 today we shall look at the effect of the flow of corrosive environment on the corrosion of metals. So far what we discussed was related to the environment being it is static and in several cases at least in the industry the environment need not be static. The environment you know there is a flow. There are so many complex flow conditions that exist in the in the industrial scenario. The corrosion of metals are different if the if the environment you know flows in relation to the metallic structures. So, this topic so assumes very greater importance. So, we will discuss in detail what are the kinds of flow that affect the the metallic structures from the corrosion point of view and what are the mechanisms involved. The factors that control the flow assisted or you know velocity affected corrosion and then also we look at the control measures. So, here we will say that flow assisted I am going to separate out like this and or erosion, cavitation, corrosion. Now, as I told you there can be a relative movement please notice that when you say the environment we talk about not the benign one it is a corrosive environment. Now, let us try to understand what do you mean by relative movement first of all. The structures can be static can be what happens the environment moves I will say. So, in example is a pipeline carrying water carrying waste types of chemicals and an example could be heat exchangers. The structure is static the environment moves maybe at different velocity, different pressure, temperature all this happen. The structure moves. Can you give me an example? Anybody? You can say shape, but the velocity is not ok you can say shape . We can talk about an aircraft also stage craft in fact, aircraft moves at a very high speed. Other examples, industrially agitators right in a in a in a turbine right a turbine blades it could be a gas turbine or it could be hydraulic turbine gas engine anything else? Impullers they move propellers you know. So, these are about the two types of structures I would like to say ok. Now, let us look at the the environment. What is mean by the environment here? Of course, we have talked about the fluids only one has to be a fluid I mean corrosion environment is a fluid. It could be a single phase could be a multiphase. Generally two phases can happen commonly. It is a simply a water a sea water an example. This is a sea water plus some sand right can happen. In a in a in a thermal power plant for example, you talk about a steam generators you may have a gas steam you may have water in a gas turbine there can be corrosive gases there can be particles. So, when you say a fluid it is not necessarily a liquid ok. It can be a corrosive gases it can be liquid, but the solid is not necessarily a corrosive. In most cases, you do not even consider them as corrosive. The role is to give an impact we will see this later how they can really affect the corrosion of structures. If you look at you know in the fluid in the fluid conditions when the when the fluid moves how the corrosion rate of the structure or metallic structure can change with respect to the velocity. We have seen in you know somewhere right I think the role of velocity on corrosion corrosion rate right. We have seen before can anybody recollect? Yeah. Yeah, we talk about a concentration polarization right. When there is a concentration polarization when increase the velocity the corrosion rate increases it is simply a diffusion control process actually there. If there is no diffusion control process the velocity does not play any significant role. You can go back and see your you know the first part of the course in thermodynamics and kinetics of electrochemical corrosion. If you plot this something like that the corrosion rate initially increases you know gradually like that a steep increase in corrosion then again it slopes off like this. There is there is you see here there is a velocity called a critical velocity above which the rate of corrosion steeply increases with velocity right. When you when you when you visualize let us say a pipeline suppose visualize it and if the fluid moves travels in the pipeline the movement of the fluid is parallel to the surface. What do you call that? You call a laminar flow. If the fluid is increased what happens now? It is no more parallel to the surface it is going to hit the surface ok. What do you call this? Now, the laminar flow turbulent flow they all depends this is described by Reynolds number and there are equations we will not get into that kind of discussion here ok. But it is it is enough to understand that the velocity or I put other way around the natural flow decides the kind of corrosion you can happen. The laminar flow or a turbulent flow that depends on velocity density the dimensions of the pipeline so many factors. So, the rate of corrosion over here the natural corrosion over here is different from that of over here can happen. So, you can say that in this case you can divide this something like this is going to be a turbulent and it is going to be laminar and major part of the laminar flow I mean we can say that can be you know the corrosion rate can be controlled by the diffusion processes. If it is essentially a corrosion is a diffusion controlled right. Some of you would have the study effect if you can recollect that right. If you recollect if it is a cathodic reaction say diffusion controlled right this is called I L and the velocity increases we talked about this in the first part of the class. So, it is a diffusion controlled process and the corrosion can assist and the mass transport is important. You see later how the corrosion can get affected right. Now, the depending upon the kind of flow conditions exist in a pipeline the types of corrosion can be broadly classified into three types ok. Nature I call as types of corrosion which are induced by the flow. One is called as flow induced corrosion. This is essentially related to increase in mass transfer. Mass transfer of what? The mass transfer of the corrosion product. How quickly it can get removed from the surface? Earlier we talked about the diffusion control process of what? In that case what is what is diffusion control there? If you remember when we said here what was the mass transfer control here? It is the cathodic species that is mass transfer control. We will see here in this case it is not necessarily the cathodic species reduction species mass mass transfer control. It is oxidized species which are mass transfer control I will just talk about soon. Next is erosion corrosion here it is mechanical damage mostly of surface films. So, there is an impact there is there is a real erosion taking place right in the in the sense of mechanical factor. The other one is is a cavitation corrosion it is majorly mechanical if you can say is minor ok put within quartz ok is minor. Now, we have classified the the corrosion occurring on a metal into three different categories primarily because the mechanism of corrosion in all these cases are different when the mechanisms are different the solutions are going to be different ok. So, we are not going to apply the same solution all the case of course, I mean there can be an overlap of properties and the protective measures, but distinctly they are different you can see that. So, now, let us look at the the first type of thing I mean that is the flow assisted corrosion flow induced or flow assisted you know you can you can call what are this flow induced corrosion or flow assisted corrosion what is this? What does it do? What it does is that the flow that happens in the system mostly in the pipelines the assist the solubility of corrosion products then reduce film formation. So, it can enhance corrosion it can happen in single or two phase conditions two phase yeah two phase flow I would say. Let me try to explain what we mean by this all these factors here. The flow assisted corrosion was you know was first a problem were found in thermal power plants here formed in thermal power plants and they made a survey in 1990 1997 of 63 power plants 63 plants utilities. In in all these cases they found you know quite an extensive you know plants suffered the flow assisted or flow induced corrosion. Now, you know in in power plants what do you use? I mean to to I mean you generate steam right and steam is used to run the turbine and so that means you are going to use water it is essentially a pure water ok. So, we are not discussing that probably in the next course on industrial corrosion control we will talk about corrosion of boilers and all the steps. The generation of steam means you use pure water there are certain specifications depending upon the pressure of the boilers they maintain the pH they maintain the the purity levels conductivity and so on so forth. They normally use steels ok. How does the corrosion resistance of the steel occur there? Primarily because by controlling the pH you can do that. When you do that they form a film on the surface. See otherwise you can imagine that steel is you know in a in a typical boiler the temperature can be about to be to 280 degree Celsius ok and in economizes maybe 130 140. The temperature of the steel tubes are in this range and use water and you know it will undergo corrosion right. See you you know very well and you have a lot of steel structures here when rain comes you see a flash just couple of hours you see is all done turning that it is brown. How is it possible for the steel to resist corrosion here? By controlling the environment that means you change into into a in alkaline pH and the film formation becomes a dominant mechanism of corrosion prevention of steel. So, that means, these oxides are generally a magnetic oxide right. This is what is the magnetic oxide Fe 3 O 4 is a magnetic oxide formed and this oxide prevents the corrosion of steel structures. See please look at you have this formation. When does it form? This will form only when the solubility of Fe 2 plus and Fe 3 plus right. This constitute Fe 2 plus and Fe 3 plus am I right? The solubility of this one is less than concentration of ions right. So, here what happens? You have Fe 2 plus and Fe 3 plus ions are coming by dissolving. If the concentration exceeds at a limit they form the precipitate they form a protective oxide please understand this ok. So, that is how generally these metals are prevented. Now, if I have a flow what happens? They swept away these ions from the surface. These ions are not going to get enriched and so, the film formation becomes very low right. So, high velocity high flow rate means they are I mean the low concentration of Fe 2 plus and Fe 3 plus taking place. That means, the metals start dissolving more and more ok as you increase the velocities. Here that is what it means. Here the velocity increases the dissolution process by sweeping away the corrosion products. That means, the film formation becomes very low in that cases. So, that is the primary mechanism of corrosion of steel by flow induced corrosion process. How do they appear? How do they appear? This is the photographs taken on a steel pipeline I mean you know of a superheated water. You see the left side has got a single phase flow here the two phase flow. You can see that the surface is quite rough ok. Because of the dissolution process the surface become rough. See they are all they are dissolved in in water. Here it is they call it they call it as tiger stripes. It looks like a appearance like a tiger stripe. So, they they they they named as tiger stripes. Please understand that these these are not very high magnification they are lower magnification they are not micro structures ok. This is the kind of appearance of the surface they are affected by flow assisted or flow induced to the corrosion process. Now, these are very important it occurs in the power plants in several places boiler feed water pump, place of occurrence, feed water pump, tube sheets, tubes in HP heaters, heated drain lines, economizer inlet tubings, piping to economizer heads, de aerated cell. Now, there are several places I think I just give some illustration that they are important for thermal power plant. Of course, it can happen to a nuclear power plant as well right. So, you have seen now the mechanism of flow assisted corrosion. We have seen the importance of the flow assisted corrosion ok. Now, let us look at what are the factors that affect the flow assisted corrosion. Obviously, you always have a material component, you have the environment conditions, then you have what is called as hydrodynamic conditions. So, people have modeled this actually. In fact, this is one of the serious problems in nuclear power plants ok. They have modeled this flow assisted corrosion. The factors that involve our material I have seen the environment and the hydrodynamic conditions existing in a in a in a in a flowing system. Now, in the materials mostly people talk about the chromium content later. The environment we have the pH, temperature ok the oxygen content and the hydrodynamic condition means it is all related to mass transfer coefficient right, how the mass is being shifted from the surface to solution. So, there are going to be a complex parameters ok. So, if you look at this it is very interesting now. In a pipeline the hydrodynamic conditions might change depending upon what? Depending upon the diameter, the velocity, the shape for example, it is a U bend or maybe it is a elbow ok. These locations are affected by severely by this flow assisted corrosion, but these are straight forward pH, temperature and oxygen content and increase in chromium content is supposed to be better from the resistance point of view. Chromium, chromium increases, increases in chromium lowers flow assisted corrosion and say that if it is more than 1 weight percent chromium, there will be no flow assisted corrosion. I do not say no and it says I would say I think significantly lower. It does not occur in stainless steel and you know stainless is are not used very extensively you know because it is expensive process you know materials. So, in economizes many of the units we discussed here they are not made up of stainless steels, they are earlier they made up of simply carbon steel. Now, people go for chrome steels ok, one chromium or even two you know two quarter chromium kind of, but one chromium is is quite common materials. Now, people use in order to prevent the flow assisted corrosion of metals. How one percentage chromium? Yeah it is a good question you you just give me just some time we will discuss now you will see how this one percent chromium is sufficiently enough to form a stable oxide on the surface ok. Let us look at the environment now ok. First the temperature and the velocity these two are very critical to plot temperature versus the corrosion rate you will find like this. And very interestingly the temperature in these range these are different velocities v 1, v 2, v 3 and v 4 and v 1 smaller than v 2 smaller than v 3 smaller than v 4. When the velocity increases the flow assisted corrosion increases, but note notice here it increases a steeply at this temperature with temperature rises and then again there is a drop in in the corrosion rate. So, it is a very unusual thing that you notice for any corrosion process. It is not same as we discussed earlier you know you remember earlier we discussed that the effect of temperature on the corrosion rate of water remember I think some of you. I said that when you rise the temperature above 85 or so the corrosion rate of water drops because the oxygen escapes from the surface right from the from the water. You know boiler that does not happen right the oxygen cannot escape from that nor the oxygen is present in this water at all. So, what happens is that essentially if you rise the temperature what happened to solubility? In some cases the solubility increases some cases the solubility decreases. What do you call the solubility called? Have you heard of this retro agressive solubility? So, the metal starts precipitating after certain certain temperatures. So, when rise the temperature the ability of the system to to precipitate to form protective film increases. So, the rise in temperature increases the kinetics of metal dissolution assume that there is no film on the surface. The rise in temperature will only lead to increase in the corrosion rate am I right? Assume that no film is formed and if you rise only the temperature the corrosion rate only will increase with temperature. But so, there are two factors the rise in temperature increases the corrosion rate, but the rise in temperature also increases the scale forming tendency. So, the corrosion rate starts falling like this. So, you have an optimum value here wherein the corrosion rate is highest and then it starts falling ok. So, this is a typical of in a in you find in terminal power plants they are they are operating for the utilities actually ok. And this temperature is in the range of this temperature is in the range of 129 to 149 degree Celsius for a single phase flow and it is in the range of 149 to 199 degree Celsius for two phase flow. So, what are the factors that that then can control flow assisted corrosion, velocity, the temperature right the pH. Now, all of them should be seen in in in in in light of the ability to form an oxide film or to damage the oxide film please look at that ok. So, those factors which assist the formation of the film will lower the flow assisted corrosion and those factors which dissolves the film will increase the flow assisted corrosion. So, you do not need chromium to form a passive film in steel. How does the film is formed? The film is formed by maintaining the pH. So, pH is is a primary thing for that. So, the role of chromium is just to assist the film formation. So, it is not going to be a pure chromium rich passive film that you see in stainless steels ok. Here the same the film will be here having rich in iron oxide is Fe 3 O 4, but the chromium assists the formation of a stable Fe 3 O 4. So, the kind of film formed here is not same as the kind of film formed in stainless steels. The chromium is only assisting it. If if the pH is not properly maintained I do not think that chromium will be sufficient to form any film at all actually ok. So, that is from we have seen from only from that point of view. And so, the one percent chromium is good of course, you increase the chromium content is even more better, but of course, cost increases. In industry every penny is important you do not want to keep increasing the MOC better materials because that is going to cost more ok. Now, what else can improve the improve the film formation interestingly is the role of dissolved oxygen ok. So, whatever you discussed is is supported by this dissolved oxygen content, deo content versus the corrosion rate. Now, so it is this is called as polished surface this is pre filmed. Please notice that generally the boilers are passivated you know it is called a treatment called passivation treatment people do they call alkali treatment in order to form the film ok. If you have a film here it is even better. So, please notice that the oxygen content helps to decrease the corrosion rate because the oxygen will what does oxygen do? It it stabilizes the it increases the corrosion rate no doubt, but if it increases the corrosion rate beyond certain level it will increases the metallic ions on the surface and it forms a film. It is very similar to critical condensity right. When it is a critical condensity at that point of time you have more metallic ions than the passive film starts forming. So, you need a critical amount of metal ion concentration on the surface in order to form the protective oxide film. So, the oxygen content it does help to do this. This means it is a very interesting here the corrosion control in this case is done by injecting some oxygen content in the boilers ok. And they it is it is about it is about 50 p p p please look at ion it is not ppm level this is 50 p p p levels. So, they lower lower the the flow assisted corrosion ok. So, we will not get into discussion of the mechanism of that there are a lot of papers available those who are interested you can can read. Please notice this water treatment is not common for all the boilers. The normal boilers we talked about they do not use this, but now we use boilers such as hope you people might have aware of this super critical boilers they give this treatment. They give the this called as oxygenated treatment in order to in order to reduce the flow assisted corrosion. So, I just summarize now what we have seen so far so that you have some clarity. You have flow assisted corrosion what are the factors that affect flow assisted corrosion? It is a material the environment involved and of course, the flow conditions or hydrodynamic conditions. The mechanism here is is is what the flow assist what? Simply the electrochemical dissolution process notice no mechanical damage it is simply that the flow conditions help to sweep away the corrosion products and so, the corrosion is increasing. They do not allow the film formation or or lowers the film formation on the surface. So, how do you control this? Control measures chromium alloyed maintaining the optimum pH very important thing the design right. Why you say design? The hydrodynamic conditions existing on the surface depend upon flow conditions right. So, you could it could it could change. If you are going to have a sharp bends you are going to have more flow assisted corrosion. If you avoid that it may reduce if reduce a velocity then also the erosion I mean sorry not erosion corrosion flow assisted corrosion would could decrease. So, the design of the system also is is equally important and of course, the corrosion allowance you do not want that as the primary way of preventing it, but that still is we are not going to totally eliminate flow assisted corrosion, but incorporate the flow assisted corrosion rates in the design. Especially you know in a unit some places inlets for example, or maybe the bends elbows if you incorporate that you know the corrosion allowance to lower the flow assisted corrosion. What next topic which is erosion corrosion? Let us straight away bring out the difference between erosion corrosion and the flow assisted corrosion ok. Here no mechanical damage the erosion corrosion mechanical damage is predominant. It happens very widely across several industries. The flow assisted corrosion mainly I would say related to the the power plants whether it is a nuclear or fossil fired by I mean power plants ok. So, you see this I now in almost the pipelines the the the locations I would say the locations or where you see them the erosion corrosion are the pipelines especially on the bends and elbows. You see this you can see that them in the heat exchangers especially the inlet. You see them in the reaction vessels you will see the agitators grinders for example, see in the in the the mining industry that grinding is very very common right. And sometime even this over is transported as a slurry along the pipelines. So, you see huge amount of erosion corrosion taking place in the in the pipelines. Again you will see that you know impullers and gas turbines and hydraulic turbines, nozzles they are all quite common examples of erosion corrosion taking place. It is an example of the erosion corrosion please do not use the term flow assisted corrosion here. It is now erosion corrosion. What you see is a this is a reactor vessel reactor vessel it is a valve actually it is a valve. Today it is there is a reactor there is the inlet here this is the inlet for this is actually the sulfonic acid reaction vessel here. This is a the kind of autoclave you want to call it reactor you call it. Through this the sulfuric acid is is is pumped. This is the surface of the reactor the sulfuric acid travels like this ok. You can see the mark you can see the marking here. Severe corrosion here taking place right. Next to that there is not much of a corrosion. For two reasons of course, one is the velocity see here. The second is then the sulfuric acid get diluted right. I mean the sulfuric acid get diluted with the corrosion rate of the sulfuric acid also comes down sulfonic acid here. So, you can also see some kind of flow patterns are you able to see this here? I hope you were able to see this flow pattern here. So, these you know these are all having specific features to show that there is certain amount of uniqueness in the way the metal gets damaged they are called fingerprints ok. This reactor did not even last for about 6 months within 6 months you know the reactor starts leaking case. This is the earlier one was a stainless steel. It is a stainless steel what you seen here it is not a carbon steel it is a stainless steel. This one is a copronical alloy it is copper 10 percent nickel and this is a tube for an heat exchanger. In the tube the seawater was flowing. Look at this the velocity was hardly about 1 meter per second was the velocity. The external I mean on the cell side the process fluid it essentially is hydrocarbon you do not see any corrosion on the on the cell side. I hope you can see the marks here right you see the marks here marks marks and I hope you can able to see some mark here also right. This is an inlet the inlet of we need a changer. It is a very interesting and there are several tubes in the heat exchanger within about few months of commissioning some tubes started leaking few months it was in a in a refinery located in Mumbai. So, they change the tubes you know within another one month the heat exchanger started leaking again and again changed the tubes. Almost every month they had to shut down the plant. See when you shut down the plant because this is a critical heat exchanger if this exchanger does not operate then the whole refinery has to shut down you know if you shut down the refinery for a day you know it just runs in the corrosive piece it is not just lacks the corrosive piece. The material is not very expensive compared to the production loss you know you see this. They were using seawater and seawater you know seawater stainless steel will have some problem because they may pitting corrosion or you may have crevice corrosion and cooper nickel alloy is supposed to be good because they do not pitch and all right. And cooper nickel is is suddenly better from the point of view of what you call as dealloying as compared to brass we seen it you seen yesterday right. Little bit of discoloration you can see this little bit of green color that has happened because of the nickel dissolution process. But nevertheless 1 meter per second is hardly a velocity and is failing so frequently. So, that was one of the problem we will probably see you know what could be the problem you know in subsequent discussion. When we start analyzing the erosion corrosion then we come back to this problem and see what was the real reason for the erosion corrosion of this particular heat exchanger. Now what I was trying to say was the inlet of the heat exchanger is generally undergo the erosion corrosion in fact, that is termed as inlet corrosion. So, when there is a turbulent it mechanically damages you see you know it it removes mechanically ok. But then what is the difference between erosion and erosion corrosion? So, I give an example suppose there is a pipeline and you are passing let us say nitrogen gas in the pipeline maybe there are certain solid particles ok and the tube gets damaged. So, it is a purely erosion damage. You put a bit of moisture in the system and you say that gas is no more dry the solid particles are no more dry you just have some water how it becomes erosion corrosion why because you have a corrosion component coming over here. So, erosion corrosion you know it is it is it is a it is a conjoint action of the mechanical erosion factor and assisted by the corrosion factor. So, when you talk about erosion corrosion then you look at the mechanical property of the material as well as the corrosion property of materials ok. So, that makes a difference in the flow assisted corrosion we never talked about mechanical property of materials we talked about the film formation film stability. That is why I said the mechanism are different and so, the way you prevent corrosion also going to be different here. So, what are the factors controlling erosion corrosion affecting erosion corrosion? Here also we talk about the surface films. They are important talk about velocity, impingement, turbulence etcetera. They are related to the physical state of the environment velocity, impingement, turbulence they are not talking about chemical state of it right we are not talking about the pH of that corrosiveness of that is a physical state of the environment ok, metallurgy of this. The metallurgy of the alloy will also decide the surface films you know it is not necessarily they are independent ok, but metallurgy here we generally mean in terms of the other mechanical properties such as hardness, toughness all kind of stuffs. So, let us look at the surface films. Let us take the the first one. Please notice the film offers a barrier for corrosion it also takes the impact takes impact of the environment. So, what should be the property of the film or properties of the film which can provide a better erosion corrosion assistance? What are the properties? It should be the film should be it should be hard isn't it? Then what happens? It should be adherent, 3 what happens? It should be resilient right and if you want it more explicitly or be less brittle should be dense and less porous. So, those alloying elements that added to the alloy that can promote all these properties will ensure that the alloy has a better resistance against erosion corrosion problems ok. So, these are the important thing that is the guiding factor for all our development also right. So, please notice it is not necessarily a very thick film now sometimes thick film can be a problem isn't it? A thick film is generally the oxide films are better compared to the metal. When the film becomes thicker and thicker what happens to the resilience it becomes very bad. When the film is thin it is more resilient against any kind of elastic deformation right. And in erosion corrosion you are not going to get into too high impact any cavitation it does happen. So, you are talking about a resilient within the elastic limit most of the deformation would occur in erosion corrosion ok. Let us look at some examples of the surface film formation happen. The surface film formation would also depend upon the nature of environment it is not just only metal also ok. Suppose, I take a stainless steel I have a reducing environment what happens to the film forming tendency of a stainless steel decreases right because you need oxidizing conditions that could be seen very well in the case of let us say type 316 stainless steels corrosion in ferrous sulphate plus sulphuric acid slurry. Please notice ferrous sulphate is reducing condition is not oxidizing condition. Ferric sulphate is oxidizing condition. If you use 316 stainless steels and if you can look at the corrosion rate again with respect to the temperatures the corrosion rate of that somewhere about 5000 MP y something like that. In the static conditions ferrous sulphate plus sulphuric acid slurry in the static condition this is not flow condition where if you increase thevelocity for something like that. So, I think it is about some I do not know about 3 meters per second state is going. So, it is it is going very very high corrosion rate steeply also changes with respect to temperatures. But you add 316 stainless steel and add some copper to that the corrosion rate you know. So, when you add a copper tostain the steel the film forming tendency becomes very high. In fact, if you if you if you look at the material selection for sulphuric acid applications the stainless steels have copper in it. So, copper helps to form stable passive film. So, this is a significant drop a significant drop in the in the corrosion rate and these copper additions are all what they are all in terms of what 0.01 to 0.02 weight percent not a very large amount of copper content you can happen. Similarly and what people have seen seawater a copper a brass the copper forms cuprous chloride and it forms cuprous oxide in derated seawater and cuprous oxide is more stable. So, what happens? So, copper zinc is better. So, what we are trying to say is that the nature of the film that forms on the material decides the erosion largely the erosion corrosion resistance of the material. The other example is steel in pure water it is where they studied the pH at 50 degree Celsius and the velocity is about 39 feet per second is the velocity here and this around about 6, 10 pH. Now, the reduction in the corrosion rate both the places are largely related to ferrous hydroxide and ferric hydroxide formation. Here from granular you know Fe 304 oxide formation again there are a lot of internal stresses. So, the point that we need to emphasize here is that the nature of oxide forming on the metal surface bears a significant influence on the erosion corrosion of metals. So, let us stop our discussion for the time being.