1 The Problemand its setting1.1 General background andimportance of the studyIn today’s world the degradation of metal hasbecome a major problem, it occurs due to hot corrosion and erosion. Thecomponents of incinerators, turbines, etcetera are made up of metal which getsdegraded as a result of electrochemical reactions. The main damage to thesemetal components of boilers, diesel engine, turbines etcetera is due to theoxidation process in which an oxide layer is formed over the surface of themetals as the metal surface comes in contact along with some salts along withoxidizing gas at higher temperature. A faster type of oxidation is the hotcorrosion which occurs at higher temperatures when salt contaminants like vanadiumcompounds and sulphates combine to form deposit of molten. This corrosion hasresulted in loss of billions of dollars to many countries like USA, Canada,etcetera. Each and every type of metal is subject to corrosion at higherdegrees of temperature which can result in heavy losses to environment andhuman life.
There are various methods to prevent corrosion like fuel cleanliness,washing of hot components, coating etcetera. In this research we are going to discuss a type of thermal spray coatingwhich is HVOF coating in which is the latest technology that makes use of the oxygenand fuels( Hydrogen, propane) at higher pressure. Basically in this processoxygen and fuels are repeatedly pushed into combustion chamber which givescombustion products at high velocity and powder particles are introduced in thegas stream get accelerated to fast speed. This coating is wear resistant andvery dense. The velocity at which the powder is sprayed is near about 560m/secand mostly all metals and some other ceramic materials are coated with thistechnique. This technique is much efficient and cheaper than other techniques. HVOFcoating technique is used in wide variety of applications in hydro powerplants, diesel engine, doctor blades and many more.
1.2 Thestatement of the problem and its sub problemsThis research proposal will investigate and performexperiment that which material either Cr3C2-25(Ni-20Cr) or Ni-20Cr ismore efficient in saving ASTM-SAE213-T22and ASTM-SAE213-T91boiler tube. In this basically the powder is put into thenozzle which is then heated and speed up finally it results in high speed of560m/s which is then coated on the workpiece which saves the workpiece fromcorrosion.Afterthat we would compare the efficiency of both the materials individually on bothboiler tubes in oxidation and salt environment.Problem and sub problem The problem can be classified onto 2 sub problems whichare:-1. Comparison of the efficiency of Cr3C2-25(Ni-20Cr)and Ni-20Cr on ASTM-SAE213-T22in oxidation and salt environment.
2. Comparison of the efficiency of Cr3C2-25(Ni-20Cr)and Ni-20Cr on ASTM-SAE213-T91in oxidation and sat environment.Therefore the research question can be framed as :-Which coating either Cr3C2-25(Ni-20Cr) or Ni-20Cris more efficient in saving ASTM-SAE213-T22and ASTM-SAE213-T91boiler tube steel from hot corrosion at 950 degree celcius? 1.3 Thelimitations 1. HVOFcoating equipment is extremely costly. 2. For HVOIFcoating an experienced technician is required. 3.
This HVOFtechnique is impossible to coat from inside of the component. 4. Thismethod takes more time for deposition of coating than other spraying methods.
1.4 Theassumptions1. The temperature of the furnace should be kept constant.2. The coating which is to performed in oxidation and saltenvironment is to be done in ideal conditions.
2 The review of the literatureH. Sidhuet. al.
(2007) did a thorough study on corrosion and he concluded that the mainreason for corrosion is sulphur gas which is there in fuel and gas along withmetal surface. Ni-Cr, WC-CO, stalite-6 alloys were preferred for coatinghowever Cr2O3was utilised as substrate material. HVOFcoating of 30µm was used at 900?C for 50 cycles in the molten salt (Na2So4-V2O5)environment the XRD shows formation of oxides of Fe, Cr and Ni.
The efficiencyof Cr2C3-NiCr is best than NiCr and EDAX displays thatNiO was formed in higher proportion.H. Singh et. Al.
(2005) revealed that due to acceleratedoxidation occurs in metal and alloys when they come in contact with fuse salt.In this case the coating techniques involved was plasma spray coating and thesubstrate material used were Ni-Cr, NiVr-AIY, Ni3Al and stallite 6.The coating was porous up to 2-4 %. XRD gives results that oxides of Fe, Cr, Niand Al were made however EDAX gives information that that Cr2O3,NiO and M2O5 and stallite 6 show high quality of corrosion resistance whereasNiCr-AlY is the best against corrosion.
H. Sidhu et. al. (2006) said that Ni-Cr alloys were utilisedfor coating as they reveal high amount of hardness.
ASTM-SAE213-T22and ASTM-SAE210 were used as material to be coated according to HVOF technique.The result of the rigidity was in scale of 200-280 HV and 1.5-3% was itsporosity.XRD displayerd the development of W2C and Cr3C2.The WC-CO coating’s rigidity was more than Cr3C2-NiCrwhereas it was opposite in case of porosity.
M.Ramesh et. al. 2013 made conclusions that Ti alloys have high strength toweight ratio, raiused temperature resistance and weariness resistance so because of these properties they can beused at higher temperatures.T31 was preferred for substrate use whereas Al2O2+COCrAlTaYwas a choice for coating material in the environment of salt of Na2SO4V2O5at 900?C for 50 cycles.In this coating utilized was HVOF and the weight gain act according to theparabolic relation with time. XRD displayed the inference that on substratematerial surface of T31 Ti oxides were formed however on HVOF surface thedevelopment of oxides of CO, Cr, Al started.
Finally he rate of corrosion decreasesas was there in uncoated part.V Sharmaet. al. 2015 explained that the efficiency of the waste incinerator,turbines and boilers should be high for good energy transfer but the efficiencyrelies upon high temperature at which oxidation and hot corrosion occurs so tomaintain the efficiency coating is important. In this the coating is done with thehelp of D-gun process in which the powder for coating is Ni-Cr whereas thesubstrate material utilised is supermi-750 for 100 cycles in the saltenvironment of Na2SO4V2O5 at 900?C. The main phase of the Niis the result given by XRD.
Fe, Ni, and Ni-Cr oxides were formed when barecompound was there as compared to coated alloys. In case of SN750 which is asuper alloy the corrosion rate was less.Sandeep et.
al. 2014 states that the occurrence of hot corrosion in metals is because thehot gases, salt, etcetera are there. At 800?C for 50 cycles when K2SO4NaCl isthere the substrate material was Inconal-625 whereas the coating materialutilised was Cr2O3.
The coated Incconal-625 resulted insmall amount of corrosion because coating of plasma was done however in case ofuncoated Inconal-625 caused in it the maximum of corrosion after thataioxidation. 300 µm was the thgickness which was foundlater. EDAX showed that Cr was mainly there in inner layer.T.Sidhuet.
al. 2006 studied the HVOFtechniques thorouhghly and he finally conclude that this technique is used invarious fields like petroleum, aircraft and many more for coating componentsand there can be no problem with this technique in harsh conditions. In his studyof salt environment of Na2SO4V2O5 in cyclic conditions at 900?C he used NiCr-BSi coating material and the subsrate materialwhich he took in his study was supermi 75, 600, 607, 718 and superfer 800h.This study concluded that NiCrBs coating was very much efficient as it reducedthe amount of corrosion also the coating thgickness was 250-300 µm moreover V2O5,TiO2, Al2O3 and MnO weredeveloped in little quantity.
The rate of porosity was in the range under 1.4%.R.
Bhatia et.al. 2014 revealed that HVOF technique is the techniques which has manyadvantages like it protects from corrosion, thick coating and has good amountof hardness, also it has fair amount of adhesion value.
In his study at 850?C of the environment of saltof Na2SO4V2O5coating of NiCr-Cr3C2 was applied and T91 was thechoice for substrate material. XRD displayed the inference that Cr and Nioxides were made whereas according to EDAX Fe2O3 wascreated. The amount of the porosity was under 2% whereas thickness was nearabout 390 µm. The main oxides that save the substrate from corrosion are Cr andNi oxides.J.
Picaset. al. 2005 deeplystudied that HVOF method is an important method to save the nanostructure hefurther continued his research and concluded that coating of Cr3NiCrcan refine wear resistance. The coating material preferred in this study was Cr2C3and Cr2C3 -7NiCr whereas simple steel was a choicefor substrate material. SEM confirms that CrC75-NiCr25 had large amount ofmicro hardness and was more protective than CrC60-4NiCr however the latter wasmore dense and homogeneous than the former.Hazoor S et.
al. 2008 revealed that in salty and gaseous conditions most of the metals andalloys disintegrate and get corroded. HVOF techniques was used for coating hecorroded part. Here the substrate material used was ASTM-SA210 whereas Ni-Cr, WC-CO,etcetera were availed as coating material. For 50 cycles at 900?Cfor Na2SO4-V2O5 300 µm coating was applied. XRDgives the results that Fe, Cr and Ni oxides were created. To conclude it wasobserved that C2rC3-NiCr was the better performer amongall.
The corrosion resistance of C2rC3-NiCr was greaterthan WC-CO whereas smaller than Ni-Cr.
