Ionization behavior of deep level donors in passive film formed on surface of stainless steel in 5% salt solution

Abstract

Deep level donor’s ionization behavior of passive film formed on the surface of stainless steel was investigated by Mott-Schottky plots. It is indicated that transformation process of deep level donors’ ionization behavior of passive film on surface of stainless steel can be divided into 4 stages with rising immersion time. At the initial immersion stage (10 min), Fe(II) located in the octahedral sites of the unit cell is not ionized and the deep level does not appear in Mott-Schottky plots. At the second stage (9–38 h), Fe(II) located in the octahedral sites starts to be ionized, which results in deep level donors’ generation and density of deep level donors almost is constant with augmenting immersion time but the thickness of space charge layer is more and more thicker with rising immersion time. At the third stage (48 h–12 d), density of deep level donors rises with increasing immersion time and the thickness of passive films space charge layer decreases. At last stage (above 23 d), both the space charge layer’s thickness and density of deep level donors are no longer changed with increasing immersion time. In the overall immersion stage, the shallow level donors’ density is invariable all the time. The mechanism of deep level donor’s ionization can be the generation of metal vacancies, which results in crystal lattice’s aberration and the aberration energy urges the ionization of Fe(II) in octahedral sites.