The Passivation of Alloy 800 in Basic Solution

Abstract

The passivation of Alloy 800 in 0.01 mol.dm^-3 NaOH has been studied at 20 and 200 degrees C using various electrochemical techniques including poten tiodynamic polarization, cyclic voltammetry, potentiostatic current transients, galvanostatic reduction and AC impedance spectroszopy. The alloy passivates spontaneously and rapidly in basic solution at 20 C. The passive film growth is a self-inhibiting process whose rate decreases exponentially with increasing film thickness. The major driving force for film growth is the high electric field present within the film. The thickness of the passive layer increases linearly with the polarization potential for given passivation time and is of the order of a few nanometers after 2x10 seconds. The film grows mostly by an increase in the thickness of a chromium-rich layer near the metal/oxide interface. At 200 degrees C, film formation and dissolution occur concurrently, although film formation dominates. The polarization resistance is low at 200 degrees C indicating that the films are less protective than at 20 degrees C and are probably porous or cracked.