Hi Chendi

you are right as far as the Magnesium element (Mg) would not reach a passivation stable status, for example in seawater, in order for one to measure the Mg passive  potential.   The Mg is known to be used in the cathodic protection method, for corrosion protection, by a sacrificial anode in a galvanic cell .   So to answer your question,  Indeed, the oxide film on the surface of magnesium alloy is not stable.   As a result, it is difficult to obtain a passivation potential from the potential dynamic curve or anodic polarization curve.

In my opinion M. Curioni, in his article "The behaviour of magnesium during free corrosion and potentiodynamic polarization investigated by real-time hydrogen measurement and optical imaging "gives a way to investigate the potentiodynamic polarization of magnesium by real time measurement of the amount of hydrogen evolved and imaging of the corroding surface. This approach is very helpful to determine the passivation current and potential of the magnesium alloy.

Best regards 

Dear Mohammed

The suggested paper is only estimating the corrosion rate of the Mg.   nothing was said about the passivation potential of Mg In the 3.5% NaCl solution.

Yes Prof. Khaled

Thank you for your kindly comment. As I said, the approach of the suggested paper is very helpful to determine the passivation current and potential of the magnesium alloy which can easily adopt graphically from the polarization plots of figure 6. This paper successfully establishes full polarization plots with stable passive film and as mentioned in the discussion part of the article, the film is "a chloride-containing oxide/hydroxide layer".

Best regards

Dear Mohammed

Figure 6, the polarization dynamic curve, of Mg in 3.5% NaCl solution consists of not only a zigzag curve (shifting between right & left on instantaneous basis), but also, the figure consists of two so called passive regions.

In my humble opinion, one can't relay on such an instantaneous zigzag curve to extrapolate the passivation current & the corresponding  passivation potential.

I could be wrong but I doubt such curve can even be reproducible.

Thank you for sharing you thoughts with us on the above matter.

Thank you Prof. Khaled

I have got your idea.

Best regards

This paper successfully establishes full polarization plots with stable passive film and as mentioned in the discussion part of the article, the film is "a chloride-containing oxide/hydroxide layer". This approach is very helpful to determine the passivation current and potential of the magnesium alloy.

Dear Saban

Figure 6 , the anodic polarization curve of the Mg alloys in the 3.5% NaCl solution, in the provided paper comprises of two plots.   One plot is the black one & the other is the gray one.   The black plot represents  the actual obtained data of the experiment on the Mg alloy in the 3.5% NaCl solution.The gray plot represents the calculated data of the simulated electric circuit of the solution/Mg interface system in the 3.5% NaCl solution.   

So, in order to determine the passivation current & the corresponding passivation potential of the Mg alloy in 3.5% NaCl solution from the calculated data of the simulated electric circuit of the solution/Mg interface system, one must have sound data of the experiment on the Mg alloy in the 3.5% NaCl solution.   In the case of Figure 6, the black plot (actual obtained experimental data ) was not only a zigzag curve (shifting between right & left on instantaneous basis), but also, the figure consists of two so called passive regions.

In my opinion, one should not simulate (by theoretical fitting) such experimental data (black plot) just because he/she has a software would do so.   In other words, the software could  simulate any experimental data we wished for  even if the actual experimental data were not sound.