 we shall continue our discussion on the weld decay. I have shown in the previous lecture that when you weld a stainless steel, intra granular stress corrosion cracking does not occur adjacent to the weld fusion zone. Just immediate vicinity of the weld fusion zone that is no decay and it occurs away from the heat affected zone I mean I am sorry it occurs at heat affected zone which is away from the diffusion zone. This is a kind of welding done on a stainless steel used for I think this is wrong actually this is a 304 stainless steel weldment and this is 316 stainless steel weldment is carried out. It is the kind of welding that people do to make the structures actually ok. And you can see that this is the fusion zone you see this fusion zone here ok. And and immediately next to the fusion zone you do not see any kind of weld decay ok. And but if you just move away from the fusion zone you will get you move away from the fusion zone in the heat affected zone you see kind of problems that you normally you get in this actually. Again this is not a L actually it is containing 316 stainless steel. Yeah very close to that you do not really see any kind of weld decay. This is kind of recapitulating what we saw in the morning. We looked at the various weld techniques in the morning and we said that the sensitization behavior of a given stainless steel could change based on the welding technique for a given composition of the stainless steel that is basically happening because of what why does why does the different technique would lead to different sensitization welding technique. If the heat it is the heat input given by the welding techniques are different and so the extent of sensitization differs in depending upon the welding technique. The other important factor that is required to be understood is the thickness of stainless steel let me call a sheet. Suppose you want to weld the thickness of the stainless steel plate sheet. Now if if I have go for a thicker sheet or thicker plate if I want to weld at one go right if I want to weld at one go the amount of heat that is deposited per unit area is going to be quite large, is not it? You are going to build up layers right we build up layers and so the more amount of heat is now deposited onto the unit area and so it takes long time to cool. It takes long time to cool and what will happen then it is possible that the cooling rate is is lower than the critical cooling rate. Required to avoid the sensitization. So, that can be depicted by the simple schematic diagram of time versus the the temperature for sheets of different thicknesses. They are welded at one go ok. I use thickness 1, thickness 2, thickness 3 and thickness 4. I have little bit you know I have this enlarge it is not going to be that wide actually ok I just enlarge for convenience to make it. Now can you tell me in this case what order the thickness is increasing from the right right. The thickness 1 what represented here is is the least thickness, the thickness 4 is the most thick one right. In practice we need to weld the thicker plates thicker tubes you know in a nuclear industry for example, the tubes are one inch above thickness are welded. Now you are going to deposit heat more, but you need avoid sensitization right. How to do that? How do you avoid sensitization in that case? You understood my question? You cannot avoid I mean you cannot say that all the time I have only thin thin sheets I have to go for thick sheets right. So, if you are going for thick sheets and even to weld how do I avoid sensitization? How do you avoid it? Can at least that is some some guys have already taken a welding course right. Yes it is a good answer right. So, you do not deposit a one go you deposit once you continue the layer, allow it to cool then what you do? You deposit the second layer allow it to cool. So, each time when you weld you are allowing the weldment to lower its temperatures. In which process you are able to maintain adequate cooling rate so, that sensitization is avoided. So, we do what is called as multiposs welding. In some cases if it is a tube for example, ok you want to weld a tube what do one can do? The circumferential weld right. Suppose there is a tube and you want to weld a tube and you want to weld two tubes suppose you want to weld them what you can do? First you can weld and you can have water inside you start welding then what happens? Then the this is a good coolant not only that it also builds up nice compressive stresses ok. So, it is in that advantages for minimizing sensitization it also minimizes stress corrosion cracking you will see later that when you normally weld a material you get a intense stress we are not discussing this point here ok. So, there are various ways people weld in aerospace industries also they normally cool it using a copper blocks you can put a copper block and then start welding the copper block extract the heat. There are several ways of you know lowering the the quantity of heat and then increase the cooling rate. So, that you can avoid sensitization of weldments. So, these are the some factors which I think is is are important in welding of stainless steels. There is one more problem that is concerned with stabilized grade stainless steels and that problem is called as niprene attack. It occurs in stabilized grade austenitic stainless steels we talked about two grades of that one is type 321 other one is type 347 and in this what you do in this case you try to type 321 it is titanium containing right how much titanium you normally add anybody remember it is about 8 times the carbon content is equal to the titanium added in this case. In this case tantalum plus niobium add together ok is equal to 10 times the carbon content. So, you can have 0.08 weight percent carbon and accordingly you can add titanium you all know that the the the titanium combines with carbon and no free carbon available when you weld it there is no sensitization. But, niprene attack is is a special case that happens when you weld thick plates. So, you weld it as I told you thick plates require multipause welding without which you are going to lead to sensitization right. And in this case this is a any one of that it could be a 321 or 347 what the form was the attack occurs very close to fusion zone into the the base metal. So, at the base metal side you will have very close to this happens ok very close it is called KLA very close it happens. Very interestingly KLA does not occur in low carbon stainless steels you need to understand this. And please notice they do not occur at the regular heat affected zone that happens you know you know in a normal 304 stainless steel it occurs very close to the to the fusion zone in the in the base metal. Why does it happen? In order to understand this you need to understand the the thermo cycle right. If you weld a single pulse weld ok if you do a single pulse weld what is the thermo cycle for that temperature right it goes up it cools like that single pulse. That means, but if I have to cover it at one go ok and I measure this temperature here ok. It measure anywhere the temperature ok any here or here anywhere ok it it occurs the the the highest temperature may change, but the cycle is going to be same. If I am going to do a multi pulse welding suppose let us assume that this is the place I want to notice here. I am going to put a thermocouple here and I do a single welding. If I am going to put one more layer over here what happens? Again look at this this zone very close to the close to the fusion zone this is the first pass the second pass. In the first pass when I weld here when I weld at this location the temperature goes to very high maybe around 1200 1300 may go it does not melt please look at next to the fusion zone it does not melt, but the temperature goes much higher than the weld decay zone. Maybe the temperature may be around about 200 to 1400. When I move up here suppose I start welding here this location is going to follow something like in heat affected zone for this area. When I start moving up this somewhere move up what happens this the distance is sufficient enough for me to consider this as heat affected zone. So, this temperature is in the range of what in the range of this probably it goes around about 800 degree Celsius and this temperature region may be 450 and 850 degree Celsius. So, here it is acting like a heat affected zone for this because it is sufficiently far away from this location right. This is the heat affected zone for this right. If I move away from this this becomes heat affected zone for this is not it the distance is somehow same am I right or not? So, what happens now? So, this is the reason why the knifeline attack occurs. Now, let us look at what is happening to the to the metallurgy of this particular weldment in this case and this case. We need to understand stability of the carbides. Cr 23 C 6 is a carbide that forms what is the temperature region? It is between say 450 and 850 degree Celsius right and this is a gamma phase. What are the carbides you have here? You also have you have Cr 23 C 6 can be there you can have titanium carbide, tantalum carbide and niobium carbide. Look at this all carbides. What is the temperature on about 1200 understood or not? The titanium tantalum niobium carbides form in the temperature region of 850 and 1200 degree Celsius. The chromium carbides form between 450 and 850 right. Above 850 what happens to chromium carbide? Dissolves. So, what is happening to titanium carbide and all? It starts forming. This carbide dissolves and these carbides form and above this temperature only gamma is there and no tantalum carbide niobium carbide all these stuffs are not there ok understood. Now, let us look at the weld thermal cycle right. In a multipause welding what you do? This is temperature time not ok I am going to draw 1200 it is about 1400 I am going to put it about 1200 ok. So, what is happening here? The stabilized grade stainless steel consists of what? It consists of tantalum niobium or one of these carbide they are present in that right. When you rise this temperature what is happening to these carbides? They all dissolve now right they dissolve ok that is they dissolve. When you again rise it to this temperature what happens now? This is in the range of 450 and it will say. So, what happens now? Suppose you re-eat again what happens now? What forms? Chromium carbide. Chromium carbide forms. Please notice in a weldment ok in a weldment I just come back to this place here in a weldment this is the melting temperature here it goes beyond the melting temperature. This place very close weld fusion zone in the base metal does not melt lower than the melting point, but the temperature is sufficient for the titanium carbide to dissolve. In the heat level zone what happens here? In the so called weld decay zone what happens here? Here the temperature is between 850 and 450, but nothing happens because the titanium carbide still remains as a titanium carbide. It does not dissolve and so chromium carbide also will not form. So, here you have no issue right here it is dissolves here. When it dissolves please do not understand no sensitization. Chromium carbide form because you are reheating it you are reheating it by welding process. If you do you see for example, if you take 3 to 1 you do a single pass welding what will happen? There will be no sensitization why? Titanium carbide dissolves, but does not mean the chromium carbide forms automatically that means, still the carbon is in solid solution chromium carbide does not form. So, if you happen to weld 3 to 1 single pass welding a thin plate suppose you do that then you have no issue. Because the thick plate because you are welding at different you know heights the lower one becomes iterator zone for the upper ones and so the chromium carbide starts forming. So, these places the chromium carbide starts forming. When the chromium carbide starts forming then you get sensitization and that is why it is very close to this. Please notice if you if you move away from it if the temperature is lower than 1200 nothing happens titanium carbide is still stable. So, that means, you are not going to get any sensitization here nor here and you get only very close to this ok. So, that is the reason why it happens. In the in the in the in the in the thermal cycle basically the first one leads to dissolution of titanium carbide. The second one when it becomes iterator zone for the subsequent welding chromium carbides form and so it leads to what is called as sensitization taking place and that is the problem that happens the stabilized grade stainless steels. It does not happen in low carbon stainless steels because what why because the low carbon stainless steels carbon is not there to form chromium carbide at all and any any way you are going to cool it down. So, in in the case of low carbon stainless steels you will not get nitrogen attack you get nitrogen attack only in the case of the stabilized grade that too when you weld the thick plates you do not weld the thick plates you will not get this kind of problems ok. Any questions any if you have any doubts? Why does TIC titanium carbide not dissolve at lower temperature never temperature range curve? Yeah titanium carbide does not dissolve. Titanium carbide is stable you know titanium carbide is stable at ambient temperature does not form. See if you hold the the thestabilized grade stainless steel at this temperature what is the likely phase formation? Only titanium carbide forms chromium carbide does not form for some reason I do not hold here I first take it to this temperature right I hold it here I dissolve all of them then I bring it to this temperature what will happen chromium carbide will form sensation will occur. Whereas, when I move from this temperature to this temperature chromium carbide does not form instead you get titanium carbide or tantalum or niobium carbides all the carbon is gone when you lower the temperatures the chromium does not get enough carbon to form chromium carbides. So, this is why the stabilized grade stainless steels or resistance against sensitization and that is the reason why it is you see why why is they called stabilized grade stainless steels? The stabilized grade stainless steels are made in this manner please note down you normally hold the stabilized grade stainless steels above 1200, 1250, 1235 depending upon the carbon content you hold it here that means, in which case all the carbides dissolve then you bring it down this temperature region somewhere around 850, 900 you you hold it here then what happens all the carbon combines to form titanium carbide in 3 to 1 tantalum niobium carbide in 347 then you cool it that is the way I use your you use your stabilized grade stainless steels. The stabilized grade stainless steels have carbides what carbides are they they are titanium carbide in one case tantalum and niobium carbide in another case if they do not have it then you weld it you still are going to have problems ok. So, stabilization treatment means the formation of these carbides in these stainless steels that is why these stainless steels are always given stabilization treatment. What do you mean by stabilization? Just fix all the carbon so that no free carbon is available for subsequent sensitization process ok. So, is is it clear actually to you any if you have any questions ok. So, you should know now the difference between weld decay and nifron attack. Weld decay occurs in in stainless steels having higher carbon content whereas, nifron attack occurs only in the stabilized grade stainless steels. The second issue is that it depends upon the thermal history if you do not reheat it then what happens you will not get any sensitization. The third difference is the weld decay occurs quite away from the fusion zone and nifron attack occurs very close to the fusion zone there are three distinct differences occurring here. So, these are the important factors. I am going to ask another question to you suppose I had taken a stabilized grade stainless steel it is not very thick but it is it is thin I welded only once but I am going to apply to a process where the temperature is held in the range of 500 degree Celsius. The liquid is kept at 500 degree Celsius. So, what will happen? I have I have two cases I have taken 304L stainless steel I weld only once only the thin the thin tube 321 also is a thin tube I only weld only once only single pass right. After welding I am putting to a process where the environment is held at the temperature of 500 degree Celsius. What would happen to 321 and what would happen to 304L please let me know. So, I am going to leave that question to you. You please think over let us discuss it in the next class I want you to get into the into the problem and by doing so you will understand the basics of of weld decay and nifron attack ok. These two are important things. So, we have reasonably covered what is called as sensitization and weld decay and you also know how do you avoid, avoid or prevention of weld decay or sensitization ok come on come on tell me. How do you avoid it? Low carbon stainless steels. Yeah 2 yeah stabilize grade for example, 321 and 347 stainless steels. Third, if you can solutionize if can be done it is not possible in practice in a in a structure you are going to do a heat exchanger and how are you going to do it. But a small you know small sections small components yeah it still can be done ok. One of the important thing that which which we need to consider in weld decay is carbon absorption. So, I have taken let us say a type 304L stainless steel suppose I have taken this and the guy has welded is found nicely sensitized. What he did he did not do was he did not do degreasing. The surface adds some carbonaceous matter right grease or something and welded it picks up carbon quite a bit. So, carbon pickup in low carbon stainless steels or quite easy it happens. So, you need to be extremely careful in welding low carbon stainless steels this is process related ones ok. Two, it also happens in the castings suppose I go for type 304L stainless steel casting and I got a casting. So, very interestingly the surface sensitized center looks fine and as you start moving inside the extent of sensitization decreases. Please note it is casting. How many of you are familiar with the foundry technology here? Foundry you prepare a mold right you prepare a mold. In the mold you pour the 304L stainless steels the carbon content is 0.03 or lower ok. The surface is getting like this. What do you think happens? If the mold has carbonaceous matter right if it has got carbon right in the in the material right it can pick up. So, the mold depends on the mold right. So, but that is why it is only on the periphery not in center you can shave off it can happen or better way is that go for a like a ceramic mold or something which does not give away the carbon content. So, you see sometimes it is it is happening and so you must know how do you tackle the problem. Among all the corrosion issues two types of problems we have developed extensive testing. One is on sensitization weld decay the other one is on stress corrosion cracking several type of tests you see that ok. Industry they are very important. I will be very brief here and you can always go to the standards and you can read ok. You see one can take about an hour or so even more also because it is a very important topic. But we do not have time. So, we will not go into in details about this ASTM A262 is the standard. And this standard has type A, B, C, D, E and F. So, we have type A, type B, ASTM A262 A, ASTM A262 B they call it like that various types of tests people do that. The prominently people use A type A it is called as oxalic acid H test. You might have done a test in the lab right what you have done there ok. You have applied you used 10 percent oxalic acid right it is it is 10 percent oxalic acid you would have applied you made this sample into a anode right. It is an anode and what did you do? You looked at the you applied a current of 1 ampere right 1 ampere per centimeter square for about 90 seconds. And you you looked at the at the specimen in the microscope you would have got somewhat similar to the structures. Please notice that there is a magnification supposed to be between 250 and 500 that is the reason behind that ok. Very low magnification you you might not able to see grain boundaries properly very high magnification you would not see grain at all you know so magnified actually ok. So, you would not see many number of grains. So, it is that range it is seen. Now, you can see the step structure we discussed already right and then the this is called dual structures and then you have a disk structures. What is the difference of a disk structure? Anybody if you observe the sample in the major vacation 250 and 500 at least one grain is completely enveloped with the disk structures at least one grain completely enveloped. So, not necessary that all of them are completely enveloped only one at least one is sufficient to do that. Please do understand you know you you you guys have to be precise in your thing otherwise you are going to do some some advice with someone I think he will be really pathetic kind ok. You have to be precise in the things. Now, this is your your disk structure because it is not completely enveloped I am sorry it is a dual structure it is not completely enveloped the disk structure and this. Please look at I just wanted to point out one you may able to see here right. Can you see this this grain can you see this here? Similarly, can you can you can you see here? What do you see here? What do you see here? What do you see here? What do you see here? Unhealed twins right the twins you see there this is not sensitized. We talked about low angle grain boundary high angle grain boundary right you see there the unhealing twins are not at all sensitized. So, it is a grain boundary ok. So, they are least sensitive to sensitization. So, that is another story all together is that you can see here also you see you can see this right you see this ok. You can see that there is you see you see here also ok. Now, this test what you need to understand primarily I am not going to you know tell everything what you need to understand is what is the purpose of these tests? ABC this one is talks about you know it talks about only the the you know various carbides. Please notice when you apply one ampere it goes to a very high trans passive region you know and so, the carbides also dissolve or this particular potentials. And this one is talks about acceptable or suspected acceptable means there is no sensitization if it is a dual or diss structure. That means, you may do further test with B, C, D, E, F to confirm whether the material is good enough for application. The oxalic acid H test shows dual structure or diss structures you need to do this test actually. Now, each of these tests I again I know there is no time to see here they are they are all of different kind. For example, this one we use nitric acid this is nitric acid plus HF it is a modified test of this it is slightly milder. So, I want you to people go through this and these guys they talk about carbides they also talk about sigma phases all this and all kind of things ok. So, you need to be looking at ok. So, in order to so, in order to so, this is very very aggressive test right sigma phases and carbides and you know and again chromium depletion region that is the primary thing in all cases right. So, it is it is it is even if you have a sigma phase you will start attacking. So, they are modified to this test say D is better than that. So, each of them have some reason why people carry out different type of tests. I think if people should you know when you have time go through in details and get to know why different types of ASTM test are are carried out. What I would like to focus on here is the quantification of of this test actually ok. Quantification of IGC please notice we are talking about IGC of stainless use only quantification of this and we use electrochemical potentiodynamic reactivation test. You might be also doing this if you are not done ok. This is also part of your lab ASTM. Now, what is the basis of this test? What is the science behind here? The science began here is you choose an electrolyte that passivates the grains, but does not passivate chromium depleted grain boundaries. Please notice this ok. Only chromium depleted grain boundaries it does not passivate, but that means, it is not sensitized grain boundaries as well as the grains they remain passives right. So, selection of this electrolyte should be such that this character is given to the electrolyte. Now, the electrolyte has got two components one is sulfuric acid other is potassium thiocyanate, potassium thiocyanate is given right. This one is to passivate this is to de-passivate. You know sulfur containing compounds are detrimental for passivation. We have seen it during our studies on pitting right. So, passivate and the de-passivate. Now, this is a de-passivator. If I am going to choose this concentration very high concentration it will de-passivate everything grains, grain boundaries and all of them right. So, I need to optimize this concentration. I need sulfuric acid because the sulfuric acid is the what passives the stainless steel. So, the concentration of sulfuric acid this are optimistic chosen and in this case of stainless steels 304 and all we will use 0.5 molar sulfuric acid plus 0.01 molar potassium thiocyanate. Now, how they got this thing by iteration process? There is no real scientific you know extrapolation that we can do or glance a shear extrapolation, shear iterations to happen and so, they arrive at this particular concentration for this stainless steel. The same thing is not applicable for duplex stainless steel nickel based alloys no they are not applicable. This is applicable only for the 304 and probably 316 and so, you need to change the composition in case the alloy alloys are changed. This is called as the EPR test electrochemical potential dynamic reactivations EPR test ok EPR test. There are two types of things that people do here. One is called as single loop test. What is done here is I am not describing to you the scan rate and bubbling all kind of stop set though ok I do not do to that. In a single loop EPR test what you do is you start in the beginning you start from 200 millivolts with respect to sachs radiative caramel electrode from there you come down ok. So, this is how it is done here. This is the sensitized case. If it is not sensitized please correct the things ok. If it is not sensitized you get like this. When you hold it at plus 200 millivolt and completely passivating please see that. Here in this case if you look at the alloy you may everything passivates complete passivation both grain boundary and grain. But here what happens? Here passivates grain boundary does not passivate. So, the current that is flowing here is essentially due to what due to the dissolution of the grain grain boundary. Now, you can calculate the number of coulombs right what is coulombs current multiplied by the time gives you the coulombs you know the coulombs of current by integrating this whole curve you get coulombs. Now, you know the grain boundary area then you can calculate the extent of sensitization. So, P a is given as q the coulombs upon what upon grain boundary area which is this is nothing, but coulombs per unit area right centimeter square right. And grain boundary area you know you guys have done the metallography studies you know how to calculate the grain boundary area you calculate that from the ASTM grain size number right. So, that is is equal to AS 5.095 divided by 10 power minus 3 exponential 0.347 x where AS sample area for which you get the current x is the ASTM grain size at 100 x. This you might have studied in the you know in your metallographic courses right. So, you know the grain boundary area and you can find out the q value and you can find out the P a ok. And P a is is indication of the extent of sensitization. Now, people use this technique for qualifying materials in the nuclear industries ok. And I give you some indication of this P a versus P a value ok and and what you call implicate what is what are the implications implications or interpretations that if P a is going to be less than 2 unsensitized. Please notice these are all empirical one right there is no fundamental principles in that no pitting 2 to 5 slightly sensitized pitting and IGC attack after granular attack at takes place 5 to 15 sensitized pitting attack of entire grain boundaries greater than 15 heavily sensitized. So, these are in fact, continually used in the nuclear industries to to qualify the material for for you know applications in the boiling water reactors and all these stuffs. This test is a little more complicated there is one more test which is more simple more simple is double loop EPR test this is what you might be doing in the in the laboratory this also called as DL EPR. In this case unlike single loop EPR where you rise the potential to the passive region and then you bring it to ECR. The double loop EPR test what is done here is you start from ECR you go up to plus 300 millivolts with respect to saturated caramel electrode and what happens you reverse the scan when it goes like that is forward forward forward reverse and this is called as I forward this is called as I reverse. Actually you do not have to know the area of the sample and you can simply use any area and you can plot only current not necessarily current density ok. Now, please notice when you scan from ECR up to this here up to this point grains and grain boundary they are all dissolved. Here what happens grains and grain boundary passivate and what happens here grains remain passive grain boundaries depasivate if sensitized ok. Now that means, this current if you can integrate corresponds to what corresponds to the dissolution of the grain boundary and you integrate this one will correspond to then to the overall material right. So, you do not have to integrate also what you can do is you can simply take these values like the degree of sensitization can be written as I reverse I forward is the fraction of this one right. This gives you the fraction of the grain boundaries that suffer inter granular corrosion. So, this also used as one of the quantitative test to quantify to what extent the sensitization has occurred ok. If there are no questions I think with this now we come to end of inter granular corrosion and weld decay of stainless steels ok. I just want to move on to another type of inter granular corrosion unless you have any questions or clarification to be sought. Let me go to the next important material which is aluminum alloys. In principle I think the inter granular corrosion can occur in all all materials. Aluminum is one of the you know material which is very susceptible to inter granular corrosion. Next is your maybe your zinc alloys you know you have zinc die castings you will use right and magnesium alloys also can undergo inter granular corrosion, but those things I am not going to discuss now. We will end our discussions with aluminum alloys because it has got wider implications in industries. Now, in aluminum alloys the inter granular corrosion turns to a different you know connotation which is called as exfoliation damage or exfoliation corrosion ok. You can call it. You know what is mean by exfoliation right. Let us say exfoliation what does it mean generally? We call it layer by layer removal right. It is kind of plane by plane removal kind of thing. In aluminum alloys this this inter granular corrosion takes into especially 2000 series aluminum alloys and 7000 series aluminum alloys. These two alloys are they are all high strength alloys right they are high strength alloys. They are used in the as a rolled sheets extruded you know components ok. When you roll and then extrude what happens? You get you get a special kind of microstructures. This is a montage optical montage of one of my students you know PhD students who worked on this alloy actually. And you see the how the grains look at at different you know directions actually right. This is the rolling plane right. This is rolling plane and this is the thickness right. Thickness short transverse and this is the long transverse ok long transverse. Now, look at the grains now ok. Look at these grains. They are all well elongated right. I hope you are able to see this elongated grains. Can you see that? Can you see this elongated grains? And these grains are called as pancake. You put you stack the pancake and you slice it, see at the sides. You see it nice. It is intact with pancake only right. You you take a material and you you roll it, you compress several layers are going to be there right. They called pancake structures. In the explanation what happens is that these grains just you know delaminate just one after another they get removed from the surface from subsurface ok. So, that is that is that is one of the specific problem that happens in the in the high strength aluminum alloys. Now, why do they really happen? How many of you studied aluminum alloys? Physical metallurgy aluminum alloys. How do you get the strength of aluminum alloys? Aging right. It is called precipitation hardening treatment right. You you age aluminum alloys right. You age aluminum alloys what happens? The excess solutes which are super saturated they dissolve and form precipitates right. And age hardening is of course, a big subject altogether. But to make it simple the two types of aging are important for us. One is called as a peak aging other is called as a over aging treatment. What is peak aging treatment? The peak aging treatment is the treatment at which the hardness of the alloy becomes the maximum. The tensile strength becomes maximum. And you over age you continue the aging treatment or you rise the temperature it will do whatever the strength falls down, hardness falls down maybe 10 to 15 percent or whatever ok. You normally do that in order to increase the resistance against stress corrosion cracking. We are not talking about right now. I just want to give an idea why you do over aging? Why you are losing your strength? You are losing your strength because at peak aging alloy are prone to stress corrosion cracking and so, they do over aging. In over aging look at this the precipitates become coarse. You see the precipitates very very very coarse precipitates and the grain boundary precipitates also become coarse. These are the grain boundary precipitates you see here is very fine and these are grain interior precipitates are very fine and so, it has has high strength. In this case because of coarsening effect strength it becomes low. Not only that the composition of these precipitates also change. In this case this alloy is is actually is a 7010 aluminum alloy is called AA 7010 alloy and this is essentially what are these precipitate? They are all zinc and magnesium precipitates and certain amount of copper is present here right and they are all almost equilibrium precipitates are there. Over here it is yeah it has again zinc and magnesium and have quite large amount of copper ok. So, the precipitates become relatively noble here as compared to this. Why? You take aluminum and compare that to magnesium magnesium is active. You can even zinc also active because zinc does not form a passivation. When you add copper to it the precipitate becomes noble. So, they become noble precipitates with respect to the matrix. In this case the precipitate become active compared to matrix. That means, they can undergo selective dissolution here there is no saturated dissolution here taking place. So, the inter granular corrosion occurs more in the case of peak age alloys as compared to the over age alloys. I hope you understand the point right it happens here. Now, the problem that happens here is that when they are they are pancake structures when they have pancake structure like this when corrosion occurs on the surface may be due to pitting whatever the corrosion proceeds inside the proceeds inside like that the proceeds like that along these things now right. It can proceed like this like this also like this can proceed like this. When the corrosion proceeds here it forms let us say zinc dissolves magnesium dissolves what happens now? Now, the volume of these products higher than the volume of the metal dissolved is not it because it has got a hydroxide all this right is higher. When this volume is more what will happen now in this case what will happen? It is going to strain it is going to lift the thing now it is going to lift up now that means, the grain starts opening there are going to be now internal stresses in addition to corrosion happening because of preferential dissolution of the grain boundary precipitates it is also going to exert certain stresses and so, there is going to be delamination along the grain boundaries. The grains start lifting from from the from the material below the subsurface. So, it is exfoliates they delaminate actually. So, that is what really happens in the case of in the case of these materials. If you want to see how it happens I can show you a diagram how these guys can happen see here ok. Now, this is a peak age you can see the corrosion has started from the surface it gone inside it it created so many corrosion products it got lifted please see this is not corroded corroded this volume of the product it got got lifted actually. In the overage condition it does not happen why it does not happen? Because the precipitates here are noble compared to this when they are noble the precipitates do not dissolve so, much and so, the grain boundary becomes stable. So, as compared to this that means, exfoliation corrosion occurs only in the case of peak age conditions they do not they do not happen in the case of overage conditions because of this particular thing. Not only that in the overage conditions one more thing happens actually you see in this case if you look at closely the grain boundary precipitates they become very coarse they are all separated look at this is this one precipitate separated so that means, they are separated now. So, there is a even when it it corrodes less, but even when it corrodes they are not good connect so, they are not connecting the grain boundary is not getting connected completely here whereas, in this case because the precipitates are very fine get connected what happens? So, the the the the the the the crack or whatever no corrosion products accumulates it starts lifting in this case actually right it has happened. So, because of the nobility of the precipitates because these precipitates are well isolated the grain boundaries do not continuously corrode and lift actually. So, the overage alloys are more stable as compared to the peak age alloys ok. So, that is the reason why the peak age alloys are better off compared to overage alloys compared to peak age I mean I am sorry there is a reason why the overage alloys are better off compared to the peak age alloys and peak age alloys or you know. So, in any case if you do not use peak age alloys for for actual application because peak age alloys are more prone to stress corrosion cracking those days people you know you were using at all. So, so it is very essential for us to understand the the the the chemistry of the precipitates and how do they dissolve. So, people have done extensive studies on that and those people want to do a MTech project or maybe a piece of the work you can you can you can read more and understand them. For the time being it is enough for us to understand that the grain boundary precipitates are very important this chemistry is very important and even more important is the alignment of the grains they are pancake structures ok and so they lift it and it is leading to exfoliation or corrosion. Now, there are different types of tests that people do and one test is called as X code test ok. And there is also ASTM standard there is also ASTM standard which is used as X code test where use highly concentrated soaring chloride plus potassium nitrate and plus nitric acid and you see if it is if it is a if it is peak age thing and you see the surface is quite rough and overage is less less tough actually and you know this is a different kind of alloy system that you have. So, when you do the X code test and you can take a cross section and you can see them in the in the in the microscope you see lifting of the grains as I have seen in the as I shown in the previous slide actually ok. And this test is considered to be very aggressive there is also modified X code test ASTM things are available for you you can use modified X code test to do that ok. So, that is about the exfoliation and intergranular corrosion of aluminum alloys. There are there are several factors the way you temper the surface hardness rolling conditions there also affect the exfoliation corrosion. There are certain good research papers some of you are interested you can you can read them and revise for this course I think it is sufficient for you to know that this is one of the serious problems. See what happens in aluminum alloys in aircraft industries how do they join the aluminum alloys are joined by riveting process. So, they drill they drill a hole right they drill a hole and then they are riveted using other aluminum you know metal right. And this hole you know look at this this is a hole here right. So, hole hole is exposed now the grains are all now exposed you see the like pancake structure you know I hope you will able to understand it right. You take a sheet and drill a hole and all these surfaces are now exposed to getting attacked in the horizontally. So, they get lifted ok and there are very beautiful pictures available I cannot show you here because of copyright problems, but you guys can read and see it is a very nice you know illustration of how a grain boundary attack and lift the you know various grains up and causing the weakness it we can it weakens the structures actually ok. Well with this any questions anybody. How IGC of stainless steel is different from that of aluminum alloys? Yeah and ok. So, very good question actually right. The intragranial corrosion in stainless steel the mechanism is different from the intragranial corrosion in aluminum alloys I did not in fact, dwell on it right. The in aluminum alloy what is the element that passivates the matter itself? The major element aluminum itself is passivating right. So, when there is a precipitation taking place what is the precipitation taking place what is the precipitation here? The precipitation in this one is what is zinc and magnesium. So, the passivating element remains in the grain boundary area I mean basically the matrix passivation does not get affected for what is happening instead the precipitates which form on the grain boundary are selectively attacked here because they are relatively active compared to the aluminum here. So, the mechanism in stainless steel is the chromium which is passivating is getting depleted and so, grain boundaries are getting attacked right. Here the passivating element is aluminum that is not getting affected rather what is happening the active elements is segregate and form precipitates in the grain boundary and so, they get corroded at all. So, it is not the depletion of passivating element causes problem here it is the formation of the of the active phases and the grain boundaries causing the problem. So, the mechanism in both the cases are different actually. Any questions? If there are no questions further on this and I still given you one problem please think over it and come out with the answer when you meet next time. So, thank you very much.