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Mehanizem degradacije dušikovega nerjavnega jekla v kombinaciji napetostne in erozivne korozije : doktorska disertacija

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  • "To delo se opira na tezo, da se vpliv dušika v nerjavnih jeklih manifestira na kemične procese in trdnostni nivo matrice pred konico razpoke, kjer je prisotna elastoplastična deformacija matrice. Izkušnje iz prakse pa kažejo, da ima dušik vpliv tudi na splošno korozijsko odpornost. Toda v pogojih napetostno korozijskega pokanja, ki je podprto še z erozivno korozijo, se prepletajo procesi, ki v začetni fazi lahko potekajo le na vmesni površini med kovino in korozivnim medijem, to je v elektrokemičnem dvosloju, z nastankom korozijskih jamic, ki predstavljajo inicialna mesta za razvoj napetostno korozijske razpoke, pa se tovrstna problematika preseli na dogajanja v jamicah in razpoki. Takšni dogodki so posledica specifičnih elektrokemičnih reakcij na teh lokacijah, kinetika teh procesov pa je odvisna od substitucijsko (Cr, Ni, Mo...) in intersticijsko (N, C raztopljenih legirnih elementov v avstenitni ali dupleksni, avstenitno - feritni fazi, od mikrostrukture, možnosti iniciacije in rasti korozijskih jamic do kritične velikosti, ki omogoča razvoj razpoke, in nenazadnje, od procesov repasivacije iniciranih poškodb. Nove generacije nerjavnih jekel, ki so legirana z dušikom, lahko predstavljajo ključ za tovrstne izzive, zato je v tem delu problematika napetostne korozije v kombinaciji z erozivno korozijo vezana na avstenitno, z dušikom legirano nerjavno jeklo Cronifer 1925 hMo (delež dušika je 0,22 mas. %). Za potrditev določenih predpostavk je bila vključena še dupleksna, z dušikom obogatena nerjavna litina FeCr22Ni7M03N, pri kateri znaša delež dušika 0,2 masna odstotka. V tem raziskovalnem delu lahko izpostavimo naslednje ugotovitve: Ekvivalent odpornosti proti jamičenju je za dušikovo jeklo Cronifer 1925 hMo povsem v bližini visoko legiranih zlitin, to pa pomeni, da je to jeklo trdno zasidrano v področju, kjer imajo te vrste materiali dobro odpornost proti jamičasti koroziji. Toda bolj kot ekvivalent odpornosti proti jamičenju, je pomembna kritična temperatura jamičenja. S pomočjo galvanostatske anodne polarizacije smo definirali to temperaturo jamičenja. Dušik izrazito dviguje kritično temperaturo jamičenja Cronifer 1925hMo jekla. Ta je ob boku visoko legirani zlitini Nicrofer 6020 hMo. Primerjalno AISI 316L jeklo pa ima neprimerno nižjo kritično temperaturo jamičenja. Aktivacijska energija, ki je potrebna za nastanek jamic (iz jamic se razvijejo napetostno korozijske razpoke), je za dušikovo jeklo zelo visoka in znaša 726 kJ mori. To pomeni, da dušik sodeluje pri formiranju stabilnih pasivnih filmov, toda kasnejše raziskave so pokazale, da dejansko kreirajo korozijske procese NH/ ioni. Študij elektrokemičnih korozijskih procesov v korozijski jamici (ti procesi so enaki tudi v napetostno korozijski razpoki) kaže, da dušik dviguje pH vrednost elektrolita znotraj jamice. Takšni procesi so možni z reakcijo N3- ionov na površini kovine, s H+ ioni iz medija, ob tvorbi NH/, kar omogoča hitro pasivacijo ali repasivacijo korozijskih poškodb v pasivni plasti. NH4 + ioni odbijajo H+ ione iz elektrodnih površin, N3- ionipa Cl- ione, ki so najpomembnejša komponenta v agresivnem kloridnem mediju. S tem je bilo oteženo ali celo preprečeno nastajanje topnih in agresivnih soli kloridov, ki predstavljajo izvor kislosti na elektrodnih površinah. V elektrokemičnem dvosloju smo predstavili delovanje dušika oziroma N3- ionov na razmere v dvosloju in na procese polarizacije korozijskih reakcij, ki so nedvomno posledica delovanja dušika. Študij erozivne korozije dupleksne avstenitno - feritne nerjavne jeklene litine razkriva, da dušik omogoča hitro repasivacijo poškodovaninih mest v pogojih dinamičnega fluida, ki s svojimi trdnimi delci bombandira površinsko plast litine. Podobno se obnaša tudi Cronifer 1925 hMo jeklo. Deformacijsko utrjanje zelo tankega površinskega sloja, ki je posledica nastanka dvojčične strukture in dislokacijskega gozda ter tvorba NH4 + ionov, predstavlja kombinacijo, ki izrazito dviguje prag občutljivosti litin in jekel z dušikom proti tradicionalno neugodni degradaciji korozija - erozija. Raziskave vpliva dušika na lomno žilavost z asistenco korozivnega medija, dopolnjujejo rezultate raziskav napetostne korozije. Z deformacijskim utrjanjem avstenitne matrice pred konico utrujenostne razpoke in zaradi elektrokemičnega delovanja amonijevih ionov v sami razpoki, dosega Cronifer 1925 hMo jeklo neprimerno višji mejni faktor intenzitete napetosti kot AISI 316L jeklo, ali Nicrofer 412L hMo jeklo. Zaradi tega pa se pomika hitrost rasti inicirane mikrorazpoke v dušikovem nerjavnem jeklu proti vrednostim, ki so manjše od 1'10-10 ms-J. Ugotovili smo, da ima dušik velik pozitiven. učinek na procese napetostne korozije. Podaljšuje čas iniciacije jamice in zavira njeno rast do tiste kritične velikosti, pri kateri se inicira transkristaina napetostno korozijska mikrorazpoka. V sekundarnem področju korozijske krivulje raztezanja razpoka raste s hitrostjo okoli 1'10-10 ms-J , kar predstavlja po kriteriju raziskovalcev v svetu, mejno vrednost rasti razpoke, pod katero razpoka ne bo napredovala. Pri Cronifer 1925 hMo jeklu je bilo to doseženo z deformacijskim dvojčenjem v elastoplastični coni tik pred konico razpoke, z nastajanjem dislokacij oziroma dislokacijskega gozda v tem področju in medsebojnim trenjem dislokacij, katere intersticijsko raztopljeni dušik uspešno sidra. Dušik, ki znižuje energijo napake zloga, sicer posredno omogoča drsenje po drsni ravnini (111), toda posledica tega je utrjanje matrice in njenih kristalnih mej. To pomeni, da se pred konico razpoke povečuje trdnostni nivo, skozi tako utrjeni material pa težje nastaja mehanski zdrs, ki je potreben za napetostno korozijski proces. Sumarno lahko zaključimo, da dušik v nerjavnih jeklih in nerjavnih jeklenih litinah dviguje odpornost proti različnim oblikam degradacije, torej tudi proti tisti, ki je kombinacija napetostne in erozivne korozije, ta pa je v tej nalogi osrednji problem."@sl
  • "This work is based on the hypothesis that the influence of nitrogen in stainless steels is manifested in the chemical processes and hardness level of the lattice just in front of the crack tip, where elasto-plastic deformation of the lattice occurs. At the same time experience from practice has shown that nitrogen has an effect on general resistance to corrosion. However, under conditions of stress corrosion cracking, which may be supported by erosive corrosion, the processes which initially occur only at the interface between the metal and the corrosive medium, i.e. in the electrochemical double layer, are combined together with the occurrence of corrosion pits, which represent locations for the initiation of the development of stress corrosion cracks, and in this case the problem is now transferred to the events taking place in the corrosion pits and in the stress corrosion crack. These events are the consequence of specific electrochemical reactions at these locations, and the kinetics of these processes depend on the substitutionally (Cr, Ni, Mo...) and interstitially (N, C dissolved alloyed elements in the austenitic or duplex, austenitic- ferritic phase, as well as on the microstructure, on the conditions for the initiation and growth of corrosion pits up to the critical size which makes possible the development of a crack and, last but not least, on the processes of repassivation of the initiated damage. The new types of stainless steels, which are alloyed with nitrogen, represent the key to the above-described challenges, so in this work the question of stress corrosion in combination with erosive corrosion has been linked to the austenitic, nitrogen-alloyed stainless steel Cronifer 1925 hMo (which contains 0.22 % by mass of nitrogen). In order to confirm the validity of certain assumptions, the duplex, nitrogen-enriched stainless steel FeCr22Ni7M03N (which contains 0.2 % of nitrogen by mass) was included in the investigations. Within the framework of the research project the following findings can be highlightedČ The pitting resistance equivalent of the nitrogen-enriched steel Cronifer 1925 hMo was found to be very close to that of highly alloyed steels, which means that this steel is firmly located in the region of steels which have a high resistance to pitting corrosion. However it appears that the critical pitting temperature is of greater importance than the pitting resistance equivalent. This pitting temperature was determined by means of galvano-static anodic polarization. It was found that nitrogen significant1y raises the critical pitting temperature of the Cronifer 1925 hMo steel. This stands next to the highly alloyed steel Nicrofer 6020 hMo. In comparison, the steel AISI 3l6L has a much lower critical pitting temperature. The activation energy which is necessary for the occurrence of corrosion pits (it is from these pits that stress corrosion cracks arise) is, in the case of the nitrogen-enriched steel, very high, and amounts to 726 kJ mori. This means that the nitrogen participates in the forming of stable passive films, although later research showed that actually NH4 + ions create the corrosion processes. A study of the electrochemical corrosion processes in the corrosion pits (these processes are the same in the stress corrosion crack) has shown that nitrogen raises the pH value of the electrolyte inside the pit, and such processes are possible with the reaction of N3- ions on the surface of the metal with H+ ions from the medium, with the formation of NH4+. This makes the rapid passivation or repassivation of corrosion damage in the passive layer, possible. The NH4 + ions repel the H+ ions from the electrode surfaces, and the N3- ions repel the crions which are the most important component in aggressive chloride media. In this way the occurrence of dissolvable and aggressive chloride salts, which represent the source of acidity on the electrode surfaces, was hindered or even prevented. The influence of nitrogen, i.e. of N3- ions, on the conditions in the electrochemical doublelayer was studied, as well as its influence on the processes of polarization of corrosion reactions, which are undoubtedly the consequence of the action of nitrogen. The results of the study of the corrosion of the duplex austenitic-ferritic stainless steel revealed that nitrogen makes possible the rapid repassivation of damaged locations under the conditions of a dynamic fluid, which bombards the surface layer of the steel with its hard particles. The Cronifer 1925 hMo steel behaves in asimilar manner. The deformation hardening of the very thin surface layer, which is the consequence of the twinned and dislocation forest, represents, together with the formation of NH4 + ions, that combination of factors which significantly raises the threshold of sensitivity of alloys and nitrogen-enriched alloys and steels against the well-known detrimental effects of erosion - corrosion. Investigations into the influence of nitrogen on fracture toughness, with the assistance of a corrosive medium, add to the results obtained in the case of stress corrosion. By means of deformational hardening, due to the action of nitrogen, of the austenitic lattice in front of the tip of the fatigue crack, and because of the electrochemical operation of ammonium ions in the crack it self, the Cronifer 1925 hMo steel achieves a much higher stress intensity factor Kiscc than that corresponding to the AISI 316L and Nicrofer 4221 hMo steels. At the same time the rate of growth of the initiated micro-crack moves towards values which are smaller than 1.10-10 ms-1. It was found that nitrogen has a strong positive effect on the processes of stress corrosion. The time needed for the initiation of pits is lengthened, and the growth of such pits to the critical size where a trans-crystalline stress corrosion micro-crack is initiated is slowed down. In the secondary region of the corrosion elongation curve the crack grows at a rate of about 1.10.10 ms.l , which is widely recognized by researchers as the limit value for the growth of a crack, i.e. the limit below which a crack will not propagate. In the case of the Cronifer 1925 hMo steel this was achieved by twinning deformation in the elastoplastic zone just in front of the crack tip, with the occurrence of dislocations and a dislocation forest in this region, and the mutual friction of the dislocations, which are successfully anchored by the interstitially absorbed nitrogen. Nitrogen, which reduces the stacking fault energy, also indirectly makes possible slipping along the slip plain (111), but the consequence of this is hardening of the lattice and of its crystal boundaries. This means that the hardness level of the lattice is increased in front of the crack tip, and thus it is more difficult for the mechanical slip which is needed for the stress corrosion process to occur through such hardened material. It can be concluded that in stainless steels, and in non-stainless steel alloys, nitrogen raises the level of resistance to different types of degradation, and thus also to that type of degradation which is a combination of stress and erosion corrosion, which has been the main subject of this investigation."@sl

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  • "Mehanizem degradacije dušikovega nerjavnega jekla v kombinaciji napetostne in erozivne korozije : doktorska disertacija"@sl