Different sealing processes for a plasma electrolytic oxidation coating (PEO) on AA6082

Aluminum alloys of the 6xxx family (Al-Mg-Si) are famous for their lightweight and other good properties such as mechanical, thermal, and electrical properties, as well as corrosion resistance. In almost all neutral environments, they spontaneously generate a passive, stable oxide film that protects the underlying metal from subsequent severe corrosion phenomena. However, this film is thin (a few nanometers thick) and insufficient to ensure passivity in the various fields of aluminum application, especially if the environment is alkaline, acidic, or containing aggressive species. Therefore, electrochemical treatments are carried out to create a thicker oxide layer, well adherent to the surface, capable of resisting attacks from aggressive species. Among these, there is Plasma Electrolytic Oxidation (PEO), an emerging plasma-based process that allows the creation of a thick oxide layer using high electric potentials and environmentally friendly alkaline electrolytes - free from harmful substances for humans and the environment. The structure of the oxide created through PEO is notably porous, due to the plasma discharges that occur during the process along with the liberation of gases. Therefore, it is necessary to seal those pores to hinder the passage of corrosive species. Some of the sealants could have two-fold effects, not only physically blocking the pores, but also providing inhibiting phenomenon for active corrosion protection. This work aims to fine-tune the electrical parameters for obtaining a thicker oxide layer and to compare some immersion sealing methods performed on AA6082 substrates treated with PEO. Mainly cerium nitrate and in-situ synthesis of LDH (Layered Double Hydroxide) structure sealings were performed. The analysis of each sample is based on characterization tests to study the morphology, composition, and elemental distribution of the oxide before and after sealing, through SEM, EDS, and XRD maps. Electrochemical tests are conducted to evaluate susceptibility to corrosion phenomena, stability, and integrity of the treated oxide in aggressive environments, through Electrochemical Impedance Spectroscopy (EIS), Linear Polarization Resistance (LPR), and Potentiodynamic polarization tests (PDP). Both sealing processes showed much greater corrosion protection than the bare PEO coating. Their protective behavior is similar since both LDH and cerium nitrate sealings exhibited polarization resistance (R_p) and corrosion current density (i_corr) in the same order of magnitude. Both R_p and i_corr are three orders of magnitude higher and lower respectively compared to the PEO base. Keywords: aluminum alloy, PEO, corrosion resistance, cerium, LDH

Different Sealing Processes for a Plasma Electrolytic Oxidation (PEO) Coating on AA6082 TESI DI LAUREA MAGISTRALE IN MATERIALS ENGINEERING AND NANOTECHNOLOGY INGEGNERIA DEI MATERIALI E DELLE NANOTECNOLOGIE Author: Muhammad Ibrahim Adam Ahmed Student ID: 10817658 Advisor: Prof. Marco Ormellese Co-advisor: Dr. Federica Ceriani Academic Year: 2023-2024   Abstract Aluminum alloys of the 6xxx family (Al-Mg-Si) are famous for their lightweight and other good properties such as mechanical, thermal, and electrical properties, as well as corrosion resistance. In almost all neutral environments, they spontaneously generate a passive, stable oxide film that protects the underlying metal from subsequent severe corrosion phenomena. However, this film is thin (a few nanometers thick) and insufficient to ensure passivity in the various fields of aluminum application, especially if the environment is alkaline, acidic, or containing aggressive species. Therefore, electrochemical treatments are carried out to create a thicker oxide layer, well adherent to the surface, capable of resisting attacks from aggressive species. Among these, there is Plasma Electrolytic Oxidation (PEO), an emerging plasma-based process that allows the creation of a thick oxide layer using high electric potentials and environmentally friendly alkaline electrolytes - free from harmful substances for humans and the environment. The structure of the oxide created through PEO is notably porous, due to the plasma discharges that occur during the process along with the liberation of gases. Therefore, it is necessary to seal those pores to hinder the passage of corrosive species. Some of the sealants could have two-fold effects, not only physically blocking the pores, but also providing inhibiting phenomenon for active corrosion protection. This work aims to fine-tune the electrical parameters for obtaining a thicker oxide layer and to compare some immersion sealing methods performed on AA6082 substrates treated with PEO. Mainly cerium nitrate and in-situ synthesis of LDH (Layered Double Hydroxide) structure sealings were performed. The analysis of each sample is based on characterization tests to study the morphology, composition, and elemental distribution of the oxide before and after sealing, through SEM, EDS, and XRD maps. Electrochemical tests are conducted to evaluate susceptibility to corrosion phenomena, stability, and integrity of the treated oxide in aggressive environments, through Electrochemical Impedance Spectroscopy (EIS), Linear Polarization Resistance (LPR), and Potentiodynamic polarization tests (PDP). Both sealing processes showed much greater corrosion protection than the bare PEO coating. Their protective behavior is similar since both LDH and cerium nitrate sealings exhibited polarization resistance (R_p) and corrosion current density (i_corr) in the same order of magnitude. Both R_p and i_corr are three orders of magnitude higher and lower respectively compared to the PEO base. Keywords: aluminum alloy, PEO, corrosion resistance, cerium, LDH Abstract in Italiano Le leghe di alluminio della famiglia 6xxx (Al-Mg-Si) sono famose per la loro leggerezza e altre buone proprietà come proprietà meccaniche, termiche ed elettriche, nonché resistenza alla corrosione. In quasi tutti gli ambienti neutri, generano spontaneamente un film di ossido passivo e stabile che protegge il metallo sottostante da successivi gravi fenomeni di corrosione. Tuttavia, questo film è sottile (pochi nanometri di spessore) e insufficiente per garantire la passività nei vari campi di applicazione dell'alluminio, soprattutto se l'ambiente è alcalino, acido o contenente specie aggressive. Pertanto, vengono eseguiti trattamenti elettrochimici per creare uno strato di ossido più spesso, ben aderente alla superficie, in grado di resistere agli attacchi di specie aggressive. Tra questi, c'è l'ossidazione elettrolitica al plasma (PEO), un processo emergente basato sul plasma che consente la creazione di uno spesso strato di ossido utilizzando alti potenziali elettrici ed elettroliti alcalini ecologici, privi di sostanze nocive per l'uomo e l'ambiente. La struttura dell'ossido creato tramite PEO è notevolmente porosa, a causa delle scariche di plasma che si verificano durante il processo insieme alla liberazione di gas. Pertanto, è necessario sigillare quei pori per ostacolare il passaggio di specie corrosive. Alcuni dei sigillanti potrebbero avere effetti doppi, non solo bloccando fisicamente i pori, ma anche fornendo fenomeni inibitori per una protezione attiva dalla corrosione. Questo lavoro mira a mettere a punto i parametri elettrici per ottenere uno strato di ossido più spesso e a confrontare alcuni metodi di sigillatura a immersione eseguiti su substrati AA6082 trattati con PEO. Sono stati eseguiti principalmente nitrato di cerio e sintesi in situ di sigillature di strutture LDH (Layered Double Hydroxide). L'analisi di ciascun campione si basa su test di caratterizzazione per studiare la morfologia, la composizione e la distribuzione elementare dell'ossido prima e dopo la sigillatura, tramite mappe SEM, EDS e XRD. I test elettrochimici vengono condotti per valutare la suscettibilità ai fenomeni di corrosione, la stabilità e l'integrità dell'ossido trattato in ambienti aggressivi, tramite spettroscopia di impedenza elettrochimica (EIS), resistenza alla polarizzazione lineare (LPR) e test di polarizzazione potenziodinamica (PDP). Entrambi i processi di sigillatura hanno mostrato una protezione dalla corrosione molto maggiore rispetto al rivestimento PEO nudo. Il loro comportamento protettivo è simile poiché sia le sigillature LDH che quelle in nitrato di cerio hanno mostrato resistenza alla polarizzazione (R_p) e densità di corrente di corrosione (i_corr) nello stesso ordine di grandezza. Sia R_p che i_corr sono rispettivamente tre ordini di grandezza superiori e inferiori rispetto alla base PEO. Parole chiave: lega di alluminio, PEO, resistenza alla corrosione, cerio, LDH