Optimization of corrosion resistance of high purity alumna ceramics

U radu su oblikovani sirovci visokočiste aluminijeve oksidne (Al2O3) keramike hladnim izostatičkim prešanjem. Ispitan je utjecaj konvencionalnog sinteriranja (električna peć) i nekonvencionalnog sinteriranja (hibridna mikrovalna peć) na mikrostrukturu, gustoću, mehanička svojstva (tvrdoća, žilavost, indeks krhkosti) i otpornost na koroziju Al2O3 keramike. Otpornost na koroziju Al2O3 keramike sinterirane konvencionalnim (EP-Al2O3) i nekonvencionalnim (MP-Al2O3) postupcima određena je izlaganjem različitim koncentracijama nitratne kiseline (0,5 mol dm-3, 1,25 mol dm-3 i 2 mol dm-3) te različitim temperaturama (25, 40 i 55 °C) do 240 sati. Potom su određene masene koncentracije iona Al3+, Ca2+, Fe3+, Mg2+, Na+ i Si4+ eluiranih iz Al2O3 keramike atomskom emisijskom spektrometrijom uz induktivno spregnutu plazmu. Otpornost na koroziju Al2O3 keramike procijenjena je na temelju (i) mjerenja masenih koncentracija eluiranih iona, (ii) analize stupnjeva otapanja iona, (iii) određivanja konstanti brzina korozije, (iv) određivanja energija aktivacije i (v) postignutih mikrostruktura u dva postupka sinteriranja. Za procjenu parametara otpornosti na koroziju u promatranom eksperimentalnom prostoru odabranog sustava „ispitni uzorak - korozivni medij“ korištena je metoda odzivnih površina. Izlaganje Al2O3 keramike korozivnom mediju provedeno je prema Box-Behnken planu pokusa. Potom je provedena analiza varijance EP-Al2O3 i MP-Al2O3 keramike kako bi se odredila značajnost pojedinih članova modela te su određene regresijske funkcije. Postupkom eksperimentalne optimizacije određeni su uvjeti postizanja maksimalne otpornosti na koroziju konvencionalno i nekonvencionalno sinterirane Al2O3 keramike. Visokočista EP-Al2O3 keramika, s većim kristalnim zrnima i porama te manjom relativnom poroznošću, pokazuje bolju otpornost na koroziju pri višim koncentracijama HNO3, dok je MPAl2O3 keramika, koja sadrži sitnija zrna i pore te višu relativnu poroznost, kemijski postojanija pri nižim koncentracijama HNO3. Kiseli korozivni medij nije bitno utjecao na tvrdoću ispitivane Al2O3 keramike. Dobivenim rezultatima potvrđena je hipoteza o mogućnosti proizvodnje visokočiste Al2O3 keramike konvencionalnim i nekonvencionalnim postupcima sinteriranja s povoljnom mikrostrukturom za primjenu u kiselom korozivnom mediju.In this thesis, high purity alumina (Al2O3) ceramic green bodies were compacted by cold isostatic pressing (CIP). Impact of conventional (electric kiln) and nonconventional (hybrid microwave kiln) sintering on microstructure, density, mechanical properties (hardness, fracture toughness, brittleness index) and corrosion resistance of alumina ceramics was investigated. Corrosion resistance of alumina ceramics sintered by conventional (EP-Al2O3) and nonconventional methods (MP-Al2O3) was determined by its exposure to different concentrations of nitric acid (0,5 mol dm-3, 1,25 mol dm-3 and 2 mol dm-3) and different temperatures (25, 40 and 55 °C) up to 240 h. Afterwards, mass concentrations of Al3+, Ca2+, Fe3+, Mg2+, Na+ and Si4+ ions eluted from Al2O3 ceramics were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). Corrosion resistance of Al2O3 ceramics was evaluated by (i) measurements of mass concentrations of eluted ions, (ii) analysis of degrees of dissolution of ions, (iii) determination of corrosion rate constants, (iv) determination of activation energies of the corrosion processes and (v) analysis of microstructures obtained by means of two sintering methods. Response surface methodology was used for optimization of parameters within experimental “sample-corrosive media” area. Exposure of Al2O3 ceramics to corrosive media was conducted according to Box-Behnken design. Afterwards, analysis of variance of EP-Al2O3 and MP-Al2O3 ceramics was carried out to define the significance of each factor of the model and to determine regression functions. Optimization was used to determine the parameters to achieve maximal corrosion resistance of conventionally and non-conventionally sintered Al2O3 ceramics. High purity EP-Al2O3 ceramics, containing bigger grains and pores, as well as lower relative porosity, showed higher corrosion resistance at higher concentrations of nitric acid, while MP-Al2O3 ceramics, containing smaller grains and pores, as well as higher relative porosity, showed higher chemical resistance at lower HNO3 concentrations. Acid corrosive media did not affect significantly the hardness of examined Al2O3 ceramics. Obtained results confirmed hypothesis of this research that is possible to produce high purity alumina ceramics by conventional and non-conventional sintering methods with favourable microstructure for application in acid corrosive media