Steely Glaze: Layered electrolytes control corrosion

Metals may someday rely on gossamer guardians. Ultrathin organic coatings offer a new way to slow the corrosion of steel, recent experiments suggest.

Salt and moisture catalyze corrosive reactions of oxygen with metals.

Microscopic defects on metal surfaces create sites where salt ions carry electric currents and trigger these oxidizing reactions.

Traditional anticorrosion coatings such as paints and resins work by keeping salt and metal physically separated. However, the coatings often don’t adhere well to the charged surfaces of metals, so they’re prone to peeling and flaking.

Organic films known as polyelectrolyte multilayers, on the other hand, tightly adhere to charged surfaces. That’s why these coatings could offer metals superior corrosion protection, according to Joseph B. Schlenoff and Tarek R. Farhat of Florida State University in Tallahassee.

Polyelectrolyte multilayers are gel-like films of alternating layers of positively and negatively charged molecules. Opposing charges pair up, holding adjacent layers together. A positively charged bottom electrolyte layer also sticks to the negatively charged metal surface. Researchers have identified possible applications for these multilayers in the manufacture of products including contact lenses and drug-bearing capsules.

To see whether the films could also inhibit corrosion, Schlenoff and Farhat deposited dozens of alternately charged layers of polyelectrolytes on millimeter-wide stainless steel wires. The process created a 70-nanometer-thick film on each wire. The researchers immersed these treated wires, as well as untreated ones, in saltwater. Then, to accelerate natural processes that might oxidize the steel, the Florida State scientists created electric potentials between the water and steel.

The treated wires suffered considerably less corrosion at all voltages, the researchers report in the April Electrochemical and Solid-State Letters.

Schlenoff suggests that manufacturers could inexpensively line pipes with anticorrosive multilayers by alternately pumping solutions of oppositely charge polyelectrolytes through the pipes before they’re used.

The study shows for the first time that polyelectrolyte multilayers may have applications in corrosion control, says Gero Decher of the National Center for Scientific Research in Strasbourg, France, who is also investigating multilayers for anti-corrosion applications.

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