Korrosionsverhalten hochchromhaltiger,ferritischer, chemisch beständiger Stähle

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

Neue Erkenntnisse auf dem Gebiet der ferritischen rostfreien Stähle

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

Der Einfluß von Stickstoff auf die korrosionschemischen Eigenschaften lösungsgeglühter und angelassener austenitischer 18/10 Chrom-Nickel- und 18/12 Chrom-Nickel-Molybdän-Stähle - II. Interkristalline Korrosion in Kupfersulfat-Schwefelsäure-Lösungen und in siedender 65% iger Salpetersäure

Influence of nitrogen on the corrosion behaviour of solution treated and annealed austenitic 18/10 chromium-nickel and 18/12 chromium-nickel-molybdenum steels. - II. Intercrystalline corrosion in copper sulfate sulfuric acid solution and in boiling 65% nitric acid Heat-treated steels containing 18.5 Cr, 11 Ni or 17.2 Cr, 13 Ni 2.6 Mo were investigated in the Strauß-Test (also in more rigid version) and the Huey-Test. The parameter studied is the precipitation behaviour and its influence on susceptibility to intercrystalline corrosion. The two methods of investigation differ with respect to the information obtainable; the influence of nitrogen e. g. can be recognized in the Huey-Test only. Nitrogen shifts the lower borderline of the grain disintegration field toward higher annealing temperatures and longer annealing times; the Z-phase appearing in the nitrogen and molybdenum containing steels influences the upper borderline and produces a significant increase of corrosion rate in nitric acid. The results are interpreted in terms of the chromium depletion theory.     

Untersuchungen über den Einfluß der Wärmebehandlung auf die Korrosion titanstabilisierter ferritischer Chromstähle in siedender konzentrierter Salpetersäure

Investigations into the influence of the thermal treatment on the corrosion of titanium-stabilized ferritic chromium steels in boiling concentrated nitric acid Titanium-stabilized ferritic chromium steels with about 17% Cr are, after quenching from high temperature, susceptible to grain boundary corrosion in boiling nitric acid; this corrosion is attributable to the chemical dissolution of the titanium carbonitrides coherently segregated at the grain boundaries. This carbide corrosion can be reduced by heat treatment within the temperature range around 850°C. In this process, the segregated carbo-nitrides are formed-in (in-formation annealing). The influence of quenching (0.5–5 hours, 900–1200°C/W[L]) and subsequent in-formation annealing (1–30 hours, 850°C/W) on the corrosion rate in boiling 65% nitric acid has been investigated in detailed on two steels of types X 8 Cr Ti 17 and X 8 Cr Mo Ti 17. In addition, the corrosion behaviour of titanium-stabilized and unstabilized ferritic chromium steels with about 17 pC chromium have been compared with each other.     

Erweiterte Diskussionsbemerkungen zum Einfluß von Silicium und Molybdän auf die Spannungsrißkorrosionsempfindlichkeit austenitischer und austeno-ferritischer Chrom-Nickel-Stähle

Amplified discussion of the influence of Silicon and Molybdenum on the stress corrosion cracking proneness of austenitic and austeno-ferritic chrome-nickel steels Tests have been carried out in boiling magnesium chloride, calcium chloride, NaCl, water (150–200°C) and hot steam (500°C); in the two last-named cases, the tests were carried out with and without the addition of chloride and oxygen. The following materials were tested: steels with (per cent.) 17 and 17.5 Cr, 12–15.5 Ni, 0 and 2.5 Mo, 0 and 4 Si, less than 0.03 C, as well as steels with 20 and 21 Cr, 8—10 Ni, 0 and 2.5 Mo, 0 and 3 Si, 0, 1.5 CU, less than 0.05 C. Silicon has a favourable effect on the stress corrosion cracking behaviour in magnesium chloride and also in calcium chloride, but a detrimental effect in NaCl and water. Its probable effect is to inhibit the extension of the crack by crystallographic obstacles and, later, by the formation of a surface film of poor conductivity (where Mo also plays a part). The attempt is made to interpret this behaviour on the strength of electro-chemical investigations; in this connection, renewed critical reservations are made in respect of the test in magnesium chloride.     

Transkristalline Spannungsrißkorrosion an austenitischen Manganstählen in chlorionenhaltigen Angriffsmedien

Transcrystalline stress corrosion cracking of austenitic manganese steels in corrodent containing chloride ions Experimental investigation with steels (0,5% C, 20% Mn, 0.2—2.7% Cr) in the shape of U-bent specimens in seawater (RT and boiling) under loads between 30 and 45 kp/mm². The electrochemical behaviour of chromium-free types (i.e. below 0.5% Cr) at RT is identical to that of ferritic shipbuilding steels (equal break-through potential, then uniform attack). Addition of N, Ni, Co and Cu have little influence on the potential behavior. In boiling sea-water, however, transcrystalline corrosion takes place, accelerated by high tensional loads and anodic polarization, and slowed down by cathodic polarization. Additions, in particular of Co and Ni, reduce specimen life, perhaps by reducing yield strength. At higher Cr contents (1.7–2.8%) susceptibility to intercrystalline corrosion begins to supersede susceptibility to transcrystalline cracking. On the basis of ideas concerning the active-passive behavior under the influence of gliding phenomena the author shows analogies to the stress corrosion behaviour of austenitic stainless steels.     

Spannungsrißkorrosion Mo-freier und Mo-haltiger Stähle in Calciumnitrat-Lösung und Natronlauge unter zeitlich konstanter Kraft und unter CERT-Bedingungen

Stress corrosion cracking of Mo-free and Mo-bearing steels in solutions of calcium nitrate and sodium hydroxide under constant load and under CERT conditions The threshold stress a which was obtained for a low-alloy steel (0.08% C, 0.01% Mo) under free corrosion conditions and chemi- cal conditions corresponding to DIN 50915 in boiling 60% Ca(NO₃)₂ solution in constant load SCC experiments was con- firmed by electrochemically controlled experiments (potential range 0 to 0.90 V). The susceptibility to SCC is increased with the potential becoming more anodic. However, the result of the corrosion test according to DIN 50 915 is valid also at more anodic potentials, i.e. under more severe corrosion stresses. For the determination of threshold stresses by constant load SCC experiments, the criterion “life time > 1000 hrs” is reliable and should be generally preferred to the criterion “crack length 1 = 0”. In boiling 60Y0 Ca(NO₃)₂ solution, also such steels are suscep- tible to SCC under CERTconditions which, under constant load conditions (σ = const.) are resistant. The mode of mechanical loading highly influences the SCC response. Critical potential ranges of SCC were evaluated by CERT experiments. The steel composition (Mo content) has no effect on the cathodic limiting potentials (EH=0 V), but on the extension of these ranges to the anodic side. Under CERT conditions, the favourable influence of molybde- num on the resistance against SCC is not noticed in boiling 60% Ca(NO₃)₂ solution but under less severe corrosion stresses (55% Ca(NO₃)₂ solution, 75 °C). With the given stress-induced SCC system “teel/Ca(NO₃)₂ solution”, threshold stresses could not be evaluated by interrupted CERT experiments, a method which otherwise was successfully used with strain-induced SCC systems. The critical potential ranges of SCC are in the transpassive ranges of the corrosion rate vs. potential curves obtained with unloaded specimens. Critical potential ranges of susceptibility to SCC were evaluated by CERTexperiments also in boiling 35% NaOH. The potential of highest susceptibility to SCC is always found between Ea = -0.80 and -0.70 V and is not affected by influencing factors of the medium (concentration, temperature), and by the steel composi- tion (Mo content).The unfavourable effect of Mo on the resistance to SCC in NaOH is reflected by the extensionof the potential range of increased susceptibility to SCC to more anodic potentials. The critical potential ranges of SCC are in the transition range from the active to the passive state of the corrosion rate vs. potential curves. The potentials of the highest susceptibility to SCC are identical with the minima of the corrosion rate vs. potential curves which occur after passing the potential range of active corrosion. The anodic metal dissolution at the crack tip which is obviously essential for the SCC system “steel/Ca(NO₃)₂ solution” is of minor importance for the system “steeVNaOH solution”. The results of metallographic investigations of the tested SCC specimens are presented. In distinct potential ranges, the Bainite phase (calcium nitrate solution), and the Bainite or Perlite phase (sodium hydroxide solution) are preferentially dissolved. These corrosion phenomena are stress induced, and are not observed with specimens free from external stresses.     

Corrosion and deposition of magnetite in ferritic steel steam generator tubes under acid chloride conditions

Abstract

Corrosion of a tube made up of 9Cr–1Mo, 2·25Cr–1Mo, and 2·25Cr–1Mo–Nb ferritic steels and containing artificial defects has been investigated under realistic steam generator conditions (355°C, 17·6 MPa) with acid chloride fault water chemistry (2 mg/kg HCl). Four regions of corrosion and magnetite deposition behaviour were observed on the tube surface. In non-heat flux regions, magnetite deposition was affected by mass transfer and probably also by surface potential. In low heatflux regions (<660 kW m^?2) observed increases in the rates of magnetite deposition and corrosion wereprobably due to rises in the degree of iron supersaturation and HCl concentration, brought about by boiling. Enhanced HCl concentrations in the normal heat flux region (660 kW m^?2) prevented magnetite deposition and caused an increase in corrosion of the three steels. Increases in corrosion and magnetite deposition were also observed at the weld between the 9Cr–1Mo and 2·25Cr–1Mo steels. In defects, accelerated corrosion was seen only in the 9Cr–1Mo steel and was confined to the top 0·15 mm. It is concluded that the corrosion behaviour observed in this work is determined by the residence time and concentration of solutions of HCl on tube and defect surfaces.     

Oberflächenpotentialprofile von metallen

Surface potential profiles of metals Using a microcapillary method in connection with simultaneous metallographic observation surface potential profiles have been determined with the potential probe in alcoholic acid solutions on cast iron, CrNi aciers steels 188, Cr steel with 17% Cr (welded) and an AlCu alloy. It was possible to distinguish cementite from ledeburite, not however, from austenite. The potential of 188 steel under pitting conditions decreases as a linear function of pit depth. The appearance of potential minima in a weld of ferrite steels is evidence of a intercrystalline corrosion in that particular medium. The precipitation of CuAl₂ in AlCu alloys at 150 °C at the grain boundaries forms an element with the grain boundary as the anode giving rise to intercrystalline corrosion.     

Erfahrungsaustausch und Schadensfälle. Schweißschlackenkorrosion bei hitzebeständigen Chrom-Nickel-Stählen in Strahlheizrohren

Corrosion by weld slag of refractory chromium nickel steels in radiative heating tubes The elevated sulfur concentration in the weld slags on refractory steels with nickel contents between 10 and 48% has given rise to considerable corrosion and the steels were destroyed by the low melting NiS eutectic. This corrosion is more severe in atmospheres resulting from incomplete combustion than in oxidizing atmospheres. Corrosions have been encountered in nonwelded material, too; in this case the formation of NiS can be attributed to the presence of CaO in the blast furnace gas; the incrustations then formed behaved like the welding slag and absorbed sulfur from the gas. It is recommended in such cases to weld not with wire electrodes but to use the TIG method.     

Korrosionsprobleme in chlorhaltigen Medien: Lösungsmöglichkeiten durch einige nichtrostende Spezialstähle

Corrosion problems in chloride containing media: possible solution by some stainless special steels The increasing water pollution forces the chemical industry to use water with increasing chloride content for cooling and other purposes. This trend brings about increasing corrosion danger, in particular pitting, stress corrosion cracking and corrosion fatigue as well as crevice corrosion. The present paper deals with some steels characterized by resistance to these specific corrosion phenomena. A steel containing (%) 21 Cr., 7.5 Ni, 2.5 Mo, 1.5 Cu, to 2 Mn, to 1 Si and 0.06 C is particularly resistant to stress corrosion cracking. It contains 30 to 50% ferrite in an austenitic matrix. Even in Mg chloride solutions it may be kept under a load of 7 kg/mm² without stress corrosion occurring (with a steel of the 18 10 CrNiMo type the admissible load is only 2 kg/mm²). A steel containing (%) 25 Ni, 21 Cr, 4.5 Mo, 1.5 Cu, to 1 Si, to 2 Mn, and 0.02 C has a broad passivity range and is resistant to general corrosion in acid reducing media and phosphoric acid of all concentrations. A ferritic steel containing (%) 26 Cr. 1 Mo and minor additions of C, Mn, Si, Cu, Ni and nitrogen is resistant to stress corrosion cracking in neutral chloride solutions and general corrosion in oxidizing and neutral media, even against hydrogen sulfid and organic acids; it is beyond that lergely resistant to pitting in chloride solutions.     

Zum Korrosionsverhalten nichtrostender austenitischer Chrom-Nickel-(Molybdän,Kupfer)-Stähle in nahezu wasserfreier Essigsäure

Corrosion of stainless austenitic steels in almost anhydrous acetic acid As-welded samples and looped specimens from 5 differently alloyed stainless steels were tested for up to 246 days in 99,5% to 99,95% acetic acid at 118°C (boiling temperature/normal pressure) and at 150°C; the chloride content was varied between < 1 and 100 ppm. Pitting corrosion – of shallow depth, however (approx. 0,1 mm) – was already observed at surprisingly low chloride concentrations. Only the following were found to be resistant to pitting corrosion:

• – stainless steels 1.4439 and 1.4539, containing approx. 4,5% molybdenum, in 99,5% acetic with < 1 ppm chloride at 118 and 150°C,
• – stainless steels 1.4439 and 1.4539 in 99,9% acetic acid with < 1 ppm chloride at 118°C, and
• – special stainless steel X 2 CrNiMoCuN 20 18 6, containing approx. 6% molybdenum, in 99,5% acetic acid with > 3, < 10 ppm chloride at 118 and 150°C.

Looped specimens and ground as-welded samples showed no sensitivity to transcrystalline, chloride-induced stress corrosion cracking at any of the concentration ranges. High surface-removal rates can be expected if air has access to the specimens; under this condition pitting corrosion and general corrosion may overlap. Contamination of acetic acid with chlorides must be prevented under all circumstances.     

Die Lochkorrosion austenitischer Chrom-Nickel- und Chrom-Nickel-Molybdän-Stähle in schwefelsaurer Bromidlösung und ihre Inhibition durch Nitrationen

Pitting corrosion of austenitic chromium nickel and chromium nickel molybdenum steels in sulfuric acid containing bromides, and its inhibition nitrate ions In acidified bromide solution CrNi steels are attacked under pitting when a certain critical potential has been exceeded; this potential is higher than in the case of chloride containing solutions. Bromides are, consequently, less active than chlorides, but the pit density is considerably higher under idential corrosion conditions. While the pitting corrosion in chloride solutions can be considerably reduced by molybdenum addition to the steel, this effect is but little pronounced in the case of bromide solutions (with Mo additions up to 4% the potential is displaced by 0.2 V toward positive values). Mo additions around 2% are even dangerous since the pitting density is considerably increased in that range. Similar to the conditions in chloride solutions corrosion in bromide solutions is inhibited by nitrate additions; the potential limit is considerably higher in the bromide solution; this phenomenon points to stronger adsorption of bromide ions at the metal surface.     

Potentialabhängigkeit der Korrosion Mo-freier und Mo-haltiger Stähle in Calciumnitrat-Lösung und Natronlauge

Effect of potential on corrosion of Mo-free and Mo-bearing steels in solutions of calcium nitrate and sodium hydroxide The effect of potential on the corrosion behaviour of three low alloy steels with different carbon and molybdenum contents (0.08 C, 0.01 Mo; 0.08 C, 0.98 Mo; 0.18 C, 1.10 Mo) was investigated in boiling 60 wt.% Ca(NO₃)₂ solution (DIN 50 915) and in boiling concentrated NaOH solutions (20 and 35 wt.% NaOH) by potentiodynamic and chronopotentiostatic polarization measurements (i/E curves) and chronopotentiostatic mass loss measurements (corrosion rate v vs. potential E curves). In Ca(NO₃)₂ solution, i/E measurements give no information about the effect of potential on the anodic dissolution. For the materials investigated, v/E measurements indicate the existence of potential ranges with pronounced differences of the corrosion response. It can be differentiated between active, passive, and transpassive ranges, and also a potential range of secondary passivity was established. Transpassivity and secondary passivity are markedly pronounced with the molybdenum bearing steels but not with the steel free from molybdenum. There are no hints to the occurrence of intergranular attack in the specimens which are free from of internal and external mechanical stresses, whereas such hints could be derived from the shape of the i/E curves. Nevertheless, under mechanical stresses (constant load, CERT conditions) the materials are susceptible to intergranular SCC. The conception that intergranular SCC of low alloy steels in Ca(NO₃)₂ solution is connected with a break-through potential of grain boundary corrosion and hence is to be interpreted as an intergranular attack which, under mechanical stresses, runs in a modified form as SCC with intergranular crack path, cannot be maintained in such general terms. A pronounced active/passive behaviour is observed in NaOH. The effect of potential on anodic metal dissolution which is derived from v/E curve is also established by i/E measurements. For molybdenum bearing steels, the active potential range is somewhat extended to more positive potentials. Manifestations of localized attack, e.g., intergranular corrosion, do not occur. From the investigations, no hints to the cause of the deterioration of the resistance to intergranular SCC in caustic solutions by molybdenum can be derived.     

Einfluß von Passivierungs- und Kohlenstoffschichten auf austenitischen CrNiMo-Stählen auf die Beständigkeit gegen Lochfraß und Spannungsrißkorrosion

Effects of passivation and carbon films on austenitic CrNiMo steels on their piting and stress corrosion resistance The influence of passive film and combinations of a passivation and a carbon layer on the resistance to pitting and SCC of austenitic CrNiMo steels has been investigated in physiological sodium chloride solution (Tyrode solution) at pH 6.9 to 7.4 at 37 ± 1°C. The passive film was obtained after electrolytic polishing in H₃PO₄ + H₂SO₄ + C₆H₅NHCOCH₃ + oxalic acid + corrosion inhibitor CS by treatment with 40% nitric acid the carbon film was obtained by CVD. Impurities in the steel (non-metallic inclusions) and the different metallic phases were investigated and the chemical composition of the passive film was determined by quantitative analysis. The resistance to pitting of the steel with and without passive film was determined potentiodynamically in Tyrode's solution at 37 ± 1°C. The resistance to SCC was determined in Tyrode's solution at 37 ± 1°C, in neutral glycerole and in boiling magnesium chloride solution at 154 ± 1°C and evaluated in terms of K_σ and K_τ. The corrosion damage was investigated by optical and scanning electron microscopy. The investigations have revealed that the different surface conditions considerably improve the pitting and SCC resistance of the steels in the media used in this work, so that they make possible the use of these materials as surgical implants.     

Messerlinienkorrosion von automatisch hergestellten Schweißverbindungen aus korrosionsbeständigen Stählen

Knife-line corrosion of weld connections of corrosion resistant steels, produced with automatic welding equipment The investigation of submerged arc weldings (obtained with automated equipment) and electro-slag weldings (obtained by manual methods) of a steel of CSSR production and of AISI 316 L steel have revealed, that the behaviour of both is practically identical, i.e. that both of them are not susceptible to knife-line attack (testing in standard solutions, i.e. sulfuric acid/copper sulfate and 65% nitric acid). In the case of the submerged arc weldings the damaged zone is some what narrower, but in either case the cause of attack in the adjacent region is due to overheating to a temperature close to the solidus line. An important influence must be attributed to heat treatment: after sensitizing, the weld metal, too, may become susceptible to intercrystalline corrosion, while after certain types of heat treatments even sensitizing has no deleterious effects.     

Inkristalline Korrosion der Nickellegierung NiCr 20 TiAl

Intercrystalline corrosion of the nickel alloy NiCr 20 TiAl The corrosion behaviour of NiCr 20 TiAl shows a clear dependence from the carbon content. In Strauss solution the alloys containing 0.007% C are not susceptible to intercrystalline corrosion, while with carbon contents of 0.036 to 0.078% intercrystalline corrosion occurs similar to that in 18-8 steels. In boiling nitric acid the corrosion susceptibility further depends from the heat treatment. After cooling from the temperature of solution annealing in air high weight losses are found which even increase after ageing, while they are reduced by quenching in water and then can be found with the alloys having higher carbon contents only. The intercrystalline corrosion is attributed to a formation at the grain boundaries.     

Besonderheiten im Korrosionsverhalten hochchrom- und molybdänhaltiger Legierungen in heißer 92, 5%iger Schwefelsäure

Peculiarities in the corrosion behaviour of high chromium and molybdenum containing alloys in hot 92.5% sulfuric acid In laboratory tests at temperatures above 50°C unusual high corrosion rates of passivating stainless steels and nickel alloys containing more than 26% Cr were observed in 92.5% sulphuric acid. In order to investigate the cause of this phenomenon further corrosion tests and additional chemical analyses were performed. The H₂SO₄ concentration tested displays a relative maximum of the electrical conductivity, the reason being a stronger dissociation of the sulfuric acid. Electrochemical investigations revealed an enhanced activity of the cathodic reactions which lead to higher corrosion rates. The cathodic reactions are strongly dependend on alloy constitution with special emphasis on the contents of Cr, Ni and Mo. Mo containing stainless steel show potential oscillations (of the open circuit potential) between ?50 and +550 mV_H. These alloys corrode under development of SO₂ (reduction of H₂SO₄ molecules) and formation of several sulfur compounds with different oxidation numbers (6+ and 2?). Alloys with chromium contents above 26% develop additionally hydrogen gas due to a lower hydrogen overvoltage of these alloys. With increasing nickel content the overvoltage for the reduction reaction of H₂SO₄ molecules will be lowered. This fact results in an elevation of the exchange current density for the Alloy NiCr45 and therefore to the highest corrosion rate observed. Alloy B-2 shows the best resistance, i.e. very low corrosion rates. Obviously high levels of molybdenum can compensate the influence of nickel on the overvoltage of the reduction reaction or even hinder the cathodic reaction.     

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 Prüfung von Schweißplattierungen mit austenitischen Bandelektroden der Art X2 CrNiNb 2413 auf Beständigkeit gegen interkristalline Korrosion

Testing of weld-claddings with austenitic band electrodes of the type X2CrNiNb24 13 for resistance against intercrystalline corrosion Two-phase, austenite- and ferrite-containing weld claddings of the 24 Cr-13 Ni-type may become susceptible to intercrystalline corrosion after heat-treatment in the 550°C temperature range due to a sensitization of the austenite. Thus, both austenite/austenite- and austenite/ferrite grain boundaries are attacked with the attack starting from the austenite phase in the latter case. Testing the resistance to intercrystalline corrosion in a sulphuric acid/copper sulphate media in the presence of copper turnings according to DIN 50914, the attack of grain boundaries ferrite/austenite becomes evident only if the specimens are bent after testing. Non-bent specimens do not show any indication of grain boundary attack by light-optical or even electron-microscopical metallographic examination. At non-bent specimens, the attack may become recognizable with considerably prolonged (several hundred hours) testing time. Referring to the investigations performed, the mechanism of intercrystalline corrosion of two-phase, ferritic-austenitic stainless steels and the influence of heat-treatment on susceptibility to intergranular attack are discussed.