Abstract:  The localized corrosion of structural materials under hydrostatic pressure is extremely critical for deep-sea industrial applications.Stainless steels are among the most widely used materials in deep-sea environments, but encountered significant degradation in corrosion resistance because of the hydrostatic pressure effect. However, the corrosion mechanism of stainless steels at hydrostatic pressure remains unclear. Here, the effect of hydrostatic pressure on passive films of 316L stainless steel was systematically studied using electrochemical polarizations, high-resolution TEM and XPS. Results indicated that hydrostatic pressure deteriorated the stability of the passive films by reducing the pitting incubation time. TEM and XPS results suggested that the deterioration of corrosion resistance at hydrostatic pressure resulted from the decrease in Fe(III)/Fe(II) ratio and Cr(III)-oxides as well as the structural inhomogeneity of the passive films. Additionally, the decrease in mechanical strength of the passive film also plays a role in degradation of corrosion resistance of the stainless steel under hydrostatic pressure.