable energy consumption. They furthermore found that, while assuming an input of 1.65 kg SF6/1000 kg Mg, the global warming potential (GWP100) increased by the substitution. Furthermore, an increase was found for the acidification potential and a reduction was found for the human toxicity potential. In estimating the life cycle GWP100, greenhouse gas emissions linked to changes in ecosystem carbon stocks, which occur in mining [51], and CO2 emissions originating from dolomite (about 3.7 kg CO2/kg Mg) were not included by Raugei et al. [33], neither were their counterparts in the primary steel life cycle. Carbonates such as limestone and dolomite are inputs in primary steelmaking and give rise to CO2 emissions. On the basis of data provided by Mapelli et al. [81], it may be estimated that this emission would amount to about 1.85 kg CO2/2.5 kg primary steel. In line with the assumption made by Raugei et al. [33], 2.5 kg steel may be replaced by 1 kg Mg alloy. Taking account of a lower recycling rate for magnesium alloys (instead of 75%) would make the life cycle non-renewable cumulative energy consumption and the global warming and acidification potentials of Mg substantially larger [66,72]. The data discussed so far provide no support for claims that current Mg alloys originating from the Pidgeon process are green, ecofriendly or environmentally friendly when they replace primary conventional mild steel in cars. A silicothermic plant producing magnesium metal in Brazil (RIMA) uses eucalyptus biomass as an energetic input [26]. No comprehensive LCA of this plant, taking proper account of land use change linked to eucalyptus plantations and the sequestration of carbonaceous greenhouse gasses after their emission by the plant, has been found. So, there is no firm support for characterizing this plant as green, ecofriendly or environmentally friendly.
As pointed out in the Introduction, the alternative to silicothermic reduction for the production of Mg is electrolysis of anhydrous MgCl2. For the latter process, two cradle-to-gate LCA studies focusing on the emission of greenhouse gasses linked to energy inputs are available. No LCA studies have been found for the electrolysis of anhydrous MgCl2 covering the other interventions presented in Box 2. This does not provide a firm basis for characterizing electrolytically produced magnesium as green, ecofriendly or environment(ally) friendly, if compared with primary conventional mild steel.
The first cradle-to-gate study regards an electrolytic magnesium production facility operated by Dead Sea Magnesium (DSMag) in Israel. In this case, MgCl2 is derived from carnallite. The facility is powered by natural gas using a co-generation plant. Ehrenberger [26] estimated energy-linked greenhouse gas emissions (GWP100) of the DSMag plant at 14 kg CO2 equivalent/kg Mg, taking into account co-product credits. The data for this estimate were collected for the study of Ehrenberger et al. in 2013 [36]. Since then, there have been changes in the technology used, and on the basis thereof, the cradle-to-gate carbonaceous greenhouse gas emissions have been estimated at about 7 kg CO2 equivalent/kg Mg, taking into account co-product credits [82]. Both values are much lower than the relatively recent estimates of the cradle-to-gate emissions of carbonaceous greenhouse gasses linked to the energy inputs involving the Pidgeon process: 21.6–27 kg CO2 equivalent/kg Mg [26,32,83]. For comparison, it may also be noted that, based on data regarding a Chinese primary steel plant with a blast furnace and basic oxygen furnace, the energy input-related cradle-to-gate greenhouse gas emission for 2.5 kg primary steel (that could be replaced by 1 kg magnesium alloy) may be estimated at about 6.5 kg CO2 equivalent [84]. Filkin and Co [82] did not include data about the use of fluorinated cover gas by the DSMag plant. V. Kotlovsky (chief technologist of DSMag, personal communication) estimates the emission of the cover gas HFC 134a to be about 0.4 kg CO2 equivalent/kg Mg. Using the estimate of Filkin and Co [82], this would lead to an overall GWP100 of about 7.4 kg CO2 equivalent/kg Mg.
There are environmental impacts of the Dead Sea Magnesium plant beyond the emission of greenhouse gasses. For instance, the supply of carnallite, which is recovered from water from the Dead Sea in large evaporation ponds, is linked to substantial environmental burdens. The coastal aquifer of the Dead Sea is subject to the intrusion of brines from these ponds, and a large project for long-range transport of massive amounts of halite (NaCl) accumulated in these ponds is planned [60]. The planned halite disposal operation will have a longstanding environmental footprint [60]. End-brine of carnallite processing is discharged in the Arava river [60]. Furthermore, the electrolysis of MgCl2 generates substantial amounts of organochlorines [53].
Ehrenberger [26] reported a global warming potential linked to cradle-to-gate energy consumption for an electrolysis-based Mg metal production plant in Qinghai (China), using a brine with MgCl2 generated as a by-product of fertilizer (potash) production. The plant of Qinghai Salt Lake Magnesium (QSLM) was at one time operational, be it not at full capacity (Ehrenberger [26]). The plant has since been subject to bankruptcy and remediation; a restart of production at low levels is planned for the second half of 2024 [85]. When operational, the QSLM plant reportedly used mainly hydropower and furthermore solar power, wind power and cogeneration [26]. Taking into account co-product credits, the cradle-to-gate GWP100 linked to the energy consumption of the Mg metal production process involving the Qinghai plant operating at full capacity was estimated at about 5.3 kg CO2 equivalent/kg Mg [26]. It should be realized that the greenhouse gas emissions linked to hydropower are characterized by a large degree of uncertainty due to the large variability in reported greenhouse gas emissions [86], which has not been included in the estimate by Ehrenberger [26]. Ehrenberger [26] apparently valued the input of brine at 0 kg CO2 equivalent/kg Mg. However, this input may also be allocated a value > 0 kg CO2 equivalent/kg Mg, linked to its energetic share in the common pathway with potash production, as the brine is a co-output of potash production used in commercial production. Still, it would seem that, notwithstanding uncertainties, production of 1 kg Mg by an operational Qinghai plant would have much lower cradle-to-gate carbonaceous greenhouse gas emissions linked to energy consumption than the Pidgeon process (estimated at 21.6–27 kg CO2 equivalent/kg Mg), a conclusion in line with [14]. As no data about the use of fluorinated cover gas in the Qinghai plant could be obtained, the cradle-to-gate GWP100 of the emissions linked to the QLSM plant cannot be established.
As pointed out in the Introduction, Liu et al. [14] characterized magnesium from the Qinghai electrolysis plant (China) as ‘generally a green product’. Additionally, the 2019 award of excellence in the environmental responsibility category of the International Magnesium Association [87] went to the Qinghai electrolysis plant for producing green magnesium. The term ‘environmental’ commonly has a wider scope than the emission of greenhouse gasses. Also, the characterization ‘generally a green product’ [14] may well be understood as having a wider scope. There are indeed aspects of the Qinghai electrolytic magnesium production plant, beyond the emission of greenhouse gasses, that are relevant to the environment. Electrolysis of MgCl2 using graphite anodes has been found to generate substantial amounts of persistent organochlorines released to cell off-gasses, mainly highly chlorinated aliphatic and aromatic compounds (up to about 0.16 g/kg Mg), including small amounts of chlorinated dibenzodioxins and chlorinated dibenzofurans [53]. Fresh water scarcity linked to the exploitation of minerals present in Qinghai salt lakes has been identified as a problem by Kong et al. [62]. The brine from which MgCl2 is derived contains large amounts of chloride and sulfate salts, boron and low concentrations of lithium [88,89]. I have been unable to obtain information as to the fate of brine-derived residues after MgCl2 production. Available data provide no firm basis for the comparative characterization of Mg alloys made from magnesium produced in an operational Qinghai salt lake magnesium plant as green, if compared with magnesium obtained by the Pidgeon process or conventional mild steel.
The ceteris paribus condition used in the life cycle assessment does not apply in the real world. Kim et al. [90] have argued that lightweighting by Mg allows for resizing the powertrain of internal combustion engine vehicles (ICEVs) as the weight of cars is reduced. They calculated that such resizing leads to reduced life cycle greenhouse gas emissions linked to energy consumption, if compared with the cars in which conventional mild steel is not replaced. Though the actual calculation of the reduction by Kim et al. [90] has received criticism [91], it would seem correct that resizing of the powertrain would, ceteris paribus, lead to a reduction in life cycle greenhouse gas emissions. However, there are other developments in the real world that have the opposite effect on sizing the powertrain of ICEVs. Increased use of lightweight materials in cars is not necessarily linked to a reduced overall weight of cars which would favor lower fuel use/km. Kawajiri et al. [92] have shown, for Japanese and US cars with internal combustion engines, that between 1996 and 2014, notwithstanding an increased use of lightweighting materials, the overall weight of cars increased. In a study regarding the Volkswagen Golf, Danilecki et al. [93] concluded that car weight increased about 46% over a 30-year period, whereas the application of magnesium alloys showed an increasing trend [1,94]. Furthermore, the International Energy Agency [95] noted the existence of a long-term trend of an increase in power for cars. These findings are at variance with resizing of the powertrain as discussed by Kim et al. [90]. There is, furthermore, the real-world matter of ICEVs being replaced by battery electric vehicles (BEVs) with another power train. Using data regarding the USA, Kim and Wallington [96] found that lightweighting electric cars was less effective in reducing the greenhouse gas emission/km in the use stage of the vehicle than in the case of cars with internal combustion engines. BEVs are also expected to substantially further reduce the emission of greenhouse gasses linked to the use stage of cars in the coming decades, due to a ‘decarbonizing’ shift in energy sources (International Energy Agency [97]. This in turn will reduce the climate benefits of replacing mild steel by magnesium alloys in the use stage.
On the other hand, a policy focused on rigorous material efficiency, including lightweighting, may lead to lower fuel use of cars/km, as shown by Pauliuk et al. [98]. To the extent that lightweighting indeed leads to reduced energy consumption of cars in their use stage, this reduction can give rise to rebound effects that have an upward effect on energy consumption [99,100]. Rebound effects were not considered in the study of Pauliuk et al. [98]. There is the possibility of a direct rebound effect (for instance due to larger distances driven) and of economy-wide rebound effects. Most studies concerning cars have regarded the direct rebound effect of improved energy efficiency, quantifying the erosion of expected energy savings. Dimitropoulos et al. [99] reviewed these studies and concluded that, on average, the short-run direct rebound effect was in the order of 10–12% and that the long-run direct rebound effect was in the order of 26–29%. The economy-wide rebound effects regarding improved energy efficiency of cars are less studied and more uncertain. Widely different results have been reported for economy-wide rebound effects for improved energy efficiency of cars in their use stage [101,102]. A review of available studies regarding all types of transport by Sorrell et al. [103] concluded that overall rebound effects may erode 15–50% of the expected energy savings from improved energy efficiency in transport.
Box 3 summarizes the consequences of removing the ceteris paribus assumption for the environmental benefits of lightweighting in the use stage of cars.
Box 3. Reasons for reduced environmental benefits of lightweighting in use stage of cars when the ceteris paribus assumption is removed.
-
Notwithstanding the use of lightweighting materials, the overall weight of cars tends to increase.
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If compared with ICEV, environmental benefits in the use stage tend to be reduced in battery electric cars.
-
When there are energy efficiency gains in the use stage, there are rebound effects.
All in all, in the real world, the environmental benefit of lightweighting in the use stage of the car is reduced if compared with the benefit found in life cycle assessments using the ceteris paribus assumption, e.g., [33]. The data discussed here provide no support for the claim that magnesium alloys applied in cars are ecofriendly, environmentally friendly or green when these terms mean that the life cycle environmental burden of Mg alloys involving the Pidgeon process is less than the life cycle environmental burden of the primary conventional mild steel that is replaced. When magnesium originates in the electrolysis of anhydrous MgCl2, available environmental data are insufficient for making firm claims that Mg alloys applied in cars are ecofriendly, environmentally friendly or green, if compared with primary conventional mild steel. It can also be concluded from studies discussed in this section that the current life cycles of alloys based on primary magnesium are not green, ecofriendly or environmentally friendly in the sense that they do not burden the environment. Section 5 will deal with the question of whether current Mg alloys are a minimal burden to the environment.

5. Reducing the Life Cycle Environmental Burden of Magnesium Alloys

As will appear in what follows, there is scope for technologies that can or may reduce contributions to the life cycle environmental burden of Mg alloys. More options aimed at reducing the life cycle environmental burden of Mg alloys are likely to emerge. In view thereof it can be stated that current magnesium alloys are not ecofriendly, environmentally friendly or green in the sense that their environmental burden is minimal. A number of options aimed at reducing the life cycle environmental burden of Mg alloys have been proposed. In many cases, these options are currently in the research and development stage, which precludes confident quantitative estimates of their actual environmental benefits when commercially applied. Comprehensive LCAs are available for photovoltaic solar panels and wind power [104,105], but for many of the options discussed below, environmental assessments using a comprehensive LCA methodology are unavailable. Actual minimization of the life cycle environmental burden by the implementation of options with environmental benefits shown by comprehensive LCAs may allow for the use of the characterizations of green, ecofriendly or environmentally friendly in the sense of a minimal burden to the environment.
In view of the environmental burden of mining magnesium resources, it has been proposed to use secondary resources such as (oxidized) refractory MgO bricks, aged ferro-nickel slag, (aged) abandoned magnesite and mine tailings containing Mg for the production of magnesium metal [106,107,108]. As to the primary production of Mg metal, several proposals have been made for reducing the environmental burden of thermal reduction processes. Fu et al. [109] discussed options to improve heat transfer in the retort for silicothermic reduction in the Pidgeon process. which might substantially increase energy efficiency. Xu et al. [27] have proposed relative vacuum continuous primary silicothermic magnesium production. This includes replacing the switching between the atmospheric pressure and vacuum that characterizes the Pidgeon process with a silicothermic reduction under relative vacuum and changes in the calcination method. Xu et al. [27] estimate that in this way coal consumption in the Mg production plant may be cut by about a third. Wada et al. [110] studied a laboratory-scale variant of the Pidgeon process based on heating with microwaves and suggested that in this way energy consumption could be reduced by nearly 70%. When this approach is scalable and when microwaves are generated using solar and/or wind power, the emission of greenhouse gasses could indeed be much reduced. Guo et al. [9] have suggested, on the basis of laboratory experiments, an aluminothermic process in flowing argon that might allow for an unspecified improvement in the energy efficiency. It has furthermore been proposed to replace FeSi-based reduction by vacuum carbothermal reduction, using coking coal as a reductant, which might allow for substantial reductions as to the inputs of minerals and energy [111]. This proposal has been characterized as environmentally friendly [111]. Actual application of vacuum carbothermal reduction awaits a commercial way to suppress the reverse reaction, a problem for which, as yet, no acceptable solution has been found [9]. Another proposal intended to reduce the environmental burden of thermal Mg metal production is the use of concentrated solar heat. Halman et al. [112] suggested the use of concentrated solar heating in the Pidgeon process. Chuayboon and Abanades [113] and Najafabadi et al. [114] studied the use of concentrated solar heat for the carbothermal reduction of, respectively, MgO and dolomite. The practicalities of fitting the intermittent and variable input of solar irradiation and the demand for heat in pyrometallurgical process cycles [29] were not addressed in these studies.
Hazardous outputs of the Pidgeon process to air such as small particles, fluorides, persistent organochlorines and polycyclic aromatics can be reduced by eliminating uncontrolled emissions to air and using advanced technology in emission control [115,116]. It has furthermore been proposed to process slags originating from the Pidgeon process by applying them in fertilizers, ceramics, construction and building materials, such as cement [29,48,117], the latter being characterized as green and environmentally friendly [117]. The application of magnesium slag in fertilizers is linked to a heavy environmental burden [80]. Applications in ceramics, construction and building materials tend to be dispersive and the indefinite sequestration of (eco)toxic contaminants present in these applications of slags cannot be guaranteed, which rather would suggest (bio)hydrometallurgical treatment before application [118,119].
The use of electrolysis of MgCl2 to produce Mg provides an option for reducing major contributions to the environmental burden by the use of suitably located photovoltaic panels and/or wind power, replacing fossil fuel-based power generation. Another option regards a reduction in the energy input for generating anhydrous MgCl2 by the solvent-free process proposed by Motkuri et al. [120]. Implementing these options can reduce greenhouse gas emissions and acidification potential linked to energy inputs in the production of magnesium metal [104,105]. A proposed MgCl2 electrolysis plant of Alliance Magnesium in Canada, which is intended to become operational by late 2025, will use MgCl2 obtained by HCl leaching of magnesium silicate originating from mining tailings containing serpentine [107]. The electrolysis plant is to be powered with at least 90% hydroelectricity [107]. The process is estimated to have a cradle-to-gate greenhouse gas emission for Mg metal production linked to the input of energy <5 kg CO2 equivalent/kg Mg [26,107], not taking into account uncertainty regarding greenhouse gas emissions linked to hydropower [86]. The Alliance Magnesium process also aims to convert non-Mg parts of the mining tailings into useful products such as magnetite, SiO2, NiCl2 and MnCl2 [107]. If this would indeed be the case, it is likely that there would be substantial co-product energy credits. On the other hand, the process might generate a substantial co-output of chlorinated organics [53].
Telgerafchi et al. [121] have investigated a process based on molten salt (MgF2-CaF2) electrolysis of MgO with a liquid Sn cathode, a carbon anode and with gravitation-driven multiple effect thermal system distillation to separate Mg from (reusable) Sn. Estimates made by Telgerafchi et al. [121] suggest that per kg Mg produced, depending on the source of MgO, the cradle-to-gate greenhouse gas emission linked to energy consumption and anode emissions would be <4 kg CO2 equivalent and <1 kg CO2 equivalent for MgO precipitated from brine. It should be noted, though, that this process may give rise to emission of the potent greenhouse gasses CF4 and C2F6, as in the case of the electrolytic production of aluminum and rare earths [122]. The Mg product output of the process is expected to contain 0.5% Sn (Telgerafchi et al. [121]). The functionality thereof in automotive applications and functional recycling of such Sn present in end-of-life cars were not addressed by Telgerafchi et al. [121]. Jeoung et al. [108] have proposed a process based on molten (MgF2-LiF-KCl) salt electrolysis of MgO and vacuum distillation and have tested the process at the laboratory scale. Jeoung et al. [108] characterized this process as green when secondary magnesium resources are used. No definition of green was presented and Jeoung et al. [108] did not provide data as to the cradle-to-gate energy input and the life cycle environmental burdens of the halogen salts used in the process.
The use of SF6 as a cover gas for molten Mg has, as noted before, a strong impact on the global warming potential of Mg alloy life cycles [33]. SF6 may be substituted by other fluor compounds. HFC-134a has found commercial applications as a cover gas in Mg metal production and processing of molten Mg [16,36,123]. HFC 134a has a GWP100 which is (molecule for molecule) roughly a factor 20 lower than the GWP100 of SF6 [34]. Jiang et al. [34] have proposed the use of hexafluoropropene as a cover gas for molten Mg. Hexa- fluoropropene has a GWP100 that is (molecule for molecule) roughly a factor 10.000 lower than the GWP of SF6 [34]. In a German smelter producing magnesium from Mg scrap, SO2 is used as a cover gas [26]. Also, SO2 is suitable as a cover gas for casting AM60 and AZ91 Mg alloys [124]. The DSMag electrolysis plant in Israel has as long-term goal to replace fluorinated cover gas by SO2 (V. Kotlovsky, personal communication). It has furthermore been shown that adding CaO to Mg alloys that can be used in cars may enable processing by melting, with or without, a lower amount of fluorinated cover gas [37,125].
Closed-loop recycling of Mg and its alloys can be increased. This can, ceteris paribus, reduce the consumption of natural Mg resources, inputs of fresh water and energy, and the life cycle emissions of greenhouse gasses and acidifying substances [16]. Dudek et al. [69] found that AZ 91 alloy casting scrap, thermally treated for the removal of organic contaminants, could partially replace virgin alloy constituents in producing a AZ 91 alloy of good quality for casting. Mg alloy machining chips may be recycled in a closed loop, using solid-state technology [123,126]. Car design for disassembly and recycling [127,128] and actual disassembly of end-of-life cars aimed at generating an output of magnesium alloys separated by alloy type would provide a major contribution to increased closed-loop Mg alloy recycling.
A problem associated with magnesium scrap that may currently be derived from end-of-life cars by separating the light metals output from shredders is the presence of contaminants such as Co, Cu, Fe and Ni that can be dysfunctional as they can increase corrosion and reduce the hardness of Mg alloys obtained by pyrometallurgical recycling [65]. These elements can originate in Mg alloy coatings, in pigments, in welding of magnesium alloys to steel and in incomplete separation of light metals in shredders processing end-of-life cars [21,129,130]. Removal of Fe at a rate of 96.2% by a mixture of chlorides at 670 °C has been demonstrated [131]. Pyrometallurgical processes using additives that can strongly reduce the concentration of Cu and Ni have been reported, be it without the recovery of these rare elements [65]. Varieties of vacuum distillation of magnesium alloy scrap to purify magnesium [132,133] may be an alternative for pyrometallurgical processing. A problem with purification by vacuum distillation is the similarity of Mg and Zn volatility [132]. Directional condensation has been suggested as a solution for this problem [132]. It has also been suggested that alloying elements present in distillation residue may be recovered by further vacuum distillation [133].
Implementing options for minimizing the environmental burden of magnesium alloy life cycles, confirmed by comprehensive LCAs, may make magnesium alloys more attractive when comparing the life cycle environmental burdens of Mg alloys and primary conventional mild steel. However, this does not necessarily mean that magnesium alloys applied in cars will become green, ecofriendly and environmentally friendly if compared with primary steel applied in cars, as there are also options to make primary steel more attractive. To the extent that lightweighting may reduce the life cycle environmental burden of cars, there is the option, which is also currently applied, to achieve lightweighting by low-density, high-strength steel [72,134,135]. And there is scope for reducing the environmental burden of primary conventional mild steel used in cars. One option is the replacement of coal as reductant for iron ore by H2 [136,137,138]. When H2 is generated by wind- and/or solar-powered electrolysis of water, this replacement may allow for a substantial reduction in the environmental burden of steel in installations currently used in primary production [136,138]. A stronger reduction in the environmental burden would be possible when such reduction by H2 is combined with melting of reduced ore in an electro-smelter powered by solar and/or wind energy [138,139,140]. There is also scope for reducing the presence of rare elements in residues of iron and steel production and for steel recycling leading to increased functional use of rare elements [141]. Hazardous emissions into the air from iron and steel production can be mitigated by eliminating uncontrolled emissions and using advanced control technology [115,116].

6. Conclusions

When sustainability is defined as remaining within safe planetary boundaries for mankind, current life cycles of alloys based on primary Mg applied in cars are not sustainable. They contribute to transgression of the planetary boundaries for emitted N compounds, radiative forcing at the top-of-atmosphere and biosphere integrity. When sustainability is defined as conserving natural capital for transfer to future generations, the application in cars of current alloys based on primary Mg is also not sustainable because current life cycles of these alloys reduce natural capital available for future generations. This is linked to substantial reductions in stocks of not-abundant natural resources generated in slow geological processes, long-lasting negative impacts on ecosystem services and the contribution of greenhouse gasses and persistent hazardous compounds to environmental burdens.
In view of available data, the current life cycles of alloys based on primary Mg are not environmentally friendly, ecofriendly or green in the sense that they pose no, or a minimal, environmental burden. A characterization of environmentally friendly or green for magnesium alloys may also mean that the life cycle environmental burden of alloys based on primary Mg is less than the life cycle environmental burden of the primary conventional mild steel that is replaced by these magnesium alloys. In the latter case, available environmental data do not support the claim that Mg alloys made from magnesium originating from silicothermic reduction are currently environmentally friendly, ecofriendly or green. When magnesium originates in the electrolysis of anhydrous MgCl2, available environmental data are insufficient for making firm claims that Mg alloys applied in cars are ecofriendly, environmentally friendly or green, if compared with conventional mild steel. There are options for substantially reducing contributions to the environmental burden of magnesium alloy life cycles and more are likely to emerge. Actual minimization of this burden by the implementation of such options may allow for the use of the characterizations green, ecofriendly or environmentally friendly in the sense of a minimal burden to the environment.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sector.

Acknowledgments

The comments of three anonymous reviewers are gratefully acknowledged.

Conflicts of Interest

The author declares no conflicts of interest.

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Sustainability 16 07799 g001
Figure 1. Process flow for the Pidgeon process [26,29].
Figure 1. Process flow for the Pidgeon process [26,29].
Sustainability 16 07799 g001
Table 1. Environmental matters with proposed planetary boundaries leading to a safe operation space for humanity and current relations between these boundaries and planetary environmental burdens [39,40,41].
Table 1. Environmental matters with proposed planetary boundaries leading to a safe operation space for humanity and current relations between these boundaries and planetary environmental burdens [39,40,41].
Environmental Matter for Which a Planetary Boundary Has Been ProposedCurrent Relation between Proposed Planetary Environmental Burden and Boundary
Flows impacting phosphorus (P) and nitrogen (N) cycles.Flows for P and N compounds transgress the planetary boundary.
Biosphere integrity.Human appropriation and reduction in net primary production (of biomass) transgress the planetary boundary.
Climate change by greenhouse gasses.Radiative forcing at top-of-atmosphere exceeds the planetary boundary.
Fresh water consumption.The planetary boundary is currently above actual consumption.
Land system change: amount of forested land remaining.Current land system change transgresses the planetary boundary.
Novel entities: percentage of hazardous man-made chemicals, released without adequate safety testing.The percentage of hazardous man-made chemicals without adequate safety testing presumably transgresses the planetary boundary.
Ocean acidification.Current ocean acidification is below the planetary boundary.
Ozone layer depletion.Current ozone layer depletion is less than the planetary boundary.
Table 2. Interventions linked to magnesium alloy life cycles that contribute to the transgression of planetary boundaries and negatively affect natural capital for transfer to future generations.
Table 2. Interventions linked to magnesium alloy life cycles that contribute to the transgression of planetary boundaries and negatively affect natural capital for transfer to future generations.
Interventions Linked to Magnesium Alloy Life Cycles
That Contribute to the Transgression of Planetary Boundaries		Interventions Linked to Magnesium Alloy Life Cycles
That Negatively Affect Natural Capital for Transfer
to Future Generations
-
Emissions of NOx
-
Emissions of greenhouse gasses
-
Land use change due to mining and dumping of slags
-
The consumption of fossil fuels and rare elements
-
The emission of greenhouse gasses
-
Land use change due to mining and dumping of slags
-
Emission of persistent hazardous substances
-
Fresh water consumption in water-stressed areas
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Reijnders, L. Are Magnesium Alloys Applied in Cars Sustainable and Environmentally Friendly? A Critical Review. Sustainability 2024, 16, 7799. https://doi.org/10.3390/su16177799

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Reijnders L. Are Magnesium Alloys Applied in Cars Sustainable and Environmentally Friendly? A Critical Review. Sustainability. 2024; 16(17):7799. https://doi.org/10.3390/su16177799

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Reijnders, Lucas. 2024. "Are Magnesium Alloys Applied in Cars Sustainable and Environmentally Friendly? A Critical Review" Sustainability 16, no. 17: 7799. https://doi.org/10.3390/su16177799

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Reijnders, L. (2024). Are Magnesium Alloys Applied in Cars Sustainable and Environmentally Friendly? A Critical Review. Sustainability, 16(17), 7799. https://doi.org/10.3390/su16177799

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