Einfluß der Oberflächenbehandlung nichtrostender Stähle auf deren chemische Beständigkeit insbesondere gegen Spannungsrißkorrosion

Influence of the surface treatment of stainless steels on their chemical resistance, in particular to stress corrosion cracking The stress corrosion resistance of austenitic stainless steels shows a pronounced dependence from the surface treatment. Grinding with a coarse material makes the surface very susceptible to this type of corrosion. The susceptible can be largely removed by subsequent picking, provided the treatment removes a layer about 3 μm in thickness. Grinding affects the structure to a depth of about 200 μm, but particular conditions appear to prevail in the above mentioned thin surface zone, so that in particular transcrystalline stress corrosion may occur. The susceptibility to stress corrosion cracking can be tested with a solution containing (%) 0.06 acetic acid, 1 acetaldehyde and 100 ppm Cl ions (as CuCl₂) when the corrosion susceptibility is to be evaluated in comparative terms. Pitting corrosion by mixed acid, too, can be largely prevented by deep picking of ground material.     

Beitrag zur Spannungsrißkorrosion austenitischer Stähle

Contribution to the problem of stress corrosion cracking of austenitic steels It is shown on the basis of a stress corrosion cracking theories that the incubation phase is the factor determining the useful life of an element because in this phase protective layers are destroyed and crack nuclei are formed. Testing corrosion susceptibility in boiling magnesium chloride solution has the shortcoming that the solution is highly aggressive so that minor changes of the surface condition which may yield a considerable improvement of material behaviour in less aggressive conditions do not come to bear. Another shortcoming of MgC1₂ testing are divergent literature data concerning concentration so that there is also a scatter of boiling temperatures which has a pronounced influence on test results. Likewise an important role can be attributed to foreign ions and silicates from glas in the solution. It is more advantageous to use less concentrated solutions (up to 35%) because they enable clear current density potential curve to be obtained. An important factor concerning the test method is the previous investigation of specimens with a view to a residual stresses due to cold reduction. Under certain conditions the time to failure may be longer under cycling loads.     

Untersuchungen zur Spannungsrißkorrosion austenitischer Stähle in kalten Chloridlösungen

Investigation on stress corrosion cracking of austenitic steels in cold chloride solutions At ambient temperatures of about 25°C austenitic chromium nickel steels can suffer stress corrosion in media with a concentration of both hydrogen ions and chloride ions exceeding 1 mol/L or in strongly concentrated chloride solutions. With the aid of constant strain rate testing and with U-bend specimens the parameters of this types of corrosion were investigated. Contrary to the situation with the acid solutions, the reproducibility of test results with the concentrated chloride solution was found to be low. The formation of martensite by cold working was found to be essential. The corrosion susceptibility decreases with increasing nickel content. Stress corrosion takes place within small critical potential ranges without a limit to more positive potentials as this is the case of stress corrosion with hot media. These potential ranges are widened with increasing cold working especially in the case of strongly grinding or cyclic loading in the plastic range. Corrosion cracking was observed with solutions of MgCl₂, LiCl, with a less amount with NaCl, but not with ZnCl₂. The concentration of chloride ions have an effect for solutions with a given kind of salt, but not for different salt solutions.     

Untersuchungen zur interkristallinen Spannungsrißkorrosion unlegierter Stähle in Ammoniumcarbonatlösungen

Investigation into the intercrystalline stress corrosion cracking of unalloyed steels in sodium carbonate solutions The steel St 36 is characterized by a very pronounced susceptibility to intercrystalline corrision, even under constant load. This sensitivity is limited to a potential zone of about 100 mV situated at the beginning of the passivity range (between ?600 and ?500 (SCE)). While a plastic deformation is not necessary for the initiation and propagation of cracks the propagation is accelerated at high loads. There is, however, no critical elongation rate which would give rise to preferential stress corrosion cracking. It is not possible to maintain the hypothesis according to which the stress corrosion cracking susceptibility would decrease at sufficiently low elongation rates. On the base of the results obtained one may assume the existence of disturbed zones in the oxide layer and the nucleation of cracks at such sites (possibility of “pre-existing-paths” according to parkins). The crack propagation would then be a function of the ratio of rate of advancement of the crack tip (which is assumed to be active) and the rate of repassivation of the crack walls, this repassivation being due to the formation of magnetite.     

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.     

Korrosionsverhalten nichtrostender ferritisch-austenitischer Stähle in Natronlauge

Corrosion performance of duplex steels in caustic solutions The corrosion performance of stainless ferritic-austenitic steels (DIN 1.4362, DIN 1.4462, Uranus 50 and Ferralium 255) was investigated in boiling sodium hydroxide solutions. It was found that these steels are corrosion resistant in NaOH solutions up to 30 mass-% with corrosion rates lower than 0.01 mm/a. The mass loss rate increases with increasing NaOH concentration, reaching a maximum in 60 mass-% NaOH solution. Small flow rates (e.g. 0.08 m/s; rotating disc) increase the corrosion rate by a factor of 6 with respect to stagnant conditions. However, further increase of flow rate yields no additional enhancement of the corrosion rate up to 1 m/s. Under conditions of free corrosion no stress corrosion cracking was observed at constant deflected samples or at tensile probes tested to failure under CERT-conditions in boiling 20–70% NaOH solutions. The corrosion attack is uniform in NaOH solutions up to 30 mass-%. In more concentrated NaOH solutions the dissolution rate of the austenitic phase is slightly higher than the ferritic phase. The corrosion products contain more nickel than the base material.     

Spannungsrißkorrosion nichtrostender austenitischer Chrom-Nickel-Stähle bei Raumtemperatur

Stress corrosion cracking of austenitic chromium-nickel stainless steels at ambient temperature For the chloride-induced SCC with transgranular crack path in austenitic 18Cr10Ni stainless steel, a critical temperature between 45 and 50°C exists. This critical temperature, however, is valid only for the passive state of the steel in nearly neutral, chloride-containing aqueous environments. In the active state, SCC with transgranular crack mode can occur at temperatures down to ambient temperature. The active state is caused by highly acidic, high-chloride containing aqueous corrosive media. Adherent aqueous films with these properties can grow on the surface of structural components inside swimming-pools when the water is disinfected by addition of chlorine. Under these conditions, failure of austenitic CrNi and CrNiMo stainless steels by SCC with transgranular crack path at ambient temperature is possible and actually occurred. SCC with preferentially intergranular crack path can also occur at ambient temperature when austenitic stainless steels with a sensitized microstructure are used. Under these conditions, the corrosion attack is caused by non-specific aggressive environments, e.g., adherent aqueous films not containing chloride ions. The crack mode, intergranular or mixed, depends on the stress level.     

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.     

Einfluß der Oberflächenbeschaffenheit auf die Beständigkeit nichtrostender,austenitischer Chrom-Nickel-Stähle gegen transkristalline Spannungsrißkorrosion

Influence of surface conditions on the resistance of stainless austenitic bromium nickel steels to transcrystalline stress corrosion Grinding weld seams may result in an increased susceptibility to stress corrosion cracking of the areas treated in that way. This susceptibility may be eliminated by an ultimate pickling step; the thickness of the layer to be removed is 0.15 to 100 μm, depending on surface roughness. The susceptibility to stress corrosion cracking of the ground zones can be determined, however, only in the boiling 42% MgCl₂-solution; no stress corrosion cracking could be produced in solutions containing from 10 to 3O% MgCl₂ or 10 to 2O% and even 40% CaCl₂. Another possibility to eliminate stress corrosion cracking susceptibility is an ultimate sand blasting which produces compressive residual stresses in the ground surface.     

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 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).     

Spannungsrißkorrosionsverhalten rostfreier Stähle nach kathodischer Vorpolarisation

Stress-corrosion cracking behaviour of stainless steels after cathodic prepolarization The stress-corrosion behaviour of an austenitic chromium-nickel steel has been studied in boiling MgCl₂-solutions after a cathodic prepolarization. Surface changes during cathodic prepolarization. Surface changes during cathodic polarization were identified by preliminary experiments which revealed that the changes depend from the pretreatment of the specimen. Cathodic polarization shifts the critical stress-corrosion potential toward more anodic values; stress-corrosion cracking occurs on passive surfaces only. The life of the steel is the longer the more negative the protective potential is during specimen loading. In order to produce stress-corrosion cracking it is not necessary to produce a mechanical destruction of the passive layer as a consequence of slipping phenomena during steel straining as required by the “film rupture” hypothesis. Even when the deformation of a fresh passive layer produced under constant load is prevented the time to failure of the specimens is not increased by comparison to specimens with a heavily strained passive layer. On the other hand no reduction in the time to failure has been observed with heavy hydrogen evolution at the steel surface.     

Homogenität und Korrosionsbeständigkeit hochlegierter Stähle

Homogeneity and corrosion resistance of high alloy steels A major number of case histories in the chemical industry are due to local corrosion the origin of which can be attributed to the inhomogeneity of the steels produced according to conventional melting process. Special processes such as electro slag remelting may give rise to a considerable increase in structural homogeneity of corrosion resistant alloys. Typical examples are increased resistance to nitric acid, Streicher's solution, seawater or reaction mixtures of urea synthesis. These results clearly demonstrate the superiority of the material which is largely free from inhomogeneities such as segregations which give rise to local corrosion phenomena.     

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.     

Untersuchungen der Spannungsrißkorrosion austenitischer Stähle durch saure Chloridlösungen bei Niedrigen Temperaturen

Investigation of stress corrosion cracking of austenitic steels in acid chloride solutions at low temperatures Tests were carried out on materials 1.4301, 1.4571, 1.4439 and 1.4558 in cold hydrocloric acid (c(Cl^?) = 1.5 mol/L and c(H⁺) = 1 mol/L, a few tests also at 0.1 and 0.01 mol/L). Chronopotentiostatic tests yielded data on active corrosion, passivity and pitting corrosion. CERT tests (10^?6s^?1, a few tests also at 2 · 10^?7s^?1) showed superposition of general corrosion on stress corrosion under free corrosion condition, while the rest potential was relatively negative in the active range. Oxygen purging has only a minor effect. The extent of cracking decreases with decreasing c(H⁺). In the case of cathodic polarisation straining induced surface notches occur which can be attributed to hydrogen induced effects. In the case of anodic polarisation pittings are generated without any crack initiation. Characteristic features of stress corrosion increase with decreasing strain rate. It follows from the results that high acid concentrations are necessary for stress corrosion cracking in the active state to occur. CERT tests cannot be used as an accelerated test for this kind of stress corrosion cracking. Few CERT tests carried out in warm NaCl solution did not show any stress corrosion cracking though these solutions are known to cause stress corrosion cracking after long periods. Furthermore, predamage in the form of pitting does not alter the situation.     

Die Inhibition der Aktivkorrosion nichtrostender Stähle in Säuren durch Kohlenmonoxid

The inhibition of the active corrosion of stainless steels in acids due to carbon monoxide The active corrosion of stainless austenitic chrome-nickel steels in H₂SO₄ is effectively inhibited by CO below the boiling temperature of the acid. The potential range of active corrosion is narrowed down, and the passivation current density as well as the maximum dissolution rate are reduced. In the boiling solution, pitting corrosion occurs in certain potential ranges, which can be reduced or completely prevented by increasing the Mo-content of the steels.     

Korrosion nichtrostender Stähle und Nickelbasislegierungen in Salpetersäure-Flußsäure-Gemischen

Corrosion of stainless steels and nickel-base alloys in solutions of nitric acid and hydrofluoric acid Reactions involving nitric acid may always result in the contamination of this acid with fluorides. In highly concentrated nitric acid, the presence of small amounts of HF will substantially reduce the corrosion of metallic materials. Mixtures consisting of hydrofluoric acid and hypo-azeotropic nitric acid on the other hand will strongly attack: the metal loss will markedly increase with increasing HNO₃ and HF concentrations as well as with rising temperatures. The investigation covered 12 stainless steel grades and nickel-base alloys. With constant HNO₃ content, corrosion rates will rise linearly when increasing the HF concentration. With constant HF concentration (0.25 M), corrosion rates will increase rapidly with increasing nitric acid concentration (from 0.3 M to 14.8 M). This can best be described by superimposing a linear function and a hyperbolic function that is reflecting the change in the HNO₃ content. Alloys containing as much chromium as possible (up to 46 wt.%) will exhibit the best corrosion resistance. Alloy NiCr30FeMo (Hastelloy alloy G-30) proved to be well suitable in this investigation.     

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).     

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 nichtrostender Vergütungsstähle mit rund 13% Chrom

Corrosion behaviour of stainless heat-treatable steels with approx. 13% Cr Hardness and weight losses in 5% and 20 acetic acid of the stainless heattreatable steels X 15 CrMo 13, X 20 Cr 13, X 20 CrMo 13, X 35 CrMo 17 and X 22 CrNi 17 have been examined as a function of tempering heat and tempering time. The basic trend of the weight loss curves is determined by the segregation of chromium carbides and the loss in chromium associated with it. The absolute magnitude of the weight losses in the hardened condition, in the segregation condition leading to maximum proneness, and in the annealed condition mainly depends on the chromium and molybdenum contents in the matrix, as metallurgical investigations have shown. With isotherm tempering, the commencement of the weight loss rise is largely independent of the corrosive medium. But the position of the maximum and the end of corrosion proneness depend on the type of corrosive medium. The dissolution current density determined in 10% sulphuric acid at 60° C and + 700 mVE shows the same correlation with the tempering method as the weight losses in 5% nitric acid at room temperature. A diagram shows, for all the steels investigated, the beginning and end of corrosion proneness as a function of tempering heat and tempering time. The notch impact strength of the steels in the range of corrosion-proneness is low and greatly fluctuating. As far as the practical application of the steels is concerned, it is important to avoid this range of corrosion proneness, indicated by the diagram.