Surface Studies of Ultra-High Strength Drilling Steel after Corrosion Fatigue in Simulated Sour Environment

Abstract

Crack growth behavior in compact tension carbon steel samples under cyclic stresses in a deaerated 5% NaCl solution buffered with NaHCO3/Na2CO3, pH=12, in contact with 0.83 MPa H2S, was investigated at 20 and 85ºC, respectively. Microscopic investigation of the crack that propagated at 20ºC shows the features characteristic of corrosion fatigue whereas the crack that propagated at 85ºC exhibits features characteristic of stress corrosion cracking. Corrosion products were found inside the cracks at 20 and 85ºC. These microscopic results provide the experimental evidence that the corrosion products were plugging the cracks at both temperatures. Energy and wavelength dispersive spectroscopy analyses detected Fe and S-rich corrosion products formed inside crack at 20ºC, and Fe and O-rich products at 85ºC. This indicates that H2S played a lesser role in the degradation process at higher temperature. This result can be associated with lower dissolution of H2S in the solution at 85ºC than that at 25ºC.