On the role of grain size variation on pitting corrosion and passive film behavior of Alloy 600H

Abstract

In this work, the effect of grain size variation on the pitting corrosion as well as the passivation behavior of the Alloy 600H has been critically assessed by various electrochemical techniques. The grain size modifications have been achieved by a series of thermal and thermo-mechanical processing. The potentiodynamic polarization study has revealed that the coarse-grained specimens exhibit higher corrosion and pitting resistance than the fine-grained ones. This has been attributed to the greater extent of random high-angle grain boundary (RHAGB) surface area in the fine-grained specimens, which acted as favorable locations for pit initiation. The electrochemical impedance spectroscopy (EIS) result has indicated the formation of more imperfect (nonstoichiometric) passive layers in fine-grained specimens due to the predominant presence of high energy and defective RHAGBs surface. Moreover, the Mott-Schottky analysis has revealed that the fine-grained specimens generate higher defect densities at the interfaces, which leads to passive film instability due to increased mobility of cation interstitial and cation vacancies at the interfaces.