LAUSR

The destruction of Ti passive film cannot occur if fluorine ions (F − ) are below a critical concentration in an open electrolyte with oxygen acting as a passivating agent. However, in an occluded crevice, few F − combined with oxygen deficiency environment may synergistically affect the crevice corrosion of Ti alloys. Herein, the role of few F − , which is far below the critical concentration in open solution, in the crevice corrosion of Ti-6Al-4V alloy was studied using a galvanic coupling technique combined with Kelvin probe force microscopy, atomic force microscope, and X-ray photoelectron spectroscopy. The results indicate that the F − dominate the crevice corrosion process of Ti alloys by occupying oxygen vacancies to form fluoride-titanium-oxygen compounds. A multicolored product film is generated in the crevice area, and its colors vary in the order of black-blue-purple-light yellow from the crevice edge to the interior, corresponding to the corrosion degree from severe to weak. Multicolor is formed by the interference phenomena due to the different surface roughness, thickness and chemical composition of the product film. Graphic abstract