Understanding Selected Critical Corrosion and Electrochemical Factors for the Improvement of Sacrificial Mg-Based Anodes for the Cathodic Protection of an Mg-Al-Zn Alloy

Research on magnesium (Mg) alloys has become widely popular in recent times for use in structures as a lightweight substitutional metal, as an anode material for batteries and fuel cells, as well as biodegradable implants amongst other applications. While research on Mg alloys has never been greater, the broader use of Mg has been limited in part by its poor intrinsic corrosion resistance and insufficient protection strategies which currently consist of primarily barrier type coatings.
The overall goal of this research is to produce a multifunctional metallic coating for Mg alloys which will act as (1) a corrosion barrier, (2) provide sacrificial anodic based cathodic protection in the event of a scratch or defect in the coating, and (3) provide ionic inhibitor release from the coating to heal the scratch. An ideal coating for this application must possess a low intrinsic corrosion rate, be able to supply a large amount of cathodic polarization to the metal to be protected, be non-polarizable to resist detrimental positive changes in galvanic couple potential, and contain a reservoir of inhibiting ions for storage and environmentally triggered release. Achieving a Mg-based coating with these characteristics is difficult primarily due to the limited number of alloying elements which can produce a negative electrochemical potential with respect to the substrate, manifestation of the negative difference effect (NDE) and anodically enhanced cathodic kinetics which complicate an understanding of the Mg corrosion mechanisms with respect to traditional electrochemical theory, as well as the limited number of elements which can provide inhibition of Mg while maintaining an anodic corrosion potential. This dissertation seeks to provide an understanding of selected factors which predominantly govern the performance of Mg-based anodes and how these factors can be manipulated to produce optimized cathodic protection of Mg alloys via a tunable alloy system.
As a first step, the range of cathodic potentials (i.e., the minimum level of cathodic polarization) necessary to achieve practical cathodic protection of Mg alloy AZ31B-H24 (Mg-3Al-1Zn) was explored. Cathodic potentiostatic polarization of AZ31B-H24 was performed over a range of potentials in various environments and the AZ31B-H24 dissolution rate was characterized by gravimetric mass loss measurements and analysis of the dissolved species in solution via inductively coupled plasma optical emission spectroscopy (ICP-OES). The results indicate that a one order of magnitude or greater reduction in the dissolution rate by cathodic protection of AZ31B-H24 is difficult to achieve at cathodic overpotentials of less than 100 mV below the open circuit potential (OCP). This was attributed to the persistence of self-corrosion of Mg alloys related to the negative difference effect. As such the origins and manifestation of the negative difference effect of Mg was explored for methods to minimize this detrimental effect which is an impediment to realization of cathodic protection.
The negative difference effect is distinguished by an anomalous increase in hydrogen evolution reaction (HER) rate with increasing anodic polarization. The NDE was studied by anodic potentiostatic polarization of high purity Mg in solutions of 0.6 M NaCl, 0.6 M NaCl saturated with Mg(OH)2, 0.1 M MgCl2, 0.1 M Na2SO4, and 0.1 M TRIS. These solutions provide a wide range of electrolytes and pH to enable the study of dissolving Mg anodes under a range of conditions. The dissolution behavior was characterized by combined gravimetric mass loss measurements, hydrogen evolution gas capture, charge measured by the potentiostat, and solution analysis ICP-OES. The fate of Mg and other impurity elements was elucidated through characterization of elemental enrichment on dissolving Mg surfaces by Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering Spectroscopy (RBS) while dissolution films were characterized by Raman spectroscopy. The results indicate that the NDE is a strong function of noble element enrichment and the nature and stability of the dissolution film which forms. Thus, alloying elements which can stabilize the dissolution film and decrease cathodic kinetics such as Sn should be explored for a sacrificial coating.
Next, the behavior of simulated sacrificial cathodic protection was performed by galvanic couple testing via zero resistance ammeter testing of AZ31B-H24 coupled to commercial Mg alloy WE43 (Mg-4Y-3RE, RE = Nd, La) and commercially pure Mg. The results were characterized by gravimetric mass loss measurements, the galvanic couple potential, galvanic couple current density, and enhanced cathodic kinetics. These studies revealed that the most effective sacrificial anode based protection of AZ31B-H24 was achieved with WE43 which was attributed to this alloy being able to resist anodically enhanced cathodic kinetics and its lower fraction of self-corrosion during galvanic corrosion relative to CP Mg coupled to AZ31B-H24. This validated the conclusions of the first task. With this knowledge, the choice of alloying elements for production of a tunable sacrificial coating was explored.
La3+ and Gd3+ were investigated to inhibit corrosion of AZ31B-H24 through simulated release from a coating enabled by seeding solutions with LaCl3 or GdCl3. These elements possess corrosion potentials at or below that of high purity Mg which suggests that they may be able to provide and sustain a negative corrosion potential with respect to AZ31B-H24 when these elements are alloyed with Mg. The concentration of La and Gd inhibitor ions were varied between 10-4 M to 0.2 M while maintaining a total [Cl-] of 0.6 M via compounding additions of NaCl. The results characterized by gravimetric mass loss indicate that almost a 2x decrease in corrosion occurred when the inhibitor ion concentration was below 10 2 M but the corrosion rate increased rapidly at with increasing inhibitor ion concentration at or beyond this level. Furthermore, both the anodic and cathodic kinetics increased as a function of inhibitor ion concentration which suggests a limited concentration of inhibitor in Mg would be beneficial to corrosion resistance. The dissolution trajectory of dissolving Mg as a function of pH was accurately calculated using a novel method to produce and analyze chemical stability diagrams. These analyses were successful in providing a scientific explanation for the solution inhibitor concentration effect.
The final chapter of this dissertation investigated the effect of Sn concentration of solid solution Mg-Sn alloys on the intrinsic corrosion resistance and sacrificial cathodic protection of AZ31B-H24. The corrosion resistance of Mg-Sn alloys was demonstrated to increase with increasing Sn concentration due to reductions of the hydrogen evolution reaction (HER) rate on the dissolving Mg surface which is speculated to occur via the low exchange current density for HER of Sn and the enrichment of Sn on the dissolving Mg surface which reduces the fraction of sites available for fast HER. ZRA testing revealed more than an order of magnitude decrease in dissolution rate of AZ31B-H24 with Mg-Sn sacrificial anodes. The final ranking of new improved sacrificial anode materials compared to CP Mg and WE43 follows the trend Mg-5Sn > Mg-1Sn > Mg-10Sn > WE43 > CP Mg where Mg-5Sn produced the lowest dissolution rate and greatest degree of protection of AZ31B-H24 when galvanically coupled. This is attributed to the low intrinsic corrosion rate of Mg-Sn alloys and the reduced cathodic reaction rate on the Mg-Sn alloys surface.
The synthesis of the dissertation has provided, for the first time, a framework for improving sacrificial anode based cathodic protection of Mg alloys based on a comprehensive understanding of selected critical factors which govern the behavior of Mg corrosion. Furthermore, the novel use of RBS and PIXE to analyze the composition of corroding Mg surfaces was successful and presents an opportunity for implementation in the field of corrosion research. Technologically, a new coating alloy concept for sacrificial anodic protection of a commercial Mg alloy was identified.