Papers by Keyword: High Temperature Corrosion

Abstract: In the industrial production of primary aluminium process, the high temperature gaseous and the molten cryolite-alumina salt electrolyte will corrosion anode steel stubs. High temperature corrosion products of the stub are analyzed by SEM-EDX. Corrosion products consist of the mixed F, O, Na and Fe, etc. From computer simulation, high temperature corrosion of anode stubs will lead steel stubs current to inconsequence, cell voltage to high, anode consumption to inequality, etc. The current efficiency, current consumption and life of electrolysis cell will change to poor.

Abstract: High temperature corrosion of metal material was found on the side water-walls of a swirl-opposed firing supercritical boiler. The gas atmosphere near the side wall has great relationship with the high temperature corrosion. Test holes were installed on the side walls to form an experimental system. The gas compositions near the side walls were analyzed under different combustion conditions. High CO concentration is detected near the side walls. The effects of the operation states of burners on the CO concentration near side walls were tested. High over-fire air quantity results in strong reducing atmosphere of the test area. The adjusting method to control the atmosphere is suggested.

Abstract: Corrosion experiments were carried out with metals 20G, 15CrMoG and 12Cr1MoVG under the simulated atmosphere (N₂-5%O₂-1500μL/L HCl) of superheaters in waste plastic boilers to choose appropriate materials preventing high temperature corrosion. Corrosion dynamic curves were plotted by mass gain per unit area. Metal specimens after corrosion tests were analyzed by SEM-EDS and XRD. The results show that 20G has the poorest anti-corrosion abilities among the three materials; the corrosion process is involved in activation oxidation of chlorine. Addition of Cr in alloys can improve their corrosion resistance in the mixed atmosphere.

Abstract: The new experimental creep resistant ferritic steel of the 9Cr-3Co-3W type was oxidised at 650 °C in air and wet air. The oxidation kinetics was measured by intermittent weight measurement. The scales formed were analysed using techniques of XRD, SEM and EDS. The results showed that the oxidation rate was more than a magnitude faster in wet air than in air. The oxidation kinetics in air obeyed the parabolic rate law of oxidation only in a limited oxidation period of up to 1726 h whereas it did not follow any power rate law of oxidation in wet air. The steel cannot form a protective Cr2O3 scale either in air or in wet air at 650 °C. Instead, the scale formed in air consisted of an outer (Fe_0.6Cr_0.4)₂O₃ layer and an inner Cr-rich (Fe,Cr)₂O₃ layer containing Cr₂O₃ particles, but in wet air it consisted of an outer Fe₃O₄ layer and an inner (Fe,Cr)₃O₄ layer.

Abstract: The application of separated over-fire air (SOFA) in coal-fired boiler results in significant decrease of NOx emission but increases the trend of fireside water wall high temperature corrosion and incomplete combustion of pulverized coal. In order to meet the balance between low NOx emission and high boiler thermal efficiency, the main factors of air staging including SOFA flow, SOFA jet velocity and SOFA jet height were experimented on a 600MWe boiler. At the same time, the main components of flue gas beside the water wall were measured to evaluate the trend of high temperature corrosion. By comprehensive analysis, the optimum running mode of SOFA is concluded: (a) 75% of normal SOFA flow is suggested to reduce the trend of high temperature corrosion; (b) higher SOFA velocity is better for higher boiler thermal efficiency; (c) higher SOFA jet height is beneficial to the reduction of NOx emission.

Abstract: Under reducing conditions, H₂S, which will result in the high temperature corrosion of fireside water wall, is predominant sulfur compounds discharged from the coal. Influences of temperature and oxygen on the transformation mechanism of H₂S were studied by chemical kinetics. It is concluded from the study that the main productions of H₂S vary with the reaction conditions, COS is mainly produced in highly reducing conditions and SO₂ is dominant production under oxidizing atmosphere. Peak concentration of S₂ only can be obtained at mildly reducing conditions and moderate temperature; meanwhile, most of H₂S is oxidized to SO₂ when the temperature is above 1300K. A turning point is found when the concentration of O₂ is about 2%, and simulation results comply with experimental measurements.

Abstract: The high temperature corrosion behavior of Si-containing alloys consisting of Cr-Si-Ni and CoNiCrAlY-Si alloys fabricated by spark plasma sintering technique was investigated in the liquid phase of Na₂SO₄ + 25.7 mass% NaCl at temperatures ranging from 923-1273 K. The purpose of this study is to develop excellent corrosion resistant alloys for coating applications. Our experimental results show the CrSi₂ alloy with 10 mass% Ni content and the CoNiCrAlY alloy with 30 mass% Si content are the most promising materials for applications in this atmosphere. This is due to the formation of a protective SiO₂ and Al₂O₃/SiO₂ scale, respectively. The formation of a dense and continuous oxide layer composed and/or consisted of SiO₂ plays a significant role in hindering the inward diffusion of chlorine and sulfur to the alloys substrate. Particularly, the corrosion mechanism of Cr-Si-Ni alloys and the influence of Ni addition on the corrosion resistance of CrSi₂ alloy are discussed in the present paper.

Abstract: High durability, quality control and prediction of deterioration are key issues for the thermal spray coatings applied to fossil fuel and waste burning boilers. Small electrical resistance inspection (SERI) has been developed and applied for boilers to quantitatively evaluate the relationship between the deterioration mechanisms and the coating defects as well as life-times of coatings. Changes in the electrical resistance (ER) of coatings showed a good correlation with the deterioration causes and also with the bonding strength of the coatings. On the other hand, new processes such as thermal spraying and automatic fusing (SFU) process by high frequency induction heating have been developed to manufacture highly durable coatings. These SFU coatings, which are 0.5~3 mm in thickness and contain chemically bonded and dense NiCrSiB alloy layers have been found to have excellent durability which is almost equal to that of 625 weld overlays used in many WTE plants. Applications of these technologies are in progress to realize coatings with high cost performance, high efficiency and easy maintenance.

Abstract: The objective of this study is to develop an excellent corrosion resistant alloy for high temperature coating applications. The Si-containing alloys consisting of CoNiCrAlY and CrSi₂ alloys with varying Si and Ni content respectively were prepared by spark plasma sintering (SPS) technique. The corrosion behavior of these alloys was investigated in the gas phase of air-(Na₂SO₄+25.7mass%NaCl) at elevated temperatures of 923, 1073 and 1273K. The results showed that CoNiCrAlY alloy with 30mass% Si content and CrSi₂ alloy with 10mass% Ni content were the most effective materials for application in the gas phase of air-(Na₂SO₄+25.7mass%NaCl) due to the formation of protective Al₂O₃/SiO₂ and SiO₂ scale, respectively. Therefore, it is realized that CoNiCrAlY-30mass% Si and CrSi₂-10mass% Si coating are very effective for improving of high temperature corrosion resistance of STBA21 steel.

Abstract: Recovery of energy from biomass and various waste–derived fuels by combustion has become important due to reduction of detrimental CO₂ emissions. Biomass does, however, release significant amounts of chlorine and alkali metals, as e.g. HCl(g), KCl(g), KOH(g) and NaCl(g), into the gas phase during combustion. The alkali chlorides may cause deposits on superheater tubes, which interfere with operation and can lead to corrosion and/or blockage of the gas path. To prevent and diminish the problems mentioned above, better and more detailed knowledge of the reactions between potassium chloride and the tube materials during combustion is needed. These materials commonly contain, among other metals, chromium, which is thought to protect the rest of the material since it forms a very dense but thin oxide layer on the surface of the tube material. It has been suggested that the reaction between solid or partly molten KCl and chromium oxide is the one responsible for starting the complex series of corrosion reactions. In this work, the overall reaction between potassium chloride and chromium was studied through partial reactions with compounds known to participate to the overall reaction or to be formed during it. The reactions were studied in synthetic air by heating sample mixtures in a DTA/TGA (Differential Thermal Analysis/ Thermogravimetric Analysis) apparatus. Selected samples were also studied and analyzed with a scanning electron microscope equipped with an energy dispersive x-ray analyzer (SEM/EDXA). Under the used conditions both potassium chloride and potassium chromate reacted with pure chromium and chromium oxide. In the case of chromium, chromium oxide was formed via the formation of potassium chromate. In reactions including chromium oxide as reactant also potassium dichromate was detected.