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.     

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.     

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.     

Felduntersuchungen mit feuerverzinkten Stahlrohren – Auswertung von Langzeit-Daten

Field corrosion tests on hot dip galvanized steel pipes – Assessment of long term data The degree of metal loss and its time dependence of hot dip galvanized steel pipes in flowing waters were correlated with the water parameters. A good correlation was observed with CO₂ concentration of the water. The higher the initial corrosion rate the higher is in many cases the decrease in corrosion rate with time. This is explained in terms of increased iron oxide content in the protective layers formed after the dissolution of a significant amount of zinc layer. Except phosphates the other water parameters (chlorides, sulphates, nitrates and TOC) do not exhibit any influence. The favourable influence of the iron oxide rich protective layers on the corrosion rate may get lost in waters very rich in carbon dioxide.     

Wasserstoffinduzierte Spannungsrißkorrosion an unverzinkten und verzinkten Baustählen

Hydrogen induced stress corrosion cracking of non galvanized and galvanized construction steels The processes of atmospheric corrosion and corrosion in collected water which may lead to hydrogen induced stress corrosion cracking of high-strength reinforcing steels in casing tubes before injection with concrete are discussed. Hydrogen uptake during corrosion occurs in weakly acid solutions as well as in neutral or alkaline aqueous solutions. The hydrogen uptake by proton discharge in acid solutions decreases with increasing pH of the electrolyte. Hydrogen can also be absorbed in neutral to weakly alkaline solutions if steels are plastically deformed and water reacts with the fresh iron surface. In alkaline solutions, hydrogen uptake is possible if, at the generally passivated steel surface, localized corrosion (pitting or crevice corrosion), local galvanic cells and a sufficient decrease in the pH of the pit electrolyte occurs. In the case of galvanized steels with damaged zinc layers, hydrogen uptake may result from the cathodic polarization of the free steel surface by zinc dissolution. The absorbed hydrogen interacts with the microstructure of the steels and weakens the bonds between the iron atoms. The influence of the microstructure of high-strength steels on the fracture behaviour is discussed on the basic of the so-called decohesion theory.     

Ergänzungsversuche zum Einfluß des Werkstoffs CuNi20Fe auf das Korrosionsverhalten verzinkter Rohre in warmen Wässern

Supplementary experiments concerning the influence of CuNi20Fe on the corrosion behaviour of galvanized tubing in warm waters The influecne of CuNi20Fe, a material used for constructing boilers, on the corrosion resistance of galvanized tubes in mixed installations was investigated in hot- water-pilot-plants. The investigations were made with Berlin tap-water, pH=7,0 t-65°C. The results demonstrate that under these conditions the dissolution of copper ions due to the corrosion of “Nickelbronze” is sufficient to induce pitting in the galvanized tubes. Further investigations show, that the corrosion rate of CuNi20 Fe without heat transfer in water of pH = 7,4 is largely independent form temperature in the region of 65–90°C.     

Der Einfluß unterschiedlicher Nitratgehalte auf das Korrosionsverhalten von verzinkten Stahlrohren

The influence of different nitrate contents of drinking water on the corrosion behaviour of galvanized steel pipes At the water works Biblis and Alsbach of Südhessische Gas und Wasser AG, Darmstadt, field tests were performed over a period of 2 1/2 years to describe the influence of different nitrate contents in drinking water and the corrosion behaviour of different qualities of galvanized steel pipes. The results obtained showed, that weight loss and corrosion rate measurements give no information on this subject. Optical inspection and flushing tests could prove, that selective corrosion occurred in the water with higher nitrate content. Galvannealed pipes showed local corrosion attack after a certain exposition time in water with higher nitrate content, depending on flow velocity. Air blown galvanized pipes and galvannealed pipes in water with higher nitrate content additionally produce „Zinkgeriesel”︁ during flushing. The selective corrosion attack also takes place to a very small extent in galvanized steel pipes, where quality is in accordance to the German Standard DIN 2444.     

Felduntersuchungen mit feuerverzinkten Stahlrohren in Wässern

Field tests on galvanized stell tubes in drinking waters Specimens cut from galvanized steel tubes were exposed to cold water for up to 5 yrs. in field tests. The testing conditions were similar to those in a real domestic installation. Three different kinds of zinc layer have been investigated: commercial standard layer according to DIN 2444, layer consisting predominantly of zinc-iron alloy phases (galvannealed) and a foamy layer produced by blowing out with air after hot dipping. The tests have been performed in water works with 11 different water qualities which are thought to be representative of the drinking water supply in the Federal Republic of Germany. Besides visual examination, mass loss and electrochemical measurements have been carried out. The testing results demonstrate the predominant influence of the water characteristics. Especially, the content of CO₂ controlls the corrosion rate of zinc. The influence of the zinc coating quality as well as of the service conditions was less important, possibly due to a relatively high flow rate of the water, if the influence of the flowing conditions and contact with copper alloys at joints are not considered. Electrochemical measurements show a limited applicability of Stern-Geary equation for the system galvanized stell/water, using an appropriate constant.     

The erosion‐corrosion resistance of uncoated and aluminized 12% chromium ferritic steels under fluidized‐bed conditions at elevated temperature

In this paper, some results from a study of the erosion‐corrosion resistance of uncoated and aluminized 12% chromium steel in a fluidized‐bed rig are reported. The aims of the research are to establish and compare the erosion‐corrosion resistance of these materials for possible applications as heat exchangers in future power plants, and to obtain an increased understanding on their behaviour and mutual superiority in a range of conditions. Damage to the uncoated 12% chromium steel occurs by an oxidation‐affected erosion process under all the studied conditions, with spallation of scale being the primary mechanism of material wastage. At a temperature of 550°C, the uncoated steel follows the typical angle‐dependence of a brittle material, while, at temperatures above 550°C, it follows an angle‐dependence that is more typical of a ductile material. This change in the angle‐dependence with temperature is related to characteristics, i.e. uniformity, adhesion and density, of the formed oxide scales. The rate of material wastage increases with increase in speed and temperature, due to the development of thicker, more uniform and more dense oxide scales, that promote more severe scale spallation. The erosion‐corrosion behaviour of the aluminized 12% chromium steel changes in the temperature range from 600°C to 650°C. This is due to a shift from a brittle‐like to a ductile‐like angle‐dependence and to a more rapid oxide scale build‐up at temperatures above 600°C. At an impact angle of 30° and at 550°C and 600°C, the prevailing erosion‐corrosion process for the aluminized steel is oxidation‐affected erosion. At 650°C and 700°C for an impact angle of 90°, the primary erosion‐corrosion mode is essentially erosion‐enhanced oxidation. The results of the study have also demonstrated that the Al₅Fe₂ coating deposited by pack aluminization offers enhanced protection against erosion‐corrosion at shallow impact angles at 550°C and 600°C and at steeper impact angles at 700°C.     

Influence of tensile stress on corrosion behaviour of high‐strength galvanized steel bridge wires in simulated acid rain

Corrosion behaviour of the high‐strength galvanized steel wires under tensile stress was researched by electrochemical polarization and salt spray test (SST) using simulated acid rain as electrolyte. Electrochemical polarization and SST results showed corrosion rate rose significantly with increasing tensile stress; white grains were observed by SEM after polarization, while cellular and dendritic crystals appeared on the rust layer after SST. XRD and TG‐DTA results revealed (Zn(OH)₂)₃ · ZnSO₄ · 5H₂O was the main corrosion product, and traces of Fe₂(SO₄)₂O · 7H₂O, Fe₂(SO₄)₃, Fe₂O₃ · H₂O were also detected. A three‐stage corrosion process for the galvanized steel wires during SST was proposed.     

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.     

Einflußeiner Verzinkung auf die Korrosion von mit Zementmörtel beschichtetem Stahl in NaCl-Lösung

Effect of galvanizing on the corrosion of steel in concrete immersed in NaCl solution Galvanized or pickled steel sheet specimens were embedded in portland cement mortar of various water cement ratios and curing conditions and then wholly or partially immersed in 0.5 M NaCl solution for 1 to 5 yrs. Free corrosion potentials and electrical resistances have been measured. Immersion conditions and the presence of zinc have a significant effect on the corrosion resistance of the embedded steel sheets. The potentials of the wholly immersed specimens are very negative. Thus, these specimens cannot act as cathodes in corrosion cells, and the steel sheets within the mortar do not corrode. The partially immersed specimens, on the other hand, show very noble potentials. Also in the case of galvanized steel sheets the potentials are shifted to the same positive values in the course of exposure time. Thus, all these specimens can act as cathodes in corrosion cells. Localized corrosion generally occurs at the water/air line. In the case of pickled specimens the mortar is cracked due to growing corrosion products. In the case of galvanized steels the corrosion is retarded significantly. The test results are discussed in detail with respect to practical problems of cell formation, internal and external protection of pipes as well as the corrosion resistance of reinforced concrete.     

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.     

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.     

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.     

Korrosionsverhalten von feuerverzinktem Stahl bei Berührung mit Baustoffen

Corrosion behaviour of galvanized steel in contact with building materials Summary of the corrosion behaviour of galvanized steels in concrete, plaster, construction materials with magnesia cement, wood, heat insulation materials and sand including constructive influences, too. Zinc is practically attacked under conditions only when stable protective layers neither can develop nor when they are destroyed according to their environment and construction materials. The particularities of galvanic elements are dealt with.     

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.     

丝束电极研究镀锌层存在点缺陷的锌/钢电偶腐蚀行为

采用3种不同配置比例的锌/碳钢异材质丝束电极,模拟镀锌层存在不同大小点缺陷时的热浸镀钢,研究锌/钢电偶在不同腐蚀阶段的电位和电流密度分布.结果表明,在钢丝与锌丝面积比为1:120、9:112和25:96的3种情况下,锌丝均能给钢丝提供足够的阴极保护,且锌丝之间存在明显的电位、电流分布不均现象,主要阳极区先随机地在邻近钢...     

Korrosionsverhalten von verzinktem und nichtrostendem Stahl sowie von Nickelbasis-Legierungen in Schwimmhallenluft

Corrosion behaviour of galvanized and stainless steel as well as nickelbase-alloys in the atmosphere of indoor swimming pools U-bend specimens of a galvanized steel, nine different stainless steels and two nickel-base alloys were exposed to the chloric atmosphere of the reservoir for overflow water of a swimming pool for max. 68 months. The visual and metallographic examination of their corrosion performance has shown that none of the metals remained completely stainless. Only the galvanized steel and the stainless steels W.-Nr. 1.4529, 1.4539 and 1.4575 remained relative free from rust and absolutely free of pitting and crevice corrosion. None of the metals showed stress corrosion cracking.