dear

While doing ppotentiodynamic polarization your surface got altered

In  EIS it in the OCP

no meaning in comparing Corrosion current densities

 you can relatively  compare the behaviour of the alloys-- all alloys are passive compared to Ti

please compare the passivation currents in the passive region

Thank you Prof. Muralidharan for your permanent support. 

the potential at which you've done the EIS test is important! I think if you select a higher potential, you will see more consistency between your data! 

Thank you Dr. Masoud for your interest, generally the EIS tests performed at OCP.

And that what I did.

Let me tell about EIS

 if you want information  about corrosion do the experiment in OCP

if you want in passive  behaviour do it at the potentials above Flade potential

If you want to know trans passive dissolution ( like cr alloys) do it just below oxygen evolution

Dear Dr. Yasser

        Why you performed the two techniques, you can depend on the one of them and can you calculate the polarization resistance using the values of corrosion current densities and Tafel slopes from potentiodynamic polarization and compare it with the Rb.

Rp= [bc*ba]/ 2.303 * icorr * (bc+ba)

Best wishes

Rana A. Anaee

Thank you Dr. Rana.

Your corrosion CD  ( use the word current density) is of the order of nA/ sqcm beter use the EIS or use charging curve method ( see my papers)

We also suffered many problems in getting our sample measurements to follow any of the then-prevailing theories and models.  In addition, none of the measurements correlated to any field performance results.  Being in industry, our interests were focused on the latter, and concerned with the former only in so far as it guided our quest to understand actual field performance. 

Fortunately, one of my colleagues (Rhea Groseclose) discovered that it was the width of the passivation region that a given metal exhibited in a given electrolyte that correlated well with subsequent corrosion testing in that medium, after a coating had been applied and cured.  See "Characterization of the Variability in Corrosion Resistance of Steel Using Electrochemical Techniques," Groseclose, Frey, and Floyd, J. coatings Technology, July, 1984, pp. 31-41, Vol. 56, no. 714. 

Two decades later, with other colleagues, we reaffirmed that behavior and added individual components of waterborne coating, plus completely-formulated WB coatings to the electrolyte to show their impact on subsequent coated behavior.  "Using DC Electrochemical Techniques to assess the relative corrosiveness of water-based coatings and their ingredients," Floyd, Tatti, and Provder;  J. Coatings Technology, Research; vol 4 no. 2; 2007.  These results were used to guide formulating efforts away from inherently corrosive components to more passive ones.  Again, the key measurement was the width of the passivation region between Ecorr and Eb (breakdown potential; aka pitting potential). 

Later work also showed that these results applied equally well to aluminum, magnesium, zinc, and various alloys of those metals, since those metals now feature prominently in the automotive industry.  [not yet published]

I suggest that you try this approach.  But first, you will need to decide what real-world test for corrosion resistance you wish to emulate.  That will determine the nature of the electrolyte solution that you will use in the electrochemical test.  Meanwhile, to be blunt, don't waste your time trying to resolve disagreements with various model behaviors.  The literature tends to be quite circular and contradictory on most points; but more importantly, none of them guided our efforts towards better-performing coatings for a variety of metals.

Separately, "Using electrochemical impedance spectroscopy to predict the corrosion resistance of unexposed coated metal panels," by Floyd, Avudaiappan, Gibson, Mehta, Smith, Provder, and Escarsega, Progress in Organic Coatings, 2009, no. 4, shows the considerable difficulty in extracting much useful information from the considerable experimentation necessary in conducting AC impedance testing for coated panels.  While some information relating to barrier performance could be extracted, information relating to electrochemical corrosion was elusive.  Primary reason:  error bars in all measurements were quite large, which made it difficult to observe any signal information above the considerable noise present.  Separately, it was confirmed that the choice of specific electrolyte mix was crucial for determining relationships with downstream performance testing results.   

Thank you so much Prof. F. Louis Floyd for this useful explanation.

I think I will follow your advice, Thank you again. 

Thanks Prof F.Louis

i am discussing an uncoated meat/ alloys

My experience  with painted netals goes back to 1980's i used WIENKERR bridge (LCR bridge) i followed Capacitance and resisitance with time of immersion

 and  followed  water uptake and subsequently paint delamination

Later years we got PAR systems; no doubt science of evaluation of painted coatings improved. Lots of reviews have come in organic coatings journals

 our discussion only now on highly corrosion resistance alloys in Ringer's solution

please read as metals  / alloys , painted metals

You can get the Rp from polarization and the Rp from impedance and compared the corrosion rates of each one and make the relation. 

There is a simple relation: Rct =  B/icorr,  where B is a constant depends on the metallic material being studied and the corrosive medium used.

ok

In order to answer your question regarding passivation, I need to see the actual plots of current vs. potential in the anodic direction for at least 500 to 1000 mv from the rest potential.  That's the only way that I can reliably assess passivation behavior.  The shape of the actual curve is important in interpreting the results. 

Please consult the references previously cited, since they discuss the relevant issues in some detail, together with example curves..