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 von Natriumchlorid auf die Oxidation von hochlegierten Chrom- und Chrom-Nickel-Stählen

Effects of sodium chloride on the oxidation of high alloy Cr- and Cr-Ni-steels The effects on the oxidation were investigated of solid NaCl deposits on the oxide scales of the technical steels X10 CrMoV 12 1, X15 CrNiSi 20 12 and X10 CrNiSi 25 20 at 700°C. Independent of the alloy composition the presence of NaCl(s) initiated a markedly accelerated Fe₂O₃ growth on the surface of preoxidized samples, under formation of voluminous, nonprotective layers. Below these scales on the metallic matrix in all cases chloride was detected. The oxides grow according to the mechanism of active oxidation in which chlorine plays a catalytic role. The presence of the chloride at the oxide/metal interface reduces the adhesion of the oxide scale and leads to spalling upon cooling to room temperature. The effects observed are independent of the alloy composition, however, the thickness of the oxide scale is decisive which means the diffusion distance for the gaseous iron chloride.     

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.     

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

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

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.     

Untersuchungen über den Einfluß von Schwefelwasserstoff und Schwefeldioxyd auf die Korrosion chemisch beständiger Chrom-Nickel-Stähle in schwefelsaurer Natriumsulfatlösung

Investigations into the influence of hydrogen sulphur dioxide on the corrosion of chemically resistant chrome nickel steels in a sulphuric acid chrome nickel steels in a sulphuric acid solution of sodium sulphate By measuring the current/potential curves and determining the weight losses, the influence of H₂S and SO₂ on the corrosion of an 18/8 Cr? NI steel and of an 18/8 Cr? Ni stell with 2pCMo and 2.8pC Cu in sodium sulphate solution has been investigated. H₂S and SO₂ have the effect of shifting the rest potential towards the electronegative side, compared with a solution flushed with nitrogen, enlarging the potential range of active dissolution, and greatly increasing the dissolution, and greatly increasing the corrosion rate in the active zone. From the results of the measurements, it may be concluded that the metal dissolution is catalyzed by hydrogen sulphide ions and probably by reduction products of the sulphur dioxide. This catalyzing effect is not confined to the zone of activation overpotential but also occurs in the zone of the active plateau, the shape of which is largely determined by the migration phenomena. SO₂also has the effect of increasing the dissolution rate in the passive condition.     

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.     

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.     

Einfluß von Chrom,Molybdän und Stickstoff auf die Korrosionsbeständigkeit des Ni-freien,austenitischen Edelstahles Macrofer 2515MoN (W.-Nr.: 1.4653)

Influence of chromium, molybdenum and nitrogen on the corrosion resistance of the Ni-free, austenitic stainless steel Macrofer 2515MoN (German Alloy No. 1.4653) Nitrogen alloyed, Ni-free, austenitic stainless steels comprising of more than 1 wt.-% nitrogen are a new group of alloys with promising properties. They show a very interesting combination of high strength and toughness with a high corrosion resistance. This combination of properties make the alloys not only suitable for fasteners but also for parts for medical and dental applications. This work shows the influence of chromium, molybdenum and nitrogen on the corrosion resistance of Fe25Mn-alloys in media typical for the above mentioned applications. According to these results Fe25Mn-alloys with appr. 20 wt.-% chromium, about 3 wt.-% molybdenum and appr. 1,3 wt.-% nitrogen have an excellent corrosion resistance in Ringer solution, artificial saliva and artificial sweat. The critical pitting temperature (CPT) as well as the critical crevice temperature (CCT) with 61°C respectively 37°C tested according ASTM G 48A provided significantly higher temperatures when compared to the commercially well established Ni-austenite X6CrNiMoTi17-12-2 (German Alloy No. 1.4571).     

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.     

Korrosionsbeständigkeit von Installationsteilen aus nichtrostenden austenitischen Chrom-Nickel-Stählen bei erhöhter Temperatur

Corrosion resistance of installation elements of stainless austenitic chromium-nickel steels at high temperatures The test results presented prove the statements made in DIN 50 929 Part 2. In accordance with DIN 50 930 Part 4, the corrosion probability for stainless steels increases when chloride containing water evaporates on warm material surfaces, whereby chloride ions concentrate. Under these conditions, ferritic chromium steels and austenitic chromium-nickel steels can suffer pitting corrosion, austenitic chromium-nickel steels also stress corrosion cracking. In the latter case, the corrosion cracks start from pits. The molybdenum-containing material no. 1.4571 withstands pitting and stress corrosion cracking in wet, chloride-containing environments at 90°C over some weeks. With increasing exposure time and at temperatures above 45°C, however, corrosion damage cannot be excluded. Then, according to DIN 50 929 Part 2, coating of the external surfaces of installation components, e.g., tubes, is required. The coating must be thick, free from pores and holidays, and resistant to heat and aging.     

Untersuchungen über den Einfluß der Mikrostruktur auf die interkristalline und Kornflächenkorrosion von reinen Aluminium-Zink-Magnesium-Legierungen in 1 M Natriumchloridlösung

Investigation into the influence of the microstructure on the intercrystalline and grainphase corrosion of pure aluminium-zinc-magnesium alloys in an 1 M sodium chloride solution Potentiostatic polarisation tests of homogeneous pure aluminium-zinc-magnesium 2- and aluminium-zinc-magnesium 3-alloys as well as of a pure aluminium zinc-magnesium 3-alloy with two step ageing and quench-interruption treatments were carried out in an airsaturated 1 M sodium chloride solution at a temperature of 303 K. The pure aluminium-zinc-magnesium 3-alloy contents (weight percent): zinc 4,76; magnesium 3,00: impurities 0,08; balance aluminium, the pure aluminium-zinc-magnesium 2-alloy: zinc 4,60; magnesium 2,00; impurities 0,011; balance aluminium. The more negative breakdown potential in the homogeneous aluminium-zinc-magnesium-alloy than in the pure aluminium is caused by the zinc. According to the heattreatment the aged aluminium-zinc-magnesium 3-alloy shows above the breakdown potential grainphase corrosion and/or intercrystalline corrosion. In contrast to specimens with G.P.-zones or ν-precipitates in the matrix the specimens with ν′-matrix precipitates are grain-phase corroded along deep parallel streaks. This observation is attributed to the favoured formation of the ν′-precipitates on the (111)-planes in the matrix. The intercrystalline corrosion is more marked in specimens with a wider precipitate free zone, i.e. with a higher concentration of zinc and magnesium in the precipitate free zone.     

Über den beschleunigenden Einfluß von Schwefeldioxid und Wasser auf die atmosphärische Korrosion von verrosteten Eisen

The accelerating effect of sulphur dioxide and water on the atmospheric corrosion of rusty iron The atmospheric corrosion process of rusty steel was observed in the laboratory in an atmosphere with 1, 10 and 100 p.p.m. SO₂, respectively, at a temperature of 30°C. It was found that the correlation of the corrosion rate with humidity can, in the range between critical humidity and nearly 100 per cent. Relative humidity, be represented by a rising quadratic parabolic equation. The differences in the three SO₂ concentrations had no influence on the corrosion kinetics, which is explained by the fact that, under the testing conditions, the rust was fully saturated with SO₄^2?. On the strength of these and earlier results, the authors submit a new working theory concerning the atmospheric corrosion of already rusty steel, introducing the theories of Heusler and Florianovitsch-Kolotyrkin into the sphere of atmospheric corrosion.     

Einflußeiner Verzinkung auf die Korrosion von mit Zementmörtel beschichtetem Stahl in NaCl-Lösung

Effect of galvanizing on the corrosion of steel in concrete immersed in NaCl solution Galvanized or pickled steel sheet specimens were embedded in portland cement mortar of various water cement ratios and curing conditions and then wholly or partially immersed in 0.5 M NaCl solution for 1 to 5 yrs. Free corrosion potentials and electrical resistances have been measured. Immersion conditions and the presence of zinc have a significant effect on the corrosion resistance of the embedded steel sheets. The potentials of the wholly immersed specimens are very negative. Thus, these specimens cannot act as cathodes in corrosion cells, and the steel sheets within the mortar do not corrode. The partially immersed specimens, on the other hand, show very noble potentials. Also in the case of galvanized steel sheets the potentials are shifted to the same positive values in the course of exposure time. Thus, all these specimens can act as cathodes in corrosion cells. Localized corrosion generally occurs at the water/air line. In the case of pickled specimens the mortar is cracked due to growing corrosion products. In the case of galvanized steels the corrosion is retarded significantly. The test results are discussed in detail with respect to practical problems of cell formation, internal and external protection of pipes as well as the corrosion resistance of reinforced concrete.     

Einfluß von Phosphaten und/oder Silikaten auf die durch Kupferionen ausgelöste Lochkorrosion in verzinkten Warmwasserrohren

Influence of phosphates and/or silicates on copper ion induced pitting of galvanized hot water tubing In connection with studies on the copper-induced pitting of galvanized steel tubes the inhibition by phosphates and/or silicates should be investigated. The investigations were carried out in Berlin tap water, at temperatures of 65°C. Inhibitors used were o-phosphate, p-phosphate, combinations of o/p-phosphate and phosphate/silicate as well as two silicates. All these inhibitors reduced the general corrosion of galvanized steel and copper. Best results were obtained with p-phosphate. Protective layers on both metals were produced not before a time of 150–200 days. Number and depth of pits were positively influenced by nearly all the inhibitors used. But only the combination phosphate/silicate lead to a complete prevention from copper-induced pitting. O-phosphate showed the lowest efficiency. These investigations once again pointed to the importance of the phase boundaries air/water/tube or deposits/water/tube as nuclei for pitting in tubes. A second layer containing calcium and inhibitors was formed on the primary layer of zinc corrosion products. These secondary layers are responsible for corrosion protection by the inhibitors.     

Einfluß der Zusammensetzung von Messinglegierungen auf die Entzinkung und interkristalline Korrosion in Ammoniumchloridlösung

Influence of the composition of brass on dezincification and intercrystalline corrosion in ammonium chloride solutions The intercrystalline corrosion of As-containing α-brass is not restricted to a defined brass composition but is found also with brass types containing, in addition, AL and Sn. In the case of two-phase alloys of this type the β-phase is preferentially corroded, in Particular when the brass contains As and the β-phase forms a coherent network. On the basis of the results obtained a hypothesis has been derived as to mechanism the partial processes; this hypothesis is confirmed by result obtained with macroelements of As-containing and As-free brass. According to this hypothesis the intercrystalline corrosion of α- brass can be ascribed to the inhibition by As of the dezincification, so that no regions more anodic than grain boundaries are formed. This is why the boundaries are attacked preferentially.     

Der Einfluß von Bakterien auf die Eisenkorrosion in Brack- und Meerwasser

Influences of bacteria on iron corrosion in brackish and sea waters The corrosion of steel in seawaters is subject to seasonal variations; the corrosion maxma being in spring and autumn. The variations are not attribuable to changes in water temperature, salt concentration or oxygen content but are due to the co-action of biotic and abiotic factors, the former givin rise to temperature dependent variations of corrosion. The activity of bacteria becomes bovious only at temperatures above 10 °C; at these temperatures metablolism is activated and the products of matabolism may attack iron (or bac-     

Einfluß der Mikrostruktur und des Elektroden-potentials auf die Ausbreitungsgeschwindigkeit von Spannungskorrosionsrissen einer reinen AlZnMg3-Legierung in 1 M NaCl-Lösung

Influence of microstructure and electrode potential upon the velocity of stress corrosion cracks of a pure AlZnMg3-alloy in an 1 M NaCI-solution The influence of microstructure and electrode potential upon the stress corrosion crack velocity in a fully aged pure aluminium-zincmagnesium 3-alloy with two step ageing and quench interruption treatments was investigated in airsaturated 1 M sodium chloride solution at 303 K. For this, propagation of the crack tip was observed microphotographically under potentiostatic conditions and under constant load using specimens precracked by fatigue. The crack propagation at the corrosion potential is accelerated by denser matrix-precipitates, narrower precipitate free zone, the presence of great T-grain boundary precipitates and minor sensitivity to intercrystalline corrosion and occurs to judge from the look of the fracture surfaces without considerable metal dissolution. In contrast to the alloy, which shows no intercrystalline corrosion, the crack propagation in the alloy with intercrystalline corrosion is obviously faster above the breakdown potential than at the corrosion potential. The results suggest that the crack propagates essentially mechanically. As mechanisms are discussed a crack growth by anodic dissolution of grain boundary precipitates or of zinc- and magnesium-rich grain boundary regions and mechanical seperation of the intermediate regions and propagation by local hydrogen embrittlement of the crack tip.     

Einfluß einer Feuerverzinkung und PVC-Beschichtung von Bewehrungsstählen und von Inhibitoren auf die Korrosion von Stahl in gerissenem Beton

Influence of galvanizing and PVC-coating of reinforcing steels and of inhibitors on steel corrosion in cracked concrete Cracked concrete beams of concrete quality B15 and B35 with carbonized cracks were exposed in artificial seawater, under frost and deicing salt conditions and in industrial climate. The reinforcement was composed of black steel, galvanized and PVC-coated steel. For the purpose of additional protection the concrete was partly mixed with an inhibitor Ca(NO₂)₂. The concrete cover was 1,5 till 5,0 cm. It was found that the inhibitor protects satisfactorily only in the case of the higher concrete quality, cover 1,5 cm and not to high crack width. A protective effect of galvanizing is given by not to high chloride contents (< 1,5% relative to cement) and crack widths. The chosen PVC-coating failed because of chemical instability in the alcaline medium concrete.     

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.