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.     

Untersuchungen der Spannungsrißkorrosion austenitischer Stähle durch saure Chloridlösungen bei Niedrigen Temperaturen

Investigation of stress corrosion cracking of austenitic steels in acid chloride solutions at low temperatures Tests were carried out on materials 1.4301, 1.4571, 1.4439 and 1.4558 in cold hydrocloric acid (c(Cl^?) = 1.5 mol/L and c(H⁺) = 1 mol/L, a few tests also at 0.1 and 0.01 mol/L). Chronopotentiostatic tests yielded data on active corrosion, passivity and pitting corrosion. CERT tests (10^?6s^?1, a few tests also at 2 · 10^?7s^?1) showed superposition of general corrosion on stress corrosion under free corrosion condition, while the rest potential was relatively negative in the active range. Oxygen purging has only a minor effect. The extent of cracking decreases with decreasing c(H⁺). In the case of cathodic polarisation straining induced surface notches occur which can be attributed to hydrogen induced effects. In the case of anodic polarisation pittings are generated without any crack initiation. Characteristic features of stress corrosion increase with decreasing strain rate. It follows from the results that high acid concentrations are necessary for stress corrosion cracking in the active state to occur. CERT tests cannot be used as an accelerated test for this kind of stress corrosion cracking. Few CERT tests carried out in warm NaCl solution did not show any stress corrosion cracking though these solutions are known to cause stress corrosion cracking after long periods. Furthermore, predamage in the form of pitting does not alter the situation.     

Einflußgrößen der H-induzierten Spannungsrißkorrosion bei niedriglegierten Stählen

Parameters influencing the hydrogen-induced stress corrosion cracking of low alloy steels Wet H₂S corrosion of steel leads to surface blistering, internal cracking at sharp edged inclusions (HIC), and in stressed specimens to transgranular micro cracks transversal to the main stress. The latter grows to SCC. HIC resistant material displays high purity of non-metallic inclusions. The critical stress level for SCC of lien pipe steels with yield strengths up to 600 N/mm² is proportional to the yield strength. In H²S containing media different grades of low alloy and rolled steels show no correlation between life time and yield strength. In general, the susceptibility for H-induced SCC increases with decreasing pH and with cathodic polarisation, whereas the temperature dependence displays a minium of life time at 20°C.     

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.     

Wasserstoff-induzierte Spannungsrißkorrosion von Stählen durch dynamisch-plastische Beanspruchung in Promotor-freien Elektrolytlösungen

Hydrogen induced stress corrosion cracking of steels subjected to dynamic loading involving plastic deformation in promotor free electrolytic solutions Plain carbon steels and low alloy steels suffer internal cracking and a relatively high embrittlement when they are subjected to dynamic loading involving plastic deformation in any type of electrolytic solution where there is a simultaneous cathodic hydrogen evolution. These conditions can be encountered in service in the case of cathodic polarisation and free corrosion in acids if the mechanical stresses lead to plastic deformation, e.g. at notches. There is an upper limit to the potential range in which internal cracking occurs. This limit is independent of the yield strength (300 to 500 N mm^?2) of the materials tested and lies at U_H = ?0.5 V in oxygen free salt waters. It is more negative in oxygen bearing electrolytic solutions or in alkalaine media. Materials containing hard transformation products such as martensite and bainite are more susceptible to cracking in the regions of these hard transformation products. There is only a small decrease in susceptibility with increasing temperature. Ultra high strength, quenched and tempered steels with yield strengths > 1000 N mm^?2 undergo hydrogen induced stress corrosion cracking even when they are subjected to static loading in the elastic region. The critical potential is very negative and shifts to less negative values with increasing strength of the material. Stainless steels with stable austenitic microstructure are resistant to this type of corrosion. However, if the mechanical deformation can lead to the formation of martensite surface cracking and brittle fractures occur. The hydrogen induced damage decreases with increasing temperature.     

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.     

Untersuchungen zur Schwingungsrißkorrosion von Stählen in Meerwasser

Investigations of corrosion fatigue of steel in seawater Corrosion fatigue of two medium strength steels was studied using CT-specimens at 10, 1 and 0.2 Hz. The threshold values of the amplitude of stress intensity factor, ΔK₁, were lowered in synthetic seawater and in hydrogen gas. The advance of the cracks per cycle in general was maximum at the lowest frequency. The maximum was obtained at low amplitudes ΔK₁ in seawater at corrosion potential, at middle amplitudes in seawater with cathodic polarization and at high amplitudes in gaseous hydrogen.     

Ein neuer Verfahrensansatz zur Früherkennung wasserstoffinduzierter Spannungsrißkorrosion

A new approach towards the early recognition of H-induced stress corrosion cracking The investigation show that diffusion of hydrogen into ferritic stedel leads to changes in the micromagnetic measured values. This can be explained by the interaction of the magnetic structure with locally occuring fields of residual stress. In principle an early recognition of H-induced stress corrosion cracking is thus possible.     

Elektrochemische Meßmethoden zur Untersuchung der Spannungs- und Schwingungsrißkorrosion

Electrochemical measuring methods for studying stress-corrosion cracking and corrosion fatigue An apparatus is described for the investigation of local corrosion phenomena with simultaneous static and dynamic loading. The apparatus enables measurements to be made of potential variations in time, interdependance of current density and potential, and current density variation at constant potential. It is shown that stress corrosion cracking of carbon steel in boiling Ca(NO₃)₂, solution and of soft iron in NaOH is a clear local corrosion process. Analogous results are obtained with austenitic stainless steel in an activating medium. Corrosion fatigue of passive austenitic steels can be produced in SO, saturated water or in ferroammonium sulphate solution. Between corrosion fatigue and stress corrosion cracking there is a number of analogies but there are also differences. Stress corrosion cracking strongly depends from material composition but may be produced by almost any medium, while corrosion fatigue is possible in certain media only, but is not related with a particular metal composition.     

Untersuchungen zur H-induzierten Rißkorrosion — Teil 1: Kathodische Wasserstoffabscheidung und Wasserstoffpermeation bei unlegierten Stählen

Investigation into the hydrogen induced cracking corrosion.– Part 1: Cathodic hydrogen evolution and hydrogen permeation of unalloyed steels From stationary and instationary polarization measurements no characteristic data of hydrogen absorption in media with/without promotors can be obtained. Permeation measurements provide that Volmer-Tafel mechanism is valid at low current densities. Diffusion constant and hydrogen concentration can be evaluated from permeation measurements. Due to hydrogen traps at inside defects and by electrochemical surface effects permeation values can deviate. With increasing degree of cold-rolling the diffusion constant decreases, whereas the hydrogen concentration increases leading to a maximum permeation rate at about 15% cold-rolling.     

Untersuchungen des Stofftransports und der chemischen Reaktionen im System Polyäthylen-weich/Schwefelsäure–Zur Frage der Spannungsrißkorrosion in Kunststoffen–

Investigations on mass transport and chemical reactions in the system LDPE/H₂SO₄–The question of stress corrosion cracking in plastic materials– Investigations on mass transport and chemical reactions in the system LDPE/H₂SO₄ reveal different kinds of damage and chemical attack both with bulk specimen containing internal stresses only and with pipes examined in creep tests under pressure. Evaluating light microscopy and electron scanning microscopy pictures it was found that the microstructure specific to the material as well as the macrostructure determined by the production process influence the character of damage. These structural parameters combined with internal or external stresses and a critical medium lead to a kind of failure which for polymers is called “stress corrosion cracking”.     

Autoklaven-Untersuchungen der Spannungsrißkorrosion von Fe-Cr-Ni-Legierungen in NaCl/Co2/H2S-Medien

Autoclave investigation of stress corrosion cracking behaviour of Fe-Cr-Ni alloys in NaCl/CO₂/H₂S-environment In oil and gas production, the corrosion problems increase as the depth of the reservoirs increases. The oil and gas products contain chloride-rich waters and mixtures of H₂S and CO₂ at high pressures and temperatures. Materials that can be used under these conditions are only high strength high alloy steels and nickel base alloys. These materials must be assessed for corrosion resistance under these conditions. The environment contain chloride ions and hydrogen sulphide, which are known to be critical components for SCC. With the aid of autoclave experiments, the fields of corrosion resistance for the materials no. 1.4462, 1.4563 and 2.4618 were determined as a function of temperature and hydrogen sulphide pressure. The base environment was a 5 Molar sodium chloride solution at 20 bar carbon dioxide. While the corrosion resistance of the duplex steel, material no. 1.4462, decreases markedly as the strength of the material and the hydrogen sulphide pressure increase, the two austenitic materials are completely resistant up to 300 °C and hydrogen sulphide pressure of 15 bar. Only at 300 °C and high partial pressures of hydrogen sulphide the material no. 1.4563 did fail, when stressed to stress levels higher than the YS. The crack path was predominantly transgranular with minute fractions of intergranular cracking. The microstructure appears to have no effect. All results indicate that a mixed mechanism of hydrogen- and chloride induced SCC is operting, while a corrosion enhancement due to interaction of both critical components takes place.     

Einflüsse des Phosphors auf die Interkristalline Spannungsrißkorrosion von Kohlenstoffstählen

Influence of phosphorus on the intergranular stress corrosion cracking of carbon steels The effects of phosphorus on the intergranular stress corrosion cracking were studied for steels with 0.15% C and 0.4 or 2% Mn, the phosphorus contents were 0.003, 0.03 and 0.05% P. Constant strain rate tests were conducted at constant potentials in 55% Ca(NO₃)₂ at 75°C, in 5 N NH₄NO₃ at 75°C, and in 33% NaOH at 120°C. The strain rate was 10^?6/s. Different grain boundary concentrations of phosphorus were established by varying the annealing time at 500°C, they were determined by fracturing the samples in UHV and analyzing intergranular fracture faces by Auger-electron spectroscopy. In the nitrate solutions the toughness, i.e. resistance against stress corrosion cracking of the steels is somewhat decreased with increasing P-content within the range of the corrosion potential, that is ?300 mV_H to ?50 mV_H in NH₄NO₃ and ?80 mV_H to ?50 mV_H in Ca(NO₃)₂. It is shown that this effect is caused by phosphorus segregated at the grain boundaries. At potentials above ?50 mV_H the relative toughness is very low for all tested steels, also without stress intergranular attack is observed. In NaOH the steels are most sensitive against stress corrosion cracking at ?700 mV_H, here the phosphorus content or segregation shows no effect. At potentials varied in both directions the toughness increases and a somewhat negative effect of phosphorus becomes visible. In all tested electrolytes the effect of phosphorus is restricted to a small potential range. There is no effect of phosphorus in the range of the minimal toughness, thus the steels are sensitive anyway, with and without phosphorus segregation. Therefore low phosphorus contents in carbon steels cannot guarantee resistance against stress corrosion, most decisive are the potential and the electrolyte.     

Spannungsrißkorrosion an CuZn-Legierungen in sulfathaltigem Leitungswasser

Stress corrosion cracking of CuZn alloys in sulfate containing tap water Failure analyses of water-pipe fittings show inducing of stress corrosion cracking in α-brass by sulfates. In order to clarify this effect corrosion tests in 10% Na₂SO₄-solution and tap water were compared with results already published in literature.     

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.     

Untersuchungen über die transkristalline Spannungsrißkorrosion austenitischer Chrom-Nickel-Stähle in heißen Chloridlösungen

Investigations into the transcrystalline stress corrosion cracking of austenitic chrome-nickel steel in hot chloride solutions To obtain a further clarification of various details in respect of the transcrystalline stress corrosion cracking of austenitic Cr-Ni steels, tensile strength tests were carried out with X 5 CrNi 18 9 in hot, highly concentrated MgCl₂, solutions. Tests without extraneous currents showed that, depending on the test conditions, the relationship between stress and service life can be formally described by an exponential function. Down to stresses of 2 kg/mm², there was no discernible threshold below which stress corrosion cracking does not occur at all. By means of potentiostatic holding tests at different stresses, it was possible to plot a potential/tensile stress/service life diagram. The occurrence of stress corrosion cracking shows a distinct threshold on the side of negative potentials; on the side of positive potentials, however, the occurrence of stress corrosion cracking is increasingly accompanied by pit corrosion. Strain measurements carried out during the tests yielded, in a non-destructive way, a sufficiently exact criterion for the division of the total test period into incubation period and tearing period. The length of the incubation period depends on the tensile stress as well as on the specimen potential. As regards the tearing period it was found that the growth rate of the cracks is formally proportional to the stress at the bottom of the crack. The proportionality constant, in its turn, is a linear function of the test potential. By means of extrapolation, it is possible to deter-mine the minimum potential below which stress corrosion cracking can no longer occur. Electro-chemical measurements showed that, in the tested solutions, it is the hydrogen reduction which represents the vital catholic part-process. The tested material is in the state of passive stability; its rest potential in the non-stressed condition lies at the lower threshold of the pit corrosion range. Tensile stresses, or the deformations caused by them, have the effect of shifting the minimum potential for heterogeneous corrosion into the negative range.     

Untersuchungen zur Rißkorrosion hochfester Baustähle in Meerwasser

Studies of stress corrosion of high-strength structural steels exposed to seawater Seawater is liable to cause stress corrosion cracking. The risk of cracking may increase as the strength of the steel rises. The report describes the results of a number of test series with welded highstrength structural steels. Long-time exposure tests were performed in natural seawater of the North Sea near Helgoland. In constant deflection tests the influence of the maximum hardness in the heat affected zone on the corrosion behaviour and the effect of chromium as an alloying element were investigated. The potential risk attributable to welded joints containing undiscovered cracks was determined by fracture mechanics tests. Proof was given that under static stress conditions, flawless welded joints on high-strength structural steels up to S890Q grade are not susceptible to hydrogen-induced stress corrosion cracking. Maximum hardness of 400 HV in the heat affected zone also do not have a negative effect on steels containing up to 1% chromium. Steels with more than 1.5% chromium are endangered if the maximum hardness exceeds values of 350 HV. A susceptibility to stress corrosion under slow strain and creep conditions can be excluded from the potential of free corrosion up to the potential of cathodic protection with zinc. High-strength tensile structural steels do not differ in this regard. Limitations on the use of high-strength structural steels in the submerged area of offshore structures are shown by the fracture mechanics tests with welded joints. The results of the tests show that under free corrosion conditions in seawater high-strength steels up to S500Q do not react more sensitively in terms of corrosion crack growth than the proven steel grade S355N. The choice of steel grade may therefore be guided by other criteria such as the fracture mechanics or fatigue characteristics and processing behaviour.     

Beitrag zur Spannungsrißkorrosion austenitischer Stähle

Contribution to the problem of stress corrosion cracking of austenitic steels It is shown on the basis of a stress corrosion cracking theories that the incubation phase is the factor determining the useful life of an element because in this phase protective layers are destroyed and crack nuclei are formed. Testing corrosion susceptibility in boiling magnesium chloride solution has the shortcoming that the solution is highly aggressive so that minor changes of the surface condition which may yield a considerable improvement of material behaviour in less aggressive conditions do not come to bear. Another shortcoming of MgC1₂ testing are divergent literature data concerning concentration so that there is also a scatter of boiling temperatures which has a pronounced influence on test results. Likewise an important role can be attributed to foreign ions and silicates from glas in the solution. It is more advantageous to use less concentrated solutions (up to 35%) because they enable clear current density potential curve to be obtained. An important factor concerning the test method is the previous investigation of specimens with a view to a residual stresses due to cold reduction. Under certain conditions the time to failure may be longer under cycling loads.     

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.     

Spannungsrißkorrosion von unlegierten Stählen in flüssigem ammoniak

Stress corrosion cracking of unalloyed steels in liquid ammonia In this work is reported on investigations about stress corrosion cracking of steels in liquid ammonia aimed at ascertaining the parameters and the ranges within which they are effective, finding the mechanism responsible for the damage of spherical pressure vessels and developing protective measures. The investigations are based partly on corrosion tests in which welded Jones samples of various unalloyed and low alloyed steels in different states of heat treatment were immersed in ammonia for long periods in a spherical ammonia storage vessel. For laboratory tests we used an apparatus specially developed to enable round tensile test bars to be tested in liquid ammonia under CERT conditions at the open circuit potential and under electrochemical control. In these tests selectively added impurities were determined quantitatively by gas chromatography. The results of the investigations show that the oxygen content of the ammonia has a decisive influence on the initiation of stress corrosion cracking in unalloyed steels. Even low O₂ concentrations (ßß2 ppm) lead to a risk of stress corrosion cracking. Addition of water in sufficient quantity inhibits the stress corrosion cracking. The water concentration needed to inhibit corrosion differs according to the material concerned (StE 355: > 500 ppm; H I: > 650 ppm; StE 460: > 1000 ppm H₂O). The different stress corrosion behaviour of the investigated steels under residual and operating stresses shows their different sensitivity. So in absence of stress relieve heat treatment only Jones samples of StE 355 remained crack-free. Investigations into the influence of potential on crack formation showed that the damage follows an anodic crack formation mechanism, since it is capable of being reinforced anodically and inhibited cathodically. Similarly the inhibitive effect of water is reduced by anodic polarization. But stress corrosion cracking cannot be induced by anodic polarization alone, i.e. in the absence of O₂. As a practical conclusion it may be said that the presence of oxygen – even in traces – must be avoided. Addition of water inhibits stress corrosion cracking; these additions are therefore recommended wherever they are possible. Containers should be made of StE 355 and they should be stress relieve annealed. Cathodic protection is considered appropriate, but the practicability under operating conditions has to be still investigated.