Satu Tuurna

High Temperature Oxidation Behaviour of Boiler Steels under Simulated Combustion Gases

The results presented here are a part of the study aiming at control and optimisation of inservice performance of boiler materials and development of simulation tools for high temperature corrosion and oxidation of steels under service conditions (EU FP5 OPTICORR Project). The high temperature oxidation behaviour of three ferritic steels was studied at temperatures of 500 and 600°C under different synthetic atmospheres simulating combustion environments. Multi-sample exposure tests in combination with SEM/EDS and XRD techniques pointed out a significant influence of the temperature and gas composition on the morphology and growth rate of the scale. The results showed that oxidation kinetics and mechanism are strongly dependent on gas species such as H2O, HCl and SO2. The presence of HCl in moist air at 500°C and 600°C accelerated the oxidation process of the steels studied. In contrast, the presence of SO2 suppressed this process. The SEM/EDS studies suggested that in HCl containing atmospheres the corrosion mechanism is active oxidation. The reduction in oxidation rate in SO2 containing atmospheres could be due to the presence of sulphates at the metal/scale interface which probably influences ion transport through the oxide scale.

Effect of Al Enrichment by Pack Cementation of FeCr Coatings Deposited by HVOF

A great contribution to CO2 emissions comes from coal fired power generation. Combination of carbon capture sequestering technologies with sustainable biomass conversion constitutes a decisive boost in limiting rise in global temperature. Co-firing alternative materials with pulverized coal and using oxy-fuel combustion conditions (oxy-fuel co-combustion) is a very attractive process for power industry. Materials with both high mechanical properties and high environmental resistance are required by such advanced combustion systems. One approach to improve high-temperature oxidation/corrosion resistance is to apply protective coatings. In the present work, low and high Cr content Fe-based alloys have been deposited in order to investigate the influence of Cr content on coating protective performance in oxy-fuel co-combustion conditions. Grade 91 steel has been assumed as reference substrate. Effect of Al enrichment on coating environmental resistance has also been analyzed. Activities have been performed within the framework of Macplus Project (Integrated Project co-founded by the European Commission under the 7th Framework Program in the Energy area).

Supercritical Oxidation of Boiler Tube Materials

The advantage of using supercritical water systems for power generation is based on the increased thermodynamic efficiency when operating at higher temperature and pressure. High efficiency in power generation is not only desirable because of economical reasons but also for enhanced environmental performance. Steam oxidation has become an important issue for steam power plants as operating temperatures increase from current to 650#xb0;C and even higher. To achieve these higher steam values new materials are needed. This paper presents results of the oxidation performance of potential new alloys (FeCrAlY, NiCrAl and Sanicro 25) in a supercritical water autoclave environment at 650#xb0;C/250 bar.

The Effect of Water Vapour and Sulphur on Corrosion Mechanisms of Steels

It is well known that water vapour accelerates oxidation; however different gas conditions and material compositions affect the mechanism. The paper addresses this issue from two different application areas; biomass and kraft recovery boilers. In these applications water vapour and sulphur are simultaneously affecting the corrosion mechanism, though the mechanisms are different. Low-alloyed steels were exposed to an atmosphere containing different amounts of water vapour at temperatures of 420, 550 and 600°C. Under oxidising conditions increasing water content generally accelerates oxidation. However, presence of SO2 in moist atmosphere retards oxidation at high temperatures. The phenomenon is seen at low temperatures with higher chromium contents. Stainless steel 304L was tested in an atmosphere containing hydrogen sulphide and carbon monoxide with and without water vapour at a temperature of 440°C to simulate elevated kraft recovery boiler furnace conditions. The tests showed that water vapour in the test atmosphere produces a protective spinel oxide on the metal surface. In tests without water vapour, the initial scales at metal surface were different sulphur compounds and intensive sulphidation occurred. The effect of water vapour on the sulphidation mechanism is addressed in the paper through the described tests and thermodynamic modelling.