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The enhanced catalytic activity of hydrogen evolution reaction on anodically polarized Mg surfaces, commonly referred to as the Negative Difference Effect, has been the topic of intense investigation in recent years. However, the cause of anodic H 2 remains unclear. To determine the primary source of H 2 evolution on dissolving Mg polarized at anodic potentials, an in-situ scanning vibrating electrode technique (SVET) analysis during galvanostatic polarization, coupled with gravimetric H 2 volume collection and potentiodynamic polarization experiments, were carried out on ultra-high purity Mg (99.9999% Mg). The combination of these methods provided solid evidence that the evolution of hydrogen on dissolving ultra-pure Mg is primarily associated with the regions dominated by the anodic reaction. Although local cathodes corresponding with the dark corrosion film formed during anodic dissolution were revealed by in-situ SVET, they appeared to play a minor role in the process.

The determination of corrosion rate for Mg and Mg alloys by electrochemical methods is complicated by several factors, including the intense hydrogen evolution (HE) that accompanies Mg dissolution, so hydrogen gas collection is often used. In the present paper a new gravimetric method for hydrogen gas collection, originally developed by Curioni, is evaluated as an alternative to the common volumetric method. The gravimetric method is based on the buoyant force exerted by the H 2 produced by dissolving Mg when the gas is accumulated in a submerged container. The accuracy of the method is investigated using a Pt electrode and its suitability for the study of Mg corrosion behavior is examined using a high purity Mg specimen. The suitability of the new experimental set-up for the study of anodic HE (also termed Negative Difference Effect (NDE)) is also evaluated. The gravimetric method exhibits higher sensitivity to HE detection than the volumetric method both in the absence and in the presence of an external polarization. Real-time HE detection during dynamic polarization was found to allow clear and rapid assessment of the effect of polarization on HE. Possible artifacts and experimental limitations of the gravimetric experimental method are discussed. Magnesium (Mg) and Mg alloys are materials that combine a very low density with good mechanical properties. These characteristics make Mg alloys excellent candidates for a wide variety of applications, from automotive transportation 1,2 to orthopedic biodegradable implants.3-6 However, Mg alloys exhibit an elevated reactivity in aqueous environments, resulting in very high corrosion rates that have limited their use so far. For this reason, intense research is being carried out with the aim of improving the corrosion resistance of these materials. Accordingly, the determination of the corrosion behavior of Mg and its alloys under certain experimental conditions of interest represents an important tool to evaluate the suitability of Mg alloys for different applications.The corrosion rate of metals can be assessed by different means, the most common being weight loss and electrochemical methods. Despite being widely utilized in corrosion rate determination, these approaches have some limitations.7 Weight loss measurements are simple and well established but inaccuracies can result from the cleaning process (i.e. overestimation or underestimation of the corrosion rate as a consequence of an insufficient or excessive cleaning after immersion, respectively). Moreover, weight loss measurements provide only an average corrosion rate and this rate will often change during the total time of experimentation. Electrochemical methods, mainly potentiodynamic polarization (from which Tafel fitting or extrapolation analysis can be performed), linear polarization resistance (LPR) measurements and electrochemical impedance spectroscopy (EIS), are reliable and reproducible methods that provide, unlike weight loss measurements, information not only about the instantaneous corrosion ra...

The increase in the rate of hydrogen evolution (HE) on dissolving Mg surfaces with increasing anodic current density or potential, which is sometimes called the negative difference effect, has been the topic of much discussion in recent years. A review of the very recent contributions to this subject is given in this paper. Increased catalytic activity of the corrosion product layer, either from the accumulated impurities or from the Mg oxy-hydroxide itself, is shown to have a minor influence on the anodic HE observed on dissolving Mg at high anodic current densities and potentials. Al exhibits similar characteristics during anodic polarization in concentrated HCl, although the anodic HE rate on Al is less than on Mg. Possible mechanisms for the anodic hydrogen are provided and implications in the area of intergranular corrosion and environmental cracking are discussed.