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.     

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.     

Einfluß einer Wärmenachbehandlung auf die mechanischen Eigenschaften und das Korrosionsverhalten nichtrostender Stähle

Influence of an ultimate heat treatment of the mechanical properties and the corrosion behaviour of stainless steels Unfavorable structures as may arise during steel processing and may impair their behaviour can be eliminated by an ultimate heat treatment. Cold hardening can be removed by heating to about 1000°C for about 1 min; dissolution of chromium carbide precipitated requires 3 min at 1030°C and dissolution of sigma phase about 10 min at 1050°C. The treatment, however, will not be successful, unless the temperatures mentioned are achieved throughout the whole workpiece and a sufficiently high colling rate is maintained after the treatment.     

Interkristalline Korrosion ferritischer Chromstähle mit rd. 17 Gew.-% Chrom nach Glühen im Temperaturbereich um 500 °C

Intergranular corrosion of ferritic 17% chromium stainless steels after heat-treatment in the 500 °C temperature range After stabilizing heat-treatment at 750°C, the non-stabilized, ferritic 17% chromium stainless steel Mat.-Nr. 1.4016 (X8Cr17) still contains a sufficient high concentration of carbon dissolved in solid solution, that after heat-treatment in the 500 °C temperature range carbides rich in chromium of the M₂₃C₆-type are precipitated, causing a relatively weak pronounced susceptibility of the steel to intergranular attack. The susceptibility to this type of attack can be detected by testing specimens in the sulfuric acid-copper sulfate-test with increased concentration of sulfuric acid as compared with the DIN standard 50914, followed by metallografic examination of the specimens. The susceptibility to intergranular corrosion of the material investigated occurring after heat-treatment in the low temperature range, which until now is unknown, is described in terms of a Rollason-diagram as it is commonly used for austenitic chromium-nickel stainless steels. As it is to be expected, the stabilized 17% chromium steels Mat. No. 1.4510 (X8CrTi17), 1.4511 (X8CrNb17) and 1.4523 (X8CrMoTi17) are resistant to intergranular corrosion after heat-treatment at low temperatures.     

The influence of molybdenum on the intergranular corrosion of a pure ferritic steel with 17% chromium

The influence of adding 3% Mo on the restoration of the intergranular corrosion resistance of a pure ferritic steel with 17% Cr was investigated on specimens quenched from 1050°C and aged at 700°C (from 5 min to 15 h). The tests were conducted in two standard oxidizing reagents currently employed for intergranular corrosion studies (Streicher and Huey tests). Whereas the sensitivity to grain boundary corrosion of a steel without molybdenum decreases in both reagents with the holding time at 700°C, the grain boundary corrosion of a 3% Mo steel, on the contrary, increases progressively. The behaviour of the steel without molybdenum is well accounted for by the theory of chromium depletion wereas that of the steel with 3% Mo can be explained by the increased rate of electrochemical dissolution of chromium carbides in which part of the chromium is replaced by molybdenum. This interpretation then gives a new significance to the Streicher test.     

The influence of molybdenum on the intergranular corrosion of a pure ferritic steel with 17% chromium

The influence of adding 3% Mo on the restoration of the intergranular corrosion resistance of a pure ferritic steel with 17% Cr was investigated on specimens quenched from 1050°C and aged at 700°C (from 5 min to 15 h). The tests were conducted in two standard oxidizing reagents currently employed for intergranular corrosion studies (Streicher and Huey tests). Whereas the sensitivity to grain boundary corrosion of a steel without molybdenum decreases in both reagents with the holding time at 700°C, the grain boundary corrosion of a 3% Mo steel, on the contrary, increases progressively. The behaviour of the steel without molybdenum is well accounted for by the theory of chromium depletion wereas that of the steel with 3% Mo can be explained by the increased rate of electrochemical dissolution of chromium carbides in which part of the chromium is replaced by molybdenum. This interpretation then gives a new significance to the Streicher test.     

Korrosion in der Wärmeeinflußzone geschweißter chemisch beständiger Stähle und Legierungen und ihre Verhütung

Corrosion in the heat-affected zone of welds in chemically resistant steels and alloys, and respective preventive measures As to weldability, chemically resistant steels and alloys can be classified as follows: (1) weldable without any restriction; (2) weldable only with reduced wall thickness; (3) weldable only with ultimate thermal treatment. These restrictions are due to the precipitation of chromium carbide and intermetallic phases at the grain boundaries; this effect gives rise to a Cr and Mo depletion and, finally, to intercrystalline corrosion susceptibility. In view of the fact that weldability requires the delay of the precipitation of chromium carbides at 650°C for at least 1 hour, and of intermetallic phases at 900°C for at least 10 min it is possible to reduce the carbon content and/or to add stabilizing elements (Ti, Nb). It should be taken into account, however, that the precipitation behaviour is not a function of carbon concentration, but rather of carbon activity which, again, depends from the overall composition of the respective alloy. This activity is increased by Ni and Si, while Mn and N function as decelerators and, consequently, contribute to weldability. In the case of the steel X 3 CrNiMoN 17 13 5 the addition of N inhibits not only the carbide precipitation but also the precipitation of the Chi-phase (at 950°C). In the case of the alloy NiMo 16 Cr the gradual reduction of the contents of secondary constituents has resulted in a practically pure ternary system characterized by high precipitation resistance: carbides are precipitated only after 5 hours at 800°C.     

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).     

Ausscheidungs- und Korrosionsverhalten unstabilisierter und stabilisierter 18/10 Chrom-Nickel-Stähle nach kurzzeitigem sensibilisierendem Glühen

Precipitation and corrosion behaviour of unstabilized and stabilized 18 10 CrNi steels after short term sensibilizing annealing The authors have studied the precipitation behaviour of the steels X5 CrNi 18 9, X10CrNiTi 18 9 and X 10CrNiNb 18 9 after solution and sensibilization annealing, and the intercrystalline corrosion behaviour in the Straußtest. On the basis of the results obtained it was possible to determine the and carbonitrides and to define the grain segregation ranges. In the case of the unstabilized materials the behaviour is a function of the formation of chromium depleted zones. However, it is not absolute value of the depletion at the carbid/austenite interphase which controls corrosion, but the depletion in an adjacent zone of measurable thickness. Beyond that the Huey test allows the corrosion susceptibility to be determined in those regions where no continuous chromium depleted zone exists. In the case of stabilized steels the corrosion behaviour depends in addition from carbonitride precipitation, since these compounds are heavily attacked by the boiling nitride acid.     

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.     

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.     

Elektronenmikroskopische Untersuchung der Carbidausscheidungen in korrosionsbeständigen Stählen

Electronmicroscope study of carbide precipitation in stainless steels Extraktion replicas and thin metal foils can be used for the preparation of specimes for electron microscope investigation. Extraktion replicas enable the size of precipitated carbide particles and their distribution in the metal structure to be established. With the thin metal foils the precipitation mechanism and orientation relations between precipitate and base material can be studied. The experimental techniques are explained for two applications. In this connection the relation has been confirmed between chromium carbide precipitation at grain boundary and intercristalline corrosion of austenitic CrNi steel. It has been found that it is not carbide morphology but the continuity of the carbide network which plays the predominate role. In addition, direct evidence has been obtained of the fact that the knife-line corrosion found in the vicinity of weldseams is mainly due to Ti carbides occuring in Ti-stabilised steels.     

Aging of cast Ni-base heat resisting alloy

Abstract

The behaviour of a Ni-base heat resisting alloy during aging was studied by mechanical testing and by optical and electron microscopy in samples aged for up to 6 weeks at 750°C. The as cast microstructure consists of an austenitic matrix and a network of two types of primary carbide (chromium and niobium). As aging proceeds, the main changes detected were precipitation of secondary chromium carbides and decomposition of primary niobium carbide in Ni rich silicide. The primary chromium carbides were largely unaltered. The mechanical tests indicate that these phenomena degrade the properties of the material.     

Selektive Korrosion an Verbindungsschweißen von Stählen

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

Erfahrungsaustausch und Schadensfälle. Grain boundary attack of stainless in ammoniacal atmospheres

Grain boundary attack of austenitic stainless steels in media containing ammonia in high concentrations cannot be attributed to chromium depletion. In the particular conditions existing in urea manufacture reactors the solubility of metal oxides in the reaction medium is particularly important; this effect comes to bear in particular at high nickel contents. On the base of an evaluation of the behaviour under industrial conditions it may be concluded that the corrosion manifests itself in particular at the grain boundaries, while the grain as such is not affected. In order to combat this type of corrosion the ammonia; water ratios should not exceed 6:1; an alternative solution would consist in increasing the temperature at the points where intercrystalline attack had been found.     

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.     

Influence of Ti,C and N concentration on the intergranular corrosion behaviour of AISI 316Ti and 321 stainless steels

Intergranular corrosion behaviour of 316Ti and 321 austenitic stainless steels has been evaluated in relation to the influence exerted by modification of Ti, C and N concentrations. For this evaluation, electrochemical measurements – double loop electrochemical potentiokinetic reactivation (DL-EPR) – were performed to produce time–temperature–sensitization (TTS) diagrams for tested materials. Transmission (TEM) and scanning electron microscopy (SEM) were used to determine the composition and nature of precipitates. The addition of Ti promotes better intergranular corrosion resistance in stainless steels. The precipitation of titanium carbides reduces the formation of chromium-rich carbides, which occurs at lower concentrations. Also, the reduction of carbon content to below 0.03 wt.% improves sensitization resistance more than does Ti content. The presence of Mo in AISI 316Ti stainless steel reduces chromium-rich carbide precipitation; the reason is that Mo increases the stability of titanium carbides and tends to replace chromium in the formation of carbides and intermetallic compounds, thus reducing the risks of chromium-depletion.     

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.     

Untersuchung der Potentialabhängigkeit der interkristallinen Korrosion eines sensibilisierten ferritischen Chromstahls mit rund 17% Cr

Investigations into the relationship between the potential and the inter-crystalline corrosion of a sensitized ferritic chromium steel with approx. 17 per cent, chromium Specimens of a ferritic chromium steel with 0.10 per cent. C and 17.43 per cent. Cr were used in order to ascertain, in the stable-annealed and sensitized condition, the relationship between the potential and the corrosion rate, polarisation current density and depth of penetration of the inter-crystalline corrosion attack. The tests were carried out in external potentiostatic arrangement over the potential range from E_H = + 250 mV to + 1250 mV in boiling 2 n H₂SO₄. All tests were extended over a standard period of 24 hours. Stable-annealed specimens showed no signs of corrosion attack whilst sensitized specimens showed inter-crystalline corrosion of the active-passive and passive-passive types. In the potential range of the active-passive type, the depth of penetration of the grain boundary attack decreases with increasing potential whilst, within the range of the passive-passive type, the depth is independent of the potential. The critical potential marking the boundary between active-passive and passive-passive corrosion is interpreted as the passivation potential of the zones with the greatest chromium deconcentration. With increasing sensitization, this boundary is shifted towards more positive potentials. The potential of the Strauss solution lies within the potential range most favourable for the test of the inner-crystalline corrosion of the active-passive type. The Strauss test in accordance with Steel-Iron Testing Standard 1875/61 is fully adequate for proving the grain decomposition resistance with non-stabilized 17 pC ferritic chromium steels.     

Korrosionselemente durch Gefügeheterogenität in nichtrostenden Chrom-Nickel-Stählen

Corrosion elements set up by structure heterogeneity in stainless chromium-nickel steels Transformation processes and precipitation of carbides could be the reason for corrosion elements in austenitic stainless steels. Examples are the intergranular corrosion and the selective martensite corrosion. The intergranular corrosion is based on a change of the electrochemical properties of the grain boundary regions by impoverishment of chromium. This “classical intergranular corrosion” takes place at all potentials. Resulting from the impoverishment of chromium after colling to very low temperatures, or by cold deformation, an α′-phase can be produced. This phase is only attacked in the active region of the current density-potential-curve. Both the intergranular corrosion and the corrosion of martensite are caused by local anodes. A selective attack of grain boundaries is also observed in the transpassive region. The intensity of this attack increase with precipitation of carbides. During the sensitivisation cathodes are produced, which cause a polarisation in the intergranular corrosive regions. Therefore, this attack of grain boundaries is not only connected with the formation of local anodes but also influenced by the formation of local cathodes.