Corrosion of magnesium alloys: the role of alloying

Aluminium (Al)

Al is the most common addition to Mg, by virtue that it is (relatively) cheap, light, soluble, and considerably improves strength of Mg (i.e. from ∼70 to ∼250 MPa). ⁷⁸ Additions of Al below the solubility limit tend to reduce the anodic kinetics of Mg. The addition of Al to Mg ennobles the corrosion potential, with Mg–Al alloys tending to be ennobled by ∼100 mV_SCE compared to pure Mg, in Cl⁻ environments. ^{65,66,79–81} The Mg–Al alloys nominally have the lowest corrosion rates of the commercial Mg-alloys, ¹³ particularly AZ31 (Fig. 6), which gives an optimum balance between physical and corrosion properties for Mg-alloys to date. Although Al is soluble to ∼12 wt-% in Mg, this is dependent on temperature, and the room temperature solubility is much lower, with alloys richer in Al than AZ31 showing the presence of β-phase (Mg₁₇Al₁₂). The extent of β-phase will depend on the time–temperature history of the alloy, with the relative proportion of β-phase for the same given composition also varying rather widely depending on alloy cooling rate or subsequent heat treatment (i.e. whether sand-cast, die cast, or high-pressure die cast). ^82–92 Above ∼3 wt-%, Al additions tend to enhance the cathodic reaction, which although further ⁹³ ennobles E _corr, is associated with an increase in corrosion rate. In open circuit conditions, β-phase serves as a local cathode. As such, cathodic kinetics of the following alloys increase in the order of AZ31<AZ61<AZ91≤AM60. Al has been reported to increase the susceptibility to stress corrosion cracking in cases where β-phase exists in appreciable fractions. ⁷⁸ It is also noted that the AZ alloy system undergoes solid-state phase changes at moderate temperatures, and hence its utility in more demanding modern applications is leading to the proliferation of Al-free Mg alloys.

Silver (Ag)

The effect of Ag additions on the corrosion of Mg was studied by Hanawalt more than 70 years ago, who suggested a tolerance limit for Ag in Mg to be ∼0·5 wt-%. ¹⁵ Above this concentration the mass loss rate increased monotonically from ∼1 mg cm⁻² day⁻¹ for 1 wt-% Ag, to ∼12 mg cm⁻² day⁻¹ for 5 wt-% Ag. In contrast, the effect of Ag on accelerating the corrosion of Mg is therefore as dramatic as that as Ca on Mg. ¹⁵ In more complex alloys, however, lower level quaternary additions of Ag (to <∼2 wt-%) in the Mg–RE–Zr family of alloys are reported, because Ag improves the age hardening response and thus provides substantial increment in strength. ¹³ However, it is prudent to note at this point that although Ag has been added to the high strength Mg alloy family (which can be considered ultra high strength when considering specific strength), this has not been an alloy design with corrosion considerations in mind. None the less, addition of elements such as Ag that have the ability to modify nucleation/precipitation is an avenue that can be exploited. For example, trace additions of Ag (∼0·1 wt-%) to AZ91 were recently shown to stimulate an interaction between Ag and the Mg₁₇Al₁₂ phase, providing an increment in hardness without any significant loss of corrosion properties, i.e. a slight increase in cathodic kinetics is counterbalanced by decrease in anodic kinetics. ³³ High concentrations of Ag led to increased corrosion rate because of co-formation Mg₄Ag precipitates that can stimulate microgralvanic corrosion. ^34,94 In addition, micro-additions of Ag (<0·5 wt-%) to Mg–Zn–Ca systems facilitate grain refinement and in cast and wrought alloys. ⁹⁵

Arsenic (As)

The effect of As on Mg corrosion was explored on the basis of the potential to kinetically limit the cathodic reaction, with As known to be a cathodic ‘poison’ preventing hydrogen recombination ⁹⁶ and restricting the completion of the reaction in equation (2). The metallurgical influence of As has been recently reported by Birbilis et al., ⁹⁷ whereby relatively small additions of sparingly soluble As (∼0·37 wt-%) result in formation of Mg₃As₂ phase. In spite of this second phase presence, a decrease in Mg corrosion rate (five times lower than that of pure Mg) was determined by simultaneous hydrogen collection and 7 day mass loss tests, as well as lower cathodic kinetics determined by potentiodynamic polarisation. The reduction in corrosion rates was associated with the ability of As to slow down the cathodic reaction rate of pure Mg.

Bismuth (Bi)

The addition of Bi to Mg–Al alloys has the effect of refining Mg₁₇Al₁₂ and is accompanied by co-formation of needle-shaped Mg₃Bi₂ particles, even for Bi concentrations below the solubility limit. ⁹⁸ When added to AZ91, Südholz reported that the presence of Mg₃Bi₂ ennobles the corrosion potential of the alloy ³⁴ and is shown to bring about the acceleration of both the anodic and cathodic reactions rates. ⁹⁹ In spite of the negative influence on corrosion, Bi-containing particles seem to have a positive effect on enhancing tensile and creep properties via restricting grain boundary sliding. ¹⁰⁰

Calcium (Ca)

In the binary context, Ca additions in low levels (less than 0·35 wt-%) are essentially inert. ¹⁰¹ In general, however, Ca additions dramatically increase corrosion rates in Mg when added near to, or above the solubility limit (of ∼1·35 wt-%). Ca-containing Mg alloys result in exceptionally high corrosion rates. According to Hanawalt, the mass loss rate of Mg–Ca binary alloys increases from ∼1 mg cm⁻² day⁻¹ for 0·5 wt-% Ca to about 6 mg cm⁻² day⁻¹ for 5 wt-% Ca. This is almost six times higher than the corrosion rate of Mg–5 wt-% Al. ¹⁵ In addition to possessing the highest corrosion rates of any candidates for structural alloys ever reported, Ca-containing Mg alloys dissolve to yield a voluminous corrosion product that is insoluble. ³⁹ Owing to the biocompatibility of Ca, however (along with Zn), Mg–Zn–Ca alloys are being explored as potential biomaterials. ²² More recently, very low levels of functional Ca additions are being explored in Al-free Mg-alloys, ¹⁰² in attempt to exploit the fact that Ca is one of the few soluble elements in Mg. Provided that Ca concentrations are kept below the solubility limit to avoid Mg₂Ca formation, the corrosion rate will not sharply accelerate, while solid solutions of Mg–Ca can display appreciable ductility. ^103–105

Copper (Cu)

The addition of Cu to Mg and Mg-alloys is generally avoided because of the insolubility of Cu, and the resultant formation of Cu islands in the microstructure. Cu is highly detrimental for corrosion, ⁷⁸ rationalised on the basis that Cu has a high exchange current density, and hence an efficient cathode. The tolerance limit of 0·1 wt-% was established by Hanawalt, however, the limit is sharply reduced to 0·01 wt-% when Al and Mn are present in the alloy composition. ¹⁵ When added to AZ series alloys it forms an Mg–Al–Cu–Zn phase, which is also a relatively strong local cathode resulting in local corrosion. ¹⁰⁶ In general, Cu is avoided in the production of Mg-alloys, with measures in place to avoid Cu contamination and pick-up. Unfortunately, from a corrosion perspective, Cu is, however, a popular element in the formation of Mg metallic glasses, ¹⁰⁷ where the influence of Cu on the resultant corrosion mechanism has been described as incongruent dissolution and discussed further below.

Cerium (Ce)

When added as a binary addition (i.e. Mg–Ce) the formation of Mg₁₂Ce occurs in increasing volume fraction with Ce additions, which can monotonically accelerate corrosion by enhancing cathodic kinetics. ²⁴ In spite of the chemical reactivity of Ce, Mg₁₂Ce is more noble than Mg, and Mg₁₂Ce sustains the cathodic reaction at higher rates than Mg over the range of potentials typical of Mg-alloys. On the other hand, Ce additions to Al-containing Mg-alloys can refine microstructure by formation of Al₄Ce or Al₁₁Ce₃ intermetallics. ^108,109 These compounds are, however, more deleterious for corrosion than Mg₁₂Ce. Rare earth elements are typically not combined with Al-containing Mg alloys, with the exception of the commercial alloy AE44. In the more modern wrought series of Mg alloys, RE and Al are not combined, because of the ease of formation of Al_xRE_y compounds that reduce alloy ductility. Some reports suggest that Ce contributes to the surface film and stabilises Mg hydroxide. ¹¹⁰ Südholz, however, revealed via XPS analysis that RE elements were not seen to dope the surface oxide of Mg–Al alloys. ²⁸ The notion of oxide modification of Mg-alloys is an area of mixed reports, and one that is an open question. Irrespectively, elements with thermodynamic ability to replace Mg-oxides/hydroxide generally do not offer a wider window of passivity than Mg. ¹¹¹

Erbium (Er)

In favour of more cost effective RE elements, Er has not been well studied in Mg. In one study, however, Er, when added in 2–3 wt-%, was found to decrease corrosion rates in Mg–2 to 3Al (wt-%) alloys compared to commercial AM60 [nominal Mg–6Al–0·13Mn (wt-%)] when tested in borate buffer solution. ¹¹² The assumption was made that Er facilitates an apparently enhanced protective effectiveness to the surface oxide giving rise to what was described as a pseudo-passivation effect by incorporation of Er into the Mg(OH)₂. However, there is still a lack of evidence to support this assumption. Furthermore, the testing of Mg in a buffer is likely to impose an unrealistic scenario of surface film evolution, which is nominally influenced by natural surface alkalinisation.

Iron (Fe)

Fe is the most common impurity in Mg-alloys. Fe contamination can be present because of the quality the starting ‘pure’ Mg as well as because ofpickup from the casting process. Owing to its low solubility in Mg (∼0·001 wt-%) Fe largely remains in its pure (body centred cubic) form. To date, the literature reports a number of tolerance limits for Fe, which is nominally restricted to as low a level as economically feasible. ^13,113 The notion of an Fe tolerance limit dates back several decades, where the proposed limit of 150 ppm by weight was commonly accepted. ^114,115 Recent works have focused more specifically on the role of Fe, as its ability to be scavenged by sequestration to AlMnFe particle formation is not possible in emerging Al-free Mg alloys. ¹⁷ The role of thermal history on the Fe tolerance limit has also been recently discussed on the basis of calculated phase diagrams. ^116,117 The experimental validation and robustness of the related thermodynamic databases for Mg–Fe remain open questions without further empirical work, particularly because it was seen that calculated phase predictions can vary from the structures characterised by electron microscopy. ¹⁷

Gadolinium (Gd)

Of the elements that Mg may be alloyed with, Gd is unique on the basis that it has high solubility (>10 wt-%) in Mg, forming binary Mg–Gd solid solutions over a wide range of compositions. Such solid solutions can be used as a basis for further loading of the matrix with solute elements – in the case of wrought Mg alloys, or for carefully controlled precipitation processes in the family of Gd-containing precipitation-hardened Mg-alloys. ^118–121 In regards to this latter class of alloys, corrosion characterisation remains scarce. The addition of Gd in Mg–Al alloys results in the precipitation of Al₂Gd and Al–Mn–Gd phases that consumes Al and reduces the volume fraction of Mg₁₇Al₁₂ phase. ¹²² These phases may contribute to a minor decrease in the total rate of cathodic reaction. ¹²³ This tendency is commonly observed for Mg–Al alloys modified with REs. However, the effect of Gd is more complex because it leads to more heterogeneous microstructures and diminished Al content in the Mg matrix. Therefore long term corrosion tests show a greater corrosion for Mg-alloys containing Gd. ¹²⁴ Other studies have also indicated that the influence of Gd on corrosion is generally defined as detrimental. ¹²⁵

Mercury (Hg)

Alloying with Hg is nominally only reserved for the production of high voltage anode materials for sea water batteries. ¹²⁶ The Mg₃Hg phase can form in binary Mg–Hg alloy at concentrations of Hg at around 4–6 wt-%. ^126,127 Thermomechanical processing of Mg–Hg alloys can promote the redissolution of Mg₃Hg into the Mg matrix, which subsequently retards microgalvanic corrosion and thus lowers corrosion rates. ¹²⁸ Feng compared the electrochemical response of Mg–5Hg (wt-%) and Mg–6Hg (wt-%) in 3·5% NaCl solution, and concluded that Hg accelerates anodic reaction rates and leads to very negative corrosion potentials (E _corr, Mg–5Hg∼−2·3 V_SCE), in fact representing the most negative corrosion potential reported of any Mg alloys. This was concomitant with high corrosion rates with i _corr of Mg–5Hg being ∼27 mA cm⁻², compared to ∼20 μA cm⁻² for pure Mg. ¹²⁷ The role of Hg in structural Mg alloys has not been explored.

Holmium (Ho)

The limited research to date regarding Ho has suggested that a Ho addition of 0·24 and 0·44 wt-% can decrease the rate of corrosion in an Mg–Al alloy (AZ91D) by decreasing the volume fraction of Mg₁₇Al₁₂ owing to the formation of Ho-containing intermetallic phases with Al. In one study, inspection of the reported data indicate a minor decrease in the cathodic reaction rate, along with as much as 10 times lower mass loss rates compared to that of the base Mg–Al alloy. ¹²⁹ This, however, is likely to lead to decrease in alloy strength and may be manifest as the same outcome as lowering of the alloy Al content. It has been posited that Ho presumably contributes to more uniform and compact layer of corrosion product because of the higher content of Al present in the surface film; however, more evidence is needed to support such assertions.

Lanthanum (La)

As a binary addition, La forms the Mg₁₂La phase ²⁴ when added above its solubility limit. This phase is a more efficient cathode than pure Mg leading to an overall increase in corrosion rates (from 10 to 30 μA cm⁻² at concentrations varying from ∼0·5 to ∼3 wt-% La). ²⁸ However, as an elemental addition to Mg–Al alloys, La modifies the alloy microstructure (in a manner analogous to Ce), forming needle-like Al–La compounds and alters Mg₁₇Al₁₂ from discontinuous to continuous with a fine polygonal-shape. ^130,131 Liu et al. reported a drop in corrosion rate (i _corr) of AZ91 from ∼603 to ∼3 μA cm⁻² in 3·5% NaCl solution when 0·5 wt-% of La was added. This phenomenon was attributed to a refined microstructure and allegedly more protective corrosion product film. It should be noted, however, that the reported i _corr of the base AZ91 was much higher than any other reports for AZ91; while the evidence for any La in the surface film was also not provided. In a separate study, Südholz also tested the effect of 0·3 wt-% of La on corrosion rate of AZ91E. In that study, however, the reduction in the i _corr value was only by 0·3 μA cm⁻² (from 7 μA cm⁻² of AZ91E to 6·7 μA cm⁻² of AZ91E–0·3La). ²⁸ It is generally observed that any excess addition of La, above the solubility limit, reduces corrosion resistance because coarse Al–La precipitation in Mg–Al alloys. ^130,132 In the context of Mg–RE earth alloys, containing only Mg and REs, the presence of La in concert with Ce and Nd will lead to a different morphology and microchemistry of the second phase; transitioning from a lamellar eutectic to a divorced eutectic. ¹³³ The relative proportions of La, Ce and Nd are important in the ultimate corrosion rate, as depicted in a combined electrochemical and microstructural study. ¹³³ It was determined that Nd is less detrimental than La (or Ce) on the corrosion of Mg.

Lithium (Li)

Mg–Li alloys because of their ultra-lightweight and superplastic behaviour remain promising materials for many critical engineering applications, ¹³⁴ in spite of limited use to date. The microstructure of binary Mg–Li alloys depending on the Li content may consist of either a single hexagonal α (HCP) phase, at 0–5·7 wt-% Li; two phase – α and body-centred β (HCP and BCC) at 5·7–10·3 wt-% Li; or a single β (BCC) phase at Li concentrations greater than 10·3 wt-%. ^135,136 Therefore, high additions of Li (>10·3 wt-%) can produce a uniform β phase that is cubic. Li is the only researched element able to impart a change in the crystal structure, which has major implications for ductility. Early work by Frost in 1955 and a later published report from Battelle in 1964 indicate superior corrosion resistance of binary Mg–11Li alloy compared to the alloys with lower Li content, for example, the mass loss rate of the alloys measured in 3% NaCl solution (at 35°C) over 8 days was Mg–2Li (4·92)>Mg–4Li (4·44)>Mg–9·3Li (0·77)>Mg–11Li (0·57), mg cm⁻² day⁻¹. ^135,137 The greater corrosion resistance of Mg–11Li alloy was attributed to the BCC structure. More recent studies have also validated aspects of the early research by reporting that Li additions below the bcc transition increase the corrosion rate of Mg (from 19 μA cm⁻² for pure Mg to 45 μA cm⁻² for Mg–8Li ¹³⁸ ). Li was also seen to increase the corrosion rate of an Mg-alloy, from 12 mg cm⁻² h⁻¹ for AZ80 to about 200 mg cm⁻² h⁻¹ for AZ80–1·95Li. ¹³⁹ In addition, over long exposure periods the corrosion rates of Mg–Li alloys are more likely to remain constant or decrease much more slowly in contrast to AZ31 for instance, for which corrosion rate decreases with time. ^135,138 Although some information on the effect of Li on bulk Mg corrosion exists, there is still a gap in regards to electrochemical effect of Li on corrosion kinetics of Mg and its alloys.

Manganese (Mn)

The influence of Mn additions on Mg has been studied in detail as far back as almost a century. ^15,25 Its role is therefore well characterised and generally well understood. As a binary addition to Mg, Mn has been reported to show no significant effect on corrosion rate at concentrations up to ∼5 wt-%. ¹⁵ Mn is essentially always added to the Mg–Al and Mg–Al–Zn systems (i.e. the commodity Mg-alloys). Compounds such as Al₈Mn₅, Al₆Mn or Al₄Mn are formed, and the addition of Mn helps to reduce corrosion rate via the incorporation of lowly soluble metals in Al–Mn intermetallic phases. The classic example of this behaviour is the incorporation of Fe into Al–Mn–Fe. ¹⁴⁰

It has been reported that the amount of Mn to effectively mitigate the effect of Fe must be in the range that satisfies the maximum Fe/Mn ratio of 0·032, whereas greater ratios will sharply increase corrosion rates. ⁶ Although Mn is known to improve corrosion resistance, a low ratio between Al/Mn leads to higher cathodic potency. Therefore corrosion rate gradually increases with an increase in Mn content, ^25,141 independent of the Fe sequestration aspect of Mn. More recently, the work of Gandel et al. reports the corrosion behaviour of Mg–Mn binary alloys where the corrosion rate i _corr reduces from 40 to about 22 μA cm⁻² in 0·1M NaCl with Mn content ranging from 0·2 to 2 wt-%, respectively. ²⁰ The role of Mn additions in Al-free (wrought Mg alloy) systems is likely to be a productive avenue of future work. Of such works, Mn has been recently observed to ‘encapsulate’ Fe in Al-free magnesium alloys, ²⁰ which is a very positive result on the basis that Mn may remain an effective sequestration tool for Fe even in the absence of Al.

Neodymium (Nd)

In binary Mg–Nd alloys, the Mg₃Nd phase is formed. ²⁴ Mg₃Nd is a more efficient cathode than pure Mg and therefore serves as a local cathode leading to continually increased cathodic kinetics (and hence corrosion rates) with an increase in Nd content. ²⁸ It is noted that the increase in corrosion arising from Nd is less than that arising from Ce or La. This is an important point, as the Mg₃Nd phase also possesses a much higher atomic per cent of RE than either Mg₁₂Ce or Mg₁₂La, which are the phases that form in the case of the latter. From a corrosion standpoint, if RE elements are preferable in isolation, Nd is thus the most suitable; unfortunately, however, Nd is also the most expensive of the common REs and its concentration is therefore often limited. In AZ and AM alloys, Nd forms an Al–Nd phase as well as replacing the Al–Mn phase with an Al–Mn–Nd intermetallic compound. ¹⁴² Nd was noted to improve corrosion resistance of Mg–Al alloys by minimising the effect of galvanic coupling that exists in Nd-free AZ or AM alloys. ¹⁴³ In addition, Nd was reported to be beneficial in contributing into formation of protective surface oxide film consisting of Mg and RE, ¹³² however, this is one of many such claims that requires further investigation. The replacement of, or contribution to, more robust surface films is a common claim in many Mg-corrosion studies, however, the finding is often speculative and ultimately not proven. As with Ce and La, the influence of Nd on Mg–RE alloys was captured in a recent combined study. ¹³³

Lead (Pb)

In the binary context, Pb was historically reported to be quite inert to Mg and not detrimental for corrosion when added up to 5 wt-%. ¹⁵ Generally, Pb is added to Mg for production of anode materials, with the AP65 alloy having a nominal composition of 5 wt-% Pb serving as a common example. ¹⁴⁴ The addition of Pb to structural Mg alloys has not been explored, perhaps on the basis that Pb has a significantly higher density than Mg (logically negating the light-weighting opportunities).

However, generally speaking the effect of Pb on corrosion of Mg-alloys has received contradictory reports overall. On one hand, Pb was reported to allegedly modify the microstructure of AZ series alloys by means of refining and a more homogenous distribution of Mg₁₇Al₁₂. ^145–147 Another example of the beneficial effect of Pb was found in Mg–10Al–12Si alloys, where the additions of 0·2, 0·5 and 1 wt-% of Pb led to lower cathodic reaction rates and subsequently resulted in a reduction of mass loss rate from 1 g cm⁻² day⁻¹ for 0 wt-% of Pb to 0·4 g cm⁻² day⁻¹ with 1 wt.% Pb. ¹⁴⁵ In contrast to this, works have suggested that low levels of Pb (≤0·5 wt.%) to AZ91 stimulated more rapid corrosion rates, with the i _corr of Mg-alloys (from 9 μA cm⁻² of AZ91E to 20 μA cm⁻² of AZ91–0·5Pb). ³⁴ This finding is consistent with what was reported earlier by Hanawalt, i.e. addition of Pb to Mg–Al–Mn system results in higher corrosion rates (exceeding 0·2 mg cm⁻² day⁻¹ at ∼0·1 wt-% Pb and 0·001 wt-% Fe) because of complex interactions between Pb and inherent Fe impurity. ¹⁵ Pb additions are not common in structural Mg alloys, and its role in Mg-alloy corrosion is not well established, in spite of its use in sacrificial Mg anodes.

Praseodymium (Pr)

In Mg–Al alloys, the addition of Pr leads to the formation of Al₂Pr in preference to Mg₁₇Al₁₂. Increasing Pr loading results in grain size reduction concomitant with an increase in the volume fraction of secondary phases. ¹⁴⁶ Salt spray testing has indicated that corrosion was reduced with increased Pr concentration from 1 wt-% to maximum of 4 wt-% in a base Mg–4Al–0·4Mn alloy. However, there are few corroborating reports of the role of Pr in Mg alloy corrosion, and of the limited works, there are some unfounded claims that relate to ‘compactness’ of the corrosion product layer increasing with Pr content and an improvement in corrosion resistance attributed to the potential of Al–Pr phases to passivate over the wide range of pH. ¹⁴⁸ Such aspects would need further validation by independent methods.

Antimony (Sb)

Owing to the insolubility of antimony (Sb) in Mg, second phase formation is expected, and the second phase will appear in a large volume fraction. It was seen that the rod-shaped Mg₃Sb₂ compound forms along grain boundaries when Sb was added to AZ91. ³⁴ As an example, alloying of AZ91 with 0·5 wt-% Sb leads to a significantly faster cathodic reaction rate and results in increased overall corrosion rate by ∼50%. ^34,149 The limited work to date clearly indicates that Sb has also been noted to have negative effect on the corrosion resistance of the AZ91D, ⁹⁹ where 0·4 wt-% Sb was reported to increase the alloy corrosion rate from 8·6 to 19·91 mm year⁻¹. Sb, however, is a known cathodic poison, and based on the recent findings on the poisoning effect of As, ⁹⁷ the revisiting of Sb presents an avenue of future work.

Scandium (Sc)

The effect of Sc on corrosion properties of Mg and its alloys is yet to be fully explored, however, the information available to date reports that Sc additions to AZ91 alloys refine microstructure through the formation of the Al₃Sc compound that suppresses Mg₁₇Al₁₂ phase formation. ¹⁵⁰ Sc has also been noted to comparatively retard cathodic kinetics, resulting in the reduction of the corrosion rate of AZ91E in 0·1M NaCl from ∼8 μA cm⁻² to 2 μA cm⁻² at 0·1 wt-% of Sc. In contrast, a higher Sc content was shown to not be as effective. ³⁴ Sc is an extremely expensive metal (>US$100 g⁻¹), which significantly limits its wider use, and therefore few studies exist. The comparatively low catalytic activity of the Al₃Sc compound in isolation was reported by Cavanaugh. ¹⁵¹

Silicon (Si)

There exist commercial Mg–Si alloys, the most common of which are AS21 and AS41 with Si additions of ∼1 wt-%. Si additions to Mg result in formation of magnesium silicide (Mg₂Si) intermetallics. Depending on the shape and distribution of this intermetallic, it may have a minor or major influence on enhancing the corrosion rates of the commercial AS alloys. A recent report has revealed that Mg₂Si acts as a local cathode in Mg alloy AS31 and thus promotes the dissolution of surrounding matrix. ¹⁵² Fine and evenly distributed polygonal-shaped Mg₂Si was reported to be more inert to corrosion of AZ91 alloy compared to coarse Chinese script-like Mg₂Si. ¹⁴⁹ It is however, important to mention that Si has an adverse effect on corrosion in the presence of Fe contamination, ^15,113 which was recognised early in the twentieth century. As such, besides the AS series of alloys which are used in some automotive applications, ¹⁵³ Si is not a typical alloying element in Mg alloys and its level in commercial purity Mg is very carefully controlled.

Samarium (Sm)

Samarium is known to be beneficial in improving the mechanical properties in binary Mg alloys, ¹⁵⁴ however, there is a limited number of studies regarding the effect of Sm on corrosion of Mg. Wu and co-workers reported that additions of about 1 wt-% Sm to AZ92 led to formation of an Al–Mn–Sm and Al–Sm phase, along with a more refined β (Mg₁₇Al₁₂) phase, accompanied with a decrease in the β volume fraction which contributed to decreased corrosion rate of AZ92 by ∼40% in 3·5% NaCl. ¹⁵⁵ Moreover Sm has also been reported to refine the microstructure of Mg–Al–RE alloys and somewhat decrease the mass loss rate from 2·6 to 1·95 mm year⁻¹ in 3·5% NaCl when added at 0·5 wt-%. ¹⁵⁶

Tin (Sn)

Tin has a tendency to form Mg₂Sn along grain boundaries. The volume fraction of Mg₂Sn increases with Sn content. ¹⁵⁷ As Sn has a low exchange current density it does not enhance cathodic kinetics, but leads to a significant increase in anodic activity – akin to the notion of anodic activation – which Sn also imposes when added to Al. ¹⁵⁸ The anodic activation phenomena in Mg alloys is not understood nor studied at the nano-scale to date. In the work of Wang et al., additions of 1–3 wt-% Sn to AP65 alloy decreased the corrosion potential of the alloys by about 120 mV in 3·5% NaCl and increased the i _corr from 6·31×10⁻⁴ to 7·9×10⁻¹ mA cm⁻² at 1–3 wt-% Sn. ¹⁵⁹ This is synonymous with anodic activation. In an early study, binary additions of Sn to Mg were reported to be negligible for corrosion for concentrations up to as high as 5 wt-% Sn. ¹⁵ It is therefore worthwhile to mention that some misinterpretations of the role of Sn in Mg binary alloys exist. For instance, the recent report by Ha suggested that Sn increases cathodic reaction rates and thus increases hydrogen evolution rate which in turn promotes passivity in Mg–Sn alloys (2–8 wt-% of Sn added). ¹⁶⁰ This effect was not demonstrated by the presence of a passive region or a passive current density. As such, the effect of Sn appears to be more deleterious in alloyed Mg, than pure Mg, either alone, or depending on the interaction of Sn with the elements present in the Mg alloy. More specifically, the experimental alloy of Mg–5Al–1Sn exhibits the i _corr value of about 51 μA cm⁻² in comparison to i _corr of Mg–7Sn that is about 27 μA cm⁻² in 0·1M NaCl. ⁵⁰ The mechanism of anodic activation is not understood, but its manifestation has been depicted rather markedly in Fig. 4 for the example of heavy Sn loadings.

Strontium (Sr)

Strontium has low solid solubility in Mg (∼0·1 wt-%) and in binary context forms Mg₁₇Sr₂ along grain boundaries. ²⁷ Sr significantly increases cathodic reaction rates and corrosion potentials of Mg that results in high overall corrosion rates. Xia reported the corrosion rate (i _corr) of commercially pure Mg of about 20 μA cm⁻² in contrast to 38 and 60 μA cm⁻² when 0·2 and 2 wt-% of Sr, respectively, were added. ²⁷ In the Mg–Al–Zn system, when added above solubility limit, Sr decreases grain size forming the and binary eutectic Al₄Sr, ¹⁴⁸ Mg₁₇Sr₂ and Mg₂Sr. These particles lower the volume fraction of Mg₁₇Al₁₂ and homogenise its distribution. ¹⁶¹ Sr is reported to improve corrosion resistance of AZ91 by a purported retardation of anodic activity concomitant with an increase in corrosion potential (E _corr) by about 150 mV and reducing the i _corr by ∼50%. ¹⁶² This agrees with results by Fan where the additions of up to 2 wt-% Sr led to ennoblement of E _corr by ∼80 mV_SCE and decrease in the i _corr of AZ91D alloy from about 24 to 12 μA cm⁻². ¹⁶³ More so, the recent study by Xia reveals an increased cathodic reaction rates and ennoblement of corrosion potential of binary Mg–2Sr alloy compared to pure magnesium (E _corr, Mg––2Sr = −1·64 V_SCE, E _corr, Mg = −1·725 V_SCE). ²⁷ Subsequently the i _corr of Mg–2Sr increased from 21 of pure Mg to 58 μA cm⁻², whereas the i _corr of Mg–0·2Sr increased to 38 μA cm⁻².

Titanium (Ti)

The literature indicates that Ti has so far only been added to AZ91 alloys. As a result, the effect of Ti on other alloy systems is not known. However, it is reported that Ti addition in AZ91 alloy, progressively improves corrosion performance (to the reported maximum values of 0·8 wt-% Ti). This effect of Ti is associated with (1) changes in morphology and distribution of β phase from coarse and semi-continuous to fine, uniform and rodlike, and (2) an increase of Al in the solid solution of α-phase. ¹⁶⁴ Südholz tested the effect of 0·1 wt-% Ti addition to AZ91E in 0·1M NaCl and found that although anodic activity was slightly increased in the presence of Ti, the cathodic reaction rate was somewhat decreased. However, this did not have a significant impact on the overall corrosion rate i _corr. ³⁴

Candan and co-workers measured the corrosion potential and corrosion rate of AZ91 alloy with 0·2, 0·3, 0·4, 0·5 wt-% Ti additions in 3·5% NaCl solution, and reported somewhat unrealistic corrosion potential values (E _corr) and corrosion current densities (i _corr) for commercial and alloyed AZ91 alloys (i.e. E _corr, AZ91 = −0·821 V_SCE and E _corr, AZ91–0·5Ti = −0·980 V_SCE, i _corr, AZ91 = 153 μA cm⁻² and i _corr, AZ91–0·5Ti = 43 μA cm⁻²). ¹⁶⁵ Such high potentials are synonymous with potentials in which the cathodic reaction would be oxygen reduction, and not water reduction – and cannot be realistic or relied upon. The corrosion potential of AZ91 alloy is typically around −1·5 V_SCE.

Yttrium (Y)

In binary Mg–Y alloys, Y additions monotonically increase corrosion rate because of the increasing volume fraction of Mg₂₄Y₅ and a concomitant enhancement of cathodic kinetics. ¹⁶⁶ An increase in corrosion rates reported from 17 to 65 μA cm⁻² when 2–18 wt-% Y was added to Mg. Y as an elemental addition to AZ series alloys forms Al₂Y phase, refines α (Mg) grains and contributes to a lower fraction of, and more homogeneous distribution of β (Mg₁₇Al₁₂). As a result, the corrosion properties of AZ91 have been reported to improve with Y additions ranging from 0 to 0·8 wt-%; ¹⁶⁷ the mass loss rate of AZ91 dropping from 7·5 mg cm⁻² day⁻¹ to about 0·5 mg cm⁻² day⁻¹ at 0·8 wt-% Y. This was not mechanistically analysed in detail by the authors, however, the electrochemical response showed slightly slower cathodic reaction rates. The improvement in corrosion was mainly because of the refined microstructure, and a change in the morphology of the Mg₁₇Al₁₂ phase from continuous to discontinuously dispersed with a concomitant decrease in its volume fraction. ¹⁶⁸ However, in contrast, with heavier loadings of Y, the work of Südholz indicated a significant increase in corrosion when 2 wt-% Y was added to AZ91E (an increase in i _corr from 8 μA cm⁻² at 0 wt-% Y to about 60 μA cm⁻² at 2 wt-% Y in 0·1M NaCl). ³⁴ The notion that Y is problematic for pure Mg, along with its ability to serve as a local cathode, ¹⁶ has resulted in the conclusion that Y is deleterious to the corrosion of Mg. That said, however, the high solubility of Y in Mg has resulted in increased use in the modern age hardenable alloys, such that Y is a major component of the commercial alloys WE54 and WE43. In addition, the magnetron co-sputter deposited non-equilibrium thin film Mg–22 at-% Y alloy was reported to exhibit the lowest corrosion rate (i _corr = 0·9 μA cm⁻²) in 0·1M NaCl (pH 12) compared to commercial purity Mg and bulk WE43 alloy. ¹⁶⁹ This significant reduction in corrosion rate was attributed to passivity enhancement by Y via incorporation in the surface film, and also because of this method of alloy fabrication not resulting in formation of Mg_xY_y phase and Y remaining in solid solution. ¹⁶⁹ Physical vapour deposition (PVD) techniques are both challenging and not up-scalable, but have been able to reveal comparatively corrosion resistant Mg-based thin film alloys and coatings, ¹⁷⁰ indicating how important microstructural heterogeneity (and retention of solid solutions) can be in limiting the kinetics of corrosion, ¹⁷¹ unfortunately this requires specialist fabrication due to the limited solubility of metals in Mg prepared by conventional casting.

Zinc (Zn)

Zinc is the second most frequently used alloying element in Mg alloy production after Al. In the binary context, Zn forms Mg_xZn_y phases (depending on precise composition) that provide and age hardening response. ^27,172,173 However, this phase serves as a local cathode to the Mg matrix and has been reported to be responsible for overall increase in corrosion rates due to accelerated cathodic reaction rates ^22,88,174 when present in concentrations above ∼1 wt-%. Although Hanawalt established a threshold limit for Zn of up to 2·5 wt-% in his early study in 1941, ¹⁵ Kirkland reported a substantial increase in corrosion rates when Zn concentration rises from 1 to 3 wt-%. ²² In addition, increasing the Zn content has been shown to lead to a higher susceptibility to stress corrosion cracking. ¹⁷⁵ Zinc is often used in combination with Mg–Al and Mg–Zr–(RE) alloys and hence the role of Zn in synergy with other elements has been described along with those specific elements.

Zirconium (Zr)

Zirconium has low solubility in Mg and does not form intermetallic phases with Mg. ¹⁷⁶ Zr is added to Mg because of its potent ability to refine the grain size of Mg alloys, improving both the casting quality and mechanical properties of Mg. Usually Zr is added to Zn- and RE-containing alloys such as ZE and ZK series alloys. Corrosion of these alloys is generally more rapid than the AZ (Al–Zn) class of alloys. It has been reported that corrosion resistance may be improved by heat treatment, ¹³ while there are even reports that Zr addition up to 0·42 wt-% may have positive influence (i.e. decrease) corrosion of Mg-alloys. ¹⁷⁷ However, of the systematic studies to date, they have indicated that Zr is rather detrimental for Mg corrosion. ^20,178 A systematic study of Zr additions was also able to indicate that the assertions made regarding the role of Zr in higher order (multi-element) alloys, is difficult to ascertain – as the corrosion response may be dictated or overwhelmed by the role of the other elements (present in a much higher proportion).

There are reports that suggest Zr can purify Mg-alloys by combining with Fe to form Fe₂Zr which settles to the bottom of the melt (by gravity) resulting in higher purity castings. This potential beneficial effect, however, was also recently described by Gandel, who revealed that overall, the influence of Zr on corrosion was by in large negative and unique. ²⁰ It was indicated (from carefully prepared binary alloys) that Zr imparted an ‘activation’ effect where it could accelerate the anodic reaction by destabilising the surface film. This led to rapid general dissolution rates corroborated by SEM. Furthermore, the ratio of soluble to insoluble Zr was also a key factor (as following the limited solubility in the Mg matrix, any excess Zr phase separates to form pure Zr). While a potent grain refiner, Zr content should be balanced so as to not be deleterious to corrosion. Zr is nominally not added to Mg–Al alloys as it combines with Al to form an Al₃Zr intermetallic that negates the grain refining effect of Zr. Even in commercial Zr-containing alloys such as ZE41, the presence of Zr particles was seen by electron microscopy to be responsible for the accumulation of corrosion damage. ^179,180 Neil and co-researchers proposed the corrosion mechanism of ZE41 that initiates with localised attack of the regions beside Mg₇Zn₃RE intermetallic, followed by deep attack at Zr-rich regions.

Other elements (Au, B, Ba, Be, Cd, Co, Eu, Ga, Ge, In, Nb, Ni, Se, Tb, Th, V)

Boron as a trace addition to AZ series alloys from 0·008 to 0·032 wt-% provides significant grain refinement, which is attributed to the presence of AlB₂ particles that serve as Mg grain nucleation sites. ¹⁸¹ Studies suggest that B improves the fluidity and die casting properties of AZ alloys, ¹⁸² however, the presence of 0·05–0·2 wt-% B in AZ31 and AZ91E alloys leads to higher corrosion rates by a factor of about 2 ^34,183 compared to the B-free base alloy.

Barium has been noted to improve the age hardening response of Mg–Zn alloys, with dispersed particles of Mg₇Zn₂Ba forming within grains and at grain boundaries. ¹⁸⁴ Ba has, however, been shown to have an adverse effect on corrosion properties of AZ91. The only corrosion study related to Ba additions to date indicates the addition of 0·1 wt-% Ba to AZ91E, ³⁴ increased corrosion rates increased by ∼50%, concomitant with a decrease in E _corr of ∼25 mV_SCE.

Beryllium has an ability to control melt oxidation of Mg during melting, casting or welding. ^185,186 However, because of its low solubility and effective grain coarsening effect, the addition of Be is nominally restricted to the parts per million range (∼<30 ppm). ⁷⁸ In addition, there are significant safety risks with Be such that it is not presently commercially viable.

Cadmium additions to Mg have not been widely explored, however, an early report suggests that additions of Cd up to 5 wt-% do not have a detrimental effect on corrosion. ¹⁵ More recently, a study by Xu indicated decreased corrosion rates with Cd additions, concomitant with a reduced corrosion potential of ∼20 mV when 2·04 wt-% Cd was added to Mg. Hydrogen evolution measurements showing Mg–Cd lowered corrosion rate three times (3·44 mm year⁻¹ with 0 wt-% Cd v 1·06 mm year⁻¹ at 2·04 wt-% Cd). ¹⁸⁷ In a separate study by Südholz, the addition of 0·2 wt-% Cd to an AZ91E+0·2Au alloy resulted in a lower overall corrosion current density compared with similar alloys including AZ91E+0·05Au and AZ91E+0·001Au (i _corr, AZ91E–0·2Au–0·2Cd = 3·1 μA cm⁻²; i _corr, AZ91E–0·05Au = 5 μA cm⁻²; i _corr, AZ91E–0·001Au = 13 μA cm⁻²). ³⁴ In the binary context, addition of Cd may also improve tensile strength and provides an increment in elongation. ¹⁸⁷ As Cd is 100% soluble in Mg it does not form any intermetallic compounds and as a binary addition to Mg, is perhaps in no way detrimental to corrosion performance; unfortunately, however, Cd is hazardous to human health and its widespread use as an alloying element in Mg will remain unlikely. ¹⁸⁸

Cobalt is mainly used as an alloying addition in the production of lightweight load sensitive materials with sensory properties, allowing for on-line monitoring of mechanical forces applied to components made of Mg–Co. ¹⁸⁹ Co has no solid solubility in Mg and forms intermetallic compounds of the types Co₂Mg and Mg₂–Co_x in binary Mg–Co alloys. Micro-additions of Co (∼0·1 wt-%) result in extremely high corrosion rates of Mg, with the corrosion rates being comparable to those Mg alloys loaded with common impurities such as Fe, Ni. ¹⁵ In the early study of Hanawalt, the tolerance limit of Mg for Co was reported to be 0·017 wt-%, the concentrations exceeding this limit drastically increase corrosion rate of Mg. ¹⁵ More recently, Co additions have also been studied in the pursuit of ultra high strength Mg alloys. ¹⁹⁰ Such alloys have been largely based on the Mg–Zn system with both heavy and complex multi-alloying to stimulate heterogeneous microstructures. Co-containing Mg alloys display among the highest corrosion potentials of any Mg alloys ever reported, owing to the significant enhancement of cathodic kinetics that Co imposes. As such, the more noble potential values are linked with very high rates of corrosion. For Mg–8Zn–1Co, an E _corr of −1419 mV_SCE and i _corr 194 μA cm⁻² were recorded. This i _corr is about 20 times more rapid than AZ31. The same alloy with 0·02 wt-% Ti added, resulted in a decrease of i _corr to ∼70 μA cm⁻², which is the same i _corr measured in the case of more Zn, in a Mg–10Zn–1Co alloy. Finally, the lowest i _corr recorded for a Co-containing alloy was in Mg–8Zn–1Co–0·5Ag–0·3Ca, with an i _corr of 47 μA cm⁻² and retention of an ennobled potential of about −1416 mV_SCE. These are comparatively higher i _corr values, than all the commercial Mg alloys as measured under similar conditions.

Dysprosium assists grain refinement and solid solution strengthening in Mg–Dy binary alloys, while it enhances compression and tensile properties to ZK alloys when added below the solubility limit. ^191–193 As Dy has a high maximum solid solubility in Mg (25·34 wt-% ⁹ ), it largely dissolves in the Mg matrix with only low concentrations departing the matrix in precipitation of second phases, which increases in volume fraction with an increase in Dy content. It is reported that corrosion of Mg–Dy binary alloys is of the filiform-type and localised, and the corrosion rate does not notably increase or decrease with differing Dy contents between 5 and 20 wt-%. ¹⁹⁴ Although the Mg–Dy system appears to be promising for research on the basis that Dy is not studied in complex alloy systems, Dy did not appear to adversely influence corrosion, while Dy has excellent promise for improvement of mechanical properties due to age-hardening; however, Dy is a rather heavy and expensive metal to be used industrially. ¹⁹⁵

Gallium as a binary addition forms rod-shaped Mg₅Ga₂ along grain boundaries. ¹⁹⁶ Mg–Ga alloys were found to have good ductility and owing to a good response to aging, show substantial increases in tensile strength compared to pure Mg (i.e. UTS = 180·1 MPa, TYS = 107·4 MPa, ϵ = 8·2% for Mg–2 wt-% Ga). ¹⁹⁶ However, the low melting point of Ga (27°C), its relative rarity, and the embrittlement risk to Al-alloys posed by Ga makes such additions unsuitable for commercial structural alloy use. On the other hand, Ga has been pursued as an alloying addition in Mg–Hg alloys that are used as a material for sea water batteries. Low concentrations of Ga (1 wt-%) show a reduction of the anodic reaction rate (which is increased owing to the Hg) improving the overall corrosion resistance of the Mg–5Hg system (i _corr, Mg–5Hg = 26·98 mA cm⁻², i _corr, Mg–5Hg–1Ga = 2·34 mA cm⁻²). ¹²⁷

Germanium forms rod-shaped Mg₂Ge precipitates in Mg binary alloys and can effectively refine the grain size of Mg. ¹⁹⁷ Of the few corrosion studies regarding Ge additions, it was seen that additions of 0·5–2 wt-% Ge in Mg increase the cathodic kinetics accompanied by an ennoblement of E _corr by about 100 mV. Ge was observed to suppress the anodic reaction rates and thereby reduce the overall corrosion rate when tested in 3·5% NaCl. ¹⁹⁸ The lowest overall corrosion rate (relative to pure Mg) was achieved at 1·5 wt-% Ge, claimed to be an order of magnitude lower than that of Mg. The explanation for this reduction in corrosion rate was given as being a result of the finer grain structure that promoted more uniform corrosion product film formation, ¹⁹⁸ however, such a mechanism is in need of further validation.

Gold because of its relatively high cost has not received much attention as an alloying element for Mg. The literature review shows that the trace amount of gold in AZ91E alloy can improve creep response, ¹⁹⁹ with only small increases in corrosion current density with 0·001, 0·05, 0·1 wt-% Ag added (AZ91E∼6 μA cm⁻², AZ91E–0·001Au∼8 μA cm⁻², AZ91E–0·05Au∼5 μA cm⁻², AZ91E–0·1Au∼12 μA cm⁻²). ³⁴

Indium additions were studied several decades ago by Hiroki, who reported the corrosion of Mg–47 at-% In both dry and humid environments. ²⁰⁰ Although the corrosion was not measured either electrochemically or by mass loss, a negative effect of In on accelerated corrosion of Mg was established. According to the proposed mechanism, the Mg–In alloy surface immediately oxidises once exposed to dry air. MgO film forms on the surface and it contains isolated In. As the melting point of In is very low (156·6°C) the In crystals continue to grow at room temperature. These crystals were posited to induce the stress in the corrosion product film that subsequently results in the film breakdown. In humid environment (30°C, 70% RH), the corrosion product was composed of the mixture of Mg(OH)₂ and isolated In crystals. The isolated In is purported to serve as a local cathode to Mg. The role of In is in need of further research, particularly at lower alloy loadings and non-binary systems.

Lutetium: The influence of Lu on Mg or Mg-alloys has not been explored yet. The recent study by Samaniego is the only report on the effect of Lu on Mg-alloys. In particular the low level of Lu (0·21 wt-%) was added to AZ31 alloy. Lutetium incorporates into Al–Mn constituent particle, however, this does not change the Volta potential of these particles. Lu was found to reduce the corrosion rates of AZ31 in the as-cast condition. This was achieved by reducing the cathodic kinetics of Mg. The overall corrosion rate was therefore decreased from 12·3 to 7·25 μA cm⁻², the mass loss rate from 0·36 to 0·14 mg cm⁻² per day. XPS analysis showed no appreciable amount of Lu on the alloy surface implying that Lu does not promote a more protective corrosion film. Although Lu seems to have a positive effect on corrosion resistance of AZ31 alloy, it negatively resulted in larger grain structure of this alloy leading to an increase in grain size from 100 to 700 μm. ²⁰¹

Molybdenum is not a typical elemental addition into Mg because of its insolubility. However, low-level additions of Mo to AZ91E (0·1 wt-% Mo), studied by Südholz, indicated that microalloying with Mo leads to a significant increase (about a factor of 10) in the corrosion current density values i _corr. ³⁴

Nickel is regarded as an impurity in Mg alloys and has an adverse effect on corrosion. A tolerance limit of only 0·0005 wt-% Ni is reported for Mg. ^15,78

Niobium is not a common alloying addition for Mg. Nb is not soluble in Mg and its alloying would only be possible via methods such as magnetron sputtering, limiting its utility to that of thin films. One attempt has been made to add Nb to Mg composites. ²⁰²

Palladium has recently been explored as an alloying addition to heavily alloyed versions of biodegradable Mg–Ca–Zn alloys. ^203,204 Pd additions (∼2, 6 at-%) to Mg–Zn–Ca amorphous alloys were reported to retard the anodic reaction rate in simulated body fluid. However, given the insolubility of Pd and its high exchange current density, this effect is concomitant with enhanced cathodic activity leading to higher corrosion current densities (i _corr, Mg–23Zn–5Ca = 1·7 mA cm⁻²; i _corr, Mg–23Zn–5Ca–2Pd = 2·1 mA cm⁻²; i _corr, Mg–23Zn–5Ca–6Pd = 2·7 mA cm⁻²²⁰³), however, it should be noted that the baseline corrosion current density of the Mg–23Zn–5Ca alloy is very high to begin with, at least an order of magnitude higher than that of pure Mg in the same solution. ³⁹

Selenium has not been widely used in Mg alloy production, likely because of its toxicity. A study conducted by Südholz indicates that small amount of Se (0·1 wt-%) in AZ91 alloy can reduce the corrosion current density values (i _corr) from ∼8 to 2 μA cm⁻². ³⁴ The solubility of Se in Mg is unknown, and its use in conventional alloy production is hazardous.

Terbium has a high solubility in Mg (24 wt-%), however, the use of Tb in Mg alloys is not widespread. Binary Mg–Tb alloys show a good age hardening response and the hardness values increase with an increase of Tb concentration (up to 93 HB at 20 wt-% Tb). ²⁰⁵ One study by Neubert suggested that Tb has a deleterious effect on corrosion of a Mg–4Tb-2·5Nd alloy. ²⁰⁶ That study reported accelerated cathodic kinetics with addition of 4 wt-% Tb and higher overall corrosion rate in 300 ppm NaCl solution compared to WE43 (where i _corr, Mg–4Tb–2·5Nd∼7·4 μA cm⁻² and i _corr, WE43∼2. μA cm⁻²).

Thorium is the most effective elemental addition for imparting high-temperature strength and creep resistance up to 350°C. In addition, it improves casting and welding properties. Unfortunately, because of the radioactive nature of thorium, it is not commercially viable, ¹¹³ and the influence of Th on corrosion is not widely reported.

Vanadium in the context of influencing corrosion of Mg or Mg alloys is yet to be reported. However, V-containing Mg-alloys have been studied, with trace additions indicating notable grain refinement for both Mg–Zn ²⁰⁷ and Mg–Al alloy systems. ²⁰⁸ In the AZ series alloys, V forms Al₃V phase that apparently results in high grain boundary density and refines grains and the distribution of β-phase (Mg₁₇Al₁₂).