The increasing rate of hydrogen evolving from magnesium (Mg) surfaces under anodic polarization was characterized through simultaneous hydrogen volume collection, mass loss, potentiostatic and potentiodynamic polarization, and inductively coupled plasma optical emission spectroscopy (ICP-OES). This is distinct from the literature in that all four techniques are not often performed in the same test. Results indicate Mg dissolves as Mg 2+ with anodically induced increases in hydrogen evolution rates due to increases in the water reduction reaction rate. In order to contribute to a mechanistic understanding of the cause for anodically induced increases in hydrogen evolution, quantitative surface spectroscopy for accurate chemical analyses of the Mg surface subject to dissolution was carried out via post exposure Rutherford Backscattering Spectrometry. Surface enrichment of transition elements other than Mg were confirmed, which provides a foundation for understanding the origin of enhanced hydrogen evolution. Magnesium (Mg) alloys remain attractive for potential weight reduction in the automotive and aerospace industries.1-3 With the growing demand for Mg products, the corrosion of Mg in aqueous electrolytes remains an important technological issue. 4 Commercial Mg alloys possess corrosion potentials less than −1.4 V SCE in a wide variety aqueous environments, where the hydrogen evolution reaction (HER) is the primary cathodic reaction and any transition metal (TM) contaminants can support HER without the thermodynamic tendency for dissolution. 5Rapid rates of Mg corrosion occur in aqueous environments of near-neutral pH compared to other engineering metals on the basis that: (1) Mg is not thermodynamically stable below pH ∼11, (2) there is no mass transport limitation on the cathodic reaction (since water, which is available in abundance, is being reduced, not oxygen), (3) the lack of a protective (passive) film, and (4) the non-polarizable nature of Mg also contributes to very high rates of anodic dissolution, since low overpotentials correspond to high current densities. This latter point is the kinetic reason why galvanic coupling of Mg to other, more noble, engineering metals such as steels is particularly detrimental. Several attempts have been made to increase the intrinsic corrosion resistance of Mg alloys, 6-14 however design of improved Mg alloys requires a fundamental understanding of the corrosion mechanisms. One widely studied phenomenon of Mg corrosion is the so called negative difference effect or NDE (i.e., increased rates of the HER at increasing anodic polarization) which confounds the understanding of dissolution mechanisms, establishment of the Tafel law, and other issues. 8,[15][16][17][18][19][20][21][22][23][24][25][26][27] Several explanations of this anomalous behavior have been proposed among which include impurity element enrichment, formation (and disruption) of a partially protective film, metal spalling, univalent Mg based anodic dissolution, and magnesium hydride based models as revie...

The inhibition of corrosion on AA2024-T351 in NaCl solutions, mitigated by either in-situ permanganate ions (MnO 4 − ) or permanganate pretreatment, was examined. Both room temperature pretreatment and solution phase additions were studied as a function of inhibitor concentration. The roles of the inhibitor during anodic and cathodic polarizations were investigated. Inhibition of corrosion at open circuit corrosion potential (OCP) under conditions where anodic and cathodic reactions are coupled was also examined. The oxidation states of the manganese oxides that contributed to protection were determined using potentiometric electrochemical reduction and in-situ Raman spectroscopy. The thermodynamics of the Mn-water system was also considered over a range of concentrations. Permanganate was shown to be both an anodic and cathodic inhibitor, and an inhibitor of copper replating at OCP.

Scratch depassivation of AA2024-T351, AA7075-T6, and 99.999% aluminum in molybdate (MoO4 2-) and chromate (CrO4 2-) containing NaCl solutions was used to examine electrochemical repassivation kinetics over a range of potentials and inhibitor concentrations. Single frequency impedance measurements were taken in the capacitive region of frequency response. Capacitance measurements enabled the observation of the effects of inhibitor additions on oxide growth as protective oxide films formed on the scratched electrode surface. Chromate was found to suppress scratched electrode current transients at high potentials on both AA2024-T351 and AA7075-T6. Capacitance measurements revealed the rapid growth of a thick, protective oxide when concentrations as low 5 mM CrO4 2-E were in solution. Molybdate did not suppress transient current density in AA2024-T351 or AA7075-T6. In some cases the addition of molybdate increased the total charge passed after the scratch. However, capacitance measurements in the presence of molybdate indicate the rapid growth of a passive film.

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