Korrosionsverhalten hochchromhaltiger,ferritischer, chemisch beständiger Stähle

Corrosion behaviour of high chromium ferritic stainless steels Ferritic steels developed for seawater desalination and containing 20 to 28% chromium, up to 5% Mo and additions of nickel and copper have been tested with respect to their corrosion behaviour, in particular in chloride containing media. The materials in the sensibilized state were tested for inter-crystalline corrosion susceptibility in the Strauß-, Streicher-, nitric acid hydrofluoric acid- and Huey-Tests. No intercrystalline corrosion was encountered in the case of the steels with 28% Cr and 2% Mo. The resistance to pitting was assessed on the basis of rupture potentials determined by potentiokinetic tests. The resistance of the steels with 20% Cr and 5% Mo or 28% Cr and 2% Mo is superior to that of the molybdenum containing austenitic types. Addition of nickel yields a significant increase in crevice corrosion resistance; the same applies to resistance in sulfuric acid. In boiling seawater all the materials tested are resistant to stress corrosion cracking. No sign of any type of corrosion was found on nickel containing steels after about 6000 hours exposure to boiling 50% seawater brine even under salt deposits.     

Elektrochemische Methode zur Untersuchung der Anfälligkeit korrosionsbeständiger Stähle für interkristalline Korrosion

Electrochemical method to study the intercrystalline corrosion susceptibility of stainless steels In continuation of earlier work concerning a method suggested for the electrochemical evaluation of the intercrystalline corrosion susceptibility of corrosion resistant steels a potentiodynamic polarization method has been worked out which is based on the principle of the reactivation from the passive state. This method has been tested on austenitic chromium nickel and chromium nickel molybdenum steels with different carbon contents. In order to check and modify the sensitivity of the new method parallel tests of intercrystalline corrosion susceptibility have been carried out in a standard solution containing copper chips. The susceptibility to intercrystalline corrosion of the individual steels has been evaluated on the basis of the intercrystalline corrosion regions in function of temperature and duration of sensitization (so called Rollason curves). The ratio of the reactivation charge to the charge in the active range of the polarization curve (at increasing potential), expressed in per cent, appears to be an optimum quantitative criterion to asses corrosion susceptibility. On the basis of a comparison with the results obtained in the standard solution susceptibility to intercrystalline corrosion must be accounted for starting from a definite value of this ratio (in the case of chromium nickel steels e. g. at values above 18%). The sensitivity of the method may be modified for the individual types of steels either by modification of the test conditions, in particular temperature and potential change rates, or by the selection of the electrolyte or of its chemical composition.     

Korrosion in der Wärmeeinflußzone geschweißter chemisch beständiger Stähle und Legierungen und ihre Verhütung

Corrosion in the heat-affected zone of welds in chemically resistant steels and alloys, and respective preventive measures As to weldability, chemically resistant steels and alloys can be classified as follows: (1) weldable without any restriction; (2) weldable only with reduced wall thickness; (3) weldable only with ultimate thermal treatment. These restrictions are due to the precipitation of chromium carbide and intermetallic phases at the grain boundaries; this effect gives rise to a Cr and Mo depletion and, finally, to intercrystalline corrosion susceptibility. In view of the fact that weldability requires the delay of the precipitation of chromium carbides at 650°C for at least 1 hour, and of intermetallic phases at 900°C for at least 10 min it is possible to reduce the carbon content and/or to add stabilizing elements (Ti, Nb). It should be taken into account, however, that the precipitation behaviour is not a function of carbon concentration, but rather of carbon activity which, again, depends from the overall composition of the respective alloy. This activity is increased by Ni and Si, while Mn and N function as decelerators and, consequently, contribute to weldability. In the case of the steel X 3 CrNiMoN 17 13 5 the addition of N inhibits not only the carbide precipitation but also the precipitation of the Chi-phase (at 950°C). In the case of the alloy NiMo 16 Cr the gradual reduction of the contents of secondary constituents has resulted in a practically pure ternary system characterized by high precipitation resistance: carbides are precipitated only after 5 hours at 800°C.     

Widerstandsfähigkeit hitzebeständiger Stähle gegen Aufkohlung

Cementation resistance of heat-resistant steels The cementation process in atmosphere used for the cementation of unalloyed and low-alloyed steels has been examined in 79 melts of Cr and CrNi steels. A characteristic feature was the simultaneous cementation and oxidation of the steels, with characteristic differences depending on the steel composition. These differences can be explained by the different composition and structure of the oxide layers which appear at the outset and may, in certain circumstances, inhibit the carbon diffusion into the interior of the metal. Sometimes, however, an inner oxidation may also occur below a cemented zone. In principle, elements capable of improving the heat resistance — e.g. Ni, Si, Cr, Al — have a favourable effect whilst Mn has a markedly unfavourable effect. The favourable effect of Ti, recently observed, is probably connected with the grain-refining effect.     

Elektrochemische Untersuchung der Lochkorrosion austenitischer CrNi-Stähle mit rd. 0 bis 4 Gew.-% Mo unter besonderer Berücksichtigung der Meßverfahren und der Meßschaltung

Electrochemical study of the pitting corrosion of austenitic Cr Ni steels containing from about 0 to 4% Mo, with special regard to the measuring arrangement and method The authors have pointed out that the pitting potentials are no constants of the particular system, depending only on the material and the corrosion medium; it is always necessary to take account of the potential influences emanating from the measuring system and the particular experimental method. With respect to comparative tests the potentiostatic determination of U_b and the galvanostatic determination of U_p are particularly useful. In the case of short time tests the galvanostatically determined value of U_p yields a well defined limiting potential below which pitting occurs. This value, however, does not account for varying properties of Mo containing steels; in this case it is necessary to make potentiostatic experiments yielding inequivocal values of U_b which cannot be obtained reliably when potentiodynamic tests are carried out.     

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.     

Legierungstechnische Möglichkeiten zur Erhöhung der Korrosionsbeständigkeit Verschleißbeständiger Duplex-Stähle

Alloying variants increasing the corrosion resistance of wear-resistant duplex stainless steels Due to their good corrosion properties, duplex stainless steels are used in many fields of chemical industry as well as power stations, marine anti environmental engineering. Precipitation-hardened variants and alloys with slightly increased carbon content are available for handling media causing simultaneous corrosion and wear. This improvement in wear properties, on the other hand, reduces their corrosion resistance and thus limits their actual fields of applications. Also, the wear resistance achieved by these steels is far inferior to that of white iron, which, however, can only be used in water with close-to-neutral pH-value. So far, handling chemically aggressive media either entailed short service lives or required expensive special materials. Many development activities are therefore aimed at improving the combination of tribological and electrochemical properties of duplex stainless steels. Different chemical compositions and types of hard phases result in varying corrosion and wear properties which are described on the basis of a test melt and a new-developed alloy.     

Herstellung und Eigenschaften stickstofflegierter,korrosionsbeständiger Stähle und Sonderstähle mit niedrigen Kohlenstoffgehalten

Production and properties of nitrogen alloyed, corrosion resistant steels and special steels with low carbon contents Alloying with nitrogen has favourable influence in particular on the mechanical properties of CrNiMo steels (X 2 CrNiMoN 17 12, materials No. 1.4406, X 2 CrNiMoN 17 13 5, materials No. 1.4439 und X 2 CrNiMoN 22 5, materials No. W.-Nr. 1.4462). This comes to bear when ambient temperature and low temperature strength and toughness are concerned. With respect to the corrosion behaviour the data concerning the effect of nitrogen are contradictory. It has become clear that nitrogen improves pitting corrosion resistance; this applies, however, only to pit initiation but not to pit growth. Stress corrosion cracking is not delayed by nitrogen but different results have been obtained with different media: while the duplex steel X 2 CrNiMoN 22 5 is attacked considerably faster than the corresponding nitrogen-free steel in 42% boiling magnesium chloride solution the time-to-failure of both steels are comparable in 30% boiling MgCl₂-solution. The nitrogen alloyed steels can be welded by all known welding procedures, provided fully austenitic welding rods are used.     

Die Bedeutung der metastabilen Lochkorrosion bei hochlegierten Stählen

The importance of metastable pitting corrosion in the case of high alloy steels A suitable computer-aided experimental method allows during potentiostatic tests to recognize and quantitatively treat current transients. The current transients result from metastable pitting phenomena below the pit propagation potential. It is possible under certain conditions to detect metastable pitting which in the SEM appears in the form of small (approximately 1 μm), in most cases hemispherical pits. A detailed study of metastable pitting has brought about fundamental knowledge about the mechanism of pit initiation, stable pit growth and repassivation in pitting and crevice corrosion processes.     

Untersuchung der Korrosion hochlegierter Stähle in praktisch wichtigen Halogenkohlenwasserstoffen mittels einer Radionuklidmethode

Investigation into the corrosion of high alloy steels in industrially important halogenated hydrocarbons, using a radionuclide method The corrosion behavior of some steels (1.4021, 1.4439, 1.4462, 1.4571) was tested in 1.1.1-C₂H₃Cl₃ and C₂HCl₃. There is no weight loss to be observed in dry and water saturated halogenated hydrocarbons. In the two-phase systems with water the hydrolysis product HCl attacks the steels. Stabilizers against hydrolysis in the halogenated hydrocarbons effect no inhibition of corrosion. In stabilized 1.1.1-C₂H₃Cl₃ first corrosion attack is found after incubation times, after which the stabilizer is used up. If stabilizers are removed partially incubation times decrease. The stabilizers in C₂HCl₃ are more stable than in 1.1.1-C₂H₃Cl₃ but they are easier to be removed. Refined C₂HCl₃ shows immediate weight loss in the water phase and in the gas phase. UV-irradiation increases the corrosiveness of both two-phase-systems. The double bond of C₂HCl₃ is violently attacked with result of rapid decomposition and high corrosion rates. Stress corrosion cracking was not to be found on steels 1.4439 and 1.4571 in both halogenated hydrocarbons.     

Lochkorrosion an Nichtrostenden Stählen

Pitting corrosion of stainless steels Stainless steels can get pitting corrosion in halide containing solution, which make them a big risk in industrial production. Many investigations were made in the past in order to understand processes involved in pitting corrosion, pit initiation and pit growth. Results about the influence of alloying elements, their contents, the state of the structure, the condition of the surface, the content of chloride, the temperatures, the pH-value, the velocity of flow and of the oxidizer on the chloride induced pitting corrosion of passive stainless steels are presented. Electrochemical measurements and the application of surface analytical methods (SEM, SAM, XPS) with high lateral resolution are carried out. A part of the samples received a diffusion annealing in order to obtain reproducible results. Pitting Resistance Equivalents (PRE) – Pitting Index – with different multipliers are given and discussed critical. An electrochemical method for selecting materials without susceptibility to pitting corrosion are also presented.     

Untersuchung über die Schwingungsrißkorrosion rost- und säurebeständiger Stähle in Schwefelsäure hoher Konzentration

Investigation into corrosion fatigue of stainless steels in high concentrated sulfuric acid Four commercial stainless steels (mat. Nos. 1.4539, 1.4586, 1.4571 and 1.4462) were tested in 96% and 90% sulfuric acid of 80 °C as to their corrosion fatigue strength by subjecting notched round bars to the rotary bending test. At the same time, the efficiency of anodic protection was examined. With all four steels, the alternating bending strength obtained in the air is essentially reduced. The semiaustenitic steel Nr. 1.4462 shows a lower corrosion fatigue strength than the three austenitic steels. Although making the corrosion fatigue strength of steels better calculable, anodic protection proves insufficient to completely prevent notched round bars from corrosion fatigue in concentrated sulfuric acid.     

Korrosionsuntersuchungen an chemisch beständigen Stählen mit Hilfe von stationären Reaktivierungsmessungen (Umschaltversuchen)

Corrosion investigations on chemical resistant steels by means of steady stade electrochemical reactivation tests (change over tests) A method to quantify the degree of sensitization on selective corrosion behaviour is the electrochemical potentiokinetic reactivation method (EPR‐Method). Due to the potentiokinetic character, however, the application of the EPR‐testing method leads to some difficulties. The corrosion is influenced by kinetic effects and electrolytic secondary reactions and shows a huge dependence on time. Therefore a steady state electrochemical reactivation method to quantify the degree of sensitization on selective corrosion behaviour would lead to a reduction of the mentioned problems and the interpretation, comparability and reproducibility of the achieved results would be more simple and clear. This paper presents the first measurements and results which point to a suitable and promising steady state electrochemical testing method.     

Widerstand der Schweißverbindungen kohlenstoffarmer und mit Molybdän legierter korrosionsbeständiger Stähle gegen interkristalline und Messerlinienkorrosion

Resistance to intercrystalline and knifeline corrosion of welds in low carbon stainless steels containing molybdenum Welds in steels containing (%) 0,019–0,080 C, 1,00–1,56 Mn, 0,03–0,92 Si, 0–0,026 P, 0–0,018 S, 10–13,60 Ni, 16–20,50 Cr, 0–3 Mo, 0–0,057 Ti, 0–0,87 Nb, prepared by electroslag and automatic submerged arc welding are as a general rule not susceptible to intercrystalline and knifeline corrosion; a certain susceptibility encountered with the Mo containing types can be largely eliminated by sensibilizing at 650 °C. Differences in the corrosion behaviour between base metal and weld seam could not be encountered; in some cases the weld metal turned out to be even more resistant. The test solutions used were: sulfuric acid + Cu sulfate + Cu, boiling, 48 hrs, and 65% nitric acid, boiling, 3 · 48 hrs.     

Korrosionsverhalten nitrocarburierter Stähle

Corrosion behaviour of nitrocarburized steels Diffusion coatings have been used increasingly to modify surface properties of various machine components for several applications. In general, these coatings are used to improve the wear behaviour. Frequently, it is desirable that the corrosion behaviour will be improved at the same time. In this investigation, the corrosion behaviour of various diffusion coating-substrate-combinations has been studied. The coatings developed by three different nitrocarburizing processes, namely saltbath, gas and plasma nitrocarburizing, were conducted on five various steel substrates. These substrates were St 52-3, Ck 45, 42 CrMo 4, 30 CrNiMo 8 and × 20 Cr 13. The thickness of the compound layers and their porosity were measured using optical microscopy. The structure of the compound layers was characterized using an X-ray diffractometer and their surface roughness by a stylus profilometer. The corrosion test was carried out using a salt-water spray test. The predominant corrosion mechanisms have been evaluated using scanning electron microscopy. It was found that the employed nitrocarburizing processes have improved the corrosion resistance of all tested coating-substrate-combinations comparing with that of the base materials. In general, the corrosion resistance increases with an increase of the compound layer's thickness and after an oxidation process. The best coating-substrate-combinations to improve the corrosion behaviour were salt-bath nitrocarburized tempering steels, whereas the thin layers of the plasma nitrocarburized specimens were the worst.     

Neue Erkenntnisse auf dem Gebiet der ferritischen rostfreien Stähle

Recent developments in ferritic stainless steels The pitting resistance of ferritic stainless steels in HCl is visibly improved by Mo, in particular in the case of vacuum-melted material. In this context the ratio Cr:Mo = 25:2 is superior ta Cr:Mo = 17:3; addition of Mo prevents, beyond that, crevice corrosion. Ti increases resistance in the Strauß test but not in the Huey test, while Nb turns out to have a positive effect in either test. Steels containing Cr: Mo = 17:l are certainly still susceptible to pitting, but no longer to stress corrosion cracking in boiling MgCl₂, solution; stress corrosion cracking is not observed in 55% boiling Ca(NO₃)₂, and 25% boiling NaOH, but after annealing at 980 °C intercrystalline corrosion takes place. The test duration required for establishing cracking susceptibility is considerably shorter with ferritic than with austenitic steels (100 and 1000 to 2000 hours respectively).     

Elektronenmikroskopische Untersuchung der Carbidausscheidungen in korrosionsbeständigen Stählen

Electronmicroscope study of carbide precipitation in stainless steels Extraktion replicas and thin metal foils can be used for the preparation of specimes for electron microscope investigation. Extraktion replicas enable the size of precipitated carbide particles and their distribution in the metal structure to be established. With the thin metal foils the precipitation mechanism and orientation relations between precipitate and base material can be studied. The experimental techniques are explained for two applications. In this connection the relation has been confirmed between chromium carbide precipitation at grain boundary and intercristalline corrosion of austenitic CrNi steel. It has been found that it is not carbide morphology but the continuity of the carbide network which plays the predominate role. In addition, direct evidence has been obtained of the fact that the knife-line corrosion found in the vicinity of weldseams is mainly due to Ti carbides occuring in Ti-stabilised steels.     

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.     

Gegen die Spannungsrißkorrosion beständige nichtrostende Manganchromstähle

Stress-corrosion resistant stainless manganese chromium steels The following conclusions may be drawn from the results of investigations into the stress corrosion cracking of austenitic and austeno-ferritic MnCr steels (19–22Mn, 13–18Cr, additions of Mo, V, Nb, Ti, N, B): Addition of nitrogen gives rise to a decrease of stress corrosion cracking resistance in magnesium chloride, sodium chloride with potassium dichromate and water at high temperatures. The same applies to the influence if nickel on corrosion in magnesium chloride and water, and for molybdenum in magnesium and sodium chlorides. From among laboratory melts the type 05 Mn 19Cr 13 had the highest resistance, followed by its modifications with additions of boron, vanadium, molybdenum, titanium, niobium and nitrogen. From among the semi-technical melts the nitrogen containing steels turned out to be least resistant, too. During further investigations the chromium level of 13% turned out to be insufficient to prevent pitting in sodium chloride solutions including seawater.