Galvanic Corrosion between Magnesium Alloys and Steel

Abstract

The corrosion behaviour of pure magnesium, cast Elektron 21-T6, and extruded Elektron 43-T5, galvanically coupled to mild steel, has been investigated. The coupling current and potential were measured in selected concentrations of sodium chloride solutions in order to establish the effects of the environment and the alloy type on the galvanic corrosion behaviour. The time evolution of the galvanic currents was related to the results of potentiodynamic polarisation and to the corrosion morphologies revealed on specimens comprising a steel bolt and washer after salt spray testing. During the galvanic coupling, the measured current for pure magnesium was less than that for Elektron 21; the highest current was measured for Elektron 43, suggesting the highest corrosion rate. These results were in agreement with the behaviour expected from the individual potentiodynamic polarisation curves. Concerning the corrosion morphology of the galvanic couple, for Elektron 43, the corrosion close to the washer was relatively deep but extended only about 10 mm. For Elektron 21, the corrosion was shallower but advanced further (up to 15 mm). For pure magnesium, a relatively deep and laterally spread attack (approximately 20 mm around the bolt) was developed. These results suggest differences between observed corrosion in galvanically coupled assemblies and corrosion rates expected based on coupling currents alone.