Your data are not conflicting.  See J. Coatings Technology:  Research, 4(2), June, 2007 by Floyd, Tatti, and Provder, "Using DC Electrochemical Techniques to Assess the Relative Corrosiveness of Water-Based coatings and Their Ingredients."  This article is designed as a tutorial on how to run and interpret this kind of test.

Ecorr is the rest (open circuit) potential of a given metal in a given liquid environment, and is simply a material property in a given environment.  The Icorr is a crude attempt to say something about the corrosion process as one artificially pushes the potential either positive or negative relative to its rest potential.  It's a waste of time to over-study or over-interpret this value. 

Instead, concentrate on the flat region that develops as one pushes the samples in the anodic direction:  the E value where the curve first turns flat is the passivation potential, and the E value where it resumes its path to the right is the breakdown potential.  The gap in between (delta E) is a direct measure of the passivation behavior of the given metal in the given liquid environment.  Metals or environments can be directly compared by this simple width in E values between onset of passivation and onset of further corrosion (latter also known as pitting potential).  It does not matter where these E values are, but rather how great the difference is between them. 

In your case, the red line has not run far enough to see its breakdown potential, so you cannot identify a delta E.  However, it is clearly greater than for the black line, which means that the alloy represented by the red line is more resistant to the medium you are using than is the alloy represented by the black line. 

The reference I offered you shows how to conduct and interpret these kinds of tests, and offers a complete set of references to get you up to speed.

Thank you so much Dr. F. Louis Floyd for this interesting reply!

The red curve for the Ti-6Al-4V also appears to have some instrumentation issues that would invalidate any computer generated Tafel slopes used to identify Icorr and Ecorr.  Take a look at the question at:

https://www.researchgate.net/post/Has_anyone_experienced_a_change_in_the_current_at_constant_potential_in_PDP?_tpcectx=qa_overview&_trid=53c9ae4fcf57d781688b460e_1"

to get an idea of how to remove the horizontal jumps in current density that appear in both the anodic and cathodic curves.

Dear;

   At first at all, you have determined icorr instead Icorr, since this farmer is corrosion current density and latter the corrosion rate. Considering those polarization curves, although Ecorr was dispaced to noblerpotential side, it corresponding icorr ia at least one decade higher than other examined alloy sample. This behavior is intimately associated with the resulting microstructure array, which is resulted of an applied processo route.

In my last suggestion (answers/email), I have been considered the electrochemical behavior as a function of the resulting microstructure array. For instance, when a rapidly cooling rate is applied, a more homogeneously and finely distribution second phase are attained, which depending of the electrochemical behavior (galvanic couple) the corrosion resistance can significantly decreased. 

On the other hand, considering these two red and black potentiodynamic polarization curves, it can be said that RED LINE has a corrosion current density of about 2E-9 micro A cm-2 against of about 5e-7microA cm-2 for the BLACK line. Although its corresponding corrosion potential clearly evidenced a displaced to nobler-side favoring the BLACK LINE, the icorr is predominant to predict the corrosion resistance performance of these two examined samples. Additionally to polarization curves, I hardly suggest to carry out EIS plot (e.g. Nyquist and Bode diagrams) in order to attain more detail concern to the corrosion resistance of these alloys. On the other hand, when the medical (implantation) is highly considered, it should also be hardly suggested to take attention into the micro encourage on the surface of the component alloy since the hydroapatite coating and osseointegration can be improved.

dear ALL

E corr  is a thermodynamic quantiy while corrosion C.D is the rate of corrosion

please see my paper on the origin of corrosion potential published  in iB.Electrochem

contact Mr Gunasekaran librarian Central electrochemical research institute karaikudi india

www.cecri..res.in

Firstly, pls understand that the corrosion current density is a kinetic value and potential is a thermodynamic value. Generally, corrosion rate is proportional to the corrosion current density (Icorr). This means that the corrosion current value is more important in determining the actual corrosion rate of a material. From your figure, the black curve (your specimen) has the more positive potential (noble) but higher current density. Means the standard specimen is still better in terms of corrosion resistance, even though with lower potential.

Dear all,

Current density is highly related to the corrosion rate. so I prefer the current density to the potential since it is a thermodynamic value.