Hot dip galvanised steels with thermal treatment or galvannealed steels are used in automotive industries because of their properties of weldability, workability and corrosion resistance. The galvannealed steels show three iron-zinc phases: zeta, delta and gamma. The distribution and concentration of iron-zinc phases affect the quality of the zinc layer and can produce stamping and welding problems. In the present work, the optimisation study of the electrochemical stripping process of industrial galvannealed steel was performed. The effect of electrolyte, pH, current density and scan rate was evaluated. The best resolution of the potential levels in the electrochemical dissolution curves was obtained for the zinc sulphate heptahydrate and sodium chloride aqueous solution, with a current density of 2 mA cm 22 and pH 4?5. Each sample surface obtained by using electrochemical stripping was analysed using X-ray diffraction and scanning electron microscopy.
The corrosion of galvannealed steel revealed three different Fe-Zn surfaces on the steel substrate, and each surface showed a different corrosion behaviour in a given environment. In order to evaluate the corrosion behaviour of each revealed surface, the electrochemical impedance spectroscopy (EIS) technique was performed in acid, alkaline and saline media containing chlorides and sulphates. The Mossbauer spectroscopy was also used to elucidate the iron phases on each surface, and these results identified the delta phase on all surfaces obtained after coulometric dissolution of galvannealed steels. In the hydrochloric acid and sodium chloride solution, the outer surface richest in zinc showed the highest polarisation resistance among the surfaces obtained using coulometric stripping of galvannealed steel, and the corrosive process occurred on the electrode surface. In the alkaline solution, the inner layer showed the highest polarisation resistance among the surfaces obtained after dissolution of galvannealed steel, and the equivalent circuit fitted to the EIS data is characteristic of a corrosion of a porous layer.
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