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Concept for Corrosion Resistance : sacrificial anode cathodic protection galvanic...

Concept for Corrosion Resistance : sacrificial anode cathodic protection galvanic corrosion , or. , ↳ Explanation for galvanized steel :En has a lower or more negative standard electrode potential than Fe in the · This results EMF series in En protecting the alloy by acting as the anode (corroding more favourably than the underlying Fe. Role of Chloride the initiation of stainless steels Chloride ions initiate stainless steels (Cr ions in pitting corrosion in pitting corrosion by breaking down the passive : in Oxide) localized in region a. Al due to : Al reactivity is is considered a reactive metal but has a large range of passivity on the Pourbaix diagram , which is the source of corrosion resistance. What crack problem?: A sharp crack. From the related equation (D) and decrease tip radius (v) causes the maximum long an increase in crack length a in the crack increases the maximum stress at the crack tip.. Ductile plastic deformation and deformation at v.S brittle notch sensitivity : In ductile materials significant energy is absorbed due to the the crack tip blunts the crack tip ; thus ductile materials are less sensitive to presence of a notch. of Length a middle crack : La Difference between high and low cycle fatigue : In high cycle fatigue the stress is below the yield stress where as in low cycle fatigue the stress is above the stress general yield. nickel plate: If diffusion rate of Why temperature increase is a problem in temperature were increased then the oxygen into the metal would increase. for force for Why cold work is required recrystalization to occur : Cold work provides the driving or internal strain energy necessary recrystallization to occur. after The effects of (dislocations) Why ductility increases recrystalization : ductility increases because removing or reversing the cold work will remove to dislocation motion barriers subsequent. force Driving during recovery : Internal energy strain or cold work. Role of dislocations heat treatment : For relief of internal strain of dislocations to during a recovery recovery , there is some energy by rearrangement lower energy configurations , and a small reduction in amount of dislocations. interstitial diffusion Why interstitial diffusion is more rapid than vacancy : Small atoms diffuse by , the spaces between atoms in a crystal lattice , rapid compared substitutional diffusion which requires atom which is to vacancy or , exchange with adjacent (and activation to and break. bonds) vacancies an energy exchange banite doesn't appear carbide phase Bainite Why on an iron-iron diagram : is a non-equilibrium or metastable phase. proeutectoid ferrite formed above Difference between entectoid and pro-entertoid ferrite in hypoeutectoid steel : For a hypoeutectoid steel , the is the entectoid temp below the entectoid temp. Upon cooling -, pearlite is formed , which contains entectoid ferrite as one of its constituents. Al resistant?: Al of (or formation) Why is considered corrosion is considered a reactive metal but has a large range passivity oxide on the Pourbaix diagram (potential and pH) which is the source of corrosion resistance. Corrosion based on DG : If DGL0 corrosion will occur , if AGCO corrosion will not. occur What material will have lower temp ?: The have lower temp the bond (EO) a metting shallow energy well will a melting - because energy is smaller. Cold work Four strengthening mechanisms : strengthening precipitation strengthening , , solute strengthening , and grain size. ↳ key objective : Make dislocation motion more difficult. Age hardening-yield strength increases then decreases : Material starts with solute atoms * - in solution. Precipitation (or nucleation initially small) particles be cut by dislocations. * particles - can - occurs , are Precipitation continues (or occurs) particles and better at growth get larger stopping dislocations. - , ↳ maximum strengthening > of far apart Particle growth continues smaller particles shrink at expense large particles particles - - now , , and dislocations can get past more easily. BCC ductile - - brittle transformation : BCC metals are not close packed they , do not have close packed planes (CPP). Dislocation metals in BCC need caused At not to by CPP this thermal to not energy move get over the barrier there being low temps energy is -.. available , hence the material becomes brittle , but at high temps. the BCC structure appears CDD , has more thermal and is ductile. energy ↳ FCC metals are close packed with CPP and (PD (direction). For cross-linked For plastic polymers have higher strength than linear : plasticity in polymers , we need sliding of chains past each other , and sliding is made more difficult by cross-linking (strong covalent bonds between chains) than if there just linear (weak VDW bonds between. chains) were polymers hydrogen or Effective elastomer low Elastomers work. Want them to be able to coiled chains uncoil when stressed and then coil = cross-linking : by having highly back let of the stress eachother up when you go. Without crosslinking the chains would slide past deformation when you apply stress , so you would get permanent , but you only want a small amount of the (too much would make. cross-linking so that uncoiling can still occur. a rigid polymer Polymer brittle at lower temp : Due to the glass transition temp. Polymers are flexible because of the rotation around the. This needs bond thermal At low temps insufficient thermal to allow the rotation chain energy.. there is energy , so polymer becomes rigid. metal (forms passive film/oxide) which resistance Based the Pourbaix the T reactive metal : Ti is a passive increases corrosion. on diagram , passive oxide (TiO2) forms over the entire pl range in water at 25% Role of Reaction of decrease (into Fe hydrolization in pitting corrosion : Fe with water produces protons within the pit which results in pl Fe soluble region). Noble metals water conditions (or require. oxide) highly oxidizing potential Noble metals corrosion resistance : are immune to oxidization in to pure a Gold-coated steel oxidization : The steel (Fe) will start locally in the scratched Based underlying to oxide region. on the EMF series · Au and Fe are difference this of Au likely to have a large potential means the corrosion Fe will be enhanced due to coupling with. - ↳ galvanic corrosion. Stainless Steel Corrosion Resistance : Due to formation of layer consisting of Cr-rich surface oxide. (Cr203) The layer/oxide passivity or a passive a passive barrier film that of the acts as a protects the alloy or the passive layer slows the corrosion rate alloy. Three factors for stress environment susceptible material, stress/mechanics cracking corrosion : corrosive. , for for Cold work required recrystalization : A minimum amount of cold work is required recrystalization because it provides internal strain force for energy (driving recrystalization to occur. force for Surface Driving grain growth : energy or grain boundary energy Yield stress Yield stress of metal will decrease to of during during grain growth boundaries which act due removal grain growth : a grain , as barriers to dislocation. movement C-shaped C-curve due to balance between the number of stable nuclei formed and the rate of diffusion diagram curve on TTT : is a for attachment of atoms from force-few stable fast allowing growth A high temp-low driving nuclei -. of attachment force diffusion rate-frequency increases. A low temp-large driving - many stable nuclei-slow diffusion rate. At the the diffusion rate and force kind of balance. nose driving are in a Martensite transformation : diffusion-less transformation and speed of sound upon (transformation does very rapid Martensite is is quenching a , not depend on time - therefore horizontal). Solute atoms Stress field : There stress field associated with presence of solute atoms If there will be a is a. they are larger than host , compression stress field for smaller atoms this would be tension field. The larger atom will in their vicinity and , a attract to tension stress fields of dislocations atoms attract field , and smaller to the compression stress. Moving dislocation from the solute required. away atoms will move the system to a higher energy state hence , a higher stress is & Recovery there is some relief of internal strain energy by dislocation motion however there are virtually no changes in grain structure. - , of strain-free forms Recrystalization A new set grains the microstructure - material ductile. during recrystalization mechanical characteristics becomes softer and > There is no change in during recovery while the more - , Primary instantaneous deformation (initial dislocations formed S - > Secondary - > Steady state creep rate - dynamic stability creep Strain rate Tertiary > rapid acceleration in creep rate-damage occurs - area If reheat and hold the 100 % material spheroidite force surface" by pearlite reducing the microstructure you produced will be. The driving is reduction in the - a the X-FegC interface boundary. The of martensite centered (bct). diffusionless crystal structure is body tetragonal Martensite transformation is which results in cooperative (slight) movement of of their This strain and the a lot atoms with respect to neighbors. in turn results in shear crystal structure will be elongated along one of it's dimensions.