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Selecting effective corrosion inhibitors from the vast chemical space is not a trivial task, as it is essentially infinite. Fortunately, machine learning techniques have shown great potential in generating shortlists of inhibitor candidates prior to large-scale experimental testing. In this work, we used the corrosion responses of 58 small organic molecules on the magnesium alloy AZ91 and utilized molecular descriptors derived from their geometry and density functional theory calculations to encode their molecular information. Statistical methods were applied to select the most relevant features to the target property for support vector regression and kernel ridge regression models, respectively, to predict the behavior of untested compounds. We compared the performance of the two supervised learning approaches and assessed the robustness of our data-driven models by experimental blind testing.

Corrosion protection systems based on hexavalent chromium are traditionally perceived to be a panacea for many engineering metals including magnesium alloys. However, bans and strict application regulations attributed to environmental concerns and the carcinogenic nature of hexavalent chromium have driven a considerable amount of effort into developing safer and more environmentally friendly alternative techniques that provide the desired corrosion protection performance for magnesium and its alloys. Part I of this review series considers the various pre-treatment methods as the earliest step involved in the preparation of Mg surfaces for the purpose of further anti-corrosion treatments. The decisive effect of pre-treatment on the corrosion properties of both bare and coated magnesium is discussed. The second section of this review covers the fundamentals and performance of conventional and state-of-the-art conversion coating formulations including phosphate-based, rare-earth-based, vanadate, fluoride-based, and LDH. In addition, the advantages and challenges of each conversion coating formulation are discussed to accommodate the perspectives on their application and future development. Several auspicious corrosion protection performances have been reported as the outcome of extensive ongoing research dedicated to the development of conversion coatings, which can potentially replace hazardous chromium(VI)-based technologies in industries.

Although hexavalent chromium-based protection systems are effective and their long-term performance is well understood, they can no longer be used due to their proven Cr(VI) toxicity and carcinogenic effect. The search for alternative protection technologies for Mg alloys has been going on for at least a couple of decades. However, surface treatment systems with equivalent efficacies to that of Cr(VI)-based ones have only begun to emerge much more recently. It is still proving challenging to find sufficiently protective replacements for Cr(VI) that do not give rise to safety concerns related to corrosion, especially in terms of fulfilling the requirements of the transportation industry. Additionally, in overcoming these obstacles, the advantages of newly introduced technologies have to include not only health safety but also need to be balanced against their added cost, as well as being environmentally friendly and simple to implement and maintain. Anodizing, especially when carried out above the breakdown potential (technology known as Plasma Electrolytic Oxidation (PEO)) is an electrochemical oxidation process which has been recognized as one of the most effective methods to significantly improve the corrosion resistance of Mg and its alloys by forming a protective ceramic-like layer on their surface that isolates the base material from aggressive environmental agents. Part II of this review summarizes developments in and future outlooks for Mg anodizing, including traditional chromium-based processes and newly developed chromium-free alternatives, such as PEO technology and the use of organic electrolytes. This work provides an overview of processing parameters such as electrolyte composition and additives, voltage/current regimes, and post-treatment sealing strategies that influence the corrosion performance of the coatings. This large variability of the fabrication conditions makes it possible to obtain Cr-free products that meet the industrial requirements for performance, as expected from traditional Cr-based technologies.

Diverse novel energy conversion and storage systems have developed rapidly in the last few decades to address the growing demand for sustainable and environmentally friendly energy sources. In this respect, aqueous Mg-air batteries have drawn much attention for their high energy density, cost-effectiveness, and environmental friendliness. A primary Mg-air battery can theoretically provide a relatively high voltage and specific energy density of 3.1 V and 6.8 kW.h.kg-1, respectively [1]. However, the high wasteful self-corrosion rate of Mg alloys during discharge in aqueous electrolytes and fouling of the anode surface with corrosion products significantly reduce the delivered voltage and the specific energy of the Mg-air battery. These drawbacks are exacerbated by the anodic polarization due to the well-known phenomenon called Negative Difference Effect (NDE). Recently, pioneered by Höche et al. [2], the use of Mg complexing agents in the battery electrolyte has revolutionized the performance of Mg-air batteries by mitigating the mentioned shortcomings. However, the understanding of the enhancement mechanism, which may differ for different complexing agents, is at its nascent stage. In this work, sodium salt of ethylendiaminetetraacetic acid (EDTA) is used as the additive for the electrolyte of a primary Mg-air battery with a commercially pure Mg anode [3]. The electrolyte pH was taken as the main parameter to explore the effect of EDTA on the discharge performance of the Mg anode. The battery voltage was improved at all tested electrolyte pH values, which ranged from 7.0 to 11.0. The utilization efficiency (UE) of the discharged Mg anode is also enhanced by the presence of EDTA in the electrolyte at pH of 11, resulting in a 10-fold increase in the full-cell battery discharge time as compared to the reference battery in NaCl electrolyte without EDTA. Different methods, including EIS, local pH and dissolved O2 measurements, and H2 evolution tests were employed to study the interaction of EDTA with the Mg anode surface during discharge. Several mechanisms of enhancement in the UE have been concluded, including suppressing the chunk effect, weakening the detrimental NDE, and promoting the formation of a more protective layer on the Mg anode surface. The findings of this study provides an improved understanding of the interaction of the complexing agents with the Mg anode in Mg-air batteries, which further helps finding the tuning parameters to optimize its performance. [1] T. Zhang, Z. Tao, J. Chen, Magnesium-air batteries: From principle to application, Materials Horizons, 1 (2014) 196-206. [2] D. Höche, S.V. Lamaka, B. Vaghefinazari, T. Braun, R.P. Petrauskas, M. Fichtner, M.L. Zheludkevich, Performance boost for primary magnesium cells using iron complexing agents as electrolyte additives, Sci Rep, 8 (2018) 7578. [3] B. Vaghefinazari, D. Snihirova, C. Wang, L. Wang, M. Deng, D. Höche, S.V. Lamaka, M.L. Zheludkevich, Exploring the effect of sodium salt of Ethylenediaminetetraacetic acid as an electrolyte additive on electrochemical behavior of a commercially pure Mg in primary Mg-air batteries, Journal of Power Sources, 527 (2022) 231176.

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Apresentam-se exemplos da aplicação de micro-sensores potenciométricos e amperométricos para estudos de corrosão. Resume-se a base teórica de cada técnica, comparam-se as suas vantagens e limitações e discute-se as potencialidades para estudos no domínio da corrosão. The use of potentiometric and amperometric micro-sensors for studies in corrosion is illustrated. A brief theoretical background of each technique is presented, the advantages and limitations are compared and the potentialities for studies in the corrosion field are discussed.