So, let's get started.

I think the time is already 4.16.

So, am I audible at the back?

Okay, so let's continue what we started looking in the pre-recorded lecture.

We are talking about this electrochemical corrosion measurement techniques and we looked

at potentiodynamic polarization and we also briefly looked at the linear polarization

resistance method to measure the corrosion rates.

Today we will be continuing on that line and introduce you to a new technique.

It is not really new technique for this class that is electrochemical impedance spectroscopy.

So before that, I would need to start with the perspective from an electrical engineering

systems perspective that we have some dynamic system here in corroding reaction.

If you take a beaker of electrolyte and immerse your material, it is in a dynamic steady

state with the cathodic reaction that is happening on the surface, which is usually hydrogen

evolution reaction or the oxygen reduction reaction and the metal dissolution.

So they are always equal to each other.

So it is in a dynamic steady state.

So it can qualify as a system to which we can apply a stimulus and get a response out

of it.

So all these corrosion testing methods that was discussed earlier or whatever we are

going to discuss in this lecture and next couple of lectures is going to be having

a particular stimulus and there should be the you apply it to a corroding system and

you get a response out of it.

For example, if you are applying some potential steps as the input, you are going to get

current transients because when you apply a small step of voltage, it stops corroding

and you stop applying the voltage, it comes back to the steady state.

That is how you get these current transient.

And if you ask, use the potential stat to apply a particular current, if you ask the

system to corrode at a certain rate, if you apply an anodic current, then it will want

to get an anodic net current value.

So if you apply that, the potential needs to change because it cannot attain that current

at OCP.

At OCP, the net current is zero.

But if you are applying a net current, say 5 milliampere centimeter square, it cannot

happen at the OCP.

It needs to go up because usually anodic reaction happens above the open circuit potential.

So if you apply plus 5 milliampere centimeter square current and as a square wave here,

you are going to get something like a response in terms of potential.

And the same goes for this potential ramp.

If you ramp it fast, you are going to get something called as cyclic voltammetry, which

is not used really in corrosion systems because we don't want to go such high, fast scan

rates.

This potential ramp was essentially what we were doing when we did the potentiodynamic

polarization and LPR at a very limited range.

We were applying this potential ramp.

We are going back up and going down here.

This is a typical polarization curve for a material that is undergoing pitting corrosion,

where you are applying a positive potential.

It is corroding and then it initiates here and you come back and it repassivates.

We will discuss about pitting and passivity and pitting corrosion in future lectures,

but this is how the cyclic polarization response will look like.