Über den Einfluß des Sauerstoffgehaltes,des pH all und der Strömungsgeschwindigkeit auf die Korrosion feuerverzinkter und unverzinkter unlegierter Stahlrohre in Trinkwasser

Influence of oxygen content, pH and flow velocity under corrosion of hot-dip galvanized and bare piping of unalloyed steel in potable water Sections and weighed ring specimens from galvanized and nongalvanized tubes had been exposed to flowing tap water with different flow rates and oxygen concentrations of 0.01–4 mg/l at pH 7.2 and 8.0 for 3.5 ys. Specimens taken at different testing times had been investigated with respect to data of protective layers, corrosion manifestation and weight loss. The corrosion rate is nearly constant in the first weeks (initial corrosion), later it follows a power law (stationary corrosion). These two phases of corrosion are determined by the testing parameters differently. Thus, results from short-time experiments of some weeks cannot inform on the stationary behaviour. In oxygen free tap water remarkable corrosion occurs due to the acids H₂CO₃ and HCO₃^?. The corrosion resistance is supported by high oxygen content, high pH, flow rate and by galvanizing.     

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.     

Langzeitverhalten feuerverzinkter Bewehrungsbänder für Bewehrte Erde Bauwerke

Long‐term behaviour of hot‐dip galvanized reinforcing strips for reinforced earth constructions For reinforced earth constructions the following parameters are found as a solution to ensure the corrosion prevention of strips for a longtime period: ? use of specific nonbinding grounds, ? preventing the penetration of salt from outside, ? choosing a zinc‐galvanized coverage with a minimum‐thickness of 100 μm. Now a corrosion induced failure of strips is almost impossible considering a building‐relevant period.     

Über den Einfluß der Strömungsgeschwindigkeit und der NaCl-Konzentration auf die Sauerstoffkorrosion unlegierten Stahls in Wässern

Influence of flow velocity and sodium chloride concentration on the oxygen corrosion of unalloyed steels in water Corrosion of mild steel in aerated water normally leads to pitting, and to an enrichment of anions within the pits. Even HCO₃^? ions can support localized corrosion. The intensity of this is remarkably increased by chloride ions. In equilibrium water with Ca²⁺ and HCO₃^? ions and about 40 ppm free CO₂ protective rust layers are formed after about 500 hs exposure not depending on flow rate and salt concentration. These layers decrease the mean corrosion rate to about 0.1 mm/a, but do not prevent localized corrosion. Protective passive layers with extremely low corrosion rates are formed in flowing water with Ca²⁺ and HCO₃^? ions and about 40 ppm free CO₂. The minimum flow rate can be estimated for a 3/4 inch pipe to lie between 0.35 and 1.5 m/s. Investigations with water containing 40 ppm free CO₂ without Ca²⁺ at pH 4.7 and with NaHCO₃ (pH 7.2) show at low pH high corrosion rates – as expected –, and at pH 7.2 formation of protective layers with poor reproducibility in comparison with the situation in equilibrium water. Thus, Ca²⁺ ions support the protective properties of rust layers. HCO₃^? ions are necessary for the formation of protective layers because these are not formed in pure NaCl solutions.     

Zerstörung von Reinstaluminium,Feinzink und Plexiglas durch Strömungskavitation

Destruction of high purity aluminium, zinc and plexiglass by flow cavitation Cavitation in cold and warm water produces a strengthening of aluminium during a first incubation period, although fine cracks are already formed which induce erosion. It is for this reason that cathodic protection remains without effect. The same applies to Zn which undergoes a limited translation (because of its lower plasticity) and twin formation and, later on, erosion. Plexiglass absorbs water, while cracks starting at weak spots in the structure induce erosion and fatigue cracks are formed later on. The second stage of cavitation is characterized by high weight losses, connected with strong changes in the material surface, while in the third stage decreasing weight losses can be found. This decrease is attributed to the fact, that the water formed by bubble implosion in cracks cannot flow out easily and thus forms a sort of protective cushion.     

Zur Schwingungsrißkorrosion feuerverzinkter Schweißverbindungen unter Dauertauch- und Wechseltauchbeanspruchung

Corrosion fatigue of hot dip galvanized welded joints with permanent and alternate immersion Zink-coatings as corrosion protection of fatigue loaded welded constructions are of increasing importance. As corresponding design concepts in general are not available, corrosion fatigue tests with constant and variable amplitudes at filled welded joints are carried out under permanent immersion in artificial seawater or under alternate immersion in 5%-NaCl-solution including a lot of material, geometrical and fabricating parameters. The analysis of the results yields reliable informations with respect to the influence of hot dip galvanizing on the corrosion fatigue of welded joints.     

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.     

Werkstoffprüfung bei hohen Strömungsgeschwindigkeiten

Material tests at high-velocity flows A corrosion test rig was developed making possible visual observation of the behaviour of metal when exposed to corrosive environments, such as aggressive media at high or low velocities. Pressures up to 15 bar, temperatures up to 150 °C are possible. Fluid velocities range from 5 to 80 m/s, or even to 100 m/s for short runs. Boiler water, seawater and various acids are the media used. Test runs with different test materials using hot brine as electrolyte have shown, that the behaviour of the material can be evaluated after a short run time.     

Das Korrosionsverhalten von verzinkten und unlegierten Stahlrohren in Trinkwässern mit unterschiedlicher Härte und erhöhtem Neutralsalzgehalt

Corrosion of galvanized and unalloyed steel tubes in drinking water of different hardness and increased neutral salt content The corrosion of galvanized and unalloyed steel tubes in drinking water of different hardness was studied at the Rheinau water works, Mannheim. The criteria to describe the influence of the water quality parameters were the decrease of the metallic zinc layer, the composition of the corrosion products and a visual qualification of the coatings. The main parameters influencing zinc corrosion are pH-value and flowrate. Comparison with other data shows no significant effect of buffer intensity and neutral salt concentration. The layer of corrosionproducts in unalloyed steel tubes showed qualitative and quantitative differences with different hardness. The most important effect can be seen in the different contents of calcite in the coating. Low buffer capacity decreased the quality of the corrosion product layer. The results lead to the conclusion, that softening should he limitated to a certain level, otherwise corrosion problems will increase.     

Elektrochemische Aspekte der Erosionskorrosion in partikelhaltigen Strömungen

Electrochemical aspects of erosion corrosion in particle containing liquids Flowing, particle containing liquids induce various effects on metal surfaces according to their corrosivity. Electrochemical measurements demonstrate in which way the parameters of erosion corrosion are acting. As corrosion systems eight ferrous materials are chosen which are exposed to a solid containing quencher liquid of a flue gas desulphurization plant.     

Kavitations-Korrosion,Schadensbilder und Mechanismen – Vergleich zwischen Strömungskavitation,Kavitation durch rotierende gelochte Scheibe und Schwingungskavitation

Cavitation corrosion, damage phenomenology and mechanisms–comparisons between flow cavitation corrosion, cavitation by rotating perforated disc and vibration cavitation In a joint test programme involving different cavitation test devices (vibrator, rotating disc, flow channel) the influence of imploding cavitation bubbles on aluminium was investigated. The behaviour of the material was described in terms of macroscopical (mass loss-testing time and similar curves) and microscopical (investigations by SEM) methods. Lots of similarities in the kinetic of destruction and in the appearance of the damaged material could be found.     

Einfluß mechanischer Spannungen auf die Zerstörung durch Strömungskavitation

Influence of mechanical stresses on the destruction by flow cavitation The destruction produced by the formation of cavitation bubbles are enhanced in the presence of mechanical stresses. One may conclude therefrom that such effects will be encountered in the presence of residual stresses, too, e.g. in the case of cast steel pieces or in the vicinity of weld seams. Consequently the cavitation resistance may be improved by a thermal treatment (provided of course the treatment does not give rise to the formation of a corrosion sensitive structure). The destructions produced by cavitation are losses characterized by a specific crystallographic orientation. Erosion effects which are connected with cavitation on the other hand are not effected by mechanical stresses.     

Schwingungsrißkorrosion von Baustahl in Offshore-Konstruktionen bei unterschiedlichen Frequenzen und Umgebungsbedingungen

Corrosion fatigue of structural steel in offshore structures at different frequencies and environmental conditions The effect of frequency on the corrosion fatigue in seawater was investigated by use of welded and unwelded bending specimens of plate material (20 mm, StE 460) by performing mainly endurance orientated constant-amplitude tests. The test frequency was varied in a wide range (10; 1, 0,1; 0,01 s^?1). The tests were performed in air as well as in artificial seawater at free corrosion, at a cathodic protection (U_H¹ = ?650 mV) and at an extreme polarization (U_H = ?850 mV). A general grading of the endurance – given by N_fr – with respect to the frequency was evaluated in the expected sequence at free corrosion only. Thereby the frequency effect is smaller as was expected and is intensively pronounced only at the lowest frequency of 0,01 s^?1. But this frequency already results in smaller N_fr – values of welded specimens at air as well as at an extreme cathodic polarization U_H = ?850 mV and higher loading. The results of air and free seawater-corrosion show the usual small scatter whereas those of the cathodic potentials U_H = ?650 and ?850 mV are significantly larger. At U_H = ?650 mV some results mark a significantly better behaviour than in air, the other ones are inside the scatter of the results in air. At the extreme cathodic polarization (U_H = ?850 mV) some results are situated in a domain of significant shorter endurance than in air; this results from negative influence of the increased cathodically emerged hydrogen at this potential. The phase of the fatigue crack initiation is shortened by free seawater corrosion compared with an air environment; it is enhanced by a cathodic protection with U_H = ?650 mV.     

Wasserseitige Möglichkeiten zur Reduzierung der Kupferabgabe aus Hausinstallationen in das Trinkwasser

Reduction of copper loss in potable water in domestic installations by water treatment In the scope of examinations for water-supply companies the effects of water treatment processes on the corrosion behaviour of materials which are used for water distribution (unalloyed steel, asbestos-cement, galvanized steel and copper) were tested by the Rhenian-Westphalian Institute of Waterchemistry and Watertechnology (IWW), Institute at the Gerhard-Mercator-University-GH Duisburg. Experimental studies have been made with copper sections and weighed ring specimens. The testing conditions were similar to those in a real domestic installation. Specimens taken at different testing times have been investigated with respect to mass loss measurements. The testing results demonstrate the possibility to estimate the concentrations on copper transported into the sewage sludge by domestic sewage. The testing conditions are described and the dominant influence of the water characteristics with respect to the water treatment processes are discussed. The experiments have shown that the central dosage of inhibitors, especially phosphate-products, is able to reduce the load of copper in domestic sewage in water systems with high concentrations of natural salts. The deacidification associated with water softening and slow-decarbonisation is qualified to reduce the load of copper in water systems with high concentrations on carbon dioxide.     

Einfluß steigender Phosphat-Silikatgehalte auf die Loch- und Kontaktkorrosion verzinkter Stahlrohre sowie die Korrosion von Kupfer in Warmwasser-Mischinstallationen aus beiden Werkstoffen

Influence of increasing phosphate/silikate contents on the pitting and general corrosion of galvanized steel tubing and the corrosion of copper in warm water mixed installation systems In hot tap water (65°C) the influence of a mixture of phosphate/silicate inhibitor on the general, the pitting and the galvanic corrosion of galvanized steel tubes and the general corrosion of copper in mixed installations of both metals was investigated. Increasing concentration of inhibitors decreases the general corrosion rate of galvanized steel and copper. A worth mentioning reduction of pitting and galvanic corrosion of steel could be reached only with high concentrations of 5 mg/l P₂O₅ and 30 mg/l SiO₂. Galvannealed tubes are much more sensitive to pitting corrosion than galvanized ones. Referring to this they could not be inhibited.     

Strömungsabhängigkeit der Lochkorrosion eines Cr-Ni-Stahles in NaCl-Lösung. Teil II: Versuche mit Ultraschall

Flow dependence of the pitting corrosion of CrNi steel in NaCl solution Test (with CrNi steel 19 9 in 1 N NaCl) with supersonic agitation have confirmed the results obtained with the rotating disc electrode. Supersonic treatment increases the pitting potential for stable pitting and reduces maximum corrosion current density. The treatment has but little effect, however, on the pitting potential for repassivating pitting corrosion. Cycling tests under galvanostatic conditions show that the effect supersonic action comes to bear almost immediately. Switching on or off the supersonic source results in immediate repassivation or loss thereof. Similar to experiments on rotating discs the pits are very small and, consequently, constitute practically no corrosion risk.     

Über das elektrochemische Ansätzen verschiedener Gefügebestandteile und von Mischkristallen unterschiedlicher Zusammensetzung

Electo-chemical first-etching of different structural constituents and of mixed crystals of different composition Heterogeneously composed material consists of different phase or of mixed crystals with locally varying composition. The different phases or different mixed crystal zones show a different electro-chemical behaviour. This difference in the electro-chemical behaviour is utilized in metallographic etching in order to ascertain. During etching in the transpassive zone, it is often possible to bring out even minor differences in composition.     

Zur Diffusion und Wechselwirkung von Wasserstoff in Spannstählen unterschiedlicher Wasserstoffanfälligkeit – Zusammenfassung experimenteller Werte und quantenchemischer Erkenntnisse

On the diffusion and interaction of hydrogen in prestressing steels of different sensitivity against hydrogen – summarized experimental data and quantumchemical results Investigation of the influence of hydrogen on prestressing steels used as building materials shows that material involved in practical damage by hydrogen-influence is characterized by a higher hydrogendiffusion coefficient already of the not tensioned material. Responsible for this is a specific influence of inner surfaces coming into existence by local material defects. Their metastable reaction with hydrogen may lead to local decohesion of the metallic bond and so cracks may be induced if the steel is tensioned.     

Korrosionsuntersuchungen zur Entwicklung von Dekontaminationslösungen für Nuklearanlagen – Teil III: Das Korrosionsverhalten ferritischer Baustähle mit unterschiedlichen Kohlenstoffgehalten in Ameisensäure-Formaldehyd-Lösungen ohne und mit Inhibierung

Investigation of corrosion in the development of decontamination solutions for nuclear systems – Part III: The corrosion behaviour of ferritic steels with different carbon contents in formic acid-formaldehyde solutions with and without inhibitors In decontamination solutions of the type formic acid-formaldehyde, ferritic steels undergo heavy pitting corrosion or shallow pitting corrosion. The addition of formaldehyde to formic acid solutions scarcely influences the temperature dependency of the corrosion rates, the solution specific corrosion rates, however, increase with increasing carbon content of the steels. The high corrosion rates, especially of the steel 21 CrMoV 5 11, can be reduced by about two orders of magnitude by adding inhibitors as thiourea, ethyl thiourea, and phenyl thiourea to the formic acid-form-aldehyde solutions, but pitting corrosion is unavoidable.     

Strömungsabhängigkeit der Lochkorrosion eines Cr-Ni-Stahles in NaCl-Lösung. Teil I: Versuche mit der rotierenden Scheibe

Flow dependence of the pitting of a CrNi steel in NaCl solution. Pt. I: Experiments with the rotating disc The pitting of the CrNi steel (German designation X 5 CrNi 18 9, Werkstoff Nr. 1.4301) in NaCl solution (1 and 3 N) at 25 °C has been investigated with the aid of the rotating disc, with a view to its dependence from the number of revolutions and the thickness of the diffusion layer. Potentiokinetic and potentiostatic experiments have shown, that the pitting potential for stable pitting is a function of flow in a more pronounced way than the potential for repassivating pitting corrosion. The pitting density increases as the number of revolutions increases and the thickness of the diffusion layer decreases, while the corrosion limiting current density and the average and maximum pit radii decreases in the same conditions. This goes to show that the sit locations are repassivated when exceeding a critical size which decreases with increasing number of revolutions. Contrary to the conditions prevailing in stagnant solutions where the pit density is almost independent from time, new pits are generated permanently despite repassivation under conditions of flow. Consequently, there is a apparent increase of pit density with time.