 we shall look at the crevice corrosion of metals and in details today. Before we start discussing about the crevice corrosion further, it will be worthwhile to recollect what we have been discussing on crevice corrosion ok. So, it is a recap. Now, we will look at the mechanism. First of all the crevice corrosion the origin of that is due to the formation of a crevice which can happen because of let us say when you have joints in the metal structures such as a flange joint or a rivet or an expansion joint or threaded joints as you see in the fasteners. These are coming out of the the design parameters ok. You can also arise out of the the other reason where like you have some deposition taking place because of the environment is not it. We also call that as under deposit corrosion taking place. So, these are the factors that leads to the formation of the crevice. Now, why does the crevice corrosion occur? Can anybody recollect? What is the basic requirement? Yeah. Is that the is the primary reason for that? Yeah. Is that what it is? What is the primary reason? Why does it start? Primarily in crevice corrosion. So, the formation of the differential erasion cells right. The basic is that there is a concentration of oxygen between the two locations starting point. So, the point is that the starting point comes because of differential erasion. Why would the differential erasion lead to this problem? First of all, what kind of metals the crevice corrosion occurs? The passive metals. So, only in the passive metals you know so called passive metals the crevice corrosion occurs. So, in such a situation what is the role of differential erasion means what? For example, there are location where the oxygen concentration is very high, there are location where the oxygen concentration is very less. So, what happens due to this? The limited current density may be yeah, the limited current density might increase when the oxygen concentration is more. So, what happens? Which one suppose you have more limited current density will it become active or passive? Passive. So, when you have higher oxygen concentration the metals tend to become passive. The one where the oxygen concentration is less the metal tend to move towards active potentials. So, the the crevice you know starts you know corroding over a time period because again we say that it is diffusion controlled. When you say crevice it is it is it is no convection right the dimension is so small there is no convection and so the diffusion process and so the oxygen within the crevice with the time steeply decreases and so that becomes an anode that is the starting point of that. So, what happens again? In the crevice why would the corrosion? So, in the crevice what drives the crevice? What is the difference between a crevice and the outside the crevice? One is oxygen concentration to start with happens ok. Subsequently what happens? When the corrosion proceeds the crevice establishes its own environment different from what you find externally right. So, it establishes an environment. So, it is it is a localized environment if you can browse through your notes you will see that right. In the crevice what is happening over a time period you will see that what happens? It forms it creates an acidic environment if the environment has chlorides the chlorides migrate towards the crevice. So, here what happens the pH drops significantly and you have chlorides. So, the passive metals they start depasivating actually ok it becomes totally active the corrosion occurs. So, it establishes a localized environment which is different from the external surfaces because external surfaces are open there is a convection taking place and so, there is the pH is not going to drop at all ok. So, the localized environment that is the reason for that. So, this leads to growth and stability of the crevice. So, we have seen these factors right. Since it is it is it is a crevice what happens that is no or poor convection of the electrolyte. So, migration only due to what migration only due to diffusion. So, this is a it is a kind of basic mechanism we saw in the last class. Now, we know that this is a industrial problem that we need to find a solution to the problem. In order to find a solution to the problem you must know what are the factors or the parameters that affect the crevice corrosion right. So, we need to we need to understand it is very interesting that the foremost important factor is the crevice itself. So, geometry of the crevice one important factor. Second factor is the environment. Of course, when you have an environment it also decides the electrochemical factors. The fourth methodology of this right. So, these are the factors broadly I would say that influence the crevice corrosion of metals right. So, let us see this in details one after another. Let us take the crevice. When I say crevice I I was meaning that it is geometry. Why the geometry must be important? What is the geometry important? This is right. Because the crevice corrosion more often is a transport control and the transport through the diffusion process. So, the geometry will play important role because that is going to decide the how the diffusion will occur within the crevice ok. In fact, you would notice that you know the metal resolution for ok. In fact, more resistant metals more corrosion resistant metals susceptible to crevice corrosion. So, no wonder that you know some cases the corrosion occurs, some cases corrosion does not occur the environment may be the same, but if the crevice geometry the crevice dimensions change then the extent of crevice damage could change ok. For example, I would say you know 904 L is a stainless steel ok. It has they say 20 percent chromium and 25 percent nickel, 4.5 percent molybdenum and 1.5 percent copper. It is supposed to be far better than 316, 317 stainless steels ok. You see the high amount of nickel, I am the chromium, but it is is prone to crevice corrosion. But 70, 30 what is this? It is a copper nickel alloy is not prone. So, very interesting thing ok. Now, let us take let us take a crevice and see what kind of dimensions will affect the crevice corrosion right. One dimension is the crevice width right and this is the crevice length right. Suppose, I have an environment here, this side where will the crevice corrosion occur and will the depth of attack is going to be same throughout ok. So, when you look at this, let us just look at this and you please tell me ok. Let us take the double view width. If the width is increasing, what will happen to crevice corrosion? The crevice corrosion will decrease. So, it increases the crevice corrosion. Why does it happen? Yeah, the stagnation is reduced. In other words, there can be you know more convection. So, because of that the oxygen concentration within the crevice would increase if the width of the crevice is increasing. This is an irony of that right. If you talk about a flange joint suppose, I take a flange and join like. Suppose, I take a flange joint right. Suppose, I make a flange you guys might have known the flanges right. Suppose, there is a flange joint suppose, I want to tighten this out. So, the mechanical engineering point of view what do you expect to happen? You expect that the gap should be decreasing then only it is going to be leak proof let us vibration. So, typically these mechanical joints are expected to be as tight as possible. The crevice width has to be as small as possible then only that becomes an effective joint otherwise it becomes a leak right. So, that means, inherently the good joints will suffer more crevice corrosion than joints which are which are loser, but you cannot help it you have to have tight ones right. So, this is a real problem in practice. So, width is is is important one. If the you know you can also talk about other one a metal and a metal interface ok 1, 2 a metal and a non-metal interface like for example, a rubber for example, which of the two joints will suffer more corrosion. I have a metallic gasket I put otherwise I have a metallic gasket and other case I have you know other rubber gasket and more you right why why do you think so. I mean the just I mean it is right I mean rubber rubber will have problem, but that what is the reason for that? Suppose if I have to tighten this right with a metal and metal and metal and rubber what will happen from the point of view of this crevice width? Where the more you know where the width of the crevice is more metal and metal or metal and rubber? So, so the crevice dimension also depends upon what kind of joints you are going to have and metal and rubber the crevice gap is going to be small and so you would have more crevice corrosion compared to metal and metal things ok. So, the two suffers more. Let us take the other effect of that. Now, the here is the environment is the length through which where do you think the crevice corrosion here will be the more this is the crevice right this is the crevice. Where do you think the crevice corrosion will be more yeah? The middle ok. So, at least you can say that the crevice corrosion at the mouth of the crevice is not going to be there at all. Why why do you think so? The oxygen there will be reasonable amount of oxygen present there crevice mouth will passivate right. So, here you find when you passivate right why? Because the oxygen partial pressure is is is ok is good. The oxygen partial pressure decreases from the mouth to the interior and after sometime it becomes almost it not going to vary at all right. So, what will happen is a crevice mouth is going to be a cathode and all these areas are going to be now the cathode drives the corrosion of the anode right is not it? The cathode drives the corrosion of the anode. Now, what also have can happen is that if the distance between the anode and cathode is increasing what will be the effect of cathode on the anode keeping other parameter same right. Keep other parameter same ok assume that the the chemistry of the electrolyte inside this crevice is same and this is the anode and this is cathode, but if I move away from here inside what will be the effect of this cathode on the anode. The cathode drives the anode right and because there is a potential difference between the anode and the cathode right. So, that is this is going to be relatively positive and this is going to be relatively negative. Now, when I start moving from here to this what will happen to this driving force? It will decrease right. So, the corrosion rate decreases because of this ok. The corrosion rate due to cathodic influence decreases, but the oxygen constant developed oxygen concentration the oxygen concentration in this case concentration decreases right from here to this, but the potential right potential decreases. So, that means, somewhere in between you are going to have high corrosion rate after that the corrosion rate will start decreasing. So, you will find that somewhere here ok more attack because as you move away from this the effect of the cathode on the anode decreases. So, this is like galvanic cell right galvanic corrosion right. So, you will have more just below the mouth of the corrosion. So, this leads to just below the mouth of the crevice corrosion the corrosion is more. Now, that depends upon what? That depends upon the of the electrolyte. What is rho? Rho is is equal to resistivity of the electrolyte. So, very interesting one you know the resistivity is 0 then also there will be no no crevice corrosion resistivity is too high is confined to the mouth of this actually ok. So, essentially it depends upon the resistivity of the electrolyte as well. So, put it in simple terms the oxygen concentration decreases from the mouth the interior the galvanic interaction between the cathode and the anode decreases from here to this. So, there are two opposing factors somewhere in between maximum crevice corrosion occurs. So, this is an important one ok. So, that is why if you just look at the some data. So, this is a typical gasket corrosion ok and this is your stainless steel plaid this is of course, steel you know the shell here is cladded with a stainless steel. Now, it is here you have the environment right you see the attack here the attack is happening somewhere here right. It does not happen too far away does not happen very close to the it does not happen to the mouth it happens somewhere in between. So, the crevice corrosion normally occurs below the mouth of the crevice because that is optimized initiation taking place because you need a electrochemical driving force between the cathode and the anode on one hand the other hand the oxygen solubility I mean also the content of oxygen also decreases as you move from the mouth of the crevice to the center of the crevice. So, that is that is how you see that the crevice corrosion always occurs below the mouth of the crevices. I hope you understand and this one is very important actually ok. You are talking about this one or another? Yeah this is you know you know see it is a shell actually you know the shell it is not a tube it is a shell. Yeah this is this is a gasket no I mean see this is the you know in a heat exchanger you know in a heat exchanger if you just look at the heat exchanger this is called what this is called as a header shell and you have not exhaling biting here you will have of course tubes all these tubes are going over like this ok. So, in a this in a so, this is the what we are seeing here is this joint. So, this what you are seeing here is this joint it is a flange you see these are place where you put the bolts and you fasten that actually right. So, it is a it is a header that that is suffered the corrosion and ok and through this what happens now through this you have a seawater right. Now, the seawater here is flowing through the cell side the tube may be some other process liquid no it may be some hydrocarbon or whatever. So, here the the coolant is a seawater that seawater is flowing through the headers yeah and and then through the shell and the tube I think there are some process liquid of in a in a in a given refinery actually ok. So, what we are referring is this this joint ok which is a a flange joint taking place is a is a fine yeah. The corrosion has occurred because of seawater because of seawater this was collided with 2507 2 plus grades in the steels ok. You will see later that when you talk about pitting corrosion this duplex in the seal has got high resistance to pitting corrosion ok what I said and you clad it because you you cannot use a whole thickness of duplex in the seal is expensive right you you just go for a 3 millimeter clad because the basic carbon steels is cheaper for us to do that. So, that is about the about the crevice. The next important thing is the environment. When you say environment we normally refer the bulk we are not referring the environment in the crevice because the environment in the crevice is generated by the crevice by itself that depends upon the the metal you are dealing with of course, it has a bearing with the bulk electrolyte ok. But still you could have vastly different electrolyte in the crevice as compared to electrolyte composition in the bulk actually ok. So, when you look at the environment it is the oxygen content pH, chlorides present in the in this actually the temperature if you want you can add agitation or otherwise. The other related parameters what is that like diffusion convection, but all relate interrelated to each other ok and biological factors. When I say biological or fouling you can put in the word generally we can say that when you decrease the pH crevice corrosion increases. Similarly, the temperature crevice corrosion chloride crevice corrosion increases. Yeah, there may be some diffusion increasing ok, but that is not that is not going to be in common thread with a the passive film destruction the passive film would destroy. In fact, if you look at titanium let us let us take this case of titanium. Titanium in ambient seawater resistant to crevice corrosion. If rise the temperature at high temperatures titanium is prone. The reason being the chloride attacks the film the passive film and exposes the bare metal and so what happens the crevice corrosion occurs. So, we will see this the role of metallurgy in in crevice corrosion we see later how does how does the the metallurgy of the alloy can influence the crevice corrosion we can see later how I mean we will see later. At this point it is sufficient to to understand that the passive film stability depends upon the temperature in addition to the environment. Chlorides attack the passive film. Where as the temperature the putting potential drops significantly. So, the film gets damaged and so metal becomes active. So, that is going to be crevice corrosion occurring. Essentially in the crevice if you are able to make the metal passive very stable then what happens? You may have differential aeration, but still you will not get a crevice corrosion at all right. So, the crevice corrosion also depends upon you know in addition to deaeration and all, but if the metal can be made inherently resistance to corrosion then the crevice corrosion resistance can be very high also we will do that ok. That is why the metallurgy being developed actually ok. So, titanium generally is very good you see what application no problem does not pit and all, but the temperature is increased. What is what does it mean? Assume that I use an heat exchanger in a seawater, seawater is used to cool the heat exchangers right. Suppose the temperature of of the seawater is ever says 70, 80 degree Celsius or maybe 100 degree Celsius probably the titanium will also start undergoing crevice attack or the temperatures or in some situations where the the it is not necessarily seawater right. It can be you know a gas turbine operating in the naval ship or ship for example, you use titanium alloys and then if salts go and deposit the temperature of these components 120, 250 Celsius they undergo crevice corrosion. So, the crevice corrosion would also depend upon the stability of the passive film and that depends upon the environment, pH in fact, same thing if you if you increase it if you lower the pH the passive film stability comes down. So, crevice corrosion is now increasing at all. So, they are of course, interrelated to each other that that we can we can say ok. So, these are the parameters. The other important parameter in the solution is the resistivity. So, very interesting thing resistivity is a very interesting one. We just made a very passing remark earlier right look at the resistivity. Let us take the case of the you guys are all familiar with the polarization curves of if the halogen concentration is more start passivating, if the halogen concentration is little less right this is the crevice mouth and this is your crevice. Can I say that? So, what does it mean? So, this is the place where this is the place where the potential is let us say relatively negative place potential is relatively positive ok. This corresponds to this this corresponds to this correct or not? What moves the potential run here to this? It is oxygen that makes the potential to move from here to this. If I have an external cathode, if I rise the potentials can this move up or not? Assume that there is no oxygen present, if I have a external cathode I apply a voltage can this move up or not? That is what you do in anodic protection right you just move it. So, that means, if this is the cathode can this cathode make this anode to move up to this? Can it move? In principle yes right. If I have a same metal one is positive another negative, this guy will start lifting this up and this guy will start lifting this down right moving down both are possible right. If this guy has to have negative here positive here then there has to be a resistance between these two you know between these two places there has to be a resistance. The resistance is 0 or less what happened now? The anode and cathode will become almost like similar only. So, this guy also start passivating in principle possible. That means, the resistance of the electrolyte is very important. If there is no resistance, substantial resistance the crevice corrosion will not exist. One of the reasons why crevice corrosion occurs is that it happens in the low resistivity electrolyte ok. So, if that means, if the resistivity increases the crevice corrosion increases because it keeps the anode and the cathode alive all the time they do not allow them to make them one unit right not possible to happens. So, this is one of the important parameters in having the crevice corrosion. In fact, people model crevice corrosion they take this into account in terms of how the crevice corrosion occurs. So, in principle if I give a resistivity you give all this I can even find out at which place the the crevice corrosion is going to be maximum right. So, these are the things that you know that happens in in a in in typical crevice corrosion of of metals. I hope you understand these concepts here. The resistivity of the electrolyte is another important parameter in deciding the extent of crevice corrosion that metal can have. In addition to chemistry of the electrolyte we talked about like the pH the chlorides and you know in addition to that the resistivity is equally important ok. I hope you follow what I mean you are saying you have anybody has a question no problem. So, that is about the the environment and ok. Now, let us look at the the electrochemical parameter we have been just discussing now so far right. Let us look at the electrochemical parameters in the ok. Let us take this electrochemical parameters it happens in the passive system right. So, that means, you would have now I am going to draw a few schematic diagrams to show how the electrochemical parameters become very important in deciding the crevice corrosion resistance of metals ok. This is alloy one. Now, this is the outside the crevice ok. This is inside the crevice. Tell me what happens? In the case of alloy two, the passivation potential is lower not higher yeah. The passivation potential is is is you know is low it is or the the the range over which passivation occurs is much much more as compared to alloy one right. Alloy one passivates here and alloy two passivates here right. It in fact, it has it has it passivates even at a more negative potential. I want to answer from you people. Now, this is given this you know now you know the mechanism how the crevice corrosion gets initiated for example. Now, what happens the alloy one alloy two? It is not complex very simple yeah you start telling more and more I mean what what more you find here in this. You take the case of alloy one alloy two what different does it make? If you form a crevice with alloy one and you form alloy with crevice crevice two ok I mean I mean you form a crevice with alloy two for example, ok both both are formed as crevices right. So, what happens? In alloy two what happens? In alloy see this is the this is the cathodic curve within the crevice the cathodic curve outside the crevice here right. So, alloy one alloy two now you should must must be easy for you to tell how the alloy one and alloy two will behave outside the crevice and inside the crevice. Outside the crevice what happens to alloy one and alloy two both are passivating right within the crevice what happen yeah. Alloy one alloy one will remain in the we will we will go to we will go to active region right what happened alloy alloy alloy two then alloy two is still remain in the passivate state right. So, that means, why does it happen you look at this now what is why does it happen where you know this polarization because what are these things use the right terminologies yeah. So, the so the critical condensate right is a critical condensity at I c of the alloy one and I c of the alloy two it is simple right it is not even a magic. So, it is a simple one. So, that means, the the electro chemical parameter influences similar to what the Rothe has been talking about suppose if I move this the passive potential to a higher and higher you will see that the crevice corrosion tendency will keep increasing now actually or the ability to passivate at negative potential makes the alloy more stable. So, the two parameters that are important one is I c if I c increases what will happen what will happen to crevice corrosion corrosion. Increase. Increase right crevice corrosion increases similarly e-passivation increases the crevice corrosion again increases you want to make it more and more lower. So, the electro chemical parameters are are are very important in deciding whether the corrosion crevice corrosion will occur or not if it occurs or what rate they really occur. Having said that the metallurgy are related to each other right then because the metallurgy cause the composition what are the compositions the all compositions that lead to passivation makes the alloy prone to crevice corrosion. But the compositions that makes the film stable alloy resistant to corrosion and makes it you know and makes it spotlessly passive if you have all these characters then what happens then the crevice corrosion resistance increases. So, this is this general thing we can we can talk about right. So, those elements which are doing the job ok you will able to see that these things really can happen. So, there are sudden some empirical rules which talks about this ok and you say that elements like chromium, molybdenum, nitrogen in stainless steels and nickel base alloys they all help crevice corrosion resistance. I just give one example free in all these cases moly content increases ok from 1 to same is to oppose with with the chromium and and also with the nitrogen ok. They all can change the passivation behavior quite significantly and and so. So, if you look at the many alloys they are developed like you know if you compare 316, 304, 316 is better than 304 and 904L is better than 317 and 254 SMO is better than 904L. So, 654 SMO is better than 254 SMO. So, all these are happening because of modifying the composition of of the stainless steels ok in order to improve the passivation behavior and more so, the critical condensity and as well as the overall resistance of the alloy to corrosion corrosion ok. So, so we have seen now the the electrochemical parameters of of the corrosion of the metals and these electrochemical parameters are related to the metallurgical parameters of course, they are all related to the environment actually right and see for example, you take 304 and you carry out polarization studies in let us say about let us say about 500 ppm of chloride solution of pH 7 and and you determine the critical condensity passive condensity pitting potential all the stuffs and then you lower the pH from 7 to let us say about 3 you are going to have increase in critical condensity increase in passive current density and also increase in the passivation potentials and so, it also depends upon the environment as as much ok. So, as much as the alloy chemistry they are all interrelated to each other and so, so, it cannot be seen in isolation right. It has to be seen together when you talk about material selection for corvus corrosion resistance. So, if you have an idea about the the environment then you can probably predict you know what kind of materials can be used for corvus corrosion resistance for a given application. The next thing that we need to be looking at is what is called as the prevention. You can now start you know your own things right because you know the principle and mechanisms you know the factor controlling the risk corrosion. So, writing I mean finding out preventive measures are not that difficult. The first and foremost is is that you can look at the the right alloy combinations. If you cannot change the engineering you know design or if you can avoid joints such as rivets flanges etcetera. They form crevices and all that. Then what do you what do you need to do go for welding if the design promise. This is the crevice farmer right. What else form the crevices we said about fouling right. So, avoid fouling use filters and if you are going to use the mechanical joints then you are going to use gaskets it must be solid non-absorbent gaskets. See you know about you know that asbestos right. Nowadays people do not use asbestos this is this is carcinogenic right. You take asbestos ok and you take a Teflon. What is Teflon? This is a commercial name of PDFE right. What is PDFE? Fluoride ok poly etcetera. Fluoride. Fluoride ethylene right ok. So, here asbestos they absorb water moisture whatever kind of thing here it does not. You can use a sealant right. If you have a problem you can go for a sealant. If you sealant then all these gaps can be filled up. This is done in aircraft industries use a nice sealant. Well, you can keep adding many kind of variations that you can do. I do not want to spend more time on this because you should be able to devise methods by yourself. I will go to the next method is the testing and evaluation and whatever. I would recommend you to go through this article by R. M. Hain Evaluation of Kravitz Corrosion. This is an ASM International ASM Metal Sandbook Volume 13, 9th edition. It is published by ASM International right. You will also have it in your library ok. Article by it is a very nice article which talks about the complexities involved in designing the Kravitz Corrosion. Now, you need to understand the first of all the concept. What is that here? You have to form a Kravitz. When you say a Kravitz that it has its own geometry all this Kravitz gap. So, there are several variations that you get and you may choose a sample which is not flat versus the sample which is quite flat. So, do you think the results will be similar? The sample with the flat and sample to become rough sample will the Kravitz attack will be similar or different? So, where you think the Kravitz attack will be more? So, the Kravitz gap you know may be more or much more flat sample than on the rough sample ok. So, the tight Kravitz will increase the Kravitz corrosion rate and the loose Kravitz will make the rate of Kravitz reduced. I can form a Kravitz between two metals or I can form a Kravitz with a a normal like a Teflon. So, a proper designing of Kravitz corrosion setup is very important ok. The formation of Kravitz and is one thing. The second is selection of the environment and you can also look at the the standard you know ASTM G 48 talks about the it is it is talking about measuring the critical critical Kravitz temperatures. Here it talks about measurement of critical Kravitz temperatures ok and here the solution is 6 percent ferricure solution. The pH is about 1.2. Why do you take ferric chloride? You take ferric chloride because it is a good oxidizer. You can aggravate, you can initiate the corrosion process much quicker, much faster than simply immersing in say soaring chloride solution. So, ferric chloride is is a good oxidizer. It is it is an accelerated test that people do and generally the temperature they chosen are 22 plus R minus 2 degree Celsius and 50 as 2 degree Celsius and they expose for about 72 hours ok. And then they look at the weight change. So, simple test actually. What is done here? You know you you you can in principle you can you can do setup you know in in your your own labs right. What you can do is see for example, you can you can take a stainless steel you know disc like that. I can have a Teflon disc on this side, Teflon disc on the other side I can do that. I can have a a rubber band holding like that. The rubber band is tight enough to secure and it also forms a Kravitz right. So, you you know a rubber band like this ok. This is the steel and this is your Teflon disc. So, immerse in in ferric chloride solution and the temperature of your own interest ok. And measure after 72 hours see what happens if there is a corrosion or not. If there is no corrosion that means at that temperature the material is you know resistance to Kravitz corrosion. Now, they also made a little bit more sophisticated setup that is called as multi Kravitz assembly ok. Where in a single experiment you can have many Kravitz's formed. See here in this case you have only two Kravitz's right one on the right and another one the left you have, but I can also have a multi Kravitz assembly by taking a Teflon disc and I make various grooves right. I can just groove like this see this is the groove here right. So, I can make several grooves like that ok. You can make as much as this is called as this is called as a plate 2 right. So, you can have about 20 plateaus you can have 20 plateaus you can have ok. 20 plateaus means you have about 20 20 grooves that you have and what you can do you can take a sample and you can put one assembly here other assembly here. So, you will have going to have about 40 grooves that means you are going to have about 40 Kravitz's are formed in one single test right. So, each other this is the Kravitz right this is the Kravitz here it is a potential Kravitz because the liquid can go through this go through this. So, here is a Kravitz here is a Kravitz here. So, when it when it sticks on the surface ok and they form the Kravitz. So, there are about 20 Kravitz's formed one side one face of it other face of these stainless steel sample you have 20 more you have 40 grooves are formed and you can now expose it in the in the in the ferric lawyer solution ok and you can you can get the attack you know see done. This is something like in experiment done in in our own lab here by one of the PhD students you see here ok. Yes this is the disc now you can see this here these are all the attack these are the attack portions see the attack portion these are the attack portions because of the Kravitz's see the attack, attack is small and whereas, there is a groove there is no attack. So, this is a groove there is no attack this is the plateau region this attack you see here the plateau the attack is on the on or broader compared to the grooves ok. So, you see kind of see here the attack attack here there is no attack here. So, the Kravitz is taken place in this cases it was done in the in the lab and you can see this alloy is 19 chromium 18 manganese 0.699 nitrogen is you know the attack is much less you can see here you do not see much attack here 1 or 2, but 14 chromium 8 manganese is attack is quite severe and 316 the standard of attack is less now. So, that means, you can able to evaluate the Kravitz corrosion resistance of the alloy, but here see here here it just means exposed only for 24 hours not for 72 hours actually. So, this is no no modern fast food when you are going to develop an alloy, but then if you are going to follow the ASTM standard you just strictly follow the ASTM standard then only you can compare between the results obtained in the different laboratories now. So, these are the kind of things that you can see and which are very useful in determining the Kravitz corrosion resistance of these alloys, but it will be very interesting. Now, 304 stainless steels can suffer Kravitz corrosion at sub 0 temperatures can imagine the Kravitz corrosion is much more severe even compared to putting corrosion other forms of corrosion. So, sub 0 temperature means you can imagine that you know even even below the ice you have some chlorides the 304 stainless steels can suffer the Kravitz corrosion of course, the time taken for Kravitz corrosion is is going to be more actually. Now, so people use this as a criteria for the Kravitz corrosion now this is called people also use term which is called as this is called a Kravitz corrosion index. So, CCI is given as the multiplication of these two and airs corresponds to the number of site attack number of sites attacked and D is the depth of attack. So, this is a kind of index people use to quantify the Kravitz corrosion resistance of the metals actually ok. This one more I before I close it is the test is electrochemical test. You have known a potential dynamic polarization curves are obtained right in the laboratory to look at the passivation behavior of the passive metals. Now, in this case what you do is the specimen what what people who in this case in the specimen you have you have a Kravitz here right. Suppose you have a sample of here in this case it is a it is about 16 mm diameter and you have a Teflon cover this with a Teflon here PDFE right. And this is your electrode in you carry out potential dynamic polarization in the in the anodic side only not in the cathodic side anodic side do that. And if there is a Kravitz corrosion and what is done here ok to do a potential dynamic polarization and you reverse this ok and this is called as hysteresis and higher this higher the character higher is the alloy prone to Kravitz corrosion. So, what is done here is here before you do the scan you hold the sample for about 1 hour in the medium that will start establishing the Kravitz actually Kravitz corrosion right. And then scan anodic scan you do 0.6 volts per hour scan they do that and reverse the scan 5 5 milli amperes. So, so that means, this current is what this current is is is equal to 5 milli amperes ok. So, it is a log i not small i because the sample here you have taken for the dimension of sample here the sample diameter and diameter is equal to 16 millimeter right. So, so the current density is going to be different. So, this this hysteresis is an indication you compare the different alloys like alloys is to lie 276 or maybe 304. And so, you compare with that alloy and see how the newly developed alloy performs 304 of course, is not an alloy meant for Kravitz corrosion resistance C 276 is supposed to be the one of the best alloys from the Kravitz corrosion resistance point of view ok. So, this is how we can do that, but you can always have a several variations you can carry out the potential static layers to know. Basically, you have a Kravitz and and how the Kravitz affects the electrochemical parameters. So, with this have you have any questions are there? Yeah. Yeah, you see one example I had the hysteresis character right I mean you can have something like that you can have something like this ok. So, this alloy is not passivating at all look at this and look at this and look at this and this alloy is more prone to attack. We will see this little detail when you when you talk about the pitting corrosion ok. So, the area of the hysteresis you see here as increases the alloy becomes more prone to pitting corrosion. I am sorry yeah pitting corrosion as well as Kravitz corrosion anyway they are not I mean you will find later that the the alloys you know resistance to Kravitz corrosion would also be resistance to pitting corrosion as well ok. So, ok. So, with this we will close the discussion related to Kravitz corrosion and yeah let us see if you have any questions think about it. In next class when we meet we will discuss on this topic.