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1.
INHIBICIJSKE LASTNOSTI NEIONSKEGA SURFAKTANTA TRITON-X-100, PRI POVIŠANI TEMPERATURI
Gregor Žerjav, 2009, undergraduate thesis

Abstract: S klasično potenciodinamsko metodo smo preučevali vpliv neionskega surfaktanta TRITON-X-100 na korozijo feritnega nerjavnega jekla X4Cr13 v 1M H2SO4 pri petih različnih temperaturah. Kritično miceljno koncentracijo uporabljenega surfaktanta smo določili na osnovi merjenja površinske napetosti. Inhibicijsko učinkovitost smo izračunali pri različnih temperaturah in sicer v območju od 25 oC do 45 oC. Na podlagi meritev ugotavljamo, da učinkovitost inhibicije raste z večanjem koncentracije surfaktanta TRITON-X-100, medtem ko z naraščanjem temperature le–ta upada. Adsorpcija surfaktanta TRITON-X-100 sledi Flory-Hugginsovi adsorpcijski izotermi. Z uporabo termodinamskih enačb smo izračunali termodinamske parametre (ΔGads,, ΔHads , ΔSads). Nenazadnje smo izračunali tudi arrheniusov parameter in aktivacijsko energijo. Spreminjanje vrednosti aktivacijske energije v odvisnosti od koncentracije surfaktanta kaže na spremembo mehanizma adsorpcije surfaktanta TRITON-X-100 na površino jekla in sicer iz fizikalne adsorpcije do kemisorpcije.
Keywords: neionski surfaktant, inhibitorji korozije, žveplova(IV) kislina, Flory-Hugginsova adsorpcijska izoterma, kritična miceljna koncentracija (CMC), termodinamski parametri
Published: 09.05.2020; Views: 599; Downloads: 39
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2.
Določevanje dobe trajanja nerjavnih jekel v pogojih napetostno korozijskega pokanja
Marko Tandler, Leopold Vehovar, Valter Doleček, 2001, original scientific article

Abstract: Pregled literature kaže, da še ni enotno priznane razlage za proces napetostne korozije. Zaradi tega in kompleksnosti napetostne korozije pa je tudi zahtevno določiti dobo trajanja konstrukcij, izpostavljenih napetostno korozijskemu pokanju. V zadnjem času se je pojavila nova metoda določevanja t.i. korozijske krivulje raztezanja, ki omogoča te vrste napovedi. Z njeno modifikacijo nam je uspelo natančno določiti primarno, sekundarno in terciarno področje. Na osnovi odvisnosti log i$_ss$-log t in log sigma-log t so bili konstruirani obratovalni diagrami, ki dajejo možnost napovedi časa do nastanka razpoke, njene stabilne rasti in časa do zloma materiala. Poznanje laboratorijskih rezultatov in korelacija teh s podobnimi iz prakse omogoča uporabo obratovalnih diagramov za napoved dobe trajanja vitalnih delov strojne opreme ter drugih konstrukcij v različnih vejah industrije.
Keywords: korozija, napetostno korozijsko pokanje, korozijske krivulje raztezanja, obratovalni diagrami
Published: 16.03.2017; Views: 520; Downloads: 64
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3.
Determination of major phenolic acids, phenolic diterpenes and triterpenes in Rosemary (Rosmarinus Officinalis L.) by gas chromatography and mass spectrometry
Maša Islamčević Razboršek, Darinka Brodnjak-Vončina, Valter Doleček, Ernest Vončina, 2007, original scientific article

Abstract: A gas chromatographic-mass spectrometric (GC-MS) method for the simultaneous identification and quantification of seven major phenolic and terpenic compounds in Rosmarinus officinalis L. was developed. The compounds were identified as trimethylsilyl (TMS) derivatives of phenolic acids (caffeic and rosmarinic acid), phenolic diterpene (carnosic acid), and pentacyclic triterpenes (ursolic, oleanolic, betulinic acid and betulin). These compounds have been identified by retention time and comparison of mass spectra. The procedure involves ultrasonic extraction using solvent mixture of tetrahydrofuran and ethanol. Extracts were fractionated by size exclusion chromatography (SEC) after purifying on graphitised carbon. The fraction with phenolic and terpenic compounds was derivatised prior to GC-MS analysis using N-methyl-N-trimethylsilyl trifluoroacetamide (MSTFA) as a derivatisation reagent. The derivatisation process was optimized regarding temperature and reaction time. The linearity of the method was tested in concentration range 4-25 mg L$^{–1}$. The correlation coefficients ($r^2$) were in the range of 0.997 to 0.999. The average recoveries for all compounds ranged from 80 to 82%. The GC-MS technique is specific and sensitive, and can be used for simultaneous identification and determination of a wide range of phenolic and terpenic compounds in different plants even at trace levels.
Keywords: Rosmarinus officinalis, diterpenes, triterpenes, size exclusion chromatography, gas chromatography, mass spectrometry
Published: 21.12.2015; Views: 1088; Downloads: 84
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4.
Analiza elektrokemijskega šuma, generiranega z različnimi korozijskimi procesi
Mirjam Bajt Leban, Valter Doleček, A. Legat, V. Kuhar, 1998, original scientific article

Abstract: Analizirali smo elektrokemijski šum, ki ga povzročijo korozijski procesi na površini elektrod. Tokovni in napetostni šum, ki skupaj sestavljata elektrokemijski šum, sta bila merjena med tremi vzporedno postavljenimi elektrodami iz nerjavnega jekla AISI 302. Srednja - skupna elektroda, ki je bila z ostalima dvema povezana v tokovni in napetostni vir, je bila obremenjena s konstantno natezno silo. Elektrokemijski šum smo merili v vodnih raztopinah z različnimi koncentracijami NaCl in HCI. Med meritvami smo z računalniškim vizualizacijskim sistemom posneli spremembe na površini elektrod, ter jih primerjali s posameznimi fluktuacijami elektrokemijskega šuma. Za karakterizacijo izmerjenih signalov, predvsem pa, da bi razločili med posameznimi tipi korozije, smo uporabili različne matematične metode v frekvenčnem (analiza spektralne gostote moči) in faznem prostoru (rekonstrukcija atraktorjev, določitev fraktalne dimenzije in maksimalnega Ljapunovega eksponenta). Raziskava je pokazala, da z merjenjem elektrokemijskega šuma in njegovo analizo lahko ločimo med splošno korozijo, ter različnimi tipi lokalne korozije. Pri lokalnih oblikah korozije detektiramo preboje pasivnega filma, njegovo repasivacijo in rast jamic. Odkrijemo lahko tudi začetek napetostno - korozijskega pokanja, ne moremo pa oceniti hitrosti propagacije posameznih razpok, oziroma napovedati pretrga preskušanca
Keywords: Lyapunov eksponent, računalniški vizualizacijski sistem, napetostno korozijsko pokanje, spektralna analiza, korozija, kaotična analiza, elektrokemijski šum, fraktalne dimenzije
Published: 10.07.2015; Views: 623; Downloads: 28
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5.
Excess molar volumes of binary liquid mixtures of cyclohexane-carbon tetrachloride and toluene-benzene at various temperatures
Aljana Petek, Valter Doleček, 1998, original scientific article

Abstract: Excess molar volumes of binary mixtures of cyclohexane-carbon tetrachloride at 288.15, 293.15, 298.15 K and toluene-benzene at 293.15, 298.15, 308.15 K have been determined using a vibrating tube densimeter. Flory's theory of liquid mixtures has been applied to calculate the excess entalphy of cyclohexane-carbon tetrachloride at 298,15 K. The calculated value for the equimolar mixture is in fairly good agreement with experimental results.
Keywords: physical chemistry, molar volumes, measurements, excess enthalpy
Published: 10.07.2015; Views: 900; Downloads: 62
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6.
Primerjava sočasnih meritev elektrokemijskega šuma in mehanskih parametrov med procesi napetostno-korozijskega pokanja
Mirjam Bajt Leban, Valter Doleček, Andraž Legat, Viljem Kuhar, 2000, original scientific article

Abstract: Namen našega dela je bil določiti značilnosti elektrokemijskega šuma, povzročenega z napetostno-korozijskim pokanjem. Želeli smo predvsem ugotoviti poveznanost značilnih elektrokemijskih fluktuacij z mehanskimi spremembami zaradi procesov pokanja ter tako posredno tudi oceniti primernost te metode za detekcijo in študij različnih oblik pokanja. Elektrokemijski šum smo merili med počasnim enakomernim nateznim obremenjevanjem delovne elektrode (modificiran Slow Strain Rate Test), izdelane iz nerjavnega avstenitnega jekla tipa AISI 304 v žarjenem ter toplotno neobdelanem stanju. Izrazite nenadne fluktuacije napetostnega in tokovnega šuma smo opazili po preseženi napetosti tečenja. Vse se časovno popolnoma ujemajo z nenadnimi spremembami raztezka natezno obremenjene elektrode, kar dokazuje zanesljivo možnost detekcije pokanja. Iz izmerjenih sprememb raztezka smo določili hitrost rasti razpok ter jih primerjali z amplitudami fluktuacij elektrokemijskega šuma. Dokazali smo, da obstaja med vrednostmi teh dveh količin linearna odvisnost. Poudariti je treba, da samo poznanje elektrokemijskih razmer ter spremljanje mehanskih sprememb, povzročenih s pokanjem, še ni dovolj za ovrednotenje mehanizmov napetostno-korozijskega pokanja, pač pa bo za to v prihodnosti treba vključiti še fraktografsko analizo prelomnih površin.
Keywords: elektrokemijski šum, napetostno-korozijsko pokanje, interkristalna korozija, vodikova krhkost, hitrost pokanja, metalografska analiza
Published: 10.07.2015; Views: 722; Downloads: 87
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7.
Volumetric properties of ethanol-water mixtures under high pressure
Aljana Petek, Darja Pečar, Valter Doleček, 2001, original scientific article

Abstract: Densities of ethanol-water mixtures have been measured using a vibrating tube densimeter and a different arrangement high-pressure experimental set-up. Measurements were carried out at 298,15 K in a pressure range from 0.1 MPa to 5MPa. Partial molar volumes, excess molar volumes and coefficients of isothermal compressibility were calculated. The molar volumes of alcohol and its partial molar volumes in mixtures with water are found to decrease monotonously with increasing pressure. Excess molar volumes are negative at all pressures. The numerical P-V relations at each composition are correlated satisfactorily as a function of pressure by the Hayward equation.
Keywords: chemistry, high-pressure vibrating tube densimeter, excess molar values, isothermal compressibility
Published: 10.07.2015; Views: 784; Downloads: 50
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8.
Impedance measurements on the passive layers of stainless different heat treated steels
Mojca Slemnik, Valter Doleček, Miran Gaberšček, 2002, short scientific article

Abstract: The behavior of differently heat-treated X20Cr13 steels in 0.1 M H2SO4, was studied using the electrochemical impedance spectroscopy. Impedance spectra were recorded at constant passsive potentials of 0.2, 0.4 and 0.6 V vs. SCE, respectively. At low passive potential, 0.2 V vs. SCE, impedance spectra typical for an active - passive transition were observed. The assumption has been made, that they are the result of the adsorption process, caused by boundwater, which blocks the passive layer of steel and leads to corrosion rate reduction. The process is the most significant at the oil and the air-quenched, but less at the tempered steel. When more noble potentials are approached the impedance spectra become typical for passive metals and the value of resistance and capacitance differ from each other with respect to the heat treatment.
Keywords: korozija, nerjavna jekla, impedanca
Published: 10.07.2015; Views: 1031; Downloads: 25
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9.
Nucleation and growth of a passive film on AISI 420 martensitic stainless steel under potentiostatic conditions
Regina Fuchs-Godec, Aljana Petek, Valter Doleček, 2002, original scientific article

Abstract: The kinetics of the early stage s of passive film formation on AISI stainless stell in mixtures of 0.1M ▫$H_2SO_4$▫ and 0.1M ▫$Na_2SO_4$▫, with pH-values of 2.42, 2.85 and 3.33, were studied at different anodic potentials. The potentiostatic step method was used as an electrochemical technique in which the anodic current density was measured as a function of time. For analysis of the measured data, the theory of "topochemical reaction" was applied for the charcterization of the exponent n in that part where the steepest current decay was observed. Through this theory, which includes the Erofeev equation, the number of conversion steps could be obtained, as well as the number of directions in which the nuclei grow. It was found that this theory can be successfully applied to the passivation process of AISI 420 stainles steel.
Keywords: corrosion, corrision resistance, martensitic stainless steels, passive films, potentiostatic step method, topochemical reactions
Published: 10.07.2015; Views: 985; Downloads: 65
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10.
Determination of the critical pitting temperature and the critical ion ($Cl^-$) concentration inducing pitting of AISI 304L stainless steel in 0.5 M $H_2SO_4$
Regina Fuchs-Godec, Valter Doleček, 2004, original scientific article

Abstract: Corrosion of AISI 304L stainless steel in aqueous solutions of 0.5 M $H_2SO_4$ + 0.01 M KCNS + x M NaCl was studied at different temperatures (25, 40, and 50 °C). The critical ion ($Cl^-$) concentration (CIC) and the critical pitting corrosion temperature (CPT) were determined on the basis of characteristic corrosion parameters ($i_{crit}$, $i_{pass}$, $E_{pp}$), which were obtained through potentiodynamic scanning experiments. The activation energy of the process wasfound to be equal to -42.0 (1 ± 0.08 ) kJ $mol^{-1}$. As the temperature increased, the pitting potential, $E_{pp}$, was shifted towards more negative values linearly with the reciprocal of temperature, when the concentration of added NaCl was above the CIC. In contrast, at concentrations very close to the CIC, the straight lines changed slopes. This result may indicate of the approximate boundary between metastable pitting and the beginning of stable pitting.
Keywords: austenitic stainless steel, critical ion ($Cl^-$) concetration, critical pitting corrosion temperature, metastable pitting
Published: 10.07.2015; Views: 712; Downloads: 69
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