[Music] [Music] let's start a lecture 39 so we are left with two more lectures and in these two lectures remaining lectures we'll consider some of the case studies or where corrosion happens in case of metallic object and we'll try to understand those corrosion events with the help of mixed potential theory and in fact in the last lecture we saw how this may expect mixed potential theory can explain the corrosion event of in case of zinc in HCl and then we try to draw even straggle which is basically plot between potential and log of current density and then we try to consider the activation polarization part only in the beginning and later on we'll also see some more examples considering the concentration polarization in this particular so that means we'll look at some of the case studies or events of corrosion considering mixed potential theory we'll try to understand through this theory as I have mentioned that we'll be concentrating on the activation polarization part initially and try to draw even's diagram but later on also we'll look at concentration polarization part also so we have two situations one is nita equal to beta log I by i0 another one is Nita equal to 0.05 9 1 n log 1 minus I by I max so these are this is basically concentration polarization this is activation and in the concentration polarization we would consider cathodic polarization now in this particular Junction we would like to mention that we would consider cathodic polarization not the anodic polarization because the anodic polarization event is very uncommon there are two possible reasons one is of course when you consider anodic polarization there should be depletion of metal ions in the interface around that interface due to corrosion event but since it's a notic polarization so metal is going into iron form so there would be enough supply of metal for the formation of metal ion so the depletion of metal ion is unlikely so that means depletion is unlikely and second part is when it is during of course during a Nordic polarization since during an unique polarization is actually going to met align in the second part during a Nordic polarization there could be a possibility of passivation this passivation can be explained on the basis of Pourbaix diagram so if you let's say some metal Eve Ursus log I if I consider the Pourbaix diagram let's say it looks like this and this is metal oxide part this is metal ion part and this is metal part let's say if I am operating at this pH sorry this is not log I axis this is pH axis so if I operate at this pH axis as I go upward that means if I go on the analytic side or the positive side Netta is positive so then initially it is in the metal part and then kindly finally it goes to metal oxide part so then you have actually metal oxide forming on the metal surface and which can lead to passivation so at a large over voltage or positive over voltage there could be possibility of passivation also but the main reason is this depletion of metal line is unlikely in case of an OD polarization and this concentration polarization happens due to depletion of concentration of ions but in this case it's unlikely that said anodic polarization we would not be able to consider we will see some of the examples considering cathodic polarization and of course in the beginning we'll consider an activation polarization in the last lecture at the end we started talking on the effect of galvanic coupling of a metal object which is active with an with an Noble component and that's what we started looking at if I see the last part we started looking at combination of zinc and platinum in acid media so now you have two beaker and in one beaker you have a block of zinc another beaker you have as BA clubs block of zinc which is connected to block of platinum and they have the same area which are exposed to the acid medium this is acid medium same condition temperature pressure both the cases in that case we have to draw events diagram and let's draw the events diagram log I this is e and we start with zinc and there because the anodic reaction is zinc dissolution and here platinum is noble and zinc is active component the zinc dissolution is anodic reaction and hydrogen evolution is the cathodic reaction when we have mixed potential theory to be active now it is this is basically eyes 0 for H 2 gas evolution let us say this is on zinc surface in this case this is the case what we are considering now this is 0 equal to and this e is equal to minus 0.76 volt and this is for zinc plus plus zinc and this is for hydrogen h2 and it is lying here for zinc this is i0 for zinc on zinc surface hi zero for zinc on zinc surface and then we see a mixed potential this is my Ecore and this particular current this particular current is I call abs Inc when there is no galvanic coupling between zinc and Noble component which is platinum now once we have hydrogen evolution on platinum surface as we know that i 0 of hydrogen on zinc is less than equal to less than less than equal to I 0 of hydrogen evolution on platinum since this value is close to 10 to the power minus 2 to 10 to the minus 3 ampere per centimeter Square and here it is 10 to the power minus 10 to 10 to the power minus 11 ampere per centimeter square so this is the difference in their values when we have this particular let's say this particular situation now now hydrogen evolution would also take place on zinc surface as well as platinum surface and since platinum is noble so platinum will act as cathode and zinc will act as anode but hydrogen evolution can't take place on both the surfaces now if I consider hydrogen evolution on platinum surface its exchange current current density is pretty high at this point so this is i0 hydrogen on platinum surface and easy Rho of course it would be same zero because we are considering hydrogen evolution and then it would also have its own cathodic polarization and it is cutting here now question is would this be my new over new mix potential no because as per the mixed potential theory we see that there will be a possibility of existence of a mixed potential where there should not be no net current flow in this in the system now in order to have this second criteria to be criterion to be fulfilled we must have summation of IC should be equal to summation of ia so that means in the skate in the system we must have to see that total cathodic current density is equal to total anodic current density now here we have two sets of cathodic reactions one is on zinc surface another is on platinum surface and one set of anodic reaction so this is two sets of cathodic and one sets of anodic here I am considering two sets means this is happening on platinum surface this is happening on zinc surface but zinc is dissolving from the zinc surface only Platinum would not dissolve because it's a cathodic component now that case since there are two cathodic reactions in order to fulfill this criteria that total cathodic current density should be equal to total anodic current density I should add up these two polarizations now in order to add up since it's a log scale so it will go at this point because I have to add at every potential every potential I should add this one and this one these two current densities and get to the new current density which is this one similarly here we have to add this point as well as this point to get to the new current density which is this point which is new IC which is IC let's say this is IC prime equal to IC hydrogen on zinc plus IC hydrogen on platinum and at this point it's basically IC hydrogen on zinc plus IC hydrogen on platinum so like that we have to add up all the cathodic reaction now if we add up then if we connect them then we will see that the a depart would follow the red line and now if I look at this point this point here I have ia which is this one and if I try to see the total I see so this is these are the two points where I could see that at this point the circle point IC IC equal to ia and this IC has two parts one is this I see another one is this IC so I see on hydrogen evolution on zinc surface plus IC hydrogen evolution on platinum surface then I could see that at this point at this point I have two hypotheses of mixed potential theory are met first one is it is reaching to a mixed potential and then at that potential I could see that this condition is fulfilled that means when this condition is fulfilled so since IC and IA they are flowing opposite directions and of equal magnitude there will not be no net current density so now this particular potential would be new Ecore and corresponding current density which is IA is basically I called nu now when there were no platinum connection between zinc there there was no connection between zinc and platinum only this case I had current density for the corrosion is this one this particular current density was the corrosion current density but as we have connected to platinum which is a noble component I could see that the current density for the corrosion has gone up enormously because you remember this is a log scale and also one more thing since it's a log scale value so when you add this particular polarization with this particular polarization that the current densities for the cathodic reactions it will not shift love wider distance because the scale has to be maintained that's what it there is a small increase on the right side of this point so that small increase is coming because of this log scale nature of this x-axis now you could see that this case one when we have connection of an active component with a mobile component the corrosion rate of the active component goes up enormously this is the explanation why it goes up with the help of events diagram now let's look at a second case this is case one I would say this is case one case one is effect of galvanic coupling and with active plus noble this is the galvanic coupling considering but remember when we we have seen this galvanic coupling we have said that that they have the similar same area which is exposed to the acid medium the area is not changing the galvanic that active component has got the some area let's say 1 centimeter square then Platinum would also have the same area now interestingly if we have another metal let's say if it is connected to another component in let's say instead of platinum we add latinized patronym that case the corrosion rate would further increase in that time what happens this particular current density would seep to switch to some other current density let us say at this point because current density exchange current density for hydrogen evolution on platinum splat is higher than that on the plane platinum surface so again further if I follow the same way what we have done in case of platinum this one would be the new mix potential and we could see that the corrosion current density has gone up where this line indicates the addition of this current density the current density following this particular polarization of hydrogen evolution on zinc surface and hydrogen evolution on platanus patina following this particular Arrowhead if I see the green arrow head this particular polarization so that this particular line this blue line is nothing but the addition of this one and this one so then only at this point I could see I see summation equal to ia is equal to I Corr so that way we have discussion or the explanation of increase in corrosion rate when we combine an active component with a noble component now let us consider the second case study case 2 where we consider the effect of presence of Occident or we call it oxidizer and that should be strong oxidizer for example fe t flash presence on the corrosion rate of active metal now again let's draw events diagram now whenever we have let's say this is an acid medium so if it is an acid medium we have hydrogen evolution reaction equal to e 0 equal to 0 here and let's say this is my easy row of this some value let's sum this value is in the minus direction now they will have their own mix potential this is my mix potential now Fe plus 3 plus a equal to Fe plus 2 this has got a very large ez row this is almost about close to zero point seven seven volt if I remember correctly now if I try to see this is positive zero point seven seven equal to e 0 Fe plus 3 Fe so let's say its position is here this is I 0 Fe plus 3 Fe plus plus on metal surface because this is in the ionic form in the solution so you have a system like this this is a acid this is the methyl and on top of it I have hydrogen cathodic reaction as well as this reduction reaction so and dissolution reaction is 2 plus this is my anodic reaction and these two are cathodic reaction so now you could see that this entire corrosion process is divided into a 1 anodic reaction and 2 cathodic reactions and both these cathodic reactions are taking place on metal surface so that's what we are considering is 0 for Fe plus 3 2 Fe plus 2 on the metal surface so it would have its own activation polarization cathodic as well as anodic now since this particular point which is I 0 M 2 plus M on metal surface and hydrogen this is i0 hydrogen on metal surface since these two potentials are lying below this potential so then everything will be coming between these two extreme that mixed potential would come in between these two extreme depending on the situation where I have the neede cathodic current density is equal to the net and OD current density so that particular point we have to find out so now without this Fe plus 3 presence I would have gotten this particular point to be my new mixed potential and corresponding corrosion current density this is I Corr this is ecor1 let us say this is 1 now once we have this presence of Fe plus 3 it is a strong oxidant that means it will have a very large very high tendency to get reduced and there would be strong cathodic polarization because it's in very strong cathodic reaction and remember all this individual redox reactions or half-cell reactions would have their own anodic as well as cathodic polarization so this is anodic polarization for hydrogen evolution this is cathodic polarization for methyl and this is the cathodic as well as anodic polarization for if he plus 3 to Fe plus 2 so now it would come like this and once it reaches to this particular plane [Music] immediately it is seeing that there is another cathodic reactions which is hydrogen evolution reactions so now as per our concept in the previous example we have add up the total cathodic current densities so now we have to add up that total current densities so had it gone through the actual path it should have been like this but since there is one more cathodic reaction which is hydrogen evolution it would add up since this is in log scale so this adding will not go very far so it will be small increment as per the value at different potentials so then we add up and we get to this and then if we extend it we could see that at this point which is nu Ecore let us say condition 2 where this is my I Corr case 2 and then if we extend this particular line I have two current densities cathodic current densities these two so now let's say this is I h2 on metal surface or rather I say hi h2 cap I see of h2 and this is I see Fe plus 3 Fe plus 2 and at this point I could see that IA equal to I Corr equal to IC and this IC is basically addition of hi CH 2 plus hi c Fe plus 3 Fe plus 2 at e equal to e core to the condition 2 so then we could see that again the corrosion rate of the metal is going up and since it's a log scale so this corrosion rate increment of that metal is very very high and interestingly one more thing if you look at this carefully had there been no Fe plus 3 the corrosion rate would have been here and this should have been my the rate of hydrogen evolution on metal surface but once we have Fe plus 3 the corrosion rate is increasing but the rate of hydrogen evolution on zinc surface is decreasing as compared to the rate what it should have been there without Fe plus 3 presence in the solution so then you could see that the metal ion on the metal hydrogen bubbling rate bubble formation is reduced somewhat and this reduction is coming because of depolarization effect what is depolarization depolarization means what should have been the polarization now that polarization amount or the over-voltage has reduced had it been only m and hydrogen reductions metal riddle dissolution and hydrogen evolution reactions my over voltage would have been this much now with the presence of Fe plus 3 the over voltage for hydrogen evolution is this much so this is when Fe plus 3s presents and this is without Fe plus 3 so we could see that the over voltage for hydrogen evolution reaction has gone down or decrease due to the presence of Fe plus 3 that particular effect we call it depolarization effect on hydrogen evolution because of presence of Fe plus 3 so that's what Fe plus 3 is called depolarize ER and in fact the same effect was seen in this case also you could see that when platinum when the Platinum is connected hydrogen evolution rate on the zinc surface is reducing so platinum is also acting as a depolarization for hydrogen evolution on zinc surface so now we have seen two examples case 1 when we connect active component with a noble component and the second case when we have a presence of strong oxidizer and both the cases I could see that the corrosion rate of no active metal is increasing to a great extent so let's stop here we will continue our discussion considering few more case studies thank you very much [Music] [Music]