For the corrosion is important only the corrosion potential, current of the corrosion is less important.

For example, a corrosion-resistant metal chromium is passivated with Cr2O3 and has a high corrosion potential. If you exceed this voltage, chromium oxide is destroyed at the surface and chromium begins to corrode rapidly (high corrosion current). Iron is not resistant to corrosion because its corrosion potential is low, but its corrosion current can be lower than that of chromium.

Such metals as iron, cobalt, and nickel are more noble than chromium (smaller corrosion currents), but chromium is more resistant to corrosion because its passivation is better (higher corrosion potential).

@Vadim Verlotski thanks for the speedy response! Thanks for the insight. However, I did not understand the iron example too well. Do you think you could restate it. Because I learned from my corrosion course that galvanic series dictates which materials corrode more than others. But, this galvanic series is based on corrosion potential. So I'm a bit confused when you say iron is more prone to corrosion not due to lower corrosion potential but lower corrosion current. Correct me if I am wrong 

 Also, the corrosion rate (C.R) equations are based on corrosion current. So If I have two coatings with C.Rs 10mm/yr and 15mm/yr. The coating with a CR of 15mm/yr has higher corrosion current, right ? . So based on just CR would I say this coating is more prone to corrosion ? If this coating also has a higher corrosion potential than the other coating with 10 mm/yr CR how should I compare them 

I try to describe the example with iron and chromium differently:

Let's take two metal pieces: one piece - steel (iron), second piece - chromium. Put these two samples in the cold dilute sulfuric acid and see what happens:

Iron begins to corrode: Fe + H2SO4 = FeSO4 + H2 and hydrogen bubbles rise upwards. The sample of chromium does not react with cold sulfuric acid because it is protected with a thin layer of oxide. But when we heat the acid and wait a couple of minutes, we see the following: suddenly bubbles also rise from the chromium sample, and more strongly than from the iron sample! The explanation is simple: in the warm sulfuric acid the protective oxide of chromium is destroyed and it begins to corrode: Cr + H2SO4 = CrSO4 + H2. Since chromium is less noble than iron, the reaction proceeds faster than that with the iron.

The same is true for the electrochemical corrosion of iron and chromium: if the passivation of the chromium is destroyed by overvoltage, it will begin to corrode faster than iron (higher corrosion current or greater corrosion rate). But if this potential is not exceeded, Chom does not corrode at all. In this sense, it is important for corrosion that chromium has a very high corrosion potential.

@Vadim Verlotski got your point now thanks. 

Other folks, any suggestion on the shape of the anodic branch of the curve ?

Hello,

Irrespective of corrosion potential, corrosion rate can be evaluated based on corrosion current density value. Corrosion potential changes quickly with change in environment whereas corrosion current density changes slowly as the corrosion proceeds. Accumulation of corrosion products on surface also indicates lower values of corrosion potential but clear idea will be obtained by corrosion current density.

Short answer:  your curve is a classic example of passivation caused by whatever environment you included in that particular run.  The flattening behavior is the passivation.  The subsequent resumption of current increase with higher potentials represents a breakdown of the passivation layer, and the resumption of corrosion.

A good summary can be found in the following paper:

"Using DC electrochemical techniques to assess the relative corrosiveness of water-based coatings and their ingredients," by Floyd, Tatti, and Provder; J. Coatings Technology:  Research, vol. 4, no. 2, pp. 111-129, 2007.  The journal is published in the UK.

This clearly describes and illustrates the anodic region of a DC scan and what the various shapes represent. 

Good luck with your research.

Lou Floyd

The protective properties of any ceramic coating system are related strongly to their integrity. The higher the integrity means that the coating is more adherent, pore free or limited, coherent, crack free, more stable, ect. The coating 2(red) performed less amount of connected porosity and less adherent to the substrate than coating 1(black). So, this coat performed more positive corrosion potential in the same time the anodic reaction area enlarged to under coat. Please remember that the resistance through connected porosity is much less than the coating matrix. These resistances connected in parallel mode, and then the resultant connected in serial mode with the electrolyte under coat. This means that the only porosity amount in coating controlled the corrosion potential and the coating bounding strength to the substrate with the porosity amount controlled the corrosion current density

Best regards

I fully agree to the comment of Mohammed Hussein j. H. Al'Atia. If you measure the electrochemical behavior of passive coatings like ceramics or polymers on mild steel, the corrosion current, bur also the corrosion potential is strongly dependent on the integrity of your coating. This includes: Number and size of voids, diffusion within pores and within your coating, thickness and adherence of the coating,.....

To asses this, I recommend to use Electrochemical Impedance Spectroscopy. Some general information can be found in the attached link.

http://web.phys.cmu.edu/~yfeng1/EIS%20papers/EIS%20reviews%20and%20papers/Evaluation%20of%20protective%20coatings%20-%20a%20new%20approach.pdf

I also agree with the above answers, that the 2% 'GO' curve (red) shows passivation and de-passivation in the concave portion of the curve. This, and the generally higher current density indicates a lower quality coating, likely with higher open porosity. This is not too weird, as any increase in thickness of a ceramic layer means slower and less efficient escape of solvents. Are you using a sol-gel proces? Optimization of this step appears to be a way forward, for a thicker coating.

The more noble resting potential however is more difficult to explain. Perhaps it can be explained by the thicker coat, providing an initial barrier against the solvent. If you run the tafel sweep again on both coated electrodes, you might see a more similar resting potential between the two cases. Might be interesting.

Corrosion current is more detrimental for corrosion. It reflects the dynamic trend.

to be exact, local current density shows the extent of corrosion, and I guess the red one has the characteristic of passivasion, in which its current keeps constant even reduced with increasing E, despite its larger tendency of corrosion

Thank you all for the valuable comments! Was really helpful 

Corrosion current is a kinetic value and potential is thermodynamic value. Corrosion rate is proportional to corrosion current (corrosion current density). Corrosion potential can shows corrosion tendency but not corrosion rate.

I absolutely agree with Stefan for general corrosion.

In terms of localized corrosion (pitting due to failures in the coating) you can not determine true corrosion rates, since you dont know the real corroding area, i.e. you can not calculate current densities. What you can do to compare such coatings is to determine the critical (pitting) potential (potential, where current exceeds a critical value in the passive region), which is more positive (better) in the red curve. Note, that polarization speed is also very important when comparing pitting potentials!

Interesting, and I essentially concur with all of the comments above. However, I would like to make an additional comment. Never forget that corrosion is a steady state condition, and that the corrosion potential is not a property, but a dynamic situation. It is the result of the reaction kinetics satisfying the equation for i(total)=0. My point is that the cathodic branches of the two curves intrigued me. Why do they not merge as the potentials become more negative? Assuming that the oxidizer content, sample size, stirring, etc are all identical, this then implies to me that one coating has either reduced the kinetics of the cathodic reactions, or reduced the area where cathodic reactions can occur. This would lower the corrosion rate and potential; but of course, increasing the anodic activity will also lower the corrosion potential. However, if it was only the change in the anodic branch, I would expect the "corrosion potential" to be close to where the cathodic branch of the red curve crosses the anodic branch of the black curve. Never forget that the corrosion rate of steel in seawater is almost always rate limited by the cathodic reaction and by the mass transport of oxidizers (dissolved oxygen) to the surface. This is why in a galvanic couple you paint the cathode (which is not corroding) instead of the anode.

Painting cathodes is safer from the perspective of anti-corrosion practice. If the corrosion current density is proportional to the potential difference and the total resistance of the system then the spot damage to the anode coating would lead to very intense corrosion. I agree that corrosion in seawater is a complex phenomenon influenced by the formation of variable aeration cells, chloride ions and others.

The surface area in the localized corrosion is unknown. So, i suggest you use other parameters such as critical potential. Also, the corrosion rate can be used instead of corrosion current density.

From the thermodynamic view, the corrosion potential can depend on. But in viewpoint from kinetic reactions, the corrosion current must be considered in corrosion rate calculation.

Hi there,

Good question, you can judge from the corrosion rate " net of electrochemical reactions " anodic or cathodic"" , in other words, Corrosion rate is proportional to corrosion current ( i.e.,corrosion current density). In general, Corrosion current is a kinetic value and potential is thermodynamic value. Corrosion potential can shows corrosion tendency but not corrosion rate as it shown from the figure.

Regards,

Dear Hind H Al Rushud

You're right, my slip. However, I will insist that the corrosion potential does not tell us anything about the speed of corrosion processes. In addition, if there was no current flowing, there would be no corrosion process and with corrosive potential, the sample immersed in water or exposed to weather conditions corrodes :)

Generally, corrosion rate is proportional to the corrosion current density (Icorr). I think the corrosion current value is more important in determining the actual corrosion rate of a material.

Hi

The corrosion potential i the driving force for possible corrosion. In contrast, the corrosion current is the actual corrosion rate, i.e., mass lost per unit area per unit time.

Corrosion potential (Ecorr) gives the information of corrosion tendency of the solution only. the corrosion rate is proportional to corrosion current density. we can calculate the corrosion rate by corrosion current density.

the corrosion potential the tendency for corrosion under the specified conditions is expected to decrease on purely thermodynamic grounds when corrosion potential moves by certain effects in the noble direction. The reverse of this statement is also true for the case where the corrosion potential shifts in the active direction.

Dear Haider,

Corrosion current is more detrimental to corrosion.

Corrosion potential and corrosion current are two important parameters which connect the fundamental electrochemistry and the practical corrosion behavior of metals. In simple terms, corrosion potential is the characteristic or property of metal and nonmetal surfaces to lose electrons in the presence of an electrolyte. During the process of corrosion, two electrodes are formed spontaneously, a cathode and an anode.

Corrosion current is the dissolution current at the corrosion potential.

High corrosion potentials are good. The system will rather take up electrons than loose electrons, so a reduction is more likely. If the corrosion potential is dropping, itmeans that the sample is oxidizing and negative charges are accumulating in it.

In an acid, the anodic partial process represented by intersects the cathodic partial process at Ecorr, the corrosion potential. The current corresponding to Ecorr is icorr, the current between the local anodes and cathodes cannot be obviously measured by conventional means, such as by placing an ammeter.

Corrosion current is a kinetic value and potential is thermodynamic value. Corrosion rate is proportional to corrosion current (corrosion current density). Corrosion potential can shows corrosion tendency but not corrosion.

Hope this is helpful for you.

Ashish

Agreed with @ Qhatan Adnan Yousif