 Welcome to the in the lecture 6. In the previous class, we discussed the application of electrochemical kinetic concepts to corrosion. The basis for application of the electrochemical concepts to corrosion is the mixed potential theory right. Mixed potential theory is the basis ok for application of a electrochemical concepts to corrosion. And the mixed potential theory talks about freely corroding material freely corroding say I call it as a metal. When I say freely corroding no external current no external potential is applied on the metal. You just immerse it and let it corrode and in that condition whatever corrosion rate that occurs given by corrosion current density and the corresponding corrosion potential they are all dictated by the mixed potential theory. So, in the freely corroding material essentially what happens I oxidation is equal to I reduction. When I say I oxidation means there can be more than one reaction of oxidation there can be more than one reactions of reduction process. So, the total oxidization is equal to total reduction rate. So, that is the mixed potential theory. We discussed this yesterday using you know a metal with an oxidizer wherein we said that if you if you if you talk about a metal go back to your nodes and see that now I we have two independent equilibria in a corrosion system right. So, you represent two independent equilibria in the in the system for a metal you have an equilibrium potentials and you also have what you call this exchange current density right you have this and the other equilibrium that we talk about let us say something like maybe hydrogen evolution reaction whatever you can we can talk about. Now, you have I naught H plus H on the metal that we are talking about is happening and this is what this is E equilibrium potential of H plus H this is over D equilibrium potential of what it is metal N plus and M. So, we said that they independently polarize following its own tuple kinetics this is the tuple kinetics for oxidation the tuple kinetics for the reduction process ok. Let us say M going as M N plus plus N electrons and in this case you can say M going as M N plus plus N electron. Similarly, you can write for the hydrogen as H plus sorry sorry this is H going as H plus plus electron oxidation and you have reduction process corresponds to H plus plus electron use wise to half hydrogen and you find at only one place that is the rate of oxidation is equal to rate of reduction and corresponding to that potential we call them as E card. So, we are talking about the application of the electrochemical parameters such as I naught, E equilibrium potentials, the tuple slopes that we have here ok. How they will they govern the E card and the I card values? In the last class we also saw two examples of iron and zinc and we said that in in acid solution zinc will corrode at a lower rate compared to iron right. The reason was what was the reason for that? The exchange condensate is different ok. The exchange condensate in this case differs very significantly and so, despite zinc having a relatively negative equilibrium potential it will show a lower corrosion rate compared to iron. The reason was the exchange condensate we saw in the in last class. I hope you have seen the the notes. Let us move further in understanding the corrosion of metals. Now, we look at the role of a oxidizer. Let me give you the picture. I take a bicarb and I take let us say hydrochloric acid take that and I immerse in iron piece. Now, assuming that the they are in the standard state ok, iron is in equilibrium with let us say Fe 2 plus of unit activity and the hydrogen gas here is 1 is equal to 1 atmosphere ok. It is possible for you to calculate the corrosion right right. If you are given X n condensity, taffel slopes, you can compute the equilibrium potential all you can do that right. Can you graphically as well as solving equation can you find out ikor value in this? Can you or can you not? Is it possible or not? Yeah. I want to clear you know response. I just take iron piece in put in in hydrochloric acid and I say that activity of Fe 2 plus here is unit activity and I also say that the hydrochloric acid is of unit activity. The partial pressure of hydrogen is equal to 1 atmosphere and I will give you X n condensity values. I give you taffel slope. Is it possible for you to determine the ikor ikor? Is it possible or not? Yes. Yes ok. Now, what I am going to do is to this I would add to this to this I would add ferricoyans add to this and the ferricoyan concentration the activity of Fe 3 plus can be considered as unity. Now, I need to know what happens to the corrosion rate of iron and I would like to calculate what is the corresponding corrosion potentials right. So, we need to calculate ikor in in HCL plus what plus ferricoyal solutions. I need to calculate ikor in HCL plus ferricoyans additions here. Is it possible to calculate or can we able to visualize how to get this based on what is called as the mixed potential theory right. Now, how to proceed? What to proceed here? How to proceed here? First of all, you need to identify the equilibrium first how many equilibria are present in this system right. So, without let us say without the addition of say ferric ion means I add ferris ferric chloride without the addition of ferric chloride. What is what is the equilibrium we will have start with? Yeah, one will be H plus plus electron gives you hydrogen. What will be the other equilibria? Fe 2 plus square electron gives you Fe. Now, for this I can calculate I can calculate E I can calculate E if I know the activity of the ions. I can also know maybe experimentally otherwise the X-ray in gun density or what Fe 2 plus Fe and here H plus H on ion right. So, now, I can able to first of all determine the corrosion rate of ion in hydrovortic acid. Can you do this graphically with events diagram? So, how to do that? Can you please try yourself first given the tuple slopes right. You you you just draw the events diagram so that you can obtain E car and I car right. You assume a standard state ok the standard state here you can assume E equals to E naught you can also assume X-ray in gun density of H plus H is equal to X-ray in gun density of ion 2 plus ion is equal to 10 power minus 6 ampere centimeter square. And you can also assume that beta or equal for assumption is equal to you can assume let us say about 100 millivolts you can compute right. Can you draw it yourself and get E car and I car values? Suppose you have drawn a similar diagram like this ok. So, what is the additional equilibria now I have? Suppose I add to this ferric ions I would have another equilibria Fe 3 plus plus electron gives me Fe 2 plus right. Would I have this or not? When I add ferric chloride to this solution I already have Fe 2 plus ions it can establish one more equilibrium which is Fe 3 plus plus electron giving you Fe 2 plus right. I have given you a handout if you take a handout there and see what is the corresponding standard potential for this yeah. It is E naught for this is equal to plus 0.771 what right. So, what I do I represent the same equilibrium I mean in the same diagram I represent this equilibrium in this diagram. So, you represent that diagram equilibrium there can you put this in here. So, the value comes somewhere here and of course, I can have a diagram going like this. What is this one correspond to? What will be this line correspond to? What reaction? Reduction. Reduction right. This will be 3 plus plus electron gives you and what will be this one will be Fe 2 plus going as Fe plus electron here and this corresponds to your exchange current density and we have. Now, there are three equilibria in the system 1, 2 and 3 having different equilibrium potentials and they have different exchange current densities you can also use the different tappel slopes you want it is not necessarily the same slope it is easier to operate here. So, I have assumed the tappel slopes are going to be same it is not necessary at all. Now, what does the mixed potential theory say? It says the total amount of oxidation is equal to total amount of reduction now. So, what I do now? I need to find out what are the corresponding oxidation reactions and the reduction reactions now. Let us take this let us take the now let us take let us start from here at any given potential this is the reduction reaction reduction. When you cross this potential this notice let us say you are going somewhere here suppose you are coming to this particular potentials. The total amount of cathodic reaction is given by the summation of this current and this current. If I take this metal and hold this metal at this particular potential what are the reaction will occur? Reduction of hydrogen gas I mean hydrogen ions into gas and reduction of Fe 3 plus into Fe 2 plus both will happen on this metal surface when you start moving below this potentials. So, the total current will be is equal to sum of this current right and when you cross when you cross this when you cross this potential how many reduction reactions will occur? Below this potential how many reduction reactions will occur? 3. 3 right. So, you have one more current adding to this is understood? So, this corresponds to what? This corresponds to I of what? I of Fe 3 plus 2 Fe 2 plus plus I of H plus 2 hydrogen plus I of Fe 2 plus 2. I have all the 3 reaction taken part simultaneously with the same diving force. Since Fe 3 plus 2 Fe 2 plus have high potential and are also nobler than H plus 2 H 2 what will happen in every cases? Yeah. So, we are coming to that ok. We are we are coming to that we are coming to what will happen to Fe 3 plus Fe 2 plus hydrogen gas evolution that is what we are going to arriving at actually. So, this is one step towards learning what would happen ultimately. You got it? We are reaching that point, but we are trying to go step by step to show how that point is reached. So, that you have clarity in terms of the electrochemical kinetics. I hope this question is clear and I hope you are clear as to what we are doing. We are simply trying to relate the current versus potential if you have multiple reactions actually right. So, please notice every every case the equilibrium will behave its own manner. This one this is not going to change this is not going to change nor this is going to change. But what will happen is the total amount of reaction that are occurring are going to be changing depending upon the potentials you got it or not ok. So, you find that the the number of reactions that will happen will increase it becomes 2 here it becomes c here. Similarly, you can also consider what will happen to the anodic reaction right. Let us look at the anodic reaction this you can extrapolate this you can extrapolate this where will the anodic reaction start increasing? It will start increasing on this point got it? So, you will find the anodic reaction for this is going to be this. So, what is this one corresponds to? This corresponds to both i corresponds to i e f e going as f e to plus plus 2 electron plus what? i h going as h plus plus electrons am I right correct or not correct? If I go to this potential if I if I cross somewhere here suppose I draw the a line here and beyond this point what happens you have one more what reaction oxidation reaction will take place. So, it is a very simple way of putting things here. Now, look at this diagram there is only one place the mixed potential theory is valid tell me where it is? All other places the mixed potential theory is not valid for the mixed potential theory to be valid the rate of oxidation is equal to reaction I mean total oxidation rate is equal to total reduction rate. So, tell me where in this diagram the mixed potential theory is applied can you tell me that you have red color and you have blue color and you have black color yeah red color intersecting with the black line here ok. So, only here you find the mixed potential theory is valid here only. In all other places if you if you just you just hold you know at any any q suppose you take this potential here right what happens the rate of oxidation is more than the rate of reduction all will happen here only here rate of oxidation is equal to rate of reduction. So, that is going to be your changed what? Changed Icar values. So, this is going to be your Icar hydrochloric acid and SCL added and Icar SCL plus C3 plus I hope you are not simply taking down the nodes I hope you are following whatever I am discussing here. We are simply applying the mixed potential theory right and any given potential you find out you take any potential you want right maybe you take this particular potential you find out what are the reactions are occurring is there only one reaction occurring here. So, is there only one current? No yeah I know the current is concerned only one current, but you have how many you have two cathodic currents here. So, sum it up ok. So, you just have to find out at any given potential how many reactions are occurring and add up that current that gives you total current total anodic current and total cathodic current or the total oxidation current and total reduction current right. Are you clear about this? Can I move? So, I will read out this for your clarity so, that we can understand this diagram a bit better. Let me draw only the relevant portion of the Evans diagram ok. This corresponds to drawn this diagram. This is your Icar in the absence of ferric chloride. So, Icar let me now tell you what happens. Let us visualize this diagram again. We have a bicar, hydrochloric acid and you have put the steel here put the iron for simplicity ok. If I measure the Icar of this that corresponds to this one am I right? If you can know the Icar of that that should corresponds to this agreed. Now, the rate of hydrogen evolution on the surface please notice the road the rate of hydrogen evolution on the surface is given by this point corresponding to this current the hydrogen evolution occurs. The same point also corresponds to the Icar I mean high resolution of the metal right. If ferric chloride is added if you add ferric chloride to this ok. So, what do you observe on the metal surface? Will there be increase in hydrogen evolution? Decrease in hydrogen evolution? Increase in the corrosion current? Or decrease in the corrosion current? Increase in the Icar or decrease in the Icar? Please note down I would like to know the following. What happens to the hydrogen evolution rate on the metal surface? What happens to the Icar? What happens to Icar? Now, let us go to the question number 1. What will happen to hydrogen evolution on the metal surface? Can you look at the diagram and tell please look at the diagram you know what is the corresponding Icar right? What is the corresponding Icar for that? Will the Icar remain the same when I add ferric chloride solution to the what will happen to Icar? So, potential move off. So, then what happens? Then what will happen to hydrogen evolution? Please see there. Please look at the diagram. It decreases right. At this particular potential the hydrogen evolution on the metal surface is decreases it is decreases from where? It decreases from the hydrogen evolution rate is decreasing. What happens to corrosion rate? Is it decreasing? It is increasing right. The corrosion rate is increasing ok. So, please follow the diagram ok. If you follow the diagram you would not go wrong. Why does it decrease? It decreases because the over voltage for hydrogen evolution reaction. What is the over voltage for hydrogen evolution reaction in this case? Can you how do you find out? Without ferric chloride this is my Icar right. Can I find out the over voltage for hydrogen evolution? What is the over voltage? Equals to Icar minus E E naught E E equilibrium right. Now, when you applied when you add ferric chloride what is happening to over voltage? It is decreasing. So, the hydrogen evolution reaction rate has to be decreasing it should be consistent. Then oh I increase the corrosion rate of iron hydrogen evolution is decreasing. How is possible? Because the ferric ions are getting reduced to ferrous ions. The electrons are now taken by other species to do cut it actually here. Of course, law of conservation of charges have to be there it cannot happen. You cannot increase the corrosion rate and there is no corresponding cathodic reaction to what happened. It only says the hydrogen evolution rate is decreasing, but the Fe 3 plus to Fe 2 plus is increasing. It is going to increase from here to this. The over voltage for that is increasing from here to this is happening or not right. So, that is the important thing you should understand in the electrochemical concepts ok. Any questions here? Is it looking confusion? So, this is a an important and also the most one of the most difficult events diagrams that you can think of ok. You will see one more complex events diagram later, but this is one of the most complex events diagram. It is very simple. What have you done here? When I said corrosion, I identified what are the possible equilibria right you started from there. So, identify the equilibria right. You identify the equilibria first of all. If you identify the equilibria, then you should have electrochemical parameters for that. Equilibrium potentials, exane condensate, table flow all the required. Use the parameters to draw events diagram. If there are more than one oxidation or more than one reduction equilibria possible, represent them in the events diagram. Follow the mixed potential theory. What does it mean? You can also find out any any given potential what is the total amount of oxidation and what is the total amount of reduction right. You construct events diagram for that. Then at one point you will see that the mixed potential theory is valid wherein the rate of total oxidation is equal to the rate of total reduction. That is the place the metal will corrode and that is the corrosion rate that will be the corrosion potentials. So, it is a very simple concept is only thing is you need to go systematically to get this particular you know e car value and i car values. Let me go further into the system. Let me take another example of corrosion of metal say in seawater. So, an example it is only an example here. What does seawater consist of salt solution NaCl solution? What is the cathodic reaction here predominant cathodic reaction? It has got what oxygen? Suppose I take a metal say I am having M a metal immersed. So, the picture is that I have a seawater I am immersing a metal in this. Why did it take seawater? What is the cathodic reaction here in this case? What is the likely cathodic reaction in this case anybody recollect? The cathodic reaction here is oxygen plus what is the anodic reaction you can say M going as this is your cathodic oh I am sorry the cathodic and this is why did I give this system because we remember that we talked about oxygen reduction reaction is diffusion control right. So, when it is diffusion control we will see how the corrosion rate changes or what are the factors that control the corrosion rate of metal when the system is under diffusion control ok. It is before I go into that it is very interesting to see we talked about sodium chloride here. Sodium chloride does not figure out anywhere here can sodium chloride directly take part in corrosion process in oxidation reduction can it take place? It cannot take place chloride cannot take an electron sodium ions are you know they are so electro negative it cannot happen. So, NaCl directly does not take part in the corrosion process. We will see later how the sodium chloride affects the corrosion you wait for some time it is indirectly affecting the corrosion process. No electrochemical reactions are possible with sodium chloride dissolved in water neither it can take electron nor it can accept electron ok because of the electrochemical potential this particular equilibrium will have. Let us come to this. Now, what is the diffusion control process ok how do I represent this? I have two equilibrium one for oxygen other for the metal ok. For a metal it is also in that I have here this is activation controlled here and what is this? This is and you can also know what is the equilibrium potential exchange condensate equilibrium potential for that exchange condensate for this all you can represent in the evens diagram. What does Icar depend on? What is this current called by the way what is this current called? IEL. IEL right this is called as IEL is called as limiting current density. So, Icar depends on what? Depends upon the diffusion current density or limiting current density IEL. For argument sake if I change a metal another metal N let us say with this is let us say N going as N somewhere say N plus only plus N plus electron gives you that ok. So, what happens? This is the other metal. So, what happens here? It has got a different equilibrium potentials it may have a different exchange current density what happens to corrosion rate what do you see? So, limiting current. So, the corrosion rate does not change as long as the corrosion is controlled by diffusion process that is the cathodic reaction here. So, you may have a relatively noble metal relatively active metal for example, I take stainless steel a technical carbon steel may all corrode at the same rate if it is a diffusion controlled corrosion process. Can I make this statement? Is it correct? Not correct. I would respond is it correct or not correct? You will have any conviction or not? No? What will happen now? Please tell me in this case if I change the equilibrium potential of the metal or exchange current density by change will the corrosion rate of the metal change or not? It will not change ok. So, you will not change you do not see that things are happening at all ok. So, this is what is important and this is we call them as diffusion controlled process. We have seen so far the metals which are actively corroding there are some metals they show passivity ok. So, we should understand the concept of passivity and it was Michael Faraday I suppose all of you been knowing him right. He did it in 1840 he did some nice experiments ok to show that the metals undergo passivity all of us know about that I think you know I do not think the any of you guys were not aware of passivity you know stainless steel and carbon steel the difference is in the corrosion rate and that corrosion rate is achieved because of stainless steels showing passivity. So, let us try to understand systematically what is mean by passivity? Passivity means the reactivity comes down reactivity falls that means, the corrosion rate of the metal drops under certain circumstances under certain circumstances. So, that we should know how what kind of circumstances are we talking about. He did some nice experiments on Michael Faraday you people know that nitric acid right it is a very highly oxidizing acid what it did was it took a beaker and you fill that with concentrated HNO3 and it dropped a steel piece, but highly oxidizing solution they should corrode very heavily ok. Observe no corrosion no I do not say no corrosion I mean say no observable corrosion ok, no observable corrosion rate you cannot see that surface looks reasonably you know uncorroded somewhat shiny you can say. Then it did another experiment it took in this case dilute nitric acid steel you found lot of hydrogen gas coming. So, there are hydrogen evolution. So, you see corrosion process corrosion is visible. Then it is wondering what is happening at all? You made a scratch here it took a glass rod you made a scratch here like that then you found corrosion occurred, but again it stopped. Then you did another experiment ok. Barren you have beaker dilute nitric acid you put a steel here steel steel from test 1 is test test 2 and test 3 ok and you transfer this one ok that means, you transferred sample here for no corrosion no observable then you made a scratch corrosion occurred. So, when there is a scratch here you had a scratch here the corrosion start occurring lot of hydrogen starts evolving from that. When you when you made a scratch here it did not happen corrosion occurred, but again stopped in this case it is happening in this case it is continues to happen from the beginning. So, you concluded that film forms film forms in test 1 called as passive film. So, that was a an experiment and you showed that steel will corrode less in concentrated nitric acid you corrode more the dilute nitric acid that is in unusual right when increase the oxidizer concentration the corrosion should increase only, but does not happen. So, you call this as a passivity. Now, passivity is a big subject and I will just give you a brief account of that you form a film as a films or barriers barriers for what barriers for corrosion they form an electrical barrier for corrosion right it has happened and this films is about 30 to 100 angstram thick you know what angstram is it is 10 power 10 meter as a 10 power 10 angstram is equal to 1 meter. Now, what is this this passivity really means what does it mean from electrochemical point of view? Now, I consider a metal in a solution and I apply this potential I apply eta is equal to negative. So, positive if I apply a positive eta what is eta is a over voltage. Now, I follow the current and you will see. So, I start with an equilibrium potential and I have a corresponding exchange current density right this is the equilibrium potential for the handle here. When I apply a over voltage it is positive what will happen to current? Current is supposed to increase like that and this behavior is called what what relation does it follow here? Tafel relation right it is a Tafelian behavior it increases after some time the current drops then again it goes like that again goes like this. See there is an active dissolution the dissolution rate decreases and what happens here? Here it is a passivation call it and again what happens here? Here again dissolution this is called as trans passive dissolution or we will see later it is also called as pitting. The metal undergoes active passive trans passive transition right as a transition active passive trans passive transitions. This is how the current is moving this is a schematic of a metal exhibiting passivity. As you rise the potential you see the driving force for corrosion increases but then again decreases again increases and this metal is supposed to be showing active passive trans passive transitions. They are very important for us most of the engineering metals you want to make them corrosion resistance one way to do is to make them passive or you want to reduce the corrosion rate you have to go for a noble metal. Noble metal is not possible you cannot use copper you cannot use silver you cannot use gold but if the metal can show passivity then they are very good. Such metals are what like stainless steels and alloy titanium chromium some of these metals and alloys are very much useful because they exhibit passivity. Now, for as from this diagram we need to know how from this diagram we understand the corrosion of the passive metals. Before we go this I would like to you know tell you what are these parameters really mean. This is the maximum current the metal reaches right when you polarize anodically and this current density is called as I critical current density the maximum current beyond which the metal starts passivating and this current density is called as IP passivation current density and this potential above which the metal starts passivating and this potential is called as passivation potentials. We can use P as passivation potential and this potential what you see here where the current steeply increases this potential corresponding to this is called as pitting potential or also called as E trans passive potentials. Now, let us look at the behavior of this active passive trans passive transition how it changes it would change depending upon the metal depending upon the environment right. So, passivation behavior suppose I take a metal that exhibits passivation let us say I am getting this passivation in sulfuric acid ok. The concentration of sulfuric acid here is let us say 0.01 molar sulfuric acid if I increase the concentration if I increase it further. So, if I can change the concentration of the environment the passivation characteristic change you know what are the characteristics the critical current density, passivation condensity, the trans passive potentials things will change. This could change other like even temperatures you can also have let us say chlorides suppose I have chlorides could change. In essence the passive passivation characteristics depend upon what? Depend upon the metal the environment they decide what they decide IP, EP and EP it are also called as E trans passive potentials and you can also change your condensity. So, the metal and the environment ok can influence these parameters they are not unique you know kinetic parameters. Now, let us go to the next question for us in such a case what decides the corrosion rate? What decides the corrosion rate in system? Follow the same principle right. So, you know that the metal is passivating right. This is an anodic reaction maybe let us say you have a metal going as m n plus plus n electron this may be your your cathodic reaction for that this is your I naught m plus you know m n plus right m n plus m this is the equilibrium potential how do you get corrosion rate for this? What do you need for that? You need a cathodic reaction. So, what is the corresponding cathodic reaction? If the corresponding cathodic reaction is represented I can get the corrosion rate. Assume that the cathodic reaction follows this. Assume that I have a cathodic reaction it follows this. Assume that I have a cathodic reaction of course, I can ask a cathodic reaction you can imagine I mean you have no restrictions about that ok. Now, you can have a cathodic reaction of this kind, this kind, this kind, this kind all are possible then for you now to determine E car and I car it is not a difficult. What is the criteria for identifying E car and I car? What is the criteria? Mix potential theory. Mix potential theory the rate of the total oxidation rate of the reaction is equal to the rate of the total reduction rate of the system. If it is if it is governing you get E car and I car is as simple as that. What would be the the nature of the anodic curve and cathodic curve? That is the criteria for you to determine E car and I car values. So, we will see this in the next class how you can determine these values and how such diagrams are useful in real life situations ok. So, I stop here ok.