 Welcome to the discussion on Frating Corrosion. This is a is a special type of you can call a corrosion occurring when the structures are loaded heavily and the interface between the mating parts the structures are not supposed to be moving against each other. So, in the structural design what you do is for example, you say riveting right riveting is is a mechanical joint. Similarly, you can talk about bolting or you could see a tie plate that you see in the rails right been grabbing the rail with with the with the wooden or the concrete slabs. Now, these are essentially the structures which are joined by by a load ok, but it is easier said than done right. When you when you apply a when you have bolt for example, a bolted a structure there can be a vibration. When the vibration occurs the interface they start moving you know at certain distance it could be as low as a angstrom a nanometer distance. So, there is a relative movement of the interface which are heavily loaded which are not meant for moving they are supposed to be held tight. In that case there is a rubbing action that is you can call it here if you want to call ok, but in this case we call it as flooding damage because in the wear process the movement between the mating surfaces are quite large right. Let us say in in the in the ball bearing within the rays you have the balls they start moving ok, it starts moving may be few meters they start moving. The gear box gear starts you know moving there is a damage, but that we call as a wear process, but a bolted interface because of vibration it moves it giggles right. That movement is very small and when it moves and we also have exposure to the air or a corrosive environment that damage we call them as a fretting damage. So, the fretting damage the fretting corrosion refers to loaded interface first of all which are not supposed to against each other and they are exposed air is an is a corrosive medium here is there is an oxygen present ok and then they start vibrating. And the distance of vibration the distance of movement I would say is of the order of 10 power minus 8 centimetres is close to angstrom imagine. When these things happen interface suffer damage called as fretting damage fretting corrosion call it. How does how does it look like ok appearance it appears as pits appears as grooves ok and covered with corrosion products. Now the fretting you know damage and fretting corrosion is also called as you know friction oxidation called also as a wear oxidation we call as false brinnelling and may be called as chaffing. Now look at this the the the term brinnelling is arising out of the fact you know the you know brinnelled hardness you know brinnelled endender right. When you when you carry out a hardness tester using brinnelled endender you see kind of what hemispherical shape right brinnelled is spherical bulk right. So, the appearance of that end end is very similar to the fretting damaged locations the pits that you talk about. So, that is why they they called as a false brinnelling it is arising out of the appearance that you see that. The friction oxidation and wear oxidation we will see later that it is more related to the mechanistic aspect of fretting damage how the fretting damage occurs in in in reality actually. Now the importance of that it is very important for engineering structures it can be in aerospace industries, transportation industries it can happen in a you know in a in auto wheel for example, it happens in rails it could happen in a heat exchanger where you are securing bolts and then you know when the when the when the liquid pumps you know it pumps the thing then start vibration can take place. So, it happens in several engineering structures it can happen. It can also happen in bio implants that you do for for orthopedic applications right you do that. I give some examples to get a feel for you know the fretting damage that can happen in engineering structures and as well as in the bio plants ok. The one of the classic case of you know defining the fretting damages is is plus fitted ball bearing rays onto a shaft you know you know it is not it. Suppose you have you have a shaft for example, we have a shaft here and right this is the sectional view of bearing right. This is the ball and this is the rays right this whole thing is called a rays and this is plus fitted right. So, in this shaft rotates what happens the rays the balls starts you know I mean starts moving right the whole rays starts moving actually around the balls right of course, balls are located if not going to move in a place. So, the rays is going to move, but this plus fitted place this is supposed to be intact it is not supposed to be losing at all, but in practice it would happen ok. However, well that you you fitted depending upon the loading conditions for a time period you see that there is going to be vibration and relative movement. And the failure that occurs here within this interface we call as fretting corrosion the failure that happens here what you call this we call a wear failure. If there are some corrosive liquid here so, happens then becomes a corrosive wear. So, the difference between this and this place is that the movement relative movement between these two are not very large, but here relative movement is quite large. The other example clear example that one can also look at is the axles of an automobile assume that I have a small car it is manufactured at the one of the automobile company. And they transport these cars to one place to other place before it is being sold to show rooms for example, how do they transport? They transport through trucks right. They transport through the trucks. When they transport through the trucks the axle does not rotate completely rather what happens now it jiggles it just starts vibrating back and forth. On the other hand when you drive the same car the axle undergoes complete rotation. The failure that happens during the second one is called wear failure. The failure that happens during the first one is called as impending damage. So, the difference is that the relative movement of this happens. In fact, one of thereal problems which is in fact, a burning problem that happens for the steel industries. And I do not know how many of you have seen some of these trucks carrying coils you know hot road coil cold road coils that have been wounded and kept on the truck right. They are rounded once I have seen there are few tons. These coils sometimes are used for automobile companies you know thin they are thin sheets right. They are coiled and they are transported from the steel company to automobile company or any user companies. What happens when the when the truck moves the old coil you can see that they start you know vibrating there is just something like bumping you can see that. The load is few tons not simply very low load there are few I mean very high loads. So, when there are high loads between this sheets the coil interfaces you will see a clear damage and that damage is nothing, but a fratting damage. You will see all pits you know like a biting pits you see the things and and it by this is one of the perennial problem for these steel manufacturers ok. So, in essence to say that whenever we have a loaded interfaces very heavily loaded interfaces and they are under vibration causing a relative movement between the interfaces and you have exposed to the to the corrosion environment like air they are going to cause a premature failure the damage and such a damage is called as fratting damage. Now, I just give an example how this can happen in in a implant. In a body implant suppose you have broken you know leg or something. So, it is for let us say a long bone you have a support of a let us say stainless steel rod supports on the leg for example, and you start walking now ok. What happens now is going to rub there is going to be rubbing between the bone and the metal implant. When happens then the bone wears out then what happens now you see that now then there is going to be you know fratting damage on the metal. Please notice that the fratting damage is not just confined to only metallic structures. It can happen on a ceramics suppose I have two ceramic interfaces right it can happen. Very interestingly people have noticed fratting damage even on the noble metals. So, that means, it looks that the corrosion is not the main requirement for fratting damage. Though though corrosion assists the fratting damage, but corrosion is not really a requirement basically because you talk about noble metals having suffering the fratting damage. Ceramics suffering the fratting damage. So, it is just not combined only to the metals it can also happen to the non metals as well actually. So, there are three basic requirements ok for the the basic requirements are the interface must be loaded to there has to be vibration or repeated ok relative motion between contacting surfaces. Third the load and the movement are sufficient enough to cause right. So, it is a mechanical damage they are taking place. The contact surfaces you know when you see when the surfaces are rubbing for example, ok what really happens you know the rubbing it leads to seizing just see just sticks you know when when you start moving fast you can stick and then move right it is called seizing. You can also have galling you know what the galling means when they are the this stick and when you remove it it just comes off from the surface right it just comes off from surfaces. It happens in the soft material soft material. For example, you have a zinc and you know zinc is a zinc bolts for example, fasteners you apply a coating you you you you you you have with the threads you suppose you you just screw them and what happens zinc will stick and just come out ok. So, the the galling comes when we have softer material in the stick and then when then move further apply the load it just comes off ok. So, the fretting damage leads to seizing and as well as the galling and then finally, what happens they are all related to materials removal materials removal from the surface. So, we have now so far described what is mean mean by fretting damage and and where it occurs what are the requirements for a fretting damage. If you need to control fretting damage we need to discuss. In the mechanism there are two types of mechanisms ok. One is called as wear oxidation the other mechanism is oxidative wear. You will see why we are proposing the mechanism. What do you mean by mechanism? So, how are the process are taking place? How are the physical processes are occurring right? What are steps involved ok? What are the route it will be taking place? So, talks about the path. Now, how do you many cases find out the path? You may have to analyze suppose there is a process you might analyze you know after the end of the process and then try to build you know a theory assumption to describe the starting point, end point and then see how things have happened right. When you when you do that then of course, you can also do a controlled experiments and to prove that a path that you describe is right or not. Essentially, you need to look at the experimental evidences to support this theoretically you can do another point of it actually. Experimentally you need to have a data to support the mechanism. In this case when you have a metallic surface the end product is always an oxide particle. You can create an oxide particles by two means. One I start with the virgin metal I mean right nicely and because of rubbing action you get a very fine particles, powders of the metal because they are very fine on exposed to the atmosphere they turn into oxides one thing look at it. The other theory is that what oh look most of the metals they are covered with oxide layer right what really comes in contact with with surface is not really metal pure metal. So, when you have a contact surfaces when you apply a load these oxides fragment and become powders because your end of the experiment you see what on the surface oxides. So, two possible roots are possible ok. So, so that is how it means that wear oxidation mechanism an oxidative wear mechanism both are successfully explaining the experimental observations right. So, that is what happens. So, you want to do that schematically you can do this you can say that ok I have. So, the first is the wear oxidation mechanism. So, you have the contacting surfaces you know the contacting surfaces are not atomically smooth right they are not atomically smooth what how do they do you always have hills and valleys right. So, you have always you always have asperities right this you have hills and valleys something like that. So, I can just discuss this and I can draw schematically something like this. So, they are under load am I right these are under load. When they are under load you have oxide free metallic surface what happens it leads to cold welding they weld each other some kind of you know local fusion takes place between these two surfaces and you apply a load a shear load onto this. Now, what happens because you know you do not need too much of load in order to fragment this asperities contact points right. So, what happens now you will see you see that of course, in the new things have come and it creates of course, a newer surface and you have fragmented particles on the surface right. These are what are these these are fragmented metallic particles. When the they we have very fine fragmented particles and surface is very reactive. So, they get what they get all these get all they get oxidized this is the one mechanism ok. The other mechanism is what is oxidative wear it assumes oxide film coverage and it rightly so right if you know most of the engineering materials you know I am engineering alloys they are all flown to oxidation at room temperatures ambient temperatures. And so, you can you can you can visualize a similar thing these are all oxides right these are all oxide films and they are the interface is under load of course, and there is in a vibration or shear that takes place. So, as a consequence what happens now you have oxide particles ok. And when you generate a oxide particles fresh surface is created that surface gets oxidized and so on and so forth right. So, this process repeats right. The presence of these oxides are the interface they damage the surface very severely why because these are all hard particles and they are the reason for having a pit like appearance right. They are they are quite hard particles and because the relative movement is not too much what happens these product accumulate within the interface. See if it is it is moving very you know very far away it is possible that many of these corrosion products oxide products are removed from the surface. Because they are the movement is very confined these oxide particles remain in the interface and start accumulating the damage with the time that is something we should be understanding. So, you should be able to have some some broad idea about what is wear, what is fretting and what is fatigue right. In fatigue also there is vibration there is a load, but there is no contact interface it all happens because of the mechanical deformation the load is vibration takes place ok. In wear process the relative movement is so large can happen ok of course, there is a surface damage taking place. In fretting damage vibration is there the relative movement is very very small that is that is to be understood actually. So, both these mechanisms the end product as you see here are are similar and so, the both the mechanisms are considered to be operating or is also possible it is not just one of the mechanisms both of them can and indeed can operate as possible in real situations. So, we have seen now the mechanism then we must know how the fretting damage very interesting thing how do you control this it can be considered as similar to wear process can you right. So, if it is considered similar to wear process how do you control wear lubrication right. So, you can apply lubrication we will do that what people do they apply oils apply low viscous oil and in order to retain this oil what people do is they do fast fading fast fading makes the surface more rough and in the in the in the in the surface you you put oil it retains a longer time otherwise in a smooth metal surface you apply oil that just goes away right. So, people do a fast fading treatment and they also called as parkerizing with the term people use to called parkerizing treatment. We can do exactly a reverse of this make the surface then what happens movement is arrested well you do not have control over movement and all and you want to choose a material. So, what should be the property of the material should be hard material ok select hard material or what you can do you can also choose hard and the soft material. What does it mean I can use a gasket right I can use a gasket gasket also arrests the movement of the face ok. So, sometimes people use lead you know when you when you have a lead poured in between surfaces what happens lead does not allow to but then nowadays lead is being banned you cannot use lead so much because of its toxicity ok. You can also do one thing I can apply a very high load ok increase the load when you increase the load what happens the relative movement decreases right increase the load to arrest. So, it reduces the slip if you cannot do that what you do reduce the load you do not find any other any other corrosion mechanism where solutions are given exactly opposite right. You can see the solutions are exactly opposite, but you do understand what happens right. So, you can also reduce the load so that ok you have vibration fine, but then when the load is reduced then the damage becomes less you can do that right. If at all not possible to arrest the movement increase increase the relative movement right. So, you can also reduce fitting damage. So, in the lighter way in if somebody has a question any answer give you a mark ok. So, they are all born out of the understanding of the phenomena happening the contact surfaces which are loaded ok. So, it is a very nice you know kind of the sinks and so that I think they have seen a very I mean a briefly we have it is not a mechanical engineers of course, deal with this very extensively they are in fact a problem for many of the industries. So, the environment is also having an impact right. So, can we if you can use an you know if you use vacuum conditions you can do that, but in practice we we cannot do that right. If we can do a hermetically sealed situations you can happen, but that can hardly possible. But then when you do hermetically sealed conditions fitting damage people seal occur right the rate of you know the damage would be reduced because the air is not there. So, that can be purely mechanical damage can happen actually ok. But nonetheless yes if you reduce the environmental severity you could lower the corrosion rate. In fact, the applying this lubrication oil it has two purpose one it lowers the friction, the second it excludes the surface from the air and other corrosive environment right it does both the purposes actually ok. So, this is one way of isolating the material from the environment. Any any other questions? Sir can we do this grounding that is active shaft grounding can we do this? Soft grounding you were talking about in ship right yeah that is called active shaft grounding you know that probably is is reduce you know signatures you know that happens there and also reduce the what you call as stray current corrosion that happens in the in the shops right because the current you know this is all cathodically protected right. So, the current will start leaving at one place and entering in another place right. So, the active shaft grounding is to maintain the you know a path through which the current will flow it does not go into the electrolyte it goes through the grounding path. It also reduces the signatures developed because of continuous rotation taking place. That of course, is slightly different subject all together happening in the especially they are required for naval ships we do not worry too much about what happens in the the commercial ok. If there are no further questions I think we shall close the discussion today and we shall continue our discussion on stress corrosion cracking in the next class ok.