Ein neuer hochlegierter Nickel-Chrom-Molybdän-Stahl für den Chemie-Apparatebau

A new high-alloyed nickel-chromium-molybdenum steel for the chemical process industry A new austenitic nickel-chromium-molybdenum steel (Nicrofer 3127 hMo/VDM alloy 31) is combining the advantages of highchromium alloyed materials, i.e. an excellent resistance to corrosive attack by oxidizing media, with a molybdenum content of more than 6%. The nickel content is comparatively low. This way, it was possible to create a highly corrosion resistant material in using only small amounts of expensive alloying elements. The new material exhibits an excellent resistance to pitting and crevice corrosion in neutral and acid aqueous solutions which makes it superior to the known 6% Mo stainless steels and nickel-base alloys like G-3. Therefore, it lends itself to applications as e.g. flue gas desulfurization, concentrating of diluted acid, pulp & paper, phosphoric acid, acetic acid and hot seawater as a promising new material for the future. The new alloy is readily weldable.     

Oxidation und Aufkohlung hochlegierter Werkstoffe für Crackrohre. Teil 1: Das Oxidationsverhalten in Luft

Oxidation and Carburization of High Alloyed Materials for Cracking Tubes. Part 1: The Oxidation Behaviour in Air The oxidation behaviour has been studied of cast materials (German Materials Numbers 1.4848, 1.4857, 2.4813), an experimental melt of 25/20/5 CrNiSi steel and wrought materials (German Materials Numbers 1.4301 and 1.4841) in dry synthetic air between 800 and 1300°C. Only the oxidation of the materials 1.4848 and 1.4841 follows an approximately parabolic law, with the other materials more complex kinetic laws are found. The oxidation rates of all the cast materials are rather similar; the scale layers successively formed on the base material are Cr₂O₃ and MnCr₂O₄. The latter is characterized by clearly distinguishable iron contents. The Cr₂O₃ layer contains SiO₂ inclusions, in particular near the metal boundary. Above 1000°C internal oxidation of silicon is found. An exception to this is the 25/20/5 CrNiSi experimental melt where, because of the high Si content, a continuous SiO₂ layer is formed consistently between base material and Cr₂O₃ layer. The material 1.4301 exhibits locally increased oxidation at temperatures above 1050°C; in this case oxide nodules are formed. The oxidation rate of material 1.4841 at temperatures above 1100°C is higher for the fine grained than for the coarse grained condition; this phenomenon is attributed to the fact that at these temperatures pronounced grain growth occurs which impairs the formation of a protective layer. The scale layers formed contain considerable amounts of iron and the Cr content in the metal at the metal/oxide boundary decreases at a considerable higher rate with increasing temperature than in the case of the coarse grained material 1.4841 and of the cast materials containing about 25% Cr. Below 1100°C the differences in the behaviour are rather small and the oxidation rate is comparable to that of the cast materials. A transition from internal silicon oxidation to external SiO₂ scale formation is found with materials 1.4848 and 1.4841 at 1100°C in H₂/H₂O mixtures below the partial pressure of oxygen corresponding to the Cr/Cr₂O₃ equilibrium.     

Entwicklung neuer korrosinsbeständiger Werkstoffe für den Chemieapparatebau

Qualification of new corrosion-resistant materials for chemical plants and equipment The publication quotes the “superaustenite” X 2 NiCrMoCu 25 20 5 (Cronifer 1925 LC/Alloy 904 L, 1.4539) as an example how to distinctly improve the resistance to local corrosion in acid and neutral chloride-containing media by reducing the sulfur content and increasing the content of nitrogen, while simultaneously influencing the mechanical characteristics in a positive way. As a result, a new material for chemical plants and equipment, Cronifer 1925 LCN/Alloy 904 LN is presented. In a second part the article explains how by its reduced carbon content and change in alloying constituents the Ni alloy GiCrFeMoCu (Nicrofer 4823 hMo/Alloy G-3 (2.4619)) might be interpreted as a progressive modification of the more conventional NiCr 21 Mo (Nicrofer 4221/Alloy 825 (2.4858)) type ein order to meet new requirements. The report specially deals with the susceptibility to sensitization in processing as well as with the resistance to local corrosion in acid chloride-containing media, thanks to which Nicrofer 4823 hMo can be ranked between austenitic steels and very highly alloyed Nibase materials as to its application fields.     

Einfluß der Wärmebehandlung auf den Verlauf der Stromdichte-Potential-Kurven bei einem 13%igen Chromstahlguß

Influence of the heat treatment on the trend of the current density/potential curves with a 13 per cent. chromium steel casting With decreasing chilling rate after hardening, there is an increase in the segregation of Cr carbides along the delta ferrite where the Cr content is reduced. The degree of this reduction is reflected in the current density/potential curves where, with decreasing chilling rate, a gradually more pronounced second current density maximum is encountered which disappears, gradually if the specimens are tempered at a temperature ranging from 550 to 600° C. A further proof for the reduction in the Cr content is obtained from an electron-microscopic observation: it is not the carbides themselves but a small seam in the delta ferrite immediately adjacent to the carbides which is particularly corrosionprone. In practice, it follows that heat treatment conditions with a maximum of current density at about + 100 mV_H are less capable of passivation and therefore less reliable in application.     

Asbestverstärkte Phenolharze als Werkstoffe für den chemischen Apparatebau

Asbestos-phenolic resin laminates as primary materials of construction for chemical plant and equipment The application potential of asbestosphenolic resin laminates is determined by their processing properties which are favourably influenced e.g. by chrysotile fibres, or by the chemical resistance which is obtained by another species of asbestos, crocidolite, which is also used for acid resistant packings. The combination of these fibres and special phenoplastic materials has yielded materials which can be machined like wood and which can therefore be used for a number of applications. Some examples are stirrers for reaction vessels, tube components and tubes with moulded-on flanges, container lids, valves, stopcocks, pumps. The chemical resistance of the resin can be increased considerably by curing at high temperature and pressure. The resins are also completely resistant to solvents at practically any admissible working temperature. The resistance to sulfuric acid extends to 90°C, above that temperature and - in fuming acid at room temperature - the resin is sulfonated.     

Oxidation und Aufkohlung hochlegierter Werkstoffe für Crackrohre – Teil 2: Das Aufkohlungsverhalten in sauerstoff- und kohlenstoffhaltigen Atmosphären hoher Kohlenstoffaktivität

Oxidation and Carburization of High Alloyed Materials for Cracking Tubes – Part 2: The Carburization Behaviour in Oxygen and Carbon Containing Atmospheres with High Carbon Activity Carburization tests have been carried out with the casting materials (German Designation) Werkstoff-Nr. 1.4848, 1.4857, 2.4813 and an experimental 25/20/5-CrNiSi melt, and with the wrought materials Werkstoff Nr. 1.4301 and 1.4848 in CO-CO₂ mixtures with a_c = 1, and in humid natural gas with a_c > 1 between 1000 and 1200° C. At temperatures below appr. 1050° C oxide layers formed are analogous to those formed in air (substrate/Cr₂O₃/(Mn, Fe)Cr₂O₄) which prevent carburization. Carburization takes place only after the protective oxide layer has been destroyed. In this context several mechanisms of destruction must be considered, i.e. (a) by chemical reaction (transformation of oxide into carbide, reduction of oxide), (b) by mechanical stresses (temperature fluctuation, growth of graphite layers, creep deformation). Reduction processes are encountered with Fe-rich oxide layers only. Scale layers rich in chromium oxide are transformed into carbide above appr. 1050° C in gases with a_c = 1, depending on the CO partial pressure. The rate of this transformation strongly depends on carbon activity in the gas phase. When graphite is deposited on the surface the transformation occurs at a high rate, while it is rather slow even at a_c = 1 when no graphite is deposited. Deposited graphite may have a mechanically destructive effect, too, because it penetrates into cracks and pores in the oxide layer and thus produces spalling of oxide particles. When the surface of the cast material is porous particles of the substrate may become detached, too. This goes to explain the poor behaviour of tubes with as cast surfaces in cracking reactors. The positive effect of silicon is due to the formation of protective silica layers between metal and Cr₂O₃ and metal and external carbide layer respectively. In order to form a continuous silica layer the Si content in the material must not be below a cricital value.     

Werkstoffe für die Salpetersäure-Industrie

Materials for the nitric acid industry Particularly demanding requirements are imposed on components exposed to nitric acid. Where metallic materials are concerned, attention must be paid not only to general corrosion, but also to intergranular and selective attack in many cases. A distinction is made between three sectors for which different materials are employed:

• a Subazeotropic and azeotropic nitric acids not contaminated with oxidants,
• b Subazeotropic and azeotropic nitric acids containing oxidants, such as chromates, and
• c Superazeotropic, primarily highly-concentrated nitric acids.

The advantages and disadvantages of the materials used in the three sectors, namely austenitic chromium-nickel-(molybdenum) steels, high-silicon austenitic chromium-nickel steels, iron-silicon casting materials, glass-lined steel, glass, aluminum (alloys), titanium, zirconium, tantalum-niobium alloys, tantalum and platinum, are illustrated. Special features to be considered in corrosion tests on steel grades resistant to nitric acid are also described.     

Neue Aspekte für den Schutz von Bauwerken durch Fassadenanstriche

New aspects of the protection of buildings by facade coatings For the life of buildings, the moisture conditions are of decisive importance. Because of the ubiquitous nature of moisture, complete freedom from water is impracticable. The aim of building protection measures must therefore be to prevent the entry of water as much as possible while, at the same time, facilitating the outward diffusion of moisture. Historically, the facade coatings used for this purpose can be divided into three “generations”. Those of the first generation had virtually no protection effect as they were not elastic enough to follow the thermal movements of the building components (lime wash, oil paints). Those of the second generation, based on plastics, were able to impede the entry of water; but they also impeded moisture diffusion and were, moreover, sensitive to alkaline building materials. These drawbacks no longer apply to the “third-generation” coatings which absorb no water, disactivate the capillary ducts of the plaster, are resistant to hydrolysis, and dirt-repellent.     

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.     

Grenzfälle für den Reaktionsablauf bei der gleichzeitigen Verzunderung und Entkohlung von Eisen-Kohlenstoff-Legierungen

Limiting cases of the reaction kinetics during simultaneous scaling and decarburization of iron-carbon-alloys Limiting cases for the reaction progress of simultaneous scaling and decarburizing of iron-carbon alloys in oxidizing atmospheres are discussed. The rate laws for diffusion-controlled carbon oxidation and simultaneous parabolic scaling are presented. These laws are compared with experimental results. The results agree well with the theoretically predicted ones. Further more, the rate laws for diffusion-controlled carbon oxidation and simultaneous time-linear scaling are derived in order to create a basis for quantitative interpretation of experiments of the oxidation of iron-carbon alloys in carbon dioxide, which will be presented in a later publication.     

Qualifizierung metallischer Werkstoffe für die Eindampfung von Abwässern aus der Rauchgasentschwefelung

Qualification of metallic materials for evaporation of waste water from flue gas desulfurization plants The ecologically-minded processing of waste water from the wet scrubbing of flue gases of coal-fired power plants to produce environmentally acceptable products is carried out in a two-step evaporater operating in closed loop mode. The evaporating process leads to high concentration of chlorides in the two evaporation steps: up to about 100 g/l in the 1st step and up to about 350 g/l in the 2nd step. Therefore in case of metallic design of the evaporation equipment materials of construction with exceptional resistance to chloride induced pitting are required. The corrosion resistance of the high-alloyed stainless steel Alloy 31 (X1NiCrMoCu32-28-7 – UNS N 08031) and of the NiCrMo-alloys Alloy C-276 (NiMo16Cr15W – UNS N 10276) and Alloy 59 (NiCr23Mo16Al – UNS N 06059) including their weldments were to be tested for this application both in the laboratory and in field tests. In addition the behaviour of Alloy 59 heat exchanger tubes had to be determined in field tests under heat-transfer service conditions. The critical pitting corrosion temperatures of the 3 materials after having been GTAW welded under uniform conditions with FM 59 (ERNiCrMo–12) filler were determined in potentiostatic tests in model solutions imitating concentrated waste water products as they may occur in practice, using 5 K temperature intervals. As to be expected the critical corrosion resistance limits of the materials lie at 85 °C at chloride concentrations of 100 g/l Cl⁻ for the Alloy 31 and of 300 g/l Cl⁻ for both the Alloy 59 and the Alloy C–276 respectively. Field tests in waste water evaporation units of flue gas desulfurization plants of coal-fired power stations are carried out as immersion tests with the welded materials and as heat-exchange experiments using longitudinally welded tubes of Alloy 59 (2.4605). The immersion tests over a period of 32 months show the Alloy 31 (1.4562) to be a corrosion resistant construction material for tubes and containers in the first evaporation step, whereas the Alloy 59 (2.4605) and the Alloy C–276 (2.4819) have to be used for the second evaporation step, where the chloride contents are much higher. The Alloy 59 is to value as the most resistant material according to its lower tendency to crevice corrosion. The heat-exchange experiments over a test period of 9 months cause to expect the Alloy 59 to be a suitable construction material for heat-exchanger tubes in both evaporation steps in comparison to graphite which is more succeptible to mechanical destroying.     

Die Korrosionsbeständigkeit metallischer Werkstoffe für Automobilbremsleitungen

Corrosion resistance of metallic materials in automobile brake lines A statistical evaluation of the data compiled by the Swedish Automobile Inspection Authority and covering corrosion effects in brake fluid lines enables the following recommendation to be made:

• (a) Copper-brazed steel tubes, though very common, have no sufficient corrosion resistance; the same applies to welded or seamless drawn steel tubing.
• (b) The best suited materials are phosphorus deoxidized copper and stainless steels (about X 5 CrNiMo 18 12). The lower fatigue strength of copper must be compensated by suitable design; as compared to the stainless steel, copper is advantageous from the cost side.
• (c) Copper as well as stainless steel tubing can be corroded at fittings and fastening points; these problems must be eliminated by suitable design to be decided uon in each particular case.

     

Korrosionsbeständige Werkstoffe für Tanks in gasbetriebenen Fahrzeugen

Corrosion-resistant materials for tanks in gas-driven vehicles Compressed natural gas is increasingly used as drive for internal-combustion engines world-wide. The employed tank material consists of composite pressure tanks made of heat-treated steel or aluminum. Tanks of steel have to be resistant against stress corrosion cracking, tanks of aluminum respectively against intergranular corrosion. This paper describes the necessary acceptance tests.     

Korrosionsprognose für den Bereich der Schweißnaht

Forecasting the corrosion susceptibility in the weld zone Forecasting the corrosion susceptibility of a weld seam requires the knowledge of as much data as possible of the material-electrolyte system. Important data are structural changes in the material as a consequence of heat flow during welding. It is therefore important to know the precipitation conditions in materials intended for use in welded structures. A particularly useful tool in corrosion forecasting is the potential probe curve the basic shapes of which and conclusions to be drawn therefrom are described. Its validity is checked using ship steels welded by different methods and being exposed to seawater. It is shown that less noble portions in the potential probe curve are a necessary but not a sufficient condition for selective corrosion. The curves agree with practical experience only when the welded specimens are exposed to flowing seawater. On the other hand, regular shapes of the curve may be taken for a necessary and also sufficient condition for the impossiblility of selective corrosion in the weld zone. As to steels the variety of possible material/electrode combinations may be divided into two large groups: unalloyed steels in aqueous, aerated and near neutral, and stainless steels in neutral and acid media.     

Neue Werkstoffe für die Chemische Verfahrenstechnik mit besonderen Anforderungen an den Apparatebau

New materials for the Chemical Process Industry with special requirements for the equipment fabricator The introduction of new materials to the Chemical Process Industry normally is based on laboratory data which do not include sufficient knowledge with respect to fabricability into plant equipment. Therefore, on the one hard, new materials require more fabricability tests and evaluation of manufacturing procedures than materials which have already been introduced to the industry for long and, on the other hand, they require more supervising attention. Insofar every new material sets special requirements for the equipment fabricator as long as it is new. Additionally new materials may require special knowledge or care during manufacturing which both have to be learned from fabricability tests and evaluation of manufacturing procedures. Examples presented in this paper are the newly developed alloys 31, 700 Si and 602 CA. In case of alloy 31 sigma-phase is precipitating over a very broad range of intermediate temperatures. Whereas this phenomenon surprisingly does not cause any welding problems with this material, the absolute need for solution annealing at 1150°C (2100°F) – 1180°C (2160°F) has to be respected without any exceptions. Otherwise resistance to pitting and crevice corrosion may he reduced. Alloy 700 exhibits reduced weldability becoming apparent by a rapid loss of ductility at intermediate temperatures due to precipitation of silicides. This reduced weldability has to be taken into account already when plant and equipment design is being made as special requirement more to design than to manufacturing personnel in this case. Alloy 602 CA, for competitive cost of manufacturing, requires special high-performance welding processes well adapted to the material and the application considered as e.g. SAW or GTAW hot-wire and the availability of a suitable flux or of the corresponding equipment.     

Untersuchungen über den Einfluß der Wärmebehandlung auf die Korrosion titanstabilisierter ferritischer Chromstähle in siedender konzentrierter Salpetersäure

Investigations into the influence of the thermal treatment on the corrosion of titanium-stabilized ferritic chromium steels in boiling concentrated nitric acid Titanium-stabilized ferritic chromium steels with about 17% Cr are, after quenching from high temperature, susceptible to grain boundary corrosion in boiling nitric acid; this corrosion is attributable to the chemical dissolution of the titanium carbonitrides coherently segregated at the grain boundaries. This carbide corrosion can be reduced by heat treatment within the temperature range around 850°C. In this process, the segregated carbo-nitrides are formed-in (in-formation annealing). The influence of quenching (0.5–5 hours, 900–1200°C/W[L]) and subsequent in-formation annealing (1–30 hours, 850°C/W) on the corrosion rate in boiling 65% nitric acid has been investigated in detailed on two steels of types X 8 Cr Ti 17 and X 8 Cr Mo Ti 17. In addition, the corrosion behaviour of titanium-stabilized and unstabilized ferritic chromium steels with about 17 pC chromium have been compared with each other.     

Erosion und Korrosion of metallischer Werkstoffe in feststoffhaltigen Wässern

Erosion and corrosion of metallic materials in particle containing aqueous solutions The combined effect and mutual influence, respectively, of erosion and corrosion of metallic materials in particle containing flows was investigated. Materials under investigation are carbon steel of different hardness levels, ferritic chromium steel, austenitic stainless steel and brass. Corrosive medium was deionized water, acid (pH 4.5), alkaline (pH 10) and chloride (3% NaCl) solution. Quartz particles were used as abrasive at various particle concentrations, grain sizes and flow velocities. For comparison, tests were performed with pure water and under conditions of idle corrosion. Mechanical or corrosive influences dominate depending on material and test conditions, in some cases, synergistic effects of both mechanisms cause an increased mass loss.     

Korrosionsverhalten ausgewählter Werkstoffe in Lithiumbromid‐Lösung für den Anwendungsfall Absorptionskältemaschinen

Corrosion behavior of selected materials in lithium bromide solution for the application absorption chillers Absorption chillers on the basis of lithium bromide‐water are interesting systems for efficient and energy‐saving supply of coldness. The heating energy for the propulsion of the chiller could result ideally from renewable energies, e.g. solar energy, or waste heating energy from industry. The chiller can work as a multiple‐stage process for increasing the energy efficiency. For the three‐stage‐plants, in the generator temperatures of up to 200–220°C are necessary. The factors high temperature, high halogenide concentration (lithium bromide) and the presence of water vapour lead to a very high corrosive exposure of the materials. In that reason the task of the present report was the consideration of commercial materials and inhibitors concerning their suitability for the application in triple‐effect‐chillers. The measurements were carried out under identical conditions, but in unmoved solution and excluding the contact with air. The result of the investigations showed that stainless steel has been identified as a promising material for this application. Furthermore we have found out that the type of the inhibitor and the interaction of different phases have a strong influence on the corrosion processes.     

Oxidation und Versagen von EB-PVD-ZrO2- Wärmedämmschichtsystemen für Gasturbinen unter Hochtemperaturbeanspruchung

Oxidation and failure of EB-PVD-ZrO₂- Thermal Barrier Coatings for turbine applications during high-temperature-load The damage behaviour of an EB-PVD (EB-PVD = electron beam physical vapour deposition) zirconia thermal barrier coating system was investigated during thermal and combined thermal and mechanical loading. The oxidation of the bond coating which causes the growth of a thin alumina layer at the interface bond coating/thermal barrier coating was investigated in annealing experiments. The kinetics of the diffusion controlled oxidation can be described by a power law. The growth of the alumina layer gives rise to a critical stress state at the interface bond coating/thermal barrier coating and leads to spallation of the ceramic layer, especially during long-term-operation. The determined crack densities and crack opening distributions in the ceramic coating after isothermal high-temperature creep experiments show a strong dependance on creep rate and temperature. Measurements of the porosity of the ceramic layer indicate aging by sintering effects during high temperature load. The sintering effects could also be observed by microscopic methods.