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1.
Effect of potential on corrosion of Mo-free and Mo-bearing steels in solutions of calcium nitrate and sodium hydroxide The effect of potential on the corrosion behaviour of three low alloy steels with different carbon and molybdenum contents (0.08 C, 0.01 Mo; 0.08 C, 0.98 Mo; 0.18 C, 1.10 Mo) was investigated in boiling 60 wt.% Ca(NO3)2 solution (DIN 50 915) and in boiling concentrated NaOH solutions (20 and 35 wt.% NaOH) by potentiodynamic and chronopotentiostatic polarization measurements (i/E curves) and chronopotentiostatic mass loss measurements (corrosion rate v vs. potential E curves). In Ca(NO3)2 solution, i/E measurements give no information about the effect of potential on the anodic dissolution. For the materials investigated, v/E measurements indicate the existence of potential ranges with pronounced differences of the corrosion response. It can be differentiated between active, passive, and transpassive ranges, and also a potential range of secondary passivity was established. Transpassivity and secondary passivity are markedly pronounced with the molybdenum bearing steels but not with the steel free from molybdenum. There are no hints to the occurrence of intergranular attack in the specimens which are free from of internal and external mechanical stresses, whereas such hints could be derived from the shape of the i/E curves. Nevertheless, under mechanical stresses (constant load, CERT conditions) the materials are susceptible to intergranular SCC. The conception that intergranular SCC of low alloy steels in Ca(NO3)2 solution is connected with a break-through potential of grain boundary corrosion and hence is to be interpreted as an intergranular attack which, under mechanical stresses, runs in a modified form as SCC with intergranular crack path, cannot be maintained in such general terms. A pronounced active/passive behaviour is observed in NaOH. The effect of potential on anodic metal dissolution which is derived from v/E curve is also established by i/E measurements. For molybdenum bearing steels, the active potential range is somewhat extended to more positive potentials. Manifestations of localized attack, e.g., intergranular corrosion, do not occur. From the investigations, no hints to the cause of the deterioration of the resistance to intergranular SCC in caustic solutions by molybdenum can be derived.  相似文献   

2.
The influence of cyclic loading on stress corrosion cracking of Mo-free and Mo-bearing steels in solutions of calcium nitrate and sodium hydroxide The resistance of five unalloyed or low-alloy steels differing essentially in their Mo-contents, against SCC was investigated by electrochemically controlled SCC experiments in boiling 60 wt.-% Ca(NO3)2 solution (EH = 0.15 V) and 35 wt.-% NaOH (EH = -0.75 V). For characterizing the SCC response, threshold stresses OG (criterion: crack depths 1 = 0) were evaluated with mechanical loading by alternating stresses (saw tooth characteristic, strain rates ε = 3.4. 10?6 or 8.7. 10?7s?1, load cycle frequency 5. 10?3 or 3. 10?3Hz). If the threshold stresses obtained by constant load SCC experiments are near the YS or in the plastic range (Ca(NO3)2 solution), the threshold stresses are lowered considerably by cyclic loading. If, however, the threshold stresses obtained under constant load conditions are in the elastic range (NaOH), there is only little influence of cyclic loading on the oG values.  相似文献   

3.
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.  相似文献   

4.
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.  相似文献   

5.
Investigation into the intercrystalline stress corrosion cracking of unalloyed steels in sodium carbonate solutions The steel St 36 is characterized by a very pronounced susceptibility to intercrystalline corrision, even under constant load. This sensitivity is limited to a potential zone of about 100 mV situated at the beginning of the passivity range (between ?600 and ?500 (SCE)). While a plastic deformation is not necessary for the initiation and propagation of cracks the propagation is accelerated at high loads. There is, however, no critical elongation rate which would give rise to preferential stress corrosion cracking. It is not possible to maintain the hypothesis according to which the stress corrosion cracking susceptibility would decrease at sufficiently low elongation rates. On the base of the results obtained one may assume the existence of disturbed zones in the oxide layer and the nucleation of cracks at such sites (possibility of “pre-existing-paths” according to parkins). The crack propagation would then be a function of the ratio of rate of advancement of the crack tip (which is assumed to be active) and the rate of repassivation of the crack walls, this repassivation being due to the formation of magnetite.  相似文献   

6.
Effect of molybdenum on the stress corrosion cracking behaviour of low alloy steels in nitrate solutions under slow strain rate testing conditions CERT tests ($ \mathop \varepsilon \limits^. = 10^{ - 6} {\rm s}^{ - {\rm 1}} $) were carried out in NaNO3 solutions (0.1… 5 mol/l and 25… 90ßC) to determine the critical parameters for stress corrosion cracking. The steels tested were 15 Mo 3, LStE 36 and two laboratory heats with 0.08% C and molybdenum concentrations of 0 and 1%. All cracked specimens were examined metallographically. Different kinds of corrosion attack with and without intergranular features were observed. The critical potential range for intergranular attack is defined by two critical potentials. The negative critical potential is about UH = ?0.2 V. It is hardly affected by the test parameters and the material. All free corrosion potentials lie in this range. All freely corroding specimens failed by intergranular cracking in 5 M NaNO3 at 90°C. Only the coarse grained material without Mo showed intergranular cracks at even lower concentrations of NaNO3. The positive critical potential varied widely over 0 to 0.5 V, depending on the test parameters and the material. Furthermore, in all cases a second critical potential range of intergranular attack was found at about UH = 0.8 V. The potential range of UH = 0.5 to 0.8 V for resistance against intergranular attack disappears with increasing temperature, as with the test conditions according to DIN 50 915 in boiling Ca(NO3)2 solutions. Furthermore, this was observed for the heat without Mo at already 90°C. With respect to environmental parameters, the effect of NaNO3 concentration is very small but the effect of temperature is markedly high. The materials can be better characterized by critical temperatures rather than by critical potential ranges. Unified ranking of the materials with respect to their resistance to intergranular attack is not possible because of its dependence on the potential. However, on the basis of the extent of the domains in which the materials are resistant, it is possible to differentiate among the materials. Their resistance to intergranular attack increases in the following order: Heat without Mo → LStE 36 → 15 Mo 3 → heat with 1% Mo. The effect of Mo is explained in terms of its association with high proportions of bainite in the microstructure.  相似文献   

7.
Passivation behaviour and stress corrosion cracking of iron-maganese-chromium alloys in sodium chloride solution Electrochemical experiments with MnCr steels (20–28% Mn, up to 12% Cr) in 3% NaCl solution. High Mn contents reduce the passivation tendency, while increasing Cr contents broaden the range of passivity. The formation of surface layers is due primarily to a direct reaction with the solution (good adhesion, high protective value) and, secondarily, to precipitation from the solution (porosity, low protective value). The tendency to form secondary layers increases as the Cr content is reduced. In oxygen containing solution there is a pronounced corrosion in the pitting range. At low Cr contents, stress corrosion cracking is mostly transcrystalline, at higher Cr contents (8–12%) it is intercrystalline, in particular when Cr carbide precipitations are present at the grain boundaries. In the range of transcrystalline corrosion the susceptibility to selective corrosion extends beyond the pitting potential. At higher Cr contents there may be pitting without any indication of stress corrosion cracking.  相似文献   

8.
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.  相似文献   

9.
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 pH 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 pH 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.  相似文献   

10.
Present state of knowledge concerning the stress corrosion cracking of unalloyed and slightly alloyed steel The author presents a synopsis of the behaviour of steels in media which give rise to stress corrosion cracking: calcium nitrate, alkalis, hydrogen sulphide + ammonia + HCN, as well as systems containing hydrogen sulphide + ammonia + carbon dioxide + HCN. The tests showed that temperature, concentration, pH and the composition of the alloy have an influence. In particular, an increasing C-content has a favourable effect, whilst nitrogen contents are unfavourable throughout. It is especially in alkalis that certain admixtures (potassium permanganate or some metal oxides) are apt to have a stimulating effect whilst others (nitrates, sulphates, phosphates, silicates) act as inhibitors. In the case of the multicomponent systems referred to above, stress corrosion cracking appears to be confined to certain quantity ratios.  相似文献   

11.
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.  相似文献   

12.
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.  相似文献   

13.
Corrosion performance of duplex steels in caustic solutions The corrosion performance of stainless ferritic-austenitic steels (DIN 1.4362, DIN 1.4462, Uranus 50 and Ferralium 255) was investigated in boiling sodium hydroxide solutions. It was found that these steels are corrosion resistant in NaOH solutions up to 30 mass-% with corrosion rates lower than 0.01 mm/a. The mass loss rate increases with increasing NaOH concentration, reaching a maximum in 60 mass-% NaOH solution. Small flow rates (e.g. 0.08 m/s; rotating disc) increase the corrosion rate by a factor of 6 with respect to stagnant conditions. However, further increase of flow rate yields no additional enhancement of the corrosion rate up to 1 m/s. Under conditions of free corrosion no stress corrosion cracking was observed at constant deflected samples or at tensile probes tested to failure under CERT-conditions in boiling 20–70% NaOH solutions. The corrosion attack is uniform in NaOH solutions up to 30 mass-%. In more concentrated NaOH solutions the dissolution rate of the austenitic phase is slightly higher than the ferritic phase. The corrosion products contain more nickel than the base material.  相似文献   

14.
The influecne of carbon and nitrogen on the sensitivity of low-alloyed steels to stress corrosion Application of stress to steels under the influence of certain media produces stress-corrosion cracking. This can be examined through experimental methods only, by which the specimens are kept under constant stress. The cracking time required for the specimen is then a measure of resistance of stress-corrosion failure. In this way it is possible to investigate the influence of alloying metals and the heat treatment on susceptibility of steels, by considering the respective strenght. Experiments were carried out to determine the effect of carbon content and the combined effect of carbon and nitrogen in a series of medium frequency and vacuum steels. It is found that increasing carbon content improves the resistance of specimens, taking also the higher strength into account. A higher nitrogen content shortens the cracking time. This shortening of resistance due to nitrogen content was much more note worthy in the case of high-carbon steels than in the case of low-carbon steels which were already sensitive in this respect. Comparing an open-hearth steel with the above steels, it seems as if there are other alloying metals which make the steels susceptible to stress-corrosion cracking. The effect of these alloying metals shall be investigated further.  相似文献   

15.
Stress corrosion cracking of austenitic chromium-nickel stainless steels at ambient temperature For the chloride-induced SCC with transgranular crack path in austenitic 18Cr10Ni stainless steel, a critical temperature between 45 and 50°C exists. This critical temperature, however, is valid only for the passive state of the steel in nearly neutral, chloride-containing aqueous environments. In the active state, SCC with transgranular crack mode can occur at temperatures down to ambient temperature. The active state is caused by highly acidic, high-chloride containing aqueous corrosive media. Adherent aqueous films with these properties can grow on the surface of structural components inside swimming-pools when the water is disinfected by addition of chlorine. Under these conditions, failure of austenitic CrNi and CrNiMo stainless steels by SCC with transgranular crack path at ambient temperature is possible and actually occurred. SCC with preferentially intergranular crack path can also occur at ambient temperature when austenitic stainless steels with a sensitized microstructure are used. Under these conditions, the corrosion attack is caused by non-specific aggressive environments, e.g., adherent aqueous films not containing chloride ions. The crack mode, intergranular or mixed, depends on the stress level.  相似文献   

16.
Creep effect on stress corrosion cracking of austenitic CrNi steels in boiling 35% magnesium chloride solution Potentiostatic and potentiodynamic polarization curves of steel X 5 CrNi 18 9 in 35% MgCl2-solution at 120° C do not show significant differences. Important for SCC tests is a narrow potential region before the onset of the potential of pit nucleation. SCC-experiments were carried out using two different techniques:
  • (a) loading in the electrolyte
  • (b) prestraining in air at 120° C; after transient creep the SCC test was initated.
Applying the technique described under (b) a decrease of SCC-susceptibility according to the crack nucleation expected in the slip dissolution model does not take place. Accordingly the dependence of time to failure on potential as well as on stress is found to be almost similar. Only in the absence of pitting a SCC threshold stress exists with its value in the range of yield strength. On the other hand SCC failure in the elastic region (< 0.1 σy) is observed when crack nucleation starts in corrosions pits.  相似文献   

17.
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.  相似文献   

18.
Stress corrosion cracking of steels in liquid ammonia The apparatus developped for the investigation of stress corrosion cracking of steel in liquid ammonia under controlled electrochemical conditions is described. The parameters of the experiments were set by a computer which also stored and correlated the experimental data. Cylindrical samples of the welding steel W. Nr. 1.0143 and of the steel STE 355 (W. Nr. 1.0562) in liquid ammonia containing ammonium chloride or lithium perchlorate as the electrolyte developped cracks only at negative electrode potentials in the regions of active dissolution and hydrogen deposition. Other parameters including contamination of the solution by air were unimportant. The results are explained by hydrogen induced stress corrosion cracking. No embrittlement was observed with passive samples. However, ultimate tensile strengths in liquid ammonia were clearly lower than at air also for samples breaking without formation of cracks. Experiments with notched sheets resulted in sharp, essentially transcrystalline cracks. Passivation of these samples was difficult in the region of the notch indicating the danger of anodic stress corrosion cracking.  相似文献   

19.
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.  相似文献   

20.
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.  相似文献   

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