In the last few years, ultrafine metals, with a crystalline grain below the micron, have gained particular interest due to their high mechanical strength combined with adequate ductility. Through severe plastic deformation techniques (Severe Plastic Deformation, SPD) it is possible to obtain ultra-fine grained materials. Among these, the technique of the Equal Channel Angular Pressing (ECAP) is very widespread because it allows an ultrafine structure throughout the volume of the material, which retains the same cross-section after pressing and its free from any residual porosity. Furthermore, ECAP offers the possibility to extend this process even to an industrial level. Aluminum alloys are widely used for different technological and industrial applications, due to their lightness, good mechanical properties and their low cost. On the one hand, in the literature, the mechanical properties of the materials processed by severe plastic deformation techniques have been widely discussed, on the other hand many uncertainties and controversial results remain on the corrosion behavior of these materials. In fact, these processes that lead to grain refinement also cause other metallurgical and microstructural changes: precipitation of the phases, distribution of impurities, surface finishing, etc. As a result, the corrosion behavior of the materials that have undergone these processes can be influenced by these changes. The works present in the literature in this area are also contradictory, even those related to the same alloy, because these changes occur in a complex and different way. In this research the corrosion behavior of two aluminum alloys, AA6012 and AA5083, subjected to the ECAP process was studied. In particular, the influence on the corrosion behavior of aluminum alloys by ECAP and cryogenic and thermal treatments carried out before or after severe plastic deformation was evaluated. The analysis of the corrosion resistance of aluminum alloys was carried out at room temperature by means of electrochemical characterization in aqueous solutions at the same chloride concentration (0.1 M Cl-), but at different pH (pH 2 and pH 6.5). For this purpose, cyclic potentiodynamic polarization, Tafel curves, polarization resistance and impedance spectroscopy electrochemical tests were performed. The experimental results show that the ECAP process deteriorates the corrosion behavior of the alloys in acid environment to a small extent, result that is clear before the aging of the alloy and in short exposure tests. In the neutral environment, on the other hand, neither the ECAP process nor the thermal treatments and the cryogenic treatment have influenced the corrosion resistance of the alloys. The cryogenic treatment, carried out before ECAP, allows a recovery of corrosion resistance of AA6012 alloy only before natural or artificial aging.
Negli ultimi anni i metalli ultrafini, ossia con un grano cristallino al di sotto del micron, hanno destato molto interesse per la loro elevata resistenza meccanica abbinata a una adeguata duttilità. Attraverso tecniche di deformazione plastica severa (Severe Plastic Deformation, SPD) è possibile ottenere dei materiali metallici ultrafini. Tra queste, la tecnica dell’Equal Channel Angular Pressing (ECAP) è molto diffusa perché permette di ottenere una struttura ultrafine in tutto il volume del materiale, il quale mantiene la stessa sezione trasversale dopo pressatura ed è privo di ogni porosità residua. Inoltre, l’ECAP offre la possibilità di estendere questo processo anche a un livello industriale. Le leghe di alluminio sono ampiamente utilizzate per differenti applicazioni tecnologiche e industriali, per la loro leggerezza, le buone proprietà meccaniche e il loro basso costo. Se da un lato, in letteratura, le proprietà meccaniche dei materiali processati attraverso le tecniche di deformazione plastica severa sono state ampiamente discusse, dall’altro rimangono molte incertezze e risultati controversi sul comportamento a corrosione di questi materiali. Infatti, questi processi che portano a un affinamento dei grani provocano anche altri cambiamenti a livello metallurgico e microstrutturale: precipitazione delle fasi, distribuzione delle impurità, la finitura superficiale ecc. Di conseguenza, il comportamento a corrosione dei materiali che hanno subito questi processi può essere influenzato da questi cambiamenti. I lavori presenti in letteratura in questo ambito sono anche contrastanti, persino quelli relativi alla stessa lega, perché questi cambiamenti avvengono in maniera complessa e talvolta differente. In questa ricerca è stato studiato il comportamento a corrosione di due leghe di alluminio, la AA6012 e la AA5083, sottoposte al processo ECAP. In particolare, si è valutata l’influenza sul comportamento a corrosione di queste leghe da parte dell’ECAP e dei trattamenti criogenici e termici effettuati prima o dopo della deformazione plastica severa. L’analisi della resistenza a corrosione delle leghe di alluminio è stata effettuata a temperatura ambiente tramite una caratterizzazione elettrochimica in soluzioni acquose alla stessa concentrazione di cloruri (0,1 M Cl-), ma a differenti pH (pH 2 e pH 6,5). A questo scopo, sono state effettuate prove elettrochimiche di polarizzazione potenziodinamica ciclica, curve di Tafel, misure di resistenza alla polarizzazione e impedenza elettrochimica. Dai risultati ottenuti è emerso che il processo ECAP peggiora in piccola misura il comportamento a corrosione delle leghe in ambiente acido, risultato evidente prima dell’invecchiamento della lega e nei test di breve esposizione. In ambiente neutro, d’altra parte, né il processo ECAP né i trattamenti termici e il trattamento criogenico hanno influenzato la resistenza a corrosione delle leghe. Il trattamento criogenico, effettuato prima dell’ECAP, influenza il comportamento a corrosione della lega AA6012, solo prima di invecchiamento naturale o artificiale, permettendo un recupero della resistenza a corrosione rispetto a effettuare il solo processo ECAP.
Studio della resistenza a corrosione di leghe di alluminio sottoposte a deformazione plastica severa / Vicere', Annamaria. - (2020 Mar 13).
Studio della resistenza a corrosione di leghe di alluminio sottoposte a deformazione plastica severa
VICERE', ANNAMARIA
2020-03-13
Abstract
In the last few years, ultrafine metals, with a crystalline grain below the micron, have gained particular interest due to their high mechanical strength combined with adequate ductility. Through severe plastic deformation techniques (Severe Plastic Deformation, SPD) it is possible to obtain ultra-fine grained materials. Among these, the technique of the Equal Channel Angular Pressing (ECAP) is very widespread because it allows an ultrafine structure throughout the volume of the material, which retains the same cross-section after pressing and its free from any residual porosity. Furthermore, ECAP offers the possibility to extend this process even to an industrial level. Aluminum alloys are widely used for different technological and industrial applications, due to their lightness, good mechanical properties and their low cost. On the one hand, in the literature, the mechanical properties of the materials processed by severe plastic deformation techniques have been widely discussed, on the other hand many uncertainties and controversial results remain on the corrosion behavior of these materials. In fact, these processes that lead to grain refinement also cause other metallurgical and microstructural changes: precipitation of the phases, distribution of impurities, surface finishing, etc. As a result, the corrosion behavior of the materials that have undergone these processes can be influenced by these changes. The works present in the literature in this area are also contradictory, even those related to the same alloy, because these changes occur in a complex and different way. In this research the corrosion behavior of two aluminum alloys, AA6012 and AA5083, subjected to the ECAP process was studied. In particular, the influence on the corrosion behavior of aluminum alloys by ECAP and cryogenic and thermal treatments carried out before or after severe plastic deformation was evaluated. The analysis of the corrosion resistance of aluminum alloys was carried out at room temperature by means of electrochemical characterization in aqueous solutions at the same chloride concentration (0.1 M Cl-), but at different pH (pH 2 and pH 6.5). For this purpose, cyclic potentiodynamic polarization, Tafel curves, polarization resistance and impedance spectroscopy electrochemical tests were performed. The experimental results show that the ECAP process deteriorates the corrosion behavior of the alloys in acid environment to a small extent, result that is clear before the aging of the alloy and in short exposure tests. In the neutral environment, on the other hand, neither the ECAP process nor the thermal treatments and the cryogenic treatment have influenced the corrosion resistance of the alloys. The cryogenic treatment, carried out before ECAP, allows a recovery of corrosion resistance of AA6012 alloy only before natural or artificial aging.File | Dimensione | Formato | |
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