Corrosion Behavior of Pure Magnesium with Low Iron Content in 3.5 wt% NaCl Solution

Abstract

The microstructure and corrosion behavior of pure magnesium, with 25 ppm Fe but a high corrosion rate, have been studied in this work. It was found that Fe-rich particles, containing also Si, distribute non-uniformly in the Mg matrix and they are spaced at a distance of 100-1000 μm. Before the initiation of localized corrosion, Fe-rich particles play a key role in determining the overall corrosion behavior by supporting hydrogen evolution. During corrosion propagation, Fe-rich particles could be oxidized once they were detached from the Mg matrix and, consequently, lose their cathodic activation. Once the corrosion has taken place over majority of the surface, the effects of crystallographic orientation of the underlying metal dominate the last stage of corrosion, which propagates parallel to the (0001) Magnesium alloys are of great value for industrial applications due to their low density and high strength-to-weight ratio. However, a major deficiency is their comparatively poor corrosion resistance when exposed to aqueous and humid environments.1,2 Consequently, there is a demand to develop magnesium alloys with improved corrosion resistance. 2,3The equilibrium potential for magnesium oxidation is well below the equilibrium potential for hydrogen evolution over the entire pH range of practical interest. As a consequence, the cathodic reaction of hydrogen evolution largely dominates over oxygen reduction during corrosion in an aqueous environment. For this reason, the presence of elements with low overpotentials (or high exchange current density) for hydrogen evolution is likely to produce detrimental effects on the corrosion resistance due to increase in the hydrogen evolution rate. Hanawalt et al. 4 found that four elements (Fe, Ni, Cu, and Co) had a very profound effect on accelerating the corrosion rate of Mg in chloride-containing aqueous environments even at concentrations below 0.2 wt%. Subsequent studies have confirmed that the most critical factor affecting the corrosion behavior of Mg is the presence of impurities.3

Self-healing plasma electrolytic oxidation coatings doped with benzotriazole loaded halloysite nanotubes on AM50 magnesium alloy

“…In the areas where the pitting corrosion has not been initiated, needle-like crystalline precipitates can be found developed on the surfaces of both PEO and BTA-HNT-PEO coatings, indicating uniform degradation of the coating matrix due to the corrosion attack. Similar precipitates comprising a mixture of Mg(OH)2, MgO and MgCl2 have been observed by Yang et al[50] in the case of uniform corrosion of Mg in NaCl solutions. However, such needle-like corrosion products were not present on the surface of the HNT-PEO coating, indicating that the pitting corrosion could have played the major role in its degradation.…”

“…However, after the PDP scan and subsequent immersion, |Z| f0 came to a level which was higher than that after 1 h. This is likely to be due to the formation of a deposited corrosion product at the sites of pitting corrosion induced by the anodic polarisation in the PDP test. It should be noted that irregularities in corrosion behaviour are commonly observed for pure Mg and Mg alloys in NaCl solutions and temporal increases in corrosion resistance could appear after few hours to several days of immersion, lasting for up to several hours until the breakdown of the deposited film of corrosion products occurs [13,50]. It is therefore conceivable that such periodic behaviour will persist for the samples with dramatically decreased during the fourth hour (Fig.…”

Hydrogen Evolution Ability of Selected Pure Metals and Galvanic Corrosion Behavior between the Metals and Magnesium

“…Therefore, we deduce that impurity elements, such as Fe, may be the cause for the high corrosion rate of the reported Mg-Al alloys [27]. Effects of Fe in magnesium alloys have been studied and it is confirmed that Fe can significantly increase corrosion rate by forming Ferich particles in m agnesium alloys [30][31][32]. Generally, less than 9 wt% Al is alloyed in Mg in commercial Mg alloys [27,33].…”

Application of Side-View Imaging and Real-Time Hydrogen Measurement to the Investigation of Magnesium Corrosion

“…Once the potential has increased sufficiently, the original film undergoes breakdown and corrosion proceeds locally. As soon as this occurs the potential becomes relatively steady and hydrogen streams composed by very fine bubbles are observed at the interface between the silvery and the dark areas 8 .…”