Passivity and passivity breakdown of zinc anode in sulphate solutions

Abstract

The electrochemical behaviour of zinc in different concentrations of Na 2 SO 4 (pH = 6.0) was investigated using the potentiodynamic anodic polarization single sweep and cyclic voltammogram techniques. The anodic portion is characterized by one distinct peak corresponding to Zn(OH) 2 or ZnO. This is followed by a passive region up to a certain potential; the passive current suddenly rises steeply without any sign of oxygen evolution. This denotes the breakdown of the passive film and initiation of pitting corrosion. It was found that the breakdown potential depends on the sulphate concentration, type of aeration, scan rate, solution temperature and pH. The pitting initiation may be explained through the adsorption of SO 4 2± anion on the oxide film formed.This decreases the repair efficiency and causes further metal dissolution. From the cyclic voltammogram of zinc in different concentrations of Na 2 SO 4 , it was found that the change in the integrated anodic charge, Áq a , which is taken as a measure of the extent of pitting, varies linearly with concentration of SO 4 2± anion.

Studies on the pitting corrosion of lead in carbonate media

“…gives the same polarization curves as the solution free of aggressive anion. This indicates that ClO 4 anion does not interfere with the mechanism of passivity of the Pb electrode (Abd El Haleem, 1979;Abd El Haleem and Abd El Aal, 1982;Abd El Aal, 1999). A similar finding was reported during the study of the pitting corrosion of lead in NaOH by the author (Abd El Wanees et al, 1991a;.…”

“…However, the film dissolution is counterbalanced by film formation. The rate of these two processes is nearly the same at the steady state of polarization, so as to keep the thickness of the passive film nearly constant (Abd El Haleem, 1979;Abd el Haleem and Abd El Aal, 1982;Abd El Rehim et al, 1996, 1997Abd El Aal, 1999, 2000. X-ray diffraction analysis showed that the passive layer consists of PbO (Abd El Rehim et al, 1997).…”

“…The behaviour is plausibly indicative of hysteresis effects (Horvath et al, 1975). The presence of the current in the positive section of CVs with positive value, while the potential of the working electrode is decreasing, may contribute to the continuous propagation of pitting in the already formed pits (Abd El Haleem and Abd El Aal, 1982;Abd El Rehim et al, 1996;Abd El Aal, 1999). Finally, the current reaches practically zero value at definite potential.…”

“…higher ClO 4 anion concentrations also cause a gradual increase in the currents at potentials in the passive region that are more active at higher concentration of ClO 4 anion. This is due to the destruction of the passivating oxide film and initiation of pitting corrosion (Abd El Rehim and Mohamed, 1998;Abd El Haleem and Abd El Aal, 1982;Abd El Aal, 1999). The potential at which the current increases is usually called pitting corrosion potential, E pitting , whereby the passive film breaks down locally due to the presence of aggressive anions that are responsible for the initiation of pitting corrosion;…”

Electrochemical studies of the galvanic coupling zinc/pipeline steel in the presence of an applied alternating current

“…It is obvious that the main chemical composition is ZnO, containing a small amount of Zn(OH) 2 , which may come from incomplete dehydration process. We then realize that the corrosion products of zinc under AC have no obvious difference with that reported in the case of no AC .…”