On-site greywater reuse (GWR) and installation of water-efficient toilets (WET) reduce urban freshwater demand. Research on GWR and WET has generally overlooked the effects that GWR may have on ...municipal sewer systems. This paper discusses and quantifies these effects. The effects of GWR and WET, positive and negative, were studied by modelling a representative urban sewer system. GWR scenarios were modelled and analysed using the SIMBA simulation system. The results show that, as expected, the flow, velocity and proportional depth decrease as GWR increases. Nevertheless, the reduction is not evenly distributed throughout the day but mainly occurs during the morning and evening peaks. Examination of the effects of reduced toilet flush volumes revealed that in some of the GWR scenarios flows, velocities and proportional depths in the sewer were reduced, while in other GWR scenarios discharge volumes, velocities and proportional depths did not change. Further, it is indicated that as a result of GWR and installation of WET, sewer blockage rates are not expected to increase significantly. The results support the option to construct new sewer systems with smaller pipe diameters. The analysis shows that as the penetration of GWR systems increase, and with the installation of WET, concentrations of pollutants also increase. In GWR scenarios (when toilet flush volume is not reduced) the increase in pollutant concentrations is lower than the proportional reduction of sewage flow. Moreover, the results show that the spatial distribution of houses reusing GW does not significantly affect the parameters examined.
► Effects of greywater reuse & water-efficient toilets on sewer systems were modelled. ► Five scenarios of greywater reuse and three of water-efficient toilets were studied. ► Increase in pollutant concentrations was lower than the reduction of sewage flow. ► The results support the option to construct new sewers of smaller diameters. ► Spatial distribution of greywater reusing houses does not have a significant effect.
This book is a review of corrosion and thermal protection topics associated with coatings applied in high temperature applications, namely catalytic converters, combustion and steam environments, and ...gas turbines. Information on corrosion influencing mechanisms and their impact are reviewed.
New generation heat-resistant ferritic–martensitic steels are of great interest as superheater materials in fossil fuel power plants. The oxidation resistance of these chromium steels (9wt.%) in ...water vapor containing environments above 600°C still requires improvement. One major problem is the breakdown of the chromia layer by formation of volatile chromium species CrO2(OH)2 promoted by water vapor in the atmosphere. Both, the fire-side as well as the steam side of the superheater tube can be affected due to high contents of water vapor in these atmospheres.
With the goal to operate ferritic–martensitic steels under such conditions, enrichment of the substrate with chromium and manganese was carried out using three different pack cementation processes: pure chromium coating, 2-step chromium and manganese process and simultaneous deposition of chromium and manganese.
All three surface-treated alloys show exceptional stabilities in steam environments, as illustrated by oxidation exposures in argon/50% water vapor at 650°C for 3000h. The pure chromium coating on the P91 substrate revealed the best oxidation behavior in steam.
•Improvement of the oxidation behavior of ferritic–martensitic steels in steam environments•Development of protective diffusion coatings with pack cementation method•Investigation of uncoated and coated P91 in steam environments to demonstrate the improved oxidation behavior
Hot corrosion of pack (zero/medium) Pt-modified aluminide and slurry (zero/low/medium) Pt-modified silicon-aluminide coatings exposed to a Na2SO4-60 mol%V2O5 deposit at 700 °C was evaluated using the ...deposit recoat technique. Thickness loss metrology was utilized to study corrosion penetration into the coatings. The corrosion products and remaining coatings were characterized by means of SEM, EDX, XRD and EPMA. The aluminide coating and medium Pt-modified silicon-aluminide coating showed the worst and the best corrosion resistance, respectively. Enhanced hot corrosion resistance of the Pt-modified silicon-aluminide was attributed to microstructure of the coating with a two-phase PtAl2-NiAl outer layer and formation of refractory silicides.
•Pitting corrosion mode of Pt-aluminide changes to uniform in the presence of Si•Pt, Si and (Pt,Si) limits nickel transient oxidation of aluminide coatings•Pt, Si and (Pt,Si) enhances vanadic hot corrosion resistance of aluminide coatings•Deeper presence of PtAl2 in Si-aluminide coating enhances its lifetime
Recent experimental investigations have widened the understanding of metal dusting significantly. Microscopic observations have been used to dissect dusting mechanisms. Iron dusts by growing a ...cementite surface scale, which catalyses graphite nucleation and growth. The resulting volume expansion leads to cementite disintegration. Cementite formation on iron can be suppressed by alloying with germanium. Nonetheless, dusting occurs via the direct growth of graphite into the metal, producing nanoparticles of ferrite. This process is faster, because carbon diffusion is more rapid in α‐Fe than in Fe3C. Austenitic materials cannot form cementite, and dust via formation of graphite at external surfaces and interior grain boundaries. The coke deposit consists of carbon nanotubes with austenite particles at their tips, or graphite particles encapsulating austenite. TEM studies demonstrate the inward growth of graphite within the metal interior. It is therefore concluded that the dusting mechanism of austenitic materials like high alloy Cr–Ni steels and Ni base materials is one of graphite nucleation and growth within the near surface metal. In all alloys examined, both ferritic and austenitic, the principal mass transfer process is inward diffusion of carbon. Alloying iron with nickel leads to a transformation from one mechanism with carbide formation to the other without. Copper alloying in nickel and high nickel content stainless steels strongly suppresses graphite nucleation, as does also an intermetallic Ni–Sn phase, thereby reducing greatly the overall dusting rate. A surface layer of intermetallic Ni–Sn Fe‐base materials facilitates the formation of a Fe3SnC surface scale which also prevents coking and metal dusting. Current understanding of the roles of temperature, gas composition and surface oxides on dusting rates are summarised. Finally, protection against metal dusting by coatings is discussed in terms of their effects on catalysis of carbon deposition, and on protective oxide formation.
Slurry aluminide coatings have been well known for a number of years and are widely used to protect metallic surfaces from oxidation and corrosion. A variety of commercial slurries is available to ...aluminize the surfaces of nickel-based superalloys, however, they have several disadvantages. They contain environmentally harmful substances such as chromates or halides which are used as binders or to activate the diffusion species. Additionally, up to now slurry coatings were only used to produce precipitate-rich coatings, with a microstructure comparable to high aluminum activity coatings produced by chemical vapor deposition (CVD) like pack cementation.
In this work these limitations have been overcome by carefully designing the powder composition of the slurry. Three different nickel-aluminide coatings were developed and applied onto CM-247 nickel-based alloy and Pt-diffused CM-247. By addition of chromium we achieved coatings with a similar microstructure and Al-content (of around 40at.%), comparable to the state of the art low-activity coatings produced by CVD. Co-deposition of silicon was also achieved in a single step, maintaining the low-activity structure by combining Al–Si powder and chromium. Furthermore, this coating procedure was combined with a platinum electrodeposition step in order to produce single phase Pt-modified low-activity aluminide coatings.
► Low activity nickel aluminides for the first time produced by slurry route ► Platinum modified nickel aluminides produced by slurry route ► Overcome expensive techniques as CVD ► The produced slurries are non-harmful. ► Coating manufacturing process was described.
This study focuses on the characterization of oxidation-induced microstructural changes in TNM®-B1 alloy due to the dissolution of oxygen during exposure at 900 °C for up to 1000h and investigates ...their correlation with the microstructural subsurface changes as well as their effect on the mechanical properties. For this purpose, the change in the phase distribution is measured during oxidation. In addition to measuring the embrittlement during exposure, the fracture strain and nanoindentation hardness of particular phases are assessed to reveal the impact of oxidation and of the resulting microstructural changes on their mechanical behavior. The subsurface embrittlement was found to be directly related to the oxygen content and to significant changes in the phase distribution of the oxygen-affected zone.
The transformation of βo-phase into α2 is proposed as an easily accessible indicator for oxygen uptake depth into the TNM alloy. Finally, the mechanism of embrittlement in the α2-phase is discussed.
•The subsurface embrittlement is directly related to the oxygen.•Oxygen dissolves into the α2-and βo-phase; γ does not show increased oxygen.•Oxygen significantly changes the phase distribution; reduces the βo-phase.•The transformation of the β-phase into α2 can be used as indicator for oxygen uptake.
High temperature materials require both, high oxidation resistance and mechanical strength. Owing to their excellent high temperature strength, Ni-based superalloys are extensively used in turbine ...engines. In order to enhance the oxidation resistance and thereby extend the lifetime, their surface has to be modified by the enrichment of stable oxide formers. Mainly NiAl-diffusion coatings are used to protect turbine components, serving as an Al-reservoir for the formation of a thin, protective scale. Besides their oxidation resistance, the mechanical behavior of such coatings is crucial for the integrity of the system. Under service conditions, compositional changes will occur due to two mechanisms: outward-diffusion of Al to form the oxide and interdiffusion with the substrate. Such chemical changes lead to a change in the mechanical behavior of the coating and thus the coated system. In this study, the compositional and microstructural changes, which occur during the thermocyclic exposure, are correlated with the mechanical properties of NiAl diffusion coatings. Prior to and after thermocyclic exposure at 900, 1050 and 1100 °C for durations up to 1000h four-point bending flexural tests with in-situ acoustic emission measurement are used to determine the fracture strain of the coating. The fracture strain increases due to Al-depletion during cyclic exposure and can be correlated with the Al-concentration in the diffusion zone. Moreover, elastic modulus and hardness of the coating zones are determined by nanoindentation. Both show a decrease in the single-phase sub-stoichiometric β-NiAl with lower Al-content, increasing again within the two-phase stability region.
•Microstructural degradation of NiAl coatings during thermocyclic exposure.•Activation energy for the SRZ growth was found to agree with the tracer diffusivity of Al in Ni3Al.•Fracture strain, indentation hardness and elastic modulus after the exposure at RT correlate with the Al-depletion.•Crack penetration depth decreases with increasing area fraction of the γ′-phase.
This book is a review of corrosion and thermal protection topics associated with coatings applied in high temperature applications, namely catalytic converters, combustion and steam environments, and ...gas turbines. Information on corrosion influencing mechanisms and their impact are reviewed.
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