Abstract: This paper discusses and analyses the electro-chemical corrosion of 17-4PHMO (OCr17Ni 4Cu4MO2Nb) steel in 1.8% H2SO4acid and 4.5% Nad solution. The test revealed - that the sample steel under normal heat treatment in Cl- and H+ solution having a stable passive film on its surface can be punctured when its anodic potential is higher than 20mV. Due to a relative positive Epp value, self-passivating at the site of corrodent Pitting arising from the punctured passive film, the passive ' film will become perfec again. Besides the microscopic structure of this steel after heat treatment is described, the relationship between heat treatment and its electrochemical property is also shown in this paper. It is concluded that pitting corrosion of this steel is hot liable to appear in acid solution with higher content of NaCl.