Heißkorrosion bei gleichzeitiger Einwirkung von Phosphaten und Vanadaten

Hot gas corrosion with simultaneous action of phosphates and vanadates Phosphate containing additives can act as inhibitors against the corrosion by vanadium pentoxide, provided the phosphate contained in the melt is maintained at sufficiently high levels. Too low phosphate contents enhance corrosion because the conductivity of the melt is then increased. The V^IV content of the melt is not a suitable criterion of the aggressivity which is rather low in the presence of high phosphate contents and at high V^IV:V^V ratios.     

Schutz von hochschmelzenden Metallen und Graphit gegen Oxydation und Heißkorrosion bis 1500°C durch aufgesinterte MoSi2-Schichten

Protection of refractory metals and graphit against oxidation and hight temperature corrosion up to 1500 °C by sintered MoSi₂ coatings MoSi₂ coatings applied by plasma spraying or isostatic hot pressing protect Ta and Nb against oxidation by forming a glassy coating of SiO₂. Diffusion of Si to the bulk metal is slow because of the low mobility of Si in the intermediate layers of Me₅Si₃ which contain Nb and Ta; the oxidation rate depends from the slow oxygen permeation through the glass layer. A 200 μm silicide layer on Ta would, accordingly, provide some thousand hours' protection at 1500 °C. Silicide coatings on graphite, however, must be protected against carburization by applying a diffusion barrier; investigated for this purpose. The useful life data calculated on the basis of the diffusion data have been registered during oxidation tests too: silicide coated Ta and Nb withstood oxidation by air at 1500 °C for 1000 hrs, and at 1200 °C for 4000 hrs; carbon with an intermediate TiC layer resisted oxidation at 1500 °C for 400 hrs. Under temperature cycling conditions it is indispensable to adjust the coefficient of dilatation of the glassy coating to that of the base material by adding GeO₂.     

Korrosionsverhalten von ungeschütztem und feuerverzinktem Stahl bei der Lagerung von Heizöl

Corrosion behaviour of unprotected and hot-dipped galvanized steel for fuel oil storage Investigations have been done with unprotected and galvanized steel under fuel oil storage conditions. By adding sodium chloride as corrosive agent and a commercial corrosion inhibitor conditions for localized corrosion of unprotected steel were simulated. Results show that no localized corrosion occurs with galvanized steel.     

Permeation von Wasser und anderen kleinen Molekülen durch Polymere

Permeation of water and other small molecules through polymers Corrosion at the interface of a metal and a polymer coating requires transport of water and oxygen through the coating. Permeation of small molecules is determined by their solubility S and their diffusion coefficient D. Whereas at low contents of the dissolved molecules in the rubbery state S and D do not depend on concentration c, they do so strongly in the glassy state. In this case S decreases with increasing c whereas D increases such that the product, i.e. the permeability D − S, remains constant. In the present study a model will be presented which explains these experimental findings. Water molecules are an exception as a solute/solute interaction of the dipolar molecules plays already a strong role at low concentrations.     

Passivierungsverhalten und Spannungsrißkorrosion von Eisen-Mangan-Chrom-Legierungen in Natriumchlorid-Lösung

Passivation behaviour and stress corrosion cracking of iron-maganese-chromium alloys in sodium chloride solution Electrochemical experiments with MnCr steels (20–28% Mn, up to 12% Cr) in 3% NaCl solution. High Mn contents reduce the passivation tendency, while increasing Cr contents broaden the range of passivity. The formation of surface layers is due primarily to a direct reaction with the solution (good adhesion, high protective value) and, secondarily, to precipitation from the solution (porosity, low protective value). The tendency to form secondary layers increases as the Cr content is reduced. In oxygen containing solution there is a pronounced corrosion in the pitting range. At low Cr contents, stress corrosion cracking is mostly transcrystalline, at higher Cr contents (8–12%) it is intercrystalline, in particular when Cr carbide precipitations are present at the grain boundaries. In the range of transcrystalline corrosion the susceptibility to selective corrosion extends beyond the pitting potential. At higher Cr contents there may be pitting without any indication of stress corrosion cracking.     

Elektrochemische Prüfung der Wirksamkeit von Heizöl-Inhibitoren

Electrochemical testing of the effectiveness of fuel oil inhibitors The potential/time and current density/potential curves for unalloyed steel were obtained in NaCl solutions with different concentrations of a certain type of inhibitor. The results show that the inhibitor acts as a passivator. A comparison with the results of conventional static tests shows that even a 99 pC inhibiting effect against uniform corrosion does not constitute an effective protection against pit corrosion. An effective protection is only obtained if the steel specimen can, like a platinum electrode, be polarized up to oxygen formation without any penetration of the Cl^?-ions.     

Wasserstoff-induzierte Spannungsrißkorrosion von Stählen durch dynamisch-plastische Beanspruchung in Promotor-freien Elektrolytlösungen

Hydrogen induced stress corrosion cracking of steels subjected to dynamic loading involving plastic deformation in promotor free electrolytic solutions Plain carbon steels and low alloy steels suffer internal cracking and a relatively high embrittlement when they are subjected to dynamic loading involving plastic deformation in any type of electrolytic solution where there is a simultaneous cathodic hydrogen evolution. These conditions can be encountered in service in the case of cathodic polarisation and free corrosion in acids if the mechanical stresses lead to plastic deformation, e.g. at notches. There is an upper limit to the potential range in which internal cracking occurs. This limit is independent of the yield strength (300 to 500 N mm^?2) of the materials tested and lies at U_H = ?0.5 V in oxygen free salt waters. It is more negative in oxygen bearing electrolytic solutions or in alkalaine media. Materials containing hard transformation products such as martensite and bainite are more susceptible to cracking in the regions of these hard transformation products. There is only a small decrease in susceptibility with increasing temperature. Ultra high strength, quenched and tempered steels with yield strengths > 1000 N mm^?2 undergo hydrogen induced stress corrosion cracking even when they are subjected to static loading in the elastic region. The critical potential is very negative and shifts to less negative values with increasing strength of the material. Stainless steels with stable austenitic microstructure are resistant to this type of corrosion. However, if the mechanical deformation can lead to the formation of martensite surface cracking and brittle fractures occur. The hydrogen induced damage decreases with increasing temperature.     

Übersicht der elektkrochemischen Methoden zur Untersuchung von Wasserstoffversprödung und Spannungrsrißkorrosion

Review of electrochemical methods of studying hydrogen embrittlement and stress corrosion cracking In this review the authors presents the features common to both types of failure and the features by which the two types of failure are different from each other. In this context it is emphasized, that the hydrogen absorption - essential for embrittlement - can be regarded as a side reaction accompanying the particular overall reaction, and that the particular reaction conditioned and the structure of the metal play an important part. Different methods for investigating the hydrogen context and the state of the hydrogen (atomic, ionic, molecular) in the metal are described, and soo are methods for studying internal stresses due to hydrogen absorption. As to stress corrosion cracking it is shown that the processes involved are not a separate mechanical and a separate chemical one, but that the joint action of both is important. In this context it is shown, however, that this view can be considered to be established in th e case of austenitic steels only.     

Korrosionsverhalten von Gasturbinenwerkstoffen unter strömenden Heißgasbedingungen

Corrosion behaviour of gas turbine alloys under high velocity burnt fuels The aim of alloy development in the field of nickel based superalloys for flying and land based gas turbines is to enhance significantly the mechanical properties at high temperatures thus leading to a higher temperature capability. The higher temperature capability of the structural elements of gas turbines results in an increased efficiency, a lowered fuel consumption and less emissions. To achieve an increased high temperature capability, however, surface degradation of the material must be adjusted adequately, hence corrosion resistance has to be improved. Additional to the isothermal and cyclic oxidation tests which are performed in stagnant air the oxidation behaviour of alloy 2100 GT and alloy C‐263 was investigated by means of burner‐rig‐experiments under high velocity burnt fuels. In the burner rig test facility the sample is exposed to a hot gas stream of burned natural gas with gas velocities in the range of 60 m/s to 150 m/s. The metal temperature of the sample can be adjusted in the range of 900°C to 1200°C. In the tests described in this paper the gas velocities were chosen to be 60 m/s, 100 m/s and 140 m/s. The test duration was 1 h and 10 h. The test temperature was kept constant at 1000°C. After 1 h of testing both alloys showed mass gain which was significantly higher for alloy C‐263. After 10 h of testing the mass loss of alloy C‐263 was enhanced with increasing gas velocity. Alloy 2100 GT showed only at the highest gas velocity a mass loss. The examinations by means of SEM and light‐optical microscopy of the oxide scale and of the microstructure showed that alloy 2100 GT has a dense adherent alumina scale and suffers no internal oxidation even under burner‐rig‐test conditions. Alloy C‐263 forms a mixed chromia and Cr‐Ti‐mixed oxide scale. The chromia is evaporated with increasing gas velocity, leaving (Cr‐Ti)O₂‐needles on the surface. In the isothermal and cyclic oxidation tests alloy 2100 GT shows an excellent oxidation behaviour up to 1200°C with a corrosion rate of less than 0.1 mm/a. The aluminium content of app. 3 wt.‐% which is remarkably high for a wrought alloy leads to the formation of a thin dense and adherent alumina scale. Alloy C‐263 is a chromia former which is not suitable for temperatures higher than 1000°C.     

Innenschutz von Lagerbehältern für Heizöle und ähnliche Medien mit Zinksilikaten

Internal protection of storage tanks for fuel oil and similar media by zinc silicates The internal surfaces of storage tanks for fuel oils or motor fuels must be protected against corrosion by such media (cause: type and pollution of the stored medium) and against rust formation (by reaction with condensed water and oxygen) by coatings (with silicate or plastic based coating materials) or linings (foils or inserted plastic bags). At the present time zinc silicate appears to be the most favourable type of internal tank coating material, having the widest range of applications and the lowest susceptibility to failure.     

Verhalten von thermoplastischen Kunststoffen gegenüber aggressiven Medien – Einfluß der Morphologie am Beispiel von Polyethylen hoher Dichte

Behaviour of thermoplastic materials in aggressive Media – Influence of morphology on the example of high density polyethylene To a certain degree the morphology of thermoplastic intermediate products can be influenced either by using different fabrication techniques (molding, extrusion) or in case of a special technique e.g. by varying the cooling conditions. Plates and pipes produced from unpigmented PE-HD granule of a certain batch, differing in morphology only were exposed to a physically active medium (p-xylene), a chemically active medium (50% chromic acid) and a surface-active medium (2% wetting-agent). Apart from immersion-tests at varying temperatures creep-experiments on plates under constant load were carried out. The registered differences in sorption, creep-compliance, cracking etc. are discussed in context with the different morphological structures (e.g. crystalline phase).     

Heißkorrosionsverhalten von SiC und Si3N4im Brennergas

High temperature corrosion behaviour of SiC and Si₃N₄ in burner gas The behaviour of the materials has been studied in the following media: hot air (1400 °C), gaseous combustion products of CH₄, molten salts (Na₂SO₄, Na₂CO₃, NaCl) and molten vanadium pentoxide, combustion gases into which solutions of the above salts are injected (the specimens were heated to 1200 °C and then after a short cooling period sprayed with salt spray), combustion gases with addition of H₂S and HCl. In the salt melts SiC is rapidly destroyed under alkaline conditions while it is highly resistant to NaCl and V₂O₅. Material obtained by pyrolytic deposition from the gas phase is considerably more resistant then sintered material. The resistance is about the same for Si₃N₄. Generally the resistance is due to the formation of a vitreous layer on the ceramic surface. In the hot gases containing salt spray the materials are also highly resistant (super alloys tested under identical conditions are destroyed within a few hours while the ceramic materials show no trace of corrosion even after 200 hours). The influence of H₂S and HCl is very negligable. Materials having low porosity are superior to porous (e.g. sintered materials).     

Zerstörung von Reinstaluminium,Feinzink und Plexiglas durch Strömungskavitation

Destruction of high purity aluminium, zinc and plexiglass by flow cavitation Cavitation in cold and warm water produces a strengthening of aluminium during a first incubation period, although fine cracks are already formed which induce erosion. It is for this reason that cathodic protection remains without effect. The same applies to Zn which undergoes a limited translation (because of its lower plasticity) and twin formation and, later on, erosion. Plexiglass absorbs water, while cracks starting at weak spots in the structure induce erosion and fatigue cracks are formed later on. The second stage of cavitation is characterized by high weight losses, connected with strong changes in the material surface, while in the third stage decreasing weight losses can be found. This decrease is attributed to the fact, that the water formed by bubble implosion in cracks cannot flow out easily and thus forms a sort of protective cushion.     

Schwingungsrißkorrosion von Baustahl in Offshore-Konstruktionen bei unterschiedlichen Frequenzen und Umgebungsbedingungen

Corrosion fatigue of structural steel in offshore structures at different frequencies and environmental conditions The effect of frequency on the corrosion fatigue in seawater was investigated by use of welded and unwelded bending specimens of plate material (20 mm, StE 460) by performing mainly endurance orientated constant-amplitude tests. The test frequency was varied in a wide range (10; 1, 0,1; 0,01 s^?1). The tests were performed in air as well as in artificial seawater at free corrosion, at a cathodic protection (U_H¹ = ?650 mV) and at an extreme polarization (U_H = ?850 mV). A general grading of the endurance – given by N_fr – with respect to the frequency was evaluated in the expected sequence at free corrosion only. Thereby the frequency effect is smaller as was expected and is intensively pronounced only at the lowest frequency of 0,01 s^?1. But this frequency already results in smaller N_fr – values of welded specimens at air as well as at an extreme cathodic polarization U_H = ?850 mV and higher loading. The results of air and free seawater-corrosion show the usual small scatter whereas those of the cathodic potentials U_H = ?650 and ?850 mV are significantly larger. At U_H = ?650 mV some results mark a significantly better behaviour than in air, the other ones are inside the scatter of the results in air. At the extreme cathodic polarization (U_H = ?850 mV) some results are situated in a domain of significant shorter endurance than in air; this results from negative influence of the increased cathodically emerged hydrogen at this potential. The phase of the fatigue crack initiation is shortened by free seawater corrosion compared with an air environment; it is enhanced by a cathodic protection with U_H = ?650 mV.     

Einflüsse von Werkstoff und Verarbeitung auf die Spannungsrißkorrosion von Spannstählen

Influence of material and processing on stress corrosion cracking of prestressing steel In prestressed concrete constructions the highstrength prestressing steels perform essential bearing effects. The alkaline layer of concrete or mortar protects the steels against corrosion and guarantees a permanent load capacity. If the corrosion protection as a result of poor workmanship is not guaranteed from the beginning, or is lost because of lacks of construction in the course of time, or the steels are predamaged during handling, stress corrosion cracking and failure of steel and construction may occur. Also an application of unsuitable materials (prestressing steel, injection mortar, concrete) can alone or in combination with the before mentioned influences favour stress corrosion cracking. In the contribution the correlations and typical failures are discussed.     

Einfluß von Oberflächenstruktur und chemischer Zusammensetzung auf die Lochfraßkorrosion von Ni-Einkristallen

Influence of surface structure and chemical composition on the pitting corrosion of nickel monocrystals With the aid of current density potential curves the pitting corrosion of nickel monocrystals (99.45–99.99 Ni) has been investigated in solutions containing 0.5 M each NiC1₂ and NiSO₄, using the pretreatment (chemical and electrochemical polishing) as a discriminating actor. As is shown by experimental results pitting is originated only at dislocations, not, however, at holidays in the passivating layer. This phenomenon is even more pronounced in the presence of impurities which may give rise to the formation of local elements. The low number of pits as compared to dislocation density can be explained in terms of special dislocation configurations required for starting pit formation.     

Ein elektrochemisches Meßverfahren zur Kurzzeitprüfung von Korrosionsinhibitoren für Heizöl

An accelerated electrochemical method for the evaluation of inhibitors for fuel oil The authors present an electrochemical method requiring but a relatively reduced experimental setup: glass container with specimen of carbon steel, counter electrode of 1818 steel with calomel electrode as a reference, direct current source up to 42 V, resistance, synchronous motor, tube voltmetre, multiple recorder. The polarizability is taken as a characteristic of the inhibition quality and improves as inhibition quality is improved. The polarizability can be determined by potentiokinetic or galvanokinetic measurements, the latter yielding a more detailed graduation of inhibitor activity because of the size of the anodic polarization range. The test solution used was the aqueous phase settling from an emulsion of NaCl solution in petroleum.     

Verbesserung der Heißdampf-Korrosionsbeständigkeit von Incoloy Alloy 800-Rohrmaterial durch verformende Oberflächenvorbehandlungen

Improvement of the resistance to corrosion by superheated steam of Incoly Alloy 800 tube material by surface deformation treatment Specimens of Incoloy 800 (%, 45.8 Fe, 31.7 Ni + Co, 20 Cr, low additions of Cu, Mn, Al, Ti, Si), having undergone a surface deformation treatment (rolling, bending, sand blasting, needle peening, emery grinding) were exposed to superheated steam (500 and 1000 hrs, 600 and 620 °C, 1, 70 and 145 atm, 0.03 and 3.5 m/s). The weight loss measured has been evaluated as a function of the surface quality. It has been shown that the corrosion losses are inversely proportional to the degree of deformation, corrosion being reduced by 95–96% at 90% deformation. In this context the bending deformation, sand blasting and needle peening are most efficient, while rolling or emery grinding give rise but to deformations in a very thin surface layer which have no “selfhealing” properties. The composition of the corrosion products depends from the degree of deformation: increasing deformation gives rise to a decrease of the iron content, while the contents of nickel, chromium and manganese are increased; this phenomenon is responsible for the protective effect which is due to the formation of chromium oxide coatings.     

Der Korrosionsschutz von Heizöllagerbehältern innen,der Anstrich von Betonauffangwannen u. a.

Corrosion protection of the inside of fuel oil storage tanks, coating of collecting vessels built of concrete, etc. The corrosion protection of fuel oil tanks assumes special importance because of the requirement laid down by the authorities in respect of the protection of phreatic water. The paper deals with the causes of corrosion and with the possibilities of preventing such corrosion through cathodic protection, inhibitors or interior linings. In dealing with the latter, the different possibilities, methods and materials are discussed in some detail.