Erosion corrosion

Transcription

We see that velocity as well as surface films that two are important parameters or factors, in case of erosion corrosion, and we have seen the example of lade pipe. The lade pipe will operate long a period while handling dilute H 2 SO 4, but when the concentration of H 2 SO 4 is increased, the lade pipe fails in a very short time. Because when it is in contact with dilute H 2 S O 4, it forms Pb O or and Pb SO 4 surface film, and when it is expose to concentrated H 2 S O 4, the Pb SO 4 or Pb O layer that is dissolved, and that leads to a farther attack. And more severe corrosion that can happen in case of lade pipe. Now, there is one more example, let us say the example of copper; the copper and brass, copper and brass and that time they are handling. If they are handling Na Cl solution, sea water solution, that time the copper forms Cu o copper, the copper forms Cu Cl 2 layer, which is black colored, black and yellow brown film of copper, copper chloride that forms; whereas, brass it forms dark grey film of Cu o. So, this Cu o, because of this Cu o formation, it gives you the brass gives much better protection towards against a erosion corrosion than copper, because these can this is a very; this is much stable oxide layer or the surface film compare to this and so this can operate better to give better resistance against erosion corrosion. Now, this is another example; this is case one case studies; this is a case study, why brass can give a better erosion corrosion resistance than copper, while handling chloride solution. Now, let us also see, what happens in case of aluminum. In case of aluminum, when it handles nitrate solution or nitric acid, when it is handling nitric acid this is. So, this time, if you see the corrosion rate as a function of this is C R. C R means corrosion rate as a function of velocity till it reaches around 4 meter 4 feet per second, up to that point corrosion rate is moderate and off course, it is not that aggressive corrosion. But it crosses 4 feet per second, then actually corrosion rate increases. So, actually it happens like this the corrosion rate increases. Now, this is due to the fact that up to this limit. Aluminum forms nitric nitrate surface layer aluminum nitrate, as well as aluminum oxide now, this aluminum nitrate and aluminum oxide until unless it reaches 4 feet per second. Those surface or surface films stay with the substrate, but once it reaches beyond 4 feet per second. Then the corrosion rate increases to a great extend, because the oxide layer is removed due to the velocity of an affect or erosion effect. So, this is another example. So, we are talking about the erosion corrosion. Now, this is about velocity surface film. Then, we can have the third parameter, which is or the factor that is the turbulence or some time it can be impingement effect now, in case of turbulence or impingement effect. We have already seen that, if you have a larger pipe and then suddenly, if you would like to the system from the larger pipe, your system will the water of or the solution flow will go in to smaller dimension pipe. That time, this portion is more vulnerable, and that portion the erosion corrosion would be maximum, because when the water is flowing this way, the solution that time it develops lamellar flow. And once reaches here, because of this conviction in the cross section. We have lot of turbulence that is developing in this section. So, because of this turbulence, corrosion would be very a rapid, and after some time, once it goes in to the smaller cross section, after some time water again develops lamellar flow. So, these parts are not the problem, the problem would be felt the erosion corrosion problem will be felt more in this section. Because you have turbulence affects. Now, another example is let us say, the water you have some bent in the pipe line. Now, the water of the system solution is moving this way, and suddenly it basically takes a sharp turn. And this problem would be more severe and soft turn. So, the water actually heating the surface first, and then it takes the soft turn. And at the same time, it develops turbulence here. Now, if the solution contains lot of dust particle then those dust particles will hit this surface. So, that is these surface is this part is getting impingement effect. The water flow is basically impinging against this facing wall. So, this section you have very severe erosion corrosion so; that means, you have turbulence at the same time, you have impingement. There could be other possibility of turbulence. Let us say the pipe design somehow; there is a extra, this part the dimension has increased. And the system has become like this a pipe has a basically a dent, a large dent. And if this is happening, then also then water can have turbulence in this section. So, this part will be having more corrosion, and that is due to erosion corrosion. So, this is basically effect of turbulence, effect of impingement, and this is also one example of turbulence. So, turbulence can increase the erosion corrosion rate. Now, fourth factor is basically the galvanic effect. For example, one case study, that is let us say 316 stainless steel. And if this is connected to organically couple to lade; this is 316 stainless steel while it handles high velocity H 2 S O 4. There will be combine action of erosion, as well as galvanic effect. Because 316 is the surface film is cathodic to the base material, at the same time lade is also cathodic. So, you have, let us see you have surface film is form like. This is my surface film somehow, due to the erosion effect, you have this surface film is shift away. And this base metal is exposing to the solution or the corrosive solution. And you also have contact with lade so this one; this one; both to a lact cathodic with respect to this section. So, this section will be anodic, there will be further dissolution until unless another surface film forms and stops the rapid dissolution. So, the galvanic effect would be another serious parameter or factor that can control or that can governed the erosion corrosion rate. And finally, we have fifth one off course, the material as well as metallurgy. As we have talked about erosion corrosion. The surface film, importance of surface film, the surface film the material which has highly adhesive stable, and strong surface film that will behave or that will have much better erosion corrosion compare to the material, which has a less stable and less adherent surface film. Now, that is what the material would be another important issue. And generally, if let us say for example, nickel chromium compare to nickel. This nickel chromium would have much better erosion corrosion resistance compare to nickel, and it will better compare to iron since nickels. If you choose nickel it has better corrosion, inherent corrosion resistance compare to iron. So, that is what the depending on the situation. We can choose different materials, one such example, and this is the material part, and second thing is metallurgy. How we can change the property of the material by doing change in processing composition or heat treatment. So, we can change the property, and finally, we can change the microstructure also. So, all these things to put together, if we change the metallurgy of the materials, in such a fashion, the surface film become much more adherent. Let us say normal 18 8 stainless steel, in that, if we add molybdenum. Molybdenum improves the adherence and stability of the surface film. So, if we add molybdenum in to weight, it will act better, it will have a better erosion corrosion resistance. Now, another thing is, this is metallurgy change, we can metallurgy change by simply changing the composition little bit or addition of different along element. That improves the stability of the surface film, even we can think of having a better hardness, surface hardness. Better surface hardness means, that it has inherent better were resistance higher hardness higher, where resistance. And also it is suppose to give a better erosion corrosion resistance. So, hardness is another important parameter. So, the material or metallurgy of this both things is important issue. If you would like control an erosion corrosion property of the material. So, now, we have surface film, velocity, then we have turbulence, then we have galvanic effect and finally, we have material and metallurgy. So, now, once we have understood this different parameters or factors that can govern erosion corrosion property of a material. Then definitely we can think of some protection mechanism. Now, for example, first of all, it come the material we can choose something which has inherent better erosion corrosion property. For example we can have iron chromium alloy. So, if you would like to have a protection against erosion corrosion, protection; protection, so or control erosion corrosion that time first is better material. Let us say, we can chose iron chromium 80, 20; nickel chromium 80, 20. These are alloys have a very good erosion corrosion resistance. And some time we can even chose the modified stainless steel after addition of molybdenum, which also helps in giving a very good erosion corrosion resistance. So, better material is one issue. Then second thing is better design by simply changing the design, we can avoid erosion corrosion effect. For example, if we would like to come to this part; that means, if we have a soft turn or if we have a bend like this, if we have a dent like this, then we have the tendency of erosion corrosion. So, in order to avoid that, what we can do instead of having a soft turn, we can put a bent with a gradual change in angle and. So, we are making a bent with a smooth angle instead of soft angle change. So, if we make it smooth. So, the turbulence effect or the impingement effect will be avoided to a great extent. Now, other design criteria would be, and also if we have a dent like this, try to avoid that dent try to make it flat. So, the turbulence the local turbulence part is avoided. Now, always we are seeing that this part of the section. Let us say, this is my pipe thickness. If this is my pipe thickness, then we see that this part is not that much affected due to erosion effect or impingement compare to this part. So, one thing is making it very smooth change in angle. Another process would be since; this part is always having some impingement effect. Let us make it thicker at the center. If we make it thicker at this end, only this part. So, what happens we can have though the erosion effect would be there, gradually this part is removing small part would be removing. But since it has a much higher cross section that is what this pipe line or the bend section will operate for longer duration compare to the smooth section thickness of the pipe thickness. So, making it thicker would help, this is simply the design part, we are not changing any material or something any or we not changing in velocity remains same the materials remains same. And also the angle remains same, but simply by putting extra material at that part the where impingement effect this maximum. So, we can improve erosion corrosion resistance. So, we can talk about this design factor later, when we come to the protection mechanism on the basis of design. But with respect to the erosion corrosion, this is one example. Now, third is environment alteration. Environment alteration means we have the velocity. We have a solution which is moving at a very high speed in the pipe line system. Let us say this so somehow, if the solution contain small dirt particle emulsion. The effect of impingement would be much higher compare to the situation, where these dirt particles are not present. So, that is what, if we have some sort of filter. So, that, we can filter out all those dust particles. Then we can avoid impingement effect to a great extent. Now, this is filtering. Now, second case we can ad inhibiter in to the system. We will talk later inhibiter in a greater depth, but inhibiter there are many types of inhibiter, these are basically chemicals. So, these chemicals are added, sometimes it is added to improve the film forming ability. So, what happens, if we add a film forming inhibiter? Let us say this particular section is one particular film is broken. Let us say this part is broken now, somehow if we add some sort of inhibiter, it will enhance the film forming ability. So, the more rapidly the film will form, and it will avoid the erosion corrosion effect. And off course, we all know that if we add oxygen, dissolve oxygen in to the system. That will act positively towards erosion corrosion; that means, if we have more oxygen, it will enhance the erosion corrosion rate. So, somehow if we can get rebuff this oxygen or dearation, if we can perform. So, we are basically changing the environment then off course, if we can think off some better coating better coating material, which will with stand this erosion effect. And finally, we can apply cathodic protection so; that means, we can make the system cathode. And avoid the dissolution of metal in to the solution in the form of irons. So, these are basically the protection or control mechanism, in case of control processes or control ways to control erosion corrosion. So, this is about erosion corrosion. Now, let us get in to one more form of corrosion. That is also falling under erosion effect that is called cavitation damage and the cavitation. If you are seen this erosion effect is basically, there is a combine action or corrosion at the same time. There is a combine action of force; the cavitation also is basically the combine action of pressure and corrosion. So, we will see the cavitation effect, cavitation damage. Cavitation damage happens due to formation of bubble. Let us say on a free surface, you have a bubble formation and the rest of the system is basically solution. And the bubble forms, and after some time due to this bubble forms due to the lowering of pressure. And after some time, if the pressure at this zone is increased, the bubble collapsed or the bubble burst. And due to the bubble burst there would be corrosion. So, this corrosion is happening. For example, if we have a surface film. Let us say, we have a surface film now, the bubble is forming, and after that it is collapsing due to increase in pressure. So, that time this locally the surface film is destroyed. This is my surface film, if the local is a surface film is destroyed. So, this base material is exposed to the corrosive solution, and there would be dissolution at this part. Now, once we have dissolution there, then gradually there would be a formation of another passive film. So, this passive film again forms. Now, since we have a crack or lines sort of thing. So, there again another bubble will form so because actually it is forming a notch there. So, around that notch a bubble can nucleate, while it is the system is going under low pressure position. And when it is again coming back to the high pressure section, the bubble will again collapse. So, due to this formation, and collapse of gas bubble, the damage is happening in the material. And that is basically the cavitation damage. And this type of damage, we always come across in the form of pits. But the nature of appearance of those pits, a little different compare to the pitting corrosion. Here the pits for example, if we have a propeller bled, and they are, let us say this part is low pressure zone; this is experiencing low pressure zone. And then here, you have bubble formation, and then when it goes to the high pressure section. So, the bubble will collapse, and there would be formation of pits like this. At this surface would look very rough appearance of those pits. So, that is basically a significance of cavitation damage, pitting type pits there would be large pit formation at the same time. The section would be very rough, and this happens in case of propeller ship propeller. Then impeller pump impeller, we can where actually the system, this kind of situation it handles high velocity fluid. And this accompanies pressure. So, the situation where have we have very high velocity fluid at the same time The pressure is changing alternatively, that case we can come across this cavitation damage or that case during low pressure zone bubble will form the this cavitation will cavitation will form. And then the bubble will burst, and giving rise to pressure on the base material, and that lead to corrosion. Now, and this is the system, where we can see gravitation damage, and it is becoming very rough, and they are a lot of pit formation. Now, let us get in to the fundamental of cavitation damage. If we would like understand the fundamental of cavitation damage. Let us see, what is basically in the theory behind it, why bubble forms and why bubble burst? Now, if we see that, let us say this is my surface, and the material is a passivity material. That means, we have passive film formation as we have said that the passive film on the surface. There could be little bit of scratch, and on those portions, when it goes through low pressure zone, there we have bubble formation. And second stage the bubble will burst. So, this is my surface film. So, bubble is bursting, and due to that lot of pressure is exerted on the material, and due to this high pressure, and that pressure can reach up to 60000 psi pound per square inch. So, at that amount of pressure, what it comes on this passive film, there could be localize passive film breaking, there could be localize passive film breaking. Now, this material is expose to the solution corrosion happens, and until unless a phrase passive film forms. So, there would be little extra dissolution, and the phrase passive film is forming. So, this is the passive film again forming. Now, second this after this another bubble, and this bursting is happening. This is low pressure zone, and this is high pressure. Now, again it will go to low pressure zone, because of this high velocity fluid, and the pressure change it will again come to the low pressure section. So, then again this zone, there would be another bubble formation, and then this is again low pressure. So, when it comes to high pressure, this will crack again; this will again burst so for the little more extra damage. So, the damage part would become like this till it forms passive layer. So, gradually the damage part would go on increasing so; that means, this will go low pressure, bubble will form high pressure, bubble will collapse or burst. And then again it will go to low pressure zone, and since we have already a notch here. So, here bubble will form, and then when it comes to the high pressure zone. The bubble will again collapse, and there would be further little more extra corrosion. Because when the bubble collapses that time little more of this passive film also breaks. So, like that way, it will go on. So, bubble formation, bubble collapse, passive film breaking down a passive film, dissolution, further passive film formation. Again, when it goes to low pressure zone, bubble will again form, and then again it will burst when it comes to high pressure zone. So, gradually that time also the passive film will break, and there would be extra dissolution till the passive film again forms. This is with respect to the passive film forming material, but it can also happen. In case of active non passive film formation is forming material, and that time the material, if it is non passive forming material. And when this bubble is bursting that time they do the exertion of pressure. There would be local slip ban formation and due to this local slip ban formation, there could be a dent also, and due to this, there would be zone with no pressure, no slip ban. And there would be zone with slip ban, this is with slip ban. So, this part would be vulnerable for corrosion. And this part will dissolve at a much rapid speed, and then again this entire cycle will continue. So, in case of non passive film forming material, that time the cavitation damage happens due to the formation of slip band, where the bubble burst. When the bubble burst, it exerts pressure on the material, and that pressure can be so high that there be localize deformation. And localize deform part which contains larger amount of dislocation, and that becomes very active. And so dissolution starts from there, and the rest of the material there will be very little dissolution. So, wherever you have bubble bursting phenomena on non passive film forming material. So, this section let us say, we have bubble bursting phenomena. So, there would be localized deformation. At the same time, there would be extra corrosion from that part. Because these part has a much higher energy due to a much higher dislocation content, because this has deformed locally. So, this is because this is for the material, where it is non film forming material, this is non film forming material. So, how the cavitation damage happens in case of non film forming material? Now, let us check, why there is a formation of bubble, when it goes through low pressure zone. And why there is a bursting of or imposing of or this is formation of bubble. And this is the bursting of bubble or the collapse of the bubble, why there is the formation of bubble, and why there is collapse of bubble. When it is at low pressure zone the formation happens, and high pressure zone collapse happens. Now, for that let us say, if the system is handling water or let us say in section centrifugal pump. In case of end section, that case we have the pump is basically nothing but like this. So, you have this is the pump part. So, here we have a small section, which is called eye of the pump. And this is basically the facing the pump now, actually if you see the cross section then it looks like this. So, actually it will look like this. So, you have eye portion here, and the impellers or inside. So, impellers are basically like this, these are basically the impellers, these are the impellers. So, the impellers are rotating like this. And the water is basically suck through this eye portion, and due to this centrifugal action, due to the movement of this impeller, the water goes to this part. Now, we will always see that the bubble formation would be in this zone, the back of the impeller pump impeller blades. So, if we have the impeller like this, we let us say the impeller part, and if it is moving this way. So, the bubble would be always forming on the back end of the blades. This is observation, and this is the section zone, and this is discharge zone. Now, why the bubble is forming on the back of this impeller blade, for that we need to take help of phase diagram what are phase diagram. Now, if you see the water phase diagram. Water phase diagram shows like this, and this is temperature; this is pressure, and here we have the triple point, and the triple point. This is let us say, atmosphere, and the triple point water pressure is up the order of 0.006 atmospheres. And the temperature is 0.01 degree Celsius, and the triple point is the point where liquid, vapor, and solid. All 3 phase cohesive phases coexist. Now, if you see the phase diagram, it looks like this. This is liquid, this is vapor, then you have vapor, and if you see this part, it is like this. So, this section we have solid. So, the here we have a liquid phase, and this is H 2 o or water phase diagram, and this axes is pressure, and this axes is temperature. Now, these lines indicate the equilibrium between 2 phases, liquid and vapor at different temperature and pressure. Similarly, this line indicates equilibrium between solid and liquid. And this line is basically the equilibrium between solid and vapor, and from the definition of melting point of water as well as boiling point of water. We know that we have to see the temperature corresponding to 1 atmosphere. So, this is my 0 degree Celsius. And this is my 100 degree Celsius which corresponds to 1 atmosphere pressure. Now, if the transmission is like this, that is solid to liquid, we call it melting. And if it is liquid to solid, then it is freezing. Similarly, if liquid to vapor, we call it vaporization. And if it is vapor to liquid that time it is called condensation. Now, similarly this part, if it goes from solid to vapor, that time it is called sublimation. And if the vapor goes from solid, that time it is called deposition And, let us say, if we are at this point, that time vapor phase would exist. And if the solid is taken from this to this, then all solid would become vapor. So, here sublimation would happen similarly, if we take a vapor and take it to this level. We can make it solid similarly, if I am here, and if I take the vapor to this level without change in the temperature. Here I am without changing the pressure I am just changing the temperature. I can make the vapor to solid or I can make the solid to vapor. Similarly I can make vapor to solid by changing the pressure, without changing the temperature. So, I can have this sort of phase transformation, if I know those positions of those lines, and if I know the pressure temperature information for water system. Now, since in case of corrosion cavitation damage, we are handling water base solution. Now, there let us say I am here and. So, let us say I am at this point on this line at this point, which are 1 atmosphere pressure and the temperature. Let us say its 50 degree Celsius. So, my common observation would be at 1 atmosphere pressure. If my temperature is 50 degree I will not have any vaporization, since I am in the liquid zone now, somehow if we can reduce the pressure. So, I can take it to this level, once I reach to this line. And these lines indicate the equilibrium between vapors, and liquid I will start forming bubbles at this point. If I reduce the pressure at the same temperature which is 50 degree I can achieve to this point. And then where I can form vapor. And once vapor forms that will form boil that will form bubbles in the system. And actually you are reducing the boiling point to a much lower temperature by reducing the pressure. So, if we have a system like this, where I have liquid, and let us say the pressure is 1 atmosphere. So, that time the liquid, and if the temperature is 50 degree I will not have any bubbling. But if I change the pressure to this level, let us say this level is around depending on this phase diagram. If this pressure is p dash, if I reach this pressure to p dash, which is much lower pressure compare to 1 atmosphere I will see that there is bubble formation fine. So; that means, I will have boiling at much lower temperature, and that is inherent with respect to H 2 O system, since it follows phase diagram with respect to pressure and temperature. So, I will see boiling at much lower temperature, if I would lower down I have 5 decrease the pressure. This boiling off course, we can also achieve, if I increase the temperature at 1 atmosphere pressure. So, this case without changing this pressure, if I take the temperature to 100 degree Celsius; I will see that again the bubble is forming. So, keeping pressures and increasing temperature I can form bubble or keeping temperature same. If I decrease the pressure I can form bubble. And decreasing pressure and forming bubble at the operational temperature that is basically the culprit for this cavitation damage. Actually, we decrease the boiling temperature by reducing the pressure and that is why bubble forms. Now, let us say the situation becomes like this, if the situations becomes like this. Let us say this is my system, and this is my peas turn. Now, here the pressure is 1 atmosphere, temperature is 50 degree Celsius. And during at then this is liquid H 2 O. I will not have any bubble formation, but if I start decreasing the pressure. I will start seeing the bubble formation, and this will also try to go out. So, this position would change. So, from this position, it will go to this position, because I am reducing the pressure. So, we have this bubble will go up, and it will be stored here. And this is basically vapor and this is liquid. And in the liquid, we have all the bubble. And the pressure has gone down to p dash which is less than 1 atmosphere. And this p dash corresponds to this point. Now, if we start increasing the pressure. So, from p dash, if I start going back to 1 atmosphere pressure, then what would happen? This pressure seems are increasing the pressure, when I pull it out I decreasing the pressure in case piston arrangement. And if I increase the pressure by pushing the pressure the piston in side, that time again I will come from this position to this point. So, this section will go up there will be no this section. Since, initially we have started with this part only which is the piston lower part is touching the liquid surface. So, that point, if I go back to 1 atmosphere at 50 degree I will reach to this level. And when I reach to this level, what is the system? That system is the phases simply liquid without vapor. Now, that time since I am increasing the pressure, what would happen to those bubbles? Let us say those bubbles are also sticking to the walls, what would happen to those bubbles? Those bubbles try to collapse. So, there would be bubble collapse due to increase in pressure, and this is happening due to this phase diagram concept. So, when I decrease pressure at a particular operation temperature bubble will form. If the condition satisfies; that means, if the condition with respect to this phase diagram satisfies now, again at that temperature, where the operation is going on? So, that point if the pressure again increases, those bubble will bubbles will collapse. And when the bubble collapse there would be huge pressure exerted on the surface. Now why that happens? Let us say I have a balloon like this, and this balloon is resting on the flat surface. Now, if I start putting some weight here. Let us say this is W 1, this will be compressed. So, this will become like this. So, if I start increasing the weight, let us say I put another W 1 load. So, actually the total load becomes W 1 plus W 2. So, that time there is a possibility of bursting of that balloon. And if the balloon burst, what would happen to those loads? Those loads suddenly will fall on the surface. And there would be sudden pressure on the surface, same thing happens. Let us say, if you have a bubble like this. And then the water is around the bubble, and this bubble will collapse, the water column will suddenly fall on those metal surfaces. And there would be huge shock wave creation on this metal surface, when these bubbles collapse. So, water column is coming and heating this surface. And this there would be creation of shock wave. And that shock wave would be so severe there could be local deformation. And that deformation can lead to cracking of passive film or it can lead to local deformation. And increase in dislocation density, and increase in corrosion dissolution rate. So, this is a sort of relation between phase diagram, bubble formation, bubble cracking and introduction of shock wave and which lid to the cavitation damage.