 we shall continue the discussion on uniform corrosion. And the last class we saw that what are the parameters those affect the uniform corrosion. We saw that the nature of the electrolyte, the temperatures, the velocity and these are the factors that affect the corrosion of the uniform corrosion of metals. We gave some illustration yesterday that the relation between the corrosion of the metal and the environment many time is not straight forward with respect to the concentration of the corrosive agent. It is possible that you may increase the concentration of the corrosive agent, the corrosion rate might drop actually. We gave an example of sodium chloride how when you increase the concentration in water the corrosion rate goes through a maxima. And in this case the mechanism is simple that the the oxidizer concentration decreases. The oxidizer here is what is oxygen solubility decreases when you increase the sodium chloride concentration. We also recalled in the previous class that a similar example lies with the corrosion of steel in nitric acid. There if you increase the concentration of nitric acid the corrosion rate increases and then subsequently decreases. The mechanism there was different there it is related to the passivity of the metal when you increase the concentration of nitric acid the passivity increases. So, the underlying mechanism of corrosion in each case is required to find a correlation between the concentration of the corrosive agent and the corrosion tendency of the metal. So, this is a kind of general relation we saw. But you know there are just illustrations there are so many examples and there are so many actual cases in the industries and you know you should be able to apply them conveniently and see how the corrosion rate of a given metal changes in each of these cases. The second the parameter that I would like to talk is on the temperatures ok. The temperature if you rise the temperature of the environment what do you expect to you have to happen? What will happen to corrosion rate? The rate of reaction will increase that is what the Arrhenius law says right. The Arrhenius law says that for every rise of 10 degree Celsius the chemical reaction rate doubles actually right. But there are situations is true also ok. But there are situations that you do not follow the same principal actually. I just give only illustration one illustration because there are several different cases that the different tendencies are happening around. For example, let us look at the corrosion of steel in water. Suppose I take a I take a beaker and I take pure water simply water here and immerse it steel sample and I just rise the temperatures ok. I rise the temperature by heating. If I can measure the corrosion rate of the steel with respect to the temperature, if I make a plot temperature versus the corrosion rate. What you might expect is the corrosion rate go you know goes like that. But interestingly the corrosion rate starts falling like this. This is the room temperature. The temperature increases in this manner. Can you explain this? It is a neutral water ok. Can you explain this? Is it possible to have like this? So, what is the cathodic reaction here? In neutral water which of course, is open to atmosphere here yeah. This is oxygen reduction reaction right. You the cathodic reaction here is oxygen will combine at all forms 4 minus. So, now you tell me what will happen? Of course, water will vaporize, but assume that water is not vaporizing. What happens? Anybody has an idea? Yeah this is relative to dissolved oxygen right. Even water vaporizes what happens? The remaining water will have same corrosion unless you have some salts and all. To take a pure water you evaporate the character of the water remains the same right. It is oxygen solubility with respect to temperature that counts. So, what happens when rise the temperature? The solubility decreases right. So, there are two opposing processes. One the rate of reaction is expected to increase with respect to temperature, but this oxygen solubility decreases right. So, the oxygen solubility decreases. So, the corrosion rate is decreasing. So, it is about approximately is about 80 degree Celsius. You see that the corrosion rate drops significantly. What happens if the same temperature effect of this? What happens if the system is closed? The system is closed not you know open to the atmosphere ok. Say you have a beaker and I just close it here and I have water up to this levels have a steel and this is water, this is steel and you are going to heat it now. So, what do you think will happen in this case? What will happen to corrosion rate with temperatures? The corrosion rate will keep increasing right right increases this is the corrosion rate versus the temperature. So, it is a closed system. So, in this case the oxygen does not escape where is the pressure. So, it remains in the system the corrosion rate increases with respect to temperatures. So, this is different from what happens in the closed system, the closed system will decrease. So, this is your open system ok. So, the examples given here are illustration they are not exhaustive by any nature ok. Just take the other parameter then we will we will move on to the next aspect of the uniform corrosion. Next is the velocity. So, what are the effects of velocity on corrosion of a metal? Especially if it is diffusion control ok, if it is if it is activation control, it is diffusion controlled. So, what do you think will happen in both these cases? So, in a diffusion control process the corrosion rate will increase. So, what will happen to the activation control case? Yeah no change will happen right. So, this is the case 1 the case 2 the case 1 no change the case 2 corrosion rate increases. So, probably what would happen is something like this you will find that the corrosion rate might increase and then probably it is going to remain constant can you can you. So, I am marking as the region A the region B. What is the difference between region A and region B? A is diffusion controlled and B is activation controlled. So, it is a transition from activation control I am sorry a transition from diffusion control to the activation control where the corrosion rate does not change. Now, let us move on to the next case where if the system is passivating let us say the system is passivating say if the metal shows. So, what do you think will happen in this case? Everybody is diffusion controlled. Suppose assume that you know something somewhere here it is diffusion controlled ok. So, what do you think will happen? How ok if I plot velocity against let us say the corrosion rate how do you think will vary? The first increases and then there will be ok first I think you see there is increases and then there will be sharp decrease and could remain about the same. Agreed any any doubts anybody has here? Can anybody tell me what will be this corrosion rate corresponds to in terms of current current density? This corrosion rate it corresponds to IP right the passivation current density right. Now, let us get into the the next aspect of quantification of the corrosion rate. Please again understand we are not talking about quantification of various forms of corrosion we are talking about quantification of the corrosion rate in the because a uniform corrosion rate. You cannot you cannot say that there is only one way of quantifying corrosion rate in all the forms of corrosion we talked about in the previous class ok. So, this corresponds to a uniform corrosion rate. So, how do you quantify this? Any suggestions? Weight loss measurement right. So, the weight loss is state forward one what is mean weight loss? So, what how do you what is the unit for this? Yeah how we got meals per year? How do you visualize the weight loss? What kind of how do you do a test in the lab for example, how do you how do you determine the corrosion rate? Yeah. So, you you you take a known dimension of the specimen right. You merge it in a electrolyte of interest corrosive environment and for a known period and then take it out and again measure the weight you see the change in the weight of the samples. Now, what you are going to measure is the measurement what you are going to do here is is the weight loss what you are going to measure right. That is a given time of exposure as a T given time of exposure of course, you need to always qualify you know what is the temperature you know if you change the temperature then everything will will change. So, you have measured the weight loss and what else you know you should also be knowing dimensions right you know the surface area. The state forward use of this data is going to be what w t i surface area is not it. What is the unit here in this case unit is going to be mass the time upon right or not ok. And there is a problem with this unit what is the problem in this unit? When I say there is a problem in the in the unit I am talking in terms of is utility in material selection material design you know for a given application. Suppose, I give this unit to someone ok let us say gram per centimeter square per day. Suppose, I give a unit for example, I say gram per centimeter square per day ok. Suppose, I give a unit like this this unit may not be of any relevance in actual situations. You cannot compare this in terms of life of a given component. For example, I may use lead I may have aluminum right and if both have the same value and if used as a pipe of the same thickness. Assume for argument sake that ok it is 5 milligram per decimeter square per day is the is the corrosion rate I am giving 5 milligram. 5 milligram per centimeter square per day for lead and aluminum in both cases the thickness of the pipe is 3 millimeter thickness aluminum and under lead the diameter of the pipe may be about say about 10 centimeter diameter pipe ok. If the unit of corrosion here both cases is 5 milligram per centimeter square per day which of these two pipes you will corrode will leak or with the leak at the same time is the question clear to you. If I if I have a corrosion rate of 5 milligram per centimeter square per day and the pipe of 10 centimeter dia thickness 30 millimeter thickness do you think the both the pipeline will will leak at the same time the decibel will will matter right. So, which will leak first hm. So, aluminum will leak first is it not for a given weight ok the aluminum will have larger volume larger thickness aluminum will will corrode first. So, this unit is not going to be useful in real applications you should have a unit which is in terms of the thickness right. So, you should have a thickness. So, the weight loss measurement you know you should be used to calculate thickness. So, what happens now? So, you have weight loss is known you know the thickness you know the I am sorry you know you know the time here ok and you know the surface area what you should do you should also thickness what is the rho here is the density. So, this will be what is the unit of this please look at this it is it is the dimension what is the dimension it is the is the length or ok you can say length or thickness dimension per what per. So, you can you can use it is you know you can able to you know formulate any equation that you like you know it is not it is not that these equations are to be taken from the book. So, if you want to calculate the corrosion rate in terms of millimeter per year you can able to substitute all this you know time in terms of year surface area in terms of in may be in the centimeter square and density may be gram per centimeter cube and similarly the weight also can be given in terms of grams. So, it is possible for you to calculate and arrive at an equation of this kind here this is going to be corrosion rate of the metal in terms of mm per year you can also have it for mm per year whatever ok. I small change here please make a delta w here. So, w in terms of gram, rho in terms of gram per centimeter cube, area in terms of what in terms of area in terms of centimeter square time in terms of hours you can have time in terms of you know if you want day then automatically things will change ok. This is the way the corrosion rate can be very determined the weight loss measurements. We have also seen in the in one of the classes that why the weight loss measurement is not a very convenient one right. The question that comes is how long how long do one immerse specimen. Now, this is given as m p y is a different unit upon the rate must be 2000 by m p y terms of hours ok that gives you the duration of exposure to the to the environment. If the corrosion rate is very very less then you need to expose for longer time because what happens there will be insignificant weight loss if the corrosion rate is understood it is just a thumb rule ok. It is just a thumb rule you will see a reasonable you can find out you know you can do that. For example, if the you just substitute if the corrosion rate is 5 m p y 20 m p y or 100 m p y what happens to the corrosion rate in that mean number of hours right you can do that right. So, you can see that there will be a significant weight loss will happen in in relation to this equation 2000 upon m p y actually ok. Yeah this is a thumb rule there is no derivation is done to get this equation here who? For example, if I if I if I keep it as 1,900 hours by m p y I do not think somebody is going to question it over all actually right. The idea here is this gives a some should see when I say thumb rule what does it mean? It means a kind of approximation that you will attempt to do in order to get a useful information right. So, thumb rule actually comes from experience ok. It is simple to understand right corrosion rate is on the denominator right. Automatically the time required for exposure is reduced when the m p y increases. What is this? Again what is the other method of doing it? We also discussed earlier ok. The other one was on electrochemical technique. There are two methods we saw anybody can recollect. What do you determine here? You determine the icard corrosion current density. So, you you determine this corrosion current density. So, one of the methods one is a taffle extrapolation. We have seen this in detail when we discuss about the modern theories right. I am just only recollecting what we discussed earlier right. You can do two kinds of experimentation. One is potentiostatic, the other is galvanostatic. You might have done have you done this experiment any of you so far. What you normally do is you normally use a potentiostatic or potentiodynamic test wherein you apply a given potential you measure the corresponding current. In this case we use a cell called a corrosion cell. The corrosion cell consists of what? The corrosion cell consists of three electrodes is a working electrode. You have again a counter electrode, you have a reference electrode. Please notice you normally choose you choose a small area of about 1 centimeter square you mask the remaining things with a resin right. You mark the remaining things with the resin ok and you expose about 1 centimeter square area. You mask this with the resin here. As we noticed earlier and you pass the current between the counter electrode and the working electrode. So, what happens? The potential on the working electrode is increasing. You measure the potential of the working electrode in relation to the reference electrode. So, you have here a potential stat and now it is they are connected to a potential stat or at the galvanostat. In fact, a single electronic system you can operate in either mode a potentiostatic mode or galvanostatic mode. So, you could able to obtain a right. You have seen this earlier also right. Please go back and refer to your notes. So, you can draw a tangent to this taffle line and you draw a tangent to this taffle line here and this corresponds to right. What is this current called? Icar and this corresponds to Icar and this slope is what is the slope? What type of slope is called? Beta. Beta C right and this is beta A. If one wants to measure the exchange current density for the cathodic reaction how do you get that? How do you get I naught? How do you get I naught from here in this one? What do you call that? In this graph how do you get? The graph is given to you now we need to do ok. What do you need in order to get that? What do you need? What do you need to get I naught? E potential. Yeah, you cannot use all the terminologies there is only one you can use right ok. So, you need to use equilibrium potential right. So, you cannot just please do not use all the words it has no meaning at all ok. So, you can extrapolate this to an equilibrium potential. Suppose this is your equilibrium potential and you could extrapolate this and you would get I naught here. You can also get I naught for the corresponding anodic reaction and so you can get this. From if I given I car how do I determine the corrosion rate? So, I car is given how do I get the corrosion rate? Let us say corrosion rate may be in mm per year or or MP y or what unit that you want. How do I get this? What is the method? Is there any relationship that can be used to determine the corrosion rate? Anyone? What is that law that says that current is equal to what ok? See is a vanadise law right. So, you know that 96500 columns if the current is passed either you can dissolve metal or anything ok of 1 gram equivalent weight. What is 1 gram equivalent weight? It is equivalent to atomic weight by the valency. So, if you pass 96500 columns of current you can either deposit this amount of metal or you can dissolve this amount of metal depending upon whether the current is anodic or cathodic actually right. Now, if I car is what you are getting this let us say I car is in terms of let us say ampere let us say per centimeter square is what the current is all about. How do I calculate now? How do I calculate per year for example? What is the first step? How do I convert this into coulombs? So, to convert this into coulombs what you do that? So, you have to. So, you have to find out what is the current that is flowing for 1 year right. So, you have to have how many days? In a day how many hours? In an hour how many minutes? In a minute how many seconds? This gives you the coulombs of current that is flowing if the I car is given there ok. So, how do I get the corrosion rate from this? So, I can calculate what is the how many equivalents of weight the metal corroded for this many amount of coulombs right. So, how do how do you get this? So, weight of metal dissolved equals to what? Equal to I car. So, this is the weight right. This is the weight of the metal that is lost in 1 year ok and 1 year in what is the so, what is this 1 year lost right. What is the unit here? Can anybody tell me? This is ampere per centimeter square right this is this is the weight also gram whatever ok. So, what is the what is the is coulombs right. So, the whole thing is in coulombs coulombs per centimeter square coulombs weight. So, what you are having here is what is weight per centimeter square per year right no. So, you will get here weight ok per centimeter square per year. So, how do you get the thickness? So, I divide this by weight centimeter square per year at the density right. So, what should be the equation now the corresponding equation is going to be I car 365 to 2460 material by atomic weight 96500 coulombs into the number of electrons n number of electrons flow density of this. And if it is an alloy what happens if it is an alloy the atomic weight is not very easy there. So, you need to calculate the equivalent of that ok. I am just giving this equation here I want it to analyze and understand that actually ok because we are not going to have time. So, for an alloy ok. So, equivalent weight of an alloy for an alloy it can be obtained by atomic fraction multiplied by upon sigma of that. So, it is summation of all of them right. I have the number of number of species for example, I have a brass of copper 70 and 30 zinc you should be able to do that right. You can you know the weight percent from there you can convert into a atomic percent. From atomic percent you can get the fraction here and you can able to get this value without much of problem. I will leave this for you to work out when you have time if you have any questions we can discuss you know later actually. So, I do not want to take too much of time in this class actually. So, please do work out when you have time you have any questions you let me know about it ok. So, the the the the electrochemical techniques you can use Tafel. What is the other technique electrochemical one? Is it used to determine the high car anybody? Yeah. Linear polarization right. So, that is called stone, Geary ok equation also called as linear polarization technique. Here what do you do? You apply a very small amplitude of voltage or current right. You apply delta V across what? Across across across E car. So, it must be plus or minus about 10 mill volt you apply and you measure the current is your E positive E negative I positive I negative ok. What is the slope? Slope is equal to R p. Now, R p can be related to I car. What is that? R p is equal to 1 upon I car beta A plus beta C. Please check this equation is correct or not ok ok. So, you can able to you can able to use this techniques in order to determine the corrosion rate of the metals now ok. So, so far we have seen the basic reactions that are occurring in a uniform corrosion. What are the possible cathodic reactions? We also saw what are the parameters that affect the uniform corrosion. Then we also looked at how to determine the corrosion rate of metals from uniform corrosion point of view. Now, comes how do you tackle this problem? How do you from a engineering point of view how do we control the corrosion of metals or how do I design a system so that I have a life which I need 20 years or 50 years whatever time. How do I do this? So, that is the question now. So, how do we control uniform corrosion? One I can choose a proper material selection you can do that. You can also look at the coatings we can also see the use of corrosion inhibitors and 4. We can also use electrochemical techniques. What I mean by this? Here you can use or employ cathodic protection you can also employ anodic protection. So, let us try to address each of them and see how we can minimize the corrosion and how to increase the life of the structures ok. We will see now. Let us look at the material selection. What is the basis of material selection? Would you like to choose the best corrosion resistant material all cases? Let us say oh titanium does not corrode. Let me suggest that ok everywhere ok go and tell them no ok let us use titanium heat exchanger ok use titanium or cars use titanium this way to do that. What is the criteria for that? Yeah first is the cars I mean of course, availability is one more thing. Design is other one yeah. Fabrication suppose you are going to do a fabrication suppose you want weld not weldable. So, the selection of material depends upon several criteria cost is one important criteria no doubt about it. In addition to that availability the ability to fabricate the shapes that you are really interested actually the cost of the system you forgotten very important one what is the expected life of the component. There is there are some more important issues which are not so obvious the issues are like safety involved. For example, if you have a very high risk like a nuclear power plant there the cost is not going to be a primary issue. The safety is going to be involved actually here. So, you might choose a better material so that you do not have run the risk out any kind of failure and the consequences are very serious. And so, when you when you talk about material selection there are several factors that has to be considered the cost of course, obviously a very important one. As a general criteria if you choose materials depending upon the necessity of this. Suppose, I choose a material for let us say valve say one example or I choose a material for a tank or I choose a material for let us say a body implant or use it for a pharma industries. Now, look at this let us look at the case one and the case two take an example here. What will be the criteria? You know the corrosion rate now see corrosion rates probably maybe you can say no you may have MPY use now still people start using the you know you still continue to use unit like MPY you can also use MM barrier ok. Now, let us take the case of a valve under tank. When you choose a material for valve and tank what will be the corrosion rate criteria? Would you like to have the same corrosion resistance for valve as that of the tank? What happens? I wanted to talk do you choose which is very critical here valve or tank valve is not it? Because even there is a small corrosion occurring in the valve leak will occur, but in a tank even it corrodes it is not going to have immediate thread for any kind of leakages will not happen at all. So, selection of material for valve is more critical than that of the tank. Same thing you can talk about a pipeline a pipeline a water pipeline is not so critical, but it is a gas pipeline is very critical and if there is a pipeline let us say iron gas commission they have pipelines in the offshore it is too remote. Now, even there is a leak it is very difficult to repair that actually. So, maintenance is another important criteria which you need not talk about. If you are able to maintain very easily then you can have a bit of a chance taking a low corrosion resistance material, but it is not easily maintainable you do not have access to it then you have to go for material which is more resistance to corrosion. So, valve let us say about we talk about corrosion rate is about in the range of 5 MPY the corrosion rate. It is this structures you have somewhere in the range of 5 and why is this a tanks and here the problem is not structural integrity the problem here is more of tolerance right. The body will not tolerate a bit of nickel ions getting segregated due to corrosion. You may have implant mechanically stronger no problem. So, here the corrosion rate is because of the contamination issues. So, here the corrosion rate has to be lower than 1 MPY. If the corrosion is going to be greater than say 50 MPY or 20 MPY not acceptable even in normal cases unsatisfactory why it can contaminate the product. You can replace it, but replacing is not acceptable here you know. So, sometimes you can replace component not necessarily that you go for excellent corrosion resistance at all right it talks about that. So, we use the term what is called as life cycle cost. This is the parameters industry use in deciding the selection of materials. The life cycle cost of course, does not involve directly the safety issues. You can integrate safety issues in the cost if you want I mean then taking care of. So, generally when people talk about life cycle cost they do not look at the consequences of corrosion in terms of safety and all these other issues. So, material selection is based on this right. Now, the thing that you would like to know is how do you select materials? I classify the corrosion resistance of the material. We can look at two one is based on the nobility. A metal which is relatively noble would have better corrosion resistance right. Also look at from the point of view of passivation tendency. So, broadly you can you can you can see that the tendency of the metal to corrode depend upon two properties the nobility and the passivation. I give an example here right let us take a nobility. We are talking about engineering materials we are not talking platinum gold and all let us not worry about that right. Suppose you take iron based material you have nickel based one and copper based one the nobility increases. So, iron based alloys are inferior compared to nickel based alloys and of course, copper based alloys are one of the good. For example, you take copper and zinc and copper and nickel can you guess which will be having better corrosion resistance copper nickel right. So, you should also able to get a feel for how the corrosion performance of the metal will behave based on the alloying elements. So, nobility there are other complexities we are not talking about the microstructure may complicate the issues, but generally the alloying elements if they are relatively noble you can say that yes is better material ok. The other example is stainless steel you have what iron nickel chromium iron manganese chromium you know they are right this is inferior this is inferior. So, it is based on the nobility you can say that which metal which alloy is better or which alloy is going to be inferior you can talk about. But you have the limited options here the alloys which are passivating are far better. So, in this case stainless steels right. So, titanium alloys or zirconium alloys or we can say tantalum based alloys. Chromium cannot be used as a structural material because it is quite brittle right. So, stainless steels titanium alloys zirconium alloys tantalum alloys they are all based on the passivity. What is the criteria here? If the IP is is low then corrosion resistance increases this is the criteria for that. I just give an example in stainless steel people have determined the corrosion rate of iron in atmospheric corrosion corrosion rate of iron versus the the chromium talk about. So, somewhere here why things are closed about 11 weight percent chromium and you call this as stainless steels this is for iron right this is for iron ok. So, selection of materials of course, you can look at the there are a lot of handbooks are available you know you can look at this you know the data available from there they are used to for material selection purposes is done. But it is better to have a broad idea about how the metals behave with respect to the corrosion. Again it depends upon the environment the same thing cannot be said that the alloy will behave same manner which change the environment conditions ok. So, with this I think we will close it for the day we will continue this discussion next week and I would need probably one more class to complete the uniform corrosion of metals.