Tin is widely employed in the manufacture of food containers and its electrochemical behaviour in the presence of organic acids has attracted considerable interest.

In most cases the anodic dissolution of tin in these acids exhibits active or passive behaviour.

In the active dissolution region in acid media, tin dissolves as Sn2+ which easily oxidized to Sn4+

• Sn = Sn2+ + 2e-
• Sn2+ = Sn4+ + 2e-

Consequently, Sn4+ hydrolyze in acid media forming the highly insoluble Sn(OH)4 which precipitates on the surface giving rise to a passive film.

• Sn4+ + 4H2O = Sn(OH)4 + 4 H+

The stability of the passive film increases with its irreversible dehydration to SnO2

• Sn(OH)4 = SnO2.H2O + H2O

Non of the acids cause pitting corrosion of tin metal without the addition of aggressive anions. Here, preliminary experiments indicates the aggressive action of acetate anion towards the passive film formed on tin. Several papers have reported an increase in the rate of corrosion of the steel when acetate ion (or the anions of other weak acids) is also present in the CO2 saturated sodium chloride brine.

Here the corrosion behaviour of tin is studied in acetate buffer solutions (pH 4.5).

The potentiodynamic anodic polarization curve in 0.1M acetate solution exhibits two anodic peaks (stannous and stannic species) prior to the passive layer formation region which is followed by pitting corrosion.

The effect of scan rate on the potentiodynamic behaviour of tin in this solution was studied. It showed that the corrosion process in the potential range of peaks is mass transport controlled.

Pitting corrosion was confirmed by light microscope images.

The impedance spectra, at potentials of passive layer and pitting formations, exhibit a high frequency conductive semicircle and a low frequency inductive loop. The results showed a decrease in the electrode impedance as the applied potential approached the pitting potential.