The effect of Mn and Zr additions on Fe impurities and the corrosion performance of Mg

Manganese (Mn) and zirconium (Zr) are two common alloying additions in magnesium (Mg) alloys. Both of these elements, while having low solubilities in Mg, each serve a specific purpose when added to Mg. Mn is often added to improve the extrudability and formability of Mg alloys and in aluminium (Al) containing Mg alloys to produce the Al8(Mn,Fe)5 phase which is able to remove iron (Fe) impurities to dramatically improve the corrosion resistance. Zr is incorporated in Mg mainly due to its unique ability to act as a grain refiner to greatly reduce the grain size and hence improve the mechanical properties of Mg. The effect of Mn alone on the corrosion of Mg and subsequent Fe impurity levels in the absence of any Al alloying addition is not well documented. Furthermore, the independent role that Zr has on the corrosion of Mg and Mg alloys has also not been reported thoroughly. In this study, Mg alloys containing various levels of Mn, Zr and Fe alloying additions in binary, ternary and quaternary combinations were produced and examined via SEM, EDX and EBSD techniques. The corrosion rates and morphologies of Mg alloy samples were examined via electrochemical polarisation and immersion testing. It was discovered that Zr additions, while beneficial in being able to remove Fe impurities, has a negative impact on the corrosion rates of Mg. Zr is able to increase both the anodic and cathodic reaction kinetics of Mg, thereby increasing the corrosion rate. Zr dissolved in the Mg solid solution was shown to act as an ‘anodic activator’, increasing the anodic reaction kinetics and the rate of Mg dissolution. Zr not dissolved in solid solution was present as Zr particles embedded in the Mg matrix. These Zr particles were efficient local cathodes, enhancing the cathodic reaction. The difference in electrochemical potential between these, essentially pure Zr, particles and the surrounding Mg matrix lead to the formation of micro-galvanic couples at open circuit, which increased corrosion rate. Mn was observed to slightly decrease the cathodic reaction kinetics of Mg when included as an alloying addition. This is rationalised on the basis that Mn will interact with Fe, and not on the basis that Mn can support reduction reactions at lower rates than Mg. This notion also appreciates that there is no Mg metal that has 0% Fe. However, beyond Mn additions of ~2 wt.%, Mn particles which formed in the Mg matrix increased the corrosion rate through the formation of micro-galvanic couples with the Mg matrix. While Mn additions in this study were ineffective at removing Fe from the alloy system, Mn was found to be capable of rendering Fe impurities less detrimental to Mg. Mn additions were found to increase the tolerance limit of Fe in Mg. As such, higher levels of Fe were necessary to increase the cathodic reaction kinetics required for increased corrosion rates. It was observed that the Mn additions form an intermetallic phase with the Fe impurities. At low Fe levels these phases appeared to be Mn particles with Fe dissolved in solid solution within these particles. At higher Fe levels, there were large Fe particles encapsulated by a layer of Mn. It is proposed that this interaction between Mn and Fe decreases the electrochemical potential difference between the Fe impurities and the Mg matrix in a similar manner to the Al8(Mn,Fe)5 phase observed in Al-containing Mg alloys, thereby, decreasing the driving force to increase the cathodic reaction kinetics through micro-galvanic coupling. This work has elucidated the interactions and effects of Mn and Zr additions on Mg and has shown that Zr is inherently detrimental to the corrosion resistance of Mg and that Mn can interact with Fe in Mg to reduce the impact that Fe impurities have on Mg alloys in the absence of Al.