Das Verhalten von Großbaustählen in Meerwasser

Behaviour of construction steels in seawater The chemical composition of steel has a considerable bearing on their corrosion in seawater. Systematic laboratory studies have shown that addition of Si, Cr and Cu, but also Al and P yields considerable improvements of the sea water resistance of construction steels. Long term tests in natural seawater (in harbours, bays and straits) close to the surface and at depth up to 90 m have revealed that the seawater resistance of steels containing about 1% Si, 3% Cr and 1% Cu may be 3 to 6 times (depending on exposure conditions) the resistance of unalloyed construction steel. This behaviour may be attributed to the fact that by forming sulfates Si and Cr inhibit the growth of bacteria which would enhance corrosion, and to the fact that Si, Cr and Cu give rise to the formation of a dense rust layer which slows down the exchange phenomena between seawater and corrosion products. The use of such steels may constitute an alternative to the use of unalloyed steels with an appropriate corrosion protection system.     

Allgemeine Beständigkeit von Schweißnähten unter Berücksichtigung der Spannungs- und Schwingungsrißkorrosion

General resistance of weld seams with a view to stress corrosion cracking and corrosion fatigue The corrosion of welds is due to thermal effects during welding which give rise to structural changes and, frequently, compositional changes in the transition zone. The welded material is rapidly cooled and may thus be heterogeneous and may present residual stresses resulting in increased susceptibility to selective and stress corrosion. The manganese content is of high importance in low alloy steels, as well as residual martensite or austenite embedded in a ferrite matrix. Low ferrite contents are generally beneficial because they counteract high temperature cracking; however, ferrite contents should be hept below 10% in order to prevent the formation of a continuous network giving rise to selective corrosion. Corrosion susceptibility may also be produced by carbide or carbonitride precipitation in austenitic and ferritic steels and nickel base alloys. Weld zones in aluminium alloys are attacked in rare cases (e.g. by HNO₃) and the susceptibility of Ta, Zr and Ti depends from the properties of the protective atmospheres.     

Schadensfälle bei osteosynthetischen Implantaten und ihre Ursachen

Case histories with osteosynthetic implants and their causes Osteosynthetic implants are made of CrNiMo steels (predominantly in Europe) and–overseas–also of CoCr alloys. Following reports on considerable numbers of failures it is pointed out that there is an absolute necessity for a binding regulation concerning the selection of materials for osteosynthetic implants; some characteristic cases are described in this connection. The alloyed steels are on principle absolutely resistant to body fluids; due to low local oxygen levels or to inhomogeneous distribution of alloy constituent, however, there may be element formation with subsequent local corrosion. Likewise it is found that incompatible metals are combined so that contact corrosion occurs. NaCl solutions which are used for cleaning areas of operation may give rise to pitting corrosion while steels with elevated carbon contents are frequently destroyed by intercristalline corrosion. Stress corrosion cracking on the other hand appears to be improbable; failures attributed to this type of corrosion are most frequently fatigue failures. On the basis of perience gained so far it is recommended that CrNiMo steels containing less than 0,03O% C and more than 2,1% Ma be used exclusively for this type of application.     

Spannungsrißkorrosion von hochlegierten Manganstählen in wäßrigen Chloridlösungen

Stress corrosion cracking of high alloy manganese steels in aqueous chlorides In tensile tests made without applied current in aerated solutions a stabilization of the austenitic structure by increasing Mn and N contents yields increased times to failure. The potential-time-to-failure curves determined by potentiostatic tensile tests reveal a compley joint action of constitution and passivation behaviour of the steels. The intercrystalline stress corrosion cracking of the steel X 40 MnCr 19 with chromium carbide precipitations at the grain boundaries can be attributed to an electrochemical differentiation of the chromium-depleted grain boundary region. Steels of this type are characterized by a pronounced sensitivity to intercrystalline stress corrosion cracking the precipitation annealed state, and by a certain sensitivity to transcrystalline corrosion cracking after solution annealing. Low carbon Mn steels containing up to 4 % Cr are susceptible to transcrystalline stress corrosion cracking irrespective of the heat treatment. As to the temperature dependence of times-to-failure, constitution and layer formation have different effects. Increasing the Cr content to 8 % gives rise to a transition from stress corrosion cracking to pitting type corrosion. In terms of electron optics, an increased chromium content gives rise to a changed dislocation pattern, so that there may be an effect of the type of gliding processes on stress corrosion, The increased stress corrosion resistance of MnCr steels containing at least 8% Cr may be due to the lower height Of the gliding step and to an increasing tendency to repassivation of damaged surface layers.     

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.     

Nachweis der Korrosionsbeständigkeit von Lagerbehälterwerkstoffen gegenüber wassergefährdenden brennbaren und nichtbrennbaren Lagermedien durch die Bewertung von Korrosionsdaten

Proof of the corrosion resistance of tank materials with regard to water-polluting inflammable and non-inflammable storage liquids by evaluating corrosion data The general protection requirements and arrangements determine in regards to the anxiety principles of the water conservation law a respective construction and observation of the storage tanks with the storage of water pollution liquids. The proof of the corrosion resistance of the tank materials in relation to the long-term influence of the storage liquids belongs to the general protective arrangements. Suppositions for the proof of the corrosion resistance are the definition of the used materials, welding seams and liquids. Liquid-materials-combinations are evaluated as qualified when the decrease of the wall-thickness caused by uniform-corrosion does not exceed 0,1 mm per year and local corrosion in form of pitting corrosion or stress corrosion is not expected. In the DIN 6601 which includes the socalled positive-liquid list are evaluated water-polluting liquids in regards to their material resistance with carbon steels and austenitic stainless steels at a temperature of max. 30°C on the base of relevant literature, user experience of producers or users over a time of min. five years and laboratory tests. The positive testify is only valid for a lot of liquids if special conditions related to liquids or related to the working conditions are fulfilled. For liquid-materials-combinations not mentioned in the positive-liquid list is the proof of the corrosion resistance through separate expert opinion necessary.     

Austenitische gegen Spannungsrißkorrosion in konzentrierten Chloridlösungen beständige Manganchromstähle

Austenitic manganese chromium steels resistant to stress corrosion cracking in concentrated chloride solutions The testing of stress corrosion susceptibility in MgCl₂ and CaCl₂ shows that the classical Austenitic CrNi and CrMnNiN steels are not sufficiently resistant to this type of corrosion. On the other hand MnCr steels exhibit good resistance even in hot solutions. The resistance, however, is considerably deteriorated by addition of Ni even in small quantities; with Ni contents up to 0.5%, however, the time to failure is still considerable. This fact is important since such nickel contents must be counted with because of production conditions in metallurgy. On the basis of experiments including investigation into mechanical properties, microstructure, metallic phases present and stress corrosion resistance (under constant tensile load) in MgCl₂ solution (35%, 115–120 °C) the authors selected out of 62 types of steels tested four experimental austenitic MnCr steels meeting the following complex of requirements: high resistance in MgCl₂ solution, sufficiently high resistance to intercrystalline corrosion, certain resistance to pitting and practically monophasic (austenitic) structure.     

Optimaler Nickelgehalt von stickstofflegierten austenitischen Stählen

Optimum nickel content of nitrogen alloyed austenitic steels The authors have studied the effect of a reduction of the nickel content on certain properties which are important with a view to practical applications. The mechanical properties are but little affected by varying nickel contents. As to corrosion resistance, no noticeable influence has been found on pitting, crevice corrosion and acid resistance; the resistance to stress corrosion cracking is a little reduced, but these steels are not considered for such type of service. On the base of the results obtained the nickel content in certain standardized Swedish steels has been reduced: consequently, the steels SIS 2370, 2371, 2374 and 2375 contain – instead of originally 8, 9, 10,5 and 11% – now 7, 8, 9 and 9.5% nickel respectively.     

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.     

Zur Spannungsrißkorrosion austenitischer Chrom-Nickel-Stähle

Contribution to the stress corrosion cracking of austenitic chromium nickel steels The authors present a summarizing treatment of the transcrystalline stress corrosion cracking of chromium nickel steels in chloride solutions. The factors having a bearing on the susceptibility to this type of corrosion during operation, temperature and p_H of the medium. Residual stresses may be due to heat-input (during welding) or to grinding which, in addition, may give rise to notch effects. The medium generally used to test stress corrosion susceptibility (MgCl₂ solution) acts according to different mecanisms, depending on its concentration which, in term, has an influence on p_H and the boiling temperature. Quite generally it may be said that stress corrosion cracking always presupposes the existence of a lower of stresses, the amount of these stresses depending from the structural factors and from the surface condition.     

Selektive Korrosion an Verbindungsschweißen von Stählen

Selective corrosion at welded assemblies of steels The paper deals with the possible selective corrosion phenomena at the most important types of weld assemblies of steels, viz. those of unalloyed ferritic ship building steels, austenitic manganesechromium steals, aluminium-killed boiler steels an well as chemically resistant ferritic and austenitic chrom-nickel steels. With unalloyed ship building steels, a strong corrosion element is liable to occur between not fully descaled plates and scaleless welding material, where the latter is particularly liable to be attacked. But even where scalefree plates are welded, a local element may be set up, depending on the killing intensity and on the filler metals, between plate and welding material in such a way that, again, the welding material is liable to be dissolved first. In the case of austenitic manganese-chromium steels, exposed to sea water, intercrystalline corrosion takes place at a certain distance from the weld as a result of chrome-carbide segregation. In the case of welded assemblies of aluminium-killed steels, intercrystalline fissuration is particularly liable to occure in a more or less marked decarbonised zone at the transition. With ferritic chromium steals and austenitic chrom-nickel steels, the welds are decisively influenced by the segregation of chrome-carbides at the grain boundaries which results in a proneness to intercrystalline grain decomposition. Measures are discussed by which the segregation of chrom-carbides during welding can be avoided. Finally, mention is made of the effect of delta-ferrite on the corrosion behaviour of austenitic welding material.     

Korrosion nichtrostender austenitischer Stähle in (kondensierender) chloridhaltiger Salpetersäure

Corrosion of austenitic stainless steels in (condensing) nitric acid containing chlorides The corrosion parameters involved in the behaviour of austenitic stainless steels and nickelbase alloys in chloride containing nitric acid are being stated. Investigation of the corrosion resistance of the austenitic stainless steel 1.4306, ESR grade for application in nitric acid, in nitric acid condensates containing small amounts of chlorides. The condensates were formed from boiling nitric acids of molarities 6 to 10 containing 15 to 45 mg chlorides/1. The calculated corrosion rates of < 1 μm/y can be considered rather small. Especially in cases of insufficient wetting of the heat exchanger surfaces, however, local roughening and pitting corrosion is to be expected, mostly under rust-colored, chloride-containing deposits. The intensity of pitting corrosion increases with increasing chloride contents of the nitric acid solutions that evaporate. The sealing surfaces consisting of steel 1.4306 and PTFE did not exhibit any crevice corrosion. Nitric acid grade ESR-1.4306 definitely does not suffice requirements as described in this paper. It is suggested to use stainless steels which exhibit an improved resistance against chlorides and, at the same time, a high resistance against nitric acid attack. The following steel grades may be considered: X 1 CrNi 25 21 (Mat. No. 1.4335), X 2 CrNiMoN 25 22 2 (Mat. No. 1.4466) and/or X 1 NiCrMoCuN 31 27 4 (Mat. No. 1.4563).     

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.     

Die Korrosionsbeständigkeit der nichtrostenden Stähle an der Atomosphäre – Auswertung von Versuchen bis zu 10jähriger Auslagerung–

Corrosion resistance of stainless steels at the atmosphere – Evaluation of the results of weathering tests up to 10 years' duration – The evaluation of results obtained in the course of weathering tests up to 10 years' duration has confirmed the conclusions drawn after one year's exposition. A CrNiMo steel (Werkstoff-Nr. 1.4401) can be used even in the most severe conditions; slight corrosions found in marine atmospheres can be attributed to unpolished surfaces. The 17% Cr steel is considerably attacked in industrial and marine atmospheres and should not be used for the external parts of buildings. 18/8 steels are perfectly resistant to urban atmosphere, with the exception of those portions which are not exposed to the cleaning effect of rain waters. In all the cases studied the corrosion resistance of the steels has been improved by electropolishing. Electrochemical studies have further revealed that the pitting corrosion susceptibility decreases in the same order as the atmospheric corrosion resistance.     

Eigenschaften des schweißbaren,hoch korrosionsbeständigen Stahles X 2CrNiMoN 25 22

Properties of the weldable and corrosion resistant steel X 2 CrNiMoN 25 22 In view of the limited application of con- ventional CrNiMo steels the authors have investigated potential applications of the above mentioned steel. An optimum com- position would be (%) 0.02 C, 25Cr, 22 Ni, 2 Mo and 0.1 N. The low carbon content is a vital factor and the nickel content must not exceed 22% because higher contents would give rise to very pronounced precipitation. The new steel ist largely resistance to boil- ing nitric acid it is considerably superior to conventional steels, The high break-through potential is indicative of high pitting resist- ance. Improvements are equally remarkable with respect to mecanical properties and weldability     

Untersuchungen zur Lötbrüchigkeit hochlegierter Stähle

A study of the liquid mentla embrittlement of high allowy steels The action of zinc on austenitic stainless CrNi steels at high temperatures may give rise to a type of intercrystalline corrosion called “soldering brittleness” (liquid metal embrittlement) which can be attributed to the formation of a low melting phase rich in nickel. These phenomena have been known for a long time in connection with other metals and alloys and have been described by several authors. As to liquid metal embrittlement of high alloy steel, however, no data have been available so far.     

Die Oberflächenbehandlung von Schweißverbindungen hochkorrosionsbeständiger 6% Mo-Stähle und Nickel-Basis-Legierungen

Surface treatments of high alloy 6 Mo stainless steel and nickel alloy weldments High alloy stainless steels (6% Mo) and a high nickel alloy (alloy 625) weldment have been tested in order to answer the question whether post-treatment of the weldment has an effect on the corrosion resistance, especially on pitting corrosion. Therefore, the critical pitting temperature of weldments was tested in acidic chloride solution (standard tests). As a result grinding with rough emery paper as well as sand blasting lowers the localized corrosion resistance in the weldment area, while pickling has a positive effect, especially after blasting. Pickling can be done either by a solution of nitric + hydrofluoric acid or by a commercial pickling paste. In any event pickling is recommended as a final surface treatment for high alloy stainless steels and nickel alloys, especially in case of prevailing highly corrosive conditions such as pitting and crevice corrosion.     

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.     

Die Bedeutung des Oxalsäure-Tests für die Prüfung der Korrosionsbeständigkeit nichtrostender Stähle

The meaning of the oxalic acid etch test for testing the corrosion resistance of stainless steels In the oxalic acid etch test according to ASTM A 262 practice A, precipitations of phases rich in chromium and molybdenum which can occur in stainless steels, are preferentially dissoved. The behaviour of such phases in the oxalic acid etch test was investigated taking precipitations of carbide M₂₃C₆, s?-phase, χ-phase and Laves-phase in stainless steels AISI 304 L and 316 L as examples. The chemical composition of these was evaluated with a scanning transmission electron microscope (STEM) by EDS. With coarser precipitations, it was possible to support this analytical method by EDS of metallographic cross sections in a scanning electron microscope (SEM). In oxalic acid, critical threshold potentials exist above which the above mentioned phases are preferably attacked, furthermore critical pH values, below which no selective attack of the precipitated carbides and intermetallic phases occurs. The numerical values of the threshold potentials as well as the critical pH values were evaluated. When testing stainless steels in the oxalic acid etch test, the steel specimens are polarized to a highly positive potential in the very trans passive range. In this potential range the corrosion rate of stainless steels increases with increasing chromium content, while in the active and passive range the corrosion rate decreases with increasing chromium content. Other than the nitric-hydrofluoric acid test, the copper-copper sulfate-sulfuric acid test, and the ferric sulfate-sulfuric acid test, the oxalic acid etch test does therefore not indicate any chromium depletion. Hence, an intergranular attack also occurs when precipitations of carbides rich in chromium are present at the grain boundaries of austenitic stainless steels with the carbides being precipitated without any chromium depletion of the areas adjacent to the grain boundaries. Sensitized austenitic stainless steels which are susceptible to intergranular corrosion due to the precipitation of chromium rich carbides and chromium depletion of the areas adjacent to the grain boundaries, can suffer intergranular SCC in high temperature aqueous environments when additionally critical conditions with respect to the mechanical stress level and the oxygen concentration in the environment are given. For the detection of sensitized microstructures, the oxalic acid etch test must be valued critically due to the dependence of the corrosion rate on the chromium content mentioned above, and is obviously by far less suited than the conventional tests for establishing resistance to intergranular corrosion in sulfuric acid-copper sulfate solutions with additions of metallic copper (Strauß test, severe Strauß test).     

Der Einfluß des Titangehaltes auf die Aktivkorrosion ferritischer Cr-Stähle und austenitischer Cr-Ni-Stähle in Schwefelsäure

The influence of the Titanium content on the active corrosion of ferritic Cr steels and austenitic Cr-Ni steels in sulphuric acids Non-stabilized and Ti-stabilized steels of types X8Cr17, X8Crti17, X5CrNi189, X10CrNiTi189 and X5CrNiMoTi25 25 are compared with each other in respect of their behaviour in the potential range of active corrosion in sulphuric acid. With ferritic 17 per cent. chromium steels, no Ti influence was detected. With more highly alloyed austenitic Cr-Ni or Cr-Ni-Mo steels, the titanium had the effect of narrowing down the potential range of active corrosion and reducing the corrosion rate which facilitates the passivation capacity of the steels. These phenomena are particularly marked with the steel known as X5CrNiMoTi25 25. With this steel, the corrosion rate in the active-passive transition range with titanium contents from 0.41 to 0.66 per cent. and about 0.66 per cent. C ist increased in the sensitivized state. Within a narrow potential range of about 200 mV, intercrystalline corrosion is encountered which decreases with increasing Ti content and is completely prevented if the C content is reduced below 0.03 per cent. The potential range of the inter-crystalline corrosion is more negative than the test potential of the Strauss test normally used for testing the grain disintegration resistance.