 we shall continue discussing the chapter on intergranular corrosion of metals and alloys. In the previous class, I have highlighted the the importance of understanding the intergranular corrosion of metals and alloys because when we use poly crystalline material, the grain boundaries they become very you know active because of high energy areas. And also these high energy grain boundary areas also attract the chemical species to lower the energy. For both these reasons, the grain boundaries are selectively attacked. When the grain boundaries are selectively attacked, the strength of the material becomes very low leading to loss in load bearing capabilities. If you have a higher stress applied on the structure, this intergranular corrosion can also lead to intergranular stress corrosion cracking. And we drew the reference to stainless steel because the stainless keys are very widely used and especially when you weld with the stainless steels, the weldment especially at the interpretive zone becomes prone to intergranular cracking, intergranular corrosion we call famously as weld decay. So, before I also went into discussing the the the sensitization or the role of heating on on the intergranular corrosion, we looked at the the the classification of various types of the stainless steels. And then we went on to say that the normal or conventional 304 stainless steels they have a carbon content much higher than the the thermodynamically allowed solubility limit. So, and so, when you are going to do the heating, then what happens if it leads to at least to the precipitation of these carbon. So, we need to understand the the mechanism of the intergranular corrosion in in stainless steels. Before I do that, I I thought I will just introduce a book which I should have done it in the yesterday. I introduced this book to you. This is you know worthwhile to to go through this book. It is a very detailed book on intergranular corrosion of steels and alloys. The author is by V. Chihar and the title intergranular corrosion of of steels, materials, science, monographs, 18, Elsevier publication, New York. It is reasonably old book 1984. And I also have written a chapter on localized corrosion where I have in reasonable detail discussed the intergranular corrosion of stainless steels and also some reference to aluminum alloys. If you want, you can refer this as well mechanisms and integration and monitoring. This book is edited by U. K. Moseley and Paul Devraj and published by Narosa, Narosa Publishing House New Delhi 2008. The page is 149. Actually in this book you will also have a detailed discussion about the other localized forms of corrosion that includes pitting, crevice corrosion, selective leasing etcetera actually. You can you can refer that as well. Let us now discuss about the mechanism of IGC of stainless steels. Please notice that the the mechanism of of IGC of stainless steels is not same as the the mechanism of intergranular corrosion of other kinds of alloys actually ok. So, you cannot just generalize this mechanism everywhere and we will see that as an example for aluminum alloys. Now, we have we have seen that let us say a typical let us say 304 stainless steel and we have seen that it has got iron 18 percent chromium 8 percent nickel and about 0.08 carbon content ok and the carbon is super saturated. They are in the solution annealed conditions we call also called mill annealed condition. Rise the temperature of the stainless steel sheet or plate to about 1050 degree Celsius where the carbon is highly soluble at that temperature. Then quench it rapidly so that the carbon is retained in the solid solution. Now, if you hit this stainless steel in the range of let us say about 450 to 850 degree Celsius and allow sufficient time ok. Then what happens is in the stainless steel you have say in this case you have iron chromium and nickel and there is carbon. Please notice I put carbon within the bracket because it is small quantity and in fact, it is in the solid solution it is mostly in the intersecial positions. This one is there. Now, what happens when you heat it in the range when you heat it in the range of 450 to 850 degree Celsius what happens is you have iron nickel remains here you have say 6 forms here. So, the carbon combines preferentially with chromium and forms chromium nitride I am sorry chromium carbide ok. And why does the chromium carbide form over the say iron carbide or nickel carbide? Because affinity is more that is given in terms of the free energy change. So, the the chemical potential of this is is very negative ok. So, the free energy change for that is so negative and so it forms preferentially as chromium carbides. And when this forms obviously, the matrix is depleted not only this there is one more important factor here that is C R 23 C 6 formation. You can also have other kind of carbides you can be M 7 C carbides can happen you know what kind of carbide can happen or M 4 C can happen there are other kinds of carbides can also form. So, these these carbides they nucleate heterogeneously and they nucleate heterogeneously because you know the nucleation is the most difficult in the phase transformation and they nucleate along the grain boundaries. Why the grain boundary use additional energy to overcome the nucleation barrier right energy barrier for nucleation? So, what it means suppose I draw as a grain here and the chromium carbides are formed over here preferentially right. So, that means, the chromium has to migrate so as the carbon to form the chromium carbide on the grain boundary. If you look at the diffusivity of of chromium and carbon right will they be similar or different yeah different. So, which will be having a faster diffusivity? The carbon because of the smaller size carbon diffuses at the fast rate whereas, the chromium diffuses at a lower rate. So, what are the consequence of that? What are the consequence of that? The consequence of that is what are the consequence of that? So, that means, the the diffusivity of chromium is much smaller than the diffusivity of carbon. Please notice the chemical formula right. What is the chemical formula of the carbide? It is 23 chromium and 4 carbon that means, approximately 1 carbon takes away 4 chromium that means, that means, you need to supply more chromium in order to form carbide chromium carbide, but the diffusivity of chromium is lower compared to the the diffusivity of the carbon. So, what are the consequence then? The consequence of that is if I draw the grain boundary, if I draw the chromium content this is the carbide here, see the chromium content will draw significantly here. Of course, it is it is not a correct way of writing this here, it should be in fact, it goes high here. So, you find that see look at the the the chromium carbide has got higher chromium right. And so, there is a huge depletion of chromium along the boundaries because of the fact that the diffusivity of chromium is lower compared to the diffusivity of the carbon here. And the carbon content can go below the required level for stainless steel. What is the required level for a stainless steel? We talked about yesterday, it is about 11 percent something like that 10.8 something ok. So, if it if the carbon, if the if the chromium content goes below the the required level for making it a stainless steel then what happens to passivity? Passivity is lost. So, that is that is the real problem. Now, people have examined it we in fact, also examined this in old lab. Of course, here please notice it is not a 304, 4 it is a 304 L actually ok. The 304 L has got low carbon. We will talk about the story a little later ok. But look at this even when the carbon content is very low and you heat it in the range of 675 degree Celsius for 24 hours long time, this is the Transmission Lateral Microscope image and you see the carbon the chromium carbide formation and in in the grain boundary area. This is the grain boundary area I hope you are seeing this right and he has mapped the chromium content next adjacent to the to the chromium carbide here and along the chromium carbide. The one what is referred here see here ok corresponds to this see that the chromium level see you know at this you know at this magnification you know there is uncertainty in the composition right. You know what is the chromium content here is supposed to be 18 percent, but what you report here is about 24 percent. So, there are limitation in the experimentation inherently that. So, do not worry about the absolute values here, but what you could see is that over here the 0 represent the grain boundary right. The grain boundary the chromium content falls very sharply it is about 10 you know 10 percent right. You just do not worry about the absolute values, but look at the difference 24 close to that close to 13. So, about 10 about 10 percent reduction in the chromium content occurs the grain boundary because of the chromium depletion taking place you see this here ok. So, there is a problem of the chromium depletion around these area. You also done a scan here along the across the the the grain boundary where you have a chromium carbide precipitate now you see here the chromium carbide as higher chromium content it is understandable right. So, there is more more more more chromium here because of the chromium carbide formation. The point I am trying to convey here is that when you have chromium carbide formation the depletion of chromium occurs close to the grain boundary area and it can go to so lower level that the alloy may not be able to pass away. So, losers is stainless steel characteristics completely. So, the chromium is is in our what happens suppose I draw this schematically here this is the grain boundary here see chromium percentage this is the grain boundary it moves like this here ok. You know very well the corrosion that depends upon the chromium content what will happen. So, you are going to have somewhere in this region what happens there is going to be severe attack of the grain boundaries. I show you some pictures yesterday right where the sensitization has led to deep grain boundary attack. So, the width of this attack what you see here depends upon the extent of depletion that occurs if the depletion is narrow what happens this attack becomes narrow the depletion is broader the attack becomes wider actually see that ok. So, essentially it is a chromium depletion that leads to the sensitization of the grain boundaries. So, this theory is called as chromium depletion mechanism. So, the mechanism is fairly simple it is not very complicated right. Now, we need to understand what are the governing factors for intergranular corrosion? There is a cause and there is a effect here right. What is the cause? The cause is subjecting to some heat treatment leading to chromium carbide formation the effect is the corrosion along the grain boundaries that is what. So, the primary reason is the chromium carbide formation along the grain boundaries. So, now, can you quantify can you understand the intergranular corrosion in more details ok. As I told you it is the temperature and the time both are important. Before I go in details how many of you are from the north metallurgy background? Quite a few of them. Have you heard of the time temperature transmission diagram in a few? So, called the TTT diagrams ok. So, let me just briefly cover this aspect of this ok and so, that you get a real feel of that. Let me tell you how you get the TTT diagram for sensitization. Suppose you take a stainless steel let us say let us take type 304 stainless steel. I want to establish time temperature transmission diagram they called as TTT diagrams. In this case I called as time temperature IGC diagrams. How do you establish this diagram ok? What one does is 304 stainless steel is taken maybe say piece let us say about take some long strip of that ok take long strip of 304 yes stainless steel. Subject them to heat treatment for different temperatures let us say T1, T2, T3, T4 this is the temperature for different time intervals say T1, T2, T3, T4 etcetera keep doing that actually ok. So, you can take this material and subject them to heat treatment in the approximate temperature region let us say one can start with let us say maybe start with say about 850 degree Celsius and be down to less to about let us say about 400 degree Celsius something like that different temperature intervals and time intervals do the heat treatment on that. Then you subject them to the intragranular corrosion to intragranular corrosion either are various solutions available ok. Maybe you can take a hydrochloric acid SCL you can take maybe you know 10 percent SCL you can take and you can you can boil that expose the sample and take the sample out in this case and you can bend the sample over a mantral right you can bend it over a mantral. When you bend it over a mantral what will happen now? There is a tensile stress here. So, what will happen now? When the grain boundaries are attacked very selectively the grain boundaries will open up this is the grain boundary right and you can measure the depth of attack and the depth of attack is a is related to the intragranular corrosion right. If there is if the grain boundaries are sensitized more and more chromium orbits are formed more chromium depletion has occurred then what happens then the grain boundaries become susceptible to corrosion. Now, you can have the depth of attack and you have you have done a big matrix of you have done a big matrix of test right various temperatures and time. Now, all these cases what you can do is you can you can plot of attack versus the time you can do it for different temperatures and you will notice that. So, you can have several temperatures and in this case what happened T 1 is greater than T 2 is greater than T 3 greater than T 4. What you notice from here? There is time for initiation for intragranular corrosion right. So, you can find out this and you can plot you can plot the temperature versus the time for start of IGC this is for the start of IGC right. So, what happens goes like that as a temperature is decreased what happens to the time for initiation of IGC increases right start increasing like that. Now, if you rise the temperature you also get like this it is very interesting now ok. You see when you rise the temperature also again the time for initiation of IGC increases right. This is a little a metallurgical concept I think those guys who study phase transformation they will understand much easier ok. And and above this temperature look at this above this temperature gamma is stable ok the gamma as now this is also gamma stable gamma stable here here what happens here it is the what is this? This is the initiation of chromium carbide formation starts right am I right in this So, now you can understand that when when lower the temperature the time taken is is is is is increasing, but again you rise the temperature again the time taken is increasing here right. What is the reason for that? Those who studied metallurgy should be able to tell this yeah yeah ok. You can also say that under cooling well under cooling again you guys should know what under cooling means right. The energy of energy. Yeah. So, the free energy change the free energy change for that is is actually is increasing is increasing like this ok is increasing right from here down to there. So, it should go one way only ok. The reason for that is is related to two factors if you can recollect any of you it depends upon the nucleation it also depends upon the growth process. And growth process requires higher temperature because the diffusion becomes faster the nucleation requires more under cooling a lower temperatures. So, there is a compromise between these two right. So, you find that a high temperature diffusion is faster, but the but the nucleation becomes slower. So, you find that again the time for transformation is is increasing actually. So, it goes this is a typical we call as T T T diagram time temperature the transformation diagram. What is transforming here? The gamma is transforming into this one of course plus gamma here. Please notice if you look at this diagram what is very clear here? The chromium carbide starts forming only and around these regions there will be no chromium carbide formation. How do I interpret this? At any given temperature suppose I take this temperature I take this temperature unless you cross this line the alloy will not get sensitized ok that is true for all cases. You can also plot what you can plot please look at you can also plot this is the maximum attack right. I can also plot the maximum attack maximum right this is IGC maximum. But again please notice it goes to a maximum again falls down here that means, if you hit for longer time what is happening to IGC? Decrease why does it happen? Why should on for longer annealing the IGC should drop chromium carbide exists there right that does not dissolve. So, you are allowing enough time for the neutralization of the depleted zone right. Now, the chromium earlier the carbon was diffusing faster and chromium could not catch up with that and so, there is more depletion along the grain boundary. So, with with a longer interval the depleted region gets neutralized the the there will be because there is a concentration gradient the chromium will start moving towards the depleted regions and get neutralized. So, the IGC decreases decreases and decreases and so on please understand that. So, what does really imply? It implies that what is the implication of that in in alloy making? What is the implication of this alloy making? I will come back to this related what is the implication of that? What are the implications? Chromium combines with carbon and forms CR 23 C 6 23 7 carbon 6 carbon. What is the carbon content in the alloy? It is about 0.08 8 percent right. The carbon takes away the chromium can I simply increase the chromium content let us say 40 percent instead of 18 percent chromium I added to the alloy 40 percent chromium. Do you think the synthesization will will go away? Because I am compensating with more more chromium. So, no it will not go because it is not that we do not have enough chromium to combine with carbon. Even you even if the if the all the carbon present in the stainless steel combines with the chromium the remaining chromium in the in the overall composition is still higher right. So, overall composition of chromium in the alloy is still higher to call it as stainless steel. The problem here is that the chromium depleted along the grain boundary area that is what the concern that is happening because of the diffusivity difference not because of the stoichiometry difference. So, that diffusivity problem still arises and so, by just increasing the chromium content of the alloy you cannot simply control the intragranal corrosion right. So, that is simply not possible right. So, it is the diffusion kinetics that are responsible for the chromium carbide formation on the grain boundary and the consequent chromium depletion around the grain boundaries. So, this curve essentially means that I have enough chromium in the in the material no problem and over a time period the depleted regions they they become replenished and so, the the grain boundaries become resistance to the intragranal corrosion which of course, this these are the things are not possible in real life situations ok. We will see later theoretically yes, but practically they are not useful at all actually. So, the factors that control IGC can be seen from this diagram. If I take say 304 stainless steel I keep it at let us say 1050 degree Celsius I hold it here longer time. I quench it fast in fact, I should quench it faster than this. If you quench it a little slower let us say cooling rate 1, cooling rate 2 and cooling rate 3. The cooling rate 2 is what is the cooling rate it is d t upon is the cooling rate right. Now, the 1 2 is is the critical cooling rate a cooling rate higher than the 2 will avoid sensitization. The cooling rate lower than this will lead to sensitization of the stainless steels. So, this diagram sets a limit for various thermal processes that can lead to either sensitization or that can avoid sensitization right. So, the temperature, the time both are important. So, when I said that it is between 450 and 850 please understand that ok. If it is not 450 just like that somebody has a question to you oh can we not be 430 what will be your answer will will be sensitize at 430 yes it could 860 cannot be yeah possible because it is a time and temperature are equally important. So, these numbers are indicative and that also depends upon the alloy it is not going to be same for all kind of alloys we will see later actually why it is ok. So, that means, you need to understand what are the things that govern the time, temperature, transformation diagrams. So, you understood this anybody has any question here? This diagram is very relevant because we are going to talk about weld decay and how do you avoid weld decay or how do you make new alloys all of them based on this diagram and so, you need to understand this one more clearly. So, it will be no doubt as per this diagram is concerned. See a typical 304 stainless steel right we discussed earlier that it has about 0.08 weight percent carbon or in fact, in you know if you are going to have 18 chromium 8 nickel I can go as much as 0.1 weight percent carbon I can go as much of that because at high temperature carbon solubility in the austenitic matrix increases high temperature right. So, dissolved single phase gamma phase. Now, if we quench it very fast then what happens? Now, or if you or you cool it very slowly what happens? Suppose, you you held it at 1050 degree Celsius and all the carbon is in the in the soluble state, you cool very slowly. So, what will happen now? The carbon will come out because the solubility of carbon in austenite decreases with temperature right because the room temperature solubility of carbon in this alloy is about 0.028 percent. So, 0.028 percent is what maximum the alloy can dissolve carbon at ambient temperatures. So, what will happen excess carbon? It is like this is very similar to you know it is it is similar to soaring chloride you take water and and add soaring chloride it dissolves right. You keep on adding more what happens after sometime soaring chloride does not dissolve right. So, rise the temperature what happens to soaring chloride? Dissolve. Dissolve, I cool it down again you start crystallizing from that right. So, it is a phase rule nothing different from that. You know in this case when carbon is forced to dissolve at the high temperature when you cool it down thermodynamically it is going to form another phase because it is not soluble. Now, how does the carbon come out? Carbon does not come out as carbon it comes out as chromium carbide precipitate because of this is this gives you the free energy change for that is going to be very negative ok. So, that what happens here? Now, if you cool it very fast then what happens you do not allow the carbon to diffuse it is frozen right and so, at at room temperature you may have even 0.1 weight 0.158 percent carbon or 0.08 weight percent carbon all of them in the solid state and the frozen no precipitates. But when you heat it again what happens you are giving energy for the carbon atom to move right and the move around what happens? It forms chromium carbide because of the phase rule that dictates. So, in in in 304 stainless steel when you are the so, called millenials stainless steels what they have done they have held this stainless steels are played whatever at that temperature 1050, dissolve all the carbon you cool it rapidly and so, what you get is a is a nice 304 can passivate no chromium carbides. Now, if you are talking about the industry guy what rate you should cool it that is given by this diagram right. This diagram tells that this is the lowest cooling rate that is tolerable. If you are going to lower the cooling rate you know below this then what happens then you are going to have chromium carbide dissipations. So, that dictates what should be the cooling rate that can that can avoid the sensitization of 304 stainless steels. So, this is a an important thing that we should be understanding ok. So, let us take this further and see how this the factors controlling sensitization. What is the factor? The first and foremost is the carbon content. If you have higher carbon content what will happen to the kinetics? We will have it will be faster right. 1, 2, I use a very crude term carbon get up. If you can remove the carbon activity in the alloy what happens? It decreases the kinetics carbon get up or which can change the activity of carbon in the alloy. These are changing the activity of carbon. You can also control this by controlling the nucleation. If you can control the nucleation of chromium carbide formation then I can control the sensitization process. So, broadly these are the three ways you can control the sensitization of stainless steels. Let me take the effect of carbon. It is not a rocket science right. If you increase the carbon content you will see the kinetics will increase. Let us take stainless steel let us say 304 ok. It has got let us say 0.08 the carbon content, the 0.05, 0.03 and 0.08 percent carbon. If I have to draw a TTT diagram for this. So, I am drawing for let us say 304 I am drawing this ok. It is about I am keeping here about let us say 400 degree Celsius maybe around about 850 you know. So, what will happen to 0.05, 0.03, 0.02 or you can even have 0.01 for example, moves were right. It is fantastic right. Please look at even these like these also coming down. Why? Because solubility of you know because it is just dissolved you know there is no much carbon. So, it dissolves at low temperatures completely ok. So, you find that this moves towards right side. The whole lot of stainless steels to avoid sensitization were developed based on this concept right 0.08, 0.03. This is this is called as 304 L, this is called as this is called 304 extra low carbon ok, extra low carbon. The nuclear industries even 0.03 is not allowed, not tolerated ok. People go for 0.02 or 0.015 something like that people go for them. So, the development of low carbon stainless steels have a basis. The basis is what? The time, temperature, sensitization, diagram for that actually. There is a problem in removing carbon. What is the problem? Now of course, making is is expensive and what is other one? The strength goes off ok. So, low carbon stainless steels are a expensive lower strength. So, somebody is making a pressure vessel automatically the cost goes off both accounts ok one related to the expense and other one related to the tank levels. So, can we retain the carbon and then prevent sensitization right. So, what we will talk about is the second concept adding the getters. What are they? Titanium, niobium, the titanium added 1 this stainless steel is called as 347 excuse me 321 and this is called as 347 stainless steel. So, two types of stainless steels have emerged these are called as stabilized grade. What are the principle here? The principle is very similar to the sensitization. If you take a 304 chromium carbide forms in preference to the iron carbide in preference to the nickel carbide, but you add a third element another element which is forms much stronger carbide then automatically carbon will go to other element. So, the titanium forms retaining carbide. So, as niobium carbide and tantalum carbides. So, you allow the carbon in the material to interact with them and form these carbides. So, what happens to the activity of carbon in the system? The activity of carbon in the system falls very low and so, the sensitization becomes extremely slow ok. So, if you can if you can plot this 304 304 L and this is going to be your 321 and 347. In fact, 321 and 347 are much more resistance to sensitization than 304 L actually, but the carbon content, the carbon activity of the stainless steel drops very low because of the association of titanium carbide, carbon carbide and tantalum carbide into this and they are called as the stabilized grade stainless steels ok. Now, there is something which is little not straightforward little indirect ok that we need to understand because you will see that some are always unexpectedly cause sensitization even though the carbon content of these stainless steels are similar right. Those who study metallurgy it will be easier, but some of other people also can try to understand actually. The activity of an alloying element in an alloy does not depend upon its own concentration, it also depends upon the other elements present in the alloy. For example, carbon activity may depend upon chromium content, it may depend upon the manganese content, nitrogen content and so on so forth. And so, when you talk about sensitization it is just not you count only the carbon content in the alloy, you have to look at the effective activity of carbon so, that it does not get sensitized. So, the work has been done actually there is an arcade diol from Ajikar Kalpakam I had done some nice work, I just make it very brief here. The carbon activity is given in terms of what is called as chromium effective chromium content. Please notice when you add more chromium content the activity of carbon comes down. So, the use of term which is called as chromium effective which is given as chromium plus 1.45 moly minus 0.19 nickel minus 100 carbon plus 0.13 manganese minus 0.22 silicon minus 0.5 aluminum minus 0.20 cobalt plus 0.01 copper plus 0.6 1 titanium plus 0.34 vanadium minus 0.22 tungsten plus 9.29 nitrogen. It is very difficult for you to remember all of them, but I want you to appreciate this ok. There are certain elements they will favor sensitization, there are certain elements which retard sensitization that is what I want to make the point here right. What are the elements that favor sensitization are the ones where you see with the minus sign here say nickel obviously, carbon right. Silicon, aluminum, cobalt and tungsten. So, you may have a same carbon content in the alloy, but by chance nickel content is more. Now, what happens? The alloy with higher nickel content will sensitize, the alloy with lower nickel content may not sensitize when you are in the borderline case ok. Similarly, if you are going to add let us say manganese in the system molybdenum let us say nitrogen they will suppress sensitization of the alloy here. Where does this problem come? It comes in practice also ok, especially you know the in the nuclear industries. Assume that somebody has got 304 L or R R X L O carbon. Assume that you have 18 chromium, you have let us say 8 nickel and 0.02 carbon. If the alloy is not controlled it goes for 18 chromium 10 nickel 0.02 carbon ok. What happens? This may sensitize. You may think that the carbon content is similar why not? No problem not 18 10 nickel nobody adds 18 nickel put 10 nickel ok. So, if it is a 10 nickel by mistake is added or you feel very happy because nickel is expensive ok, but this might sensitize and this guy may not sensitize actually or assume that other way around 0.05 carbon 0.05 carbon this borderline this guy does not sensitize here for sure he is going to sensitize. So, you know going into more details about sensitization it is also important to understand what are the associated chemistry of the alloying elements that will affect the intergranular corrosion when you go heat treatment at all. See please notice that you know if it is all solutionized nicely and they do not have any problem right, but when you sensitize it I think some of them may be having some alloys may have a faster kinetics some of them may not have sensitize at all actually. One more thing that I just want to discuss and then we close for today's discussion ok. That is about the nucleation you know about the grain boundaries we call them as two times high angle grain boundary or a low angle grain boundary. It is also called as special boundaries sometimes they are called special boundaries, sigma boundaries they are called in metallurgical terms. What is the difference between a high angle grain boundary and low angle grain boundary? Anybody here exposure to this? In the high angle grain boundary look at the high angle grain boundary look at the energy from the point of view. See you have a interface right high angle grain boundary as high energy and low angle grain boundary as energy. Now, the nucleation occurs atrogenous nucleation right nucleation occurs at the grain boundary because energy of the grain boundary is higher compared to within the grain actually right gives interface. Now, what happens now that means, if you look at the nucleation. So, what happens the nucleation I call this one called two is is is is easier in one in in one than in. So, if you are going to now make the alloy with a low angle grain boundary then the alloy will not undergo will undergo sensitization, but will undergo sensitization at a much lower kinetics ok. So, the the low angle grain boundaries are also called as twin boundaries right ok even twin boundaries called low angle grain boundaries. So, if I have to plot time these are a log actually unfortunately I have not made there properly ok. Time is always log here fortunately please correct your earlier slides ok. Normal grains solve say twin boundaries. So, sensitization. So, you can also change because change the grains so that you can you can control the sensitization of of the alloy actually. Well, I think we will continue I do I think it is not over yet we we can talk about it more in the next class ok. And for the time being we will end our discussion related to the the grain boundary natures.