Corrosion behavior of oil tube steel in simulant solution with hydrogen sulfide and carbon dioxide

Electrochemical measurement techniques, X-ray diffraction and scanning electron microscopy were applied to investigate the corrosion behavior of N80 tube steel in simulant static solution with carbon dioxide (CO₂) and hydrogen sulfide (H₂S) at a temperature of 100 °C. Sweet corrosion occurred when a very small partial pressure of H₂S was added. At this condition, uniform corrosion was found. The added H₂S only accelerated the general corrosion rate. Sour corrosion was primary as the partial pressure of H₂S increased to 0.010 MPa. The general corrosion rate decreased quickly, but severe pitting was found. The corrosion scale, mainly composed of coarse grains of mackinawite (FeS_1-x), was loose and brittle. In sour corrosion, general corrosion rate decreased slowly and pitting became slight with increasing partial pressure of H₂S because the primary corrosion product, fine grains of pyrrhotite (FeS_1+x), made the scale more compact and continuous.