Erprobung verschiedener metallischer Legierungen für die Abwasseraufbereitung hinter Rauchgasentschwefelungsanlagen

Evaluation of different metal alloys for waste water treatment of flue gas desulfurization plants

1 Vortrag anläßlich der ACHEME '94 Frankfurt/Main, 5.-11. Juni 1994.

Desulfurization of flue gases from coal-fired power plants is carried out by wet scrubbing processes in 90% of the cases. Using milk of lime or lime stone suspensions as a sorbent, said processes not only remove sulfur dioxide but also all other flue gas pollutants. The sulfur compounds are mostly separated in the form of calcium sulfate (gypsum) in the solid-liquid separation process whereas the water-soluble components, mainly calcium chloride are discharged with the scrubber solutions. Since the salt load of effluents to be discharged into municipal sewer systems or surface waters is limited by legislation, the FGD scrubber solutions must be treated by evaporation, salt modification or resource recovery. For evaporation crystallization of these extremely corrosive FGD scrubber solutions which – depending on the process – can lead all the way to a product suitable for depositing, an economic equipment concept based on metallic construction materials was to be investigated. In a parameter study, electrochemical test methods were used to determine the influence of the pH value, the Fe³⁺, F^? and SO₂ contents in a solution containing 200 g/l CaCl₂ + 30 g/l NaCl at 80°C on the resistance to pitting corrosion of a stainless steel and various NiCrMo alloys. In addition, electrochemical and wet corrosion tests (e.g. crevice corrosion) were conducted using original FGD scrubber solutions with 43–50% dry substance from the power plants Reuter-West, Oberhavel and Rudow of the BEWAG, Berlin. Of the materials tested, NiCrMo alloys 2.4605 (NiCr 23 Mo 16 Al, Alloy 59), 2.4602 (NiCr 21 Mo 14 W Alloy 22) and 2.4819 (NiMo 16 Cr 15 W, Alloy C-276) proved to be resistant under the conditions of the third evaporation crystallization stage, with the corrosion resistance decreasing in the above order. The stainless steel 1.4529 (X 2 NiCrMoCu 25 20 6) and the NiCrMo alloy 2.4856 (NiCr 22 Mo 9 Nb, Alloy 625) experienced pitting corrosion in nearly all test solutions at redox potential. Also the alloy 2.4610 (NiMo 16 Cr 16 Ti, Alloy C-4) was not always sufficiently resistant. In the evaporation crystallization process, care should be taken to ensure that the pH does not fall below 5, that the redox potential of the FGD scrubber solutions remains below the transpassive dilution or pitting potential and that crevices are avoided, if possible.     

Untersuchungen zur Prüfung geschweißter NiCrMo-Legierungen für Rauchgasentschwefelungsanlagen auf selektive Korrosion

Investigation on testing of welded NiCrMo-alloys for flue gas desulfurization plants on selective corrosion The use of NiCrMo-alloys is mandatory in places where a very high corrosion resistance is required, as well in oxidizing as in socalled reducing media. All standardized tests for examination on intercrystalline corrosion, however, are providing strongly oxidizing conditions only. Therefore test methods have to he found for examination on selective corrosion in less oxidizing media as they are prevailing in aqueous flue gas desulfurization media of fossil fired power plants. Having this object in mind the anodic polarization behaviour of the NiCrMo alloys C-276, C-4 and 59 has been examined in sulfuric acid solutions with additions of SCN^? and Cl^? as structure sensitive ions. The material conditions selected for being examined were the solution annealed and sensitized wrought alloys as well as weldments made with matching filler and varied heat-input during welding. In the active-passive transition range the various alloy, conditions display in fact a different behaviour with respect to selective and intercrystalline corrosion but only under test conditions which have to be closely controlled and which are differing from alloy to alloy. These tests, on the one hand, provide the less oxidizing character as prevailing in aqueous flue gas desulfurization media but are much more corrosive than in practice on the other. The different sensitivity of the various materials and material conditions to selective corrosion as shown in this type of test corresponds with practical experience. In case of a heat-input during, welding which exceeds 8 kJ/cm a tendency to increased selective corrosion is most pronounced with Alloy C-276 followed by Alloy C-4. With Alloy 59 an influence of heat-input during welding can be seen only at extremely low or extremely high heat-input rates of 6.2 or 15.1 kJ/cm respectively. Alloy 59 proves to be the less sensitive NiCrMo-alloy also with respect to welding parameters. Though the test conditions experienced so far represent a first approach to a test method which conforms with real world conditions in flue gas desulfurization plants the test results do not allow a forecast of the materials' corrosion behaviour during such type of service. However, an evaluation in terms of a ranking should he possible.     

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.     

Corrosion behaviour of alloy 31 – UNS N08031 – under conditions of oil & gas production

The corrosion behaviour of alloy 31 (UNS N08031‐31Ni – 27Cr – 6.5Mo – 1.2Cu – 0.2N – bal. Fe) was tested in laboratory and field tests in seawater with and without additions of CO₂ and/or H₂S in slow strain rate tests, and in SSC (Sulphide Stress Corrosion) tests according to NACE MR0175. The results demonstrate a high resistance of alloy 31 to localised corrosion. Due to the high chromium and molybdenum concentration, its resistance to pitting and crevice corrosion in chloride‐contaminated seawater is significantly higher than that of alloy 28 and alloy 825 and it equals that of typical nickel base alloys like alloy 625. Alloy 31 is not sensitive to chloride‐induced stress corrosion cracking, either with or without H₂S, or sulphide stress cracking. Alloy 31 is approved for sour gas applications up to LEVEL VI in NACE MR 0175. The combination of properties makes alloy 31 an attractive choice for components in oil and gas production including wirelines, umbilicals, tubing, piping and topside application.     

Nicrofer 5923 hMo,ein neuer hochkorrosionsbeständiger Werkstoff für die Chemische Industrie,die Umwelttechnik und verwandte Anwendungen

Alloy 59, a new highly corrosion resistant material for the chemical process industry, environmental pollutioncontrol and related applications

1 Vortrag anläßlich der ACHEMA '91, Frankfurt/Main, 09.–15. Juni 1991.

A new nickel-chromium-molybdenum alloy, Alloy 59 has been designed to withstand severest corrosive conditions as encountered in chemical process industry and today's environmental pollution control systems. This alloy, which is composed of about 59% nickel, 23% chromium and 16% molybdenum is compared to the common NiCrMo-alloys Alloy C-276, Allo C-4, Alloy 22 and Alloy 625. Conditions of corrosion testing have been varied between oxidizing and reducing mineral acids combined with differing halide contaminations. Furthermore, corrosion data in hot concentrated sulfuric acid and in technical relevant solutions of flue gas desulfurization plants are given. The general resistance to corrosion in sulfuric acid and in hydrochloric acid is highlighted in isocorrosion diagrams. Alloy 59 has an excellent resistance to uniform and localized corrosion in all these environments. The new Alloy 59 clearly outperforms the other NiCrMo-alloys C-276, C-4 and 22 as well under oxidizing conditions as in the strongly reducing hydrochloric acid environment. The time-temperature-sensitization-diagram demonstrates the alloy's excellent thermal stability and reveals an improvement compared to Alloy 22 or the well-known Alloy C-276. Alloy 59, therefore, can be used in the as-welded condition without any additional solution annealing treatment. The excellent corrosion resistance is not impaired if the alloy is processed or fabricated to equipment even in larger sections. Weldability with a matching filler metal is without problems as demonstrated i.a. in the varestraint test.     

Titan und Titanlegierungen – Korrosionsbeständige Werkstoffe in Rauchgasentschwefelungsanlagen (REA)

High performance materials for use in flue gas desulfurization In scrubber mixing zones, only Titanium alloys and Alloy C-276 are resistant to corrosion attack. Under acidic deposits in the quench zone Titanium Grade 7 is the only metal totally resistant to localized attack. “Wallpapering”, the use of thin corrosion resistant cladding, is a promising fabrication technique that reduces the high initial cost of using alloys like Titanium and Alloy C-276.     

Vergleichende Untersuchung der Schweißzusätze Alloy C22 und Alloy 59 Zur Frage der Einsetzbarkeit des Schweißzusatzes Alloy 59 für das Schweißen von Nickellegierungen der C‐Reihe bei Anwendungen in der chemischen Prozessindustrie

A comparative investigation of weld filler materials Alloy C22 and Alloy 59 In the chemicals processing industry all nickel alloys of the C series are utilised in vessels and pipelines. Due to ever increasing corrosion demands these materials are being replaced by higher alloys such as Alloy 59. The present research aims to establish whether all available base metals can be welded with a filler material such as Alloy 59.     

Effect of temperature on the galvanic corrosion of a high alloyed austenitic stainless steel in its welded and non-welded condition in LiBr solutions

Galvanic corrosion generated by the coupling between the austenitic stainless steel Alloy 926 (UNS N08926) and the welded Alloy 926 has been studied by means of electrochemical methods. The materials have been tested in highly concentrated LiBr solutions at different temperatures. The effect of Li₂CrO₄ as corrosion inhibitor has also been evaluated. Galvanic corrosion has been studied under open circuit conditions using a zero-resistance ammeter (ZRA). Results have demonstrated the poor severity of the coupling between the Alloy 926 and the welded metal in the studied conditions. The probability of localized corrosion increased with temperature and concentration, as the galvanic current density and galvanic potential data demonstrated.     

Über die transpassive Auflösung von Nickel-Basis-Legierungen und Edelstählen in sauerstoff- und chloridhaltigem Hochtemperatur-Wasser

Transpassive dissolution of nickel-base alloys and stainless steels in oxygen and chloride containing high-temperature water The corrosion behaviour of different nickel base alloys and stainless steels (2.4605 [alloy 59; NiCr23Mol6Al], 2.4633 [alloy 602 CA; NiCr25FeAlY], 2.4819 [alloy C-276; NiMol6Crl5W], 2.4856 [alloy 625; NiCr22Mo9Nb], 2.4606 [alloy 686; NiCr20Mu16], 2.4646 [alloy 214; NiCrl6AlFe] and 1.4401 [UNS S 31600; X5CrNiMol7122]) was investigated in oxygen and chloride containing high-temperature water (temperatures up to 600°C; pressures up to 38 MPa; oxygen concentration 0.48 mol/kg; chloride concentrations up to 0. 1 mol/kg). All alloys show a similar corrosion behaviour, depending on temperature. At temperatures below about 150°C, only slight intergranular corrosion was observed. At higher temperatures (between about 150 and 300°C) pitting was detected. Most of the original surface in this temperature range remained unattacked. At higher temperatures, morphology of pitting changed towards shallow pitting and the whole surface is penetrated. The high general corrosion observed in these areas can be attributed to transpassive dissolution of the alloys' protecting chromium oxide layers with following dissolution of the alloy. At supercritical temperatures, corrosion decreased drastically, and only transpassive intergranular corrosion was detected. The observed decrease of ion-induced corrosion phenomena can he attributed to the change of physical and chemical properties of water (solubility of ions). Corrosion in neutral and alkaline solution was significantly less. Both pitting and transpassive dissolution shifted towards higher temperatures or were not detected respectively.     

Imposed potential measurements to evaluate the pitting corrosion resistance and the galvanic behaviour of a highly alloyed austenitic stainless steel and its weldment in a LiBr solution at temperatures up to 150 °C

Pitting corrosion resistance and galvanic behaviour of Alloy 31, a highly alloyed austenitic stainless steel (UNS N08031), and its weldment were studied in a heavy brine LiBr solution 1080 g/l at different temperatures (75–150 °C) using electrochemical techniques. The Mixed Potential Theory was used to evaluate the galvanic corrosion between the base and welded metals. Cyclic potentiodynamic curves indicate that high temperatures make passivation and repassivation of pits difficult, because the whole passivation range and the repassivation potential values decrease with temperature. The critical pitting transition occurs between 100 and 125 °C.     

Applications and experience with a Superaustenitic 7Mo stainless steel in hostile environments

A new superaustenitic stainless steel is described in terms of chemical composition. mechanical properties and corrosion resistance. Results obtained by using this highly alloyed stainless steel, EN 1.4652, in a number of hostile environments are reported. Applications include neutral and acid chloride containing environments such as those existing in e.g. – pulp bleach plants, – ventilation systems of pharmaceutical plants, – equipment for flue gas cleaning in power plants and municipal waste incineration plants, – equipment for recovery of chlorinated hydrocarbons, – sea water piping, – nuclear power plant condenser tubes. The paper also contains laboratory corrosion test results positioning this steel in relation to other highly alloyed stainless steels and nickel base alloys, mainly in terms of its resistance to chloride induced localised corrosion.     

Einfluss der Halbzeugform auf die Korrosionsbeständigkeit hochlegierter Ni‐Cr‐Mo‐Werkstoffe

Effect of semi‐finished products on the corrosion resistance of high‐alloyed Ni‐Cr‐Mo materials The corrosion resistance of different semi‐finished products of six superaustenitic steels and nickel based alloys in the condition of delivery was investigated in some typical standard corrosion tests. The resistance of sheets, plates, strips, seamless tubes and welded tubes to intercrystalline corrosion was tested according to ASTM G 28 methods A and B, as well the resistance to pitting corrosion according to ASTM G 48 method C. The nickel based alloys 625, C‐276 and alloy 59 are resistant to the FeCl₃‐test according to ASTM G 48 method C and therefore a differentiation of these types in regard to their localized corrosion resistance was achieved only in the more aggressive ‘Green‐Death’‐solution. The laboratory experiments confirmed that the corrosion resistance is identical for all semi‐finished products and that it shows only a slight dependence of the surface condition of the materials tested. Additionally, some typical industrial and practical applications of the six high performance materials are presented to demonstrate the excellent corrosion resistance in the manufactured condition.     

Der Einfluß von Chlorid auf das Korrosionsverhalten von Nicrofer 3033 (1.4591) in mittelkonzentrierter heißer Schwefelsäure

The influence of chloride on the corrosion behaviour of Alloy 33 in medium concentrated hot sulfuric acid In the concentration-temperature field of 15 to 80 mass-% p. a. grade sulfuric acid and temperatures of 50 to 90 °C in which Alloy 33 is either corrosion resistant without and with small additions of Fe³⁺, the influence of chloride concentration on the degradation of its corrosion resistance has been determined. The evaluation of the critical chloride content has been carried out by gravimetrical means in tests of 7 days duration. For comparison materials used in sulfuric acid applications as Alloy 20 (2.4660) and Alloy 31 (1.4562) have been examined in form of random tests in the above mentioned concentration-temperature field. Based on polarization tests the influence of chloride ions in sulfuric acid solutions could be evaluated for the conditions of presence and absence of the oxidant Fe³⁺. The chloride concentration, which can be tolerated, diminishes with increasing acid concentration and temperature. If the oxidant Fe³⁺ is present in the sulfuric acid solution, higher chloride concentrations can be tolerated. Alloy 20 (2.4660) will in comparision to Alloy 33 (1.4591) be activated at significantly lower chloride concentrations. Compared to Alloy 33 (1.4591) the range of application of Alloy 31 (1.4562) is mainly restricted in Fe³⁺ containing sulfuric acid.     

Localized corrosion of alloy 22 in the potential Yucca Mountain repository environment

The U.S. Department of Energy (DOE) has indicated that it may use Alloy 22 as the waste package outer container material for the potential high-level waste repository at Yucca Mountain, Nevada. Additionally, a drip shield, made of titanium grade 7 and titanium grade 29, may extend the length of the emplacement drifts to enclose the top and sides of the emplaced waste package. Localized corrosion in the form of crevice corrosion could be one degradation process that may adversely affect the waste package performance. This paper will summarize the work conducted to evaluate the effects of environmental conditions relevant to the potential Yucca Mountain repository, metallurgical states (e.g., mill-annealed and welded plus solution annealed), and similar and dissimilar metal crevices on the crevice corrosion susceptibility of Alloy 22. This work also evaluates crevice corrosion propagation behavior resulting from contact with Alloy 22 or titanium grade 7.     

Der Einfluss von Oxidationsmitteln auf die Korrosionsbeständigkeit von Nicrofer 3127 hMo in niedrig‐ und mittelkonzentrierter Schwefelsäure

The influence of oxidants on the corrosion resistance of Alloy 31 in low and medium concentrated sulfuric acid The isocorrosion lines < 0.1 mm/y were determined for Nicrofer 3127 hMo (1.4562, Alloy 31) by weight loss measurements after 24 h immersion in areated sulfuric acid of technical and p.a. grades in the concentration range up to 90% – both without and with 1 g/l chloride. The free corrosion potential indicates that the superior corrosion resistance of Nicrofer 3127 hMo in the technical acid grade can be attributed to the content of oxidants in the acid. The presence of oxidants leads to metastable passivity in technical sulfuric acid with concentrations ≥ 20% at temperatures that can be 10–50 K higher than in p.a. acid depending on the acid concentration. For sulfuric acid concentrations up to 10% the content of oxidants in the technical acid is too small to improve the corrosion resistance of Nicrofer 3127 hMo as compared with that in p.a. acid. Similar behaviour was observed for Nicrofer 3127 hMo in sulfuric acid containing chlorides. However the activating effect of the chlorides limits the region of corrosion resistance to lower temperatures in comparison with the chloride free‐acid.     

Microbially influenced corrosion on stainless steels in waste water treatment plants: Part 1

Abstract

Field tests on stainless steels have been carried out at five waste water treatment plants for one year. Three grades of stainless steel, i.e. AISI 304 (UNS S30400), AISI 316 (UNS S31600), and duplex 2205 (UNS S31803) were tested in the final settling tank in the plants. The time dependence of the open circuit potential (OCP) was measured for all coupons. Ennoblement of the OCP, similar to that reported from investigations in sea water, was found in one of the plants. Waters from three of the exposure sites, containing dispersed deposits from exposed coupons, were chemically analysed. Pitting corrosion was observed after the field test on steel grade AISI 304 in three of the five plants, and on AISI 316 in one plant. No corrosion was found on 2205 in any of the plants. Laboratory measurements of the OCP were carried out for the AISI 304, AISI 316, and duplex 2205 steels in water collected from one of the plants. Cathodic polarisation curves were recorded in waste water from the same plant. T he cathodic reaction rate increased at the highest OCP. Simulation of the ennoblement was carried out by potentiostatic polarisation in a 600 ppm chloride solution. The current response indicated corrosion of welded AISI 304 material and of AISI 304 and AISI 316 steels in crevice assemblies after a long induction time. Part 2 of this study presents the results of further testing and a risk assessment design.     

Effect of aqueous LiBr solutions on the corrosion resistance and galvanic corrosion of an austenitic stainless steel in its welded and non-welded condition

The corrosion resistance and galvanic behaviour of a highly alloyed austenitic stainless steel (UNS N08031) and a nickel-base alloy (UNS N06059) in its welded (GTAW) and non-welded condition were analysed in LiBr solutions by means of electrochemical measurements. Samples microstructure was studied by SEM and EDX analysis. The alloys considered showed passive behaviour and they were able to repassivate after the breakdown of the passive film. The pitting resistance of the materials increased as the LiBr concentration decreased. With regard to galvanic corrosion, the welded metal was always the anodic element of the UNS N08031-welded metal pair.     

Werkstoffe für SCWO-Prozesse zur Oxidation von chlorhaltigen Schadstoffen in überkritischem Wasser

Materials for SCWO processes for the oxidation of chlorine containing residues in supercritical water The oxidation of harmful residues in supercritical water (SCWO – Supercritical Water Oxidation) is an efficient procedure for converting hazardous substances into environmentally friendly products. Due to the usually high chlorine concentrations, however, the reactor material is subjected to a highly corrosive medium at high pressures and temperatures. For the corrosion studies an experimental set-up was used, that allows to subject the materials to these conditions. The nickel base materials Alloy 625 and Alloy 686 were studied at 500°C and 46.5 MPa up to 320 hours. The model fluid consisted of HCl+H₂O+NaCl+methanol with H₂O₂ as oxidant. Alloy 625 forms complex scales consisting of alternating Cr-Mo-Nb-O mixed oxide layers and chloride layers, mostly NiCl₂. Alloy 686 forms similar corrosion scales and shows after 320 h additionally stress corrosion cracking. The ICP-AES analysis of the fluid in function of time shows time intervals with high Ni concentrations alternating with time intervals with high Mo and Cr concentrations, which indicates that alternating mechanisms control the corrosion process.     

Effect of temperature on the corrosion resistance and pitting behaviour of Alloy 31 in LiBr solutions

The corrosion resistance and pitting behaviour of Alloy 31, a high-alloyed austenitic stainless steel (UNS N08031), is studied in two heavy brine LiBr solutions (850 g/l) with and without corrosion inhibitor (lithium chromate) at different temperatures (25 °C, 50 °C, 75 °C and 100 °C) using electrochemical techniques. Cyclic potentiodynamic curves indicate that UNS N08031 is less pitting corrosion resistant and it reduces its repassivation properties as temperature increases. Comparison between the results obtained in LiBr solutions with and without inhibitor suggested a decrease in the inhibitor efficiency of lithium chromate at high temperatures.     

Korrosionsuntersuchungen an ausgewählten metallischen Werkstoffen im System Essigsäure/Esigsäureanhydrid

Corrosion tests of selected metallic materials in the system acetic acid/acetic anhydride The corrosion behaviour of the ferritic austenitic stainless steels X2 CrNiMoN 22 5 (UNS S31803) and X2 CrNiMoN 25 7 4 (UNS S32750), the nickel base alloy NiMo16Cr16Ti (alloy C-4) and the titanium grades Ti2 (Grade 2) and Ti2Pd (Grade 7) was investigated by immersion tests in boiling mixtures of acetic acid and acetic anhydride in the presence of air. All materials tested were corrosion resistant in acetic acid and acetic anhydride but were corroded rapidly by the mixtures with a corrosion loss up to 1 mm/a, except the nickel base alloy (alloy C-4). It was corrosion resistant in all solutions with a corrosion loss not exceeding 0.01 mm/a. Electrochemical tests show that all materials tested exhibit stable passivity in glacial acetic acid and active corrosion in the presence of 10% acetic anhydride.