Rice University graduate student M.A.S.R. Saadi coats a compound of sulfur and selenium onto steel to test its ability to protect the surface from biotic and abiotic contaminants. Photo: Jeff Fitlow/Rice University.An insulator of sulfur and selenium made with flexible devices in mind may have found its true destiny: as an anticorrosive coating for steel.
The compound, developed by the Rice University lab of 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.
At the same time, the material had its creators thinking: what else can it do?
“Even before we reported on the material for the first time, we were looking for more applications,” said materials scientist Muhammad Rahman, principal investigator on the study and an assistant research professor of materials science and nanoengineering in the George R. Brown School of Engineering at Rice University. “So we thought, let’s put it in salt water and see what happens.”
“Atop all that, we found the viscoelastic coating is self-healing,” adds Rice graduate student M.A.S.R. Saadi.
The results of experiments on the material at Rice and the South Dakota School of Mines and Technology could be a boon for infrastructure – buildings, bridges and anything above or below the water made of steel – that requires protection from the elements. Ajayan and his colleagues report the results in a paper in Advanced Materials.
According to the researchers, the sulfur-selenium alloy combines the best properties of inorganic coatings made of zinc and chromium, which bar moisture and chlorine ions but not sulfate-reducing biofilms, and polymer-based coatings, which 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 sulfur-selenium alloy. Then, with a plain piece of steel for control, they sank both steel samples into seawater for a month. Whereas the bare steel rusted significantly, the coated steel showed no discoloration or other change. The coating proved highly resistant to oxidation while submerged.
To test against sulfate-reducing bacteria, which are known to accelerate corrosion by up to 90 times compared with 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 Rice compound also performed well compared to commercial coatings with a similar thickness of about 100µm, easily adhering to steel while warding off attackers.
Finally, the researchers 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 two minutes when heated to about 70°C (158°F) and could be folded just like the original film. Pinhole defects were healed by heating them at 130°C (266°F) for 15 minutes. Subsequent tests with the healed alloys demonstrated their ability to protect steel just as well as pristine coatings.
“If you give the alloy a poke, it recovers,” Rahman said. “If it needs to recover quickly, we assist it using heat. But over time, most thick samples will recover on their own.” He added that the lab still needs to test whether thin layers of about 100µm will heal without assistance.
The lab is now tweaking the material for different varieties of steel and looking into coating techniques. “The first target is structures, but we’re aware the electronics industry faces some of the same problems with corrosion,” Ajayan said. “There are opportunities.”
This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.