 Welcome for the part 2 discussion on galvanic corrosion. The part 1 discussion on galvanic corrosion we raised certain issues of like can we predict galvanic corrosion or not if you have 2 metals. If you can predict can we determine the galvanic corrosion rates and if these 2 questions are answered then we said that you will let you know what are the factors that control the galvanic corrosion. And if you know what are the factors or parameters that control corrosion that are applied corrosion then it is easy for us to propose methods to avoid or to prevent the galvanic corrosion of the metals. We also need to look at the evaluation of the material called application against the galvanic corrosion. We have seen the first aspect clearly in the earlier class. Can we predict if 2 metals will undergo galvanic corrosion or not? What is the criteria for that? What is the criteria to show that ok I have given you let us say stainless steel and maybe a steel between these 2 one will be galvanically corroded. What is the criteria for that? It is a potential. It is a potential. What is their potential called? It is a galvanic potentials or the corrosion potentials. So, we seen in the last class that the galvanic potential is the primary reason for making one metal as an anode the other one as a cathode. We also just started discussing on how do you really predict the rate of corrosion. We look at that is the Evans diagram alright. We look at the Evans diagram taking an example of a clearly a noble metal and an active metal. We took for example, I think the platinum I think right. We took platinum and then and then used the Evans diagram to find out what happens if I if you can couple them galvanically with the other active metal you know. We gave only schematic thing there. We did not give really actual values. I think we used for steel I suppose steel iron is talked about. Now, we also said that in the case of platinum the the the the equilibrium that we deal with there is the we I would say maybe H plus H H equilibria or maybe oxygen and hydroxide or water equilibria for example, we are not really concerned about the platinum iron and platinum equilibria because too far from that and then we also looked at the equilibria of what? The equilibria of of the active metal in this case may be iron and iron ions and the corresponding taffle lines and and see how the mixed potential theory will ultimately lead to the the ecore value of the platinum and the steel when they are coupled. What is the corresponding ecore value? How does you get it? The same the mixed potential theory is valid here the total amount of cathodic reaction on the system is equal to the total amount of what is that other reaction? Ionode reaction. So, that is the criteria. If that is the criteria then you know what is the corresponding ecore and icore values. We also said that if you are going to shorten platinum versus steel the ecore value of the couple I mean the ecore value of of the iron goes up and and so the anodic dissolution increases. We also say that the hydrogen evolution on steel or iron decreases whereas the hydrogen evolution platinum significantly increases. So, these are observations that we made. In practice we do not normally see a noble metal being shattered with the active metals. We normally see metals which are generally active in nature we are not talking about it ok. Say some examples practically you will see you know you may have let us say steel you may have stainless steels or you may have cupronic alloy you can have steel and you know we were talking about one of the prevalent place of galvanic corrosion is what is the heat exchangers. In heat exchangers you can have a steel shell the stainless steel tube you may have cupronical tube and steel as a shell you can have we can also have titanium and you can have steel titanium stainless steel this kind of combinations are quite common ok. And so you are going to talk about really two active metals when they are involved how does the corrosion takes place. Now I am going to now talk about here between two metals which is let us say I have called iron here which is somewhat like steel I talk about a steel combined with zinc. In practice it is not the situation here you have a galvanized coating but I do not think zinc is used as a structural material you know there may be some zinc digas things but otherwise structurally zinc is not used as a good material of construction. I taken zinc for one important reason to show that Icar is not important in the galvanic corrosion you have two metals like a steel and zinc if I have assumed that the steel has got high corrosion rate and zinc has got low corrosion rate that is not going to decide the galvanic interaction what is going to decide is the galvanic potential that decides which is going to be made anode which is going to make it a cathode. So that is the reason why I chosen to illustrate how the galvanic interaction occurs between say iron I can call a steel and the zinc. Now how do you proceed with you proceed in this very simple similar manner like you draw two Evans diagrams right the Evans diagram for what Evans diagram for the steel the Evans diagram for zinc which is first you need to make them right but when you making this be clear about what you what in the Evans diagram what are the things required you need what are like exchange condensity required tough lines are required tough so required the equilibrium potentials are required okay these are the things that are required actually. So what you can do is you can construct Evans diagram for zinc and iron separately or independently and find out between these two which will undergo more severe corrosion and when you shot them when the galvanic interaction occurs right what happens to these two metals okay and so I think you can you can work independently now actually listen you can start drawing the diagrams but keep in mind the one important factor what is the difference between zinc and iron we discuss I think which will undergo more corrosion zinc will undergo more corrosion or iron will undergo more corrosion let us say in analysis solution which of the two or maybe in a let us say in a soaring chloride solution where the hydrogen reduction reaction is the cathodic reaction okay other is the metal oxidation you tell me which of the two metals you expect to corrode more iron why yeah so the exchange condensity for hydrogen equilibrium with H plus on zinc is much lower as compared to the exchange condensity for H plus H on iron it is about 10 power 3 times difference okay so so then you can now we can draw okay a typical in or I would say a schematic difference diagrams representing iron and seal and show how the corrosion taking place I think you should draw parallely and and see how you are you are getting this these things now. So I am just going in a very simple manner and there is nothing very complicated here right so I represent now the the tariff aligns for hydrogen reduction reactions okay I will do that so which is what which one corresponds to iron and which one corresponds to zinc the right one corresponds to the exchange condensity for Fe 2 plus Fe and and zinc and zinc 2 plus are almost the same 10 power minus 6 or so so it is not going to change much actually so I just drawn a you know kind of representative diagrams okay which one will corresponds to zinc which is the one corresponds to iron lower so this corresponds to zinc right I naught zinc 2 plus and zinc and what is this this corresponds to now you can find out what is the corresponding corrosion rates right and this is the zinc right what corresponds to iron the iron comes somewhere here see please again label that because you have to label these things otherwise it will confusing you corresponds to and this corresponds to electrons right and these two are corresponding to what this is corresponding to arrow here this corresponds to how do I proceed further how do I know when you do a galvanically shorting them what will be the resultant corrosion rate of iron and zinc how do you proceed I have add the oxidation of both the oxidation curves and I have add the reduction curves corresponding to both the metal surfaces right so I do that okay so what you can do is you can just add this to this corresponds to what this corresponds to hydrogen evolution on both zinc and iron correct and similarly I can do this corresponds to total anodic so what is happening now it is this intersection point of this the mixed potentiary is now is valid right so I draw a line connecting all of them and I draw this line what is this potential curve this is of E car iron and zinc couple right this is the I car of iron zinc couple total corrosion rate is increasing actually right I hope this lines are clear to you now so can you tell me that means what happens I take a zinc I take zinc and I take iron here I determine this corrosion potential of that corrosion current of this now I put them together I just shot them like this when I shot it will the potential of iron and zinc will be similar or different the similar because is conducting is conducting highly conducting so the potential of these two are going to be similar right that potential of this corresponds to what is given in the event diagram as E car iron and zinc this is what you are seeing this year at that particular potential what is the current that is flowing on steel and zinc is given by this diagram now can you can you get from the diagram the diagram what is the one that corresponds to the dissolution of iron what is the one that corresponds dissolution of iron and what is the one that corresponds to the dissolution of zinc can you people tell me in this what is happening there yeah so the line the potential line intersects now where is intersect it intersects all the lines it intersects the cathodic reduction curve of iron zinc anodic current of iron and as well as zinc right so that particular intersection point gives you what is the corresponding reduction reaction oxidation reaction in both cases ok so what happening now now what is happening now you see what is happening to hydrogen evolution on zinc on zinc the hydrogen evolution on zinc is getting reduced to this value am I right it is reduced from here to this is values getting reduced right the hydrogen evolution is decreasing from here to here but what is happening corrosion the corrosion of zinc is moving from here to this value am I right so zinc is now getting oxidized more so this is your this is the icorr of zinc when when coupled with iron right no I am sorry nothing wrong actually yes it is wrong actually right yes correct only yeah I am sorry it is correct yes correct so it is that of the zinc here is correct right so this is the zinc dissolution rate given here and this is the dissolution rate of the this is the dissolution rate or icorr of zinc actually right so the iron corrosion rate is decreasing from from this point to this point am I right and the zinc corrosion rate is increasing from here to this ok so what do you call this process you discuss about one protection before the electrochemical protection of a metal right what do you call this the sacrificial protection it is it is a cathodic protection of the metal by sacrificial action zinc is getting dissolved at the expense of iron right the iron is getting protected here ok so look at these diagrams it is a very busy slide I would say busy diagram actually you have several information that you can draw from here if in fact you understand this you understood the mixed potentiary in completion I do not think you will have problem because it is the most complex events diagram that you can think of actually ok now you know that the the the dissolution of of of iron is is is getting reduced from here to this zinc is increased from here to this ok is increasing here ok no sorry zinc is it is moving from here to this actually ok so so so this is what happens now you you you look at the the kinetic factors that influence the corrosion of zinc what influences the corrosion of zinc in this can you just tell me more each of them influence other one right iron influences zinc zinc influences iron right you have iron has got two table kinetics one is the anodic kinetics corresponding to iron dissolution please understand you have a cathodic kinetics corresponding to H plus giving rise to hydrogen iron has got two kinetics for the zinc to dissolve which of the two kinetics influence very severely yeah hydrogen evolution iron is severely affecting that means the beta C the beta C of hydrogen evolution and iron is severely affecting this agreed then look at from the zinc point of view which the kinetics of zinc is affecting the galvanic corrosion of zinc zinc has two kinetics couple kinetics one is hydrogen evolution on zinc right other is a zinc dissolution zinc to zinc 2 plus right two double slopes beta C for zinc dissolution and beta A for the I am sorry beta beta C for the hydrogen evolution on zinc and beta A which is the oxidation of zinc taking place beta is ok so the beta A of zinc is getting influenced more please notice the beta C of the noble metal I can generalize it when I say noble I mean relative noble so the beta C of the relative noble metal beta C of the relative noble metal and beta A of the relatively active metal decide the galvanic corrosion rate can you say that actually the beta C of the the act of the of the relative noble metal beta A of the relatively active metal decides the galvanic corrosion rate right similarly the I naught of the I naught for the cathodic reaction of the noble metal is more important than the I naught of the cathodic reaction of the relatively active metals similarly you talk about I naught of the metal oxidation to metal reduction you talk about it is more for the active metal than for the relatively noble battles so this this this diagram this diagram what you see here this diagram see here it talks about what are the governing factors for controlling the galvanic corrosion can you predict the corrosion rate can you do it if I give you this these all the double parameters exchange currency is it possible for you to calculate by solving the equation what will be the corrosion rate of of of iron before coupling with zinc and after coupling with zinc is it possible to do that by solving the equations are not possible to do that I want to answer from you possible possible so it is possible to calculate the corrosion rates based on the mixed differential theory based on the Evans diagram what will be the corrosion rate of the galvanic couple okay in two different metals are existing at all all we need are the independent governing equations yeah that is right you can add one more complication what happens suppose I have also in the system it is unique question when when metal is corroding in water you still have so you need to add one more complication to that right add one more to this you can solve the equation right you can write algorithm and solve it it is not that difficult for you to do this in fact people do it is not that in academic exercise people do in the field how the corrosion are taking place in the galvanic conditions right so it is possible people do it because there are practical problems existing in the field okay so they use these governing equations to solve the galvanic interaction between the two different metals are done actually okay to say then what kind of a life is possible all this you can do this with this equation I hope you are able to get this clarity in this Evans diagram if not you please have a look at this go through more closely if you have any problem you please do let me know okay if you understood this Evans diagram I think you have understood almost all the complex Evans diagrams that are possible in the corrosion processes of metals whether galvanically taking place taking place by correct process fitting process all all are following the Evans diagram the kinetics of that actually okay if you are clear and if you have any questions we can move forward okay anybody has any questions please let me know yeah how do we get the line corresponding to the total anodic reaction in the given graph huh what do you do see the current varies with respect to the potential right so what I do let's take the case here let's take this one this corresponds to what this corresponds to iron dissolution this is the equilibrium potential when I move the potential to higher value suppose I move the potential I move it to here for example I move to this place okay this corresponds to this how do I get this the travel line right eta is equal to plus beta log I by I cannot I can calculate I know equilibrium potential I know applied potentials I can calculate right so I get this line I get this line now at this given potentials suppose let's take this particular potentials I know this is the amount of current for and dissolution this is the current for zinc dissolution there is a total current I am going to get suppose I take iron and take zinc together and pass current apply same potential by using a meter that is a total current I will get it so I simply added these two okay and in a graphical way but you can also do it by solving the equations by applying various potentials right you know the equilibrium potentials you can keep on at different potentials you can able to do that okay so graphically you can do this you can also do by having writing a proper algorithm and and getting these values without any problems okay so that is what I have done it okay see I have not added here please see here below this is only one current and above this only the current goes up right so I have just done a simply a simple common I would say should understanding that how this will work at all actually yeah see please here this is a log scale the log scale this is you know may marginal means it is a log value right assume that it is 10 power minus 4 and this 10 power minus 3 you added what happens so you will be you know 10 power minus 4 10 power minus 3 is going to be a thousand thousand one for example okay so so in graphically speaking it is a log scale if it is a linear scale you would have just pushed equally right so it is a log scale here that is why you are getting this small variation otherwise it is a significant variation that you are seeing that okay of course compared to this this is small variation no doubt because a log scale at all okay so please notice that this is a log scale that is why we we have added here okay only small in fact if you look at more closely you may not even go to this valley it must be just touching the line only because you know it is 10 power 10 power 3 times higher so he is going to go into that the third decimal point so it is going to be less only actually okay so basically you know it is a log scale that is what it is yeah there is a cell you move okay so this is how you understand and you can predict the calculate the the corrosion rate of of of the of the metals let us make it a little bit more complex assume that the system is passivating you know the passive systems are you know can you give an example of a passive passive system stainless steel in maybe in sulphuric acid okay titanium cannot be okay let's make the the case of a stainless steel in a natural solution it is derated it is not aerated so what happens in that case it undergoes what it undergoes active passive transitions you've seen somewhere before but is different way of interpreting today okay so stainless steel the aerated acids I am not talking about pitting at all just talk of passivation okay let's take this what I am going to do okay so this is corroding here the metal is undergoing this is let us say I naught H plus H on stainless steel okay this is an active passive dissolution and you guys know that it is it is an I car this is your to make it pictorially I say take in a material acid they say sulphuric acid they will do sulphuric acid stainless steel so this is the car if I measure with a reference electrode and I get this e car and if you carry out polarization diagram all you get something like this actually what I will do I am going to now insert a platinum and I am going to short this what do you think will happen now in order to know what to do happen what you should do now I should draw platinum so I should do one for platinum what I have done for this I do one for platinum okay I do something so what do you do you just some of these two so what do you think happens now corrosion decreases right the corrosion rate significantly decreases because of the galvanic action right so you can see that that the corrosion rate moves from there to this is due to what it is due to I car of stainless steel understood for some reason assume that the line is going to line somewhere here assume that line is going to somewhere here what happened what will happen increases so depends upon what extent the exchange condensate is increasing the exchange condensate is not increasing adequately you could have a situation where the corrosion rate could increase right let us go to the situation here now is moved from here to this does it does it reflect something we discussed earlier is it cathodic protection is the potential is moving cathodic it is moving anodic right the potential is moving anodic so it is similar to anodic protection in anodic protection you are applying an external current by a by a by a potential stat here you just use a platinum the platinum can enable okay the the fascination of metals possible I am not saying doing but it is possible that you can do if this current is adequate enough to cross the nose of the right nose of this particular particular polarization curve that does of course is critical right so it is possible to do this okay and this is happening in order here this happens in the case of titanium and platinum it happens in the case of titanium and platinum we can take a titanium the reason is that the EP is quite low for that is very low the critical condensate for titanium to pass version becomes very small so you do not need so much of so much of current required okay so the platinum it does help so titanium platinum is a well-known example of industrial use industry it is used really obviously used industrially I just give a different okay people do this called as titanium platinum alloys okay here small quantities of type of small quantities platinum is alloyed you make an alloy the metallurgy is no right you take titanium put platinum to that you melt it and this undergoes persuasion how does it go I have let us say okay and I this is your titanium alloy and I have very small amount of platinum that I have how does it work initially in this case okay as I told you before I have this initially what happens you get something like that now as the titanium dissolves what happens platinum gets enriched okay now what happens as a titanium getting enriched what will happen to this this line this line will start alright this will move as titanium gets corroded and platinum gets enriched so what will happen in this case at beyond certain point the platinum will spontaneous beyond certain point the titanium will spontaneously pass it I do not need to pass any external current okay because of this so you do not put too much of platinum in the beginning you put small platinum okay as the titanium dissolves enrichment of platinum occurs and and so the the cathodic curves are shifting towards right side and ultimately need to pass fashion corrosion right isn't it with mechanical action the titanium and platinum will go together right it do not remain platinum also it just get knocked off because platinum is noble it does not dissolve it remains on the surface and so it gets enriched okay so and this is called as this is called this term for this is called as noble metal alloying mostly sulfuric acid reduction reaction is hydrogen only in sulfuric acid right what is reduction reaction is a hydrogen reduction reaction actually right yeah yeah of course you can you can put a platinum and I just simplified all this actually okay so this you can say if you want is called as a noble metal alloying and so this the principle is like this only so what do you need to look at here is analyze the situation okay how does the galvanic interaction works okay it could work if it is going to be purely noble metal and active metal two different active metals or you can have a noble metal and a passive metal the passive metal and the active metal you know you can have all kind of combination they do not have to worry we just have to draw the immense diagram and then find out what is the resultant galvanic corrosion can happen in the metal actually okay so that is the way you should proceed in calculating computing the corrosion rate of the galvanic couples okay is it is it clear to you okay so far we have seen the influence of electrochemical factors on the galvanic corrosion of two dissimilar metals if I can summarize what are the factors that one should look into when you are computing the galvanic corrosion I think that will be very useful the following factors electrochemical factors we have seen three different cases active metal and a noble metal two active metals in another case a passive system and a noble metal combinations if I summarize okay what are the governing kinetics in each case we can say that the cathodic kinetics of what metal you think which of the two the noble or active metal will will influence the galvanic corrosion the cathodic kinetics of which one the noble metal okay of noble metal to the anodic kinetics of which of the two okay I will put it as active or I called as relatively active that way you can even call this noble or what you call it or relatively noble right noble or it could be relatively noble when you say this kinetics what do you mean by that what are the parameters you think the broad right kinetics means what are the parameters you think what are the things that will take into account the kinetic parameters are what one the exchange current density to what is that it is the equilibrium potentials or you may even call it as a corrosion potentials what else third table slopes right. So, so you if you can look for these informations with respect to the relatively active metal and relatively noble metal and you get these parameters you can you can calculate what can be the galvanic corrosion rate of the relatively active metal right. So, that can be can be done without much of a difficulty any any questions here okay.