Loading
Notes d'étude
Study Reminders
Support
Text Version

The Effect of Concentration Polarization of Cathodic Reaction

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

    +

Let’s start lecture 30.And the broad topic of course passivation and mix potential theory, we have consideredone case studies like increase in concentration of oxidant and then after reaching to a finalhigh concentration decrease in the concentration gradually in the same path and then also wehave seen the influence of mechanical disturbance of the passive film after reaching the regionor the concentration.Where the mixed potentials that particular concentration polarization and this cathodicpolarization plot is cutting across the anodic polarization plot at two or three locations.So, there we could see that if we do a mechanical disturbance of the passive film instead ofmaintenance of activity, we can increase the corrosion rate because of the breakdown ofthe passive layer.Now, if we recall.So, lecture 30Topic it is same as last one passivation and mixpotential theory and here will be now try to see, we will try to see the few more casestudies and before we go to the case study second case study let’s analyse Faraday’sexperiment.The Faraday’s is experiment on passivationof steel in HNO3.Now, in that experiment, if we go back and see that experiment initially iron block isput in a concentrated HNO3 which is around 70% concentration.So, there initially there will be a little bit of bubble formation on the iron surfaceand then immediately the bubble formation vanishes.And then if we see the weight loss due to the corrosion effect, we see that there isnegligible weight loss.Now, after that what was done in the same beaker if we add water and then keep dilutingthe HNO3 solution we see that still there is no noticeable change in our weight loss.Now, if we go to a very dilute HNO3 solution, the system shows no weight loss.So, that point we say that initially when it was dipped in acid solution concentratedas its solution it went into passivation and then it remains passive when we change theconcentration from high to low.Now, after that in the dilute HNO3 solution, if we shake the particular beaker or if wedo with a glass slide, if we just scratched the top surface of the Iron object,We could see that the vigorous bubbling starts and we could see that the weight loss is noticeablenow, due to corrosion.So, this is the phenomena which can be solved after seeing after understanding the effectof concentration of oxidant from high concentration to low concentration and addition to thatis the mechanical disturbance of the passive player.So, in the last case what happened if I go back to the last one.If I try to see that when we see the blue one.This blue curve at the 6 point when we reduce the concentration from let’s say concentration8.Concentration 8 it’s spontaneously passivity, but as we decrease the concentration whenit reaches to concentration six.Concentration 6 if we do not disturb the passive current density will be the corrosion rate.But if we disturb the surface with the mechanical effect scratching or other means, then thecorrosion rate immediately moves to the point.Which is this one which is nothing but i critical.So, this exactly happens in case of HNO3.Now in case of HNO3 the reaction the oxidation this reduction reaction we can say that NO3minus H plus 4H plus 3 equal to NO plus 2H2O and that is what when we scratch the ironblock when it was taken from a high concentration to low concentrated HNO3.We see the NO gas that will might this nitrogen oxide gas is present there.So, that bubbling is nothing but the because of this this particular gas this is a gasphase that happens and this E0 E node for this oxidation reaction.So, I can say that HNO3 minus NO, I can say because here it is.So, this is the value is plus 0.96 volt with reference to SHE and if we consider E0 foriron equal to minus 0.44 volt with reference to SHE which is standard hydrogen electrode,okay.So, if we see the interaction,Iron has got its own active passive transition mode.This is the iron plot active passive polarization plot log i which is ampere per centimetresquare.Now, when the concentration is very high the cathodic polarization line is cutting at thislocation.So this is E equilibrium for this reaction.Let’s say r1 is a reaction r1 and this is E equilibrium of iron i zero of iron overiron surface.This is i zero of reaction one over iron surface.Because that reduction process is also taking place on the iron surface.So, I could see that it is actually being spontaneous passivity and if I see the passivepolarization plot, the polarization plot will look like this, okay.Now, if we decrease the concentration, so now, I am changing the concentration or let’ssay this particular location that is i zero does not change.So, it goes like this and then it comes here like this.So, okay.So, if we see those concentration, so, as this potential is decreasing, the concentrationof oxidant decreases and here oxidant is of course, this one.So, this oxidant decreases and if it decreases, I could see that when it decreases to let’ssay, C1 this is C0 initial concentration, this is C1 this is C2 and C1 and C2 if wedo not disturb it will maintain those passivity, okay.Now, at this location at this concentration.If I scratch the surface, then where will be the point where mix potential should bemaintained so, the point will move from this location to this location.So, this location comes when we have mechanical scratching, but if we do not scratch the systemwould maintain i passive which is nothing but i corr.So, it will maintain passivity i corr is of course very low point when it is equal tothe passive current density.But once we disturb because, when it is C2 that time, it is actually going through a3 locations where ic equal to ia, but it will maintain this point if it is not disturbedif it is disturbed, it will come to this is a similar discussion what we had in the previouscase where we change the concentration of oxygen from high to low and then coupled withmechanical disturbance.So, we could see that the corrosion rate.Initially it was here and then it is moving to this particular equation, this is i corrosionprime.So, i corrosion prime is very large compared to i corrosion close to ip.So, we could see this particular reaction would happen at a very high rate, which isequal to this one.So at this location ic, ic of this particular reaction 1 equal to i corr, right.So that was that corrosion rate increases.If we scratch the iron block in the dilute nitric acid solution after it is passivated,remember after it is passivated if it is not passivated.If we put it in dilute nitric acid then of course it will corrode at this particularrate, okay.And when it reaches to the concentration C1 they not only the passivation happens so,that is what when we put the iron block in the HNO3 solution, the concentrated HNO3 solution.Initially we do see a little bit of bubbling because that time this particular potentialand this potential, they are trying to drag both of them and to a particular mix potentialwhich is nothing but this and then only passivation appears.So, initially when it when this particular potential is dragged to higher location, itwill move in this fashion then only it reaches here.So, before that when it moves like this, until unless it reaches to this location.You will have a little bit of vigorous gas formation which is this that means vigorousthis particular reaction rate would be higher because the anodic dissolution rate is alsohigher.Since it is moving in this location in this particular pattern.So, this will follow this anodic polarization like so, this is the understanding of faradaysis experiment on passivation of steel HNO3 with reference to mix potential theory.So, let’s take up case 2.Case 2 let’s see the effect of concentration polarization of cathodic reaction on the passivityof an Active Passive metal.So, we have seen in case of a condition where the cathodic reaction is concentration controlledand anodic reaction is activation control that will dissolving metal is going throughactivation control mode.We could see that constant corrosion rate at certain point it becomes constant.But that case if this is voltage, so, if this is my, so this is iL1 iL2 this is iL3 IL4and it happens when velocity increases, we have seen that, that anodic this ox this ismy ox plus e equal to red.This is the reduction process and it is reaching for limiting current density at a differentlocation with increasing velocity.So, 1 velocity let’s say velocity 1 is less than velocity 2 like that it moves.And that case Delta 1 would be greater than delta 2 greater than delta 3 greater thandelta 4 and this delta is nothing but iL equal to nF DnFC0 by delta.This is the basic equation and since we are considering e which is 1 electron.So, this will be 1, fine.So, now, this delta which is the boundary layer diffusive layer so that it decreasesas we increase the velocity.So, now, in case of active metal if this is the condition,so, now so, this is iL 5, so, V 5 and then delta 5, so, all other factors are same, ifwe start with the same concentration every case and if it is at a temperature T whichis let’s say room temperature, so, this will be the situation and then if we try tosee the corrosion rate as a function of velocity.So, these are the points where I could maintain mixed potential theory.But once it crosses V4 velocity.Once we go to V5 or V6 if we have iL 6, which is delta 6 this is V6 it does not matter thatwith all other velocities apart from V4 the corrosion rate would be at this point.So, the corrosion rate increases and then it remains constant.Now, this is the situation where active portion of the dissolving metal.So this is M minus ne equal to M n plus so, this is polarization ia.So, in case of that corrosion rate initially increases and then it remains constant.But, what will be the situation if this interaction is with active passive metal.So, in that case the situation would be little it would be a little it the situation wouldbe very different.So, this is log i, this is voltage in this is case where active we are considering activemetal and in this case this is active passive metalin that case.Let’s see what will be the situation.So, if this is the active passive anodic plot.And of course, all the points are there.So, this is Ecp equal to Epp in this particular situation this is i critical.This is E equilibrium of that metal.This is i zero metal over the metal surface.Now, if we see a cathodic plot let’s say this is the cathodic plot and now, this isthe iL 1, now after the very high current density.This is ic this is ia and this corresponding to ox plus e equal to reduction.This particular reduction process and here it is reaching limiting current density.So, if we, this corresponding to some V1 speed of the V1 velocity of the solution.So, if we reduce the V1 then of course, the this particular factor will increase.So, if we increase that all other factors are remaining same.So, this will achieve at a lowerso this is iL2, iL3, iL4, iL5, iL6.So, this when we, 6 velocities we are considering and accordingly I could see that V1 here inthis particular case, V1 has the highest speed.So, V1 equal to V2 equal to V3 or since we are taking italics, so, this is Roman we areconsidering.So, V4 greater than the V5 greater than V6.So, correspondingly Delta 1 would be less than delta 2 less than delta 3 less than delta4 delta 5 Delta 6.So, this will be the situation.And we are considering iL equal to DFC0 by delta and C0 is the concentration of oxidant.Now, interestingly iL 4 is exactly equal to i critical.Now if we start from iL 6, let’s say this concentration is started from iL 6 as we areincreasing the concentration to V1.So, then if it is a freshly prepared surface with no oxide surface on the soft on the metalsurface oxide layer on the metal surface.So, that case as we have discussed in case of the case one were case study one whereconcentration of oxidant is increasing.So, the mix potential point the corrosion rate will be in these locations.Now, once we cross the velocity 4 to velocity 3 that means, the velocity is increased from4 to 3 immediately the corrosion point will reach here.So, and then after that if we keep increasing the velocity iL does increase.But my passive region would be always at this location.And remember when we have this kind of situation that the passive film is very strong in natureso that with that increasing velocity the passive film does not break down we can easilyat the same time even it breaks down still immediately it starts reforming because allthe time I have to maintain this particular point where ic equal to ia.So, the corrosion rate would be with velocity if I see this is corrosion rate,if I try to see the corrosion rate when it was 6 and starting from 6, so, this is 6 so,velocity is increasing like this, okay.So, at this point the corrosion rate would be here then corrosion rate in case of 1,6, 5 it will be increasing 6, 4 it will be here this point which is nothing but I criticaland then once we reach 3, the corrosion rate would drop down to the passive region.Corresponding to the passive current density.So, this is ip since this is ip and then if we increase the speedthe corrosion points would be this point only all the time.So, the distribution or the variation of corrosion rate would be this one.So, I could see that the corrosion rate varies initially it increased initially it increasesand then once it crosses i critical i will have the existence of ip which is a very lowcurrent density.Or this particular zone is nothing but stable passivity.So, this is the analysis and now if I try to see the concentration if I if the velocitydecreases in this way velocity is increasing.So, if it is this is decreased, now, the pattern would be when it reaches to IV, if the oxidelayer does not get dislodged, my corrosion rate point would be when the velocities correspondingto V4 will be here.Now, after that even if we go to 6 which is the lowest velocity.If the oxide layer is not getting dislodged.I will keep that passive layer.So, the when we return the return point that was the velocity is decreased gradually from1 to 6, I could say that every place I will get passivity, because the surface is notgetting disturbed, but if the surface is disturbed, let’s say at 4 point if it is disturb, thiswill reach to this or at 3 see 5 point if it is disturbed it will reach to this or 6point if it disturbs it reached to this.So, this way we can analyse, the effect of concentration polarization on the Active Passivemetal corrosion rate.So, let’s stop here.We will continue our discussion on the Active Passive metal.And we will take two more instances one is the influence of alloying on the attainmentof passivity in some critical metal like titanium.And the second part is, we have to also talk about anodic protection of metal.Where in the anodic side if you polarize, we do get a lower corrosion rate and protectthe structure.So, let’s stop here.We will continue our discussion in the next lecture.Thank you.