Researchers from Rice University (Houston, Texas, USA) and the South Dakota School of Mines and Technology (Rapid City, South Dakota, USA) have developed an insulator of sulfur and selenium that they say can be a boon for infrastructure—including bridges, buildings, and anything above or below the water made of steel—that requires protection from the elements.
Developed by Rice University materials scientist Pulickel Ajayan, the insulator compound proved itself more dielectric (i.e., insulating) than most flexible materials and more flexible than most dielectrics, making it a good candidate for components in electronics like bendable cellphones. But it then occurred to Rice researchers that there may be more additional applications for this material.
“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. “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 and co-lead author M.A.S.R. Saadi.
As the researchers point out in their Advanced Materials study, the sulfur-selenium compound combines the best properties of inorganic coatings like zinc- and chromium-based compounds that bar moisture and chlorine but not sulfate-reducing biofilms, as well as polymer-based coatings that protect steel under abiotic conditions but are susceptible to microbe-induced corrosion.
In the first test of the material, the Rice University lab coated small slabs of common mild steel with the sulfur-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 changes, but the bare steel rusted significantly, according to the researchers. They also reported that the sulfur-selenium coating proved highly resistant to oxidation while submerged.
In a subsequent test, the researchers studied the effects of sulfate-reducing bacteria on coated and uncoated samples by exposing them to plankton and biofilms for 30 days. Such bacteria are known to accelerate corrosion up to 90 times faster than abiotic attackers, yet researchers calculated a 99.99% inhibition efficiency for the coating.
Researchers found that the Rice compound also performed well compared to commercial coatings with a similar thickness and exhibited notable self-healing properties. After cutting a film in half and placing the pieces next to each other on a hotplate, the researchers saw separated parts reconnect 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 proved their ability to protect steel just as well as pristine coatings.
“If you give the alloy a poke, it recovers,” Rahman says. “If it needs to recover quickly, we assist it using heat. But over time, most thick samples will recover on their own.” Rahman adds that the lab still needs to test whether thin layers will heal without assistance.
According to Ajayan, his lab is tweaking the material for 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,” he says. “There are opportunities.”
Source: Rice News, https://news.rice.edu.