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Passivation and Mixed Potential Theory - Case Study

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And today’s discussion will be some case studies based on passivity of a metal andits interaction with cathodic polarization line and we will follow will be followingmixed potential theory.So, lecture 29 is basically the topic broad topic is passivation and mixed potential theory.So, the case one the effect of concentration of oxidant and mechanicaldamage of the passive film or rather I would say effect of oxidant on the passivity andmechanical damage of the passive film.So, when we try to do that and we will be considering activation control mode of cathodicpolarization This is one consideration and second thing is the dissolving metalhas Active Passive anodic polarization.So, when we have these two situations.Let’s see how we can get the idea of corrosion rate as well as mixed potential.When we change the concentration of oxidant.Let’s the oxidant is ox plus e equal to reductant and then corresponding E ox by redequal to E node ox by red plus RT by F. Since, here we are considering one electron ln concentrationof ox and concentration of red when you say this if ox concentration increases then ofcourse, E ox red increases and similarly if it reduces this will also reduce.So, first we will try to see the increase in concentration of ox and then the secondpart this is the first part.And then second part will see the decrease in concentration of ox after reaching thefinal concentrationin part 1 and when we see the second part that time we will also see the effect ofmechanical damage.Now in order to do that.We have to draw mixed potential theory.So, let’s say this is going to be my E versus log i plot this is ampere per centimeter squarethis is in volt now, let me first draw the anodic part of the passive Active Passivemetal and here we are considering E transpassive Ecp and that means, here we could see thatEcp as well as Epp are at a same position these are at the same position as per thisparticular diagram this is i zero of M over M.This is E equilibrium of M. Now, this is the diagram this is the anodic part where I canhave metal minus ne equal to metal n plus.So, this particular process is the anodic process and this particular line indicatesthe anodic line this is i critical, okay.So, now, when we have this situation let’s try to draw the diagram for cathodic linecathodic polarization part.So, this is anodic polarization part because this way anodic part.Now if we go let’s start with concentration C1 so the concentration let’s say the C1and that time this is the anodic.This is the polarization part.So, this is E1 corresponding to concentration C1 of the oxidant and this is i zero of oxover metal surface.Now here we are considering here considering i zero ox over metal surface remains constantfor all the concentration beta c which is the activation the tafel slope for the cathodicpolarization.This is fixed for ox plus e equal to red this reaction.So, if we keep changing the concentration let’s say concentration first increasesthough this particular point will sift upward.So, it will sift like this.So, this is the second composition this is the third composition, thisis fourth, this is fifth, this is sixth, and which is let’s say just crossing i critical.This is the way it is moving this is the seventh, this is eighth let’s say this is nine andthis is 10 and this is 11.So, we have taken 11 compositions.So, 1,2,3,4,5,6,7,8,9,10,11 compositions we have taken so correspondingly we have thispoint is shifting this point will safe but there will be no change in the i zero thatwas the assumption we have taken and E equilibrium value E ox equilibrium of course this is Eox that will keep on increasing because we are increasing the concentration and thatvalue is increasing that value is increasing and beta remain same.So, that is what every lines are parallel, but they will have interactions accordingto at the way they are cutting across the anodic polarization part of the dissolvingmetal.So, what are those points, so, this is 1 point second, third, fourth, fifth, sixth, and thenwe have other connections also this is 1,2,3,4,5 So, these are the connections or the crossingpoints, red colored circles.Now, if we try to see the corrosion rate.So, those red points are those points where I could see that mixed potential theory ismaintained.So, the red circled points, I could see that at that potential ic equal to ia but as wehave said our initial condition was a metal was freshly prepared and it does not haveany oxide layer to start with.So, it will only see the circle point when the concentration of oxidant is increasing.And those circle points should remain should be considered.Those points should be considered which are basically the connection between the activeparts of the anodic polarization portion so this is the active part So, this particularregion is the active part and of course, these two region so, thepassive part, so, this is the passive part and beyond this, we have transpassive, fine.So, as we are using the fresh metal.So, we have to consider this point because it will only consider the cross-section pointsof the active regions.So, the corrosion rate if we try to plot corrosion rate plot would be with concentration if wetry to plot.So, this is corrosion rates CR means corrosion rate.So, when we have C1 this is 1,2,3,4,5,6,7,8,9,10 and this is 11.So, now, if we consider this, so, the corrosion rate is at this point that say this is 1 point.This is the let’s say to start with this is the point now, as were going with a higherand higher concentration.I will see that the concentration is increasing at the same time corrosion rate is increasingbecause I could see that the corrosion rate was here and then when the concentration haschanged from C1 to C2 corrosion rate has gone to this like that way the corrosion rate isincreasing.So, 2 let’s say this is the point 3, this is the point this 1,2,3,4,5 all though areconcentration of oxidant concentration.So, then 4, I will have this point so 4, is if I try to see the 4 is this one then 5 isthis one.So, 5 is still further increasing it has gone to this now 6 when the concentration has beento C6 which is this particular line, we are considering that is just crossing the c criticali critical it just crossing i critical.So, since it is the fresh metal to start with the corrosion would be red would be here notat this location because we are starting with the fresh metal to begin with.So, now at six the corrosion rate would be equal tois equivalent to i critical, okay, fine.Now, once we cross 6 once we go to 7, I could see that mixed potential is located at thisparticular location, fine.So, now corrosion rate suddenly drops and that drop-in corrosion rate.So, this is corresponding to i passive ip.So, it will be similar to ip so, if I try to C7, so, 7 the corrosion rate has move toip.So now, if I again start increasing my and then it at this particular location, it willachieve stable passivity because only one location it is plotting across that anodicportion.And now, if I increasing increase the concentration for this if I go to 8 and then 9, so, 9 isbasically the location where the transpassivity still just starting.So, if it is just starting so, till 9 it will maintain the same corrosion rate.And then once we go to 10 this the corrosion rate will be the mixed potentially satisfiedin the transpassive zone and I could see that the corrosion rate is increasing again.So, corrosion rate would be here because i small i nothing but the rate of dissolution.So, this is the location I could achieve and 11 it is still further increases it is increasingso, it will be here.So, what will be the plot nature.So, the plot nature as we increase the concentration from C1 to C11 the plot nature would bethis will be the plot nature.Now, once we do this after that.If we decrease the concentration from 11 till 1, okay.If I try to do that, what will be the situation?Now, if I go to 11 to 1, if I draw with a different color, let’s say I draw it witha blue color.So, in that case 11 the point would I am just drawing by the side of by just below thisparticular line.So, this is 11 so, it will be here if I do it with this color this will be here and then10 it will be similar to this will be here when I go to 9.I could see is it just crossing the transpassive, passive to transpassive zone, the corrosionwill be same as that particular current density of that anodic portion, 8 I could see thatagain that passivity is achieving.So, 8 I will be the corrosion rate this.7 the corrosion rate would be here because it is at this location.Now, when I go to 6, what will be the situation if I go to 6 that mean at this location aswell as this location.2 points that particular polarization plot for the cathodic polarization is crossingthe anodic portions, but since the passive layer has already formed, if the passive layerhas already formed, if we do not disturb the passive layer, this passivity will maintainSo, it will not reach to the i critical rather it will try to stay at i passive because weare not disturbing the passive layer.So, if it does not disturb, so, then in case of concentration C6 the current density wouldbe at this location.So, current density would be same as this one, okay.So, now 6 if I go to 5, so 5, I could see the 3 points it is crossing that this particularconcentration, but since we are not disturbing the passive layer, the current density willremain here at this location.Similarly, it will be 4 in this case it will be there and 5 if I consider the concentration3.I could see that it is just at the i passive to active condition zone.But, still it is having a cut point at the i passive section and it is not disturbed.So it will still remain 5 also it will be here and 4 also it will be here in the closedin the i passive zone and 3 also, if I see 3, this will also stay here, but once we cross3, i go to 2, then I could see that the Active Passive at this mixed potential theory ismaintained at this location.So, the corrosion rate should increase in case of when a go from 3 to 2.So, the corrosion rate would be at this location and then if i go to 1, the corrosion ratewould be at this location.So, the corrosion would be here.So, now what will be the plot the plot nature would be if I go if I see the reverse trend,when I change the concentration, it will follow this track and then it will move like thisand then go down.So, the plot nature would be like this.So, in the forward scan going to increase the concentration.The movement would be like this and the reverse scan, when we change the concentration afterreaching that C11 the path the plot pattern will be like this.So, in this case, I am not disturbing the surfacenot disturbing the passive film.Now, if we disturb what would happen let’s say after reaching 6 that means 6, we havethis point which is basically the corrosion rate when we return from C11 to C6.At C6 initially the point is here without disturbing condition disturb condition, butif we now disturb let’s say scratch it, the surface is scratched, okay.So, once the surface is scratched, I am breaking open the passive film.So, once we break open the passive film the system would try to achieve the corrosionmixed potential corrosion rate at this location, fine.So, instead of remaining here the C6 the concert the point would reach to this point.So, the movement would be this once we scratch the surface after reaching 6 C6 concentrationand then once it is passive layer is broken, then after that, if you decrease the concentration,it will all is follow this track, fine.So, then it will follow this track it will follow remember these pointsand this point both are the same location, but just to give you an indication that howthe movement happens that for I have drawn it little away from the black spear blackpoint black circle, okay.So, the location would be so, the movement would be this way.Now, instead of 6, if I scratch it at after reaching C5.So then the concentration then the movement would be this way.So, it will follow this and this and this and after reaching C4 if we scratch it againit will till C4 it there will be nothing there will be passivity with a maintenance of passivity,but at C4 concentration I’m scratching it the moment would be this way.So, the corrosion rate will move to this location, fine.So, like that way, this is the effect of scratching.So, the blue color is basically the scratching effect or we can say this the mechanical effectmechanical disturbance of the passive film.So, this is the way it should operate in case of Active Passive metal when we change theoxidant concentration from low concentration to high as well as high concentration to lowconcentration.So, let me stop here.We will continue our discussion on this very aspect.And will take up two more instances where or case studies where we could see the influenceof Active Passive metal and when we have when we consider mixed potential theory.So, let’s stop here.Thank you.