PF Newbie & Electro-chemical behaviour question?

[Crispin]

Hi All,

I am currently studying a MSc from UMIST in materials & corrosion science. I'm doing this via distance-learning, so kind of "self-tought", with some tutor support via e-mail.

I picked physics as my science subject in school, so have little, if no chemistry background, and certainly have not studied in the area of science for a loooong time, (I'm now 35, & have a full-time job as a mechanical integrity engineer for a petro-chemical company).

I am going to pop in & out of this forum with some of the more difficult concepts/reading material if that's OK, and hopefully I can learn from you guys, and equally, if there are any threads that I can add some value to, I will certainly do so as well.

I joined the site a while ago, and have just posted a few threads in this time, but as using the site goes, I am a relative newbie.

OK then, here goes with a question;

Throughout the recent material we have been studying the effects of electrode behaviour when cathodic & anodic overpotentials are applied. An extract from a reference book;

"The magnitude of polarisation is usually measured in terms of overpotential n, which is a measure of polarisation with respect to the equilibrium potential Eo of an electrode. This polarisation is said to be either anodic, when the anodic processes on the electrode are accelerated by changing the specimen potential in the positive (noble) direction, or cathodic, when the cathodic processes are accelerated by moving the potential in the negative (active) direction".

Also, in some recent material I have read, relating to a time record from an electrochemical noise measurement, showing positive spikes of potential 50mV above the average potential of +200mV (referenced against SCE).

"if the potential is going more positive, it implies a burst of cathodic activity, not a pitting event", and also, "The potential certainly suggests that the steel is passive, but the spikes are positive-going, which is not consistent with an anodic event(which would make the potential go negative by virtue of it's effect on the cathodic reaction on the passive surface"

Are these not contradictory?, or is it that because in the first instance it talks of an applied potential shift affecting the polorisation, where as in the second, it is the shift that the reaction causes, so the two are opposite to each other?

Anybody able to explain a little?

Thank you

Crispin

 

[Rustypipe]

Crispin
Good luck with the MSc, the distance learning is a hard route and you miss out on a lot of great social stuff but avoid the Uni politics and egos so its a balance I guess.

Yes I would agree with your last statement first... one is passive i.e. electrochemical noise (a listening device, like a radio)

and the other (pushing potentials around) is an active / passive device (like a telephone) signals sent both ways. (I am now sure there are many uni scientists sticking pins in my wax doll for that).

Active passive...The magnitude of polarisation is usually measured in terms of over potential n, which is a measure of polarisation with respect to the equilibrium potential Eo of an electrode. This polarisation is said to be either anodic, when the anodic processes on the electrode are accelerated by changing the specimen potential in the positive (noble) direction, or cathodic, when the cathodic processes are accelerated by moving the potential in the negative (active) direction".

Whereby you push it away from the reference electrode potential hold and measure (either positive or negative) and see what it does to the amps (aka... current or electrons).

Note: (bumper sticker for your car... corrosion engineers do it with electrons...)

electrochemical noise measurement, showing positive spikes of potential 50mV above the average potential of +200mV (referenced against SCE).

Electrochemical noise guys sit quitely and listen a lot and try not to influence the electrons at all in a very zen way. Its very ...Can electrons be involved in corrosion at anodic and cathodic sites ...Answer only if they want to be...

Therefore what your seeing with the burst of cathodic activity is a mixed potential electrode at its "open circuit potential" where allsorts of exciting things happen, at a very micro level, were cathodic and anodic "happenings" are taking place across the electrode.

Electrochemical noise guys get really excited by these "bursts" and have posters of Les Callow and Karel Hladky on the wall. (hint google Hladky's work on electrochemical noise).

Hope this answer helps

Rusty (Ex Imperial Corr MSc)

 

[Crispin]

Hope this answer helps

Rusty (Ex Imperial Corr MSc)

Yes it does Rusty, & many thanks for the informative explanation, I understand now (it's sort of the difference between pushing or pulling on the same electrochemical property, when the reaction creates the shift it's one sign, when the shift is imposed upon it, it's the opposite sign, makes sense).

All that remains now is to go buy that car sticker...! lol

Thanks again, you've helped me out there

Crispin