The Effects of Oxygen Concentration and pH on the Galvanic Protection of Tin

SVET is used to investigate rapid detinning processes affecting commercial tinplate (produced by TATA steel) in the malic acid solutions, commonly found in tinned food products. In malic acid concentrations ≥ 0.1 mol dm^-3 tin was found to become sacrificial with respect to iron, reversing the polarity predictions on the basis of tabulated E_o values. The tin dissolution mechanism in aerated solutions is shown to be cathodically controlled by the mass transport of oxygen to the exposed steel cathode [1]. The sweeping removal of the tin coating is represented in Figure 1 which shows SVET-derived current density maps, the localised anodic dissolution of tin intensifies with an increasing cathodic steel surface area supporting the oxygen reduction reaction.

Particular emphasis is given to the effects in low partial pressures of oxygen replicating the conditions occurring inside a food can. A novel, Environmentally controlled Scanning Vibrating Electrode Technique (E-SVET) is described which allows the measurement of localised corrosion current flux under conditions of controlled oxygen partial pressure. Experiments undertaken using the E-SVET show that reducing the oxygen partial pressure significantly reduces the detinning rate due to the limited oxygen supply for the cathodic oxygen reduction reaction. Polarity reversal and oxygen reduction kinetics are confirmed using global electrochemical techniques such as RDE and OCP [2,3].

[1] S. Geary, H. N. McMurray and A. C. A. de Vooys. ECS Transactions, (2013), 50, p37-51.

[2] M. S. Abdel-Aal and M. H. Wahdan. British Corrosion Journal, (1988), 23, p25-28.

[3] V. Jovancicevic and J. O'M. Bockris. Journal of The Electrochemical Society, (1986), 133, p1797-1807.

Figure 1. 3D SVET-derived maps showing the distribution of normal current density J above a detinning 2.5gm^-2 non-reflowed tinplate sample in 0.1 mol dm^-3 malic acid at (a) 3 hours, (b) 3.9 hours and (c) 4.8 hours and (d) 5.1 hours of immersion.