 We will continue to discuss the cathodic protection method for the control of corrosion. We saw earlier that in a corroding metal which are exposed to environment suppose you have an environment the metal is getting oxidized electron, this is the metal and they release the electrons on the surface. There is a cathodic reaction and which takes away these electrons these electrons will flow and there can be some species in oxidizing species which would accept these electrons they become reduced. So, in this case the rate of oxidation equals the rate of reduction the potential exhibited by the metal is equal to what? The metal is equals to the E corr or the corrosion potential. Now, if you are going to supply electrons to this surface, if you are going to supply electrons to the surface, if you are going to supply electrons to the surface, then what will happen in that case the reduction reaction rate rate increases right, the oxidation rate decreases. So, thus is a simple lead chattelier principle and that is how the cathodic protection works. Now, we need to understand this more in detail about what are the factors that control or that are required in cathodic protection of a given metal. This we can understand by looking at the Evans diagram right. We know how the Evans diagram is you know can be constructed right. So, let us construct the Evans diagram I think all of you could do that right and you can do the you can draw an Evans diagram. What is the Evans diagram here? You have so, log i versus the potential right and how do I get E corr? So, you represent the cathodic reaction starting from its equilibrium potential somewhere here and the reduction reaction and you also have somewhere your in oxidation reaction. So, this is a metal N plus plus N electrons right and this corresponds to what? This corresponds to oxidizing species accepting the electrons and getting reduced. And what is the E corr? This is the intersection point you guys know and this is your corresponding I corr. So, what you are doing actually you are forcing electron onto the metal surface. How can you do that? By making the metal surface negative right. So, you are going to make the metal surface is negative. That means, you are polarizing the metal negative to the corrosion potential am I right. So, you are you are moving from down from here you have to you have to move down right. So, you have to bring down the potential from this point to this point. What is this? This is the called as the equilibrium potential for what? For the metal equilibrium and this is the equilibrium potential for your oxidants and reductant got it. Now, I have to move in order to reduce the current corrosion rate I need to move from here to this point by applying an external voltage apply an external voltage I do this. Ideally, the metal stops corroding when if I release this particular potentials at this particular potential the metal becomes totally immune to corrosion. If you have to make it immune to corrosion I need to supply the current. Do I need to supply current? See at this point at E car I do not supply any current agreed, but when I move here the rate of reduction reaction is higher than the rate of oxidation reaction. That means, the remaining current has to come from an external source agreed. So, this is the current that is to be applied. So, this is the current and this is I applied to completely protect the a steel or metal I can put any of them actually ok structures. So, this is the difference in current right the cathodic current minus anodic current gives you the net current I have to apply externally to bring the potential from this point to this point that is the principle of cathodic protection. How do I apply the current? I apply the current what should be the then the case if you have a metal if you want to bring down is the potentials right and I need to have and this is going to be your cathode. What kind of current I apply? Is it AC current or DC current? DC current ok. So, apply ok. So, this is a on the negative terminal this is the positive terminal and this is what this is a can be rectifier it can be rectifier if you want you can call it as a rectifier. What are happening now? What is the direction of the flow of current? The direction of the flow current is opposite to the direction of the flow of electrons. Now, if this anode now current flows like that systems agreed. So, rectifier now provides the current to the anode and the anode sends the current toward to the to the medium. The medium could be a soil or any electrolyte in general may be sea water. So, the purpose of the anode here is to send current in the electrolyte which corrodes this is the metal structure and the role of rectifier is to provide the current to provide the voltage. Now, what should be the voltage in this case? The voltage in this case as for the theory is it is equal to the equilibrium potential of the structure. So, we see that whether that is should be you should do it or not we will see later. So, this is one form of cathode protection. So, this is called as the impressed current cathode protection because you are using an external source rectifier and the role of anode is only to supply current to the soil and from the soil or from the electrolyte the current enters the cathode or the steel and renders this metal into a cathode is what it does ok. So, that means, electrons start flowing like that right electrons flow here the current starts flowing in this direction agree. So, this is called as a impressed current and or a impressed current cathode protection system. You can also have you know the galvanic series right you have seen it before or you have seen I have given you a table before there is a series or you have a EMF series. If you look at that series the corrosion potential of the metals and alloys are given in some order. That is in the table you will see that the potential the corrosion potential decreases from the top to the bottom. Top most is platinum and bottom most given there is a magnesium there. Suppose, you take the case of steel and just take the case of let us say zinc to take this case right which is relatively noble steel. The steel is noble and zinc if I can short steel I short them up here. The potential of the galvanic potential of zinc is normally considered as minus 1.1 volt and the corrosion potential of steel in you know in see water is considered as something like minus 0.6 volts with respect to saturated caramel electrode. This is relatively noble and this is relatively active. So, what will happen now? Zinc will dissolve as zinc ions the electrons will start flowing and enter this steel like this of course, it is go like this only ok. The current will start I mean the current moves in the directions the negative ions starts moving in the direction. So, to put it in this way zinc dissolves as zinc 2 plus plus 2 electrons and these electrons are available for cathodic protection. What is the driving force for that? The driving force for this is you can consider E cell is the driving force is equal to what is equal to minus 0.6 volts minus of minus 1.1 volt. So, what is the value? 0.5 volt approximately ok. So, you have that much driving voltage for the system to sustain. So, zinc is able to provide you a driving force. So, that the steel can become cathode is made as a cathode and this process is called as. So, you have two types of cathodic protection system. One is called impressed current cathodic protection. The other case is sacrificial anode cathodic protection system. Let us look at this cathodic protection a little bit more in depth to try to understand this. Let us go back to this diagram. When you do a cathodic protection of the structure, no doubt that is a reduction in the coefficient rate. But what is happening to the cathodic reaction rate? It increases right. The cathodic protection the cathodic reduction reaction rate is increasing. Assume that this cathodic reaction is let us say H plus plus electron gives you hydrogen right. Assume that the cathodic reaction is this one. So, what will happen now? Lots of hydrogen gas will evolve on the metal surface and it can lead to hydrogen embrittlement we will see later right now we are not talking about it. If you apply a coating the coating will get damaged because of the gas evolution. So, it is not a right thing to do that you can apply as much of voltage as possible. You apply a very high voltage from here to this the chances of hydrogen evolution rate is very high. So, in practice we do not normally keep this the the cathodic protection potential at or is equal to the equilibrium potentials. So, it is kept in a way that is more practical and more appropriate from engineering perspective of this actually. So, that becomes a criteria for cathodic protection understood. So, from otherwise ideally speaking you can make the metal immune to corrosion by bringing the potential from e car down to equilibrium potentials. The disadvantage is that the hydrogen evolution will takes place and the metal may become embrittle if you have a coating the coating also get disbanded. So, in practice you do not do that. So, what we do in actual is is is as follows. So, you come up with a criteria which is called as cathodic protection criteria. The idea here is that if I can reduce the corrosion rate by 10 times the life of the structure increases by 10 times right. Assume that without a cathodic protection the steel corrodes steel corrodes in a soil at at let us say at a rate equals to let us say about 20 MPY right. So, you can design the structure right. Suppose I want a pipeline for example, I have a pipeline I can design the thickness based on the pressure and all right. If you can bring down the corrosion rate to 2 MPY assume that with this I have about 5 years is a life. If I bring down the corrosion rate from 20 to 2 MPY it becomes. So, I am not completely stopping corrosion, but it is ok from the point of view of engineering applications. So, the criteria of corrosion or the criteria of cathodic protection is based on that particular concept. So, the following criteria is is is generally used it is it is actually called minus 0.85 volts with respect to copper saturated copper sulphate electrode. Very widely used criteria it has some limitations we will not discuss now actually ok, but let us just take it as as a normally used criteria for the cathodic protection. That means, after applying cathodic protection I should get a potential which is equal to minus 0.85 volt with respect to copper saturated copper sulphate electrode. This is a reference electrode that is understood as is adequate protection to serve the required life of the particular pipeline. And here you minimize the the the hydrogen evolution you minimize the hydrogen damage all is done. In fact, if people are going to use high strength steels you know in the case of a ship and all in fact, they do not even go this much slightly making little more anodic because in the sea water if you make it more negative the hydrogen evolution occurs the then there will be cracking taking place. So, there are some variations within this defined a criteria based on the applications, but most of the pipeline application this criteria is is followed. Now, let us look at the the difference between the impressed current cathodic protection system the sacrificial anode cathodic protection system. What are the what are the advantages and disadvantages of each of the systems? And I think we can we can look at it two systems. Bottom sir minus 0.85 volt. So, even if the soil is changing whether it is aerobic, anaerobic the criteria remains same it is generalized criteria. Yeah, yeah whether soil is aerobic or anaerobic the criteria is almost the same. The only difference that happens is that if the soil has got a microbial corrosion then you add another 100 millivolts you becomes minus a 950 millivolts. And where the problem is is uncertain for example, see what happens since you you have started that I can I can clarify. In some cases the corrosion potential itself is equal to minus 0.85 you measure with respect to the copper copper sulphate ok. So, then the people use a criteria called as 100 millivolt criteria. What we do is when I apply a cathodic protection the potential drops you know assume that it is minus 0.85 it may go to minus 0.95 volts right. You turn it off you know and when you turn it off then there is a shift from the protected potentials to the unprotected potentials ok. And that if it is 100 millivolt difference then we consider that the pipeline is or tank is well protected. Now, the criteria is very simple you know the Tafel equation right. What does the Tafel equation say? All of us know that eta is equal to what eta is equal to let us say in the case of anodic thing beta log I upon I naught or you can say I car whatever you can call it ok. You can call I car here. What is eta here? Beta is equal to E applied minus E car. Now, if you substitute here 100 millivolt isn't it? Substitute here 100 millivolt. Assume that the Tafel slope is is about 100 millivolt ok. Then what happens? This ratio becomes 1 you can see here now assume that the beta is 100 millivolt and the shift is 100 millivolt right isn't it? The eta is equal to shift is 100 millivolt right. So, if shift is eta is equal to 100 millivolt and beta is assumed as 100 millivolt what happens? Then you find log I upon I car is equal to 1 that means, from from this this you have moved ok. How much you have moved? You have moved about 10 times am I right or not? So, that is how you can able to you can able to see that 100 millivolt criteria means it is it is about 10 times you are lowering the corrosion rate of the metal. If you make it let us say 300 millivolt it will be 1000 times you are reducing the corrosion rate provided the Tafel slope is equal to 100 millivolt of course, that is assumption that you are making it actually ok. So, there are other criteria people use it and we are not discussing right now you will you know if you take the other course advances in design and control of corrosion there will be detailed discussion on the cathode protection engineering you will see the merit and demerits of various criteria. It is not that this criteria is is ideal criteria for that for simplicity it is ok to understand that minus 0.85 volt with respect to copper copper sulphate is normally used across the various various you know structures and soils and so on ok, but yes there are issues which I think should be taking care of ok yeah. So, let us let us go into the comparison look at ICCP impressed current cathodic protection system you also have sacrificial anode cathodic protection system. It is first of all it is a a complex system you need a rectifier and system like monitoring them in practice it is a very very complicated feedback systems are there it is very complex and it is it is cost high this is simple cost low I am talking in relation to that each other right. This is for long term we want to apply a long term cathodic protection as I said 20 years 25 years like that people go for ICCP this is for short term application. The third is a very important criteria if you talk about a pipeline allowing in a very highly resistant soil right soil has got resistance. If the soil has got high resistance I need a driving force has to be higher in order to pass the current from the anode into the soil and then on to the pipeline right. So, it requires higher driving voltage if the soil is low resistance I have less problems. So, high resistant soil I can use this soil ok can be applied. The rectifier can give you 50 volts right 20 volts I mean there is no restriction as it is in terms of capacity right because high driving force driving voltage availability ok. Can be used only in the low resistant it is not possible always. 4 anodes generally are stable stable they are like they are like let us say graphite iron silicon platinum titanium insoluble anodes these anodes are sacrificial anodes they are like zinc, magnesium and these alloys they have aluminum. Aluminum only in sea water applications aluminum forms a pacifism right. So, you cannot be used everywhere ok. Aluminum and aluminum alloys people use actually aluminum people use aluminum zinc indium system aluminum zinc tin systems these are the alloys they use for sacrificial anodes. There are more differences we will not be discussing about for example, there is something called interference current stray current corrosion all they are in ICCP, but they are not there in the they are not there in the in the sacrificial anode cathodic protective system, but I think we will we will not discuss those into into details of that. One thing I just want to say here because it is not out of place to talk about is that cathodic protection and coatings, pain coatings I have put it more specifically pain coatings are complementary they help each other right. You apply coating for what to reduce corrosion, you apply you apply cathodic protection to reduce corrosion the why you should have coating as well as cathodic protection. Something which has been that is how holes are there that time also. The second one is more correct you you do not find any pain coatings which is 100 percent impervious to water you know there are certain defects this system. So, coatings yeah they do protect, but they are not going to protect completely first of all. First reason that so, it is going to aid the coating in preventing corrosion to a large extent ok. So, the reason is the coatings are generally or defective. The second important point the current requirement for cathodic protection is significantly brought down you would not believe you know from amperes you can bring down to milli ampere about 1000 times and even more you can bring down the current requirement. So, coatings the current required for cathodic protection I think we can keep discussing more and more I think because in engineering you know it is you know what you saw is only the science of it right. The engineering is is complex if you want to install a cathodic protection then you have to define how much current is required, how many anodes are required, how to distribute them, what is the what is the rate capacity of the rectifier several issues are there. So, that is we call a engineering that is not discussed now ok. And those who are interested you can look at the book yeah see I mean yeah I mean that is not called as a cathodic protection ok. In the paints also you add a zinc as a pigment zinc powders zinc flakes are added as pigments and the zinc you know when they sit on the steel surface it can cathodically protect the structure you know that is what right. You you say good question now for example, suppose I have a pipeline and I apply a paint on this sorry this is the paint and in the paint if I am going to add you know zinc particles sufficient quantities ok. Now what happens now the paint is we saw in the morning that it is a barrier it does not allow the water to permeate just like that it is a barrier ok. But nevertheless the water will permeate the corrosive species like chloride sulfate may permeate over a time period the water will ingress here. The second level of defense the first level of defense against corrosion here is a barrier by the paint the physical barrier. The second level of defense is that ok if the water comes over here the zinc now will act as a sacrificial thing. So, it is a mini cathodic protection system right isn't it at a micro level they are operating. So, the life of the paint coating is increased significantly because of the presence of zinc in the in the in the paint and they they called as these these paints are called as zinc rich organic paints. Now the issue is we should have enough zinc powder. So, the all the zinc powder should be touching each other right otherwise if the zinc powder is not sufficient and then the cathodic protection may not be operating. So, there are critical amount of zinc powder like 80 percent is added. So, that this zinc is effective cathodic protection. Here we do not call a cathodic protection in true sense of it ok. But the principle of cathodic protection is very much applied here and of course, is a conjugate with the physical barrier offered by the paint system yeah it is it is what it does work. So, it is use the same concept to develop a new paint any any questions any of you ok. Let us go into the into the next aspect of the the the controlling the uniform corrosion of metal through electrochemical means and this is we call as anodic. It is not very difficult to understand how and when anodic protection can work. What is anodic protection? You are you are rising the potential towards the anodic direction right. The potential is increased towards the anodic direction. If you increase the over voltage isn't it right this is what you do what do you expect to happen in a normal metal the corrosion will increase right. But if the metal is passivating if you rise the potential to the passive region what will happen to the corrosion rate? A decrease in which case what will be icon icon equals to IP right. So, by rising the potential to the passive region and you are moving the icon which is going to be equal to high passivation. So, this is a simple principle right. So, that means, the system has to be passive ok. So, it is for the passive system. Only passive systems you can able to do this. The secondly, high passivation has to be lower than ICAR otherwise no use. If ICAR is going to be smaller than IP it is not going to help you at all right. Let us look at the Evans diagram. Let us look at the Evans diagram for a passive system and see how the anodic protection system works right. So, let us draw the Evans diagram. I suppose you can you can draw yourself and and see how it can be done. The system is not passivating right what happens? You have a cathodic curve and you get the ICAR. I can do it provided I rise the potential right. So, if I can rise this potential to this level assume that I am going to have I take it to this level now ok. If I hold the metal at this particular potential the corresponding cathodic reaction and the corresponding anodic reaction rate right. You at this particular potential suppose I have a metal right metal is immersed in a let us say in sulphuric acid somewhere right. Say in sulphuric acid from ICAR I am going to rise it by applying a positive potential and negative potential for this right. This is let us say a steel maybe. If I rise it to this look at this the ICAR has moved up, but what happened to the ICAR just come down to this value. So, corrosion current density has significantly reduced by rising the potential from ICAR up to the passive region. So, many interesting things are happening here right. Look at this now if I have to bring down the same corrosion rate as I do for passive system for example, here. If I have to do it by cathodic protection, suppose I have to do it with the cathodic protection right I can bring down the corrosion rate either by making it anodic passive or I can bring down the potential and make it cathodic right. Both the cases I can bring down the corrosion rate, but what is the difference there? ICAR. Yeah, ICAR is the same both cases. The current required for cathodic protection is lesser more than anodic protection. The effective amount of current I need to supply to make it anodically protected which is more. Cathodic. Cathodic is more anodic more. Now, the current is going in this direction right then log scale please see this. Assume that this is let us say 10 power minus 6 assume that is going to be 10 power 1 ok. Now, you tell me where you apply more current cathode right. So, you need only a few micro amperes here is generally you need only a few micro ampere per centimeter square. Here we need few milli amperes isn't it? You see you have to move in the in the in the log scale then you understand that right this is higher value. So, cathodic protection generally requires higher current compared to the anodic protection right is one difference right. So, one difference is that is let us look at the difference between these two systems comparison right. Let us go to the anodic protection. To reach the eye passivation do we have to surpass eye critical in anodic protection? How do we do it? Yeah ok that that is it is a good question ok that that is that is a good question and and I just come here this is a very good question actually. Let us let us look at this process you need to reach this place how do you reach it by making this as a cathode right. So, you need a cathode this is the cathode. In cathodic protection the counter electrode is a anode the anodic protection the counter electrode is a cathode right. So, how much current do I pass here? How much current do I pass here? Before I reach here I need to pass a current you should be higher than the critical current density then only I can able to reach this this this cathode should supply enough current so that I reach here, but once I reach here what happens? Maintaining. The maintaining that becomes easier. So, it is a good question. So, the capacity of the the the the DC source in fact, I do not call it DC source we call this as a potential stat we will come to this later. This has to have the capacity to cross the critical current density otherwise you will not able to achieve the anodic protection at all, but after reaching that place then I think there is no problem ok you need less than the current ok that is something we should be you should you should you should you should understand that. So, when you talk about anodic protection and cathodic protection the one of the one of the first and foremost is the anodic protection is applicable only when the system is fascinating, but the cathodic protection is applicable to almost all systems we are no issue at all actually ok. So, only for passive systems I want to go once a further and say that IP has to be lower than ICAR that is not sufficient it has to be other criteria for that it can be applied for any system. The other important thing is generally the current required is small current need is let us look at this a bit more closely this diagram. I have say suppose I am holding the potential of the cathode somewhere here, I start moving little bit up and down what happens? The battle will not corrode more if in this case if I do not hold the potential properly then I end up corroding at a very high rate than the and the ICAR values right. So, the equipment required for that is called a potential stat ok. You need a potential stat ok here you need a potential stat and it is normally expensive rectifier is sufficient. Now, the fourth point is you cannot make it immune cannot make it immune to corrosion can can make it right can make the metal immune to corrosion that is because cathodic protection is a is a thermodynamically stable system, but anodic protection passivation is a kinetically hindrance. So, you cannot completely make the metal immune to corrosion but that is not really required in actual service applications. Now, the question now is is anodic protection been used at all? It is now used actually and because it took you know when you talk about a potential stat the potential stat was first constructed by a person called Adileno ok. It came much later as compared to rectifier and other constant current sources. So, anodic protection is a relatively a new engineering concept that happened as compared to the cathodic protection system ok, but nevertheless it is used in fact, it is used in sulfuric acid manufacturing plants. They use it for heat exchangers can be used for sulfuric acid tanks is used for steel used for stainless steels. It is used for steel and stainless steels, but very limited applications compared to the cathodic protection systems. But indeed used ok. So, this brings us to the end of discussion or you know lectures on uniform corrosion and if you have many questions or any clarifications we can we can have we can discuss. How much area can be covered by the ICCP and Satisfacial Anodic Method? What is the number and size of the anodic required in both the cases? Actually as I have been telling you cathodic protection is a is an engineering. Now, when you talk about ICCP generally if you compare the structures be it a pipeline or a tank or a ship hull the anode is a point source. You can say it is infinite cathode or a very very finite anode size. Now, that is where the location of the anode it matters. The current emanates from the point source and goes into the soil like a wave right like radially it goes and and and enters the pipeline. And if we compare to sacrificial anodes the number of anodes required for ICCP is less, but in the sacrificial you need more anodes because the driving voltage required or offered by the sacrificial anodes are very small ok. But there are cases where the anodes can run parallel to the pipelines. There are very very isolated cases. For example, you have a pipeline going through a rocky terrain right and the current cannot go through the rocks. So, what people do is they use what is called as horizontal anodes buried in a very thin anode buried parallel to the pipelines ok. Because you keep it outside the current cannot penetrate to the rock actually there are certain segments people do it. Otherwise the number of anodes for ICCP are very very few and the size also very very very small actually. No questions ok. So, then let us close the discussion today and we shall continue the class that we can see when you can do that. Can an inhibitor be added as alloying element? Inhibitors are added to the electrolyte added to the corrosive environment right. But when you say alloy I do not know whether using alloy is a general term anything you mix together you call as alloy right. Are you talking about mixed inhibitors? Ok. Yeah, inhibitors can be added as a mixed. See we we discussed in the morning that it can be anodic inhibitor and by cathodic inhibitor you can design a inhibitor which is having a mixed function of anodic and cathodic or you can add an two inhibitors one is having an anodic character other is a cathodic character you can do that ok. So, that is done in many many you know systems in cooling water systems people add a combination of anodic inhibitor and cathodic inhibitors ok and you know that they are they are very common. In fact, that becomes more effective in this case. See zinc for example, people add zinc which is the cathodic inhibitor you know zinc compounds and the molybdenum is an anodic inhibitor right a system people add you know zinc phosphates and and you have a molybdenum you can add it to that actually ok. So, combination of that are quite common do that actually. They are in fact, very effective and when you choose again inhibitors depend upon the temperatures sometimes the temperature of the system can be quite large in a cooling water system the temperature of the system can go to 50 to 60 degree Celsius. The stability of the inhibitors are also very important in boilers it becomes even more higher ok. There are so many variations in tailoring the inhibitors ok yeah I mean you are right we can have such variations. How does the functionality and mechanism of the inhibitors vary with the working conditions? See the electrolyte would have a bearing on that on any of this ok. Now, basically the inhibitor they we have seen in the morning right either they form an adsorbed layer on the surface and how do they get adsorbed? Either you will have a positive charge on the on the you know on the molecule you know or you can have a native charge. Here these charges are all what they are I would say the polar groups you know kind of things. So, they get attached to the to the you know to the metallic sides you know you can attach to a cathodic side if for example, the molecule is relatively having a positive character right and you have negative character it may get into an anodic side. So, both both are possible to do that and in fact, there are certain models you know people people you know they they look at the density function theory and all this they use it to to to first of all understand second of all even to tailor a new molecule to do that ok. And there also people talk about adsorption isotherms like a Langmuir isotherm, Tempken isotherm you might be knowing where they look at the activation energies for adsorption more is activation energy for adsorption I mean I mean more is a frequency change for adsorption and they absorb what better. So, these calculations that people make in order to characterize this inhibitors when they tailor the new inhibitors for various systems. Yeah they they they depend on the on the metal they also depend upon the electrolyte significantly that anything ok. So, thank you.