 Today, we will continue our discussion on corrosion protection methods as applicable to uniform corrosion. We listed in the last class that the following methods can be applied to prevent uniform corrosion. It involves uniform corrosion, uniform corrosion prevention methods involves material selection and the protective coatings, inhibitors and we said that in the retrochemical techniques. The retrochemical techniques we have two methods one is a cathodic protection, other one is the anodic protection. In the last class we discussed in details about the methods of selecting materials depending upon the requirements like how critical a component is depending upon that you choose a corrosion resistant material actually because you know if you are going to increase the corrosion resistance of a material then the cost of the material also goes up. So, the application of the corrosion resistant material also depends upon what depends upon how critical the component is. And we also looked at the two types of metals and alloys one based on the noble character, the other one is based on whether the metal can offer the passivity. In both cases the criteria of the material corrosion resistance we have seen in the last class. We will continue this and the next important aspect of uniform corrosion control is the application of protective coatings. See if you look at from the point of view of wider application of a single technique for corrosion protection, protective coating stands first. It is applied at ease it is applied you know very versatile and in most applications it is possible that you can use protective coatings. And here we are not going to go in detail we are going to give very broad outline we will be having a course in the second semester which is called as MM 650 it is devoted to protective coatings. So, a detailed course will be offered on that those who are interested will attend that course and you will get a much deeper understanding of protective coatings. We just give an outline here. Again for want to read more on this you can refer these books. One is ASM Handbook Volume 5 10th edition Faheo 1994. This covers almost all kinds of coatings metallic non-metallic polymer coatings and various types of things it is on surface engineering. The other book you want more towards the paint coatings polymer coatings you can read Charles G. Munger Corrosion Prevention by Protective Coatings. It is a it is a NAIS publication 1984 it is its Texas and this is more on paint coatings organic coatings. Whereas, the first book covers in detail various aspects of surface engineering that includes coatings as well ok. So, those who are really interested to go more in detail I would suggest you refer this these two books actually. Let us go to the coatings broadly you can classify the coatings as metallic paint or also called as organic coating. You can like to add this along with that as the conversion coatings. The conversion coating is not really a full-fledged you know standalone coatings many times and the conversion coating helps probably other coatings. In few cases the conversion coating can be a standalone coating. Let us look at briefly what are these types of coatings. If you go to metallic coating you can classify them based on the the way you apply the coatings. You can have electro deposition, you can have hot dip, you can have thermal spray. In fact, I want to add along with electro deposition which is also variant of it is called electro less deposition. So, these are the broad classification based on the way you apply. You also have other you know you have some physical vapour deposition and you know there are several other magnetrons portraying. Widely used relatively in large quantities these are the type of coatings used for corrosion resistance application. Otherwise you have a variant of several types of applications. We are not going to be focusing on that ok, but these are all generally used in large numbers. If you ask me the most widely used of course, is is is a hot dip coating this thing. The second one is your electrode deposition. The third is thermal spray. The thermal spray is becoming more and more popular now actually the time the the the extent of application of thermal spray coating increases. Now, if you look at you go a little more into details about these coatings, you would say you can have relatively active metals or you can call as anodic coatings and you can also have relatively a noble coatings. This is you can call as cathodic coatings. Most of these classifications are in relation to the steel you know not necessarily to steel, but steel is very widely used and required protection. So, when we normally talk about anodic and you know cathodic coatings we refer only with respect to the substrate, the substrate here is steel. If you change the substrate it is possible that what was anodic to steel may not be anodic to some other matter. Now, suppose you take steel as the substrate which is widely used you have zinc coatings, cadmium coatings. These are all relatively the anodic coatings nickel, copper, silver, gold, tin. These are all relatively noble coatings. The active coatings they go by sacrificial action. We will see this later. What is in the sacrificial? The noble coatings mostly is as sort of barrier you can say. You do not allow the corrosion to occur because they are relatively noble the rate of corrosion of this metal or relatively relatively less compared to the substrate. Each of these coatings have their advantages and disadvantages. Noble coatings sometimes can cause it can cause a galvanic corrosion. We will see this later, but the surface can look very bright. You give a nickel coating surface looks very bright. To this you can also add chromium, you can also add chromium to this. Chromium is also is widely used ok. In these metallic coatings you see zinc and aluminum are used as hot dip, hot dip. Even tin can be used as hot dip coatings. And the metals high melting metals difficult to hot dip difficult to coat by hot dip process. Because the melting point is so high the substrate might warp, substrate may melt all this can happen. Zinc and tin and aluminum are widely coated by hot dip process ok. Now the high melting metals, now these metals so high melting metals how can you coat? How do you coat? Yeah. Either do thermal spray yes or you can do electrode deposition you can also use electrode as deposition. But again there are some restriction in all cases it is not that you can you can do it, but I just give a broad picture about. Ok. So, if you talk about electrode deposition you want to have an example of this you can do copper, you can have silver, gold, chromium you can do that. That way you can even coat tin, you can coat cadmium, you can coat zinc also. They may not be high melting metals, but you can coat them by electrode they cause electrode galvanizing right electrode galvanizing is quite common. So, that way zinc is a little unique you can coat by thermal spray, you can coat people do by thermal spray, people coat by electrode deposition, people coat by hot dip process or all are possible with zinc actually. Electroles deposition here you coat you can coat generally copper, coat you can coat silver, you can coat let us say gold you can also coat nickel. Thermal spray widely you can do you can use again zinc when I have aluminum that way most metal that you can do actually you know zinc and aluminum is a very widely used coatings. Complex alloys are done by the thermal spray, necrolene and cochrolet all this kind of high temperature coatings are done, stellate coatings are done. So, there are other coatings people use thermal spray as a technique to do that ok. So, they are high temperature hard coatings it also give you high corrosion resistance. So, that is also possible. I do not think we should get into discussion about too much discussion about what these techniques are. You know electrode deposition means what you make the substrate as a cathode, you pass a DC current, you get the coating on this on the substrate. The thermal spray you melt it and throw it at high velocity and they go and get deposited on the surface. The electrode deposition you do not pass current, but what you do normally you add a reducing agent in the bath ok. So, that reducing agent reduces copper, silver or gold or nickel on the surface and forms coating. It is very important if the reduction takes place in the bulk and no use everything becomes a powder. So, so they are very catalytic surface catalyzes. So, most of the coating occur on the surface by electrolysis deposition. So, these details you will not go into you know in this course now the other course you will read them a lot. Now, let us about the metallic coatings is get into the paint coatings mostly organic coatings. There are several coatings lots of coatings, but the important are alkate acrylic coatings. You have epoxy polyurethane nowadays you have silane coatings and each of these coatings they have their uniqueness. But these coatings they act as a physical barrier. See they are inert mostly they are inert by nature. They are supposed not to allow water in the system, but you know organic coatings are not impervious to water it slowly permeates and so it limits the life of these coatings ok. However, it is ok to say that these paint coatings are stormed as the barrier type of coatings they do not allow the water to permeate and reach the substrate ok and so it is isolates the environment it isolates the water. Now, in the in the in the in the paint coatings how it is done? If you look at a paint coating generally it consists of minimum of two layers one you must have a primer you have a top coating. Sometimes you may have more layers you can also have intermediate layer intermediate layers you can have intermediate layer or it can be layers it can be have many many thing you can have ok. The purpose of the primer the primer the purpose is to provide adhesion first and foremost is it provides adhesion to the substrate. The primer may also have some additives to this there are some additives, but if you have them I can say they can inhibit corrosion you may have some inhibitors they may have some kind of you know additives like like zinc and all like that they may have they can have sacrificial action. Primer is in fact, it is a key in the painting process the top coat gives you may be other properties it might give you like sometimes you know chipping resistance it may give you resistance. There are other properties in fact, the paints if you take it is very versatile because you could have n number of properties at your disposal you can have different color for example, can be anti graffiti you know write something you do not stick on those surfaces or you could have water repellent properties several properties are tailored on the top coatings. The intermediate layer also can have different properties because you to augment the corrosion the intermediate layers are given. So, the primers when you talk about these primers can be based on alkyd primer, acrylic primer, epoxy primers, polyurethane primer all these kind of primers ok. So, these primers you know are the key without applying primers I do not think you are going to apply a coating at all this is the key in the the the paint coating of any metal substrates. In the conversion coatings we have like phosphates in the phosphate and you also have chromate conversion coating right. The phosphate as several classification take a phosphate for example, this you have simply a iron phosphate coating can have zinc phosphate is the manganese phosphate. You can say overall the protection ability of the phosphate coating increases from iron phosphate to zinc phosphate to manganese phosphate ok. The protection ability protection increases from iron phosphate to zinc phosphate to to manganese phosphate. What do they really do? What do you mean by conversion coating? What do you what is happening here? You are converting the substrate into a phosphate. For example, if I have iron the iron is converted to iron phosphate this is an insoluble first insoluble salt it provides resistance against corrosion and sometimes the phosphate also can act as a inhibitor. These phosphates are also given as a pretreatment before you coat to apply a paint coating ok. It can be pretreatment can be pretreatment for painting you do in the in the automobile for example, in automobile the automobile industry they they apply a phosphating treatment even in the case of aluminum alloys zinc alloys zinc they give a like a chromating they do that and then they apply a painting. So, they also enhance the adhesion strength of the paint coatings. In the case of manganese sulphate it can be a standalone coating it can be can be a standalone can be a standalone whereas, iron phosphate and zinc phosphate are not standalone you again over that you apply a paint coating. Chromating there is an issue here chromating we people use a chromium 6 plus people use it very effective what is the problem it is or you know it is almost now banned many cases. So, people start using CR 3 plus or is being used and even this is now not allowed in the case of steel, but it is allowed in the case of used for like aluminum, magnesium, zinc for these cases they use. Otherwise the chromate is one of the most efficient least expensive type of conversion coating for most of the metals. In the same line you have one more type of you may call it as conversion coating if you wish you call them as anodizing. What is anodizing? It is done for aluminum alloys aluminum and it is alloys. So, they convert the surface what do they do into an oxide. Now oxide is already corroded right is a corroded product it is if it forms a film which is adherent resist the corrosion. Again it is not very simple and you know you need to anodize it there are pores you need to seal them it is a technology of anodizing should be understood properly. I am just giving you various options available for us to for use for corrosion prevention of alloys. But again anodizing cannot be applied widely right is used for it is used for aluminum alloys. You can use for titanium alloys for example, but the most widely used is is is aluminum. In this also there are two cases you know one is called also called as hard anodizing. It is used for the wear resistance it is quite hard aluminum is aluminum is very hard. So, in order to get a similar thing people also go for we call as plasma electrolytic oxidation. It is done for aluminum very nicely you know people have also developed for magnesium it gives you extremely hard coating. The main purpose here is to provide hardness wear resistance corrosion is secondary in this case ok. Now, we have seen you know a sketch of different type of coatings that that are available for us. What I would like to now this discuss with you is what are the requirements of the coating when do you get a good coating ok. The first and foremost is adhesion it is a big subject actually. The way the adhesion is developed it varies from the coating to coating. If you talk about electrode opposition or you talk about hard dip coating the bond between the coating and the substrate is metallic. But in the case of a paint coating the bond more often comes by a mechanical interlocking. The surface is rough the paint goes and then locks by itself. So, in all these cases surface preparation becomes very important because it is a surface that connects the substrate and the coating. The surface is not clean the surface is not properly prepared I do not think you can ever get a good coating at all. So, surface preparation is an important aspect of this. A lot of standards are there for that to to to to understand to follow. The second aspect of that is the nature of the coating you see each coating they have their own properties inherent properties right. If you say a zinc coating it is sacrificial take a copper coating it is noble coating. You take a paint coating for example, epoxy coating if you take it is resistance to chemical resistance it has got very adhesion. But it is not good from UV resistance point of view. But if you take acrylic it is good from UV resistance point of view or everything is a tough. So, nature of the coating also plays important role in terms of its performance of that actually. The third the application method the same coating can be applied by different means it can be hard dip coating or it can be electrode deposition coating zinc. For example, a hard dip coating hard dip galvanized coating and electrode deposition coating the formability. Suppose you take the metal and then you know sheet and you bend it the formability of electro galvanized coatings are far superior compared to the formability of hard dip coatings. Similarly, if you take a paint coating you can apply through brush you can apply through spray I mean you can do electrophoretic coatings several ways that you can apply the coating the performance of these coatings are going to be different. And the substrate you are going to decide about it and I call a structure I say macro structure macro. If you are going to have a electrode deposition of sharp edges different intricate components are different you cannot quote them say the substrate nature the way substrate is there. Of course, the performance of the coating also depends upon what it depends upon the environment and you can also add there is a performance. All this will be discussed much in more detail in the in the mm 6 by 0 course when you when you take in the next semester. There were coatings you have any anybody has any questions it is more informative rather than going much into details yeah. You said that we do not use Cr 3 plus for steels why is it so? See what happens is no all said and done we do not want chromium per se for use in any structures. Why why we do not use why you do not use chromium 3 plus chromium 6 plus steel is that we have other alternatives you can have phosphates right. In fact, the European Union now even banning the phosphate also now they are going to non-phosphate based siren based you know which consists of titanium containing silicon containing all these kind of things. So, most of this you know the regulations come from the environmental issues. Now, until you find a solution people still start using that. They have found alternative for iron steel to use and aluminum is still is a problem. I think there are certain deadlines even for that they have to replace chromium totally. With respect to aluminum and magnesium or all actually. So, it is a matter of developing technology to find alternatives ok. Chromium is bad either way you can do that good question ok. So, let us move to the next topic which is yeah please. We are using chromium in making steel also that and stainless steel that is also forming a layer of chromium oxide on the top of stainless steel. It is a good question actually yeah I mean chromium, nickel these are all not very good. In fact, nickel is even more dangerous actually ok. If you wear a ring of nickel you know you see skin will be very allergic to that actually. The difference between the stainless steel and the coating is it forms a very very thin oxide on the surface. You know already that you know the thickness of the passive layer are less than 100 angst times and then they do not very easily dissolve ok and they just remain on the surface quite long actually. And if you calculate the rate of dissolution corrosion of steel they are they are not they are not in microns per year something like that. So, the extent of corrosion that it can happen is very very less ok, but nevertheless ok it it it can cause a problem especially there are cases where people do not want to use the body implant for example, we discussed the other day ok. That we are seemingly careful about all this actually, nickel you do not want to have nickel. Some of the stainless steels people replace nickel with manganese and nitrogen and high chromium content. So, there are of course issues ok and then we only have to see how best that you can tackle them into that actually. Any other questions? So, let us go to the next topic thank you these are inhibitors. Now, inhibitors are added or compounds they can be organic compound or it can be inorganic compound added in small quantities. But what happens? But bring out bring out a significant reduction in the corrosion rate. For example, if you really want to not to worry too much about environmental issues if you add a few ppm of chromate like potassium dichromate and all like that add ok. The corrosion rate of of steel in acid can be reduced by by even 98 percent. There are several several inhibitors like you have amines, azoles even you know alcohols. In amines there can be aliphatic aromatic ring over so many you know kind of amines are there a secondary amine, tertiary amine so many kinds of compounds are available to that. You can also have mole updates, you can have zinc case also ok, you can have silicates. These are you know organic kind of compounds there are organic compounds. In fact, if you look at some of the publications I know there are thousands of publications where people develop lot of inhibitors. Nowadays people talk about green inhibitors, they extract them from the plant and start using them and so they they are in fact used in specific applications. When I say specific application where the environment is confined obviously you know what do you mean by confined? For example, I I I want to pickle steel right what is this called descaling right. You want to do descaling is one of the very widely used operation. You have an oxide you know a magnetic oxide or a wood site you know or maybe hematite formed on the steel I want to remove this scale. You can do a mechanical removal you can do or you can do a chemical removal you can do that. And chemical removal is called pickling you normally people use acids right they use acid. The acid dissolve what are it dissolves what? Dissolve the scale right. But you can start attacking the steel you do not want the steel to be attacked. So, when add an acid sorry to acid we add a inhibitor attack on metal is reduced. The other example is cooling water systems in industry that is there are heat exchangers your refrigerator is heat exchanger right. So, they add they add inhibitors to it to reduce the corrosion of metals by water ok. They add lots of inhibitors in fact, and the cooling water this cooling water here is it is a recirculated. Sometimes inhibitors are added to coatings also a paint coating you can add sodium chromate ok they add to it yeah I will I will come to that it is a good question ok. I will come to that we add a very small quantity how does it you know lower the corrosion rate of a metal ok. So, it has a limited application of course, it is not very wide application, but they are critical applications. If you do not use you know inhibitors in cooling water system you cannot use stainless steels use stainless steels you cannot use copper based alloys you cannot use in some cases if people use even carbons still they use. So, it is not possible to use them, but mind you that this water is recirculated. Similarly, boiler you know applications ok the water is being recirculated you cannot use corrosion of structures and see what can you do that you cannot add it because it is simply not possible to change the environment at all actually. So, in a limited you know applications inhibitors are very successfully employed to prevent the corrosion of metals. And the question is what is the mechanism or what are the mechanisms it is not only one mechanism. One the do by adsorption who is film formation it could be scavenging action scavenging you can remove the species responsible for corrosion ok. I will give some example here to to understand ok understand how this mechanism in practice really function. So, let us look at the adsorption. Let us take an amine how many if you know amine it has got N H this may be some R alkyl group something like it is it is a primary amine right. If you take amine what is special about this nitrogen molecule what is special about this molecule I am sorry what is special about this nitrogen atom yeah. It is for a lone pair of electrons are there there are lone pair of electrons are available. So, it is relatively charged. So, it can adsorb onto the metal surface you can have a similarly sulphur kind of thing you have oxygen sample there are several kinds of of of the functional groups most of them are 4R in nature they can get adsorbed on the metal surfaces. When they get adsorbed on the metal surfaces on the metal surfaces there are several thing you know now what happens that depends upon the finitely change for adsorption adsorption or absorption both you can call about it it is more negative it is going to adsorb much more effectively and why it happens. So, that is why a lot of chemistry people develop so many molecules you know bigger size molecules means it is easier to adsorb on the metal surfaces is this is talks about steric is talks about polarization several factors not worry about it ok. They get adsorbed when they adsorbed you can adsorb on the anodic side or on the cathodic side or can adsorb on both. Now, assume that it is it is now adsorbing on the anodic side can I find out by some test it is getting adsorbed on the anodic side you now you are expert now in electrochemistry you know how to carry out tests you know all this right you know polarization how do I know the molecule is adsorbing on the anodic side yeah yeah it is activation polarization. So, what happened in activation polarization you are there, but do I want to be a little bit more specific it control for example, it absorbs on the anodic side what happened to anodic reaction well what will happen anodic kinetics what will happen increase or decrease it decrease right if the anodic kinetics are decreasing. So, how will be revealing electrochemically you know all this right you have seen a lot of diagrams and all you have drawn what happens yeah you know it current of course will decrease corrosion rate will decrease right but how but how do you know it is a anodic inhibitor and how do you know it is a cathodic inhibitor how do I find out come you guys know quick you do a test in the lab and then come out with the results oh this is anodic inhibitor what is the parameter that will tell you that it is an it is anodic inhibitor yeah table slope agree with him which table with the table slope you think will happen which one there are two table slopes right. Yeah beta. Beta here right. So, it is the anodic table slope that is going to be changing that will it will increase or decrease you think it will it will increase right it will increase because the current will not be the current is not going to be more the current will be decreasing right. So, the slope will be. So, if I if I plot if I plot if I look at a at a events diagram and if I get a I get a I have this is without inhibitor ok I have added an inhibitor and I get this like this what is this inhibitor called what is this inhibitor called what is this inhibitor called are you sure see he has got the answer you see right it is cathodic reaction it is cathodic reaction right. So, it is a cathodic inhibitor it inhibits a cathodic reaction right. So, it is a cathodic inhibitor right on the other hand if the slope goes like this what happened this what is this inhibitor called an anodic inhibitor if I add an inhibitor I get both anodic and cathodic both are getting slopes are increasing what will happen you call that as a mixed type inhibitor. So, it is possible for us to to to quantify and the extent of inhibition it is also possible for us to know the mechanism through which the inhibition is really occurring agreed or not agreed I hope you will able to recollect your image diagrams right. So, please understand this now now what is the consequence of that look at this if it is a cathodic inhibitor what happens to e car e car goes up or goes down goes down anodic inhibitor the goes up, but at the same time i car also decreasing right. So, so this is how you try to understand the role of inhibitors and how the inhibitors you know function for different types of metals understood or understood ok. Now, it is a film forming is one more type an example of this is is benzo triazole and this is copper inlet some acid maybe in copper and hydrochloric acid. The benzo triazole is a compound it is organic compound ok you you you dissolve it in fact, it is very difficult to dissolve in aqueous solvent you dissolve it in some alcohol or something and add small quantities in hydrochloric acid what can happen is they form a very very thin film very thin film is formed very thin film of benzo benzo triazole. There are several types of azoles tolly triazole and macabre benzo triazole kind of compounds and these are all sulfur compounds sulfur containing containing aromatic compound. So, they form a barrier for corrosion of course, the sulfur also absorbs it does both ok, but it forms a nice film on the surface. The next one is a is a is scavenging I would say scavenger you want to call it and it is done in the case of boilers. For example, oxygen is is a is a molecule which is going to be involved in the the corrosion process you you all know what it can be done. You can add compounds like hydrazine hydrazine and this is I think N 2 H 2 it combined with oxygen and forms nitrogen and plus water this happen sodium sulfite what does it happens it becomes sodium sulfate. So, there are several other efficient compounds which are added to water to remove the oxygen content they are called as scavengers. Well, I have one more topic to be covered which is anodic protection and cathodic protection I am not sure whether we can finish it, but let us start with the principle ok. Let us start with the principle of let us take this this is electrochemical technique electrochemical methods we call it. The two methods one is cathodic protection two is the anodic protection. They work diametrically opposite way in preventing the corrosion of the metal. We have seen how the electrochemical reaction occurs for corrosion right. Let us take the let us take the case of cathodic protection. If you I think we will be also having a having a course where we are going to discuss in detail, but I will be very brief here that is mm 712 ok advances design there we will be discussing in detail I think you know about 12 13 lectures we will be talking in detail about the cathodic protection. What we will be talking about here is a very brief discussion on the cathodic protection principles only we will do that. And again you want to read more in detail if you do not attend the course suppose you want to read you can read this book Peabody s Control of Pipeline Corrosion Second Edition it was edited by edited by R L Bjarne Satie and it is again N S publication Texas 2001. It is a nice book mostly focused on pipeline related cathodic protection and it also deals with the coatings as applicable to pipelines and you do not of course, talk about cathodic protection of the tangs and all these stuffs are not there. But it is a very nice book you can look at the basics you can get a better picture about what the cathodic protection is. Let us start understanding an electrochemical corrosion ok let us recollect that what you said. We said that in the electrochemical corrosion a metal is involved exposed to the environment a metal is getting oxidized in the environment maybe some N electrons are released. So, you have N electrons are released on the some species from the environment ok some species from the environment they will take this electron and they get reduced. Some oxidized species they take this electron and get reduced into reduced species and this is a metal. This is the corrosion process. The when it is corroded the metal exhibits a potential which is equal to what is that potential called what is that potential when metal is corroding if you measure that potential using a reference electrode what is that potential called? Corrosion. It is a corrosion potential right. So, it is a corrosion potential at that potential the rate of oxidation of the metal equals to the rate of reduction of the species involved in the corrosion process. The metal is quite neutral right am I right metal is metal surface is quite neutral because the number of electron released are consumed by the species from the environment. Now, what we are going to do is suppose I am going to now I am going to pump electrons here I am going to pump electrons on the surface I am going to put more electron I am going to flood the surface with electrons. Look at this diagram if the surface is flooded with electrons what would happen to this corrosion process? There are two there are two reactions here right one reaction is metal going as m n plus plus n electrons right. The other reaction involved is some species take away this electrons like this. When I am flooding this surface with electrons what do you think will happen? Yeah I want you to spell out clearly tell me yeah. Polarize. Yeah it does polarize the reaction this is reaction 1 reaction 2 it polarizes right. So, what will happen to the corrosion what happened to these reactions yeah what will happen to reaction 1 what will happen to reaction 2? The reaction 1 will. It will give slow down when you have more electron that means, as well forget about electrochemistry as per lead chattelier principle the reaction will revert back because you are supplying more electrons here. So, the reaction will turn towards like this what will happen to second reaction will increase actually. So, you find the metal surface the rate of corrosion increase decreases whereas, the other reduction reaction rate increases. So, when you put this more and more electrons on the surface that is what really happens and that process is called as cathodic protection. Now, the there are this is simple the simple concept is very simple I have more electrons on the surface and so, automatically the oxidation of the metal is getting reduced. But of course, the cathodic reaction which is you know which normally takes electrons the rate of that reaction will increase and this is the essence of cathodic protection of metals. So, I stop here ok.