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.     

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.     

Hochtemperaturkorrosion hochlegierter Stähle unter simulierten Müllverbrennungsbedingungen

High temperature corrosion of high alloy steels in simulated waste incineration environments The corrosion of high alloy steels was investigated in discontinuous exposures in N₂-O₂-HCl-SO₂ at 600°C with and without deposits, taken from a waste incinerator. Additions of 500–3000 vppm HCl to N₂-O₂ leads always to accelerated corrosion due to the formation of Cl₂. The Cl₂ diffuses through cracks and pores of the oxide scale and mainly FeCl₂ is formed at the metal/scale interface; by evaporation and outward diffusion through the oxide scale it is oxidized to Fe₂O₃ (“active oxidation”). After addition of SO₂ the corrosion process is retarded by sulfide formation on the metal-chloride. At 600°C the main corrosion process under deposits is “hot corrosion” caused by a sulfate melt. In N₂-O₂ oxide scales are dissolved by basic fluxing, the corrosion products are PbCrO₄ and zinc-rich spinel. In N₂-O₂-SO₂ the oxide scales are dissolved by acidic fluxing as sulfates; under these conditions Cr₂O₃-layers are sufficiently resistent. In N₂-O₂-HCl, (K, Na) Cl is formed in the deposits and therefore catastrophic corrosion occurs by Cl₂. After addition of SO₂ to N₂-O₂-HCl the corrosion rate is decelerated and acidic behaviour of the melts is induced.     

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.     

Optimierung des Schweißprozesses hochlegierter Stähle und Verbesserung der Korrosionsbeständigkeit der Schweißverbindungen

Optimization of the welding process of high alloyed steels and improvement of corrosion behaviour of welded joints The optimization of welding processes is necessary to obtain a good durability of the welded joints connected with a minimization of the corrosion performance. Welding processes represent a considerable influence of the material. The formation of precipitations, strong structure changes, increasing of the residual stress and not at all undefined surface layers are possible. All these changes have a great influence on the corrosion behaviour. Particularly tempering tarnish changes the passive layer which is decisive for the corrosion resistance. But also surface treatment methods can influence the corrosion behaviour. Therefore both the welding process and an “after‐care” coordinated with the respective welding process had to be optimized. The optimization of the welding process was carried out by variation of the energy per unit length and the use of different protective gases. For a selection of a surface treatment method it has to be taken into account that an obvious remove of the tempering tarnish doesn't lead to an improvement in the corrosion behaviour. Traces of the working tool which can have a negative effect on the corrosion behaviour often remain on the surface. The influence of these different parameters on the corrosion property could be proved by electrochemical and surface analytical examinations. The investigations were carried out at specimens of two typical representatives of high alloyed austenitic steels and at welded joints, which had different surface treatments.     

Beitrag zur Kenntnis der Lotsprödigkeit einiger Kupfersorten und Legierungen sowie zweier hochlegierter Stähle

A contribution towards the understanding of liquid metal embrittlement of some coppers, copper alloys and two high-alloy steels The tendency to liquid metal embrittlement in the following materials has been investigated: Three varieties of pure copper, two age-hardenable copper alloys, an extruded brass, an 18/8 stainless steel and a 12% chromium steel. The specimens were exposed to each of the following two commercially available brazes: L-Ag 15 P between 660 and 800°C, and L-Ag 42 Cd between 600 and 750°C. During the tests the brass remained totally unembrittled, whereas the 12% chromium steel was only slightly embrittled in the L-Ag 42 Cd at high temperature. All other materials failed the exposure tests. At the normal brazing temperature the less aggressive solder L-Ag 15 P produced intercrystalline attack only of the 18/8 stainless steel.     

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.     

Elektrochemisches Verhalten von Aluminium und Möglichkeiten des Korrosionsschutzes in der Praxis

Electrochemical behaviour of aluminium and possibilities of practical corrosion protection The characteristic passivity of aluminium is shown by potentialcurrent density diagrams of materials AlMg2Mn0.8 and AlMgSi0.5 in seawater and in special hard water. These diagrams indicate the value of the passivity area, which is limited in anodic direction by the pitting potential. In the corrosion-system the pitting potential is influenced by the alloying elements in aluminium and also by the inhibitors in the corrosive medium. Zinc in aluminium diminishes the passivity area and sodium chromate in the special hard water enlarges the passivity area. Comparison of the corrosion behaviour of AlMg2Mn0.8 and high purity aluminium with respect to only the pitting potential does not permit conclusions to be drawn about the corrosion probability since pitting corrosion can only occur when a critical value is exceeded. The corrosion-sensitive aluminium-recycling material can be cathodically protected by reducing the potential below this value with help of galvanic anodes.     

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.     

Über das Verhalten einiger CrNiMo-Stähle beim Einsatz im chemischen Betrieb

The behaviour of some CrNiMo steels in use at chemical plants A report is given about the behaviour of some highly alloyed CrNiMo steels in use in inorganic-chemical plants. The observations are supported and supplemented by results of potentiostatic tests. In the presence of mixed acids, the corrosion resistance of the steels greatly depends, e.g., on the SO: Cl^? ion ratio. In the presence of Cl^? ions and at higher temperatures, exceeding about 70°C, the resistance is largely influenced by the specifically orientated analytical and structural pattern of the steels. Attention is drawn to the detrimental influence, especially in cast metals and welds, of the concomitant element, silicon, which — if encountered in increasing quantitities — favours the segregation of several corrosion-promoting phases. Examinations of case of damage in practical operation, supported by potentiostatic tests with CrNiMo steels, with and without copper contents, have shown that the presence of copper is apt to reduce the corrosion resistance in media containing hydrochloric acid or chlorine ions. On the other hand, a copper content may be beneficial in sulphate solutions free from chlorine ions.     

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.     

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.     

Hochlegierte nichtrostende Stähle und Nickelbasislegierungen im Chemie-Apparatebau

High alloy stainless steels and nickel-base alloys in chemical equipment construction The new ferritic chromium steels and ferritic-austenitic steels, high alloy special austenitic steels and nickel-base alloys presented in recent years have by now yielded some years' positive industrial experience in several applications. The group of austenitic steels containing 6 wt.-% molybdenum are at the threshold for large size application in chemical equipment construction. The results of the first comparative investigations with other high alloy steels are rather promising. The spectrum of nickel-base alloys will certainly be complemented by Hastelloy alloy C-22, so that this alloy will be used in addition to titanium alloys in contact with strongly oxidizing media.     

Untersuchungen zum Einfluss des Stickstoffs auf das Lochkorrosionsverhalten hochlegierter austenitischer Cr‐Ni‐Mo‐Stähle

Investigation of the influence of nitrogen on the pitting corrosion of high alloyed austenitic Cr‐Ni‐Mo‐steels Austenitic stainless steels (18% Cr, 12% Ni, Mo gradation between 0.5 to 3.6%) had been gas‐nitrided. By stepwise removal, samples could be prepared with various surface content of nitrogen up to 0.45%. The susceptibility against pitting corrosion of these samples had been tested by two methods: – determination of the stable pitting potential in 0.5 M NaCl at 25°C – determination of the critical pitting temperature in artificial sea water (DIN 81249‐4) The influence of nitrogen to both determined parameter can be described well by PRE = Cr + 3,3 · Mo + 25 · N That means for the investigated steel composition and the used corrosion system there is no influence of molybdenum on the effectiveness of nitrogen.     

Einfluß der Elemente Phosphor,Mangan, Chrom,Molybdän und Vanadium auf das Verhalten niedriglegierter Stähle bei Spannungsrißkorrosion

Influence of elements such as phosphorus, manganese, chromium, molybdenum and vanadium on the stress corrosion behaviour of low alloy steels A series of such steels having graduated alloy addition contents were obtained experimentally in an electrical furnace and were tested under permanent tensile loads in calcium nitrate solution after different heat treatments and at different relative loads. The heat treatment has a very pronounced bearing on corrosion susceptibility: overheating produces the highest sensitivity annealing at 750°C the — relatively — highest resistance, while normalizing occupies an intermediate position. P has but little effect on corrosion susceptibility so that its influence can practically be neglected. Mn seems to have generally a beneficial effect while Cr — even in amounts up to 2% — is unable to improve corrosion resistance. In the case of V there is some improvement in some cases only while Mo generally produces increased corrosion resistance. In this connection, a new method for evaluating experimental results has been described, which has been developed for the practice of steel assessment. The experimental results confirm, that a steel may be designated ?resistant”? at a certain load when its useful life under tensile load in alkaline media is 240 hrs. Extension of the test duration does not yield an improvement in evaluation accuracy.     

Potential-Sonden-Messungen und Potential-Profile über Schweißverbindungen korrosionsbeständiger Nickel-Basis-Werkstoffe und Sonderstähle

Potential probe measurements and potential profiles across weldments of corrosion resistant nickel-base alloys and special stainless steels A great number of potential profiles was measured across weldments (SMAW, GTAM, GMAW, Plasma) of the highly corrosion-resistant nickel-chromium-molybdenum alloys 22, C-276, C-4, of the nickel-chromium-iron-molybdenum-copper alloy G-3 as well as of the highly corrosion-resistant austenitic steel Cronifer 1925 hMo. After several tests had been made in a high-ohmic electrolyte solution of 90 vol.-% of methanol + 10 vol.-% of 10^?3 M HCl ( χ = 18 to 20 μs/cm) the tests were continued, for reasons of easier handling, in deionized water whose conductivity was adjusted by using 0.1 M HCl to χ = 10 to 20 μs/cm. It clearly proved that the potential profiles of various weld joints were dependent on the electrolyte. The potential profiles measured across weldments of the NiCrMo alloys which were made with matching filler metal revealed only insignificant or no differences between the potentials of the base material and the weldment. Consequently, in chloride-containing electrolytes it is very unlikely that matching NiCrMo weldments are subject to a preferred corrosion attack. This corresponds to practical experience. The potential profiles in the aqueous HCl electrolyte across weldments of the 6% Mo stainless steel Cronifer 1925 hMo and of the NiCrFeMoCu alloy G-3 welded with a NiCrMo-filler metal showed a potential drop of up to 420 mV over the weldment when compared with the base material. These potential differences can be explained by means of current density-potential curves, showing the electrochemical reactions of base material and weldment with the electrolyte to be different. Nevertheless, the corrosion resistance of the weldment is not inferior to that of the base material.     

Korrosionsverhalten einiger hochlegierter Stahl und Hartgußsorten in Waschwässern von Rauchgas-Entschwefelungs-Anlagen

Corrosion behaviour of some cast stainless steels and high alloy white irons in scrubber solutions of flue gas desulfurization plants Weight loss and electrochemical measurements have been used to determine the ranges of applicability of cast austenitic stainless steel Werkstoff No. 1.4408, of two special cast ferritic-austenitic stainless steels NORIDUR® 9.4460 and NORICLOR® NC 24 6 and of two high alloy Cr and CrMo white irons in scrubber solutions of Flue Gas Desulfurization (FGD) plants. Whereas the Werkstoff No. 1.440 8 cannot be used due to its insufficient resistance to general and localized corrosion, NORIDUR® 9.4460 can be used in scrubber solutions with pH > 2.5 and chloride concentrations up to 80 g/l, NORICLOR® NC 24 6 with 5% Mo even in liquids with pH > 1.5 and chlorides up to 100 g/l. At lower pH-values both duplex stainless steels show active corrosion of either the austenite or the ferrite depending on the contents of hydrochloric acid in the solution. At higher chloride concentrations pitting occurs on the passive materials. The CrMo white iron NORILOY NL 25 2 with 25% Cr and 2% Mo can be used in scrubber liquids with pH > 3.5. As the ferritic matrix is cathodically protected by the precipitated carbides, there is no sensitivity of this alloy to chlorides. In liquids with pH < 3.5 there is selective corrosion of the ferritic matrix. For practical application of all these cast alloys the limits for purely corrosive attack have to be modified to assure. resistance to a superposition of corrosion, erosion/abrasion and cavitation on parts exposed to real flow conditions in FGD scrubbers.     

Inter- und transkristalline Spannungsrißkorrosion austenitischer Mn- und CrNi-Stähle in Meerwasser

Intercrystalline and transcrystalline stress corrosion cracking of austenitic Mn and CrNi steels in seawater The MnCr steels which were originally used for the construction of special ships turned out to be susceptible to intercrystalline stress corrosion cracking in seawater; later on, transcrystalline corrosion susceptibility was found, too; this latter type of corrosion appears in cold seawater and is not due to a sensilizing annealing. This type of corrosion was also found with austenitic CrNi steels in chloride solutions of higher temperature, not, however, at room temperature. The author has made an effort at defining the susceptibility regions of the particular austenitic steels. According to the results obtained it is necessary always to take account of the possibility of stress corrosion cracking when Mn based austenitic steels are used, while austenitic CrNi steels can be considered to be resistant to this type of corrosion. Sensilizing annealing, too, has a bearing on the Mn containing steels only, while the influence of temperature, potential, specimen diameter and stress does not reveal any difference between the two types of steels.     

Der Einfluß von Kupfer auf das Korrosions und Passivierungsverhalten von Eisen-Chrom-Legierungen Teil I: Elektrochemisches Verhalten in schwefelsauren Elektrolyten

The influence of copper on corrosion and passiviation of iron-chromium-alloys Part I: Electrochemical behavior in sulfuric acid solutions The kinetic of dissolution and passivation of alloys type FeCrXCuY (X = 5; 7; 9; 11%; Y = 0; 0.5; 1%) was investigated in 1N H₂SO₄ (pH 0.3) and 0.5 M Na₂SO₄ (pH 3), using rotating disc electrodes (RDE) and hydrodynamically modulated rotating ring disc electrodes (HMRRDE). Instationary current density-potential curves (10 mV/s) were measured. For both types of electrodes the transients of current density after pulse polarisation into the passive range were investigated. The selfactivation of one hour prepassivated electrodes (open circuit breakdown) was investigated by potential scans. The dissolution of Cu bearing samples led to the formation of a visible film of copper lowering the current densities in the active and prepassive range as well as the critical current and charge densities of passiviation. Copper accelerates similar to chromium the decline of current transients after pulse passivation, whereas the partial current for film formation was increased as deduced from studies with the HMRRDE, leading to the conclusion of a larger film thickness. However, according to the accelerated open circuit breakdown of copper bearing samples, films on these alloys are less stable than expected, which is related to a faster removal of the oxide film. The results demonstrate an ambivalent effect of copper in the passive state of these alloys.     

Zur pH-Abhängigkeit der Aktivierungs- und Passivierungspotentiale 17%iger Chromstähle

The dependence on the p_H value of the activation and passivation potentials of 17%-chromium steels For a 17%-chromium steel, the active-to-passive transition was investigated potentio-kinetically and potentio-statically (stationary test) in diluted sulphuric acids, at 60° C, with pH -values ranging from about 0 to 2. The observed values greatly depend on the measuring method. In some cases, the potentio-kinetically observed values are found to be considerably on the low side, the error being due to the stationary condition failing to materialize, or due to cathodic currents. Stationary, potentio-static measurements showed a correlation of about ?58 mV/pH for the passivation potential and about ?106 mV/pH for the activation potential.