 Welcome to the last lecture of this course, Cathodic Protection Engineering. In this lecture, we shall cover anodic protection and then we shall provide an overview of the lectures delivered so far. If you look at it, anodic protection in fact, does not have much in common with the cathodic protection engineering which is the main objective of this course. If one wish to find any similarity, we can say that both are electrochemical techniques which are employed for preventing corrosion of metallic structures, there ends a similarity. In fact, typical cathodic protection engineers they may not find this lecture very relevant. Then the idea of including this lecture in this course lies not only this lecture being academic, but also considered to be important to plant personnel where anodic protection engineering is important in some cases. Now, let us look at today's lecture anodic protection and overview of this course. This lecture as I told you before has two parts, the part one involves understanding the principles of anodic protection, then we shall examine what are the applications these anodic protection can be applied in the field and what limitations one faces when anodic protection is applied to various engineering structures. Then we will conclude this lecture by giving an overview of the course we have seen so far. So, let us look at the first part the anodic protection of structures. Let us start with giving a perspective to anodic protection where it started with and how it become a technology. The anodic protection as is origin in the discovery of passivity of metals by Lomano in the year 1738. Later it was Michael Faraday who suggested that the film formation is the main reason for passivity of metals. However, the anodic protection as an engineering concept can be attributed to C. Edeleno who first published a paper in Nature in the year 1954. He has shown using a potential stat that the alloy 304 stainless steel held at different potentials in sulfuric acid undergoes different date of corrosion. What are shown here in the left side this diagram is the variation of the corrosion rate with the applied potentials. As you notice that in sulfuric acid the corrosion rate increases as you increase the applied potentials it reaches a maximum value and then the corrosion rate decreases it reminds almost steady over a potential range and beyond a critical potential the current increase the corrosion rate increases. So, the corrosion rate seems to be related to the applied potentials. In fact, this forms the basis for the anodic protection of engineering structures and it became a technology in the year 1970. If you see later this curve has a resemblance to anodic polarization curve wherein you plot potential versus current density that you will see in the next slide. So, let us look at the principle of the anodic protection. What is plotted here is a well known potential versus the log I diagram wherein you notice that as you increase a potential as you increase a potential potential. Now, the metal source increase in current reaches a critical value and then it reminds constant up to this point and then again the current increases. This is a schematic of a polarization diagram of a pass varying metal which shows active resolution here and then passive resolution here and a trans passive resolution over here. Now, if you immerse this metal in an acidic solution or any corrosive medium it attains a corrosion potentials defined by a cathodic kinetics and given by this particular line here you see this the cathodic kinetics it is the equilibrium potential for the cathodic reaction and as it polarizes towards a negative potentials the current increases. You notice that at this point the cathodic current density is equal to anodic current density and the metal exhibits a potential called as a car and correspondingly there is icard which is called the corrosion current densities. Now, it is possible to reduce the corrosion rate of this metal by rising this potential to somewhere here in this region. So, let us look at the difference between the anodic protection and the cathodic protection and how it these two things are appearing. Suppose if you are interested in cathodically protecting these structures you bring down the potential from icard to a relatively more negative value in which case one has to apply a current density is given by this value that is the eye protection that is the current required to protect cathodically is the difference between the cathodic current density and the anodic current density. Please notice that this difference in the current is quite significant because it is in the log scale right. Let us look at how the anodic protection is done. In the anodic protection you rise the potential of the metal from the corrosion potential to the more positive value to hold the metal in the passive region right. And the current required to anodically protect this metal is given by the difference in the current density between the the anodic current density here and the cathodic current density here. Please notice again that it is in the log scale the current required to protect the structure anodically is much smaller because the the current is the difference between the passive current density and the cathodic current density at that particular potential. So, there is a huge difference in terms of the current requirement for the anodic protection and the cathodic protection of the engineering structures. What is more important is that even though the current required to protect the structure anodically is small the it is necessary that the system should able to provide a current so that it can overcome this current which is called as critical current density. We will see this later. So, we looked at the principles of the anodic protection and the cathodic protections and it is necessary to understand how they are different in practice. Cathodic protection is a thermodynamic concept and so, it is applicable to all systems. You can simply bring down the potential towards an active value a corresponding decrease in the corrosion rate occurs. However, anodic protection is possible only when the metals on the corrosive medium they exhibit a passivity. So, it is restricted to certain systems where the metals can exhibit a reasonable passivity. As we discussed the metal is made cathode the metal is made anodic here. Again the cathodic protection can give rise to complete immunity from corrosion whereas, there is a residual corrosion occurs in those structures which are anodically protected. But in fact, in reality even in the cathodic protected structures the corrosion is not completely stopped and so, there seems to be not much of a problem in adopting the anodic protection of the structures if possible. As I have seen earlier the current requirement is high for the cathodic protection systems as opposed to just a few micro amperes per centimeter square required for a few tens of micro amperes per meter square required for the anodic protection of metal success. This in fact, is compared to a few milli amperes per centimeter square required for cathodically protecting the structures. The other problem that happens in the cathodic protection is when you are going to make the metal cathode there is a possibility of hydrogen embrittlement because hydrogen can enter into the metal because of cathodic polarization. This in another problem as far as the anodic protection is concerned because you are rising the potentials towards a positive direction the metal has higher tendency to corrode and only thing the resist corrosion is the passive film formation. If there are any species that can attack the passive film then there is a great chance of localized attack like pitting can takes place or a curved corrosion can takes place on these alloys or structures. Now, the other important difference is it is more tolerance towards potential control. I can have a varying potential ranges. The only thing is the extent of cathodic protection can change whereas, in the case of anodic protection there has to be stick control of potentials if you move towards the active directions the corrosion indeed will increase it will not get reduced. As you have seen that the anodic protection is restricted to certain systems I have shown you a list of them like sulfuric acid, phosphoric acid, nitric acid and nitrate solutions and organic acid and caustic solutions and corresponding metals are listed here ok. So, that means, the anodic protection is much limited as applicable to or in contrast to the cathodic protection of engineering structures. Let us look at the sulfuric acid wherein anodic protection is very much utilized. We have taken sulfuric acid because it is the largest chemical produced worldwide and in fact, it is in a economic indicator how much sulfuric acid is produced is an indirectly an indicator of the economy of a country. So, it is very relevant that we take sulfuric acid as an example for the anodic protection of the engineering structures. At the end the anodic protection of these structures in sulfuric acid plants are the following the storage vessels, process reactors, heat exchanges and transportation vessels for corrosal liquid. So, a limited applications wherein the anodic protection can be applied. Let us look at how it is important to have anodic protection with respect to sulfuric acid. This is a typical polarization curve ok. It is a schematic of polarization curve of a metal showing active passive transition and it illustrates how the alloying elements and the nature of sulfuric acid can affect the passivity and so how the the alloys select and how and so how we need to change the alloy in order to reduce. So, this will illustrate how depending upon the nature of sulfuric acid we need to change the alloy chemistry. Let us look at this. Let me now describe this polarization curve here. Again what is very important is the passivation potential and the critical current density. As you notice that unless the system crosses the critical current density the metal cannot achieve the passivity ok. So, if you have a cathodic reaction that can substantially increase the reaction kinetics it is possible that the metal can reach a corrosion potential that will lie in the passive region ok. And so it will exhibit a corrosion current density which is equivalent to the passive current density. If the concentration of this oxidizer increases then you notice that it is detrimental then the mixed potential will lie in the trans passive region and so the corrosion rate increases. So, in order to reduce the corrosion rate we need to look for alloys which shows an extended passivity. So, that is done by alloying elements using chromium for example, the alloy 310 shows very high resistance to corrosion over a range of concentration of sulfuric acid. If I look at the other side if the oxidizer concentration is very small the corrosion rate indeed is increasing right. In order to reduce the corrosion rate we need to suppress the critical current density and how it is done? It is done by adding alloying elements such as molybdenum and even chromium. But lower in the critical current density it is possible to reduce the corrosion rate. So, alloy selection is one of the means of reducing the corrosion of sulfuric acid in a typical plant. We do not really need to do anodic protection all along, but there is an advantage we will see that later. So, based on this concept various kinds of alloys are being considered starting from carbon steel with silicon, cast ions, stainless steels, cast stainless steels, higher austenitic stainless steels it means containing higher amount of chromium, nickel and molybdenum. One such alloy is alloy 20 CB type alloys and we also look at iron, chromium, nickel base alloys, nickel base alloys, zirconium and titanium tantalum. As you notice that these alloys as you move from the top to bottom that the ability to resist corrosion is increasing because they have extended passivity and also lower passive current density. As you seen earlier in the polarization curve that the corrosion potential can lie in the active region, the passive region and the trans passive region depending upon the conditions of sulfuric acid. And so, we define the sulfuric acid as reducing, mildly oxidizing and strongly oxidizing conditions. So, the nature of sulfuric acid indeed affects the corrosion rate of the metals. Some cases it is advantages, some cases it can lead to more corrosion. Some examples of the range of corrosion rates the sulfuric acid causes is given here is let us say you talk about sulfuric acid in the concentration range of 97 to 100 percent in the temperature range of 70 to 120 degree Celsius you can see that the corrosion rate of steel it can be greater than 2.4. There are cases where the steel can corrode even as much as 2000 millimeter per year ok. If the sulfuric acid goes to about 60 percent and 70 percent concentration and same is the case with the cast iron and the ductile ions. 304 L is relatively better than steel, but it also can exhibit a range of corrosion rates depending upon the concentrations as I told you earlier that the corrosion rate of 304 L stainless steel can be as much as 2500 millimeter per year in 60 percent sulfuric acid. And 3.16 is slightly better as compared to 304 L, but however it can reach the corrosion range which is not practicable to use 3.16 L for any of the sulfuric acid applications. Now when the acid concentration is such that it has become a reducing conditions that is when the concentration of the acid is lower it is mostly reducing conditions then the alloy is having high moly content is used because in reducing conditions you see that you need to reduce the critical current density and molybdenum reduces the critical current density. So, these alloys are applied in reducing conditions are low concentration sulfuric acid. So, the application of stainless steels for low acid concentration is very difficult because the acid becomes highly corrosive. The advantage of sulfuric acid is that the sulfuric acid is a good conductor of electricity and therefore, it has very high through power. If you install a cathode the current can reach at a distance as much as 4.5 to 6 meter length and so if you have a typical heat exchangers if you keep a cathode on both the sides of the heat exchangers you find that heat exchanger can be completely protected using anodic protection. So, because of high through power of current the anodic protection can be used for complex structures on the shell size of stainless steels. The stainless steels again as you said that depending upon the the temperatures and the concentrations the anodic protection is really applied. Generally the anodic protection is applied to 93 percent sulfuric acid and somewhere between 98 and 99 percent sulfuric acid in these temperature ranges because the corrosion rate can be reduced to 0.01 in the range of 0.01 to 0.1 millimeter per year as you seen they are really higher at this concentration of sulfuric acids. The beneficial effects of anodic protection as seen here the corrosion of steel structures in sulfuric acid leads to contamination of sulfuric acid to an extent of to 5 to 20 ppm per day ok. By installing anodic protection the concentration the quality of sulfuric acid can be significantly increased that means you are going to produce high purity acids or even during transportation of sulfuric acid the anodic protection helps to retain the purity of sulfuric acid. And it can be applied over a range of components say heat exchangers, shell and tube type, spiral and plate type and and so anodic protection is integral part of sulfuric acid manufacturing and transportation processes. As I told earlier you can see that the corrosion rate can be significantly reduced from 5 millimeter per year to 0.025 millimeter per year for sulfuric acid in the range of 96 to 98 percent up to a temperature of 110 degree Celsius and at a very low cost. If there is no anodic protection then you may have to apply stainless steels or nickel based alloys which are very highly expensive. And it is indeed being applied to large storage tanks of like 20000 tons using the anodic protection and because of anodic protection the maintenance cost comes down and there is no need to apply any coatings. However for a smaller tanks anodic protection is not viable people go for phenolic coatings which resist the corrosion of sulfuric acid. There is some information that you might be needing in order to understand the anodic protection of engineering structures you need various types of reference electrodes. This has been listed here there is no need for me to go in details there are a range of electrodes. Generally if you are going to use let us say sulfuric acid try to avoid contamination with the chlorides they try to use mercurous sulphate, mercuri mercurous sulphate electrodes can be much useful and so depending upon the nature of the chemicals you have a range of reference electrodes which are applied in practice. There are different types of cathode available in fact as opposed to the cathode protection where we use anodes which disintegrate over a time period the cathodes by nature because we are applying a cathodic current they are very stable. The range of cathodes starting from platinum down to copper are being used for various applications. Typical anodic protection system that consists of a potentiostat as you seen here which is connected to a cathode and a reference electrode and and of course it is an internal protection of the tanks. To summarize the anodic protection what you have seen so far it is obvious that only passive systems and the passive system depends upon metallic and the environment are amenable to anodic protection that too only when the passive condensate is significantly lower than the corrosion current density of the metallic structures. If it is amenable for anodic protection you need significantly lower current as compared to what you apply in the case of cathodic protection engineering and also you avoid expensive alloys and coatings in the case of anodic protections. You will see later that cathodic protection by default requires coatings as opposed to cathodic protection anodic protection does not require a coatings. The another problem with the anodic protection is that anything in the system that damages passivity can be detrimental to protection of metallic systems. For example, if there are chloride contamination you see that the passivity deteriorates the pitting potential drops and rise in potential towards anodic direction can be very detrimental it can lead to pitting it can lead to crevice corrosion. The other important thing about anodic protection is it requires a potential stat that should hold the structure at a given potential intact. If there is a drift in potential towards inactive direction the metal will start corroding at a higher rate. So, potential stat is an important part of anodic protection. Before I end this course let me give you a brief overview of what we have seen so far. We all know that corrosion of engineering metals are spontaneous. No external force or energy is required to cause corrosion of engineering metals whether exposed to chemical environment. The other important point that we should keep in mind is aqueous corrosion follows electrochemical kinetics. Broadly it follows Butler-Walmer relationship in comprehensively over a range of potentials. It follows taffodil relationship in a narrow range of potentials. In fact, the range of potentials we talk about in the cathodic protection the aqueous corrosion follows the taffodil relationship. Buried pipelines and storage tanks suffer different types of corrosion on both their sides. When they are exposed to soil it undergoes different types of corrosion like differential aeration corrosion, microbial corrosion, even stress corrosion cracking and the like. And within the tank the corrosion is depends upon the nature of the process fluid that is being stored. So, as a case in the case of pipeline the nature of the process fluid or the fluid that is being transported decide the corrosion of these structures internally. The soil when it comes to soil as you noticed before it causes different forms of corrosion on the buried structures. Because the the the structures are so large like the pipelines or the storage tanks and because they suffer different types of corrosion cathodic protection is one of the most predominant means of controlling corrosion. As opposed to the various methods available to control corrosion in a typical plant structures. The important thing that one should understand and one should remember when you talk about cathodic protection is the cathodic protection criteria. The instant of potential which we called as minus 0.85 volt with respect to copper saturated copper sulphate electrode. And the 100 millivolt criteria which talks about the true polarized potentials. The instant of potential discounts any possible IR drop between the reference electrode and the structure whose potential is being measured with respect to the soil. So, these two criteria are very important. The first criteria that is is instant of potential is reasonably easier as compared to the measuring the true polarized potentials. When it comes to cathodic protection the surveys are very very relevant and important. They involve soil resistivity, structures structure to soil potential, line current, drain current, DCVG that is differential current voltage gradient and ACVG that is alternating current voltage gradient to monitor the effectiveness of the corrosion. The DCVG and ACVG are mainly employed to look at the coating damages in the buried structures without being excavating the the soil. One of the most important thing in the cathodic protection is the anode ground bed. The anode as you know is something like a heart for the cathodic protection. It pumps the current into the soil and the current then travels through the soil and then enters the pipeline. The current so entered in the pipeline brings down the potential to the required levels. Establishing the anode ground bed involves understanding or calculating the anode resistivity with various governing equations we have seen actually. And in the case of sacrificial anode we have two more parameters the anode potentials and the anode capacity. The anode potential indicates the ability of the anode to drive the required current onto the structures and the capacity of anode talks about the longevity of the anodes to protect the structures. Let us look at the governing equations. The two kind of equations I would like to recall one that is related to the current flowing within the soil and the current flowing in the metallic structures. And these two are governed by simple ohm's law. But the relation between the piped soil potential and the current are of different kind. And this relationship is related to either Butlermer or the Taffel relationship which is logarithmic in natures. What does it mean? Even a small variation in the potential can cause an exponential shift in the in the current current density and vice versa. So, that means even a small shift in the potential that applied on the metal surface can either increase the cathodic protection or if you are not able to shift the potential towards cathodically it might undermine the cathodic protection of the structures. The other important aspect of stray current and AC indifferent corrosion is the other important aspect of cathodic protection is the stray current and the AC interference. These two factors affect the corrosion of the structures very significantly. In fact, for stray current it can cause as much as few centimeters loss in thickness barrier. As far as AC current is concerned it does not significantly affect corrosion as much as a DC current does. However, beyond certain level of AC current a part of the AC current is converted to DC current by a phenomena called Faraday electrification and that can induce corrosion of the structures. How do you determine whether this structure is suffering stray current corrosion? The surveys that we talked about like current flowing in the pipeline, pipe to soil potentials can be used to determine if a structure is suffering from stray current corrosion. There are various methods to avoid to minimize the stray current corrosion that involves increasing the resistance for the current flow in the soil or you can relocate the anode bed, try to redistribute the current by having many number of anodes or in the worst case use metallic bonds that is avoid the electrolytic conduction mechanism which induces the corrosion of structures. We have also seen the role of coatings in the cathodic protection of the engineering structures mainly it lowers the current requirement for cathodic protection. It also causes lower attenuation which means making the cathodic protection very efficient over a long distance. When it comes to coating the following properties are very important the resistivity, adhesion, cathodic disbondment or the major properties that one should look at in selecting the coatings. Rectifier is the source of current for the ICCP system and the selection of rectifier depends upon the anode ground bed resistance and the current required to protect the structures. The purpose of rectifier is to provide current, regulate them so that the required pipe to soil or structured soil potential is maintained. There are two topics which are not so closely relevant to the cathodic protection to be covered one of that is the internal corrosion of oil and gas pipeline. But if you look at the internal corrosion that also affect the structural integrity of the pipeline or even the soil atoms. While the cathodic protection minimizes the corrosion of engineering structures that are buried in the in the electrolyte buried in the soil or immersed in the any electrolyte the internal corrosion is of different kind. If the cathodic protection does not affect the internal corrosion of any storage tanks or the pipelines it depends on the kind of chemicals that are being handled. In the case of crude we all know that crude and even the natural gas they are non-corrosive but the impurities like sulphates but the impurities like water, hydrogen sulphide, carbon dioxide, chlorides they are responsible for corrosion. Notably and those pipelines used in the oil and gas the metallurgy does does not significantly affect corrosion accepting the stress corrosion cracking. So, the only way of controlling internal corrosion is application of inhibitors, coatings or even lowering the impurities such as water, hydrogen sulphide, carbon dioxide, chloride and and so on. And of course, similar relationship can be formed in storage tanks depending upon the process fluid the corrosion rate can change. The last part of the lectures we covered anodic protection as I told you earlier the anodic protection has least resemblance to cathodic protection accepting the fact that the anodic protection and cathodic protection are the techniques which employ electrochemical principles to prevent the corrosion of structures. In the anodic protection as opposed to cathodic protection we use a relatively positive potentials so that the structure is held in the passive region. The other important aspect of anodic protection that one needs to consider is that only passive systems one can apply anodic protection when I say passive system it reverts to both the metallic as well as the environment. The combination of metal and the environment would only decide whether a given system will will pass away or not. One of the most important aspect of the anodic protection is that it requires very low current density as compared to those required for cathodic protection. One more difference if you want to say that between the anodic protection and the cathodic protection is cathodic protection always involves an additional coating of structures whereas, in anodic protections no coatings are really applied. So, I given you a overview of the course that we have seen so far and I do hope that you get a perspective of this course. It is not giving you the finer details about how to implement the cathodic protection, but this course is expected to give you a perspective the basic understanding of cathodic protection, the basic understanding of corrosion processes, the basic understanding of anodic protection, the internal corrosion of the pipelines and the gas pipelines. And I do hope that this helps you to understand better the cathodic protection of anodic structures and if you have any doubts and questions you are free to contact me and this email that given here and I wish you all the success in engaging yourself in cathodic protection engineering. Thank you for attending this lectures.