THE BEHAVIOR OF PASSIVE FILMS ON CARBON-STEEL IN SULFURIC-ACID-SOLUTIONS

A kinetic study of the passive state of carbon steel in H2SO4 solution s was performed based on impedance measurements. Complex impedance dia grams with only two capacitive components were obtained in the potenti al range 0.6-1.4 V/SCE. The parameters of the passive electrode (R(HF) , C-HF, alpha and R(m)) were obtained as a function of potential. The product IR(m) of the passive current I and the transport resistance R( m) in the passive film was found to be proportional to the film thickn ess. The thickness of the passive film increased with increasing polar ization potential at relatively low potentials, but decreased with inc reasing degeneration of the outer layer of the film at high potentials . The polarization potential is lower than both trans-passive potentia l and the oxygen evolution potential.