Zur Spannungsrißkorrosion austenitischer Chrom-Nickel-Stähle

Contribution to the stress corrosion cracking of austenitic chromium nickel steels The authors present a summarizing treatment of the transcrystalline stress corrosion cracking of chromium nickel steels in chloride solutions. The factors having a bearing on the susceptibility to this type of corrosion during operation, temperature and p_H of the medium. Residual stresses may be due to heat-input (during welding) or to grinding which, in addition, may give rise to notch effects. The medium generally used to test stress corrosion susceptibility (MgCl₂ solution) acts according to different mecanisms, depending on its concentration which, in term, has an influence on p_H and the boiling temperature. Quite generally it may be said that stress corrosion cracking always presupposes the existence of a lower of stresses, the amount of these stresses depending from the structural factors and from the surface condition.     

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.     

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.     

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.     

Influence of heat flux on localised corrosion of stainless steel under boiling conditions

The article deals with the influence of heat flux on localised forms of corrosion of stainless steels under the conditions of phase transition on a metal surface. The observations focused on the influence of the initiation and propagation of pitting corrosion and stress corrosion cracking. Pitting corrosion was tested in the environment of 0.6% FeCl₃ + 0.3% EDTA + 0.1% HCl, the susceptibility to stress corrosion cracking was verified in 35% MgCl₂. The tests proved a negative influence of heat flux on the initiation and propagation of non‐uniform corrosion.     

Untersuchungen über den Ablauf der Spannungsrißkorrosion an austenitischen Chrom-Nickel-Stählen in Magnesiumchloridlösung Mitteilung aus dem Forschungsinstitut der Mannesmann AG

Investigation into the cinetics of stress corrosion cracking of austenitic chromium-nickel steels in magnesium chloride solution Experimental investigation with the steels (German designation) W.-Nr. 4301 (X5CrNi18 9), 4401 (X5CrNiMo18 10), 4449 (X5CrNiMo17 13), 4541 (X10CrNiTi18 9), 4550 (X10CrNiNb18 9) and 4580 (X10CrNiMoNb18 10) in 42% MgCl₂ solution at 144 and 130°C. The known distribution of stress corrosion cracking between an incubation and a crack propagation period has been confirmed (the first taking about 10% of the whole specimen life). The cracking rate is proportional to the potential at the base of the crack which, in turn, depends from the surface potential. Nb and Ti have no bearing on the cracking rate. Addition of Mo displaces to more noble potentials the potential limit below which there is no stress corrosion cracking and reduces cracking rates when differences between the potential limit and a given potential are equal. The influence of temperature is restricted to the crack propagation period. A co-action of increasing Ni contents in connection with the Mo addition cannot be excluded.     

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).     

Kommen oberflächliche Deckschichten und Materialversprödungen als Ursachen für das Auftreten von Spannungsrißkorrosion in Betracht

Could surface layers and material embrittlement be among the causes of stress corrosion cracking? Neither the hypothesis claiming rupture of the surface layer nor the embrittlement theory yield a sufficiently consistent explanation of the susceptibility to stress corrosion cracking, since the latter can occurr even in the absence of such layers (e.g. brass in copper nitrate and copper tetrammine salts). An indispensable condition for stress corrosion cracking to occurr is the possibility that cathodically active zones are formed by corrosion products at crevices and cracks; such active zones amy be formed e.g. by Cu₂O or by dissolved and redeposited noble metals. This phenomenon would also account for the specific action of certain agens: local formation or deposition of corrosion products which are not dissolved again (as e.g. noble metals may be dissolved with complex formation). The susceptibility to stress corrosion cracking is increased by mixed crystal formation, because this process enhances reactivity at grain boundaries, inner defects and creeping zones. In the case of alloys containing no noble metals, however, it is difficult, to predict susceptibility to stress corrosion cracking.     

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.     

Allgemeine Beständigkeit von Schweißnähten unter Berücksichtigung der Spannungs- und Schwingungsrißkorrosion

General resistance of weld seams with a view to stress corrosion cracking and corrosion fatigue The corrosion of welds is due to thermal effects during welding which give rise to structural changes and, frequently, compositional changes in the transition zone. The welded material is rapidly cooled and may thus be heterogeneous and may present residual stresses resulting in increased susceptibility to selective and stress corrosion. The manganese content is of high importance in low alloy steels, as well as residual martensite or austenite embedded in a ferrite matrix. Low ferrite contents are generally beneficial because they counteract high temperature cracking; however, ferrite contents should be hept below 10% in order to prevent the formation of a continuous network giving rise to selective corrosion. Corrosion susceptibility may also be produced by carbide or carbonitride precipitation in austenitic and ferritic steels and nickel base alloys. Weld zones in aluminium alloys are attacked in rare cases (e.g. by HNO₃) and the susceptibility of Ta, Zr and Ti depends from the properties of the protective atmospheres.     

Bildung und Einfluß von α'-Martensit auf die Spannungsrißkorrosion von Chrom-Nickel-Stählen

Formation of martensite and influence of the latter on stress corrosion cracking of chromium nickel steels Martensite formation depends from alloy composition, deformation and undercooling. By contrast to an industrial alloy corresponding to (DIN) X 10 CrNiTi189 a pure Fe 19 Cr 10 Ni alloy becomes partially martensitic upon deformation and quenching to ?190 °C. Undercooling and subsequent deformation give rise to the addition of deformation and quenching martensite. The formation of α′-martensite is largely suppressed by addition of 2% Mo. Since corrosion resistance in aqueous MgCl₂ solutions increases with the α′-martensite contents, cracks grow predominantly along the martensite plates. Addition of molybdenum reduces stress corrosion resistance because of the suppression of martensite formation. In this type of alloy cracking is intercrystalline nature.     

Spannungsrißkorrosion an austenitischen Chrom-Nickel-Stählen bei aktiver Korrosion in chloridhaltigen Elektrolyten

Stress corrosion cracking on austenitic chromium nickel steels during active corrosion in chloride containing electrolytes Austenitic stainless steels may suffer from stress corrosion cracking (SCC) in chloride containing environments not only in the passive state of the materials and at elevated temperatures, but also under the conditions of active corrosion at ambient temperatures. This type of active SCC was investigated for 18/8 CrNi-steel in sulphuric acid-sodium chloride solutions by potentiostatically controlled experiments with stressed specimens. Critical potential ranges of susceptibility to SCC were evaluated. Comparison with potentiodynamically measured current-potential-curves shows that these critical potential ranges of SCC are identical with the potential ranges of active dissolution of unstressed specimens. Also in boiling 42% MgCl₂, active SCC is observed in a narrow potential range more negative than the critical limiting potential of passive SCC. This range becomes more narrow by addition of Mo, but is extended by Ni. Incoloy 800 with about 32 wt.-% Ni shows exclusively active behaviour in the potential range interesting for testing the material in 42% MgCl₂. Active SCC is at last caused by the formation of an incomplete protecting layer of adsorbed chloride ions which allows local differences of anodic dissolution, hereby in the presence of mechanical stresses making SCC failures possible. At potentials sufficiently negative, the thickness of the adsorption layer can increase and a macroscopic salt-layer is formed. The chemical composition of such a layer, formed in magnesium chloride, was investigated. The layer consists nearly exclusively of nickel chloride.     

Beitrag zur Spannungsrißkorrosion austenitischer Cr-Ni-Stähle in MgCl2-Lösungen verschiedener Konzentration bei definierten elektrochemischen Außenbedingungen

Contribution to the stress corrosion cracking of austenitic CrNi steels in MgCl₂, solution of various concentrations at defined electrochemical conditions SCC tests in aqueous MgCl₂ solutions of various concentrations, temperatures and aeration conditions have been used to determine the influence of sample pre-treatment, oxygen supply at the materials surface, type of loading and the effect of various metallic coatings on the useful life of austenitic CrNi steels. In addition, feasible ways have been studied for achieving a better reproducibility of results, e.g. by lowering MgCl₂ concentration or by introducing defined preset potentials or impressed currents. The behaviour of the individual steels in various MgCl₂, solutions has been studied on the basis of current density-potential curves.     

Kritische Grenzbeanspruchungen zur Erzeugung einer Spannungsrißkorrosion von Werkstoff Nr. 4301 in siedender 42%iger MgCl2-Lösung

Critical limit stresses for generating stress corrosion cracking of material No. 4301 in boiling 42% MgCl₂ solution By means of potentiostatic reversal tests on a specimen tensile bar of material No. 4301 developing stress corrosion cracking in boiling 42% MgCl₂ solution, the attempt was made to determine the critical limit potential. The strain rate was measured to serve as a reference basis for the corrosion rate. The results show the existence of a limit potential dependent on the tensile stress as well as the existence of a critical tensile stress dependent on the potential. A re-examination of earlier test results showed no incompatibilities with these findings.     

Correlation between the degree of sensitization and stress corrosion cracking susceptibility of type 304H stainless steel

Austenitic stainless steel 304H is extensively used in the super heater tubes of power boiler due to its superior mechanical properties at elevated temperatures. However, its relatively high carbon content increases the susceptibility to sensitization and subsequent stress corrosion cracking.This work is concerned with investigation of the sensitization and stress corrosion cracking (SCC) of austenitic stainless steel grade 304H. Electrochemical potentiokinetic reactivation (EPR) test was used to evaluate the degrees of sensitization (DOS) of the studied alloy at various temperatures and periods of time. DOS increased with increasing sensitization time and temperature. This was confirmed by microstructure examination after EPR test. Boiling magnesium chloride (MgCl₂) test was used to evaluate the susceptibility of 404H stainless steel to stress corrosion cracking. DOS and test stress level had negative effects on time to failure in boiling MgCl₂. The correlation between DOS and SCC was also discussed.     

On the resistance of duplex steel to stress corrosion cracking

The paper gives the results of tests carried out into the stress corrosion cracking in duplex stainless steel (Type 02Cr22Ni5Mo3N, W.Nr. 1.4462); this grade is characterized by high resistance to intergranular corrosion, while resistance to stress corrosion cracking may be impaired by temperature and mode of loading. This behaviour requires special attention. The tests included testing in 35% MgCl₂ solution under constant load at 120°C, the drop evaporation test using a 0.1 mol · 1^? NaCl solution and the slow strain rate test in 35% MgCl₂ solution at 120°C.     

Notched tensile tests of cold-rolled 304L stainless steel in 40 wt.% 80 °C MgCl2 solution

The effects of cold-rolling (20% thickness reduction) and sensitization treatment (600 °C/10 h) on the microstructure, tensile properties and susceptibility to stress corrosion cracking of 304 stainless steel in 80 °C MgCl₂ (40 wt.%) solution were investigated. The increase in hydrogen traps, which retarded hydrogen diffusion to the strained region, accounted for the low loss in notched tensile strength (NTS) of such a cold-rolled specimen, as compared to the solution-treated specimen in the corrosive environment. By contrast, the high NTS loss of sensitized specimens in MgCl₂ solution was attributed mainly to the formation of stress-induced martensite near grain boundary regions.     

Vergleichende Betrachtungen zur Spannungs- und Schwingungsrißkorrosion

A comparative study of stress corrosion cracking and corrosion fatigue On the basis of experiments (without external current, potentiostatic, anodic polarisation) in MgCl₂, with CrNi steel 18/9 a mechanism is postulated for stress corrosion cracking; according to this theory the effect Of a primary movement of dislocations is increased by more difficult repassivation and a notch effect due to increasing surface roughness. The same applies to corrosion fatigue, but effects are more localized here.     

Einflüsse bei der Prüfung der Spannungsrißkorrosion

Influence factors in the testing of stress corrosion cracking The results of the tests of stress corrosion cracking of austenitic stainless steels in MgCl₂ solution are significantly dependent on the type and method of the test. The testing method used in the Research Institute of the Schoeller-Bleckmann Steelworks Ltd. is discussed, and attention is drawn to the influence of a number of test conditions such as the preparation of the MgCl₂ solution, the preheating of the test vessel with water, the surface condition of the specimen, the diameter of the specimen, the covering of its shaft, the time at which the load is applied, and the advance potential applied. The correlation of the service life with stress, concentration and temperature is used as an example in order to show that the test method is apt to affect not only we service life but also the extent and, type of these correlations. It is therefore only in conjunction with the exact test conditions that the test results can be regarded as fully significant.     

Sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure

An investigation has been undertaken to establish the effect of surface preparation method on the susceptibility of a 304 stainless steel to stress corrosion cracking under simulated atmospheric corrosion conditions. MgCl₂ was deposited onto four-point bend specimens, which were then placed in a chamber with a relative humidity of 45% and temperature of 60 °C. These test conditions were designed to reflect external exposure of stainless steel components in industrial plant, including nuclear reactor components, situated in a coastal region, but with the severity of the exposure conditions enhanced to allow discrimination of the effect of surface preparation in a short timescale (up to 1500 h). Four surface preparation methods were evaluated: transverse grinding, longitudinal grinding, transverse dressing using an abrasive flap wheel, and transverse milling. For each case, surface topography, surface defect mapping, near-surface microhardness mapping, residual stress and electron back-scattered diffraction measurements were undertaken. Stress corrosion cracks were observed for the ground and milled specimens but not for the dressed specimens, with cracks apparently originating at corrosion pits. The density of cracks increased in the order: transverse ground, milled and longitudinal ground, with the cracks notably much smaller in length for the transverse ground condition. The propensity for cracking could be linked to the high residual stress and apparent nanocrystalline microstructure at the surface. There was a greater propensity for pitting to initiate at local defect sites on the surface (laps, deeper grooves). However, the tendency was not overwhelming, suggesting that other factors such as more general roughness or the distribution of MnS inclusions had an influence, perhaps reflecting the severity of the environment.