 Welcome to the last lecture on this course M M 7 1 3 Aquus Corrosion and Discontrol. Today, I thought I will just talk about in general how do you really you know manage corrosion. What are the broad guidelines that one would follow in controlling corrosion. Before we do that it is better to review what we have seen so far. We said that we broadly categorize this course into two parts. The first part was on thermodynamics andkinetics of corrosion. The second part deals with the various forms of corrosion and in the thermodynamics and kinetics primarily we focused on two questions. The first question was can we predict if a metal can undergo corrosion or not and we used the electrochemical concepts to predict the corrosion because that is much easier than using the free energy concept. Then for quantification we looked at the the kinetics of the corrosion and wherein we started with the simple Tafel relationships the relation between the over voltage and the current as an electrochemical reaction would have. Then use that the Tafel relationship to to calculate the e core value and i core value that is corrosion potential and corrosion current densities. And then we went on applying this to a complex systems of corrosion processes wherein we can use the concepts to predict corrosion in a complex situations where you have multiple cathodic reactions, where there are flow velocities, where there are some additives, where there are galvanic corrosion. In fact, we started applying this to all forms of corrosion we have been talking about when we talked about the different forms of corrosion. Having understood thehow we can able to look at the production and the quantification of corrosion. We looked at the various forms of corrosion as applicable to industries. We looked at the various forms of corrosion from the point of view of industrial applications normally. And we we we this classify them as various forms of corrosion primarily because the way you control corrosion would be different depending upon the mechanism of corrosion we talk about. And so, we we looked in details for various forms of corrosion. The mechanism of corrosion we looked at we looked at the parameters affecting the corrosion. We looked at the corrosion control measures test methods. In all these cases we also saw the illustrations and to highlight how they are important and what are the signatures that these failures leave behind. So, all this we looked at over the last 33 lectures we saw that. The one thing that we need to look at is the corrosion control is a major domain beyond understanding prediction quantification of that and understanding the various forms of corrosion because it is occurring in industries because it is an industrial problem. So, you must have a clarity in how do you approach the corrosion problem. The corrosion problem I would like to see as a perspective as a control as a perspective. Anyone who who talks about corrosion control should start with the drawing board right in the project stage. In the project stage it has to be integrated in the project stage. The no point in start worrying about corrosion control when you have you know commissioned the reactor and process is on because that is going to be very expensive process. So, start with the design of system as you do you know as a mechanic engineer does for taking care of various other aspects of a reactor or a structure and so on. You should integrate this in the design itself. How do you integrate when you decide you integrate in the design the whole thing then comes of course, the fabrication right in the actual fabrication of the component and I think now you must be aware of having understood various forms of corrosion. How the fabrication also can influence the corrosion right a welding problem could be one of the things a dissimilar metals that you can come into contact with your problems you know. So, the fabrication also we should look at how the corrosion can influence be influenced by this fabrication. The plant commissioning is another one which is equally important in order to in order to reduce the corrosion process. There are cases where the people started the project by the time ended the several component got corroded actually ok. So, it is important to look at the plant commissioning stages also right and then comes the operation of the particular unit, the particular equipment ok or particular industry for example, you know in here we should now take care of those aspects of that. Now, I am going to now look at in each every case broadly you know again it is hard to go in much details let us go a little bit more broadly how you look at these four aspects of design fabrication and you also talk about plant commissioning and operation of that. But in talk about a design the first thing that comes is what is the function of that right. What is the function? What function does it really do? So, that is the major thing is it is it a heat exchanger is a pipeline is a conveyor belt it could be a simply a crusher can happen in a cement factory what are the function of that. Why it is important is please notice the material selection that you talk about does not primarily consider corrosion as the requirement in many cases. So, the material selection depends upon the functional requirement of particular industry component at all actually ok. So, that is why you need to look at what is the function of the plant you need going to be there. If you know the function then you can also start looking at the environment what is that environment you should review the environment there. What kind of feed feedstock being used? What are the input from there? What are the pH of that and what is the chloride content and what is the water content in that? What are the organics present here? What are the temperatures? What are the pressure? All this we need to review and when you start reviewing this and you know what the function of the plant is then you can look at the control measures. The control measures must sync with the main function of the plant otherwise you know nobody is going to listen to you right ok. Just because it is corrosion resistance you cannot use the particular material at all. Now, corrosion control measures we have seen before I just want to just you know highlight to you what kind of corrosion control measures. It could be a material selection you can talk about could be a coating could be use of inhibitors you can go for cathodic protection systems. In some cases even anodic protection systems you can use and of course, when you talk about corrosion control monitoring is equally important. So, you can use the corrosion control I mean corrosion monitoring probes for online monitoring probes are used actually ok. So, probes also can be because you know when you want to prove you should have a provision in the reactor right. If you know provision in the reactor tomorrow you cannot just put a probe there it does not happen. So, start in the design stage. If you have this idea about it then they are incorporated into the design of the particular system actually. So, the design stage you take care of them then you of course, you talk about the fabrication and this is here inspection are very important and people follow the course. Codes are very important I think we are not dealing with this right now, but as we go along and serve in the industry we know there are so many codes API codes for example, all petroleum things ASME boiler codes you are going to be there. So, these codes would be enforced in all the stages even design stage the fabrication stage commissioning stage these codes will be a part of corrosion control exercise that you have. Coming to operation it is very important to adopt the corrosion control measures you know you see in the design stages itself you decide know what you do or am I going to go for cathodic protection or am I going to go for inhibitors or am I going to add I mean you know I am I am going to you know go for a cathodic protection for example, coatings. So, this whatever that you decide they have to be properly adopted in the in the in the in the practice maintenance of the environment and process control. See I have seen industries where people have designed for certain you know operations in terms of temperatures pressure feed feedstock they use and tomorrow they change it because the feedstock they get are different. So, definitely for example, they may get accrued may be from somewhere middle east there is certain amount of hydrogen sulphide present, certain amount of carbon dioxide present they change the crude because because the cost becomes low, but no more the constituents the corrosion constituents are same. So, it changes. So, that means, you need to take care of that you know if you take. So, if you are going to do that then you should see how we are going to manage it otherwise the reactor is going to prematurely fail. I have seen cases where people want increase the production by rising temperature and pressure just fails in a week it work for 20 years just failed in one big time actually. So, the maintenance of the environment and process control systems are very important look at the how much it can have excursion ok that is something very very essential ok. Course inspection is a part of this actually inspection and maintenance ok they are all part of the systems you need to look at you you know look at the inspection inspection team should be aware of of of the corrosion issues how to do that. Inspection itself is a big subject where do you inspect ok is a big issue for example, I have a long pipeline of 1000 kilometer long am I going to inspect every inch of the pipeline. So, people sometimes who use models to look at risk based inspection we will see you later ok where you have more risk you focus on that location that is based on the models based on the mechanisms and way I know how you predict where you think the corrosion becomes very severe. It happens in nuclear power plants, it happens in thermal power plants, it happens in many systems where the the the the issues are very critical they do that. So, it is a broad philosophy of of corrosion control and we should really enforce that. Now, what we are not seen ok. So, when you when you what also you do is it is very interesting to see here when you do when adopt corrosion control measures you do inspection you do maintenance you learn new things. See the problem with the corrosion is it is a time dependent process it is very difficult to have a test which can simulate exactly the operating conditions and get a data and short time. So, many times you do an accelerated test either you over predict or you have underestimate the corrosion. So, when you do operations you know how the systems are working. So, what happens you plow back ok you you you you plow back what learn what you learn from this from the operations and put it back into the designs. The next generation of design tests in the account the experience and you also in fact, use it to define your models ok. So, that the predictive models becomes more useful at all. So, this what I think is done we will go to spend some time on material selection with some time. See material selection we have seen it under various forms of corrosion. So, when is a galvanic corrosion pitting corrosion, crevice corrosion all this you know erosion corrosion and even sensitization we have seen how we do a material selection for a given corrosion problem. I am going to give a broad perspective here and you know so, that we save some time you know if you take a fortuna book it is a big chapter on material selection actually ok it deserves to be given, but it is introductory course and we do not have much time. So, I am going to be a bit more brief in this. Somebody has you as a corrosion engineer what is the material what is the basis of material selection? You simply jump into thing that oh corrosion is the issue. So, I am going to choose a material which gives you a best corrosion assistance that is not the case ok that is not the case. There are so, many factors that decide what material to be selected. I have given some of them here, but not it is not an exhaustive list by no means. It makes to feel that how we look at material selection that is the reason why we are given this, but no way it is a comprehensive ok. So, first and foremost in material selection process as you seen before you look at the operating conditions, you review the environment and you foresee possible failure mechanisms. Will it suffer a stress corrosion cracking? Will it undergo selective leaching? Will it undergo sensitization? Because you have now expert that should be much easier for you to do that ok. So, if you know the operating conditions and it is possible for you to predict what kind of corrosion mechanism corrosion failure it can it can operate a given situations. So, the material selections that is the main hinge ok and that is point. If it having a problem you have to search alternative material or you have to find some other way of overcoming the problem ok, but you need to recognize what is the kind of corrosion problem even operating conditions can happen in a plant. The second issue is the product requirement right. Sometimes the engineering allows certain amount of corrosion we have seen it also earlier you know when you select materials or the the corrosion allowance we talked about right. You can you can put a corrosion allowance, but then the product says no you cannot allow any corrosion to take place ok. Because it may be a drinking water it it gave me for a pharmaceutical industries or some kind of chemicals you you make you do not want to be contaminated. So, there is another requirement which is a product requirement. The product requirement demands a high purity I think corrosion resistance becomes utmost important. You select the material properly ok, so that you can bring down the corrosion rate. But sometimes what happens you bring down the corrosion rate by just applying a stainless steel the stainless steel cracks because that environment the temperature content is such that it cracks. So, you may bring down the overall corrosion you know rate by choosing stainless steel, but you are going to have a newer problem. In which case what you do you probably go for a carbon steel you give a lining you can give a glass lining and work actually ok. So, that is how it works. So, the material selection process is a evolution process it has minute I mentioned to that actually ok. There one more issue somebody in an iron gas industry they have offshore platform there is a drilling just they are they are doing a drilling there ok and then taking a oil and I want to choose a material. What is the problem there? The problem is the replacement becomes a problem right even I say that I want replace a replacement cost is much more than the cost of the component actually. So, which means you better go for better materials that is another rider comes over there ok. So, where you cannot monitor where you cannot inspect where you cannot replace it quickly you have no choice first to go for a better material with respect to corrosion actually ok that is another factor that comes into picture. Design and fabrication oh you choose a good material, but you cannot weld it very nice cast iron ok you have iron you know and 14 percent silicon best corrosion resistance you know you can have it you cannot weld it actually right. So, there are other factor that comes into picture is can you really fabricate what is the other other requirement that you have ok that comes into picture here in the setting this maintenance. In some cases I can afford to inspect maintenance you know once in 6 months some cases I cannot afford to do it for 3 years 4 years ok. So, then what happens then obviously, the choice of material the cost is not going to be a consideration there at all actually ok. If you are going to give additional production measures then you can reduce the cost. A ship hull is an example where carbon seal is used right it is not a great material, but is used in sea water applications how you apply a coating you carry out a cathodic protection it does work. So, you can also look at additional production methods used and so, we can go for a cost effective material can happen at all ok. Because all this we are doing because there is one factor many times you know you need to consider can be overriding also is the cost of the material actually. The cost is more then of course, you are nobody is going to listen to you. The cost again should be seen not in the cost of material the cost should be seen in a broader sense like life cycle cost. If I use a carbon steel where it worked for 2 years, if I use a stainless steel you can work for 20 years and you cannot compare the cost in that manner. How do you compare it? You compare it as a life cycle cost ok and of course, you know suppose you do a good painting painting the very painting process is very expensive you know or it can be equally costing as a paint itself ok. So, then you go for better paints because it is not going to be easy. All are fine I want to use materials it has a big embargo right you are not getting the material in India now right what you do about it right. So, materials availability is equally important one or I come out of the fantastic design for a mechanical engineering point of view or this gives you better efficiency guess everything, but what happens? There is no material available to withstand that operating conditions. One is availability the ability of the materials to function the way that you want it actually right. So, there is another dimension to the to the problem ok, selection materials from purely from mechanical engineering point of view chemical engineering point of view does not work because that material may not. In fact, there are several applications the critical thing is what material right, you cannot withstand very high temperatures I want to have 1500 degree Celsius it does not happen ok. So, material availability one more thing ok all of them I have fantastic low cost I produced only yesterday I have only small data that I have I want to fly tomorrow I want to use it in the aircraft I think they are think very much the data availability how robust this data can count a lot in terms of in terms of material selections. So, that is somewhere very cautious. So, that is why people what the people do is the time that is develop a particular material and come to the production or usages or may it may be 10 years 5 years it takes because the data because of problem. If the data is not available you have to be extremely careful ok in terms of you know selecting the material or if you selected I have to have some way of monitoring that more properly ok. So, these are some basis for material selection please again they are not by any means all exhaustive you can add more to it because as you start practicing you know there are so many critical issues which decide which add to the material selection issues. So, that is what I think you should you should consider in this actually. Now, when you are talking about material selection we always you know you know see people look at differently at different people material selection in my experience like that if you are going to talk about you know any dechanger a mechanical engineer talks about from the point of view transfer ok he looks at heat transfer. When a corrosion point of view it is a different issue together. I can tell you suppose I use a copper based alloy and the copper based alloys have high thermal conductivity as compared to carbon steels as compared to stainless steels. But you know what happens to be the with the copper alloys? Copper alloys with the time it may not corrode more, but it starts falling it just falls. When it falls it forms a scale and those scales are what they are you know you can hinder the heat transfer and so what happens then it becomes a problem. So, the material selection when you talk about we had look at various properties not this is only corrosion actually you know. In fact, I see in this happening more with the with the with the people the people would have a background of metallurgy actually and would be involved in define they do happen and get into problems. So, we have seen it also in last about 33 you know lectures we have seen ok and in bits and pieces we have seen you know how the the material properties affect, but I give you a bit more comprehensive manner. Heat treatment can you give an example where the heat treatment can be a problem if for corrosion yeah? Yeah. Yeah, sensitization it could be a problem. We are going to use aluminum alloys high strength aluminum alloys heat treatment could be a problem peak age you would use it is going to get embrittle in the environment right. So, heat treatment could be a problem and we are going to use a modern state exchange the steel or steel if you do not temper it you are going to undergo stress corrosion cracking and embrittlement can happen ok. So, heat treatment is is is something one looks at it hardness macro on micro again macro in overall sense micro means suppose you weld what happens to the to the weldment thus the hardness increase you can affect stress corrosion cracking and embrittlement. Of course, tensile properties toughness ductile brittle transient temperatures stability against temperatures you know again corrosion is what we are looking at actually. So, these are the properties that you watch over for in overall you know you know in the overall aspect in designing a particular reactor particular vessel and so on. So, come to materials ok. We generally I put here metals I am not covering the non metals please understand that there is no way you can exclude other materials ok for the construction purposes ok. There are plastics and there are ceramics being used there are non metals are being used actually ok. And, but what I listed given list is is kind of broad you know you know broadly they are used I would say and I have given you can see here the cast ions steels low alloy steels stainless steels nickel base alloys copper base alloys aluminum alloys titanium alloys zirconium alloys tantalum tantalum is not used as an alloy normally used as a element only ok. Now, these are the materials generally used and again the criteria for choosing any of this you have seen before what is the kind of criteria that we talk about ok. They should all apply over here ok. So, that you can optimize the material selection for a given application in this list now ok. Let me just give you a broad thing about how materials are being used in the things now. I just touch upon first the ferrous group of metals based on iron ok. How do we really use actually? So, let me start with the with the the ferrous group of of the metals which I think ferrous groups are very widely used along all the alloys we listed now ok. And, in the ferrous group the basic material is is a cast ion ok. And, you know what a cast ion is it has a large amount of carbon in that and and so, the cost is is less and can be used as a structural material. But, what is the limitation of this? What is the limitation using cast ion? Why why cannot you use everywhere cast ion? Yeah? In brittle. In brittle can be there are of course, some cast ions and ductile also. So, you cannot fabricate it very easily you know the fabrication becomes a problem. Even workability is a problem you cannot draw into a sheet you cannot draw them into wire that is the problem that you see in the case of the cast ions. So, what you do is you remove the carbon in the cast ion you get into the carbon steel. The carbon steel is a basic structural material. Even if you look at many of the important industries, refineries, chemical process industries, power plants, the major constituents of materials is still a carbon steel. We may talk about stainless steel all this yeah they are important, but the major structural material still is the carbon steel. So, it is the advantage you can fabricate you can weld lot of advantages of that what is the problem with that? The problem is it is a poor corrosion resistance it is a basic material. The corrosion resistance of that is it is not that great. So, how do you improve corrosion resistance? You add chromium, but you cannot add chromium when you have large amount of carbon. So, remove carbon you add chromium you get into a stainless steel that is called as ferritic grade stainless steels. It has got a reasonable corrosion resistance as it exhibits passivity ok, but what is the problem? The problem that lies here is that is difficult to weld you know if you can weld, but the weldability is little bad. It has got you know poor resistance against higher than the embrittlement. So, you want to improve upon this what do you do? Add to it a nickel right add to this a nickel. When add nickel to it you get a different class of stainless steel which is nasternetic grade stainless steel. There are two types here 300 series and a 200 series. The 200 series is is having more amount of manganese and the 300 series consists of nickel as the alloying element. You know nickel is required to stabilize the authentic phase. When you use this one it has got high strength you can weld very easily I mean no doubt about weldability is very good, but what is the problem here? The problem with the stainless steel is it is prone to pitting, it is prone to crevice corrosion of course, it is prone to stress corrosion cracking as well ok. Now, in order to minimize or lower the pitting tendency crevice corrosion tendency what you do? Add to it molybdenum to this you get 316 stainless steel, you get a super austenitic grade stainless steels, you get a even super ferritic stainless steels. So, you get a another variation of stainless steel, they are reasonably resistance against pitting corrosion and crevice corrosion. In order to avoid pitting corrosion you have other alternative alternative is what copper base alloys ok. So, we will not be covering here copper base alloys, but you have an option of going to copper base alloys. But this copper base alloys and high moly containing stainless steels are generally resistance against pitting and crevice corrosion actually. Now, when you take austenitic grade stainless steel generally they prone to sensitization we know about that right. So, how do you control this? You minus carbon right you get a new variety of stainless steels which is 304 and 316L low carbon stainless steels you know how we can reduce that. And this this this you know the the one here the ferritic ferritic here and austenitic here this is the two class of stainless steels are used between these two austenitic is very widely used ferritic of course, are used for limited applications they do that. So, so low carbon stainless steels are of course, resistance to the sensitization and, but then you are not addressed the issue of stress corrosion cracking so far ok. So, how do you address this? You go into duplex grade stainless steels right. Duplex grade stainless steel here means it consists of a ferrite phase under austenite phase how do you get it? If you go from austenite you lower the nickel content right you increase the chromium content you get into duplex grade stainless steels ok. And so, so this how you develop this this kind of stainless steel which is resistance against stress corrosion cracking I think, but nevertheless the alloy is still prone to what is called as the crevice corrosion. You can also go for high nickel alloy which is in conal actually the in conal gives you high resistance against the SEC, but the in conal is expensive ok. So, you you can strike a balance between austenitic grade stainless steels and the in conal alloy in terms of cost. So, you go into duplex stainless steel and you can have SEC resistance offered by the duplex stainless steels actually. It costs more as I told you and and it is resistance to SEC, but you can look at the duplex stainless steels they are still prone to crevice corrosion and pitting corrosion because you removed nickel content this actually right. So, add nickel add more chromium you go into what is called as super duplex stainless steels 2507 we talked about this 25 chromium 7 nickel that we have ok. And it has got resistance SEC it is resistance to pitting corrosion crevice corrosion, but it is not real resistance you cannot use and see water application still there are some issues with the duplex stainless steels. So, recently they have come out with the better grade of duplex stainless steel which is called as hyper duplex stainless steels add further nickel we have further nitrogen we have further chromium to it and again maintain the phase balance and this this alloy is offering resistance against SEC and pitting and crevice corrosion resistance. So, they are being used in sea water applications ok course when you when I do all this happily like a tree going from cast iron to super duplex stainless steels to hyper duplex stainless steels and to nickel I mean conal alloys you have to keep in mind the cost also goes up from the bottom most to the top most. So, the corrosion control also comes in the cast, but the bottom line is what bottom line is the life cycle cast the safety involved here see there are certain things as I told you when I talk about material selection I did not discuss about safety involved environment involved there are so many issues which we also talk about many when we deal with the the material selection. So, this is about the the ferrous group of materials used you know in industries and lots of developments are taken place over the time period. The next class of material which is called aluminum light or light metals actually. Light metals from the corrosion point of view they are not may used as I told you that material selection depends upon what? Depends upon the functional requirement. Light means it is required for certain applications like transportation application I want to be light ok, but otherwise some applications you do not care it should be light at all. So, aluminum is used in the transportation industries and it is also used where you do not want to spend too much money and it want to be corrosion assistance you know this is a possible rule that. So, let us look at this aluminum alloy aluminum it is a light metal ok and you have good corrosion resistance you know it forms a stable oxide film in the atmosphere. So, it is resistance to corrosion ok that is real good thing, but it has a low strength. You know very very pure aluminum you know what is the strength of it? It can be 50 to 60 mega pastels so low right. So, you want to increase the strength further what you do? We add these alloying elements such as copper, zinc and copper, magnesium you know the various alloying elements are added and there are several different alloy systems have been developed 2000 series and 7000 series aluminum alloys or high strength aluminum alloys. When you add these elements when you increase the strength again strength goes up ok. We we talked I think other day may reference to peak age and over age kind of things. When you add the strength you know when add these elements the strength goes up the problem is there the problem is it is prone to pitting, prone to exfoliation, prone to stress corrosion cracking is prone to hydrogen embrittlement actually. Especially exfoliation, stress corrosion cracking and hydrogen embrittlement are related to the microstructures ok and and microstructure depend upon alloying elements and the heat treatment. So, if you have high strength well the service life is not going to be great because we are prone to these kind of corrosion problems. To overcome this you give a heat treatment and that heat treatment is called as over aging treatment and over aging treatment ok. There is a loss in strength that is one of the problems, but it is resistance against SCC resistance against exfoliation and hydrogen embrittlement are all it is good, but there is a loss in strength, but there is a problem. What is the problem? The aluminum alloys cannot be used for elevated temperature applications right. Why? It precipitation dissolves now the strength is lost. So, there is a limitation in using high strength aluminum alloys at elevated temperatures and I am not talking about high temperature I am talking about the elevated temperature itself is not going to be ok. So, in order to overcome this people go for the titanium alloys is used in gastrobeins ok very very extensively used in gastrobeins in medium temperature range actually around 250 to 400 maximum people use titanium alloys ok. Titanium alloys have high strength at high temperatures when it is high temperature please look at as medium temperature in the range of 250 to 400 Celsius you can use ok and, but again it is limited temperatures ok. I cannot take it 600 800 it is not possible ok you can do that. There is also some problems you will you will talk about titanium alloys ok. What are the problems with the titanium alloys you cannot use it for storing dry chlorine gas you have seen before right ok. It either titanium alloys are very good as long as it is a aqueous condition. If it is a non aqueous condition the titanium alloys I think are not going to be great actually. And, you want to raise the temperature further you go to super alloys they have high strength and reasonable high temperatures you can go to 800 Celsius and all ok. And, again moderate 800 you can do you want to go beyond that what happens super alloys are not having strength. And, also what happens they also suffer oxidation hard corrosion you studied the course on high temperature corrosion right. There you have a little idea about how these super alloys are vulnerable to various forms of corrosion at high temperatures. In order to overcome this people give what is called a thermal barrier coatings. The thermal barrier coatings it is a composite coating right you may also have a bond code and you can also have a ceramic code. It increases oxidation resistance it also reduces the surface temperature of the super alloys component because thermal barrier means it offers a reduction in the temperature because of the insulation property of that. So, it is used in gas turbine applications ok. But, what is the problem there is a limited life you know that right it has to be limited life and we need to refurbish it actually right. There are some life for these blades and again you would re coat it or I think you need to do that. And, nowadays people are looking at simply ceramic blades, they are now looking at ok ceramic blades and where it is supposed to have good toughness. And, of course, ceramic blades may have you know less problem I mean there may be more problem of oxidation and chemical resistance attack you know that could be a problem, but they are things are going on in some direction of that. In fact, G is supposed to come with engines with the ceramics replacing some of these super alloys for high temperature applications. You need high temperature in order to make the turbine you know efficiency higher. So, there is about the light metals then we can look at the other kind of alloy the major alloys are used or the copper base alloys ok. They are used in the applications the copper base alloys are used and it is resistance to pitting resistance to crevice corrosion that is very good actually. But, what is the problem is this the low strength. What increase the strength of this alloy with zinc? When alloy with zinc what happens you get a brass now brass has high strength. What is the issue there? It undergoes desincification certain processes. So, how do you overcome that? You add tin you have antimony, arsenic, phosphorus ok. These things are added to reduce these in these equation and leads to admirality brass. The admirality brass you know it resist desincification, but you want to increase the zinc more it does not help right. And so, it is prone to erosion corrosion. You cannot withstand higher velocity in the pipelines and you want to increase zinc then what happens? You can have erosion corrosion resistance, but it is suffering desincification problem. So, what you do? You go for you know you go for nickel addition to copper and you get copronical alloy here. The copronical alloy is resistance to erosion you can go to 3 meters you can even yeah 3 meter per second is the limited limit of reach the the the the the the material can resist. See again please look at when I say 3 meter per second you are going to have a very corrosive liquid 3 meter per second is not I mean valid. We talked about normal water maybe sea water ok. So, these are to be seen in a in a you know within certain within qualification line ok. You know otherwise you cannot blindly just use this these numbers everywhere actually they are only indicative. And copronical alloys are prone to ammonia and prone to chlorination you cannot do a very high chlorination actually the systems and and so, you go for titanium alloys. So, if you have very high chlorides and ambient temperatures you go for titanium alloys, but titanium alloys have the problems the dry chlorine gas you cannot be stable and its resistance pitting crevice corrosion erosion corrosion damage all this can happen. So, I have given broadly 3 class of materials and how it can be used and what are the issues involved here in a in a in a very brief manner ok to get a feel for the the materials properties and the selection criteria for them. Any of you have any questions here? Ok. So, hope things are clear. Now, I am going to go to the next aspect of this. Now, how do I look at material? How do I look at materials development actually ok. Now, I have given you an axis here it is an x axis time needed for the defects to cause the failures. Now, the time needed to cause the defect to failure has to be higher than the service life of a component right. If I decide the service has to be 10 years the defect should take may be 11 years to cause the damage then the component integrity is fine otherwise what happens the component will start failing ok. So, the time needed to cause a failure will depend upon the kind of defect that you have. The things that go without saying is the nature of environment stresses strain I I am not talking about it ok. I am just comparing that the environmental behavior is similar and what will happen if I change the size of the defect. Now, we look at the the degree of the impact and the defect size. If I start relating these two you get a very nice picture about the corrosion control on one side material development on the other side of it actually ok this is. Now, what could be the bigger defect what could be major problem? The problem is the plant itself the location of the plant itself you know. Suppose, you have you have established a plant just at the seashore or a creek where the sea sea breeze all the time just knocks the plant ok. You have huge amount of chlorides coming to and there can be frequent problems of one component turn all the components. In fact, I have seen a case in one of the refinery over here you guys know what is called a cooling tower right we use cooling tower. Cooling tower is used to to run the heat exchangers right the the chill the water by counter current that water is now sent to the heat exchangers actually. How does the cooling tower work? Water goes from the top you know from the they can somewhat I atomize it on the bottom you blow the air the air takes away the heat ok and then water gets cooled down. But when the when the when the air goes from the from the bottom like this ok it also carries moisture it carries water and when you when assume that that is in the center of the unit center of the plant you can imagine that water away wind goes it is going to carry and all the surrounding areas suffer huge amount of corrosion. So, plant location is a very important thing and wherever it is possible this to be considered. If you have a problem I think you are sure that the time for failure is very short I given here the time to failure the time to failure is this this much it can happen quickly. If you plug that particular problem assume that that you have overcome the problem and you have a problem in the designs of that it can be catastrophic stress concentration happening stress corrosion cracking can happen within 4 months within 3 months ok. So, I think if there is a 40 design you find then then again you will you see that the time to failure is is less. If I fix the problem of design you got into problem of fabrication you did not properly fabricate and I think again what happens then the time of a failure is extended, but still is not too far. If you fix the problem of fabrication if you do not look at maintenance inspection all this kind of stuffs in the industry again it is going to be premature failure right it again its time is increasing for failure, but then again fails operation maintenance of that. All these are part of the engineering practice they are in the plant practice operating practices ok. It is not related to metallurgist the guy who makes the material you may get a better material, but you do not do things properly I think it is going to make a failures in this case. Assume that all these are taken care of in this case and the life of this can be extended further into that actually then what happens then you get into issues related to materials actually. If all I am doing this well I mean you do not use a right material then is going to cause the problem. The material can have a defect it could be porosity you have welded build as a porosity you casted as a porosity ok and it can big force inclusions can cause the problem it can fail this year. See the time has improved from here to this, but again is a premature failure. But if you take care of this large inclusions and all, but you let the small pores small cracks peep in in the material then what happens then again it is going to now fail prematurely that, but you have traveled quite this quite a quite a far distance in terms of time you have done it actually. But if you can get rid of that, but you can look at fine inclusions this is of course a manufacturing process have a we talked about earlier what we talked about the hydrogen blistering where the inclusions are part of it right. The inclusions can be there they are fine inclusions actually they are not very coarse inclusion they are fine inclusions they can cause problems in the system. But you can take care of the fine inclusions and if you do not take care of microstructures ok the microstructure is at a next level of dimension wise right. You do not take care of microstructure I have not done a over aging I done a peak aging you finished ok it can fail much much earlier it can happen. So, you take care of microstructures but you have done some pore rolling there are dislocations present to the materials can cause stress corrosion cracking. So, if you do not you know and phase distribution again you know this is another thing whether it is it is a whether it is a grey cast iron or white cast iron the phase distribution can be one problem fix that you get into dislocations other dimensions of that again the life is is is traveling like this here. So, fix the problem of dislocations then we look at elements at the atomic levels atomic level alloying elements ok. Allowing elements point defects all of they are going to be there. This had to be engineered now it is very important. It is very important when you talk about a long term implication I give an example where long term means you never imagined actually. Can you imagine that I need to have a a a storage vessel lasting for 10,000 years have you guys ever imagined about it? You need it to store nuclear waste. The nuclear waste takes long time to you know it is in this decay right there is radiative decay it takes about 10,000 years or so in order to succeed all this decay and need to contain them in a vessel. Now, what happens now? Now, you you have improved the life and all, but 10,000 years is really a pretty pretty long time and nobody is going to be here to see witness that actually right. But assume that I have taken very nice nickel based alloy or alloy of very high purity. I say sulphur is sulphur is reduced so much that is 0.00 may be put 3 zeros and and and they say about 1 sulphur ok. You are very happy because impurities are controlled atomic level everything is controlled now. But now imagine that the metal corrodes over a time period. Now, when metal corrodes what happens now? Assume the nickel corrodes, iron corrodes, chromium corrodes everything comes may be very very low rate no problem. But what happens? The sulphur gets enriched in the surface. One atomic layer of sulphur on the surface is sufficient to simply depasivate all of them. It is possible for you to calculate how much metal is required to be corroded before I enrich the surface with sulphur ok. So, that means, it could be a problem. We never anticipated that this sulphur can be a problem because they are looking very good. So, when you talk about life longer life I think what was counted so far is not that you know is not enough you need to count more than that. So, designing alloys at this level is very important in order to have a longer life of this component. And these are all we call them as material development ok. The material developments depends on in the in the life of the component that we expect from here. But again if you engineer materials at that level then what happens? The cost also goes up. But then there are situations that cost is not the just the criteria safety becomes very important. So, the material development is ongoing process and the corrosion is also an ongoing process. The issue of corrosion was not there today, but you cannot say the issue of corrosion will not be there tomorrow because we talk about environmental issues, we talk about sustainability, we talk about depicting resources and so, the corrosion becomes very critical in all these cases. Now, coming to cost of corrosion it is it is very important when you look at what the cost of corrosion means. Now, when you talk about material selection also you know it starts from that you know. Now, when you talk about cost of corrosion I mean what is the how much it takes for me to reduce corrosion right or what is the. So, if I am going to look at material is something very expensive material ok or cost more, but that only adds to the initial cost of corrosion control right. But you look at what are the cost of corrosion you know that should be added to it right. If you imagine that I am going to have a frequent maintenance, frequent shutdown, then the time for value of money is increases. I can say that I have now working on two projects with industries. The problem started sometime it started problem in April or so, I think ok and the person calls you and says this is a corrosion problem. Even today the investigation goes on ok it is almost now what from April to now how many months are over? It is almost about 7 months are over. 7 months are over we are not got in root cost of the problem at all actually. There are so many constraints involved downtime. So, you do not know what is cost of that ok. And there is one more case this is this is a pharma company is very well known internationally known pharma company we produce as drugs very very much actually there are three reactors all shut down ok and now it is going on. Now it is so, when you talk about cost of this it is not seen only in that perspective of it what is the downtime that is involved. And some cases you cannot afford the downtime where it is for example, it defends. Can I can I can I keep the readiness of the this this this thing I have to keep them ready right. I cannot have like the ok I am just losing the profit fine, but here it is not the question. The question is is it really readily ready for use. So, some cases you cannot really afford to that actually ok. So, when you talk about cost of corrosion I listed few of them, but again it is not only that I want you to think broad that is why I am just giving this here actually ok. There is no way an indication of that I have added everything to the cost of this actually think more than materials more than fabrication and more than you know safety involved. There are several issues involved in defining what the cost of corrosion could be actually. So, it could be a problem. And so, people look at the corrosion control in in many ways and you know and one is called as the asset integrity management ok. You know see you know I have an asset now and I want to see this is functional ok. So, this is a very important subject people nowadays are aware of it actually and they do it in a very periodically. They see that the assets are you know are good functional ok. And how do I manage this? And again inspection becomes important part and again inspection sometimes very difficult in aircraft what portion what portion you see that actually you know. So, that means, you you go for what is called as risk based inspection. Now, when you talk about risk based inspection then becomes mechanism becomes important, then tariff lines becomes important right and thermal lines becomes important ok. If you can able to measure everything physically then they are not important right because I can go and measure it and all right. So, where they are important is that when it comes to protection when you want to see how I can be able to manage things with you know for that I need models, I need understanding basic understanding of that ok. So, that is a very important thing that happens in the in the in the in the in the you know in in keeping the assets you know working actually you know and then do all kind of inspection and you better know. Now, again this is a big job here you have inspection, you collect the data, how to analyze the data actually? I just measure potential right. How do you analyze this? Unless you understand the science of it, otherwise you say a potential it has gone from minus 500 to minus 400 oh it is going to go like this I mean what? Sometimes going from minus 400 to minus 500 minus 400 to minus 500 may be good, sometimes reverse may be good depends upon what happens there at all actually ok. So, there are lot more in in in in managing the corrosion control of any equipment ok and again important is the management, how the particular management visualizes the corrosion as issue. Sometime you know you you see that it is a pipeline failure, 15 flows died, gas got you know gas fire and corrosion everybody talks about it and then again subsides again till another failure occurs they again work up and then start looking at it ok. But I think things are no more same because there are stricter environment regulations and controls as it goes I think the corrosion failures are not affordable anymore. So, that becomes very important. So, with this I think I would like to stop the my discussion on this particular course. I do hope you guys had some broad perspective about the the the basics of corrosion and you know and how do you really tackle at the introductory level actually ok. So, thank you very much and I hope that you continue to to work on the corrosion control thing and read so that you get more and more enriched on this topic. So, thank you very much.