Anforderungen an Stähle für die chemische Industrie

Properties required for steels in the chemical industry The materials used in the chemical industry must not only be chemically stable but must also have certain physical and other technological properties. Moreover, in choosing the materials, it is always necessary to take the cost factor into account which is also determined by servicing requirements and repair proneness. From these aspects, the authors deal with the development of modern stainless steels which is illustrated by some practical applications. Among these are petroleum distillation; petroleum desalination; hydrating cracking; production of synthetic gas and ethylene in petrochemistry; production of melamine from urea and of acetic acid from methanol and CO and butanol, respectively; production of phosphoric and nitric acid as well as urea in the fertilizer industry; preheaters; boilers for the regeneration of pulps in the cellulose industry. The economic aspects are discussed with the aid of numerical examples, using a nomogram.     

Die Korrosionsprobleme der Klein- und Mittelständischen Industrie - eine Herausforderung für die Gesellschaft für Korrosionsschutz

Corrosion problems in small and medium-sized companies - a challenge for Gesellschaft für Korrosionsschutz The Gesellschaft für Korrosionsschutz (GfKORR) attaches high priority to solving problems related to materials and corrosion encountered in small and medium-sized firms. In this connection historical trends are presented and existing activity in the fields of corrosion and corrosion protection in various institutions is analysed. It is demonstrated that, compared with large-scale industries, small and medium-sized industries have the greatest recourse to service organizations. Statistics, derived from the work of the Karl-Winnacker-Institut and the Bundesanstalt für Materialforschung und -prüfung, indicate the types and the distribution of corrosion projects. Implications for the future work of the society are given. In particular this entails the promotion of production and product-integrated corrosion protection which is applicable during the developmental stage of processes and products. It is intended that this approach to problems should replace additive corrosion protection which only takes effect when the damage has occurred.     

Einfluss der Oberflächenbehandlung auf den Fotoeffekt und die Potentialverteilung an Kupferrohren

Influence of a surface treatment on the photo effect and potential distribution of copper tubes Copper tubes have been used in drinking‐water installations since long times. This study will show the influence of a blasting treatment of the inner surface of tubes to the photo effect. Furthermore the soldering process was simulated at untreated and blasted copper tubes and the dependence of the free corrosion potentials as a function of the position was registered with a normal reference electrode and with a scanning kelvin probe. Within a time range of 10 days after exposition in synthetic drinking water a different behaviour of the free corrosion potential and the photo effect can be seen for the untreated and blasted tubes. The potential profile of the untreated inner surfaces of the copper tubes showed inconstancies in the heat affected zone which possibly could be the starting point of pitting corrosion, whereas the blasted samples do not show these inconstancies.     

Zur Herstellung und Anwendung von Ionenaustauschermembranen für die Alkalisalzelektrolyse

Manufacture and application of ion exchange membranes for alkali salt electrolysis Up to 30% by weight of styrene-divi-nylbenzene have been grafted on non-crosslinked high-pressure polyethylene films. Addition of isooctane in different proportions, yields variable degrees of porosity. Sulfonation yields cation exchange membranes, while chlormethylation followed by amination yields anion exchange membranes. In order to characterize the membranes thus obtained, their ion exchange capacities, specific conductivities and permselectivities were determined. The cation exchange membranes were used as diaphragms in the KCl and sodium sulfate electrolysis. Current efficiency and chloride ion diffusion were determined as functions of voltage, current density and degree of porosity and were correlated with the properties cited above.     

Korrosions- und Verschleißeigenschaften von Werkstoffen für die chemische Verfahrenstechnik

Corrosion and wear behaviour of materials for chemical engineering It has been estimated that 7.4 percent of all corrosion related failures are mechanically assisted (i.e. involve wear), and that such failure consume, or render useless, 37 million pounds of highly alloyed materials per year. Despite this, relatively little is known of the basic wear behaviour of alloys commonly used in the chemical processing industries, and little attention has been paid to the design of alloys for conditions involving simultaneous corrosion and wear. The objectives of this paper are three-fold. First, the various types of wear are described, and the alloy characteristics required to combat each of these types are discussed. Second, new wear data are presented for several alloys commonly used in the chemical processing industries. Third, a comparison is drawn between these well established materials and a newly introduced cobalt-based alloy designed specifically for corrosion/wear service. The conclusions of the paper are that hardness is a poor indicator of performance for most of wear, that the nickel-chromium-molybdenum alloys are considerably more resistant to cavitation erosion and galling than the austenitic and duplex stainless steels, and that the cobald-based alloy possesses exceptional resistance to slurry erosion, cavitation erosion, and galling.     

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.     

Einfluß der Oberflächenbehandlung nichtrostender Stähle auf deren chemische Beständigkeit insbesondere gegen Spannungsrißkorrosion

Influence of the surface treatment of stainless steels on their chemical resistance, in particular to stress corrosion cracking The stress corrosion resistance of austenitic stainless steels shows a pronounced dependence from the surface treatment. Grinding with a coarse material makes the surface very susceptible to this type of corrosion. The susceptible can be largely removed by subsequent picking, provided the treatment removes a layer about 3 μm in thickness. Grinding affects the structure to a depth of about 200 μm, but particular conditions appear to prevail in the above mentioned thin surface zone, so that in particular transcrystalline stress corrosion may occur. The susceptibility to stress corrosion cracking can be tested with a solution containing (%) 0.06 acetic acid, 1 acetaldehyde and 100 ppm Cl ions (as CuCl₂) when the corrosion susceptibility is to be evaluated in comparative terms. Pitting corrosion by mixed acid, too, can be largely prevented by deep picking of ground material.     

Oberflächenbehandlung als Korrosionsschutzmaßnahme von nichtrostenden Stählen

Surface treatment as corrosion protection measure of stainless steels The pickling behaviour of several stainless austenitic steels and of one steel with ferritic/austenitic grain structure were investigated in pickling solutions of different compositions based on hydrofluoric acid. Because of uncertainties in practical applications, the influence of temperature, time and acid content on the mass loss is of high interest. In another series of experiments, aqueous solutions of citric acid were tested for their suitability as pickling chemicals for the materials X 6 CrNiTi 18 10 (AISI 321) and X 6 CrNiMoTi 17 12 2 (AISI 316 Ti). Finally, the pickling procedures based on nitric acid/hydrofluoric acid mixtures were compared with mechanical cleansing methods and with pickling procedures based on aqueous citric acid solutions as well, to elucidate their influence on the corrosion resistance of the treated materials. The valuation followed a pitting corrosion test in sodium chloride solutions of different concentrations after Herbsleb and Schwenk. Pickling with hydrofluoric acid solutions is superior to other cleansing procedures, if corrosive environments are present. The ecologically beneficial citric acid solutions are only able to remove the annealing colours from stainless steels.     

Eine universelle Einrichtung für die Prüfung der atmosphärischen Korrosion von Metallen und Schutzüberzügen

Universal accelerated weathering cabinet for weathering tests of metals and protective coatings The authors have developed a accelerated weathering cabinet with visual observation of specimens during the test. The temperature can be controlled between 0 and 50°C by a current of warm air and a thermostat, while a humidification device enables the relative humidity to be controlled between 50 and 95%. Aggressive additions such as sulphur dioxide, aerosols, salt spray etc. are added through a spray system. Sulphur dioxide concentrations can be controlled between 0.1 and 10 ppm and 1 to 100 ppm respectively.     

Inhibitorwirksamkeit beim Langzeitschutz von Stahlbehältern für die chemische Oberflächenvorbereitung gegen örtliche Korrosion durch salzsäurehaltige Prozesslösungen

Inhibitor efficiency in long‐time protection of steel tanks for the chemical surface preparation against local corrosion by process solutions containing hydrochloric acid The efficiency of prevailing acid inhibitors is examined by age hardening heavy tank‐steel plates in technically usual hot‐galvanizing solutions for 1000 hours. With acid inhibitors local corrosion emerged as shallow pit formation first and foremost in hydrochlorid acid pickles (20 g/l HCl) at ambient temperature as well as in cleaners containing hydrochloric acid (10–30 g/l HCl) at 40°C when other conditions also applied. Above all, local corrosion was produced if the inhibitor concentration became too low (0.2 g/l) in connection with a minimum hydrochloric acid concentration (10–30 g/l). However, oxidizing agents like iron(III)‐ions (5–10 g/l), atmospheric oxygen and free chlorine (100–1000 mg/l) lead to local corrosion, too. Local corrosion did not emerge in rinse baths (2–10 g/l HCl) and fluxing material solutions of zinc chloride and ammonium chloride (pH value: 2.0–5.5). Here uniform corrosion developed. Acid inhibitors turned out to be very effective against uniform corrosion in the examined long‐time period (inhibiting values up to 99%).     

HASTELLOY® Alloy C-22 –ein neuer und vielseitig anwendbarer Werkstoff für die chemische Industrie

HASTELLOY® alloy C-22–a new and versatile material for the chemical process industries HASTELLOY® alloy C-22 is designed based on a critical balance of chromium, molybdenum, and tungsten contents. Such a balance provides unique corrosion resistance in the Ni-Cr-Mo system to oxidizing acids, localized corrosion, and non-oxidizing acids. Alloy C-22 also possesses thermal stability equivalent to HASTELLOY alloy C-4 in high heat input welding procedures. Applications of HASTELLOY alloy C-22 demonstrate its ability to solve difficult plant corrosion problems in services where other high performance alloys have failed.     

Die Oberflächenbehandlung von Schweißverbindungen hochkorrosionsbeständiger 6% Mo-Stähle und Nickel-Basis-Legierungen

Surface treatments of high alloy 6 Mo stainless steel and nickel alloy weldments High alloy stainless steels (6% Mo) and a high nickel alloy (alloy 625) weldment have been tested in order to answer the question whether post-treatment of the weldment has an effect on the corrosion resistance, especially on pitting corrosion. Therefore, the critical pitting temperature of weldments was tested in acidic chloride solution (standard tests). As a result grinding with rough emery paper as well as sand blasting lowers the localized corrosion resistance in the weldment area, while pickling has a positive effect, especially after blasting. Pickling can be done either by a solution of nitric + hydrofluoric acid or by a commercial pickling paste. In any event pickling is recommended as a final surface treatment for high alloy stainless steels and nickel alloys, especially in case of prevailing highly corrosive conditions such as pitting and crevice corrosion.     

Grundlagen für die Wahl von Anstrichsystemen in der chemischen Industrie der Volksrepublik Polen aufgrund einer Klassifikation der in der Atmosphäre von Chemieanlagen auftretenden angreifenden Mittel

Principles of coating system selection in the chemical industry of Poland, based on a classification of the aggressive media present in the atmosphere of chemical plants According to the Polish standard PN 71/H 04651 five classes of agressivity are defined in industrial atmospheres of chemical industries; these aggressivities are obtained by combining 15 aggressivity factors (acid, basic and neutral components as well as solvents and vapours) and four humidity classes according to an additive principle. On the basis of this classification the coating materials for chemical plants are assembled in corresponding protection classes. The individual coating materials include drying oil base materials and phthalate resins; drying oil base materials, chlorinated rubber and phthalate resins; drying oil base materials, vinyl and phthalate resins; chemically resistant chlorinated rubber; chemically resistant vinyl; coating emulsions based on styrene acrylic acid copolymeres; chemically resistant epoxy resins; heat resistant silicone resins; heat and chemically resistant phenoplastics; epoxy-tar and epoxy-asphalt; combinations of metal and paint coats. The ranges of application including longterm maximum temperature, aggressivity class and required coating fitness are summarized in tables.     

Die Bedeutung von Rostschichten für den Ablauf von Korrosionsreaktionen bei niedrig legierten Stählen

Influence of rust layers on corrosion reactions of low alloy steels Rust layers formed on iron base alloys are electrochemically highly reactive. Due to a polarisation of these oxide scales solid state reactions are induced, by which the 3-valent iron oxides are reduced. These redox reactions allow the dissolution of iron without the reduction of oxygen and therefore increase the corrosion rate, if the corrosion conditions vary periodically or if the material is coated by a polymer layer. Due to the oxide reduction electron-conducting oxides are formed, which determine the rate of the oxygen reduction. This causes the loss of adhesion for polymer coated and oxide covered iron substrates.     

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.     

Einsatz von Oberflächen- und Mikroanalysenverfahren bei der Untersuchung von Inhibitoren

Surface- and micro-analytical methods in the investigation of Inhibitors ESCA, AES, ISS, TOF-SIMS and LAMMA investigations concerning the composition and thickness of the protective layers formed when Cu, Ni and Co in water are inhibited with interface inhibitors (benzotriazole, tolyltriazole, mercaptobenzothiazole and 2-(5-aminopentyl)-benzimidazole), or unalloyed steels, likewise in water, are inhibited with interphase inhibitors, such as phosphonotricarboxylic acid, are reported. The layers formed by interface inhibitors are only a few monolayers thick. Information on the molecular structure of the layers is provided particularly by LAMMA and TOF-SIMS. Where Cu and Ni are concerned, quasi-polymeric molecules of the metal(I) benzotriazole type are formed, whereas in the case of Co, complexes of higher valency occur also. The same molecular structures are also observed on oxidized metal surface. The inhibitors layers cannot be removed from the surface of the metal with solvents, and they are also stable during prolonged exposure to air. The interphase inhibitors form substantially thicker layers. The mechanism of the protective layer formation is discussed with references to a variety of parameters.     

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.     

Richtlinien für Werkstoffauswahl und Korrosionsschutz in der chemischen Industrie

Criteria for the selection of materials and corrosion protection in the chemical industry In chemical technology, the technical requirements to be met by materials as well as the protection measures against corrosion become more differentiated. On the other hand, owing to the technical progress in the sphere of materials number of alternative solutions to cope with these specifications is becoming available. The selection process, which has thus become more difficult, must lead to an overall solution representing the economic optimum; but must not become too expensive in the planning of new plant or in the corrosion protection of existing plant. A distinction is made between, and a brief outline given of, several stages in the material selection process, viz. formulation of requirements; selection methods prior to testing; laboratory tests; utilisation of technical testing plants; tests under operating conditions; determining the correct solution. The economic calculations designed to determine the most favourable technical solution call for numerous technical data (e.g., the corrosion rates under different corrosion conditions) as well as economic data (especially the capital and annual costs depending on the material). The calculation and estimating methods suitable for this purpose are described in detail. In order to reduce the costs incurred through corrosion, money must be spent on certain protective measures. This relationship is illustrated by the phenomenon of cost substitution. Finally, a synopsis is given of all the practicable methods suitable for determining the optimum conditions.     

Die Bedeutung des Oxalsäure-Tests für die Prüfung der Korrosionsbeständigkeit nichtrostender Stähle

The meaning of the oxalic acid etch test for testing the corrosion resistance of stainless steels In the oxalic acid etch test according to ASTM A 262 practice A, precipitations of phases rich in chromium and molybdenum which can occur in stainless steels, are preferentially dissoved. The behaviour of such phases in the oxalic acid etch test was investigated taking precipitations of carbide M₂₃C₆, s?-phase, χ-phase and Laves-phase in stainless steels AISI 304 L and 316 L as examples. The chemical composition of these was evaluated with a scanning transmission electron microscope (STEM) by EDS. With coarser precipitations, it was possible to support this analytical method by EDS of metallographic cross sections in a scanning electron microscope (SEM). In oxalic acid, critical threshold potentials exist above which the above mentioned phases are preferably attacked, furthermore critical pH values, below which no selective attack of the precipitated carbides and intermetallic phases occurs. The numerical values of the threshold potentials as well as the critical pH values were evaluated. When testing stainless steels in the oxalic acid etch test, the steel specimens are polarized to a highly positive potential in the very trans passive range. In this potential range the corrosion rate of stainless steels increases with increasing chromium content, while in the active and passive range the corrosion rate decreases with increasing chromium content. Other than the nitric-hydrofluoric acid test, the copper-copper sulfate-sulfuric acid test, and the ferric sulfate-sulfuric acid test, the oxalic acid etch test does therefore not indicate any chromium depletion. Hence, an intergranular attack also occurs when precipitations of carbides rich in chromium are present at the grain boundaries of austenitic stainless steels with the carbides being precipitated without any chromium depletion of the areas adjacent to the grain boundaries. Sensitized austenitic stainless steels which are susceptible to intergranular corrosion due to the precipitation of chromium rich carbides and chromium depletion of the areas adjacent to the grain boundaries, can suffer intergranular SCC in high temperature aqueous environments when additionally critical conditions with respect to the mechanical stress level and the oxygen concentration in the environment are given. For the detection of sensitized microstructures, the oxalic acid etch test must be valued critically due to the dependence of the corrosion rate on the chromium content mentioned above, and is obviously by far less suited than the conventional tests for establishing resistance to intergranular corrosion in sulfuric acid-copper sulfate solutions with additions of metallic copper (Strauß test, severe Strauß test).