 Today, we shall discuss an important topic which is pitting corrosion. Pitting corrosion of metals is it can cause a severe damage to the structural integrity because the rate of corrosion in the pit can bevery high it can be about a million times higher corrosion rates as compared to the surrounding matrix. So, the pitting corrosion isis very insidious can cause accelerated corrosion the localized areas of a metallic component. It can ultimately can lead to perforation it can be a leak. Sometimes the pit can act as as initiator for say stress corrosion cracking it can be initiated for simple fatigue or it can be even corrosion fatigue. You know the reason because when you have a pit which is very localized it can be a stress riser you know the stress intensity at that location can be quite high and so, the crack can initiate and grow even below the yield point of a metal. So, pit is in fact, is very detrimental can bring down the component life quite significantly leading to premature failures. I just want to show some illustrations how these pit can really look like you do not how clear it is to you it is a it is a long shot and taken on you know facade you know this is stainless steel this is you can see that there is a stainless steel here right you can see here is a stainless steel and you see some fine darts appearing right. These fine darts are actually pits and these pits appeared within about 6 months to 1 year of commissioning of this particular structure because it was at the seafront and the seafront you know that you have a lot of chlorides being carried by the wind and the chlorides deposit on this stainless steel structure that leads to pitting corrosion and this is a this is a it is a it is a civil engineering structure you know it is you would in the same location you will see here is here slightly better I suppose you can able to see a pit here right you can able to see a pit around here and a pit around here and you see pits here right. So, and of course, you just tiny pits some somewhere around here you can see lots of tiny pits which are visible to the naked eye ok and again this 316 stainless steel you could see the pitting happening within 6 months to 1 year ok. They become very severe where they were welded see that here right right I hope you were able to see this reddish brown spots ok these parts they do correspond to severe pitting. Two kinds of problems here one is the loss in the structural integrity otherwise the cosmetic appearance of the stainless steels right you have reddish brown color and especially you are talking about you know household appliances or the facades of a house and you know which are so which are all can cause you know you know can appearances which are not really acceptable. Now these are all macroscopic view on pits right and if you see in the microscope sometimes these pits appear to start with the microscopic levels actually. You you guys are all know right you have done already you know earlier you have seen that what is polarization and how the polarization of passivating metals look like when you analytically polarized you get a passive region and you rise the potential further you get into pitting regions ok and if you look at the samples after the anodic polarization and you will see a very microscopic pits here ok. You see these pits are very microscopic natures many of them are all hemispherical in natures ok that is how the pits starts actually ok. In fact, the pit could start at the at the submicroscopic level actually the submicroscopic means less than more micron actually can happen. So, these these are the kind of pits happened in the case of a stainless steel which is 304 L stainless steels and this stainless steel was you know plasma ion implantation technique ok where you implanted nitrogen in the stainless steel and you can see here that it is untreated right and treated with with with nitrogen and you can see that the the the number of the depth of pit here is reduced as compared to over here. So, in this case the nitrogen addition has improved of course, the treatment was done at different temperatures at a higher temperature you can see that the pits become more. Essentially this is a a PhD work ok wherein they converted the surface so that it can have higher pitting resistance and higher passivation. In that case they have added nitrogen by plasma source ion implantation or also called as plasma ion you know plasma immersed in ion implantation technique ok. So, essentially I want to show that these are all can be microscopic you can start with you can be even submicroscopic levels. These pits start and then they grow and and you know they can start damaging. Sometimes the pit also can have a very complex morphologies we will see later actually. You can see here these pits these are all corresponding to a stainless steel where nickel was substituted with the manganese right and why is why people substitute nickel with the manganese because the manganese is cheaper and also you can stabilize the austenitic phase. So, it is a it is a it is a an equivalent of a 304 stainless steel but very low nickel content with more manganese content. You see here the pitting behavior is is quite severe you know see this is see compare these two now you know and you just see this the left side corresponds to the the manganese substituted one actually replace nickel with that it is kind of a nickel now though the one with the nickel is certainly far better compared with one replaced by manganese actually. As I told you the pits can be very very complex you know depending upon the microstructures you know. So, this is a slightly you know irregular morphology of the pit that you see here and this is essentially it is a it is a composite right. It is a it is a A356 aluminum alloy actually wherein you add silicon carbide particles added is essentially used for the rotor disc you know for the for the for the automobile applications you know you know the brake disc actually you know is is normally made up of cast ions but cast ions are very a very high density. So, they they want to be substituted with with aluminum but then aluminum is suffers very high severe I would say where you know where problems. So, what happens is they added with they added with with silicon carbide the silicon carbide gives you reinforcement and so the wear resistance now. So, you can see that the pits are more complex it takes the contour of some of these silicon carbides ok. Look at this shape of this this follows the contour of the silicon carbide particles. So, you can you can have a pit which can be irregular pit based on the microstructure of the material actually that means the metallurgy also plays a very important role in pitting of this pitting of the steels and other alloys. So, so what we would like to say is that the pit can start at microscopic some microscopic level then grow and then perforate the structures that is why they are quite deleterious. Look at this corrosion at the microscopic level right that means there is no significant change in the weight change in the mass of the structures. So, there is going to be no significant change in the mass of metallic in pitting corrosion. So, the test like mass loss test has no meaning right and why does it really originate pitting? There are lots of studies you know there are probably hundreds of papers which are devoted to understand the pitting mechanism pitting characteristics and so on. And I do not think we have now the last word in understanding the pitting mechanism so far. Of course, a significant progress has happened in understanding pitting corrosion, but still there are several steps of pitting we do not still understand. In this course, I am going to be a little brief because it is the first course and we are not going to too much of science of the pitting corrosion. But however, if you are really interested to know to more you know I would recommend strongly this article by G S Frankel you know this article is it is a review and it is published in the general law of the electrochemical society. It is a it is a very good review ok. It covers quite a bit of fundamentals of pitting corrosion. To start with can pitting occur in all cases in all corrosion system? The answer for that is it happens only in the passive system in the passive metals. If the metals do not passivate there would be no pitting. Passivation is an important criteria. First the second you need aggressive anions you need aggressive anions. So, even when the metal is passive it may not pit unless there are specific aggressive anions. I give some examples. What are the metals you know of which are passivating in wide environment yeah stainless steels yes then aluminum titanium alloys. There are so many other metals right you have for example, you can have tantalum zirconium you know there are several metals which are passivating. It is also possible that you can passivate even pure iron and I given pH conditions right you have seen the pure bed diagram. It is not necessary that you know the stainless steel can passivate in all cases not necessarily. But predominantly yes from the engineering point of view these metals they exhibit passivation. Now all of them they can undergo pitting they can undergo pitting ok. For example, stainless steels pit in chloride media. So, as aluminum titanium chlorides at ambient temperatures no at high temperatures yes, but you can have like bromide for example, you can pit. So, passivation is an important criteria for pitting to occur and predominantly these metals they suffer pitting depending upon how severe the environment is and what kind of when you say environment it means the temperatures it means the nature of the the anions that we we think about and both are included. As I told you pitting corrosion is not so far well understood. Now if you if you take a let us say if you consider a polarization curve you if you know of this this is very familiar to you right and this is the potential we call them as e-pit or also called as yes pitting potentials. Now if you look at the voltage here let us say typically a 316 stainless steel in chloride medium maybe the e-pit could be about plus 200 millivolts versus saturated caramel electrode right pitting potential in say 3.5 8 percent NaCl solution. But actually if you look at the driving force for pitting it is very enormous. Now what is this potential? This potential is is with respect to the solution right the potential of the metal with respect to the solution as measured by a reference electrode in this case a saturated caramel electrode. So, I am measuring the potential of this metal in relation to the solution it is a 200 milli volt. But if you look at the film thickness when I say film thickness I mean the passive film thickness right they are in the order of nanometers. The metal is highly conducting this film you can consider as approximately you can say is insulating or you may consider as a semi conducting in both you know. So, it is a barrier for the flow of the current is it not when I apply a voltage the current is not rising the current remains steady state here it is called a passive current density right this is this is your IP passive current density because it is the film is offering a barrier resistance. But if the film thickness is in nanometers and even you apply let us say 0.2 volt what is the field across the film? The field across the film turns out to be about 10 power 6 to 10 power 7 volt per centimeter what is the field voltage by the distance always the voltage is applied right. Now, look at this here so, it is a very high field that means, the metal is about to have a electrical breakdown right can electrical breakdown can happen right. So, it is in the threshold of of the electrical breakdown one aspect of it. The second aspect of it is the passive current density is of the order of what it is of the order of 10 power minus 6 to 10 power minus 7 amperes centimeter square. But you look at the active metal resolution at the bare surface surface it is about 1 to 10 amperes per centimeter square. Do you understand this? Do you understand this concept? The passive current density at these locations right you measure it they are all in terms of micro amperes. If the film breaks and if you expose a bare metal the current density over here is the order of amperes 1 ampere to 10 amperes per centimeter square right. So, how many times the metal corrodes here as compared to the corrosion of metals in the passive state? How many times it is about? 10 power 6 is about a million times corroding at higher rate. So, if you look at the film breakdown if the due to some reasons the dissolution at that location becomes very severe the rate of corrosion can be of the order of 10 power 6, 10 power 5 depending upon what extent the bare metal is exposed. So, this pitting corrosion becomes very very severe very very insidious and it happens at the microscopic level I hope I have conveyed the points clear it to you to understand here ok. So, this is a very important type of failure that is why lot of work has gone lot of research has gone to understand the pitting corrosion of metals. Now, let us look at the pitting what it consists of? It consists of passive film breakdown then we called as metastable pitting third we call them as a stable pitting. Let me explain to you what do I mean by these three events ok. These are the three events simplified events of pitting corrosion the passive film breakdown. Now, let us take the case one what do you mean by passive state? What do you mean by? So, what is happening in a passive metal? You get a definite passive current density there is a film formation, but even then there is a definite current flowing across the film. So, there is a metal dissolution it is not that there is no metal dissolution the metal dissolves, but dissolves at a lower rate. So, the passivity is a is a dynamic process film breakdown and passivation it continuously happens right. The only thing is the rate of you know repassivation or whatever and you can call repassivation you use a term repassivation may be more appropriate ok. So, so if we can quickly heal when the current comes down ok. Now, they are all happening at a at nano levels I mean to say nanowheel talk about spatial right nanoscopic levels. Spatially the occur the atomic and the nano levels the metal atoms dissolve and there is a repassivation they are taking place. But what can happen is if if a rise of potential the potential is is increased if the potential is increased or if some aggressive and or present film breakdown dominates you may call this as initiation. As I told you the initiation process is still less understood the film is getting damaged the film is getting broken actually ok and the bare metal is exposed. When it is exposed then what happens it leads to metastable pit you know what the what do you mean by metastable anybody? It is between a stable and unstable state right a stable state and unstable state in between the state is called as a metastable state. It can you can either go to a stable state or you can move to a unstable state that is called a metastable state ok. In the metastable pit what happens the damage is quite visible. So, what do you what do you mean by that? If you normally observe the anodic polarization curves of let us say stainless steel you may aluminum alloys some cases even titanium alloys you start from the e car right this is your e car ok and you rise the potential you go to the critical condensate then passive state current apparently looks very stable current right. When it is not increasing the current starts fluctuating and this is called as metastable place. Why I called a metastable here? The pits are visible pits are visible to a microscope you just see them in the microscope they can be one micron you know half a micron visible right. As opposed to the film breakdown right as opposed to film breakdown that is initiation process no visible change is seen. You do not see them anything right in the in other cases you see that that the dynamic process of metal dissolution and then repassivation taking place and see them in the microscope you would not see any features, but over here you observe in the microscope you see microscopic size pits appearing disappearing appearing and disappearing that means, what it becomes truly metastable and some of these pits may become stable later right. So, you see a metastable pit here ok which are observable in the microscope and this metastable pits pits or microscopic in nature ok some of them becoming stable pit and many of them repassivate it goes to the original state of passivity. Now, what does it mean? You when you start moving from the corrosion potentials up towards the noble side ok noble side you see the current start fluctuating and then the current becomes here you see here what happens here the current is steadily increasing and this is becomes a stable pit at this particular potential it means a stable pit. So, far e pit you describe is nothing, but the pit that forms become stable ok the pit does not disappear above the particular potentials and e pit and above the chances of the pit remaining stable is very high below e pit the chances of pitting becoming stable become the rest. In fact, it becomes less as you start moving down from here or so to say it is possible to have a pit even at this particular place over here over here over here, but what what does really mean in real terms the real in in real terms the probability of the pit that is formed here becoming stable is so, less compared to the probability of the pit so, formed becoming stable. So, this probability is quite large here as compared to this that means, the concept of e pit becoming a unique number is not correct the pitting can occur below the pitting potential as well at e pit and above for sure the pitting will take place hope I am not I am making the point clear to you. So, the the the concept of saying that e pit corresponds to pit initiation is is a little bit you know not a right terminology to use all we can say is that yes the pit will start stabilizing at the particular potentials ok ok. I am I am a clear to you actually may clear ok. So, this is something very important to look at it. In fact, this has led to some concept of what is called as induction time. Actually just to make some remarks I would say pitting corrosion pitting tendency I would say e is a function of applied potentials the chloride or anion concentration then the time this is going to be factors ok. e where here e is applied potential chloride is the air concentration chloride or other anions time is the time of exposure. Now, if you take for example, let us draw this diagram of polarization curve suppose you take these places these points suppose you take and I hold the sample at that particular potential time versus the the current log i you can call it log i current. If I hold at potential corresponding to the point d the current increases see what happens this is your let us say this is your I p this current is the base current is I p here. Here what happens the current will remain here and then this is going to be c c this is going to be b the increase in current corresponds to the pitting of of the of the metal and this is given some you know gross schematic here ok. You do not worry about the values just look at the the relative positions of these you know current versus time curves. So, what does it mean? If you are going to hold it at d the pit starts immediately it does not take more time if you hold it if you reduce the potentials it takes longer time and and so on b and and so as a right. So, which means that the chances of metal pitting in an environment you know it depends on what it depends upon not only the potentials also the time factors ok. So, this is something you should be looking at actually. So, now, the pit becomes a stable right. What makes a transition from metastable to stable pit? Here the there is a lot of similarity between a crevice corrosion and a pitting corrosion here ok. What makes the pit stable right? The pit can repassivate that means it can form the film that means, the pit becomes unstable or the pit will continue to dissolve and grow that means, the pit becomes stable right. So, so what makes this pit stable? Now, there are several you know theories are there we will make it very brief and somewhat simple. Let us take a let us take a surface let us take a surface this is the passive film. The cathodic reaction and the anodic reactions they occur all through right. The cathodic reaction may be considered as here the you can have a cathodic reaction. Generally, for a stainless steel to passivate the cathodic reaction has to be what? Has to be you put an acid if you are going to give you an acid. So, cathodic reaction is generally the oxygen reaction that only makes a potential in the passive region right. Otherwise, it do not make the it do not render the metal passive you need to shift it to the passive region. So, the cathodic the cathodic current I mean sorry the the the cathodic reaction must have noble potential. So, all through you have a metal dissolution and as well as the oxygen reduction reaction taking place like this. Now, if there is going to be a a pit formed assume that there is a pit formed. If the pit is covered by corrosion products passive film whatever can happen, then it becomes an occluded cell. What is an occluded cell? The occluded cell there is no convection there is only diffusion process right. So, it is only it is only a diffusion process here. Now, in the in the in the in the in the pit the so called metastable pit assume that it is a metastable pit ok. It is a metastable pit. There is a pit formed here it is covered with some corrosion product or something. The metal continues to dissolve below this cover ok. Here the metal dissolves continuously right. To dissolve continuously it will it will generate various cations. For example, if you have a M go into solution as M N plus plus N electron this becomes an anode. Now, the surrounding area becomes cathode and over here oxygen content drops. Why does the oxygen content drops? Because it is covered with a with a corrosion product. Now, you have this metal ions generated here right and these metal ions when they are generated what happens now? You have now the generation of H plus ions. So, this is also going to promote chloride ion migration chloride or maybe iodide or whatever you know depending upon the anions present in the electrolyte and they can migrate to this and you can form hydrochloric acid. It is somewhat similar to crevice formation am I right? It is somewhat similar to crevice corrosion formation. Now, the question is when will the pit be stable? The pit will be stable if you form the acid right pits become stable pitting ok become stable if the pit is acidified. How does acidification is occurs? This is possible metal ions the cations accumulate in the pit or in the metastable pit or occluded cells. I hope you understand now. See here you are not forming external crevice here we are you know the crevice itself is formed by the corrosion process. Now, in order that the metal to dissolve continuously that pit the occluded location has to be acidified and should maintain that acidity. Now, how how is it possible? The acidity is possible is is first of all created by it is created by the dissolution by by the hydrolysis of the metal ions. Look at this metal ions will interact with water hydrolyze and form metal hydroxide plus acid H plus and H plus in turn will attract more chlorides and form this ones. Assume that this metal ions can easily migrate outside. I form a pit you have metal ions they just go out of the surface what will happen? Here is the metal ion that coming here metal ions are formed here right. Assume that this metal ions do not get accumulated they just go away from the surface get into the electrolyte. So, what will happen? Increase or decrease? Decrease. Why will you decrease? No, you are right pitting will not occur why it is not happening? No, what does the metal ions first of all do if they stay here what do they do? They form. They form ions right they form hydrogen ions they hydrolyze if these ions are quickly moving away from surface what will happen to acid formation? The acid formation reduces the chloride ion migration also reduces and so, the pit becomes unstable ok. So, if M n plus migrate faster the pit becomes unstable or I put other way around you need. So, or to say accumulation cations are needed if they are not accumulated they are washed away from the surface. Now, a pitting will not be stable that surface becomes again passive. Now, let us look at this I made this little bit longer this is the or length of the pit or is the pit length. Now, please look at here the metal resolution occurs the electrons will travel here and they may combine with with oxygen and you can form hydroxide electrons also can travel here combine with oxygen here and you can and it can form hydroxide what are what are you can form. So, you have an anode here and here you are going to have a cathode I think most of you are engineers now right I am going to put a question to you. I want more cations to be here because when you have more cations it will hydrolyze and form acid it also can lead to migration of chloride ions in the into the pit. Now, what what makes now look at there are two processes one is the dissolution of merlions giving rise to this and it is what is the other process. The other process of the other process is the migration of these ions from the pit to the outside the pit. So, now, you tell me what are the governing factors that will make the pit stable of course, gravity is one thing ok assume that gravity is there ok gravity is going to be everywhere now what are other factors in terms of pit dimension the other one. Depth of the pit. The length of the pit and the rate of dissolution right. So, the product length of the this one and I the distribution current is is a parameter used to describe the pit stability. Please look at they are multiplied if r is higher I can have lower current density why because diffusion path is longer. If the diffusion path is shorter then I must have a higher dissolution rate in order to make the pit stable right. So, the pit stability this is the criteria used ok by a great guy called Gallivale. So, this is a one of the important criteria in deciding the in deciding the whether the pit formed in a metal will happen or not ok. So, I think I will not get in too much of things beyond this particular point actually ok. Only one thing I want to make a mention that I think this is important that probably you like to know and then we move on to the respect of that. Now, there are various metallic cations let us say aluminum iron 2 plus 3 plus whatever you may have let us say chromium. So, very interestingly that each cations they produce different pH. They are decided by the k value. For example, AL 3 plus k for that we decide what can be the pH. Those ions use acid yes acid that is not correct. So, the criteria of pitting is. So, to summarize what we have seen so far pitting occurs in passive metals film breakdown localized corrosion causes pitting. Now, you have a criteria called metastable pitting which can occur even below a pit right can occur even below the pit. Pit stability depends on what depends on the pit chemistry. The pit chemistry is not promoting is not good enough then pitting will not occur right. So, the pit stability depends upon the pit chemistry and pit chemistry in turn depends on what pit chemistry depends upon depends upon the diffusion parameters which means cation accumulation right which is it defined by the criteria called r into i. So, please go through this I mean ok and we can discuss this.