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.     

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.     

Korrosionsbeständige Werkstoffe für Tanks in gasbetriebenen Fahrzeugen

Corrosion-resistant materials for tanks in gas-driven vehicles Compressed natural gas is increasingly used as drive for internal-combustion engines world-wide. The employed tank material consists of composite pressure tanks made of heat-treated steel or aluminum. Tanks of steel have to be resistant against stress corrosion cracking, tanks of aluminum respectively against intergranular corrosion. This paper describes the necessary acceptance tests.     

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.     

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).     

Qualifizierung metallischer Werkstoffe für die Eindampfung von Abwässern aus der Rauchgasentschwefelung

Qualification of metallic materials for evaporation of waste water from flue gas desulfurization plants The ecologically-minded processing of waste water from the wet scrubbing of flue gases of coal-fired power plants to produce environmentally acceptable products is carried out in a two-step evaporater operating in closed loop mode. The evaporating process leads to high concentration of chlorides in the two evaporation steps: up to about 100 g/l in the 1st step and up to about 350 g/l in the 2nd step. Therefore in case of metallic design of the evaporation equipment materials of construction with exceptional resistance to chloride induced pitting are required. The corrosion resistance of the high-alloyed stainless steel Alloy 31 (X1NiCrMoCu32-28-7 – UNS N 08031) and of the NiCrMo-alloys Alloy C-276 (NiMo16Cr15W – UNS N 10276) and Alloy 59 (NiCr23Mo16Al – UNS N 06059) including their weldments were to be tested for this application both in the laboratory and in field tests. In addition the behaviour of Alloy 59 heat exchanger tubes had to be determined in field tests under heat-transfer service conditions. The critical pitting corrosion temperatures of the 3 materials after having been GTAW welded under uniform conditions with FM 59 (ERNiCrMo–12) filler were determined in potentiostatic tests in model solutions imitating concentrated waste water products as they may occur in practice, using 5 K temperature intervals. As to be expected the critical corrosion resistance limits of the materials lie at 85 °C at chloride concentrations of 100 g/l Cl⁻ for the Alloy 31 and of 300 g/l Cl⁻ for both the Alloy 59 and the Alloy C–276 respectively. Field tests in waste water evaporation units of flue gas desulfurization plants of coal-fired power stations are carried out as immersion tests with the welded materials and as heat-exchange experiments using longitudinally welded tubes of Alloy 59 (2.4605). The immersion tests over a period of 32 months show the Alloy 31 (1.4562) to be a corrosion resistant construction material for tubes and containers in the first evaporation step, whereas the Alloy 59 (2.4605) and the Alloy C–276 (2.4819) have to be used for the second evaporation step, where the chloride contents are much higher. The Alloy 59 is to value as the most resistant material according to its lower tendency to crevice corrosion. The heat-exchange experiments over a test period of 9 months cause to expect the Alloy 59 to be a suitable construction material for heat-exchanger tubes in both evaporation steps in comparison to graphite which is more succeptible to mechanical destroying.     

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.     

Werkstoffe für die Salpetersäure-Industrie

Materials for the nitric acid industry Particularly demanding requirements are imposed on components exposed to nitric acid. Where metallic materials are concerned, attention must be paid not only to general corrosion, but also to intergranular and selective attack in many cases. A distinction is made between three sectors for which different materials are employed:

• a Subazeotropic and azeotropic nitric acids not contaminated with oxidants,
• b Subazeotropic and azeotropic nitric acids containing oxidants, such as chromates, and
• c Superazeotropic, primarily highly-concentrated nitric acids.

The advantages and disadvantages of the materials used in the three sectors, namely austenitic chromium-nickel-(molybdenum) steels, high-silicon austenitic chromium-nickel steels, iron-silicon casting materials, glass-lined steel, glass, aluminum (alloys), titanium, zirconium, tantalum-niobium alloys, tantalum and platinum, are illustrated. Special features to be considered in corrosion tests on steel grades resistant to nitric acid are also described.     

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.     

Einfluss der Halbzeugform auf die Korrosionsbeständigkeit hochlegierter Ni‐Cr‐Mo‐Werkstoffe

Effect of semi‐finished products on the corrosion resistance of high‐alloyed Ni‐Cr‐Mo materials The corrosion resistance of different semi‐finished products of six superaustenitic steels and nickel based alloys in the condition of delivery was investigated in some typical standard corrosion tests. The resistance of sheets, plates, strips, seamless tubes and welded tubes to intercrystalline corrosion was tested according to ASTM G 28 methods A and B, as well the resistance to pitting corrosion according to ASTM G 48 method C. The nickel based alloys 625, C‐276 and alloy 59 are resistant to the FeCl₃‐test according to ASTM G 48 method C and therefore a differentiation of these types in regard to their localized corrosion resistance was achieved only in the more aggressive ‘Green‐Death’‐solution. The laboratory experiments confirmed that the corrosion resistance is identical for all semi‐finished products and that it shows only a slight dependence of the surface condition of the materials tested. Additionally, some typical industrial and practical applications of the six high performance materials are presented to demonstrate the excellent corrosion resistance in the manufactured condition.     

Verbundrohre mit guter Korrosionsbeständigkeit bei hohen Temperaturen

Composite tubes with high corrosion resistance at high temperatures This paper describes studies on the compatibility of alloys used in composite tubes exposed to 600–800 °C. The alloys were chosen on the basis that one (ironrich, clad alloy) has good high temperature strength and the other (nickel-chromium cladding) has good high temperature corrosion resistance. The interdiffusion of elements from one alloy to the other has been studied using the microprobe analyser. The interdiffusion of major alloy elements (e.g. iron) is affected both by the chromium content of the cladding and the zirconium content (added to improve hot workability). The diffusion of carbon from the clad alloy into the cladding is controlled by the identity of the carbide forming elements in the clad material. The mechanism of these compositional effects is discussed.     

Untersuchung zur Korrosionsbeständigkeit von Porzellan und Steinzeug

Investigation into the corrosion resistance of porcelain and stoneware The author communicates the results of corrosion tests made with porcelain and stoneware in contact with acid and alcaline media (sulfuric acid, phosphoric acid, nitric acid, hydrochloric acid, NaHO₂ KHO) under rigid conditions resembling those encountered in practice (oboiling temperature, flow velocities upto 1 m/s). while the acid resistance can as a rule be considered very good there is considerable attack by alcaline media. In this context there is a close interdependence between the attack and the composition of the materials and the nature of the glassy and crystalline phase present. Aluminated materials show good resistance somewhat reduced.     

Entwicklung neuer korrosinsbeständiger Werkstoffe für den Chemieapparatebau

Qualification of new corrosion-resistant materials for chemical plants and equipment The publication quotes the “superaustenite” X 2 NiCrMoCu 25 20 5 (Cronifer 1925 LC/Alloy 904 L, 1.4539) as an example how to distinctly improve the resistance to local corrosion in acid and neutral chloride-containing media by reducing the sulfur content and increasing the content of nitrogen, while simultaneously influencing the mechanical characteristics in a positive way. As a result, a new material for chemical plants and equipment, Cronifer 1925 LCN/Alloy 904 LN is presented. In a second part the article explains how by its reduced carbon content and change in alloying constituents the Ni alloy GiCrFeMoCu (Nicrofer 4823 hMo/Alloy G-3 (2.4619)) might be interpreted as a progressive modification of the more conventional NiCr 21 Mo (Nicrofer 4221/Alloy 825 (2.4858)) type ein order to meet new requirements. The report specially deals with the susceptibility to sensitization in processing as well as with the resistance to local corrosion in acid chloride-containing media, thanks to which Nicrofer 4823 hMo can be ranked between austenitic steels and very highly alloyed Nibase materials as to its application fields.     

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.     

Korrosionsbedingte Veränderungen von Festigkeitseigenschaften metallischer Werkstoffe

Changes in the strength properties of metallic materials due to corrosion Corrosion produces changes in the strength properties of metals and such changes can be assessed by mechanicophysical testing of materials. The residual strength after corosion is proportional to the weight loss, e.g. for uniformly corroded copper and when the specimens are tested in the tensile ruptured. The elongation at break, however, shows an unexpectedly strong decrease already at low corrosion levels. In the case of intercrystalline corrosion the strength properties depend not only from the reduction of the load-bearing cross section, but, additionally, are determined by the notch effect of corroded grain boundaries. This fact is shown on potentiostatically precorroded specimens of a CrNi steel 18/9. At transpassive potentials this material is attached by intercrystalline corrosion even after solution annealing. Because oft he low intensity of the intercrystalline attack in this range of potentials there are, however, no significant differences in the strength behaviour of the steel after different sensibilization treatments. The fatigue resistance of the CrNi steel is becoming worse as intercrystalline corrosion is advancing into the depth of the metal structure. Chromium carbide precipitation have a favourable effect because they block slip planes and thus improve fatigue resistance.     

Das Aufkohlungsverhalten verschiedener Werkstoffe für Pyrolyserohre

Carburizing behaviour of various materials for tubular pyrolysis furnaces Chromium-nickel cast alloys with 25% Cr, 20–35% Ni, 0.4% C and silicon-contents ranging from 1.0 to 3.1% were pack-carburized for 200 h at 900–1150°C. Silicon reduces carburization most distinctly at 1050°C. At this temperature alloys with 1% Si heavily carburize, alloys with 2% Si very little carburize, and alloys with 3% Si do not carburize at all. It has been proved that alloys in carburizing atmospheres should be alloyed with 2.0 to 2.5% silicon. Aging tests up to 5000 h at 600–1000°C showed no significant aging temperature. Maximum aging seems to occur at 750°C and takes place during the first 500 h only. Up to a silicon-content of 2.5% there are no differences in the aging behaviour of the tested material. Stress rupture tests show that even with Si-contents of 3%, the underline of the scatter band is reached, and there are no concerns about silicon up to 2.5%.     

Asbestverstärkte Phenolharze als Werkstoffe für den chemischen Apparatebau

Asbestos-phenolic resin laminates as primary materials of construction for chemical plant and equipment The application potential of asbestosphenolic resin laminates is determined by their processing properties which are favourably influenced e.g. by chrysotile fibres, or by the chemical resistance which is obtained by another species of asbestos, crocidolite, which is also used for acid resistant packings. The combination of these fibres and special phenoplastic materials has yielded materials which can be machined like wood and which can therefore be used for a number of applications. Some examples are stirrers for reaction vessels, tube components and tubes with moulded-on flanges, container lids, valves, stopcocks, pumps. The chemical resistance of the resin can be increased considerably by curing at high temperature and pressure. The resins are also completely resistant to solvents at practically any admissible working temperature. The resistance to sulfuric acid extends to 90°C, above that temperature and - in fuming acid at room temperature - the resin is sulfonated.     

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.