A test procedure was designed and employed for exposure and degradation testing of well cements in a simulated downhole environment of offshore deep drilling. A Class H cement sample was crushed to coarse grains and was exposed to 5 % mass NaCl(aq) solution in contact with CO2 at 100 °C and 10 MPa for 200 hours. Key ionic species were identified from the aqueous phase from the testing to develop the cement synthetic pore solution (CSPS) recipe. The CSPS was then used for corrosion characterization of high chromium well casing steel at the same conditions of the cement exposure for 60 hours. The corrosion potential in the measurements ranged between-0.4 and-0.5 V vs. SHE. The average corrosion rate of the steel samples was on the order of 0.1 mm/year, representing a worst-case scenario for degradation of the cement material, but significant for critical steel components like a well casing.