 We started discussing the concept of passivity in the last class and you know and passivity is a important property that is exploited to prevent corrosion of metals. We also saw that the passivity means it is the formation of an oxide film on the metal surface and this oxide film is not very thick usually it could lie between 30 angstroms to about 100 angstrom thick but they are quite impervious they resist the flow of current and so, it offers excellent resistance to the corrosion of metals. So, that is why the passivity becomes very important. We gave some examples also where the passivity is very much exploited in engineering applications of metals stainless steel is an obvious example where the passivity is very much utilized the other metals are the titanium, zirconium even for that matter aluminum they are all your tantalum. These are all the metals they show a lot of passivity and so, they offer extremely high resistance to corrosion. Now, we looked at a qualitative description of what a passivity is. We also started defining the passivity from the point of your electro chemical concepts. We started looking at the so, called Evans diagram for a metal showing active, passive, trans-fascia transition right. That is if I can draw a schematic diagram in the log current density and the potential and if you see a typically a metal in a environment this is your equilibrium potential will show an active dissolution it shows an active dissolution and then it started reverting the current when you apply the voltage. So, the current almost remains constant and again the current increases with voltage applied voltage. This is a transition where the metal initially undergoes active dissolution and then this is the the passivity and this region we defined as trans-fascia dissolution. We also defined the other parameters which are important for us they are like what we call this I c it is critical current density and you call this as the I p be the passive current density right and you call this as potential which is called as T p trans-fascia potential. You can also call as E p it which is called as a pitting potentials and this is the potential above which the metal passivates this is called as a passive potential E p ok. So, the metal will will will if I increase the voltage or the potential on the metal the current increases and again start decreasing here current remains steady then again current increases this how this metal starts behaving. So, in order to understand how the metal in actual situation behaves you need to first understand how this particular plot will change. If I change the environment if I change the environment how does this change? So, we will see how does this change if I change the environment. Assume that I am going to change the concentration of the acid right. If concentration of an acid is changed is increased let us say. So, what is going to happen to this? So, again you need to draw the potential versus the log plot right and assume that you have a sulphuric acid of point not 1 molar sulphuric acid ok. Suppose I have point not 1 molar sulphuric acid. So, this is 1, 1 is point not 1 molar sulphuric acid. If I make it say 2 molar sulphuric acid I am sorry if I make it as let us say point 1 molar sulphuric acid you will see the active dissolution will increase this corresponds to I am just giving a schematic I am just giving how the trend is looking like. If I increase let us say the concentration to 1 molar then it is possible that this guy will go something like that. So, these parameters are all depend on the environment it may also depend on temperature. Suppose I increase the temperature I may get a similar effect can you just tell me what is happening to the anodic polarization? If I increase the concentration from point not 1 molar to point 1 molar to 1 molar sulphuric acid what are the changes happening? Can you point out what are the changes electrochemical parameters what current density can be very specific the critical current density changes right. So, I c increases one factor what more is happening E p is also increasing then ok yeah the third E t p are E p p is right now we do not worry about whether E t p are E p but is also it is what is happening it is decreasing right what more is happening the I p the passivation current density is increasing. I am not really worried too much about the exchange current density at this point of time ok let us not worry about it. You see these are the changes they are accompanied due to the change in the environment right what will happen to the corrosion rate that we are not discussed what will happen to corrosion rate have you discussed we are not discussed now. Please notice this is we are talking only about the metal dissolution right let us say this is your metal m is going as m plus plus n electrons. If you want it you can also write another one as m plus plus electron gives you m if you want you can do that ok. This is not going to give you corrosion rate right how do I get a corrosion rate I need to have what I should have I should have high car how do I get high car what I look for I look for the second equilibrium this if the metal is getting oxidized there has to be a corresponding reduction reaction right without that you will not happen right. So, I am looking for what I am looking for another equilibrium that will drive the corrosion of the metal actually ok. So, I again go to the so called mixed potential theory apply this mixed potential theory and find out what is happening to corrosion rate right. So, let us look at this now and see how interesting these features are. I draw again schematically a polarization curve showing active passive transition ok you can also do yourself right. So, what I do now I want to just look at the possibilities three distinct possibilities there can be several possibilities three distinct possibilities I will consider ok. Now, I can draw a cathodic kinetics following this graph it can follow like this ok. So, I have on this m going as m plus plus electron here maybe I would say this is the H plus plus electron gives me hydrogen molecule like this here. I can also have another you know characteristic lines like this I can have one more. So, this is I can have one more line this is line 1, line 2 can also have one more something like that let us say line 3 they are distinctly different. You can now we can draw so many in between right they represent three different type of you know phenomena can happen on corrosion. You can look at the intersection points you can label this maybe you can start from A B this is C D E F of course, I I I think you know this is the C here right this is the C that corresponds to this. The case of line 1 the mixed parallel theory is applicable and happens only at one place right they are seemingly clear. Now, I know what will be corresponding corrosion current density I can know that I can know the corresponding corrosion potentials. If I take the line 2 the mixed potential theory is as well applicable for the location intersection point C D and F am I right. All three places the mixed potential theory is very much valid the line 3 which means they are different kinetics you have only one point where the mixed potential theory is is valid all other places is not valid. So, if you know where the mixed potential theory is applicable I can find out the car value I can find out the corresponding I car value am I right. So, the point you know C D and M can also happen in practice the point B can happen in practice point G can also can happen in practice. If you want to give an example for what corresponds to 1 the one typical system corresponds to titanium in de aerated sulphuric acid dilute sulphuric acid. System 2 corresponds to chromium in de aerated dilute sulphuric acid. System 3 stainless steels titanium chromium etc in in aerated acid solutions it could be mostly acid it could be let us say dilute sulphuric acid. Why why why what happens in the aerated condition can anybody tell Alzin is there right you have one more one more reaction what is the reaction here the third corresponds to Alzin 4 H plus 4 electron gives you water this is the equilibrium that you have. So, the cathodic reaction is quite having higher kinetics. You see now how you know the same material like titanium and chromium can behave differently depending upon the kind of environment that you deal with ok. Now, let us look at this a little bit more carefully. Let us take the line line 2 the kinetics 2. You can have the the mixed-paren theory you can have a E car corresponding to this one you can also have an E car corresponding to this you can also have an E car corresponding to this. Now, we need to understand why does it happen. Let us take the case of G the metal you know in this case in this case of 3 this is this is called as spontaneous passivity. The metal here undergoes spontaneous passivity because why? Because I just immerse let us say a stainless steel in dilute sulphuric acid I do that and I have air which is now this stainless steel is spontaneously passivating there is no problem because the potential is going to the passive region. When you remove air in the case of titanium the current you know it does not exceed beyond this particular value. So, in order to passivate the metal suppose I need to reach this place please notice the metal has to be polarized you have to move up and move up and move up and reach this point then what happens? Then automatically the current starts falling down. So, you need to cross this point what you call this point called as critical condensity unless you cross this the metal will not passivate. The metal will start passivating only when you cross this. To cross this point look at this the metal undergoes oxidation there has to be equivalent amount of reduction then only the metal will corrode am I right? You take this particular potential here at this particular potential if the metal resolution rate is higher than the reduction rate what happens? At this particular point if the metal oxidation rate is higher than the reduction rate of the environment what will happen? The system will not sustain it will come down. Or if the rate of reduction is higher than this what happens? The metal starts tending towards up. So, in order to in order to cross this particular point the rate of oxidation here must be lower than the rate of reduction then only it is possible to cross. That means, the cathodic kinetics should be higher than the anodic kinetics then only this can move. Let us take this line here I have stopped updating this line you see at this particular point the rate of reduction is going to be higher than the rate of oxidation and so the potential starts moving up. So, this is the requirement for that. So, in this case the metal can either stay here stay here. So, it has an option of 1 and 2 the point D is is unstable the point D is never happened. When will when will will when you move over here in case the concentration of this piece is slightly increased the guy will move over here otherwise they stay here only. So, the system 2 it is called as unstable system whereas, the system 3 the system 1 they are all the stable systems. Stably active corrosion stably passive systems whereas, the system 1 can show either e car here or you can show any car here. So, both things are possible in this case. So, system 2 is not acceptable because it is uncertain system 3 is ideal you want that the system 1 is predictable is better to have system 1 than to have system 2 ok. So, this how you look at from the engineering perspective ok which of these things can happen and these are systems where chromium in de aerated sulfuric acid is uncertain you may have high corrosion rate it may have low corrosion rate ok both are possible. We will see this in the subsequent discussion and how these things really happen. So, we have now seen in detail the application of the mixed potential theory for the corrosion of metals when they are actively corroding when they are passivating we also saw that how we can apply the mixed potential theory if the system is under diffusion control and I suppose you are now in a conversion of it the understanding the kinetics and if you any of you have any any questions nothing we can discuss and then we can move forward. What is the possibility of CDF solely depend on potential? Yeah whether the metal will go to CDRF we consider D is a little unstable what does it mean by that actually actually if you if you scan this potential little fast you will see that this whole line starts moving up if you scan slowly it will move down ok. So, this whole of this this horizontal line is an unstable line in reality you would not get the intersection point D, but what you will get is the intersection point F and the intersection point C these are the point you can get it actually ok. This also would would depend upon the passive condensity you know if the you know for example, if the passive condensity is going to be little higher for example, you will get this uncertainty is much more ok much more can happen. So, IP value is equally important we will we will see subsequently ok and how this if for example, if you lower the IP value the reducing agents required to maintain the passivity is very low. So, you will find that once you reach this state that this state becomes quite quite stable ok. So, if the kinetics of this reaction is going to be either here or here ok initially it may start passivating you know, but but I think the passivity is going to be quite unstable because you need to reach the the critical condensity ok. Above which only the metal will move up in the potentials ok. So, you will see that this you know system sometimes exhibit the two EECOR values EECOR value of this and EECOR value of this whereas, this system 3 and system 1 would always show only one EECOR value and I would probably show you at later time when you talk about experimentation how these systems are indeed showing two different EECOR values when you carry out transferment actually ok. Because the system is not becoming totally stable because of the poor the cathodic kinetics for this the system any other questions ok. So, let us move on to the next topic of application mixed potential theory in practical situations. So, let us take a system where you add an oxidizer you have seen this earlier anybody recollect you talked about an oxidizer and in the case of active metal and we said this is the corrosion rate will increase right. So, we saw this before you know for active metal for active metal we saw that when add an oxidizer the corrosion potential moves up the corrosion rate indicated by ICAR goes up we also made one more observation what was it? The hydrogen evolution reaction in acid will come down actually these observations are made right any of you recollect that actually ok. So, you know if you recollect that we see it is simple we are talking about some metal some may be you know may be iron may be you can say going as a few 2 plus plus 2 electrons and we were saying that say iron in an acid right and this was what this corresponds to what corresponds to H plus plus electron gives you half hydrogen molecule right and you have a corresponding reduction reaction right this is your H going as H plus plus electron. We said that we are going to add an oxidizing agent something like your this corresponds to what this corresponds to Fe 3 plus plus electron gives you 3 2 plus right listen you remember that. So, what did we do in that case we added the yeah. So, we added both the cathodic reactions somewhere here and we say this is your your mixed potential point right where you have rate of oxidation is equal to rate of reduction we also said right. So, we said that when you add an oxidizer the rate of corrosion is increasing right we saw this earlier hope you will able to recollect this right. Now, let us look at this in the case of in the case of a passive system and see how the mixed potential theory is revealing actually right let us look at this. So, I can add an oxidizer to this right suppose without an oxidizer this is let us say iron is corroding as Fe 2 plus plus 2 electrons it is iron is immersed in say it is iron N sulphuric acid iron sulphuric acid and to this I add let us say ferrous sulphate I am going to add ferric sulphate sorry. So, this I am going to add a ferric sulphate ferric sulphate means I am going to have in this Fe 3 plus ions right add to this. Now, without addition of these ions I would draw a cathodic line like that ok. What does this correspond to? This corresponds to the corrosion rate of the metal ena in acid and this potential corresponds to ecar and I am going to add to this certain amount of ferric ions what is going to happen now? You will have so, they have different concentration of the oxidizers here C naught is no without oxidizer I add concentration 1, concentration 2, concentration 3, concentration 4, concentration 5, 6, 7, 8 any of you who have not followed this diagram please let me know. See what are these line corresponds to the cathodic kinetics occurring as a solution containing what? Containing ferric ions am I right? This is only with the C naught corresponds to the corrosion of iron in sulphuric acid without the addition of ferric ions. So, the C 1, C 2, C 3 all this corresponds to gradual addition of different concentration of ferric ions I keep adding. You imagine an artificial you know imaginary experiment right. In an imaginary experiment you take a bicar and you take a sulphuric acid and you dip a ion piece and you can measure the ecar value easily right. But of course, it is very difficult to measure icar we will see later how to do that, but icar also is some way you can able to determine actually. I keep adding different extent of ferric ions and each of these lines corresponds to the cathodic kinetics for that addition of the ferric ions in the solution agree. So, these are not simply lines these are all corresponding to the different kinetic equations. They are different taffle lines that you have right they are different taffle lines. If you have this now I can find out what will be the corresponding corrosion potential and the corresponding corrosion current density. So, I can start plotting this I can plot thelog icar I plot versus the concentrations can I. So, a 0 concentration I know what the corrosion rate is somewhere it starts here at 0 concentration that is 0 concentration then what I have I have 11. So, I have added different concentration. So, I would find out what is the corresponding corrosion rate for each addition of the ferric ions. So, if I add C 1 concentration the corrosion rate increases. So, I would approximately make this point here right this corresponds to what. So, you can also label this if you want there are so many lines is the problem A, B, C, T, E, F ok that is H. So, there are several points that you can label here in the polarization curves ok. So, this corresponds to what this corresponds to A right this may corresponds to B and what next 2 corresponds to what C and C 3 corresponds to the D the 4 corresponds to E right and C 5 corresponds to what corresponds to C 6 corresponds to D here. Please notice when you keep adding I am not moving here because to move this particular point or to reach this particular point the system has to cross this critical condensity. So, when I add these ferric ions it will not cross for example, if you add C 4 C 4 corresponds to this it cannot go beyond E. So, the corrosion rate only increases to 2 from D to E it would not reach J natural reach M also ok. So, it you can go only up to this. So, when adding the concentration higher quantities you know the current increases it does not go beyond these values, but when you add let us say C 7 what happens? When I add C 7 what happens now what happens to C 7? It decreases to this value ok it C 7. So, it it becomes now corrosion rate lower than this it comes close to this. What is this called? P C 8 remains the same C 9 remains the same C 10 remains the same ok. Then C 11 what happens? C 11 it again increase where does it go it goes somewhere close to D actually right it goes somewhere close to D here yeah oh ok also ok. Now so, you can see that the corrosion rate increases it decreases here it remains stable and again it increases. So, what does this line tell you is that the gradual increase in corrosion current density on the addition of oxidizer until it reaches completely the passive state then there is a short decrease in the corrosion rate current corrosion current density it remains almost the same again it goes to trans passive region it goes here ok. Now, let us look at the reverse situation after reaching this particular point I dilute the electrolyte by using water or I reduce the concentration of the ferric ions gradually from C 11 to C 1 I reduce the concentration of ferric ions gradually from C 11 to C 1. So, what happens to the corrosion rate of that metals? So, after reaching this place I start reducing the concentration of ferric ions. Now, when I start moving from here to this place what will happen now? It will again follow the reverse path it will follow the reverse path here right it will again follow the reverse path here it follows again same thing again follows again follows. So, from here to please C 7 we are very clear it is almost reversible it has regained its classivity. Now, from C 7 you go to C 6 now let us go to C 6. C 6 you have the options of having E car decided by this point or decided by this particular point at 2 and this H is a unstable point we do not worry about it and you can have either E car here or here. How do you decide whether the E car will remain here I will come over here? Do you think the E car will remain here or do you think it will shift to this place yeah? It will it will remain there because to maintain this anodic reaction you do not need more amount of oxidizer. So, once once you reach passivity the amount of oxidizer required to maintain this passivity is much less because the rate of anodic reaction here is much less compared rate of anodic reaction here. So, once I have the passivity the passivity remains stable because the rate of oxidation is equal to rate of reduction and you do not need too much of oxidizer for that. So, that will happen until you reach the point C 3. So, until you reach the point C 3 ok, the passivity will remain the same. Then what happens at C 2? At C 2 you will go back to C go back to C and then you will follow same line understood for the you can label what these are all all these lines correspond to what corresponds to 3 plus plus electron gives you. I will give another example then we will further into the other topics of electrochemical kinetics. So, let us look at the velocity. We have seen this earlier how the velocity can affect corrosion it need not affect corrosion. You tell me a case where the velocity may have effect on corrosion it may not have an effect on corrosion. If the corrosion is dash then velocity has no effect. If the corrosion is dash velocity has an effect yeah. So, if the corrosion is diffusion controlled then the velocity has an effect. If the corrosion is activation controlled velocity has no effect right that has to be very clear about. We will see later velocity has one more effect later we call erosion corrosion that is a different point we will talk about later. Now, let us consider a case where the corrosion is under diffusion control. So, diffusion control process and metal undergoes active dissolution then what happens? You simply draw very easy right you just have to go be do it very systematically represent the the kinetic diagrams a metal undergoing only active dissolution, but the cathodic reaction is what is diffusion controlled. This you have seen it and this corresponds to what to I L am I right and it also corresponding to the I car. So, if I increase the velocity what is suspected here what will change what will change which one the anodic curve will move yeah the cathodic curve will move why should it move the I L is inversely proportional to the diffusion layer thickness. The diffusion layer thickness decreases when you increase the velocity of the fluid am I right. So, if we increase the velocity ok. So, velocity 1 then you have velocity 2 3 velocity 4 velocity 5. So, you can plot the I car versus the velocity. So, what happens? For the 0 velocity you have this corrosion rate coming here what happens when you increase the velocity it is increases then what happens why it is not why is that the velocity has no effect here yeah it has become activation control right. So, both are act happening in series when the diffusion is is not controlling it becomes totally an activation control process ok. So, for the active metal right let us go to the passive metal and see what happens can I move ok. So, what do you have to do you have to draw anodic polarization curve showing the active passive and trans passive dissolution and also represent a cathodic curve exhibiting what exhibiting the passivity and you change the cathodic reaction kind of take with increase in velocity find out the mixed potential in all these cases and you can find out the corresponding I car right. So, there you go and see what happens you can start plotting and see how things are different in this case. So, what do you think will happen can you make a guess before you go into this what can happen with increase in velocity yeah. So, it is possible that the reduction rate can cross or go beyond the critical current density right. If you increase the velocity the I L can increase and go to an extent where the I L is going to be greater than I C. Of course, there is one more caveat there the caveat is the equilibrium potential of the cathodic reaction has to be much higher than the passivation potential just imagine what happens right. So, then what happens if you have I L becoming greater than I C what do you think will happen yeah it will become spontaneous in the passivating right the I car will will drop I car will drop. So, let us draw this and see if it can happen at all. So, I am also drawing along with you right this is a anodic polarization curve showing active passive trans passive dissolution right and I would go for a cathodic reaction whose equilibrium potential is quite relatively positive. Then I represent the cathodic kinetics and if I increase the velocity so, 0 velocity v 1, v 2, v 3 and v 4 and you can know the corresponding limiting current densities you can also plot log I car right 1, 2, 3 is it ok. Now, I want to say that we are not going to stop using this mixed potential theory from now. No, we will be applying the mixed potential theory when you start discussing let us say galvanic corrosion, crevice corrosion, pitting corrosion. In all these cases we will apply later and see how they are useful in understanding the various forms of localized corrosion. So, we will revisit it and to appreciate how they are useful to understand the concepts of various forms of localized corrosion. And for the for the time being we stop here we move on to next topic of what is called as how to use electrochemical principle or electrochemical technique to determine the corrosion rate. The corrosion rate determination you might have started discussing and doing experiments in the lab to measure the corrosion rates. One simple experiment is to use gravimetry study where you may choose a sample of known dimension known area right known surface area. You immerse it in a given electrolyte a corrosive medium you may call whatever for a given time you measure the loss in weight using a balance and you know the time of exposure you know the loss in weight you would know the density and you will be able to calculate the loss in thickness right. But when you do these experiments it is a very time consuming it is very time consuming when the corrosion rate becomes very low. Have you started this experiment any of you who are the guys who have done the experiment what experiment did you do? Wind-close metal. What material? Mild steel. Mild steel. What environment? SCL. SCL. SCL. So, what did you observe suppose you put you just a dip steel in SCL what concentration what have you observed did you observe anything visually did you see anything what happened? It is very important that you know experimentation is observation if you do not have observation you cannot do research you know a very important thing. You would have noticed a large amount of hydrogen evolving on the surface there may be a solution turning into reddish brown color right may be ferric ions may be formed whatever. Suppose I give you how how long did you immerse? But 24 hours. 24 hours. In 24 hours you are lucky you are able to see an appreciable loss in weight that you can measure in a balance. I give you a titanium and I give you let us say soaring chloride solution of 3.58 percent. I want you to determine the corrosion rate after 24 days you just measure the change in weight be so insignificant you may not measure you know appreciable you know weight loss very difficult to do this. In fact, when you do an experiment the time of immersion there is a thumb rule you know what is the thumb rule 2000 upon MPY gives you the hours MPY is in terms of corrosion rates you will see later you know all the units in detail ok. So, lower the corrosion rate higher is the time required to expose in order to get a reasonable you know weight change metal like titanium will take a lot of time. So, this technique is not going to be very good technique. So, you have to find a technique that gives you corrosion rate may be in you know 1 hour, 2 hours, 3 hour, 4 hour depending upon the things. Not only that we measure the corrosion rates we also to try to understand the mechanisms that is where the electrochemical techniques they come in handy ok. So, electrochemical techniques are that way more powerful than the weight loss measurements. One it can be quite quicker, second is you can understand the mechanisms you can look at the tuple slopes all these kind of stuffs. Now, in order to interpret the the so called electrochemical data we call as you say DC polarization technique and you need to understand the plots. How do you really understand what is the point of origin to interpret this data? The point of origin for that is a mixed potential theory. So, what we will be seeing now is how do you correlate a typical potential static or potential dynamic polarization experiment to the real electrochemical kinetics ok. So, we are trying to know correlate between these two that we will see in the in the next class I do not know where you know in the next class we will start now I do not think we will have time to do that. And and so, that is a very important thing ok. So, we will see in the in the next class. So, right now I think we will close our discussion if you have any questions please let us discuss ok.