(Invited) Selective Response of Magnesium Corrosion Inhibitors Incorporated in PEO Coatings

Abstract

Corrosion inhibitors are the integral components of protective coatings used by aerospace, automotive, and consumer goods industries. Despite recent achievements in discovering new, effective magnesium corrosion inhibitors [1,2], the quest to replace the highly effective but carcinogenic Cr(VI) from corrosion inhibiting processes continues and has now become urgent. Plasma electrolyte oxidation (PEO), also known as Micro-Arc Oxidation (MAO), is widely used to form coatings on magnesium with reasonable wear and corrosion resistant properties. Unexpectedly, we found out that not all of the corrosion inhibitors, efficient for a bare magnesium alloy, can effectively inhibit its corrosion once loaded in PEO coating. In an effort to unravel the mechanism behind this phenomenon and to formulate the criteria for selecting inhibitors that maintain their efficiency in PEO coatings, a number of inhibitor-loaded PEO coatings have been synthesized. Several electrochemical and surface analysis techniques were used to thoroughly characterize the reference PEO, those loaded with the corrosion inhibitors, and compare all PEO-based coatings with the Chromate Conversion Coating as an industrial reference.Electrochemical Impedance Spectroscopy (EIS) was the main method to rank the corrosion protection properties of the inhibitor-loaded PEO coatings. The inhibitors were ranked based on their ability to suppress the corrosion onset of inhibitor-loaded PEO coating. ToF-SIMS and XPS characterization exposed that specific magnesium corrosion inhibitors can promote fast formation of conversion layer that assures further corrosion inhibition. This, however, does not occur on the ceramic-like surface of PEO that, at first approximation, can be represented as MgO/Mg(OH)2. On the contrary, effective adsorption or precipitation inhibitors typically maintain their inhibiting efficiency once loaded in PEO coatings.In particular, one of the salicylate derivatives, 4-methyl-salicylate, shown to be prone to chemisorption on Mg [3,4], revealed high inhibiting efficiency on both: bare and PEO treated magnesium. The SVET measurements coupled with time lapse optical microscopy revealed the initial stages of PEO degradation followed by the outbreak of filiform corrosion and its decay observed only for the sample protected by the inhibitor-loaded PEO coating. The reference PEO sample resisted the corrosion outbreak for ca. 45h, which is only 10 hours less that the inhibitor-loaded PEO sample, but once the filiform corrosion started it remained in its active phase for the duration of the measurement, 94 hours. Surprisingly, the sample protected by the Chromate Conversion Coating revealed persistent, albeit moderate, corrosion activity throughout the measurement. In contrast, the sample containing the corrosion inhibitor, 4-methyl-salicylate, demonstrated the active corrosion protection evident from the decay of the initial filiform corrosion. The second event of filiform corrosion also faded away after some hours of activity. This clearly demonstrates the active corrosion protection provided by the inhibitor-loaded PEO coating. The inhibitor loaded PEO coating revealed a corrosion protection superior to that provided by the industrially accepted Chromate Conversion Coating.The criteria of an effective corrosion inhibitor for PEO-coated magnesium will be discussed based on the inhibitor interaction with the bare magnesium and PEO coating. Acknowledgement The authors acknowledge the financial support of the following projects: H2020 Clean Sky 2 “Almagic” Grant Agreement Number 755515, and HZG IDEA project MMDi. Di Mei thanks China Scholarship Council for the award of fellowship and funding (No. 20160704051).