The initiation of pitting corrosion of type 304 stainless steel under a thin electrolyte layer containing chloride ions has been investigated. 1 M MgCl2 solution layers with four different thicknesses of 1000, 500, 125, and 25 mu m were placed on the stainless steel electrode surface and then dried at different rates at 300 K. During drying, the corrosion potential E-corr was monitored to determine the time when the pitting corrosion commenced. A solution resistance R-s of the thin electrolyte layer was simultaneously measured to monitor the concentration of chloride ion. From both monitoring data of Ecorr and Rs, critical concentration of chloride ion [Cl-](pit) for pit growth under thin chloride solution layers during drying was determined. The pitting corrosion mechanism of type 304 stainless steel during the drying process is discussed on the basis of these results. (c) 2005 The Electrochemical Society.