Investigating the Use of Mg-3Pb Alloy for Cathodic Protection of Mg Alloys
Authors: García-Corredora Marta , Reyes-Riverol Raul , Garcia-Galvan Federico R. , Barranco Violeta , Galvan Juan Carlos , Jiménez José Antonio , Fajardo Santiago 
Published: February 2024

Magnesium alloys possess many interesting properties, including high strength-to-weight ratios, good castability, and excellent biocompatibility. However, its use has been limited because they are also highly reactive and susceptible to corrosion in aqueous environments. As a result, the development of effective corrosion protection strategies for Mg alloys represents a significant challenge for many industrial applications, particularly in the aerospace, automotive, and biomedical industries.

Sacrificial cathodic protection is a commonly used strategy to enhance the durability of metallic materials. This method involves the use of an active metal (known as the sacrificial anode) in galvanic contact with the metal to be protected from corrosion, which acts as the cathode. Due to their low corrosion potential, Mg alloys have traditionally served as sacrificial anodes to protect more noble metals such as steel, aluminum, and zinc. However, the growing interest in Mg alloys for industrial applications has led to the development of effective corrosion protection alternatives for Mg alloy components. Among them, the use of Mg alloy coatings as the anode material for the sacrificial protection of other Mg alloys is an interesting strategy. This approach offers several advantages such as minimized galvanic corrosion due to their comparable electrochemical behavior, as well as enhanced adhesion between the anode and the substrate.

One of the key requirements for a metallic material to be used as a sacrificial anode is that its electrode potential must be more negative than that of the metal being protected. Additionally, the anode material must be chemically stable in the environment in which it will be located. These conditions are largely dependent on the composition of the anode material. Although recent studies have aimed to enhance the cathodic protection properties of Mg alloys for their use in industrial applications, such work remains restricted. This is due to the limited number of alloying elements with lower standard reduction potentials than Mg that are stable in solution, thereby decreasing the anode corrosion potential. In addition, Mg exhibits enhanced hydrogen evolution rates with increasing anodic polarization, which contradicts expectations of standard electrochemical kinetics and leads to significantly increased rates of self-dissolution. This phenomenon is termed anomalous hydrogen evolution (HE).

This work investigates the potential of Mg-3Pb alloy as possible sacrificial anode material for the corrosion protection of three industrially important Mg alloys: AZ31, AM60, and AZ91. For that purpose, a Mg-3Pb alloy was casted in-house and its microstructural characteristics and electrochemical behavior were studied. Furthermore, galvanic corrosion between the Mg-3Pb alloy and the industrially relevant Mg alloys, AZ31, AM60, and AZ91, was evaluated in 0.1 M NaCl solution. The choice of Pb as an alloying element was motivated by its extremely low exchange current density for the HE reaction (i0,H2,Pb) on the order of 10-12 A/cm2. This was expected to result in reduced HE rates under cathodic polarization, which shifts the corrosion potential (Ecorr) to more negative values, and during anodic polarization, hindering anomalous HE and reducing self-dissolution.

The microstructural characterization of the Mg-3Pb alloy revealed that it consisted of large α-Mg grains with the presence of a small amount of secondary phases, including Al, Si, Mn, Fe-containing intermetallics and oxide particles. Additionally, scanning electron microscopy revealed the occurrence of remarkable Pb segregation at grain boundaries.

Electrochemical measurements showed that the Ecorr associated with the Mg-3Pb alloy was consistently lower than those of the AZ31, AM60, and AZ91 alloys during 24 hours of immersion in the test solution. Moreover, potentiodynamic polarization confirmed that the Mg-3Pb alloy exhibited the lowest cathodic kinetics. In terms of anomalous HE, which is directly related to the self-dissolution rate of the Mg alloy anode material, galvanostatic polarization at different anodic current densities showed that the HE current densities were linear with respect to the applied current density. Additionally, the charge associated with the HE was in the range of 40–50% of the applied anodic charge, which is consistent with previous findings for high purity Mg.

Finally, ZRA measurements indicated that the galvanic current of the Mg-3Pb alloy remained anodic for 24 hours of immersion with no polarity reversal, indicating that the Pb-containing alloy served as sacrificial anode for the AZ31, AM60, and AZ91 alloys.

In conclusion, the use of Mg-3Pb alloy in cathodic protection systems for Mg alloys has proved to be a viable option in 0.1 M NaCl solution, offering new possibilities for its use in various applications.

Funding: This work was supported by the State Research Agency (MICINN), the Spanish National Research Council (CSIC) and European Regional Development Fund (ERDF) under the project RYC2019-027006-I (AEI/FEDER/UE).