 Welcome to the next chapter on the forms of corrosion. And today we shall be discussing on what is called as microbial corrosion. What I have shown here, it is a pipeline got corroded due to microbial corrosion. To be more specific the presence of sulfate reducing bacteria and the ion bacteria were implicated in the corrosion of this particular pipeline. The pipeline lasted only for about 6, 7 years. This pipeline was transporting water from a river and this water went to a water treatment plant treated and then distributed. The corrosion occurred between what I would say the water treatment plant and where the water was taken out from the river. We call them as a raw water side. So, this is the kind of problems you normally encounter. There is other problem which we analyzed you know sometime back as corrosion of a pipeline you know carrying cooling water. And it was lasting about 5 to 7 years old. The cooling water here was treated sewage water. And you see here you know you can see very clearly the kind of appearance you see you see here. This kind of silver silver is color let us see there you see the silver is color here. The silver is color that you see here are all due to the microbial corrosion more specifically it was due to surface reducing bacteria. And such bacteria can lead to premature failure see it is a pipeline. And in fact, this pipeline carries water from the chiller and goes to the heat exchanger and you know in fact, it was used for one of the the Pfizer hotel where the air conditioned system you know you know the air conditioned systems how it works. And and this is this pipeline is carrying the cooling water. Now, I do not know how much you can able to see visualize this here this is small leak you see here ok. There are a lot of punctured holes coming from the pipeline and and so there was as a problem. Now, the the microbial corrosion as we see it is it is a problem to several industries is just not only some industries having problems. It happens in in so many industries these problems really noticed it could it could happen even in the aircrafts especially in the fuel tanks. It can happen in the nuclear and thermal power plants. It happens in the marine atmosphere and it can happen in underground pipelines, sewage handling and treatment industries, metalworking industries. See I have just listed a few examples actually you know it is just not only confined to this chemical process industries. You know chemical process industries is a generic name it could be a fertilizer, it could be petrochemical, it could be various types of chemicals being produced. In all these industries you notice that microbial corrosion is one of the should I call a mechanism of corrosion that causes a premature damage to the structures. Now, what I am going to do is I am going to give a very brief discussion you know deliberation I would say on microbial corrosion because this involves quite a bit expertise related to biology ok. So, I think you know we are not so comfortable in dealing with the biological situations. But nevertheless as a corrosion specialist you should understand broadly what kind of mechanism it operate in microbial corrosion and what are the types of microbes they cause corrosion, how do you control them and how do you monitor them. So, that is what I am going to discuss you know in this particular class. The first and foremost that you would like to know is the microbes are involved. The microbial corrosion in a way is not a mechanism by itself ok. The microbes presence can affect various forms of corrosion. What do you mean by that? It is a pt corrosion, stress corrosion cracking, uniform corrosion, even the intergranular corrosion. Many of these forms of corrosion can be accelerated by the presence of microbes. Now, how are these microbes really work on it ok. What is the mechanism through which these microbes really affect the metals? Now, the microbes first of all know they produce organic inorganic acids they produce this acid. How do they produce as a metabolic pipe product? What do they metabolize? They metabolize related to carbon containing compounds nitrogen and phosphorus these are all the nutrients. They take these nutrients they metabolize and the byproduct of that could be an organic acid or an inorganic acid. What are these acids? The organic acids are acetic acid, citric acid could be a succinic acid, lactic acid etcetera. They are the prominent acids they form. Now, the these organic acids are not as strong as inorganic acids right, but nevertheless they can bring down the pH of the environment significantly and especially so, in the localized areas of the metal surface. We see how these acids are confined to the surface. They are not getting affected by the external environment bulk water is not getting affected. These acids are secreted at the surface of the base metal substrate and so, they cause corrosion of the metals. Inorganic acid as much as 10 percent of sulphuric acid you can you can you can produce and they produce how they produce it is it is a I just tell this called as sulphur oxidizing bacteria. We will talk about in details about the nature of bacteria subsequently. So, they can produce these acids and these acids can lower the pH of the environment or the surface and so, they in turn cause corrosion. Now, you see that bacteria per se are not going to eat the metals. They create in a environment and that environment in turn is very corrosive in nature. This is one type mechanism. The other one is they can produce sulphides under aerobic conditions. This is done by sulphate reducing bacteria. Now, how does how does this sulphide affect corrosion? Let us say it forms iron sulphide. The iron sulphide are conducting I mean conducting means you are electrically conducting compounds. They can lead to galvanic corrosion. So, assume that you know I have metal sulphides that there is a one patch where you have iron sulphide formed the neighbouring metal becomes anode the iron sulphide becomes a cathode there is a corrosion or you have iron sulphide which is porous allows the water to enter into it. Then there is going to be a galvanic corrosion between the iron sulphide and base metal. Assume that the iron sulphide is quite continuous impervious and nothing happens. In fact, corrosion rate will drop, but that is does not happen in practice. So, you normally see that the presence of sulphides cause the corrosion of the metals more so, steels it can happen. You can also introduce new redox systems redox reactions I would say oxidation reduction reaction. Generally what is the redox system we normally you see for corrosion of metals either the oxygen reduction reaction, oxygen with the hydroxide equilibrium or H plus hydrogen equilibrium right these redox systems. It can introduce one more and you can cause the corrosion process that is another cathodic reaction that will also assist the corrosion of the metals. You have seen in mixed potential theory right the more redox processes more corrosion can really can can happen. You can produce concentration cells cells either of chemicals and R p oxygen there. I can have oxygen partial pressure variation between two locations. One place is rich in oxygen content other place is poor in oxygen content there is a cell and so, there is going to be anode and cathode can lead to the corrosion process. Now, what what it does do what what does it do? In some cases they can depolarize the cathodic reaction. I think you guys are very familiar with electrochemical concepts of kinetics right. If it is depolarizing what happens to the corrosion rate increases right the overworld rate decreases and so, the corrosion rate increases. In example, here is just take this suppose you have iron it gets oxidized it is in anodic reaction right and you have water can get split into that can form hydroxide can form hydrogen. It could be a cathodic reaction right water splitting water reduction could be a cathodic reaction. If I can accelerate this reaction if I can accelerate the cathodic reaction then what will happen to anodic reaction will increase provided the cathodic reaction is the rate determining step. If this is slow step if you can fasten this particular reaction then the corrosion reaction overall increases. So, what is happening now suppose I have a sulfate and I can combine with this what happens it can form with SRB sulfate reducing bacteria. Please notice that SRB takes away hydrogen to combine with sulfate sulfate to form sulfides. So, that means, this is what is this called this is called as a depolarization please look at it is not an electrochemical reaction right the hydrogen is already in the atomic form. So, so this in fact, facilitates this one. So, this is called depolarization process. Now, what is this can happen this iron 2 plus can combine with sulfur sulfide can form iron sulfide right as a corrosion product. See notice here now sulfate reducing bacteria is not really doing anything to the metal directly it just facilitates corrosion indirectly by depolarizing the cathodic reaction and producing iron sulfide which also can catalyze through a galvanic action. So, this is going to be a problem if you have sulfate reducing bacteria in the system ok. So, this is another kind of mechanism that can cause the corrosion of metals. We talked about introducing a new rock systems you know. Now, suppose if the environment has traces of manganese ions it can accumulate right it can accumulate Mn 2 plus and form Mn 2 plus and Mn 4 plus redox system. This is another type of bacteria right. So, it can now introduce increase the rate of corrosion quite significantly. The bacteria can also form biofilms it is also called as slime it can form on the on the on the surface. Actually it is a byproduct of the metabolic product you know metabolic processes and these are all called protein rich polymers. If you have a metal it is also called as the biofilm now can consist of an acid we talked about before it can consist of iron sulfide. So, it can be corrosion products and it could be a metabolic products to sit on this biofilm right. Now, assume that I have some acid at local places or I have iron sulfide at local places. So, here I have let us say organic acid here I may have iron sulfide what happens? Now, the corrosion is now confined to that particular place because the biofilm it separates the environment you know this is your environment. The biofilms are formed as a result of the metabolic activity of the of the microbes, but in any case you see some kind of fouling happening on the metal on the metal surfaces if you have some organic products in water right maybe that you know some kind of fouled leaves for example, ok. And many of this the organic products can sit on the metal surface a few layers can form and in fact, that only invites the microbes you see the microbes will not go to the surface unless otherwise they find some nutrients in the surfaces. These nutrients are what they are consists of carbon nitrogen phosphorus compounds they are nothing, but organic you know organic substances actually. So, there is a relation between the cleanliness of the water or the environment and then relation to the microbial activity if the water is quite clean and everything in the microbial activity comes down. So, that apart when you have a organic fouling then you have microbes colonizing this have the microbes colonize it they secrete the extracellular substances as a byproduct they all form in the surface and these are all called biofilms. And this biofilm plays a critical role in in corrosion of the metal. First of all it localizes the environment here it can also here is environment. Now, I am going to magnify this biofilm and I make it quite bigger right. This is the please look at this is a membrane type you know the biofilm allows the water to diffuse through it is not it is not going to be a barrier like you know for water. The water will be there anyway that means, the water will be in fact, the metal will be wet completely it is not that metal will be totally dry at all this metal will be totally wet. The biofilm in fact, the major fraction of the mass is due to water only. So, water is going to be there. So, water is your water your air there is a dissolved oxygen present here d o is a dissolved oxygen content right right. Now, please look at as a biofilm what will happen to the concentration of oxygen as you move from here to this what happens? The oxygen is let us say here is 100 percent dissolved oxygen here right and as you move down here here least oxygen content in the water you can form a concentration cell with respect to the oxygen content. So, what happens now here that means, this place is totally an aerobic this place is an aerobic right and this place is what aerobic that means, you have reasonable amount of oxygen present here here the oxygen content is reduced. So, this leads to a different situations for the microbes to persist to live and the microbes live here they are called as aerobic bacteria and these bacteria called as an aerobic bacteria. So, it is possible that you have somewhere here an aerobic bacteria you have aerobic bacteria sitting on the metal surface. This is very dangerous you know combination for the corrosion of metals we will see that shortly ok. So, the idea of showing this right now is how these biofilms can localize the processes the corrosion products the metabolic products on the surface can cause localize corrosion thing and so that leads to severe amount of corrosion like a pitting leaking can happen at these places. In fact, if there are stresses it can lead to stress corrosion cracking as well ok. So, there are cases where stainless steels suffered stress corrosion cracking because of microbial presence because it gives an acid there are chlorides and there are stresses and in fact, there is a going to pitting and so that leads to stress corrosion cracking of metals whereas, as I told you before microbial corrosion is not a corrosion any different from the normal types of corrosion it only microbes assist the various forms of corrosion that we saw so far. Now, in practice what in fact, it happens I think if you can recollect my the photo I shown before this is very interesting actually right. If you see I do not know how visible it is is visible to you here this one so all this they are all called as tubercles called tubercles right your lumps you can see lumps of them you break it break open it you see the black here and this black is nothing, but this anaerobic thing because in over there the oxygen content is very less and the sulphur reducing bacteria remain there and create hydrogen sulphide and then what and then they create what is called as iron sulphide lead to corrosion process. So, is this what happens when you have you know microbes present in the water. Let me touch upon a few things about the microbes what they are present here ok. Again as I told you it is a huge classification I will make it a little bit more simpler here. Probably you can classify them as fungi, algae and you can also as a microbes. In fact, the fungi is known to induce corrosion in aluminum alloys happens, but these are predominantly the problem and these microbes can be classified as aerobic and an aerobic and sulphur reducing bacteria is among this. There are of course, several subclassifications I just give only two examples and what are their biological name one is called as de-sulfo vibrio ok. One and you also have de-sulfo is called de-sulfo to mac column ok. So, these are these are all affected this affect the this all affect the steels strain the steels etcetera in fact, actually all of them reduce sulphate to sulphides right. They have all this what they do is they convert that into sulphide they do that. This anaerobic I think I made a mistake I am sorry for that these are all this is actually is anaerobic bacteria ok. Please make this change it is anaerobic bacteria and this is a aerobic bacteria. The aerobic bacteria some of them are called as oxidizing bacteria in general terms ok and these are called as thio bacillus. Some of you might be aware of this thio bacillus and all you know some of you be knowing about bio leaching some you might have studied in the mineral beneficiation process you know they are used these things. So, thio bacillus is one gallinola is one another one gallinola anyway. So, these are all you know the one which thrive in the aerobic conditions these guys thrive in the in the anaerobic conditions. The situation where I talked about in normal pipelines or you know in industries you can also have on the top aerobic bacteria on the bottom they can have anaerobic bacteria right and they can they can synergize the corrosion process. See for example, you have iron bacteria here iron bacteria oxidizing bacteria sulphur oxidizing bacteria ok. What it can do? You can take sulphur converted to sulphate SRB what it can do convert this and the cycle again continues ok in the process. So, they can keep on you know the corrosion can continue even though you may not have large amount of sulphates with the limited sulphates the corrosion can continue and abetted in the actual situations. Now, let us go into how do we how do you prevent microbial corrosion. There are several ways that you can do first and foremost is to keep the system free from microbes and also clean. What do I mean by keep the system free from microbes? It is not possible always, but where it is possible you should do that. A good example is hydrotesting people do hydrotesting ok. So, this is what pipelines heat exchangers any pressure vessels that we talk about is designed to take a certain pressure right. Before you put them into surveys you need to pressurize it the best way to pressurize is with water they cannot do with the gas because it can be dangerous. So, when you use water there are many cases the heat exchangers hydrotested and even before commissioning the cubes started leaking primarily because the water that they used contain the microbes. So, what you can do in this case use sterilized water. I can also add along with us some bio-sides. We should be talking about this galionela right. This is what is very common in water and it can cause corrosion problem. In the pipelines people use cathodic protection underground pipelines. You do not have much control over the soil nature right. The soil you know you are going hundreds of kilometers of pipelines you are laying and you have no control over that it is not possible to keep the soil free from microbes. So, you do cathodic protection. In fact, the microbes can also degrade the coatings the paint coating that you apply microbes can disintegrate lead to corrosion and yeah we can do that of course, where possible we can do that yes. You can also increase increase the pH in the range above 9 to 10. So, what does it do? The aerobic bacteria will not live. So, the cathodic protection when you do it has two purposes. One of first of all it does not allow metal to corrode. Secondly, in the cathodic protection it will create a hydroxide is not it? There is going to be hydrogen evolution. So, both the way it is beneficial to that. In a closed system you can use biosides. Please look at when I said closed system and you know and what are these closed systems? It is a cooling water system right open recirculating system. It is a cooling water system. In fact, it is invariably they all use this and these biosides some of them are could be like chlorine gas, ClClO2, chlorine dioxide, activated bromine, ozone and so these are all they are all oxidizing right. They simply they are all called as they are all called as oxidizing agents. People also use what is called as enzymatic poisons. They simply attack the protein structure of the microbes and so these this may die. You can also use surface active agents. An example is a quaternary ammonium compounds they are used actually ok. So, they can penetrate the lipid cells you know and then they simply damage the walls of microbes and so they also you know kill the bacteria. The next is how do you evaluate? Actually it is rather difficult to exactly prove that the microbials are the reason for corrosion of the structures. Many times it can be done indirectly. For example, you take the material and you expose the same conditions in the sterilized water and if the corrosion of the metal is better corrosion rate is you know lower then we can say that it is suffered microbial corrosion. Alternatively you take the sterilized water and you dip the metal you add microbes and find out that the corrosion is is anyway different at all. Or the visual examination you just look at it and see there are any microbes present at localized areas. So, from the point of view of diagnosing whether the particular failure is due to corrosion or not is many times is indirect of that. You can take the corrosion product you can analyze for the presence of the various microbes. So, it is it is a kind of indirect evidence that you normally collect show that the microbial corrosion is in action. If you really want to monitor where corrosion occurs or not what people do is people install the coupons. The coupons are installed in a system you take out periodically see whether there are microbes or not. Very interestingly when you talk about microbes they are sessile microbes they only cause corrosion. When I say sessile what I mean by that the microbes which stick to the surface the one that flows in the water they may not really cause any corrosion. So, suppose you take water and you found that there are microbes present in water it does not automatically indicate to you that the system is suffering microbial corrosion. Alternatively you take water you analyze for microbes there are no microbes are present you cannot say that microbial corrosion does not exist because some microbes are sitting on the walls sessile bacteria. So, it is always a difficult exercise to analyze evaluate the microbial corrosion of any systems. But nevertheless you can use coupons you can analyze the deposits for what you deposit for what for the presence of microbes. You can also do what is called as chemical oxygen demand they called as COD correct. And what is this chemical oxygen demand? It is oxygen required for what for the but degrading the various biological systems it could be organic things also right. So, the oxygen is required to degrade them actually. Why are they important? Because these are the systems which are used as a nutrients by the microbes if I am having too much of COD that means, the system is full of nutrients available for the microbes to to survive. It talks about the quality of the water that can support the microbial activity. So, it is a indirect way of saying yes that can be microbial corrosion. Next is called as BOD it talks directly about the living organisms microbes. The microbes require the oxygen to survive. So, you can also look at determine the BOD content. I am not going to talk about how we are going to do that, but it is enough to say that these are some of the ways to assess the probability of microbial corrosion happening in a given systems. Again as I told you and the when you talk about the bacteria you know the only s l bacteria is important. The bacteria that move around which is called as planktonic bacteria ok. That is they are not that important, but more often people only determine the planktonic bacteria in the systems. Well, I think I have tried to give you a broad overview of of the the microbial corrosion, but it is just the beginning I think you guys can lead further and you know and I am sure that will be very useful because when you talk about a corrosion problem, microbial corrosion is one of the causes of corrosion of metals ok. So, ok. So, with this we shall now complete almost all aspects of activist corrosion of course, in the introductory level of course.