 Welcome back to this discussion on stress corrosion cracking. And we have been talking about the factors affecting stress corrosion cracking. In that we saw two aspects in reasonable details the tensile loading conditions, tensile stresses. Then we talked about the the environment, the natural environment how it can affect the stress corrosion cracking. Towards the end of the previous class, we discussed about the metallurgy and there are several factors in the metallurgy that that affect that influence the stress corrosion cracking. And one of that we saw was on the alloy composition. And we just took an illustration of how the nickel can affect the stress corrosion cracking of stainless steels. We also see similar kind of things happening in the copper base alloys as it is. You know the copper, pure copper is relatively resistance against stress corrosion cracking. By the way earlier there was a a kind of understanding that pure metals do not undergo SCC. Now that is metals can also undergo and also suffer SCC. So, earlier you know they were considering that the pure metals are not formed SCC. It has been shown that copper, pure copper, pure aluminum they do suffer stress corrosion cracking. But yes the the susceptibility index you recall right. I think it is not that severe compared to the alloys. So, even the copper system when you add copper you know we talk about copper zinc alloy system. Add more zinc as zinc content increases the stress corrosion cracking increases. On the contrary if you take aluminum copper alloy systems especially we talk about 7000 series aluminum alloys addition of copper addition is beneficial. So, it is always useful to understand how the chemical composition of an alloy can can affect the stress corrosion cracking. The other important thing in the metallurgy is the crystal structure. We make illustration only related to ferrous system. You take a stainless steel, stainless steel you know we have seen the classification of stainless steel when we when we talked about the sensitization of stainless steels. And you have a BCC they are resistant to SCC whereas phase centered cubic that is our austenitic grade stainless steels are prone to. So, when I say SCC you always have to associate with the environment right like the chlorides. So, even in the case of austenitic grade stainless steels chlorides in a sea water per se it is not prone to SCC it undergoes pitting no doubt. But you lower the pH in austenitic range then I think the chlorides promote SCC or raise the temperature that when the temperature goes beyond 50, 60 degree Celsius the chlorides are prone to SCC and whereas the ferritic stainless steel is not prone to stress corrosion cracking. And again you can have a duplex stainless steels and you have alpha plus gamma that is your duplex stainless steels also also resistant to resistant to SCC. So, the crystal structure they do play a role in in terms of offering the SCC resistance. And I will highlight the the importance of the microstructure I will I will take an example of aluminum alloys ok aluminum copper or aluminum zinc magnesium aluminum zinc magnesium copper systems. This aluminum you know alloys are known for one one special properties you know. And most of the aluminum alloys the strength are derived from where from the precipitation hardening. Especially the the 2000 series alloys and the 7000 series alloys they are all age hardenable. They are age hardenable alloys right you know through the precipitation the strength increases. You all know that aluminum alloys when you talk about aging we call them as what you call them as under aging peak aging and you also have called as over aging right. The strength of the aluminum alloy increases from the solutionized conditions to under aging strength increases the maximum in the peak age condition. And subsequent aging treatment what happens the strength drops down over aging conditions the strength drops down. So, if you if one plots the time of aging versus the strength or hardness do that out of the strength lie the strength goes something like goes like that and that this is your peak aging over aging this is your under aging treatment right. If somebody looks at the this is the strength this curve belongs to this. If I look at the resistance of the alloy to stress corrosion cracking what happens? You will find ok the time to failure which also is indication of the stress corrosion cracking right. So, at the peak age condition the alloy becomes the most susceptible right. And again when you do a over aging the SCC resistance increases. So, when you increase the SCC resistance by over aging you are going to have a drop in the strength level. So, there is you know some compromise on the available strength for structural applications. Without going much into details you can say that in the peak age conditions conditions you have what is called as a coherent precipitate. And in the over age conditions you are going to have semi or incoherent precipitates. Again a lot of metallurgy involved about coherency semi coherence and all ok. So, so when you have a coherent precipitate that means you know to put in simple terms when you have when you have a precipitation it is a matrix right. It is a matrix you have and you form a precipitate if the precipitate the crystals the crystal you know if the crystal structure the lattice parameters a good match you call as a coherent. So, this that kind of you know coherency that happens in the system leads to more strength you know you guys sort of you know and when you talked about the strength in mechanisms I think some of you might have read you know how the peak age conditions the strength increases say right. And because the precipitates are plenty and the space between the precipitates are less your coherency strain is difficult for the dislocations to move through and so the strength goes up. But same can lead to stress corrosion cracking because of the fact that this leads to more of the planar you know let us say the planar you know dislocation formation in the system. The idea here is to show that microstructure plays a role. The same is the case when you talk about martensitic steel when you coincide you form a martensite is more prone to stress corrosion cracking. When you age it you temperate the stress corrosion cracking resistance increases of course we are going to lose certain amount of strength. So, the microstructure in drawn plays a role and that is to be to be considered in designing any alloys. This location structure and again not going much in details planar slip more stress corrosion cracking cross slip more resistant more resistance to SCC take place. Similarly, the stacking fault energy decreases SCC tendency increases ok comes next to the grain size lower the grain size better is SCC resistance. So, fine grains are better. So, these are you know kind of not in very detail, but a broad outlook about the role of metallurgy on the stress corrosion cracking of alloys that we talked about. I want to spend some time on on the mechanism not in detail, but broadly we can look at the mechanism here. I think I have given a reference to stress corrosion cracking book in the beginning right on the class I think this edited by me and Tetsu Soji. There are two articles by Stan Lynch and you know he has given nice review article on corrosion mechanism stress corrosion cracking mechanism and also hydrogen embrittlement mechanism those who are interested can go through that in details here I am going to be very very brief. The mechanisms there are of course, several mechanisms slip assisted active path dissolution ok and the crack is assisted by the slip process. When you deform a material you will have a slip plane right and you have dislocation on that. When you deform it what happened to the dislocation? The dislocation goes to the surface and form a step right. Now, if I have a film a passive film if I have a passive film could apply a tensile load what happens? An application of tensile load a tensile stress dislocation reaches the surface. And then form form right this disrupts the passive film. So, passive film on the surface at that level gets disrupted then what happens now? The metal starts dissolving if I plot the current versus the time you have a current normally is equal to passive current density I P you are getting. If the dislocation disrupts the surface current goes up and again it gets repassivated. Now, the Q that you give the Q is is equal to the amount of metal dissolved. So, you can able to correlate to Faraday's equation right you can do that you know the current you know the area. So, you know how far the crack would advance in one step. Repassivates what I what it could happen again? This is the I again can you can. So, every time the crack growth is assisted by the slip step. The tensile load the tensile stress would assist the dislocation movement to the surface and so, the crack starts growing. So, this is one model. Now, look at this. Now, if I am going to have a coplanar dislocation what happens? The slip steps form are the same location the crack growth rate becomes more advanced. What is coplanar dislocation? The dislocations lie on the same plane and you deform it then one after another the dislocation leads the surface and form steps. I am not again going to talk about the too much details about the merits and demerits of this particular models this is quite essential. All one you can say here is if this model the crack is expected to do grow continuously not the discontinuous manner ok. You would not see that because every time the metal dissolves it just moves at the atomic levels. Next is called film induced cleavage model. The film induced cleavage model primarily hinges on dealloying like you have copper zinc alloy system right. The copper zinc dealloy system what happens? The zinc dissolves and copper gets deposited onto the metal surface. So, you would expect this is the dealloyed layer ok and you apply a tensile stress. Now, the dealloyed layer is considered as dealloyed layer is considered to be brittle. So, when you stretch this sample when apply tensile stress and if this dealloy layer is brittle when fractures what happens on tensile loading the dealloy layer fracture and what happens? The energy is the energy is is deposited onto the. So, what happens now? The crack now see when you when you have a when you have a brittle film it fractures and you deposit the energy is so fast even the ductile material will turn into a brittle material right it depends upon how fast how what is the strain rate. So, to high strain rate the even the so-called ductile materials will behave as a brittle material. So, now what happens? Now the crack advances, but again what happens? Dealloying occurs and again fracture of the dealloyed layer again what happens? Crack growths. So, the crack growth here is discontinuous. So, when you observe the brittle surfaces you when you observe the the metallic surfaces which suffered stress corrosion cracking you normally see the cracks are not continuous it just advances stops advances stops. So, this model explains how you have a discontinuous crack growth happening in the ductile materials because of stress corrosion cracking. So, the the event the precursor event here is a dealloying enrichment of the noble metal which of course, is brittle in nature on tensile loading the fracture because they are brittle and the energy that is released in the fracture process is deposited onto the crack front the crack advances. So, every time the sequence repeats and the crack propagates that is why it is called as film induced cleavage model. I think these two models I think is is good enough for us I think I will stop discussing more on the stress corrosion cracking models. If you want more you can refer that particular review article for details. Any any of you have any questions? Now, let us go to the next important topic of how do we control the stress corrosion cracking? How do you do that? So, what do you do? Can you give some ideas? We can we can choose the right material that is select right material. Yeah, choose right materials right select appropriate material. Of environment. Yes, control of environment that means, elimination elimination of critical species what more lower the stress levels we talk about removing the residual stresses how people do this one. That is not removing the residual stresses what you do in steel in steel what you do is called as yes subcritical annealing treatment below the hydratoid transmission temperature right. What else you can do? Yeah, of course, I mean in the alloy development you can reduce the grain size all probably will come into alloys you think. Anything more you can do? You can do father to protection add inhibitors right. We can also do you said short peening right. Short peening in fact, is practiced that gives the compressive stresses. What more you can do? You can also apply coatings where possible. So, that brings us to the end of the discussion on stress corrosion cracking and any questions. In the aluminum copper system especially in the 7000 series alloys. The copper containing 7000 alloys if you look at the crack growth rate the crack growth rate very significantly get reduced by increase in the copper content. Of course, there are I mean quite a bit work on it and people have been you know discussed in detail about that ok. I do not know what I mean this is subject by itself we need to see what really happens you know because you see the copper essentially makes the precipitates also noble actually which are otherwise you know assist the stress corrosion cracking of the aluminum alloys ok. So, then we get into mechanism of SCC had in battle band in aluminum alloy itself ok. We have done some quite a bit of work on this system and you know we can discuss probably offline you know that is more in detail section. Any questions?