 Welcome to the 16th lecture on Aqueous Corrosion and its control. Now, we are discussing on the Galvanic Corrosion Part 3 right. We are looking at the Galvanic Corrosion Collet. Now, after looking at the electrochemical parameters that govern the Galvanic Corrosion, we need to look at overall factors which can influence the Galvanic Corrosion of dissimilar metals ok. So, I term it as factors affecting Galvanic Corrosion. The more obvious one is the nature of the environment. The second is location Galvanic attack. The third we are using two different metals, two different structures having different chemistry, different electrochemical parameters and the area ratios are very relevant very important ok, area ratios anode and cathode. So, we shall go in details and try to understand. If you understand the factors controlling Galvanic Corrosion, it will be much easier for you to devise means of controlling the Galvanic Corrosion. So, that is the whole basic idea of analyzing the factors those can influence the Galvanic Corrosion. Let us start with the nature of the environment. We can say the severity of the environment would increases the Galvanic Corrosion also increases. The vice versa is also true. If the environment is quite mild then the Galvanic attack becomes relatively slow. Now, we can have just few illustrations to demonstrate how the severity of the environment is going to be influencing. I give two examples. One is the Corrosion in Magnesium Sulphate which is relatively a mild environment and Corrosion in Sodium Chloride solution which is relatively a severe environment ok and take the two metals which are steel and the zinc. If you can look at the corrosion rate of these two metals in uncoupled conditions and Galvanical coupled conditions you will see how the corrosion of the zinc which is more active would change significantly if the environment becomes very severe. This is a data taken from the Fandana book. You can just have you know feel for it. Take the case of 0.05 molar Magnesium Sulphate zinc which really does not dissolve and corrode steel corrodes. When you Galvanical couple these two you notice that zinc corrodes at a higher rate and steel in fact shows a marginal improvement in the weight in fact the weight gain. But you notice that the weight gain is not because of growth of steel, growth of iron. It primarily happens because when you make a metal cathode you know the pH goes up and some deposition occurs. So, if you clean them up you would not see there is any significant improvement increase in the corrosion rate. So, this rise in the weight is not anywhere related to the metal deposition here. As opposed to this you take molar NaCl solution both the steel and zinc corrodes. See both corrode in fact the corrosion rate is almost the same primarily because it may be diffusion controlled right. The diffusion controlled it does not depend upon the corrosion tendency of a metal right. In the couple conditions you will see that zinc corrodes at much higher rate and steel it does not corrode at all. So, the corrosion rate of the active metal here I mean relatively active metal here it is a zinc. It is very much dependent upon the how severe the environment is and the environment here sodium chloride proves to be very aggressive in nature. There are also cases where the galvanic tendency of the metal changes. If you change the temperature or concentration it is possible that at a given temperature and pressure or a concentration. One metal act as active, other one act as a noble. Slightly change the temperature the metal which is active turns into a noble metal. These things are having practical relevance importance. Let me give some examples. Let us take the case of steel versus zinc here. And all of us know that galvanized steels and the zinc coated zinc you know it sacrifices and then steel is getting protected right. But if the temperature of the coating is increased if the temperature goes beyond 80 degree Celsius zinc becomes and iron becomes iron as steel becomes active. And this is called as galvanic reversal. Such a phenomena has a practical problems. One is we have seen that you know the galvanic corrosion of steel you know if the steel is galvanized steels what does it mean? Here steel corrodes. One more example which is relevant we will discuss little later actually is application of zinc as sacrificial anodes in a in a cooling water system not possible beyond 80 degrees. Why? The zinc starts passivating it loses its protective nature in which case you may have to use magnesium or anyway the zinc is not going to work. There are few more examples which is important in fact very interesting. You take a tantalum and platinum this is is a passive system which is let us say sulfuric acid and you do not see no galvanic no galvanic until 110 degree Celsius temperature of the sulfuric acid. You have no problem you can have platinum and tantalum coupled together you do not give any galvanic corrosion rate at all. But if you rise it above or equal to 200 65 degree Celsius ok and tantalum corrodes and you will see a galvanic current of about 100 milli amperes square feet. The tantalum turns into a active metal. The galvanic corrosion was not operating at all. One more practical example is you know the tin coated steel confinus for storing the food right it is a beverage cans ok. If the food I mean if some of let us say juice for example, orange orange juice or apple juice decomposes and give rise to organic acid it attacks tin tin complexes the tin becomes active otherwise tin is a noble metal right compared to steel ok. That is not going to work now steel I mean your tin starts corroding because of the complexing of this actually ok. So, such kind of galvanic reversals are of importance in industries and should be taking care of or should be studied very well before you select the system. Let us look at the location of the galvanic attack. Take an example yeah in which case see when you guys have tin you are in the case of zinc you are talking about yeah see you take in the case of zinc ok. When you rise the temperature the zinc forms nice adherent zinc hydroxide zinc carbonate which are somewhat similar to passivation and you know that when the metal is passivation and the corrosion potential moves up the corrosion rate drops whereas, in high temperature steel does not get protected it does not form a protective oxide film ok. So, the corrosion potential of zinc becomes higher compared to the corrosion potential of steel because steel does not form any passivance. Please understand the galvanic corrosion depends primarily on e-car it does not depend on corrosion rate at all. The first and foremost condition is the potential difference. Tafel, slope, exchange, gun density all of them are important, but if the potential difference between these two metals do not exist then there will be no galvanic corrosion or the one which exhibits relatively active corrosion potential will act as a as an anode ok. So, the potential what happens? So, surface changes would bring in this this kind of effect. Tantalum the similar situation rise the temperature sulfuric acid the passivation is no more stable it just goes away. Tantalum is another reactive material right how does the corrosion resistance for Tantalum affected somewhat similar to stainless steel or titanium alloys. In fact, better than titanium alloys the oxide film is very stable ok and that is why Tantalum is used in most aggressive environment although it is very expensive, but oxide films are kinetic resistance right rise the temperature the film breaks the film dissolves and so, the galvanic reversal occurs in this case right ok. Thank you for the question. Now, look at the location of the galvanic attack it is it is very important relevance for us actually ok. Let us take an example of let us say okapa you have a steel and it is exposed to the environment and you know that this is anode right and electrons move like this and there is going to be maybe H plus or oxygen whatever comes and hydrogen is getting liberated. Agreed? This what happens? Now, you tell me where will the galvanic attack be severe in this case the most severe yeah or the interface right. The galvanic corrosion will be more severe at the interface why it is more severe at the interface the answer is correct but why is it more severe you consider. The potential difference between this point and this point remain the same that is going to almost independent of that ok what more is coming to picture here if the resistance offered by electrolyte right the resistance offered by electrolyte. So, the current flows like this what electrons flow this way and current flows in the electrolyte. So, if the current has to flow from this end to the other end for most end the resistance increases the quantum of current that reaches this place will be less compared to the quantum of current that reaches to the interface because the resistance depends on what? Resistance depends upon the rho L upon A gives you the resistance. The resistivity of the electrolyte the length over which the current is going to flow of course, area which the current is going to flow cross section of orange current is going to flow related to resistance. So, as this the distance increases the quantum of current is going to flow reduce. So, the amount of current corrosion is going to be decreasing from this point to this point. So, that means, if the resistivity of the electrolyte is increasing what is going to happen? Assume that there are two electrolytes one with a higher resistivity the other with the lower resistivity and you will see the attack and what kind of attack difference you will find the same copper and steel one with a higher resistivity other one with lower resistivity. What kind of differences do you see by just observing them that is true, but between the two yeah. So, when the resistivity is more the localized attack is more and the attack is confined to the interface right more than the attack or the interface phase becomes. In fact, this is one of the characteristics of galvanic corrosion ok. If you want to diagnose a problem if you see in the industry that there are two different you know set of metals which are electrically shorted and you want to show that there is a galvanic corrosion the first criteria is that what the interface should have a deeper attack. In fact, attack will be something like that this is your active metal you are relatively no more metal. On the on the other hand assume that I have pipeline and pipeline I got you know the two different metals in the pipeline ok and I have pipe. So, one side is relatively active other side is relatively noble the fluid is going inside and it is corroding. If the corrosion is away from the interface at a severe would you call it as a galvanic corrosion and it happened in the active metal right would you call it as a would you call it as a galvanic corrosion. Do you get my question? The pipeline carrying of corrosive fluid the pipeline has got you know two parts part one is active metal part two is noble metal and they are electrically shorted as interface. The pipeline leaks the found that leak has occurred at the anodic member of pipeline, but you also notice that the leak is not at the interface it is about about say about about 10 centimeter away from that or 5 centimeter away from this would you call that as a galvanic corrosion you have to give an answer. Why do you call a galvanic corrosion then? If the resistance is offered then you should be confined more to us when you say resistance means what higher resistance then you should be more confined to the interface yeah liquid is moving that is what the problem we get in industry right. So, what do you think if the corrosion has to occur by galvanic mechanism there is no other way, but the interface has to corrode at higher rate is there any other way you can do that when the resistance of the of the of the electrolytes you know of the electrolytes increases from the cathode to the anode has to move away from it the current will not flow more we expect current to flow more at higher distance and less current at the interface the physics does not permit it has corroded no doubt, but it is not due to galvanic corrosion at all it cannot happen you need to diagnose what the problem is there may be other form of corrosion that has happened, but yes certainly it is not a galvanic corrosion ok. So, it is very important that we need to know the fingerprints you need to know the characteristics of the corrosion failure the characteristics of corrosion failure is it happens in the active metal it happens more at the interface if it happens for example, at the noble metal you would not call a galvanic corrosion maybe something maybe erosion has taken place metal maybe noble, but it is softer it happened maybe there are some microbial corrosion taking place. So, it is very important that you need to understand the fundamental and then try to relate to the failure it cannot be having a conflict with the basics ok ok. So, it is not really a galvanic corrosion in which case it may be different then you need to diagnose and find out the root cause why the corrosion has taken place there and this is in fact, is a fingerprint of galvanic corrosion. The environment is is less severe the guru attack become less only, but you cannot say that corrosion occurs far away and so, there is it is a galvanic corrosion. Let us go into the next important aspect of it the area effect it is very easy to understand and it is a very impacting in designing structures to avoid galvanic corrosion. Let us look at the mixed potential theory and see whether it helps us. You know that total cathodic current is equal to the total anodic current right it cannot really change. Now, what is this is equal to what this is equal to current density multiplied by the area of the cathode is equal to right current density at the anode and area of the anode agreed ok. You know what it is right A represents what A represents area and other variables that you are aware of right. Let us rearrange this equation equation 1 equation 2. What do you understand from this equation? If the area if the cathode area of the cathode is becomes very high the corrosion current density on the anode is going to be I. So, that means, you can say that if A cathode tends to become infinity I A is going tend to become when I say I A here I mean the increase in the corrosion rate because of galvanic action. On the other hand A A tends to become infinity I A tends to become 0 due to galvanic couple understood. So, what does it really mean in practice? If you are an engineer you want to construct certain component what should be the design criteria? Yes you should take care that that the component you are thinking about you are choosing a material you have to choose two metals you have no choice but to do that. You should always ensure that the anode is larger and the cathode is smaller. The vice versa could be a real problem ok and this can be also seen by the mixed potential theory as well ok. Let us look at the mixed potential theory see whether it is correct or not. I am putting log capital I here I am not using here small i capital I means what current right ok and assume that this is steel steel. So, it is iron going as Fe 2 plus in acid right and I have theory ratio of iron and platinum or equal to 1. I am not showing the cathodic curve corresponding to steel because the hydrogen evolution on steel is lower compared to hydrogen evolution or you can show it here it is ok. I naught H plus H on steel ok then you can use platinum. Platinum right this is area ratio is let us say is 1 centimeter square you know steel platinum 1 centimeter square steel ok 1 centimeter square I make platinum it is about 1000 centimeter square. When I make a 1000 centimeter square you can please make I think it is a small change here you cannot make this I naught here should be capital I ok because we are talking about the area we are not talking about the normalize it we are talking about actual current right. So, if we are going to have multiply the area of that we like this ok this is 1000 times ok. So, the corrosion rate would would increase because of the galvanic action. And this has some practical implications and I just repeat the illustration given in Fontana's book it is not too concentrated, but it is you know it is an reasonably concentrated sulfuric acid. See very highly concentrated sulfuric acid ok you do not need anything the steel is it offers very high resistance to corrosion, but moderate concentration and dilute you know you know dilute acids the steel corrodes heavily in in sulfuric acid. So, how do how does the how does the steel becomes resistance to corrosion you apply a coating you call a lining actually right you give a epoxy lining. The difference between a lining and a coating is the following the coating is normally given in order to prevent the corrosion of the metal. So, the lining is given not only to prevent the corrosion of the metal, but also to see the product does not contaminate right. So, you know I mean you may allow metal to corrode ok because you can replace it, but the sulfuric acid has got too much of iron corrosion products is no use. So, the lining is generally meant to you know prevent corrosion to the extent that the corrosion products they do not contaminate whatever process fluid that you are talking about. Now initially a tank was steel tank was was used and they had this was the epoxy lining this is sulfuric acid you are working well ok. It gave us to few few years of service ok there is no problem, but what happens you know there is always a mechanical damage you know people want to clean the tank. So, when you start cleaning the tank and the bottom of the tank more often suffered the corrosion ok severe corrosion, but lasted a few years 7, 8 years lasted and they found that the corrosion is more often on the bottom is more due to mechanical damage due to corrosion damage. They thought that they can line this tank with stainless steel stainless steel is harder and mechanically it is quite sound and you know we do not have to worry about mechanical damage. When they want to do a lining of stainless steel then they thought ok let them let us not line completely the tank because this cost more see many other tanks are very very big right. You know you can save money by reducing the you know the lining and it is happening anyway only at the bottom of it. So, they started giving a lining in in this manner they gave a lining and the top portion the steel and it was a steel the tank and it is coated it is not really well drawn anyway it is a paint this is painted. How do you line it and this is a place you weld right welded and you can see ok you can make it more neat stainless steel lining stainless steel lining is done interestingly this sulphuric acid now tank failed very shortly and it failed where failed at the weldment. Now you see here you spend stainless steel very high expensive lining despite that the life is very short very interestingly the guy diagnosed saying that the painting is bad and so the tank failed because the painting occur I mean the corrosion occurred at the paint and this place is welded here right and you weld it and painting is always difficult at the at the at the painted location right normally you grind very nicely and then you paint so the person said that oh the failures happened because of the improper painting of that and they went on to say that you should we should redo the painting but some wisdom prevailed and try to look at why it is failed why it failed before I go into that answer can you anybody guess or find out why does happen fantastic ok the paint can never be impervious always porous when the paint is not impervious it is porous now what happens they are very fine finite place small locations you have an anode not only that you have a lining which is stainless steel lining which is a cathode that cathode is of infinity area compared to very finite anode below the coating defects so very highly accelerated corrosion taken place right so the reason is simple what is the reason the reason is defective coating then what happens you have AC upon AA tend to becomes infinity why it failed so there are several such cases why coatings you know are to be given only on cathode if you cannot coat both right or you coat completely suppose had the guy has coated completely both the anode and cathode then what happens there are defects the paint coating the cathode also very small in number right small in area no problem so this leads to the understanding that if you have dissimilar metals you coat both the anode and cathode if you cannot coat then what happens the coat only the cathode so if you can coat only the cathode then what happens then you will have a very very finite area of the cathode the galvanic corrosion becomes very minimal and you would not have any problem at all so coat both anode and cathode or if not possible coat only the cathode and not the anode so such kind of surprising you know conclusions that will come out of understanding of the parameters affecting the corrosion ok so we have seen now the factors affecting corrosion and I think you should be able to now find out are able to propose how to prevent or mitigate galvanic corrosion yeah I want a very quick answer quickly you should move forward yeah how do I mitigate galvanic corrosion yeah avoid use of dissimilar metal that is simply not possible ok many cases you have seen in the case of intake changer not possible if I have to use then I have to use I use what the first criteria what do I see look for first I look for I aggregate I look at it but I if I can choose metals yeah so I look at the galvanic series right look at the galvanic series metals closer in the galvanic series what more I do that yeah I can also look at higher anodic and ratios ok where is it is possible look for higher anodic to cathodic area ratio then what happens I can isolate electrically right I can insulate insulate electrically please look at where possible you know it may not be possible everywhere ok what else I can do coating right coating it is an important lesson here right what is the lesson here coat yeah both coat both anode and cathode if not possible the cathode so we also had one more lesson in terms of effect of environment on galvanic corrosion what is that more severe is the environment more severe will be the galvanic attack so how do I reduce the corrosivity of the environment yeah what is that inhibitors right use inhibitors they are done in cooling water systems people add inhibitors what more you can do yeah you can use the third one use third metal active to both this is used to much in industry yeah this is done mostly in industries I just given this figure you have seen before right and it is an heat exchanger this is the this is called a header or water wall boxes header it is these tubes generally are stainless steels that is the galvanic corrosion here how do I control galvanic corrosion you can in these places you can use zinc anodes zinc anodes zinc anodes ok when I weld these places with the zinc anodes what happens now the steel though active to stainless steel it is rendered noble with respect to the zinc zinc corrodes so so you you know a couple of years no problem ok and you can replace the zinc anodes periodically and it is done in industries ok it is not uncommon it is done widely practiced in order to reduce the corrosion of corrosion of the the active metals ok it is done so use the third metal that means you have ok this is your cathode so anode what happened you are going to use a1 and a1 is a1 is anodic 2 and c is what c is a cathodic so you can have you can replace this a1 all the time I know whenever you you find this anode is dissolved so it is a sacrificial one ok you can also do one thing you can also you know 7 make this anodic parts easily replaceable you can design that right you please notice we have given some 7 8 like this right 6 7 not necessarily all of them will work it depends upon the particular application you might use a combination of one or two or one or more the idea is that the cost being down and there is a reliability safety in the system ok so that is an engineering aspect of what kind of technique can be used to avoid the galvanic corrosion of metals so we are talking about mitigation let us go to the last important one that is how do you test for galvanic corrosion so what are the testing that you would like to do the most simple test what will be that I just give two metals I want to know if they are compatible or not or I give three metals you find out which is which are the two are most compatible I want to do a very quick test ok what test will be useful you you studied all of them all parameters now three metals are given to you or I have given you let us say half a dozen metals are given to you you do not have much time I just want to choose two of them otherwise all six of them are satisfy the requirements like what are our requirements I want to choose yeah yeah first of all you can determine the galvanic potentials right is like I call it as e car actually ok e car in a given environment you just measure it right you choose those two metals which lie close in e car values right so e car is one important parameter it does not give you the corrosion rates right it does not give you corrosion rates so you want to determine the corrosion rate in the in the galvanic systems what parameter do you use will I car of each of them will work it will work I give you I car of metal A I car of metal B will that be useful do you think can use that yeah will that be useful not useful if useful how it is useful if not useful tell why it is not useful yeah so I car by itself is not useful it cannot give you a direction which of the two will undergo corrosion first of all it does not even tell you which of the two will be anodic in nature or cathodic in nature so ok not useful ok I can do a weight loss method right I can do a weight loss method what I can do I can short both of them right and I can determine the corrosion rate right I can just short or both metals determine the corrosion rate this is time consuming takes more time right what I can do if I want to use a Faraday's law ok and I just give you a clue right and I want to find out you know what kind of corrosion rate you know I just give you half an hour assume that system is stable ok in half hour times I want to find out the galvanic corrosion rate what parameter do you measure assuming that the system becomes stable of course what do you want to measure I can measure galvanic current or not I can I measure the galvanic current can I can I just have metal 1 little 2 only problem here is I should use a zero resistance ammeter no otherwise what will happen the ammeter itself will offer resistance the current flow will be reduced right so use a zero resistance ammeter can do that ok. So, it is a quick method it may not take as much time as it will happen in the case of weight loss but these two techniques suffer one problem what is the problem the galvanic corrosion rate depends on what it depends only on two different metals a difference anything anything more of course that is taking care of here right anything more that will affect the corrosion rate of of the active metal environment is known that is given to you you use environment only yeah tell me what is that area is going to change right if you are going to change the area then the rate of attack of the anode is going to be different right. So, there is going to be one more effect which is an area effect. So, which means when you do a test you are to ask a question what is the area ratio in the actual field I need to choose that area ratio otherwise the corrosion rate so measured will not reflect the corrosion rate or given metal in a given component right. So, area effect is a very important one so you may have to do several tests to find out what happens to the corrosion rate the area effect is changed ok. So, that can be circumvented by a simple technique which is called as the electrochemical technique and use Taffel extrapolation. I hope you understood what I am trying to say how do I how do I determine I say that I carried only one test for the anode and one test for the cathode so called cathode and I use the data for all variations in the area right can I I give only one data for anode I mean so called anode both cathodic curve and anodic curve and I give one anodic curve and one cathodic curve for the noble metal. Now, I say please use it for different area ratios possible not possible. So, I carry out electrochemical experiment right I just carry out an electro chemical experiment scan from one potential to another potentials I get this ok in terms. So, this is the data I get current density versus E this this is a data I give it to you from this can you determine the galvanic corrosion rate if the area ratios are changed assume that the cathode is becoming 100 times more ok in terms of area. What happens? So, you would what ok so, what do you do in that case? So, you multiply this by 100 times right. So, you do not plot high small i you plot current here log current versus this and so, this you multiply by 100 times it moves over here or is anode is I mean sorry ok this is metal A metal A in this case is anode here, but if the I am sorry you see this is the cathode in this case right ok. So, cathode here is anode here right. So, you multiply this by 100 times it moves here if the anode is is 100 times you know larger in size then you multiply this current by 100 times then you start moving up here. The same curve can be used to determine the galvanic corrosion rate of metal under different area ratios all you need to do is you need to get only one polarization curve for the different metals and then you can find out the the galvanic corrosion rate of the metals ok. So, so much easy. So, what I mean is that this is where you understand how the advanced diagrams how the electrochemical conducts will help you in many ways you know ok even testing also you can make it a very effective testing ok and you have any questions. So, with this we will complete our discussion on the galvanic corrosion rate of metals. We saw in the beginning that the galvanic corrosion primarily occurs because of the galvanic potential difference between the two different metals they are in service. The galvanic corrosion does not depend upon the equilibrium potentials of course. And we also looked at the cases like active metal and noble metal and two different active metals in the passive systems. And we see the electrochemical parameters such as X i in condensity, Tafel slopes, the equilibrium potentials or the factors that control the the galvanic potential the galvanic corrosion condensity ok. The extent of corrosion depends upon the nature of environment like how corrosive it is. So, when the environment is changing its properties you rise the temperature and all these issues then there can be change in the galvanic behavior a one metal can turn into a the metal which was behaving earlier as a noble metal can turn into active metal. And the other important thing is the the fingerprint the signature of the galvanic corrosion is very clear. It occurs at the anode cathodic interface and occurs at the anodic side. In fact, it is a groove it is not even a state one. Occurs state one I mean something like that then there is something more than that actually ok. Assume that I have you know metal 1, metal 2 it occurs at the anodic side, but occurs fully as a groove it is not like a you know it is not like a v groove then it is not really in my view it is not going to be a truly a galvanic corrosion there may be some segregation of something happening at the interface. So, the fingerprints of galvanic corrosion need to be understood ok. The other important area which you need to to look at is that if you have a component where you are fabricating the two different metals the anode area has to be always larger and the cathode area has to be always smaller. And and we have also seen how the Evans diagrams can be used to to determine the galvanic corrosion rate of metals with a simple experimentation. And with this we will end our discussion on the the the galvanic corrosion of metals. The next class I think we will go on discussing about the crevice corrosion ok. Thank you very much.