2022 Volume 62 Issue 4 Pages 750-757
The pitting corrosion behavior of SBHS500 steel in boric-borate buffer solutions containing chloride ions was investigated by macroscale and microscale polarization, immersion tests, optical microscopy, and scanning electron microscopy. Calcium sulfide inclusions (CaS) existed in the SBHS500 steel. When the specimen was immersed in a boric-borate buffer solution (pH 8.0) containing 10 mM NaCl for 24 h at 25°C, the steel matrix was not corroded. However, partial dissolution of the CaS inclusions was observed. Pitting occurred after the wet-dry corrosion test, and calcium and sulfur were detected near the center of the pit. From the results of the microscale polarization measurements, the pitting initiation sites for the SBHS500 steel were determined to be CaS inclusions. No pitting was observed at the microscale electrode area without inclusions. In a boric-borate buffer solution containing 10 mM NaCl, the depassivation pH at the microscale electrode area without inclusions was 6.0. The depassivation at the microscale electrode area with the CaS inclusions occurred at an approximate pH of 6.6. The CaS inclusions in the SBHS500 steel were found to trigger the depassivation of the steel matrix surrounding the inclusions.
