Untersuchungen zum Verhalten von Aluminium bei kathodischer Polarisation

Investigations into the behaviour of aluminium under cathodic polarization Characteristic discontinuities in the cathodic current density-potential curve (traced for high purity Al and Fe-containing Al in buffered Na-chloride solution), relations between the current density and the inflexion point of the curve and the buffer capacity of the electrolyte, as well as the heavy corrosion observed above a critical current density yield the conclusion that a cathodic rupture occurs because of the alkaline reaction of the liquid boundary layer. The cathodic reaction is thereby transferred from the oxide-solution boundary to the metal-solution boundary and the formation of hydroxide takes place not by direct hydration, but via hydroxo complexes which are precipitated at the electrode because of the pH prevailing there and then block the electrode. In the case of the Fe-containing Al there is an enrichment of Al₃Fe which results in increasing hydrogen evolution at constant potential.     

Untersuchungen der Korrosion von Aluminium in Wasser bei hohen Temperaturen und Drucken

Research on the corrosion of aluminium in water at high temperatures and pressures The stationary corrosion rate i_corr of aluminium is measured electrochemically in a 10^?3 m sodium bicarbonate solution at temperatures between 100°C and 200°C using a V4A high pressure loop, the result being Thus, the effective activation energy is 15 kcal/mole. The stationary thicknesses of the oxide layer on aluminium are calculated as a function of the temperature from the corrosion rates and the weight changes of the specimen. The results are compared to the thicknesses measured microscopically.     

Untersuchungen zum Korrosionsverhalten von AlLi-Legierungen

Corrosion behaviour of aluminium-lithium alloys The AlLiMgCu alloy 8090 was studied in its texturated fine grained version “A” and in its recrystallized coarse grain structured version “C” in different artificially aged conditions in reference to several other AlLi alloys, each in its heat treatment condition of practical interest, and to the convetional alloy 2024 T3. The subject of research was the general corrosion behaviour of semifinished AlLi products, particularly sheet material, under alternate and permanent immersion conditions in neutral 3.5% sodium chloride solution; the stress corrosion behaviour was studied under constant load in the long transverse direction according to ASTM G44 and G49. The underaged conditions, which are the relevant conditions for technical application of the 8090 “A” and “C” sheets, showed an approximately equivalent or even better corrosion behaviour in comparison to the lithium-free alloy 2024 T3 in the corrosion tests with unloaded specimens. The threshold above which the AlLi alloy 8090 in some heat treatment conditions is attacked by stress corrosion cracking within the 30 days lasting constant load test depends on alloy composition, testing direction, grain size, stretch-forming, artificial ageing condition, surface pretreatment and the specimens' dimensions.     

Untersuchungen zur Korrosion und zum Korrosionsschutz von meerwassergekühlten Rohrkondensatoren aus Kupferbasislegierungen-Versuche mit rotierenden Proben

Investigations on the corrosion and corrosion protection of seawater cooled condensor tubes of copper-base alloys - Experiments with rotating samples Experiments have been done on rotating disc and cylinder electrodes of CuZn20Al2, CuNi10Fe and CuNi30Fe in 3 w/o sodium chloride solution and in artificial sea water at 25 and 40 ° C. Erosion corrosion of sea water cooled tube condensors should be simulated and studied. The corrosion mechanism of copper base alloys in clean and polluted sea water was intensely studied with particular consideration to the influence of ammonia, sulfides and chlorine. The effectiveness of iron sulfate dosing was tested. With the rotating samples under test no erosion corrosion could be detected on free corroding specimens.     

Untersuchungen zum Emaillieren von Nickelbasislegierungen

Investigations concerning glass lining of nickel base alloys Frequently, handling of highly corrosive products requires the selection of glass-lined equipment in chemical engineering. Normally, carbon steel or low-alloy steel for elevated temperature application is used as base metal. The experience shows, however, that in case of any damage to the glass-lining the base metal will corrode rapidly, thus necessitating an immediate shut-down. Since extreme demands for equipment availability may require a failure-tolerant performance of the equipment I-gal-autoclaves built from various nickel-base alloys were lined with different glass-types and investigated for service performance, especially with regard to mechanical and resistance properties. The paper summarizes the results.     

Untersuchungen zum Einfluß von Sulfationen auf die chloridinduzierte lokale Korrosion am Stahl 1.4541 bei Temperaturen bis zu 280°C und gleichzeitiger Werkstoffdehnung

The influence of sulphate ions on stress corrosion cracking for stainless steel AlSl 321 with constant extension rate in aqueous chloride solution at temperatures up to 280°C CERT (Constant extension rate test: ? = 3·10 ⁶ s ¹) was used to study the inhibition effect of sulphate ions on stress corrosion cracking. Smooth tensile round specimens (AlSl 321) have been tested in solutions with sulphate (10 ²m Na₂SO₄) and chloride ions (10^?3m NaCl. 5. 10 ³m NaCl) at 150°C and 280°C. The presence of sulphate in chloride solutions increases the time to fracture and the reduction of area in comparison to pure chloride solution. The reason is the lower concentration of corrosive hydrogen near the fracture surface in comparison with tests in chloride solutions. The stress corrosion cracking is completely inhibited by the ratio of 10:1 for sulphate and chloride ions in the solution. The fracture surface investigated by Scanning Electron Microscopy has shown a cleavage type of fracture in chloride solutions and a ductile fracture in sulphate/chloride mixtures. The diffusivity of corrosive hydrogen is increased at 280°C in comparison to 150°C. Therefore at 280°C the corrosive hydrogen is able to diffuse into the inner part of the specimen and to influence the fracture mode.     

Elektrochemische Untersuchungen zur Korrosion von Nickelüberzügen

Electrochemical investigations into the corrosion of nickel coatings The corrosion behaviour of pure rolled sheet nickel, electrodeposited nickel and electroless Ni-P-alloy-coatings is investigated by electrochemical methods. Some mechanisms of corrosion and passivation in 0,1 N H₂SO₄ and deionated water - both with addition of several anions - are described.     

Untersuchungen zum Einfluß von Sulfid- und Sulfationen auf die Korrosion des Eisens in alkalischer Lösung

Investigation into the influence of sulfide and sulfate ions on the corrosion of iron in alcaline solution The corrosion behaviour of Armco iron as well as that of a technical high-strength steel was investigated in sulphide - and sulphate-containing electrolytes of pH ～ 12.6. Current density - potential curves show that iron, after successful passivation, remains passive and protected against corrosion in a solution saturated with CaS, CaSO₄ and Ca(OH)₂ at potentials up to 800 mV(EH). Passivation experiments conducted by changing from the active region to various anodic potentials yielded a critical potential region around 300 mV(EH), above which corrosion appeared. Up to E_H = 200 mV complete passivation was observed. The same critical potential region was observed in experiments of repassivation after mechanical damaging of a protective passive layer. Additional tensile stresses of 85%· σ_0,2 caused no stress corrosion cracking at a potential of 500 mV(EH). After damaging the passive layer, through scratching of the stressed wires fracture occured at and above potentials of 300 mV(EH). At 200 mV(EH) repassivation was observed even under tensile load. In a sulphate-free Na₂S/Ca(OH)₂ solution of the same sulphide ion concentration no stress corrosion cracking of the samples occured at any potential up to 800 mV(EH) even after scratching the steel samples. The observed stress corrosion in sulphide - and sulphate-containing electrolytes is to be ascribed to the action of sulphate and not to sulphide ions.     

Untersuchungen zum Einfluß des gelösten Siliziumdioxids auf die Korrosion von Chemieemail in Salzsäure

Investigations on the Influence of Dissolved Silica on the Corrosion of Chemical Service Glass Enamel in Hydrochloric Acid At high temperatures chemical service glass enamel is being attacked by mineral acids more than expected in regard to the ion exchange theory. Taking as an example the corrosion test results of a glass enamel in 20% hydrochloric acid at 140 °C one can demonstrate that under adverse conditions all glass constituents will be dissolved almost equally during attack. This result is surprising in respect to the glass component silicon-dioxide, however, its solubility in the acid is increased to 70 ppm by the high temperature. This limit essentially determines the degree of the acid attack on the enamel. If the silica concentration of the attacking acid is low and fairly low amounts add because of short reaction times or high volume to surface ratios then the corrosion rate will be high. If the attacking acid already contains higher concentrations of silica the glass enamel corrosion will be strongly inhibited. On the basis of the quantitative results it is possible to recommend a procedure for testing glass-linings at 140 °C in 20% hydrochloric acid. For this application, too, the corrosion situation in the vapor phase and liquid phase of a technical size chemical reactor is being discussed.     

Eigenschaften von Elektrolyt-Filmen bei der atmosphärischen Korrosion

Properties of electrolyte films formed through atmospheric corrosion An investigation has been carried out into the composition of the electrolyte films which are formed on non-metallic materials (glass) as well as on metals (Cu, Zn, Fe) in an atmosphere containing SO₂. Fresh as well as pre-corroded specimen were used. It was found that the SO₂ absorbed in the solution is very rapidly oxidized into SO if the electrolyte film contains dissolved particles of the corrosion products. With 1 to 55 ppm SO₂ in the atmosphere, the change in the pH value of the electrolyte is but small and does not vary with the SO₂ partial pressure.     

Elektrochemische Untersuchungen zum Korrosionsverhalten von Duplexgusswerkstoffen in Rauchgasentschwefelungsanlagen

Electrochemical investigations on the corrosion behaviour of duplex cast stainless steels in flue‐gas desulphurisation plants This paper deals with results from corrosion examination on duplex cast stainless steels from original components in the flue‐gas desulphurization (FGD) of the VEAG‐power stations Boxberg III and Jaenschwalde under practice and laboratory conditions. In practice the mentioned components have a better corrosion resistance in the FGD medium of the Boxberg power station. The same results were obtained by laboratory examinations, getting a shift in critical potentials of a few hundred millivolts. The tests in a by‐pass of the FGD confirm these results in tendency. However, partially significant differences to laboratory examinations were established on specimens with pre‐damaged original surfaces. This can be traced back to the more severe test conditions in the FGD, like solids and flow. The repassivation behaviour of the materials under investigation was examined by means of electrochemical noise analysis. The values of the free corrosion potentials in the Jaenschwalde power station were higher than potentials, necessary for a safe repassivation of occurring pits. This cause the failures on the components. Remedy steps are discussed in this paper in detail. The free corrosion potential in the Boxberg power station is about 200 mV below the repassivation potential. Because of that, short‐time fluctuations of system parameters leading to enhanced corrosion risk can be tolerated.     

Berücksichtigung der Korrosion bei der Konstruktiven Gestaltung von Chemieapparaten

Taking account of corrosion in the design of chemical apparatus The selection of materials is of prime importance in the design of chemical plant because it is necessary to take account, in addition to mechanical stresses, of corrosive influences, in particular where local corrosive attack is to be expected. When using dissimilar metals suitable measures must be taken to prevent contact corrosion. The design must be free from crevices as far as possible in order to avoid crevice corrosion. This requirement must be given due consideration in connection with weld design, too. Discharge devices must enable the plant to be completely emptied in order to prevent shutdown corrosion. Flow velocities must be limited in order to prevent any intensification of corrosion by turbulence, or to avoid erosion corrosion. Simultaneous mechanical stresses and corrosive attack must be taken account of at the design stage in order to prevent stress corrosion cracking and corrosion fatigue. suitable countermeasures include reduction of stress levels, avoiding stress peaks, mitigation of corrosive effects, utilization of more resistant materials, making use of suitable thermal treatments and using protective coatings. Feasible anti-corrosion design ist demonstrated on some construction details.     

Korrosion von CuZn-Legierungen in Chloridlösungen bei potentiostatischer Polarisation

The corrosion of CuZn alloys in Chloride solutions with potentiostatic polarisation The intercrystalline corrosion susceptibility of brasses can be studied either by potentiostatic methods in NH₄Cl+HCl solutions with polarisation to +80 mV, or – without polarisation – in NH₄Cl+HCl+CuSO₄ solutions where the potential of +80 mV is attained without external polarisation. Tests carried out with macroelements formed either by brasses having different structures (α, β/α), or identical structure (α) but different zinc contents (21 and 32% respectively) have shown the preferential attack of the β phase at potentials lower than those required for grain boundary dissolution, and the more anodic character of the alloys of higher zinc content.     

Untersuchungen zum Zusammenhang von Stofftransportgeschwindigkeit und Abtragsgeschwindigkeit unter Erosionskorrosionsbedingungen

Investigations on the correlation between mass transfer rate and mass loss rate depending on conditions of erosion corrosion Interactions of flow conditions, mass transfer rates and mass loss rates are investigated in two phase flows depending on conditions of simulation of erosion - corrosion. The velocity and the particle dose of the flow are varied. The limiting current density technique is applied to measure the mass transfer rate. By means of a potassium ferric/ferrocyanide redox system the limiting current density is measured at a platinum electrode. At mild steel and austenitic CrNi-steel the mass loss rate is gravimetrically determined. The mass transfer rate is a characteristic number of erosive treatment in two-phase flows. In certain conditions bistabilities are observed in both mass transfer rate and mass loss rate.     

Korrosion von Klebverbindungen

Corrosion of adhesive bonds Today the topic of the adhesive bond technology is no more the realization of a strong adhesion directly after curing the adhesive and before the bond has been exposed to any environmental load. As a rule this is possible even in the case of oiled steel sheets for the body-in-white in the automotive industry. For plastics which are difficult to bond (e.g. polyolefines) suitable surface pretreatments are available. The mechanical loads at “normal” temperatures can be controlled by the static and dynamic strength of the adhesive and by the design of the adhesive bond, even if there is a high expenditure for performing all the tests, because for most of the applications the available data basis is insufficient. Most important for the reliability of the adhesive bonding technology is the long-term durability under environmental loads, e.g. moisture. Three aspects of the durability have to be differentiated depending on the location and on the way of the influence of the medium: • diffusion-dominated durability of the polymer • adhesion-dominated durability • corrosion-dominated durability     

Untersuchungen zum Verhalten von Hochtemperaturwerkstoffen gegenüber Aufstickung

Investigations of nitridation behaviour of high temperature materials Furnace rolls and inner tube walls of industrial bright annealing furnaces are often subject to attack by nitrogen atmospheres. In order to select appropriate materials for application in nitriding atmospheres, three commercial stainless steels (AISI 314, alloy DS, alloy 800H) and four nickel base alloys (alloy 45-TM, alloy 600H, alloy 601H and alloy 602CA) with different concentrations of nickel, chromium, silicon and aluminium were exposed to both N₂/H₂ gas atmospheres at temperatures of 1000°C, 1100°C and 1200°C. The impact strength, the mass change due to nitrogen pick-up and the depth of internal nitridation were determined after exposure. At 1000°C nitrogen pick-up and loss of impact strength, was low for all alloys investigated. At 1100°C and 1200°C, however, all alloys suffered internal nitridation. Both internal nitridation and loss of ductility were more severe in the iron base alloys than in the nickel-base alloys. The corrosion attack by nitridation decreased with increasing nickel content. The highest resistance to nitridation was found in the nickel base alloys 600 H and 602 CA.     

Entstehung von Sulfatnestern im Rost bei der atmosphärischen Korrosion von Stahl I

Occurrence of sulphate nests in rust during the atmospheric corrosion of steel. I. A pure FeSO₄, solution (with ³⁵S) was used to generate, on steel surfaces, a continuous layer of FeSO₄, through evaporation. The samples thus treated were exposed to the action of air of different humidity. Within three hours, the sulphate ions tend to concentrate on the surface of the sample and to penetrate into the metal locally, causing nest-like concentrations of corrosion products. The number of the nests increases with the FeSO₄, concentration up to a maximum. Between the frequency and the size of the nests, there is a correlation which depends on air humidity. With increasing air humidity, the size of the nests increases whilst their number decreases.     

Die Entstehungs- und Ausbreitungskinetik von Sulfatnestern bei der atmosphärischen Korrosion von Stahl

Kinetics of formation and propagation of sulfate nests on steel The atmospheric corrosion of steel starts at individual sites, the number of which increases until the steel surface is covered by a rust layer. The initial distribution of corrosion sites seems to be connected with active sulfide inclusions (in particular MnS). The manganese sulfates formed are hydrolised, so that sulfuric acid is formed which initiates the formation of sulfate nests. The ferrous sulfate formed in such nests is again hydrolised and this reaction controls the rate of sulfate nests propagation. Good rust protection requires the removal of such sulfate nests which can be achieved only by removal of the rust layer (either mechanically or by pickling); mere rinsing or the use of rust stabilizers are not sufficient.     

Einsatz von mathematischen Methoden bei der Untersuchung der Korrosion von Eisen-Nickel Legierungen in schwefelsauren Medien

Application of mathematical methods to the study of the corrosion of iron-nickel alloys in sulfuric acid media Mathematical methods have been used for planning and optimizing experimental work aimed at the investigation of the corrosion of ironnickel alloys in sulfuric acid media and in the presence of chloride ions. The calculated regression equations of the second order adequately described the relation between corrosion rate (i_con), pH, chloride ion concentration and alloy composition. A comparison with the results obtained by classical methods demonstrates the applicability of the mathematical methods.