 in the last class we started discussing to find out if we can predict whether a metal will undergo corrosion or not. We will continue that one to make this concept clear. Before we do this, I like to summarize what we discussed in the last class. We said that for any reaction be it a corrosion or be it a chemical reaction or a physical transformation the first criteria for the spontaneity the reaction to occur spontaneous the corrosion is the free energy change. The free energy change delta G has to be negative this is what we saw in the last class. But if you talk about a corrosion measuring the free energy change is more difficult for every corrosion processes. So, what we did? We looked at the corrosion processes especially the aquas corrosion processes in detail. We said that the electrochemical corrosion the electrochemical reaction is the basis for aquas corrosion right. You may take for example, zinc reacting with hydrochloric acid giving rise to zinc chloride and plus the hydrogen gas. This may look like a simple chemical reaction, but in a sense e is an electrochemical reaction right. If you recollect we said that you can write this as zinc interacting with 2 plus ions in the solution and then zinc is getting oxidized and the hydrogen ions are getting reduced to form this. So, there is an oxidation process there is a reduction process. So, you can separate this into zinc going as zinc 2 plus plus 2 electrons and 2 H plus plus 2 electron giving as the hydrogen process and we call this an oxidation and we call this as reduction process. So, if you assume that that this is in a electrochemical process, we go for electrochemical criteria. It is easy to measure the potential of in electrochemical systems. How do you measure this potentials? How do you relate it to the free energy change? We also saw that we use the Nernst relationship. What is the Nernst relationship? It is between the free energy change delta g is related to minus n f e is what we saw in the last class. So, you can relate the free energy change to the potential using this the Nernst relationship right. Now, for a chemical reaction you have a relationship delta g is equal to delta g naught plus RT ln activity of the products upon the activity of reactants you saw that right. For any reaction we can write like this and using the equation 1 into 2 do this you can convert this into E equal to E naught minus RT by n f ln activity of the product upon the activity of the reactants. And this is the the Nernst equation right. This is the Nernst equation. How do you read this equation? Let us take the example given here zinc right. Let me look at zinc immersed in in zinc ions right. This immersed in zinc ions right this is the zinc metal the solution. It establishes in equilibrium right wherein zinc goes as zinc 2 plus and again it comes back. In the process the electrons are getting exchanged between zinc ions and the zinc metals. The potential is so established between the zinc and the solution containing zinc ion we call them as in equilibrium potentials right. And this potential E is called as what is called as equilibrium potentials. Please understand it is neither oxidation nor reduction right. The reaction occurs forward the reaction occurs backward. So, you cannot call this potential as either an oxidation potential we do not call either as a reduction potential we call them as what? We call them as equilibrium potentials. This is a very important thing in understanding the electrochemical equilibrium. So, what I mean by that if I write an equilibrium in this manner or you write like this both of them would exhibit what? Both of them would exhibit same potential irrespective of what? Irrespective of the way you write it. So, the potential is sign invariant we saw this in the last class. So, how do I get the equilibrium potential? What is the criteria for that? Can anybody recollect? How do I get the equilibrium potential? If I give you a concentration of zinc let us say 0.1 molar suppose I give 0.1 molar I substitute in the equation right. But equation will be always written in such a way that E is equal to ok E naught plus 40 by N f L n activity of oxidants upon reactivity of reductants. So, only when you write like that you will get V value correctly otherwise E value could be changing ok. So, this is the the most important thing in determining the equilibrium potential for any electrochemical system. We discussed this in detail in the last class right and if you have any questions on this no questions. So, let me proceed now. Now, let us go into this the next equation right like we have seen E is equal to E naught plus 2.303 RT by N f and log of the activity of oxidants upon the activity of reductants. You know what R is right? R is what? R equal to the gas constant which is equal to 8.314 joules per mole K per Kelvin joule or kilo joule joule it is a joule it is ok. You can also see you can represent as kilo calories and T is is a temperature in in Kelvin right. What is F? F is equal to Faraday constant is given as roughly 96500 coulombs. If temperature is assumed to be is equal to 25 degree Celsius I have 270 convert that into it what happens is equal to 298 Kelvin then you substitute everything in this here 2.0303 RT by F. So, 2.303 RT by F turns out to be 0.059. You can calculate this at leisure time and you see that the number that is given is the correct number or not. So, now, using this you should be in a position to calculate the equilibrium potential for any electrochemical system right be it Fe 2 plus E is in equilibrium with Fe 3 plus zinc is in equilibrium with zinc 2 plus ions no matter what the equilibrium is you should be in a position to determine clearly what the equilibrium potential is in calculation. How to measure this? How do you measure this? How do you measure this? Is it a measurable quantity or I say no no it can be only calculated can you measure this? Right. So, I should be able to measure this. We have seen in the last class again that it is not possible to measure the absolute value right. If you are going to use another probe in the in the electrolyte for example, if I consider an electrochemical system some metal take for simplicity same zinc is in equilibrium with zinc ions where to measure it I need what? I need a probe. If a moment I put a probe here and I measure with the volt meter and this probe also will have an equilibrium. So, you are going to measure only a potential difference you cannot actually measure the absolute values it is not possible right. So, how does one solve the problem? The problem is solved by considering this equilibrium H plus plus 2 electron giving rise to hydrogen in the standard state. You consider E for that is considered as a. So, we consider that as a 0 and all the potentials measured are in relation to hydrogen right. So, in relation to hydrogen that you can measure it. So, you need an electrode you need an electrode which is also in equilibrium. So, how do you visualize this? Can somebody visualize an experiment measurement of equilibrium potential yeah yourself. So, let us say I want to measure the electrochemical potential of ion immersed in Fe 2 plus ions. What do you do? I take a beaker I put Fe 2 plus ions of the given concentration I dip ion electrode and other side what I will have? I will have an hydrogen electrode. The hydrogen electrode in what state? The standard state. What is the standard state? The activity of H plus ions equal to unity the hydrogen partial pressure is equal to one atmosphere and I use a voltmeter. What is that voltmeter? An high impedance voltmeter the measured potential between these two are called as the equilibrium potential of the ion Fe 2 plus system very simple. But in practice if I have to I have to measure the potential by use a hydrogen electrode it is very complicated. Why it is complicated? What is the hydrogen electrode? I will just define to you what hydrogen electrode is ok. I would say you would have some beaker made up of let us say glass I immerse a platinum electrode here right. And then what I do? I take acid let us say hydrochloric acid where in the activity of H plus is equal to 1 I pass through this hydrogen gas. What should be partial pressure of hydrogen here? The partial pressure of hydrogen here has to be the potential so measured equal to standard potentials. If I have to do this in the lab it is very cumbersome not very easy right. Can you do this very easily? Not very easy. So, you are going to use reference electrode which are easily can be made very easily. And these reference electrodes are what? They are all some of them called as Kalamal electrode ok. And you can also have silver silver chloride electrodes. You can also have copper copper sulphate electrode. There are several of them are there right. What is the Kalamal electrode? It consists of mercury and macros chloride like this. And these electrodes have potentials depending upon the concentration in this case concentration of chloride concentration of chloride here. And if you are going to use a saturated Kalamal electrode saturated Kalamal electrode ok. The potential is plus 0.2444 volts with respect to standard hydrogen electrode. Similarly, you can also have silver silver chloride and the chloride is saturated. Then the potential is is equal to plus 0.197 volt ok. I just give an example and there are several electrodes you should refer this in standard books ok. In fact, there is a book on reference electrodes only. How to make this reference electrode in the in the lab? You can you can construct this things yourself only. So, these values you can obtain from any books in a standard books. So, there is no problem ok. So, far I suppose you are clear about how do you calculate a reference? I am sorry how do you calculate equilibrium potentials? You know how to measure the equilibrium potential of the systems ok. Then how do you use this potential to predict if the corrosion occurs or not? That is given by this concept what is called as E cell is is equal to what? E cathode minus E anode here right. This is for the cathode and this is for the anode and by convention this has to be always positive. It cannot be negative at all it has to be only positive right. So, use this criteria. So, this is the criteria this is the criteria. Now, what is the corrosion? In corrosion you have two reactions one is oxidation we call as anodic and other is a reduction which is what which is the only thing is we do not know which will be cathode and it will be anode that we do not know. Now, let us take this example and see how this can be used to predict if the corrosion occurs or not. I have given you a handout ok. Please take this handout. What is given there? What is given is the standard oxidation reduction potentials. It is also called redox potentials. I would simply call it as standard potentials. What is the standard potential? Yeah. So, let us go back to this equation. To go back to this equation what you have seen before you go back to this equation here right. Let us take this equation one of this you take this equation here. When the activity of the of the the reductant the activity of the oxidant or or you can take this equation the activity of oxidant and reductant they are unity the equilibrium potential equal to standard potentials. Please remember this. So, it is a special case of the equilibrium potential right is it is essentially equilibrium potential the special case of the equilibrium potential right that I think is is what we should do. So, what I shown here take this table ok. Now, in the table let us take two cases take the case of say copper take the case of let us say iron 2 and I am going to take copper and I am going to immerse in sulfuric acid. So, I am also going to take iron and immerse in sulfuric acid ok. So, two cases iron in one normal sulfuric acid ok and copper in one normal sulfuric acid. Some of you who are not familiar with this normality, molarity I think you should read and maybe discuss with yourself and it will be hard for me to to discuss here actually ok. So, please get yourself familiarized with the with these units of concentrations how do you really represent them actually ok. Just take two cases. So, I am going to have put it in this form of a diagram take here I am going to immerse copper here one normal sulfuric acid and I am going to have iron in one normal sulfuric acid. Please notice they are de-erated de-erated means these are all de-erated no dissolved oxygen present in both the cases. Now, we need to predict if the corrosion occurs. I make your life more simpler for you I will say that the partial pressure of hydrogen here is equal to 1 atmosphere and the partial pressure of hydrogen here equal to 1 atmosphere I make it quite simple. Now, you tell me how do we really solve the problem? Let us take this the table where you have standard potentials given for the host of equilibrium and iron and copper equilibrium also given in this. Please tell me how do we solve this problem? So, what you have to do first of all first of all you have to write the corrosion equation right. What will be the corrosion equation in both the cases? Let us say in the case of iron what will be the corrosion equation? What do you think happens? If you put iron in in sulfuric acid what do you think will happen? With corrodes what ox what oxidizes? Iron will oxidize. Iron will oxidize as Fe 2 plus and hydrogen will as if if iron corrodes. Similarly, what will happen in the case of copper? So, copper should get oxidized as copper 2 plus or maybe copper 2 plus and this plus iron should get reduced as hydrogen gas if copper is. So, you start with the 2 equilibrium 2 equilibrium in this case. What are the equilibrium? Let us take the case of iron for example, what are the 2 equilibrium involved here that leads to corrosion? You have to start with 2 equilibrium right. What are the equilibrium that are involved here right? Before corrosion there are 2 equilibrium coming to picture. One is iron is in equilibrium with the Fe 2 plus ions ok. Again to make it simple I want to make this one more variation here. You immerse iron in Fe 2 plus and the concentration is equal to 1 molar right. Similarly, in this case I have a copper and copper 2 plus equal to 1 molar I assume this make assumption right to make it more simpler for you ok. So, how do I start with? How do I start? So, first of all you calculate E cell right. So, before calculate to get E cell what do you need? Yeah you need the equilibrium potential for 2 independent equilibria right isn't it? So, you need to calculate E cell to calculate E cell you need what? You need the equilibrium potential for 2. What are the equilibria here in can you write? What are the equilibria involved in the case of iron? Can you please write here in your papers what is equilibria involved here? So, involved are Fe 2 plus plus 2 electron gives you Fe is 1 equilibria. What are the other one? 2 H plus plus 2 electron gives you hydro is 1 equilibria right. So, equilibrium 1 equilibrium 2 for this right. What is for copper? 2 plus plus 2 electron gives you copper and 2 H plus 2 2 electron gives you hydrogen. So, it is for 1 the 2. Now, these are the equilibria now one should become cathode other should become am I right or not ok. So, let us take the case of iron and try to solve the problem. So, what is the equilibrium potential for Fe 2 plus plus 2 electron Fe with respect to this problem minus point why you take minus 0.44 why did you take that? Yeah why did you take that? You were right I mean why did you take minus 0.44? Reduced also will they have same potential only same will sign will change if I am going to take iron as getting oxidized or reduced is the equilibrium potential sign invariant or sign variant right invariant right. So, minus 0.44, but why not minus point something else why did you choose you are right why did you choose you look at the problem there yeah you are talking about a standard here you see iron is immersed in activity is equal to 1 that is why you are taking minus 0.44 if the concentration is different for example, if I say it is 0.1 molar would you have minus 0.44? So, what happens in that case you have to now calculate that actually right you have to calculate it please understand. So, you need to calculate that in order to get the values. So, I made the life simple by assuming that both equilibria are in the standard state. So, we are using the standard potentials. So, let us assume that both copper is corroding and iron is corroding let us assume that please find out whether corrosion occurs or not. Can you please make a calculation and see assume that assume copper and iron corrode. So, what happens? Can you find a E cell if you assume that that iron is corroding and copper is corroding what happened to E cell? Since E cell of iron is positive therefore, iron is corroded. It will corrode. But E cell for copper is negative therefore, copper is not corroded. It will not corrode right. So, it is possible that you can come to a right conclusion. You may assume whatever you like you may you may even assume that iron is not going to corrode and say that it is E c minus E a then you will get into a negative sign. So, it is possible to to compute and say that the metal corrode or does not corrode. I hope each of you label to make these calculations E cell is equal to E c minus E a. So, what happens? We assumed that iron is corroding. So, what happened minus of minus 0.44 and what is E c? It is 0 E cell is equal to plus 0.44 volt. So, iron corrode ok. In this case you take this this case E cell equal to what? Is equal to you assume that copper is corroding that means E cell in this case is equal to 0 minus of plus how much 0.337. So, E cell is equal to minus 0.337. So, is is is a volt. So, copper cannot corrode. So, understood ok. So, it is possible for us to predict if a metal will undergo corrosion or not. Now, measurement of electrochemical potential is much easier. It is very easy you go to lab and in about 5 minutes you can measure it ok. All you need to have is a voltmeter of high impedance value, you need a reference electrode and the metal that you are interested to measure the metal or other system whose potential required to be measured. So, I suppose you people now are now clear in terms of the first concept of how do we really predict if the metal will undergo corrosion or not. To extend this argument I want to pose a problem to you. The problem is I take tin ok, in in water of pH 7 ok. Two conditions take water and immerse tin here. In another case I take water I have tin I close this here. It is open ok. It is what I mean in this case it is closed no oxygen present no oxygen present ok and the water of pH is about 7. So, the pH of the water here you know the water pH I make the following assumptions. What are the assumptions here? The assumptions are large amount of water pH does not change with corrosion. Two in the closed system in the closed system what you see here the space between water and the lid and the top lid is very small and you have large amount of of tin in the system. You understood the problem ok. I have given you pictorially how they are looking like. One beaker where I put tin I have taken in water of pH 7. Other case I have taken tin in water, but it is closed and this space is very small and it is no oxygen present. I have two questions for you the question is will tin undergo corrosion or not ok. One ok now the questions are question will tin undergo corrosion. The second thing is can tin continue to corrode irrespective of the time that time of exposure. You understood the problem? How do you address this problem? How can you solve this problem? The first question for example, what I can give you is I can give one more information for you. I can other information that I like to give you what information that you need because it is not sufficient. The other additional information is SN is corroding if at all if it corrodes it is going to be as SN 2 plus. The concentration of SN 2 plus always remains 10 power minus 6 moles per litre. The partial pressure of hydrogen can be considered as say 10 power minus now this I am not going to change this ok. This is left out ok ok. Now what do you think will happen? The partial pressure of hydrogen may vary I put this way. I did not part the partial pressure of hydrogen may vary or you start with let us say you can start with or let us say 10 power minus 1 start with 10 power minus 1 atmosphere ok. I put this I will go back to this problem. I have defined this problem in this manner and I have tin here. I have tin exposed to water. One case vessel is open to atmosphere and the other case vessel is closed. The space between the water and the top lid in this case is smaller. The pH of the water remains same all the time at 7 and SN 2 plus in this case is 10 power minus 6 year minus 6 year moles per litre right and and of course, there is no oxygen percent. If that is the case then can you say how these systems are going to behave in terms of corrosion are they going to behave corrosion with a time same manner or it is going to be different if so how they are going to be different or to start with can you think that tin will undergo corrosion or not. So, that prediction you should be able to now do that. I think you are you know having enough understanding of what electrochemical equilibrium is you understand what the corrosion is with the equations that you have the relationship that you understood you should be able to tell how these systems are behaving in the corrosion point of view. Now, I just want to know I just want to give this your assignment for you and I want to see this answer when you come to the next class ok. So, please do solve the problem and you know send it by email and the TA will be able to go through and you all can discuss please everybody discuss, but do not copy your answers ok. Understanding is free you should always discuss generally you will be able to understand the concept ok, but solve the problem yourself independently do not do that ok. So, please do this and tell me how these systems will respond with prospective time ok that way we can do that and based on this I will give you some more problems and I think we can we can work on that ok to make your understanding better. Before I move on to the next topic of how do you calculate the corrosion rate? We talked about to predict corrosion rate. How do you calculate corrosion rate? I want you to understand one concept ok. See they are very elementary you know I am not supposed to discuss in this course, but they are very vital if you need to understand corrosion that is the concept of pH what is pH yeah yeah it is related to the activity of hydrogen ions in a solution. So, it is a concentration of hydrogen ions in solution please I am going to use activity and concentration in a synonymous manner because otherwise our life is going to be complicated you cannot solve any of the problems ok. So, it is related to hydrogen concentration how is it related to yeah it is negative logarithm of of H plus S equal to please understand this please look at. So, when I when I say pH 5 and pH 9 please understand the question when I say pH 5 and pH 9 how many times the hydrogen concentration changes 10 to the power 4. So, it is very important people think you know the corrosion of steel is happening at 4 they will get surprised it does not happen at at at pH 9 it is only 5 it is not 5 it is a 5 orders are difference in the hydrogen concentration corrosion involves reduction of the H plus ions. So, please get that feel of it what you are talking about you also try to get the feel of what is molar and what is equivalent weight please understand these are the terms you should know I am not going to talk about here atomic weight, molecular weight, equivalent weight these are the terms I think you people should for a time period get familiarized with and even the concept like solubility product they are chemical terms, but they are very very useful for us to understand why corrosion occurs in some conditions why corrosion does not occur in some other conditions right. See please look at we are scientists as much as engineers you need to understand why why is not there I think there is no point in studying m tech corrosion course there is no do's and don'ts for you guys it is for you to get analyze tell what happens based on what you see is environment you go to industry not simple you know you find the reaction occurs some reaction vessel corroded you know and you say you know this company we are using another company says it is failing here they rise the temperature only by you know 5 degrees start cracking unless you understand the relation between the parameters that affect corrosion you cannot really give a solution to the problem ok. So, you need to understand interrelation between corrosion and various factors affecting the corrosion processes. So, we will end our discussion related to what is called as the prediction of corrosion. Now, comes can we calculate then we calculate corrosion say I have immersed zinc in let us say hydrochloric acid you know how to compute whether it corrodes or not whether it corrodes or not use in this equation. Now, in zinc in hydrochloric acid what will be rate of corrosion ok. So, what will be the rate of corrosion of zinc? So, question can you make a calculation question what are the governing equations what are the governing principles for that that is our next step. Now, let us look at the corrosion in details right let us look at the corrosion here I am looking at corrosion like this I am illustrating this corrosion process in simpler terms right. Before corrosion occurs zinc is in equilibrium with the zinc ions right see that hydrogen is in equilibrium with the anions both of them establish potentials what do what do they call that potential called equilibrium. Now, does it corrode here does zinc corrode here does not corrode rate of forward reaction equal to rate of backward reaction. Will there be any hydrogen evolution here no hydrogen evolution because hydrogen ions combined with electron form hydrogen gas and hydrogen gas in turn give electrons and form H plus they are equilibrium conditions. So, I define this as H plus use you this H and define this as zinc 2 plus this 2 electron and zinc this here this should happen now right. And if they are in the standard state for our assumption the voltage e for that is equal to 0 0 and the standard state e is equal to minus 0 point and I short this what happens I use an electrical cable to connect these two what will happen the potential of this is 0 0 and potential of this is minus 0.4 now I am just shorting these two so, what do you think will happen when you when you take a copper wire I have a resistance in the in the copper wire I have a resistor ok. The resistance is infinite and this side I I have voltage which is say minus 0.4 and this side the voltage is 0 0 and now I lower the resistance I I make it 0 light. So, what will happen current will the current flow or not the current will start flowing the moment I remove the resistance the current starts flowing am I right or not. So, you have an infinite resistance they are not connect talk to each other. Now, they have independent potentials the independent equilibrium that you have right and I short this with a wire this will start moving towards what happens let us look at this I draw like this this is called a potential axis so, potential axis this is 0 0 this is H plus H and this is minus 0.44 volt volt this is F u 2 plus and F u 2. I remove resistance what will happen this potential starts moving towards this and this will start moving towards this will it happen or not is it not. So, the 0 the potential of the hydrogen electrode will tend towards positive I am sorry the potential of the hydrogen electrode tend towards negative and that of zinc will tend towards positive move towards like this and the current will start flowing am I right. If I put an ammeter here I can measure what is the amount of current that is flowing I can use Faraday's laws and I can determine the rate of corrosion can I can I can I not right. So, if I can use an ammeter here I measure the current and I know the current I can use the Faraday's laws I can determine the corrosion measure it called measurement. Can I calculate it without measuring using ammeter can I determine what will be the amount of current that is flowing between this and this. If there are governing equation then that governing equation will tell us so, what should be the amount of current flowing here and that current can be used to calculate the corrosion rate based on the Faraday's equations. So, what we are going to do in the in the subsequent lecture is to find out the governing equations here between the voltage and the current. So, please notice when the potential is start moving from here to this the current is going to move more the potential is moving more will be the amount of current that is flowing in the system. So, my idea is can I get a relation between the potential that moves up coming down and the current that is flowing on this electrode on this electrode. If I know it I can calculate it I do not have to go and do an experiment in the laboratory it will be right I do not have to do that. So, our task now is to find out the governing equations and then use that equations to compute the corrosion rate of metals. Am I clear? Am I clear to you? Ok that is what we are going to do that. So, then we can answer this question can we calculate the corrosion rate? This of course, is a it is a very involved you know derivations. If you take the Bakris and Reddy book on modern electrochemistry the kinetics are very well derived and nice things are there and there is called as Butler-Walmer equation. I am not going to derive the equation and those who are interested please read that book read other books ok as to how the Butler-Walmer equation is going to be derived. I am going to make a very simplistic approach so that you know you are from somebody from you know metallurgy background somebody from electrical engineering background somebody with mechanical background. So, I do not want you to flood with the equations, but yet I want you to appreciate the governing relationship the governing equation so that you can able to calculate yourself tomorrow ok. So, that is our next task of how do we really compute the corrosion rate of the metals. Am I right? Actually, am I clear to you? So, far what we discussed are you people clear? So, let us go to the next step of how we compute the corrosion. Let me start with the equilibrium condition right. Let me start with the equilibrium condition. What is the equilibrium condition? Let me take a simple electrochemical equilibria. What is that equilibria? Let us take this equilibria H plus plus electron gives you hydrogen gas. So, very known to you. In the standard state, in the standard state what is the E value for this? Are you clear? This is an equilibrium. What does it mean? The hydrogen releases electrons it forms H plus H plus accept electron and forms hydrogen gas. So, it is in equilibrium condition. So, how do I pictorially put this? I put this pictorial like this. This is the a platinum wire a metal and I have H plus I have hydrogen gas. Now, here now what is happening? Please look at it. If I able to visualize if I have the ability to go and see through my eyes what is happening at the interface right. This is the interface right. What is the interface? This is by platinum metal and this is the acid right. So, what is happening? One reaction is going as H plus. The other one H plus goes like this electrons are there the electrons come over here only ok right and it it becomes what it becomes now the hydrogen molecule. Please look at the rate of forward reaction equal to rate of backward reaction. Can I say this? So, the rate of forward reaction is equal to the rate of backward reaction right. What is the forward reaction? The forward reaction here is is what is H going as H plus plus electrons. What is the backward reaction? H plus coming as as what is the unit for rate of reaction? When you have to guys know what is the unit for rate of reaction? R equal to is given as rate of reaction is equal to what? Is equal to moles per unit time. Now, we are going to do it in a area going to be unit area am I right. What are the moles? We have given you before how many units of hydrogen is getting transformed to X plus. So, mole per unit time per unit area is what is going to be not you simply say area here not unit area it is area right mole time area gives you this. Now, what is happening here? Please look at please you have to be with me now. In this reaction electrons are released in this reaction electrons are accepted. So, what does it mean? It means what is the direction of current in normal electrical engineering? What is the direction of current? The direction of current current flow right direction of current flow is equal to reverse the flow of negative charges. Why when it charges it can be electron it can be any ion it can be anything right. So, it is the direction of flow of current is is is the opposite of the direction of flow of negative charges. Now, please look at this interface we are talking about electro chemical reaction. Now, look at this interface now. Now, current in this case current how does the current flow? The current is flowing from the solution to the metal here the current is sorry. In this case the current is flowing from the metal to the solution in this case the current is flowing from solution to the metal right am I right or not? Is it ok? I am not ok. I am not seeing any response from you people. So, it is it is is it is electro chemical equilibrium. What is electro chemical equilibrium? There is a flow of current associated with the reaction rate. Now, now when hydrogen gas is converted to H plus H plus ion moves from the metal surface to the solution it moves like that. So, it carries a current in this direction. When H plus ions are getting reduced it carries a charge in the opposite direction that means, the current is now flowing like this. So, that means, the equivalent of that is going to be I ok like this here I is going to be direct. Can I write like this ok? In the first case I is in this direction in this case in this case I is in the this direction. Current moves differently because the charges are different in this case. Am I right or not understood? Now, let us put all of them together. What do you understand? You understand that at the interface there is no net oxidation, there is no net reduction, but the current is flowing back and forth. It goes forward, it goes backward, net amount of flow is 0. So, net amount of reaction is 0. So, no corrosion, no reduction, no oxidation taking place. System is under equilibrium condition. Am I right or not? So, system is now defined as under equilibrium condition. How do I calculate I? Can I calculate I here? Is it possible to calculate I? I can calculate I if I know the rate of reaction. How do I do that? I is equal what is equal for I? I equal to what? Moles ok, pair time, pair area, multiply by what? A number of electrons into paradigm. Am I right? I equal to the moles per unit area, the number of electrons am I right or not? This is I right. So, R is equal to I upon n. Have I made clear to you or not? Too much too much of electrical engineering, too much of electrochemistry I suppose this would be easier right. Now, I have converted this rate of reaction into current ok using this equation now. So, I upon n number of electrons per Faraday is your this all you might have studied in 10th or you know 11th and 12th and all I suppose ok. So, what I am trying to say now the rate of reaction is related to the current here ok current here. Please notice in this case what happens? The rate of forward is equal to the rate of backward and so, the I is equal to I naught yeah oh I am sorry yeah ok I I go back now in the case ok. It is good, it is good that you guys are telling that yeah. Now, we have seen now the rate of reaction R is given as I upon n f under equilibrium what happens? The rate of oxidation is equal to rate of reduction ok. Then what happens? Then and what happens now I naught by ok that means, I tends to become I naught. What is I naught? I naught is defined as exchange current density. Suppose you guys are now getting clear pics about what is an electrochemical interface? What is an electrochemical interface? I stop here now. What is an electrochemical interface? The electrochemical interface is where there is oxidation and reduction taking place right am I right or not and so, the current flows back and forth. If the rate of forward reaction equal to rate of backward reaction then the system is under equilibrium conditions. At that condition the amount of current transfer is equal to exchange current density ok. Ok let me just draw here ok. Just take the case of hydrogen gas converted to H plus. Now, what is happening? Now, the electrons are released on the surface right. Now, please look at hydrogen here is moving here as H plus what is the what is the what is the direction of current? This is the direction of current it is going please current is a vector now please look at current is a vector ok. Now, H plus moves here and accepts on electron. What happens? It goes out as a gas. What is the direction of current here? I is moving now this I and this I must be same or same it should be different it should be same if the system has to be under a equilibrium and so and this I is equal to I naught ok if the system is under equilibrium condition and this I naught is related to rate of reaction through this equation right I naught upon N and F look at this ok. So, today I think we should stop here. The exchange condensity is an equilibrium parameter please understand. The exchange condensity is a an equilibrium parameter similar to what? Similar to the equilibrium potentials. Can I say that? The exchange condensity is a parameter similar to equilibrium potentials. The only problem is it is not easy to compute I naught we will we will we will try to talk later, but this is a difficult subject. It is not easy to compute I naught that is the real difficulty that we have, but it is enough for us to say that I naught represents the equilibrium existing of the interface between the solid surface and the electrolyte divided into we are we are discussing ok. So, with this I will stop here we will continue in the in the next class ok and I hope that in a in one or two classes we should able to get an answer how do we compute corrosion rate for a given metal ok. Because you know that corrosion is a non-equilibrium state it is a spontaneous one right. So, first you define what equilibrium state is then you start deviating from the equilibrium then you are going to get into corrosion right. So, first definition is how do I define in equilibrium condition. So, define in equilibrium condition then I think I will know how it deviates and that deviation is going to be now computed ok. So, today I will stop here please do go through these nodes actually and understand what you are discussing. As you see that the complexities of electrochemistry will keep building up ok and it will be very easy provided you understand step by step ok these concepts ok. So, concepts is the key without concepts is very difficult for you to make any any calculations or computations because corrosion is there are so many variety of forms of corrosion ok different environments different metals so many of them ok. You cannot give a table and then tell you know please make a computation you must know the governing equation and then try to use the equation so that the corrosion rate can be calculated for any system that you are interested actually right. So, that is a purpose of the this this particular lecture ok. So, please go through that and you please solve the problem which I given you so that that gives you a clear understanding of how we predict a metal will undergo corrosion or not ok. So, today we will stop it.