Metastable pitting corrosion of 304 stainless steel in 3.5% NaCl solution

•The pit metastable growth was in situ observed by 3-dimensional microscope.•The theoretical calculated value and measured value of pit radius were different.•The pit would turn from cone-shaped to dish-shaped during its growth.•The average lifetime and maximum of lifetime had different change trend.•The ohmic potential drop was existed between the pit surface and bulk solution.