In this study, the impact of galvanic coupling of magnesium to steel on the corrosion rate, surface morphology, and surface film formation was investigated. In particular, experiments were performed to examine and quantify the role of self-corrosion (also called negative difference effect (NDE) or anodic hydrogen) during the corrosion of galvanically coupled Mg. It was found that galvanic coupling at high cathode-to-anode area ratios resulted in high rates of corrosion that impacted hydrogen evolution on the Mg surface. Self-corrosion accounted for, on average, approximately one-third of the total observed corrosion. The self-corrosion fraction varied with time and was found to reach values in excess of 50%. Surface film formation was observed, and approximately 30% of the Mg lost to corrosion was found in the film at the end of our experiments. The surface morphology observed during galvanic corrosion was dramatically different from the filiform structures associated with free corrosion of our samples, but showed similarities to the morphology observed previously for anodically polarized samples. Film formation appeared to slow the rate of self-corrosion with time. These results complement previous studies of Mg corrosion and add important insight into the role of hydrogen evolution on the Mg surface during galvanic corrosion. New environmental regulations, good mechanical properties, abundant availability and reasonable cost have generated renewed interest in the wider use of magnesium (Mg), especially for automobile and aerospace applications.1-4 There has also been growing interest in the use of Mg for batteries and in hydrogen generation systems. 5-8A key obstacle to the use of Mg is its corrosion susceptibility. Consequently, several previous studies have focused on understanding the corrosion mechanisms that control the dissolution rate and surface morphology of Mg during corrosion. Both disk-shaped and filiform corrosion have been observed for unalloyed Mg, depending on the concentration of impurities in the metal and the concentration of Cl − in the electrolyte. [9][10][11] In both cases, the morphology is strongly influenced by the cathodic reaction, which limits the rate of corrosion. 11,12The situation changes dramatically when Mg is coupled to another metal because of the increased cathodic area that significantly diminishes limitations due to the cathodic reaction on the Mg surface. Although most hydrogen evolution occurs on the coupled metal, hydrogen evolution on the Mg surface does not cease. The purpose of this paper is to investigate the impact of galvanic coupling on Mg corrosion that is associated with hydrogen evolution on the Mg surface, and the extent to which such coupling influences both the morphology and rate of Mg self-corrosion.In this paper, we use the term "self-corrosion" to refer to the dissolution of Mg that is not associated with the galvanic current that passes between the coupled metal and the Mg, similar to the definition used previously by others in galvanically coupled st...
New environmental regulations, good mechanical properties, abundant availability and reduced cost have generated renewed interest in the wider use of magnesium (Mg), especially in the automobile and aerospace industries [1]. The main obstacle to the use of Mg is its susceptibility to corrosion. Fortunately, its corrosion rate is limited to some degree by the relatively slow rate of hydrogen evolution on the surface of magnesium. However, the rate of corrosion increases significantly when Mg is galvanically coupled to another metal, such as steel, which coupling leads to increased rates of hydrogen evolution [2]. Although galvanic coupling of Mg results in increased corrosion, self-corrosion does not stop entirely and varies throughout the corrosion process. In this study, we examine the relative roles of self-corrosion and galvanic corrosion for magnesium that is galvanically coupled to carbon steel. As described previously [3] 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 (EpoThinTm2 Epoxy System, Buehler) for testing. After casting, samples were cross-sectioned with a diamond saw, and each 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. Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) was used to directly measure the amount of Mg dissolved during the experiment. The difference between the total corrosion, as measured by magnesium dissolution, and the total amount of galvanic corrosion (from the integral of the galvanic current) provided the amount of self-corrosion. We have already seen that the fraction of self-corrosion at the end of 10 minutes was roughly one-third of total corrosion [3-4]. However, fraction of self-corrosion does not remain constant over the entire experiment. As seen in Fig. 1, the fraction of self-corrosion is initially low for galvanically coupled Mg, but subsequently increases to a peak value, after which the rate decreases once again. The galvanic portion of the corrosion steadily decreases with time, possibly due to increased solution pH [3-4]; however, the self-corrosion behaves quite differently. This is also reflected in the surface morphology where galvanically coupled samples experience disc-shaped corrosion while self-corrosion of uncoupled samples is filiform in nature (see Fig. 2). Previous attempts to model Mg corrosion have noted a discrepancy between the measured and predicted corrosion rates [5-6]. One possible reason for this might be the influence of the changing rate of self-corrosion, which represents a significant fraction of the corrosion rate for galvanically coupled systems. In this study, we combine experiments and numerical simulations to investigate the roles of self-corrosion and galvanic corrosion for Mg in NaCl solutions. In particular, the influence of the active surface area and changing hydrogen reaction rates are explored. Acknowledgments The authors wish to thank CD-adapco for their generous funding of this work. References Kulekci, Mustafa. The Int. J. of Advanced Manufacturing Tech. 2008: 851-65. Williams, G., N. Birbilis, and HN McMurray. Electrochemistry Communications 2013: 1-5. Banjade D., Harb J. Electrochemical Society Spring Meeting 2015-Abstract. Banjade D., Harb J. “The role of self-corrosion during the dissolution of galvanically coupled magnesium-To be submitted manuscript” Jia, J. Materials and corrosion 2005: 56-7 2005. Deshpande, K. Corrosion Science 2010: 3514-22. Figure 1
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 (EpoThinTm2 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 Kulekci, Mustafa. The Int. J. of Advanced Manufacturing Tech. 2008: 851-65. Williams, G., HN McMurray, and R. Grace. J. of Electrochem Soc. 2008: 155-7. Williams, G., R. Grace. Electrochem. Acta 56. 2011: 1894-1903. Figure 1
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