Kritische Werte von Einflußgrößen für die Spannungsrißkorrosion von Austenitischem 18 Cr-10 Ni-Stahl in Chloridlösungen

Critical values of Influencing parameters for stress corrosion cracking of austenitic 18 Cr-10 Ni steel in chloride solutions In critical systems “material and corrosive environment”, stress corrosion cracking can only occur with the stress exceeding critical values (“classical” systems of SCC) or the strain rate being within a critical range (“non-classical” systems of SCC) and, furthermore, with certain parameters of the critical system, especially potential, temperature and concentration of the specific corrosive medium being present in the environment, also exceeding critical values. Based on hints given in the literature and on own investigations, the present knowledge about critical values of the influencing parameters mentioned above, is described. Above all is shown by open-circuit corrosion experiments that for stress corrosion cracking to occur with the critical system 304 SS/boiling 42% MgCl₂, a threshold stress does exist at very low stress levels of about 50 Nmm^?2. This low threshold stress can only be found when residual compressive or tensile stresses are completely avoided in the unaxially loaded tensile specimens used for the experiments. The experimental results are also influenced by the chemistry of the corrosive medium. A MgCl₂ solution already boiling for a longer time is less aggressive than a just prepared solution. In isothermal exposure tests performed with U-bend specimens showing a triple phase boundary material/aggressive medium (CaCl₂ solutions of various concentrations)/air, a critical threshold temperature below which no SCC occurs was found. This critical temperature does not depend on the chloride concentration over a broad concentration range between 1 and 100 g/l chloride ions and runs between 40 and 45 °C. This critical temperature was found somewhat lower than that measured when investigating SCC with ground flat specimens in diluted NaCl (0.1 g/l chloride ions). For U-bend specimens with triple phase boundary, the critical temperature for SCC to occur increases when lowering the chloride concentration (0.1 g/l Chloride ions) but decreases slightly at increasing concentrations. The system parameters potential and temperature influence each other and their value depends furthermore on the alloy composition of the austenitic CrNi steels, i. e. on the chemical resistant properties of these materials.     

Untersuchungen zur transkristallinen und interkristallinen Spannungsrißkorrosion austenitischer Chrom-Nickel-Stähle in heißen Magnesiumchlorid-Lösungen

Investigations Into transgranular and intergranular stress corrosion cracking of austenitic stainless steels In hot magnesium chloride solutions The stress corrosion cracking (SCC) of austenitic stainless steels in hot magnesium chloride solutions is known to be transgranular. Therefore the slip-step-dissolution model is most favourable when explaining the failure mechanism. Constant load and constant extension rate tests (CERT) show that both methodes are almost equivalent. Moreover constant extension rate tests in more concentrated magnesium chloride solutions at 135°C reveal a small potential range of intergranular stress corrosion cracking more negative than the range of transgranular SCC. Observations of crack nucleation and crack propagation make plain that crack nucleation is a localized corrosion process. Pitting produces crack nucleis in the elastic range whereas cracks start along slip lines after plastic deformation. Fractography of specimens which failed by intergranular and transgranular SCC show macroscopically brittle fracture surfaces. Therefore a model is proposed which explains crack propagation by hydrogen-induced intermitted cracking at high-stressed sites at the crack tip.     

Corrosion‐technical properties of high‐strength stainless steels for the application in prestressed concrete structures

Experience with prestressed concrete over about half a century has indicated that the corrosion resistance of conventional prestressing steel does not always satisfy, especially the prestressing steels are susceptible to chloride attack (de‐icing salts) and hydrogen (hydrogen‐induced stress corrosion cracking). On the other hand corrosion agents, such as chloride, condensation water, can penetrate in the concrete and arrive at the surface of steels. Hence, corrosion damage of prestressing steels can happen and, in the extreme cases, the prestressed concrete structure collapsed resulting from the failure of the tendon. In this paper, consideration is made to use high‐strength stainless steels as prestressing tendon with bond in concrete. The high‐strength stainless steels of qualities 1.4301 (X5CrNi18‐10), 1.4401 (X5CrNiMo17‐12‐2), 1.4436 (X3CrNiMo17‐13‐3) and 1.4439 (X3CrNiMoN17‐13‐5) with sequence of increasing austenite stability were investigated. For application in prestressing tendon with bond in concrete the cold‐drawn high‐strength stainless steel of quality 1.4401 is an optimal proposition regarding its satisfactory resistance against pitting corrosion and stress corrosion cracking (SCC) in structure‐related corrosive conditions. The lower alloyed steel 1.4301 has an insufficient resistance against the chloride‐induced corrosion because of the lack of molybdenum and the content of deformation martensite due to the strong cold‐drawing of its unstable austenitic structure.     

Untersuchungen zur Spannungsrißkorrosion austenitischer Stähle in kalten Chloridlösungen

Investigation on stress corrosion cracking of austenitic steels in cold chloride solutions At ambient temperatures of about 25°C austenitic chromium nickel steels can suffer stress corrosion in media with a concentration of both hydrogen ions and chloride ions exceeding 1 mol/L or in strongly concentrated chloride solutions. With the aid of constant strain rate testing and with U-bend specimens the parameters of this types of corrosion were investigated. Contrary to the situation with the acid solutions, the reproducibility of test results with the concentrated chloride solution was found to be low. The formation of martensite by cold working was found to be essential. The corrosion susceptibility decreases with increasing nickel content. Stress corrosion takes place within small critical potential ranges without a limit to more positive potentials as this is the case of stress corrosion with hot media. These potential ranges are widened with increasing cold working especially in the case of strongly grinding or cyclic loading in the plastic range. Corrosion cracking was observed with solutions of MgCl₂, LiCl, with a less amount with NaCl, but not with ZnCl₂. The concentration of chloride ions have an effect for solutions with a given kind of salt, but not for different salt solutions.     

Inter- und transkristalline Spannungsrißkorrosion austenitischer Mn- und CrNi-Stähle in Meerwasser

Intercrystalline and transcrystalline stress corrosion cracking of austenitic Mn and CrNi steels in seawater The MnCr steels which were originally used for the construction of special ships turned out to be susceptible to intercrystalline stress corrosion cracking in seawater; later on, transcrystalline corrosion susceptibility was found, too; this latter type of corrosion appears in cold seawater and is not due to a sensilizing annealing. This type of corrosion was also found with austenitic CrNi steels in chloride solutions of higher temperature, not, however, at room temperature. The author has made an effort at defining the susceptibility regions of the particular austenitic steels. According to the results obtained it is necessary always to take account of the possibility of stress corrosion cracking when Mn based austenitic steels are used, while austenitic CrNi steels can be considered to be resistant to this type of corrosion. Sensilizing annealing, too, has a bearing on the Mn containing steels only, while the influence of temperature, potential, specimen diameter and stress does not reveal any difference between the two types of steels.     

Korrosionsprobleme in chlorhaltigen Medien: Lösungsmöglichkeiten durch einige nichtrostende Spezialstähle

Corrosion problems in chloride containing media: possible solution by some stainless special steels The increasing water pollution forces the chemical industry to use water with increasing chloride content for cooling and other purposes. This trend brings about increasing corrosion danger, in particular pitting, stress corrosion cracking and corrosion fatigue as well as crevice corrosion. The present paper deals with some steels characterized by resistance to these specific corrosion phenomena. A steel containing (%) 21 Cr., 7.5 Ni, 2.5 Mo, 1.5 Cu, to 2 Mn, to 1 Si and 0.06 C is particularly resistant to stress corrosion cracking. It contains 30 to 50% ferrite in an austenitic matrix. Even in Mg chloride solutions it may be kept under a load of 7 kg/mm² without stress corrosion occurring (with a steel of the 18 10 CrNiMo type the admissible load is only 2 kg/mm²). A steel containing (%) 25 Ni, 21 Cr, 4.5 Mo, 1.5 Cu, to 1 Si, to 2 Mn, and 0.02 C has a broad passivity range and is resistant to general corrosion in acid reducing media and phosphoric acid of all concentrations. A ferritic steel containing (%) 26 Cr. 1 Mo and minor additions of C, Mn, Si, Cu, Ni and nitrogen is resistant to stress corrosion cracking in neutral chloride solutions and general corrosion in oxidizing and neutral media, even against hydrogen sulfid and organic acids; it is beyond that lergely resistant to pitting in chloride solutions.     

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.     

Einflüsse von Werkstoff und Verarbeitung auf die Spannungsrißkorrosion von Spannstählen

Influence of material and processing on stress corrosion cracking of prestressing steel In prestressed concrete constructions the highstrength prestressing steels perform essential bearing effects. The alkaline layer of concrete or mortar protects the steels against corrosion and guarantees a permanent load capacity. If the corrosion protection as a result of poor workmanship is not guaranteed from the beginning, or is lost because of lacks of construction in the course of time, or the steels are predamaged during handling, stress corrosion cracking and failure of steel and construction may occur. Also an application of unsuitable materials (prestressing steel, injection mortar, concrete) can alone or in combination with the before mentioned influences favour stress corrosion cracking. In the contribution the correlations and typical failures are discussed.     

Corrosion induced failure mechanisms of prestressing steel

Rarely occurring fractures of prestressing steel in prestressed concrete structure can, as a rule, be attributed to corrosion induced influences. The mechanism of these failures often is not well understood. In this connection it is difficult to establish the necessary recommendation not only for design and execution but also for building materials and prestressing systems in order to avoid future problems.This paper gives a survey about corrosion induced failure mechanisms of prestressing steels with a particular emphasis on post‐tensioning tendons. Depending on the prevailing corrosion situation and the load conditions as well as the prestressing steel properties the following possibilities of fracturing must be distinguished: • Brittle fracture due to exceeding the residual load capacity. Brittle fracture is particularly promoted by local corrosion attack and hydrogen embrittlement. • Fracture as a result of hydrogen induced stress‐corrosion cracking. • Fracture as a result of fatigue and corrosion influences, distinguishing between corrosion fatigue cracking and fretting corrosion/fretting fatigue.     

Korrosion niedrig legierter Stähle in flüssigem Ammoniak

Corrosion of low-alloyed steels in liquid ammonia Corrosion behaviour and stress corrosion cracking of the base material, the welding and the heat affected zone in liquid ammonia at room temperature of the low-alloyed steels 13 Cr Mo 4 4 (W. Nr. 1.7335) and 15 Mo 3 (W. Nr. 1.5415), as well as the un-alloyed steel StE 460 (W. Nr. 1.8905) for comparison were investigated. According to polarization curves, the corrosion current density in the passive state usually was lower for the welding than for the base steel. In the active state the welding of the low-alloyed steel corroded faster than the weldings of the unalloyed steel. In the region between the welding and the heat affected zone of the base steel corrosion was nonuniform. CERT-experiments revealed susceptibility to stress corrosion cracking in the orders StE 460 < 15 Mo 3 < 13 Cr Mo 4 4, and base steel < welding < transition zone. The highest susceptibility was observed at electrode potentials in the middle of the passive region. Additions of water exceeding about 100 ppm favoured stress corrosion cracking. The susceptibility decreased again at concentrations ≥ 1000 ppm. Oxygen increased the susceptibility to stress corrosion cracking only at high concentrations not occurring in practice. The presently investigated low-alloyed steels are not superior to the unalloyed steel from the viewpoint of stress corrosion cracking.     

Pitting, crevice and stress corrosion cracking studies of cold drawn eutectoid steels

The role of stress corrosion cracking in the failure of cold drawn eutectoid steels in a concrete environment was investigated. Constant strain tests in chloride containing solutions at pH 10, 12 and 14, revealed that pitting and crevice corrosion could be predicted from potentiostatic determinations on unstrained specimens. Since failure of the tendons is by yield at the point where the cross-section has been reduced by pitting, hydrogen embrittlement does not lead to a synergistic interaction between stress and corrosion so that failure may be ascribed simply to pitting or crevice corrosion caused by the presence of chlorides.     

高强钢WOL试样应力腐蚀开裂的金相跟踪观察

高强钢WOL恒位移试样应力腐蚀的金相跟踪观察表明:高强钢的应力腐蚀在微裂纹出现之前,先在缺口前端出现滑移线,滑移线的出现是由于氢引起的,位错运动形成滑移台阶.氢促进局部塑性变形,当局部塑性变形发展到临界状态时,缺口前端就出现微裂纹.     

Assessment of test methods for evaluation stress corrosion cracking susceptibility of prestressing steels

Prestressing steels can be exposed in prestressed concrete structures in construction stage (ungrouted duct) to environmental conditions causing formation and growth of hydrogen induced cracks with brittle fracture of the steel. The risk of this hydrogen induced stress corrosion cracking can be minimized by appropriate treatment, but there is a requirement for construction to approve only prestressing steel having no enhanced susceptibility to stress corrosion cracking. The paper describes the historical development of the different test methods proposed in the past and gives an overview on the general requirements for a suitable test method to assess the hydrogen‐induced stress corrosion susceptibility of prestressing steels. Since 1982, based on investigations on construction sites and transfer of the results into laboratory tests, a test method is available enabling realistic estimation of suitable application possibilities of prestressing steels. This so called DIBt‐test is approved to distinguish between prestressing steels susceptible to hydrogen induced stress corrosion cracking and those suitable and therefore approvable steels at practical environmental conditions. Furthermore the test enables estimation of corrosion risk for newly developed prestressing steels with higher strength.     

Untersuchungen zur Spannungsrißkorrosionsbeständigkeit nichtrostender Stähle mit unterschiedlichem Verformungsgrad

Investigations on stress corrosion cracking resistivity of stainless steels with different degree of cold straining Within the context of the elaboration of a new approval for stainless steels for civil engineering applications the question arised whether cold straining could adversely affect the resistance against stress corrosion cracking. By means of experimental tests with different stainless steels it should be clarified whether cold straining increase the stress corrosion cracking susceptibility under high chloride loads like e.g. in swimming hall atmospheres. The investigations were carried out using the high-alloyed steels 1.4462 and 1.4529. As reference material with known susceptibility against stress corrosion cracking the steel 1.4301 was used. The tests were performed using U-bent and tension specimens with saturated MgCl₂ salt spots. Indications for an increase in stress corrosion cracking susceptibility of the cold drawn steel were not found.     

Korrosionsverhalten hochchromhaltiger,ferritischer, chemisch beständiger Stähle

Corrosion behaviour of high chromium ferritic stainless steels Ferritic steels developed for seawater desalination and containing 20 to 28% chromium, up to 5% Mo and additions of nickel and copper have been tested with respect to their corrosion behaviour, in particular in chloride containing media. The materials in the sensibilized state were tested for inter-crystalline corrosion susceptibility in the Strauß-, Streicher-, nitric acid hydrofluoric acid- and Huey-Tests. No intercrystalline corrosion was encountered in the case of the steels with 28% Cr and 2% Mo. The resistance to pitting was assessed on the basis of rupture potentials determined by potentiokinetic tests. The resistance of the steels with 20% Cr and 5% Mo or 28% Cr and 2% Mo is superior to that of the molybdenum containing austenitic types. Addition of nickel yields a significant increase in crevice corrosion resistance; the same applies to resistance in sulfuric acid. In boiling seawater all the materials tested are resistant to stress corrosion cracking. No sign of any type of corrosion was found on nickel containing steels after about 6000 hours exposure to boiling 50% seawater brine even under salt deposits.     

Dehnungsinduzierte Spannungsrißkorrosion an Stählen

Strain-induced stress corrosion cracking of steels Characterization of mechanical influencing factors and their effects on the corrosion of low alloy and high alloy steels. Stress- and strain-induced corrosion of steels. Influence of strain on the occurrence of stress corrosion cracking of low alloy steels in nitrate-, carbonate- and sodium hydroxide solutions and of high alloy steel in magnesium chloride solution. Significance of critical strain-rates.     

Die Bedeutung einer Deckschichtbildung bei der Spannungsrißkorrosion

The importance of the formation of a cover with stress corrosion cracking Many metals and alloys, when exposed to corrosive agents, from covering layers which have a considerable influence on the corrosion process. This applies, in particular, to covering layers with protective effect. The significance of the formation of electro-positive cover films in stress corrosion cracking is illustrated by the example of the formation of cracks in unalloyed steels in nitrate solutions and alkali solutions as well as in austenitic steels in solutions containing chloride and in sulphuric acid. As with pit corrosion, the incomplete or disturbed formation of such layers is the cause of the occurrence of localized corrosion where, under the influence of tensile stresses, the local anodes become the points where the cracks originate. Tension crack corrosion must therefore be regarded as a special problem of the corrosion of metals with incomplete cover films.     

Die wirtschaftliche Verwendung von hitzebeständigen Werkstoffen in Prozeßöfen der Petrochemie

Rational utilisation high temperature materials in petrochemical furnaces While tubular furnaces in petrochemical industries were originally manufactured of cast materials, advantage is being taken now increasingly from wrought materials processed by hot deformation. These materials are characterized by a high degree of homogeneity and enable long tubular material to be obtained without transverse weld seams; in addition, they allow the use of additional specific alloying elements. In the EDC furnaces (ethylene-dichloroethylene) for the manufacture of vinyl chloride use is made of the high corrosion resistance of nickel alloys; these materials possess a remarkable resistance in chlorinated media, in particular at temperatures exceeding 600°C. Another property of importance in such conditions is the resistance to stress corrosion cracking produced by the formation of hydrogen chloride and chlorides. In the enthylene production furnaces operating at temperatures above 800°C steels are used with 32.5% Ni and 21% Cr; they resist carburazation and are creep resistant. These materials are also used for reforming furnaces and in particular for the catalyst tubes where high resistance to creep cracking is indispensable.     

Über den Einfluß verschiedener Chloridionenkonzentrationen auf die Aktiv-Passiv-Korrosionsempfindlichkeit von Stahl in gesättigter Kalziumhydroxidlösung

Influence of various chloride ion concentrations on the active-passive corrosion susceptibility of steel in saturated calcium hydroxide solution The corrosion of steel in concrete is caused mainly by the presence of chlorides. This effect was investigated by measuring potentiodynamic current-voltage-curves and by potentiostatic experiments of stressed (55 or 77% σ_B) plain steel electrodes in saturated Ca(OH)₂-solution with varying chloride concentrations. The results show that down to Cl′-concentrations of 100 mg/l there is a significant statistical probability of corrosion within 48 hours, and that for longer times the threshold concentration may be even lower. The microscopical examination of corroded samples shows that under the experimental conditions only pitting corrosion is the cause of failure and that this pitting is promoted by the applied stress.     

Wasserstoffinduzierte Spannungsrißkorrosion an unverzinkten und verzinkten Baustählen

Hydrogen induced stress corrosion cracking of non galvanized and galvanized construction steels The processes of atmospheric corrosion and corrosion in collected water which may lead to hydrogen induced stress corrosion cracking of high-strength reinforcing steels in casing tubes before injection with concrete are discussed. Hydrogen uptake during corrosion occurs in weakly acid solutions as well as in neutral or alkaline aqueous solutions. The hydrogen uptake by proton discharge in acid solutions decreases with increasing pH of the electrolyte. Hydrogen can also be absorbed in neutral to weakly alkaline solutions if steels are plastically deformed and water reacts with the fresh iron surface. In alkaline solutions, hydrogen uptake is possible if, at the generally passivated steel surface, localized corrosion (pitting or crevice corrosion), local galvanic cells and a sufficient decrease in the pH of the pit electrolyte occurs. In the case of galvanized steels with damaged zinc layers, hydrogen uptake may result from the cathodic polarization of the free steel surface by zinc dissolution. The absorbed hydrogen interacts with the microstructure of the steels and weakens the bonds between the iron atoms. The influence of the microstructure of high-strength steels on the fracture behaviour is discussed on the basic of the so-called decohesion theory.