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.     

Sonderstahle für den Apparatebau mit sehr guter Korrosionsbestandigkeit und erhöhter Festigkeit

Special steels with superior corrosion resistance and strength for chemical equipment manufacture Stainless steels are among the materials most predominantly used in chemical plant engineering. During the past few years, quite a number of special steels of this type have been developed to meet the eve1 increasing demands in this field. This paper deals with two of them. VEW A 963 is an austenitic CrNiMo steel containing about 6.3% molybdenum which shows superior resistance to the attack of media with a high chloride content, in other terms, excellent resistance to pitting and crevice corrosion. Stress corrosion cracking tests in NaCl solution with crust formation show steel VEW A %3 to be definitely superior to CrNiMo grades with up to 4.5% molybdenum. Corrosion resistance in acids, too, is very good. The steel possesses good weldability, and suitable filler metals are available in the form of coated electrodes and inert gas welding wire. Big scale production of steel VEW A 963 in the form of sheet and plate, bar, forgings and seamless tube is possible. VEW A 905 is an austenitic-ferritic CrNiMo steel with manganese and nitrogen additions which permit to obtain a minimum 0.2% proof stress of 590 Nlmm² in the solution annealed condition. The micro- structure shows more or less equal parts of austenite and ferrite which hardly change with rising temperature. This is of great importance for welding: there is no grain coarsening in the heat affected zone, nor increase of ferrite content. An electrode type of the same composition is available. VEW A 905 has good resistance to pitting corrosion and to the attack of a variety of acids. Particular emphasis should be laid on its excellent resistance to chloride induced stress corrosion cracking. VEW A 905. too, is available in the form of sheet and plate, bar, forgings and seamless tube produced on a large scale.     

Die Oxidation von Aluminium-Magnesium-Legierungen in Sauerstoff bei erhöhter Temperatur

The oxidation of aluminium magnesium alloys in oxygen at elevated temperatures The oxidation behaviour of alloys of high purity with 0.17, 1.55 and 4.6% Mg has been determined by gravimetric and electron optical as well as photo electron spectroscopy. The investigations have been carried out in pure oxygen at temperatures between 300 and 600°C. On the alloy containing 0.17% Mg spinel crystals are formed at first below an amorphous oxide layer; these crystals, however, do not grow to form a continuous layer. Magnesium oxide is formed at a later stage. The oxide growth generally obeys a parabolic law. The alloys containing 1.55 and 4.6% exhibit a complex sequence of kinetic laws at low temperatures: logarithmic, sigmoidal, linear and with progressively growing rate. The crystalline phase formed is exclusively magnesium oxide. On the strength of the findings of photoelectronic spectroscopy a mechanism is proposed for the diffusion of magnesium through the amorphous oxide layer.     

Zum Kontaktverhalten von feuerverzinktem und unverzinktem Bewehrungsstahl in Beton bei erhöhter Temperatur

Contact behaviour of galvanized and ungalvanized steel in concrete at higher temperature Electrochemical measurements at 60 °C on steel and galvanized steel embedded in cement mortar with and without chloride show that there is no potential reversion between steel and zinc during the test time of 30 weeks. The contact current between steel and zinc is always anodical related to zinc. The current is continually diminishing with time, its value is slightly higher in mortar containing chloride. Contacted steel and galvanized steel therefore leads to a higher zinc loss at least in the starting time. Without contact there is only a small corrosion attack on zinc in both cases of chloride free and chloride containing mortar.     

Schutzmaßnahmen gegen interkristalline Spannungsrißkorrosion von Gashochdruckleitungen aus Stahl bei erhöhter Betriebstemperatur

Protection of high-pressure steel pipelines for the transmission of gas against stress corrosion cracking at high temperature Grade X 60 line pipe sections were capped to pressure vessels and were placed in concrete pits where they were exposed to controlled corrosion conditions. The corrosive conditions were represented by a sand environment containing NaHCO₃, a wall temperature of 70°C and pipe-to-soil potentials of U_H = ? 0.43, ? 0.55 and ? 0.68 V. The pipes were polyethylene coated to DIN Standard 30 670 without prior surface treatment (no mill scale removal) or after the removal of the mill scale by brushing or sandblasting. The coating included holidays of different sizes to allow disbonding. These pressure vessels were tested by pressure cycling. The strain rate for the cycles was 10^?7 sec^?1 whereas strain release was much faster. The test programme included 4800 cycles between 44 and 87% YS, 1390 cycles between 66 and 87% YS and 5000 cycles between 79 and 87% YS. The total duration of the test was 9830 hrs. Following the completion of the pressure cycles, the pipes were examined. No corrosion defects were discovered in the holiday areas. In the cases of the pressure vessels where mill scale had been removed by sandblasting or brushing, no corrosion was found below the disbonded coating, either. Minor crack formation which was not revealed by conventional test methods was, however, established below the disbonded coating of the pressure vessels where the mill scale had not been removed. The scanning electron microscope showed corrosion pitting type strain induced cracking of a depth of up to 0.2 mm. In a limited number of cases these cracks could be considered intercrystalline stress corrosion cracking. The paper discusses what action must be taken to prevent such stress corrosion cracking. It concludes that stress corrosion cracking in West German high-pressure gas transmission pipelines is impossible if the pipelines are designed, built and operated in accordance with existing standards and codes of practice.     

Schwingungsrißkorrosionsverhalten des austenitischen Stahles X 6 CrNiMoTi 17 12 2 in 3%iger NaCl-Lösung unter Umlaufbiegebelastung

Corrosion fatigue behaviour of the austenitic steel X 6 CrNiMoTi 17 12 2 in 3% NaCl-solution under rotating bending load The fatigue behaviour of steel X 6 CrNiMoTi 17 12 2 (German material-number 14571, comparable to AISI 316 Ti) is investigated under rotating bending load with a frequency of 50 Hz. The fatigue strength in inert medium (thermostat oil) at room temperature is 315 N/mm². In 3% NaCl-solution of pH 7 at the same temperature the corrosion fatigue limit for N = 10⁷ reached only 94% of the mentioned value. A higher temperature (70 °C) caused a further reduction to 89% and an additional decrease of the pH-value (pH 3) lowers the dynamic strength to 86% of the reference value. The free corrosion potential, recorded during all corrosion fatigue tests, remains in the passive region for all specimens, which do not fail under the mechanical-corrosive load. The potential of specimens, which fail under a higher mechanical load, announces their rupture by a sudden decrease to negative values, which starts at least 3·10⁴ cycles (10 minutes) before.     

Untersuchungen über die Ursachen der Spannungskorrosion bei nichtrostenden; C-stabilisierten austenitischen Cr-Ni-Stählen

Investigations into the causes of stress corrosion cracking with stainless, C-stabilized austenitic Cr-Ni-steels With the deformation of specimens (steel with 17 per cent. Cr, 10 per cent Ni) in boiling MgCl₂ solution, the potential is shifled to the negative side, at a rate which increases with the expansion rate. This phenomenon is ascribed to the fact, that, with slow expansion, the active zones are again apt to be passivated. It is only when major cracks are already present that the expansion rate at the bottom of the crack becomes so high that no passivation takes place any more and that the potential remains continuously below that required for stress corrosion cracking. Between stres corrosion cracking and pitting corrosion, there is a correlation inasmuch as the pitting has a corrosioninhibiting effect. The hydrogen and chlorme ions which accumulate in the pitting have the effect of shifling the potential to the negative side so that, during expansion, the potential is no longer much below the negative potential required for stress corrosion cracking. This interpretation is derived from the different behaviour of specimens which were exposed to loads after brief polarisation (causing pitting corrosion), and directly.     

Untersuchungen zur Spannungsrißkorrosionsgefährdung von Spannstählen (Stäben) bei erhöhter Vorspannung

Investigations on stress corrosion cracking of prestressing steels (bars) at increased prestressing level The application of prestressing methods using bars lead to additional bending stresses or internal stresses when plastic deformation takes place at curvatures. These stresses are superimposed by the well-planned prestressing. The probability for hydrogen-induced stress corrosion cracking in corrosive media (especially water in the metal sheath) is considerably increased in the presence of internal stresses and if the prestressing level is enlarged. The permissible prestressing levels of the EU guidelines are higher than the values fixed in DIN 4227 with the same permissible radii of curvature. In order to find out if there is an increased risk of stress corrosion cracking, exposure tests were performed with prestressing bars grade St 900/1030 and St 1080/1230, respectively, in an electrolyte simulating metal sheath conditions. All specimens withstood the 2000 h lasting exposure test without fracture which means that the risk of hydrogen-induced brittle fracture could be excluded under practical conditions provided that the current state of the art has been considered. Tensile tests after the exposure revealed a reduction of strain values due to the exposure. Fracture analysis by means of SEM did not show any signs of embrittlement for the materials tested.     

Korrosionsuntersuchungen an nichtrostenden,austenitischen Stählen im System Essigsäure/Essigsäureanhydrid

Corrosion tests of austenitic stainless steels in the system acetic acid/acetic anhydride The corrosion behaviour of the austenitic stainless steels X6 CrNiMoTi 17 12 2 (mat.no. 1.4571, AISI 316 Ti) and X2 NiCrMo-Cu 25 20 5 (mat.no. 1.4539, alloy 904 L) was investigated by immersion tests of welded specimens in acetic acid and acetic anhydride and in mixtures thereof, as well. The tests were carried out at 80°C and at the boiling point of the test solutions. The test equipment was either open to the atmosphere or the mixtures were deaerated with nitrogen. The tests show that in the presence of air the mixtures are more corrosive than the neat media. At 80°C in aerated solutions only alloy 904 L (mat.no. 1.4539) performed well, except in a mixture of 70% acetic acid and 30% acetic anhydride. At the boiling point of the test solutions the corrosive attack on both stainless steels increases considerably; the corrosion rates are in the range of 1 mm/a (1.4571) and 0.6 mm/a (1.4539). Deaeration with nitrogen decreases the corrosivity of the solutions to the stainless steels. Electrochemical tests show the detrimental influence of acetic anhydride to the passivation process of stainless steels tested in glacial acetic acid.     

Mechanismus des Rißwachstums bei der Spannungsrißkorrosion des austenitischen Stahls X6 CrNiTi 18 10 in wäßriger Chloridlösung bei erhöhten Temperaturen

SCC crack growth mechanism of austenitic stainless steel X6 CrNiTi 18 10 in aqueous chloride solution at elevated temperatures The SCC crack growth mechanism of steam generator heat transfer tubes from stainless steel X6 CrNiTi 18 10 under internal stress conditions at elevated temperatures is discussed. Based on crack tip characterization by means of Scanning and Transmission Electron Microscopy and the evidence of hydrogen originated throughout the corrosion process a crack propagation model is presented. The results refer to a microcrack induced gradual crack growth caused by local hydrogen embrittlement. Microcrack growth has been observed due to slip-band decohesion. The crack growth rate is mainly influenced by the stress state near the crack tip and the hydrogen evolution throughout the corrosion process.     

Einfluß der Spannung auf die Empfindlichkeit eines austenitischen Chrom-Nickel-Stahles gegenüber Spannungsrißkorrosion

Influence of stress on the sensitivity of an austenitic chromium-nickel steel to stress corrosion cracking The transcrystalline stress corrosion cracking of a chromium-nickel steel (German designation X 5 CrNi I8 9) in 35 % magnesium chloride solution at 150°C exhibits a well defined limit at the 0.01 %yield stress. In the range of intermediate stresses the stress corrosion resistance increases as the load is increased. This behaviour may be interpreted in terms of the time required for the formation of stable cracks. This mechanism apparently follows the gliding step-repassivation model involving a reduction in height of the gliding steps and the influence of a cathodic protection due to the reduction of the mixed potential.     

Eigenschaften des schweißbaren,hoch korrosionsbeständigen Stahles X 2CrNiMoN 25 22

Properties of the weldable and corrosion resistant steel X 2 CrNiMoN 25 22 In view of the limited application of con- ventional CrNiMo steels the authors have investigated potential applications of the above mentioned steel. An optimum com- position would be (%) 0.02 C, 25Cr, 22 Ni, 2 Mo and 0.1 N. The low carbon content is a vital factor and the nickel content must not exceed 22% because higher contents would give rise to very pronounced precipitation. The new steel ist largely resistance to boil- ing nitric acid it is considerably superior to conventional steels, The high break-through potential is indicative of high pitting resist- ance. Improvements are equally remarkable with respect to mecanical properties and weldability     

Untersuchung der Eigenschaften von nach einem neuen Verfahren hergestellten Phosphatüberzügen

Investigation into the properties of phosphate layers obtained according layers obtained according to a new process Phosphate coatings were obtained by anodic phosphatizing in sodium phosphate solutions (2.5-5% phosphate, PH 4.2-5.4, current density 2.5-5.0m Amps/cm²). The coatings (coating weight 7.3–20 g/cm₂) consist of ferrous/ferric phosphates, the Fe²⁺: Fe³⁺ ratio depending from bath composition. he hardness of the coatings is lower than the hardness of coatings from Zn and Mn phosphate baths and their absorption capacities for chromic acid, mineral oil and paints are higher because of their fine crystalline structure. With respect to corrosion behaviour they are superior in NaCl and equivalent to Zn and Mn phosphate coatings in moist atmospheres, while the latter seem to be superior in the salt spray seem to be superior in the salt spray test. The minimum coating thickness required for subsequent painting is 8/m² subsequent oil impregnation or cold working require higher weight.     

Das Korrosionsverhalten von verzinkten und unlegierten Stahlrohren in Trinkwässern mit unterschiedlicher Härte und erhöhtem Neutralsalzgehalt

Corrosion of galvanized and unalloyed steel tubes in drinking water of different hardness and increased neutral salt content The corrosion of galvanized and unalloyed steel tubes in drinking water of different hardness was studied at the Rheinau water works, Mannheim. The criteria to describe the influence of the water quality parameters were the decrease of the metallic zinc layer, the composition of the corrosion products and a visual qualification of the coatings. The main parameters influencing zinc corrosion are pH-value and flowrate. Comparison with other data shows no significant effect of buffer intensity and neutral salt concentration. The layer of corrosionproducts in unalloyed steel tubes showed qualitative and quantitative differences with different hardness. The most important effect can be seen in the different contents of calcite in the coating. Low buffer capacity decreased the quality of the corrosion product layer. The results lead to the conclusion, that softening should he limitated to a certain level, otherwise corrosion problems will increase.     

Das Ausscheidungsverhalten von hochlegierten austenitischen Stählen mit 6% Molybdän und sein Einfluß auf die Korrosionsbeständigkeit

Precipitation behaviour of high-alloyed austenitic steels with 6% molybdenum and its influence on the corrosion resistance The high-alloy austenitic steels with 6 to 7% Mo, 20 to 21% Cr and 18 to 25% Ni are increasingly used in seawater and chemical applications. This is due to the excellent resistance to pitting and crevice corrosion in chloride-containing neutral and acidic environments. It is the high chromium and molybdenum content which provides the excellent corrosion behaviour but, at the same time favors the tendency to precipitation of intermetallic phases. Therefore, time-temperature-precipitation diagrams have been established for two steels with 6% Mo, 21% Cr, 25% Ni, 0.14 and 0.19% N and for one steel with 6% Mo, 20% Cr, 18% Ni and 0.21% N. The corresponding time-temperature-sensitization diagrams (in accordance to SEP 1877/II) and time-temperature-pitting diagrams (testing in 6% FeCl₃ solution) have been evaluated as well. Precipitation of intermetallics occurs rapidly especially in the range between 700 and 1000°C. In case of the 18% Ni steel and the 25% Ni/0.14% N steel grain boundaries are covered to a large extent with precipitates after only 15 min at 850 or 950°C. In case of the 25% Ni/0.19% N steel precipitation is considerably slower. The precipitates are interpreted to be chi-phase. After very long annealing times additionally small amounts of Laves phase appear. Neither carbides nor nitrides were observed. In spite of the rapid precipitation, sensitization in terms of the 50 m?m grain boundary penetration criterion is observed not before 0.7 h at 850 °C and not before about 2 h at 800°C in case of the 25% Ni/0.19% N steel. After about the same times of annealing also the critical pitting temperature as observed in the FeCl₃-test is dropping below 50°C. Therefore, when welding according to established rules and recommendations, no deterioration of the corrosion resistance in the heat-affected zone is to be expected. If high heat inputs will occur during manufacturing because of hot forming operations or welding of heavy sections, or if more severe test conditions are a requirement, a steel with 25% Ni and about 0.2% N (UNS N 08925, Cronifer hMo) is recommended due to its retarded precipitation and sensitization behaviour when compared to steels with only 18% Ni (UNS S 31 254). Additionally, the steel with 25% Ni has an increased resistance to general corrosion in acids. Notch impact strength of the materials under consideration is increased by the initial precipitation of the intermetallic phases and decreases only after longer times of annealing below the ductility of the solution annealed material.     

Zur Lochkorrosionsneigung von austenitischen CrNiMo-Stählen in konzentrierten Ammoniumrhodanidlösungen

Pitting corrosion of austenitic CrNiMo-steels in concentrated ammoniumrhodanide solutions Quasipotentiostatic and potentiokinetic polarisation measurements at various 18 Cr-10 Ni steels with molybdenum contents up to 4,3% were performed in 25 and 45% ammoniumrhodanide solutions. It was found that pitting corrosion is caused by incomplete passivation in the potential range of –300 to +250 m V _H. At these potentials the formation of stable passive layers is hindered by the formation and local oxidative dissolution of sulfidic layers. Above +250 m V _H rhodanide ions act in these weak acidic ammoniumrhodanide solutions as agents which destroy passive layers, comparable with chloride ions. The limiting potentials for stable pitting corrosion, obtained from potentiostatic experiments, are shifted from –300 to –150 m V _H with increasing molybdenum content of the steel. The least tendency of pitting corrosion was found for that steel with the highest molybdenum content.     

Strömungsabhängigkeit der Lochkorrosion eines Cr-Ni-Stahles in NaCl-Lösung. Teil II: Versuche mit Ultraschall

Flow dependence of the pitting corrosion of CrNi steel in NaCl solution Test (with CrNi steel 19 9 in 1 N NaCl) with supersonic agitation have confirmed the results obtained with the rotating disc electrode. Supersonic treatment increases the pitting potential for stable pitting and reduces maximum corrosion current density. The treatment has but little effect, however, on the pitting potential for repassivating pitting corrosion. Cycling tests under galvanostatic conditions show that the effect supersonic action comes to bear almost immediately. Switching on or off the supersonic source results in immediate repassivation or loss thereof. Similar to experiments on rotating discs the pits are very small and, consequently, constitute practically no corrosion risk.     

Korrosionsbeständigkeit von CrNi-Stahlguß (Typ 18/9 bzw. 18/13) mit erhöhtem Siliciumgehalt in konzentrierten Salpetersäurelösungen

Corrosion resistance of chromium nickel cast steel (types 18/9 and 18/13) within increased silicon content in concentrated nitric acid solutions The corrosion resistance of CrNi cast steel, types 18/9 and 18/13, with different carbon (0.035-0.12%) and silicon (appr. 4%) contents was investigated in nitric acid (27-96%) eventually containing Cr⁶⁺ ions between 25 and 100°C. In this connection mechanical properties and structure of the above cast steels were determined, too. It has been found that in the case of 80% nitric acid solutions and of nitric acid solutions of higher or lower concentrations but containing Cr⁶⁺ ions the cast steel must have a silicon content of appr. 4% together with a carbon content as low as possible. The results of the laboratory tests have been confirmed unter industrial conditions.     

Entwicklung einer mathematischen Beziehung für die Passivierung eines austenitischen Schweißguts an Luft und unter einem Elektrolyten

Development of a mathematical correlation for the passivation of an austenitic weld material in air and under an electrolyte Differences in data concerning the electro-chemical potential of stainless steels are mostly due to the fact that the measured potential of a metal is generally a function of the duration of exposure to atmosphere and of the duration of the test. In order to improve the reproducibility of the potential/time curves for austenitic weld materials, the influence of the duration of the exposure of the specimens to air has been experimentally determined before and during the testing period. The weld material contained, in one case, 65% Ni, 15%Cr and 30% Nb and, in another case, 19% Cr and 10% Ni. The potential/time curves were plotted without current, by means of the microcapillary method. The results were used for the compilation of a passivation equation containing a proportionality factor which, though showing a scatter over a certain range, assumes a different and characteristic value for different materials With the aid of the curves thus obtained, the correlations between potential, duration of test and duration of air exposure can be found.     

Korrosionsschäden am Förderband eines Ofens zur Reinigung von mit organischen Chlorverbindungen kontaminiertem Erdboden

Corrosion damage to the conveyor belt of a furnace for the removal of soil from organic chlorine compounds This paper reports the corrosion behaviour of metallic materials which were used in components of a conveyor belt of a decontamination furnace or of which specimens were attached to the belt for testing purposes. The furnace was used for decontaminating soil containing organic chlorine compounds. A total of 1 ferritic and 8 austenitic steels, 9 nickel base alloys, 2 east steels with 15 % Si and 15 % Si + 5 % Cr respectively, and 3 aluminised steels were tested and afterwards investigated by metallographic and micro-analytical methods. Most of the materials failed due to external and/or internal corrosion, preferentially along grain boundaries. The 2 cast steels revealed fairly good corrosion resistance. The material 2.4061 (LC-Ni 99.6) displayed surprisingly good behaviour, however it remains to be seen whether the inward migration of O and C along grain boundaries causes embrittlement. The materials 2.4610 and 2.4831, containing 14-17 % Mo and 8.5-9,5 % Mo respectively, showed good resistance. The reason for this good corrosion resistance could not be clarified. One reason may be the relatively high Mo content but other materials with a comparable Mo content, like materials 2.4856 and 2.4663, were significantly attacked. It appears necessary to check the long-term behaviour of these two materials.