Zum Kontaktverhalten von feuerverzinktem und unverzinktem Bewehrungsstahl in Beton bei erhöhter Temperatur

Contact behaviour of galvanized and ungalvanized steel in concrete at higher temperature Electrochemical measurements at 60 °C on steel and galvanized steel embedded in cement mortar with and without chloride show that there is no potential reversion between steel and zinc during the test time of 30 weeks. The contact current between steel and zinc is always anodical related to zinc. The current is continually diminishing with time, its value is slightly higher in mortar containing chloride. Contacted steel and galvanized steel therefore leads to a higher zinc loss at least in the starting time. Without contact there is only a small corrosion attack on zinc in both cases of chloride free and chloride containing mortar.     

Untersuchungen über den Einfluß des Zinks auf das Korrosionsverhalten von Spannstählen

Investigations on the influence of zinc on the corrosion behavior of high strength steels Corrosion and fracture behavior of hot dip galvanized high strength steels with and without damaging zinc layer was studied in deionized water, in tap water, and in saturated calcium hydroxide solution through which nitrogen, air, and carbon dioxide were bubbled separately. Hydrogen permeation measurements have been carried out for steel specimens which were cathodically polarized by means of the zinc layer. It has been observed that the hydrogen activity reached to a maximum of 1-2 in neutral solutions and 4-10 in saturated calcium hydroxide solution or in dilute acid solutions containing carbon dioxide, before decreasing to low values. The decrease of hydrogen activity is due to the formation of surface layers. Constant load tests by application of a load amounting to 100-110% yield strength caused no hydrogen induced brittle fracture within 300-400 h neither with nor without damaging zinc layer. At a constant strain rate of 5· 10^?7/s only 8 mm cold drawn steel wires were found to be almost unsusceptible to hydrogen that was absorbed during the test. Under notched conditions, all the steels investigated showed macroscopic brittle fracture.     

Über das Korrosionsverhalten von Zink und feuerverzinktem Stahl in erwärmtem Wasser

Corrosion behaviour of zinc and hot dip galvanized steel in warm water The corrosion rate of zinc and hot dip galvanized steel in continuously flowing tap water is but little influenced by water temperature. The slight decrease of the corrosion rate which has been found at higher temperatures (60°C) can be attributed to the preferential formation of zinc oxide at these temperatures, this oxide being less soluble than the hydroxide formed at lower temperatures. The potential shift toward more noble values at higher temperatures does, however, depend from water composition and may eventually start as early as at 35 °C; in certain critical potential ranges the uniform corrosion may even be changed into pitting. Addition of phosphate to the water may reduce either the corrosion rate or it may restrict the potential shift (in this latter case the appearance pitting is prevented). The measurement of the electrode potential (not of the polarization resistance) thus yields information on the danger of pitting corrosion. The quality of the zinc surface seems to be important in those cases where the ZnFe alloy layer protrudes to the zinc surface.     

Korrosionsverhalten feuerverzinkter Baustähle in überwiegend sandhaltigen Böden

Corrosion behaviour of galvanized steel in mainly sandy grounds In mainly sandy grounds with different portions of fine parts < 0,06 mm (2,5–20,4%) specimens of ungalvanized and galvanized steel were stored outside and in the laboratory. Besides the composition of the ground, the salinity and the temperature of the ground were varied too. The corrosion rate and, for ungalvanized specimens, the behaviour to pitting corrosion were determined. Furthermore the factors characterising the corrosion behaviour such as specific resistance of soil and corrosion potential were investigated continuously. The loss in weight of metal was much greater for ungalvanized than for galvanized specimens and increased for ungalvanized specimens with an increasing portion of fine parts in the ground. Additions of salt at the beginning of the tests produced an increased amount of metal wastage, but for galvanized specimens they only had an influence upon initial corrosion. The increased removals of material started since contents of 3 · 10^?3 MolCl^? + SO/kg. If salts were added to the ground after 2 years (after the formation of a surface layer), they increased the wastage of material for ungalvanized but not for galvanized specimens. Apart from ungalvanized bars in the soil with a fineness portion of 20,4%, corrosion, after an acceleration at the beginning, slowed down owing to the formation of a surface layer. Ungalvanized specimens were attacked by a strong pitting corrosion and that more in aerated than in dense and, thus, water-containing grounds. The additions of salt accelerate more an uniform corrosion of material than a pitting corrosion. As for galvanized specimens after a local removal of zinc under extreme conditions the steel base had been hardly corroded away. The parts free of zinc were protected cathodically by the still existing zinc. The corrosion of steel depends upon the temperature: by increasing the temperature from 4 to 20°C increases of corrosion up to 100% were stated. As for galvanized surfaces temperature has only a small influence upon corrosion.     

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.     

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 Langzeit-Korrosionsgeschwindigkeit des passiven Eisens in anaeroben alkalischen Lösungen

The long-term corrosion rate of passive iron in anaerobic alcaline solutions Gas generation is an important issue in safety assessments of low and intermediate level radioactive repositories. In this connection the hydrogen production from corrosion of passive iron in saturated calcium hydroxide, in dilute alkali hydroxide and cement porewater solutions has been determined. The measurements were performed manometrically using fusion sealed glass cells, the measurement periods being between 275 and 560 days. In 0.1 M and 0.04 M alkali hydroxide solutions the initial hydrogen generation rate was 12 mmol/m²yr corresponding to a linear corrosion rate of 64 nm/yr. The reaction rate decreases with time. The smallest value obtained after 330 days is 0.3 mmol/m²yr corresponding to 1.5 nm/yr. The influence on iron of the saturated calcium hydroxide solution and the calcic porewater solutions differs from that of the alkali hydroxide solutions. At pH 12.5 the hydrogen generation rate remains practically constant up to breaking off the experiment, the value being about 1 mmol/m²yr corresponding to 5 nm/yr.     

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.     

Potentiodynamische und potentiostatische Messungen an gespannten Drahtelektroden in gesättigten Ca(OH)2-Lösungen mit verschiedenen Zusätzen

Potentio-dynamic and potentio-static measurements on tensioned wire electrodes in saturated Ca(OH), solutions with different admixtures The influence of mechanical tensile stresses on the corrosion behaviour of unalloyed steel wires has been examined in saturated calcium hydroxide solutions with and without additions of chloride or chromate ions to simulate the practical conditions encountered with steel reinforcement in concrete. In order to account for the influence of oxygen dissolved in the corrosion medium, the tests were carried out in an air-saturated solution as well as in a solution free from oxygen. The tests revealed a considerable increase in the dissolution rate of iron in the active potential range as well as an increase in the passive current density with increasing tensile stress. Also with increasing tensile stress the stress corrosion cracking potential is shifted towards the negative side. The test results indicate that the thickness of the passive film increases with the tensile stress. All these effects are even more marked in air-saturated solution.     

Das Korrosionsverhalten von feuerverzinktem Stahl in kalten und warmen Modellwässern in Abhängigkeit einer CO2- und/oder O2-Spülung und von der Kalkhärte

Corrosion behaviour of hot dip galvanized steel in cold and warm model waters with gassing by CO₂ and/or O₂ and as a function of lime hardness In oxygen-or carbon dioxide free waters the corrosion is due to the evolution of hydrogen from water; this corrosion is accelerated by higher chloride contents. It is therefore advisable to keep chloride contents below 150 mg/l Cl^?. In the presence of oxygen the zinc coating is attacked by oxygen; here, too, the influence of chlorides is quite evident. The presence of a certain water hardness gives rise to the formation of protective layers. When the water contains carbon dioxide the corrosion is of the acid type; it is inhibited, however, by protective layer formation. It should be borne in mind, that the hardness of water has a favourable effect only in cold waters, while it is very deleterious in warm waters. Water flow gives rise to accelerated corrosion. Pitting can be prevented by oxygen elimination or by the formation of protective layers. This layer formation is optimum under lime-carbonic acid equilibrium conditions.     

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.     

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.     

Das anodische und kathodische Verhalten des Eisens in sulfathaltigen Elektrolyten. Chaire d'Electrochimie,Faculté des Sciences,Strasbourg, France

The anodic and cathodic behaviour of iron in sulphate containing electrolytes The formation of Fe₂(SO₄)₃ on passive iron at p_H = 1 appears probable from a thermodynamical point of view. At high SO₄^2? concentrations the equilibrium system contains but low concentrations of Fe³⁺, and no Fe²⁺ ions, a fact showing the relatively elevated stability of the Fe₂(SO₄)₃ layer on passive iron. In slightly acid solution (p_H = 4) the passivity of the iron is determined by iron oxide layers. The formation of FeSO₄ from metallic iron and sulphate ions is restricted to the transpassive zone (p_H 4 to 7), in alkaline solutions even to the active zone. In the p_H region 2 to 14 the passive layer on iron has about the same composition in the systems Fe|H₂O + SO₄^2? and Fe|H₂O.     

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.     

Einfluß des pH-Wertes,des Sauerstoffgehaltes und der Strömungsgeschwindigkeit von kaltem Trinkwasser auf das Korrosionsverhalten und die Schutzschichtbildung bei feuerverzinkten Stahlrohren

Influence of pH-value, oxygen content and flow velocity of cold drinking water on corrosion behaviour and surface layer forming of galvanized steel tubes During a test program of 28 months determinations of weight loss, metallographical examinations and chemical analysis data of surface layers showed clearly that in the chosen variation range mainly the pH is important for the longterm behaviour and the expected working life of galvanized steel tubes. Lower pH-values of 6.9 lead to increased rates of Zinc attack and to inhomogene, disturbed surface layers, whereas higher pH-values of 7.9 show decreased Zinc loss and favour the forming of well protecting layers. Oxygen content and flow velocity become important only in the early stage of Zinc corrosion. Additional electrochemical measurements gave a good impression of Zinc corrosion rate development, corrosion attack morphology and properties of surface layers.     

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.     

Schwingungsrißkorrosion feuerverzinkter Schweißverbindungen im bauteilähnlichen Maßstab bei wechselnden Korrosionsbedingungen in chloridhaltigem Medium

Corrosion fatigue of hot-galvanized component-like welded joints in chloride medium at changing corrosion conditions The project was focussed on the investigation of the fatigue behaviour of welded specimens of hot-galvanized plate material of steel RSt 37-2, St 52-3 and St 70-2 as frequently used in steel constructions which were mended by thermal zinc-spraying in the fillet area and tested at changing corrosion conditions. In addition, the fatigue behaviour of welded specimens with additional PVC coating of both non-galvanized and galvanized types at changing corrosion reaction was also investigated. The established findings can be summarized as follows:

• – At constant amplitude loading, hot-galvanized precludes very markedly the drop in endurance which would occur under corrosion. By contrast, an increase of fatigue strength in the corrosive medium due to galvanizing is only conditionally discoverable at variable amplitude loading. In regard to the protection against corrosion, welding of hot-galvanized parts brings an improvement at variable amplitude loading only if a given thickness of the zinc coating depending on the material is not exceeded. Too thick coating results in the formation of pores at the subsequent welding and reduces the fatigue strength in spite of the later zinc-spraying.
• – The structural steels RSt 37-2 and St 52-3 in hot-galvanized state are better suitable for welding than the more notch- and corrosion-sensitive St 70-2.
• – Soldering brittleness could not be established as cause of the failure.
• – The application of PVC protective layers secures high rates of increase or the fatigue for the non-galvanized specimens.

     

Zur Korrosion von Eisen und Aluminium in wässerigen Lösungen von Ammoniak und Kohlensäure – 2. Mitteilung: Elektrochemische Untersuchungen

Contributionto the corrosion of iron and aluminum in aqueous solutions of ammonia and carbonic acid. 2nd Communication: Electrochemical investigations Corrosion potential-time curves and current density-potential curves as well as polarization resistances and other electrochemical methods have been used to characterize the corrosion behaviour of aluminum and iron base materials in an aqueous solution of 11% ammonia and 7% carbon dioxide (leaching solution applied in hydrometallurgical nickel raffination) at 50 °C. For comparisonal purposes the behaviour of the same materials has been investigated in ammonia, sodium carbonate and sodium hydroxide solutions at constant pH. Despite of the relatively elevated pH value aluminum is protected against corrosion in the ammonia-carbon dioxide-water-mixture through selfpassivation (fast repassivation of mechanically destroyed passive layer). In contrast, the stability of iron and mild steel in the same solution is seriously impaired by easy activation. Permanent passivation is only achieved through anodic protection by means of an external current source.     

Untersuchungen zum Aktiv/Passiv-Übergang von nichtrostenden Chromnickelstählen in organisch wäßrigen Medien. Teil 3. Resultate der Impedanzmessungen und Passivierungsmodell einer Legierung

Investigations into the active/passive transition of 304 stainless steel in organic media containing water and hydrogen chloride Part 3. Results of the impedance measurements and passivation model of an alloy Steady state polarization curves and electrode impedances were measured during the active/passive transition of type 304 stainless steel in dearated ethanolic solution containing hydrogen chloride and different amounts of water. The passivation potential and the critical current density for passivation strongly depend on the water content of the solution. The impedance measurements in the active/passive transition show the same sequence of diagrams independent of the water content of the solution. They indicate the onset of passivation before the maximum current density and show two time constants related to two different passivating species on the alloy surface. The experimental results were interpreted on the basis of a reaction model with parallel dissolution and passivation mechanism of the iron and the chromium compound of the alloy. The resulting total surface composition (related to the steady-state polarization curves) can be described with a reaction model of iron–the alloy behaviour is that of pure metal. The fundamental passivation reaction is described as a potential dependent equilibrium between adsorbed Me(II)- and passivating Me(III)-hydroxide, water molecules being directly involved in the formation of this primary passivating film. In the case of stainless steel this primary passivating film mainly consists of chromium (III) adsorbates. Finally, a general model for the passivation is proposed: The passivation of a pure metal or of an alloy can be understood as the coupling of the stepwise deprotonation of the water molecules at the interface metal/solution and the formation of a high cation charge density in this adsorbed hydroxide/oxide film to build up the passive layer. The effect of water content, pH, adding of passivating species to the solution or the alloying with chromium on the passivation potential and the critical current density thus can be explained.     

Effect of Fe-Zn alloy layer on the corrosion resistance of galvanized steel in chloride containing environments

In this study, the effect of Fe-Zn alloy layer that is formed during galvanizing process on the corrosion behavior of galvanized steel has been investigated. The galvanostatic dissolution of galvanized steel was carried out in 0.5 M NaCl solution to obtain the Fe-Zn alloy layer on the base steel. The alloy layer was characterized to be composed of FeZn₁₃, FeZn₇ and Fe₃Zn₁₀ intermetallic phases, which constitute the zeta, delta1 and gamma layers of galvanized steel, respectively. It was observed that the alloy layer has similar cathodic polarization behavior but different anodic polarization behavior compared to galvanized steel. The anodic current plateau of alloy layer was up to 100 times lower than that of galvanized coating. Corrosion test performed in wet-dry cyclic condition has shown that the alloy layer has lower corrosion rate as compared to galvanized steel. From the results of corrosion test of alloy layer and base steel, it was concluded that Zn²⁺ has positive effect on the protectiveness of the zinc corrosion products. The measurement of surface potential over the alloy/steel galvanic couple has confirmed the galvanic ability of alloy layer to protect both the alloy layer itself and the base iron during initial stage of atmospheric corrosion.