 Today we shall look at the another important form of corrosion called as crevice corrosion. It is also called as under deposit corrosion, sometimes called as waterline corrosion if you want to call it a gasket corrosion. All these different names are given for these forms of corrosion. So, crevice corrosion is a is a is a common name. So, it is called as also as under deposit, sometimes called as gasket corrosion ok. And is what similar to that is called as waterline corrosion. In all these cases there is something very common, it is a common mechanism. This particular type of corrosion problem is industrially very important ok. I show some examples of how the crevice corrosion can affect the performance of a component. What you have seen here is is a flange joint of an heat exchanger ok. The heat exchanger is made up of the carbon steel, you can see this. And the carbon steel inside was cladded with a duplex stainless steel. So, it is a stainless steel and it is a flange joint and use a use a use a gasket here right, use a gasket here. And the medium of exposure in this case was see water. Now, you see the corrosion right. The exposure is inside here, the attack is slightly inside. See this is inside, it is not at the mouth of this joint ok. So, these are the typical we call gasket corrosion or crevice corrosion joint corrosion problem. You have the other kind of joint you will see in the case of a heat exchanger. We probably brought out this heat exchanger in one of the previous classes. Now, you have the tube and tube sheet joint right, tube and tube sheet joint. And this location is the tube and tube sheet joint here just expanded. See please look at this a mechanical joint it is just expanded. You have a you have a tube, you have a tube ok. Surrounding that is a tube sheet you join them by expanding it this it bites actually. And there is an air line gap. Now, look at this only the place where it is joined that is undergoing corrosion. This is a stainless steel it is used in the one of the high pressure heaters in a boiler operations actually. Thermal power plants these are joints. What you see here? This is something I call as under deposit corrosion right. I hope you were able to see this spots here ok. These are the spots the fouling occurred. What does it mean by fouling occurred? Some kind of you know organic pill or products accumulated on the surface of this. When they accumulated look at this below the deposit only more corrosion. And the surrounding area there is no corrosion at all actually. Inside you can see that you know inside this T you see this no corrosion here ok. So, wherever there are deposits and below the deposits it can cause corrosion of in this case the stainless steel ok. This is a a common industrial problem ok. And it can cause premature failure of the industrial component actually it can happen. And I just want to introduce a new book and if you people you know can have a look at that. This book is an Introduction to Metallic Corrosion by U R U Anse. I think you know all the name is very familiar to you right. The Evans Diagrams. And it is published by Edward Arnold 1981 Great Britain is actually it is a third edition. I just introducing this book to you because you will see quite a bit of industrial illustrations importance. Not much on thermodynamics and kinetics the way other books are dealing with. But it is a nice illustration exposures you will see. There are some typical experiments in the laboratory to simulate the various forms of corrosion. So, when you people have time I think you can you can look at this book actually. I will give you some nice illustrations as given in this book to show how this corrosion can occur. If you take a take a lead take a lead you immerse it in a in potassium chloride solution and you place a lenticular lenticular glass like this. Keep a lenticular glass like this of course, in the glass you do not find liquid ok. You allow the lead to corrode for some time and then you remove this the if you remove this glass and observe the corrosion of the lead. You notice somewhat similar to this here no corrosion taking place and surrounding this region corrosive attack. And here onwards all you can case is you can see the passivity. There is a corrosive attack and also you see you know over this place also see take lead dioxide. It is formed. This is the plane view the plane view of the lead surface. You take a cross sectional view of this same thing you take a cross sectional view of this sectional view right. Now, course here you will see quite a bit of a a dioxide. Here it is not corroded this is your uncorroded area is uncorroded area and this area all passivated area. So, it is very interesting observation right where the the lenticular glass touches the lead quite you know I mean it touches I mean is the contact is so good it replaces the liquid completely. So, the no corrosion between the liquid and the metal. So, it does not corrode, but slightly away from it there is a airland gap and in the airland gap I think corrosion takes place ok. And this this is the this are all the corroded areas area you see that. So, there is a very very fine gap and there is a very fine gap that exists between the lenticular glass on the lead over there is a corrosion and the gap is quite large there is no corrosion. And if the glass touches and replaces the potassium chloride solution completely of course, there is no corrosion, but in between this is the place where the airland gap the corrosion takes place. So, airland gap is is a critical issue in corrosion of the lead here. This one more illustration is very nice ok. In another case you can take a steel take a steel and you have a drop of potassium chloride solution right you have a drop of potassium chloride solution with water. And to this you add what is called as peroxide indicator. And what is a peroxide indicator? The peroxide indicator is prepared by mixing phenophthalene with potassium ferricyanide. So, the solution consists of consists of what? It consists of a dilute solution of potassium chloride and the peroxide indicator and it consists of phenophthalene and potassium ferrocyanide. The very interesting thing happened in this case and if one watches closely you will see that for a time period here you will you will notice the blue coloration and here this location this location you will see the pink coloration pink color ok. So, this is a pink color here pink and you have a pink color. Anybody with a with a chemistry background you will understand this. When you have a phenophthalene and you have a pink color what does it really means the time. When you get pink color in a solution containing phenophthalene exist yeah this is acidic this is alkaline. So, it is alkaline. So, when you have alkaline condition only you get phenophthalene giving rise to pink color. So, the edges in the periphery of the on the droplet completely ok. If I if I take a if I take a a plane view of that ok. If I see this as a plane view the droplet you will see the blue color and this is your pink color ok. The pink happens because you have hydroxides and blue happens because you have a few to plus ions. If you have few to plus ions the interact with the ferric cyanide potassium ferric cyanide you get a blue color and you have a hydroxide then what happens you will get a pink color. How is that possible? How it happens? It happens because this area becomes a cathode. What happened to this area this area becomes you take a steel you put in sodium chloride or potassium chloride solution the anode is where you have an oxidation in this case a steel will get converted into Fe 2 plus and the electrons are released the electrons are consumed by a cathodic reaction. If the water is little you know neutral or little alkaline what is the cathodic reaction the cathodic reaction is going to be or the cathodic reaction here it could be oxygen plus electron 2 H 2 O even rise to 4 O H minus. So, it becomes alkaline here you have metal getting dissolved here. So, what does it mean? Here the ion is going out as Fe 2 plus the electrons are released here they go to this place you have a cathodic reaction here here you are going to have cathodic reaction on the center you have anodic reaction. The reason is very simple what is involved in the cathodic reaction? What is the species involved in the cathodic reaction? Yeah, it is the oxygen present here. So, the oxygen so, oxygen is a cathodic reaction in the peripheral or the periphery of the drop you know you have more access for oxygen there oxygen will enter from the air and go to the periphery and you start moving from the periphery to the center the oxygen concentration decreases. So, most of the cathodic reaction is confined to the periphery and the anodic reaction automatically is centered around the drop. So, you find that in a droplet you have separate anode and separate cathode spatially. So, it is not a uniform corrosion anymore it is a localized corrosion it is essentially this is this is in fact, called as differential aeration corrosion the the oxygen concentration at the center and at the periphery is different. So, this leads to the other form of the corrosion called as water line corrosion. So, concentration cells so, the key issue here is the concentration cell what is the cell formed? The cell formed because of the change in the oxygen part cell pressure in the electrolyte. So, that is a key for the the localized corrosion. This can be very well demonstrated actually ok it can be very well demonstrated. Let us look at the typical stainless steel which exhibits active passive trans passive behavior ok. Let us just take a anodic polarization curve like this this is your you know a schematic of anodic polarization curve showing active passive trans passive transition. The e-car for this particular system would depend upon the cathodic reaction. Assume that the the steel is exposed to water which is a neutral water. The major cathodic reaction will be what? The oxygen reaction reaction. I consider water containing large amount of dissolved oxygen the water containing other case less amount oxygen there. So, I am going to draw the cathodic curves case 1 and case 2. One case the oxygen content is more other case oxygen content is less. Can you tell me which of the two has lower oxygen content? Case? Case 2 how do you say that? Yeah right, but how do you say that? What is the basis for that? Why does it pass away really? Why should a need cross i critical? Yeah if the oxygen content is less what does really happen? What is related to actually your oxygen is less the amount of oxidation amount of reduction may be less. But can you be a little bit more specifically you have seen some relation before? Yeah. So, the limiting current density is a factor that talks about right isn't it? You all know that limiting current density is you know you know the concentration of the bulk of the species it is that ok. So, when the concentration of oxygen is less the limiting current density less and so, you will get e car of corresponding to this and you get e car corresponding to this depending upon the oxygen content ok. Of course, we all know that when there is more oxygen content if more passivation takes place all these things ok. So, on a metal surface coming back to the point on a single metal surface you know and one place you have more oxygen content another place less oxygen content the place having less oxygen content would not be tending to passive. So, one place where having high oxygen content will tend to become passive actually because because of what? Because the you need a sudden you know current density to cross the i c value which is i critical you need to do that. So, this also was demonstrated by another nice simple experiment actually you know. What has what was done was if you take a semi permeable membrane ok, the permeates for oxygen there and take this electrolyte suppose you suppose you take a stainless steel take stainless steel and you close this, but what you do you bubble one side with the oxygen there another side you bubble with nitrogen right. So, this is electrode 1 electrode 2 ok electrode 1 and electrode 2 take care. And also you can connect this with an ammeter you connect with the ammeter there ok. So, what will happen now? You have oxygen here and so, what what do you think will happen? Will that be a potential difference between E 1 and E 2 ok. So, what happens if there is a potential difference and you short that what will happen? The current will flow. So, how does the current will flow here tell me current will flow from E 2 to E 1 or E 1 to E 2? The current will flow from E 2 to E 1 right. And so, the E 1 is going to act as a is going to act as a cathode in current flows agreed as these things. So, what do you understand from here? If in a metal see the potential of the of the the the E equilibrium potential for the for the oxygen system ok. What is this this one E equal to E 0 plus 0.059 upon 4 here right and the partial pressure of oxygen ok upon what? The partial pressure of OH minus to the power 4 yeah yeah thank you I think you must have locked here locked here ok. So, when the oxygen partial pressure increases the E increases tend to be more noble. And so, it sets up what is called as concentration cell that leads to corrosion of metals. Now, this is only one reason for crevice corrosion, only one reason for gasker corrosion, but you will see that that is not the only reason for that ok. You will see the environment plays a very important role we will we will talk about soon. But the main initiative comes from the concentration cell because the concentration cells make one place anode then other place as as a cathode and making as you know localize anode and localize cathodes at macroscopic level. Please look at macroscopy why called macroscopy it is a gasket they are few millimeters apart it is not microscopic like the microstructures you see the we see the situations. These are as I told you important problem because you know you know there are you know you have a rivet joints for example, right you join the metal with the rivets and flanges sometime hem joints and these joints would have some gap ok. So, the like you have rivets have flanges you have in fact threaded bolts and deposits you know the idea is you must have an air line gap. This gap is sufficient for the liquid to penetrate insufficient for convection within this gap there is only a diffusion process ok. So, there is only a diffusion of species. So, what does it mean? If the gap becomes more suppose see these these gaps are all in in the range of few microns may be about the say 4 to 10 micrometers is the the crevice depth crevice width I would say width of the crevice. If you are going to be a few millimeters what happens then the oxygen can migrate much easily through convection process though they do not form a cell at all due to concentration difference because it does not exist. So, one of the requirement for crevice corrosion is there should be a gap and that gap has to be very small and there should be no diffusion taking place in the system ok. So, that is the basic requirement for a crevice corrosion. See what are geometry what are fabrication that you do? See there are cases where people weld it you know welding right you talk about a TIG welding. In the TIG welding if the fusion is not very good if there are air line cracks the air line cracks can become a crevice corrosion it is not necessarily you have an external joints like that ok. So, that kind of things you know can lead to crevice kind of attack in very many industrial component. Suppose I have a oxide scale I did not remove oxide scale I make it loose can lead to form a crevice corrosion taking place on the metal. But nevertheless you need to understand one important thing for crevice corrosion to occur the metal has to be passive. Please understand this why? If the metal is not passive and you visualize the situation the crevice corrosion may not occur why may not occur? I have same gap the gap is same, but it is not a passive metal like a carbon steel suppose I take little more acidic environment I take it and say there is no crevice corrosion crevice corrosion more often occurs in stainless steels you know where the passivation is predominant. Why does not occur when the metal is not passivating? Yeah see when you when you when you when you when it does not passivate increase in oxygen content will only increase the corrosion rate. A decrease in oxygen content will only decrease the corrosion rate is not it? Is it correct or not? In a carbon steel you know where it does not passivating when you increase oxygen content what will happen to corrosion rate? It will not decrease it will only increase because the increase in oxygen content always rises the electrode potential that is for sure right no matter what the metal is that is stainless steel or a steel the e-car will always go up, but in the case of stainless steel when the e-car go up it goes up there is a chance that it passivates, but in a carbon steel when the e-car is going up then what happens? The corrosion to only we increase it is not going to decrease at all. So, the the crevice corrosion is more applicable to the passive metal, aluminum alloys yes true it has happened magnesium alloys crevice corrosion can occur ok. So, crevice corrosion also is has to be metal has to be passive in order to form the you know the crevice corrosion. So, we have looked at the broad contours of crevice corrosion. Let us look at the mechanism of crevice corrosion. You can broadly say that the crevice corrosion involves one is the initiation the two is growth initiation of the crevice and growth of the crevice. And the initiation of crevice is not so well understood as the growth of the crevice corrosion. Now, let me try to explain to you with some schematic what are the things involved in the crevice corrosion of stainless steels initiation process. Let us let us look at some thought experiments wherein I take a metal I take a metal which is a stainless steel it forms a passive film. Please notice this passive film is thin is not it we we have been talking about the passive film thickness in the order of angstamps about 40 angstamps to 100 angstamps like that. They form a passive film and it is exposed to the to the environment and then you have exposed to the air. So, when a stainless steel is exposed to the air and environment what do you expect? You will expect that to be passivated right it is supposed to be passivating. So, it is passivating now. If one measures the potential at say the three locations you measure the potential of this ok. E you measure the potential of this it will be in passive range or active range what do you normally expect stainless steels nicely aerated condition environment will be spontaneously passivating. So, the potential will be generally in the passive range may be something like 3 volt something I am just giving some approximate number there is nothing you know unique about this numbers. If I measure the pH of this it will be neutral or slightly alkaline may be let us say about 7.5 7.5 right. So, that is the pH of the solution at location 1, location 2 and location 3 you get these things. Now, what we do over this you keep a nice flat glass plate ok. Let us look at what happens now ok. Let us now take a this is a stainless steel and I will just place a small glass plate over this it is a water. So, I measure the E value and the pH value at time t equal to 0 right at time t equal to 0 I measure these values just after placing. So, what would happen? It will almost show the same values right it will show right. Now, the corrosion reaction will occur see when the metal is passivating does not mean there is no corrosion only thing rate of corrosion is less the anodic reaction rate is less. So, there is a corrosion. So, what will happen now? When there is a corrosion metal get oxidized the reducing species gets reduced right like in this case you may add oxygen present they get reduced. The oxygen consumption over here continues, but what happens to the replacing of this? It does not get replenished because of crevice the oxygen content with the time decreases over here. Whereas, the oxygen content over location here and location here remains in the steady state it does not change at all whereas, the oxygen content here decreases. So, what will happen to the potential here? The potential will drop with the time. So, at time t equal to some x some x value ok. Now, what happens now here? Here this may become still 0.3 volts and this may become let us say 0 point say minus 0.3 volts and this is the pH may still remain the same it may remain the same. So, what happens now in this case? Now, the potential over here drops it goes relatively to a more negative potential and this is remain the same. So, this now tend to become an anode and this becomes now what? The cathode now earlier the almost the same. So, what happens now with the next thing that can happen is ok is t equal to y and e would this n will remain the same here may become little bit maybe a little bit more negative. Now, what happens now over here the iron gets suppose I blow this area I magnify this I magnify this what happens iron go as plus square electrons right and this electron from here go to this go over here all right. And here the auxin will tend to become OH minus here the auxin will tend to become. So, it establishes a cathode here the cathode here and anode here. Now, what can happen? The one more thing can happen here what is that Fe 2 plus can combine with water can form ferricide oxide and 2 plus this is possible to happen case. So, this place tend to become more acidic and tend to become more positive. So, what happens now it becomes acidic and also electrically what electrically positive am I right? You have more positive charges on this on this on this location as opposed to that with the outside as relatively more every charges right. So, what will happen to pH now? The pH it might remain almost the same outside it remains almost the same or it can become maybe it can become maybe nine something like that because the pH is increasing because of a cathodic reaction here the pH may go less about the pH can be about 4 can be pH can be about 4 that becomes relatively acidic. Please notice what is happening that it is electrically positive. So, in order to neutralize this location what will happen? The negative ions from the surrounding areas like chloride it migrates to the to the chloride bulk chloride it goes to the crevice chloride it goes to the the crevice chloride. So, what happens now? The H plus Cl minus can form hydrochloric acid agreed understood. So, the initiation part if I summarize the initiation. So, if I summarize the initiation initiation leads to local anode and local cathode. It leads to drop in pH on the anode and say small rise in pH ok. If chlorides are present the chlorides are present they migrate to anode which is a crevice ok. Then what happens? Then is going to be formation of acid leads to film breakdown etcetera. So, that is in a active dissolution. So, let us now take. So, we have seen that the initiation process is is driven by the oxygen partial pressures and then is accelerated by the presence of aggressive ions such as the chlorides. How does the growth occur ok? How does the growth occur? Let us look at the situation where the steel glass plate and the passive film and here what happens the metal ions M the electrons travel here and what happens? The chlorides migrate to this place chlorides at this place you have H plus over here right. Now, look at this situation that this place is now called as occluded cell it is confined is fully diffusion controlled. So, no convection taking place ok. Now, it becomes an autocatalytic process. Please notice that the anode sorry the anode over here and the cathode they remain with the time we said in the uniform corrosion the anode and cathode will will alternate. The time this remains anode this becomes a because the cathode and they establish a distinct potential anodic potential and cathodic potentials. If you would expect this cathode to polarize this anode so that this become passivation right isn't it? Now, can you see that we saw the galvanic corrosion before right you have two different metals with the different potentials what happens the galvanically is shorted the anode will tend towards the cathode and cathode will tend towards anode. So, you it can drive it can move in a crevice that does not happen because the resistance between the cathode and the anode would remain it does not allow the anode to ever become a cathode. So, in the crevice there is a resistance for ionic I am talking about ionic resistance. The the electronic conductivity is very good in the metal no problem, but there is an ionic resistance taking place and so the anode the anode and the cathode would remain on the time ok. So, one of the conditions of the crevice corrosion is that it occurs more in the highly resistive corrosive environment. When resistance is more in the electrolyte what happens the anode and cathode are well separated they do not become a single entity at all. So, that is a property of the environment that will promote the crevice corrosion as we see later. So, the the growth of a crevice people have been modeling quite a bit on that actually ok and if this is a very important problem for the naval applications in fact, even the aircraft applications where the aluminum alloys undergo crevice corrosion at the joints and all the steps ok ok. So, a lot of work has gone into understanding of crevice corrosion and modeling the crevice corrosion kinetics of metals ok. So, this you know completes our discussion related to the mechanism of crevice corrosion. Any of you have any questions we can we can discuss ok. If it is not there I will complete another similar interesting thing about the water line corrosion ok. Here some inhibitors may cause, but overall the inhibitors lower the corrosion rate of the metals right. Does the increase in the conductivity of the electrolyte cause an increase in the uniform corrosion? Uniform corrosion will always less when you increase the resistivity of the any environment. When the conductivity is increased the uniform corrosion will will increase. But when the conductivity of the electrolyte is is is lowered it promotes more of uniform corrosion more of yeah more of the localized corrosion than the uniform corrosion of metals. Does inhibitor change the conductivity of the electrolyte? Of course, there are you see the inhibitors does not reduce or alter the conductivity of the electrolytes generally generally you can say. But it can you know there are different mechanisms you saw before right. You saw that it can form a film on the surface. So, it covers if there is an anodic inhibitor you can very well go and absorb onto the anodic site and it can bring down the corrosion rate on the systems. So, the inhibitors overall will bring down the all forms of corrosion actually. In fact, you you will see you know even you can bring down even pitting corrosion crevice corrosion and erosion corrosion. Basically, but then it may not be so effective right. It will bring down the uniform corrosion much much effectively as compared to crevice corrosion. But yeah it could reduce the crevice corrosion to certain degree can do that. In carbon steels is it possible that crevice corrosion starts occurring because of the presence of airline gaps? No you see you are you are talking about the crevice corrosion with respect to carbon steel okay. So, the carbon steel as it you know as I could say that if the oxygen content is is quite quite large okay and you you will always see that the corrosion rate is increasing only okay. And in fact, that is the reason why wherever the carbon steel is involved you always lower the oxygen content there possible where is in a boil water treatment or in a cases okay. So, the the crevice corrosion in fact, the crevice corrosion of carbon steels never exist. If you can exist probably it can happen in a case where you are tending to make these the carbon steel passive by increasing the pH. If you look at the poor bed diagram the stainless steel I mean the carbon steel also can passivate in the range of about 12 to 4.5 and all like that okay. There is possible but in fact, the problem with the carbon steel is that if you add the oxygen content little bit more start a pitting actually you know it does not get into get into that you know because the breakdown of these oxides films on an iron is so easy okay. So, you slightly increase the potential beyond certain values it can break down and lead to pitting. So, carbon steels they do not undergo any crevice corrosion that is why whenever you have chlorides we simply do not have any problems. Whereas, when you have some fasteners such kind of things used for marine environmental conditions stainless steel stainless steels of most of the varieties they face a what is called as the crevice corrosion problem primarily because they passivate and the chlorides can destroy the passive film you know quite effectively. Now, as you noticed in the mechanism of initiation and growth now the growth of the crevice would much depend upon the environment assume that there is no chloride present in the environment okay. So, the growth of the crevice is almost negligible okay. So, you may have a concentration cell setup and make the one place anode where the oxygen content is low other place where the oxygen content is higher you may have have a cathode okay it does happen but the growth of the crevice is is is much slow. So, that means, the time taken for the failure of the component would depend upon the chloride concentrations like if the the chloride concentration of certain environments. So, let us take let us say water some water may have 100 ppm of chloride and some cases you can have 1000 ppm of chlorides okay. You will see later that 316 may not undergo crevice corrosion in water containing 1000 ppm of chlorides whereas, the 304 stainless steel might undergo crevice corrosion in in water containing 1000 ppm of chlorides, but in 100 ppm yeah none of them both of them they do not undergo crevice corrosion. Even though there may be a oxygen cells are formed at all right, but to sustain this okay you also have to have a aggressive environment okay is is important for that okay. So, yes the is primarily driven by the oxygen partial pressures difference between two locations, but is even more important to see the how I know the what kind of environment that you deal with that decides what the life is and that decides also the selection of materials for a given applications you know as it is not increasing chloride more and more you are going to choose a better and better materials to resist the crevice corrosion. So, oxygen is one one part of it, but aggressiveness of the environment is is equally important. So, then we will stop the discussion for today and we will talk about the the so called you know water line corrosion okay see what does this the water line corrosion really means actually okay. So, we will discuss in the in the next class okay. So, thank you and