CORROSION, PASSIVATION AND SEMI CONDUCTIVE PROPERTIES OF LEAD, ZINC AND STEEL ALLOYS USING MOTT-SCHOTTKY PLOTS

Passivation of lead and lead-tin alloys is investigated at an anodic potential of +0.7 Volts Vs Hg/Hg₂SO₄/K₂SO_{4 sat} in 0.5M H₂SO₄ solution using e.i.s

In these conditions p-type semi conductive lead monoxide (α-PbO) is formed and provokes a premature capacity loss due to the electrical passivation of the metal, acting as an electrical barrier at the grid surface, hindering the recharge of the battery.

The experimental impedance data have been simulated by an electrical equivalent circuit that includes:

- the space charge capacitance C_sc describing the semi conductive properties of PbO and represented by a constant phase element (CPE) in order to take into account the dispersion of the capacitance value.

-the diffusion impedance Z_D where:

Related to transport of anions O^-2 in a finite region having a definite thickness and modulated by two parameters (Y₀ and B) indicating that the PbO growth is due to a solid state diffusion under local electric field.

From the analysis of the impedance results the diffusion coefficient D is determined.

The effect of low and high concentration of Sn added on the thickness of PbO is discussed in terms of the internal electric field and the composition of the conducting oxides precipitates in the grain boundaries of PbO generating electrical short circuits through the layer.

Mott-Schottky plots have been drawn, the concentration of the charge carriers determined and the semi conductive and corrosion properties of the oxides interpreted.

Finally, electronic properties including the effect of doping are discussed as to their relation to the corrosion protection of the substrate for other passive systems like zinc in alkaline solution where Zn_1+xO is formed or stainless steel concerning the effect of added molybdenum.