Korozijska odpornost nerjavnega jekla THERMA 4828 in titanove zlitine TIMETAL Exhaust XT : magistrsko delo

Broder, Kim (avtor), Medved, Jože (mentor), Milošev, Ingrid (komentor)

V magistrskem delu smo preiskovali korozijsko odpornost nerjavnega jekla THERMA 4828 in titanove zlitine TIMETAL Exhaust XT. Eksperimentalno smo preučili visokotemperaturno oksidacijo pri temperaturah 700 in 950 °C in korozijo v vodnem mediju. Najprej smo vse preiskovane vzorce ustrezno pripravili za oba dela preizkusov. Visokotemperaturno oksidacijo prvega dela vzorcev smo izvedli v napravi za simultano termično analizo (STA 449 Jupiter podjetja NETZSCH). Rezultate kinetike oksidacije smo pridobili na podlagi preizkusov v STA napravi in jih primerjali z linearnim, parabolnim, oziroma kubičnim matematičnim modelom. Vzorce smo po oksidaciji prerezali, jih metalografsko pripravili in s pomočjo digitalnega svetlobnega mikroskopa izmerili debeline nastalih oksidnih plasti. Za tem je sledila še analiza s pomočjo vrstičnega elektronskega mikroskopa (SEM), kjer smo z detektorjem EDS (energijske disperzijske spektroskopije) zaznavali kemijsko sestavo vzorcev in oksidnih plasti. Pri izračunih s programsko opremo Thermo-Calc smo modelirali deleže posameznih oksidov in njihove kemijske sestave pri določeni temperaturi. Korozijsko odpornost pa smo vzporedno s preizkusi visokotemperaturne oksidacije preučevali še z elektrokemijskimi meritvami, ki smo jih izvedli s sistemom, ki sestoji iz korozijske celice, potenciostata in računalnika s programom Nova 2.0. Drugi del ustrezno pripravljenih vzorcev smo namestili v korozijsko celico; kot korozijski medij je bila uporabljena 3,5 % NaCl raztopina. Rezultate meritev smo predstavili kot krivulje potenciala odprtega kroga OCP (angl. open circuit potencial) v odvisnosti od časa, v obliki Nyquistovih in Bodejevih diagramov, ki predstavljajo elektrokemijske impedančne spektre in kot potenciodinamske polarizacijske krivulje. Na podlagi interpretacije rezultatov meritev visokotemperaturne oksidacije smo prišli do zaključka, da titanova zlitina TIMETAL_XT oksidira hitreje kot nerjavno jeklo THERMA 4828 in tvori debelejšo oksidno plast v primerjavi z nerjavnim jeklom. To potrdijo tako oksidacijske krivulje mase v odvisnosti od časa, kjer je največja sprememba mase zabeležena pri visokotemperaturni oksidaciji titanove zlitine pri temperaturi 950 °C kot tudi rezultati mikroskopiranja. Elektrokemijske meritve pa so pokazale, da je titanova zlitina korozijsko bolj odporna v kloridni raztopini kot nerjavno jeklo. To smo ugotovili na podlagi OCP meritev, ki so pokazale počasnejšo stabilizacijo za titanovo zlitino, kar kaže na postopno nastajanje zaščitne oksidne plasti. Potenciodinamske polarizacijske krivulje prav tako niso zaznale preboja oksidne plasti pri titanovi zlitini, medtem ko je bil pri nerjavnem jeklu ta preboj jasno viden.

• Jezik: Slovenski jezik
• Ključne besede: avstenitno nerjavno jeklo, titanova zlitina, visokotemperaturna oksidacija, kinetika oksidacije, termodinamsko modeliranje, elektrokemijske meritve
• Vrsta gradiva: Magistrsko delo/naloga
• Tipologija: 2.09 - Magistrsko delo
• Organizacija: NTF - Naravoslovnotehniška fakulteta
• Kraj izida: Ljubljana
• Založnik: K. Broder
• Leto izida: 2025
• Št. strani: XVII, 102 str.
• PID: 20.500.12556/RUL-168058
• UDK: 669
• COBISS.SI-ID: 232258819
• Datum objave v RUL: 27.03.2025

Sekundarni jezik

• Jezik: Angleški jezik
• Naslov: Corrosion resistance of stainless steel THERMA 4828 and titanium alloy TIMETAL Exhaust XT : master's thesis

Izvleček

In the master's thesis, we discussed the corrosion resistance of stainless steel THERMA 4828 and titanium alloy TIMETAL Exhaust XT. We experimentally studied high-temperature oxidation at test temperatures of 700 and 950 °C and corrosion in an aqueous medium. First, all examined samples were properly prepared for both parts of the tests. The high-temperature oxidation of the first set of samples was performed using a simultaneous thermal analysis device (STA 449 Jupiter from NETZSCH). The oxidation kinetics results were obtained based on tests conducted in the STA device and were compared with linear, parabolic, and cubic mathematical models. After oxidation, the samples were sectioned and metallographically prepared, and the thicknesses of the resulting oxide layers were measured using a digital optical microscope. This was followed by analysis using a scanning electron microscope (SEM) where an energy-dispersive spectroscopy (EDS) detector was used to determine the chemical composition of the samples and oxide layers. In calculations using the Thermo-Calc software, we modeled the fractions of individual oxides and their chemical compositions at specific temperatures. In parallel with high-temperature oxidation tests, corrosion resistance was also examined using electrochemical measurements. These were conducted with a system comprising a corrosion cell, a potentiostat, and a computer running Nova 2.0 software. The second set of properly prepared samples was placed in the corrosion cell, with a 3.5% NaCl solution used as the corrosive medium. The measurement results were presented as open circuit potential (OCP) curves over time as Nyquist and Bode plots representing electrochemical impedance spectra and as potentiodynamic polarization curves. Based on the interpretation of the hightemperature oxidation measurement results, we concluded that the TIMETAL_XT titanium alloy oxidizes faster than THERMA 4828 stainless steel and forms a thicker oxide layer compared to stainless steel. This was confirmed both by the oxidation mass curves over time where the greatest mass change was recorded in the hightemperature oxidation of the titanium alloy at 950 °C and by the results of microscopic analysis. Electrochemical measurements indicated that the titanium alloy exhibits greater corrosion resistance in chloride solution than stainless steel. This was determined based on OCP measurements, which showed a slower stabilization for the titanium alloy, indicating the gradual formation of a protective oxide layer. Potentiodynamic polarization curves also did not detect oxide layer breakdown in the titanium alloy whereas a clear breakdown was observed in the stainless steel.

• Ključne besede: austenitic stainless steel, titanium alloy, high-temperature oxidation, oxidation kinetics, thermodynamic modeling, electrochemical measurements