image: This illustration shows how Shewanella algae biofilms create local oxygen-depleted microenvironments and use flavin-mediated extracellular electron transfer to reduce titanium’s protective oxide layer, resulting in passive film destabilization and accelerated corrosion.
Credit: ©Science China Press
Titanium is widely used in ships, offshore structures, aircraft and medical implants because it is remarkably resistant to corrosion. Its durability comes from an ultrathin layer of titanium dioxide that forms naturally on the metal surface. This oxide film acts like protective armor, separating the metal from the surrounding environment while blocking the electron transfer needed for corrosion reactions.
Yet one question has puzzled scientists for decades: How can microorganisms corrode titanium if its protective oxide layer is almost electrically insulating?
Researchers have now uncovered an unexpected answer.
A study led by Dr. Jiaqi Li at Northeastern University, under the supervision of Prof. Dake Xu and Prof. Derek R. Lovley, shows that marine bacteria do not attack titanium directly. Instead, they first weaken its protective oxide layer through a previously unknown bioelectrochemical process.
The team focused on Shewanella algae, a marine bacterium known for its ability to transfer electrons outside its cells. Although the surrounding seawater remained oxygen-rich, bacterial biofilms continuously consumed oxygen and created microscopic oxygen-depleted zones at the metal surface. These hidden anaerobic microenvironments allowed extracellular electron transfer to occur even under bulk aerobic conditions.
The key player turned out to be reduced flavins, naturally produced by the bacteria. Rather than acting as corrosive chemicals, flavins function as electron shuttles. They carry electrons generated during bacterial metabolism to titanium's protective oxide layer, gradually reducing stable titanium dioxide into lower oxidation-state titanium oxides. As the passive film becomes thinner and accumulates structural defects, it progressively loses its ability to protect the underlying metal, making titanium increasingly vulnerable to localized corrosion.
“We were surprised to find that the bacteria were not simply changing the surrounding chemical environment,” said Dr. Jiaqi Li, the study’s first author. “Instead, they directly transferred electrons to titanium’s protective oxide layer, fundamentally altering its chemical state and weakening its corrosion resistance.”
To verify this mechanism, the researchers combined advanced electron microscopy, surface chemical analysis, electrochemical measurements, genetic engineering and first-principles calculations. Together, these complementary approaches consistently demonstrated that flavin-mediated electron transfer destabilizes the titanium dioxide passive film. Bacterial strains engineered to produce higher levels of flavins caused even greater passive film reduction and more severe corrosion, providing direct evidence for the central role of flavins.
The discovery challenges the long-standing view that microbiologically influenced corrosion of titanium is driven primarily by acidic metabolites or other corrosive chemicals released by microorganisms. Instead, it identifies extracellular electron transfer as a direct pathway through which microbes can modify the electronic structure and chemical state of titanium's passive film.
“This discovery changes our understanding of how microorganisms interact with passive metals,” the researchers said. “Rather than only preventing biofilm formation, future corrosion-resistant materials may be designed to make the outer surface more electrically insulating.”
The findings have important implications for industries that rely on titanium under long-term service conditions, including offshore engineering, marine transportation, energy infrastructure and biomedical implants. By revealing a previously unknown bioelectrochemical corrosion pathway, the study provides a new framework for developing next-generation titanium materials with improved resistance to microbiologically influenced corrosion.