Titanium and its alloys offer a great opportunity and reliability when applied in those industrial applications where the contact with an aggressive solution, or the occurrence of too serious consequences in case failure, limit the use of other common structural materials like iron or aluminum. For those reasons common fields of Ti implementation, listed in order of market demand, are: aerospace, chemical, nuclear, and naval industry. In all cases, the contact with acidic environments can severely affect the protective properties of the thin oxide layer that naturally grows on the metal. This is particularly true when dealing with sulfuric acid often present, for example, in the stratosphere where airliners fly or during metal pickling and synthesis of several chemicals like fertilizers. Followed from those observations, electrochemical surface treatment, like plasma electrolytic oxidation (PEO), offers a great deal to improve surface properties upon the growth of a thick (even several microns) and hard conversion coating. The aim of this research is to optimize PEO surface treatments for titanium grade 2 tested for corrosion resistance in a solution of hot concentrated sulfuric acid (typically 10 %v/v at 60 °C, a common environment adopted in metal pickling applications). In the first part of the thesis, the role of the main technological parameters, like frequency and amount of cathodic polarization, was investigated according to a detailed characterization made possible upon the use of transmission electron microscopy (TEM). This allowed to probe the material structure, at the scale of the nanometers, according to the use of electron energy loss spectroscopy (EELS). Detailed structural maps were constructed improving knowledge about the barrier layer, i.e., the morphological region of PEO oxides directly in contact with the metal and responsible for the corrosion resistance, and the porous region, i.e., the oxide portion directly in contact with the external environment. It was verified that upon the use of pulsed PEO with hybrid duty cycles, repeated at high frequency (1000 Hz), it was possible to limit the amount of oxygen vacancies and to promote the formation of the most thermodynamically stable TiO2 polymorph, i.e., rutile. The latter verifications were found to be fundamental in characterizing the corrosion response of the coatings, particularly in acidic environments, where the presence of electronic defects can favour protons reduction and consequent atomic hydrogen ingress inside the oxide layer. The corrosion mechanism was investigated through the use of electrochemical impedance spectroscopy (EIS) allowing to extract proton diffusivities of oxides produced according to different PEO conditions. Accordingly, a clear correlation between technological parameters, synthesized materials, and corrosion response was established.
Il titanio e le sue leghe offrono grande affidabilità quando utilizzati in applicazioni industriali dove il contatto con una soluzione aggressiva, o il verificarsi di conseguenze catastrofiche in caso di malfunzionamento, limitano l’uso di altri materiali strutturali come l’acciaio o l’alluminio. Per queste ragioni, tipici settori industriali che considerano l’utilizzo del Ti, elencati in ordine di domanda, sono: aerospaziale, chimico, nucleare e industria navale. In geneale, il contatto con ambienti acidi può compromettere la stabilità e la funzione protettiva dello strato di ossido che spontaneamente si forma sulla superficie del metallo. Ciò è particolarmente verificato in presenza di acido solforico, per esempio durante le rotte degli aerei di linea nella stratosfera, in caso di decapaggio metallico o durante la sintesi chimica di fertilizzanti. Di conseguenza trattamenti superficiali come l’ossidazione elettrolitica al plasma (PEO) offrono una grande opportunità di migliorare le proprietà superficiali del materiale, per esempio aumentando lo spessore dell’ossido. Per questo lo scopo di tale ricerca riguarda l’ottimizzazione di trattementi PEO su titanio grado 2 al fine di migliorare la resistenza a corrosione testata in una soluzione di acido solforico ad alta temperatura (10 %v/v a 60 °C, un ambiente comune alle operazioni di decapaggio). Nella prima parte della tesi è stato investigato il ruolo dei principali parametri tecnologici, quali frequenza e intensità della polarizzazione catodica, attraverso un dettagliato studio condotto mediante microscopia elettronica a trasmissione (TEM). Ciò ha permesso di investigare la struttura del materiale alla scala nanoscopica mediante utilizzo di spettroscopia di perdita di energia di elettroni (EELS). I risultati sono stati modellati attraverso l’utilizzo di un algoritmo, scritto in linguaggio Python, al fine di construire delle mappe strutturali rappresentanti la struttura cristallina delle principali porzioni di ossido ovvero strato barriera e strato poroso. È stato verificato che attraverso l’utilizzo di un segnale pulsato con ciclo di lavoro ibrido, ripetuto ad alta frequenza (1000 Hz), è possibile diminuire la quantità di vacanze di ossigeno all’interno della struttura cristallina promuovendo la formazione di rutilo. Ciò risulterà fondamentale nella risposta del materiale all’ambiente corrosivo, dove la presenza di difetti elettronici può favorire la riduzione dei protoni con conseguente ingresso di idrogeno atomico nello strato di ossido. Il meccanismo di corrosione è stato investigato mediante spettroscopia di impedenza elettrochimica (EIS) permettendo l’estrazione delle diffusività delle cariche presenti in soluzione all’interno dell’ossido anodico, permettendo di instaurare una chiara correlazione fra i parametri tecnologici adottati durante la sintesi del materiale e la conseguente risposta in termini di resistenza a corrosione.
Pulsed anodizing treatments to increase the corrosion resistance of CP titanium in acidic environment
CASANOVA, LUCA
2021/2022
Abstract
Titanium and its alloys offer a great opportunity and reliability when applied in those industrial applications where the contact with an aggressive solution, or the occurrence of too serious consequences in case failure, limit the use of other common structural materials like iron or aluminum. For those reasons common fields of Ti implementation, listed in order of market demand, are: aerospace, chemical, nuclear, and naval industry. In all cases, the contact with acidic environments can severely affect the protective properties of the thin oxide layer that naturally grows on the metal. This is particularly true when dealing with sulfuric acid often present, for example, in the stratosphere where airliners fly or during metal pickling and synthesis of several chemicals like fertilizers. Followed from those observations, electrochemical surface treatment, like plasma electrolytic oxidation (PEO), offers a great deal to improve surface properties upon the growth of a thick (even several microns) and hard conversion coating. The aim of this research is to optimize PEO surface treatments for titanium grade 2 tested for corrosion resistance in a solution of hot concentrated sulfuric acid (typically 10 %v/v at 60 °C, a common environment adopted in metal pickling applications). In the first part of the thesis, the role of the main technological parameters, like frequency and amount of cathodic polarization, was investigated according to a detailed characterization made possible upon the use of transmission electron microscopy (TEM). This allowed to probe the material structure, at the scale of the nanometers, according to the use of electron energy loss spectroscopy (EELS). Detailed structural maps were constructed improving knowledge about the barrier layer, i.e., the morphological region of PEO oxides directly in contact with the metal and responsible for the corrosion resistance, and the porous region, i.e., the oxide portion directly in contact with the external environment. It was verified that upon the use of pulsed PEO with hybrid duty cycles, repeated at high frequency (1000 Hz), it was possible to limit the amount of oxygen vacancies and to promote the formation of the most thermodynamically stable TiO2 polymorph, i.e., rutile. The latter verifications were found to be fundamental in characterizing the corrosion response of the coatings, particularly in acidic environments, where the presence of electronic defects can favour protons reduction and consequent atomic hydrogen ingress inside the oxide layer. The corrosion mechanism was investigated through the use of electrochemical impedance spectroscopy (EIS) allowing to extract proton diffusivities of oxides produced according to different PEO conditions. Accordingly, a clear correlation between technological parameters, synthesized materials, and corrosion response was established.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/183022