Localized and uniform corrosion has always been a problem, especially in very aggressive environments. Titanium has shown a great corrosion resistance due to the spontaneous formation of a stable TiO2 oxide layer whereas oxygen is present, besides great lightness and strength characteristics. As a drawback the price is higher than other possible options, like stainless steel, and it can further rise if it undergoes particular treatments, like noble metals alloying, able to improve corrosion resistance in prohibitive environments for normal titanium (i.e. Ti grade 7 with 0.25% Palladium). Several techniques have been discovered, like Anodizing Spark Deposition (ASD), which consists of a high voltage application to the sample immersed in acid environments for a certain time. Voltage manages to break down the oxide dielectric formed previously and to ionized valence electrons of Ti-O bond, rising temperature up to 7000K for little time, generating a ceramic and porous layer. Using programmable power supplies, it is possible to choose arbitrarily potential/current input into the sample. This allows to work with either direct current (DC) regime, or alternative (AC) monopolar anodic current regime and different frequencies, or alternative bipolar anodic/cathodic current regime. This treatment may lead to varied morphology, structure and corrosion characteristics. Five samples made of commercially pure (c.p.) titanium grade 2 were treated at 160 V for 320 seconds time: the first one is obtained in DC, the second and third in AC anodic regime 25% of the time, respectively at 20Hz and 1000Hz; the fourth and the fifth in AC anodic/cathodic regime in equal time (25% - 25%), using a cathodic peak of 5%, frequencies are respectively 20Hz and 1000Hz. They were morphologically analysed thanks to FIB/TEM electron microscopes, crystallinity surveys have been carried out with diffraction patterns and EELS-XRD acquisition got recorded for phase distribution studies. Corrosion resistance has been evaluated in two different aspects: uniform corrosion evaluation in acid ambient (10% v/v H2SO4 at 60°C) for 24h; localized corrosion assessment using potentiodynamic test in 0.5 M NaBr environment at 50°C.
La corrosione localizzata e uniforme è sempre stata un problema, soprattutto in ambienti altamente aggressivi per i metalli. Il titanio presenta un’ottima resistenza a corrosione dovuta alla formazione spontanea di un ossido molto stabile nelle zone a contatto con ossigeno (TiO2), oltre che ottime caratteristiche come leggerezza e forza. Purtroppo il prezzo è molto più alto rispetto alle altre opzioni disponibili, come acciaio inossidabile, ed aumenta ulteriormente se sottoposto a trattamenti particolari: aggiunta di metalli nobili in grado di aumentare la resistenza a corrosione in maniera notevole in ambienti proibitivi per titanio non trattato (Ti grado 7 con 0.25% di Palladio). Diverse tecniche sono state sviluppate, tra cui Anodizing Spark Deposition (ASD), che consiste nell’applicare un alto voltaggio al campione all’interno di ambienti acidi per un determinato tempo. Il voltaggio riesce a rompere il dielettrico fornito dallo strato di ossido formato in precedenza e a ionizzare gli elettroni di valenza del legame Ti-O, aumentando la temperatura fino a 7000K, creando uno strato ceramico e poroso. Attraverso l’uso di generatori di voltaggio ad onda programmabile è possibile gestire l’entrata di potenziale/corrente nel campione arbitrariamente. Questo permette di lavorare su diverse frequenze e in regime di corrente diretta, oppure in regime di corrente alternata monopolare anodica o bipolare anodica/catodica. Questi cambiamenti possono alterare il campione nella sua morfologia, struttura e caratteristiche alla corrosione. Cinque campioni di titanio commercialmente puro (grado 2) sono stati trattati a 160V e 320 secondi di permanenza: il primo in DC, il secondo e terzo in regime anodico al 25% del tempo, rispettivamente a 20Hz e 1000Hz, mentre il quarto e il quinto in regime anodico e catodico in egual misura (25%-25%), con un picco catodico del 5%, frequenze di 20Hz e 1000Hz. Sono stati morfologicamente analizzati tramite microscopi elettronici FIB/TEM, effettuate diffraction patterns per la cristallinità, eseguiti test EELS con TEM Titan e XRD per la distribuzione delle diverse fasi. La resistenza a corrosione è stata testata in due diverse tipologie: ambiente acido (10% v/v H2SO4 a 60°C) durata 24h per resistenza a corrosione uniforme, test potenzio-dinamico in ambiente 0.5 M NaBr a 50°C per corrosione localizzata.
Oxide layer characterization and corrosion resistance assessment of titanium after anodic spark deposition
AROSIO, MATTIA
2018/2019
Abstract
Localized and uniform corrosion has always been a problem, especially in very aggressive environments. Titanium has shown a great corrosion resistance due to the spontaneous formation of a stable TiO2 oxide layer whereas oxygen is present, besides great lightness and strength characteristics. As a drawback the price is higher than other possible options, like stainless steel, and it can further rise if it undergoes particular treatments, like noble metals alloying, able to improve corrosion resistance in prohibitive environments for normal titanium (i.e. Ti grade 7 with 0.25% Palladium). Several techniques have been discovered, like Anodizing Spark Deposition (ASD), which consists of a high voltage application to the sample immersed in acid environments for a certain time. Voltage manages to break down the oxide dielectric formed previously and to ionized valence electrons of Ti-O bond, rising temperature up to 7000K for little time, generating a ceramic and porous layer. Using programmable power supplies, it is possible to choose arbitrarily potential/current input into the sample. This allows to work with either direct current (DC) regime, or alternative (AC) monopolar anodic current regime and different frequencies, or alternative bipolar anodic/cathodic current regime. This treatment may lead to varied morphology, structure and corrosion characteristics. Five samples made of commercially pure (c.p.) titanium grade 2 were treated at 160 V for 320 seconds time: the first one is obtained in DC, the second and third in AC anodic regime 25% of the time, respectively at 20Hz and 1000Hz; the fourth and the fifth in AC anodic/cathodic regime in equal time (25% - 25%), using a cathodic peak of 5%, frequencies are respectively 20Hz and 1000Hz. They were morphologically analysed thanks to FIB/TEM electron microscopes, crystallinity surveys have been carried out with diffraction patterns and EELS-XRD acquisition got recorded for phase distribution studies. Corrosion resistance has been evaluated in two different aspects: uniform corrosion evaluation in acid ambient (10% v/v H2SO4 at 60°C) for 24h; localized corrosion assessment using potentiodynamic test in 0.5 M NaBr environment at 50°C.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/151368