Effect of Heat Treatment and NC Ni Coating on Electrochemical and Hydrogen Embrittlement Behaviour of a High Strength Low Alloy (HSLA) Steel

As-received (AR) high strength low alloy (HSLA) steel in plate form was subjected to annealing, normalizing, and intercritical annealing (IA) heat treatments to produce steels with different microstructures having different grain sizes. The AR steel was coated with nanocrystalline (NC) Ni using the pulse current electrodeposition technique. An optical microscope is used to investigate the microstructures of the AR HSLA steel and various heat-treated states. The electrochemical behaviour of the HSLA steels was evaluated by polarization and electrochemical impedance spectroscopic (EIS) studies in 3.5 wt.% NaCl solution. Furthermore, hydrogen embrittlement (HE) and stress corrosion cracking (SCC) behaviour of the HSLA steel of various states were evaluated by a slow strain rate tensile test (SSRT) at a strain rate of 2 x10(-6) s(-1) in 3.5 wt.% NaCl solution. A local electrochemical cell was constructed, and a cathodic overvoltage of 250 mV vs. SCE (saturated calomel electrode) was applied to the tensile specimen by a potentiostat/galvanostat. Potentiodynamic polarization and EIS studies confirmed that NC Ni coating had enhanced the corrosion resistance of the HSLA steel. SSRT results indicated that the AR HSLA steel tends to affect by HE, whereas HSLA steels deposited with NC Ni coating are immune to HE. Scanning electron microscopy fractographic features of the failed samples also exhibited the respective fracture features.