Modified 13 Cr Martensitic Stainless steels (UNS S41426) are used extensively in the oil and gas industry as wellbore components in sweet and moderately sour service applications. The synergistic effects between H2S activity and temperature have a detrimental effect on the passive film stability. Electrochemical tests were performed using a 1L autoclave holding 35 bar of CO2 gas and varying H2S activity at 25°C and 150°C at a pH of 3.5. The pitting susceptibility is enhanced in regions along Ti(C,N) inclusions and regions where defective sulfide films generate local galvanic cells around regions with limited protection along the steel surface further driving the pitting process. Both DC and AC electrochemical techniques were correlated with microstructural characterization to observe the extent of passivation on the surface. Some service limits under acidic conditions are proposed based on the electrochemical results.