 The next topic of interest for us is what is called as the hydrogen damage. This is a very interesting and important subject somewhat similar to stress corrosion cracking, but the mechanism of cracking is different. I have used the term broadly as hydrogen damage as given by the Fontana book. In all these cases the main species for responsible for premature failure is hydrogen, but the way the hydrogen causes the failure is different. And so, we we we we called broadly as the hydrogen damage. Under this we could have hydrogen blistering or also called as safe wise cracking. The other one is hydrogen embrittlement. Some people loosely termitize hydrogen induced cracking called decarburization. The fourth is hydrogen attack. I would like to say that 1 and 2 are ambient temperatures, processes and 3 and 4 are high temperature failures. You say look at that again briefly not going much into details. Let us now discuss the topic of hydrogen blistering. The typical failure you know happened in one of the refineries and located in Mumbai. You can see how the sample bulged we call a blistering. Primarily because the hydrogen accumulated somewhere here ok and created the pressure and so it happened. If you look at microscopic level you see that these these are all stepwise zigzag kind of cracks. If you look at even more at a microscopic level they are micro cracks and this is actually a cross section of a pressure vessel ok. So, we will discuss this in detail why does it really happen and how do you prevent the hydrogen blistering of steels. Hydrogen blistering generally occurs in low strength steel. Let us look at the mechanism how the hydrogen can cause problem. We all of us know that you know hydrogen is the tiniest element in the periodic table ok. There is I do not think there is any other element which is smaller than that and hydrogen in the atomic form can diffuse through the metals and steel for sure you can diffuse through that. You can diffuse through the interstitials and so it can cause two types of problems. The one type of problem is hydrogen blistering. Let us take a cross section of steel here steel plate ok or a vessel it is exposed to say an acid make it simple exposed to an acid and this is steel. The steel has got a small defect and this defect is where the metal corrodes what is the reaction you get? You have iron going as Fe 2 plus plus 2 electrons H plus can combine with the electron to form atomic hydrogen that is all we know. These things happen on the surface and hydrogen ions move towards the surface and accept the electron and form hydrogen atoms. All these are all absorbed hydrogen. Please note here that these are all atomic hydrogen they absorb on the surface. The following things are possible 1 H plus no no no H the hydrogen atom can combine with the other hydrogen atom and it can form a hydrogen molecule and it can escape from the surface. That is they can simply combine and go out escape as hydrogen. The second chance is that the hydrogen diffuses through the metal. For example, it can diffuse come out as hydrogen here and diffuse come out as hydrogen here. Here they can combine and the hydrogen gas can get liberated. It can happen in many of the cases. In fact, in some pipeline corrosion monitoring they monitor the amount of hydrogen that is escaping the pipeline that related to the corrosion rate. You know is not it? Each ion that is getting oxidized in an atom you have one gas molecule is getting released right. You have two electrons and two electrons in the combine with H plus and form one gases hydrogen molecule. So, by monitoring the amount of hydrogen you can monitor the corrosion rate. In fact, people do that. So, you have this option taking place. The other option is through metal and escape. Third option is hydrogen diffuses and get trapped at the internal defects and become hydrogen molecule right. So, you will have quite a bit of in these places you can have hydrogen molecules. It can build up pressure in in the case of 3 hydrogen builds pressure advances the crack. So, what is the role of hydrogen? The hydrogen role of hydrogen is just to exert pressure. The existing defects they grow that is what they do that. In which case the fracture here is a really a ductile fracture you are not going to get any brittle fractures. The hydrogen does not interact in with the lattice at all. Now, the fracture here is ductile. The strength of the steel also less only steel quality also unclean steel. So, in addition to the pores and cracks you can also have other other trap sites especially you can have manganese sulphide stringers and especially you have it in kiln steels when rolled. When you roll it the manganese sulphides they elongate the stretch ok and if you see this in the micro in the microscope you will see they are all in the plane. These are all the planes and sulphide inclusions. Now, what happens in this case? The hydrogen get accumulated here hydrogen accumulate cause micro cracks right. So, what happens then? Micro cracks choirs they choirs and form not the cracks. So, for example, you know this this can grow this can grow this can grow and then they join and the form stepwise cracking finally, leading to blister. Please see that these cracks generally grow parallel to the surface. They grow parallel to the surface why all these stringers are aligned in a plane and that plane is parallel to the surface of maybe a pipeline or maybe a pressure vessel. So, it is a material problem first of all it is a material problem. The material has is not clean in the sense it has got pores and micro cracks. The steel has got inclusions and they especially the the manganese sulphide stringers are there actually. Now, what are the factors that affect this more poisoning of the interface? What does it mean by poisoning? Here in the sense when say poisoning here it means hydrogen is less allowed to combine and escape as molecules. If the surface for example, if surface of the steel has sulphur, arsenic, antimony and phosphorus all these are poisoning agents these are poisoning agents right. So, they so they they hold the hydrogen on the surface for quite some time. They do not allow that means the actually they retard recombination reaction. What is the recombination reaction hydrogen plus hydrogen giving rise to molecule this is retarded. When this is retarded what happens they have enough time for the hydrogen to get into the steel. And in the steel the manganese sulphide inclusions there are two problems here. They are good trap is not it? They are good trap sites because the sulphur they trap the two. The manganese sulphide is is is a long stringer what happens to the steel is toughness is transfers ductility is less. So, so that is the kind of problem that we have. So, it is a problem of surface. Now these elements like arsenic, antimony, sulphur all can come from the environment right. Now if you are going to have crude if it is a crude oil pipeline crude oil pipelines may may have and you know in this crude may have hydrogen sulphide gas this is called as a sour gas. So, crude may have some no so sour gases right and this hydrogen sulphide acts as a poison and so they are problems. They are problems in the oil and gas industries that transport crude oil and crude oil in many cases it has a hydrogen sulphide gas and so cause the problems. And sometimes these cracks can be it can be few foot length it can happen not small it can have a very long cracks. So, we have seen this mechanism how do you control this? Yes. Yes. First of all lower H2S or or poisoning elements in the the environment this may not be possible always right depends upon the crude we are going to get you can reduce of course they also use diesel prices and all they do that, but that is not going to be you know easy in all cases go for a clean steel you can also go for inhibitors. Please notice it is no corrosion no hydrogen. So, no hydrogen no blistering you can control the corrosion. And 4 when talk about coatings coatings eliminate the corrosion and so hydrogen blistering becomes very less hydrogen embrittlement occurs in high strength steels high strength steels and the fracture is brittle there are quite a few mechanisms here in this. There are quite a few mechanisms and I just name it and then move on I am because this this is very involved subject. One is decohesion and there is hell called as hydrogen enhanced localized plasticity. There are also some model which talks about it lowers the surface energy. You know you know what do you mean lower surface energy you when you create a when you when you fracture it you create a new surface. This energy required to create a new surface and hydrogen gets absorbed then what happens then the energy required to create a new surface reduces the simple mechanical model. In the decohesion you know the decohesion comes with the it lowers the bond strength it lowers the bond strength and so what happens you can cleave right the energy required to cleave the atoms they become less. So, that is called as hydrogen induced decohesion model they call H C D E you call it ok this also called as H E D E hydrogen enhanced decohesion model and help means what happens it lowers it it assists the assist the dislocation motion. The dislocation can move even at a stress level lower than the yield strength. Assume that the dislocation moves below the yield strength there will be no ductility to localize plasticity right and so it becomes brittle. So, anyway I have given you a broad I mean contours about the the mechanisms of this. There is one more mechanism is also talks about hydride formation these things are happening in the case of titanium, zirconium, even magnesium and you know some of these materials they form hydrides and hydrides are when they are brittle I need you know it refracts us to lose the ductility of this. There is also anyway there is also one more model called as aid adsorption induced dislocation emission actually ok model. So, there are a lot of things you know the aid mechanism is basically of course, that way even aid help and and H C D E are all crystallographic models actually right all involving the dislocations and as well as the cohesion in the lattice. It will be nice to compare stress corrosion cracking versus the hydrogen embrittlement. Stress corrosion cracking again I am very simplifying it I am simplifying it. Stress corrosion cracking is anodic dissolution involved in hydrogen embrittlement what is involved hydrogen evolves there may be metal dissolution there may not be hydrogen dissolution there may not be metal dissolution right. So, either there can be metal dissolution most cases it can happen because hydrogen evolution how it can come unless metal dissolves it gives electrons H plus cannot reduce to hydrogen but it is not necessarily it can happen. I can do a cathodic protection and hydrogen evolution can occur ok. So, this is anodic dissolution here there is a hydrogen evolution process occurring. So, if you look at the fontana book and just give a nice distinction between these two. So, potential and this axis is time to fail how long it takes the time to fail. If I apply a negative potential in relation to the ecard if the time to failure decreases like this is the time to failure right or every time the time to failure. So, it is the fail here this is no fail and I can also have a similar thing here fail that is no fail. Can you now guess or just analyze what you call such failure and what you call this type of failure what would happen suppose if it fails faster right this is this is a time axis right when I increase the potential positive the time to failure decreases it will be faster. So, what do you call this failure? If I can classify as SCC in hydrogen embrittlement what do you call this yeah choose between these two. So, this is your hydrogen embrittlement failure and this is going to be SCC. So, one way to differentiate is whether the metal undergoes hydrogen embrittlement or SCC is to apply a potential for apply a positive potential if the metal does not fail then you say that it is what it is prone to hydrogen embrittlement may be prone to hydrogen embrittlement. If you apply a a negative potential if it does not fail and it fails in the positive side then you can say that it is it is susceptible to SCC why when you do a cathodic polarization what is happening to metal what is happening here here what happens now over here metal dissolution decreases right metal dissolution decreases metal dissolution increases you can able to understand why things really are happening at all. There is one more way of uh distinguishing these two I thought I will just discuss that too I have not discussed to use this strain rate all the stuffs ok and in ductility strain rate is um let us say it is 10 power say minus 8 I just give something 10 power minus 1 something like that ok. Here I carry out a simple tensile test right the specimen is immersed in the environment of interest when I carry out a tensile stress tensile test I pull the sample at a given strain rate what is the strain rate how do you define a strain rate what is strain delta L upon L right and strain rate means again is going to be your time ok dt. So, it is going to be your strain rate this is going to be your strain rate what rate at which you are going to pull in relation to the of course, it also involves the gauge length right L is the gauge length and delta L is the elongation extension and of course, dt is the time ok. The metal can behave like this two types of behavior the metal can exhibit the first one how does it behave the ductility is very high at high strain rate also high at low strain rate. The other one the ductility gradually decreases and then almost becomes steady state downward it does not take off again ok. How do you explain this? If you want I can say that the case one one corresponds to stress corrosion cracking the two corresponds to hydrogen embrittlement. Now, let us discuss why I would assign the one the curve one to SCC whereas, the second curve I assigned it to hydrogen embrittlement. Well, let us just look at SCC what is SCC we discussed earlier at least in the slip assisted dissolution model right what do we discuss there ok. One I can have it is not a well drawn it is smooth curve suppose I draw it like this ok. Now, you tell me the order of susceptibility of material one, material two, material three I get it like this ok and this is scratch or when slip step forms the current increases and then decreases say amount. You tell me what will be the order of increasing susceptibility to stress corrosion cracking. What will be the order of increasing susceptibility to stress corrosion cracking? We are talking about a model which is slip assisted active dissolution model for stress corrosion cracking use that model I have tell me which one you would expect to be the highest susceptible and the lowest susceptible. Sir, the 3 will be the lowest then 4 by 19 2. 3 will be the lowest least that is right then 1 then become 2 right you are right why why you saying like this. So, the time for repassivation. So, the Q you talk about Q ok the Q you see here ok Q 3 is the lowest then you have Q 1 and becomes Q 2. So, time for repassivation becomes. So, that is a key issue. Now, there are two things are happening one you are deforming the film is broken other is the repassivation right. Now, let us look at this curve now if you pull the sample so fast the time for the dissolution is very less because I when I pull very fast how long it takes to to practice the sample it takes less time to practice samples. So, you are going to get a purely a mechanical failure. The time the sample resides in the environment during the completion of the test is very low here because the time taken to fracture is very less because of the high strain rate. So, I do not expose the environment for long time. So, it becomes totally ductile I understand this. So, when you are allowing more time for exposure as the strain rate is lowering never happens becomes a maximum here this also you can understand here. Why does it go back? It goes back because before the mechanical you know before the mechanical you know stress breaks the film the film is again reforming. So, you do not have. So, it there are two process opposing processes one is the film breakdown other is a repassivation. The film breakdown occurs because of the mechanical strain right. So, if the strain is so low ok the film is not really broken that much it is it is able to repassivate compared to the film breakdown. So, in this case it is easy to heal it is easy to heal the film and so the time so the ductility becomes more and so you get more elongation here. In this case of course, you have a bare metal the time taken for exposure is so low so low here is optimum. So, you are going to get the highest excessive susceptibility in this case taking place. So, that is why it goes to a bell curve and have the lowest ductility over here in SCC. Hydrogen embattlement is a different story together right. How does the hydrogen embattlement work? The hydrogen embattlement works SCC for hydrogen embattlement it works differently right. Assume that I have tensile sample and I apply a load here tensile load the crack starts advancing now. We have given different models in a decoction model or localize the plasticity model aid model what are model it is going to be there. In all the models I need hydrogen to act. If a hydrogen is not there then it is not going to act that means, I need hydrogen at the crack tip ok. So, the hydrogen now what happens start diffusing to the crack diffuses to the crack tip here ok. The hydrogen diffuses to the crack tip. So, hydrogen needs to diffuse and reach. So, the diffusivity of hydrogen is very important diffusion coefficient is very important. Now, whatever be the diffusion coefficient ok the two factors that would advance the crack one is the rising strain strain rate the crack advances when you apply a strain rate. Other one is the diffusion of hydrogen right. If hydrogen can reach faster than the crack advancement right by mechanical straining then what I what it can do it can it can now it can assist the crack growth right. So, when the crack front I have hydrogen it assists so, but it is not there it is going to mechanical failure. So, it depends upon the diffusion rate now look at this if I am going to now load the strain rate the concentration of hydrogen at the crack tip would be increasing if I load the strain rate right. Isn't it? If I if I pull it very slowly what happens the diffusion rate is same almost you do not worry about the strain induced diffusion and all forget about that. Assuming the diffusion coefficient is constant and if I if I strain it very slow like this then there will be ample hydrogen on the crack tip and so, the cracking occurs right. So, now look at this now look at this plot go back to this plot here as I load the strain rate you expect more and more hydrogen to crack reach the crack tip and so, the effect becomes more. It becomes steady state because after that it saturates it does not really go beyond that level. So, it becomes saturated. So, the hydrogen embrittlement tendency increases when you lower the strain rate and lowers only one way whereas, SCC the SCC you know this distress coefficient cracking goes through a bell shaped curve and you have an optimum value of strain rate that gives you the maximum SCC and this optimum value will depend upon the alloy you know it may not be the same for all the alloys it could be different for stainless steels for and it could be different for aluminum alloys and magnesium alloys and so on so forth. So, you have seen the hydrogen embrittlement behavior of the metal and what causes the embrittlement. You can say one thing that if the strength of the strength of an alloy increases the strength of alloy increases the hydrogen embrittlement tendency also increases which means if I take a steel of let us say 1000 mega Pascal the critical amount of hydrogen required to form a crack if it is going to some x value if I increase the strength level to 24 100 mega Pascal for example, ultra high strength steels then it could be much less than that it can be even one PPM level of that for a mild steel you can even have 20 PPM or 100 PPM nothing happens. So, as the strength level increases for a steel the tendency of the steel towards hydrogen susceptibility increases more and more ok. And you have fraction mechanics concept we will not enter into why does it really happen is enough to say that the tolerance become less and the strength of the metal it increases it can happen in aluminum alloys it can happen in steels it can happen in other metals as well. So, this is one thing that we should be aware of that. So, the hydrogen embrittlement I would say can be a function of strength and the concentration of hydrogen. Of course, one more called as applied stress please notice if you do not have you do not apply a stress no cracking occurs here. In the previous case and the hydrogen bliss spring the cracking occurred without any external stress here you need to have an external stress. So, you need to notice the difference right. So, external stress is required for hydrogen embrittlement to occur bliss spring not required similarly here you get a brittle fracture whereas, in the in the in the case of hydrogen bliss spring it was a ductile fracture this happens in high strength steel whereas, there it happens in the low strength steels and unclean steels. So, please see the difference between ok you want to call it as a brittle fracture this is your hydrogen embrittlement hydrogen bliss spring it is it is a ductile fracture to applied stress needed no applied stress needed. Generally in high strength steel happens in the low strength steels and both of them of course are ambient temperature fracture process. What creates hydrogen in the system you know there are what are the sources of hydrogen the hydrogen source it could be you know corrosion simply a corrosion process can lead to hydrogen to electrode deposition could be cathodic protection it could be may be during welding ok welding case what happens you have using a in humid conditions right humid conditions it can be in the steel making process several places the hydrogen can be a source. Fortunately hydrogen embrittlement is a reversible process. So, if you can degas hydrogen then that treaty can be recovered. So, people have studied you know some work on this time to failure versus the applied load on this and they have done it on a steel which is which was coated by an electrode deposition process you know when you do electrode deposition process you make the metal into a cathode and hydrogen evolves and part of hydrogen gets into the material right. So, you find that the behavior goes something like this this is asplated it is baked let us say baked you know T 1, T 2, T 3 and T 4 right what are the T 1 they are all baking time T 1, T 2, T 3, T 4 the baking time normally if it is a steel that you can have in the range of 180 degree Celsius to 250 degree Celsius if you do it and most of the hydrogen can get a escape. Some hydrogen can still remain as a trapped hydrogen that may not of course, significantly affect the hydrogen embrittlement. Please notice the hydrogen embrittlement occurs only if the hydrogen is diffusable because hydrogen does not diffuse then embrittlement does not occur why it has to go to the crack front control. How do you control this? First of all reduce the hydrogen level in the material right to what you can do reduce residual stresses same thing what do you do for HCC right of course, reduce the applied stress you can give coating all this and reduce corrosion and reduce corrosion hydrogen and take reduces reduce stress concentration there are several factors you can also look at reduce stress concentrations in the structures wherever you have. But very interestingly if you if you take if you take a ferritic steel an austenitic steel it could be steel or stainless steel it is resistant to hydrogen embrittlement prone to hydrogen embrittlement what is the reason anybody because the diffusion coefficient for hydrogen they are all changing quite significantly right it can be 10 power minus 6 centimeter per second it is a plain carbon steel and it is 10 power minus 12 centimeter actually you know diffusion coefficient comes as a square right per second right. So, you can see about 10 power 6 times you know the diffusion coefficient is less for austenitic stainless steel. So, austenites are generally more resistance to stress corrosion crack sorry more resistance to hydrogen embrittlement than ferritic stainless steels the contrary true ferritic stainless steels are more resistance to SCC and austenitic stainless steels are more prone to SCC. So, you need to understand the the the basic reasons why these materials behave in that way. So, continuing our discussion on hydrogen damage the third type of hydrogen damage is decarburization you know the steel take steel the major strength comes from where it is coming from the carbon right from carbon. Now, if you are talk about a steel at ambient temperature the solubility of carbon in the ferrite is very low right. So, the solubility of carbon is very low. So, what form the carbon present in a steel typically that leads to strength. If you take a microstructure of a carbon steel what kind of microstructure do you see you will see ferrite and what else you see you will see a polyite right the polyite consist of ferrite plus the cementite. Now,this combination gives the highest strength the cementite in fact, gives the highest strength to steel. Now, supposeso, you have essentially alpha plus polyite and the polyde is nothing, but alpha plus Fe3C. Now, suppose I take this steel and I subject it to a temperature ok a temperature greater than 200 degree Celsius and I have hydrogen in the system. Now, what would happen that steel is nothing, but iron and the carbon is present the carbon is present as the polyite. When you do this when you heat steel in in in hydrogen what is expected to happen. So, when the steel is exposed to hydrogen atmosphere the temperature above 200 degree Celsius right. Now, what can happen now thethe carbon present in the steel will react with hydrogen and it can form emitting gas. So, the carbon is essentially removed right if you do this then it becomes what it becomes an iron and it becomes a methane gas. Now, this reaction can a CH4 yeah this is a 4 hydrogen here you can balance the equation oh yeah sorry yeah to write thank you. So, it is it is a it is a methane gas is a CH4. So, the carbon can interact with hydrogen either in the atomic form or in the molecular form. If it is in the molecular form it reacts only the surface then what happens it removes the surface carbides right you have a carbide essentially you have this. Now, if the atomic hydrogen diffuses into the steel and then form methane gas it just can happen. Now, what happens the molecular hydrogen of course, the atomic hydrogen also can react with the carbides present on the surface is possible right. Now, what could really happen right if we look at the reaction essentially it is it is a diffusion of carbon right diffusion of carbon to the surface when you remove carbides because the concentration of carbon or the surface is reduced because of decarburization. So, the poured in the bulk will slowly decompose into iron and the carbon will migrate to the surface and then form surface methane gas ok. So, you you see this happening. So, you have a time period what happens you see the carbide now the now the depth depth of carbide free zone increases with the time right. As the time passes more and more carbides will decompose and come to the surface and form the methane gas. It is also possible the atomic hydrogen the hydrogen can also migrate inside and can form a methane gas in the grain boundaries these are all methane gas and they build pressure. If you build pressure in the grain boundaries what will happen to the grain boundary deform the form inter granular cracking. I I show you the micrographs of a steel containing 0.1 carbon because you have some chromium you have some molybdenum you have some vanadium in this without exposure to the high temperature and hydrogen content you can see the microstructure here you see a nice folite quite large extent of folite is seen right. On exposure to hydrogen in a holding in for about what is more than 110,000 hours you can see that the amount of folite got reduced because of the decarburization happening within the steel. So, this is one of the problems happen in industries especially it happens in hydrocarbon industries wherein you do what is called as catalytic cracking of hydrocarbons. If you crack say you you get a Indian gas right you get Indian gas. So, how do you get Indian gas? You get Indian gas from long chain hydrocarbon they are essentially liquid right and you fragment into smaller molecules they become gas and then they compress and give you as a liquid. So, when you take a long chain hydrocarbon you crack them into fine smaller molecules you what happens now you see that the hydrogen also evolves along with that. So, there is a free hydrogen the catalytic cracking is done at high temperatures because of the of the generation of hydrogen these steels they suffer decarburization. Now, decarburization leads to two kind of problems one of course is the cracking as as I have mentioned before. The other thing that can happen is can you fill up that what can happen if you remove carbon strength falls down. So, this is a a severe problem over here. So, how do we really control? How do we how can you really control? How do you control this? You need carbides to form you know I mean to to to have high strength to import high strength to steel right they are required, but the carbides interact with the hydrogen they decompose. So, how do I minimize decarburization? Any strategy? So, you have to form stable carbides right you have to form what is called as the stable carbides. You have some idea about stable carbides you know iron carbide is not that stable. Can you tell me an alternative carbide which is more stable than iron carbide could be titanium carbide could be vanadium carbide it could be as simple as chromium carbides. You know very well you know when it dealt with sensitization of stainless steels chromium carbides form in preference to iron carbide because that is the most most stable phase as compared to iron carbide. So, you allow the steel with chromium molybdenum more specially and you can also add vanadium kind of stuffs vanadium can have titanium all this kind of components, but more importantly people add chromium and molybdenum to that. Now, just look at this equation temperature right. Now, the decarburization tendency depends on what? Can you look at this equation and tell me? Present of hydrogen and temperature. Is a present of hydrogen and temperature. So, what how do you quantify this? It depends upon the partial pressure of hydrogen and the temperature of of the reaction. So, people have generated some data on this temperature partial pressure of hydrogen, this is the simple carbon steel, this is one chromium of moly steel and also have. So, when you increase the the hydrogen partial pressure to increase the temperature you see you need to have high chromium content and high moly content. Again, there is a limit beyond certain hours you see that if you increase you know beyond beyond certain partial pressure of hydrogen, if you increase the partial pressure of hydrogen again what happens? The temperature over which the reactor can be subjected can be reduced. This is the relation between the temperature of the reactor and the partial pressure of hydrogen in the reactor. If you increase the partial pressure the temperature at which the reactor can be safely operated is decreasing now. But you want to increase the temperature and increase the partial pressure of hydrogen and you go for high chromium and high moly content. So, this is a way the material is selected for the reactors where decarburization is a major problem. By the way the chrome moly steels are also known for creep resistance right. In order to have high creep resistance people go for chrome moly steels in in fact, it goes both ways ok. If you want more problems you go for 5 chromium for example, you can go for it you can go for 9 chromium. So, that the stability of the steel against decarburization increases with more chromium content. So, chrome moly steels are better materials for environments where the decarburization is a is a problem ok. So, this is about the decarburization and these curves are called as Nielsen curves. Please look at this the decarburization is a high temperature phenomena and it is a chemical reaction taking place. In the case of hydrogen embrittlement it is a ambient temperature phenomena and it is a it is a more of mechanical loss in ductility because of embrittlement taking place. It is not an embrittlement here here because of loss in carbon the strength of the steel significantly reduced and beyond that it is a high temperature process. You can have a molecular hydrogen here you can have atomic hydrogen here both can cause problems as far as the decarburization is concerned. Let me go to the last topic of hydrogen damage that is called as hydrogen attack. This again is in high temperature process. It is found to happen especially in in copper base alloys especially in copper beryllium alloy systems. Copper beryllium is used you know the strength of copper beryllium is very high. In the copper base alloys assume that you have C2O in oxide inclusions are present and if you anneal this sometime what people do is you know when you have highly rolled work hardened aluminum I am sorry copper alloy and you want to remove these the riskless stresses you anneal it at high temperature in hydrogen containing atmosphere so that the copper is not getting oxidized it is reducing atmosphere. But if you are going to have it at other temperature what it can happen is you can combine with this and you can form steam. So, hydrogen can diffuse into the metal and you can form water. Please notice the water is at higher temperature at higher temperature what happens now this will be in form of a this will be in the form of a steam. So, builds up pressure and cause. So, it is an internal cracking it is happening because of hydrogen atomic hydrogen getting into the copper base alloy. Please notice copper is noble compared to hydrogen that is why hydrogen can reduce coppercoupress oxide leading to copper and water because of high temperature the water becomes a steam because of high volume it leads to a cracking process. So, this is a you know we termed as hydrogen attack in high temperature process ok. So, any questions in this so far? There are no questions I think this brings us to end of our discussion on stress corrosion cracking and hydrogen damage of metals.