 Welcome to the discussion on inter granular corrosion. This is a very important topic. It relates very strongly to the metallurgy of the metals and alloys that suffer corrosion. Why this topic becomes important? Because we more often use poly crystalline alloys. The engineering alloys that we deal with they are all poly crystalline in H s right. So, use poly crystalline alloys are useful for many of these structural applications. When I say poly crystalline what do you mean by that? I mean you would you take this any of these metallic alloys and you grind polish in H and look at the surface with the microscope. What will you see? You see nice grains and grain boundaries. So, schematically if I have to draw and some of you you have not seen they are non-materialized this seen. What you will see is something like that you will see ok. You will see poly crystalline. Now, each of this grain you can be considered as a single crystal. They have a similar orientation of the atoms and since these crystals have different orientations at the interface what happened? You will have mismatch between the surrounding crystals. So, what happens now in that case? You will have a sort of disordered structure or disordered arrangement of atoms which means the energy of the grain boundary is higher than the grains. When I say energy I mean when I say energy of grain boundary I mean the atoms the atoms or at a higher energy state the grain boundary as compared to similar atoms located within the grains. Now, having a poly crystalline structure is good for many cases. I mean the metallurgists like it mechanical engineers like them because when you have poly crystalline material you know you have higher hardness and higher strength you have higher toughness. So, you normally want to have very fine grain size. You know of all pitch relationship you know where you can say that the the the strength is inversely related to the grain size of that. But over here we encountered a different problem. The problem is that these disordered regions have very high energy and so, they can preferentially suffer corrosion ok. So, can suffer preferential corrosion. It is it is very interesting the corrosion of the grain boundary is favored not only because of energy consideration, but also because of the kinetic concentrations. If you want to remove an atom from the lattice where the atoms are all coordinate well coordinator it is difficult to remove because they are all bonded by so many other neighboring atoms. But in the grain boundary what happens? The atoms some of them are dangling bonds they are not really connected. So, kinetically it is also easy to remove these atoms that is why removing atom from a grain boundary or removing an atom from the dislocated areas for example, you can etch right. When you have dislocations you etch it you can etch fits because because when the atoms are loosely held it is easy to remove. So, the grain boundaries are more favorable sides for corrosion. The grain boundaries because of high energy it also leads to segregation of elements. Some elements are intentionally added to improve the property of the material some of them may be impurities right. So, they segregate very heavily in fact, you know they are in fact, thermodynamic consideration they just segregate. You know very well right tin is surface sensitive right they go and go to the surfaces actually. And sometimes segregation can be as is 10 power 5 times 6 times 4 times as compared to the bulk. When the elements are segregated and these elements are going to be either anodic or they are cathodic in both the cases they can induce corrosion of the grain boundary area. If it is anodic they dissolve if it is a cathodic what they do it induces the dissolution of the grain boundary areas. So, the grain boundaries are not as corrosion resistance as the grains in a poly cast iron material. Having said that I also need to caution you that the grain boundaries need not be very highly corrosive if it is a very pure element. For example, if I take let us say tin an aluminum and whose purity is is order of 99.999999459 decimals then you know very well I think as a metal like this you polish and etch it it is not easy to etch the grain boundaries you cannot see them very easily. But you add a small alloying element it becomes very easy to etch. So, the grain boundaries no doubt are high energy sides they corrode but the extent of corrosion also depend upon the kind of segregating elements present in the material in the grain boundaries especially ok. So, that has to be very clearly understood do not say that all the grain boundaries are going to corrode very heavily and so we have problems we have we have steels for years they do not undergo intergranular corrosion. The stainless steels certain conditions we are very prone to intergranular corrosion. Aluminum alloys certain conditions are prone to intergranular corrosion you can have similar behavior for zinc alloys ok. So, this particular discussion this particular topic brings out what are the mechanisms involved in intergranular corrosion and of course, how do you avoid intergranular corrosion and we give more emphasis to the stainless steels because stainless steels are very widely used in the chemical process industries because you all you know the stainless steels are primarily developed for corrosion resistance applications. But you weld them and certain conditions they suffer corrosion we call the term a sensitization. Similarly, aluminum alloys undergo corrosion and the grain boundaries because of some heat treatments we call that as exfoliation corrosion it is one type of intergranular corrosion. So, we will discuss this in details. Now, to just give an example how the weldments can cause selective corrosion. Look at this diagram it is a macro photograph of a stainless steel welded and then exposed to the corrosive medium like soaring chloride solution in this case and you can see that the corrosion has occurred at the both this sides of the fusion zone right. You know that this is called a fusion zone it is a cross section actually this is the thick plate ok and it is a cross section this is the weldment weld fusion zone and this is the heat affected zone the heat affected zone and you see the corrosion occurs very interesting. The corrosion does not occur at the fusion zone nor it occurs very close to the weld fusion zone it occurs away from that we need to understand why it happens why not very close to this why not the fusion zone it does not happen away from the heat affected zone this is called a heat affected zone actually ok or more precisely this called as weld decay zone because stainless steel suffer extensive intergranular corrosion and the term used as weld decay and we will spend quite a bit of time on understanding intergranular corrosion of stainless steels and sometime we will spend on aluminum alloys because aluminum alloys also are prone to this type of corrosion not as extensive as stainless steels though. Now, I do understand that we have we we we have people here with a non-metallic background I suppose here for them I just give a brief review of stainless steels so that you get a clarity when you talk about the intergranular corrosion of weldments of of stainless steels. I just show some some slides here you might find this in in any standard books actually ok and I have just summarized here ok. We all know that the what makes the steel stainless steel is chromium right. If chromium is not there you do not call a stainless steel all the alloying elements may be there you know you can have nickel you can have molybdenum you can have whole host of elements, but unless you have chromium we do not call that iron alloy as stainless steel. They have some properties I am summarizing here briefly I could go quickly because most of you would be aware of this. The most alloying element is chromium here it promotes the ferrite phase I will talk about it later ok. The the stainless steels are classified based on the crystal structure ok and one of the class of stainless steel is classes of stainless steel is ferrite ferritic stainless steel and you have simply only chromium it forms a ferritic based stainless steel right and the very purpose is to improve the general corrosion resistance of the iron base alloy. The nickel is another important constituent which improves the austenite which is essentially a is a is a phase standard cubic phase here it also improves corrosion resistance carbon it is not that you added it, but it is there actually right it is there ok. But it has good properties as well you know we are going to curse all through this the carbon when you talk about the intragranal corrosion, but ok carbon is not in fact carbon is the real reason why the intragranal corrosion of stainless steel is actually causing. But then look at this it increases the strength ok you will see how important it is and if you lower the strength what happens the cost of the reactor goes up if you lower the strength of a alloy if you want to make a pressure vessel you have to go for a thicker component thicker thing then the cost goes up. So, carbon is a problem, but then we need to have we cannot simply ignore the the beneficial effect of carbon as well. Nitrogen is other element nowadays you come out with a nitrogen containing stainless steels it increases the strength improves pyrrhenic resistance you have seen before to certain extent it also improves the weld decay resistance ok. The manganese is is is added it also reduces the hard cracking when you weld the stainless steel sheets. The molybdenum again is is is a ferritic stabilizes you have seen before improves pitting corrosion as well as the crevice corrosion resistance. Niobium is added again we will see later why we add niobium it is a ferritic stabilizes it forms stable carbonite rights. Resistance sensitization again we will see this later silicon it improves wetting flow oxidation resistance all the stops titanium again as the similar effect as that of the niobium here aluminum improves the high temperature oxidation resistance copper in fact, copper is good from the point of view of application in sulphuric acids ok and sulphur is not good at all ok, but you are going to talk about a stainless steel where you need to do machinable machining then the sulphur is helps actually as well as for more solid cracking. So, I have given you a brief quick review of the various types of alloying elements that are added to the stainless steels. Now, if you look at the stainless steel when add these elements what do they do? These elements effect of elements they may change the crystal structure it is very important for us it not only change the the other properties like mechanical properties it can also change the corrosion properties. It can lead to mechanical property changes mechanical properties can affect weldability. Of course, it affects the corrosion assistance you can also look at it sometimes magnetic properties. You know for body implants right body implants what kind of stainless steels people use you want to fracture the bone for example, what kind of stainless steel people use people do not use Celtic stainless steels people do not use Duplo stainless steels people use arsenic right is because it is it is a diamagnetic right. Otherwise it is a magnetic then what happens you will have problems. So, the magnetic properties sometimes may be good, but not always you can affect this. You can also affect the thermal conductivity. This is the property mechanical engineers worry about it exchangers a carbon steel conducts heat much faster than an austenitic stainless steel. So, you have a better corrosion assistance, but the designing of it exchanger has to be taken into account the thermal conductivity of this material. So, these elements you know have the multiple effect on the properties, but at these elements they change the crystal structure and so, you have different classification of the stainless steels. Now, the just take iron and add chromium to it you get a stainless steel which is called as a ferritic grade stainless steel. What is the crystal structure of this? BCC body centered cubic structures BCC iron at room temperature as a body centered cubic structure when you add chromium chromium is a ferrite stabilizers BCC stabilizer. So, the crystal structure does not change. So, it reminds the body centered cubic structure, but you are going to add to this required amount of nickel then what happens then crystal structure changes and this is called as a austenitic grade stainless steels. The crystal structure is phase centered cubic structures why because the nickel stabilizes a austenite stabilizes a phase centered cubic structure ok. Now, you can have a combination of these two I can have we call them as duplex grade stainless steels. It has got BCC structure plus FCC structures. BCC is also termed as alpha, FCC is termed as a gamma here and this also called as alpha plus gamma structures. Generally, the volume fraction of these two phases or 50 percent each is the volume fraction. Now, if you look at the strength of an austenitic grade stainless steel and a ferritic grade stainless steel are you compare with let us say high strength lower high steels for example, or you compare with a martensitic steel. You know what is the normal strength of a an austenitic grade stainless steels let us say 304 stainless steel what is the strength level anybody has any idea about it? The UTS can be about 600 mega Pascal something like that you know. You can keep increasing more amount of nickel more chromium I do not think the strength of the stainless steel increases significantly because they are all substitutional solid solutions. So, in order to increase the strength of this one what does what gives strength in the steel the carbon. So, you have developed other kind of stainless steels which is called as martensitic grade stainless steel which gives you high strength high hardness good wear resistance. What is structure here? The body is centered tetragonal right BCT structures body centered tetragonal crystal structure is like any other martensitic steel the difference is what the difference is that it has got chromium. So, the main purpose of martensitic stainless steel is to have high strength high mechanical properties, but also have good corrosion resistance, but not as good as ferritic as not as good as austenitic or duplex, but certainly this is better than martensitic steel ok. So, it has got reasonable corrosion resistance and good mechanical properties. And so, you are not going to apply a martensitic stainless steel wear you are going to apply a austenitic stainless steel for chemical process industries because corrosion resistance is not that great, but you need to also have good mechanical properties. So, it is a good compromise between austenitic stainless steel and martensitic steel actually. You also have other class of stainless steel called as precipitation hardenable stainless steels. It is also called as ph grade stainless steels. In this case what people do they add aluminum they add copper to this the precipitates formed from the aging treatment ok. They give the strength right. So, the precipitation hardenable stainless steels they give you strength equal to or even better than martensitic grade stainless steels. But the corrosion resistance of this ph grade stainless steels are better than martensitic grade stainless steels, but suddenly it is not going to be superior compared to austenitic grade duplex grade stainless steels of course, they are going to be very expensive kind of things. These things they start they may be called you know they are called as austenitic start from austenitic phase semi austenitic and martensitic grades. I am not going to discuss more details into into this into this actually. But all I would like to emphasis here is there are different classes of stainless steels of varying mechanical properties and varying corrosion resistance. In all these cases chromium is an important element. The amount of chromium we add to it would change because keep adding chromium then automatically what happens now it is going to change the phase. So, you cannot get all this. So, so there is going to be a phase balance by adding the other relevant elements. For example, if I talk about a martensitic stainless steel I cannot get it unless I have I am going to I am going to have carbon. In a precipitation automatically stainless steel I may not have carbon at all right. I would add rather elements like aluminum and copper that precipitate that give us the corrosion resistance. In in a martensitic grade stainless steels you form a martensite you temperate it forms chromium carbide some kind of carbides the corrosion resistance drops. But that problem does not exist in precipitation automatically stainless steels because you do not have a chromium carbide like as you seen in the previous case it is the phases related to aluminum and copper and so the precipitation automatically stainless steels are superior compared to martensitic grade stainless steels. But you talk about surgical blades, cutleries all these steps I think people go for martensitic grade stainless steels. But you talk about aerospace applications some nuclear applications people go for precipitation order stainless steels even for ok because you need to go for good corrosion resistance combined with with the with the mechanical properties. It has chromium the chromium can be more also. The precipitation order stainless steel 17 pH it has got 17 chromium I will come to their point now actually why they are not chromed to stainless steel. Why for example, a martensitic grade stainless steel is not as chromed to stainless steel as you find in austenitic grade stainless steels and why the ferritic stainless steels are even more chromed to stainless steel compared to austenitic grade stainless steels ok. The extent of this is going to change because of crystal structure because of the precipitation sequence that happens in these stainless steels ok. So, that we will will see during this particular you know class we can do that ok. So, but what I want you to do is you get a clarity because especially when you work on corrosion you need to know the stainless steels classification and when to use what kind of combinations are required ok and that that should be well note of course, see I am not climbing that we cannot read understand completely in this year. I am only trying to probe you please go and read more details about this material generally you can able to able to be a good corrosion engineer actually right. Otherwise you just you know you get some bare bare information it is not possible for you to to prevent corrosion effectively. So, you have any questions here so far? Let us now discuss one related to the sensitization of stainless steels. Before we discuss the sensitization we need to understand to some extent the the stainless steel composition especially the austenitic grade stainless steel. Let us start with the with the austenitic grade stainless steel let us say let us start with let us say type 304 stainless steel right. What is the major alloying element here iron? How much is the chromium? 18. 18 chromium? 18. 8 nickel. What is the carbon content? 0.08. Of course, you have manganese and you have some trace element of silicon and sulfur and phosphorus I am not you know talking about it, but these are major and important elements as I told you you need to add manganese because otherwise there will be weldability problem right and see we are not looking at that now ok. So, you have 18 chromium 8 nickel 0.08 carbon all weight percentage. You know the 18 chromium is added it is beyond the minimum required level of chromium required to passivate and you add nickel because nickel is required to make the face austenite at at ambient temperatures otherwise austenite will appear at only at the high temperatures. The carbon the carbon is is is present ok. In those days they found difficult to remove carbon. Removal carbon was very difficult to do that and only the process like AOD what is this called AOD process anybody? Orgon. Orgon. Orgon. Oxygen decarbonization right. When the AOD process was introduced then they were able to successfully comfortably remove carbon to a large extent. Otherwise the removal of carbon was a problem ok. In fact, you look at the old nuclear reactors and they are formed. The two types of stainless steels are used one 304 as it is other one was called as stabilized grade stainless steels. They know of the problem what it is, but they were not knowing how to handle to remove the carbon. But these nuclear reactors suffered the well decay and it has been real real problems you know. Some of the companies almost become bankrupt because of these kind of corrosion problems. Now when you have this what is the issue here? The issue here is that the solubility of carbon carbon in iron 18 chromium 8 nickel at room temperature is about 0.02 weight percent. Please notice I am all these cases I refer only to the weight percent only. So, only this much only is soluble right. If you look at the phase diagram of of this this one I give a part of the phase diagram here the temperatures ok and this is the carbon percentages. These iron 18 chromium 8 nickel if you look at it ok you will see something it goes. This is the gamma phase and this is your gamma plus and Cr 23 C 6 phase the phase diagram the phase diagram goes like this only and the carbon level here approximately 0.28 percent carbon. So, when we use stainless steel let us say type 304 S stainless steel they use a term called mill annealed. The mill annealed type 304 stainless steel is being used I have been used also. So, when you say mill annealed what does it mean? It means the rise of temperature of this stainless steel to 1050 degree Celsius you hold it. The holding time depends upon the thickness of the sheet or maybe a plate or whatever then what you do they quench it. When the quench what happens now the all the carbon all the excess carbon will be in the super saturated state that means they are completely soluble and what you will see? You will see only a single phase. So, look at the microscope this one you see a single gamma phase. You will see grain boundaries you see grains all you can see that, but you would not see the presence of chromium carbide phase it is not going to be there at all. I am not saying that there are no other phases you know in other sense for example, you may have magnetic sulphide inclusions sometimes sometimes you may have some oxide inclusions can be there. I do not mean you know when I say the gamma phase is single phase I do not mean that there are other inclusions, but these inclusions are unwanted one you have no way of I mean controlling them completely ok. Some amount of inclusions are going to be there, but what I mean meant was that the it is it is the phases formed because of iron, chromium, nickel the alloying elements you know these are not going to be there at all it can be single gamma phase are going to be there. So, in this state chromium is completely in the solid solution and provides an excellent passivity actually. So, if you look at the microstructure of the stainless steel I mean as far as the austenitic grade stainless steel it will be chemically homogeneous single phase that is why the austenitic grade stainless steels are very good materials from the corrosion point of view because chemically they are homogeneous. So, chromium, nickel, carbon and iron of course, are in solid solution. So, provide excellent corrosion resistance. Suppose now you happen to keep this this stainless steel in the range of let us say you heat it steel in the temperature region of 450 to 850 degree Celsius you say that these they suffer intergranular corrosion they suffer much of very much intergranular corrosion. Please see that the temperature is a range here 450 to 850 in fact, you know it can even suffer at 400 also I mean it can happen even 400 it can happen even in a nuclear power plant it happens 300 degree Celsius, but it is kept for 40 years 50 years 60 years and all. So, you notice that the temperature is one factor the time is another factor as we go along we will understand the kinetics of of sensitization. So, it suffers intergranular corrosion. What it is? I have shown you the micrograms of stainless steel under different conditions of treatment. The top one you see here you see the low magnification of this right low magnification. This essentially is millenial kind of situation where the heated to 1050 degree Celsius and then quenched in water and then they carry out a test we called ASTM A2628 test we will see later to reveal the the grains here. And you see that I hope you will able to able to see the difference here for example, you can see this these are all grain. Now, you can able to see this slightly ok. You see this these grains it is another grain and in fact, you can see that that this grain is slightly a lowered compared to this right slightly elevated slightly elevated lowered probably this little lowered elevated. So, this we call them a step structure please look at this grain what you see here is corroded more as compared to this the rate of dissolution is more here. But, however look at the interface the interface is quite clean neat ok and very very thin kind of grain boundary is seen here in all cases right you can see very thin boundaries ok. Compare that with the one which was held between 450 and 800 do not have to worry about the temperature here just show the difference and then you carry out the test you see this here now you see a clear attack on the grain boundaries. The grain boundaries are thickly corroded actually I hope you are able to see this attack here and this is called as inter granular corrosion. The corrosion occurred selectively along the grain boundaries or preferentially along the grain boundaries lead with corrosion here. If the if the if the time is shorter you see that you see the grain boundary attack, but the grain boundary attack is not complete now you can see this they are partly attacked here please notice this is not the same temperatures the temperature times are different and so you see that there are attack, but they are not completely attacked here. This is called as a dual structure we will see later we call as a diss structures we call them step structures. So, in a millenial stainless steels would have you know features microstructure resembling this. The one which is sensitized by heat treatment process will be something like this ok and this is called as the sensitization. The alloy which is very good having very high resistance to corrosion now now suffering inter granular corrosion. What are the consequence of that? The consequence of this is is very very simple and you see this here it is you know straight forward not simple it is straight forward you can see that when the grains are getting attacked corroded like that if you corrosion occurs along this more and more what happens now? The grains will loosen and come out of the things here when the when the grains loosen. So, what happened the strength? Stain falls down. So, the strength falls down and if you have applied stress the cracking occurs along the grain boundaries we call them as inter granular stress corrosion cracking. We will talk about when you deal with stress corrosion cracking subject ok, but so, what I am trying to say that these are all prone to prone to corrosion and leading to grain falling and leading to inter granular cracking if you have stresses if there are no stresses the simply grain falling the stresses the crack starts moving and then leading to separation of the the component into two parts or many more parts actually. So, that is what really happens. So, it is not a good thing from the structural point of view not a good thing for the structural point of view ok. I think I think we can stop here and we can have the discussion in the class about the mechanism of sensitization or mechanism of inter granular corrosion of stainless steels ok. Any questions? Why 12 8 percent chromium is required in stainless steel to attain acidity? Why they are adding now ok, this is a good question. Yeah see I will tell you what happened the one we talk about 10 percent and 12 percent. How it came? The 12 percent came by examining the corrosion resistance corrosion resistance stainless steels in relation to atmospheric corrosion they exposed to the atmosphere and determine the corrosion rate right. For example, you see you would have seen this curve right you would have seen this curve corrosion rate versus the chromium content right is something like that it goes right. This diagram corresponds to atmospheric corrosion and so, that is the you know that is not a very aggressive environment. So, the definition of stainless steel came from that only of course, you have to start see you cannot say that you call a stainless steel you cannot designate a stainless steel based on the environment right. I cannot say that you know ok this is a stainless steel I call it when I use for atmosphere and I cannot call it as stainless steel when I use for a chemical process industries you cannot do that right. So, it is it is a kind of it is a kind of thumb rule only I say there is no first principle involved in in over here ok first first answer. The second answer is that the the effect of chromium would also depend upon other alloying elements right when I add for example, when I carbon the corrosion resistance come down right. Similarly, when I add nickel I would need a little bit more chromium in order to do that because the enrichment of chromium on the surface see essentially what happens it has to form a nice passive film protective film. The passive film formation would depend upon what depend upon the environment it also depend upon other alloying element. When you add other alloying element then it is possible that the enrichment of chromium on the surface is not that much and so it depends upon other alloying elements it depends upon the other environment and so so it is so that is why you always add more. In fact, you look at look at the stainless steel classification now you know ferritic stainless steels lean grade normal grade super ferritic super austenitic all this comes because of these particular reasons. Duplex stainless steels you call it lean grade duplex stainless steels ok because at bare minimum required some like storage tanks I do not need huge amount of nickel not required. So, this is not a very rigid I would say, but you need to have some definition in order to call it as a stainless steel, but that seems to reasonable which is atmosphere is a mild corrosive environment actually.