Untersuchungen von Sonderfällen der Spannungskorrosion bei homogenen,nicht übersättigten Mischkristallen und der hierbei auftretenden elektrochemischen Prozesse. Teil II: Weitere Untersuchungen zur Spannungskorrosion des Messings

Research into special cases of stress corrosion with homogeneous, non-over-saturated mixed crystals, and of the electro-chemical processes associated with it. Part II. Further research into the stress corrosion of brass Stress corrosion in brass is caused not only by basic agents which though free from ammonia, contain Cu⁺⁺ ions; the effect of these agents, however, is less marked. These agents contain, as a reaction product, Cu₂O which forms the cathodic zone required for the setting up of a marked electro-chemical process in the fissures and cracks and which, in contrast to CuO, is resistant even in acid solutions up to a p_H value of approx. 2. With Ms 70 and 63, the crack pattern is mainly transcrystalline, with peculiarly ramified and extended cracks. This is explained by the fact that Cu₂O occurs in isolated un-linked crystals and therefore does not form a uniform cover of the walls of the cracks. The walls are therefore exposed to intensified local corrosion which leads to ramifications of the cracks.     

Einsatz von Oberflächen- und Mikroanalysenverfahren bei der Untersuchung von Inhibitoren

Surface- and micro-analytical methods in the investigation of Inhibitors ESCA, AES, ISS, TOF-SIMS and LAMMA investigations concerning the composition and thickness of the protective layers formed when Cu, Ni and Co in water are inhibited with interface inhibitors (benzotriazole, tolyltriazole, mercaptobenzothiazole and 2-(5-aminopentyl)-benzimidazole), or unalloyed steels, likewise in water, are inhibited with interphase inhibitors, such as phosphonotricarboxylic acid, are reported. The layers formed by interface inhibitors are only a few monolayers thick. Information on the molecular structure of the layers is provided particularly by LAMMA and TOF-SIMS. Where Cu and Ni are concerned, quasi-polymeric molecules of the metal(I) benzotriazole type are formed, whereas in the case of Co, complexes of higher valency occur also. The same molecular structures are also observed on oxidized metal surface. The inhibitors layers cannot be removed from the surface of the metal with solvents, and they are also stable during prolonged exposure to air. The interphase inhibitors form substantially thicker layers. The mechanism of the protective layer formation is discussed with references to a variety of parameters.     

Untersuchungen über die Ursachen der Spannungskorrosion bei nichtrostenden; C-stabilisierten austenitischen Cr-Ni-Stählen

Investigations into the causes of stress corrosion cracking with stainless, C-stabilized austenitic Cr-Ni-steels With the deformation of specimens (steel with 17 per cent. Cr, 10 per cent Ni) in boiling MgCl₂ solution, the potential is shifled to the negative side, at a rate which increases with the expansion rate. This phenomenon is ascribed to the fact, that, with slow expansion, the active zones are again apt to be passivated. It is only when major cracks are already present that the expansion rate at the bottom of the crack becomes so high that no passivation takes place any more and that the potential remains continuously below that required for stress corrosion cracking. Between stres corrosion cracking and pitting corrosion, there is a correlation inasmuch as the pitting has a corrosioninhibiting effect. The hydrogen and chlorme ions which accumulate in the pitting have the effect of shifling the potential to the negative side so that, during expansion, the potential is no longer much below the negative potential required for stress corrosion cracking. This interpretation is derived from the different behaviour of specimens which were exposed to loads after brief polarisation (causing pitting corrosion), and directly.     

Untersuchung der Reaktionsmöglichkeiten von Sulfiden bei der Einwirkung von Kohlensäure und Sauerstoff auf sulfidhaltigen Zementstein

Investigation into potential reactions of sulfides during the action of carbonic acid and oxygen on sulfide containing concrete A possible sulfide content in concrete cannot be considered to be a potential source of stress corrosion cracking, it is true that during carbonate formation a small part of the sulfide is set free as hydrogen sulfide, but, on the other hand, oxygen from the air gives rise to rapid oxidation with formation of sulfur or thiosulfate. To study potential corrosion under such conditions the saturated calcium sulfide solution has shown to be unsuitable since in this case the modifications due to atmospheric oxygen are disregarded.     

Die Anwendungen der elektrochemischen Oberflächenbehandlung von Metallen bei der Herstellung von Apparaten für die chemische Industrie

Applications of the electrochemical surface treatment of metals in the construction of apparatus for the chemical industry In view of the constantly higher requirements concerning the surface quality of chemical apparatus the electrolytical polishing is gaining in importance. By contrast to the mechanical polishing processes the metal structure is preserved (no formation of a beilby layer) and the resulting surface is so smooth that electropolished surfaces are close to an ideally plane surface. Electropolishing can be used also in connection with mechanical polishing in order to remove the Beilby layer formed in the latter. Main applications are the construction of apparatus for unclear technologies and the production of pure reference surfaces for materials and corrosion testing, in particular because it is possible uniformly to polish parts of complex shapes.     

Untersuchung der Korrosionsneigung mit Hilfe des elektrochemischen Rauschens Teil 1: Phänomen,Entwicklungstrends und Probleme

Examination of corrosion propensity by means of electrochemical noise Part 1: Appearance, trends and problems A simple, sensitive and almost nondestructive measurement procedure for the examination of corrosion processes in corrosion research and assessment of corrosion propensity of metallic materials is available with the electrochemical noise diagnostic. The possibilities of application can be enlarged both on the solution of examining tasks in laboratory and on special problems for the corrosion monitoring in practice with today's knowledge about electrochemical noise at corrosion processes. So the various advantages of this measurement procedure can be used to solve a lot of problems and questions, often as a complementation of other test procedures. In this work the general bases of the understanding of the electrochemical noise, current trends and some difficulties concerning the use of electrochemical noise diagnostic are presented.     

Untersuchung des chemischen und elektrochemischen Verhaltens von zementmörtelausgekleideten Stahlrohren in kohlensäurehaltigen Wässern

Investigation of electrochemical element formation by crevices of different width in cement lined steel tubes The author describes an experimental setup for the investigation of the attack of water with aggressive carbonic acid on the cement lining of steel tubes. In equilibrium waters crevices in the cement are covered by protective layers; in aggressive waters, however, there is a critical crevice width (probably 2 mm), which must not be exceeded without the danger of corrosion. When dissolved calcareous constituents aer replaced by iron oxide in the mortar the resistance of the cement is increased.     

Untersuchungen zur Werkstoff- und Korrosionsproblematik bei der Oxidation von Schadstoffen in überkritischen Wasser

Investigations on material and corrosion problems for the oxidation of contaminations in supercritical water A selection of stainless steels and special alloys for the oxidation of organic contaminations in aqueous solutions under high pressure and high temperatures (SCWO -- Supercritical Water Oxidation) is shown. A high pressure laboratory plant for representative corrosion studies under real conditions of SCWO (temperatures above 647.35 K, pressure above 22.1 MPa) is described. The corrosion behavior of the selected alloys (X 2 CrNiMo 18 10; × 5 CrNi-Mo 17 12 2; × 5 CrNi 18 10; NiCr 15 Fe; NiMo 16 Cr 16 Ti; NiCr 23 Mo 16 Al) in operation as fittings and reactor material is presented and discussed. Especially the degradation of 4-chlorophenol with oxygen as oxidizing agent shows clearly the difference in corrosion resistance between stainless steels and nickel alloys.     

Elektrochemische Untersuchung der Permeabilität,Adhäsion und Inhibitorwirkung von Plasmapolymerschichten auf Edelmetallelektroden

Electrochemical investigation of the permeability, adhesion, and inhibition properties of plasma-polymer films on noble metal electrodes Electrodes were coated with thin acrylonitrile/argon plasma-polymer films and investigated with electrochemical techniques. The analysis of the impedance of the coated platinum electrodes in acidic electrolytes yielded the charge associated with the hydrogen electrosorption process. This is a measure for the fraction of the metal surface accessible to the electrolyte. Thus the “free” surface shortly after immersion of the coated electrodes was determined to range between 0.01–1%. Depending on the preparation parameters this fraction was observed to rise more slowly or rapidly, demonstrating differences in the stability of the metal-polymer bonds. The coating possesses membrane properties permitting the electrochemical investigation of the metal/polymer interface in presence of the intact film.     

Die Abhängigkeit der Spannungskorrosionsanfälligkeit vom Werkstoffzustand bei homogenen Kupfer-Zink-Legierungen

Interdependence of stress corrosion susceptibility and structural state of homogeneous copper-zinc alloys Experimental investigations in ammoniacal solution of copper and ammonium sulfates have shown, that — with equal load ratio (σ/σ_0,2 = 0,9) — CuZn alloys are less sensitive to stress corrosion cracking after recrystallizing annealing than after cold working. The sensitivity increases with increasing Zn content and appears to depend primarily from the dislocation structure: parallel structures are highly sensitive while increasing transition to cellular dislocation structure reduces stress corrosion susceptibility. In the cold worked alloy the crack geometry is predominantly transcrystalline, in the recrystallized state intercrystalline. Similar relations between dislocation structure and susceptibility seem to exist in the case of austenitic stainless steels.     

Änderungen der Struktur und der chemischen Zusammensetzung von Rohrwerkstoffen bei der Hochtemperaturpyrolyse

Changes of structure and chemical composition of tubing materials for high temperature pyrolysis The changes of structure and chemical composition of the surface layers of radiation tubes for hydrocarbon pyrolysis furnaces made of Cr25Ni35Nb steel were investigated after longterm service. It was found that carburization of the tube wall occurs where the primarily formed M₂O₃ oxide film with high chromium content is damaged by local spalling. The M₂O₃ oxide film spalling or deterioration is usually attributed to reduction by carbon from the pyrolysis medium. Our own investigations showed M₂O₃ oxide film deterioration to be caused by residual stresses generated during the growth of the zone beneath the oxide film, which is internally oxidised and affected by intercrystal line corrosion. During further oxidation negative effects including the dangerous carburization and furnace life is reduced by the formation of M₃O₄ oxide. This oxide contains less chromium and deteriorates the protection. The regeneration of the M₂O₃ oxide film having a high chromium content is not possible because of the lower chromium, silicon and manganese depletion in the surface-near zone, which prevents M₃O₄ oxide formation.     

Modellvorstellungen über den Reaktionsverlauf und Diskussion der Funktionen passivierender Zusätze bei der chemischen Passivierung von Eisen im pH-Bereich schwach sauer,neutral, basisch

Possible models of the reaction mechanism and discussion of the functions of passivating additions during the chemical passivation of iron in weakly acid, neutral and basic pH ranges A model has been developed describing the reactions occurring during chemical passivation of an oxide free iron surface in the pH-range weakly acid, neutral, basic. It covers the region approximately up to the Flade potential, i. e. the socalled region of incomplete passivity where Fe²⁺-species still pass into the solution. The model is based on facts known about the composition of the passive layer, about the conditions of synthesis, the composition and oxidizability of magnetite and about structural conceptions of its formation. Finally the model and experimental results are used to discuss the rǒle the constituent parts of chemically passivating systems of the type – oxidant/nonoxidizing agent – play in realizing passivation.     

Untersuchung verschiedener Einflußgrößen bei der Korrosion von Zirkonium-Vanadium-Legierungen in Wasser und Wasserdampf

A study of various factors influencing the corrosion of zirconium/vanadium alloys in water and steam Experimental investigations in water and steam at saturation (250 to 350°C) with Zr and Zr alloys (0.5 to 0.7 and 2.5 to 3.9% V) have revealed, that it is advisable to use grinding for the surface preparation of the specimens (good reproducibility). The increase in weight is linear at 250 and 350°C, the oxide scale is removed by disintegration after some time; increasing amounts of V give rise to increasing corrosion rates. Between Zr sponge and iodide Zr there is no difference as to oxidation behaviour. The rate increase with increasing V additions is attributed to the poor resistance of ZrV₂ precipitated at grain boundaries and being also responsible for the poor adhesion of the oxide layer. In pure Zr, too, higher rates of corrosion are found when two phase (α + β) are present.     

Methoden der Untersuchung von Reaktormaterialien nach Heißdampf-Korrosion in Versuchskreisläufen und Kernreaktoren

Methods for investigation of reactor materials after corrosion in superheated steam in test circuits and nuclear reactors If it is impossible to make gravimetric determinations one may use planimetric measurements under the optical microscope; the mean thickness of the oxide layer enables the metal losses to be calculated. A combined method involves the descaling of corroded specimens with subsequent determination of the alloy constituent concentration in the surface layer of the descaled metal; this method is suitable in particular when – as is the case during corrosion in steam under heat transfer conditions – part of the oxide is lost to the steam system. The alloy constituents are determined by the microprobe or by X-ray fluorescence. A semi-quantitative method is used for fuel elements; in this case the dimensions of the material are measured prior to use and changes are determined by optical microscopy after use. If the values thus obtained are compared with values determined in out-of-reactor experiments one may even assess the influence of the neutron radiation.     

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.     

Untersuchung der Korrosionsschutzwirkung von Wärmeträgern Teil 3: Elektrochemische Untersuchungen und Bewertungskriterien

Testing the corrosion protection of heat transfer fluids Part 3: Electrochemical tests and valuation criteria. The third part of the report demonstrates electrochemical criteria for the assessment of corrosion protection of inhibited antifreezes on glycol base. The measurements were carried out by means of a cylinder of revolution test plant under practice oriented conditions with iron, aluminium and copper materials. The polarisation resistance, the length of passive-range and hysteresis effects at the anodic part of current density-potential curves proved to be the most useful assessment criteria. The most reliable information concerning the inhibitor efficiency is given by a combination of all three criteria. Potentiostatic tests anodic to the rest potential confirm this further. According to this criteria the tested antifreezes offer a good corrosion protection to the metals investigated.     

Untersuchung der Korrosionsbeständigkeit von Auftragschweißungen der Hartlegierungsfülldrahtelektroden Stelly Fe12 und Stelloy 6

Investigation of the corrosion resistance of the hard-facing alloys stelloy Fe12 and stelloy 6 Cobalt hard-facing alloys are often used under complex conditions with combined wear and corrosion attack at higher temperatures. For using in equipments of atomic power plants cobalt free claddings are required to guarantee a low radioactivity level. Beside this there is an economic interest of the industry to use cobalt free welding filler metals. The problem consists in developing of cobalt free hard-facing alloys with the same wear and corrosion resistance as cobalt base alloys. The resistance of claddings of the electrode stelloy Fe12 with about 19% Cr and 10% Ni has been investigated in 0.5 M sulfuric acid at 66°C during a full immersion test in comparison with the cobalt base alloy stelloy 6. Both investigated materials have no satisfactory corrosion resistance. But according to the EPR-test they were resistant against intergranular attack (IK). The high matrix dissolution of grinded samples was disadvantageous to evaluate IK resistance by EPR-test.     

Untersuchung der Korrosionsschutzwirkung von Wärmeträgern Teil 1: Chemische Prüfungen,Screening- und Spezial-Tests

Testing the corrosion protection of heattransfer fluids Part 1: Chemical, screening and special tests As supplement to AGK Arbeitsblatt W l “Corrosion testing in heat-transfer fluids of solar heating installations” the different methods applied for testing the corrosion protection in heattransfer fluids provided with anti-freeze mixtures on glycol basis have been checked. The coupon tests corresponding to ASTM D 1384, the EMPA-Test, special tests according to ASTM D 4340 as well as the FVV-Heat-Test were part of these investigations. The investigations were carried out with steel, cast iron, two wrought aluminium alloys, one cast aluminium alloy, with copper and brass as well as with tin solder in non inhibited as well as inhibited heat carriers. The non inhibited heat-transfer fluids were composed of water with ethylene and propylene glycol, the others were made up with antifreeze concentrates prefabricated by well known producers. All investigated products provide excellent protective qualities against corrosion. The coupon test according to ASTM D 1384 has been the most effective of all methods tested up to now. This article will be continued.