Typically inductance is related to absorption of some species from the medium. The corrosion products most likely hinder the active sites to be accessed by corrosive species. Hence, if there is any absorption the resistance will be larger in the presence of corrosion products.

If the inductance is in positivy quadrant, we are in presence of the pasivity.

Otherwise, if the inductance occurring in the negative quadrant, if the presence of corrosion pitting.

Part of the answer is going to depend upon the nature of the corrosion products.  Some corrosion products are ionic semiconductors.  In this case, there were be an inductive loop doe to the gradient of ions moving through the corrosion product lattice.  For products that are not ionic semiconductors, you could get a loop due to porosity in the corrosion product that is producing ionic gradients either of corroded ions out or corrosive ions inward,

We are monitoring the evolution of the corrosion process  by means of impedance measurements  with t two Mg-Al alloys immersed in 0.6 M NaCl solution.

Immersion time has been seen to have an effect on the size of the inductive loop.

At longer immersion times, between 1 and 28 days, the length of inductive arc increases continuosly with one of  the magnesium allosy but decreases significantly with the other. .

Do you know anything about the corrosion product film on the surface, such as thickness or composition?  The EIS data would appear to support the idea that you have two different composition films.  However, it could also be that the Al levels are simply changing the ionic "permeability" of film.  If you are building a film with increasing Al2O3 content with time, that would support the idea of increasing passivity proposed in Abel Castañeda's reply.

We have quantified relative differences in the chemical nature of the corrosion layers  by X-ray photoelectron spectroscopy (XPS), No significant Al contents were detected on the non-sputtered  surface of both alloys and after sputtering.

Is the protective layer mainly made of Al oxide? If so, corrosion should be slown Down, and the resistance increased significantly as reflected in EIS. By the way, what do you exactly mean "ARC" in EIS, a term not often used. Do you mean "semi-circle" in electrochemical impedance Spectra? In addition, the electrolyte used (NaCl) should be close to "neutral" pH. Have you tried measurements in acid or/and basic solutions?

The X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDX) and low-angle X-ray diffraction (XRD) data tend to suggest .that  the corrosion product layers formed on magnesium alloys is mainly composed of mixtures of of magnesium hydroxide and carbonates with different contents. No significant contents Al contents were detected by XPS or EDX

The Nyquist diagrams obtained for both alloys immersed in 0.6 M NaCl show the presence of a capacitive loop at high frequencies (HF) and an inductive loop at low frequencies (LF). The increase in immersion time led to a size increase in the inductive loop for one of the Mg-Al alloy and a decrease for the other alloy.. 

When Mg-Al alloys are immersed in NaCl, magnesium dissolution takes place with hydrogen evolution and release of OH− ions and, as a consequence, the pH of the liquid adjacent to the metallic surface increases

your question is vague

please mention the system- be specific - otherrwise it is gossip

we can help - inductance arc has to do Rct

and the origin of the arc needs to be studied after knowing the system chemistry

The evolution of the corrosion process on the AZ31 and AZ61 alloys immersed in 0.6 M NaCl solution has been monitored by means of impedance measurements. The Nyquist diagrams show the presence of a capacitive loop in the high frequency (HF) region and an inductive loop at low frequencies (LF). Many studies in the literature, support the use of the charge transfer resistance (RCT), deduced from HF capacitive loop, to obtain information on the corrosion rate of magnesium alloys. It is normal to associate the diameter of this capacitive loop with the charge transfer resistance of the corrosion process.

As commented previously by my colleagues, regarding the inductive loop observed at LF, several processes can induce this behaviour in the corrosion of magnesium and its alloys. In the literature  it is mainly attributed to the relaxation of adsorbed species, such as Mg2+ or MgOH+ on the electrode surface and, also, to the possible dissolution of partially protective surface films (pitting corrosion) .

In the literature, it is common to observe an increase in the capacitive loop with immersion time associated with the diminishing or vanishing of the inductive loop suggesting an improvement of the protection of the surface by the corrosion products. Curiously, in our measurements with the AZ31alloy we have observed a linear relation between the diameters of the inductive and capacitive loops and both increase significantly with immersion time. It is not easy to find an explanation to this behavior..

the origin of inductance  loop is due to the interface behaving like an inductance  in the frequency region - the interface will behave differently with frequency

first of all we have to identify the possible reactions likely to occur in the frequency region

what causes the charge storage- and coil back

knowledge of the system is important

Hi Feliu,

You might send us the EIS Spectra for a look, which could make things easier.

Best luck.

You might obtain the EIS spectra in the following link:

https://www.researchgate.net/publication/280007891_Corrosion_product_layers_on_magnesium_alloys_AZ31_and_AZ61_Surface_chemistry_and_protective_ability

Article Corrosion product layers on magnesium alloys AZ31 and AZ61: ...

Only by curiosity, due to uniform corrosion becomes the predominant corrosion mode for the EIS tested AZ31 specimens after removal of the corrosion products, which show a very significant inductive loops in  the Nyquist plots, I wonder if there is an alternative reason to pitting corrosion or adsorption of chemical species to explain the presence of inductive arc in EIS diagrams

the inductance loop is due to charge storage

the film may be semiconducting, reversible and reoccurring chemical reactions 

in high frequency it contributes to resistance and low frequency it needs discussion