 And today we will be talking about an important aspect of corrosion that is called as galvanic corrosion or dissimilar metal corrosion. And the topic of the discussion is galvanic or dissimilar metal corrosion. Now, why you should be concerned with the galvanic corrosion? Why we should be worried about the dissimilar metal corrosion? If you look at the engineering components, in many cases consists of multiple metallurgy, multiple metals. They may use in combination like steel and stainless steel or they may apply as a brass the steel, titanium may be along with the stainless steels. And there are several reasons for that. For example, if you consider a pipeline carrying water and you want to have a tap where you want to take the water out and you want to regulate it. We have seen when we talked about the galvanic corrosion that the materials requirement for a given application depending upon how critical it is, how critical the corrosion resistance is. In this case you take a pipeline, if there is a small amount of corrosion there is a loss in thickness and you are not going to see an apparent leak in the pipeline. But if there is a small amount of corrosion in the tap or in the valve, then what happens? The valve or the tap starts leaking. You may not able to control this. So, while you select a steel for the pipeline, you cannot for obvious reasons choose steel for the tap and valve. Many of you know, you will have seen various fittings right. They are made up of stainless steel or made up of brasses. Because stainless steels and brasses they corrode at a lower rate. And so, the leak is avoided. Now they bring in similar problems at other places as well. I give an industrial example ok. You know of this right. What are this unit called? Heat exchanger right. If you look at the heat exchanger, the shell what you see here is in this case made up of a carbon steel. Actually it is a this is called an HB heater, high pressure heater in a thermal power plant. The water is heated to high temperatures. And so, there is high pressure. So, this is made up of carbon steel. But you make the tubes with the stainless steel because the water in the tubes flow at very high rates. And the carbon steel cannot withstand erosion corrosion. So, people go for a stainless steel. Now the tubes are stainless steels. Look at the tube sheets. This is called a tube sheet ok. This is called a tube sheet right. And this is made up of a carbon steel. Similarly look at the header for example, this is header here right header. This inlet and this outlet here. And this header is made up of carbon steels. And these tubes what you see here they are impregnated into the into the sheet. You can see this here. So, these are all tubes which are you know will are welded onto the tube sheet. So, you have a combination of carbon steel and a stainless steel being used. I give another illustration. You sometimes weld the structures. It is a stainless steel here. You weld this. The weldment the weld fusion zone we call it could have a different chemical composition as compared to the tube itself. So, there are different chemistry. Basically you can call this is metal 1. This is your weld fusion zone. And the tube we can call it as they are another metal. They are coming in contact. Please look at they are coming in electrical contact. They are coming in electrical contact with each other. So, one on the top you see here slightly a different reason where the corrosion occur here. This is a carbon steel. Another carbon steel is alloy steel tube. It is a chrome moly steel tube used in the boilers. Now, what has happened here was copper deposition in these locations. The copper deposition has happened because the water carried corroded copper metals, corroded copper metal ions. And in fact, that copper ions came some of you might know that when you when you look at the thermal power plant that is also condenser. You have a steam. The steam goes to steam turbine, it does work. The steam is getting converted into water again by a condenser. And condenser in this case was made up of cupronical. And copper dissolved and that copper carried all through and got deposited over here. They are copper metal and this is steel they led to what is called as galvanic corrosion. There are several reasons several places you see. The other obvious example that can happen is you sometimes coat steel with a nickel. You have seen in car bumpers you know you have seen in in washroom fittings to make it looking very good for aesthetic reasons. If the coating is not proper then you will see a galvanic corrosion between the nickel coating and the steel substrate. So, whenever you have two different metals coming in contact with each other. When I say contact I mean the electrical contact with each other then it leads to what is called as the galvanic corrosion. So, two so two different metals, two dissimilar metals shorted electrically can lead to galvanic corrosion. This is the origin. The origin of galvanic corrosion comes like this. You know a very famous device most of us use in a very constructive manner where the galvanic corrosion works and works for the benefit. What is that dry cell batteries? You know dry cell battery right. In the dry cell battery take it dry cell battery what is this made up of in a few it is made up of a zinc sheet casing right zinc casing. In the center you see graphite of course, you will see a cap here right you see a cap and there is a copper cap. The inside you may have something like ammonium chloride and you know and you also have manganese dioxide. How does the current get degenerated here? It is getting generated because it is shorted now ok. When these two are shorted when you short when you short these two when you short the when you short graphite and zinc happens. It delivers a current delivers a current and and voltage. Now what is happening to zinc? Just zinc remind intact. How does the current come? The current comes because zinc dissolves and gives rise to electrons. The two electrons are used to reduce manganese dioxide to manganese oxide. So, that is the voltage between difference between the zinc and this zinc and this this graphite electrode, but graphite electrode is not an electrochemical system by itself. It is manganese dioxide and manganese mn 4 plus and mn 2 plus equilibrium here. So, between these two the current flows and zinc is rendered as anode and this is rendered as a cathode and and so, the corrosion occurs. This is a well known example of the so called galvanic corrosion used very constructively. But this happens in the industries and that is not going to be good for us. You have a carbon steel, they have stainless steel. If one of these two metals corrode then there is going to be what? There is going to be a damage to the component. So, there can be leak or there can be you know it can happen. So, so dissimilar metals can lead to the corrosion actually. Now why? Why it occurs? One is say that there are differing corrosion rates and the differing potentials. So, if I have two metals you know you take any any environment ok. This is m 1 I just put environment and then this shot and one is you know. So, since they have two different potentials you know ok and and so, that can lead to the things. But we even before a shot is possible that one will have a higher corrosion rate other will have lower corrosion rate. But if you look at closely the corrosion rate differences are not the reason. No this is not not the major reason. One is not the major reason. Sometimes it may not be reason at all. The second is the reason. Now so, if you know these galvanic corrosion can occur when you have two different metals they come in contact with this. Then we need to know two things. How do we predict which of the two will undergo corrosion at a higher rate and if it undergoes at accelerated corrosion due to galvanic you know interaction what is the corrosion rate of that ok. So, we need to know this case ok ok. Can we galvanic corrosion? First question. Can we calculate can we calculate galvanic corrosion? So, what is the basis for that? What is the basis for that? So, that we will see. This the third question that comes is what are the factors right. If you know what are the factors the parameters that control galvanic corrosion then it is possible for you to methods you know to devise methods of controlling corrosion control galvanic corrosion. Of course, next is how do we evaluate galvanic corrosion? How do you evaluate them? So, let us now look into one after another and see that we can have a clarity in all this. So, that if you are doing a research or if you are developing a material or developing a processes or you are developing a monitoring technique for example, it is easy for you to do all this if you can understand all the five aspects of the galvanic corrosion. Let us take the first one. Can we predict? We all know the galvanic corrosion occurs primarily because of the potential difference. So, potential difference is a reason. We are aware of calculating the equilibrium potential right. You buy some about the potentials which is the thermodynamic property. In the standard state what you call that potential? So, the equilibrium is called as standard potentials ok. So, the one readily available data in the literature is the standard potentials and from the standard potentials you can also calculate the equilibrium potentials provided you are given the concentration or activity of the species involved there. For simplicity if you take the standard potentials and put them in some order either in the descending order of the standard potentials or in the ascending order of the standard potentials. You can list them like this right and for different equilibrium and such a list such a series is called as electromotive force series ok. This is called as electromotive force is not it? Potentially is a force a driving force for the reaction to occur ok. This is called as electromotive force. You can have you know a list I mean you can have hundreds of equilibrium not necessarily the equilibrium related to metal and metal ions you can have between the you know zone species you know. For example, there are some organic molecules there are even there are inorganic molecules Fe2 plus Fe3 plus in equilibrium can happen not necessarily metal is involved there. So, you can have an exhaustive list, but for as if you can look at you know the metals which are somewhat related to engineering applications just have a look at them it is very somewhat interesting. Look at some of them for example, let us say let us take the system like gold and the old and look at the standard potential of that it has got their olds right. When you say standard potential it is implicit it is in relation to the standard hydrogen electrode. So, I do not really have to make mention about it, but all other cases you should make mention in whose reference the potential is measured. You can also have just given few examples here ok and you can also get it from the lots of published literature ok. So, this gives you a driving force. Now, the electrode potential as a standard electrode potential is given in the decreasing order from the highest we have here is the gold and here lowest is magnesium here ok and somewhere you see here the hydrogen and you know how to interpret this data right. Suppose any metal that is lying you know below this hydrogen you immerse it in the assay solution of standard state and the ions are standard state then hydrogen is going to evolve and all this above this metal you will not get hydrogen evolution taking place. So, corrosion will not take place. So, you normally call this as noble right and you call this as active generally that is how you talk about it and the nobility increases as you move from the top to the bottom and you can compare any two the the one just lies above is noble and the one below is considered as relatively active and relatively noble ok. So, when we talk about all this we talk about the relative values not the absolute values right. If you take let us say gold in gold chloride and platinum and platinum chloride you just couple them together the platinum will be acting as a anode and gold will act as a cathode. So, you can predict which of this will undergo corrosion what is the basis normally we use a basis earlier called as E cell is equal to E cathode minus E anode it has to be positive right you recollect our discussion earlier right ok. So, it is possible to use you know this ah EMF series and and and find out which of these metals which are shorter electrically can become anode and become a cathode ok based on the equilibrium potentials ok. But can this be true can this happen in the real life situations? In the real situation do you have a an equilibrium potential or no standard potentials in the state where the metal is corroding the steel is immersed in water zinc is immersed in water it will establish a potential right what is that potential called when zinc immersed in in water and it is what is the what is the potential called that is simply called as a corrosion potential. So, it is it is going to establish a potential which is called as a corrosion potential, but what is the what is the criteria for the metal to establish corrosion potential how does that value is arrived there how does the metal arrives at value what is the basis for that. When you when you take a steel I mean you just take iron and put it in acid and at some time it will attain a potential which is steady state slowly it becomes steady state and you measure the potential and that is called corrosion potential. Why does the metal reach that particular potential? Yeah yeah see that is the criteria for that it has to satisfy the mixed potential theory the rate of anodic reaction must be equal to that of cathodic reaction that is the place where the metal will stabilize and the corrosion potential will stabilize all are the cases the potential will drift drift to us that actually that is the criteria for that ok. So, in actual real-life situations the aim of series or the equilibrium potentials they have very less relevance to predict the galvanic corrosion. So, what is the potential that is relevant actually? So, we look at what is called as corrosion potentials is relevant and equilibrium potential is irrelevant. What drives them? What drives the current between let us say zinc and iron immersed in any environment you know maybe water maybe acid? What drives actually? What potential is that? It is the corrosion potential that drives the current between these two metals. Now, when you take corrosion potential there is a problem. What is the problem? The corrosion potentially is it unique to unique to a metal actually high-tech steel? Do you think the corrosion potential of steel will be same in in all corrosive medium yeah? No. No. So, the corrosion potential of the metal will very much depend upon the environment, the temperature, the pressure, the concentration of the species, nature of the species, pH. The so many variations that you will get and you probably must have a million of million data of equal values to compare. So, that is simply not possible in practice. So, they developed a potential based on metals and alloys in pure seawater. You can use seawater also questionable right with the composition of seawater in Arabian sea maybe we have Bengal maybe different ok. It was measured in US at Ann Arbor ok. It is deep in the sea so that the pollutants and all are not going to influence the corrosion potentials. Again I do not know how many of you know even in a given environment the corrosion potential is not going to be unique like you see in the case of equilibrium potentials it can vary by few millivolts 5 millivolts 10 millivolts can really change. The way you prepare the sample sometime you prepare a very nice smooth samples sometime mirror finish. So, you are not going to have a unique equal value even a given environment even for a given metal and alloy. So, you can only look at the relative performance, relative values of the corrosion potentials. That is why we see the Fadrana books many times they give you a relative positions and not the red values. You also see the ASM handbook the corrosion values are given as a band of values you are not going to get the unique values why it is kinetics ok. Because of kinetics you are not going to have unique equal value and so there is going to be a brand of value you are going to have it. So, what I have done is the pure seawater and they list the corrosion potentials of that and that is called as the galvanic series. I can just give some illustrations here ok and and if you see in the in the published literature you see lots of data, but let us give some important you know metals and alloys platinum tops here then you get gold then you get the graphite titanium ok. You have silver you have say 188 moly stainless steel passive nickel passive, cupronickel you have copper zinc and you have tin stainless steels which are active cast iron steel ok, aluminum alloys have zinc before that I think you will get cadmium your aluminum have zinc combination. We are just moving from here like that. So, you see some interesting behavior in the in the metals and alloys in galvanic series we saw this the AMF series there are some of them very obvious you can see that ok. I hope you will you could have recollect some of them ok. I just put back here you can have a look at this galvanic series I am sorry the AMF series you see how they are looking like this here ok and titanium comes somewhere here titanium comes titanium will come in fact, the titanium will come even somewhere here actually ok. Titanium will come even below magnesium is not it? How do you extract titanium? Use magnesium to extract titanium actually the growth process ok yeah. So, titanium come even below actually one of the most reactive material is is titanium. Compare this compare this and what are the obvious differences do you see right what are the obvious differences you see in this no no differences yeah what do you see yeah. Yeah the first obvious difference you find is in the AMF series you do not see any alloys you see only pure metals right. In the galvanic series you do see alloys and metals that is one major difference what is the other difference? Of course, this is a sea water ok of course, there is that is of course, a standard state or it has to be its own ions ok that is of course, there is another implicit difference that you have anything more you see? Yes. Yes yeah we see you know some of the which are reactive in the automotive series as I said titanium will be figuring somewhere here ok. The titanium is going go up even between platinum and gold the platinum acts relatively noble as compared to the gold actually see that is a reversal taking place and even more important is the stainless steel exhibits the galvanic potentials at two locations the passive state and active state you know what the passive state is you know what active state is right. So, metal when it is passive the potential is noble when it is active the potential is it will let you be more negative potentials active potentials you know the reason why it is right why does it happen? When the metal spontaneously pass away what happens in the car does it go up or come down what happens anybody can recollect that it goes up right you if you look at the active passive trans passive polarization curve you best recollect that the car they will lie at a very you know close to equilibrium potential f if the alloy is not spontaneously pass waiting if it is pass waiting the potential will will shift up. So, obviously, that potential is noble compared to the metal exhibiting active potentials. So, stainless steel itself can form galvanic corrosion between themselves in the active state in the passive state you will see later that you know in a pit for example, it will act as a anode surrounding the pit you will act as a cathode we will see that later ok. I hope you are getting this point right. So, you please do understand this if you do not understand this you cannot predict galvanic corrosion in reality where that happens in the field ok. I hope you are able to see the differences very clearly between these things ok. So, practically we use the galvanic potentials ideally if I am choosing a alloy somebody somebody comes to you ok and they ask you to select two different materials for application. What data do you seek? I need to use steel and stainless steel or maybe somebody says oh I want to use titanium and I want to use stainless steel I need to do together you know. So, what data do you seek from that? You say that oh you use it or what do you do? Yeah. So, what is the environment you are looking at you know and that put and that environment you should get the corrosion potential that is what the right thing to do that. But if you do not have that data now I have two tables I have a table one which is a galvanic series and the table two which is a EMF series we are available readily for you which one do you use? I use galvanic series because galvanic series is more reliable than the EMF series. But do understand that there is no galvanic series which is applicable all through any environment. I give an example suppose in the rooftop you know you are going to use you know two different metals. The galvanic series for that atmosphere corrosion is different from the galvanic series that you find in seawater because some of them may pass away at something can happen ok. So, it is it is it is important that you keep in mind that the galvanic series what you will see here is seawater that should be used very cautiously only when you do not have the data corresponding to given application otherwise you need to generate the data and that data can only be employed in selecting the materials material selection. Now, let us take the case of the galvanic corrosion. What are the factors that affects the galvanic corrosion? We saw the potentials right. Now, what is that that governs the galvanic corrosion of the metals? Let us take two distinct examples a simple one and a complicated one. So, that you can understand the endochemical parameters that affect the galvanic corrosion rate of a metal. We have seen that when the galvanic potentials are different the one with the noble potential will act as a cathode the one with the active potential will act as a anode that you know that, but you do not know what rate at which the active metal will undergo corrosion. So, how do you electro chemically you understand that? So, let us use the so, called the Evans diagrams to understand this. Let us take the case of say iron and say platinum in let us say one molar hydrochloric acid. How do I get this? Can you do now I think you guys have mastered the kinetics and we discussed this for about 10 10 hours of lectures we discussed right. How do you do this? Anybody has a clue? You go apply mixed potential theory right. So, first of all you apply mixed potential theory for platinum separately mixed potential theory for steels I mean iron separately you get the E car and I car for iron and platinum in that water without coupling them galvanically you know the corrosion rate. You put them together now now you rearrange all of them actually. Again mixed potential theory is applied what do you what do you say the total anodic reaction is equal to total cathodic reaction in overall system ok. You solve that equation then you will get which undergoes what or what rate it is happening at all taking place. So, simple galvanic theory you can it can be evolved by applying the Evans diagrams ok. Suppose I take platinum let us take platinum I just take platinum here and you know the case of let us say take the platinum here one case. Now, what happens? You go for standard states for the for simplicity let us go standard state what is the equilibrium potential of platinum? This is 1.2 what is this platinum 2 plus plus 2 electron gives you platinum am I right. And what about the other equilibrium? So, what happens in this case? Will platinum corrode? Does platinum corrode? Platinum does not corrode at all. So, practically we do not use the dissolution or the platinum equilibrium when you talk about iron right because you see here now when you put platinum and a in a in a in an acid and you shrouded with a with a hydrogen you will always have the hydrogen gas dissociating into H plus and platinum will not dissolve ok. So, we do not normally use this if you see in the pattern of book you do not see that this is there at all ok. And for iron it is easier right what is this cathodic reaction here? This is your exchange condensity for what? Adrenal iron please understand that ok. So, this is your cathodic reaction H plus plus electron gives you. So, iron when iron corrodes you have iodine evolution taking place. Now, let us put this together and see how the galvanic reaction occur between platinum and these things right. So, this is a platinum separately immersed in the acid iron immersed in the acid separately. Now, if I am going to be together what happens now? Let us look at this this is iron and platinum shorted is iron separate platinum separate ok. What are the equilibria you will have to be considered there are only two equilibria to be considered one is H plus is in equilibrium with hydrogen iron is in equilibrium with Fe 2 Fe 2 plus am I right or not? Platinum 2 plus platinum does not come into picture at all will it come does not come into picture right. So, let us take the case of iron here let us take the case of iron right this is your Fe going as Fe 2 plus plus 2 electrons. Now, what is this called? Exchange condensity for H plus and H on iron please look at that ok and this if you want to put iron here you can draw this diagram for iron. This is this is your exchange condensity for Fe 2 plus and this is your E car. Now, I want to represent now this is clear right this is same thing I have written here only. Now, I want to now represent platinum in the solution how to represent now? What will happen on the platinum surface you put it in acid when I have hydrogen what equilibrium will be there in the surface of platinum H plus is right. So, what is the how do you represent here? What will be the potential there will the potential of H plus H on platinum and iron will be different or same? The equilibrium potential for H plus H on iron and platinum it will be same or different why it will be different? How do you how do you find out the equilibrium potential for H plus H here come on quick how do you find out the equilibrium potential can you find out or not? How do you find out? So, it is it is nurse equation there is not it whereas, it come into picture where is platinum or iron. The equilibrium potential does not depend upon iron or nickel whatever it is talks about the partial pressure of hydrogen and the concentration of H plus ions that is not going to change. So, the equilibrium potential of that will remain the same not going to change this is the equilibrium potential of H plus and H right, but what is going to change on platinum yeah. Come on I want to see if people talk you guys know come on what what what is going to happen can you look at your your hand out anybody has the hand out now what is the exchange condensate approximately for for H plus H on platinum 10 power minus 3 watt amperes centimeter square for iron how much minus 6 ok. So, it is over 3 orders of magnitude higher for that. So, the exchange condensate for that is going to come come somewhere here am I right? The tuple slope remain the same. So, I write the tuple slope here this is the exchange condensate I naught H plus H on platinum agreed or not agreed. So, this is on platinum. Now, what is the mixed potential theory? You sum up all the cathodic reactions and sum up all the iron reaction am I right or not? So, sum up this when sum up this becomes so small this is going to be your place where the mixed potential theory is applicable and this is going to be this what this is your i car of what of iron combined with platinum here. What happened to this one? This is my equilibrium potential sorry this is your e car of iron when you shot with platinum agreed ok agreed or not agreed ok. Now, if you agreed on it now I would ask the question now to you. Let us make the observations here now we will make let us look at the observations ok. They are an acid let us say 1 molar HCl please notice they are not shorter now just put it here. If you can closely observe the surface of iron surface of platinum what do you what will you see there? Will you able to see anything or not? Yeah there will be. Bubble. What bubble what what bubbles here will be where will there be bubble? Platinum where there will be bubble an iron right is iron there will be gas bubbles what do you have what will be the gas the gas will be. Hydrogen. Hydrogen gas I think you should have given simply one answer hydrogen gas that talks about you know if people are clear about you know ok. So, you have hydrogen evolution taking place on the on on on this will there be hydrogen evolution on platinum? No because platinum cannot get oxidized ok. So, there will be hydrogen evolution taking place. Now what happens I am going to take it out and I am going to shot this you observe now what happens where where will the hydrogen evolve? Platinum. Platinum right now the hydrogen will not evolve there will be less hydrogen evolution on iron then more hydrogen evolve on the platinum right. So, the hydrogen evolution will start shifting from here to this what happens? So, there will be more hydrogen evolution on iron upon the on the platinum the amount of hydrogen evolution on iron is less, but then the amount of hydrogen evolution on iron and platinum put together will be equal to the amount of iron corroding on iron surface can I can I say that actually? Am I making sense? The amount of hydrogen and evolved on platinum and iron if you look at it ok iron take this quantified this which will be equal to the amount of iron that is getting corroded on the iron surfaces that is how the mixed person theory is what is mixed person theory? The total amount of reduction is equal to total amount of oxidation takes place. Now in this case if I measure the potential here using a voltmeter what will happen to potential of when you shorten this the potential of this this will be same only am I right assuming that the conductivity is very very very high conductivity here when I put then this together the potential will change. Now what will happen to the potential of this system when you short it? When I do not short it the platinum will exhibit a potential is equal to what potential equal to hydrogen equilibrium whereas, iron will exhibit a potential equal to corrosion potential right. Now when I short this what will happen to potential of these two? Go up please look at your diagram only ok what about this potentials? It will increase right the potential starts moving from here to this I want you to look at the diagram if you do not look at the diagram analyze you cannot you cannot guess it I think what it guess it you have finished you have to look at the diagram see what happens here is is the potential start moving from here to this that is what happens mixed potential ok it moves up here ok the potential moves from here to this because this is the mixed potential is now because moves from here to this the hydrogen evolution on iron what happens now look at this the hydrogen evolution on iron is decreasing the amount of hydrogen evolution on platinum is more because now it is moved from here this potential it is moved to this potential. So, the hydrogen evolution on platinum is increasing because at when you when it just immerse platinum in the solution this is the potential you are going to have it and shorting the potential moves from here to this, but for iron the potential moves from here to this. So, that means, the hydrogen evolution on iron is decreasing the hydrogen evolution on platinum is increasing from here to this. So, that is what will happen I hope I think there will be an experiment in the lab I say I do not know how many of you have already done it. If you have not done it you please do observe this this particular test. I want you to observe it and see does it really happen or not happen at all ok. So, this is something you should be looking at actually ok. You clear about yeah what means you are talking about this right yeah ok. Yes it is not correct which is yeah it is I it is interesting this point that is right. Yeah. So, how do you explain this now? Suppose I take a platinum right I immerse dip it in a sulphuric acid and I shroud the sulphuric sulphuric acid completely with hydrogen let us say one atmospheric hydrogen. So, what do you think will happen there? What what do you think will happen? What you can happen is of course, you will establish an equilibrium actually ok. He will establish an equilibrium. Will this will ok will this will this go to this one? It may not go here also. In fact, this will not go here ok. Suppose assume that this guy is getting oxidized suppose you are getting oxidized right that is must be a reducing species to take. If I have platinum ions in the solution what will happen to platinum ions? The platinum ions it will get reduced. So, you visualize other situation I have platinum in the solution as platinum 2 plus and I have one more or hydrochloric acid and I am also putting hydrogen in the system. I keep putting hydrogen in the system what will happen now? Slowly platinum ions will get deposited as platinum metal will happen ok. If on the other hand there is no platinum ions in the solution what will happen? Nothing will happen the potential remain here only because if iodine dissociates as to as electron there must be some species to take electrons at all right. There is no species to take electrons at all ok. That means, nothing will happen there that is why platinum is considered as a reference electrode in this case. What is the platinum actually? Platinum immersed in acid and shrouded with hydrogen what is the equilibrium there? The equilibrium is between H plus and H there is no oxidation no reduction takes place it goes both ways right that is why platinum in acid is considered as a reference electrode where there is no net oxidation no net reduction taking place. If you add some platinum ions obviously, all the platinum ions will slowly get reduced to platinum at all actually. So, question now you are right you will not go here it will remain here only ok. But, but, but then the moment I am going to put platinum chloride this line is applicable is not it? If I put a platinum chloride what is this one actually that is why I put here platinum 2 plus plus 2 electron gives you platinum that means, the guy has got platinum ions here there is no platinum ion this guy will not even come at all actually here ok. Yeah you are right if you are going to use if you are going to read this title iron platinum in in 1 m s e l probably this fellow does not even figure here ok ok. From that point of view it is correct I just drawn is only to illustrate to you how the equilibrium will start shifting if you do not have platinum ions there will be no way you will get hydrogen dissociation as H plus ok. It will dissociate again will will will reduce there will be equilibrium establishing between platinum between the H plus and and H on the platinum surface that becomes your reference electrode ok that is what happens at all actually. Yes. Dot line the percent according to the exponential j is the final outcome right. The yeah what is this dot line correspond to yeah that is good what is this dot line what is what is this dot line means? The sum of this the hydrogen reduction reaction for this hydrogen reaction this hydrogen reaction occurs in the ion surface given by this kinetics hydrogen reduction reaction occurs in a platinum surface followed by this solid solid lines. So, what does it mean? If I hold if I hold iron at that particular potential the amount of hydrogen on iron is only this much but on platinum it will be this much but if I have both platinum and iron together the total will be this much isn't it you just I know assume that I have a platinum and platinum platinum and iron together I short it and apply a common voltage what will be the amount of hydrogen will takes place this is on the iron this is on the platinum the total will be for that particular potential that is the amount of thing that happens. Instead of platinum I am going to put gold you look at the gold what is the value of gold actually there exchange intensity is given not given there 10 to the power minus 6 right. So, 10 to the power minus 6 is coming very close to this right 10 to the power minus 6 probably you will not get a huge difference in the hydrogen evolution reaction if it is a gold ok. So, the line will shift towards this. So, the effect in fact, we will see later the effect of galvanic interaction of gold and steel will be low as compared to the galvanic interaction between platinum and steel ok because the exchange current density on gold is lower for hydrogen equilibria as compared to that of platinum ok. So, so that is what we are now looking at what the kinetic parameters what are the kinetic parameters that they will not affect this now. Before we close I want to look at this here look at what are the ok and look at this this is the the anodic tuple slope of the active metal and the cathodic tuple slope of the other one is ok is ok I think we will talk about this when we when you deal with two active metals here it is noble metal here it is an active metal. So, we will see what happens when you have two active metals how they will change ok and I think today I mean we can stop here provided you have any questions or for clarification. Now, is that meant that in general we see that iron on iron there will be no hydrogen evolution when they are. Virtually no hydrogen, but you cannot say no hydrogen. Virtually no hydrogen. Virtually no hydrogen. Will it suppress that much steel? Yeah that is because of that it is again we go back to same thing the Evans diagram only talks about the relation between the potential and the current you have a platinum here that is all. But what is the role of platinum the role of platinum means to lift the potential from here to this otherwise the platinum is not talking to the steel at all anyway right because it is lifting the potential here it is happening otherwise any other metal will lift to that where where it will happen. So, it is again the same Tafel equation is valid you moved from here to here you apply the Tafel equation you still get the value ok. So, it is platinum or something else something else does not matter at all because as long as you you know you follow the same Tafel line this is the Tafel line at all ok. On the other hand you move down the hydrogen evolution on iron will increase right ok. So, that we will see later when will you move down that we will see actually that could happen if you are going to take about a more active metal than iron things will be different we will talk about it in the next class.