Abstract: One of the most aggressive organic acids for stainless steel is formic acid. In particular, the corrosion of 316L stainless steel in aqueous formic acid solutions at high temperatures is directly related to its safe operation and production efficiency. To better understand the passivation-activation transition behavior of 316L stainless steel in aqueous formic acid solutions, an investigation was conducted at the formic acid mass fractions of 0.5%, 5%, 15%, and 30% at 90 ℃. Laboratory immersion tests with a period of 1200 hours were performed at each formic acid mass fraction to document the corrosion rates and the corrosion morphologies of 316L stainless steel, and electrochemical tests, including open circuit potentials and anodic polarization curves, were conducted in the presence of dissolved oxygen using conventionally divided glass cells with three electrodes. The influences of formic acid mass fraction on corrosion rate, corrosion morphology, open circuit potential, primary passivation potential, critical current density, passive current density, and passive film breakdown potential were analyzed. In addition, the effects of H⁺ and HCOO⁻ ions on anodic reactions occurring in the active region, the active-passive transition region, and the passive region were discussed. Due to the stability of the passive state, the laboratory immersion tests showed that at the formic acid mass fractions of 0.5%, 5%, and 15%, 316L stainless steel suffered from slight corrosion, and thus no measurable weight losses could be acquired. However, in the 30% aqueous formic acid solution, the corrosion rate of 316L stainless steel reached 1.2 × 10⁻³ mm·a⁻¹, which indicated that 316L stainless steel was in the active state and the passivation-activation transition had occurred. The corrosion of 316L stainless steel in aqueous formic acid solutions is characteristic of non-uniform generalized corrosion. According to the results of electrochemical tests, with an increasing mass fraction of formic acid, the open circuit potentials and the primary passivation potentials became nobler, the critical current densities and the passive current densities increased, and the passive film breakdown potentials shifted to negative values. It is suggested that the passivation-activation transition of 316L stainless steel in aqueous formic acid solutions may be due to the competitive adsorption between HCOO⁻ and OH⁻ ions. Therefore, formic acid mass fraction increased, anodic dissolution accelerated, the formation of passive film was delayed, and corrosion susceptibility increased. In short, the concentration of formic acid significantly influences the corrosion behavior of 316L stainless steel from passivation to activation.