7.6 Korrosion und Korrosionsschutz
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Eingeladener Vortrag
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Geothermal energy is an interesting alternative to polluting fossil energy sources. Therefore, in Belgium, two wells have been drilled for a deep geothermal power plant. However, the environment to which the installations are exposed is challenging. The geothermal brine has 165 g/l total dissolved solids (of which 90% are sodium and chlorine) and the production temperature can be up to 130 °C. To assess their suitability to be used in a geothermal power plant, the corrosivity of the artificial brine to three common construction materials was investigated with exposure and electrochemical tests. The metals under consideration are a low-alloyed carbon steel (S235JR), an austenitic stainless steel (UNS S31603) and a duplex stainless steel (UNS S31803). The carbon steel, that was found to corrode uniformly, could be considered as a constructional material if a sufficient wall thickness is chosen. The austenitic stainless steel and the duplex stainless steel demonstrate very low uniform
corrosion rates. They are however susceptible to pitting and crevice corrosion. To guarantee safe operation of the geothermal power plant, the susceptibility of the alloys to stress corrosion cracking should be tested and in situ experiments should be performed.
The composites combining aluminum and silica nanoparticles with the addition of tetramethylammonium hydroxide (Al-SiO2(T)) and butanol (Al-SiO2(B)) as mixing media have been successfully fabricated. Corrosion behavior of Al-SiO2 composites before and after exposure in 1M NaCl solution was examined using potentiodynamic polarization (Tafel curve analysis). The study was also equipped with scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and X-ray diffraction (XRD) investigations. Before exposure, Al-SiO2(T) exhibited the best corrosion resistance. Performance improvement was indicated by Al-SiO2(B) up to 10 times better than Al-SiO2(T) after exposure. The increased SiO2 content did not significantly enhance the corrosion resistance of the composites. The Al-SiO2 composites with 5% SiO2 content showed very high corrosion resistance (as the optimum composition). Furthermore, pitting corrosion was observed in the Al-SiO2 composites, indicated by the formation of corrosion products at grain boundaries. The product was affected by the presence of SiO2 in the Al matrix and the NaCl environment at 90 °C (approach to synthetic geothermal media: Na+, Cl, H+, OH-). Our study revealed the presence of γ-Al2O3, γ-Al(OH)3, and Al(OH)2Cl as the dominant corrosion products.
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The fabrication and characteristics of amorphous silica reinforced Al matrix composites are studied in this paper. The major starting materials were commercial Al powder and extracted nanoparticle SiO2 (NPS) powder from Indonesian silica sands. Two different active solutions,namely N-butanol and tetramethylammonium hydroxide (TMAH), were introduced during synthesis. Characterisations in terms of physical, mechanical, microstructural and corrosion rate examinations were also employed. Introducing the SiO2 nanoparticles into the Al matrix has decreased the density and increased the porosity of the composites. The addition of N-butanol into Al/SiO2(Al/SiO2(B)) led to broader and lower X-ray diffraction profiles than the addition of TMAH (Al/SiO2(T)). From the microstructural analysis, we found that the SiO2 particles enter and agglomerate into the opening gap of the Al sheets. Furthermore, yield strength, ultimate compression strength and modulus of elasticity tended to reduce the addition of SiO2. The corrosion rate of Al/SiO2(T) was lower than that of Al/SiO2(B) composites.
Interfacial properties related to wettability and corrosion in CO2 transport pipelines are experimentally determined by the sessile and the pendant drop methods. The contact angle of a water drop in a compressed CO2 atmosphere is analyzed on an X70 pipeline carbon steel and compared to that on a martensitic steel S41500 to elucidate the effect of corrosion process on active wetting behaviour. The measurements are performed with liquid CO2 at 278 K and pressures ranging from 5 to 20 MPa. The results show that the contact angle (CA) increases with pressure from 132 ° to 143 ° for S41500 and from 117 ° to 137 ° for X70 and decreases with drop age by 20 ° to 24 ° regardless of the pressure and of the fact that corrosion only occurs on X70, which is confirmed by scanning electron microscopy, element mapping and energy dispersive x-ray spectrometry (EDS) analysis. At higher pressure, the contact angles on both materials converge. Further, related properties like density and interfacial tension were determined. CO2 - saturated water has a higher density than pure water: At 5 MPa saturated water reaches a density of 1017 kg⋅m^(-3) and at 20 MPa 1026 kg⋅m^(-3) compared to pure water with a density of 1002 kg⋅m^(-3) and 1009 kg⋅m^(-3), respectively. In this pressure range the IFT drops from 33 mN⋅m^(-1)at 5 MPa to 23 mN⋅m^(-1) at 20 MPa.
Depending on the CO2 generating and the capture process as well as on consecutive purification steps applied, CO2 streams from different emitters may differ in their composition. When CO2 streams with different compositions are fed into a larger pipeline network, there are several aspects that must be considered: i) chemical reactions, such as acid formation, may occur within the joint CO2 stream; ii) there may be a variation of mass flow rate and CO2 stream composition within the pipeline network if the feed-in behavior of different CO2 sources changes with time. Potential impacts of changing CO2 stream compositions and mass flow rates in CCS cluster systems were investigated in the collaborative project "CLUSTER" (see also www.bgr.bund.de/CLUSTER). In this presentation, we focus on the experimental investigations of formation and condensation of strong acids and their impacts on the corrosion of pipeline steels. When SO2, NO2, O2 and H2O are present simultaneously in CO2 streams chemical cross-reactions may occur leading to the formation of strong acids such as sulfuric and nitric acid. To prevent this acid formation the concentration of at least one of these four impurities must be kept very low (e.g., Rütters et al., 2016). At temperatures below the acid dew point temperature, acids will condense, e.g., on pipeline steel surfaces. In turn, these acid condensates may trigger steel corrosion. To better understand the process of acid formation and condensation and its implications for steel corrosion, exposure tests were performed on pipeline steel X70 in dense CO2 with varying SO2, NO2 and O2 concentration under high pressure and at 278 K in an observable autoclave, in which water was added as droplets or as vapor. Further, electrochemical tests were carried out with X70 specimens immersed in 500 mL CO2-saturated synthetic condensate solution or in droplets of the same solution on the specimen’s surface. Depending on impurity concentrations in the CO2 streams, condensates consisting of different relative amounts of nitric and sulfuric acid were formed. In condensates containing both nitric and sulfuric acid, corrosion rates were higher than the sum of those of the individual acids. In addition, corrosion products and forms depended on the condensate composition. Investigations of water droplets on steel surfaces in impurity-containing dense-phase CO2 revealed the diffusion of SO2 and NO2, followed by cross-reactions forming corresponding acids. An increase in droplet size (from 1 to 5 µl) lead to higher corrosion rates. However, in comparison to measurements in bulk solution, corrosion reactions in droplets resulted in thick, high-resistance corrosion products and observed droplet corrosion rates were significantly lower. In addition, the possibility of acid droplet formation and growth in impure liquid CO2 is influenced by the wetting behavior of the acid droplet on the steel surface. Thus, the contact angle between a water droplet and the surface steel specimens in a CO2 atmosphere was investigated in a high pressure view cell following the sessile drop method. The contact angle wasand found to be larger at higher CO2 pressures (studied from 5 to 20 MPa) and at higher temperatures (e.g. 278 K to 333 K). Further, measured contact angles were larger on rough than on smooth metal surfaces. In addition, acid formation reduced the contact angle, i.e. lead to better wetting, thereby stimulating condensation that was followed by a corrosion process. These detailed insights on the complex interplay of acid formation, condensation, wetting behavior and corrosion allow a better assessment of material suitability for pipeline transportation of impure CO2 streams
Ageing at 600 °C (from 0.1 h up to 20 h) leads to the formation of precipitations at the ferrite-ferrite (α/α) and ferrite-austenite (α/γ) grain boundaries of the lean duplex stainless steel (LDSS) X2CrNiN22-2. This leads to sensitisation due to chromium depletion and decreased pitting corrosion resistance proven by the results of various electrochemical methods (DL-EPR and determination of CPT, Epit). These results were compared with the KorroPad method, which uses an agar-based gel-electrolyte for the detection of stainless steel surfaces prone to pitting corrosion. However, the standard configuration of the KorroPad showed no differentiation for the various ageing conditions. Therefore, modified versions of the KorroPad with two, five and ten times higher NaCl and potassium ferrocyanide III (K3[Fe(CN)6]) concentrations were successfully used to visualise the behaviour detected by DL-EPR, Epit and CPT. Therefore, the KorroPad method can also detect a microstructure related reduction of pitting corrosion resistance, which can drastically reduce the experimental effort to generate sensitisation diagrams for stainless steels.
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Compatibility of polymeric materials with heating oil/biodiesel blends at different temperatures
(2019)
Materials compatibility is a major concern whenever the fuel composition is changed. The question arises of whether polymeric materials are resistant to heating oil with admixtures of 10 % biodiesel (B10) and 20 % biodiesel (B20).
The polarity of biodiesel increases its solvency and facilitates permeation and extrac-tion. Solvation, swelling and/or extraction lead to changes in the physical properties and chemical changes of polymeric materials.
The objective of this research was to determine the resistance of frequently used sealing materials such as FKM, EPDM, CR, CSM, NBR, IIR, VMQ, FVMQ, PA and PUR in up to four-year aged B10 for 84 days at 20 °C, 40 °C and 70 °C.
The polymeric materials: ACM, FKM, HNBR, PA, PE; POM, PUR and PVC were ex-posed to B20 for 84 days at 40°C and 70°C in another research project.
Mass, tensile strength, breaking elongation and Shore hardness A (D) of the test specimens were determined before and after the exposure for 84 (42) days in the heating oil blends B10 and B20.
There is not determined a threshold for the reduction in tensile properties and Shore hardness in the international standards. Therefore, a threshold of 15 % was deter-mined for the evaluation of the compatibility.
Measurements of the variations in mass, tensile properties and Shore hardness after exposure of the polymers in non-aged and aged heating oil B10 showed clearly that FKM, FVMQ and PA were the most resistant materials in B10. The elastomers CR, CSM, EPDM, IIR, NBR and VMQ were generally not resistant to B10. Damage to the materials increased with higher test temperatures and the age of B10.
FKM, POM and PVC showed high compatibility in B20 at 40°C and 70 °C. ACM, HNBR and PA were evaluated as resistant in B20 at 40 °C but not at 70°C.