Galvanic Corrosion of Magnesium Alloys

Abstract

The purpose of this study was to examine the corrosion behaviour of cast Elektron 21-T6 and extruded Elektron 43-T5 magnesium alloys. The main focus was galvanic corrosion between these alloys and steel. Steel is often used in bolts and fasteners that connect different components together. While electrical contact can be eliminated by use of insulating measures, there may be situations where electrical contact does occur. The selected materials were characterised using optical and electron microscopies (SEM). Relative potential differences on the surfaces of the alloys were measured with scanning Kelvin probe force microscopy (SKPFM). Corrosion experiments included immersion and electrochemical tests. The microstructure of Elektron 21 alloy comprised equiaxed grains of the alpha-Mg matrix, Mg41Nd5 second phase at the grain boundaries, precipitates of β-phase, and Zr-rich particles surrounded with clusters of fine Zn-Zr precipitates. The microstructure of Elektron 43 alloy comprised equiaxed grains of alpha-Mg matrix and network of Mg41Nd5 second phase decorating the grain boundaries. Zr-rich and Mg24Y5 particles were identified. Bands of recrystallised grains were elongated along the extrusion direction. Relative surface potential measurements indicated that the Mg41Nd5 second phase present in both alloys had a lower potential than surrounding matrix. Similarly the Mg24Y5 phase was found anodic relative to the Mg matrix. The Zr-rich particles and the Zn-Zr precipitates had higher potentials than the matrix. Elektron 43 had the lowest OCP of -1.8 and Elektron 21 did not reach a stable potential but also remained lower than pure Mg. Potentiodynamic polarisation measurement indicated that Elektron 43 had the lowest cathodic reaction rate, and pure Mg had a higher cathodic reaction rate than both Elektron 43 and Elektron 21. Study of the cathodic reaction kinetics on pure Mg indicated that the corrosion rate increases when the surface is partly corroded. Elektron 43 was shown to have the lowest corrosion rate of 0.579 mm/year as indicated from weight loss test. The corrosion rate of Elektron 21 was 1.131 mm/year and pure Mg 86.775 mm/year. The corrosion mechanisms were proposed for Elektron 21 and Elektron 43. Galvanic corrosion had different morphologies for all the materials studied. The rate of galvanic corrosion was driven by the potential difference between the anodic magnesium material and the external steel cathode. Therefore, Elektron 43 generally suffered the highest galvanic corrosion rate. The morphology of the galvanic corrosion was different from the uncoupled corrosion. Microgalvanic interactions were overridden by the galvanic current from the steel cathode. The dissolution had a crystallographic character.

Date of Award	1 Aug 2014
Original language	English
Awarding Institution	The University of Manchester
Supervisor	George Thompson (Supervisor) & Xiaorong Zhou (Supervisor)