Effect of single and double tempering on corrosion behavior of modified CA6NM steel in 3.5% NaCl solution

Abstract

The modified Ca6NM cast martensitic stainless steel (MSS) was subjected to heat treatment to explore the effect of tempering temperature and cycle on its corrosion behavior in a 3.5% NaCl solution. The metallographic observation, open circuit potential (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were conducted to analyze the microstructure and corrosion behavior of the tempered samples. As a result of OCP and polarization test, the more positive free corrosion potential (~160 mV) and the larger passivation (~703 mV) were obtained by single tempering at 500℃ (ST-500), meaning the better stability of passive layer formation. In general, the corrosion rate decreases as the temperature rises at single tempering, reaching a maximum at 550℃ (0.004 mmpy), and then gradually rises in high temperature. Contrarily, double tempering tends to decrease with the temperature rising both in single and double tempering. However, double tempering has detrimental effects and barely increases the pitting resistance since the absence passivation. Conforming to the EIS analysis, the higher Rct value (244 kΩ) and broader arc radius directly correlate to the oxides and hydroxides layer on the surface. The SEM observations shown that a single tempering process created the perforated pit cap at low temperatures, and the pit wasn’t really visible after ST-550C.