Die Ermittlung der Beständigkeit gegen Spannungsrißkorrosion (SpRK) von hochlegierten Sonderstählen in chloridhaltigen wäßrigen Medien

Determination of resistance to stress corrosion cracking (SCC) of high-alloy special steels in chloride-containing aqueous media The 18 Cr 10 Ni(Mo) based stainless steels have been continually improved by raising the Cr, Ni and Mo contents. The behavior of these high-alloy steels towards SCC was determined in test media generally used in practice since the question of the resistance to stress corrosion cracking (SCC) had still remained unanswered to a large extent. SCC tests on U-bend samples in boiling 62% CaCl₂ solution showed a good differentiation depending on the Ni and Mo contents. With increasing Ni content, the susceptibility of special high-alloy steels to SCC is shifted towards longer service lives, alloys containing ≧ 42% by weight of Ni being resistant. High-Mo special alloy steels are more resistant to SCC than low-Mo special alloy steels. These results could be confirmed by tests carried out on circular cross section samples in boiling 62% CaCl₂ solution under constant load and potentiostatic control. The free corrosion potentials recorded for 25% Ni special alloy steel and Ni-based alloys are within the potentiostatically determined range of insusceptibility to SCC. The high-Mo special alloy steel X 2 NiCrMoCu 25 20 6 (1.4529) shows the same critical SCC potential on the anodic side as the Ni alloy NiCr21 Mo (2.4858). Superferrit X 1 CrNiMoNb 28 4 2 (1.4575) and austenitic ferritic steel X 2 CrNiMoN 22 5 (1.4462) showed that the SCC behavior was unsatisfactory in both tests as in the case of steel X 10 CrNiMoTi 18 10 (1.4571). Tests in boiling 4 m NaCl showed no SCC, not even under the aggrevated test conditions in the test set-up. The great influence of the oxygen content was demonstrated in tests carried out in the autoclave with defined oxygen and chloride concentrations. The resistance of the steels to SCC decreases under air-saturated conditions (8 … 10 ppm O₂) whereas the chloride concentration (200 and 2000 ppm Cl^?) does not exercise an important influence. U-bend samples should be given preference to Erichsen samples for SCC tests. SCC break characteristics could be determined metallographically and by scanning electron microscope.     

Untersuchungen zum Einfluß von Sulfid- und Sulfationen auf die Korrosion des Eisens in alkalischer Lösung

Investigation into the influence of sulfide and sulfate ions on the corrosion of iron in alcaline solution The corrosion behaviour of Armco iron as well as that of a technical high-strength steel was investigated in sulphide - and sulphate-containing electrolytes of pH ～ 12.6. Current density - potential curves show that iron, after successful passivation, remains passive and protected against corrosion in a solution saturated with CaS, CaSO₄ and Ca(OH)₂ at potentials up to 800 mV(EH). Passivation experiments conducted by changing from the active region to various anodic potentials yielded a critical potential region around 300 mV(EH), above which corrosion appeared. Up to E_H = 200 mV complete passivation was observed. The same critical potential region was observed in experiments of repassivation after mechanical damaging of a protective passive layer. Additional tensile stresses of 85%· σ_0,2 caused no stress corrosion cracking at a potential of 500 mV(EH). After damaging the passive layer, through scratching of the stressed wires fracture occured at and above potentials of 300 mV(EH). At 200 mV(EH) repassivation was observed even under tensile load. In a sulphate-free Na₂S/Ca(OH)₂ solution of the same sulphide ion concentration no stress corrosion cracking of the samples occured at any potential up to 800 mV(EH) even after scratching the steel samples. The observed stress corrosion in sulphide - and sulphate-containing electrolytes is to be ascribed to the action of sulphate and not to sulphide ions.     

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.     

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.     

Untersuchung des SwRK-Verhaltens des martensitaushärtenden Stahls X 5 CrNiCuNb 17 4 PH im Vergleich zum weichmartensitischen Stahl X 4 CrNiMo 16 5 1 ESU in chloridhaltigen wäßrigen Medien. Teil 1: Korrosionsuntersuchungen und SpRK-Untersuchungen zur Optimierung der Wärmebehandlung bezüglich der SpRK-Beständigkeit

Investigations of the corrosion fatigue behaviour at a super pure martensitic stainless steel (X 5 CrNiCuNb 17 4 PH) in comparison to the soft martensitic stainless steel X 4 CrNiMo 16 5 1 ESR in chloride containing aqueous media — Part 1: Corrosion investigations and stress corrosion tests to optimize the heat treatment according to the stress corrosion resistance The aim of this investigation was to improve the reliability of operation and the economy of corrosion fatigue stresses of structural elements, especially at elevated temperatures, by optimizing the material. This investigation is of great interest both to the industry and to related fields. It concerns the influence of a higher degree of purity achieved by the secondary metallurgical aftertreatment of soft martensitic steel X4CrNiMo 16 5 1 ESR (ESR-electroslag remelting) in comparison to the corrosion fatigue behaviour of the precipitation hardening steel X5CrNiCuNb 17 4 PH, which was specially heat-treated with respect to stress corrosion cracking resistance. The stress corrosion was investigated for all heat-treatments of the 17-4 PH in 22% NaCl(pH3) solution. The precipitation hardening steel was most resistant to stress corrosion in concentrated NaCl-solution after a three-stage heat-treatment. There was no improvement of corrosion fatigue resistance after metallurgical aftertreatment of soft martensitic steel compared to the untreated material. This is due to the instable passive behaviour of the material which led to crack initiation, especially during the 150°C experiments, at chloride-induced places of pitting. The investigation of the electrochemical corrosion behaviour of both materials showed that the pH-value hardly influences corrosion restistance. An increase of the salt content leads to higher pitting induction. At temperatures of 80°C in a saturated NaCl-solution the material showed no corrosion resistance. In potentiokinetic investigations, a direct transition from the active area to the pitting potential was observed. In accordance with both the corrosion fatigue and the stress corrosion cracking investigations, it was found that pitting at the martensite precipitator starts primarily around Cu-containing or oxidic inclusions.     

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.     

Zum einfluß des borat-anions auf die elektrochemischen eigenschaften des eisens in Wäßrigen Lösungen

Measurements of potential—time and current density—potential curves of iron were carried out in air-saturated and argon-flushed sodium borate solutions of different concentrations and pH to determine whether borate anions possibly influence the electrochemical behaviour of iron even in the absence of oxygen. The critical minimum pH necessary for passivating iron in air-saturated borate solutions has been established. Using these results and those taken from the literature we discuss the role of the borate anion in possibly stimulating the process of oxygen reduction and/or in influencing the process of metal oxydation, respectively. The latter influence, leading to a decrease of the critical current density, does not absolutely require the presence of oxygen. However, with the formation of hydrogen as the cathodic reaction it cannot give rise to the passive state of iron. The relation between the potential of the passive iron electrode in air-saturated borate solutions and the pH is explained by Fe(III)-borohydroxo-complexes possibly being formed on the solution side of the passive layer.     

Korrosionsbeständigkeit metallischer Werkstoffe in mittelkonzentrierter heißer Schwefelsäure

Corrosion resistance of metallic materials in medium concentrated hot sulfuric acid For the newly developed chromium-base material Alloy 33 (German material No. 1.4591) the corrosion rate has been gravimetrically determined in non aerated high purity grade sulfuric acid in the concentration range from 15 to 80 mass-% at temperatures from 50 to 90°C. This way critical limits of corrosion resistance behaviour under very unfavourable conditions of exposure result. An erratic increase of the corrosion rate by a few orders of magnitude takes place with increasing temperature according to the transition from the passive into the active state. This transition into the active state is shifted towards lower temperatures with increasing concentrations of sulfuric acid. Alloy 33 (1.4591) in the passive state possesses clear advantages against the reference materials for use in sulfuric acid: Alloy 825 (2.4858) and Alloy 20 (2.4660). Polarization measurements in agitated oxygen containing and deaerated sulfuric acid solutions show a higher tendency for passivation and a more stable passive state in comparision with the reference materials. The presence of small amounts of oxidants such as ferric sulfate, nitric acid or potassium dichromate results in a widening of the range of corrosion resistance of Alloy 33 in the studied temperature-concentration field. In addition the minimum concentrations of oxidants necessary for this behaviour have been determined by investigation of the measured corrosion rates of Alloy 33 (1.4591). Polarization tests as well as potential measurements of rest potential and redox potential serve to further explain the mode of operation of these oxidants.     

Zum Potentialverhalten der Oberfläche metallischer Werkstoffe bei milder und starker Schwingungskavitation – V. Eisen-Werkstoffe

Surface potential of metallic materials under conditions of mild and severe oscillation cavitation – V. Iron base materials Four materials – pure iron, low alloy steel, 13% Cr steel, and CrNi steel 18 9 – have been studied in 1 N NaCl solution and in artificial sea-water, the oxygen contents being varied from saturated to zero. The variations of the electropotentials enable conclusions to be drawn with respect to the influence of cavitation of corrosion. Pure iron and low alloy steel are in the active state under the experimental conditions; oscillation up to 20000 g give rise to a potential displacement toward more positive values. At higher oscillation loads there is a displacement in the opposite direction (to be attributed to the degazification of the solution). The CrNi steel is in the passive state but the type of potential displacement is about the same. At the phase boundary martensite formation takes place. The chromium steel occupies an intermediate position being active in the NaCl solution and passive in the sea-water. The potential displacement toward more negative values encountered with all the materials studied in the oxygen-free electrolyte at high acceleration can be attributed to a facilitation of the anodic partial reaction.     

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.     

Effect of Cl~- Concentration on the SCC Behavior of 13Cr Stainless Steel in High-Pressure CO_2 Environment

An effect of Cl-concentration on the stress corrosion cracking(SCC) behavior of 13 Cr stainless steel was investigated by employing electrochemical measurements and the slow strain rate tensile tests.These tests were conducted in various solutions with different concentrations of NaCl at 90℃under 3 MPa CO_2 with 3 MPa N_2.The results indicate that the passive film of the specimen formed in the 10% NaCl solution has the best protective effect on the matrix.The SCC susceptibility does not increase with increasing the chloride ion concentration,the lowest SCC susceptibility occurs when the NaCl concentration is 10%,and the specimens show higher SCC susceptibility in the 5% NaCl and 20% NaCl solutions.     

Inhibition von Niedertemperaturkreisläufen in Kraftwerken mit Lithiumhydroxid

Inhibition of low temperature circulating systems with lithium hydroxide in power stations Corrosion current density measurements of mild steel St 37 in deionized, air-saturated water between 20 and 60°C showed that addition of small quantities of lithium hydroxide (ca. 25 ppm) protects against corrosion attacks similar to pitting corrosion and is found adequate even up to 5 ppm chloride ion concentration. Constant strain rate tests of mild steel St 37 showed no indication of stress corrosion cracking (SCC) at 70°C, up to 500 ppm lithium hydroxide, and 100 ppm chloride ion concentration. Metallographic examination of specimens indicated intergranular stress cracking with cracks of 1–30 m?m depth in environments containing 2%, 4%, and saturated lithium hydroxide (with solid excess salt) at 70°C. Accumulation of lithium hydroxide should therefore be avoided. No SCC was observed in austenitic stainless steel X 5 CrNi 18 9 specimens in the above environments.     

Untersuchungen zum kritischen korrosionsauslösenden Chloridgehalt von Stahlfasern in künstlicher Betonporenlösung

Investigations into the critical corrosion‐inducing chloride content of steel fibres in artificial concrete pore solution It is well known, that reinforcement steel in concrete is normally protected against corrosion due to the high pH‐value of the pore solution of the concrete. This alkalinity leads to a passive layer on the steel surface, which prevents further corrosion. The passive layer can be destroyed by chloride ions diffusing into the concrete. The concentration of chloride in the concrete which leads to a destruction of the passive layer and therefore to corrosion of the steel is defined as the critical chloride content. Investigations in artificial concrete pore solutions show that the critical chloride content of black steel is strongly dependent on the pH‐value of the solution: the higher the concentration of the OH⁻‐ions the higher the critical chloride content. For steel fibres earlier investigations have shown, that steel fibres do not corrode in concrete even at high chloride contents. Therefore it could be assumed, that the critical corrosion‐inducing chloride content of steel fibres in concrete is distinctly higher than of conventional reinforcing steel. To verify this assumption the corrosion‐inducing chloride content of steel fibres is investigated in artificial chloride‐containing concrete pore solutions at different pH‐values. 5 different types of steel fibres, 1 lashing wire and as reference 1 reinforcing steel are investigated at 3 different pH‐value ranges. The concentration of chloride within the pore solution is gradually increased in time steps of 12 h. The beginning of corrosion is determined by current as well as potential measurements. Furthermore additional investigations are carried out with intermediate products of the fibre production (steel wires with different diameters) to investigate if the critical chloride content of the wires is increasing gradually with decreasing diameter. The investigations show, that steel fibres in artificial chloride‐containing pore solutions indicate an distinctly increased resistance against chloride‐inducing corrosion compared with conventional reinforcing steel for high pH‐values. With decreasing diameter of wires the critical chloride content increases gradually.     

Anodische Spannungsrißkorrosion eines Stahls in flüssigem Ammoniak

Anodic stress corrosion cracking of steel in liquid ammonia Steel rods W.No. 1.8907 were tested for stress corrosion cracking in liquid ammonia at about 20 °C under controlled electrochemical conditions at constant rate of elongation. Stress corrosion cracking was not observed in pure ammonia at the free corrosion potential of the steel corroding in the active state, but only upon polarization to the passive state. The susceptibility to stress corrosion cracking grew with the temperature and was negligible at temperatures below about 10°C. In presence of oxygen at pressures exceeding about 0.1 bar, stress corrosion cracking occurred at the free corrosion potential in the passive range at more negative electrode potentials than in absence of oxygen. Additions increasing the specific conductivity of ammonia and 0.15% water inhibited stress corrosion cracking. The small danger of hydrogen embrittlement was enhanced by cathodic polarization and by addition of acids.     

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.     

Zum Einfluß der mechanischen Belastung auf die interkristalline Spannungsrißkorrosion von unlegiertem Stahl in Carbonat/Bicarbonat-Lösung

Effect of mechanical stress on intergranular stress corrosion cracking of unalloyed steel in carbonate/bicarbonate solution Intergranular stress corrosion cracking of a carbon steel in 0.5 M NaHCO₃ + 0.5 M Na₂CO₃ solution at 70°C was examined by normal, interrupted and cyclic CERT testing. Within the critical potential range, the lengths of intergranular cracks increase with increasing strain rate, and decrease when the material is cold worked. The fracture time indicates that the crack velocity increases with increasing strain rate. This is also observed with tensile specimens subjected to interrupted and cyclic strain rates. Detailed information on crack propagation and threshold stress levels can be obtained by interrupted and cyclic CERT tests. Intergranular cracks arise if surface region of the material undergoes plastic deformation in excess of a definite amount. In the case of a 60°-notch specimen the plastic zone must be greater than 0.3 mm. For cyclically strained specimens the crack velocity can markedly decrease with time or can become even zero as a consequence of strain hardening. From this one can conclude that stress corrosion cracking can only occur if stress levels are very high.     

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.     

X80管线钢在近中性pH溶液中腐蚀行为研究

目的研究X80管线钢在近中性p H溶液中的腐蚀与应力腐蚀裂纹萌生行为。方法采用电化学实验和浸泡实验研究X80管线钢在近中性p H溶液中的腐蚀行为,采用慢应变速率拉伸实验研究X80管线钢在近中性p H溶液中,在自腐蚀电位和外加电位下的应力腐蚀裂纹萌生行为。结果 X80管线钢在近中性p H溶液中的极化曲线只有活化区,没有钝化区,其自腐蚀电位约为-750 m V,浸泡195天后,试样表面没有氧化膜出现,但是观察到点蚀坑。在自腐蚀电位下,X80管线钢试验表面有大量的应力腐蚀裂纹;在-500 m V阳极外加电位下,X80管线钢试验表面几乎没有观察到应力腐蚀裂纹;在-850 m V阴极外加电位下,X80管线钢试验表面的应力腐蚀裂纹很少,但是随着外加阴极电位负移到-1300 m V时,X80管线钢试验表面的应力腐蚀裂纹增多。结论 X80管线钢在近中性p H溶液中发生均匀腐蚀,但是夹杂物剥落能在X80管线钢表面形成点蚀坑。在近中性p H溶液中,在自腐蚀电位下,X80管线钢应力腐蚀裂纹萌生敏感性最强;外加阴极电位抑制应力腐蚀裂纹萌生,但是随着外加阴极电位的负移,应力腐蚀裂纹萌生敏感性增强;外加阳极电位下,由于均匀腐蚀的作用,应力腐蚀裂纹萌生敏感性较弱。     

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.     

Polarisationsmessungen an niedriglegierten Stählen in konzentrieten Nitratlösungen

Polarisation measurements on low alloy steels in concentrated nitrate solutions Current density potential curves were determined by potentiostatic measurements involving constant potentials and stepwise potential change for six low alloy steels in Ca(NO₃)₂ and NaNO₃ solutions. The instationary curves vary markedly with the polarisation technique and do not characterize the anodic behaviour of the materials. For Ca(NO₃)₂ solutions the polarisation measurements do not show any correlation with stress corrosion cracking behaviour. Similar results were also obtained in NaNO₃, solutions. Contrary to the situation in Ca(NO₃)₂ solutions, a potential range for resistance against intergranular corrosion types exists at about U_H = 0,5 V. This range is characterized by relatively high stationary current densities. For materials without this range of resistance, the stationary current densities are significantly smaller. The electrochemical properties of the system steel/nitrate solution are extremely complicated. The free corrosion potential varies between the potential ranges for passive and transpassive corrosion. For anodic polarisation, the potential ranges of susceptibility as well as resistance against intergranular corrosion types follow each other.