Zum anodischen und kathodischen Verhalten des Eisens in neutralen und alkalischen Lösungen

The anodic and cathodic behaviour of iron in neutral and alkaline solutions To investigate the behaviour of iron in the p_H range from 7 to 14 in the presence of chloride ions, the authors make use of the electrochemical affinity/overpotential diagram. At p_H =7, the activation of the iron by the destruction of the FeO primary layer is possible. With increasing alkalinity, the passive zone is reduced by the direct effect of Cl^? ions, by the oxidation of FeCl₂, and by the formation of the iron complexes FeO₄^2?. The decomposition of Fe(OH)₂ by Cl^? ions is thermodynamically possible up to p_H = 10.5 only. At p_H > 10.5, there occurs the equilibrium Fe(OH)₂ = HFeO₂^? + H⁺.     

Die instationäre Korrosion des passiven Eisens in saurer Lösung

The instationary corrosion of passive iron in acid solution The corrosion rate of the passive iron, i.e. the dissolution rate of the passivating layer, in 1 n H₂SO₄ is investigated, with the layer thickness being increased and decreased, respectively (non-stationary conditions). The corrosion rate is determined chemical-analytically as well as coulometrically on the basis of galvanostatic measurements. Both methods yield the same result. The corrosion current density, i_K, increases as the formation current density, i_S, of the layer increases, and decreases the more below the stationary corrosion value, i_K, the higher the layer reduction current density (i_S <0) is. i_K and i_S, are independent of the electrode Potential of the passive iron electrode, similar to the behaviour of the stationary value, i_K,0. This independence is explained by the semiconductor properties of the passivating oxide (γ-Fe₂O₃). There is an unique correlation between i_K and i_Ss which can be explained also theoretically by a variable overvoltage at the phase boundary oxidelectrolyte. The kinetics of the corrosion and of the layer formation is evaluated from the values of the apparent charge transfer coefficients by referring to the p_H dependence of the stationary corrosion.     

Lochfraßkorrosion des Eisens in alkalischen Chloridlösungen

Pitting corrosion of iron in alkaline chlorine solutions The author has studied the corrosion of iron in chloride solutions (0.4 M KCl) with hydroxyl ions (0.1 M KOH), used as a pitting corrosion inhibitor. The rate of pitting corrosion in alkaline media can be represented in the form of a cubic parabola whilst the number of pits is a linear function. It was found pitting corrosion depends on the potential, on the concentration, and on the ratio of the concentration of activating and passivating ions, respectively. Another important factor is the adsorption of the anions on the electrode surface. The author has formulated equations for all these relations. The depassivation potential proves to be a suitable criterion for characterizing the pitting corrosion proneness under different conditions.     

Die Kinetik der Auflösung von Aluminium in alkalischen Lösungen

The kinetics of the dissolution of aluminium in alcaline solutions The stationary dissolution rates of rotating aluminium discs were measured in alcaline sodium sulfate solutions as function of the Ph- value and of the rotational speed using the Förster eddy current test and electrochemical methods. The rates are controlled by diffusion or by heterogeneous dissolution of aluminate from the passive, oxide-covered aluminium. If the rate is diffusion-controlled, a secondary layer very probably consisting of hydrargillite is formed on the passive aluminium. The rate of the heterogeneous dissolution grows little with the Ph-value and depends strongly upon the electrode potential at unnoble potentials and weakly at noble potentials. Cathodic evolution of hydrogen increases the diffusion-limited as well as the reaction limited dissolution rates because of the excess of hydroxyl ions formed at the surface.     

Korrosion von Chemieemail in alkalischen Lösungen

Corrosion of chemical service glass-enamel in alkaline solutions Chemical service glass-enamels are high resistant, multi component silicate glasses fused onto steel yielding a compound material of mechanical and chemical stability as well. The resistance of these glass-enamels against alkali attack is investigated in laboratory tests involving gravimetric evaluation of totally glass-lined steel samples after contact with various alkaline media. Thus the influence of the following technically relevant parameters on the glass corrosion rate becomes perceptible: agitation and renewal of the medium, volume-to-surface ratio, exposure time, temperature, alkali content, glass quality; special attention is paid to inhibition effects originating e. g. from calcium(II) ions. These results may help to conceive on a laboratory scale a corrosion test for glass-linings which simulates in good approximation service conditions. In consequence, the corrosion situation in glass-lined equipment becomes more evident and appropriate measures may be considered to improve its economy.     

Elektrochemische Untersuchungen über die Korrosion des Eisens in sulfidhaltigen Lösungen

Electro-chemical investigations into the corrosion of iron in solutions containing sulphides In order to investigate the corrosion behaviour of steel reinforcements in concrete containing sulphides, measurements have been carried out of electro-chemical current potentials. The electrode material consisted of carbonyl iron; the air-less electrolyte solutions had varying contents of H₂S, HS^. and S^?, with a PH value ranging from 4 to 12.6. The tests showed that, with PH= 12.6, iron remains protected by a passive oxide film even in a solution containing sulphur. However, from about PH = 10-11 downwards, the passive film is replaced by a non-protective iron sulphide layer. But at least down to PH = 9, the corrosion rate is still very low due to the strong inhibition of the cathodic part-reaction of the hydrogen segregation. The kinetics of the hydrogen segregation due to H₂S reduction are discussed.     

Die potentiodynamische Polarisationskurve des Kupfers in Lösungen von Chloriden

The potentiodynamic polarization curve of copper in chloride solution The potentiodynamic curve obtained under conditions of high rate polarization does not only represent the dissolution of copper but also the redox reactions taking place in this rather complex system. The first part of the curve characterizes the anodic reaction resulting in the formation of CuCl^?₂ and CuCl; it can therefore be used for studying spontaneous corrosion. In addition, the fast polarization enables the influence of various factors to be assessed and fast reactions to be recognized which are not registered under conditions of slow polarization. It should be possible on this basis to study e.g. the influence resulting from structural changes in copper alloys.     

Über den Einfluß von Schwefelwasserstoff auf die Korrosion des Eisens in sauren Lösungen

The influence of hydrogen sulphide on the corrosion of iron in acid solutions A considerable change of the polarisation behaviour of iron electrodes in acidic perchlorate and sulphate solutions was observed if they where saturated with hydrogen sulfide gas. H₂S markedly increased the anodic current densities and increases the corrosion rates by a factor of ten. The anodic Tafel slopes d log i_A/dE are only about half as steep as those in the absence of hydrogen sulphide. It is probable that the SH^?-ions behave as catalysts of the iron dissolution reaction in a similar manner as the OH^?-ions, but are adsorbed almost to saturation.     

Kritischer Chloridgehalt – Untersuchungen an Stahl in chloridhaltigen alkalischen Lösungen

Critical chloride content – Investigations of steel in alkaline chloride solutions The question of the critical corrosion-inducing chloride content of steel in concrete still incites controversial debates. In a first series of trials, which will be outlined as part of the present article, the reduced corrosion system of steel/concrete pore solution was investigated. The results suggest that a log-log linear relationship exists between the molar concentrations of corrosion-inducing and passivating ions that can be expressed by the following equation: log c(Cl¯)_krit = 1.5 · log c(OH¯) - 0.245. On the basis of this relationship the results compiled in the bibliographical search were discussed and the significance with respect to the pitting potential was explained. Supplementary investigations into the time of chloride addition and the composition of the pore solution were conducted.     

Zum Verhalten von Natriumsilikat als Korrosionsinhibitor für Aluminium in alkalischen Lösungen

The behaviour of sodium silicate as a corrosion inhibitor for aluminium in alkaline solutions Even at a temperature of 80°C and even in the presence of intensive mechanical stresses on the metal by a sharp spray jet (with a spraying pressure of 30 atmospheres excess pressure at the nozzle), sodium silicate (Na₂O:SiO₂ = 1 : 2,5) is able to prevent corrosion of pure aluminium (99,5 pC) by alkaline solutions (pH 11). For this purpose, very small quantities of adsorbed silicate per unit of area are already sufficient. It is essential, however, that the SiO₂ concentration in the solution does not sink below a certain minimum value, depending on ambient conditions, so that a sufficient quantity of silicate can be adsorbed. With increasing temperatures and spraying pressures, the silicate concentration must be higher if effective corrosion protection is to be attained. The corrosion-preventing film on the metal surface is formed by non-aggregated silicate. Different metals draw different quantities of corrosion inhibitor from the silicate solutions. This behaviour is presumably governed not only by the specific adsorption capacity for silicate ions but also by the corrosion resistance of the different metals in the solution.     

Kritischer korrosionsauslösender Chloridgehalt – Untersuchungen an Mörtelelektroden in chloridhaltigen alkalischen Lösungen

Critical chloride content – Investigations of mortar electrodes in alkaline chloride solutions Although the penetration of chloride ions into the concrete is obstructed by numerous protective mechanisms, the concrete thus a complex multi‐barrier protective system represents, the chloride ions nevertheless succeed it to break through the protecting passive oxide layer of the steel, like the numerous chloride‐induced corrosion damage from building practice clearly document. In a first part results have been outlined concerning the investigations of the reduced corrosion system of steel/concrete pore solution [1]. The present article comprises results of investigations into the critical corrosion‐inducing chloride content that have been obtained by electrochemical investigations of mortar electrodes in alkaline chloride solutions. Finally the findings gained are explained with a view to the relevance they imply for building practice, and particularly for the design of new buildings to be constructed and the assessment of existing structures.     

Zur Koexistenz von aktiven und passiven Bereichen auf Elektroden in oxidierenden Lösungen. Untersuchungen an einer Zinkelektrode in phosphathaltigen Jodlösungen

The coexistence of active and passive zones on elctrodes in oxidising solutions. Investigation of a zinc electrode in phosphate-containing iodine solutions Earlier work on the coexistence of active and passive zones on an anodically charged zinc electrode in aphosphate solution has been extended to include the condition of the electrode being externally currentless and the anodic partial current being compensated by a cathodic partial current. The cathodic partial reaction comes from the iodine/iodide redox reaction at the zinc electrode. A general theoretical derivation is presented to show the behaviour, with reference to the area of the active zones, as a function of the electrolyte concentration. Two additional borderline cases are discussed, viz. electron-conducting and non-con-ducting covering layers. The theoretical investigations have been compared with the experiment; they show the applicability of the model for the system investigated.     

Die Korrosion des Bleis in tetrafluoborsauren Elektrolytlösungen

The corrosion of lead in acid tetrafluoborate electrolytes The corrosion of 99,985% lead in acid tetrafluoborate electrolytes, predominantly 1 mole/litre HBF₄ and 2 mole/litre Pb (BF₄)₂, was investigated. The corrosion rate in air saturated solution at 20°C is 35 μA/cm², which corresponds to 1 mm/a. Oxygen corrosion predominates (corrosion is rather reduced under argon). At higher temperatures, corrosion rate increases by an order of magnitude, and acid corrosion has a greater share due to a decrease of hydrogen overvoltage on lead and solubility of oxygen. Cementable additives like Cu⁺⁺, Sb⁺⁺⁺ and Pd⁺⁺ are found to enhance the initial corrosion rate by up to two orders of magnitude. However, after 10–20 hours, the original value is reestablished. Bi⁺⁺⁺ is cemented as well; the resulting acceleration of corrosion is proportional to the additive concentration and decreases but slowly. Oxidants like Fe⁺⁺⁺ or quinone accelerate the dissolution of lead as well and are consumed stoichometrically. Reducing agents like Fe⁺⁺, V⁺⁺⁺ or hydroquinone are not able to act as a mediator for air corrosion. Nitrate ions are virtually inert in the given concentration range. Consequences in reference to the lead dissolution secondary battery are discussed. Self discharge in the absence of O₂ is very low. On the other hand, it is possible by oxygen corrosion to redissolve lead, which has accumulated on the electrode. In this way, the battery can be brought back to its original state.     

Die aktive und passive Molybdän-Elektrode in sauren Lösungen

The active and passive molybdenum electrode in acid solution It follows from the anodic and cathodic thermodynamic polarization curves in the system molybdenum/aqueous solution (p_H 0 to 10) that the favourable behaviour of molybdenum in acid solutions is determined by the extension of the immunity region becomes smaller, but without simultaneous passivation of molybdenum; this fact explains the active behaviour of molybdenum in alcaline solution. The passivation of molybdenum starts as soon as the half cell reaction sets in, where The p_H-dependence of the cathodic passivation potential in acid solutions follows the equation      

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.     

Untersuchungen über die Lochfraßkorrosion des passiven Eisens in chlorionenhaltiger Schwefelsäure

Investigation into pitting corrosion of passive iron in sulphuric acid containing chloride ions Pitting corrosion of metallic materials is generally connected with presence of a surface layer giving rise to a local differentiation of the electrochemical behaviour of the metal surface. The pitting corrosion by halogen ions on passive metals is investigated using passive iron in chloride ion-containing sulphuric acid as the model system. Quantitative data are presented concerning the mechanism and kinetics of the individual processes giving rise to pitting corrosion in a chloride ion concentration range covering three powers of ten, and in the whole potential range of iron passivity, from the Flade potential to the transpassive breakthrough potential. Pit formation normally follows a linear kinetic law, the rate depending in particular from the chloride ion concentration and from the thickness of the passive layer. The growth of pit diameters follows a linear kinetic law, too; the dissolution current density in the pits depends from the chloride ion concentration. Comparative investigations carried out on active iron, and potential distribution as measured in the pits show that the metal is active in the pits, too. The heterogeneous mixed electrode condition — active pit/passive metal surface — is stabilised by resistance polarisation. The investigations so far do not permit any statement concerning the specific effect of the chloride ions.     

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.     

Über die Inhibitorwirkung der Fettsäuren bei der Korrosion des Eisens in saurer Lösung

Inhibiting effect of fatty acids on the corrosion of iron in acid media Fatty acids (C₁–C₁₂) in acid media (0.01 M HClO₄ + 0.2 M NaClO₄) have partially inhibiting and partially stimulating effects on the corrosion of iron. Short chain acids (C₁ to C₇) in concentrations below 5·10^?5 M act as stimulators, in higher concentrations as inhibitors. Beyond C₈ the stimulating effect gradually disappears. As to the inhibition mechanism the fatty acids are of the mixed type (though the anodic control predominates); they are not chemisorbed but only physically adsorbed by van der Waals forces and become desorbed when the temperature is increased.     

Untersuchungen zur Passivierung des Eisens in wäßrigen Lösungen durch das System „Sauerstoff/Phthalat”︁

Investigation into the passivation of iron in aqueous solutions of the system “phthalat/oxygen” The passivation properties of dissolved oxygen on iron and mild steel in potassium hydrogenphthalate solutions have been studied by means of potentiodynamic polarisation curves and potential/timemeasurements. It is shown that the critical passivation current in a solution of constant phthalate concentration decreases with increasing dissolved oxygen concentration and the solution pH. The transition from active dissolution to passivation associated with a potential shift toward more noble values depends on critical minimum values of pH and dissolved oxygen concentration in the solution. Under these conditions the oxygen reduction diffusion current is greater than the critical passivation current. This effect is interpreted in terms of interactions of phthalate ions on the interface iron/solution.     

Inhibierung der Korrosion von Kupfer in salzsauren Lösungen

Inhibition of copper corrosion in hydrochloric solutions Furfural is one of the best corrosion inhibitors of copper in hydrochloric acid. The inhibition effect ranges from 90 to 100 per cent. With increasing acid concentration, the quantity of furfural required for effective corrosion inhibition becomes smaller. By means of polarisation measurements, it can be found that, in 0.2 n and 1.0 n HCl, the inhibitor has an essential anodic effect and the cathode becomes depolarised whilst, in 2.0 n and 3.0 n HCl, the anodic zone is virtually unchanged and there is a considerable polarisation in the cathodic zone. The protective effect of furfural is probably due to its resin forming capacity.