An electrochemical monitoring cell consisting of two multi-holed plates sandwiched by a dielectric foil has been developed as corrosion sensors in the authors' lab. Based on the cell current and its accumulated quantity the mean or instantaneous corrosion rates of metals in gases can be measured and recorded continuously. In this paper the construction and characteristics of the sensors are described. For the carbon steel sensor in electrolytic cell,the quantitative relation between cell current and true weightloss defined as the cell's factor was studied. In addition,some applications of above sensors were illustrated and the results of on-line monitoring tests of corrosion in atmospheres or in industrial exhaust gases or inside natural gas pipeline were reported respectively.