Loading

Alison's New App is now available on iOS and Android! Download Now

    Study Reminders
    Support

    Set your study reminders

    We will email you at these times to remind you to study.
    • Monday

      -

      7am

      +

      Tuesday

      -

      7am

      +

      Wednesday

      -

      7am

      +

      Thursday

      -

      7am

      +

      Friday

      -

      7am

      +

      Saturday

      -

      7am

      +

      Sunday

      -

      7am

      +

    Hello everyone, today we have our lecture 8.Today’s topic will continue our discussion on explanation of corrosion process on thebasis of mixed potential theoryand today we will consider 2 instances I think it will be both lecture 8 as well as 9.We will consider situation: 2 cathodic reaction and 1 anodic reaction.We have already covered situation where cathodic reaction is 1 and anodic reaction is also1.So, that means single cathodic and single anodic reactions.Now here we are considering 2 cathodic reactions and 1 anodic reaction and the example beingzinc dissolution and iron ferric iron reduction and also if we put zinc in impure acid whichis deaerated that means there is no dissolve oxygen.But if we have a very small quantity ppm level content of Fe plus 3, so then we can havethese 2 cathodic reaction.So, these are the 2 cathodic reaction and this is 1 anodic.Now we have to draw mixed potential theory.We have to draw those interactions of these reactions apply mixed potential theory, soagain the plot would be E versus log i and here we are only concentrating activationpolarization.So, our equation to indicate these 3 equations should be general equation over voltage equalto the tafel slop log i versus i zero this is the general equation.If it is anodic that time there would be positive and beta a, if it is cathodic negative betac log i and here also since it is anodic in this case it would be cathodic reaction.So, since let’s see where are those equilibrium potentials.Let’s assume that the equilibrium potential of zinc let’s make it is equilibrium itis reversible potential, let’s say this is my reversible potential which is if itis E equilibrium zinc.And since zinc is very active and if we consider this to be minus 0.76 volt which is the standardreduction potential.So, even if the zinc ion content decreases that means if we do not keep the concentrationas 1 because when you are inserting into the metal, the concentration of zinc in the beginningwill not reach to 1.So, it will be less than that, so this equilibrium potential would be further down there if itis not unit concentration in that particular solution.So, anyhow, so this is if we assume that this is to be that particular situation this isassumption, completely assumption this is the potential.Now this particular reaction if we consider this is a very strong reduction reaction.So, we know E zero Fe3 plus, Fe2 plus equal to 0.77 volt which is very strong reductionpotential very high reduction potential and E zero this is zero volt.So, it means that the equilibrium let’s also consider the situation that pH let’ssay 1.If we keep ph should be 1 or 3 or 2 any pH we consider in the acidic medium consideringthat in the acidic medium this reaction would happen.So, this particular potential let’s say E hydrogen equilibrium, so depending on pHthis equilibrium potential would vary would be equal to minus 0.059 pH.So, now if I consider pH to be 1 then it should be 0.059 volt, now depending on the pH thispotential would vary that equilibrium potential.Now if this particular concentration Fe plus 3 concentration is very low still if we assumethat this potential since there is a huge gap between zero and 0.77.This is a huge gap between this reduction potential.So, it would lie this equilibrium potential Fe plus 3, Fe plus 2 equilibrium.It should lie above this point.Now we have to consider their respective i zero.Let’s say this is the point of i zero of zinc on zinc.Let’s say this is the i zero, hydrogen on zinc.Because if we have a solution deaerated HCl let’s say HCl, so there we have those ionsH plus as well as Fe3 plus and now we have zinc, zinc block.Now these 2 reactions this reaction as well as these reactions they must find some surfaceon which this reaction can happen.So, these are the surface over which this reaction would happen is basically the zincsurface.So, now that is what i zero hydrogen over zinc surface, similarly this ion is gettingreduce to Fe plus 2 that reduction process would also happen on zinc surface.So, we can write this is the point because these are the equilibrium potential and izero always lies on equilibrium potential for the respective reactions, so this pointis i zero Fe plus 3 Fe plus 2 over zinc surface.Now as per the mixed potential theory they must reach to a point where this conditionis met where summation of ic equal to summation of ia.Now if there is no Fe plus 3, so then it would have its own corrosion potential that meansthis is as per the single cathodic and anodic reactions.This is E corr and I just put this particular situation zinc because zinc is dissolved andi corr is so that means at this condition ic hydrogen on zinc equal to ia zinc equalto i corr zinc.Now the other reaction is also following its own polarization let’s say this is the polarizationof the other reaction which is Fe plus 3 plus electron equal to Fe plus 2, so this is goinglike this.Fine.Now we have to maintain this particular criteria that suggest that whenever 1 polarizationline a particular polarization line.For example, here it is the cathodic polarization the blue line is the cathodic polarizationline for Fe plus 2 to Fe plus 3 to Fe plus 2.So, whenever it experiencing another cathodic polarization those currents are to be addedup, so that means once it is coming to this much polarization.So, that means if the polarization level is this much which is plus 3 to Fe plus 2 thisis the polarization level for this reaction.So, here it is experiencing another cathodic reaction which is hydrogen reduction.So, now as per this mixed potential criteria we have to add up those currents and thenat this point we have to add total equal to ic of Fe plus 3, Fe plus 2 plus ic hydrogen,this is on zinc surface.This is to be added up and then we will take log and so that means that log point willshift to the right.Because the actual this total ic would be following a different line which should bethe current value should be at a particular potential.If I consider the particular potential that current value should be little higher thanthat because here this one and this one these are added up and then taking log, so it willcome here.So, similarly this one and this one they are to be added up and then this point will comehere and now you see that it will be parallel to that.So, then we have to draw this line, so this is the plot of log of ictotal, so the ic this ic contains cathodic current density for this cathodic reactionas well as cathodic current density for this reaction.Now if we extend this line then I have to find out that point wherethis criteria is met, these 2 criteria is met.Now if I consider there are 2 points, if I consider this point as well as this pointthen we could see that at this point only I could see that ia zinc equal to ic hydrogenover zinc plus ic Fe plus 3 Fe plus 2 over zinc and also it corresponds to a potential.So, this is E corr mixed, so this purple line actually and cross section of that purpleline at the black line for the zinc, zinc anodic line that is the point we have to concentrateon and that is the point the system would try to reach.And that corrosion rate of zinc would shift from this point to this point.Why?When we have when we do not have this Fe plus 3 in this, that means it is a pure HCl, mycorrosion rate was here.But once we added Fe plus 3 into this solution then my corrosion rate has move to this position,so that means corrosion rate of zinc has increased and the criteria for the mixed potential theoryis also met.And now ia zinc is nothing but the new corrosion rate of zinc when Fe plus 3 is there.Now interesting part here if you look at if we see this particular mixed potential thatin the new mixed potential when we have impurity in the form of Fe plus 3 I need to see thehydrogen evolution on zinc surface.Now if you see the hydrogen evolution rate when there was no impurity what is that hydrogenevolution rate.This particular point is the hydrogen evolution rate which is nothing but let’s say thisis term it has ic hydrogen on zinc surface when there was no Fe plus 3 iron in the solution.Now once we have F plus 3 let’s say this point is that is i total, so at that point,so corresponding to this point.Okay, this is the new point I would say hydrogen over zinc when there was Fe plus 3 on it.So, let me put it as prime, so this is ic hydrogen zinc prime, now what happens oncewe have this Fe plus 3 when it was pure the hydrogen evolution rate on the zinc surfacewas indicated by this particular rate.Now once we have added Fe plus 3 when hydrogen evolution rate on zinc surface has reduced,now what could be the observation.The observation is unique, so in the beginning when there was no Fe plus 3 will see lot manybubbles of hydrogen on top of the zinc surface.But once we have Fe plus 3 since the rate of this rate is less than ic hydrogen on zincsurface when there was no impurity when there was this is the case when there was impurityand this is the case when there was no impurity.So, now in this case no Fe plus 3 but once we have Fe plus 3 since the rate of hydrogenevolution is low on zinc surface.Because that time the current density corresponding to hydrogen evolution at that new E corr mixedis less.So, we would see that the hydrogen evolution on the surface is much less compare to this,so here we have Fe plus 3.Interestingly when we had no Fe plus 3 what was the polarization the total polarizationthe over voltage for the hydrogen evolution reaction.So, the total over voltage was this much let’s say this is new one this was the over voltagewhen there is no Fe plus 3.Now once we have added Fe plus 3 the over voltage has gone with reference to the equilibriumpotential of hydrogen evolution to this, so this is neeta 2.So, we can say that the over voltage 1 is greater than over voltage 2, so this is correspondingto with Fe plus 3 and this is corresponding to no Fe plus 3.Okay.So, now which one is larger, so this one is definitely larger, so that means once we haveadded Fe plus 3 the total polarization for the hydrogen evolution reaction on zinc surfacehas gone down.So, we can say that it has got depolarized, so we can clearly say this is the kind ofdepolarization effect on zinc surface due to addition of Fe plus 3 and depolarizationof what hydrogen evolution.So, actually I can say that depolarization effect on hydrogen evolution on zinc surfacedue to addition of Fe plus 3.So, we can also say that Fe plus 3 is acting as depolarizer.Further so I can say that depolarization for the hydrogen evolution reaction.Now interestingly so if we consider that the hydrogen evolution rate over zinc surfaceis the observation for zinc corrosion.That means if you say that the more the hydrogen evolution on the zinc surface more will bethe corrosion of zinc that may not be true.That means when we have a strong ion strong reduction reaction when we have a strong reduction.For example this case are oxidant the strong oxidant because Fe plus 3 in this case isa strong oxidant which has a tendency to go to Fe plus 2.So, if we have a presence of strong oxidant then there could be a situation that the zinchydrogen evolution of zinc surface would be low.But the rate at which the zinc dissolution happens would be much faster than the rateat which when the observation was the hydrogen evolution on zinc surface was high.So, this is the reason for that situation because when we have a lower polarizationfor the hydrogen evolution I have much higher dissolution rate of zinc.So, on this particular ratio let’s have some numerical problem.Okay, so I have chalk down numerical problem on this.That problem statement is like this let’s say situation where log i and this is potentialand remember this potential with reference to standard hydrogen electrode.Okay.So, that time I can put some of the so the same diagram let’s put this value is 10to the power minus 11 ampere per centimeter square, this value is i zero Fe plus 3 Feplus 2 over zinc surface is 10 to the power minus 9 ampere per centimeter square.When the there was no Fe plus 3.And if there is no Fe plus 3 iron so then this is the corrosion rate of zinc which isi corr equal to 10 to the power minus 8 ampere per centimeter square, this value this i zerozinc over zinc surface equal to 10 to the power minus 11 again ampere per centimetersquare.And the corrosion rate of zinc i corr of zinc I would say mixed I just put mixed when thatthe mixed potential theory is getting applied.So, these value becomes 10 to the power minus 6 ampere per centimeter square.And few data that is given is beta equal to 0.1 volt per decade of i the current densitywhy decade because beta can be considered as neeta by log of i by i zero.So, that is what I am taking per decade of because this is in decade, so now so betac if we consider that time it will be minus 0.14 volt per decade I simply said decadeof current density I just omit this of i rather I say this a volt per decade.And beta a equal to 0.1 there is a minus sign when we consider beta c this is decade.So, this is the data that is given and another data which is pH because will decide whatshould be the E equilibrium hydrogen.So, not this point, this is the point which is E equilibrium hydrogen that would be thispH let’s consider pH to be 3.Now we have to find out what will be E equilibrium Fe plus 3 Fe plus 2 E equilibrium zinc andic Fe plus 3 Fe plus 2 at.Okay.So no this is not the case.So, the E mix when Fe plus 3 is present E mix when Fe plus 3 is not present as wellas ic Fe plus 3 Fe plus 2 and ic H2 over zinc surface both at E mix when Fe plus 3 is present.So, these are the parameters are to be found out, so we will take this problem to our nextlecture.let’s stop here, thank you.