Magnesium alloys have an exceptionally high strength-to-weight ratio, making them extremely appealing metals for industrial applications. Magnesium alloys are some of the lightest common alloys and have high tensile yield strength, so they’re often used for smaller metal parts in the automotive and aerospace industries.
However, their relatively high susceptibility to corrosion limits their potential to be used at scale for industrial applications. Magnesium naturally oxidizes when exposed to air and water, which is why it’s usually combined with corrosion-resistant metals like zinc to prevent oxidation. Abrasion to magnesium increase the corrosion problem further.
Thus, the best coatings treatments for magnesium corrosion usually involve barrier coatings, a coating that protects the magnesium substrate from the electrolyte-rich environment via a protective layer. Magnesium coatings must prevent the electrochemical activity that naturally occurs in the world.
Magnesium coatings options
The type of coating and application needed will depend on the magnesium part itself. In the automotive industry, parts that might be magnesium include the steering wheel, cylinder head cover, intake manifold, transmission case, seat frame, and door frames. In the aerospace industry, magnesium can be found in seat components, overhead ducts, lavatory components, and throughout the galley. The inaccessibility of these components means coating must be done correctly the first time to ensure corrosion resistance.
Magnesium coatings should provide a barrier that’s uniform, nonporous, and free of cracks. As mentioned above, a natural oxide forms on magnesium alloys, which will need to be cleaned and removed in order for a coating or electroplating to be applied.
Conversion coating is one of the most commonly used coating treatments for magnesium alloys. Conversion coatings react with magnesium to become part of the magnesium component itself. Chromate conversion coating is the most popular method, but a variety of conversion coating treatments exist including chrome-pickle treatments, dichromate treatments, chrome manganese treatments, ferric nitrate pickle treatments and phosphate treatments.
Organic coatings are another option for those looking to protect their magnesium components. A typical organic coating can be applied via conventional means and cold sprays. Within the organic coating category, powder resins, high-temperature coatings and water-based coatings are options provided alkali-resistant binders and pigments are used. Magnesium oxidation produces highly alkaline byproducts, acting as a stripper for many organic primers.
Powder resins coatings are used in a variety of applications and are typically composed of an epoxy or polyester. They are also environmentally-friendly due to the lack of solvents.
High temperature coatings are often used for aerospace and engine compartment applications. Higher temperatures require resins with a higher resistance to extreme heats. According to Magnesium.com, “the best resins for service at 260 oC to 315 oC are straight silicones, followed by silicone-modified epoxies or epoxy-phenolics. In the 200 oC to 260 oC range, various combinations of epoxies and phenolics can be used.”
Water-based coatings are necessary when EPA regulations regarding VOCs (volatile organic compounds) need to be considered. Water-based resins could include alkyds, epoxies, polyurethanes and hybrid resins.
There are a lot of options out there for coating magnesium. Consult an industrial coatings expert to find the best option for your application.