 Welcome to corrosion protection engineering lectures. We have so far discussed two important aspects that is related to aqueous corrosion and the electrochemical aspects of electrochemical corrosion. Today we shall be looking at the external corrosion of pipelines. Today's lecture we will focus on the types of corrosion that affect the external buried structures. We will be looking at the role of the soil on the corrosion, microbes, metal surfaces, external structures in the vicinity of these buried structures and the applied stresses, how they affect the external corrosion of pipelines. Pipeline corrosion is unique in one aspect because the pipeline phases two types of corrosion, one externally depending upon the environment be it a soil or in offshore seawater. Internally the kind of commodities are the products the pipeline really transfers and the nature of corrosion in both the cases externally and internally are different. Both of them affect the life of the pipelines. The protective measures are different. The cathodic protection as we discussed in the last class is concerned with the prevention of corrosion or the external surfaces. The internal surfaces corrosion will be dealt with separately. So, when you talk about pipelines or even the tanks for that matter, we had to be very specific about what surfaces are we dealing with. So, today we will be talking about the external corrosion of the pipelines and also applicable to other buried structures. If you look at the year 2002 data of the US Office of Pipeline Safety, it lists the following. It surveyed the number of accident occurred on the pipelines. In that year in the US they had about 81 cases of pipeline failures for 20 different reasons actually. I will list 4 of them here which are prominent in HHS in terms of the number of cases they are the largest. Internal corrosion on the pipeline stands the topmost with the 14 cases which accounts for 17.2 percent of the overall failures. The third point the excavation damage is about 30 numbers which again accounts for close to about 16 percent of the failures. The external corrosion you notice that it is little less than that it is about 7 percent of I mean about 8.6 percent of the failures which are about 7 in numbers. The natural calamities such as flood accounts for 4 numbers which are about 6.17 percent. But look at the cost of the damages due to the 4 reasons which are listed above. The internal corrosion cost significantly about 15.23 percent of the overall cost of 24.4 million US dollars. The external corrosion interestingly the number of cases were only 7 it considers only 8.64 percent. But in terms of the percentage of cost of damages it accounts close to about 17 percent. The third point the excavation though they are large in number in terms of 16 percent it constitutes only 4.36 percent. The flood of course the natural calamity you see about 17.84 percent. What is to be noted is the cost due to corrosion is much higher than the natural calamities and also the man inflicted damages such as excavation. That is the reason why you should be worry about the corrosion control of these pipelines. In pipelines APA grade pipeline steels are very normally used for oil and gas applications which are these pipelines are cathodically protected externally. I have listed here various grades starting from API 5L, X 52 to down to X 70 grade steels. They vary in terms of the yield strength these in fact this X 52 corresponds to the yield strength given in terms of KSI. So, these steels the strength increases from say API 5L X 52 to API 5L X 70 in terms of the strength it increases. You also notice there is a gradual reduction in the ductility is as expected when you increase the strength of the steel. But what is important to notice from the corrosion perspective is that the chemical composition of these different grades of steels are not very significantly different there only a minor differences are taking place. These strengths are achieved by different heat treatments. So, when the chemical composition of these steels are not changed the corrosion the external corrosion is not going to significantly change due to the variation of the strength of the steels. In addition to this API 5L 70 grade steel that I have listed here is also very somewhat recently developed steel which is API 5L X 80 whose yield strength is about 80 KSI the ultimate tensile strength is about 90 KSI. The elongation is reasonably good about 20 percent. But what distinguishes this API 5L X 80 steel from the remaining steels is that they are micro alloyed there are deliberate addition of some micro alloying elements in order to increase the strength of the steels. And also you see the reasonably good ductility you can see it is close to API 5L X 52 grade steels. So, regard a good strength and as well as good ductility. Again you notice that the elements which normally affect the corrosion of the steel such as chromium and nickel there is not significant amount in order to change the corrosion behavior or corrosion resistance of these steels. And therefore, there are different ways we should control corrosion all the pipeline grade steels we can say comfortably that they are not a resistance to corrosion of the soil. Now, if you look at the corrosion of these steels external corrosion of these steels the corrosion is you can recall back the discussion we had earlier there are I mean in oxidation reaction which is anodic reaction where iron is getting oxidizes iron 2 plus plus 2 electrons. There are cathodic reactions I listed two of them here one is simply reduction of water leading to hydrogen other case the oxygen present in the environment may be soil may be in the water also giving rise to cathodic reaction. The point we are trying to look at here is that the corrosion involves anodic reaction and cathodic reaction and these reactions occur in the surface in differing manner. In the case of uniform corrosion where the corrosion is very uniform across the metal the anodic reaction and cathodic reactions which are listed above they occur uniformly all through the surfaces are equally all through surfaces over a time period. Even though at a given time they are spatially separated but over a time period they even out in terms of the rate of reaction occurring on different locations. But in reality such a uniform corrosion may not occur in structures which are buried in the soil it is possible that the cathodic reaction and the anodic reaction are separated spatially leading to a form of corrosion called localized corrosion. So, we will be now discussing what are the factors that affect the localized corrosion. As we discussed in one of the earlier lectures the localized corrosion is much more insidious they cause unexpected damages to the structures than the uniform corrosion. So, we shall now understand what are the different forms of localized corrosion the buried structures are really facing. One of the reasons the buried structures cause localized corrosion is the differential aeration. We have shown here pictorially two different cases on the left side this differential aeration arises because of the fact the soil there are two types of soil a soil which is loosely held especially that is surrounding the pipeline and then at the bottom of it you have a soil which is very packed very highly dense. So, when you have a loose soil the permeation of oxygen in the soil becomes quite higher and so you have large amount of oxygen content in the loosely packed soil as compared to the densely packed soils. So, the area where the pipeline comes in contact with the packed soil the oxygen content becomes less. So, as a consequence this contact point becomes an anode because the oxygen content is less over here. The remaining areas of the pipeline you have significantly large amount of oxygen content that becomes a cathode. We will talk about the reason in the next slide. All I would like to point out here is there are reasons where the pipeline faces less amount oxygen content and there are reasons where the pipeline faces higher amount oxygen content this is causing us differential aeration corrosion. This arises mainly out of the soil properties loose and densely packed soil the pipeline encounters. There is another situation where the pipeline is surrounded by things like rock or maybe a concrete block for example, they come in contact with the pipeline. So, at that location the oxygen you know fugacity or the partial pressure of oxygen there is reduced significantly as compared to the surrounding areas. So, very similar to the previous case you have you know differential aeration or differential oxygen content that renders one place anodic the other place cathodic actually. So, this is one type of corrosion one type of localized corrosion we call them as differential aeration corrosion. Let me try to explain to you what causes the differential aeration corrosion from the electrochemistry point of view. What we have seen in this this is the kind of is a schematic of the anodic polarization curve of let us say a steel buried in a soil. What you see in this diagram is is the the metal passivates it shows active resolution and then a passive resolution and then a pitting or a trans passive resolution taking place. This is the anodic polarization behavior of the steel. Now, the corrosion potential you know is is arising out of the intersection of the cathodic reaction with the anodic reaction. In this case the cathodic reaction is the oxygen reducing along with the I think it is the water here. So, H2O is something missing here. There is H2O here and this H2O oxygen electrons released from the metal even arise to hydroxide. Look at these two cases when the oxygen content is significantly higher the cathodic reaction rate is significantly higher here. On the other hand when the oxygen content is is less as you notice within the curve wise the cathodic reaction rate becomes kinetics becomes slow and so, you will find that the corrosion potential in the case of the where the oxygen content is very low it becomes lower. It intersects the active resolution portion of the anodic curve and so, the metal dissolves at higher rate. When the oxygen content is higher you notice that the corrosion potential lies in the passive region. So, in metals where you have situation of high oxygen content and low oxygen content. The low oxygen content areas not only corrode at a higher rate it exhibits relatively anodic potentials and the areas where oxygen content is more it exhibits a cathodic potentials that is a sort of galvanic cell happening. So, this is the kind of localized corrosion occurring in the pipelines. There are situations where the pipelines surfaces would have some metallic impurities such as let us say copper or sometimes you have mill scales. The mill scales are the scales that are formed during heart rolling of the steel sheets. Normally the mill scales are removed the mill scales are not removed they remain on the metal surfaces. You can have a impurity the impurity is such as maybe a copper or sometimes I would say deliberately there may be like a copper cables to which are grounded for example, onto the metal surfaces. Now, these you know the impurities such as copper and noble metal or the mill scale they exhibit a potential which is relatively noble as compared to the compared to the unrested clean or a bare steels. The bare steel exhibits about 0.5 minus 0.5 to minus 0.8 volt with respect to copper saturated copper sulphur electrode. Whereas, the the the one with mill scale it shows about minus 0.2 volts. Similarly, if you are going to have a copper on the surface it has about minus 0.2 volt with respect to copper saturated copper sulphur electrode. Sometimes you may have stainless steel fittings this is all called bimetallic corrosion. So, in such a case the mill scale and the copper or the stainless steel fittings where even the rusted places what you have seen before they act as a cathode and the unrested or sometimes you you make a scratch maybe something like a dented places they become the anode. So, there is going to be localized corrosion occurring. So, the anodic reaction is now centered around this area a cathodic reaction is centered around all these noble areas. So, that causes the localized corrosion occurring on the metal surfaces. There is yet another form of corrosion which is called as microbiologically influenced corrosion. Microorganisms exist in the soil and these organisms they colonize the metallic surfaces and wherever they colonize the metals become anode and the remaining area becomes the cathode. In fact, the current from these colonized areas they enter into the soil and comes back to the pipeline and again returns back again to the anode here ok. So, localized anode formation is part of microbiologically influenced corrosion ruckus. Microbiologically influenced corrosion becomes very important because about 20 to 30 percent of failures are due to microbiologically induced corrosion. The presence and activities of the microorganisms such as bacteria and fungi they leads to corrosion of metallic structures. The steel in fact is very highly prone to microbiologically induced corrosion. Let us look at the role of microorganisms on the corrosion of metallic structures. The microorganisms do not directly damage the metallic structures. They are not involved in corroding the structures. The byproducts of the metabolic reaction that happens on the metal surface they interact with the metal and causing the corrosion. They also in fact introduce crevices petings under the deposits enhancing the corrosion of the metals. There are several types of microorganisms that cause the corrosion. I have listed here three predominant type of microbes which are responsible for corrosion. They are anaerobic type, aerobic type and the acid producing type. Among these three the anaerobic type bacteria are very commonly present in the underground pipelines. What does the a microbe really do? Say a anaerobic microbe what does it really do? In this case called as sulphate reducing bacteria it reduces the sulphate into sulphides and these sulphides further interact with the corrosion product such as iron Fe 2 plus and form iron sulphide and again it generates H plus ions in the process. These localized area consist of iron sulphides which can depasivate the metal. In fact, in this area even the hydrogen can enter into the metal much more. The other type of bacteria called the aerobic bacteria predominant among them is iron oxidizing bacteria. It oxidizes Fe 2 plus to Fe 3 plus enhances the reaction on the metal surface. Please look at this is an oxidizing bacteria this reducing bacteria. So, it requires oxidizing atmosphere it requires the other one requires reducing atmosphere. There are other bacteria such as nitrifying bacteria they produce acid and it can it can increase the corrosion of it. For example, it can convert the iron sulphide or hydrogen sulphides back into sulphuric acid and cause the corrosion of metallic structures. I would like to explain the role of microbes on the electrochemical corrosion behavior of the metals. We shall use the Evans diagram to explain how the microbes affect the electrochemical corrosion behavior of the metal. The electrochemical kinetics for the cathodic reaction in this case H plus combining with electron even rise to hydrogen and a metal getting oxidized to metal ions both the kinetics are given here and the and this point wherein the rate of oxidation is equal to rate of reduction corresponds to the corrosion potential or the natural potentials. And you also have the icorr which is the corrosion current density. Now, if you have microbes it can alter either the anodic kinetics or the cathodic kinetics. In this case I have represented here in a simple way the altering the anodic kinetics. Please look at this the slope which we normally called as Tafel slope is decreased significantly because of the microbe. As it is decreased the corrosion rate is increased from this point to this point and the corrosion potential has decreased from a higher value to a lower value. In fact, when we talk about cathodic protection we will see that we apply much higher negative potentials in order to prevent the microbial corrosion. The reason being that wherever you see a microbial active place is very likely you have higher corrosion rate forward by lower corrosion potential or natural potential. Now, we shall talk about the role of soil ok. What I have shown here is a pipeline that goes to two different types of terrains wherein the pipeline goes through a marshy land which is wet land here and it goes through a marshy land and because it is a wet and that becomes a anodic area where the corrosion becomes higher the remaining area becomes a cathode. You can also have soil of different chemistry you can have a clay soil you can have a sand soil you can have soils of different chemical compositions for example, chlorides one place sulfates in some other place you can also have a different pH of the soil. So, that means you can have differential soil concentrations that leads to one area becoming anodic the remaining area becoming cathodic. The current flows between two places as you see here the the the area that is marked over here is called as anode and current leaves this pipeline here goes through the electrolyte and goes to the cathode again the current goes back to the anode here ok. So, these are the different types of soil natures can cause corrosion. We can similarly what you have done in the case of microbial corrosion explain using the Evans diagram how the electrochemical behavior of the steel changes because of soil chemistries. I have I have just listed here the the electrochemical kinetics of the anodic reaction of the metal you know the soil 1 and the soil 2. You can see clearly that the Tafel flow changes as the Tafel flow changes the natural potentials open circuit potentials are e-card you call it the change as a consequence you see as a change in the corrosion content series. Because of two different potentials existing in these two locations and one place becomes anode the other place becomes the cathode. There is eternal form of corrosion which is very important in the pipeline the external corrosion of pipeline is called as stress corrosion cracking. I will just talk about the basis of stress corrosion cracking. The stress corrosion cracking occurs when you have a metal exposed to the environment and this metal is subjected to a tensile stress because of the design considerations. Now when you talk about stress corrosion cracking is very important to know what are the factors that affect stress corrosion cracking. I have listed here the the different factors with respect to material environment and the in the design. When it comes to material the chemistry the chemical composition of the material the the crystal structure of the material the microstructure of the material the all can affect the stress corrosion cracking. In the environment you have different type of chemical species present the pH or I would say the even the temperature of the pipeline then affect the stress corrosion cracking over the metals. In in design the stresses it could be a residual stresses happening because of the weldment or because of fit up stresses that is happening or the stresses because of the process conditions maybe hoop stresses can affect the overall stresses present on the metal structures. So, it is very important to know how these pipelines will behave with respect to stress corrosion cracking. Now why is stress corrosion cracking very important? It is important because almost all grades of steels that we listed before and their their beat welded ones or beat seamless stainless steels they are prone to stress corrosion cracking. If it is a welded one the heat affected zone with coarse grains are more prone to SCC than the base metal. The stress corrosion cracking in in relation to the pipelines there are two types of stress corrosion cracking. One is associated with the high pH and the cracking mode in this case is intergranular actually and it is associated mostly with bicarbonates and carbonates solutions that are surrounding the pipelines and you know that when you have bicarbonate and carbonates the pH of the solution is about 9 pH and the high pH stress corrosion cracking increases with the rise in temperature it is a temperature dependent process. There is again one more type of stress corrosion cracking which is near neutral or low pH stress corrosion cracking. This type of cracking is differentiated from the high pH stress corrosion cracking in terms of the mode of failure these transgranular cracking in edges and as stated here it occurs at the low pH about 6.5 it consists of you know the solution consists of carbon dioxide as opposed to the high pH stress corrosion cracking the low pH or near near neutral pH SCC is less temperature dependence. Now we have two types of SCC we have seen they are affecting all grades of these steels we have seen earlier. Now how significant they are are they really that severe? I shown here what are the parameters that the stress corrosion cracking can really affect. The stress corrosion cracking can occur below the yield strength it can occur below the fracture toughness. So, it becomes difficult for the engineer to design the structures stress corrosion cracking because of that the steel loses its ductility it becomes like a brittle. So, stress corrosion cracking is a real problem from the point of view structural integrity of the pipelines you should be worried about. What are the other characteristics of stress corrosion cracking? The stress corrosion cracking occurs in stages there are initiation occurs at different locations actually and small small nuclei or cracks formed they coalesce and then they grow and these large cracks further join together and then the crack growth it becomes very large and the ultimate failure occurs. Now if you look at the two types of stress corrosion cracking most IGACC their intra granular stress corrosion cracking failures occurring at high pH they are due to hoop stresses the crack the crack characteristics are longitudinal in direction and they occur you can see very nicely at a stress level 46 to 76 percent of yield strength no residual stresses are found wherever people noticed high pH IGACC failures. In the case of near neutral pH SCC there are features which are localized which assist the failures these features are corrosion say corrosion gauges welto and these are the ones which which leads to stress concentration leading to the failures. So, the high pH stress corrosion cracking the neutral pH stress corrosion cracking they have different characteristics in terms of the damages. There is other form of corrosion which is called as stray current corrosion. We shall be discussing stray current corrosion in detail later and we also talk about how to control stray current corrosion. We shall give a small introduction to stray current corrosion so that you can complete the understanding of external corrosion of pipelines. The stray current corrosion occurs primarily because the cathodic protective structures be it a pipeline or a storage tank when it encounters another metallic structure. And stray current corrosion is again predominant when people use high ccp system and in this case repressed current cathodic protection system we use a rectifier the driving force for the cathodic protection becomes very large. And the current that leaves the anodes goes to the soil most of the current while it may enter into the pipeline of interest or the structure of interest which is cathodically protected. A part of the current enters a foreign pipeline and when the current enters a foreign pipeline or a foreign structures and this current travels across a pipeline because the pipeline provides a low resistance path. The current so entered has returned back to this back to the source. So, when the pipeline crosses the the other pipeline of interest which is cathodically protected close to this vicinity the current from the foreign pipeline enters the soil and then re enters the host pipeline. You know that the current wherever it leaves it is an anode and wherever it enters it makes the structure cathode. So, the foreign pipeline has two characteristics where the current enters the pipeline it is made as a cathode there no corrosion occurs. But in the foreign pipeline where the current leaves and enters the soil it is made as a anode and so the corrosion occurs over there. The current density at these locations are significantly large the damage can be quite significant. So, this is one form of corrosion that can happen when you have foreign structures surrounded by the structure of interest in host structures. And we will talk about this in detail later as to how to prevent the corrosion of the staker and corrosion of the structures. I shall come to the last topic of today's talk how much of the metallurgical factors affect corrosion. With respect to external corrosion we see that in the external corrosion with respect to uniform corrosion even for that matter localized corrosion the various pipelines various grades do not significantly change the corrosion rate. The metallurgy does not affect the corrosion they behave almost the same because the corrosion resistant alloying element like chromium nickel are not significantly added to it. So, we need to have control measures what are the control measures given? Coatings and cathodic protection are given without which all types of all grades of pipelines will suffer external corrosion. There are of course, the two types of cracking we will see later which could be hydrogen embrittlement, stress corrosion cracking and hydrogen induced cracking. But these type of failures are very much dependent on the grades of steels and they are also largely affected by the cathodic protection. If done in a reasonable manner in a correct manner stress corrosion cracking can be controlled by cathodic protection. But if you are going to have hydrogen embrittlement then cathodic protection can be a problem we will see that later too. Before we close this lecture I would like to summarize what we have seen so far. We have seen about 9 different reasons for the localized corrosion of the buried pipelines with respect to external corrosion. They involve bimetals, lack of oxygen content, differential concentration of the soil, metal impurities, mill scales, damaged surfaces because of the scars, bacterial corrosion, stress corrosion cracking, stray current corrosion. They are all different forms of localized corrosion happening on the metal. We also saw that the chemistry of the pipeline steels generally used are not significant enough to alter the uniform corrosion or the localized forms of corrosion, accepting the stress corrosion cracking of the pipelines. Thank you. We hope to see you in the next lecture.