Tungsten Oxide Maintains Strength of Steel Against Fouling

While we love ceramics and glass, there’s just no denying it—steel is one of the most important materials to modern living. So this is big—researchers at Harvard University’s John A. Paulson School of Engineering and Applied Sciences have devised a way to improve the ubiquitous steel by protecting its surface from fouling and corrosion.

While there are varying grades of steel today, the surface has remained largely unchanged—meaning that steel is still rather susceptible to corrosion and abrasion. Both disrupt the mechanical stability of steel, among other materials, and have a huge economic impact. According to the Wikipedia page on fouling, “one estimate puts the losses due to fouling of heat exchangers in industrialized nations to be about 0.25% of their GDP. Another analysis estimated (for 2006) the economical loss due to boiler and turbine fouling in China utilities at 4.68 billion dollars, which is about 0.169% of the country’s GDP.”

Harvard researchers have developed a scalable technique to give steel a metal oxide coating to prevent liquids from sticking to its susceptible surface. The new coating, a rough nanoporous tungsten oxide layer, “is the most durable anti-fouling and anti-corrosive material to date, capable of repelling any kind of liquid even after sustaining intense structural abuse.

To prevent performance degradation, aka mechanical instability, the team applied the tungsten oxide coating with electrochemical deposition. Instead of creating an even coating, the method grew tiny islands of the metal oxide floating on steel’s surface.

(Accelerated corrosion test, in which unmodified stainless steel (300 grade) (right sample) and the lower part of the TO-SLIPS sample with a 600-nm-thick porous TO film on steel (left sample) were exposed to very corrosive Glyceregia stainless steel etchant. (a–h) Images show corrosion evolution as a function of contact time.)

While that may sound like a weakness for the coating, one of the researchers points out just how valuable those islands are: “If one part of an island is destroyed, the damage doesn’t propagate to other parts of the surface because of the lack of interconnectivity between neighboring islands. This island-like morphology combined with the inherent durability and roughness of the tungsten oxide allows the surface to keep its repellent properties in highly abrasive applications, which was impossible until now.”

The material is tested by scratching it with stainless steel tweezers, screwdrivers, diamond-tipped scribers, and pummeling it with hundreds of thousands of hard, heavy beads,” according to the release. “Then, the team tested its anti-wetting properties with a wide variety of liquids, including water, oil, highly corrosive media, biological fluids containing bacteria and blood. Not only did the material repel all the liquid and show anti-biofouling behavior but the tungsten oxide actually made the steel stronger than steel without the coating.

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