Study of Corrosion Inhibitors for Mg Alloy AZ31B with Variations of PCO2 and Electrolyte Layer Thickness

Magnesium alloys have been proven as useful, lightweight, engineering materials in several applications such as aerospace transmission casings and enclosure frames for general consumer electronics.  Automotive industry has also shown much interest in this material both in the past and recent years.  Yet, the full applicability of magnesium alloys has not yet been achieved due to their active corrosion in common media.

 One way to act upon the corrosion of magnesium alloys is by corrosion inhibiting chemical compounds.  Such compounds, when effective at low concentrations, are ideal candidates for coatings-based inhibitor systems provided that they can be stored and released by ion-exchange pigment additions to the coating.  To date, corrosion inhibitor studies for magnesium alloys have almost entirely relied on techniques based on immersion of specimen in bulk solution.

 In the present work, it is shown that the corrosion product and effectiveness of several inorganic inhibitor compounds on AZ31B, such as sodium metavandate, vary with both electrolyte layer thickness and partial pressure of CO₂.  Characterization of corrosion product layers indicates tendency for the formation of magnesium carbonates rather than Mg(OH)₂ (brucite) when specimen are exposed to electrolyte layers as thick as 250µm compared to test conditions of the same CO₂ partial pressure in bulk solution.  Furthermore, it is shown that in some cases inhibitor compounds exhibit change in speciation with the changing parameters. It is well known that corrosion inhibition by inorganic ions is often particular to oxidation state and speciation of the compound such as is the case for tetrahedral coordinated vanadate species. Hence corrosion inhibitor effectiveness is altered by accessibility of CO₂ and electrolyte layer thickness.

 Lastly, synergistic effects of exposure cnditions with corrosion inhibitor compounds is discussed and examples given.   These conclusions highlight the importance of replication of likely field exposure environment in design of lab experiments for magnesium alloys.  Such information is crucial to the future development of coatings-based inhibitor systems.