Studying and comparing the erosion-enhanced pitting corrosion of X52 and X100 steels

The critical ratio of Cl- to HCO- concerning the corrosion rate has been determined with and without electrolyte erosion in 0.3 M NaHCO3 by using various concentrations of NaCl for X52 and X100 steels. The open circuit potential (OPC) was recorded for both steels in electrolytes containing 0.3 M NaHCO3 and 0.1 M NaCl without erosion, with electrolyte erosion, and with electrolyte-silica sand erosion. The electrochemical techniques like linear polarization resistance (LPR) and potentiodynamic polarization scans were used to study and compare the material resistance against synergistic impact of erosion and corrosion. Scanning electron microscope (SEM) and optical microscope were used to study the morphology of pitting corrosion in the absence and presence of erosion. The change in pitting potential due to electrolyte and electrolyte-silica sand impingement has been discussed. The microstructure and hardness of X52 and X100 steels were evaluated and discussed with respect to the erosion-corrosion behaviors. Cracks were observed at the inner wall of a pit because of silica sand impingement during erosion-enhanced pitting corrosion. X100 steel is more stable than X52 steel against pitting corrosion with and without erosion. X52 steel suffered pitting corrosion in standstill electrolyte containing chloride where as X100 not even in the presence of erosion. Pits propagate in the direction of erosion because of the impingement and disruption of the passive layer in that direction.