Korrosionsverhalten von Stahl in Zementmörtel bei kathodischer Polarisation in Meerwasser und 0,5 M NaCl-Lösung

Corrosion characteristics of steel in cement mortar under cathodic polarization in seawater and 0.5 M NaCl solution Steel specimens in cement mortar were exposed to seawater on the VDEh testing stand at Helgoland and to 0.5 M NaCl solutions in the laboratory up to three years. In seawater specimens covered with portland cement were cathodically protected by use of zinc and magnesium anodes. In the laboratory specimens in portland cement and in blast furnace cement were exposed to free corrosion and to potentials ranging from U_H = ?0.53 to ?0.98 V. In all cases small pittings were observed. Those developed in seawater were significantly dependent on potential and accompanied by a slight increase of chloride concentration. Here a normal anodic pitting corrosion must be assumed that cannot be totally prevented by zinc. The process slows down with increasing time. The pitting observed in the laboratory tests showed no systematic dependence on potential and the pits were substantially smaller. In the case of the strongest negative potential U_H = ?0.98 V the specimen in portland cement displayed pits as deep as 0.5 mm. These are assumed to be of cathodic origin. They were full of magnetite and virtually free of chloride. Because that far negative potentials, which lie in the region of hydrogen development, do almost not occur this type of corrosion contrary to literature has no technical bearing.     

Zum Einfluß von Chlorid-Ionen auf die Korrosion von unlegiertem Stahl in Wasserversorgungsanlagen

Considerations on the effect of chlorides on corrosion of unalloyed steel in plants for water supply The effect of anions of neutral salts, especially the chloride anions on corrosion of steel in tap water is discussed in terms of the theoretical aspects and explanations according to DIN 50 930 part 2. For a better understanding it is useful to separate the different corrosion effects: uniform corrosion attack, local corrosion attack and transfer of corrosion products into the water. With respect to these different effects test results are evaluated. It is shown, that neutral salts have no effect on uniform or local corrosion, whereas there is a marked effect of other water parameters. The transfer of corrosion products into the water is controlled by neutral salts as well as by alkalinity, pH, service conditions and the age of the plant.     

Einfluß der Zusammensetzung von Messinglegierungen auf die Entzinkung und interkristalline Korrosion in Ammoniumchloridlösung

Influence of the composition of brass on dezincification and intercrystalline corrosion in ammonium chloride solutions The intercrystalline corrosion of As-containing α-brass is not restricted to a defined brass composition but is found also with brass types containing, in addition, AL and Sn. In the case of two-phase alloys of this type the β-phase is preferentially corroded, in Particular when the brass contains As and the β-phase forms a coherent network. On the basis of the results obtained a hypothesis has been derived as to mechanism the partial processes; this hypothesis is confirmed by result obtained with macroelements of As-containing and As-free brass. According to this hypothesis the intercrystalline corrosion of α- brass can be ascribed to the inhibition by As of the dezincification, so that no regions more anodic than grain boundaries are formed. This is why the boundaries are attacked preferentially.     

Einfluß einer Feuerverzinkung und PVC-Beschichtung von Bewehrungsstählen und von Inhibitoren auf die Korrosion von Stahl in gerissenem Beton

Influence of galvanizing and PVC-coating of reinforcing steels and of inhibitors on steel corrosion in cracked concrete Cracked concrete beams of concrete quality B15 and B35 with carbonized cracks were exposed in artificial seawater, under frost and deicing salt conditions and in industrial climate. The reinforcement was composed of black steel, galvanized and PVC-coated steel. For the purpose of additional protection the concrete was partly mixed with an inhibitor Ca(NO₂)₂. The concrete cover was 1,5 till 5,0 cm. It was found that the inhibitor protects satisfactorily only in the case of the higher concrete quality, cover 1,5 cm and not to high crack width. A protective effect of galvanizing is given by not to high chloride contents (< 1,5% relative to cement) and crack widths. The chosen PVC-coating failed because of chemical instability in the alcaline medium concrete.     

Einfluß der Mikrostruktur und des Elektroden-potentials auf die Ausbreitungsgeschwindigkeit von Spannungskorrosionsrissen einer reinen AlZnMg3-Legierung in 1 M NaCl-Lösung

Influence of microstructure and electrode potential upon the velocity of stress corrosion cracks of a pure AlZnMg3-alloy in an 1 M NaCI-solution The influence of microstructure and electrode potential upon the stress corrosion crack velocity in a fully aged pure aluminium-zincmagnesium 3-alloy with two step ageing and quench interruption treatments was investigated in airsaturated 1 M sodium chloride solution at 303 K. For this, propagation of the crack tip was observed microphotographically under potentiostatic conditions and under constant load using specimens precracked by fatigue. The crack propagation at the corrosion potential is accelerated by denser matrix-precipitates, narrower precipitate free zone, the presence of great T-grain boundary precipitates and minor sensitivity to intercrystalline corrosion and occurs to judge from the look of the fracture surfaces without considerable metal dissolution. In contrast to the alloy, which shows no intercrystalline corrosion, the crack propagation in the alloy with intercrystalline corrosion is obviously faster above the breakdown potential than at the corrosion potential. The results suggest that the crack propagates essentially mechanically. As mechanisms are discussed a crack growth by anodic dissolution of grain boundary precipitates or of zinc- and magnesium-rich grain boundary regions and mechanical seperation of the intermediate regions and propagation by local hydrogen embrittlement of the crack tip.     

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.     

Der Einfluß des Salzgehaltes auf die Korrosion von Eisen und niedriglegiertem Stahl in belüfteten Lösungen

Influence of the salt content on the corrosion of iron and low-alloy steel in aerated solutions The influence of borate and carbonate on the corrosion of iron and steel was investigated in aerated, neutral electrolytes. Both salts were inhibiting the corrosion. But only borate in higher concentrations was able to prevent the corrosion by chloride. The convection of the electrolyte was of only small influence.     

Untersuchungen über die interkristalline Korrosion und Spannungsrißkorrosion einer reinen AlZnMg 2-Legierung in NaCl-Lösung

Investigations Concerning the Intercrystalline Corrosion and Stress Corrosion Cracking of a Pure AIZnMg 2-Alloy in NaCl-Solution In a pure AIZnMg 2-alloy diversified fully step aged or overaged in one step intergranular corrosion (IG) and stress corrosion cracking (SCC) in airsaturated 1 N NaCl-solution or in deaerated 0,1 N NaCl-solution at 303 K were investigated under potentiostatic control. The pure AlZnMg 2-alloy had the following composition (weight percent): Zn: 4.60; Mg: 2.00; Fe: 0.005; Si: 0.006; balance Al. In the two step aged AlZnMg 2-alloys the IG-sensitivity is lowered by an increase of the first step aging time, while SCC-susceptibility and the yield point are raised. As possible mechanism of SCC hydrogen embrittlement is suspectet. At potentials noble to IG-breakdownpotential an interaction of intergranular corrosion and SCC brings about crack velocities, which are to be regarded as a sum of the velocity of intergranular corrosion, which is virtually independent of tensile stress, and the velocity of crack propagation due to SCC. In the overaged AlZnMg 2-alloy IG-attack was observed although in this state of aging no precipitate free zone anodic to the matrix can be assumed. A model to explain this variation of intergranular corrosion, that proofs to be dependent of tensile stress, is represented.     

Über den Einfluß verschiedener Chloridionenkonzentrationen auf die Aktiv-Passiv-Korrosionsempfindlichkeit von Stahl in gesättigter Kalziumhydroxidlösung

Influence of various chloride ion concentrations on the active-passive corrosion susceptibility of steel in saturated calcium hydroxide solution The corrosion of steel in concrete is caused mainly by the presence of chlorides. This effect was investigated by measuring potentiodynamic current-voltage-curves and by potentiostatic experiments of stressed (55 or 77% σ_B) plain steel electrodes in saturated Ca(OH)₂-solution with varying chloride concentrations. The results show that down to Cl′-concentrations of 100 mg/l there is a significant statistical probability of corrosion within 48 hours, and that for longer times the threshold concentration may be even lower. The microscopical examination of corroded samples shows that under the experimental conditions only pitting corrosion is the cause of failure and that this pitting is promoted by the applied stress.     

Zum Einfluß der mechanischen Belastung auf die interkristalline Spannungsrißkorrosion von unlegiertem Stahl in Carbonat/Bicarbonat-Lösung

Effect of mechanical stress on intergranular stress corrosion cracking of unalloyed steel in carbonate/bicarbonate solution Intergranular stress corrosion cracking of a carbon steel in 0.5 M NaHCO₃ + 0.5 M Na₂CO₃ solution at 70°C was examined by normal, interrupted and cyclic CERT testing. Within the critical potential range, the lengths of intergranular cracks increase with increasing strain rate, and decrease when the material is cold worked. The fracture time indicates that the crack velocity increases with increasing strain rate. This is also observed with tensile specimens subjected to interrupted and cyclic strain rates. Detailed information on crack propagation and threshold stress levels can be obtained by interrupted and cyclic CERT tests. Intergranular cracks arise if surface region of the material undergoes plastic deformation in excess of a definite amount. In the case of a 60°-notch specimen the plastic zone must be greater than 0.3 mm. For cyclically strained specimens the crack velocity can markedly decrease with time or can become even zero as a consequence of strain hardening. From this one can conclude that stress corrosion cracking can only occur if stress levels are very high.     

Korrosion von unlegiertem Stahl in sauerstoffreier Kohlensäurelösung

Corrosion of unalloyed steel in oxygen-free carbonic acid solution The investigation into the influence of dissolved carbon dioxide on the anodic partial reaction of iron dissolution in 0.5 M sodium sulfate solution has revealed, in agreement with data published in literature,

• 1 ) that the transfer of acid molecules to the cathode is the step controlling the rate of cathodic hydrogen evolution in oxygen-free solutions of little dissociated weak acids. The cathodic limiting current density is, consequently, a direct function of acid concentration and not of the pH value as in the case of strong acids.
• 2 ) that the anodic dissolution of iron in sulfate solution is catalyzed by carbonic acid which gives rise to a change of the dissolution mechanism, although this change is not reflected by a change of the cathodic protection potential (?0.85 V_CU/_CuSo4) which, consequently aplies to waters rich in carbonic acid, too.

     

Untersuchungen über den Einfluß der Grundbeschichtung auf Elementbildung und Blasenanfälligkeit von mit Teerpech-Epoxidharz beschichtetem Stahl in Salzlösungen

Investigations into the influence of the priming coat on galvanic couple formation and bubble formation susceptibility of steel coated with tar-epoxy coating in salt solutions Unalloyed steel sheets with the surface conditions grit blasted, hot-dip and metal-spray galvanized, coated with 4 different primers have been top coated with coal tar-epoxy resin of 4 different thickness values. The specimens have been immersed in NaCl solutions for two years. Potential and coating resistance values as well as cell currents between the specimens and external steel electrodes have been measured. Finally the corrosion amount of the steel electrodes and of artificial holidays have been determined. The resistance values measured after 3–6 months are found essentially for the degree of corrosion protection. With decreasing resistance values the formation of blisters within the coating as well as the cell activity increase. Thus, steel at holidays and unprotected steel in contact with coated steel are anodically attacked. Excellent corrosion protection is given by coatings with coating thickness > 400 m?m as well as with Zn-epoxy or Zn-ethyl silicate primers, whereas sodium silicate is unsuitable.     

Untersuchungen über die Kathodenwirkung von Eisensulfid und Mangansulfid auf die Korrosion von Eisen in dreiprozentiger Natriumchloridlösung

Studies on the cathodic effect of iron sulfide and manganese sulfide on the corrosion of iron in a three percent sodium chloride solution In the galvanic element iron/sodium chloride solution/iron sulfide and in the element iron/sodium chloride solution/manganese sulfide, flushed with nitrogen or air, the sulfides become the cathode and the iron is anode. Upon short circuiting the electrodes a strong cathodic polarisation of the sulfides occurs. However, only if the iron is in contact with iron sulfide in the air-flushed sodium chloride solution the corrosion of the iron is enhanced because of considerable anodic polarisation. The corrosion of the iron electrode is increased corresponding to the increase in cathodic area. According to these results inclusions of iron sulfide or manganese sulfide act as local cathodes but cannot cause pitting corrosion of unalloyed steels in a neutral chloride solution.     

Einfluß des entstehenden Rostes auf den Verlauf der atmosphärischen Korrosion von Stahl

Influence of incipient rust on the pattern of the atmospheric corrosion of steel The corrosion pattern with continuous rust formation has been compared with the results obtained and accumulated at certain intervals during the same time from analogous specimens. It was found, firstly, that there is a critical value of SO₂ concentration and, secondly, that the existing rust is capable of keeping certain quantities of SO₂ away from the surface. The latter effect explains the flattening-out of the corrosion rate to constant values, as well as the fact that, in pure atmosphere, the stationary conditions are reached later than in an industrial atmosphere.     

Untersuchungen zum Einfluß von Inhibitoren auf die Wasserstoffversprödung von unlegiertem Stahl in Salzsäure

Investigations on the influence of inhibitors on hydrogen embrittlement of mild steel in hydrochloric acid The influence of four commercially available inhibitors as well as their main effective components on the corrosion behaviour of mild steel in 16 and 32% hydrochloric acid was investigated by potentiokinetic polarisation measurements, electrochemically controlled measurements of hydrogen permeation and by estimating the rate of mass loss. The inhibitors showed different, yet positive effects on the inhibition of the permeation of cathodically produced hydrogen. The best results were found with benzylchinolinium chloride as inhibitor. Particularly in acids containing hydrogen sulfide propargylalcohol turned out to be a poor inhibitor of hydrogen permeation. Under these conditions the addition of propargylalcohol to quart. ammonium salts even decreased their good effectiveness as single components. The fact that the effectiveness for the inhibition of hydrogen permeation often diminished in the course of short periods was not due to a simultanous increase of the corrosion rate.     

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.     

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.     

Einfluß des Chloridgehaltes in Trinkwasser auf die Korrosion von unlegiertem Stahl

Influence of chloride content in tap water on the corrosion of unalloyed steel Regarding the effect of neutral salts in tap water on the corrosion of unalloyed steel, little data is available, some of it being contradictory. This applies in particular to chlorides. In the present paper, disks rotating at velocities of from 0–2000 min⁻¹ and pipes through which tap water flows have been used to investigate, for various water supplies (Ludwigshafen, Leverkusen and Berlin tapwater) and as a function of the chloride concentration (0.28–15.5 mol m⁻³) and of the test period (up to 10 000 hours), the area-related linear rates of loss in weight, the fractional corroded surface areas, the through depths and pit depths, and the distribution of elements in the surface layers.     

Zementmörtelauskleidungen von Trinkwasserleitungsrohren aus Guss und Stahl

Cement mortar linings in drinking water pipes made of steel and ductile iron For more than 100 years cement mortar has been used as internal corrosion protection for water pipes of steel and ductile iron. The combination of these 2 components has proved to be a perfect solution – not only regarding the physical and mechanical aspects, also with respect to the corrosion chemical side for pipelines and construction pipes. This report gives a survey on questions that are often discussed in practical use concerning the possibilities and limits for the use of cement mortar linings in the drinking water area.     

Einflußgrößen der Fremdkathoden aus Bewehrungsstahl in Beton auf die Erdbodenkorrosion von Stahl

Factors influencing the activity of reinforcing steel as cathode on the soil corrosion of steel Factors influencing the corrosion cell established between steel/soil (anode) and reinforcing steel in concrete (cathode) were investigated in field tests extending up to 3 years. In all cases, the differences between the corrosion rates measured in the cell and under free corrosion conditions were higher than the equivalent cell current. This effect means an increase of the cathodic partial reaction by cell action (negative difference effect), and is related to better protecting layers in the case of free corrosion. As to the cathodes, the parameters of the concrete have nearly no effect. But galvanizing of the reinforcing steel has a strong influence on the cell action. Since the cell resistance is mainly controlled by the electrolyte resistance (ground resistance of the anodes) the changes in the polarization resistance of the cathodes can hardly influence the cell action. A decrease of cell currents due to galvanizing the reinforced steel is no doubt present, but the effect is too poor for proper protection against cell activities. Therefore, only conventional electrochemical methods or organic coatings on the reinforcing steel can be applied as protective measures.