Formation of lead sulphate membrane and passivation of the lead electrode

It has been established, through scanning electron microscopy, that the PbSO₄ layer formed on anodic polarization of Pb electrode in H₂SO₄ solution has membrane properties. This PbSO₄ layer screens the electrode surface and the electrochemical reaction proceeds only on that part of the surface which is free of PbSO₄ crystals. Lead ions pass through the pores of the PbSO₄ layer. Depending on the size of these pores two types of electrode passivation are observed.

Thermodynamic passivation. Pavlov proposed the following mechanism. The PbSO₄ layer acts as a semi-permeable membrane that allows H₂O, H⁺, OH^- and Pb²⁺ ions to pass through but stops the large SO₄^2- and HSO₄^-  ions. In order to preserve the solution in the pores electroneutral, the latter is alkalized. The electrode potential on open circuit is between -750 and -650 mV, and it does not change with time. P. Ruetschi measured experimentally the potential of the membrane electrode and confirmed Pavlov's mechanism, which is now known as "the mechanism of Pavlov-Ruetschi".

Kinetic passivation. The pores in the PbSO₄ layer are larger than those in the semi-permeable membrane and hence all ions can move freely through the pores. When the flow of lead ions is larger than that of sulphate ions entering the PbSO₄ pores, the reaction between them proceeds at the pore orifice near the electrolyte. To keep the solution in the pore interior neutral, it is alkalized and basic lead sulphate and/or lead oxide are formed. On opening of the circuit, the basic lead sulphate and PbO crystals react with H₂SO₄ as a result of which the pores are opened thus allowing access of H₂SO₄ to the lead surface. The electrode potential is equal to the equilibrium potential of the Pb/PbSO₄ electrode, i.e. -950 mV.

References

1. D. Pavlov, Processes of formation of divalent lead oxide compounds on anodic oxidation of lead in sulphuric acid, Electrochim. Acta, 13 (1968) 2051.
2. D. Pavlov, R. Popova, Mechanism of passivation processes of the lead sulphate electrode, Electrochim. Acta, 15 (1970) 1483.
3. D. Pavlov, B. Monakhov, Effect of Sb on the electrochemical properties of Pb/PbSO₄/H₂SO₄ and Pb/PbO/PbSO₄/H₂SO₄ electrodes, J. Electroanal. Chem., 218 (1987) 135.
4. T. Laitinen, B. Monahov, K. Salmi, G. Sundholm, Ring-disk electrode studies of soluble intermediates formed during the polarization of Pb in H₂SO₄, Electrochimica Acta, 36(1991) 953.

DSc Thesis

D. Pavlov, "Mechanism of the processes on anodic oxidation of lead in sulphuric acid solution"

Keywords: PbSO₄ semi-permeable membrane, thermodynamic passivation, pore solution alkalization mechanism, basic lead sulphates, formation of lead oxide, kinetic passivation