Behind the Science: Dissolved Oxygen Influences Magnesium Corrosion

Dr. Kate Lawrence, Editor of ChemistryOpen, talked to Dr. Eduardo L. Silva, Helmholtz-Zentrum Geesthacht, Germany, about his recent article on the role of dissolved oxygen in magnesium corrosion.

Why is your attention focused on magnesium?

Magnesium is the lightest structural metal, with two-thirds of the density of aluminum. Coincidentally, it also has mechanical properties very similar to those of human bone and is important for bone health. It is biocompatible and very prone to corrosion in aqueous electrolytes.

And you are interested in corrosion.

Yes. Corrosion is generally unwanted in most structural applications. However, it is actually the working principle for biodegradable implants. Contrary to permanent implants, their biodegradable counterparts are designed to degrade while giving place to the original tissue, while it is healing from injury.

Due to the above-mentioned characteristics, magnesium is currently the most promising biodegradable metal. However, for such implants to have the desired effect, magnesium corrosion needs to proceed in a controlled manner, i.e., be regulated according to the healing rate of the surrounding tissue.

However, magnesium corrosion can be a very complex process, and there is currently a lack of agreement among the scientific community regarding some aspects of its degradation. One of the topics requiring further research is the significant difference in corrosion rate between in vivo and in vitro measurements.

Why did you study the role of dissolved oxygen?

Usually, when we talk about metallic corrosion, this means that the metal is oxidized and transfers electrons to the reduced species, usually dissolved oxygen or water, in neutral or alkaline electrolytes. In the case of magnesium, there is significant hydrogen evolution as a result of water reduction. However, our measurements with dissolved oxygen have been showing oxygen depletion on the surface of magnesium when immersed in aqueous electrolytes. Therefore, we understood that it would be important to investigate the cause and extent of this depletion and how it can affect magnesium corrosion.

Which part of this work proved the most challenging?

Since hydrogen evolution occurs during magnesium corrosion, it was possible that the observed oxygen depletion was merely due to a purging effect by the action of H₂. After testing this hypothesis with a specifically designed experiment, we have concluded that the observed oxygen depletion is much higher on the near-surface of corroding magnesium specimens than when considering purging effects alone. It was important to clarify this in order to assign oxygen reduction as the major cause of depletion.

Which techniques did you use?

While some metals tend to undergo uniform corrosion, others—such as magnesium—are very prone to localized corrosion, and must be characterized with localized corrosion techniques. These include a number of micromanipulated techniques that roughly consist on scanning the near-surface of metals with specific microsensors under electrolyte immersion conditions. High-resolution measurements of variables such as ionic currents and localized pH can be obtained with techniques such as the Scanning Vibrating Electrode Technique (SVET), and the Scanning Ion-selective Electrode Technique (SIET), respectively. For dissolved oxygen measurements, microamperometry or a scanning micro-optode can be used.

What is the broader impact of this paper for the scientific community?

The mechanisms behind the corrosion of magnesium are still far from being universally accepted. The contribution of our work was to show that dissolved oxygen reduction occurs during immersion of the metal in an electrolyte, and, therefore, it must be taken into account for the corrosion mechanism of magnesium. These observations are expected to be most relevant in conditions where the available oxygen concentration is different from normal aeration conditions, such as in physiological environments.

Thank you very much for the interview.

The article they talked about

• The Reduction of Dissolved Oxygen During Magnesium Corrosion,
  Eduardo L. Silva, Sviatlana V. Lamaka, Di Mei, Mikhail L. Zheludkevich,
  ChemistryOpen 2018, 7, 664–668.
  https://doi.org/10.1002/open.201800076