 Today, we shall discuss an important topic which is called as selective leaching or also called as dealloying, sometimes they called as partitioning. They all mean the same. Now, why does the problem of selective leaching occur in the first hand? Why does really it happen? It happens because it is concerned with the alloys. They are not concerned with the pure metal, they are concerned with with alloys. Some examples if you want me to give you can say that copper zinc, copper nickel, they are well known examples. Now, the question is why do you really need an alloy? Why not we use a pure metal? Stink is a consideration right. So, the stink is a consideration why we go for alloys. I will give want to give an example in relation to copper zinc and copper nickel. Now, if you take a let us say a material and then the properties and the properties what are the properties is mechanical properties, maybe the yield strength, the ultimate tensile strength and you look at the elongation is the percentage elongation the units of that can be mega Pascal. And you can see the you can appreciate how the alloying helps to improve the mechanical properties. Suppose, you take a a annealed copper, annealed pure copper, copper is is very pure. If you look at the the mechanical properties, the the yield strength of that is only 33 mega Pascal. The ultimate tensile strength is 209 mega Pascal. The percentage elongation is about 60. If you take a commercial pure copper, take this, the strength increases considerably of course, the strength and elongation are many times inversely related to each other. But you take a brass which is a brass which is a copper 35 percent zinc which is a annealed annealed. You will see the strength increases quite significantly. In this case even the elongation also is improving in this case. If you take let us say copper 10 percent tin, take this you can see that the strength level is again increasing and like this. If you look at the copper nickel you know the alloy systems say copper 10 percent nickel I mean talking about weight percent, the strength level is the yield strength is 105 and the U2S is 275. The ductility varies depending upon cold worked or not. It can vary from 15 to 35 you know depending upon how they are done. It can vary quite widely. If you increase the nickel content further say 30 percent nickel, see again there is an increase in the strength level. Again it can vary in the range something like that. So, in the essence we go for alloys because we want to improve the mechanical properties especially strength of the metal. But if you look at this you have seen earlier when you talk about the galvanic corrosion when you have copper, when you have zinc has two different entities a copper block and a zinc block. If you couple them galvanically in the environment what is expected to happen? The zinc will corrode preferentially it becomes anode and the copper will act as a cathode. So, the active metal dissolves and the noble metal gets protected. They are macroscopic there is a clear entity of a copper block and clear entity of zinc block. Between these two the current flows right copper act as a cathode and zinc act as anode the current is flowing between the between them. But in a alloy let us take a copper 35 percent zinc which could be a homogeneous alloy we will talk about it later. The mix at atomic level right they are homogeneously dissolved and more so in the copper nickel systems right the copper nickel system is known for having extended solid solubility. So, at atomic level they mix very nicely, but even then the electrochemical property of copper is different from the electrochemical property of zinc or the electrochemical in the case of brass and in the case of coop or nickel alloy nickel is different from that of copper. So, you could able to see having different electrochemical properties and so you expect at atomic level exhibiting different corrosive behavior with respect to two different alloying elements. So, that is why we talk about selective leaching or we call about the dealloying. Now, to just give an example how the system can become complicated let us take this the phase diagram. This is a famous copper zinc phase diagram I think most of you might have studied in a physical metallurgy course and you will see lot of very technique transformations we call it. The copper zinc system they exhibit different phases right when the zinc content is is less you have an alpha phase as you increase the zinc content what happens now you can get a beta phase somewhere here. The composition lines between them you get alpha plus beta phase of course, you have a gamma phase you have epsilon phase you get a phase and so on so forth. From the point of view of real engineering applications the alpha phase and alpha plus beta phases are important we call them as alpha brasses we call them as alpha plus beta brasses we call them as the beta brasses the moose metal. So, these are all very familiar with you what I want you to understand is the amount of zinc varies depending upon the type of brass that you take. Now, why you take different brasses because they have different mechanical properties the alpha plus beta brass will have a different mechanical properties in fact, it could be much harder as compared to alpha brass. So, from the structural point of view we need to use different type of alloys which means they have different electrochemical corrosion properties that is the crust of the metal. Now, these are some examples you can also have various other alloy systems you know and these processes are classified depending upon which element is selectively dissolving selectively leaching. In a copper zinc system you expect what which one will will will dissolve first selectively zinc will dissolve and you call them as de-nitrification. So, a very familiar term in the case of copper nickel we call them as de-nitrification you may have somewhat similar to copper aluminum and aluminum can be with even other alloying elements you can have we call them as de-alumification ok. Suppose you have silicon we call D and so on and so forth. These elements are selectively dissolved because they are relatively anodic compared to the the next element. Sometimes selective dissolution preferential dissolution is is beneficial right. What an example iron chromium it is beneficial why they form the passive films. So, so we need to contrast you need to contrast and how this really are happening. Now, in order to use this alloys let us say copper zinc copper nickel and whatever alloys we will talk about. We need to understand the concept of selectively leaching or de-alloying we call it. In terms of mechanisms, in terms of the factors that affect the de-alloying actually and if you know both of them then you can able to know device means to control de-alloying actually. So, what we will do is we will discuss now in terms of how this mechanism of of de-alloying occurs. There are about four different types of theory important theories I would say. One, dissolution, redeposition mechanism. The second is volume diffusion. The third we will talk about surface diffusion. The fourth we will talk about percolation model. I will be very brief in discussing these mechanisms ok. And they are fairly simple they are not really that that that complex. Let us look at the first mechanism we talked about dissolution and redeposition. Let us take the case of brass. A copper zinc in the system in this alloy is exposed to the environment. We consider that both the alloying elements they undergo dissolution or corrosion process like this. This is your step number 1. The step number 2 what is expected to happen here is. Now, look at this you can have look at this in this solution your copper ions and zinc ions the copper ions the copper copper 2 plus equilibrium is nobler compared to zinc zinc 2 plus equilibrium right. So, you have copper ions here this copper ions would displace zinc ions and zinc in the lattice. So, you have copper coming over as a copper here. So, you see now the copper ions in the solution will undergo a cementation process we called it right. We called cementation process wherein the zinc transfers the electrons on to copper ions and the copper ion get deposited zinc dissolves as zinc 2 plus ions. So, this is your step 2 process. See 2 plus is the solution. Yes. See I put a pictorial like this ok. Initially both they dissolve of course, it does not go from the bulk it goes from the surface ok goes to surface. Then what happens now? What happens in this case? You have copper ions they get exchanged that leads to copper here copper here you have again copper here zinc copper zinc so on right. So, this is what is the displaced. So, it is from solution the solution consists of copper ions the copper ions being noble they replace zinc from the lattice of the alloy ok and the copper is getting redeposited and you will form a porous structure. See look at now it forms a porous structure it is not going to be a complete you know a normal metallic structure. If this is the case you know this is one one one mechanism that we talk about right is one mechanism and this theory has some limitations. For example, suppose you have copper gold for example, or you have gold silver so noble. So, you need not get you know and and in this case look at this in this case the gold may not dissolve right only copper may dissolve only silver may dissolve because copper is quite noble right. Suppose I take I give you a copper gold alloy system and put in an environment not necessarily that copper dissolve I am sorry not necessarily the gold dissolves like you have copper dissolution in the brass system because gold is noble in this case. So, without dissolution of the noble metal it is possible only the active metal can dissolve right. So, it is so. So, in this case what happens now it is saying in this case it talks about the dissolution of only the active metal the noble metal need not dissolve from the surface ok. So, it is a really a selective dissolution and not a redeposition process. Do you understand the thing now? So, so it is so this mechanism may not be valid in system like that simply copper can dissolve and and this can dissolve is done also you know you know how people how how does the goldsmith separate you know gold right. The gold has got copper and and gold right ornament for example, how does it separate? It takes this one and melts with the silver and then forms a alloy and then pours it into the aqua is a solution. What happens now? Only silver comes out ok copper of course comes out gold does not dissolve at all. So, you can have a side to dissolution process wherein noble metal need not dissolve actually ok. So, this mechanism does not is flying in those metals that those alloys where the noble metal will not dissolve at all. So, there is no question of redeposition process. So, you have the the the second mechanism which it talks about the volume diffusion. Please understand the remaining 3 mechanisms we talked about the volume diffusion, surface diffusion, the percolation model may not invoke the dissolution of the noble element of the alloy. It could say that only the active element dissolves the noble element does not dissolve in the system ok. Now, if there is a case suppose I have a bulk alloy let us say it consists of say copper and gold something like that. In the surface see you have a surface here in the surface it is possible that only copper comes out and gold get enriched right. So, it is a selective dissolution of the relatively active metal active element and noble metal does not dissolve. So, only copper comes out gold remains on the surface and there is a problem here. Can you tell me what is the problem? The problem is that you know the diffusion of the elements at ambient temperature is so low right. If this process has to continue unless the copper comes to the surface it is not possible for the dealloying to continue am I right. For the dealloying to continue the copper from the lattice has to diffuse and then come to the surface and then only there can be dealloying. If it does not happen a few atomic layers of the alloy will dissolve where the relatively active metal goes solution and then thereafter there will be no dealloying taking place. Because the diffusivity is low at ambient temperatures you might have heard of high temperature oxidation right where you talk about diffusion of aluminum diffusion of chromium to surface diffusion of silicon to surface get oxidized forms an ice protective film and all possible. But at ambient temperature the diffusion of these elements are so low you do not expect them to continue with the dissolution process. But however, what people have done they have noticed that when there is a dealloying taking place people have observed change in the phases. For example, if I take copper zinc system I take zinc rich alloy I take zinc rich alloy ok. And when it dissolve when it dissolve what happens just look at the phase diagram here right. If I take an epsilon alloy here then surface will be alpha then alpha plus beta beta beta plus gamma and gamma plus yeah gamma. So, it will go this way right because if you remove zinc from surface you will get more rich copper rich phase right. So, copper will I mean the the the brass will have copper rich on the surface because zinc is removed. So, as you move from the surface to the bulk the phases will be alpha alpha plus beta beta plus gamma and gamma and so on so forth is not it that is how it happens. So, that is how people have detected in some cases to show that there is a selective dissolution of these elements. But however, the thickness of these layers are so small. So, this theory is little bit a problem, but of course, they have proposed what is called as die vacancy theory. They say that if you have two vacancies together it is easy to to migrate. So, that is the theory that they have been proposed, but it is not been well proven across various alloy systems ok. So, the water diffusion is considered to be more difficult and it is not be very favorable for dealloying of various metals. There is something called as as surface diffusion mechanism this is very interesting. Now, I have a surface and I get dealloyed it is a dealloyed surface a dealloyed surface. When it dealloyed then what will happen to to the surface? Surface will be rough and porous there will be atoms loosely held right. Now, these atoms migrate through diffusion process because when the atoms are alone the energy of the atom because surface becomes quite large right. So, when the energy of the surface is more then what happen they try to try to agglomerate they try to agglomerate actually. So, what happen they they migrate diffusion process and then form islands of noble metal clusters noble metal you will see porous structures porous you see pits. So, the initial process is the active element dissolves the noble element remains on the surface, but subsequently what happens the noble elements the atoms they diffuse on the surface and they form clusters. In the process what happens you are going to generate a free surface again am I right? It can create a free surface because the atoms are moved and formed as a clusters. So, it creates a free surface that again and again leads to dealloy. So, this is I have been people have been modeling this and then and then trying to show that the the the dissolution is by clustering process people have shown nanostructures people have shown the porous structures ok and and and that is how you know and the dealloying continues it is it is it is not just the surface process it can go subsurface process. So, so surface the diffusion process is a result of dealloying of the active element a clustering of the atoms because of the surface energy leading to creation of the crust surfaces and subsequently again there is a dealloying and the process continues and dealloying happens eventually to a larger thickness in the in the alloy right. There is a small variation in this in this concept which is called as percolation. You know what is called percolation right this this model probably many many of you might have studied you know and and various context of that. You take a solid containing a and b any entities that you have you mix them thoroughly right you mix them thoroughly. Suppose assume that a is the major element and b is the minor element right you mix them thoroughly the b's can come in contact with each other when you exceed the concentration of b beyond a level right do you get it. So, when I when I when I have a and b I mix it you know if the content of b is very low then the all b are isolated or all the b are surrounded by a actually. But if you keep on increasing b at one level b and b will come in contact with each other ok. So, that is so that means, that makes a continuous path for the b to dissolve am I right when the b and b are coming contact with each other then it is not dissolving actually. So, the percolation model tells that beyond certain concentration the dealloying occurs. This is a very interesting theory for example, take copper and zinc less than 18 percent of zinc no dealloying if you increase the zinc content beyond that the dealloying occurs. So, the the percolation theory talks about the active elements coming in contact with each other. So, that the dissolution path is quite continuous right it goes like that. But please notice that the percolation model and the diffusion model can work together right. I can have I can have some threshold level of active element that leads to dissolution, but even then there can be diffusion of the noble element in the surface so that they can form clusters. So, the percolation model does not totally exclude the surface diffusion model actually right. So, it is quite possible. So, the percolation model explains why you need critical concentration of alloying element in order to get the dealloying without that it will not happen right. So, that is a important thing when we talk about the dealloying of materials actually ok. So, we will talk about the factors affecting dealloying you know shortly ok, but these concepts are important in order to understand the factors that control the dealloying of the metals and alloys ok. Is is it clear now actually? So, we so far saw four theories. One involving that both the noble element and the active element they dissolve in the first instance and subsequently the noble element what happens? It it deposits back onto the surface and it displays I know and then the active element goes to solution right. This got displacement process yeah. So, in each of these cases there are two processes opposite processes are after the first is the electrochemical difference in the potential between the elements second should be the thermodynamic kinetic property of diffusion. So, in this percolation thing still there is a potential difference between copper and zinc even after such a those. Very true yes. So, why is not that dealloying thing? How is this through the accounting model? The the you are saying that why do you need a percolation model right is what you are saying. Yeah because zinc here is dissolved not deposited like on the surface proper you to move around in the form fits. Yeah yeah see now now look at the model for example, the dealloying model I mean in the in the redeposition model that we talk about it is simply that at one go both elements dissolved and they get redeposited into the system right. In which case irrespective of the amount of alloying element added right what I mean talk about concentration element right the dealloying should should continue to happen right. It does not happen if you have copper zinc, but it is 18 percent or you know famous red brass. Red brass you do not get dealloying we will see later yellow brasses occur ok. The yellow brasses and red brasses are all half of brasses, but then the amount of zinc in red brass is only 15 percent. So, 15 percent zinc does not lead to dealloying any amount of time you expose to the environment nothing happens ok. So, how can you explain that? The only way that you can explain is that you need a critical concentration in order that the dealloying occurs. Now, the critical concentration almost is required that also will depend upon what may depend upon temperatures may depend upon the environment it may depend upon the two alloying elements. For example, if the alloying elements have wider potential difference right then the critical concentration may come down also actually ok the factors that you see little later. So, yes they are all going to decide what is the critical concentration or threshold concentration above which the dealloying occurs ok, but these factors are to be taken into account which that is what we are going to see in the subsequent discussions. So, the percolation model explains the need to have critical concentration of the active element in order to have dealloying process that is that is what the discussion all about actually ok. And the surface diffusion model is also proven because when you take out and see there are islands you know there are islands and there are you know we are not going to details we look at the RC Newman's work and all you know they they are shown very clearly there are nano grains ok. How we get a nano grains they get nano grains because these atoms are get clustered and form the nano grains actually ok. So, clustering they do happen ok and then since the nano grains are porous and then there is a continuous dissolution of metal really taking place at all. So, this this this is so, what is after all mechanism the mechanism is what we propose in order to explain the observations that taking place right that is what it is all about actually. And then these mechanisms should also explain the fundamental processes that are taking place for example, when you say volume diffusion the issue there the issue there is that how much what should be the thickness over which this can happen. So, this can happen. So, they also used a theory called as dive action theory, but then still it is very limited you cannot have extended dealloying of metal you cannot take place at all actually. So, that is why the you know the where the the the volume diffusion was foot forward by Howard Pickering you know in his own papers. So, you see some papers appeared in corrosion science long ago very nicely demonstrated that there are you know if you see the surface as I told you the volume diffusion is what when you dealloyed the surface you will have the least amount of active element as you move inside the active element concentration increases right that is why it that is why it is called volume diffusion of taking place right. And so, that he has proved he has shown it by using electron diffraction pattern that these phases are changing from surface to this. But the the the thickness over which they are happening are very limited and taking place at all. So, each of these theories is not that as though that there is no evidence, but there are limited amount of evidences and each theory have their own limitations to explain the completely observed phenomena in the dealloying of the metals actually ok. I hope I have made you made the point clear to you in terms of you know these models in explaining dealloying of of metals any other questions you have ok. So, let us move on to the next one actually ok. So, what are the factors controlling? Now, this is called as partitioning right. The partitioning will depend upon element A which is noble I call it potential of the noble element and the potential of the relatively less noble element we call as active element ok. I do not call anode or cathode active element right. If the delta E is is equal to E n minus E a is large then dealloying increases. What does it mean? You take copper zinc and you take take copper zinc and copper nickel. If I make if I make a a tube for a heat exchanger that carries a sea water one one case it is copper zinc, other case it is copper nickel, both cases sea water will be used. Which of the two tubes will fail earlier? Copper zinc will fail earlier ok is what I meant here. So, because the potential difference between these two are quite large as compared to this. And secondly is the composition higher the content of the active element is the dealloying. Say an example copper let us say 18 percent zinc, copper let us say 30 percent zinc and copper 40 percent zinc. This is this both are called auto brasses right. And this is called the beta brass this is called the Moons metal right. The Moons metal the famous Moons metal is copper 40 percent zinc right. So, the dealloying will increase from this to this and to this am I right. Because the delta E is going to be same you know in all cases, but the content of the active element increases from this to this to this actually. Similarly, when I also have copper 10 percent zinc sorry nickel and copper 30 percent nickel. The question then you will ask is why would people use alpha beta brasses? In sea water people use alpha beta brasses why they use? Because the alpha beta brasses they have better erosion corrosion resistance. They have better mechanical properties. So, the damage mechanism need not be always dealloying. It can be simultaneous process when there is a flow line there is velocity you can have dealloying you can also have erosion corrosion. If the erosion corrosion is more dominating then what happens? Then you will see that copper 18 percent zinc will be failing faster than copper 30 percent zinc. Because copper 30 percent zinc has got better mechanical properties. So, so we need to look at overall perspective in terms of the metal selection alloy selection for given applications ok. But what we are seeing here is understanding the science of dealloying. So, that I think you should you should try to understand. Now, in addition to this it also depends upon the environment. More oxidizing environment, more oxidizing, more corrosive dealloying. An example is chlorination. People do chlorination in cooling water systems and if you do chlorinate chlorine is an oxidizer and so the more dealloying occurs. Let me look at a slightly you know more related to alloy alloy chemistry towards the to the to the dealloying ok. Let us take that I have element A and B. B is active. So, B concentration increases you know. As you go from this curve 1, 2 and 3 the concentration of the active element increases. Now, they show some kind of passivity and you have a potential here and this is the potential we call it ok. We call the potentials and this is called as EC beyond this selective leaching occurs. So, there is a driving force for corrosion to occur right. So, for these alloys in order to find out whether they undergo dealloying how effective they are you can carry out a polarization curve ok and get this and find out what is the EC value. EC here represents the potential above which the dealloying really occurs. And the increase in passive current density here so called passive current density means that there is more dealloying is taking place. Now, if the corrosion potential exists above EC if ECOR is going to be greater than or equal to EC then what happens dealloying occurs right or not because only above EC you have dealloying you can use polarization curves. So, how do you get how do you get ECOR at that particular value? What is the basis? When will the ECOR will go here? In actual situations when do you expect that will happen? What is the criteria for that? Yeah that is right you call it trans passive I do not want called as a trans passive here you know we call here these potential above which there is a dealloying taking place. When will the ECOR I mean ECOR will go above this or what? So, it depends upon the reducing reaction right. You have one more reaction this is your anodic reaction you have a cathodic reaction. If the cathodic reaction the kinetics suppose that suppose I have this is for cathodic reaction you cannot happen. But if the cathodic reaction occurs somewhere here what happens the ECOR is going to lie above the EC. So, you are going to have dealloying here no dealloying here. So, what I mean is the environment also is important the environment is less oxidizing then the dealloying will not take place at all ok. And this this concept we have seen its own many number of times you know how the ECOR really occurs. ECOR is a mixed potential right mixed potential means it is both on the anodic as well as the cathodic kinetics ok. So, it depends upon the alloy it depends upon the environment as well that is the point that you need to be understanding at all. Now, the EC of course, it depends upon what depends upon the alloy here right. If you the EC will will will keep reducing if you are going to alloy more and more active element in system. Yeah, it is not pitting here you do not see pit here in this case that is why you know in copper in a copper zinc system and copper nickel system you do not get a pit at all actually ok. If you if you if you observe the surface here you will see the surface is enriched more with what with copper because zinc is just getting out of it. Similarly, copper nickel also nickel comes out of solution. In fact, if you take if you take a copper nickel system if you carry out a polarization here and see the solution the solution colorably turn into green why that more nickel is getting dissolved. So, you see a nice green coloration at this particular potential. You will not see you may see a nickel also here, but then the amount of coloration you see very very small here. So, that is the way you can able to show that nickel is preferentially dissolving over its particular potentials ok. Hope this pointer is is is clear to you. Now, we have seen these things that what does it really means in practical applications ok. Application where are these problems? I am not sure to what extent you can able to see this picture here ok. Let me try to see you know you see this this black coloration here they are greenish black color actually hope I do not know how far it is able to see. And this is a copper nickel alloy in seawater application in one of the refineries located in Mumbai it is about a couple of years you know through through this the water see water you know was passed through this is a heat exchanger. Now, you see the coloration here I hope you are able to see this coloration here. So, this is the coloration they will see look at this color you can able to see this color here is bright here right that is and this the black thing that you seeing here is due to the de nickelification the nickel came out of surface and form this actually ok. This is something which is which is normally you see. This is another example this is another example of how the dealloying can really affect. This is essentially it is a de zincification process the left side what you see you see here this layer is it is a dealloyed layer ok let us see this it is it is it is rich in copper. But look at this this is a layered type here it is something like a pit type you know you see here. So, you can have two different types of dealloying taking place it depends upon the the alloy it depends upon the environment you know you can have you can have a plug type it is called a plug type it is called a layered type. So, if we have like this please notice now if I remove zinc from brass what will happen is what will happen to strength? Reduce. Reduce because that is what is giving point. So, it will it will the strength will reduce actually ok and in this case not only reduces strength it also means stress concentration at this kind of pits here ok. So, this is the other kind of problem. So, these are the real problem happened, but fortunately they are not the dealloying is not as fast as as you are pitting corrosion at all, but yes it does have effect over years ok and in this case of course, this is an isolated one acidified copper solution. So, 7 days only, but in practice it may take a couple of years to get the dealloyed layers actually on the surface. Is sensitization same as leaching? No sensitization is not a leaching there right. In sensitization what happens now in sensitization you are talking about stainless steels right. In stainless steels you are taking the chromium out of the strain boundary area right. So, you are going to have simply iron, iron simply dissolves there actually is it not ok. And if you have in fact, this goes other way around if you are going to have a stainless steel and you know iron and chromium, chromium dissolves preferentially and it forms a nice passive layer stops actually. In fact, a sort of selective corrosion is happening in stainless steels ok, but only thing that happens now the chromium forms a film. So, it does not allow this subsequently the dissolution of the alloy. So, dealloying in some cases can be beneficial ok it can help, but in these cases where it is not passivating it is not good because it is going to be reducing the strength of the alloy significantly and cause the problems ok. So, applications now you see that you know you have alpha brasses are thrown to this to dealloying which is of course, beyond 18 percent alpha brasses alpha beta brasses and the alpha beta brasses all of them are thrown to dealloying actually. What people have done actually in order to avoid this the desinification process all the brasses you might be knowing what is called as admiralty brass right. People from the navy you know it is admiral who first you know formulated this particular alloy actually that is why it is called as admiralty brass. The admiralty brass it consists of tin ok it has got a tin I think it is about I think it is about 1 percent I think it is about 1 weight percent yeah. So, about 1 weight percent of tin is added to this to alpha brasses to support this they also add small quantities weight percent of what of arsenic antimony and phosphorus are added to this and they are effective in curbing the dealloying of alpha brasses. The mechanism is not completely known this is not known actually ok there are some speculations but not completely known. But alpha brasses such additions such additions not help. So, they continue to undergo dealloying same as the beta also right and for most seawater application where you want to really have a safe thing people go for copper nickel systems people go for copper nickel. So, as seawater people use that predominantly. I will just cover another topic which is relatively small and and related to dealloying and this is called as graphitic corrosion. Yeah I mean see in the solid state diffusion of all these elements you know at ambient temperature is very very very small right is extremely slow actually taking place and I do not think zinc also the diffusivity is any significant significantly higher compared to that ok. So, the diffusivity of zinc in copper is still is very low. If the diffusion is going to control the dealloying then the rate of dealloying will be very very low ok. Now, then how do you explain the rate of dealloying which is higher ok. So, that is the question that comes now ok. Now the so, the bulk diffusion is a volume diffusion model that if you are to account for the rate of dealloying it cannot be accounted based on diffusion you know diffusivity of zinc at ambient temperature. A bit of you know the the use of the the die vacancy you know that means, to say that there are two zinc atoms which are dissolving and creating two vacancies and two vacancies are move little at a faster rate and so, that assists the you know migration of zinc from the you know from bulk to this speculation the speculation is taking place. Otherwise simply based on the diffusivity of zinc in the lattice at ambient temperature you cannot account for the rate of dealloying occurring in process at all these things. So, that is where the volume diffusion theory lacks in explaining the experimental observations. So, we are going to go into surface diffusion because in surface diffusion what happens is you are simply clustering right. The reaction the dealloying is always happening in the surface zone it is not happening at the bulk. When you have let us say that zinc is coming out and your copper you may have you know a copper atom which are free atom there actually they just move and they form islands of copper right which means they are clustering that means, the free surface is that. Now, this islands also start growing like you know in two-dimensional things now. So, that means, you always have it created a free surface and through the free surface the dealloying is taking place. So, there is no bulk diffusion that is accounted for in the surface diffusion process. So, that is why you are able to talk about it and people have done that actually again human model is look at you know Monte Carlo simulation and then you know trying to see how these are happening at all all these they have done it. So, that explains one part of it. The other part it does not explain is that why you need a threshold level of the active element in the alloy in order to cause the dealloying that does not really tell ok. That is where the percolation model comes in the pictures because you see here what it means is that yeah there can be surface diffusion, but surface diffusion is not enough to account for the the dissolution rate of it right. So, in addition to this surface diffusion the percolation model which connects the active element in the the matrix it dissolves and then it goes to connect like this only ok. So, these these are all in fact, you could talk about even composites in order you know people talk about composites various properties the electrical properties of the composites for example, ok they are isolated then the electrical properties does not happen. Suppose, you have element A and element B the element A is insulator and you add element B into system actually and beyond certain level of B which is conducting element you see a nice conductivity increases because the B is is connecting the each other to the pathways actually. So, this is essentially that you are connecting them and then through the pathway they come as a channel and dissolve out of it. So, this is another way of looking at the selected dissolution of of the atoms actually, ok. So, so so they are not isolated. So, percolation model talks about critical concentration required for that, ok. But all or again related to potential difference related to the environment because the driving force for the corrosion again depends upon the other factors. What potential variation the environment that you are going to be here leak or value that shift we are all going to be a part of which talks about the rate of dissolution of the alloying, ok. So, these two things should be seen separately the factors and the mechanism or to be seen you know should be properly understood I would say I think, that is the thing. Is that answered now your questions? Ok. Any other questions people have? Ok. Now, let us go into the gravity corrosion I just want to finish up this year. People have been using cast ions, cast ion pipelines those who are not familiar with cast ion we just spend a minute, ok to to to get an idea about water types. What is the difference between a steel and a cast ion? Castable grade, ok. So, what is the composition of carbon? From 2 to 4 percentage of carbon or greater than 2 weight percentage. So, more than 2 percent 2 point something like 4 or whatever kind of weight percent if the carbon content is going to be there, ok. If the carbon is more than that you get it in cast ions. Can you give a better definition of that? It is right, right. As of iron, iron and carbon that can be casted. I can cast anything I want. I can cast. Presence of eutectic cases. You are right, yeah. So, in the solidification process it does not go through from the liquid it does not go through it does not go through this it does not go through a gamma transition, right. Isn't it? If you look at the iron carbon diagram in all the cases the liquid directly can give us to what? Give us to alpha plus F e 3 c you can give, right. You can give us, ok. You can give alpha and then you can give you give us to alpha plus F e 3 c. So, it does not go through a gamma in the phase diagram. First of all, first thing. The second thing is it can form a cementite form with alpha or it can also form alpha plus graphite, am I right. Now, of course, there are different types of cast ions you know which is very famous. Grey cast ion, white cast ion, nodular cast ion. What is the difference between a malleable cast ion and a nodular cast ion? Anything more and it is converted to malleable cast ion by heat treatment process, right. Whereas, the nodular cast ion goes directly from the liquid you get the graphite actually, right. And what is the grey cast ion? Here also you get a directly you get a you get a graphite, but the graphite is in what form? It is in the flake form actually, ok. Now, you please read those people who have want to know more about. For example, why should it form F e 3 c? Why? How is possible for you to move you know without forming F e 3 c? How you can have graphite? What are additions are added? All this you can read some books and you know there are nice books available simple books if people read it. Now, it is important for you to understand the microstructure of cast ion in order to in order to understand what is called a graphitic corrosion. This is a grey cast ion, right. What do you see these flakes? These flakes are all what? These flakes are all graphite, right. The matrix is a ferrite, sulphur matrix. This is a grey cast ion. But what you need to know is these flakes they may look little isolated. Are they isolated? They are interconnected, right. These are all something like. So, if you have a thickness suppose you have a thickness of the cast ion a flake may start from this end it may end in this end actually, right. So, these are all not isolated they are interconnected in the grey cast ion. This is a very important thing and for those who have not an idea about grey cast ion should know this. If you take a a a spheroidal cast ion it is a microstructure of this. Please notice these are graphites, graphite. These graphites are not only spheroids, spherical. And what what different does it make between the grey cast ion and this? They are not interconnected, they are well separated, right. They are well separated. That means, each of this are surrounded by the matrix, ok. It has got that good mechanical properties as compared to grey cast ions. But anyway we are not going to discuss that that aspect, but these microstructures have relevance for corrosion that I think we should be understanding at all. Please understand you have a graphite in the system, you have a alpha in the system which alpha means is ion a little bit of carbon dissolved there, ok. And so, that is what you normally see the system. So, what are the consequence of that? This alpha ion is a graphite here. I exposed to the environment, right. There is an environment here. What happens? What do you think will happen? I have taken this alloy and exposed to the environment may be like sodium chloride, solution or water, moisture something like that. So, what do you think will happen? The ion will dissolve preferential why does it happen? Yeah. Yeah. So, graphite is noble. So, if you look at the galvanic series, I hope you will able to recollect the galvanic series we have shown before. Graphite is noble, ion is. So, what is going to happen? You are going to have micro galvanic cells. It is not a typical dealloying like what you see in the in your brass or maybe in a cupronical alloy system. Here there is a distinct graphite phase and there is a distinct matrix of alpha ion and so, they have a different you know noble and active characteristics and one which is noble will act as a cathode, one which actor will be suffering corrosion, actually serious corrosion here. So, what will happen? You will see the corrosion occurring in the or the interface the corrosion becomes severe, right. In the galvanic corrosion where is the attack more or the interface ok, the interface corrodes. Now, now it corrodes like that if you utilize this. So, what do you have will happen here? In this case the corrosion suppose here the environment here environment the corrosion starts it will proceed like that, proceed like this, proceed and start leaking here because of continuous thing it leaks. On the other hand, if I take a nodular cast iron or a malleable cast iron, if you take a cross section like this I may have some graphites here, I may have some graphites here and I have here, here right. Now, the corrosion will start here, here start here. Now, what happens? They they fall off. Now, subsequently what happens to galvanic corrosion? It stops. Galvanic corrosion does not proceed further, micro galvanic corrosion does not proceed further. So, that what happens now that means, no graphitic corrosion. Now, the graphitic corrosion that occurs in the in the in the gray cast iron obviously depends on the environment right. So, this is a pipeline it is the soil the soil chemistry becomes important right. You have for example, you have an acidic soil and you have lot of chlorides the attack becomes more ok. Nowadays, they are not much of a problem because nowadays people do not use the gray cast iron that much because people have come to know there is a problem. Earlier days when there were pipelines and the pipelines you know infinitely the problem started when when they were paving the roads ok, when they start digging the roads the excavator. The shock waves that went and you know hid this pipeline cause simply the cracking right. See, what happens you know imagine that that I have a dissolution process here, what happens now? All the iron slowly dissolves there is a residue of graphite just sitting on the surface right. So, it loses its mechanical integrity at all you know. And so, even a small impact given onto the pipeline it just fractures ok. So, these were the problems you know just after second world war ok. There are lot of problems, but nowadays of course, people are very clever they do not use gray cast iron, but it is important for us to understand the science behind this and why we do not use the gray cast iron for many of the pipelines, but cast irons are good. If you compare the cast iron versus the steel, generally the cast iron corrode at a lower rate. I am talking about uniform corrosion because the cast iron may have some silicon all the stops which we are more generally more resistance to uniform corrosion, but your gray cast iron the problem comes about the localized attack that happens between the graphite and the iron matrix leading to leaking of pipelines. People in those days were storing even sulphuric acid you know using the cast iron tanks and sulphuric acid will leak from inside to outside. All structure looks very intact, but it moves out because the corrosion occurs between the graphite flake and the matrix and so, start oozing out from the surfaces. So, only way to avoid here is that not to use gray cast iron people use what is called as a malleable cast iron or the nodular cast irons ok. So, that should bring us to the end of the discussion related to the dealloying and the selective leaching and I wanted to go through this and