12 November 2021
by Andrea Gaini

A flexible, self-healing material protects steel from elements

An insulator of sulphur and selenium made with flexible devices in mind works an anticorrosive coating for steel.

Rice University graduate student M.A.S.R. Saadi coats a compound of sulphur and selenium onto steel to test its ability to protect the surface from biotic and abiotic contaminants
Rice University graduate student M.A.S.R. Saadi coats a compound of sulphur and selenium onto steel to test its ability to protect the surface from biotic and abiotic contaminants © Jeff Fitlow/Rice University

The compound developed by the Rice University, USA, materials scientist Pulickel Ajayan proved itself more dielectric (insulating) than most flexible materials and more flexible than most dielectrics, making it a good candidate for components in electronics like bendable cellphones.

‘Even before we reported on the material for the first time, we were looking for more applications,’ says materials scientist Muhammad Rahman, principal investigator on the study.’So we thought, let’s put it in salt water and see what happens,’ he explains.

Researchers claim that the results of experiments at could be a boon for infrastructure – buildings, bridges and anything above or below the water made of steel – that requires protection from the elements.

The researchers pointed out that sulphur-selenium combines the best properties of inorganic coatings like zinc- and chromium-based compounds that bar moisture and chlorine ions but not sulphate-reducing biofilms, and polymer-based coatings that protect steel under abiotic conditions but are susceptible to microbe-induced corrosion.

In the first test of the material, the lab coated small slabs of common ‘mild steel’ with the sulphur-selenium alloy and, with a plain piece of steel for control, sank both into seawater for a month. The coated steel showed no discoloration or other change, but the bare steel rusted significantly, they reported. The coating proved highly resistant to oxidation while submerged.

To test against sulphate-reducing bacteria, which are known to accelerate corrosion up to 90 times faster than abiotic attackers, coated and uncoated samples were exposed for 30 days to plankton and biofilms. The researchers calculated an ‘inhibition efficiency’ for the coating of 99.99%.

The compound also performed well compared to commercial coatings with a similar thickness of about 100μm, easily adhering to steel while warding off attackers.

Finally, they tested the alloy’s self-healing properties by cutting a film in half and placing the pieces next to each other on a hotplate. The separated parts reconnected into a single film in about 2 minutes when heated to about 70˚C and could be folded just like the original film. Pinhole defects were healed by heating them at 130˚C for 15 minutes.

Subsequent tests with the healed alloys proved their ability to protect steel just as well as pristine coatings. The lab is tweaking the material for varieties of steel and looking into coating techniques.

Authors

Andrea Gaini