The excellent properties of magnesium (Mg) and its alloys, such as high strength to weight ratio, good thermal and electrical conductivity, excellent vibration and shock absorption, castability, weldability, recyclability, biodegradability, and biocompatibility, makes them the ideal choice for many applications ranging from automotive, aerospace, defence, electronics, construction and biomedical to various consumer products. However, the susceptibility to aqueous and galvanic corrosion hinders their widespread application in several areas. Among the different methods considered to control corrosion of Mg alloys, surface coating approaches are perhaps the simplest and straightforward ones. Mg alloys, in practice, require a proper surface treatment to combat the corrosion issues [1, 2].

Among the various coating methods available, the conversion coatings are highly significant as they are mainly employed as a corrosion inhibiting base layer to the commercial barrier coating [2]. Most of the conversion coatings in practice are inorganic compounds-based such as chromates, phosphates and rare-earths. However, their use is restricted due to disadvantages such as toxicity, heavy metal contamination, non-biodegradability and cost. An environmentally friendly and economical alternative is non-toxic and biodegradable organic compounds-based conversion coatings [3]. These organic compounds with carboxylic/other functional groups can effectively chelate with the surface hydroxide layer of Mg. They can provide anchoring groups for the polymer overcoat and act as hosts for nanoreservoirs with loaded inhibitors for active corrosion protection [3, 4].

The presentation provides a comprehensive outlook on organic conversion coatings for Mg and its alloys, emphasising experimental parameters determining the deposition, mechanism of formation, effect of pre and post-treatments, composite coating strategies, and electrochemical corrosion behaviours of coated alloys. Related approaches and existing challenges will be debated.

References

1. E.F. Emley, Principles of magnesium technology, 1966, Pergamon Press, ISBN: 978-0080106731.
2. V.S. Saji, T.S.N. Sanakara Narayanan, X.B. Chen, Conversion Coatings for Magnesium and its Alloys, Springer Nature, ISBN: 978-3-030-89976-9, 2022.
3. V.S. Saji, Organic conversion coatings for magnesium and its alloys, J. Ind. Eng. Chem. 75 (2019) 20–37.
4. V.S. Saji, Supramolecular concepts and approaches in corrosion and biofouling prevention, Corr. Rev. 37 (2019) 187-230.

Figure Caption

Figure 1: A pie chart showing organic conversion coatings for magnesium alloys.