Intergranular Corrosion Susceptibility of Alloy 600 after Autogenous Tungsten Inert Gas and Laser Beam Welding using Electrochemical Technique

Intergranular corrosion and intergranular stress corrosion cracking is influenced by precipitation of chromium carbides at grain boundaries and formation of chromium depletion regions. The present study focuses on understanding the carbide precipitation and subsequent sensitization in the weldments of Alloy 600 using two different welding techniques. The effect of heat input on microstructure and IGC susceptibility was measured using electrochemical reactivation test. The SEM studies were done to evaluate the presence of chromium depleted regions. The carbide was indentified to be Cr7C3 using TEM. The laser beam weldments showed an increased resistance to IGC as compared to TIG weldments

Electrochemical characterization of oxide film formed at high temperature on Alloy 690

High temperature passivation studies on Alloy 690 were carried out in lithiated water at 250 °C, 275 °C and 300 °C for 72 h. The passive films were characterized by glow discharge-quadrupole mass spectroscopy (GD-QMS) for compositional variation across the depth and micro laser Raman spectroscopy for oxide composition on the surface. The defect density in the oxide films was established from the Mott-Schottky analysis using electrochemical impedance spectroscopy. Electrochemical experiments at room temperature in chloride medium revealed best passivity behaviour by the oxide film formed at 300 °C for 72 h. The electrochemical studies were correlated to the chromium (and oxygen) content of the oxide films. Autoclaving at 300 °C resulted in the best passive film formation on Alloy 690 in lithiated water