Degradation and Protection Phenomena of High Alloyed Fe-Cr Alloys in Hot, Multi-Component Gas Atmospheres

Abstract

In order to transition to a more sustainable and environmentally friendly future, a change in power generation is necessary. Reducing CO2 output is a crucial factor to combat climate change. A major contributor to CO2 emissions is the generation of electricity, mainly from coal and lignite combustion, emitting billions of tons of CO2 every year. Additionally, coal and lignite are fossil fuels and therefore are not replenishable in human time frames. One solution to this problem is firing existing power plants with biomass as a substitute. This has the advantage of both being a renewable energy source and significantly reducing CO2 emissions. However, the use of biomass can cause issues within the reactor by corroding the super heater tubes. This phenomenon occurs due to the chemical differences between biomass and coal, as well as the composition of super heater tubes, which were engineered to withstand coal combustion, not biomass combustion. During the firing of biomass, salt deposits form on the surface of the steel, quickly corroding the underlying metal. This hinders the efficiency and lifetime of super heater tubes, requiring replacement and causing excessive costs.

In this work, model alloys, designed after commercially utilized steels, are investigated in varying environmental conditions. One ferritic alloy (Fe-13Cr, similar to X20 steel) and two ferritic-austenitic alloys(Fe-18Cr-12Ni (similar to TP347H), Fe-25Cr-20Ni (similar to HR3C)) are exposed to experimental temperatures of 560°C. The corrosion attack is investigated using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) to determine the structure and elemental distribution of both alloy and the forming scale. Phase composition of the scale is determined using X-ray diffraction (XRD), and thermodynamic boundary conditions are calculated using FactSage.

A common phenomenon observed in biomass combustion is the formation of KCl deposits. Therefore the effect of KCl deposits on steel corrosion is investigated in chapter 2. For these experiments steels are tested in a SO2 bearing atmosphere with and without KCl deposits. Steels without KCl show good corrosion resistance in the form of a thin Cr2O3 layer. Samples with KCl deposits show a multilayered scale comprised of metal sulfides and oxides. The alloy composition also shows a large impact, with higher alloyed steels showing stronger Cr depletion and Ni bearing samples exhibiting large internal corrosion for samples with KCl.

The third chapter studies the initial growth (5h) of the scale in samples with and without KCl coating using synchrotron radiation. The initial formation of the scale is important, since some phases only exist for a short time before reacting further. This allows for a better understanding of the corrosion mechanism and how KCl impacts the scale growth. The impact of alloy composition and different microstructure of the alloys was studied. Samples without KCl coating showed a protective scale of Cr2O3 throughout the experimental length. All samples coated with KCl showed a weight gain about one order of magnitude higher than their non-coated counterpart. This increase in weight gain is linked to the higher proportion of formed Fe-oxides and higher Fe/Cr ratios in mixed (Fe,Cr)2O3. The coated samples also show a large Cr depletion towards the surface for highly alloyed samples. This prevents the formation of new Cr2O3 after possible spallation events. The results show that KCl is detrimental for all sampled steels and that highly alloyed Ni bearing steels aren't beneficial in this kind of environment, with Fe-13Cr performing the best.

The fourth chapter studies the impact of humidity on the corrosion behavior in biomass fired power plants. To simulate the impact, Fe-18Cr-12Ni, with KCl deposits, is exposed at 560°C in either laboratory air or SO2. In each atmosphere one experiment is performed under dry conditions while a second one is conducted under humid conditions. This variation in humidity represents both the possible range of humidity from different biomass sources as well as the possibility of influencing the humidity artificially by drying the biomass. In dry lab air the sample showed about double the weight gain and porosity when compared to humid air. In SO2 bearing atmosphere the difference is even more pronounced. The dry sample showed a scale width about one order of magnitude larger than the humid counterpart. Additionally, the dry sample showed internal corrosion, which wasn't observed in the humid sample. The better performance of Fe-18Cr-12Ni is linked to the chlorine compounds found in this environment. H2O allows for the formation of HCl, instead of Cl2, which has a lower reaction rate with steels. When SO2 is present, the H2O facilitates the formation of K2SO4, which together with Cr2O3 form a protective layer preventing further scale growth. These results show that artificial drying is counterproductive for biomass combustion and significantly accelerates the corrosion attack.