Die Sauerstoffreduktion aus schwach sauren und neutralen,ungepufferten Lösungen

Oxygen reduction in weakly acid, not buffered aqueous solutions The cathodic reduction of solved oxygen was investigated with the cyclic voltammetry at oxid-free, bright Pt-electrodes in slightly acid, not buffered, aqueous solutions. The limiting diffusion current density of the protons causes a plateau in current-potential curve. An influence of the anions was detected.     

Korrosion von silber in neutralen alkalisulfatschmelzen

The corrosion of silver in the ternary eutectic of (mole %) 78 Li₂SO₄, 13·5 K₂SO₄ and 8·5 Na₂SO₄ at 625°C is proportional to the oxygen pressure above the melt. The corrosion current density of completely immersed specimens is 1·4 × 16^?6 A cm^?2 in the unstirred melt under a 100% oxygen atmosphere. The corrosion rate is increased by stirring.Specimens incompletely immersed in the melt are corroded at a much higher rate than fully immersed specimens. The specimen surface protruding from the melt is covered by a thin film of the molten material. Local cells are formed where O₂ is preferentially reduced on the surface above the melt, and silver is preferentially dissolved from the immersed portion of the surface. The corrosion current density is controlled by the ratio of areas inside and outside the melt.     

Inhibierung der Korrosion von Stahl in sauren Medien durch Ammoniumalkylxanthogenate

Inhibition of steel corrosion in acid media by ammonium alkylxanthogenates The authors have investigated the inhibiting effect of ammonium alkylxanthogenates with different alkyl radicals (C₅? C₁₆) and ammonium groups substituted in different ways (alkyl radicals C₁? C₁₀, even with formation of 6-member ring structures with the nitrogen). The acids investigated included inorganic (sulfuric, hydrochloric, amidosulfonic) and organic acids (formic, acetic, lactic, citric). The inhibitors reduce corrosion losses as well as hydrogen absorption during pickling or cleaning of steel. The effect can be attributed not only to the presence of the S and N atoms, but also to the alkyl radicals which probably form some type of hydrophobic barrier on the metal surface. As to the reduction of hydrogen absorption the effect is observed after extended periods of time only, while no essential difference has been found between inhibited and uninhibited acids as to their effect on mechanical properties.     

Potentialabhängigkeit der H-induzierten Korrosion (HIC) und H-induzierten Spannungsrißkorrosion (HSCC) bei Röhrenstählen in schwach sauren und neutralen Medien

Influence of Potential on Hydrogen-Induced Cracking (HIC) and Hydrogen-Induced Stress Corrosion Cracking (HSCC) of Line-Pipe Steels in Weak Acidic and Neutral Environments In weak acidic (pH 5.5), sulfide-containing environments, high-strength line-pipe steels (0.2% yield strength 430 to 700 Nmm^?2) show a critical mechanical stress level σ_cr ≈ 60% YS above which HSCC occurs. The given value is independent of the mechanical properties. σ_cr decreases with decreasing pH (pH 3: σ_cr ≈? 30% YS), increases with increasing pH and becomes identical with the YS at sufficiently high pH. In buffer solution pH 5.5, σ_cr corresponds to a critical limiting potential for the occurence of HSCC, which depends on the mechanical stress level, ranging between U_H = ?0.42 and ?0.55 V for σ = 60% YS. At potentials more negative than a cathodic limiting potential, which ranges from U_H = ?0.8 to ?1.0 V, no HSCC occurs. This is probably due to the formation of protective layers or of an alkaline environment on the surface. This limiting potential is shifted into the positive direction with decreasing pH and does no more exist in acidic environments. With anodic polarization to potentials more positive than the free corrosion potential, uniform attack occurs resulting in failures of the specimens by reduction of area. Susceptibility to HSCC has a maximum at ambient temperature. There exists also a critical potential U_HIC for the occurrence of longitudinal cracks due to HIC which is more positive than U_HSCC. There is no connection found between U_HIC and U_HSCC nor between σ_cr and U_HIC. From the occurrence of HIC one can not derive any conclusions for the susceptibility or resistance of the given material to HSCC. As a summary, high strength line-pipe steels display no higher susceptibility to HSCC in sulfide-containing media than steels with minor mechanical properties. The HSCC-behaviour of the steels under investigation in buffer solution pH 5.5 is completely equivalent to the behaviour in stronger acidic environments (pH 3).     

Die Rolle der Begleitstoffe von Abgaskondensaten aus der Erdgasverbrennung bei der Korrosion von Wärmetauschern

Impurity constituents of flue gas condensates and their significance for the corrosion of heat exchangers This paper presents investigations concerning the acidic constituents of flue gas condensates which may be generated by the combustion of natural gas. Nature, origin and concentration of relevant constituents will be presented. Their corrosive impact on components of domestic heating facilities will be discussed taking into consideration usual materials based on aluminium, stainless steel and copper. It will be shown that condensates are considered as diluted and aerated acids with a moderately reduced pH. With respect to aluminium based alloys and stainless steels the corrosive potential of condensates is deduced from their content of strong acids. When condensing is forced due to techniques which are used in condensing boilers there is an effect of dilution active which results in a reduction of the corrosiveness. The primary corrosion mechanism of copper is related to the presence of oxygen in the condensate. Therefore the dilution of condensates is of restricted significance. Unprotected copper is known to be unsuitable for condensing conditions.     

Das Korrosionsverhalten von sauerstoffionenimplantiertem Magnesium in schwach saurem Elektrolyten

The corrosion behaviour of oxygen implanted magnesium in weekly acid solution Under atmospheric conditions magnesium forms oxide layers which consist mainly of magnesium hydroxide. The protective action of Mg(OH)₂ is minor in sulphate‐, carbonate‐ or chloride containing solutions [2, 18]. Additionally, the crystallographic misfit between magnesium and magnesium hydroxide leads to crack formations inside the hydroxide layer and the metallic surface is then exposed. By using the ion implantation of oxygen (1*10¹⁸ O⁺/cm²) a MgO‐layer can be formed and buried within the bulk metal. Pre‐tests of MgO‐layers produced by ion implantation suggest that it is successful as corrosion protection [17]. Crystallographic stresses between metal‐ and oxide phase will be reduced, depending on the gradient of oxygen concentration. The investigation in weekly acid sodium solution shows, that the oxygen ion implantation leads to a significant improvement in the corrosion behaviour of magnesium.     

Einflüsse von Legierungselementen auf die Korrosion und Wasserstoffaufnahme von Eisen in Schwefelsäure – Teil II: Korrosion und Deckschichtbildung

Effects of Alloying Elements on Corrosion and Hydrogen Uptake of Iron in Sulfuric Acid Part II: Corrosion and Formation of Surface Layers The effects of C, S, P, Mn, Si, Cr, Ni, Sn and Cu on the formation of surface layer and hydrogen uptake of iron during corrosion in 1 M H₂SO₄/N₂ were investigated using AES, XPS, SEM and electrochemical permeation techniques. Cu, Sn, P and C are enriched on the surface of iron during corrosion in H₂SO₄. Cu is enriched in the metallic form. P forms a phosphate and phosphide containing surface layer. Ni is not enriched. Cr is preferentially dissolved. Cu, Sn and Ni inhibit the dissolution of iron and thus decrease the hydrogen activity. S, P and Mn (MnS) increase the corrosion and hydrogen activity. Cr forms traps in iron which increase the hydrogen uptake.     

Untersuchungen zur passivierenden Wirkung organischer Oxydationsmittel auf Eisen in neutralen und schwach sauren Lösungen. 2. Mitteilung: Aromatische Nitroverbindungen

Investigation into the passivation effect of organic oxidizing agents on the corrosion of iron in neutral weakly acid solutions. 2nd communication: Aromatic nitrocompounds The present communication is con cerned with the corrosion inhibiting properties of aromatic nitro compounds on iron in the neutral range. Nitrobenzenes (m-nitroaniline, p-chloronitrobenzene, M-nitrobenzoic acid, ethylester, p-nitrobenzaldehyde, p-nitrobenzonitrile) in concentrations between 0,25 · 10^?2 and 0.5 · 1^?2 Mole/litre act as stimulators in airsaturated 20% methanol of pH 6.8. Their combination with a 20% methanolic boric acid/borate solution of pH 6.8 – which, as such, only maintains passivity, yields passivating systems. From these examples as well as from others involving known inhibitors it appears highly probable that the prassivation of iron by corrosion inhibiting addition in the neutral range, too, requires a communication of oxidizing and non-oxidizing agents (combination principle )able to lower the critical passivation current density.     

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.     

Kinetik der mit Wasserstoff- bzw. Sauerstoffdepolarisation verlaufenden Korrosion von Eisen

Kinetics of the corrosion of iron accompained by hydrogen or oxygen depolarisation The paper brings a critical review of research work devoted to the problem of corrosion of iron in aqueous solutions, free from oxygen as well as aerated. The relationship is discussed between corrosion rate on the one hand, on the other hand, the p_H value, the composition of the solution, the admixtures contained in the iron, the structure of the latter, and the condition of the surface of the test specimens. As regards corrosion associated with oxygen depolarisation, the paper also deals with the transport problems. The authors emphasize that the results obtained by different methods are liable to differ from each other.     

Hemmung der sauren Korrosion unterschiedlicher Kohlenstoffstähle unter Verwendung von Thioharnstoff

Inhibition of the acid corrosion of various carbon steels In the present paper the influence of carbon content in steel (0.002–1.05%C) on the corrosion rate in deacerated 2.0 M sulphuric acid containing thiourea in a concentration of 0.003-0.4% has been studied. The measuring effects of spontaneous dissolution and the potentiostatic measuring results for two different stirring rates of the disk electrode have been shown. It was found that the thiourea represses the corrosion of steel to a certain optimal concentration above which a corrosion stimulation takes place. The optimal concentration of the thiourea depends on the carbon content in steel and in a smaller degree on the stirring rate. In the absence of thiourea the anodic and cathodic polarisation curves have a run considerably depending on the carbon content in steel. The polarisation curves at the thiourea concentration of 0.010 and 0.100% show a Tafel run with an elevation practically independent on the carbon content in steel. The run of the polarisation curves as well as the metallographic observations allow to admit that the cementit and also the ferrite undergo an anodic dissolution at the presence of thiourea.     

Inhibierung der Korrosion von Eisen in Säuren mit Hilfe von organischen Schwefelverbindungen

Inhibition of iron corrosion in acid with the aid of organic sulphur compounds Tests were carried out with Armco-iron in 1 n 10%-aqueous ethanol sulphate with the addition of n-butyl, s-butyl, n-hexyl, n-octyl, n-decyl, benzyl and phenylmercaptane, o- and m-thiokresol. The purpose was to determine the inhibition effect on the loss of weight and on the embrittlement through hydrogen adsorption. The inhibition effect is probably governed by adsorption via a free pair of sulphur electrons. The sulphur charge is therefore the controlling factor which also explains the effect of the chain length which affects the electron density at the sulphur. The inhibition effect has a maximum with C₈ whilst, with larger chains, a screen effect is again apparent. The adsorption of hydrogen is first stimulated by low inhibitor concentrations, and later inhibited. But there is no clear correlation between the effect on corrosion and that on hydrogen adsorption.     

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.     

Untersuchungen zur passivierenden Wirkung organischer Oxidationsmittel auf Eisen in neutralen und schwach sauren Lösungen — 1. Mitteilung: C-Nitrosoverbindungen und Chione

Investigations into the passivating effect of organic oxidizing substances on iron in neutral and weakly acided solution I. Communication: C-nitrosocompounds and quinones Using potential/time curves the authors have found that in solutions containing 0.1 M sodium sulphate and 0.1 M KH₂? PO₄/K₂HPO₄ certain oxidizing media have rather a pronounced passivating effect. The inhibiting action of p-nitrosodimethylaniline and p-nitrosophenol comes to bear in a comparatively broad pH range while potassium quinone monosulfonate and acetylene quinone are passivators in neutral solutions only and quinone as such has a good effect only in additated solutions buffered to pH 5.2 to 6.3 α-nitroso-β-naphthol on the other hand forms a surface layer inhibiting corrosion.     

Einfluß der Hydrodynamik auf die Korrosion von Eisen und Stahl in neutralen belüfteten NaCl- und Na2SO4-Lösungen

Influence of hydrodynamics on the corrosion of iron and steel in neutral aerated NaCl and Na₂SO₄ solutions The corrosion behaviour of iron and steel in aerated neutral NaCl and Na₂SO₄ solutions is mainly determined by the transport-controlled cathodic oxygen reduction. Its reaction zone depends on the physical and chemical properties of the 3-D layer formed on the corroding surface. The influence of hydrodynamics is restricted to the transport of the dissolved oxygen to the reaction zone. The obtained results are independent of laminar or turbulent flow conditions.     

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.     

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.     

Die Oxydation von Eisen bei hohen temperaturen in Inertgasen mit kleinen Sauerstoffgehalten

Oxidation of iron at high temperatures in inert gases with small oxygen contents At temperatures > about 800°C and linear flow velocities of the gas between about 5 and 12 cm/s the oxidation of iron in N₂-O₂, Ar-O₂ and He-O₂ mixtures containing < 1% oxygen follows a linear kinetic law. Under such conditions the rate controlling step is the transport of oxygen from the gas phase to the surface of the scale layer which consists exclusively of FeO. The constant of the linear kinetic law under laminar flow conditions is proportional to the oxygen content of the gas and to the square root of the gas flow velocity, and inversely proportional to the square root of the gas viscosity, while it is practically independent from temperature. Some conclusions with reference to practical applications are presented.     

Elektrochemisches Rauschen bei der Korrosion

Electrochemical noise from corrosion On the basis of the causes, the sources of the electrochemical noise, by several examples the good correlation between different system parameters and the noise behaviour is shown. With attention of some factors very good possibilities are offered of using the noise for a kind of an early warning system during the process monitoring. It is also shown, that electrochemical noise as well as the corrosion at all is of complex nature. Ignoring certain influences and parameters can lead to dramatic false estimates.     

Zur Kinetik der Sauerstoffreduktion bei der Korrosion von Nickel in sauerstoffhaltiger Schwefelsäure

Kinetics of oxygen reduction during the corrosion of nickel in oxygen containing sulfuric acid The rate of oxygen reduction was determined for the corrosion of nickel in sulphuric acid containing oxygen. The rate depends upon the oxygen concentration and the acidity of the electrolyte. For a second order reaction with a preceding diffusion of oxygen and protons the values of the rate constant and the thickness of the diffusion layer were calculated. It could be shown that in In sulphuric acid the reduction rate is control led by the oxygen diffusion while in the weaker acids the rate controlling step is a proton transfer to the oxygen, adsorbed at the metal surface. Measurements of the dependance of the corrosion in In sulphuric and hydrochloride acid confirm these results.