 We shall continue to discussthe chapter on Stress Corrosion Cracking. In the last class I started givingthe historical perspective of stress corrosion cracking and then we also highlighted the importance of the stress corrosion cracking for the industrialcomponent safety, how it canimpact the safety of the components. And then we looked at the characteristics of stress corrosion cracking especiallyin terms of the cracking behavior. Andabout the characteristics we said thatthese cracks are brittle in nature as compared to ductile failures happen in metalsdue to mechanical loading. Andthese brittle fractures can be intragranular type or it can be transgranular type of cracking. Of course, sometimes you can also have a mixed mode of failures it you know partly intragranular and partly transgranular cracking can really happen. In all these cases they are brittle and mostly cleavage kind of cracking they they theyappear. So, continuing on the same direction the next important thing that you would like to know is what are the factors affecting stress corrosion cracking. If you recall I have given youthe Venn diagram right, where in we discussed what is stress corrosion cracking? This is SCC the phenomena that happens and the factors involved are the metal or the material, the environment or the tensile stresses. So, if you need to understand what are the governing factors we need to understand in relation to the metals the metallurgy of the material that we used for given applications. The environment thatthe material will be exposed to and the nature of the tensile stresses. So, all these are going to govern thethe susceptibility of a material towards stress corrosion cracking. And if you start with let us start with the tensile stress. Please notice that we are not talking about fatigue or not talking about compulsive we do not talk about the torsion right we will talk about the tensile stresses. The tensile stresses acting on the material can be classified into two categories one it is applied the other residual stresses. The applied stresses are you know a process specific right what kind of load you are applying externally in a chemical reactor you know it can come out of the pressure of the reactants and products involved. And the residual stresses they reside in the material because of let us say welded component you weld it and the weld zone as you could have a tensile component and somewhere in the weld component you can also have a compressive component we have. So, there is a the residual stresses acting even you remove all the external loading or stresses onto a component ok. It could be a cold work you have done a wide drawing you have done you have done some kind of sheet metal you know formed by rolling operations it can be thermal stresses acting on the material or it can also be related to corrosion product accumulation in the system right. The corrosion product accumulation can come depending upon the situation I give a illustration you guys have some exposure to heat exchangers we have seen right heat exchanger you would have tube and a tube sheet you weld it right you weld you weld it or sometimes in expansion joint actually you know mostly it is an expansion joint right you expand it. And sometimes you know you weld on one side you know normally the weld you know where it is facing outside they weld it and this side is not welded right. So, what happens the liquid goes inside the liquid goes inside ok the environment penetrate what can happen for example, you have iron the iron can convert into water it can convert into various corrosion product another product could be iron hydroxide kept on. The volume of the iron hydroxide is more than volume of the iron and corroded right. So, volume increase can increase right. Now, there is a corrosion products here and volume increases what will happen to this tube? The tube will will dent right it could be dent it can happen at dent stresses will start acting on that ok it can be very high tensile stresses. Now, these stresses can add to the applied stresses can add to the applied stresses. So, when you consider the material against you know stress corrosion cracking you need to take care of both the applied stresses and the the residual stresses acting on the structure. Now, there is a kind of I will say concept ok I will say concept called as threshold stress below which stress corrosion cracking does not occur called sigma T h there is a symbol sigma T s for that. This arises out of the fact that if you carry out a test in the lab time to fail versus the applied stresses right you apply a stress you can conduct a simple test right I I can take a a tensile sample and I enclose it you know the cell I can take the environment here you can apply stress on the sample right. This is the environment here and I can find out the time to failure simple test ok it can do and if you notice that something like that you know you get like this. When I apply a stress it fails instantaneously right what should be the stress at which the metal component fails a metal fails instantaneously. What is stress called? Fact is stress or what you call in normal term it is called as it is called as ultimate tensile stress right. But if you lower the stress level in environment the time to failure increases right isn't it as it is lower the stress the time to failure increases and there is a there is a limit ok and there is a limit and this is called as threshold of stress flow which metal is not failing. Now, this is used in some cases you know play especially in in oil and gas industries the use is concept of that ok. But you notice that the threshold stress that we talk about is time dependent right. What do you mean by that? Suppose you you know here it starts failing so you know that here it doesn't fail what is the time you wait for right. So, generally the time is say 1000 hours or 720 hours if you wait for more it may start failing ok. So, this is not an ideal parameter, but it is used as the engineering parameter to determine the you know applicability of a particular material especially in the oil and gas industries. We come back to this later right and this concept is not used in the in the nuclear industries ok and again I will talk about it later why it is not used in the nuclear industries. But nevertheless the literature as you know you see lots of materials you know people give this as a property of course, it depends upon the environment you keep change environment the sigma threshold could change. It is not a material property alone it is also environmental property ok that I think you should you should be aware of that. Now, you know for example, I would say this is your 304, 316 and 310 something like that you know you can see that you can change if you are going to use a duplex stainless steel things can still go up and the threshold stress intensity sorry the threshold stress does not I mean remain constant for all the materials it can change. Now, this this particular thing is is is very important to understand from the concept of what happens in the stress corrosion cracking. Suppose, I apply a component and for a you know an industry application how does stress corrosion cracking really work actually? Let us look at the time with the time what things are happening what will really happen with the time right. This is the time and this is you can say extension you take a material and you apply a load you know and then exposed to the environment and you observe you see that you know the extension does not happen long time that is what it takes place ok. At a at a given load at a given a given stress applied because change if the load is if the stress is you know increasing or decreasing it could move left and right kind of thing right. If you look at here what happens is you see generally between 90 percent of the time generally 90 percent of the time for initiation for crack initiation. So, this is where crack initiates and this is where the crack propagates and this crack initiation it is a very interesting thing ok. This initiation time the initiation here involves it could be a pitting assisted pitting corrosion assisted one, it could be a caravans corrosion assisted or intragranodal corrosion assisted one, it could be selective leaching. There all can be they all called as a precursors and they are involved in the processes. A pit turns into a crack an intragranodal corrosion turns into a crack into a intragranodal stress corrosion cracking. A caravans attack can lead to stress corrosion cracking within the caravans can happen. Cerebral leaching also can happen. So, these are the factors can assist stress corrosion cracking and since these are relatively slow processes and quite a bit of time is involved in the crack initiation. So, when you talk about stress corrosion cracking control you should also understand that you can control it by controlling the pitting corrosion controlling caravans corrosion, intragranodal corrosion side releasing all the stuffs. Of course, the metal can leak just with pitting only it need not be SCC you know caravans corrosion can be standalone failure can happen, but if they are loaded and if the stress levels are beyond the threshold stress levels yes they can undergo stress corrosion cracking at a later time. So, this is a very important and a very interesting thing when you talk about materials development and materials design actually. The other thing that you like to notice is that please notice is that the crack initiate and grows and finally, it is a rupture right and this rupture is a mechanical failure. Failure of the remaining portion of the specimen or a component right. Suppose, I have taken let us say about one centimeter thick component responsible that stress corrosion crack exist up to 70 percent of the thickness and remaining 30 percent of thickness it fails by war road I come to the point later ok. So, it is not necessary that the entire sample entire component fails by stress corrosion cracking. This is very important because when you talk about investigation when you take a fail component you would not see completely inter granular cracking or trans granular cracking you will see yes there are brittle cracks, but you also see some part of sample having a ductile fracture. The ductile fracture is coming out of the fact that it is a war road failure the crack advances right means the crack advances and then it leads to it leads to finally, war road failure you know. So, you look at this here right what is this actually this one leads to ultimate tensile thing right. Yes if you if you lower the cross section you keep the same applied load the stress increases and ultimately that that particular segment leads to a mechanical failure ok can happen in practice and it indeed happens in in most cases actually ok. Now that means, that means, the two types of the two types of two types of design people talk about the design that takes into account crack initiation and the design that takes into account crack propagation right. So, what do you mean by a design that takes into account crack initiation? I simply do not accept even a small crack in the component because the metal is considered to be brittle and a small crack is there and it is going to fracture. So, there is a problem right there is a problem. So, the design that takes care of crack you know initiation account they use the threshold stress as a concept right, but those design we take care of the crack propagation they use the fraction mechanics as a concept as a criteria for that right that is what they use it. Now, so, let us look at how the crack propagates again I am not going to deal with the fraction mechanics you know in details, but a bit of exposure to that is useful to see how stress corrosion cracking can affect the crack growth in any given component right. So, if if I you know what is you know what is called as stress intensity factor ok. What is stress intensity? A you know and you of course, have different modes of you know loading that you have and mode 1 of loading ok k k 1 is given by what given by y sigma and square root of pi a right you might have again you know again studied in maybe in other courses ok. And k 1 corresponds to crack tape stress intensity and sigma corresponds to applied tensile stress and a corresponds to crack length and y is a complex factor anybody requires it depends upon the geometrical factors ok it depends upon the geometrical this is a geometrical parameter. So, we are not getting into two details about it, but it is you know it is I think it is sufficient for us to get a feel for stress corrosion cracking and crack growth in a given material. Now, it is now look at here depends upon the applied stress and it depends upon the length of the crack which is related to k 1 value right. Now, you know very well that there is something called as k 1 c what is this called anybody yeah it is toughness it is a material parameter right below which a crack does not grow fracture toughness right critical stress intensity factor or the fracture toughness ok and below which the crack does not really propagate. If I carry out a test to measure the crack growth rate versus crack tip stress intensity ok. So, if I plot a is the length of the crack and so, dA by dt corresponds to crack growth rate and versus the crack tip stress intensity if I do that in a given environment of course, you will see that it goes like that. This is called as stage 1 crack growth rate it is called as stage 2 crack growth rate and this is stage 3 crack growth rate. What is special about it? What is special about it? The stage 1 and stage 2 the crack growth rates depend upon the crack tip stress intensity whereas, the stage 2 does not depend on the crack tip stress intensity. How they depends on what? So, the stage 2 that k 1 dA by dt in stage 1 stage stage 3 depend upon and so on applied stress intensity or crack tip stress intensity. Stage 2 depends on the nature of the environment ok and not on stress intensity factor. Again there are lot of you know understanding there we will not get more details about this you know those who we want to do there are good papers available and so we will not right now get into that discussion. Now, let us look at the the part and this corresponds to what? This corresponds to k 1 c and this corresponds to k 1 h c c ok. What is k 1 h c c threshold stress intensity factor for stress corrosion cracking? Please notice that if we have environment look at this plot we have environment the crack can grow at a much lower stress intensity level right much lower than k 1 c right. The k 1 c is here it grows much lower than that and in fact, this is they called as subcritical crack growth right. And you know who in the in the impressive environment you can have subcritical crack growth taking place in the material that is a disadvantage of the environment actually ok that is something is used to be used to regard. So, the people use these factors and also people use this is this is another factor and this is also please look at this here the crack growth rate now does not depend upon stress intensity and this also is used as an engineering parameter to design the life of the component. So, k 1 c c and stage 2 crack growth rate. In fact, there are designs just takes care of or uses only the stage 2 crack growth rates they do not worry about other things because if I know what crack growth below k 1 c you are not supposed to have any crack growth rate at all. Why is that the crack is growing here? Because of the environment ok. Here of course, you have a combination of quite a bit of stress intensity and the environment, but the role of environment here is much more here because see there are two things happening. The stress intensity drives the crack first of all the environment drives the crack more. When you lower the stress intensity the driving force where crack growth is decrease or increase. When you lower the stress intensity of the crack type driving force for the crack is decreasing or increasing decreasing right. So, it is so, if you move over like this the driving force for the crack is decreasing crack to grow is decreasing, but what keeps up the environment keeps up the environment is making the crack to grow. If there is no environment in this case the crack growth rate has to be almost the crack is not supposed to grow at all actually right not supposed to grow at all. So, that is that is that is the meaning of this particular plot. How how it will be without the environment actually? In fact, without the environment it will start from here only right it does not start from here it will start somewhere here only it will go from here it will start going here is not it? Without the environment the guy will start here only. In fact, this whole thing will not come into picture it will it will start going right here. Above k 1 c only the crack will start even growing or not it does not even grow the crack is there, but does not does not grow if it is less than k 1 c if there is no environment ok. So, the crack will start you know you will see in fact, it will go like this only it will go like this or like this depends upon top you know other things now ok, but it will start from here only. Yes, did I answer the question? Yes. Ok. The other parameter that also counts the stress corrosion cracking in general of material is is the ductility. The measured ductility especially on in a slow strain rate machines in a slow strain rate. I am not discussing right now strain rate test. We use a concept called a loss in please again notice all this again time dependent parameters you know if you are going to do the experiment at a very low rate very slow cooling then the ductility might even decrease quite significantly. So, it is again SCC is a time dependent process much like creep. The thing is creep occurs at a higher temperature, but here this occurs at the it can occur at the ambient temperatures. It is very similar to the creep process ok and, but happening at the ambient temperatures. SCC can occur at higher temperatures. I am not confining this to that what I am trying to see an analogy of SCC versus creep it is somewhat similar to creep it means they are all time dependent phenomena we should take into account ok. So, we we we are. So, we had some idea about. So, that means when we have this then people use a term what is called as SCC susceptibility index the term that is used and you might have if you attended the other course as I might have discussed in details is given as a parameter measured in air minus parameter measured in environment upon parameter measured in this parameter can be anything you know it could be ultimate tensile strength measured there it can be elongation reduction in area and it could not be and it can be to one call it yeah you can call it as k 1 SCC versus k 1 C yeah it is it is a k 1 C it is in air right it is in air it is in the. So, that is about the the role of tensile stresses and how the material is going to behave in terms of the you know these mechanical parameters in in in presence of the environment we saw this you know briefly what yeah from the fracture feature of that also we if you if you can recollect the we said yes it is brittle you know and you also said that you know in a in a in a in a failed sample in in a SCC whether you do it in a laboratory test or in the field you also have a ductile fracture component in the specimen ok that is happening because you know see let us let us look at like this suppose this is a noise sample right noise noise sample noise and I have the environment here and apply that the applied stress is constant you keep it like that, but as the propagates what happens the stress increases right the stress the actual stress increases on the load bearing member of the specimen right and as it moves then what happens stress can be equal to it could be k 1 C in this case in a smooth sample it can be UTS value and so the metal can fail in a ductile manner. So, the last ligament that fails would have a ductile component in the in the material actually you can see that. The next important thing that we should talk about is the environment when you say environment you know there is always some kind of general you know feeling that this specific to environment is generally that kind of you know the concept prevails and say you know to give an example stainless steel take that it could fail in the chloride medium ok. In carbon steel it could be nitrates carbonates like that you know nitrates carbonates and even fast weights. So, what I am giving is just illustration not an exhaustive list ok and you are talking about let us saycopper base alloys say the ammonia. Ammonica solution is a problem similarly when you talk about an aluminum magnesium chloride medium ok. So, there are you know some kind of you know there are experimental observations right. It appears that SCC need not be confined to specific environment. The list of environments they keep adding with the time ok. In fact, you know in nuclear industry you know that SCC is observed in stainless steels in pure water. The water quality that they use in nuclear industries is you know extremely very high quality even then stress corrosion cracking occurs. But then the time taken for SCC to occur is more, but the pure water can induce stress corrosion cracking on stainless steels at the temperatures, at that pressure and over a long period. So, the specific environment though people overall talk about it you know if you are going to be more technical I think this may not be totally correct ok. It can happen in many kinds of environments, but nevertheless environments play a significant role in terms of the life actually you know. What are these environments you say? One chemical composition of the environment right say chloride sulfates and phosphates all the stuffs to the pH of the environment. Third the temperature. So, given a broad picture right and broadly you can you can say this chemical composition and pH and the temperature they can influence the stress corrosion cracking susceptibility of the metals. Now, when you look at this broadly speaking gain let us let us look at you know our understanding of stress corrosion cracking. Right now I am not going to get into mechanisms ok. Let us look at the our understanding of the stress corrosion cracking. Now, when I apply a stress, hence a stress you have a crack a precrack is formed here. The crack grows perpendicular to the applied stress right. The crack grows perpendicular to the stress axis. Stress axis is perpendicular to stress axis it is it is a propagates you know it grows. When you have an environment the crack has to grow over the crack depth. If the crack grows for example, if the crack grows in sideways like that sideways like this the stress has to be concentrated at the crack depth. Assume that there is a huge dissolution here right. Assume there is a huge dissolution high dissolution. At the crack front the dissolution rate of the metal is very high. So, what will happen? What do you think will happen? I magnify this and here the huge uniform corrosion right it corrode so much what will happen there? The crack will blunt crack is no more sharp blunt stress concentration could be significantly reduced from that. So, what it means for stress corrosion to cracking to occur. So, for stress corrosion cracking to occur the crack walls need to be passivated the film has to be there. So, that only the crack tip the corrosion occurs the crack starts propagating along the directions otherwise the SCC will not occur. Assume that I am going to use very strongly corroding acid SCC will not occur. So, the one criteria is that the metals must passivate if they are not passivate they would not undergo stress corrosion cracking. The film has to be there in the surface and say when you take a passivation right when you take a passivation passivity depends on what you guys already studied right it depends on what it depends upon it depends on the environment it depends upon the potential and depends upon and what is this diagram called? Cooper diagram forms certain kind of broad you know indication where the stress corrosion cracking can occur for metals. So, when we when we said that the the the nitrates the carbonates and phosphates when you talk about it they are the ones that passivate the carbon steel and so, they are the ones they also promote stress corrosion cracking. If there are no applied stresses I mean they will be very happy the corrosion rate may be very small ok and because there are tensile stresses the stress corrosion cracking occurs am I right ok. So, the passivity is a a very important one. Is the passivity is a very important one? Yes, but then if you look at so, then I am going to draw this now say it is your potential the pH something like that I think I hope you know this is for iron diagram right is for iron in water. So, you have carbonates and bicarbonates right and bicarbonates you have nitrates and you have phosphates. So, these are the ones they they passivates that means, what what does it mean when you take a steel and have water and put a carbonate bicarbonate the potential of that solution automatically goes this particular place and start passivating and to phosphates it goes here in phosphates put a nitrate it goes here. So, these environments maintain certain pH in the potentials and so, these are the places where you get stress corrosion cracking happening in in these steels. People also have shown that in a polarization diagram SCC region right is a polarization diagram you see this they also found that stress corrosion cracking occurs very close to the pitting potential and very close to the active passive potential these two potential region they are happening. So, how do you imply from this? What do you imply from this? Passivity is important, but if it is going to so, absolute passivity there will be no stress corrosion cracking the passivity. So, it has to be an unstable passivity or something will should disrupt the passivity ok. So, so, the passivity disruption has to be there. So, you take a stainless steel what does chloride do? The chloride disrupts. What does oxygen do? Oxygen will take the potential towards the higher noble potentials right. You have chloride you have oxygen both of them they become they make the passive film you can see that you know there are pits and the pits they become the place of crack initiation the pitting occurs. So, the chlorides they do help to damage the film and then and when you are form a pit they become stress risers and lead to stress corrosion cracking. You know we discussed earlier that stress corrosion cracking involves initiation right and the initiation process involves we say pitting is one of the precursor event you see the pits they are responsible for stress corrosion cracking it can happen and and so, they are they are they are the problems. If this is the case they found a very nice relationship between stress corrosion cracking and the oxygen content they called dissolved oxygen versus the chloride level SCC no SCC. This we are talking about stainless steels in thermal and in nuclear power plants. So, when you use so, what how how do they how do you generate power? They use water they create steam the steam is used to from the turbine. So, the water should contain as low as chloride and the oxygen content so, that they do not undergo stress corrosion cracking it is very critical ok and you will see that the oxygen content is in the in the PBB levels ok. The chloride also is very very untraceable levels of chlorides ok. So, this are the very important things when you talk about the the material stability this is you know in in in SCC. Yes. Similarly, you also see this in the cooling waters systems the heat exchanges at the ambient temperature see these these temperatures are what these temperatures are all around about 200 to 280 degree Celsius on very high temperatures ok. In the cooling water systems the temperatures are all in the range of what? In the range of 35 to about 50 degree Celsius and you know cooling water is used you normally use the drinking water the chloride content there may be 100 ppm and 200 ppm are going to be there, but then if you are going to use chlorides and then what happens the stainless steels become prone to when you say stainless steel I mean austenitic please make this change austenitic stainless steels become susceptible to SCC above 50 degree Celsius. The other example I just give and then move on because you know there is several kind of you know cases we can discuss this illustration ok. When it comes to caustic and how we can promote stress corrosion cracking temperature carbon steel no stress relief required and carbon steel wells wells or whatever stress need to be relieved or or use stainless steels here only nickel base alloys. So, put it in a in simple terms it is necessary for us to understand the environment you know before we talk about what will be the right material when I say environment I would mean the pH chemical species temperature all the stuffs ok or to be known so, that you make right material selection. Going back we talked about let us say you know cooling water right the cooling water the temperature goes if the temperature is going to be greater than 50 degree Celsius how do the people tackle the tackle using duplex grade stainless steels use that ok. So, you could find a solution right where you cannot use it you what you do you bring down the intensity of the environment you know in boiler water you know 280 degree Celsius you cannot use duplex stainless steels. So, what happens you get it out of fluorides you can get it out of the oxygen content. So, you can have a C C in the nuclear reactors what they do in fact, they add even a hydrogen so, that the potential comes down because of the presence of hydrogen in the system. So, you can modify the environment also in order to bring down these stress corrosion cracking tendency of the metals. There are sometimes you get into a very difficult situation in the material selection it happens in the some industries like a fertilizer industry for example. The fertilizer industry say in a in a heat exchanger you use ammonia on one side and the chloride on the other side of it how do you choose the material right. If I have a chloride then I can choose a copper base alloy it does not undergo stress corrosion cracking whereas, ammonia I would go for stainless steel because it is free from stress corrosion cracking in ammonia right. But I have streams one side you know maybe in the tube let us say ammonia goes in and shell side water goes in and water has got chloride. So, how do you do that? So, what people do in this case is they use bimetallic tubes. So, these are all tubes and you have two different metals bonded actually right. One surface will have brass and inside you will have stainless steel they are bonded and so, that kind of things are being used. So, what you are trying to look at is if you know the problem you can find an engineering solution to the problem right. So, this is an engineering solution to the problem right. So, you can overcome the issue without any problems at all ok and of course, they are expensive bimetallic tubes are expensive, but that is a solution that you have so, that you can overcome the problem of stress corrosion cracking in the fertilizer industries ok. Are these bimetallic bonded together? Yeah they just yeah they bonded you can do whatever I mean it depends upon how these stream looks like right. You can have copper outside or steel inside depending upon of course, the technology that is available ok you can do that, but they are simply bimetallic tubes right. What does it mean know it is what it is right? This may be you strain the steel may be your copper base alloy yeah I will go inside this is a one fluid here fluid one here is a fluid two and so, what happens in this case I will send the ammonia outside right and the chloride containing one inside. So, I do not have any stress corrosion cracking issue at all. Any questions so far? Let me go to the next subject which is in that is it is a metallurgy of that ok. It of course, in is very difficult to cover a huge amount of things here ok and I just give a very broad outlook and move because the metallurgy that you deal with stainless steel is different from than one you deal with aluminum alloys and copper alloys so, everything goes ok. So, let us make a very broad discussion so, that you get a feel for what we mean by metallurgy in relation to the stress corrosion cracking. It could mean the alloy chemistry could mean the crystal structure microstructure when you say microstructure it means the the nature of phases nature and morphology of the phases the types of dislocation in the system grain size and the nature of the grains. So, they all can happen. Let me start with the alloy chemistry one example I will give to do that and I would take here the austenitic stainless steel or stainless steel I call it and it is stress corrosion cracking in chloride environment. Time to failure versus the percentage of the nickel content ok. You know what is the nickel content of this? 8 percent nickel almost the one we use as a workers right 304 as 8 percent nickel and its worst. Actually it is it is very difficult to separate out alloy chemistry crystal structure microstructure dislocation they are all intertwined it is it is not possible just to separate out actually you know it is not possible to do that because if I am going to add nickel and what happens the crystal structure changes from BCC to FCC. If I take if I add nickel then what happens you will see that the stacking power energy decreases. So, they are going to have different dislocation structure. So, they are quite intertwined, but still you can try to make you know you know some component of you know what really happens you know to the FCC from that perspective that point of view ok. So, the the chemistry of the alloy because you know at one point of time you said that the passivation is a very important one right. If you are going to add a very high chromium content if you are going to make the passivation very very strong then the stress corrosion cracking can come down as well actually ok. It is possible can happen you know from 304 to 360 at the some cases this they do see marginal improvement in the stress corrosion cracking of the alloy they do see that thing ok. I think we can continue in the in the next class.