The Role of Self-Corrosion during the Dissolution of Galvanically Coupled Magnesium

The use of Mg is increasing in the automobile and aerospace industries due to its light weight and suitable structural properties that lead to decreased vehicle weight and, consequently, reduced fuel consumption and greenhouse gas emissions. The main obstacle to the use of Mg is its corrosion susceptibility [1]. Mg corrosion is limited to some degree by the relatively slow rate of hydrogen evolution on its surface.  However, the rate of corrosion increases significantly when magnesium is galvanically coupled to another metal such as steel, which coupling leads to increased rates of hydrogen evolution [2, 3].

Both a disc-type morphology and filiform corrosion have been observed for Mg, depending on the concentration of impurities in the metal and the concentration of Cl^-in the electrolyte [2, 3].  In both cases, the morphology is strongly influenced by the cathodic reaction, which limits the rate of corrosion [2, 3].  The situation changes dramatically when Mg is coupled to another metal.   Although hydrogen evolution on the coupled metal dominates the cathodic reaction, hydrogen evolution on the Mg surface does not stop due to galvanic coupling.  The purpose of this paper is to investigate the impact of such coupling on the self-corrosion of Mg (i.e., corrosion that is associated with hydrogen evolution on the Mg surface), and the extent to which galvanic coupling influences both the morphology and rate of magnesium self-corrosion.   

The samples tested consisted of the exposed end of an Mg rod (99.95%, GalliumSource), 3mm in diameter, that was insulated and surrounded by a mild steel electrode.  The steel electrode was a cylinder with an inside diameter just large enough to accommodate the insulated Mg rod and an outer diameter of either 8, 12 or 16mm.  This structure (Mg surrounded by steel) was cast into epoxy (EpoThin^Tm2 Epoxy System, Buehler) for testing. After casting, the sample was cross-sectioned with a diamond saw, and the surface was polished prior to submersion in the 5 wt% NaCl electrolyte. The concentric Mg and steel electrodes were galvanically coupled by electrically connecting them external to the solution through a zero resistance ammeter, which also permitted direct measurement of the galvanic current.  An inverted graduated cylinder was used to capture and measure the volume of hydrogen evolved during corrosion. Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) was also used to directly measure the amount of Mg dissolved during the experiment. The difference between the total corrosion, as measured by hydrogen evolution and/or magnesium dissolution, and the total amount of galvanic corrosion (from the integral of the galvanic current) provided the amount of self-corrosion.

Figure 1 shows the galvanic corrosion current as a function of time.  The magnitude of the current increased with the size of the steel cathode, indicating that the corrosion process was still cathodically limited for the 8mm cathode, and to a lesser extent for the other two cathode sizes. A general decrease in the galvanic current with time was observed. The morphology of the corrosion observed for galvanically coupled samples was very different than that observed for uncoupled samples.  Self-corrosion was substantial for all galvanically coupled samples as shown in Table 1, and accounted for approximately a third of the total amount of corrosion.  The rate of self-corrosion for coupled samples was significantly greater than the free corrosion rate.  Thus, galvanic coupling of Mg to steel enhances hydrogen evolution on the magnesium surface. The results of these experiments provide additional insight into the processes that control the dissolution rate of galvanically coupled Mg. 

In summary, our work quantifies the role of self-corrosion for galvanically coupled Mg and describes the relationship between the corrosion morphology and the processes that control the corrosion rate. These results provide fundamental insights into the corrosion of magnesium and a foundation for the development of a model to describe Mg corrosion.

Acknowledgments

The authors wish to thank CD-adapco for their generous funding of this work.

References

1. Kulekci, Mustafa. The Int. J. of Advanced Manufacturing Tech. 2008: 851-65.
2. Williams, G., HN McMurray, and R. Grace. J. of Electrochem Soc. 2008: 155-7.
3. Williams, G., R. Grace. Electrochem. Acta 56. 2011: 1894-1903.