Comparison of Electrochemical and Thermal Evaluation of Hydrogen Uptake in Steel Alloys Having Different Microstructures

Using a light optical microscope (LOM), microstructural analysis is carried out on plain-carbon, DP600, and As-Quenched (As-Q) martensitic steels to identify the different phases interacting with hydrogen. Our recently developed electrochemical procedure, based on cyclic voltammetry (CV) and potentiostatic discharging method, is applied on these steel alloys having different phases to monitor H-uptake in the steels with respect to their microstructural features. The electrochemical method is capable of measuring diffusible H-concentration (including mobile hydrogen) for the steel alloys under H-charging condition, where hydrogen embrittlement phenomena can occur within in-service environments. The best practice in this procedure is to perform electrochemical H-measurements immediately after H-charging without interruption between steps to avoid spontaneous H-loss. Various charging times are investigated to estimate the time to near H-saturation for each steel alloy. To gain additional insights in our H-related findings, hot extraction measurements are performed to measure the diffusible H-concentration in the steels. A clear correlation between the results of hot extraction and electrochemical discharging methods is confirmed by a mathematical model, based on Fick's Law, predicting diffusible H-loss due to the time lag. Thus, under the used charging conditions, As-Q martensitic steel has been found to contain the lowest amount of diffusible hydrogen, with its near H-saturation reached after 4 hours of H-charging. DP600 is H-saturated after one hour of charging, while near H-saturation for plain-carbon steel is attained after 30 minutes. The fraction of mobile-H in plain-carbon steel is relatively higher than in DP600 steel.