In Electro-chemical series, Standard Reduction Potential of Zn is given -0.76 V and for Aluminum it is -1.66V. But according to galvanic series Zinc has more corrosion potential. Why it is so? Is it due to Passivation of Aluminum?
As per my understanding, I expect some correlation between both the series, as both of them are based on the tendency/potential to gain/loose electrons. Maybe my understanding about the two series is not correct, please clarify my doubt.
Thankyou
Dear Dr. Jagdeep Singh ,
the standard reduction potential is in a category known as the standard cell potentials or standard electrode potentials. The standard cell potential is the potential difference between the cathode and anode. The standard potentials are all measured at 298 K, 1 atm, and with 1 M solutions.
When solving for the standard cell potential, the species oxidized and the species reduced must be identified. This can be done using an activity series. The tables generally show the standard reduction potentials in decreasing order. The species at the top have a greater likelihood of being reduced while the ones at the bottom have a greater likelihood of being oxidized. Therefore, when a species at the top is coupled with a species at the bottom, the one at the top will become reduced while the one at the bottom will become oxidized. Below is a table of standard reduction potentials.
Different thing is the galvanic series measured, for example in seawater, for some common metals and alloys. When two metals are further apart in the list (e.g. a larger difference between the two numbers), the driving force for galvanic corrosion is increased. The most anodic (active) metals are at the top and most cathodic (noble) at the bottom.
For example, if zinc (think galvanized steel) which is an active material and near the top of the list and stainless steel, a noble metal and near the bottom of the list were in direct contact and in the presence of an electrolyte (es. water), galvanic corrosion will occur if they are regularly exposed to an electrolyte.
For more details, please see the sources:
- Standard Reduction Potential by Chemistry LibreTexts (2021)
Available at: https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Redox_Chemistry/Standard_Reduction_Potential
and
- Galvanic Corrosion by SSINA
Availaable at: https://www.ssina.com/education/corrosion/galvanic-corrosion/
My best regards, Pierluigi Traverso.
Kinetics beats thermodynamics in the practical consideration of corrosion. Passivity is one of the most effective effects for the protection of our metals. On the practical use of voltage or galvanic series, I always tell my students: know they exist, but you can hardly use them.
Dear Jagdeep
Yes Indeed ,the standard reduction potential od the Al is more negative than that of Zn in the electrochemical series. Because Al is.
a transition metal and capable to form a thin oxide-passive even in air ,therefore the Al is less corrosive than Zn in Galvanic corrosion study.
Dear Dr. Pierluigi Traverso
Firstly, I feel honored you wrote "Dr." for me, but I am not :(
Now what I could understand is:
The key difference in Standard Reduction Potential and Galvanic series measurement is the method, and scope of Testing. Standard Potential, which is measured against Standard Hydrogen Electrode or Carbon Electrode, is measured under standard condition for wide array of materials, while Galvanic Series is measured only for metals and alloys, and under sea water conditions.
My doubt is because of the following two conditions:
1. Aluminum is Anodic/Corrosive to Zinc under standard conditions.
2. Aluminum is Cathodic to Zinc under Sea water conditions.
Why they swap their positions when we change the conditions?
Also, Aluminum should be even more Anodic / Corrosive under seawater because of Chloride ions in Sea water that are detrimental to protective oxide layer of Aluminum.
Hi. I hope the following link ( section 2.4) might help you very much:
https://www.usna.edu/NAOE/_files/documents/Courses/EN380/Course_Notes/Ch02_Corrosion_Electromotive_corrected_.pdf
Dear Jagdeep,
I would say not to dwell on the titles (Dr., Prof., etc.) but I think it is better to try to answer your question.
First of all, measurements under standard conditions produce an absolute, stable and reproducible value.
What is achieved under different conditions can generate different indications. In particular, the passivation of aluminum creates a thin but protective surface film that increases its anticorrosive characteristics but it is not a material with high mechanical characteristics and therefore, very often, Al alloys are used in seawater (as you will have noticed from the graphs related to galvanic corrosion). In this case, in addition to having a different corrosive environment, the material also varies ...
Generally 5XXX (eg 5083) or 6XXX (eg 6061) series aluminum alloys are used, sometimes, but not always, painted and you know how boats of this type are protected ... with sacrificial Zn anodes. And I assure you that the system works ...
My best regards, Pierluigi.
Aref Wazwaz
Yes, I got clarity. It is clearly mentioned that Electrochemical series is not valid when oxides are there at surfaces. Means, electrochemical measurement doesn't include any effects of any oxide layer, where as galvanic series includes the actual/Practical effects.
Sector 2.7 reference
"The electrochemical series presented earlier can only be applied to oxide free surfaces at ion concentrations for which the standard potentials are valid. When metals are exposed in a more complex electrolyte such as seawater, the galvanic series may be used to help predict if corrosion of a metal is possible"
Thankyou
Pierluigi Traverso
Yes, I agree with you. We also use Zinc as a sacrificial element to extend the life of Aluminum. I was very confused about the usability of different series with respect to corrosion. Now got the clearer picture :)
Thankyou
and Thankyou everyone
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Materials Science
- Materials
- Inorganic Materials
- Metals
- Alloys
- Steel