 Hi, today we shall discuss an important topic of cathodic protection engineering that is cathodic protection criteria. In this lecture, we will discuss various types of criteria that are available for cathodic protection, what are the limitations of these criteria. Let us start with the well known diagram, the Evans diagram and how it is related to cathodic protection. We have seen this in the last couple of lectures. Now the cathodic protection criteria will be discussed in relation to this diagram. What is shown here is when you have a structure buried in a electrolyte or maybe in the soil, it exhibits a potential which is called as the natural potentials or corrosion potentials. And corresponding to this particular potentials, the metal dissolves even by this current, the corrosion current density. Now in cathodic protection criteria, we bring down this potential from the natural potentials to as low as possible. Ideally, you should bring down to the equilibrium potential as shown in the diagram. Below the equilibrium potential, the metal is completely immune to corrosion. Any potential that you hold the metal, there is a corresponding current, corrosion current. If you want to completely prevent the metal from corrosion, you need to bring down the potential to a value which is equivalent to equilibrium potentials. But that also means we need to supply a large amount of current in order to enforce the cathodic protection of the structures. In addition to that, you also see there is a significant increase in the cathodic reaction occurring on the metal surface. This is quite damaging if it leads to hydrogen evolution on the metal surface. So, on the one hand, you want to minimize the corrosion rate by bringing down these potentials as close as possible to the equilibrium potentials. On the other hand, you need to supply large amount of current density and also there is a likelihood of the metal getting damaged because of hydrogen evolution. If the pipeline is coated, there will be damage to the coating. So, in this diagram, one thing is very clear that if you are going to bring down the potential from the open circuit potentials, the corrosion rate decreases. And as you bring down, the metal is getting we call polarized in relation to the corrosion potentials. The polarization concept here in cathodic protection engineering is used in reference to corrosion potential. It is used in reference to corrosion potentials. It is not used in reference to equilibrium potential as you normally do. So, as we polarize the metal towards more negative potentials, then the metal tends to become lower and lower in terms of corrosion rate. So, from the engineering point of view, we are not looking at totally immunizing the metal from corrosion. You want to bring down to a corrosion rate to an extent that you have a desired life for the structures. And so, there are different criteria which describes the adequacy of cathodic protection of the metals. We shall see one after another what are the criteria for cathodic protection. The first and foremost criteria that have been widely used the simple one is minus 0.85 volt with respect to copper, copper sulphate electrode which is the pipe to soil potential for the metal. If you can hold the metal at this particular potential, then it is adequately said to be protected from corrosion. There is yet another criteria which is based on the polarized potential of the metal, which is a true polarized potential of the metal which is again is 0.85 volt with respect to copper, copper sulphate electrode measured. This is a another very simple criteria which is 0.3 volt shipped from the corrosion potentials or the natural potential of the metal. So, this shift is made towards the negative direction. The fourth criteria is about shifting the potential to 100 milli volt and that 100 milli volt is determined by interacting the current, the cathodic protection current and measuring the voltage decay. There is another simple method criteria, a net protective current from the electrolyte to the pipe. It is also called as a drain current measurement. It is another criteria for ensuring that the metal is protected adequately. There is another last criteria I would say is based on potential versus log i curve that you determine for a given structure. Now, we shall look at in detail what are the merits and demerits of these 6 criteria. Now, let us look at various merits and demerits of each of this criteria. Let us start with the first criteria which is minus 0.85 volt. There is pipe to soil potential measured with respect to copper, copper sulphate electrode. This criteria is a simple method, is very versatile and you can measure the potential any time. In fact, it can be measured as online. However, the main problem arises by the fact that the measured potential may not be actually corresponding to the polarized potential of the pipeline. Now, what I shown in this left side diagram is a cathodic protection system for a pipeline using sacrificial anode. The potential of the pipeline is measured using copper, saturated copper sulphate electrode. As you notice that the electrode is normally not kept just over the pipeline, it is normally kept little away from the pipeline. In such a case, we are not measuring the true potential of the pipeline. Instead, we measure the potential of the soil at the location where the reference electrode is kept. What is shown below in this diagram is the potential variation between the sacrificial anode and the pipeline which is cathodically protected. You can see that the potential drop happens within the soil and this potential drop happens within the soil because of the resistance offered by the soil. So, if you are going to keep the reference electrode somewhere here, but in these two points there is a voltage drop and this drop is related to the resistance of the soil and the extent of current that is passed in the cathodic protection of the pipeline. So, this requires to be discounted in order to measure the true potential of the pipeline. So, that is not possible in this criteria. It is possible to minimize this IR drop by keeping the reference electrode as close as possible to the pipeline. It is also you can see that as I stated before when the soil resistivity is more, the issue becomes very high and we have stray currents again it interferes with the measurements of the true potential of the pipelines. If the pipeline is well coated you would notice that because of the resistance offered by the coating, the IR drop lies very steeply between the coating on the metal surface compared to that drop the soil resistance is significantly less. So, the role of IR drop becomes less in a well coated pipelines. So, in order to overcome this particular problem we go go to the next technique. Before you go to next technique I would like to discuss with you the other important problem that one faces if one follows the 0.85 volt criteria. The plot shown on the left side is a schematic of the potential measured over a pipeline over a some distance. You will notice that for a long distance the native potential of the pipeline is well below the criteria which is 0.85 volt criteria. However, at some locations you do notice the natural potential or the native potential of the pipeline is above the 0.85 volt criteria. Now, this gives once a feeling that the pipeline over here is cathodically protected even though no external current is applied over this location. That means, simply measuring the pipeline potentials with the reference I told obtaining a value lower than 0.85 volt does not guarantee that the pipeline is protected due to cathodic protection. The other problems associated with the criteria is if the pipelines are poorly coated or the coatings deteriorated over a time period and you want to maintain the pipeline at a potential which is below 0.85 volt which is the criteria then the pipeline may need lot of current to cathodically protect. So, in such cases this criteria here becomes very difficult to apply. We will see later what other criteria can be applied if the coating is severely damaged in pipelines. Of course, in the well coated pipelines or the newly laid pipelines where the coatings are very good this criteria works very well. In order to overcome the IR drop offered by the soil between the pipeline or the structure on the reference electrode. The another criteria has been devised and this criteria is called as instant of criteria. The principle of this criteria is as follows. If a current is passed across a resistor and if you turn off the current the phase lag between the current and the potential is 0. That means, if you turn off the current instantaneously the potential drops that corresponding to the resistance offered by the circuit. So, what is done in this case is you know and a pipeline where the structures are cathodically protected and in which case we call them as on potentials. The potential measured with respect to a reference electrode when the cathodic protection is on is called as on potentials then you turn off the rectifier and then you follow the voltage change. Instantly you will see the potential drops and in the off conditions you hold it for some time and then again you turn it on. So, this can be cycled a couple of times in order to ensure that the potential so measured is consistent with that. The instant of potential is considered as the true potential of the pipeline because we eliminate the potential drop due to the soil. So, the polarized potential or instant of potential is equivalent to the native potential of the pipeline plus the cathodically polarized potentials that occurred due to passing the current. Now, how do you really measure it we have seen now you have on off cycles and the ratio between the on time and the off time can be between the off time and the on time is one-fifth to one-third ratios. Typically the on time is 8 seconds and the off time is 2 seconds. Notably there can be some spike seen some cases because of inductive effect that can be ignored. Typically the potential is measured after about 0.2 to 0.5 seconds after a turn off the rectifiers. This is for superior technique compared to the 0.85 volt criteria. The difficulty lies here is synchronizing all the systems are very difficult. For examples, if there are nearby areas there are pipelines we need to turn it off and we need to turn off all the cathodic protection stations so that the pipeline does not receive any current. So, that is one of the major disadvantage of this particular technique. The graph shown here the schematic graph shown here illustrates how important is to use the off potential measurements over the on potential measurements. What do you notice in the top line here the top curve corresponds to the potential measured with respect to a reference electrode over a distance. You notice that across the distance overall pipeline the potentials are well above the criteria which is minus 0.85 volt criteria. But you compare that with the off potentials. There are few locations you notice that the pipeline is not adequately cathodically protected. In this case is barely protected that means, the higher voltage that you notice in the on potential is essentially due to IR drop offered by the soil. So, this means that on potentials sometimes can be problematic. It may not exactly give the true potential of the pipeline. Well there are some other disadvantages apart from synchronizing the various you know stations. The access to structures sometimes the seasonal fluctuations in the potential between the testing times. Spatial fluctuations in potential between test stations. The presence of multiple pipelines as I told you I mean you need to disengage them during cathodic protection. And sometimes you may be having problems with the disbonded coatings they shield the current. So, the pipeline does not receive the current and so, there can be problems. There is yet another criteria which is 100 millivolt criteria here. And this criteria in fact, becomes very important when the pipelines are poorly coated or when you talk about bare pipelines. As we discuss when you are going to talk about the bare pipelines if you use 0.85 volt criteria you need to apply much more current. And that becomes unviable. In this criteria the following way you make the measurement. You turn off the rectifier and then follow the potentials that can be spiked in the potential you see here. And maybe because of capacitive reactants or inductance can happen. And then there is a drop in the potential taking place. And this drop in potential to this place the difference between this point on the on potential is due to IR drop. But you allow the pipeline not to receive current that means you keep the rectifier off for longer time. The pipeline starts depolarizing because the pipeline does not receive any current. The potential tends to become that of the corrosion potential or the natural potentials. So, depending upon the locations this decay may take about 4 to 8 4 to 48 hours it may take. And ultimately it reaches the potential which is the true potential of the pipeline or the native potential of the pipeline. The potential difference so measured after about 4 or 48 hours. And the instantaneous off as noted here is the true polarization the pipeline is experiencing. And if this value is greater than 100 millivolts the pipeline is considered to be adequately protected. When you say adequately protected we will consider them as required lifetime. If you want to make some calculations if you assume that the Tafel slope for the anodic reaction is 100 millivolt per decade you see that this change in 100 millivolt will lead to a reduction of corrosion current density by 10 times. And which means the life of the structure can be increased by 10 times. If the pipeline is supposed to last for 5 years without a cathodic protection and applying cathodic protection it can become 50 years. So, that is the engineering requirements. So, we are not completely stopping from corrosion but the cathodic protection is adequate for the desired life of the component. Some of the problems associated with this technique is you need to disengage all the sources of current as you do in the case of off potentials as well. And since you need to allow the pipeline to reach its natural potentials which is called as unpolarized potentials you need to it may take sometimes a week maybe longer that may not be practical in many situations. You might turn off your own structures but if there are surrounding structures which are cathodically protected it is hard to switch off the cathodic protection for all the structures. Moreover the cost of the survey is quite higher. This technique again is not applicable when you are going to talk about dissimilar metals. And in the areas where the structures are prone to intergranular stress corrosion cracking this 100 millivolt criteria is not applicable because the IGCC occurs because of the anodic dissolution we need to protect little bit more than what the 100 millivolt criteria can offer. There is a eternal criteria which is a 300 millivolt criteria and this criteria is is based on unpolarization. You move the natural potential by 300 millivolts towards the negative direction. So, you increase the current until the pipeline potential sips 300 millivolts more negative from its corrosion potentials or the natural potential. Now it is possible for one to calculate what will be the change in the corrosion current density if one applies 300 millivolt criteria. Now it is similar to what we done for 100 millivolt criterion right. We all know that the over voltage is related to slope beta beta the Tafel slope and the current between the two potentials when it was not applied when the cathode protection was applied. So, if you assume that the Tafel slope is about 100 millivolts and the over voltage that is the change in voltage from the natural potential to the polarized potential is considered as 0.3 volts then you can able to calculate and show that the current ratios between the cathode protected structure and freely corroding structures the current ratios is about 10 to power 3 times. That means, you can bring down the corrosion rate by 1000 times if you shift the potential by 300 millivolt due to cathode protection. However, there is a limitation the limitation is very much similar to the first criteria of cathode protection that is minus 0.85 volt over year 2 that is voltage drop is called as IR drop due to the soil that cannot be accounted in the 300 millivolt criteria. In fact, for that reason this criteria has been now taken off from the normal standards. But this criteria still being used for structures such as concrete structures because we there we do not know the true potential of the steel rebar. The other criteria is a net protective current from the electrolyte to the pipe. This criteria is is used mainly on locations where selectively the metals are getting corroded. The pipeline you know we have seen in earlier case when we discuss about the the external corrosion of the buried structures we said that there are certain locations there are anodes and some location there becomes a cathode the localized corrosion that do occur maybe like because of soil variations, microbial corrosion and such kind of factors. So, this criteria can be successfully applied to locally prevent the corrosion of the structures and the structures which are bare are ineffectually coated. They suffer extensively corrosion over a distance even in normal pipelines and in such cases is very difficult to apply the other criteria because it requires a lot of current in order to meet the minus 0.85 volt criteria and so one uses a net protective current from the electrolyte to the to the pipeline that is a criteria. So, if the current can enter the pipeline you assume that the pipeline corrosion is getting reduced. How do you ensure that the net current enters the pipeline? For that this procedure is applied one turns on the CP cathode protection systems places a reference electrode just over the structure and two reference electrodes on either side of the structures. If the electrode over the structure shows negative potential with respect to the other two reference electrodes then it is assumed that the current enters from the soil onto the pipeline. So, this is used in a limited areas so that you can achieve low-cries cathode protection of the structures. Limitations again we have you know that there is a current enters the pipeline, but we are not sure whether how much a current has entered whether there is adequate polarization that is causing the pipeline to avoid corrosion to the desirable extent. It is also not applicable where there is a stake current corrosion and where the soil has got high resistivity. And in such a kind of criteria one has to measure potentials at the close intervals as close as 0.5 meters in order to ensure that the narrow region which you are interested in protecting the structure a net current enters the pipeline. The last criteria that you like to discuss is based on potential versus log I plots. This actually is a polarization plot of the structures. What I have shown here is the line this corresponds to the natural potential of the structure or the natural potentials you know that rate of the anodic reaction equal to rate of cathodic reaction taking place. If one polarizes the structure towards more positive value positive potentials then what happens the reaction 1 increases the reaction rate to decreases. On the other hand if one polarizes the structure towards a negative potentials then the reaction 1 that is the metal resolution decreases and the cathodic reaction such as H plus reduction or auxin reduction increases. As you polarize more and more you increase the potential you see that the rate of the cathodic reaction is increasing. Now there is an inflection point at the point of inflection here and this inflection point is important point. What you see here is a tangent drawn across an aero zone wherein the potential and current or log current are linearly related to this and this is called the Tafel region. Above this inflection point the potential and the current follows the Tafel kinetics and when it follows the Tafel kinetics it means that one of the reaction is dominant. In this case the cathodic reaction is dominant the anodic reaction becomes very weaker that means the pipeline you have net cathodic reaction the cathodic reaction is much higher here and anodic reaction is virtually suppressed. So, Tafel region means the you know the relationship is hold valid only when in this case the cathodic reaction rate is much higher than the anodic reaction rates and so the inflection point indicates that at that particular potentials the pipeline is adequately protected. So, this is again a criteria which is you know it can be done. For this normally people install a temporary ground bed and apply the current and obtain this potential versus the current plot and by identifying this inflection point you identify the potential required to protect cathodically the pipelines. The limitations again used for limited areas such as river crossings, well crossing industrial areas. For large areas it is difficult because you cannot polarize the structure you put is too large. To summarize what you have seen so far we have seen from the Evans diagram we can in principle completely stop the metal from corrosion, but that is not practical. What is practical is to protect the structure so that it shows the desired life. For that we have various type of criteria and these criterion they have some merits and demerits you have seen, but in order to appreciate you need to understand the basics of the cathodic protection criteria in each case. If you look at all these criteria instant op potential criteria is very widely used and there is more reliable than minus 0.85 volt criteria. Though the 100 millivolt criteria is more accurate it gives you to what extent the pipeline is polarized. Thank you.