The effect of relative humidity change on atmospheric pitting corrosion of stainless steel 304L

•Humidity fluctuations affect atmospheric corrosion pit growth in stainless steel.•More humidity fluctuations lead to a larger population of small pits.•Constant humidity leads to the growth of larger individual pits.•Constant humidity tests give more severe pit growth than natural ambient conditions. Relative humidity fluctuations tend to influence atmospheric corrosion under natural exposure conditions. In this study, the effect of change in relative humidity on pitting corrosion of stainless steel under MgCl2 droplets is investigated with in situ X-ray microtomography and optical microscopy. Relative humidity fluctuations (between 33% RH and 85% RH or between 33% RH and 12% RH) tend to lead to nucleation of many small pits whereas continuous exposure at constant 33% RH leads to growth of a single pit. This indicates that natural fluctuations in relative humidity might be beneficial for preventing the growth of a large penetrating pit.