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There has been an increase in the production of bio-fuels. Organosolv delignification, high temperature ethanol/water environments, can be used to separate lignin, cellulose, and hemicelluloses in the bio-mass for bio-fuel production. These environments have been shown to induce stress corrosion cracking (SCC) in 316L stainless steel. Previous research has been done in mixed solvent environments at room temperature to understand SCC for stainless steels, but little is known about the behavior in high temperature environments. Simulated organosolv delignification environments were studied, varying water content, temperature, pHe, and Cl- content to understand how these constituents impact SCC. In order for SCC to occur in 316L, there needs to be between 10 and 90 volume % water and the environment needs to be at a temperature around 200°C. Once these two conditions are met, the environment needs to either have pHe < 4 or have more than 10 ppm Cl-. These threshold conditions are based on the organosolv delignification simulated environments tested. SCC severity was seen to increase as water content, temperature, and Cl- content increased and as pHe decreased. To prevent failure of industrial vessels encountering organosolv delignification environments, care needs to be taken to monitor and adjust the constituents to prevent SCC.

The effect of a specific type of pre-exposure heat treatment on the susceptibility of AlSI type 304 stainless steel to stress corrosion cracking was studied in terms of time for crack nucleation and rate of crack propagation. U-bend specimens were exposed to 42 weight percent magnesium chloride aqueous solution after pre-exposure heat treatments at 140°C and 154°C. The straight-line relationship between maximum crack depth and the logarithm of exposure time expressed by the empirical equation log t = log t_o + D/M was obtained. The stress corrosion constants derived from the empirical equation indicate that this type of pre-exposure heat treatment promotes crack nucleation because of the formation of less protective surface films, and retards the rate of crack propagation because of effects on internal structural changes within the alloy. / Master of Science

L’état de surface des vis de liaison des internes de cuve du circuit primaire des REP (Réacteurs à Eau Pressurisée) en 316L en service correspond à une finition d’usinage par rectification. Ces vis sont affectées par l’IASCC (Corrosion Sous contrainte Assistée par l’Irradiation). Le processus d’amorçage de la fissuration est fonction de l’oxydation externe, de l’état de surface et des interactions de la couche d’oxyde avec la localisation de la plasticité. Un des objectifs de cette étude est déterminer l’influence de la préparation de surface sur la cinétique de croissance des couches d’oxyde et la réactivité de surface en général des nuances de type 304, 316 exposées en milieu primaire des REP à 340°C. Le second objectif est de déterminer l’influence de la localisation de la déformation sur la CSC (Corrosion Sous Contraintes) des aciers inoxydables austénitiques en milieu primaire des REP. En effet, la microstructure représentative de ces nuances irradiées correspond à une microstructure à déformation localisée dans des bandes de déformation dépourvues de défauts d’irradiation. Afin de reproduire cette microstructure représentative sur le matériau modèle de l’étude (l’acier austénitique inoxydable A286 durci par la précipitation de la phase γ’ Ni3(Ti,Al)) sans avoir recours à l’irradiation, des essais de fatigue oligocyclique à Δε/2 imposée sont réalisés. Durant le cyclage mécanique (après les premiers cycles de durcissement), les précipités sont dissouts dans des bandes de glissement menant à la localisation de la déformation. Une fois les conditions expérimentales en fatigue oligocyclique permettant d’obtenir la microstructure de déformation désirée déterminées, les interactions bandes de déformation / oxyde de surface sont étudiées en oxydant des coupons pré déformés contenant des bandes de déformation et des coupons non déformés. La préparation de surface des coupons est identique. Les essais de traction lente à une vitesse de déformation de 8 x 10-8/s sont également réalisés sur des éprouvettes pré déformées et non déformées. Les résultats ont montré que la préparation de surface modifie la microstructure du métal sous la couche d’oxyde, conduisant à un ralentissement de la cinétique de croissance de la couche d’oxyde. La préparation de surface induit cependant une accélération du développement de pénétrations d’oxydes dans le métal sous la couche d’oxyde. Ainsi, sur les échantillons rectifiés, la zone recristallisée sous la couche d’oxyde est plus profonde que sur les échantillons polis (jusqu’à 1,5μm contre 500nm au maximum sur les échantillons polis) et la couche d’oxyde est plus fine que sur les échantillons polis, tandis que les pénétrations d’oxyde sont présentes sur près de 1μm de profondeur en sous couche (contre 300nm sur les échantillons polis). Nous montrons que la zone de recristallisation induite par la préparation de surface ne permet pas l’observation des interactions entre les bandes de déformation générées dans le volume par la fatigue oligocyclique et la couche d’oxyde en surface. De fait, la réactivité de surface est très importante pour l’étude de la CSC des aciers inoxydables en milieu primaire des REP. Nous avons également démontré que cette nuance était très sensible à la corrosion intergranulaire en milieu REP à 340°C. Enfin, la localisation de la déformation plastique ne semble pas favoriser la CSC sur cette nuance à cette vitesse de déformation. / The surface conditions of the 316L screw connecting vessel internals of the primary circuit of PWR (pressurized water reactor) corresponds to a grinding condition. These screws are affected by the IASCC (Irradiation Assisted Stress Corrosion Cracking). Initiation of cracking depends on the surface condition but also on the external oxidation and interactions of oxide layer with the deformation bands. The first objective of this study is to point the influence of surface condition on the growth kinetic of oxide layer, and the surface reactivity of 304, 316 stainless steel grade exposed to PWR primary water at 340 ° C. The second objective is to determine influence of strain localization on the SCC of austenitic stainless steels in PWR primary water. Indeed, the microstructure of irradiated 304, 316 grades correspond to a localized deformation in deformation bands free of radiation defects. In order to reproduce that microstructure without conducting irradiations, low cycle fatigue tests at controlled stain amplitude are implemented for the model material of the study (A286 austenitic stainless steel hardened by the precipitation of phase γ ‘Ni 3 (Ti, Al)). During the mechanical cycling (after the first hardening cycles), the precipitates are dissolved in slip bands leading to the localization of the deformation. Once the right experimental conditions in low cycle fatigue obtained (for localized microstructure), interactions oxidation / deformation bands are studied by oxidizing pre deformed samples containing deformation bands and non deformed samples. The tensile tests at a slow strain rate of 8 x 10-8 /s are also carried out on pre deformed samples and undeformed samples. The results showed that surface treatment induces microstructural modifications of the metal just under the oxide layer, leading to slower growth kinetics of the oxide layer. However, surface treatment accelerates development of oxides penetrations in metal under the oxide layer. As example, for grinded samples, the recrystallized area under the oxide layer, induced by surface treatment, is deeper than for polished sample (up to 1.5 microns vs 500 nm for the polished samples) and the oxide layer is thinner than on the polished samples, while the penetrations oxide are expands on nearly 1μm under the oxide layer (against 300nm for the polished samples). We also show that the area recrystallization resulting from surface treatment, does not allow observing the interactions between the deformation bands in the bulk generated by LCF and the oxide layer at surface. Actually, surface reactivity is strongly important for SCC study of stainless steels in PWR primary water. We also demonstrated that this grade was very sensitive to intergranular corrosion in PWR environment at 340 ° C. Finally, localization of plastic deformation does not seem to favor SCC in our A-286 grade, at that strain rate level

A thesis submitted to the Faculty of Engineering and the Built Environment, University of Witwatersrand, Johannesburg in fulfilment of the requirements for the degree of Doctor of Philosophy (Engineering), 2017 / Chromium oxide provides an inexpensive and practical means of increasing the corrosion resistance of austenitic stainless steel in most environments. However, the oxide is prone to dissolve in reducing acids and in chloride containing solutions, which compromises the durability and effective operation of structures made of austenitic stainless steel. This research project explored the use of thin ruthenium surface alloys produced by ion implantation, RF sputtering and pulsed electrodeposition (PED) to improve the corrosion resistance of AISI 304L austenitic stainless steel in reducing acids and chloride solutions via a technique known as cathodic modification. The properties of the alloyed 304L stainless steel were evaluated using a number of tools including X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), potentiodynamic polarisation, and electrochemical impedance spectroscopy (EIS). Preliminary tests in 1 M sulphuric acid showed that the ruthenium surface alloys sufficiently raised the corrosion potential of 304L stainless steel to ranges where the stability of chromium oxide is guaranteed. Surface alloys produced by RF sputtering and PED were associated with the best corrosion resistance, and protection efficiencies of at least 85%, but they spalled during corrosion exposure rendering them unsuitable for corrosion application. The corrosion of the ruthenium implanted surface alloys exhibited a strong dependence on the surface roughness of the stainless steel, with the least corrosion rates achieved on rough 304L stainless steel samples implanted with 1016 Ru/cm2 at 50 keV. Corrosion characterisation of these ruthenium implanted surface alloys was studied in various corrosive media including sulphuric acid, sodium chloride, magnesium chloride and simulated fuel cell solutions. Their corrosion rates in sulphuric acid decreased with increase in acid concentration, and exhibited non-Arrhenius behaviour in the acid solutions; corrosion rates were unaffected by increasing exposure temperature from 25 to 50°C. In 3.5 wt% sodium chloride, addition of ruthenium via ion implantation changed pit morphology from elongated to circular, indicating a diminished tendency for pits to initiate at manganese sulphide stringers. Corrosion rates of the ruthenium implanted stainless steels in the simulated fuel cell solutions were at least 69% lower than the target corrosion rate for use in polymer electrode membrane fuel cells (PEMFCs), thus presenting a possible practical application of ruthenium surface alloyed austenitic stainless steel. / CK2018

Student Number : 9904952F - MSc Dissertation - School of Chemical and Metallurgical Engineering - Faculty of Engineering and the Built Environment / This dissertation is a study of the effect of manganese, nitrogen and molybdenum on the corrosion behaviour of a low nickel, austenitic stainless steel. The trademarked steel, HerculesTM, has a composition of 10 wt% Mn, 0.05 wt% C, 2 wt% Ni, 0.25 wt% N and 16.5 wt% Cr. Eighteen alloys with a HerculesTM base composition were made with varying manganese, molybdenum and nitrogen contents, to establish the effect of these elements on the corrosion behaviour of the steel, and to determine a composition that would ensure increased corrosion resistance in very corrosive applications. The manganese was varied in three levels (5, 10 and 15 wt%), the molybdenum in three levels (0.5, 1 and 2 wt%) while the nitrogen was varied only in two levels (0.15 and 0.3 wt%). The dissertation details the manufacturing and electrochemical corrosion testing of these alloys. Preliminary tests were done on 50g buttons, and full-scale tests on 5 kg ingots. The buttons had a composition that was not on target, this was however rectified in the making of the ingots. Potentiodynamic tests were done in a 5 wt% sulphuric acid solution and the corrosion rate (mm/y) was determined directly from the scans. From the corrosion test results, it was clear that an increase in manganese decreases the corrosion rate, since the 5 wt% Mn alloys had the highest corrosion rate, whereas the 15 wt% Mn alloys, the lowest. The addition of molybdenum at 5 wt% Mn decreased the corrosion rate such that a trend of decreasing corrosion rate with increasing molybdenum was observed. At 10 and 15 wt% Mn molybdenum again decreased the corrosion rate significantly, but the corrosion rate value remained more or less constant irrespective of the increasing molybdenum content. At nitrogen levels lower than those of HerculesTM (less than 0.25 wt%) there was no change in corrosion rate as nitrogen was increased to levels closer to 0.25 wt%. For nitrogen levels higher than 0.25 wt%, corrosion rates decreased as nitrogen levels were increased further from 0.25 wt% but only at Mo contents lower than 1.5 wt%. The HerculesTM composition was developed for its mechanical properties. Microstructural analyses revealed that the 5 wt% Mn alloys were not fully austenitic and since the 15 wt% Mn alloys behave similarly to the 10 wt% Mn alloys, it was concluded that 10 wt% Mn was optimum for HerculesTM. All the alloys tested had a much lower corrosion rate than HerculesTM. Any addition of molybdenum thus improved the corrosion rate of this alloy. An alloy with a HerculesTM base composition, 10 wt% Mn, 0.15 wt% N and a minimum addition of 0.5 wt% Mo would be a more corrosion resistant version of HerculesTM. Pitting tests were done on the 10 wt% Mn ingots in a 3.56 wt% sodium chloride solution. The results showed that an increase in molybdenum increased the pitting resistance of the ingots. Immersion tests in a 5 wt% sulphuric acid solution at room temeperature on the 10 wt% Mn ingots confirmed that the ingots corroded by means of general corrosion.

Os aços inoxidáveis possuem numerosas aplicações devido à boa combinação de propriedades tais como resistência à corrosão e oxidação, ductilidade, tenacidade, soldabilidade e resistência mecânica em temperaturas elevadas. No entanto suas propriedades e desempenho estão fortemente relacionados com a microestrutura que por sua vez, no caso de peças fundidas, dependem principalmente da composição química e da velocidade de solidificação. No presente trabalho o efeito destas duas variáveis foram estudados e os resultados experimentais comparados com as previsões teóricas e modelos disponíveis na literatura. Dezesseis corridas de diferentes aços inoxidáveis austeníticos foram fundidas e suas composições químicas completas (16 elementos analisados) foram determinadas. A maioria das corridas analisadas apresentou modo de solidificação do tipo C. Foram encontrados teores de ferrita (medidos com auxílio de ferritoscopia) na faixa de 0 a 11%. A influência da composição química do aço na quantidade de ferrita delta formada foi marcante, enquanto a influência da espessura foi pouco acentuada. Dentre as numerosas fórmulas testadas para a previsão da quantidade de ferrita delta, as duas que apresentaram melhor resultado foram as fórmulas de Schneider e de Schoefer, sendo que esta última é recomendada pela norma ASTM A800. A amostra contendo cerca de 10% de ferrita apresentou uma rede quase contínua, o que pode comprometer a tenacidade da peça, caso esta ferrita venha a sofrer fragilização. Nas amostras contendo por volta de 5% de ferrita, a rede de ferrita é semi-contínua, enquanto para teores baixos (por volta de 2%), a ferrita apresenta-se como ilhas isoladas. As morfologias encontradas foram classificadas como sendo todas do tipo vermicular. Os estudos de micro-análise química dos elementos Si, Mo, Cr, Fe e Ni, efetuados na ferrita e na austenita revelaram coeficientes de partição de acordo com o previsto pela literatura. O efeito da espessura nas variações de composição foi pequeno e não conclusivo. / Stainless steel has numerous applications due to a good combination of properties such as corrosion and oxidation resistance, toughness, weldability and mechanical strength at high temperatures. However these properties and performance are strongly related to the microstructure and in the case of castings are mainly dependent of chemical composition and cooling rate. In this work the effect of these two factors were studied and the experimental results compared with theoretical models available in the literature. Sixteen heats of different austenitic stainless steel were cast and their complete chemical compositions (16 elements) were determined. Most of analyzed heats showed the solidification mode type C. Ferrite values (measured with ferritoscope) were found in the range from 0 to 11%. The influence of chemical composition on delta ferrite was strong while the influence of thickness was less accentuated. Among numerous tested formulas to estimate the quantity of delta ferrite two that demonstrated better results were the ones of Schneider and Schoefer, where the last one is recommended by ASTM A800 standard. The sample with approximately 10% of ferrite showed an almost continuous ferrite network microstructure that may deteriorate component part toughness if this ferrite comes to suffer embrittlement. On the samples with content ferrite around 5% the ferrite network is semi-continuous while for low values (around 2%) the ferrite showed isolated cores. The morphologies were classified as vermicular. The study of micro chemical analysis of Si, Mo, Cr, Fe and Ni on ferrite and austenite showed partition coefficient in accordance with values defined in literature. The thickness effect on chemical composition was small and not conclusive.

Global increase in energy consumption and global warming require more energy production but less CO2emission. Increase in efficiency of energy production is an effective way for this purpose. This can be reached by increasing boiler temperature and pressure in a biomass power plant. By increasing material temperature 50°C, the efficiency in biomass power plants can be increased significantly and the CO2emission can be greatly reduced. However, the materials used for future biomass power plants with higher temperature require improved properties. Austenitic stainless steels are used in most biomass power plants. In austenitic stainless steels a phenomenon called dynamic strain aging (DSA), can occur in the operating temperature range for biomass power plants. DSA is an effect of interaction between moving dislocations and solute atoms and occurs during deformation at certain temperatures. An investigation of DSA influences on ductility in austenitic stainless steels and nickel base alloys have been done. Tensile tests at room temperature up to 700°C and scanning electron microscope investigations have been used. Tensile tests revealed that ductility increases with increased temperature for some materials when for others the ductility decreases. This is, probably due to formation of twins. Increased stacking fault energy (SFE) gives increased amount of twins and high nickel content gives a higher SFE. Deformation mechanisms observed in the microstructure are glide bands (or deformations band), twins, dislocation cells and shear bands. Damage due to DSA can probably be related to intersection between glide bands or twins, see figure 6 a). Broken particles and voids are damage mechanisms observed in the microstructure.

Austenitic stainless steels have a major problem during solution annealing or welding in the temperature range of 500-800 &deg / C due to the formation of chromium carbide, which causes chromium depleted areas along grain boundaries. This means that the structure has become sensitized to intergranular corrosion. Susceptibility to intergranular corrosion can be determined by means of destructive acid tests or by nondestructive electrochemical potentiokinetic reactivation (EPR) tests. The EPR test, which provides quantitative measurements, can be practiced as single loop or double loop. Single loop EPR method for AISI 304 and 304L type stainless steels was standardized / however double loop EPR (DLEPR) method has not been validated yet. In this study, the degree of sensitization was examined in AISI 304L and 316L type steels by DLEPR method whose experiments have been carried out on sensitive and nonsensitive steels to examine and determine the detailed parameters / solution temperature, concentration and scan rate of the DLEPR method. In order to determine the degree of sensitization, oxalic acid, Huey and Streicher tests were carried out and revealed microstructures and measurements of weight loss by the acid tests were then correlated with DLEPR method results, as a first step towards standardization of DLEPR method for 316L steels. Best agreement was provided with test parameters which are 1M H2SO4 + 0.005M KSCN at 3 V/hr scan rate with 30 &deg / C solution temperature. It was concluded that specimens can be classified as step, dual and ditch, if the Ir:Ia ratios were obtained to be between 0 to 0.15, 0.15 to 4.0 and 4.0 to higher respectively.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Dr. Richard W. Neu; Committee Member: Dr. David L. McDowell; Committee Member: Dr. Itzhak Green.