304SS heat exchanger tubes,which have been used for only two years,suffered from leakage due to corrosion.After examining the failed heat exchanger macroscopically,it was found that corrosion occurred in the crevices between sheet of 16 Mn steel and tubes of 304SS.In this paper polarization behavior of 304SS and 16 Mn was studied by electrochemical method in hot potassium carbonate solution both inside and outside the crevice. Analyses by EDS as well as pH value test were made on the surface of corroded area and uncorroded area.It is concluded that lack of V 5+ in the solution inside crevice between sheet and tubes made the 16 Mn sheet change from passive state to active state, so active passive cells were produced between 16 Mn in passivation and 16Mn in activation. Occluded cells generated inside crevice speeded up solution rate of 16 Mn inside crevice, enabled the medium to be acidated and negative ions such as Cl - to be concentrated.With the decrease of pH value and increase of Cl -,piting corrosion of 304SS occurred. With the decrease of pH value uniform corrosion of 304SS occurred. It is put forward to delay the corrosion that pre passivation of 16Mn and welding should be used in connection of sheet and tubes.