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.     

Untersuchungen zum Einfluss des Stickstoffs auf das Lochkorrosionsverhalten hochlegierter austenitischer Cr‐Ni‐Mo‐Stähle (Teil II)

Investigation of the influence of nitrogen on the pitting corrosion of high alloyed austenitic Cr‐Ni‐Mo‐steels (Part II) Austenitic stainless steel (18% Cr, 12% Ni, Mo gradation between 0,06 to 3,6%) had been solution nitrided. By step‐by‐step removing, the samples could be prepared with various surface contents of nitrogen from 0.04 to 0.42%. In two test series the influence of nitrogen had been determined. The susceptibility against pitting corrosion of these samples had been tested by the chronopotentiostatical method. For the investigated steel composition and the used corrosion system there is no infuence of molybdenum on the effectiveness of nitrogen. The effectiveness of nitrogen can be described by the factor 25 in the PRE. By the investigation of the surfaces with the XPS analysis, it could be shown that the passivation and the pit nucleation is influenced by nitrogen. In these ranges NO_x, NH_x, and NH_z‐spectra have been detected. Bound Mo was found in steels containing molybdenum. It is assumed that the repassivation mechanisms of N and Mo work independently of each other. With the results efforts are supported to improve the pitting corrosion resistance also at molybdenum poor steels by surface nitriding or nitrogen alloying. The achieved results justify the assumption that the observed positive effect of the nitrogen may be extented to even higher nitrogen contents. A prerequisite for this is avoiding secondary phases in the matrix. The adverse influence of small particles is known well.     

Einfluss von Verunreinigungen auf die Korrosionsbeständigkeit und die mechanischen Eigenschaften von Bauzink

Influence of impurities on the corrosion resistance and the mechanical properties of construction zinc Zinc is a material from which for example roofs, rain‐tubes and gutter are produced. For a sufficient creep resistance, zinc‐titanium‐copper‐alloys are used. Aluminium and lead impurities in the zinc alloy increase the susceptibility of the alloy to intercrystalline corrosion. The assessment of a damaged rain‐tube with scanning electron microscopy and EDX‐analysis shows high aluminium and lead concentrations in the alloy which led to intercristalline corrosion and embrittlement of the alloy. The alloy which has been used for the fabrication of the rain‐tube does not fulfil the specification of titanium zinc according to EN 988.     

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.     

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.     

Der Einfluss von Oxidationsmitteln auf die Korrosionsbeständigkeit von Nicrofer 3127 hMo in niedrig‐ und mittelkonzentrierter Schwefelsäure

The influence of oxidants on the corrosion resistance of Alloy 31 in low and medium concentrated sulfuric acid The isocorrosion lines < 0.1 mm/y were determined for Nicrofer 3127 hMo (1.4562, Alloy 31) by weight loss measurements after 24 h immersion in areated sulfuric acid of technical and p.a. grades in the concentration range up to 90% – both without and with 1 g/l chloride. The free corrosion potential indicates that the superior corrosion resistance of Nicrofer 3127 hMo in the technical acid grade can be attributed to the content of oxidants in the acid. The presence of oxidants leads to metastable passivity in technical sulfuric acid with concentrations ≥ 20% at temperatures that can be 10–50 K higher than in p.a. acid depending on the acid concentration. For sulfuric acid concentrations up to 10% the content of oxidants in the technical acid is too small to improve the corrosion resistance of Nicrofer 3127 hMo as compared with that in p.a. acid. Similar behaviour was observed for Nicrofer 3127 hMo in sulfuric acid containing chlorides. However the activating effect of the chlorides limits the region of corrosion resistance to lower temperatures in comparison with the chloride free‐acid.     

Einfluß von Eisen auf die Korrosionsbeständigkeit von Nickelbasislegierungen als mögliche Werkstoffe für SCWO-Reaktoren

Effect of iron on the corrosion behaviour of nickel based alloys for SCWO plants An experimental set up to study corrosion at high temperatures and high pressures and able to simulate the conditions of supercritical water oxidation is described. On the basis of the alloys AC 66, 45 TM, G-3 and 601 H the influence of iron on the corrosion behaviour of nickel base alloys in aqueous solution under high pressures containing oxygen and chloride is shown and discussed. The corrosion resistance of the nickel base alloys is decreased with increasing iron concentration in the whole temperature range (100 °C–420 °C). The corrosion started at lower temperatures and the weight loss increased rapidly with increasing iron content. The corrosion morphology changed from uniform corrosion for alloys with less iron content to deep pits for alloys with high iron concentrations.     

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.     

Die Korrosionsbeständigkeit metallischer Werkstoffe für Automobilbremsleitungen

Corrosion resistance of metallic materials in automobile brake lines A statistical evaluation of the data compiled by the Swedish Automobile Inspection Authority and covering corrosion effects in brake fluid lines enables the following recommendation to be made:

• (a) Copper-brazed steel tubes, though very common, have no sufficient corrosion resistance; the same applies to welded or seamless drawn steel tubing.
• (b) The best suited materials are phosphorus deoxidized copper and stainless steels (about X 5 CrNiMo 18 12). The lower fatigue strength of copper must be compensated by suitable design; as compared to the stainless steel, copper is advantageous from the cost side.
• (c) Copper as well as stainless steel tubing can be corroded at fittings and fastening points; these problems must be eliminated by suitable design to be decided uon in each particular case.

     

Korrosionsbeständigkeit metallischer Werkstoffe in mittelkonzentrierter heißer Schwefelsäure

Corrosion resistance of metallic materials in medium concentrated hot sulfuric acid For the newly developed chromium-base material Alloy 33 (German material No. 1.4591) the corrosion rate has been gravimetrically determined in non aerated high purity grade sulfuric acid in the concentration range from 15 to 80 mass-% at temperatures from 50 to 90°C. This way critical limits of corrosion resistance behaviour under very unfavourable conditions of exposure result. An erratic increase of the corrosion rate by a few orders of magnitude takes place with increasing temperature according to the transition from the passive into the active state. This transition into the active state is shifted towards lower temperatures with increasing concentrations of sulfuric acid. Alloy 33 (1.4591) in the passive state possesses clear advantages against the reference materials for use in sulfuric acid: Alloy 825 (2.4858) and Alloy 20 (2.4660). Polarization measurements in agitated oxygen containing and deaerated sulfuric acid solutions show a higher tendency for passivation and a more stable passive state in comparision with the reference materials. The presence of small amounts of oxidants such as ferric sulfate, nitric acid or potassium dichromate results in a widening of the range of corrosion resistance of Alloy 33 in the studied temperature-concentration field. In addition the minimum concentrations of oxidants necessary for this behaviour have been determined by investigation of the measured corrosion rates of Alloy 33 (1.4591). Polarization tests as well as potential measurements of rest potential and redox potential serve to further explain the mode of operation of these oxidants.     

Über die Korrosionsbeständigkeit von Nickel-Tantal-Legierungen

On the corrosion-resistance of nickel-tantalum-alloys The corrosion-resistance of different nickel-tantalum-alloys in concentrated phosphoric acid for use as current-collectors in hydrocarbon fuel-cells was measured potentiostatically in the anodic range at temperatures between 100 and 140°C. An alloy with a tantalum contents of 1 wt% showed active dissolution and high corrosion rates at all potentials, an alloy with 10 wt% tantalum showed a passive range at about 0,7 V and alloys with more than 30 wt% tantalum and tantalum itself showed very low corrosion-rates and self-passivation in phosphoric acid. A temptative explanation of the results by considering electronic properties of the alloys was given.     

Oberflächenbehandlungsverfahren und deren Einfluß auf die Korrosionsbeständigkeit nichtrostender Stähle

Surface treatments and their influence on the corrosion resistance of stainless steel The surface treatments pickling, grinding and glass beading were investigated on several stainless austenitic steels and one ferritic/austenitic steel. The different surface treatments were used on two different prepared types of samples:

• 1 high temperature oxidized samples
• 2 welded samples

The quality of the surface treatments has been examined by means of potentiodynamic, ferric chloride, dip and spray tests made in series. The corrosion resistance was highly depending on the used treatments. All in all the examination showed that a higher corrosion resistance was achieved by pickling than by grinding or glass beading.     

Die Korrosionsbeständigkeit der Silicide der Übergangsmetalle

The corrosion resistance of the silicides of transitional metals The disilicides of Ti, V, Nb, Ta, Cr, Mo and W, as well as Mo₅Si₃, Mo₃Si and W₃Si₂ have, in general, an extremely high resistance to acids. However, the silicides of Mo and W have a low resistance to most acids whilst the remaining silicides have a high resistance to nitric, hydrochloric and sulphuric acid, but a low resistance to hydrofluoric acid, but a low resistance to hydrofluoric acid. Similar conditions are encountered for hydroazoic acid whilst the resistance of all silicides to NaOH is poor. The high resistance to acids is due to the formation of a surface layer rich in SiO₂. To obtain comparable results, the silicides were ground to powder which was then fused in a plasma are into fairly uniform globules.     

Korrosionsbeständigkeit metallisch und organisch vorveredelter Stahlfeinbleche in der Freibewitterung

Corrosion resistance of unpainted and coil-coated sheets under atmospheric exposure An extensive study was carried out to compare the corrosion behaviour of unpainted and coil-coated hot dip galvanised and Galfan-coated sheets under atmospheric exposure. FIat and roll-formed test pieces with various metallic and organic coatings were exposed to atmospheric weathering at five different locations for 9 years. The corrosion behaviour of unpainted specimens was determined quantitatively by measuring the loss of metallic coating thickness and by visual assessment of red rust on surfaces and bends. In the case of organically coated specimens formation of red rust and blistering on surfaces and bends was assessed visually and creepage at scores and cut edges was measured. The results show that Galfan coatings are subject to much slower weathering than hot dip galvanised coatings. It is concluded that in most applications users could select the next lower coating thickness group when using Galfan instead of hot dip galvanised sheet. Visual assessment of corrosion damage (red rust, blisters, creepage) also leads to the overall conclusion that both unpainted and coil coated Galfan perform much better than hot dip galvanised sheet in natural weathering. The advantages of using Galfan especially for building applications are thus being established.     

Über die Korrosionsbeständigkeit inchromierter Teile in See- und Industrieatmosphäre

The corrosion resistance of chromized parts in sea and industrial atmospheres According to tests carried out with screws of low carbon steel the corrosion restistance is primarily determined by the thickness of the diffusion layer. The shape of the parts plays an important role, too: the corrosion resistance is lower when notched shapes are used. The conditions of the diffusion process have but little effect on the corrosion resistance. Generally, however, the protection afforded by the chromizing layer is not sufficient under the conditions studied: even with a thickness of 80 μm the useful life does not exceed one year. The reason of this low resistance is primarily the imperfect structure of the coating which inhibits the formation of the passive chromium oxide layer.     

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).     

Das Ausscheidungsverhalten von hochlegierten austenitischen Stählen mit 6% Molybdän und sein Einfluß auf die Korrosionsbeständigkeit

Precipitation behaviour of high-alloyed austenitic steels with 6% molybdenum and its influence on the corrosion resistance The high-alloy austenitic steels with 6 to 7% Mo, 20 to 21% Cr and 18 to 25% Ni are increasingly used in seawater and chemical applications. This is due to the excellent resistance to pitting and crevice corrosion in chloride-containing neutral and acidic environments. It is the high chromium and molybdenum content which provides the excellent corrosion behaviour but, at the same time favors the tendency to precipitation of intermetallic phases. Therefore, time-temperature-precipitation diagrams have been established for two steels with 6% Mo, 21% Cr, 25% Ni, 0.14 and 0.19% N and for one steel with 6% Mo, 20% Cr, 18% Ni and 0.21% N. The corresponding time-temperature-sensitization diagrams (in accordance to SEP 1877/II) and time-temperature-pitting diagrams (testing in 6% FeCl₃ solution) have been evaluated as well. Precipitation of intermetallics occurs rapidly especially in the range between 700 and 1000°C. In case of the 18% Ni steel and the 25% Ni/0.14% N steel grain boundaries are covered to a large extent with precipitates after only 15 min at 850 or 950°C. In case of the 25% Ni/0.19% N steel precipitation is considerably slower. The precipitates are interpreted to be chi-phase. After very long annealing times additionally small amounts of Laves phase appear. Neither carbides nor nitrides were observed. In spite of the rapid precipitation, sensitization in terms of the 50 m?m grain boundary penetration criterion is observed not before 0.7 h at 850 °C and not before about 2 h at 800°C in case of the 25% Ni/0.19% N steel. After about the same times of annealing also the critical pitting temperature as observed in the FeCl₃-test is dropping below 50°C. Therefore, when welding according to established rules and recommendations, no deterioration of the corrosion resistance in the heat-affected zone is to be expected. If high heat inputs will occur during manufacturing because of hot forming operations or welding of heavy sections, or if more severe test conditions are a requirement, a steel with 25% Ni and about 0.2% N (UNS N 08925, Cronifer hMo) is recommended due to its retarded precipitation and sensitization behaviour when compared to steels with only 18% Ni (UNS S 31 254). Additionally, the steel with 25% Ni has an increased resistance to general corrosion in acids. Notch impact strength of the materials under consideration is increased by the initial precipitation of the intermetallic phases and decreases only after longer times of annealing below the ductility of the solution annealed material.     

Korrosionsbeständige metallische Werkstoffe für Rauchgas-Entschwefelungsanlagen

Corrosion resistant metallic materials for flue gas desulfurization plants Forced by environmental legislation installations for flue gas desulfurization (FGD) are presently being built to a large extent in the Federal Republic of Germany as in some other countries. Absorption by lime/limestone slurries is used in most cases for desulfurization. The components of the flue gas as well as the kind of process applied are of primary importance for the corrosive behaviour of the materials of construction. In view of pH values going down below 1 in some cases, chloride contents of occasionally more than 10% Cl^?, temperatures between 40 and 160°C and local deposits of solids it is the resistance to pitting and crevice corrosion which has to be considered in the first place while stress corrosion cracking and erosion corrosion are of minor importance. Therefore, only austenitic materials with molybdenum contents of more than 2 wt.-% have to be considered. According to the severity of the corrosive media these are predominantly alloys as e.g. Cronifer 1925 LCN (Alloy 904 LN), 1925 hMo (Alloy 904 LM) going up to the high alloyed nickel base materials Nicrofer 6020 hMo (Alloy 625) and 5716 hMo W (Alloy C-276), which exhibit molybdenum contents of 9 and 16 wt.-% and are to be used at places where corrosion is extremely severe as e.g. at the raw gas inlet. The use of such high alloyed materials plated on carbon steel has been tried successfully. Special attention has to be paid to all aspects of welding in order to avoid welds to become locations being vulnerable by corrosive attack. Therefore, welding of corrosion resistant materials in FGD units has been investigated extensively. The results of application oriented laboratory tests as well as practical experience with existing FGD units are to be considered. Correct use of corrosion resistant metallic materials will be an important contribution to minimizing repair and shut downs of FGD units and to extend their life.     

Beurteilung spezieller Fragen zur Beständigkeit hochlegierter Werkstoffe mit dem EPR‐Versuch (Číhal‐Test)

The EPR‐technique to evaluate special questions on the corrosion resistance of high‐alloyed materials The electrochemical potentiokinetic reactivation (EPR‐)test is a very useful tool to reveal microstructural peculiarities of high‐alloyed materials and especially the sensibility to intergranular corrosion even if only very localised; it is possible to quantify the results referring to a standard. As this test does not only react on chromium depletion due to carbide formation a metallographic examination of the tested surface is indispensable. Due to it's very high sensitivity this technique is highly qualified not only for research tasks but also for the evaluation of components and for failure case analysis. To our opinion this method seems less recommendable or suitable for general purposes of acceptance testing as the results may be ambiguous and, in addition, generally accepted standards are missing up to now.     

Einfluss der Oberflächenbehandlung auf den Fotoeffekt und die Potentialverteilung an Kupferrohren

Influence of a surface treatment on the photo effect and potential distribution of copper tubes Copper tubes have been used in drinking‐water installations since long times. This study will show the influence of a blasting treatment of the inner surface of tubes to the photo effect. Furthermore the soldering process was simulated at untreated and blasted copper tubes and the dependence of the free corrosion potentials as a function of the position was registered with a normal reference electrode and with a scanning kelvin probe. Within a time range of 10 days after exposition in synthetic drinking water a different behaviour of the free corrosion potential and the photo effect can be seen for the untreated and blasted tubes. The potential profile of the untreated inner surfaces of the copper tubes showed inconstancies in the heat affected zone which possibly could be the starting point of pitting corrosion, whereas the blasted samples do not show these inconstancies.