The evaluation of in-service materials degradation of low-alloy steels by the electrochemical method

The nondestructive evaluation procedure for detecting in-service materials degradation of low-alloy 2.25Cr-1Mo and CrMoV steels by the electrochemical method has been investigated. The results can be summarized as follows. (1) For 2.25Cr-1Mo steels, the peak current mainly caused by the selective dissolution of coarse carbides M₆C appears at ∼+100 mV during potentiodynamic polarization measurements in dilute sodium molybdate solution. This peak value of current density, ΔI_p, can be chosen as a reflective parameter of an amount of coarse carbides M₆C and shows excellent correlations both with shifts in fracture appearance transition temperature (FATT) caused by carbide coarsening and with hardness change. Actual operational temperature can be estimated from operational period, since the Larson-Miller time-temperature parameter (LMP) value of materials has a unique relationship with ΔI_p values. (2) For CrMoV steels, the evaluation of temper embrittlement of CrMoV cast steel by a novel electrochemical technique is described. Intergranular corrosion (IGC) occurs only on temper-embrittled samples during anodic polarization process in calcium nitrate solution. The characteristic changes in polarization curves attributed to IGC have an excellent correlation with shifts in FATT caused by temper embrittlement.