 discussing the topic ofputting corrosion. In the previous class, we discussedthe the fundamental or the basic mechanism of putting corrosion. We saw that the putting corrosion essentially occurs in what metals metals exhibiting the exacerbation. That is a important criteria. The second criteria is that the putting corrosion occurs in selective environments actually. You must have a species that should induce putting corrosion. This is the second important factors. The coming to mechanism, we said that the the putting corrosion mechanism can be considered as pit initiation, metastable pitting and then a stable pit growth. The understanding of the pitting initiation is not stillwell understood process. We saw that the factors that cause the pit initiation or you say the applied potentials which will break the passive film. The breakage of the passive film is assisted by the environment like chlorides and all kind of thing. Andbut then when you you start initiating that when the film breaks down occurs, it is not guaranteed that wherever the film has broken will lead to ultimate pitting. There will be alwaysdamage of the film, the repassivation of the film, they occur in a dynamic manner actually that we called as a metastable pitting. Of course, there is a damage to the metal. If you observe microscopically, there are submicroscopic or microscopic level pits are occurring, but these pits not necessarily stabilize. This we looked at the stabilization criteria. The stabilization criteria for the pitting is what is that the pit has to have its own chemical environment. That chemical environment of ferrocidic must have also rich in the the damage causing species such as the chlorides. So, whether the pit will be stable or not depends upon how good you may able to stabilize the environment in the in the pit. Outside the pit, you are going to have a normal chemistry what is stored pit actually. Within the pit, it is an occluded cell. Nowthere are dynamic processes, the metal dissolves, it releases cations actually, they go from the pit surface to the pit. From the pit, the metal ions migrate outside the pit, goes outside the mouth of the pit right. If they stay within the pit, they hydrolyze giving rise to what is called as H plus ions. That also leads to the migration of the chloride ions because of maintaining the charge neutrality. H plus means more positive. So, the cation hydrolysis ok which decides the pH of the environment. So, there are two processes, the dissolution process and the migration process. That is why we said the criteria for pit stability is radius multiplied by the i. That is the criteria that the pit could be stable or not. Now, of course, we can go in deeper in studying the pitting mechanisms and you know various factors. I think since it is a first course on corrosion, we will not go into depth on the mechanisms. Now, what we are going to look at today? Two aspects. One is the pitting characteristics and the factors that control pitting corrosion of the metals. What are the governing factors? The second aspect of that is how do we control the pitting corrosion and how do you evaluate at the test a material for pitting corrosion? With that I think we will be completing the discussion on pitting corrosion of metals. So, let us look at the when you call a pit, you have uneven corrosion occurring on the surface right. In fact, most uniform corrosion that you see or they are uneven, you can visually see there are rough surfaces. You see industries when you go you know the steel surfaces are covered with oxides you know ferric oxide ok. Remove it you see surfaces are undulation. Now, people mistake it as pitting corrosion. Now, you should be able to define when you call that surface undulation as the pitting. The one of the criteria is called as a pitting factor. The pitting factor is is given by ok and before I do do this I draw this diagram of corrosion leads to pitting. This is a pit you see here right and there is also uniform corrosion occurring on the metal. This is the d average corrosion, uniform corrosion you may call it. This is is the depth over which the pit has occurred d pit depth. Now, the pitting factor is given as d upon d is a pitting factor. So, it should be greater than 1 then you call a pitting. If it is not greater than 1 you do not call it as a pitting. The other way of defining the pitting is it should be a pit upon radius. When I say radius it is a it is a gross radius you are not going to have a perfect circle perfect hemispherical this thing ok. It should be you know either some people define it you know it should be greater than 1, some people call it as 2. So, you have a very shallow kind of thing you know then it is not pit. Now, this makes sense right. If the pit radius is very large and the depth of pit is very small what happens? Can you connect it to the mechanism? It is not a crevice corrosion. The migration becomes much easier right. It is is no more diffusion controlled and you can subject it to convection right. If it is so shallow the metal will just get out of the pit. It will not sustain a pit chemistry right. So, these two sustain a pit chemistry. So, that the pit becomes very stable. The people also have been following the pitting kinetics of various metals and some people have come to this kind of relationship. The pit depth is related to k t to the cube root that this ok and d means the depth of the pit right and k depends upon what k depends upon the environment. Please understand this is related to growth data. It is not related to pit initiation. The pit initiation may take weeks, it may take a month, it may take an year whatever, but this only talks about the pit growth. It does not talk about pit initiation. What is t here? The t corresponds to the time of exposure. It is necessary to understand the pit is not only one shape, you can have a complex shapes, you can have a different morphology of the pits ok. So, it is necessary to to recognize that actually. So, we see that you know there can be different morphologies of the pit. And if you want to know more details you can look at this this standard ASTM G46. It talks about identification and examination of the pits. It talks about various pit morphologies. I just reproduce some of them here schematically right. And let us say you will have is called narrow, you can have elliptical pit, you can also have wide shallow pit. You can have pits which are subsurface. You see here it just is just undercuts some, just undercuts, just subsurface pits. It grows like like a internally it grows like that you know. And there are some let me see if I have some photo here. There are some examples of how it goes. I suppose you will able to see this, this is a pit here. And pit has grown sideways and you know from the bottom start corroding up. And so, you see the pits here very fine pits. So, you see a small corrosion and there is a lateral corrosion taking place right. And from the bottom it it just starts you know corroding. And ultimately leading to you know leading to this kind of morphologies ok. You see this kind of things could happen in the in the in the systems you know. You also see a similar things here. This is all corroded and then lifted. See this is above to lift here, above to lift, just above to lift here. So, these are the portions. You know I hope you are able to see this here. You look at this ok. And it is corroded from the bottom and moves up and then it just goes away. So, you can have a very complicated pit morphologies. You can also have sometimes very strange undercutting something like that you know. Sometimes you know sometimes it follows the microstructures. If suppose you have some jazz follows all kind of things microstructurally dependent. It just takes the takes the shape of microstructures. I think I have shown this in in the beginning of the in the earlier class. Let me just see if I can picture that slide. If you see here right, these pits are all taking the morphology of this is a composite, silicon carbide composite. And so, the shape of the composites taken here. So, it can take all kinds of complex shapes. So, you can have a nice hemispherical pits and you can have complex shaped pits depending upon the alloy. Because alloy is not homogeneous all the time. It can be chemically it could be heterogeneous. And so, there can be you know uneven type of corrosion morphologies for pits. Now, let us look at the the factors that control the pitting corrosion. Because if you understand the factors controlling pitting corrosion, then you can able to know prevent pitting corrosion. Broadly, I would like to say that there are two two kind of factors one related to the to the to the environment. The second one related more to the metallurgy. Let us look at these factors briefly and let us look at these factors briefly. Let us look at the environment. You have seen the condition is you should passivate right, you should have aggressive ions. Mostly the halide ions are considered to be aggressive. What are these halide ions? Anybody remember? What are they? They are chloride, chloride, bromides. When it comes to stainless steels the most aggressive is chloride, next is bromide. And the fluoride and iodide they are not really that aggressive. Now, you know I mean you can say kind of broad I would say I think. So, I mean broadly speaking I would say ok, not really actually one can verify all these parameters. Fluoride see if you have passive film, if the passive film has to get damaged by these ions, ions have to migrate in the passive films right. They have to migrate and then form a complex or this possible. The migration depends upon what the radius of the ions. If the radius of ions is too big then migration becomes very difficult. So, if you if you take it that way fluoride is ionic radius should be smallest as compared to chloride and bromide and iodide. But even then the fluoride does not cause severe pitting as compared to chloride for two reasons. One the fluoride forms complex and so, promote more uniform dissolution. The second reason is the fluoride forms strong hydrated layers. See these ions in water are not very free they are hydrated. The water molecules you know I mean these ions are enveloped by the water molecules even you take H plus ion they are not free of. H plus ions are covered with what? Covered with water molecules. Why? Water is a polar, polar molecule right has got a positive charge and iodide charge and so, they get enveloped. So, when you have a fluoride it is getting hydrated very strongly hydrated then you do not allow the water to come out and free the fluoride ions because to remove the water molecule from the fluoride ions are more difficult because the charge density is more on fluoride as compared to the chloride. So, the two reasons make the fluorides you know less aggressive towards pitting as compared to the chlorides and iodide is the largest radius. So, it does not pitting does not cause pitting or other actually ok. Now, there is there is a kind of empirical or thumb rule that the pitting tendency is related logarithmically related to the concentration of the aggressive ions. Something like you can say E pit is equal to E minus A log chloride something considered. So, the E pit will will decrease if you increase the chloride ion concentrations A this factor depends upon the nature of the the the aggressive ions. So, higher the concentration of chlorides more susceptible the right towards pitting. It also depends upon the pH ok, lower the pH it also you know what happens now the it is it is easy for the pit to stabilize an acidic pH right. You have you have H plus ions in the solution. You also have one more thing that is the nature of the cations. Suppose I add cupric ions I add ferric chloride ions for example, ferric ions. So, what do you think will happen to pitting? When add a ferric chloride let us say I have taken sand steel and immersed in soaring chloride solution and and maybe it has taken 6 months to pit right 304. When I add a ferric chloride what do you think will happen? Will it will the will the time for pitting will increase or decrease? It do not change at all. Can you remember sometime back I gave a very an empirical you know relationship that he pit is a function of what function of chlorides, time and of course, applied potentials right. The pitting tendency I said that these are function of applied potentials concentration of the species that time of immersion right. Now, when I add a ferric chloride what happens? What do you think will happen? Any idea? What is the nature of ferric chloride? What is why why have chosen ferric chloride? It is oxygen species right the standard potential of of ferric and ferrous ions equilibrium is is quite positive right. So, when I add these ions what happens? What will happen to the corrosion potential of of sand steel inserting chloride solution? What will happen? It goes up. It goes up right. So, when the potential goes up so, what will happen to tendency of pitting? Increases right. So, when you add these ferric ions these are oxidizes so, they promote pitting because they are all oxidizes. On the other hand suppose I add let us say you know chlorine chloride or nickel chloride you know they are not really going to change the potential of the the electro potential of the corroding metal they may not have very significant effect on pitting corrosion. So, what about the temperatures? What do you think will happen to pitting corrosion? When the temperature increases so, what will happen to pitting corrosion? Diffusion will become positive more can happen actually that is only one aspect of it what also happened to the dissolution of the metal ion or the at the pit increase right. So, it is going to increase. So, when rise the temperature when the temperature is increased pitting tendency also increases. In fact, they also make some of the salts you know formation and the salts and then it stabilizes the the lower pH and stabilizes the chlorides. So, rise in temperature is always detrimental towards pitting corrosion. Now, some of these ions say chromates, silicates, molybdates and even add even hydroxides a inhibit pitting corrosion. In fact, you know for aluminum alloys you know to to to improve the pitting tendency they they give conversion coatings you know they give conversion coating with chromates ok. The chromates indeed promote resistance against pitting corrosion of aluminum alloys. Let us look at the metallurgy. Here we are not going to look at a host of materials you know say titanium undergo pitting and almost all passive metals will undergo pitting given the conditions. So, we will just confine mostly towards stainless steels very broadly used actually ok. So, let us look at the the metallurgical aspect of it. When I say metallurgy we say the composition one aspect, two the microstructure. You can also talk about the surface nature is not really metallurgy that is that way surface nature. Now, let us take the case of stainless steels and discuss these aspects in details because stainless steels are very widely used and they are prone to pitting corrosion. What are the constituents of stainless steel anybody? Of course, base alloy is iron you have nickel, you have chromium, you have molybdenum sometimes people add nitrogen, they add tungsten, they add copper you know and sometimes they can add manganese kind of stuffs ok silicon. So, let us look at how these these alloying elements affect the pitting pitting tendency of the metal. Now, the pitting tendency of the metal you could define by some electrochemical parameters right. What are the parameters anybody recollect? What are the parameters that you can use to measure the relative resistance of an alloy to pitting? What are the parameters? What is the yeah please what is that? Pitting potential. Pitting, pitting potentials and other one is called as repassivation potentials. Now, let us look at this how various alloying elements affect this these these two properties. So, schematically this is the forward scan, backward scan right. From the corrosion potential you rise the potential you get what is this called? This is called as what do you call this current density? I see I critical current density. Now, what is this called? I passive. Now, what is this one? A pit and this is called as E repassivation potentials. Now, let us look at the elements that can affect this. The elements that move up the pit are going to improve the pitting tendency pitting resistance of the stainless steels. So, what are these elements? These elements are chromium, molybdenum, nitrogen, tungsten, silicon, vanadium and nickel. These elements they increase the pitting potentials. So, when you increase the pitting potential that means, the initiation tendency of the metal to pit is reduced. What about this? If it goes up it is good. This is called repassivation potential. If it goes up these things you have molybdenum, nitrogen, tungsten and chromium. Of course, you want to know about the passive current density if it wants this to be reduced or not. The passive current density is reduced these elements chromium, nickel, tungsten and nitrogen to a certain extent. And you want to know about this one. What is this potential called? E primary passive potential. What are the factors that affect this? And you see nickel is not good it increases and so as copper increases. Which one decreases? I see now these elements like chromium, nickel, vanadium, moly, nitrogen. So, in fact, this forms a basis for development of various types of stainless steels right which are resistance to corrosion in general right fitting corrosion in particular crevice corrosion in particular ok. And this forms the basis of that. Of course, you know you cannot keep adding the way you want you know suppose I take a austenitic stainless steel I want to increase the pitting resistance I cannot keep adding so much of chromium right. If you add it what happens? The austenitic phase will turn into a ferritic phase ok. So, then you need to make a phase balance. So, you need a nickel to it right. So, the development is a different story altogether right. But the corrosion resistance story depends on this diagram ok. And in order to get different phases you compensate with the relevant alloying elements ok. That is a different story of physical metallurgy of stainless steels we will not be discussing over here. Now, if you look at here if you look at here there is a that is also an empirical equation and to to to correlate alloy chemistry to pitting corrosion that is using p or En is is the is is the is an index which is given as percentage of chromium plus 3.3 percent of molybdenum 10.5 of tungsten plus 16 percent of nitrogen. What is p or En? It is called as pitting resistant equivalent number ok. And you can say that if p or En increases pitting resistance increases. So, for applications involving large amount of chlorides let us see what are applications the stainless steels with higher p or En are being developed some of them in fact are in use ok. Now, I give some some example here. I hope you can able to see this chart here. It is this represents various austenitic grade stainless steels you can use this kind of things for even ferritic grade also you can do that. What is given here is the p or En number you know starting from let us say 15 right it is going up to let us say 16 ok. And see the stainless steels have been developed from starting from here up to this right is called as 654 SMO ok. This is this is called as 6 molybdenum you know is this higher amount of molybdenum is present here ok. Now, you cannot add a high amount of molybdenum because it becomes a predict stabilizer. So, people add more amount of nickel to that actually ok. So, this is also you have 254 SMO this 254 SMO ok this and you know 25 chromium we have actually ok. And so, you find that and you know so, the various kinds of stainless steels are being developed in order to have higher repeating resistance which means they do have higher PRN number. But the cost of this also going to go high right you have more nickel more molybdenum it is really increasing its cost of it. So, that is one of the problems of using austenitic grade stainless steels. So, what happened what happened was that ok there is a variation of this people use what is called as a duplex grade stainless steels. It is it is it is somewhere in between you know nickel based alloys and ok nickel based alloys also have very high putting resistance. Now, the cost of the duplex stainless steels with high you know at high PRN number or lower compared to the cost of austenitic stainless steels at high PRN number. The reason is very simple the nickel content in duplex stainless steels are all going to be mostly lower compared to nickel content in the austenitic grade stainless steels. So, again there is a gradual growth of duplex stainless steels with low PRN number it is going up to 40 46 and so on. Now, look at this this this duplex stainless steel can be used in seawater application where the chloride content is 3.58 percent. Similarly, you can also use you know 254 SMO ok sorry 654 SMO you can use that and in but then this is prohibitively very expensive this material here compared to the duplex stainless steel here ok. So, what I am trying to say is that the development of stainless steels were based on understanding the chemistry of the alloy towards repeating corrosion of metals actually. In all this the nitrogen plays a very very significant role. Let me just see if I can show you some some kind of yeah curves here. Let me show this polarization curve ok. And see it is it is essentially it is a 904L filler wire you know you know what is called a cladding right what is the cladding and you melt and deposit you can also deposit without melting also actually right. It is it is a layer of of in this case 904L formed on mild steel here ok this is done by welding by weld cladding. So, melt it and and apply on the mild steels it is 904L ok. So, what happens see in this case the the carbon steel why people go for cladding because you want to go for higher corrosion resistance materials you want to have a thick one it is very expensive right. So, you can go for a carbon steel or other steel having lower cost go for a thicker component you just deposit the clad layer of they say 2 or 3 millimeters that takes care of the corrosion and the the the basic substrate takes care of the strength characteristics. So, it becomes cheaper. Now, we have done some work related to 904L ok. Look at this here it starts with the this is the 904L without adding any nitrogen. When you start adding nitrogen further and further you see what happens now the pitting the pitting potential increases and even the repasses in potential also increases. Now, in this case the nitrogen content is 0.245. So, when add to the weld more nitrogen what happens the pitting resistance increases significantly please see here the weight is it is about 0.258 percent only it is not too too much. So, by adding this nitrogen you can increase pitting resistance from you know if it value from minus 400 you know it has gone up to 1.2 volt. In fact, 1.2 volt means it is not pitting actually if you look at the poor bed diagram it will be what it will be oxygen evolution reaction ok. So, essentially this does not repeat at all by just addition of small amount of nitrogen. So, our development can be can be done to improve the pitting performance of stainless steels. Of course, the you know pitting corrosion also is very widely studied from the mechanistic point of view. What I mean by that? When add molybdenum when add when you add molybdenum when you add nitrogen they are much more effective as compared to chromium naturally. So, people have studied why molybdenum is very very effective people have studied why nitrogen is a very effective because there is no last word right. Still there are multiple theories as to why molybdenum improves pitting resistance as to why nitrogen improves pitting resistance. I just summarize a few of that and then move on to the next topic. The molybdenum the role of molybdenum has been has been you know first of all they say that you know molybdenum is not found in the film not found in the passive film. It is found in the actively corroding areas. So, suppose you have a pit in the deep of the pit you can find molybdenum or you are going to keep the you are going to keep the metal in the active region and then you observe the surface surface will have more molybdenum ok. But in the passive region the passive film there is no molybdenum at all ok and and so this led to some kind of understanding that the molybdenum you know it says it blocks the active surfaces blocks the active surfaces to one one kind of hypothesis. There is also other hypothesis which which which says that the the molybdenum ok corrosion rate decreases when H plus concentration increases it is a very uniqueness of molybdenum actually right you you saw before when the pH of the environment decreases what happens to corrosion rate increases right. But molybdenum has a different characteristics when you increase the acid concentration the corrosion rate of molybdenum decreases this is called as a negative reaction order ok. Suppose you have a molybdenum on the surface and if you if you if you increase the acid concentration it is not going to corrode why because molybdenum has a negative reaction order ok negative reaction rate order and so and so this is considered as the other kind of factors. The other people called it as molybdenum is a getter for chlorides it can complex with the chlorides chlorides are the species which which promote corrosion right in the pit you have more chlorides and the molybdenum forms a complex with the chlorides and then so what happens the free chlorides are very less and so pitting is is decreased. There are also other theories talk about semiconducting theories. The passive film is considered as a semiconductor right it is like high temperature oxidation right you have oxide film it is considered as either rich in oxygen or less in oxygen non stoichiometric oxygen there right then you have either p type or a n type oxide conductors. So, similarly you can also consider passive film as a semiconductor and and what you do when add molybdenum molybdenum has let us say 6 plus you know valence is state then what happens it reduces what happens it will reduce the cation right. When you are go to add a higher valence ions in the oxide what will happen to cation concentration cation concentration decreases to make the retro neutrality and so what happens then then what happens lowers passive current makes it stable. So, there are several kind of you know theories available I have just given only few of them and there are some more theories I think you know again for this course we do not have to worry too much about the mechanism just an outline of the mechanisms are ok. The nitrogen has a similar role about nitrogen still not clear some people say it forms ammonia that means, it neutralizes, but what happens is I we have seen that the active dissolution is decreasing active dissolution is reduced due to nitrogen in the in the alloy. Now, we also have the microstructures let us go to microstructures. Suppose you have some phases which are selectively corroding like manganese sulphide it what happens it lowers pitting resistance the selectively dissolved attack actually. So, any phase which are prone to corrosion they will promote the corrosion. Similarly, segregation of elements promote pitting like given example you weld it you weld what happens now what what does happen to the material when you weld it the weld fusion zone what happens some of you guys are having metallurgy background right when you weld it by let us say fusion welding ok thick welding or amig welding you take a stainless steel and weld what what will happen if you look at the microstructure what do you what do you what do you see the difference between the welded fusion zone and in the base metal I am talking fusion zone you take a nicely rolled wrought stainless steel sheet and you weld it and the fusion zone what you get what structure is that have you heard of a cast structure and wrought structure have you heard of what kind of structure you will have in a casting suppose you melt and pour it what structure do you get in a casting dendritic structure dendritic structures you see they are very simple right see normally all your stainless steel they are all you know I mean you have molten liquid pour it and then you do you know they do hot rolling and then anneal it out they make it chemically homogeneous but when you weld it you are going to have elements which are segregation of course you do not have time to discuss those issues ok the welds or you will have chemical segregations like you may have segregation of chromium molybdenum and things like that. So, you have some place which is enriched with the chromium some place it is depleted with the chromium what will happen to pitting resistance the pitting resistance will fall because there are some regions or 4 n chromium content molybdenum. In fact, if you look at some of the literature it published you see if you see the the percentage of molybdenum versus thecritical pitting temperature ok and you see that normally particularly but temperature goes like that it is the rod and it is welded what happens I will tell you later what is critical pitting temperature. So, the microstructures play a role because microstructure leads to different chemical variations and so there are issues of of you know rendering these alloys you know susceptible to corrosion. The the other example is let us say I just give small example aluminum alloys that is very nice system to to discuss and you know how the pitting resistance of the aluminum alloys really change you know the aluminum unless stainless steel will have multiple phases to to have the strength right. In aluminum alloys alloying elements in solid state that is accepting magnesium and zinc increase a pit. So, they are generally good, but then you do not have you know you do not make aluminum alloys generally you know with the elements in solid solution you always want to have precipitates to improve the increase the strength performance actually. But from from sandwich point of view you can add so many elements like molybdenum when tungsten chromium and host of elements if you can put them in the solid solution and you will find that is a very significant improvement in the pitting resistance of these alloys actually Let us look at the other one metallurgy cold working generally cold working and cold work lowers resistance why it introduces dislocations and so there will be more corrosion surface roughness ok, smooth surface favors better passivity. Hence rough surface is prone to so you make the surface you know you know if you do electro polishing you know make it mirror like have excellent resistance to pitting at all actually. The one of the factors external factor like you know this is not related to metallurgy external factor like velocity of the environment. So, what do you think will happen to pitting corrosion? You have a stagnant condition you have flow condition what will happen to pitting corrosion pitting resistance will increase ok. So, it will increase is why does it increase because it will destabilize the pit chemistry right it destabilizes the pit chemistry is destabilized. Hence increases pitting resistance. So, we have seen you know overall the the factors that are controlling the pitting corrosion then it must be easy for you to assess methods to prevent pitting corrosion. How do I avoid pitting corrosion quickly yeah. So, you can choose an alloy choose a right alloy or what you can do you can yeah you can change the surface state to some extent it is not going to be ok for long run, but yes it is it is good. Third environment change where possible if you are going to apply a material for seawater application obviously, you have to choose a right alloy right you cannot really do much about it right, but if you are talking about a cooling water system or you know a boiler water systems you can change the environment right. You can lower chlorides you can do that add additives the temperatures then what happen velocity that means, avoid stagnance right. If you are going to talk about seawater application you know you know the very robust alloy system is what titanium right titanium forms a very strong passive film and so, it is more resistance to pitting corrosion you have of course, standalone ok the ultimate in terms of corrosion resistance and pitting resistance ok ok. So, let us go to the last topic of this which is testing and evaluation ok. There are ASTM standards for that ok. You can look at the ASTM standards and ASTM U 48 and this talks about the 6 percent weight percent ferric chloride solution. It determines the critical pitting temperatures critical pitting temperature is determined. What you do in this case is that you immerse it for about 72 hours maintain the temperature plus or minus 2 degree accurate find out the highest temperature below which no pitting occurs. The highest temperature below which no pitting occurs that is called critical pitting temperatures pitting is not formed. Second one is is also we have ASTM G 61 it is cyclic polarization. What you do here you determine E pit you determine the E repassivation potentials essentially right. See I am not going to give you all these details you can read this in the in the standard is available in the library ok. I tell you what is important here what is the scan rate higher the scan rate you get higher the E pit values first ok. So, you have to maintain this scan rate as stated in the standard 0.6 volt per hour. There are some problems you normally encounter if you lose 0.6 volt per hour sometimes you will get a crevice attack in the samples you know samples sometimes you mount it you know you have a mount here you keep the sample you know this is a sample and you may get a crevice attack you must ensure that there is no crevice attack taking place. If there is a crevice attack then E pit will be lower than the actual value why before pit occurs the attack takes place along the interface here. So, mounting the sample properly ensuring that there no attack occurs with interface between the sample and the mount is is a very important thing. In our lab we we apply a small bead of epoxy resin and so, that the interface is protected from that. There are also you know special electrochemical cells available I think we will not discuss here you can you can see them in the literature actually. What is more important is that this reversible current what we talk about here is 5 milli amperes per centimeter square. Now, if this is not maintained the reversible see for example, if I if I if I just scan like that and I just reverse it here you get like this. If I on the other hand if I reduce this current even more I get like this. So, this this re-passivation potential E E R P E P whatever you you write here E R P depends not just on the material it depends upon at what current density you are reversing the scan right. I hope you are you will able to understand why does it takes place. If you apply more current the pit becomes deeper it becomes difficult to re-passivate if the current is lower the pit is shallow it is easy to re-passivate. So, the re-passivation of the pit does not depend only on the metal, but also depends upon the depth of the pit. So, but what they have found here is if you are go to apply 5 milli amperes and above it is not go to depend upon this current. So, the in fact, you know it was done work was done in south-west research institute by Narasi serieser and group where they have shown that if you have a critical pit depth beyond which it is not going to it is not going to affect the re-passivation potential. So, that means, you need to follow this standard meticulously without which you may get a different E R P value which is not an indication of the the pitting growth resistance tendency of the metal actually. Please look at this is talks about pitting initiation and this talks about the pitting re-passivation here. Now, look at this this is called a instead is now right when you when you when you do this I think we discussed earlier in the in the class we will discuss this one or not did we discuss this one? No ok. So, then I spend a minute on this to discuss what what what it is. So, then you will understand it better. See the here you will say the pit has initiated when you advance the potential more and more what happens here new pits initiate old pits is not it. So, here the it becomes a stable pit here right. So, beyond this what happens you know the pit that is formed here is is stable, but when you start increasing the current further which means your own increase the potential also now what happens the new pits are going to form the old pits grow. Now, what is happening now when you reverse this current here please see this it is not following the same same path rather what happens now the current the reverse current is higher than the forward current right. What does why does it happen? It happens because when you have a pit is formed here this pit is not re-passivated. The pit continues to corrode even though you are bringing the potential below the e-pit the pit continues to grow because the pit is now seeing a higher chloride ions it is seeing a higher amount of H plus ions and the film is totally damaged it is not healing at all. So, when you bring down the potential from here to this. So, what is happening now? The driving force is now the driving force is decreasing the over voltage for the metal to dissolve is decreasing here. So, now what happens the dissolution current is decreasing keep decreasing here at this particular point of time you can say the pit is pitting stopped the walls become passive now. So, that is what really happens now. So, that is why this is called as a repassivation potential now ok. So, we look at the two types of testing for pitting. One is ASTM G48 where in use ferric chloride and determine the critical pitting temperatures and also we also use that particular test to quantify the pitting on scales. If you go through this standard the standard gives you a lot of u-grabs to quantify the scales based on the number or based on the depth based on the radius of that. We also looked at the electrochemical test where we use cyclic polarization to determine the pitting potential and the repassivation potentials. These two are the important criteria in ranking a stainless steel or any material against pitting corrosion in a given environment. Please again notice that the e-pit or e-pissivation potential also depends upon the environment is not just only depend upon the material. So, these two techniques are you know very widely used. Obviously, you cannot use weight loss measurements to determine the pitting because the apparent weight loss is so insignificant it does not give you any meaningful information about the pitting tendency of any alloys. The other important character of pitting corrosion is it is stochastic ok. You know what is mean by stochastic? It is more probabilistic in nature. So, that makes some kind of constraint in extrapolating any of the experimental data. So, this aspect we will discuss now ok. Now, if you expose a stainless steel or aluminum alloy or any of the alloys which are prone to pitting to a given environment for a given time then you observe the the pits that are formed on the surface. You look at the size distribution, you look at the numbers right the two two types of quantification that you can make right. So, you can look at the number of pits in the given size and see how it varies you would probably get something like this. The number of pits against the size of the pits you see a kind of humble distribution something like this you will see. What does really indicate? If you count the number of pits on the lower size and at the highest size both sides are very small in number right. The average size of pits are large in number, but if you talk about a component failure right. What does it depend upon? Suppose you have pipeline and it suffered pitting and the leak occurs. The first leak that occurs does it depend upon the number of pits or does it depend upon? It depends upon the deepest pit. So, in practice this is what is going to count for us ok. This is what is going to affect life of a component. This looks little more even more simple right. Let us look at the size effect of of the specimen tested. Does it really play a role? Now, in order to look at it we take this as a probability concept here ok. The probability of occurrence of one size of the pit ok. For example, what is the probability of this pit occurring? What is the probability? You can also look at the probability of this one right. So, I can determine what are the probabilities of occurrence of a given size of the pit? I can do that right. If I can expose the samples of various sizes and determine the probability of occurrence of any of these size pits. I assume let us say this is the the the size of the pit. Let us say it is about this assume that this is the size of the pit. Let us say about let us say D is the size of the pit. Let us say 2 D is size of the pit and 3 D is the size of the pit right. I have given approximately ok. And if I plot the probability against specimen size probability of occurrence ok. This is a 0 and you say right. And I for simplicity I also read the size in terms of unit unit sizes. Let us say the size of the specimen is one unit area the two unit area 3 and 4 like that I can have right. Unit 1 this this is the 2, 3 and maybe you can say 4 or whatever kind of thing ok. After you let us go up to 3 only. So, what I mean by that? Suppose I take this is 1 centimeter square this is 2 centimeter square 3 centimeter square right. Let me look at the probability of finding the pit of diameter D that is here. It will be very interesting that the probability of finding the pit D the sample 1 something like that goes like that. So, the size of this pit the probability of finding that size of the pit increases when the size of the sample increases. Now, you can also you can also find out similar thing for the the pit size of 2 D. So, what do you understand from this particular plots? If the size is increasing the probability of finding deeper pits is more. More. So, I mean the structure is more in reality if you are going to go for a larger structures the probability of pitting is going to be larger. The smaller structure the probability of pitting becomes less. So, it is a stochastic process. So, in a laboratory suppose you take a very small size of the sample and somebody takes a larger size sample it is possible that you may get different results not necessarily you get the same kind of results actually. So, this makes the things more complicated. There are several reasons one can attribute to that. Suppose I take a stainless steel the stainless steel consists of what consists of inclusions right the probability of finding an inclusion which affects the pitting is more than the size of the pit when size of the sample is larger. So, this is one aspect of thing then people have in fact dealt with this the the probability realistic theory of pitting corrosion you know has been um one of the important topic of research. We will not go more in detail into it I just the idea is to expose to you the complexities involved in determining the pitting tendency of in a sample. So, that is why when you do test it is been the corrosion test you give all information size of the sample environment the duration temperatures overall there are several factors that affect the corrosion behavior of the metal. In this case the size becomes even more important and relevant for us to to to consider actually. So, that is something you should you should look at it so on. And one thing I I just you know um forgot to mention about the about the prevention of pitting corrosion I think if you can go back to this slide sometime back we discussed actually ok. I please recall this discussion I just want add right. We said that the pitting corrosion can be prevented by of course, choosing a right alloy of course, that is a very very generic way of talking about it ok. And it also can you need to look at the surface state of the material how polished or the environmental changes chloride should be reduced. You can add additives you can lower the temperatures and you avoid stagnancy and all. But of course, the material selection depends upon what depends upon a given environment right you know the temperature you know the chlorides you know the pH conditions. And so, you decide what can be the alloy that is suitable for that. And for that you also have test like critical pitting temperatures, e p values please again notice these are all ranking of the materials. I do not think that these tests are going to give you any kind of life that you can supposedly you know you can predict that is simply not possible so far about the pitting corrosion or the metals. Coming back to this the first first aspect that is the choice of alloy and in it comes to same results you know that right you know that you want to increase the pitting resistance the one way to look at is the composition which is pitting resistance equivalent number is it goes high then I think the pitting resistance increases. But the other way of looking at is is you choose an alloy which is not passivating. For example, carbon steel is better than stainless steels for seawater applications right that is why ship hull is made up of what made up of carbon steels right not the stainless steel. But what is the problem with the carbon steels uniform corrosion rate is more right the uniform corrosion rate is more than that of the stainless steels. But you if you have the problem about uniform corrosion then you go for copper base alloys. What are the alloys that you you can think of you can think of brass you can think of cupronical alloys right. They are all used in seawater applications they have reasonably good uniform corrosion assistance compared to carbon steels and their own pit. And so, they are in fact, widely used for seawater applications. The other way of looking at is you have a very strong passivating metals and alloys very strong passivating that is we saw a PRA number. Similarly, you know you can look at titanium, zirconium so all can be used. Titanium is used for seawater applications strong oxide films are formed and ambient temperatures slightly elevated temperatures titanium can be very very used. So, to summarize pitting corrosion a pitting corrosion occurs predominantly in the passive metals right only the pitting you put it as passive system. I know there are metals like titanium stainless steels and you know alloys titanium they are all passive in a very you know wide variety of environments right. But you also know that you can also passivate many metals right depending upon the environment you make it pH you know slightly favorable right even steel can do magnesium can undergo passivations and in such cases they can undergo pitting. So, it is the passive system it also depends upon the environment specific anions I specially put it. Of course, cations also can influence right can cations influence pitting corrosion yeah. Cation accumulation. Of course, that is that is another part of it I am talking about the environment I am not talking about the cations dissolved in the pit and they are causing the pH change ok. I am talking about external addition of cations. If they are oxidizing agents. Yeah oxidizing agents right your ferric ions, cupric ions, mercury chloride you know all the highly oxidizing environments can can cause pitting environment of course, but then anions are very important ok. We talked about the pitting mechanisms we look at especially what initiation then we talk about metastable pitting and we talk about stable pitting right. The initiation is governed by what film breakdown am I right film start breaking down because of the anions present in the environment. Initiation mechanisms still are not you know well defined still not 100 percent clarity is there. Who I have looked at the metastable pitting right the film breakdown takes place there is a localized damage micron or submicron levels and how do you know it is metal undergoes metastable pitting? If you look at the current oscillations if you are going to apply a potential in the passive region over the sample and you monitor the current the current there is no stable current there is always a fluctuation in the current right and as you move the potential towards the pitting potentials the fluctuation increases. These fluctuations indicate that there is a breakdown of the film and then there is again healing of the surface by reforming the film and the criteria of a metastable pitting turning into a stable pitting right. What are the criteria for this? When does it become stable? Yeah, when does it become a stable pit? Pit chemistry. Yeah it is a pit chemistry right it is a pit chemistry. The pit chemistry is special right it has got more H plus it has got more chloride. So, it does not allow the metal to repassivate. So, that is governed by what salt film diffusion effects we also had a criteria called as or eye criteria we talked about right. It also depends upon the potential drop drop between what between pit pit and the pit mouth because there is a potential drop it does not allow the pit it is a growing pit that front does not allow to repassivate right. They are kept at a very more negative potentials the dissolution occurs. We talked about the mechanisms then we talked about the factors controlling the pitting corrosion right. This is environment and electrochemistry metallurgy right. We look at the temperatures, look at the pH, you look at the velocity effect for example, you have a very great influence on that. Tertiary chemistry means what it is the potential right. If you are going to move the specimen towards pitting potential it is going to break, film is going to break and you are going to have you are going to have a higher damage to the placer films. We also look at the metallurgy in term mostly we have seen in relation to stainless steels right. And how the various alloying elements affect the the two parameters pitting potential and the repassivation potentials right. We saw that various alloying elements right microstructures they do significantly affect pitting corrosion. When you say microstructures it could be all kind of things can be segregation of various phases and we talked about cold work. I forgot to summarize right and we also defined what is a pit right. A pitting factor is just not that an uneven corrosion is can be defined as a pitting right. The radius of the pit to the depth of the pit is very important. We should have you know the the radius should be smaller compared to the depth of the pit. Then only pit can be stable. Then we will look at what look at the control measures then we look at the test methods. You can some you can add one more to this in metallurgy is is a surface state. Any questions in pitting corrosion so far? Okay there are no questions we will shall close the discussion on this topic of pitting corrosion.