A transient multi-ion transport model for galvanized steel corrosion protection

A transient multi-ions transport and reaction model for galvanized steel corrosion in a 10 mol m−3 NaCl is proposed. The pH and the current density evolutions are simulated as a function of the immersion time and the results are compared with measurements. The simulations are two-dimensional and for a time up to 20 000 s. When considering no flow in the 10 mm electrolyte layer above the electrodes, it was impossible to find agreement with the measured pH profile that showed a diffusion layer. The addition of natural convection, which is mimicked by a 0.1 mm/s fluid flow towards the electrodes, leads to a much better qualitative agreement of the measured and simulated pH profile. Even in the presence of natural convection, the model predicts a low pH area only above the zinc and the zinc/steel interface. The insertion of two insulating pieces, representing the passivation due to zinc hydroxide deposit extends the low pH area up to 4 mm beyond the edges of the anode and shows that the passivating layer can still grow further. The thus obtained plateau of low pH around the anode matches closely the measurements, while the predicted high pH matches the measurements also far away from the anode.