Lecture 05

Let us start lecture 5 and the course is Corrosion Protection Methods. In our previous four lectures, we tried to understand different conditions that can influence the corrosion aspects of material. In a briefly we discussed like environmental condition, surface condition, then we talked about material aspect like composition, microstructure and then also we talked about design or geometry aspect. Now, let us talk about the classification or the different methods of corrosion protection. So the course is and this is lecture 5 and topic classification of corrosion protection methods. Interestingly, when we classify we will see that those environmental factors, surface factors, microstructure, composition as well as how to modify the surface those things are coming up. Now, when we talk about this if we see the protection methods, we can say that one is geometry modification or we can say design aspect. It is a physical design aspect, it does not depend on the material part or the condition environmental condition. We only talk about how the shape look like shape of that component look looks like or whether we can introduce something, so that we can avoid some of those corrosion processes happening on the surface of the component. Then we have second part is change of metals and when we talk about change of metals, it talks about both composition, microstructure, surface modification without changing. When we talk about surface modification, it means that without changing the bulk without changing the bulk as well as we can also introduce one more thing which is stress and here stress mean residual. And when we talk about this, it involves processing intimately processing of materials and when we talk about processing means, it can be melting or casting, it can be forging, it can be rolling, it can be shot peening. Those all are coming after the preparation of the metal, how you are we are modifying the material. So, that means once we prepare the metal definitely composition is fixed. The only possibility is the microstructure modification or surface modification as well as stress factor. The processing of materials also should be looked at in combination of change of metals. So, that means there are sub sections here. So, what are those sub sections? Change of composition, change of composition means we can introduce oxide forming elements like chromium, like silicon into steel, one such example. So, that way we can improve the passivity, passivation behavior like addition of noble metal in titanium alloys like platinum and palladium to a limited extent a very small extent. And in fact, because of that it helps in passivation of titanium alloys. So, those are the change of compositions, we will discuss more on that. Then change of microstructure, so the change of microstructure one example definitely we have discussed is stainless steel, but there are other aspects change of microstructures like one such example is if we can make it let us say a surface of some steel and the steel surface is made in such a fashion that the surface becomes amorphous. So, then definitely it will have a very high degree of corrosion resistance. Now, when we talk about change of microstructure in stainless steel itself, we can design our processing route in such a fashion, then we would not allow the chromium carbide to form. In fact, at times we add little bit of niobium or titanium, so that it actually helps in preventing chromium carbide formation, so that chromium stays in the solution all the time. What it does? It all those titanium or niobium they actually takes care of carbon in that, so that not much of carbon will be available for the formation of chromium carbide. And this titanium niobium they have a very very high affinity towards carbon, so that and then you can have actually those carbides this is a small modification those carbides will form uniformly, but at least chromium will be will be spared. So, the chromium level would always remain same along the grain boundary as well as along the grain body and it will maintain its passivity. So, we will talk more in case of aluminum alloys there are instances how to modify the microstructure, so that we can get a very good level of corrosion resistance. Then we can actually introduce stress from outside, let us say short pinning and this introduces compressive stress and that compressive stress if that material is undergoing fatigue and if there are corrosives available around that in contact with that particular metallic component. So, there is a possibility of corrosion fatigue, the compressive stress can avoid corrosion fatigue. Then we can actually go for stress relieving stress relieving process stress relieving operation. Now, when we go for stress relieving operation or we can say stress elimination. Now, in case of stress elimination we have a classic example like sensitized stabilized steel, stabilized stainless steel during the stress relief operation or elimination of stress thermal stress. If we take that particular stabilized stainless steel to a temperature between 450 to 650 or 700 again the chromium carbide can form along the grain boundary. So, that would lead to knife line attack stress relieving operation is important because it reduces the stress factor and we can have a good amount of corrosion protection, but depending on the material we have to be careful. So, like one such careful point is stabilized stainless steel heat treatment. So, we will talk about that as well as we talk about the stress relieving operation and its influence on the corrosion protection fine. So, the stabilized stainless steel is nothing but where we add those alloying elements carbide forming elements like niobium and that becomes stabilized stainless steel. So, these are the factors which is related to change of metals and it actually considers processing also. Now, when we talk about microstructure change in microstructure it could be bulk as well as surface or bulk both the things we have to consider fine. Surface means we can modify the surface by making it harder or putting some coating or doing some heat treatment making it amorphous those are possibility and bulk of course, by doing bulk processing like deformation or heat treatment like quenching we can also change the microstructure and accordingly we can modify the corrosion tendencies or corrosion effects. Now, the third point which comes change of environment. So, when we talk about change of environment there are several factors like one possibility is elimination or removal of corrosive. So, when we talk about removal of corrosive what are the corrosives for that we should know environment that is what we talked about environmental conditions. What are the corrosives oxygen, moisture, NaCl or any type of salt those are the corrosives. Now, if we remove this corrosive definitely we can improve the corrosion resistance the example simple example we can put is basically we see silica gel when you order electronic items from online platform as it comes to your house if you open the box that box if you see it contains a small packet of silica gel that is essential everywhere you will find that whenever you open those boxes. In fact, nowadays some of the food items we see that silica gel is silica gel packet is kept for example, fenugreek fenugreek this coated fenugreek the sweetened fenugreek those are actually if you open the box you will see that there are there will be a small packet of silica gel what it does it actually absorbs moisture. So, once you absorb moisture, so moisture would not allow those sweetened cover in a in a in a molten condition in a in a. So, that that can become very messy. So, but in case of electronic items those moisture if you absorb. So, that means on the metal surface moisture is not present because it is absorbed by the silica gel pack. So, you have removed the corrosive because water is a corrosive there while water is a corrosive I will come to that reaction again you will see that the electronic item is wrapped and it is wrapped in such a fashion that there is no leakage it is a kind of a sealed box and in that sealed box you will see that plastic layer is actually sticking to the electronic surface electronic item surface. So, that means it must have been evacuated and then sealed why why it is done because you are actually sucking air out of it and when you suck air out of it you are actually reducing the partial pressure of oxygen. So, at the same time even if there are some moisture there will moisture molecules water molecules you are actually also sucking them out and then you are sealing it. So, that means no more oxygen or from outside or moisture can ingress there. So, two effects one is silica gel which absorbs moisture and then another effect is evacuated and then sealed. So, you are removing two items here this as well as this now why these are corrosives. So, this cathodic reaction happens then definitely we have to supply electron in the form of dissolution of metal. So, the corrosion can take place. So, you are removing those corrosives. So, that means this is a corrosive this is a corrosive in fact this acts as an electrolyte and this is the oxidant and which actually gets reduced and then for that we need electron and that electron supply comes from anodic reaction and anodic reaction if it is iron component and this is nothing but. So, you remove corrosive you prevent corrosion Nacl or salt you take care of those salt and Nacl. For example, when you have ferric ion present in the HCl that HCl becomes much more corrosive than without ferric ion. So, that means if you take care of the ferric ion you can actually avoid corrosion to a great extent because ferric ion is a strong oxidant. So, in fact you are actually trying to remove oxidant oxidant if you remove you can actually prevent corrosion, but at times oxidant is needed we will see later on that in case of passivating metal oxidant helps we will see that. Then change of environment when you talk about change of environment we can think of adding external agent and this is very important we call it inhibitor as the name suggest it inhibits corrosion. So, adding external agent and that should be added a very small quantity it has a several aspects of it we can discuss more about that inhibitor when we talk inhibitor part separately, but there are several aspects like it can be adsorption type inhibitor there could be adsorption type what it does it actually forms a layer on top of the metal. So, that the electrolyte and the metal interaction is hindered then there could be hydrogen evolution poison what it does see if this is one of the cathodic reaction very common cathodic reaction if, but it happens in a several stages it can happen like this 2 H atom and those 2 H atom combines each other and then form hydrogen and this hydrogen would come out and when it comes out as per the last principle this equilibrium is broken. So, the reaction would always go to the front and then you will it will always need electron and that electron comes from the dissolution of metallic atom going into the ions, but if you can stop this evolution then definitely you can stop the requirement you can reduce the requirement of electron for the hydrogen and reduction and of course, that way you can reduce the electrochemical dissolution of metal. Then there could be possibility of scavenger we will talk about scavenger later on what it does the scavenger is it actually removes the corrosive reagents from the electrolyte scavengers act that way then there could be oxidizers. So, I was talking about those oxidant now if you can remove oxidant in case of active metals that way you can reduce corrosion, but in case of passive metal addition of little oxidizers like ferric salts nitrates chromate those are oxidizers nothing but oxidants. Those oxidants can help achieving passivity in some of the engineering materials like iron based material or stainless steels those kind of stuff we can have a good effect of passivation due to the presence of strong oxidants or oxidizers. Then there could be possibility of vapor phase inhibitor. So, we will talk vapor phase inhibitor separately. So, those are the different variations of inhibitors and this is a rough distribution of different inhibitors we have more detailed distribution of inhibitors as we talk about inhibitor separately. Then this is again this is actually following the change of environment. The change of environment can be possible if we have operating variable change in operating variable what are the major operating variables temperature there could be pressure. Pressure when we talk about we can say partial pressure of oxygen I am just giving an example or we can change pH by changing the pH of the system we can have good amount of corrosion resistance mostly if we increase the pH and if we can reach to the level of passivity that is good for having a good degree of corrosion protection of metals and alloys. We can actually have change of velocity change of flow velocity. In fact, some of the cases increasing velocity is good some of the cases moderate velocity is good some of the cases stagnancy is very bad like if we have pitting as we have discussed pitting or stagnancy is very bad. So, that times we will talk about one particular industrial problem where in fact, because of the stagnancy it was kept ideal for one and half months during the pandemic period and that led to a serious level of pitting and some of the blades inside the tank which were actually pumps those got into a very bad shape and then pump were. So, all those tank was redesign and then recommissioned. So, we will talk about that. So, the stagnancy is bad for pitting and crevice if we have little flow it actually helps. So, we will talk about change in operating variables. Now, if we talk about atmosphere we have more about it, but let us concentrate on these many factors. Then fourth is this is interesting this is change of metals or alloys potential. So, when we talk about potential we are talking about electrochemical potential. So, when we talk about electrochemical potential either a metal surface can be made into cathode or it can be made into. So, when we make it cathode. So, there could be cathodic protection there could be anodic protection. So, in case of anodic protection we intentionally take the potential of the metal towards the positive side and this relates to passivation we will talk about that. And when we talk about cathodic protection you are actually making the surface cathode and when you have cathode definitely you could have very good level of protection. Generally for active metals we use cathodic protection and passivation is most important if the material is not showing any passivation we cannot use anodic protection we will talk about that later on. Now, these are the two things for example, if we talk about cathode if we are means we are making the metal surface cathode. So, let us say if we have this metal which is to be protected and let us say this is one another anode this is anode. So, if we connect. So, then definitely this is cathode this is anode and through the electrolyte this is electrolyte the current will flow from anode to cathode and then cathode will be protected. So, this is cathodic protection and in anodic protection what happens if we have a metal with a passivation behavior if we can take the potential to this level in the passive zone and if we can maintain that particular potential we can have a very high degree of corrosion protection. So, these are two methods by changing the metals potential. Now, the fifth is use of coatings generally the common perception is if we want to have protection we always say that paint it, but remember painting sometime goes wrong in such a fashion that before one can realize the corrosion can increase into the material dangerously. So, that is what use of coatings before we use coating we have to understand the material aspects otherwise it will be very difficult to get a very good protection. In the coating also we have several variations it can be metallic coating, it can be non-metallic coating when we talk about metallic it can be cathodic coating, it can be anodic, it can be anodic coating and when we talk about metallic non-metallic it could be inorganic, it could be organic. So, these are variations, but there are more to it there are several other variations and when we talk about this coating it can have barrier protection that means it can also prevent the material from wear or erosion plus it can also have electrochemical protection. So, for example, zinc coating it can be used as a barrier protection as well as a galvanization usual galvanization that is sacrificial anode protection. So, this is about coating we will talk more about coatings we will spend good amount of time on coatings. I think till then let us stop here we have talked about the classification of different protection methods and now we will go into the individual protection methods in greater details from next class onwards. Thank you.