Zum Einfluß von Heißgaskorrosion auf die Haltbarkeit von Hochtemperaturwerkstoffen

Influence of hot gas corrosion on the life of high temperature materials The study of gas turbine materials shows that behaviour is influenced by stress corrosion cracking even in the creep range. The creep strength is reduced at 1000 °C by 20–50%, depending on the salt concentration in the gas. The influence of the salt concentration is negligible only ≤ 1 ppm salt and then it is possible to base calculations on creep data obtained in air. The influence of hot gas corrosion is particularly critical between 800 and 1000 °C. The influence of general corrosion can be neglected.     

Autoklaven-Untersuchungen der Spannungsrißkorrosion von Fe-Cr-Ni-Legierungen in NaCl/Co2/H2S-Medien

Autoclave investigation of stress corrosion cracking behaviour of Fe-Cr-Ni alloys in NaCl/CO₂/H₂S-environment In oil and gas production, the corrosion problems increase as the depth of the reservoirs increases. The oil and gas products contain chloride-rich waters and mixtures of H₂S and CO₂ at high pressures and temperatures. Materials that can be used under these conditions are only high strength high alloy steels and nickel base alloys. These materials must be assessed for corrosion resistance under these conditions. The environment contain chloride ions and hydrogen sulphide, which are known to be critical components for SCC. With the aid of autoclave experiments, the fields of corrosion resistance for the materials no. 1.4462, 1.4563 and 2.4618 were determined as a function of temperature and hydrogen sulphide pressure. The base environment was a 5 Molar sodium chloride solution at 20 bar carbon dioxide. While the corrosion resistance of the duplex steel, material no. 1.4462, decreases markedly as the strength of the material and the hydrogen sulphide pressure increase, the two austenitic materials are completely resistant up to 300 °C and hydrogen sulphide pressure of 15 bar. Only at 300 °C and high partial pressures of hydrogen sulphide the material no. 1.4563 did fail, when stressed to stress levels higher than the YS. The crack path was predominantly transgranular with minute fractions of intergranular cracking. The microstructure appears to have no effect. All results indicate that a mixed mechanism of hydrogen- and chloride induced SCC is operting, while a corrosion enhancement due to interaction of both critical components takes place.     

Korrosionsbeständigkeit von Installationsteilen aus nichtrostenden austenitischen Chrom-Nickel-Stählen bei erhöhter Temperatur

Corrosion resistance of installation elements of stainless austenitic chromium-nickel steels at high temperatures The test results presented prove the statements made in DIN 50 929 Part 2. In accordance with DIN 50 930 Part 4, the corrosion probability for stainless steels increases when chloride containing water evaporates on warm material surfaces, whereby chloride ions concentrate. Under these conditions, ferritic chromium steels and austenitic chromium-nickel steels can suffer pitting corrosion, austenitic chromium-nickel steels also stress corrosion cracking. In the latter case, the corrosion cracks start from pits. The molybdenum-containing material no. 1.4571 withstands pitting and stress corrosion cracking in wet, chloride-containing environments at 90°C over some weeks. With increasing exposure time and at temperatures above 45°C, however, corrosion damage cannot be excluded. Then, according to DIN 50 929 Part 2, coating of the external surfaces of installation components, e.g., tubes, is required. The coating must be thick, free from pores and holidays, and resistant to heat and aging.     

High-temperature high-pressure SCC testing of corrosion-resistant alloys

Stress corrosion cracking test methods of corrosion-resistant alloys are reviewed. The interest to write a review on this topic was drawn by demands for oil country tubular goods applicable in deep wells with high pressures, high temperatures, and the presence of H₂S, where stress corrosion cracking is one of the most critical failure modes. All conventional methods for determining the stress corrosion cracking resistance of an alloy, mainly slow strain rate testing, constant load testing (tensile, 4-PB), constant strain testing (U-bend, C-ring), and fracture mechanics (double cantilever beam sample) are covered. Considering the variety of testing solutions, the field of search is narrowed to hot (up to 250°C) aqueous chloride solutions with dissolved H₂S and CO₂ gases under high pressure (up to 200 bar total pressure).     

Nickellegierte Werkstoffe im Chemieofenbau

Nickel alloyed materials in chemical furnace construction The trend toward ever larger plant in chemical process technology as well as trends toward higher working temperatures and new designs at higher mechanical loads, and transition to other raw materials in petrochemistry have resulted in increasing demand for high nickel alloys or nickel base materials. The sealing resistance could be increased by alloying technologies, while the carburizing susceptibility could be considerably reduced. New alloys having improved creep resistance enable applications under high uniform and cycling temperature loads with the additional advantage of lower wall thicknesses. Realized and potential applications listed include: pyrolysis furnaces in ethylene production plant; tubing suspensions; transfer tubing; steam reforming plant; reformer tube; collectors; pigtails; styrene production with superheated steam as a diluent; furnace tubing in reaction furnaces for the manufacture of carbon disulfide; cracking plant for acetic acid; heavy water production from petrochemical feeds.     

Korrosion des Hastelloy C 4 und anderer metallischer Werkstoffe in heißen,konzentrierten Salzlaugen

Corrosion of Hastelloy C4 and different metallic materials in hot concentrated salt brines In a quaternary salt brine saturated at 55°C with magnesium chloride, potassium chloride and sodium chloride, polarization curves were measured in the temperature range from 100°C to 200°C at different materials in order to study their corrosion behaviour. Steels and nickel corrode in the active state. The corrosion current density of an unalloyed steel at 100°C was about 20 μA/cm² corresponding to a removal rate of 0.2 mm/a. Iron-silicon and nickel corrode significantly slower at a rate of a few μA/cm². Steady state corrosion rates of Hastelloy C 4 and of titanium at 100°C are 10 nA/cm² and less than 4.5 nA/cm², respectively. For both materials, the susceptibility to pitting grows with the temperature and with the chloride concentration. The steady state corrosion rates of passive Hastelloy C 4 depend on temperature according to an activation enthalpy of 85 kJ/mol and become about 5 μA/cm² at 200°C. Even at high temperatures steady states are attained rather slowly within about one day. The critical pitting potentials approach the corrosion potentials in deaerated solution at a rate of about 0.8 mV/K. At 200°C the critical pitting potential is only 90 mV positive to the corrosion potential. Thus, Hastelloy C 4 may be used at high temperatures, only if it is cathodically protected.     

Corrosion of aluminium,stainless steels and AISI 680 nickel alloy in nitrogen‐based fuels

Nitrogen‐based compounds can potentially be used as alternative non‐carbon or low‐carbon fuels. Nevertheless, the corrosion of construction materials at high temperatures and pressures in the presence of such fuel has not been reported yet. This work is focused on the corrosion of AISI Al 6061, 1005 carbon steel (CS), 304, 316L, 310 austenitic stainless steels (SS) and 680 nickel alloy in highly concentrated water solution of ammonium nitrate and urea (ANU). The corrosion at 50 °C and ambient pressure and at 350 °C and 20 bar was investigated to simulate storage and working conditions. Sodium chloride was added to the fuel (0–5 wt%) to simulate industrial fertilizers and accelerated corrosion environment. Heavy corrosion of CS was observed in ANU solution at 50 °C, while Al 6061, 304 and 316L SS showed high resistance both to uniform and pitting corrosion in ANU containing 1% of sodium chloride. Addition of 5% sodium chloride caused pitting of Al 6061 but had no influence on the corrosion of SS. Tests in ANU at 350 °C and 20 bar showed pitting on SS 304 and 316L and 680 nickel alloy. The highest corrosion resistance was found for SS 310 due to formation of stable oxide film on its surface.     

Neue Nickel-Chrom-Legierungen mit hoher Beständigkeit gegen Brennstoffaschenkorrosion

New NiCr-alloys with high resistance to oil ash corrosion After giving an account of the origin of oil ash corrosion the authors dwell in particular on the behaviour of the alloys CrNi 35/65, 50/50 and 60/40. In this context they show the range of applications of these materials as derived from laboratory experiments and field tests in refinery furnaces as well as the influence of temperature and firebox conditions. A comparison is discussed of corrosion values obtained under practical conditions with commercial austenitic and ferritic cast materials. Finally the authors discuss the mechanical, physical and processing characteristics of the CrNi alloys, dwelling in particular on their creep properties. CrNi 50/50 and 60/40 can be welded under certain conditions only; welding of load-bearing elements is discouraged.     

Untersuchungen zum Einfluß von Sulfationen auf die chloridinduzierte lokale Korrosion am Stahl 1.4541 bei Temperaturen bis zu 280°C und gleichzeitiger Werkstoffdehnung

The influence of sulphate ions on stress corrosion cracking for stainless steel AlSl 321 with constant extension rate in aqueous chloride solution at temperatures up to 280°C CERT (Constant extension rate test: ? = 3·10 ⁶ s ¹) was used to study the inhibition effect of sulphate ions on stress corrosion cracking. Smooth tensile round specimens (AlSl 321) have been tested in solutions with sulphate (10 ²m Na₂SO₄) and chloride ions (10^?3m NaCl. 5. 10 ³m NaCl) at 150°C and 280°C. The presence of sulphate in chloride solutions increases the time to fracture and the reduction of area in comparison to pure chloride solution. The reason is the lower concentration of corrosive hydrogen near the fracture surface in comparison with tests in chloride solutions. The stress corrosion cracking is completely inhibited by the ratio of 10:1 for sulphate and chloride ions in the solution. The fracture surface investigated by Scanning Electron Microscopy has shown a cleavage type of fracture in chloride solutions and a ductile fracture in sulphate/chloride mixtures. The diffusivity of corrosive hydrogen is increased at 280°C in comparison to 150°C. Therefore at 280°C the corrosive hydrogen is able to diffuse into the inner part of the specimen and to influence the fracture mode.     

Aufkohlung von Schleudergußrohren in der petrochemischen Industrie und Erklärung dieses Verhaltens durch Laborversuche mit Chrom-Nickel-Stählen und - Werkstoffen im Temperaturbereich von 800–1150°C

Carburization of centrifugally cast tubes in the petrochemical industries and explication of this behaviour by laboratory tests with chromium nickel steels and chromium nicked materials in the temperature range 800 to 1150°C The carburization behaviour of the tubes used under the conditions of petrochemical cracking processes depends in a first line on the temperature. Up to 1000°C carbon pickup is low, but above 1050°C heavy carbon pickup and increasing carburization depth must be counted with. This temperature dependence is due to the fact that at 1050°C equilibrium is attained between chromium oxide and carbide, so that the oxide is no longer stable and the original protective effect of the oxide layer is lost. It is for this reason that pre-oxidation of the tubes has not only favourable effects: during cooling cracks may form in the oxide layer, so that carburization of a surface layer may set in at temperatures as low as 800°C. Carburization is delayed by high Cr, Ni and Si contents: for uses at 1100°C at least 28% Cr, 32% Ni and 2.5% Si are required.     

Einfluß von Eisen auf die Korrosionsbeständigkeit von Nickelbasislegierungen als mögliche Werkstoffe für SCWO-Reaktoren

Effect of iron on the corrosion behaviour of nickel based alloys for SCWO plants An experimental set up to study corrosion at high temperatures and high pressures and able to simulate the conditions of supercritical water oxidation is described. On the basis of the alloys AC 66, 45 TM, G-3 and 601 H the influence of iron on the corrosion behaviour of nickel base alloys in aqueous solution under high pressures containing oxygen and chloride is shown and discussed. The corrosion resistance of the nickel base alloys is decreased with increasing iron concentration in the whole temperature range (100 °C–420 °C). The corrosion started at lower temperatures and the weight loss increased rapidly with increasing iron content. The corrosion morphology changed from uniform corrosion for alloys with less iron content to deep pits for alloys with high iron concentrations.     

Einflüsse kleiner Zusätze von Niob oder Cer auf Korrosion und Kriechen von Incoloy 800 in Co-H2O-H2-Atmosphären

Effects of small alloying additions of niobium or cerium on the corrosion and creep of Incoloy 800 in CO-H₂O-H₂-atmospheres In oxidizing and carburizing atmospheres at high temperatures Fe-Ni-Cr-alloys are carburized under creep conditions by carbon transfer through cracks in the oxide layer. In creep experiments in CO-H₂O-H₂ atmospheres at 1000 °C with several alloys based on Incoloy 800 the carburization could be related to the strain of the specimens. Alloying additions of Nb in the range 0.2 to 1% caused changes in the creep rate, a decrease for >0.35% and an increase for >0.35% Nb. The creep resistance for the high Nb concentrations could be improved by solution annealing at high temperatures (1200 °C). Niobium strongly decreases the carburization - this effect can be explained by the formation of an internal layer of Al- and Nb-oxides beneath the outer Cr₂O₃ layer. An alloying addition of Ce (0.06%) also has beneficial effects on the creep resistance and carburization resistance of Incoloy 800.     

Die Beeinflussung des Schwingungsrißkorrosionsverhaltens des Stahles X 20 Cr 13 durch einen Molybdänzusatz in chloridhaltigen Medien

Influence on the fatigue corrosion behaviour of the steel X20Cr13 in chloride containing media by molybdenum additions The fatigue corrosion behaviour of a 13% Cr steel (used for turbine blades) has been studied in terms of its dependence on carbide shape and molybdenum additions in aqueous NaCl solutions of various concentrations at temperatures between room temperature and 200°C. Further tests in autoclaves were carried out in order to study the surface layers formed above 150°C without mechanical loads. Addition of 1% molybdenum yields a considerable increase of corrosion fatigue strength at room temperature, while there is no influence attributable to different shapes of graphite precipitations. At 150°C the corrosion fatigue strength is largely independent from sodium chloride concentration (0,1 to 27%); this means that very small quantities of sodium chloride are sufficient to produce heavy corrosion in wet steam at > 106 cycles. A range particularly suitable for such tests is the load range between ± 150 and ± 200 N/mm² at 150°C, because these conditions approach those encountered in practice. The passivating layers are very thin (interference phenomena). Considering the increase of corrosion resistance produced by 1% Mo it is expected that 2.5% Mo will produce a further improvement, in particular with regard to pitting corrosion since rupture generally starts at the pits.     

Evaluation of SCC susceptibility of supermartensitic stainless steel using slow strain rate tests

Abstract

The susceptibility to chloride stress corrosion cracking (Cl-SCC) of supermartensitic stainless steel (SMSS) was evaluated at different temperatures through slow strain rate tests. In order to evaluate the Cl-SCC susceptibility the reduction in area and the time to failure ratio were considered. It is clear that all specimens tested in the chloride solution exhibited a generally high resistance to chloride SCC. However, according to the slow strain rate tests results, at lower temperature (5°C) the susceptibility to Cl-SCC increases compared to the other temperatures studied; also an increase in the corrosion rate was observed as the temperature decreased according to electrochemical impedance spectroscopy measurement. The corrosion mechanism observed on the material under dynamic stress was a combination of high activation resistivity with a diffusive process on the metallic interface given by a thin film from a protector layer of chromium oxides. This mechanism generates a high corrosion resistance and therefore good performance to chloride induced SCC.     

Versuche zur Natriumkorrosion austenitischer Werkstoffe bei 1000 °C

Investigations into sodium corrosion of austenitic materials at 1000 °C A series of austenitic materials with sufficiently high strength for the use at high temperatures were examined in respect to their compatibility with liquid sodium at 1000 °C. Austenitic steels with nickel contents of 12% showed the highest stability, but materials with 32% nickel and 20% chromium suffered also moderate losses of weight, however, additional internal corrosion effects. Nickelbase alloys were most severely attacked. They are less applicable for the use with sodium of very high temperatures in spite of their high temperature strengths.     

Korrosionsbeständige metallische Werkstoffe für Rauchgas-Entschwefelungsanlagen

Corrosion resistant metallic materials for flue gas desulfurization plants Forced by environmental legislation installations for flue gas desulfurization (FGD) are presently being built to a large extent in the Federal Republic of Germany as in some other countries. Absorption by lime/limestone slurries is used in most cases for desulfurization. The components of the flue gas as well as the kind of process applied are of primary importance for the corrosive behaviour of the materials of construction. In view of pH values going down below 1 in some cases, chloride contents of occasionally more than 10% Cl^?, temperatures between 40 and 160°C and local deposits of solids it is the resistance to pitting and crevice corrosion which has to be considered in the first place while stress corrosion cracking and erosion corrosion are of minor importance. Therefore, only austenitic materials with molybdenum contents of more than 2 wt.-% have to be considered. According to the severity of the corrosive media these are predominantly alloys as e.g. Cronifer 1925 LCN (Alloy 904 LN), 1925 hMo (Alloy 904 LM) going up to the high alloyed nickel base materials Nicrofer 6020 hMo (Alloy 625) and 5716 hMo W (Alloy C-276), which exhibit molybdenum contents of 9 and 16 wt.-% and are to be used at places where corrosion is extremely severe as e.g. at the raw gas inlet. The use of such high alloyed materials plated on carbon steel has been tried successfully. Special attention has to be paid to all aspects of welding in order to avoid welds to become locations being vulnerable by corrosive attack. Therefore, welding of corrosion resistant materials in FGD units has been investigated extensively. The results of application oriented laboratory tests as well as practical experience with existing FGD units are to be considered. Correct use of corrosion resistant metallic materials will be an important contribution to minimizing repair and shut downs of FGD units and to extend their life.     

Improving the oxidation resistance of γ‐titanium aluminides by halogen treatment

Intermetallic alloys based on TiAl are candidates for several structural high temperature applications but their oxidation resistance is limited to temperatures below 800 °C. In this paper the results of high temperature oxidation and creep tests will be presented and discussed. The treatment with halogens improves the oxidation resistance of these alloys up to 1050 °C. A thin protective Al₂O₃‐layer is formed after treatment with halogens instead of the mixed TiO₂/Al₂O₃/TiN scale typically grown on these alloys. This alumina layer protects the component under isothermal and thermocyclic conditions. The protective effect is stable up to at least 8760 h. Creep tests of halogen treated materials at high temperatures showed no effect on the creep behaviour. Automotive turbocharger rotors were exposed at 1050 °C in air with and without fluorine‐treatment for demonstration of real parts.     

Unterschiedliches Aufkohlungsverhalten von Schleudergußrohren aus G-X 40 NiCr 35 25 (W.-Nr. 1.4857) in einem Cracker zur Äthylenherstellung

Carburization of centrifugally cast tubes in the petrochemical industries and explication of this behaviour by laboratory tests with chromium nickel steels and chromium nicked materials in the temperature range 800 to 1150°C The carburization behaviour of the tubes used under the conditions of petrochemical cracking processes depends in a first line on the temperature. Up to 1000°C carbon pickup is low, but above 1050°C heavy carbon pickup and increasing carburization depth must be counted with. This temperature dependence is due to the fact that at 1050°C equilibrium is attained between chromium oxide and carbide, so that the oxide is no longer stable and the original protective effect of the oxide layer is lost. It is for this reason that pre-oxidation of the tubes has not only favourable effects: during cooling cracks may form in the oxide layer, so that carburization of a surface layer may set in at temperatures as low as 800°C. Carburization is delayed by high Cr, Ni and Si contents: for uses at 1100°C at least 28% Cr, 32% Ni and 2.5% Si are required.     

Untersuchung der Korrosionsbeständigkeit von Auftragschweißungen der Hartlegierungsfülldrahtelektroden Stelly Fe12 und Stelloy 6

Investigation of the corrosion resistance of the hard-facing alloys stelloy Fe12 and stelloy 6 Cobalt hard-facing alloys are often used under complex conditions with combined wear and corrosion attack at higher temperatures. For using in equipments of atomic power plants cobalt free claddings are required to guarantee a low radioactivity level. Beside this there is an economic interest of the industry to use cobalt free welding filler metals. The problem consists in developing of cobalt free hard-facing alloys with the same wear and corrosion resistance as cobalt base alloys. The resistance of claddings of the electrode stelloy Fe12 with about 19% Cr and 10% Ni has been investigated in 0.5 M sulfuric acid at 66°C during a full immersion test in comparison with the cobalt base alloy stelloy 6. Both investigated materials have no satisfactory corrosion resistance. But according to the EPR-test they were resistant against intergranular attack (IK). The high matrix dissolution of grinded samples was disadvantageous to evaluate IK resistance by EPR-test.     

Untersuchungen über den Einfluß des Gefügezustandes auf das Korrosionsverhalten hochlegierter,rost- und säurebeständiger Stähle in reiner sowie in chloridhaltiger Schwefelsäure

Corrosion Properties of High Alloyed Stainless Steels in Pure as well as in Chloride Containing Sulfuric Acid The corrosion behaviour of the high alloyed stainless steels material no. 1.4439 (X3CrNiMoN17135), 1.4539 (X2NiCrMoCu25205), 1.4503(X3NiCrMoCuTi2723) as well as the reference materials AlSI 316 L and alloy 825 was tested in diluted sulfuric acid (5, 10, 20 and 50%) at 50, 100 and 150°C. The test solutions additionally contained impurities as chlorides and cupric ions. On the material side the effect of various microstructures was checked as well: material as received (commercial production), solution annealed under laboratory conditions, cold deformed and for two selected steels electroslag remelted. Corrosion testing methods are: the immersion test will sheet coupons and the measurement of the weightloss; electrochemical testing, i.e. Current potential-and free corrosion potential-time-curves. No pitting corrosion is observed in the presence of chloride ions. In some cases the general corrosion rate is lowered if chloride ions are present. This beneficial effect of chloride ions, however, is observed only at low chloride concentrations (500 ppm). Annealing under laboratory conditions as well as electroslag remelting does not generally improve the corrosion resistance. A negative effect by cold deformation is only observed for standard stainless steel AlSI 316. Cupric ions added to the 20% sulfuric acid solution improve the corrosion resistance of all steels investigated to that extent, that they can be used in practice up to 100°C provided that the concentration of cupric ions in the solution is sufficiently high (2000 ppm). Electrochemical test results indicate that the positive effect of cupric ions is due to the shift of the free corrosion potential into the potential range of stable passivity. Copper alloyed stainless steels show the highest corrosion resistance.