noc18-mm14 Lecture 29-Exchange Current Density

[Music] [Music] okay let's continue our discussion on kinetics of corrosion and today we'll have lecture 29 and we'll continue our discussion on kinetics corrosion now if I recall our last lecture we started this problem and then there we address the first part we had to calculate the corrosion rate in the form of MD D by using the corrosion current density what is provided to us and then we found out and we have actually did little bit of conversion of units and then we got the value which is 13 point 0 1 M DT now we have to find out the corrosion in the form of MP y there are two ways one is you once you know MD D you can convert that MD D into milli inch per year or you can start with the basic equation which is corrosion rate is equal to I a M here M is nothing but the SN and F into Rho s n and Rho is the density am a SN is basically the atomic weight of tin and then we can write it as 2 point 4 5 into 10 to the power minus 6 it's basically ampere per centimeter square now I will write it directly in terms of Coulomb per second per centimeter square so this is Coulomb per second per centimeter square weight is atomic weight is one one eight point seven one gram and n is equal to 2 F is equal to nine 6500 Coulomb into Row the row of SN is equal to seven point three one gram per cc or centimeter cube so seven point three one gram per centimeter cube now I could see that this one will get we can can cancel out Coulomb Coulomb can be cancelled out and this gram gram also would get cancelled out so you are left with two point four five into ten to the power minus 6 divided by into one one eight point seven 1/2 into nine six five zero zero into seven point three one gram per second now we have to convert this gram per second into milli inch per year so one second equal to 365 a year consists of 365 days each day consists of 24 hours and then 60 minutes and each minute 60 seconds so this much here now one centimeter we can convert one inch equal to 2.5 4 centimeter so one centimeter equal to 1/2 point 5 4 I can make it milli inch by putting here thousand because one inch is equal 2000 million ch so now I will put these values here so 2 point 4 5 into 10 to the power minus 6 into 1 1 8 point 7 1 in 2 divided by 2 into 9 6 5 0 0 to seven point three one in 2000 sorry we made a mistake head should be centimeter because only this part is left out so thousand by two point five four so that is one centimeter equal to thousand by two point five for milli inch and then one second is equal to this much here so par second so that means this will be multiplied here so this much inch per year so this becomes million per year or so we could see that we can express the corrosion rate by using the corrosion current density and then we can convert it into either MTD that means gram per area part weight loss per area per unit time and also we can convert it into MP Y which is milli inch per unit barri per year so we can convert it into uniform as well as localized depending on the surface nature of that particular metal in that particular corrosion corrosive environment so now let's get into little deeper into the current density part now we have already started mentioning this particular term called i0 which is nothing but exchange current density and this has a relation with I know if I try to look at this particular reaction and if it reaches equilibrium and before it reaches equilibrium so then this is anodic this is cathodic so this direction we have cathodic reaction and this reaction sorry we have just reverses this would be cathodic and this side would be anodic now this is reduction the forward reaction and the backward reaction is the oxidation reaction or corrosion reaction now if I see the cathodic part if I see the cathodic part the rate at which electrons are consumed if we express that we can convert it in the term of IC or the current per unit time for any time or equal to charge it's not current it's basically charge per unit time is equal to current and this current is involving cathodic reaction so that's what we call it as cathodic current now if we divide this particular particular reactions let's say this is the metal surface on top of which the cathodic reaction is taking place so we can divide this area and then we will get or a a is nothing but area divided by area a so I get cathodic current density similarly on the analytic side we can get anodic current or charge in the form of electrons generated divided by time and then if I divide it by area of the anodic surface we get anodic current density i we will try to term it as IC and this one would be IA now this is IA this is IC now we will talk in terms of current density only because we have seen the rate at which this deposition or dissolution that will be directly proportional to the their mass loss per unit area per unit time or mass gain per unit time must deposited per unit time per unit area that we have already noticed not in the form of current because current will not give us the actual information because it doesn't involve the area into k it doesn't doesn't take the area into consideration now so this reactions m n + + NE m this reaction is taking place simultaneously and then they will reach equilibrium so i see i now if I consider this particular direction the current density to BIA so the reverse direction which is cathodic reaction rate is why we shall mention it in the form of minus IC now this minus sign indicates the direction this negative sign indicates direction only of current flow it doesn't tell that that current density is actually negative magnitude is positive but negative means it's flowing opposite to the an OD current so whichever you take to be positive the other one should be negative but our what we'll take it ia we'll take it as a positive current and I say we'll take it as a negative current so that means it's flowing opposite to the an OD current now when they reach and also we see that this is proportional to the deposition rate per unit area per unit time and this is proportional to dissolution rate per unit area per unit time so the rate of corrosion rate since they are taking place at a particular temperature and pressure and they will reach equilibrium because if we consider time and the rate and if we tell the srate in the form of AI then initially if we consider if we say that our particular solution has a m-- n plus iron so once I dip this particular metal into that solution at a particular temperature pressure see this metal will start depositing and at the same time opposite process would also start so now if I consider that this is the initial concentration of initial rate because since metal and concentration is very high in the beginning when we dip that particular metal into that solution containing metal plus plus n plus iron so there are lot of supply of metal in class so the rate at which the deposition would happen would be very high rate so and then gradually this one would deplete gradually but at the same time there would be a dissolution also the metal one will form from metal surface and these two opposing process will try to equilibrant or they will try to reach to a particular rate where the rate of deposition and the rate of dissolution would be equal so initially this is the rate which is let's say I see and I am putting a minus sign because I am considering the direction of flow of that particular current and it's a deposition now the opposite current would also start immediately after dessert after putting that particular metal into that solution so this is IA and then after reaching the same value they will maintain that value if we don't disturb that particular solution so if you don't change the temperature pressure that metal surface quality is same solution is same so it will maintain that particular rate so this indicates this particular value indicates i0 which is the exchange current density now interestingly and whenever we are reaching to this equilibrium in that particular situation do we get the dissolution or deposition to be different no they are same and in fact that situation is called non corroding system so that means the metal is not dissolving preferentially or the deposition is not going on preferentially so both are happening same rate so whatever metal is dissolving that particular that amount is again going back and depositing on the metal surface so that is called non corroding system and that time we see the rate of deposition or IC with a minus sign because I am considering it to be the opposite to the anodic current flow is equal to rate of dissolution which is ia and that particular situation this is called a zero or exchange exchange current density so this exchange current density is the unique situation at a particular temperature or pressure on a metal surface for that particular reduction or the half-cell reaction so remember this is for half-cell reaction because whenever we are considering this rate of deposition and rate of dissolution we are not taking part taking into consideration of other species dissolving or depositing or other reaction which is either reduction or oxidation so it's the same metal ion and metal they do these two species one is this depositing one is dissolving so one is reduction another one is oxidation this is basically with reference to the hop cell reaction if we consider that in a particular cell the two situations are taking place one is differentially reduction one is preferentially oxidation then we say that the one part becomes hop cell because this these are because the full cell consists of two half-cells that's what we have understood so this is related to the one single electrode system or the single reduction or oxidation process now before we go deep into this discussion I would like to just state the importance of this rather current density if we recall we saw that in case of galvanic corrosion if we have a copper this is copper plate this is copper plate and if we have a kind of iron rivet which is connecting these two copper plates and if we put it in NaCl solution and according to the galvanic corrosion this one will dissolve quickly and then finally the rivet would lose its strength joint strength so this is the example what we have put in while discussing the galvanic corrosion and that to wait that time we consider the area effect so we analyze this particular area effect saying that if we have a higher cathodic area in connection to a smaller anodic area then the rate at which the anodic area would dissolve or the dissolution would be faster in case of that situation rather than a situation where anodic area is larger than the cathodic area and we said that the number of cathodic reactions that are taking place on cathode area will be so high in order to meet that much of electrons supply we need to have a more and more metal line that should form from the anodic region and the dissolution becomes faster now can we prove it through I which is current density small line we can do that so this is a circuit let's say this is a circuit so you have a copper and you have an iron so now you have this is the closed circuit let's say this is copper so here the current flow is taking place this is positive this is negative because this is a note in case of Calvin Excel this will form definitely a galvanic cell this is cathode and this is through the metal this current through the metal and this is through the solution that way the total circuit is complete now if we see the area the total area of copper is so large so assume let's assume that hundred oxidation cathodic reactions are taking place some number arbitrary number were taking cathodic reactions and the cathodic reaction is nothing but Oh 2 + 2 h2o + 4 e equal to 4 o H - and in order to see this cathodic reactions you have to meet this number of cathodic reactions we need 100 into for number of electrons similarly on the analytic side we have we have to meet this total number of electrons so we have to create into 4 number of electrons so this should be created by anodic reaction and here it should consumed for this cathodic reactions and here the anodic reaction is Fe Fe plus plus plus 2 e now you see in order to meet this so now I see that here also the charge is 400 charge and here it is equal to 400 charge in the form of electron of course now in order to meet that we have to so this is let us say per unit time so that means the current this is C equal to ie which is analytic reactions and now in order to maintain that we just mentioned it as mod IC which is the I am taking the we am taking part of care of that negative sign now this is I see equal to I a now we have to take care of that area factor so the area of cathode and area of anode is very large now we know that current flow if we see this circuit this circuit the current is same entering into the metal or going out through the metal from the metal so this is same but when I try to understand the rate at which this cathodic and anodic reactions are taking place so I have to divide IC with AC and I ate with a a so these are area of anode and this is area of cathode now since AC which is the copper area in this particular situation in this particular situation you see the area that is exposed to the solution is the cathodic area copper behaves as a cathode in that situation that case area of cathode is very large now if we see this ratio then as per the ratio this should be like this because AC is much greater than I a so then I a should be greater than IC or I would say cathodic rate reaction rate is much lesser than the anodic reaction rate now if a C equal to a a then I would say that I a equal to IC or IC equal to ia and if AC is less than less than a a IC would be greater than greater than ie now interesting is in this situation we are saying that the anodic current density is very very high as compared to the situation what we have considered here so the dissolution rate of that anode would be so fast if we have a larger cathode area compared to the anodic area so the anode will dissolve in order to meet that many electrons requirement for the analytic cathodic reactions to happen on the cathode surface so this is one interesting fact is when we see that current density we are able to analyze or understand this particular phenomena more clearly or moons are better better better and scientifically it's more intriguing rather than considering from analytical point of view so that means this IC or the current density is extremely important consideration so let's stop here we'll continue our discussion on this very topic in our next lecture thank you [Music] [Music]