An insulator of sulfur and selenium made with flexible devices in mind has found another important use: as an anticorrosive coating for steel. The compound 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. But when placed in salt water, the viscoelastic coating was found to be self-healing and could protect steel from the elements.
Sulfur-selenium combines the best properties of inorganic coatings like zinc-and chromium-based compounds that bar moisture and chlorine ions but not sulfate-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 team 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 change but the bare steel rusted significantly. The coating proved highly resistant to oxidation while submerged.
To test against sulfate-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 percent. The compound also performed well compared to commercial coatings with a similar thickness of about 100 microns, 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 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.
The team still needs to test whether thin layers of about 100 microns will heal without assistance. They are tweaking the material for varieties of steel and looking into coating techniques. While the first target is structures, the electronics industry faces some of the same problems with corrosion.
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