Das Ausscheidungsverhalten von hochlegierten austenitischen Stählen mit 6% Molybdän und sein Einfluß auf die Korrosionsbeständigkeit

Precipitation behaviour of high-alloyed austenitic steels with 6% molybdenum and its influence on the corrosion resistance The high-alloy austenitic steels with 6 to 7% Mo, 20 to 21% Cr and 18 to 25% Ni are increasingly used in seawater and chemical applications. This is due to the excellent resistance to pitting and crevice corrosion in chloride-containing neutral and acidic environments. It is the high chromium and molybdenum content which provides the excellent corrosion behaviour but, at the same time favors the tendency to precipitation of intermetallic phases. Therefore, time-temperature-precipitation diagrams have been established for two steels with 6% Mo, 21% Cr, 25% Ni, 0.14 and 0.19% N and for one steel with 6% Mo, 20% Cr, 18% Ni and 0.21% N. The corresponding time-temperature-sensitization diagrams (in accordance to SEP 1877/II) and time-temperature-pitting diagrams (testing in 6% FeCl₃ solution) have been evaluated as well. Precipitation of intermetallics occurs rapidly especially in the range between 700 and 1000°C. In case of the 18% Ni steel and the 25% Ni/0.14% N steel grain boundaries are covered to a large extent with precipitates after only 15 min at 850 or 950°C. In case of the 25% Ni/0.19% N steel precipitation is considerably slower. The precipitates are interpreted to be chi-phase. After very long annealing times additionally small amounts of Laves phase appear. Neither carbides nor nitrides were observed. In spite of the rapid precipitation, sensitization in terms of the 50 m?m grain boundary penetration criterion is observed not before 0.7 h at 850 °C and not before about 2 h at 800°C in case of the 25% Ni/0.19% N steel. After about the same times of annealing also the critical pitting temperature as observed in the FeCl₃-test is dropping below 50°C. Therefore, when welding according to established rules and recommendations, no deterioration of the corrosion resistance in the heat-affected zone is to be expected. If high heat inputs will occur during manufacturing because of hot forming operations or welding of heavy sections, or if more severe test conditions are a requirement, a steel with 25% Ni and about 0.2% N (UNS N 08925, Cronifer hMo) is recommended due to its retarded precipitation and sensitization behaviour when compared to steels with only 18% Ni (UNS S 31 254). Additionally, the steel with 25% Ni has an increased resistance to general corrosion in acids. Notch impact strength of the materials under consideration is increased by the initial precipitation of the intermetallic phases and decreases only after longer times of annealing below the ductility of the solution annealed material.     

Einfluß der Legierungselemente auf die Passivierung und die Korrosionsbeständigkeit der Legierungen auf Eisen-Chrom-Basis

Influence of alloying elements on the passivation and the corrosion resistance of iron chromium base alloys The authors investigated the influence of cathodic alloying (with 0.1–0.5% Pd) on the passivation and the corrosion resistance of alloys of the system FeCr(25–100% Cr) and 25% Cr steels containing Mn, Ni, Mo and N; the experiments have been carried out in hot concentrated sulfuric acid and diluted hydrochloric acid solutions. It has been found that the addition of Pd as a cathodically active component considerably increases the autopassivation tendency as well as the corrosion resistance of the alloys under the particular conditions. Mn gives rise to improved autopassivation of austenitic and austenitic ferritic CrNi and CrNiMo steels, since it is catodically active, too. Cr steels cathodically alloyed with Pd acquire resistance to hydrochloric acid, too.     

Einfluss von Verunreinigungen auf die Korrosionsbeständigkeit und die mechanischen Eigenschaften von Bauzink

Influence of impurities on the corrosion resistance and the mechanical properties of construction zinc Zinc is a material from which for example roofs, rain‐tubes and gutter are produced. For a sufficient creep resistance, zinc‐titanium‐copper‐alloys are used. Aluminium and lead impurities in the zinc alloy increase the susceptibility of the alloy to intercrystalline corrosion. The assessment of a damaged rain‐tube with scanning electron microscopy and EDX‐analysis shows high aluminium and lead concentrations in the alloy which led to intercristalline corrosion and embrittlement of the alloy. The alloy which has been used for the fabrication of the rain‐tube does not fulfil the specification of titanium zinc according to EN 988.     

Beitrag zum Problem des Einflusses von Silicium,Stickstoff und Bor auf die Verzunderungsbeständigkeit und Warmfestigkeit austenitischer Chrom/Nickel-Stähle

A contribution to the problem of the influence of Si, N and B on the scaling resistance and high temperature strength of austenitic CrNi steels Laboratory and field rests (up to 5000 hrs in an enamelling stove) into the scaling behaviour of alloyed steels, with a particular view to a possible reduction of the Ni content. From the steels – Cr23Ni18, Cr25Ni20Si2, Cr23Ni13, Cr23Ni13Si2, Cr23Ni13Si2N and Cr23Ni13Si2B – the type Cr25Ni20Si2 has the highest, Cr23Ni13 the lowest oxidation resistance. Addition of Si has a negative effect, in particular on creep resistance at high temperatures, while oxidation resistance is improved. In cases of simultaneous thermal and mechanical loads boron additions are preferable. In cases of alternating temperature changes under mechanical stress alloying with nitrogen appears most promising.     

Einfluß von Passivierungs- und Kohlenstoffschichten auf austenitischen CrNiMo-Stählen auf die Beständigkeit gegen Lochfraß und Spannungsrißkorrosion

Effects of passivation and carbon films on austenitic CrNiMo steels on their piting and stress corrosion resistance The influence of passive film and combinations of a passivation and a carbon layer on the resistance to pitting and SCC of austenitic CrNiMo steels has been investigated in physiological sodium chloride solution (Tyrode solution) at pH 6.9 to 7.4 at 37 ± 1°C. The passive film was obtained after electrolytic polishing in H₃PO₄ + H₂SO₄ + C₆H₅NHCOCH₃ + oxalic acid + corrosion inhibitor CS by treatment with 40% nitric acid the carbon film was obtained by CVD. Impurities in the steel (non-metallic inclusions) and the different metallic phases were investigated and the chemical composition of the passive film was determined by quantitative analysis. The resistance to pitting of the steel with and without passive film was determined potentiodynamically in Tyrode's solution at 37 ± 1°C. The resistance to SCC was determined in Tyrode's solution at 37 ± 1°C, in neutral glycerole and in boiling magnesium chloride solution at 154 ± 1°C and evaluated in terms of K_σ and K_τ. The corrosion damage was investigated by optical and scanning electron microscopy. The investigations have revealed that the different surface conditions considerably improve the pitting and SCC resistance of the steels in the media used in this work, so that they make possible the use of these materials as surgical implants.     

Oberflächenbehandlungsverfahren und deren Einfluß auf die Korrosionsbeständigkeit nichtrostender Stähle

Surface treatments and their influence on the corrosion resistance of stainless steel The surface treatments pickling, grinding and glass beading were investigated on several stainless austenitic steels and one ferritic/austenitic steel. The different surface treatments were used on two different prepared types of samples:

• 1 high temperature oxidized samples
• 2 welded samples

The quality of the surface treatments has been examined by means of potentiodynamic, ferric chloride, dip and spray tests made in series. The corrosion resistance was highly depending on the used treatments. All in all the examination showed that a higher corrosion resistance was achieved by pickling than by grinding or glass beading.     

Kurzzeitprüfmethoden zur Untersuchung des Einflusses von Wärmebehandlung und chemischer Zusammensetzung auf die Korrosionsbeständigkeit martensitischer Chrom-Nickel-(Molybdän)-Stähle mit tiefem Kohlenstoff-Gehalt

Short-duration test for evaluating the influence of heat-treatment and chemical composition on the corrosion resistance of low carbon martensitic chromium nickel (molybdenum) steels The following procedures were examined and compared for their suitability as rapid corrosion tests in comparative investigations on the influence of heat treatment and other production parameters on martensitic chromium-nickel-(molybdenum) steels of low carbon content:

• –Measurement of the mass loss in boiling 20% acetic acid, and in some cases 5% nitric or 1% hydrochloric acid.
• –Immersion in sulphuric acid/copper sulphate solution (modified Strauss test).
• –Current density/voltage curves in 1 N sulphuric acid.
• –Measurement of pitting potential und activation pH in 3% sodium chloride solution (on limited scale).

The test materials employed contained 0.032–0.082% carbon, 11.6–16.4% chromium, 3.8–5.9% nickel and 0.31–1.6% molybdenum. The most suitable testing procedures were selected according to the steel type. Up to a mass loss rate of ≥ 0.05 mg · cm^?2 · h^?1 (materials with resistance inferior to steel type 13-6-1) the acetic acid test provides an excellent basis for comparison. Provided the optimal testing temperature is adopted, current density/voltage curves and immersion in sulphuric acid/copper sulphate solution yield usable results with all steel types investigated. The value of the information yielded by the test procedures is generally enhanced by metallographic characterization of the corroded surfaces; in particular the localization of chromium-depleted structure zones is made possible by this.     

Einfluß von Kohlenstoff,Stickstoff und Phosphor auf die interkristalline Korrosion und Spannungsrißkorrosion des Eisens

Effects of carbon, nitrogen and phosphorous on the intergranular corrosion and the stress-corrosion cracking of iron Experiments without stress, with constant load, or with constant strain rate were performed in boiling 55% Ca(NO₃)₂ solution (115°C) to elucidate the effects of carbon, nitrogen and phosphorous on the intergranular corrosion (IC) and stress-corrosion cracking (SCC) of iron. The original material contained 20 to 40 ppm C, 17 ppm N, and 20 ppm P. One batch of this material was normalized only, a second batch was also decarburized up to under 10 ppm C prior to normalizing, and a third batch was decarburized, and then nitrided up to 140 to 220 ppm N prior to normalizing. Some of the specimens were tempered at 550°C for one month. All normalized specimens showed no susceptibility to IC at applied potentials from 800 to 1000 mV (SHE). In contrast, specimens tempered at 550°C were very susceptible to IC, which was observed even at 200 mV after decarburization. Auger-spectra of intergranular fracture surfaces of a tempered specimen produced at liquid-nitrogen temperature revealed grain boundary segregation of P (about 10 atomic %). The as-normalized specimens fractured intergranularly within 1 to 2 h, when held at constant load of 0.8 and 0.9 of ultimate tensile strength (UTS), and 0 mV. By contrast, under identical conditions, the decarburized specimens and the nitrided specimens did not fracture in 30 d. The fracture energy tested with the constant strain-rate method at 0 mV, as compared with experiments in oil, was strongly lowered in the as-normalized specimens and significantly lowered in the nitrided specimens. The values of fracture energy obtained for the decarburized specimens were mostly similar to those in oil at 115°C. Only some of the decarburized specimens showed substantial decreases in fracture energy. However, the decarburized and tempered specimen showed an even larger decrease in fracture energy, with intergranular fracture mode.     

Einfluß von Verformung und spannungsinduzierten Ausscheidungen auf die Korrosionsbeständigkeit von siliciumlegiertem,nichtrostendem Stahl X2CrNiSi 18 15 in Salpetersäure

Effect of deformation and stress-induced precipitations on the corrosion resistance of silicon alloyed stainless steel X2CrNiSi 18 15 in nitric acid The corrosion resistance of silicon alloyed X2 CrNiSi 18 15 stainless steel with about 4 wt.-% Si in nitric acid depends on both deformation degree of the material and number and distribution of precipitations in the microstructure. After cold deformation and subsequent heat-treatment at 700°C (1292°F), the corrosion rate in 5 M HNO₃ is higher than that of non-deformed material. This result was established with specimens isolated from each other but was particularly pronounced with specimens being in galvanic contact. By deformation and subsequent annealing at 700°C, precipitations are formed at the grain boundaries the number of which increases with increasing degree of deformation. With higher degree of deformation, the precipitations are preferably arranged at slip-lines. They were identified as chromium carbide, M₂₃C₆, and silicide of the Cr₅Ni₃FeSi₂ type. It is shown by electrochemical measurements that these precipitations stimulate the cathodic partial reaction, i.e., the reduction of nitric acid. Hence, the corrosion resistance of the silicon alloyed stainless steel is impaired by deformation and stress-induced precipitations formed during annealing at 700°C.     

Einfluß von Eisen auf die Korrosionsbeständigkeit von Nickelbasislegierungen als mögliche Werkstoffe für SCWO-Reaktoren

Effect of iron on the corrosion behaviour of nickel based alloys for SCWO plants An experimental set up to study corrosion at high temperatures and high pressures and able to simulate the conditions of supercritical water oxidation is described. On the basis of the alloys AC 66, 45 TM, G-3 and 601 H the influence of iron on the corrosion behaviour of nickel base alloys in aqueous solution under high pressures containing oxygen and chloride is shown and discussed. The corrosion resistance of the nickel base alloys is decreased with increasing iron concentration in the whole temperature range (100 °C–420 °C). The corrosion started at lower temperatures and the weight loss increased rapidly with increasing iron content. The corrosion morphology changed from uniform corrosion for alloys with less iron content to deep pits for alloys with high iron concentrations.     

Der Einfluß von Schwefeldioxid,Schwefelwasserstoff und Kohlenmonoxid auf die Lochkorrosion von austenitischen Chrom-Nickel-Stählen mit bis zu 4 Massen-% Molybdän in 1 M Natriumchlorid-Lösung

The Influence of SO₂, H₂S and CO on Pitting Corrosion of Austenitic Chromium-Nickel Stainless Steels with up to 4 wt. % Molybdenum in 1 M NaCl Active corrosion of chromium-nickel stainless steel X 5 CrNi 189 (AISI 304) in H₂SO₄ is stimulated by H₂S as well as by SO₂ (extension of the potential range of active corrosion, increase of the maximum corrosion rate in the active state and of the passivation current density), but is inhibited by CO (decrease of both maximum active corrosion rate and passivation current density). It is investigated whether likewise stimulating and inhibiting effects are valid also in case of pitting corrosion of austenitic stainless steels with molybdenum contents ranging from about zero (material no. 1.4301) to 4 wt. % (material no. 1.4449), tested in 1 M NaCl (ambient temperature) saturated with the gases mentioned above. The pitting corrosion behaviour of the materials investigated is judged by their pitting potentials measured by potentiostatically controlled experiments (testing time 24 hrs). The pitting potentials are compared with those measured in 1 M NaCl, N₂-bubbled. Pitting corrosion is stimulated by SO₂, CO and H₂S, with the stimulating efficacy increasing in the sequence given before. No stimulation is found only in 1 M NaCl, SO₂-bubbled,-with the highest Mocontent. In all other cases, stimulation of pitting corrosion increases with increasing Mo-content of the stainless steels. In coarse approximation, the critical limiting potentials of stable pitting in 1 M NaCl, bubbled with H₂S, SO₂ CO, correspond to the critical potentials of repassivating pitting corrosion found in N₂-bubbled 1 M NaCl. The chemical reactions and reaction products of SO₂ in aqueous solution are discussed. The nature of the stimulating component of the corrosive medium is not quite clear. The stimulating effects of SO₂ or one of its reaction products and of H₂S on pitting are in keeping with their stimulating effect on active corrosion of the steels investigated. The stimulating effect of CO, however, is in contradiction to the results expected and cannot be explained.     

Erweiterte Diskussionsbemerkungen zum Einfluß von Silicium und Molybdän auf die Spannungsrißkorrosionsempfindlichkeit austenitischer und austeno-ferritischer Chrom-Nickel-Stähle

Amplified discussion of the influence of Silicon and Molybdenum on the stress corrosion cracking proneness of austenitic and austeno-ferritic chrome-nickel steels Tests have been carried out in boiling magnesium chloride, calcium chloride, NaCl, water (150–200°C) and hot steam (500°C); in the two last-named cases, the tests were carried out with and without the addition of chloride and oxygen. The following materials were tested: steels with (per cent.) 17 and 17.5 Cr, 12–15.5 Ni, 0 and 2.5 Mo, 0 and 4 Si, less than 0.03 C, as well as steels with 20 and 21 Cr, 8—10 Ni, 0 and 2.5 Mo, 0 and 3 Si, 0, 1.5 CU, less than 0.05 C. Silicon has a favourable effect on the stress corrosion cracking behaviour in magnesium chloride and also in calcium chloride, but a detrimental effect in NaCl and water. Its probable effect is to inhibit the extension of the crack by crystallographic obstacles and, later, by the formation of a surface film of poor conductivity (where Mo also plays a part). The attempt is made to interpret this behaviour on the strength of electro-chemical investigations; in this connection, renewed critical reservations are made in respect of the test in magnesium chloride.     

Einfluß von Molybdän und Aushärtungsgrad auf die Korrosionseigenschaften chromlegierter martensitaushärtender Stähle

Influence of molybdenum and degree of hardening on the corrosion properties of chromium alloyed maraging steels The influence of intermetallic precipitations on the corrosion resistance of maraging chromium alloyed steels with various molybdenum contents has been studied by tracing potentiokinetic and potentiostatic current density/potential curves. It has been shown that the passivation current density and the passive current density are measures of the corrosion resistance. Hardening times yielding maximum strength result in an increased dissolution current density; in this context increased molybdenum contents displace the increase of the current density toward longer times. Corrosion tests in artificial seawater distinctly show the positive influence of molybdenum which displaces the rapture potential toward more nobel values.     

Zum Einfluß von Mangan auf die Korrosionseigenschaften von austenitischen 18.10-CrNi-Stählen

Influence of manganese on the corrosion properties of austenitic 18.10-CrNi stainless steels The influence of manganese in the range of 0.25 to 1.5 mass-% on the passivation and pitting corrosion behaviour of unstabilized and Tistabilized austenitic 18/10 CrNi stainless steels is examined by determination of useful characteristical electrochemical dates using potentiodynamical polarization measurements in H₂SO₄-acidic and neutral model electrolytes. In the case of Ti-stabilized steels, a trend to an improved ability to passivation and to an increased pitting corrosion resistance is signified with increased Mn-content. This is in agreement with the austenite stabilizing effect of manganese. In the case of unstabilized steels, a significant deterioration of the passivation and pitting corrosion behaviour is observed, if the Mn-content of the steel is increased from less than 0.7 to more than 1.0 mass-%. These observations are discussed in the viewpoint of segregation of Mn-rich sulfide inclusions in the steel, which are essentially influenced by the presence of titanium in the steel.     

Der Einfluss von Oxidationsmitteln auf die Korrosionsbeständigkeit von Nicrofer 3127 hMo in niedrig‐ und mittelkonzentrierter Schwefelsäure

The influence of oxidants on the corrosion resistance of Alloy 31 in low and medium concentrated sulfuric acid The isocorrosion lines < 0.1 mm/y were determined for Nicrofer 3127 hMo (1.4562, Alloy 31) by weight loss measurements after 24 h immersion in areated sulfuric acid of technical and p.a. grades in the concentration range up to 90% – both without and with 1 g/l chloride. The free corrosion potential indicates that the superior corrosion resistance of Nicrofer 3127 hMo in the technical acid grade can be attributed to the content of oxidants in the acid. The presence of oxidants leads to metastable passivity in technical sulfuric acid with concentrations ≥ 20% at temperatures that can be 10–50 K higher than in p.a. acid depending on the acid concentration. For sulfuric acid concentrations up to 10% the content of oxidants in the technical acid is too small to improve the corrosion resistance of Nicrofer 3127 hMo as compared with that in p.a. acid. Similar behaviour was observed for Nicrofer 3127 hMo in sulfuric acid containing chlorides. However the activating effect of the chlorides limits the region of corrosion resistance to lower temperatures in comparison with the chloride free‐acid.     

Einfluß der Elemente Phosphor,Mangan, Chrom,Molybdän und Vanadium auf das Verhalten niedriglegierter Stähle bei Spannungsrißkorrosion

Influence of elements such as phosphorus, manganese, chromium, molybdenum and vanadium on the stress corrosion behaviour of low alloy steels A series of such steels having graduated alloy addition contents were obtained experimentally in an electrical furnace and were tested under permanent tensile loads in calcium nitrate solution after different heat treatments and at different relative loads. The heat treatment has a very pronounced bearing on corrosion susceptibility: overheating produces the highest sensitivity annealing at 750°C the — relatively — highest resistance, while normalizing occupies an intermediate position. P has but little effect on corrosion susceptibility so that its influence can practically be neglected. Mn seems to have generally a beneficial effect while Cr — even in amounts up to 2% — is unable to improve corrosion resistance. In the case of V there is some improvement in some cases only while Mo generally produces increased corrosion resistance. In this connection, a new method for evaluating experimental results has been described, which has been developed for the practice of steel assessment. The experimental results confirm, that a steel may be designated ?resistant”? at a certain load when its useful life under tensile load in alkaline media is 240 hrs. Extension of the test duration does not yield an improvement in evaluation accuracy.     

Der Einfluß von Cer,Zirkonium und Bor auf die Oxidationsbeständigkeit hitzebeständiger Stähle an Luft

Influence of cerium, zirconium and boron on the oxidation resistance of heat-resistant steels in air Isothermal and cyclic oxidation experiments were carried out in air to investigate the influence of the minor elements such as Cerium, Zirkonium and Boron on the oxidation resistance of heat resistant ferritic and austenitic steels like X 10Cr 18, X 10CrAl 18 and X 15 CrNiSi 20 12. In the case of cyclic experiments samples were exposed at constant temperatures for 100 h and then cooled to R. T. This cycle was repeated 10 times. The corrosion was determined as weight change and was continuously measured by a thermo-balance. The distribution of the alloying elements on the phase boundary scale/steel was examined by Scanning-Electron-Microscope. Addition of small amounts of Ce (0.3 wt-% max.) could reduce the oxidation rate in the case of isothermal and cyclic conditions. Zirkonium concentrations below 0.1 wt-% could have a beneficial effect, but at higher concentrations the oxidation rate increases with increasing amounts of Zr. Small Boron concentrations of 0.02 wt-% lead to catastrophic oxidation at temperatures above 1000°C.     

Der Einfluß des Molybdängehaltes auf die Korrosion austenitischer Cr-Ni-Stähle im Aktivzustand

The influence of the molybdenum content on the corrosion rate of austenitic Cr-Ni steels in the active condition The corrosion rate of steels X 5 CrNi 189, X 5 CrNoMo 18 10, X 5 CrNiMo 18 12 and X 5 CrNiMo 17 13 has been investigated under potentiostatic conditions in the active zone in 2 n H₂SO₄ flushed with nitrogen and sulphur dioxide. The active rest potential of the steels is in the vicinity of the active-passive transition, and the corrosion rate increase at cathodic polarisation. With increasing Mo content, the corrosion rate is reduced in the active condition, but the passivation potential and the corrosion rate in the passive condition are not influenced. In the acid flushed with SO₂, the corrosion rate is increased in the active range, and the latter is extended in the direction of the electronegative potentials. With these steels, even a pre-activation of the specimens has an influence on the test results. In the active-passive transition zone, the steels in the test solution containing SO₂ are partially passive and subject to local corrosion attacks.     

Über den Einfluß von Kohlenstoff und Stickstoff auf die Empfindlichkeit niedriglegierter Stähle gegen Spannungsrißkorrosion

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.     

Der Einfluß programmierter Schweißtemperaturzyklen auf die Gefügeausbildung und das Korrosionsverhalten von austenitischen korrosionsbeständigen Stählen

Influence of programmed welding temperature cycles on the resulting structure and the corrosion behavior of austenitic steel Studies on corrosion resistant austenitic steels subjected to simulated welding temperature cycles have shown that even after very short annealing times the corrosion resistance is decreased. In the temperature range between 700 and 900 °C carbides are precipitated, while grain growth and twinning are observed at temperatures between 900 and 1100 °C. Above 1280 °C delta-ferrite is formed as early as after 15 sec. Potentiokinetic current density/potential curves have been traced on the basis of tests in 1 N sulfuric acid at room temperature. The passivation current density has turned out to be a suitable corrosion resistance criterion.