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Hello everyone today we will start our lecture 17 and it is the same topic will be continuingexplanation of corrosion events on the basis of mixed potential theory.And we have discussed several situations and in the last 15 and 16 lecture we talked aboutgalvanic coupling between two active metals.And then we could see that one active metal corrodes heavily, the another active metalis protected.An example to this is nothing but the galvanic coupling between zinc and iron.Even we can have the same situation when we connect magnesium with iron or aluminium withiron where aluminium zinc and magnesium all will be acting as anode and iron will be actingas cathode.And this particular technique of protection of iron is called cathodic protection andwe have also seen that there are two segments of cathodic protection one is sacrificialanode, where zinc or magnesium or aluminium are connected to or is to iron piece whichis to be protected.And zinc will act as or magnesium or aluminium will act as anode and iron will act as cathode,because we have also seen that that preferentially that iron will act cathode because the cathodicreaction will take place on iron surface but on the anodic surface the dissolution of thatparticular metal takes place.And we have also seen that that particular mix potential theory that base of that particularprotection root that mix potential root is based on basically the main mixed potentialtheory.Where we could see that yes that hydrogen evolution reaction on iron surface increasesand the hydrogen evolution reaction on zinc surface decreases when we connect zinc andiron.Similarly, we have also seen that if it is oxygen reduction process that is basicallythe cathodic reaction then on iron surface oxidation reaction oxygen reduction takesplace.But on cathodic surface on anodic surface dissolution of zinc take place preferentiallyso it is not that iron dissolution is not taking place rather iron dissolution is takingplace at a much slower rate than in the situation where iron is separately placed in that electrolyte.So this, so we today we will talk about little more on cathodic protection.So the situation will be talking about effect of magnesium and zinc on iron corrosionso this particular when they are galvanically coupled.Fine and what we have seen in case of and this is also an example of cathodic protectionand remember our discussion will be only confined to the application of mixed potential theorywill not talk detail about cathodic protection that means the technical part of cathodicprotection only the mixed potential theory will be talked about.Now when we talked about cathodic protection we could see that there are two modes oneis sacrificial anode another one is impressed currentcathodic protection.So this is in short we call it ICCP and if we try to see their mix potential mode inone case we will just draw schematically you try to follow the previous lecture.So this is log I this is E in volt.We have two active metal joining, this is 1 andso this c is cathodic reaction and M1 and M2 these are the two active metals.If we have this situation.Now in one case, we talked about hydrogen evolution reaction to be the cathodic reaction.Another case we talked about oxygen evolution reaction to be the oxygen reduction reactionnot evolution oxygen reduction reaction to be the reduction cathodic reaction.So both the cases we explained our protection of iron because of galvanic coupling of ironwith an active metal more higher active metal, rather I would say that zinc corrode, zinkif we see the equilibrium M2.So this is M1 and M2 are two metals, this M2 could be zinc, magnesium aluminium theseare the and here M1 is nothing but iron.And then we followed our mixed potential theory.Fine, so this point is my point where ic equal to ia we have already realized that and thenwe have seen the mixed potential is this one and our statement was the iron would preferentiallyact as cathode.Because the cathodic reaction would take place on the iron surface at a higher rate thanthe rate of cathodic reaction on zinc.Here also you could see that cathodic reaction this is basically summation ic and this pointis nothing but respective I zero point of cathodic reaction.And this is summation of I zero point.Now this line is summation of ia point where this is ia is M 1 and M2.Now I could see that hydro if it is hydrogen evolution reaction or oxygen reduction reaction,the rate of ic of cathodic reaction whether it could be hydrogen or oxygen reduction hydrogenevolution or oxygen reduction that is at a much higher rate than the ic of c on M2 orzinc surface.So iron becomes cathode and dissolution rate if I try to see so this line is nothing butia of M2 and M2 of course we could see that it could be zinc magnesium aluminium.So the rate at which zinc dissolution or M2 dissolution would take place this is the rateso had it been the situation where zinc is separately placed in that electrolyte thedissolution of zinc would have been here so this is i corr zinc.Since we are talking about M2 so this is M2 this is i corr.So I could see that ia of this is ia point ia of zinc or I could see M2 has gone up.Whereas, the dissolution of iron has gone down.So this point is nothing but ia of M1 and had it been separately placed that M1 thecorrosion rate of M1 would have been ia of M1.So I could write it like this so this is M1 or this is M1 when it was separately addedseparately placed in that electrolyte.So this is the mode of sacrificial anode, where M1 is M2 is dissolving and M1 is protectedrather I would say that M1 is corroding less as compared to the situation where it is notconnected to connected to M2.Similarly, and so there I could see that the corrosion potential if I consider for ironis this is the corrosion potential of iron when it is not connected to zinc or the corrosionpotential of M1 so that case my corrosion potential of iron has gone to this particulardifference so I could see that iron is polarizing towards negative side with reference to thecorrosion potential or we can say the free corrosion potential.So there from it is polarizing negatively.So that is what it is called a cathodic polarization.This is also a cathodic polarization of iron and that also and because of that I couldsee that ic that the cathodic current density over iron surface is increasing so that iswhat it is also coming under the influence of cathodic protection.But when we talk about impressed current cathodic protection I could see that this theory ofimpressed current put in cathodic protection.There I am not having any dissolving electrode rather I would have an electrode which willbe just used to supply negative current to the supply, just used to have a completionof circuit.So what we are doing let’s say this is the metal object which is to be protected in anelectrolyte and that electrolyte could be soil solution or seawater.So there I could have not seawater in there we’ve to remember this impressed currentcathodic protection may not be suitable for seawater.Because this is not high resistance path.Generally impressed current cathodic protection is used where the resistance of the electrolyteis very high that is what it is mainly in soil application and then this is let’ssay the this is iron which is to be protected and in this case protected and then this isunprotected by supplying negative current.So if I try to supply a negative current to this that means I have to supply ic to this,so I have to connect it to and a cell which is supplying a DC current to the system.And then that circuit is to be complete.Since here we do not have a dissolving metal which will dissolve or sacrifice itself andthen protect the iron piece.Here we are having an electrode which will be acting as an auxiliary anode.So this is connected to this so the positive end is connected to some metal which is generallyhigh silicon steel.High silicon steel that is used silicon percentage could be almost about 16 to 17 percent sothough that steel will not dissolve rather it will just complete the circuit.And the current negative current which will be sent to the iron piece for protection byexternal power source so this is my circuit current and the current is leaving this anodeand going to metal surface.So the current in that electrolyte is leaving anode surface so this becomes anode and enteringthe cathode surface and ic is entering into the metal surface so since ic is enteringso that means we are sending electrons to the metal surface and those electrons willbe used up for cathodic reactions.This is the mode of impressed current cathodic protection.Where if this is my ic and this is my ia then this could be this is my free corrosion potentialand this will be the steady corrosion potential if electrolyte condition is maintained similar.So how can I reduce this corrosion rate so this becomes my i corr equal to ic and thisis a mod sign because ic sign is negative.Now if I try to do that I have to go down, okay.So in order to reduce the rate of corrosion of iron so if I go down then only the currentdensity will move this way.So here if I do this way if I take the potential downwards that means I have to polarize theiron surface towards negative side and that means polarize towards cathodic side.So that is what it is basically the cathodic protection.When you do that I could see that if I go to this potential this is my potential E1.So my polarization is nothing but E1 minus E corr since this is my E corr which is negative.So I can term it as neeta c, fine.So this is the cathodic over voltage.This neeta is over voltage.So corrosion current density was here and now it comes to this so the corrosion ratehas reduced.So this is also cathodic protection but here we are sending current from external sourcethat is called impressed current cathodic protection.Now the main difference between these two, these two is in the way the current is sentto the negative current is sent to the iron surface.Now in this case the negative current or the electrons are sent to the iron surface becauseof the dissolution of the more active metal which is zinc or magnesium.But here the negative current is sent by applying, by supplying current from an external DC powersource.Now if I try to see other differences now here if I see the potential difference thepotential difference is driving or not potential difference the polarization.So here is this much but here the polarization is if this is my corrosion potential of iron.when it is not connected to any acting any other active metal now after connecting toM2 which has lower equilibrium potential than iron so then in this particular situationI have this much so this much would be my over voltage.So this is neeta c for this case and neeta c for this case cannot be changed anymorebecause that is decided by the self-dissolution of the M2.Yes, it can be changed only if M 2 is changed.For example, M 2 is changed from zinc to magnesium or zinc to aluminium, so I replace zinc withaluminium, if I replace zinc with magnesium, so that case this value would change so wewill see how this value is changed because of change of anode material but in this caseI could see that the over voltage is this much in this particular case.Now if I supply more negative current so that means if ic is increased fine.Then the potential will still drop, so I can drop it further so I can take it to this potentialso now corrosion current density or the dissolution current density of iron would go from thisto this initially it was here when it was there was no current flow in the system.But once we started current started connect once we started flowing the current into thesystem so we could see that depending on the amount of current we are sending I can changethe over voltage and then accordingly I can change the rate of dissolution of iron.But so that is possible in this situation in this case but in this case it is decidedby the self-dissolution of the active metal and that can this over voltage amount canonly be changed if we change the anode material.So we have to just look at that part so that is what the situation we have considered herethe effect of magnesium and zinc on iron corrosion when they are galvanically coupled.So let us see that part, okay.So if I try to see that so let’s say in case of zinc so we draw side by cases.So one case was so this time we will just draw a possible situation so this is let ussay zinc so this is E equilibrium zinc, this is E equilibrium of iron, this E equilibriumof cathodic reactions.Here I am not specifying whether it is a hydrogen or oxygen hydrogen evolution reaction or oxygenreduction reaction so I am just simply mentioning the cathodic reaction in that situation Iwill have so this is summation of ic, okay.And this will flow this way and this will flow that way.This is my point where ic summation equal to ia summation, fine.This potential is the E mix.Now before connecting iron and zinc this was the corrosion potential ofE corrosion iron and after connecting to zinc I could see that the E mix is here.So over voltage is neeta c.So the neeta c equal to E mix minus E corr iron and correspondingly I could see thatthe corrosion rate of cathodic reaction on iron surface is increasing.And corrosion rate of sorry not corrosion rate the rate of cathodic reaction on ironsurface is increasing.But the rate of corrosion or dissolution of iron is decreasing from this point.So this is I corr of iron when it was not connected to zinc and this is the ia of ironwhen it is connected to zinc, fine.So this is the neeta c.Now how can we increase it.Now let’s say if I take magnesium and we know that if we compare E node rather reductionpotential equal to 0.44 volt.E zero of zinc plus plus E node rather minus 0.76 volt.E node of magnesium plus magnesium equal to minus 2.36 volt.So I could see that if I compares the potential so I could see that this is the most activeand this is moderately active moderately active and this is active, fine.Moderately higher, so I should not say moderately rather I would say okay so I can put it asmore active and this is less active I can say that this is less active.So that situation so this point in this case it was equilibrium point so I can change theequilibrium point.E equilibrium magnesium and this iron equilibrium point will not change.And this point if we do not change.So this is iron this is magnesium.If i consider this situation let’s say i if this is the situation.So when separately done so that time this is the corrosion rate of i corr of magnesiumand this is E equilibrium c, this is i zero of c on magnesium surface, this is I zeroof c on iron surface.This is E equilibrium iron.And then if i continue this same process I will follow.So this becomes my, this is my mix potential so summation of ic equal to summation of iathis potential is E mix and this is E corr of iron.So if I try to see the E mix.This is my new E mix.Now neeta c if I have to compare so this becomes my neeta c.So I could see that if I extend this line extend this line I could say that neeta cin case of with reference to magnesium and neeta c with reference to zinc, I could increasethat neeta c or the cathodic polarization when we connected to magnesium.When you connect it to magnesium.So that means the magnesium is highly active the most active if we compare the E zero valuesof iron, zinc and E node values of iron zinc znd magnesium.So that case I could increase the over voltage cathodic over voltage or I could say I couldincrease the cathodic polarization and that would influence.So let me complete this so this is my individual cathodic reaction so this is ic this particularline is ic of cathode equation over iron this particular line ic of Cathodic reaction onmagnesium surface.But if we compare so the since we are getting higher over voltage in case of magnesium connection.I could see that the corrosion rate is also of that iron the dissolution rate of ironafter catholically coupling with magnesium is also decreasing as compared to the corrosionrate of iron or the dissolution rate of iron when it is connected to zinc, okay.So this way we can actually increase the over voltage amount.But other ways we cannot but in case of ICCP we actually can increase it depending on thestronger or weaker power source whatever we wish to use.Let’s stop here we will continue our discussion on the mixed potential theory and analysisof corrosion events in our coming lectures, thank you very much.