Stress corrosion cracking of a 14 wt% Cr martensitic stainless steel, with commercial names PH-15Cr5Ni, FV520B or X4CrNiCuMo15-5, used for the manufacture of low pressure turbine blades, has been studied with the intention of gaining a better understanding of the processes involved, how they occur and why. Industrially this is very important as stress corrosion cracking is considered to be a delayed failure process, whereby microscopic cracks can potentially propagate through a metal undetected until catastrophic failure occurs. The aim of this work is to establish links between crack length and external factors, such as exposure time, in order to devise a method of dating stress corrosion cracks and therefore predicting their possible occurrence in-service.

Neste trabalho estudou-se o efeito do tratamento de boretação na resistência ao desgaste, corrosão e oxidação dos aços AISI 1060 e AISI H13. As amostras dos aços foram boretadas com pó a 900 e 1000°C por 2 e 4 horas. Foram realizados ensaios de microdesgaste em máquina do tipo esfera fixa, sem uso de abrasivos, nas amostras boretadas e não boretadas. A resistência à corrosão, das amostras boretadas e não boretadas, em HCl 0.1M foi avaliada por ensaios de polarização potenciodinâmica. Ensaios de oxidação do tipo quase-isotérmica foram realizados nas amostras, utilizando temperatura de 550°C ao ar. Para todas as condições de tratamento empregadas foram formadas camadas de boretos com elevadas durezas sobre os substratos. As propriedades das camadas como dureza, espessura, morfologia da interface camada/substrato e as fases presentes, foram influenciadas pelas composições dos aços. No caso do aço AISI H13, que possui maior quantidade de elementos de liga, as camadas formadas foram mais duras, menos espessas, com interface lisa, e para todas as condições de tratamento foram formadas os compostos de boro Fe2B, FeB e CrB. Já no aço AISI 1060 as interfaces camada/substrato foram do tipo dentada e para a temperatura de boretação de 900°C apenas a fase Fe2B foi formada, aparecendo o FeB a partir da temperatura de boretação de 1000°C. A resistência ao desgaste, corrosão e oxidação dos aços boretados aumentou significativamente, sendo que em relação ao desgaste, os melhores resultados foram obtidos com as amostras boretadas a 1000°C. Quanto à oxidação, os dois aços boretados a 900°C por 2h apresentaram os melhores desempenhos, e quanto à corrosão, os parâmetros do tratamento de boretação não influenciaram significativamente os desempenhos. / In this work the effect of pack boriding on wear, corrosion and oxidation resistance of AISI 1060 and AISI H13 steels were studied. Samples of the steels were pack borided at 900 and 1000°C for periods of 2 and 4h. Fixed-ball microwear tests were performed on borided and non-borided samples, without use of abrasives. The samples corrosion resistances, in HCl 0.1M, were evaluated by potentiodynamic polarization tests. Quasiisothermal oxidation tests were performed at 550°C on air. High hard layers were formed on steels for all treatment conditions employed. The layers properties, such as hardness, thickness, layer/substrate interface morphology and phases formed, were influenced by steels compositions. For AISI H13 steel, which has a larger amount of lloying elements, the formed layers were arder, thinner, with smooth interface and for all treatments conditions the compounds Fe2B, FeB and CrB were formed. For borided AISI 1060 steel, the layer/substrate interfaces showed a saw-tooth morphology and for boriding temperature of 900°C only the Fe2B phase were formed, appearing FeB for boriding temperature of 1000°C. The wear, corrosion and oxidation resistances of borided steels significantly increased and in relation to wear, the best results were btained with the samples borided at 1000°C. In oxidation case, both steels borided at 900°C for 2h showed the best performances, and with respect to corrosion, boriding treatments parameters did not significantly affect the performances.

Neste trabalho estudou-se o efeito do tratamento de boretação na resistência ao desgaste, corrosão e oxidação dos aços AISI 1060 e AISI H13. As amostras dos aços foram boretadas com pó a 900 e 1000°C por 2 e 4 horas. Foram realizados ensaios de microdesgaste em máquina do tipo esfera fixa, sem uso de abrasivos, nas amostras boretadas e não boretadas. A resistência à corrosão, das amostras boretadas e não boretadas, em HCl 0.1M foi avaliada por ensaios de polarização potenciodinâmica. Ensaios de oxidação do tipo quase-isotérmica foram realizados nas amostras, utilizando temperatura de 550°C ao ar. Para todas as condições de tratamento empregadas foram formadas camadas de boretos com elevadas durezas sobre os substratos. As propriedades das camadas como dureza, espessura, morfologia da interface camada/substrato e as fases presentes, foram influenciadas pelas composições dos aços. No caso do aço AISI H13, que possui maior quantidade de elementos de liga, as camadas formadas foram mais duras, menos espessas, com interface lisa, e para todas as condições de tratamento foram formadas os compostos de boro Fe2B, FeB e CrB. Já no aço AISI 1060 as interfaces camada/substrato foram do tipo dentada e para a temperatura de boretação de 900°C apenas a fase Fe2B foi formada, aparecendo o FeB a partir da temperatura de boretação de 1000°C. A resistência ao desgaste, corrosão e oxidação dos aços boretados aumentou significativamente, sendo que em relação ao desgaste, os melhores resultados foram obtidos com as amostras boretadas a 1000°C. Quanto à oxidação, os dois aços boretados a 900°C por 2h apresentaram os melhores desempenhos, e quanto à corrosão, os parâmetros do tratamento de boretação não influenciaram significativamente os desempenhos. / In this work the effect of pack boriding on wear, corrosion and oxidation resistance of AISI 1060 and AISI H13 steels were studied. Samples of the steels were pack borided at 900 and 1000°C for periods of 2 and 4h. Fixed-ball microwear tests were performed on borided and non-borided samples, without use of abrasives. The samples corrosion resistances, in HCl 0.1M, were evaluated by potentiodynamic polarization tests. Quasiisothermal oxidation tests were performed at 550°C on air. High hard layers were formed on steels for all treatment conditions employed. The layers properties, such as hardness, thickness, layer/substrate interface morphology and phases formed, were influenced by steels compositions. For AISI H13 steel, which has a larger amount of lloying elements, the formed layers were arder, thinner, with smooth interface and for all treatments conditions the compounds Fe2B, FeB and CrB were formed. For borided AISI 1060 steel, the layer/substrate interfaces showed a saw-tooth morphology and for boriding temperature of 900°C only the Fe2B phase were formed, appearing FeB for boriding temperature of 1000°C. The wear, corrosion and oxidation resistances of borided steels significantly increased and in relation to wear, the best results were btained with the samples borided at 1000°C. In oxidation case, both steels borided at 900°C for 2h showed the best performances, and with respect to corrosion, boriding treatments parameters did not significantly affect the performances.

The PhD thesis provides detailed description of a successful development of MgO and TiO2 particle reinforced Steel Ceramic Composites (SCC) for molten aluminum alloy applications. For this purpose, the influence of MgO and TiO2 addition and subsequent pre-oxidation surface treatment on the structure of SCCs and their corrosion resistance against long-term contact with liquid aluminum alloys was investigated. The initiation and progression of corrosion processes were thoroughly analyzed by means of newly developed DSC-aided corrosion tests, high temperature electrochemical studies and adapted wettability measurements. The gained insights led to the recognition of most important factors contributing to the corrosion, including both the electrochemical and the chemical driving forces arising between the SCCs and aluminum alloy. The evaluation of long-term corrosion resistance was performed with the help of finger immersion tests, crucible corrosion tests and subsequent SEM/EDS/EBSD and XRD analyses aiming at the determination of elements most prone to the dissolution in the liquid aluminum alloy and formation of corrosion phases. The pre-oxidized MgO reinforced SCC revealed superior corrosion resistance, being capable of withstanding more than 168 h of contact with liquid aluminum alloy.:Table of content 1 Introduction 1 2 Theoretical background 5 2.1 Wettability measurements 5 2.2 Electrochemical behavior of SCC/molten aluminum alloy material pair 8 2.3 Steel-based materials/molten aluminum alloy reaction 13 2.4 Long-term corrosion mechanisms 16 2.5 Differential Scanning Calorimetry for corrosion precipitation analysis 18 2.6 Corrosion of steel and SCCs during long-term contact with aluminum alloys 19 2.7 Protective coatings and surface treatment of steel and Steel Ceramic Composites 21 2.7.1 Protective coatings against molten aluminum alloys 21 2.7.2 Oxidation kinetics 22 3 Materials and Methods 25 3.1 Materials and Composites Manufacturing 26 3.2 Investigation of corrosion phase formation via DSC-aided corrosion tests 29 3.3 High temperature electrochemical studies 30 3.4 Elaboration of suitable surface pre oxidation for SCCs 33 3.5 Wettability Tests 34 3.6 Finger immersion tests 35 3.7 Crucible Corrosion Tests 36 4 Results and Discussion 41 4.1 Investigation of corrosion phase formation using DSC-aided corrosion tests 41 4.1.1 Determination of reference information for DSC-aided corrosion test 41 4.1.2 Influence of the sample/melt contact duration on the alteration of DSC signal – elaboration of suitable corrosion test conditions. 44 4.1.3 Investigation of 120 min contact time between 316L40TiO2 and 316L40MgO Steel Ceramic Composites with aluminum alloy on the formation of corrosion phases in the melt 47 4.1.4 SEM/EDS microscopical analysis of 316L sample after DSC-aided corrosion test with AlSi7Mg0.3 aluminum alloy for 120 min 51 4.1.5 SEM/EDS microscopical analysis of 316L40TiO2 sample after DSC-aided corrosion test with AlSi7Mg0.3 aluminum alloy for 120 min 53 4.1.6 SEM/EDS microscopical analysis of 316L40MgO sample after DSC-aided corrosion test with AlSi7Mg0.3 aluminum alloy for 120 min 55 4.2 High temperature electrochemical studies of SCCs 60 4.2.1 Evaluation of thermal and chemical stability of selected three-electrode cell materials 60 4.2.2 Differential Potential 61 4.2.3 Impedance Spectroscopy and Potentiodynamic Polarization 64 4.2.4 Microscopical analysis of WE after the electrochemical experiment 68 4.3 Surface treatment of SCCs 79 4.3.1 Dilatometry and Thermogravimetry of SCCs during pre-oxidation 79 4.3.2 Preliminary evaluation of morphology of the SCCs cross-section after pre oxidation at different temperatures and for different durations 83 4.3.3 Detailed SEM/EDS/XRD structure analysis of selected pre-oxidized SCCs 87 4.4 Wettability of aluminum alloy on SCCs 102 4.4.1 Characterization of substrates surface 102 4.4.2 Wetting angle at the drop release 102 4.4.3 Wetting angle 30 min after reaching 850 °C 104 4.4.4 Evaluation of the droplet/substrate cross-section 105 4.5 Finger Immersion Tests 107 4.5.1 Preliminary evaluation of peroxidized SCCs after immersion test 107 4.5.2 Analysis of 316L40TiO2 immersion sample pre oxidized at 850 °C for 24 h 111 4.5.3 Analysis of 316L40TiO2 immersion sample pre-oxidized at 1000 °C for 24 h 112 4.5.4 Analysis of 316L40MgO immersion sample pre oxidized at 850 °C for 24 h 113 4.5.5 Analysis of 316L40MgO immersion sample pre-oxidized at 1000 C for 24 h 114 4.6 Crucible Corrosion Tests 115 4.6.1 Preliminary evaluation of crucible corrosion test results 115 4.6.2 Analysis of 316L40TiO2 sample pre oxidized at 850 °C for 24 h after the crucible corrosion test for 24 h 119 4.6.3 Analysis of 316L40TiO2 sample pre-oxidized at 1000 °C for 24 h after the crucible corrosion test for 168 h 120 4.6.4 Analysis of 316L40MgO sample pre-oxidized at 850 °C for 24 h after the crucible corrosion test for 24 h 122 4.6.5 Analysis of 316L40MgO sample pre-oxidized at 1000 °C for 24 h after the crucible corrosion test for 168 h 125 4.6.6 Analysis of the microstructure of aluminum alloy after crucible corrosion tests 128 4.6.7 Evaluation of contamination of aluminum alloy after crucible corrosion tests 133 Conclusions 137 References 145 Appendixes 161 Appendix A: Preliminary Investigations 161 A.1: Preparation of SCC granulates 161 A.2: Evaluation of properties of composites pressed from granulates 162 Appendix B: Constructions and Designs 169 B.1. Three-Electrode Cell – for high temperature electrochemical measurements with molten aluminum alloys as reference electrode 169 B.2. Capillary System – for capillary purification technique wettability measurements with aluminum alloys 177 Appendix C: Auxiliary Investigations 184 C.1 Detailed SEM/EDS analysis of pre-oxidized SCCs 184 C.2 SEM/EDS analysis of SCCs after 96 h Finger Immersion Tests in aluminum alloy 207