Untersuchungen über den Einfluß des Gefügezustandes auf das Korrosionsverhalten hochlegierter,rost- und säurebeständiger Stähle in reiner sowie in chloridhaltiger Schwefelsäure

Corrosion Properties of High Alloyed Stainless Steels in Pure as well as in Chloride Containing Sulfuric Acid The corrosion behaviour of the high alloyed stainless steels material no. 1.4439 (X3CrNiMoN17135), 1.4539 (X2NiCrMoCu25205), 1.4503(X3NiCrMoCuTi2723) as well as the reference materials AlSI 316 L and alloy 825 was tested in diluted sulfuric acid (5, 10, 20 and 50%) at 50, 100 and 150°C. The test solutions additionally contained impurities as chlorides and cupric ions. On the material side the effect of various microstructures was checked as well: material as received (commercial production), solution annealed under laboratory conditions, cold deformed and for two selected steels electroslag remelted. Corrosion testing methods are: the immersion test will sheet coupons and the measurement of the weightloss; electrochemical testing, i.e. Current potential-and free corrosion potential-time-curves. No pitting corrosion is observed in the presence of chloride ions. In some cases the general corrosion rate is lowered if chloride ions are present. This beneficial effect of chloride ions, however, is observed only at low chloride concentrations (500 ppm). Annealing under laboratory conditions as well as electroslag remelting does not generally improve the corrosion resistance. A negative effect by cold deformation is only observed for standard stainless steel AlSI 316. Cupric ions added to the 20% sulfuric acid solution improve the corrosion resistance of all steels investigated to that extent, that they can be used in practice up to 100°C provided that the concentration of cupric ions in the solution is sufficiently high (2000 ppm). Electrochemical test results indicate that the positive effect of cupric ions is due to the shift of the free corrosion potential into the potential range of stable passivity. Copper alloyed stainless steels show the highest corrosion resistance.     

Chloridinduzierte Korrosion von nichtrostenden Stählen in Schwimmhallen-Atmosphären Teil 2: Einfluß von Hypochloriten

Chloride induced corrosion on stainless steels at indoor swimming pools atmospheres Part 2: influence of hypochlorite The work was started on the occasion of failures of stainless steel components at the indoor swimming pool atmosphere in Uster (Switzerland). Highly-alloyed stainless steels were tested at defined mechanical and environmental conditions. Therefore Ubend specimens with salt spots were examined at 40°C and 35 and 70 % rel. humidity respectively. The results of the experiments with the electrolyte magnesium-chloride (30 %) were presented in part 1. The high nitrogen alloyed steels 1.4529 and 1.4565 showed the best corrosion resistance. The highest corrosion attacks were observed at the steels 1.4401, 1.4462, 1.4439 and 1.4539. Stress corrosion cracking (SCC) was determined at the steels 1.4401, 1.4462 and 1.4439 at 35 % rel. humidity by metallographic methods. A distinctive relationship was observed between the pitting resistance equivalent and the kind of corrosion attack. Furthermore, short time experiments (< 1000 h) can be used to define a qualitative judgement over the occurrence of the kind of long time corrosion attacks at stainless steels.     

Korrosionsuntersuchungen an nichtrostenden,austenitischen Stählen im System Essigsäure/Essigsäureanhydrid

Corrosion tests of austenitic stainless steels in the system acetic acid/acetic anhydride The corrosion behaviour of the austenitic stainless steels X6 CrNiMoTi 17 12 2 (mat.no. 1.4571, AISI 316 Ti) and X2 NiCrMo-Cu 25 20 5 (mat.no. 1.4539, alloy 904 L) was investigated by immersion tests of welded specimens in acetic acid and acetic anhydride and in mixtures thereof, as well. The tests were carried out at 80°C and at the boiling point of the test solutions. The test equipment was either open to the atmosphere or the mixtures were deaerated with nitrogen. The tests show that in the presence of air the mixtures are more corrosive than the neat media. At 80°C in aerated solutions only alloy 904 L (mat.no. 1.4539) performed well, except in a mixture of 70% acetic acid and 30% acetic anhydride. At the boiling point of the test solutions the corrosive attack on both stainless steels increases considerably; the corrosion rates are in the range of 1 mm/a (1.4571) and 0.6 mm/a (1.4539). Deaeration with nitrogen decreases the corrosivity of the solutions to the stainless steels. Electrochemical tests show the detrimental influence of acetic anhydride to the passivation process of stainless steels tested in glacial acetic acid.     

Teil 3: Einfluß einer realen Schwimmhallen-Atmosphäre

Chloride induced corrosion on stainless steels at indoor swimming pools atmospheres - Part 3: Influence of a real indoor swimming pool atmosphere The work was started on the occasion of failures of stainless steel components at the indoor swimming pool atmosphere in Uster (Switzerland). Highly-alloyed stainless steels were tested at defined mechanical and environmental conditions. Therefore U-bend specimens with and without (with the original metal surface) salt spots were examined in a real indoor swimming pool atmosphere at approx. 30 °C and approx. 65% rel. humidity. Basically negligible corrosion attacks were observed at the U-bends without salt spots after 6200 h. The U-bend specimens with salt spots showed similar results determined at the lab at 35 and 70% rel. humidity respectively. The lab results were published in part 1 and 2. However slightly higher corrosion attacks were detected at the outdoor exposed U-bends in a real indoor swimming pool atmosphere. The steels 1.4401 and 1.4439 incured stress corrosion cracking (SCC).     

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.     

Ein Beitrag zum Korrosionsverhalten von austenitischem 17-12-2 CrNiMoTi-Stahl in siedenden Essigsäurelösungen - Betriebserfahrungen und Laborversuche

A contribution concerning the corrosion behaviour of austenitic 17-12-2 CrNiMoTi stainless steel in boiling acetic acid solutions - Service experience and laboratory tests Service experience of acetic acid processing industry shows that X 6 CrNiMoTi 17-12-2 stainless steel (similar to AISI type 316 Ti) can be used as construction material for handling concentrated boiling acid without facing corrosion problems if, in the medium, dissolved oxygen and small amounts of water are present, and if chloride ions are virtually absent. In laboratory tests performed on X 6 CrNiMoTi 17-12-2 stainless steel in acetic acid of 118°C containing variable extents of impurities, the steel exhibited passive behaviour in cases where the chloride ion concentration was less than 1 ppm and, at the same time, water amounts to some few tenths of a percent. The exact value of the water content necessary to achieve passivity depends on the concentration of formic acid the presence of which, at least in traces, is unavoidable. In anhydrous acetic acid, the steel corrodes in its active state, and dissolved oxygen increases the corrodes in its active state, and dissolved oxygen increases the corrosion rate. If chloride ion concentration is 1 ppm or higher the otherwise passive steel undergoes localized corrosion and the attack mostly follows crystallographic orientations. In aerated boiling mixtures of acetic acid and anhydride, the steel is boiling mixtures of acetic acid and acetic anhydride, the steel is severely corroded until the portion of anhydride reaches 70%. Beyond this, corrosion rate steeply decreases. Practically no corrosion could be found in 97% acetic anhydride.     

Chloridinduzierte Korrosion von Nichtrostenden Stählen in Schwimmhallen-Atmosphären Teil 1: Elektrolyt Magnesium-Chlorid (30%)

Chloride induced corrosion on stainless steels at indoor swimming pools atmospheres Part 1: electrolyte magnesium-chloride (30%) The work was started on the occasion of failures of stainless steel components at the indoor swimming pool atmosphere in Uster (Switzerland). Highly-alloyed stainless steels were tested at defined mechanical and environmental conditions. Therefore U-bend specimens with salt spots were examined at 40°C and 35 and 70% rel. humidity respectively. A further test series was realised with round specimens under constant load in a stress cracking testing system at the same temperature and humidity conditions. The highly-nitrided steels 1.4529 and 1.4565 showed the best corrosion resistance. The steels 1.4401, 1.4462 and 1.4539 incured the highest corrosion attacks. Stress corrosion cracking (SCC) was determined at the steels 1.4401 and 1.4462 at 35% rel. humidity by metallographic structure micrography only. A distinctive relationship was observed between the pitting resistance equivalent and the kind of corrosion attack.     

Versprödungserscheinungen an ferritischen Chromstählen und deren Einfluß auf das Schwingungsrißkorrosionsverhalten

Embrittlement conditions of ferritic chromium steels and influence on the corrosion fatigue behaviour The corrosion fatigue behavior of two ferritic stainless steels, with 18% and 28% chromium respectively, embrittled by α'- or sigma-phase has been studied in this investigation. No effect of the α'-phase on the corrosion fatigue behaviour in a 0.5 N or 4 N air saturated soldium chloride solution at 80 °C has been found. In the 4 N solution corrosion fatigue cracking of the 18% chromium steel in the solutionized as well as in the α'-embrittled condition is initiated by pitting at nonmetallic inclusions. Sigma-phase not only decreases the mechanical properties of the 28% chromium steel by heavy embrittlement but also leads to crack initiation by pitting in the 4 N solution at the interface sigma/ferrite. The pronounced decrease of the corrosion fatigue properties of the 28% chromium steel could well be correlated with chromium depletion of the ferrite matrix by precipitation of sigma phase.     

Untersuchungen zum Korrosionsverhalten nichtrostender Stähle in Phosphorsäure in Abhängigkeit von dem Chlorid- und Fluoridgehalt und einer Wärmebehandlung beim Schweißen

Investigations into the corrosion behaviour of stainless steels in phosphoric acid in dependence on the chloride and fluoride contents and a heat-treatment during welding In the present investigation the corrosion behaviour of recently developed high-alloyed austenitic (X 2 NiCrMoCU 25 20) and austenitic-ferritic (X 2 CrNiMoCuN 25 5) steel has been tested towards phosphoric acid, the composition and concentration of which was adjusted to correspond to crude phosphoric acid with varying contents of Cl^?, F^?, Fe³⁺, and SO₃. Current density-voltage curves were recorded on steel test-specimens at room temperature, 45°C, and 80°C. In addition, test-pieces of both steels, after annealing treatment between 800°C and 500°C, as well as welded specimens of similar type were examined for the effect of heat input and consequent structural alterations on the corrosion behaviour. Both steels exhibit good corrosion resistance towards pure phosphoric acid. Cl^? and F^? ion impurities render the acid more aggressive. Austenitie-ferritic steel reacts more sensitively than austenitic steel towards heat input, resulting in deterioration in corrosion resistance. This may be attributed to the precipitation of inter-metallic phases in the ferritic structure.     

Zur Korrosionsprüfung von Schweißverbindungen hochlegierter CrNiMo-Stähle und NiCrMo-Legierungen

On the corrosion testing of weldments of high alloyed CrNiMo-stainless steels and NiCrMo-alloys Weluments of high-alloyed CrNiMo stainless steels and Nicro alloys can he more susceptible to localized corrosion than the solution annealed basic material owing to segregations and precipitations in the heat affected zone, the high temperature zone and/or in the weld. To investigate these differences the FeCl₃-test (10% FeCl₃ · 6aq), the test “green death” (11.5% H₂SO₄, 1.2% HCl, 1% CuCl₂, 1% FeCl₃) as well as chronopotentiostatic tests in artificial sea water or in 3% NaCl-solution are used. In particular for testing the highest alloyed materials a CaCl₂ test was developed (4.5M CaCl₂, chronopotentiostatic test in duration of 8 to 10 hours at + 200 mV (SCE)), which can be carried out to a temperature of 115°C at atmospheric pressure. The aggressivity increases in the range FeCl₃-test, “green death”-test, CaCl₂-test. Matching and graduated over-alloyed weldments (TIG, heat input of 7 and 15.5 kJ/cm) of materials 1.4529, 1.4562, 2.4856, 2.4819 (german materials No.) are comparingly examined in various tests, of materials 1.4406, 1.4539, 1.4439 and 1.4563 (german materials No.) only matching weldments in the FeCl₃-test. In strongly oxidizing media only a highly over-alloyed performed weldment (filler material 2.4607, german material No.) produces the best corrosion behaviour, measured as the critical temperatures of localized corrosion. Measurements of critical current densities of passivation can be used for investigations of corrosion behaviour of weldments, too. Critical current densities of passivation are showing a tendency to inverse proportion to the critical temperatures of localized corrosion. Suitable electrolytes are among others 0.2M H₂SO₄ + 1M NaCl + 10^?3% KSCN, N₂-bubbled, 25 to 60°C and xM H₂SO₄ + 4M NaCl + 10^?3% KSCN (x = 0.05 to 1), 25°C, in contact with air. An influence of heat input at the welding is indicated in the test of localized corrosion, but it is only small. It is sometimes more clearly shown at measurements of passivation.     

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

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

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.     

Elektrochemische Untersuchungen zur Hochtemperaturkorrosion von Chrom-Nickel-Stählen in Alkalisulfatschmelzen bei 700°C

Electrochemical investigations into the high temperature corrosion of chromium nickel steels in alkali sulfate melts at 700°C Specimens from austenitic stainless steels, with and without susceptibility against intergranular corrosion attack, decarburized and carburized, and stabilized with Ti or Nb, have been corroded in eutectic mixture of alkali sulphates at 700°C at constant potentials. Furthermore additions of 10% NaCl and 1% SO₃ have been investigated. Any selective corrosion of both carbides and Cr depleted zones did not take place. With decreasing potentials there was a preferred attack at grain boundaries – especially in the case of NaCl additions. At very negative potentials an “active” corrosion state exists without formation of protective oxide layers with the consequence of fissured surfaces and internal oxidation. The effect is supported by NaCl addition and by Ni as alloying element.     

Corrosion behaviour of highly alloyed stainless steels in mixed chloride and fluoride aqueous solutions

Laboratory weight loss and cyclic potentiodynamic polarization corrosion tests were performed on two types of corrosion resistant alloys, a duplex alloy (ferritic-austenitic stainless steel) and two austenitic stainless steels, in mixtures of chloride (3000, 9000 and 15000 ppm) and fluoride (4800 and 15000 ppm) ions at pH 3. Two temperatures were tested, 60 and 70°C. The electrochemical results indicate that the duplex stainless steel presents high corrosion resistance. Weight loss results show low corrosion rates of the two types of stainless steels after 60 days exposure. Sonic pits-crevices were found under the corrosion crust deposits on the duplex stainless steel.     

Korrosionsverhalten des Werkstoffs 1.4539 und von Nickelbasis-Legierungen in Gaswässern

Corrosion behaviour of material no. 1.4539 and nickel based alloys in gas waters Laboratory tests with synthetic gas waters containing the gases ammonia, carbon dioxide, hydrogen sulphide and hydrogen cyanide were carried out in order to examine the influence of medium components on the corrosion of material No. 1.4539 and nickel based alloys Hastelloy C-4, C-22 and C-276. Hydrogen sulfide was identified as the decisive component for corrosion. For stainless steel corrosion rates of about 2 mm ° a⁻¹ were already found at 50°C in a critical pH-range with sulfide concentrations > 2%. As cyanide stimulates corrosion by dissolving sulfide surface layers by complexation of the iron ions, an increased material loss rate per unit area was found in the critical range with increasing cyanide concentration. The much more stable nickel based alloys only revealed considerable weight losses after being exposed in the autoclave at 100°C. The graduation of the loss rates C-22 > C-4 > C-276 can be explained by the different contents of high grade alloy elements. The testing of nickel based alloys of the Hastelloy type and of material No. 1.4539 and 1.4571 by means of the dynamic tensile test (CERT-method) revealed no risks of stress corrosion cracking in the tested media. The mechanical data, such as energy at break, elongation at rupture and reduction of area when breaking, did not indicate any change in comparison to the data found in a neutral medium (glycerine). As a function of the aggressiveness of the liquid phase, the metallographic analysis showed some traces of local corrosion in the highly deformed area, ranging from pitting to limited surface cracking. These phenomena were found to nearly the same extent for all materials.     

Inhibition von Niedertemperaturkreisläufen in Kraftwerken mit Lithiumhydroxid

Inhibition of low temperature circulating systems with lithium hydroxide in power stations Corrosion current density measurements of mild steel St 37 in deionized, air-saturated water between 20 and 60°C showed that addition of small quantities of lithium hydroxide (ca. 25 ppm) protects against corrosion attacks similar to pitting corrosion and is found adequate even up to 5 ppm chloride ion concentration. Constant strain rate tests of mild steel St 37 showed no indication of stress corrosion cracking (SCC) at 70°C, up to 500 ppm lithium hydroxide, and 100 ppm chloride ion concentration. Metallographic examination of specimens indicated intergranular stress cracking with cracks of 1–30 m?m depth in environments containing 2%, 4%, and saturated lithium hydroxide (with solid excess salt) at 70°C. Accumulation of lithium hydroxide should therefore be avoided. No SCC was observed in austenitic stainless steel X 5 CrNi 18 9 specimens in the above environments.     

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.     

Einfluß des Kriechens auf die Spannungsrißkorrosion austenitischer Chrom-Nickel-Stähle in heißer 35%iger Magnesiumchloridlösung

Creep effect on stress corrosion cracking of austenitic CrNi steels in boiling 35% magnesium chloride solution Potentiostatic and potentiodynamic polarization curves of steel X 5 CrNi 18 9 in 35% MgCl₂-solution at 120° C do not show significant differences. Important for SCC tests is a narrow potential region before the onset of the potential of pit nucleation. SCC-experiments were carried out using two different techniques:

• (a) loading in the electrolyte
• (b) prestraining in air at 120° C; after transient creep the SCC test was initated.

Applying the technique described under (b) a decrease of SCC-susceptibility according to the crack nucleation expected in the slip dissolution model does not take place. Accordingly the dependence of time to failure on potential as well as on stress is found to be almost similar. Only in the absence of pitting a SCC threshold stress exists with its value in the range of yield strength. On the other hand SCC failure in the elastic region (< 0.1 σ_y) is observed when crack nucleation starts in corrosions pits.     

Kurzzeitprüfmethoden zur Untersuchung des Einflusses von Wärmebehandlung und chemischer Zusammensetzung auf die Korrosionsbeständigkeit martensitischer Chrom-Nickel-(Molybdän)-Stähle mit tiefem Kohlenstoff-Gehalt

Short-duration test for evaluating the influence of heat-treatment and chemical composition on the corrosion resistance of low carbon martensitic chromium nickel (molybdenum) steels The following procedures were examined and compared for their suitability as rapid corrosion tests in comparative investigations on the influence of heat treatment and other production parameters on martensitic chromium-nickel-(molybdenum) steels of low carbon content:

• –Measurement of the mass loss in boiling 20% acetic acid, and in some cases 5% nitric or 1% hydrochloric acid.
• –Immersion in sulphuric acid/copper sulphate solution (modified Strauss test).
• –Current density/voltage curves in 1 N sulphuric acid.
• –Measurement of pitting potential und activation pH in 3% sodium chloride solution (on limited scale).

The test materials employed contained 0.032–0.082% carbon, 11.6–16.4% chromium, 3.8–5.9% nickel and 0.31–1.6% molybdenum. The most suitable testing procedures were selected according to the steel type. Up to a mass loss rate of ≥ 0.05 mg · cm^?2 · h^?1 (materials with resistance inferior to steel type 13-6-1) the acetic acid test provides an excellent basis for comparison. Provided the optimal testing temperature is adopted, current density/voltage curves and immersion in sulphuric acid/copper sulphate solution yield usable results with all steel types investigated. The value of the information yielded by the test procedures is generally enhanced by metallographic characterization of the corroded surfaces; in particular the localization of chromium-depleted structure zones is made possible by this.     

New stainiess steels for sea water applications. Part. 1: Corrosion and mechanical properties of ferritic stainless steels

Two experimental ELI ferritic stainless steels (22 Cr – 2.5 Ni – 3 Mo and 22 Cr – 2.5 Ni – 3 Mo – Ti) prepared in laboratory and a commercial one (21 Cr – 3 Mo – Ti) were investigated. Electrochemical and laboratory exposure tests were carried out to define the localized corrosion resistance (pitting and crevice) of such steels in chloride solution. Intergranular and stress corrosion resistance was also evaluated. Room temperature tension tests and impact tests were performed. 22 Cr – 2.5 Ni – 3 Mo – Ti and 21 Cr – 3 Mo – Ti steels are immune to intergranular corrosion whatever temperature they are heat treated at and have the same pitting corrosion resistance as a function of temperature; crevice corrosion of 22 Cr – 2.5 Ni – 3 Mo is decidely better than in the commercial 21 Cr – 3 Mo – Ti. The experimental steels were immune to stress corrosion in hot chloride environment.