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    Hello everyone, let’s start lecture 7 and topic we will now continue our discussionon explanation corrosion processes on the basis of mixed potential theory.Fine.We in the last lecture at the end we started talking on the evolution of micro structureof pearlite and how it looks another microscope, optical microscope this has relation to thearea ratio of a galvanic couple.So, let’s explain that particular micro structure, so it is actually a practical aspectof corrosion.So, how to or explaining the typical micro structure of pearlite, so this is relatedto area of cathode and anode.Now in steel the pearlite is a phase mixture in a steel and that to we get it very lowcarbon steel let’s say 0.1% carbon we do not see the pearlite much.But as we increase the carbon content let’s say if you go to 0.4 to 0.5% carbon we canclearly notice pearlite.And in the pearlite the phase mixture, why call it phase mixture because it consist of2 phases one is cementite and another one is ferrite.So, ferrite is nothing but alpha iron which is a solid solution of carbon in BBC ironand cementite Fe3C this is a compound.Okay.Now their formation happens due to nucleation and growth phenomenon, so either ferrite orcementite nucleates on a grain boundary and that to in the pre austenitic grain boundaryand then gradually by the side of it.If cementite nucleates by the side of it ferrite nucleates and then gradually this nucleationhappens at the same time side by nucleation keeps on happening at the same time you havethe growth of this ferrite as well as cementite into the austenite grain.So, that means if we have a austenite grain boundary like this, so then if cementite nucleates,this is a cementite let’s say or I would say that Fe3C.Fine.And then since cementite takes carbon from the austenite, this is austenite, austeniteit termed as gamma iron or austenite.So, since carbon diffuses and then forms Fe3C and the carbon in the Fe3C is 6.67 in termsof weight percent, so the surrounding area would be depleted in carbon.And this particular steel from the austenite it has been taken to a region below eutectoidpoint.So, that time by the side you will have a ferrite formation, this is ferrite.So, now by the side then there will be a cementite nucleation again, cementite formation, solike that where at the same time they keep growing along this direction.Fine.So, finally your structure would be, so this is cementite, this is another cementite, thisis another cementite.So, like this it forms a lamellar structure and this lamellar, so their growth may notbe absolutely a kind of a flat parallel lines, they are actual look like this.Okay.So, that time this is the look what we are getting in under the optical microscope.Now if we take of steel where let’s say 0.4% carbon steel and that is annealed furnaceannealed.So, that means I would say not furnace I mean I would say furnace is cooled.So, that means you take it to 900 degree Celsius, this particular carbon steel then wait forsometime let’s say half an hour.So that the complete austenite forms in that particular micro structure and then you allowit to cool in the furnace by putting off the furnace.So, that time you will have a slow cooling and once the temperature crosses below eutectictemperature which is around 727 degree Celsius and then you will have this politic reaction,this kind of structure evolves.So, then you do this is the austenitization temperaturefor half an hour or let’s say 45 minutes just to be on the safer side.So, that every carbon goes into the solid solution of gamma iron and then you cool itslowly furnace cooling.So, you will have a structure like this, so you have austenite ferrite grain and thenyou have pearlite.So, like this a kind of structure you will get, so where these are the pearlite and theseare alpha iron, alpha phase which is ferrite.So, these are the ferrite which has a special name called proeutectoid ferrite.So our interest is not on the detail of phase transformation.Our interest is why it looks like this there is a black strips in the bright background.Now in order to understand this we need to understand little bit of etching and thiskind of steel is etched in around 3% nital solution and nital is a mixture of nitricacid in alcohol.So, these nital solution you first have the sample you polish it go up to the mirror polish.So, that you can see your face are reflected on that polish surface and then you hold itwith a tweezer take this nital solution on a petri dish and then dip that particularsurface into the acid wait for maybe 2, 3 seconds or maybe maximum 4, 5 seconds.Depending on how much change you are facing you are experiencing on the surface it willturn into little grey little a darkish appearance that means the surface is getting attack bythat particular solution and that particular process we call it etching.And this is nothing but etchant, we call it etchant.So, now that chemical reaction, we call it chemical reaction but actually it is not chemicalreaction, it is electro chemical reaction.And because of that particular electrochemical nature of that reaction, we get to see appearancelike this under optical microscope.So, now what exactly happens, so now you have those lamellar is not it.So, you have ferrite, you have cementite like this, those are aligned like this in a particularpearlite colony.So, these are cementite and this white portions are nothing but ferrite.Okay.So, now when this surface is attacked typically if we compare this 2 phases their electrochemicalbehavior this has higher potential in that galvanic series as compare to the ferrite.So, these acts as a positive terminal and these acts as a negative terminal in thatgalvanic couple condition, so, they are actually forming a galvanic coupling.Fine.So, once they form galvanic coupling since this is negative that means this is anodeand this becomes positive that means this is cathode.So, cathode would not dissolve, anode would start dissolving but interesting part is ifwe see this particular surface this part is negative, this part is positive.So, dissolution would follow the pattern what we have experience before.So, the experience we have experience like this that in the dissolution would be morepronounce near to the galvanic couples region.So, now if I consider here also the dissolution of ferrite will be more pronounce around thisregion.Similarly, here the dissolution would be more pronounce around this region.Fine.Away from that the dissolution of course it will dissolve.But the dissolution rate would be slower because of the resistance offered by the electrolyteaway from the galvanic coupling or the point where the other line where the galvanic couplinghas been set up.So, now if they dissolve like this if we put it in a red colour, so now their dissolutionpattern would belike this.Fine.So, that means you see that this is the surface line, so everywhere that dissolution has takenplace but the dissolution is more pronounce aroundthis segment.Now if you see if you zoom 1 particular region this is the cementite.Okay.Now here we have situation like this.Okay.Or depth.Now how do we see the structure in optical microscope.We see it because the light which is falling, so you have a surface the lights are fallingand then they are getting reflected and you have an eye piece.So, this eye piece you look at and then see the structure, so basically it is a magicof reflection of light in the optical microscope.Okay.So, now when you have a situation, if it is a flat surface before etching it is a flatsurface and it shines why it shines because all the points over which those reflectionsare taking place of the light are happening at the same condition.And the energy of the light which is coming out or almost of the similar range, so thatis what there is no multiple reflection within a small region.If we consider this small region not much of multiple reflections are taking place withinthat particular zone, so that is what we are having appearance of a very shiny surface.But if I consider a situation like this, so the here the lights are falling, here alsolights are falling, so they are reflected.And since it’s a almost kind of a flat region a near flat region, so the surface look thissurfaces from this particular zones.This surfaces are this section or this section they look shiny or the bright but if I considerhere in this zone what happens.Let’s say 1 light has fallen like this, now here we have to consider are reflectionangle, now it will go like this.Fine.And when it goes like this that could be hindered by this cementite plate thickness or if thatreflection because if we consider the cross section, the cross section looks as a circularregion.On the center we have cementite.So, if it is falling away from that particular if it reflecting away from that particularcementite rod, rod like a plate like appearance.So, then it will have a reflection here multiple reflection and then again you will have aangle like this, it will go like this.Okay.So, it is not coming back to the eye piece here we have eye piece.Fine.Similarly, if another light falls like this, so you have angle like this it might go likethis.Fine.So, like that there could be a multiple kind of reflection that will take place, so thosereflections might not come back to the eyepiece.So, if they do not come back to the eye piece, so that means the energy that is being takenby those rays, I would say that the number of rays coming out from those regions, thisparticular region will be less than the lights coming out from this flat region.So what could happen?It would look like a dark region.So, if something looking dark within that it is small a pin kind of thing or the platekind of thing that would look like a dark object.Okay.So, that is why we see this particular cementite to be dark and interestingly this darknesscomes because of multiple reflection and those lights will not come out to the and it willnot go to the eye piece.So, it will look dark and this region will look bright, so this region will look darkand this region will look bright.So, if we consider here this section, so this section, they will dark they will be darkand this region will be bright.So, the and interestingly these are the regions of the dimension is pearlite of few micronsmaybe around 10 to 20 microns.So, that time, so if this is that much dimension, so then what happens.In the optical microscope the resolution is less, so it will not be able to gauge theposition of that cementite properly only it will show a kind of dark patch around that.So, that for the Final micro structure would like this where this is alpha or ferrite andthis is cementite.Fine.So, this so why we have explained this because it has a relation with the area and the resistanceaffect during electrochemical etching process.And these relates to the understanding what do we had in our previous lecture is this.So, let’s go to understanding of those 5 instances, the situation, 5 situations whatwe have talked about.Now first instance is situation or instance or situation ishere we have 1 cathodic reaction plus 1 anodic reaction.So, in this case we have already explained this before while we consider corrosion ofzinc in deaerated pure HCl.So, that case our mixed potential theory was, so this is E, this is log i and the plot looklike this.So, this is i zero for the zinc over zinc surface, this is i zero of hydrogen over zincsurface, this is ia of hydrogen, so that means H2 minus 2e equal to 2H plus, this reactionwould happen.And this line corresponds to ic H2 plus 2e equal to H2, this line corresponds to ia zincwhich is nothing but zinc minus 2e equal to zinc plus plus and this line is ic zinc whichis plus plus plus 2e equal to zinc and this is the point which is E mix which is nothingbut E corr and this point is i corr.And this current density meets the basic of mixed potential theory where it is ic equalto ia is nothing but ic hydrogen over zinc surface equal to ic zinc ia zinc.So, that means this is the complete picture of that particular mixed potential theoryfor explaining this particular situation.We have already explained it in detail I think end of fifth lecture.So, the overall reaction is overall electrochemical reaction is zinc plus HCl equal to zinc chlorideplus H2 where this is plus this is zero, this is plus plus, this is minus, this is minus,this is zero.So we have oxidation, we have reduction.Fine.So, let’s go to the situation where we have considered 2 cathodic reaction and 1 anodicreaction.Now this particular thing can be explained can be observed when which is zinc corrosionin deaerated HCl with some impurity in the form of Fe plus 3 ion.Now a deaerated means that HCl does not have any dissolve oxygen.And that particular so interestingly, so if you leave HCl in the atmosphere it will alwayshave some dissolve oxygen.Now how do we take care of that oxygen, how do we take out those oxygen dissolve oxygenfrom the HCl.So, what in the lab scale what is done is we purge nitrogen into the solution.So, the nitrogen bubbling happens and that releases dissolve oxygen out of the HCl solution.So, this impurity Fe plus 3 is a small impurity very small amount some ppm level.Okay.What would happen?Now in this case let’s first analyze what are those 2 cathodic reaction and anodic reaction.So, the anodic reaction it easy to understand it is zinc minus 2e equal to zinc plus plus.This is anodic 1 anodic.Now one of the cathodic reactions of course if it is an acid solution, so the cathodicreaction of course this is the cathodic reaction and we would we do see that hydrogen bubblingis taking place on the zinc.Now interestingly just before going into this explanation the observation that has beenobservation is typical observation is.If you consider this particular case, here also hydrogen bubbling takes place on thezinc surface and here also hydrogen bubbling takes place on the zinc surface and we doget zinc corrosion.Now the hydrogen bubbling at amount I would say that the bubble form across the surfacewould be much less in this case as compare to this case.So, we would also understand that particular observation through mixed potential theory.Now this is cathodic reaction, now where is the another cathodic reaction.The another cathodic reaction happens in this form.So, this is also cathodic reaction and interestingly this is a very very strong cathodic reactionwhy, so if we consider standard reduction potential E zero.This one is 0.76 minus volt, this one is zero volt and this one is plus 0.77 volt ok andthat to all at 25 degree Celsius and 1 atmosphere pressure.Okay, so that condition these are the potential the standard reduction potential.So, now we can see that the difference between this 2 is huge, so that means and also evenif we consider these 2 the difference is huge.So, that means this is a much stronger cathodic reaction than this, so now you could see thatthere are 2 cathodic reactions and 1 anodic reaction.And overall process of course zinc would dissolve and these 2 cathodic reactions would happenon the surface of the zinc.So, we would analyze this particular situation in our next lecture, so let’s stop here,thank you.