Schwingungsrißkorrosionsuntersuchungen an ferritischen rostfreien Stählen

Investigation into corrosion fatigue of ferritic stainless steels The corrosion fatigue behaviour has been studied under cyclic flexural load in air and in concentrated NaCl solutions. The materials of construction used in the rest were the steels X 20 Cr 13 and X 20 CrMo 13, and the recently developed steels Inconel X 744 and PV 520 B which are used for turbine blades. The results show that the two last mentioned steels containing, in addition to 26 and 13% Cr respectively, 6 to 7% Ni are characterized by much higher corrosion fatigue resistance (10⁷ cycles). In view of the differences found in the results with respect to the ratio between unnotched and notched specimens, on the one hand in the group X 20 Cr 13 and X 20 CrMo 13, on the other hand in the group Inconel X 744 and PV 520 B the necessity becomes evident of using for corrosion fatigue tests notched specimens too. Nitrided surface layers may exert a considerable influence in the case of short test durations, particularly in the case of the steel X 20 CrMo 13. At test durations exceeding some 100 hours, however, the protective effect of the nitrided layers disappears.     

Einfluß von Beschichtungen auf das Schwingungsrißkorrosionsverhalten des Chromstahls X20Cr13

Influence of coatings on the corrosion fatigue behaviour of 13% chromium steel The influence of coatings on the corrosion fatigue behaviour of 13% chromium steel has been studied. There have been selected different coating systems: Barrier coating (enamel), diffusion coatings, (aluminizing, chromizing) and anodic coating, (aluminium, zinc, tin, cadmium). The corrosion fatigue limits of coated with uncoated specimens in neutral NaCl-solution are compared. Salt-concentrations were 0,01 and 22% (? 0,38 M) NaCl at 80°C and 150°C. The tests were carried out with alternating tensions and a constant frequency of 50 Hz. Only the use of andic coatings improved the corrosion fatigue behaviour of the chromium steel.     

Einfluß der Kerbgeometrie auf das Dauerschwing- und Schwingungsrißkorrosionsverhalten des Duplexstahles X 2 CrNiMoN 22 5 3

Influence of notch geometrie on the fatigue and corrosion fatigue behaviour of duplex stainless steel X 2 CrNiMoN 22 5 3 The fatigue behaviour of the ferritic-austenitic steel X 2 CrNi MoN 22 5 3 (German number of materials 1.4462) is investigated under cyclic tension load with a frequency of 25 Hz. Four different notch shapes with stress concentration factors varying from α_k = 1. 0 to 3.9 are researched. The experiments are carried out in laboratory air and in 3% NaCl-solution with pH = 7, both at room temperature. In air the fatigue limit of smooth specimens amounts to 490 N/mm². An increase of the stress concentration factor causes a reduction of the fatigue limit to 285 N/mm² for α_k = 3.9. The corrosion fatigue limits in 3% NaCl-solution for N = 10⁷ are 20% lower than the corresponding values in air. The calculated fatigue stress concentration factors for air and corrosive environments reveal nearly identical results. The recordings of the free corrosion potential show characteristic results. For all specimens, which do not fail under the mechanical-corrosive load, the potential remains in the passive state during the whole test duration. Higher mechanical loaded specimens announce their rupture by a sudden decrease of the potential to negative values. For equal fatigue life the remaining time until rupture of the specimen is considerable longer for higher stress concentration factors.     

Einfluß von Mo-Zulegierungen auf das SwRK-Verhalten eines ferritischen Cr-Stahls

Influence of molybdenum on the corrosion fatigue behaviour of ferritic Cr steels The moving blades in the low pressure area of steam-turbines are endangered by corrosion fatigue. For industrial turbines in this section preferably the steel X20 Cr 13 is used as blade material. An effective method to raise its resistance against corrosion fatigue is to alloy molybdenum to this steel. By the results of comparative tests under dynamic load in NaC1-solutions up to 200 °C the improvements of the resistance against corrosion fatigue are shown, which can be reached by alloying 1% and 2,5% molybdenum to the X20 Cr 13 steel. Moreover, by variation of the boundary conditions it is shown that the temperature is to regard as the main parameter for the behaviour of 13% Cr-steels under alternating stress and simultaneous corrosion attack. An explanation for these effects is given by the interpretation of microanalyses of the passive films with Auger-electrons and ESCA.     

Der Einfluß des Prüpotentials und der Wärmebehandlung auf das Korrosionsverhalten niob- und titanstabilisierter ferritischer Chromstähle mit rund 17% Chrom

The influence of test potential and heat treatment on the corrosion behaviour of ferritic chromium steels stabilized with niobium or titanium By means of corrosion-chemical, electrochemical, metallographical and electronmicroscopical investigations, the influence of the electrode potential and the heat treatment on the corrosion behaviour of steels of the X 8 CrTi 17 and X 8 CrNb 17 has been determined. With ferritic 17 pC chromium steels, a distinction must be made between two different types of grain boundary corrosion:

• (1) Grain boundary corrosion on steels which were quenched at high temperatures and therefore sensitized, unstabilized or understabilized, due to the segregation of chrome-rich carbides at the grain boundaries, causing a chromium reduction in their vicinity (typical inter-crystalline corrosion), and
• (2) Grain boundary corrosion on steels quenched at high temperatures but fully stabilized, due to the chemical dissolution of the type MX carbonitrides segregated at the grain boundaries during quenching (carbide corrosion).

The heat treatment conditions conducive to carbide corrosion were determined, and the correlation of this type of corrosion with the potential was ascertained by potentio-static tests in sulphuric acid and compared with the behaviour of synthetic carbides. The different corrosion behaviour of the ferritic chromium steels quenched at high temperature and stabilized with niobium and titanium, respectively, is attributed to the different chemical dissolution rates of the carbo-nitrides segregated. The findings also provide an explanation of the corrosion behaviour of sensitization-annealed, stabilized austenitic chrome-nickel steels in acid solution.     

Untersuchungen über den Einfluß des Zinks auf das Korrosionsverhalten von Spannstählen

Investigations on the influence of zinc on the corrosion behavior of high strength steels Corrosion and fracture behavior of hot dip galvanized high strength steels with and without damaging zinc layer was studied in deionized water, in tap water, and in saturated calcium hydroxide solution through which nitrogen, air, and carbon dioxide were bubbled separately. Hydrogen permeation measurements have been carried out for steel specimens which were cathodically polarized by means of the zinc layer. It has been observed that the hydrogen activity reached to a maximum of 1-2 in neutral solutions and 4-10 in saturated calcium hydroxide solution or in dilute acid solutions containing carbon dioxide, before decreasing to low values. The decrease of hydrogen activity is due to the formation of surface layers. Constant load tests by application of a load amounting to 100-110% yield strength caused no hydrogen induced brittle fracture within 300-400 h neither with nor without damaging zinc layer. At a constant strain rate of 5· 10^?7/s only 8 mm cold drawn steel wires were found to be almost unsusceptible to hydrogen that was absorbed during the test. Under notched conditions, all the steels investigated showed macroscopic brittle fracture.     

Untersuchung zur Schwingungsrißkorrosion an unlegierten und niedrig legierten Stählen

Investigation into corrosion fatigue of unalloyed and lowalloy steels Six commercial steels – three fine-grain normalized steels, one aged steel, one heat-resistant steel and one aged high-strength steel – were tested in rotary bending test machine equipped with a corrosion test cabinet. The test medium used was 0,05 m NaCl-solution. The results show that the cycling bending strength of all the steels tested is considerably reduced in this weak corrosion medium. The influence of corrosion on mechanical strength is the greater the higher the strength of the steel. The impact of other testing parameters is almost negligible. Cathodic polarization gives rise to increased corrosion fatigue strength of high strength steels and reduces corrosion fatigue sensitivity. In the case of lower strength steels complete cathodic protection can be achieved. The cyclic bending strength of the steels is reduced under condensing humidity conditions, too.     

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.     

Über den Einfluß der Austenitstabilität von 18/8-Chrom-Nickel-Stählen auf die Verformungseigenschaften und auf das Korrosionsverhalten dieser Stähle

Influence of austenite stability of 18-8 Cr-Ni-steels on the cold working and corrosion properties of these steels The martensite formed during cold working has a negative effect on uniform corrosion only when the specimen is active in the particular medium; if so, corrosion current density increases with the degree of cold working. The martensite generated by cold working reduces on the other hand the susceptibility to intercrystalline corrosion and – in amounts up tn 2% – increases the time to failure in stress corrosion cracking (15 and 45 kp/mm²). The pitting potential is not shifted by the martensite, but pit density increases with the martensite content. In the Kesternich test no negative effect of the martensite is found. It is therefore concluded that reducing the Ni content in 18-8 steels improves workability without having a pronounced deleterions bearing on corrosion behaviour.     

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.     

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.     

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.     

Beobachtungen über den Einfluß kleiner Zusätze von Cermischmetall auf das Zunderverhalten von Stählen

Observations concerning the influence of small additions of cerium mixed metal on the scaling of steels The addition of appr. 0.1 % cerium mixed metal to steel X 10 CrNi 18 10 has practically no influence on the isothermal oxidation of the steel in air at 800 and 900°C, but improves the adhesion of the scale under temperature cycling conditions. The positive effect of cerium becomes apparent only after extended annealing or after several temperature cycles respectively. In the case of steel 10CrMo9 10 the addition of cerium mixed metal in quantities up to 0.3% has practically no effect. With cerium contents of 0.75%, however, annealing in air at temperatures around 600 °C results in a scale showing excellent adhesion and resisting even quenching in water without spalling. Cerium contents of this level, however, give rise to changes of the mechanical and technological properties of the steel so that their practical use should be out of the question.     

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.     

Der Einfluß der Umgebung auf die Rißzähigkeit und Rißausbreitung bei höherfesten Stählen

Influence of the environment on the cracking toughness and crack propagation in high-strength steels The crack toughness of four fine grained steels and one high strenght steel of different heat-treatments are investigated in 3% NaCl solution depending from the electrochemical polarisation. The steels are of commerical compositions, their strengths are in the range of 635 to 1460 N/mm². The stress intensity factors, K_ISCC for slow crack growth are determined by a method of fracture mechanics. For these investigations the DCB-specimen with an incipient crack is used. The K_ISCC-values generally decrease with increasing strength of the steels, but they show different susceptibility for crack growth under cathodic and anodic polarisation. Outdoor experiments conducted in seawater covering a period of one year confirm the results of laboratory.     

Einfluß von bituminösen Dachbelagsmaterialien auf das Korrosionsverhalten von Dachentwässerungssystemen aus Zink und verzinktem Stahl

Influence of bitumineous roof coverings on corrosion behaviour of roof draining systems made of zinc and galvanized steel In long time weathering and laboratory tests on a number of different bitumineous roof coating materials it was confirmed that they are the cause of damage on zinc roof draining systems. The acidic corrosion capacity rain_water is very increased by the formation of acids in bitumineous materials. The formation of acids is almost independent of the different types of bitumineous base materials, but can be reduced by adequate gravel or state covering. But only alkaline mineral coverings, found in a few cases, give sufficient protection against corrosion of zinc. Further important influence which cause corrosion damages is the relation of bitumineous roof area to zinc area and the particular conditions of water draining.     

Der Einfluß martensitischer Gefügeausbildung in Cr?Ni-Stählen auf das Lochfraßverhalten

Effect of martensitic structures in Cr? Ni-steels on pitting corrosion The paper describes the influence of martensitic microstructures, which were produced in austenitic CrNi steels by directed kryoforming at ?196°C, on pitting corrosion. The investigations were carried out with a series of alloyed steels with a basis content of 17% chromium and nickel contents between 8% and 20%. The microstructures were characterized by means of radiographic, magnetic, metallografic, mechanical and chrono-potentiostatic techniques which revealed correlations between martensite transformation and properties. The comparison of measurement results obtained for the strain hardening, flow and corrosion behaviour has shown similarities in the change of hardening characteristics and pitting potentials in dependence on the degree of deformation. The pitting behaviour is improved if α′-martensite is primarily formed by a sufficient degree of plastic deformation. In contrast, microstructures with ?-martensite and derived portions of α′-martensite display a less favorable pitting behaviour.     

Der Einfluß von pH-Wert und Sauerstoffgehalt von Pufferlösungen auf das Korrosionsverhalten metallischer Werkstoffe

Influence of pH and oxygen content of buffer solutions on the corrosion behaviour of metallic materials The application of solutions to the decontamination of materials in nuclear installations is based on the condition that their corrosion behaviour is clearly understood. Since electrochemical corrosion is due to cathodic and anodic partial reactions which are influenced in different ways by the pH of hte solution and the oxygen content it is suggested that the results of electrochemical experiments with buffer solutions be used as a model for predicting the corrosion behaviour of materials in other solutions. In the tests described here potentiocinetic current-potential-curves have been traced and galvanic corrosion tests have been made. The results obtained in ascorbic acid, postassium hydrogen phthalate, ammonium citrate and acetate, sodium and potassium tartrate, ammonium hydrogen phosphate, sodium carbonate, hexamethylene tetramin, ethylene diamine enable – on the basis of summarized current-potential-curves – the metals studied to be classified in four groups characterized by clear differences concerning the influence of pH on the corrosino behaviour.     

Einfluß einer Wärmenachbehandlung auf die mechanischen Eigenschaften und das Korrosionsverhalten nichtrostender Stähle

Influence of an ultimate heat treatment of the mechanical properties and the corrosion behaviour of stainless steels Unfavorable structures as may arise during steel processing and may impair their behaviour can be eliminated by an ultimate heat treatment. Cold hardening can be removed by heating to about 1000°C for about 1 min; dissolution of chromium carbide precipitated requires 3 min at 1030°C and dissolution of sigma phase about 10 min at 1050°C. The treatment, however, will not be successful, unless the temperatures mentioned are achieved throughout the whole workpiece and a sufficiently high colling rate is maintained after the treatment.     

Zum Einfluß der Chloridionenkonzentration auf HCl-gebeizten Stahlfeinblechen auf deren Korrosionsverhalten

The influence of the chloride ion concentration on the corrosion behaviour of steel plate pickled in hydrochloric acid The residual chlorides remaining on steel plate after pickling in hydrochloric acid accelerate the formation of rust on unprotected as well as on primed material; the effect is the more pronounced the higher the chloride concentration. 10 mg/m² of chloride ion appear to be a certain limiting concentration above which uniform corrosion is observed over the whole area. The surface concentration of chlorides was determined with the aid of a two-stage desorption method; the results obtained in this work enable rinsing conditions to be established which make sure that the limiting concentration is not exceeded.