 Welcome to the discussion on one of the most important topics in corrosion of metals and alloys. The topic of discussion is stress corrosion cracking. So far we had looked at the corrosion of metals from a perspective where there are no applied stresses acting on the metal. We discussed the galvanic corrosion where you had dissimilar metals coming in contact with each other electrically and we said that the the anodic member of the couple will undergo the corrosion aided by the cathodic member. We then went on to discuss the crevice corrosion in the structure. When you form a crevice because of the fabrication issues the crevices act as sites of corrosion. We looked at the the passive metals and we said that the passive film can break down under certain conditions may be a mechanical damage or it could be due to the specific ions present in the environment leading to pitting corrosion. Then we looked at what happens when there are the transformation in the material especially in stainless steels that leads to sensitization. We looked at the intergranular corrosion of the stainless steel and these relevance to welding and fabrication. The intergranular corrosion again not only confined to stainless steels you can happen in any other material more so in magnesium alloys and aluminum alloys and copper base alloys when the grain boundaries are becoming electrochemically active. In all in all these cases we also assumed that yeah there one more case we also discussed where we call as selective leaching and dealloying. In all these cases we assumed that the environment is static in relation to the material and we looked at what happens if the environment moves in relation to the structure to the material. Then we had looked at 3 different forms of corrosion and flow assisted corrosion erosion corrosion damage the cavitation damage we talked about where the environment we considered was so fluid it could be a gas it could be a liquid and of course, in all these cases you can have a suspension of the solids but essentially so fluid. We looked at one more case where the the contact surfaces are in motion ok. In the contact surfaces in this case are a solid state right they are solids one metal against another metal and one metal against another ceramic for example, when they are relatively moving and when they are under low conditions what kind of corrosion it occurs we call them as a fratting damage. The lowering condition that are acting on this on these contact surfaces only you know they are not considered in terms of what happens in the bulk the load the stresses that happens in the bulk we are not considered. In fact, these you know contact surfaces the interface the loading is the predominant factor. But you know most often the engineering metals and alloys are made for structural applications. They they bear the load right beat a ship a pipeline or a reactor could be a pressure vessel for example, or could be aircraft or I mean you can name quite a few actually. In all these cases the metallic structures are experiencing certain amount of stresses. You can also start classifying stresses in the different categories it could be a tensile stress, it could be a fatigue stress or it could be a really retortion stress you know stress can happen. So, when you have this when you also have an environment acting on the surface the failure mechanisms are different and they have a very serious consequence. In fact, you would see that the metal hardly corrodes in terms of visible changes. The loss in weight is hardly seen at all, but it can lead to cracking it can lead and as a result of which it can be leaking and you know and structural instability and there could be safety issues involved. So, what we are going to look at in this particular may be today and in a couple of days is how a metal under tensile loading condition and also exposed to the environment respond to it ok. What is the mechanism of such corrosion process and what are the factors that affect such a kind of corrosion and how do you prevent them and if there are any kind of fingerprints characteristics of that form of corrosion is also important. So, these aspects we are going to see. So, before I go into it I just want to show certain illustration how the corrosion can really affect in practice. This in fact, is an ongoing work which we have been do we are doing now. This is the reactor in fact, it is a pressure vessel I would say it is high pressure. It is meant to produce an aromatic compound you know most of you know aromatic compounds are you know as special characteristics and they are synthesized using inputs which are mostly organic and maybe sometime you may add sodium hydroxide you may add water. You are subjected to high temperature, high pressure conditions. So, the reactor that you are seeing here do not worry about all this inlet, outlet all this stuffs actually ok. Because there are you see the feed hydrogen you know when you feed hydrogen then there are lot of precautions that you have to take constantly to do it. So, there are so many ports. It was made up of 304 L stainless steel. The company wanted to increase the production though it went from a smaller reactor to a bigger reactor in just 4 months just 4 months of its inception commissioning ok. The reactor start leaking and I do not think you know when you have hydrogen you really would like to take risk of that actually. And if you if I look at it look at the inner walls of the reactor normally you do NDT right. NDT is one very versatile NDT is dye penetrant test. Some of you have to be aware of it right you apply a dye on the on the surface and you wipe them out then you can just you know you also add another elutant to the surface and and so, the the dye comes to the surface and wherever the crack the dye penetrates and comes back to this. So, you will able to see this you know the subsurface cracks even surface cracks using the dye penetrant test. When you take a cross section of that you see how the cross section looks like I do not hope you will able to see very fine cracks starting from the surface and moving towards this. It is a very thick vessel of 50 millimeter thickness. So, it is a 304 L stainless steel and you find that it just failed in 4 months. If you look at little bit more in detail at higher magnification ok and you will find the cracks or microscopic in nature and the microscopic cracks turned into a macroscopic failure ok. And you you guys now are reasonably familiar with metallurgy you see that these cracks are all branching now and you see some of them are called transgranular cracks ok. We will come back to this soon. So, 4 months time and the reactor started failing. This is a heat exchanger ok you know this is a kind of schematic of that ok. And it is called as air condenser right. You want to dry the air. So, you need to you know and you know especially when see what happens you know in industry every heat is important right. You you when you create heat you just do not want to let it out right. You take the heat out from that heat is essentially energy right. So, the air comes out of the reaction process you need to remove the heat because it costs money and this is the condenser right air condenser right. It takes the heat and it is called as a as a cooling water system ok. So, what you do here is that you can see here that there is an inlet here ok. It goes inside this outlet here and it is a horizontal heat exchanger and it is made up of 304L stainless steel. The air inlet the temperature is about 150 to 160 degree Celsius at a pressure of 110 kg you know centimeter square and you have a cooling water you know enters here and comes out air enters here and then it comes out here ok. It is a counter current process right. It is what happens the material used was a 304L stainless steel. It is a fertilizer company and the cooler in operation was for 6 to 7 years not longer. And the tubes which are inside 63 of them started leaking at all right that was a problem. So, it is all premature failure right. You just take these tubes and see the tubes you know you know in a in a microscope this is the cross section of the tube. You see here the crack started from the external thing where the water went went through the water went through this ok. So, from wherever the you know because the surface is you know is covered with water and you start you know cracking. This is called cooling water. This is not a distal water. It is of course, a pure water but not not like your DM water. Again you see a kind of branching of cracks. This is one of the characteristics of distortion cracking and if you know you know if you do a tensile testing or if you have the mechanical loading it is a failure you only see a one crack the cracks are not branching at all. The crack morphologies we will discuss shortly and this is also somewhere some kind of a trans granular cracks taking place. Look at this in a in a scanning error on microscope in in detail see the surface is the surface is is quite brittle surface right. It is a brittle surface. It is a brittle surface and those guys who are with the metallurgy background you will have you will appreciate you will have to understand what brittle fracture is and those who are not from the metallurgy background I want to show how a normal fracture should look like in scanning at a microscope ok. You see this here this is a material failed in air in the absence of environment it is a tensile failure and hope you can see this is called a fibrous failure it is called a dimple failure and dimple fracture and you can see the fibers you know the metal has nicely elongated they are they are just a flow nicely flow right. So, this is a kind of ductile fracture you see when you have when you have you know simply failure in air. I suppose that it looks like a glass this fail here ok. So, the problem in stress corrosion cracking is that it can cause a premature failure and it can lead to certainly see it it is not going to cause too much of corrosion see here now the surface ok. This is the surface internally and you do not really see much of corrosion at all I hope you were able to see this clearly now it is it is almost, but then wherever there is a crack crack that happened and subsequent place you are not seeing any any corrosion. So, visually they are not undergoing severe uniform corrosion at all and so, this corrosion is very localized and is called as a stress corrosion cracking. Now, stress corrosion cracking has been a a really a problematic thing actually. It is a it is a problematic thing and in fact, if you look at the story historical perspective of that I am not going to discuss there is paper by Sege Sipplaw I think give a reference later and he has given very nicely the history of of stress corrosion cracking actually in the year 1873 Manchester. So, a guy called Williams H Johnson he did a very interesting curious experimentation. What he did was he took steel and then he dipped in hydrochloric acid and and also in sulphuric acid both acids he did a nice just dip rate and then he carried out a tensile test. He found ductility reduced significantly. In the year 1974 another interesting person and Osbone Reynolds he did a very another interesting experiment. He found that ductility loss is can be reversible. We expose it to the environment and do tensile testing and you see there is a loss in ductility. But you just leave it for quite some time in air. He says that you do a test after a few months and major part of the ductility loss can be recovered actually. That was a very interesting experiment that was done. Now subsequently and in the year 1886 the guy called Robers Austin what he did was he took 22 carat gold drawn into a wire and then he exposed ferric chloride wire got cracked. Probably this is for the first you know such observation which talks about the so called stress corrosion cracking. The earlier observations they pertain to hydrogen damage hydrogen embrittlement which we will talk about later. So, they were all academic experiments they are more academic and the importance of such kind of loss in mechanical properties. You see what is here there is a loss in ductility taking place. You have not realized they started realizing it in the early 20th century. There was a time where there was industrial revolution taking place right lot of you know technologies were developed actually and it so happened the invention of steam driven locomotives right. The steam driven locomotives first saw the problem of stress corrosion cracking. How severe it can be right? Now it is a simple water and it generated steam and that is used to drive locomotives. Now it is used same steam is used now to generate electric power right now people have use electric powers. Now this is a problem because you see here this is very mind boggling 1865 to 1870 288 boilers in UK and in 1867 to 868 we had 441 boilers in USA exploded. It was really kind of you know and you know you see the consequence of explosion right the pressure vessel actually and the steam at that temperature pressure lot of casualties actually. However happened the steam locomotives made up of steel I mean not locomotives I am sorry the boilers made up of steel and they are fabricated using what how they fabricated using rivets. What is then you put a seat and then you put a rivet and then you you just what you are you cold compress it right there will be more stresses there will be lots of residual stresses in the system and you know when you when use water for the boilers the the pH is increased otherwise what happens the steel will corrode right. The pH used to be around about 10 or so and then what happens when when you when you boil the water you take the steam out what will happen to whatever remains in the boiler. How do you make the pH of the water higher? You add sodium hydroxide right and you increase the pH to let us say 10 and you you boil and evaporate. So, what happens now with the time the sodium hydroxide concentration increases and then it leads to cracking. So, thus the name emerged we called as caustic embellishment. The caustic embellishment now we all know that it is simple mechanism of stress corrosion cracking right, but those days they called a caustic embellishment because wherever the caustic accumulated see what happens when you when you have a sheet and then you join them like that right join them like this and you put a rivet you put a rivet here ok and this gap there will be accumulation of the accumulation of the sodium hydroxide and start giving rise to crack. Historically there are one more incident that was that also brought the attention of people seasoned cracking of brass cartridges. You know it is a bullet right and you you mount a you mount a bullet you crimp here right ok this is hard one maybe steel or something like that ok this made up of brass. So, they you know what happened was the cartridges started cracking during the winter time see they they were stressed India first actually because the British found that they found that these cartridges failed during the during winter and monsoon time you know winter monsoon where you know what happens to organics suppose you have a organics they decompose and then they generate ammonia the ammonia cause cracking of brasses. So, these are the names given to stress corrosion cracking because they are associated with certain events and they are all in fact, can come under stress corrosion cracking of of of metals. Why why is that you should worry about stress corrosion cracking? Why bother about stress corrosion cracking? We have I think some mechanical engineers right background right I think I give a material and I ask you to use that material to design some structures say maybe a pipeline or maybe a foot over bridge whatever ok. So, what input parameters do you take to design these structures all the material right of course, it has to take certain load ok. So, the loads are given to you ok. So, what input parameters of the materials you would consider yeah yield stress yield strength right then. Ultimate tensile strength. Ultimate tensile strength then you look at the ductility in terms of the elongation right these parameters of course, you can also look at a toughness which is k 1 c value you can do that right. So, you look at the yield strength of the material ultimate tensile strength of the material look at the elongation that happens at all and then you can look at the k 1 c value which is called as a toughness. So, the parameters the the mechanical parameters ok of interest to design a structure 1 could be yield strength, 2 ultimate tensile strength, 3 the elongation elongation 2 failure right failure, 4 the fracture toughness. If you design a structure wherein the load does not cross the yield strength of a material right. Suppose I use a steel and a carbon steel was what may be a 200 mega Pascal, 250 mega Pascal is the carbon steel strength right yield strength and and ultimate tensile strength may be over 400 mega Pascal something like that and the elongation may be around about 18 percent say range of things will happen. So, suppose I design a structure such that the load does not lead to a stress level beyond the yield strength of the steel what happens? It will endure, it will endure forever because it is within the elastic limit it does not undergo any plastic deformation right it is within the within the elastic limit. So, it suppose you endure or wrong. Suppose you have an environment coming in contact with this structures can fail well below yield strength it can it can happen even at about 25 percent 30 percent of yield strength and not all cases I am saying there are cases where the metal can fail are about 25 percent 20 percent of the yield strength. Now, crack can grow cracks can grow well below k 1 c cracks the toughness is k 1 c right elongation is given as epsilon sigma UTS and sigma yield strength that is because the elongation loss can be 60 to 70 percent I can know I can lose my elongation 6 to 70 percent. So, what does it mean? If I lose that elongation that is what is going to be elongation means what? That is it right the ability of the metal to manage any distortion in the structures that could happen during the load bearing things isn't it? It could happen right. So, if there are what will be a distortion and if the metal is not plastically deforming then what happens like a glass you can just simply crack. So, you can lead to a brittle that only talks about safety right the safety here is depend upon how ductile how tough they are actually. So, if it is going to be brittle then safety becomes a casualty. So, stress corrosion cracking is a problem and for the structures where you expect it could it could suffer ok it could suffer through that particular failures. So, look at what is stress corrosion cracking? It is conjoint action of tensile stress corrosive environment acting on metals can also happen in materials also. It can happen even in glasses it could happen in polymers but most of the engineering materials we you know we deal with and especially this course we discuss more on metals ok, but it is just not confined only to metals and alloys materials can happen ceramics can fail glasses can fail polymers also can undergo stress corrosion cracking because the environment can can interact with them. So, you have three players here one is material other is the the environment is the stresses. I mean I mean stresses I mean here that tensile stresses ok that is not a fatigue it is not a fatigue stress not a shear or I mean we are not talking about torsion and we are not talking about you know compression all this kind of things and all this lead to SCC. So, it is a synergy taking place please look at it is a synergy right it is it is it is not a cumulative that corrosion means this much loss in weight in air the metal undergo this much of tensile strength it is not additive it is it is a synergy and also not much loss in weight due to corrosion. Now, this is a van diagram we call as van diagram now you look at this there are three players here the material the environment I mean the chemical environment here I suppose we define the beginning of the course itself we are not talking about a physical environment we always talk about that chemical environment when it comes to corrosion and we have talked about the tensile stresses. So, in order to understand stress corrosion cracking we need to look at the material aspect the environment aspect here and we need to also address the stresses. So, all three we need to be understanding in order to get a picture about stress corrosion cracking and then to say how we can control the stress corrosion cracking. Stress corrosion cracking can be taught maybe some 10, 12, 15 lectures more actually it will be very very brief and you know at best we will take about three classes and try to see you know how we can understand stress corrosion cracking of metals and alloys. Now, let us go into the first aspect of the cracking the types of cracking whatever be the cracking whatever be the types of cracking the cracking is always brittle in nature. So, brittle is a very qualitative term it is very difficult to define what a brittle is right. I do not think this can be say anything which is 100 percent brittle and you cannot say anything as 100 percent ductile kind of things. So, it is a character of the material actually. What is the character here? What is meant by brittle as opposed to ductile? What do you mean by that? Let us look at from the perspective of what happens to the metal and you deform it actually right. Sir, can we say that brittleness is that there will be no elongation before the crack propagation can take place. So, there will be. No local elongation. Okay what is no elongation right okay what is no elongation? Local yielding at the surface. Okay there is no local yielding or surface okay more okay alright you are right. What does what is happening that means there essentially there is no atomic mobility taking place essentially in the success when you apply a stress the atoms start moving that is what you call a plastic right you call a plastic deformation right. So, plastic deformation we call it when does the plastic deformation occur when the stress applied is greater than the that is where the dislocations move atoms move you know now the mobility of atoms are related to the mobility of the dislocations right. So, only above yield point dislocations move and through which the atoms move and so there is going to be going to be plastic deformation the metal take the shape that you want it right. In the elastic limit there is no atomic mobility is not going to be there. If the metal fails in the elastic regime then it becomes a better okay why because there is no essentially atoms do not move at all. If the stress corrosion cracking in the environment facilitates the crack initiation and crack growth below the yield point then what happens the metal will behave something like a glass because they do not really move at all am I right or not. So, the fracture process depends upon at what stress level the metal fails. If it fails close to ultimate tensile stress then what happens is going to be nice full ductile. It will fails in somewhere between yield point and duty as depending upon where it fails the extent of ductility is going to change ok. So, it is a qualitative parameters I do not want to you know discuss too much on it ok, but I want you to get a feel for that right that means, if the metal can fail below yield point you can get nice brittle cracks occurring on the metals, but not necessarily always the metal fails in that manner. Now, let us look at at a microscopic level how does the crack propagate that is a macroscopic level right. At a microscopic level how does the crack propagate? We all know we deal with pollicase line material right. Now, the crack can grow along this way we call it as trans granular crack. In fact, the most of the mechanical failure happens in the air are all trans granular cracks, but they are trans granular ductile cracks ok. Here you also have you know trans granular, but then these are trans granular brittle fractures. The crack also can grow it can grow along and what is this called inter granular cracks. I will go back to the some slides which I shown you before so, that you get a clarity in terms of what is mean by trans granular cracks and inter granular cracks. I hope this is the slightly at higher magnification lower magnification this is the higher magnification. Now, I hope you were able to see the grains here ok. Now, look at this the crack grows straight you know cuts across all the grains ok. So, it is it is a trans granular crack this way is here. You see that in this scanning microscope it is it is the please look at this is the this is the this is not the crack interface right. You can you can break open this this is a crack plane this is a crack plane. It is a crack plane in the this is the plane through which the crack is moving now. Now, look at this this becomes totally totally totally trans granular cracking. And I hope you were able to see some grains here grains here there are grains here there are grains here. Now, you can see this also there is a grain here. So, the crack cuts across the grains. So, they become trans granular cracks in nature. Look at this end it is a clear inter granular cracks ok. Look at the grain here it is a grain here crack grows along the grain boundary ok. Why? The grains are got sensitized you can see this here the grains got sensitized the crack grows along this. So, it is it is a it is a surface. The surface of the specimen see that you open this you break open this you look at the crack plane you will able to see the nice grains which are not elongated right. You can look at this right these are all grains grains you see the grain facets ok. You can see the very clear grain facets grain facets the grain facets means the grains are not deformed like a sugar the sugar grains you see that right. So, they are not deformed they are they are you know I mean the corrosion has occurred selectively along the grain boundary and the crack starts moving along the grain boundary. So, the grain boundaries are relatively electro chemically active. I use a term electro chemically active because it can be an anodic can happen it can even sometime can even can have cathodic that happens in hydrogen embrittlement case. But so, if the if the the preferential dissolution takes place here ok and and so, the grains are not deformed and and you see a huge amount of intergranular cracking taking place. This is the one way of looking at it. There are cases where it is not necessarily 100 percent intergranular 100 percent transgranular it could be a mixed kind of failure can happen right. So, um so, this is what really happens. Now, all these cases the failure is called as I hope you have heard of the term called cleavage failure. What is the cleavage failure? Anybody has area. So, lattice they have right when you apply force the atoms start moving when you have dislocations you know dislocation of course, difficult for that to move. Now, when you when you pull it now ok assume that I I use a scissor I cut this bond cut this bond I cut this bond I cut this bond I cut this bond. If I cut this bond what happens now? This piece becomes separate this becomes a separate that means, atoms are intact the positions are not changed because these are weakened now. So, the cleavage fracture means essentially the bond between the atoms are broken well before the atoms could displace from their positions ok. So, this what we call in the cleavage fractures that what happened the grain boundaries what happens the atoms cleave right you have one grain another grain in between the atoms there is this cleave out can happen. That happens of course, here in this case corrosion can lead to that happens, but you also have intergranular cracking taking place in in air also right you all we talk about temperament element all these stuffs there ok. So, so, they are all cleavage fractures the atoms or or you know the bond between the atoms or or I can say poor cohesive strength it can happen and it can lead to this kind of failures. So, it is very important to understand the nature of cracking in a stress corrosion cracking failures otherwise you will get mislead you may it consider that as what it could be you know normal tensile failure ok. So, cracking more is a one of the one of the signatures of stress corrosion cracking. Now, if you do not identify that then you will not identify the problem you know let me just spend a minute and then I will I will I think I will close this today's discussion. This one or or you get a yeah if you find that a crack like this the both of this are done what is called as you you guys already studied the sensitization right it is a stainless steel they carried out ASTM A262 A test was done ok. You do not see any sensitization here, you see the sensitization taking place here right here also you have ASTM A262 A test right, but there are also sensitization taking place. So, the alloy failed by brittle fracture case 1 and the case 2, but the reason for failure are different right the reason for failure in this case it is improperly solution annealed or improperly welded whatever kind of thing here ever it was ok, but the alloy is not good enough for the environment ok. So, the way you diagnose the problem is different. In this case you say it is not the metallurgy is not adequate for the severity of the environment ok, you need to change the alloy. Here the primary cause seems to be sensitization right, the sensitization is related to carbon content welding improper solution and treatment so many factors are going to be there. So, you need to understand how the cracking take place before we can come to a conclusion what is the root cause of the problem in stress corrosion cracking ok. So, so we have seen now the the crack morphologies, the crack the natural cracking and we will end our discussion today.