Bubbles are key to new surface coating method for lightweight magnesium alloys

A team led by a researcher from Tokyo Metropolitan University has devised a new way of coating magnesium alloys to improve their corrosion resistance. Instead of costly, unwieldy, and slower coating techniques under vacuum, they used liquid-based chemical conversion coating with the addition of cavitation bubbles.

The resulting thick coating helped improve corrosion resistance to chlorides and mechanical properties. The team’s new technology is aimed at reinforcing lightweight materials in electric cars.

The findings are published in the journal Surface and Coatings Technology.

As the automobile industry undergoes radical changes to transition to electric vehicles, a quiet revolution in materials science is helping develop lighter materials that ensure the same batteries take the same cars over longer distances.

Magnesium alloys have been a major player in this shift, boasting the lowest density of all practical metals. However, it is far from perfect; there are worries around their corrosion resistance against chlorides (salts) and undesirable mechanical properties.

While magnesium-based composites have been proposed as an alternative, they are costly to produce and suffer from complicated manufacturing processes.

Another approach altogether is to apply a coating to conventional magnesium alloys, a series of methods known as plating. However, most examples of plating rely on the slow deposition of ceramic particles, giving rise to weak adhesion between the original substrate material and the coated layer.

Many processes also require a vacuum chamber; not only is this costly, the deposition of particles often requires high temperatures. This is hardly feasible for magnesium alloys, which have a relatively low melting point.

This inspired a team led by Assistant Professor Masataka Ijiri from Tokyo Metropolitan University to turn to chemical conversion coating, where surfaces to be coated are exposed to a liquid. But while liquid-based methods are cheaper and easier than plating, the layers created are often too thin, leading to poor corrosion resistance.

In experiments carried out in water with phosphoric acid, the team found that the addition of cavitation at the surface, the creation and subsequent violent collapse of bubbles, led to the formation of thick, even films of magnesium phosphate.

Both methods tested, using jets of bubbles fired at surfaces (water jet peening) and a multifunction method where the same jets are exposed to ultrasound waves (multifunction cavitation), led to the formation of protective layers with significantly improved properties compared to liquid treatment alone. They were able to demonstrate particularly marked improvement in corrosion resistance to chlorides, as shown using electrochemical tests.

While using whole magnesium composite parts may give desirable results, the cost of such parts means that a method to selectively and controllably apply coatings to cheaper magnesium alloys is a more feasible alternative for industry.

The team’s technology promises big strides forward in improved materials for the next generation of electric vehicles.