Titanium is a metal with unique characteristics, thanks to the combination of high mechanical properties together with incredible resistance to corrosion. For these reasons, researchers are attracted to its use in different types of industry, especially those that have aggressive environments, such as the petrochemical sector, or where applications require a high strength-to-weight ratio, for example in the Aerospace field. The first chapter of this thesis begins with an overview on titanium, from its discovery to its pure refining, to continue on its physical characteristics and properties. The latter in particular are linked to commercial classification. The first section ends with an in-depth study of the titanium market in relation to its applications. Despite the anticorrosive properties are of an exceptional nature, by virtue of its natural oxide film, titanium suffered in particular environments. A second paragraph, therefore, deals in detail with the forms of corrosion (uniform, localized and environment-induced cracking) and the chemical agents that attack titanium. Finally, the chapter concludes with the treatments that are applied to improve corrosion resistance: chemical and thermal oxidation, ion implantation and grain refinement, along with the various elements with which titanium can be alloyed to improve its mechanical characteristics. Finally, a review of the anodizing process, which allows the development of the titanium dioxide film to be controlled, is reported. The treatment can be traditional or by spark deposition (ASD), also called plasma electrolyte oxidation (PEO). The advantage of these techniques lies in the low cost and ease of implementation. Using a programmable power supply it is possible to control process parameters such as duty cycle, frequency and voltage. In this way it is possible to perform PEO with a monopolar pulsed regime, allowing to obtain the desired morphological and structural changes of the oxide layer and an interesting level of energy saving with respect to the DC approach. The key point of this work is the introduction of two new parameters which are the phase of cathodic polarization, replacing the rest time in the monopolar, and the intensity of this phase, expressed in cathode peak. The PEO bipolar process is thus defined. In Chapter 2, the experimental methodology used is treated in detail: the preparation and anodization of the samples, the latter performed at a controlled temperature in a solution of 0.5 M sulfuric acid with duty cycles 90%A - 10%C and 60%A - 40%C at frequencies 20 Hz and 1000 Hz and cathodic peaks 2%, 5%, 7% and 10%; the execution of corrosive tests, according to the ASTM directives (G1 and G31 for immersion tests in reducing acids and G5 for potentiodynamic tests of anodic type, in NaBr, and cathodic, in H2SO4); surface characterization in terms of morphology, structure and roughness, through the techniques of SEM, XRD, GDOES and profilometry. All the results of the aforementioned experimental investigations are discussed in chapter 3. The conditions of anodization have greatly influenced the morphology, the thickness, the quantity of crystalline phases and the corrosion behavior. The obtained data, for this reason, have been crossed to identify the parameters of interest for possible industrial applications.
Il titanio è un metallo con caratteristiche uniche, grazie alla combinazione di elevate proprietà meccaniche e incredibile resistenza alla corrosione. Per questi motivi, i ricercatori sono attratti dal suo uso in diversi tipi di industria, in particolare quelli che hanno ambienti aggressivi, come il settore petrolchimico, o dove le applicazioni richiedono un elevato rapporto resistenza-peso, ad esempio nel settore aerospaziale. Il primo capitolo di questa tesi inizia con una panoramica sul titanio, dalla sua scoperta alla sua raffinazione, per proseguire sulle sue caratteristiche fisiche e proprietà. Queste ultime, in particolare, sono collegate alla classificazione commerciale. La prima sezione si conclude con uno studio approfondito del mercato del titanio in relazione alle sue applicazioni. Nonostante le proprietà anticorrosive siano di natura eccezionale, in virtù del suo film di ossido naturale, il titanio soffre la corrosione in ambienti particolari. Un secondo paragrafo, pertanto, tratta in dettaglio le forme di corrosione (uniforme, localizzata e cracking indotto dall'ambiente) e gli agenti chimici che attaccano il titanio. Infine, il capitolo si conclude con i trattamenti applicati per migliorare la resistenza alla corrosione: ossidazione chimica e termica, impiantazione di ioni e raffinamento del grano, insieme ai vari elementi con cui il titanio può essere legato per migliorare le sue caratteristiche meccaniche. Infine, viene riportata una revisione del processo di anodizzazione, che consente di controllare lo sviluppo del film di biossido di titanio. Il trattamento può essere tradizionale o mediante deposizione ad arco elettrico all'anodo (ASD), detta anche ossidazione elettrolitica al plasma (PEO). Il vantaggio di queste tecniche risiede nel basso costo e nella facilità di implementazione. Utilizzando un alimentatore programmabile è possibile controllare parametri di processo come duty cycle, frequenza e tensione. In questo modo è possibile eseguire la PEO con un regime pulsato monopolare, consentendo di ottenere i cambiamenti morfologici e strutturali desiderati dello strato di ossido e un interessante livello di risparmio energetico rispetto all'approccio DC. Il punto chiave di questo lavoro è l'introduzione di due nuovi parametri che sono la fase della polarizzazione catodica, che sostituisce il tempo di riposo nel monopolare e l'intensità di questa fase, espressa nel picco catodico. Il processo bipolare PEO è quindi definito. Nel capitolo 2, la metodologia sperimentale utilizzata è trattata in dettaglio: la preparazione e l'anodizzazione dei campioni, questi ultimi eseguiti a temperatura controllata in una soluzione di acido solforico 0,5 M con cicli di lavoro 90% A - 10% C e 60% A - 40% C a frequenze 20 Hz e 1000 Hz e picchi catodici 2%, 5%, 7% e 10%; l'esecuzione di test corrosivi, secondo le direttive ASTM (G1 e G31 per test di immersione in acidi riducenti e G5 per test potenziodinamici di tipo anodico, in NaBr e catodico, in H2SO4); caratterizzazione superficiale in termini di morfologia, struttura e rugosità, attraverso le tecniche di SEM, XRD, GDOES e profilometria. Tutti i risultati delle suddette indagini sperimentali sono discussi nel capitolo 3. Le condizioni di anodizzazione hanno influenzato notevolmente la morfologia, lo spessore, la quantità di fasi cristalline e il comportamento a corrosione. I dati ottenuti, per questo motivo, sono stati incrociati per identificare i parametri di interesse per possibili applicazioni industriali.
Bipolar plasma electrolytic oxidation to enhance the localized and uniform corrosion resistance of titanium
LA PADULA, MARCELLO
2018/2019
Abstract
Titanium is a metal with unique characteristics, thanks to the combination of high mechanical properties together with incredible resistance to corrosion. For these reasons, researchers are attracted to its use in different types of industry, especially those that have aggressive environments, such as the petrochemical sector, or where applications require a high strength-to-weight ratio, for example in the Aerospace field. The first chapter of this thesis begins with an overview on titanium, from its discovery to its pure refining, to continue on its physical characteristics and properties. The latter in particular are linked to commercial classification. The first section ends with an in-depth study of the titanium market in relation to its applications. Despite the anticorrosive properties are of an exceptional nature, by virtue of its natural oxide film, titanium suffered in particular environments. A second paragraph, therefore, deals in detail with the forms of corrosion (uniform, localized and environment-induced cracking) and the chemical agents that attack titanium. Finally, the chapter concludes with the treatments that are applied to improve corrosion resistance: chemical and thermal oxidation, ion implantation and grain refinement, along with the various elements with which titanium can be alloyed to improve its mechanical characteristics. Finally, a review of the anodizing process, which allows the development of the titanium dioxide film to be controlled, is reported. The treatment can be traditional or by spark deposition (ASD), also called plasma electrolyte oxidation (PEO). The advantage of these techniques lies in the low cost and ease of implementation. Using a programmable power supply it is possible to control process parameters such as duty cycle, frequency and voltage. In this way it is possible to perform PEO with a monopolar pulsed regime, allowing to obtain the desired morphological and structural changes of the oxide layer and an interesting level of energy saving with respect to the DC approach. The key point of this work is the introduction of two new parameters which are the phase of cathodic polarization, replacing the rest time in the monopolar, and the intensity of this phase, expressed in cathode peak. The PEO bipolar process is thus defined. In Chapter 2, the experimental methodology used is treated in detail: the preparation and anodization of the samples, the latter performed at a controlled temperature in a solution of 0.5 M sulfuric acid with duty cycles 90%A - 10%C and 60%A - 40%C at frequencies 20 Hz and 1000 Hz and cathodic peaks 2%, 5%, 7% and 10%; the execution of corrosive tests, according to the ASTM directives (G1 and G31 for immersion tests in reducing acids and G5 for potentiodynamic tests of anodic type, in NaBr, and cathodic, in H2SO4); surface characterization in terms of morphology, structure and roughness, through the techniques of SEM, XRD, GDOES and profilometry. All the results of the aforementioned experimental investigations are discussed in chapter 3. The conditions of anodization have greatly influenced the morphology, the thickness, the quantity of crystalline phases and the corrosion behavior. The obtained data, for this reason, have been crossed to identify the parameters of interest for possible industrial applications.| File | Dimensione | Formato | |
|---|---|---|---|
|
Tesi_MLaPadula.pdf
accessibile in internet per tutti
Descrizione: Bipolar plasma electrolytic oxidation to enhance the localized and uniform corrosion resistance of titanium
Dimensione
6.23 MB
Formato
Adobe PDF
|
6.23 MB | Adobe PDF | Visualizza/Apri |
I documenti in POLITesi sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
https://hdl.handle.net/10589/152220