Effect of cyclic-step quenching on the corrosion behavior of micro-alloyed steel in simulated sea water environment

Abstract

In this work, a novel heat treatment process known as cyclic-step quenching was implemented on micro-alloyed steel and its effects on corrosion resistance in simulated seawater were determined. Samples of micro-alloyed steel were prepared and categorized into five groups named control, CSQ_1, CSQ_2, CSQ_3 and SQ. CSQ_1 samples were heated in a muffle furnace to 900 °C and held at this temperature for 10 min before cooling to an intercritical temperature of 750 °C. Subsequently, the samples were quenched in vegetable oil. The procedure was conducted for CSQ_2 and CSQ_3 samples, for which it was repeated once and twice for CSQ_2 and CSQ_3 respectively. SQ samples were heated in furnace to 900 °C, held at this temperature for 10 min and quenched in vegetable oil. Samples from each category were subjected to Potentiodynamic Polarization (PDP) and Electrochemical Impedance Spectroscopy (EIS) tests in 3.5 wt% NaCl solution. Morphology and elemental composition of corroded samples in 3.5 wt% NaCl were assessed via Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS). Results obtained from PDP showed decrease in corrosion rates with increased number of cyclic-step quench cycles from 1.12 mmpy for control sample to 0.86 mmpy for CSQ_3. Similarly, data obtained from EIS equivalent circuit fitting showed increased polarization resistances with increased number of cyclic-step quench cycles from 64.73 Ω cm 2 for control sample to 371.88 Ω cm 2 for CSQ_3. Corrosion products thinning, depression and pitting showed selective dissolution of corrosion product for CSQ_2, CSQ_3 and SQ. Increase corrosion resistance for heat-treated samples was due to decrease in cathode-to-anode ratio.