Lecture 09 : Galvanic corrosion IV

[Music] [Music] welcome back we will have lecture nine for the course ah corrosion failures and analysis and will continue our discussion on galvanic corrosion so the course is and topic galvanic corrosion now if we see that in case of galvanic corrosion its basically is defined as two metal corrosion and one example we have provided is iron and zinc s and we have seen couple of observations the observations were preferential cathode and anode and second observation was cathode also corrodes but very very small portion rate and anode corrodes at a high rate fine and interestingly one more observation we can make out of it if we see that zinc and iron situation the cathodic reaction is nothing but h two and anodic reaction was plus plus plus of course it corrosion rate is very very small its corrosion rate is very very high but it does not stop now interestingly if we consider a situation like what we had in case of daniel cell that we have one end copper is dipped in copper sulphate and copper ion activity was maintained at one and another end we had zinc and zinc activity and it was taped in zinc sulphate and activity of zinc iron was maintained at one and we develop some potential and that potential we measured by standard hydrogen electrode and here also we also maintain standard hydrogen electrode and then we measure the potential now interestingly this potential if we maintain this condition it is coming out to be 0.34 volt and here it will be minus 0.76 volt now this potential how it is coming from last equation considering this reaction and every time we are considering reduction potential thats what i have written in terms of reduction right where the reaction was interestingly this would become if we connect them and remember there should be a shall bridge to maintain electrical connectivity and here we connect this with a conductor with a and then current will flow from this copper into linked and this is positive or cathode and here it is negative for anode now if you see cathodic and anodic reactions the same ion is depositing and same metal is forming the in case of zinc surface its zinc which is going to the anodic situation when the it is connected so that means preferentially anodic reaction would happen here and preferentially cathodic reaction would happen here and interestingly here since we are not having any other species other than copper plus plus only this cathodic reaction can take place and only this anodic reaction can take place this is corrosion and this is deposition since copper has a higher reduction potential thats what it actually reduction process is preferred here now this case and this case is different so this is also galvanic corrosion galvanic corrosion of zinc but here we have ions of both the metals in the solution but in this case iron of iron is not present iron azim ions of zinc is also not present in the solution initially to start with it was only h plus but still we ended up getting this reactions so that is another important aspects because these things are coming that the because this value is is this and this is nothing but which is standard reduction potential of zinc but in practical cases we experience the situation what we had in case of iron zinc ions of individual metals may not be present in the actual actual situation or practical situation so that time we cannot judge that which type of reaction would happen from the reduction potential for example here we cannot judge from the iron reduction potential which is minus 0.44 the standard reduction potential for iron and zinc it was minus 0.76 volt standard reduction potential because those ions are not present in the solution so in this case its very busy how to decide anodes and cathodes for example if we have two metals m one and another case m two and they are in their respective ions in in connection with respective ions of that electrolyte where we have that respective ions this is m one n plus and it is m two n plus both ions are present in those electrolyte so i can decide which will act as cathode and which will act as a node if the such situation arises simply by looking at the nernst equation which is generalized form this is a generalized form we can use and we know that there is a series exist for example here if this value is greater than so then m one would be anode m two would be cathode ok and the reaction should be in this case and in this case m two so these will be two reactions that way we can judge so in so pure cases so it can possible it can be possible when it is a pure case pure species and here we can judge by looking at standard reduction potential series and that series for example if i try to point out here you will get like this a u a u three plus e zero value would be one point four nine eight volt and this is all with respect to standard hydrogen electrode this is gold two plus is basically like that way you can go up to magnesium so like that way we can have a series which is based on finding this value by putting the same ion of that particular metal maintaining that ion concentration to be activity to be one so for example here if this concentration is one this activity is one then of course potential of that particular electrode would be same as reduction potential standard reduction potential because this long part would be zero if the activity of metal ion concentration is one so like that way we can find out these values and here if i try to see this if we connect it to this two when their ions are present then this would be cathode and this would be anode similarly if i connect magnesium with nickel this should be cathode magnesium would be anode similarly if i connect nickel with copper nickel would be anode copper would be cathode fine its very easy to find out but its impossible to get in practical situations so that time we have to generate another series that is called galvanic series galvanic series and in that galvanic series we have to just find out that which one in a two metal contact those two metals could be alloy those could be metal two could be pure metal those could be pure metal and alloy all those combinations are possible even it could be same metal but the reactions would have different activity ok i will come to that so galvanic series how and it starts with this particular discussion that cathodic reaction and analog reaction if you see cathodic reaction is always this even if iron remains cathode why we say this because catholic reaction wherever catholic reaction happens it is basically cathode but that does not mean that that particular metal will have a cathodic reaction here it is not happening rather that cathode is actually actually going through anodic reactions so that is what we need to have a kind of comparative series which will not get bound by the values what we have in standard reduction potential series and these series could be different for different electrolyte let us see how you find that series ok so let us say we have a metal one m one and m two and another metal m three lets say all are m one this m one and metal m one metal and m two metal they are dipped in let us say nacl solution or seawater solution this is a m one this is m two and let us say this is aerated aerated solution and if we connect them this is a solid bridge s b s b is nothing but the salt bridge now we have to just check which way current moves if current moves from this end to this end so the current flows from this end to this end and we know that in galvanic connections current always moves from positive end to negative end so this will be cathode and this would be anode so this is anode and this would be cathode since this is a galvanic mode of situation and here the corrosion is taking place so that case m two would corrode and m one would act as this is anode m one would be the surface for cathode or cathodic reaction cathode and interestingly when we have this initial there we did not have m n plus here also we did not have m two n plus so that way we can see that by looking at the current flow we can check which one is active which one is less active so the less active metal in this series in nacl solution whatever three point five percent of sea water in this series m one would move on top of m two and here we are not putting any values you can also find out the values here by joining standard hydrogen electrode you will get some potential but we are not mentioning that we are just comparative we are making a comparative statements that we are actually which one is acting as a positive end which one is acting as a negative end so accordingly we decide which one is cathode or which one is anode similarly if we have a joining like if a condition like the same condition instead of m one if we put m one and m two lets say when we join this like this same situation what we have here if the current moves from m one to m two in the external circuit then definitely m two is cathode and m one is anode so that means we can see that if we compare m three if we compare m one and m three definitely m three is less active and m one is more active so more active one dissolves and less active metal will act as cathode where cathodic reaction happens and here so that means m three would stay on top of m one so now we have a series which is m three m one and m two so if we join then m through n m two definitely this would be positive this will be negative now interestingly what are the cathodic and anodic reactions since i have said that this is aerated so we have dissolve oxygen and if we consider nacl solution its a neutral solution so the cathodic reaction would be an anodic reaction m one minus n e equal to m one n plus sorry it should be m two ok even if it is m one no problem in that case this is joined m one and m three because m three is acting like cathode and another canonic reaction would be m two minus two e equal to any so let us let us make a generalized statement m n number of electrons are associated so it would be m two n plus that time m one and m two so in this case this is positive this is negative in this case this is positive this is negative and that's what it is followed in that galvanic series so now in the galvanic series this m one m two and m three could be metals could be alloys and could be some reactions also ok fine now but if we consider only metals so we can have a series like this in let us say ah sea water and interestingly before i get to that particular thing so that means you could see that if you change this nacl solution you can change it to some other solution you might find that m three might become active compared to m one so its not sacrosan that in one particular electrolyte one metal or alloy is less active or noble does not mean that that will remain noble in other solution ok so in order to understand the industrial applicability of galvanic series we have to see a complete galvanic series and see how we select materials in order to prevent corrosion related problems and whenever we select materials we try to select materials which are close by in the galvanic series in that particular environment ok so in order to understand that we will take this galvanic series discussion because it involves lot of engineering applicability in the next class because this particular class we have i think hardly left with 10 minutes so 10 minutes will not be sufficient to understand this galvanic series situation but at least i could tell you that what is the implications of galvanic series and standard reduction potential series because in case of standard reduction potential series we always deal with same metal same species and unit activity is maintained but in case of galvanic series its not needed so for example if we consider steel and stainless steel fine so let us say you have a stainless steel ss and three zero four standard in the standard form of eighteen eight white austenitic stainless steel austenitic stainless steel and in this case if we have another steel which is mild steel and you lets say you have made an electrical contact so you have one is mild steel another one is stainless steel and if this couple if exposed to normal portable water you would see that the ss would become cathode hardly any corrosion will be noticed there and but this will be highly corroded ok why this situation happens because if we compare the galvanic series in portable water s s would always stay on top of mild steel and their difference in potential if even if you measure the potential in the in the portable water this difference in potential is large and in fact when you connect them the large current will flow from ss to the mild steel from the through the conductor or to the external circuit and here the conductor is nothing but this and this is the electrolyte so here ions move and here electron moves ok so s s would become cathode and m s image becomes anode but here interestingly they are not pure metals these are they are highly the ss is highly alloyed made by metal and ms is less alloyed metal fine so this is very important because this tells me that we cannot have such kind of situation where highly noble and highly active metals cannot be joined for an engineering applications ok so we have to be very careful that part that we have to select materials so that their galvanic seal in the galvanic shear series they are close by but in this case they are wide apart so wide apart means it will be highly cathode and highly anode ok and if it is they are close by the galvanic effect will be hardly noticed ok so we will talk this part in our next lecture so till then let us stop here we will continue our discussion on galvanic series galvanic series as well as galvanic corrosion because this i this galvanic corrosion i consider to be one of the fundamental corrosion forms thank you [Music] you