Abstract
Anodic polarization curves and the susceptibility to stress corrosion cracking (SCC) have been studied for Haynes Alloy No. 25 (0.1% C-20% Cr-15% W-10% Ni-bal. Co) in aged (TP-1 specimen) or overaged (TP-2 specimen) conditions. The effects of overaging heat treatment of the aged specimen (TP-1) on the anodic polarization curves and SCC susceptibility were examined, and the relation between the electrochemical behavior and the susceptibility to SCC has been discussed. The results obtained are as follows: (1) The aged specimen (TP-1) has lower corrosion rates than that the overaged specimen (TP-2) in boiling 1mol/l H2SO4 solution. (2) The polarization behavior in 30% MgCl2 solution at 80°C was greatly affected by aging condition. The aged specimen (TP-1) exhibited active-passive behavior with low passive current densities in contrast to totally active dissolution of the overaged specimen (TP-2). (3) Transgranular stress corrosion cracking was found in C-ring aged specimens (TP-1) immersed in boiling 42% MgCl2 solution, while no cracking was found in overaged specimens (TP-2) without passivity in MgCl2 solution. (4) Overaging treatment of previously cold worked aged specimen (TP-1)at 800°C for more than 0.1h resulted in disappearance of a passive region in polarization curve and no SCC in MgCl2 solutions. These overaging effects can be explained on the basis of the relationship between the preferential precipitation of Cr carbides at slip lines and the concentrations of metallic Cr and W required for passivation in matrix.
