Spannungsrißkorrosion von hochlegierten Manganstählen in wäßrigen Chloridlösungen

Stress corrosion cracking of high alloy manganese steels in aqueous chlorides In tensile tests made without applied current in aerated solutions a stabilization of the austenitic structure by increasing Mn and N contents yields increased times to failure. The potential-time-to-failure curves determined by potentiostatic tensile tests reveal a compley joint action of constitution and passivation behaviour of the steels. The intercrystalline stress corrosion cracking of the steel X 40 MnCr 19 with chromium carbide precipitations at the grain boundaries can be attributed to an electrochemical differentiation of the chromium-depleted grain boundary region. Steels of this type are characterized by a pronounced sensitivity to intercrystalline stress corrosion cracking the precipitation annealed state, and by a certain sensitivity to transcrystalline corrosion cracking after solution annealing. Low carbon Mn steels containing up to 4 % Cr are susceptible to transcrystalline stress corrosion cracking irrespective of the heat treatment. As to the temperature dependence of times-to-failure, constitution and layer formation have different effects. Increasing the Cr content to 8 % gives rise to a transition from stress corrosion cracking to pitting type corrosion. In terms of electron optics, an increased chromium content gives rise to a changed dislocation pattern, so that there may be an effect of the type of gliding processes on stress corrosion, The increased stress corrosion resistance of MnCr steels containing at least 8% Cr may be due to the lower height Of the gliding step and to an increasing tendency to repassivation of damaged surface layers.     

Transkristalline Spannungsrißkorrosion an austenitischen Manganstählen in chlorionenhaltigen Angriffsmedien

Transcrystalline stress corrosion cracking of austenitic manganese steels in corrodent containing chloride ions Experimental investigation with steels (0,5% C, 20% Mn, 0.2—2.7% Cr) in the shape of U-bent specimens in seawater (RT and boiling) under loads between 30 and 45 kp/mm². The electrochemical behaviour of chromium-free types (i.e. below 0.5% Cr) at RT is identical to that of ferritic shipbuilding steels (equal break-through potential, then uniform attack). Addition of N, Ni, Co and Cu have little influence on the potential behavior. In boiling sea-water, however, transcrystalline corrosion takes place, accelerated by high tensional loads and anodic polarization, and slowed down by cathodic polarization. Additions, in particular of Co and Ni, reduce specimen life, perhaps by reducing yield strength. At higher Cr contents (1.7–2.8%) susceptibility to intercrystalline corrosion begins to supersede susceptibility to transcrystalline cracking. On the basis of ideas concerning the active-passive behavior under the influence of gliding phenomena the author shows analogies to the stress corrosion behaviour of austenitic stainless 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.     

Elektrochemisches Verhalten und Deckschichtbildung hochlegierter Manganstähle

Electrochemical behaviour and scaling of high alloy manganese steels Passivating surface layers are considered to be one of the indispensable requirements for stress corrosion cracking of metallic materials. It is shown by potentiostatic and potentiokinetic current density-potential curves that the steel X 40 MnCrN 19 in neutral aqueous chloride solutions has a passive potential region. The passivation behaviour Of precipitation hardened samples is in agreement with the chromium depletion theory. The effect of alloying on the passivation behaviour of low carbon Mn steels is studied in 3 % NaCl solution at 20 and 100 °C Increasing proportions ε-martensite reduce the passivation of susceptibility. Increasing the Mn content has the same effect. The vital factor concerning passivation behaviour, however, is chromium content. Increasing the temperature of the corrodent results in an increased tendency to form scales of steels containing less than 8 % Cr. Long-term corrosion tests have shown, that increasing the Cr content produces a continuous transition from general localized and even pitting Corrosion. Tests made without applied current in aerated solutions have shown, that the variation in time of corrosion potentials depends from the tendency to be passivated of the materials and from the oxygen content of the solutions. In oxygen containing solutions passivable steels exhibit a pronounced corrosion in the pitting region, because with such alloys anodic dissolution current densities equal to those of the limiting diffusion current of oxygen reduction are obtained only at potentials above the pitting potential.     

Comparative study of stress corrosion cracking susceptibility of Fe18Cr10Mn- and Fe18Cr10Mn1Ni-based high nitrogen stainless steels

The stress corrosion cracking (SCC) behavior of Fe18Cr10Mn1Ni(0.3–0.8)N alloys was investigated in aqueous NaCl environment by using slow strain rate test method, and the results were compared to those of Ni-free counterparts. The addition of N tended to improve the SCC resistance of Fe18Cr10Mn- and Fe18Cr10Mn1Ni-based alloys. The alloying Ni magnified the beneficial effect of N on the SCC susceptibility and, eventually, the Fe18Cr10Mn0.8N alloy was immune to SCC in 2 M NaCl solution at 50 °C. The SCC behavior of the present alloys was found to be closely related to the repassivation tendency and the resistance to pitting corrosion.     

Gegen die Spannungsrißkorrosion beständige nichtrostende Manganchromstähle

Stress-corrosion resistant stainless manganese chromium steels The following conclusions may be drawn from the results of investigations into the stress corrosion cracking of austenitic and austeno-ferritic MnCr steels (19–22Mn, 13–18Cr, additions of Mo, V, Nb, Ti, N, B): Addition of nitrogen gives rise to a decrease of stress corrosion cracking resistance in magnesium chloride, sodium chloride with potassium dichromate and water at high temperatures. The same applies to the influence if nickel on corrosion in magnesium chloride and water, and for molybdenum in magnesium and sodium chlorides. From among laboratory melts the type 05 Mn 19Cr 13 had the highest resistance, followed by its modifications with additions of boron, vanadium, molybdenum, titanium, niobium and nitrogen. From among the semi-technical melts the nitrogen containing steels turned out to be least resistant, too. During further investigations the chromium level of 13% turned out to be insufficient to prevent pitting in sodium chloride solutions including seawater.     

Stress corrosion cracking of bainite 0.3C‐1Cr‐1Mn‐1Si‐1Ni type steel in acid rain simulated solution

The effect of carbon content, heat treatment and surface treatment of high strength aircraft 0.3C‐1Cr‐1Mn‐1Si‐1Ni type steel on susceptibility to pitting corrosion, on hydrogen transport and on hydrogen embrittlement has been studied in acid rain simulated solution. Under open circuit conditions, susceptibility to stress corrosion cracking is associated with susceptibility to pitting corrosion and decreases with decreasing carbon content and at application of shot peening. Susceptibility to hydrogen embrittlement governed by hydrogen trapping is promoted by carbon segregation at boundaries of bainite laths and parent austenite grains. Decrease in the carbon content caused the decrease in hydrogen trapping resulting in increasing resistance to hydrogen embrittlement. Shot peening increased the trapping efficiency within the deformed layer, but the presence of the shot peened layer decreased the hydrogen flux entering the core and thus decreased the susceptibility of the core to hydrogen embrittlement. The shot peened low carbon steel, exhibiting mechanical properties required by the appropriate Standard and high resistance to stress corrosion cracking may be considered to replace the standard 0.3C‐1Cr‐1Mn‐1Si‐1Ni steel for the aircraft parts.     

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.     

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.     

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.     

Beitrag zum Spannungsrißkorrosions-Verhalten der AlZnMg-Legierungen

The stress corrosion cracking behaviour of aluminium-zinc-magnesium alloys The stress corrosion cracking of AlZnMg3 is a two-phase process where the preparation period may also take place without tensile stress. With decreasing pH, the service life is reduced, the reduction being confined to the preparation period. The same applies to anodic connection associated with heavy pitting corrosion where the fracture consists of stress corrosion cracking zones and transcrystalline force fracture. Even with cathodic connection, stress corrosion cracking is encountered. Since, in this case, the current density does not decrease prior to fracture, it must be assumed that the propagation of the crack may have nothing to do with the electro-chemical dissolution at the tip of the crack. Even minor quantities of water — e.g. in carbon tetrachloride — are sufficient to cause stress corrosion cracking, probably through adsorption of atomic hydrogen which has the effect of reducing the surface energy.     

Neue Erkenntnisse auf dem Gebiet der ferritischen rostfreien Stähle

Recent developments in ferritic stainless steels The pitting resistance of ferritic stainless steels in HCl is visibly improved by Mo, in particular in the case of vacuum-melted material. In this context the ratio Cr:Mo = 25:2 is superior ta Cr:Mo = 17:3; addition of Mo prevents, beyond that, crevice corrosion. Ti increases resistance in the Strauß test but not in the Huey test, while Nb turns out to have a positive effect in either test. Steels containing Cr: Mo = 17:l are certainly still susceptible to pitting, but no longer to stress corrosion cracking in boiling MgCl₂, solution; stress corrosion cracking is not observed in 55% boiling Ca(NO₃)₂, and 25% boiling NaOH, but after annealing at 980 °C intercrystalline corrosion takes place. The test duration required for establishing cracking susceptibility is considerably shorter with ferritic than with austenitic steels (100 and 1000 to 2000 hours respectively).     

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.     

Effect of W on stress corrosion cracking susceptibility of newly developed Ni-saving duplex stainless steels

Effect of W on stress corrosion cracking behavior (SCC) of Ni-saving duplex stainless steels (Fe18Cr6Mn3Mo0.4N (2.13, 5.27)W, in wt%) was investigated in 4 M NaCl solution using a slow strain rate test method. The change in the W content from 2.13 to 5.27 wt% marginally affected the tensile properties of the investigated DSSs. Alloying W clearly improved the pitting initiation resistance and repassivation tendency of the investigated alloys, but the SCC susceptibility was not remarkably decreased by addition of W. The slight enhancement in the SCC resistance of the alloy containing 5.27 wt% W was revealed to be correlated with the accelerated galvanic corrosion between the ferrite and austenite phases as a result of the W partitioning preferentially into the ferrite phase which could encourage the propagation of pitting.     

Pitting and Stress Corrosions of Aluminum Alloys; Correlation between Them

It is found that pitting corrosion evolves solely in solutions bearing monovalent anions. This fact is associated with the specific features of one-step dissolution. The pitting corrosion of aluminum typically evolves over its (100) crystallographic planes. Such crystallographic type of corrosion gives way to other types with an increase in the pitting initiation potential and the content of alloying elements. The obtained curves for the pitting initiation potential as a function of the solution strength show regions of passivity, a linear dependence, and the transition to the value of the pitting initiation potential measured in pure water. Common features in pitting and stress corrosions evolving from passive state are revealed. However, for stress corrosion, the factors which accelerate depassivation are dislocation piles-up at grain boundaries and hydrogen, which arises by corrosion reactions and weakens intergranular bonds. Under conditions when pitting corrosion shows up as local tunnels directed toward phase inclusions, transgranular corrosion cracking is observed; whereas, under conditions when pitting corrosion evolves on grain boundaries, intergranular cracking is observed.     

Austenitische gegen Spannungsrißkorrosion in konzentrierten Chloridlösungen beständige Manganchromstähle

Austenitic manganese chromium steels resistant to stress corrosion cracking in concentrated chloride solutions The testing of stress corrosion susceptibility in MgCl₂ and CaCl₂ shows that the classical Austenitic CrNi and CrMnNiN steels are not sufficiently resistant to this type of corrosion. On the other hand MnCr steels exhibit good resistance even in hot solutions. The resistance, however, is considerably deteriorated by addition of Ni even in small quantities; with Ni contents up to 0.5%, however, the time to failure is still considerable. This fact is important since such nickel contents must be counted with because of production conditions in metallurgy. On the basis of experiments including investigation into mechanical properties, microstructure, metallic phases present and stress corrosion resistance (under constant tensile load) in MgCl₂ solution (35%, 115–120 °C) the authors selected out of 62 types of steels tested four experimental austenitic MnCr steels meeting the following complex of requirements: high resistance in MgCl₂ solution, sufficiently high resistance to intercrystalline corrosion, certain resistance to pitting and practically monophasic (austenitic) structure.     

Der Einfluß der Strahlenschädigung durch schnelle Neutronen auf die Spannungsrißkorrosion (SKR) von Eisen-Chrom-Nickel-Stählen

Influence of radioactive deterioration by fast neutrons on stress corrosion cracking of chromium-nickel steels Neutron irradiation has a bearing on the mechanical properties of the pure and industrial steels investigated. The creep limit is increased while the range of uniform elongation is reduced. Under an integral fast neutron flux of about 2.5.10¹⁹ n/cm² there is a defined yield strength increasing the stress corrosion susceptibility and there are indications of intercrystalline cracking in some cases. The industrial alloy has exclusively transcrystalline cracks een after neutron irradiation; the crack density is higher than in the pure alloy.     

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.     

Warmverformbarkeit und Korrosionswiderstand von Stählen der Type 1Cr18Ni12Mo2Ti

Hot workability and corrosion resistance of 1Cr18Ni12Mo2Ti type steels Increasing the Mo content to the level required for corrosion resistance has a negative effect on the hot workability of austenitic steels. Addition of 2% Mo requires an increase in the proportion of austenite-forming elements (in particular Ni), while the Cr content must be reduced to the lowest admissible value for corrosion resistance. As has been shown by experimental work in a redox system and in a pulp digester 16–20% Cr can be considered sufficient (corrosion loss 0,01 mm/y). In bleach liquors of the textile industry, however, there is the danger of pitting corrosion. Heat treatment and hot working have a very pronounced bearing on corrosion resistance.     

Untersuchungen zum Einfluss des Stickstoffs auf das Lochkorrosionsverhalten hochlegierter austenitischer Cr‐Ni‐Mo‐Stähle (Teil II)

Investigation of the influence of nitrogen on the pitting corrosion of high alloyed austenitic Cr‐Ni‐Mo‐steels (Part II) Austenitic stainless steel (18% Cr, 12% Ni, Mo gradation between 0,06 to 3,6%) had been solution nitrided. By step‐by‐step removing, the samples could be prepared with various surface contents of nitrogen from 0.04 to 0.42%. In two test series the influence of nitrogen had been determined. The susceptibility against pitting corrosion of these samples had been tested by the chronopotentiostatical method. For the investigated steel composition and the used corrosion system there is no infuence of molybdenum on the effectiveness of nitrogen. The effectiveness of nitrogen can be described by the factor 25 in the PRE. By the investigation of the surfaces with the XPS analysis, it could be shown that the passivation and the pit nucleation is influenced by nitrogen. In these ranges NO_x, NH_x, and NH_z‐spectra have been detected. Bound Mo was found in steels containing molybdenum. It is assumed that the repassivation mechanisms of N and Mo work independently of each other. With the results efforts are supported to improve the pitting corrosion resistance also at molybdenum poor steels by surface nitriding or nitrogen alloying. The achieved results justify the assumption that the observed positive effect of the nitrogen may be extented to even higher nitrogen contents. A prerequisite for this is avoiding secondary phases in the matrix. The adverse influence of small particles is known well.