Chemical Mechanism of Chromate Conversion Coating on Magnesium

Advanced Surface Technologies

Conversion Coating

Protection of magnesium alloys and associated industrial processes from corrosion remains one of the key challenges. Multiple techniques have been developed, such as electrochemical plating (electroplating), conversion coating, anodizing and hybrid coatings – with electroplating being one such approach to increasing corrosion resistance of magnesium; while chemical conversion treatment offers another effective technique. Among them, chemical conversion treatment offers especially good corrosion protection in high value applications like high voltage applications; however there remain several aspects of corrosion protection processes which have received less attention – for instance surface pretreatment is paramount to success of coating systems.

Mg is well known for its superior corrosion behavior; however, moisture exposure can quickly deteriorate its integrity. The reason is due to spontaneous corrosion reactions occurring within its alloy composition; as a result a protective coating must be employed in order to isolate Mg from its surroundings; currently the most efficient means is chromate conversion coating which provides an economical and eco-friendly option; so much research effort has gone into developing one specifically tailored towards protecting Mg.

Chromate conversion coating mechanism is produced by immersing Mg substrates in an acidic pickling solution containing nitric acid. Once immersed, nitric acid reacts with Mg’s surface hydrolyzing into oxide/hydroxide films which then react with chromic acid in the bath to deposit Cr(OH), Cr(2O3), and K2CrO4 ions at the center of its grains – improving corrosion resistance by at least 100-fold compared with its original state; evidenced by XPS measurements.

Once initial chromate deposition begins, Mg(OH)-2+ and Mg(OH) are released and passivate the surface of Mg crystalline grains to create a dense yet porous film that reduces cracking of surface and tendency for formation of voids.

During the conversion process of chromate film production, some hexavalent chromium is trapped inside of it as part of its formulation. Hexavalent chromium forms strong bonds with other components within this matrix and is unlikely to easily leave Mg substrate’s crystal surfaces; this phenomenon likely accounts for its remarkable self-healing abilities under corrosion conditions.

The composition and morphology of chromate coatings depend on the pH in a phosphating bath, so in order to obtain high-performance coatings it is crucial to maximize this value by diluting an ionic liquid with organic solvents or by anodic polarization.

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