In this work, the passivation behavior of Nb in acid fluoride-containing solutions with pH of 4, 5 and 6 was investigated. Both electrochemical measurements and surface characterization were systematically used to explore the pH effect on the electrochemical stability of Nb in aggressive oral cavity environment. The point defect model (PDM) and surface charge approach (SCA) were used as theoretical basis to analyze the passive film growth and dissolution kinetics. As the pH value decreases, the passive film becomes more reactive and suffers more severe dissolution, leading to higher point defect density across the passive film and more pronounced surface charge behavior at the film/solution interface. Furthermore, the surface becomes rougher and the passive film is thinner with less amount of suboxide as the solution is more acid. Since high electrochemical stability of the passive films is desirable for Nb implants, the results from this work shed insight into the use of fluoride-containing prophylactic substances. (C) 2018 Elsevier Ltd. All rights reserved.