Das Korrosionsverhalten von pulvermetallurgischem Chrom

Corrosion behaviour of powder metallurgically produced chromium Powder metallurgically produced chromium of purity 99.96 percent can be processed into semifinished and finished parts of up to now restricted sizes. It is highly resistant to certain hot solutions of salts, hot carbonic acids, oxidizing anorganic acids, bases and gases (hot gases especially in combination with ashes and slags). It is shown in which fields and for what kind of functional parts pure chromium eventually could be used.     

Untersuchungen zum Korrosionsverhalten von AlLi-Legierungen

Corrosion behaviour of aluminium-lithium alloys The AlLiMgCu alloy 8090 was studied in its texturated fine grained version “A” and in its recrystallized coarse grain structured version “C” in different artificially aged conditions in reference to several other AlLi alloys, each in its heat treatment condition of practical interest, and to the convetional alloy 2024 T3. The subject of research was the general corrosion behaviour of semifinished AlLi products, particularly sheet material, under alternate and permanent immersion conditions in neutral 3.5% sodium chloride solution; the stress corrosion behaviour was studied under constant load in the long transverse direction according to ASTM G44 and G49. The underaged conditions, which are the relevant conditions for technical application of the 8090 “A” and “C” sheets, showed an approximately equivalent or even better corrosion behaviour in comparison to the lithium-free alloy 2024 T3 in the corrosion tests with unloaded specimens. The threshold above which the AlLi alloy 8090 in some heat treatment conditions is attacked by stress corrosion cracking within the 30 days lasting constant load test depends on alloy composition, testing direction, grain size, stretch-forming, artificial ageing condition, surface pretreatment and the specimens' dimensions.     

Korrosionsverhalten von Metallen in Verbindung mit Holz

Corrosion behaviour of metals in contact with wood Wood contains acetic acid ester, which hydrolyses in the presence of water to give free acetic acid. This acetic acid acts as a corrodent to several metals in contact with wood and in the vapour space near wood surfaces because of the volatility of the acetic acid. The quantity of acetic acid ester, the rate of formation and emission of acetic acid is specific for the species of wood. Moreover the rate of formation, as a hydrolytical reaction, is strongly effected by moisture and temperature. According to treatment and species, wood may retain its corrosivity for many years. Wood preservatives, especially salt solutions, are able to increase the corrosivity to a certain degree. The risk of corrosion of metals in practical services depends on environmental conditions, i.e. relative humidity and moisture content of wood. In covered buildings under indoor conditions the risk is relatively small. If the humidity and the moisture content of wood are high, corrosion resisting metals, e.g. ferritic or austenitic stainless steel, should be used.     

Korrosionsverhalten von Plasma-Auftragschweißungen aus Titan-Legierungen

Corrosion behaviour of plasma-claddings from titanium alloys This paper is dealing with investigation results of plasma weld surfacing with the titanium alloys Ti grade 2 and 12 and the correlation between structure and corrosion properties of such claddings on interlayers of copper. First the developing of a process technique was important which allows an exact heat control during welding on copper and which ensures lowest dilution degrees between the materials. It was possible to produce two layered titanium claddings on a copper interlayer with a dilution percentage of smaller than 4% by using plasma-arc-powder surfacing with pulsed direct current. Optimized titanium claddings show structures without any defect and have a good adherence to the base material whereas high degrees of dilution effect the formation of intermetallic phases and inhomogeneities of the structure. The results of various corrosion examinations have shown that protective layers produced by plasma-arc-powder surfacing are comparable with rolled reference material.     

Elektrochemische Untersuchungen zum Korrosionsverhalten von Duplexgusswerkstoffen in Rauchgasentschwefelungsanlagen

Electrochemical investigations on the corrosion behaviour of duplex cast stainless steels in flue‐gas desulphurisation plants This paper deals with results from corrosion examination on duplex cast stainless steels from original components in the flue‐gas desulphurization (FGD) of the VEAG‐power stations Boxberg III and Jaenschwalde under practice and laboratory conditions. In practice the mentioned components have a better corrosion resistance in the FGD medium of the Boxberg power station. The same results were obtained by laboratory examinations, getting a shift in critical potentials of a few hundred millivolts. The tests in a by‐pass of the FGD confirm these results in tendency. However, partially significant differences to laboratory examinations were established on specimens with pre‐damaged original surfaces. This can be traced back to the more severe test conditions in the FGD, like solids and flow. The repassivation behaviour of the materials under investigation was examined by means of electrochemical noise analysis. The values of the free corrosion potentials in the Jaenschwalde power station were higher than potentials, necessary for a safe repassivation of occurring pits. This cause the failures on the components. Remedy steps are discussed in this paper in detail. The free corrosion potential in the Boxberg power station is about 200 mV below the repassivation potential. Because of that, short‐time fluctuations of system parameters leading to enhanced corrosion risk can be tolerated.     

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.     

Das Korrosionsverhalten von sauerstoffionenimplantiertem Magnesium in schwach saurem Elektrolyten

The corrosion behaviour of oxygen implanted magnesium in weekly acid solution Under atmospheric conditions magnesium forms oxide layers which consist mainly of magnesium hydroxide. The protective action of Mg(OH)₂ is minor in sulphate‐, carbonate‐ or chloride containing solutions [2, 18]. Additionally, the crystallographic misfit between magnesium and magnesium hydroxide leads to crack formations inside the hydroxide layer and the metallic surface is then exposed. By using the ion implantation of oxygen (1*10¹⁸ O⁺/cm²) a MgO‐layer can be formed and buried within the bulk metal. Pre‐tests of MgO‐layers produced by ion implantation suggest that it is successful as corrosion protection [17]. Crystallographic stresses between metal‐ and oxide phase will be reduced, depending on the gradient of oxygen concentration. The investigation in weekly acid sodium solution shows, that the oxygen ion implantation leads to a significant improvement in the corrosion behaviour of magnesium.     

Der Einfluß von Martensit auf das Korrosionsverhalten von 18 Cr-10 Ni-Stahl

The effect of martensite on the corrosion properties of 18 Cr-10 Ni steel The corrosion properties of the martensitic phases formed in an austenitic matrix by (i) cooling to low temperature (-196 and ?269°C resp. ?320 and ?452°F) and (ii) cold working at room temperature was investigated for two chromium-nickel stainless steels of the 18 Cr10 Ni type. Austenite and martensite have the same chemical composition but different lattice structures. In sulfuric acid, both martensitic phases formed at low temperature and by cold working arc preferentially attacked in the active range of corrosion. The effect of potential on the corrosion attack was elucidated by potentiostatically controlled experiments with subsequent light-optical and SEM-investigation of the specimens. Selective corrosion attack of the martensitic phase was found down to a potential being 0.8 V more negative than the cathodic limiting potential of the active corrosion range of a fully austenitic steel, irrespective of the way of the martensite transformation. The preferential attack of martensite may be explained by the supposition that the deposition of cathodically protecting layers possibly consisting of nickel is rendered more difficult at the martensitic surface areas. In sulfuric acid, no differences in the corrosion properties between austenite and martensite were found in the passive and transpassive range. In chloride containing aqueous solutions, the resistance to pitting is not effected by martensite formed at low temperature. From this it is concluded that also martensite formed by cold working does not effect the pitting resistance.     

Einfluß von Seigerungen auf das Korrosionsverhalten von austenitischem nichtrostendem Schweißgut

Influence of segregations on the corrosion behaviour of austenitic stainless weld metal Fully austenitic stainless weld metal on solidification acquires a segregation structure. The segregation takes place with the participation of the alloying elements responsible for the corrosion behaviour. Regions of low alloy content are preferentially attacked and microelements form between them and more corrosion resistant regions. The progress of corrosion is enhanced by lamellar segregation structures and is inhibited by cellular structures. The corrosion due to segregations is particularly pronounced when the resistance in one medium is a pronounced function of an alloying element prone to segregation. This is demonstrated on the example of weld metal X 2 CrNiSi 1815 having different Si contents. The effect of the base metal/weld metal macro-element is discussed. Corrosion due to segregation takes place only when the weld metal is subject to attack in the vicinity of its resistance limit.     

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.     

Zum Korrosionsverhalten von Nicrofer 3033 in hochkonzentrierter Schwefelsäure

Contribution to corrosion behaviour of Alloy 33 in high concentrated sulfuric acid Investigations on the corrosive behaviour of Alloy 33 were conducted in 93% and 95–99% technical sulfuric acid at temperatures of 70–200°C. 6‐h‐tests with gravimetric evaluation and measurement of the free corrosion potential and 48‐h‐tests with gravimetric evaluation served to determine the influence of the sulfuric acid concentration and the temperature on the resistance of sheet samples at presence and absence of SO₂. The influence of the flow rate was examined in 6‐h‐tests via rotation of pipe samples at circumferential speeds up to 2 m/s. The potential and gravimetric measurements showed that there is a resistance with ≤ 0.1 mm/a for Alloy 33 as from about 98.5% at temperatures from 100°C. However, about 125°C there seems to be a zone at which the resistance down to about 97.5% is kept at least for a long test duration. Examinations on the flow influence indicate that there is a reactivation in the said zone with increasing flow speed. However, the flow influence is low at the other temperatures. This extension of the resistance zone about 125°C proves that the temperature sensitivity of Alloy 33 is not monotonous in highly concentrated sulfuric acid. There are corrosion maxima and minima as a function of the sulfuric acid concentration at different temperatures. If SO₂ is present, there is only a limitation of the resistance zone at low test temperatures, 80°C, 100°C in the 6‐h‐test and 70°C, 80°C in the 48‐h‐test.     

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.     

Korrosionsverhalten von feuerverzinktem Stahl in erwärmten Wasser1

Corrosion behaviour of hot galvanized steel in warm water Pipe specimens made of commercial grade galvanized steel and zinc have been tested in comparison to pipe specimens having modified zinc coatings. The specimens were corroded in 11 test lines with warm water at approx. 60 °C and cold water with continuous and intermittent flow. Dortmund town water with polyphosphate and orthophosphate additions in individual test lines was used for the tests. One test line was operated with warm water from a water heater with cathodic vessel protection according to the Guldager method. The investigation included visual assessment, determination of mass loss, and electrochemical measurements. The main results were as follows: 1. The localised corrosion tendency is not determined by the amount of potential ennoblement, but rather by the inhibition of the cathode reaction on the surface layer formed, which can be read off the cathodic current density vs. potential curve. The inhibition of the cathode reaction can be affected both by variations of the material and by variation of the water quality. 2. The tested materials with modified coatings featured throughout a poorer corrosion behaviour in warm water than commercial zinc coatings according to DIN 2444. 3. The phosphates used for the investigation induce an inhibition of the cathode reaction in zinc. Zinc-iron alloy phases in warm water fail to ensure an appropriate effect. A treatment of the water according to the Guldager process ensures a strong inhibition of the cathode reaction both with zinc and especially with zinc-iron alloy phases and induces an important improvement of the corrosion behaviour of galvanized steel in warm water.     

Das Korrosionsverhalten von Eisen and Stahl in flüssigem Ammoniak

Corrosion behaviour of iron and steel in liquid ammonia Polarization curves were measured with iron in liquid ammonia containing various electrolytes of different acidities at - 40°C and + 20°C. The dissolution rate of iron by formation of iron(II) is independent of the electrolyte composition and follows a Tafel-line of the slope RT/2.0.4 F. At positive potentials iron becomes passive. The steady state current densities are nearly independent of the electrode potential and grow with acid concentration and temperature. At very positive electrode potentials the current densities again increase exponentially with the potential. The current efficiencies for nitrogen evolution and iron(III) dissolution are of equal magnitude. Iron oxidized in air exhibits an electrochemical behavior distinctly different from the one of iron passivated in liquid ammonia. Very probably the passivating layer is composed of iron nitride with a thickness of the order of nm. Passive steel FG 32 in the soft state after heat treatment corrodes at about the same rate as pure iron, but approximately 8 times faster in the cold worked state.     

Korrosionsverhalten von Edelstahlrohren und Rohrverbindungen bei der Warmwasserverteilung in der Hausinstallation

Corrosion behaviour of alloy steel tubing and tube connections in warm water distribution systems in dwellings The corrosion behaviour of steel in warm waters essentially depends on heat transfer and in particular on the direction of heat transfer. The precipitation of water constituents responsible of water hardness may result in local chloride enrichment and, consequently, pitting and/or stress corrosion cracking. Ferritic chromium steels are not sufficiently resistant in these conditions, while addition of 2% Mo or 9% Ni may yield satisfactory results. The danger of corrosion also exists in the case of tubing embedded in mortar, if the latter contains chlorides. As to tube brazing it should be borne in mind that hard solders with a high silver content and sufficiently low point of fusion should be used; the type of corrosion to be expected at such places would be in particular the knife-line attack. In the case of zinc solders there is the danger of dezincification with subsequent pitting caused by the products of hydrolysis of zinc compounds. As to crevice corrosion on noticable sensitivity has been found under the conditions studied.     

Einfluß einer Eisenaufmischung auf das Korrosionsverhalten von NiCu30 Fe-Stahlplattierungen

The influence of iron pick-up on the corrosion properties of NiCu30Fe/steel clad materials The influence of iron pick-up on the corrosion properties of overlay and butt weldings of NiCu30Fe/steel cald materials was investigated by electrochemical tests in aerated artificial sea water (ASTM Standard D 1141-75) at 25, 50 und 80°C and by exposure to sea water in a sea water test rig on Helgoland. NiCu30Fe plates with defined Fe contents between 0% and 14% were used for comparison. On the Fe being distributed homogeneously, no negative influence of the iron on the pitting resistance was noted in aerated artifical sea water of 25°C up to an Fe content of 10%. The pitting resistance increases with rising temperature (50 and 80°C) and the Fe influence increases slightly. On exposure to sea water on Helgoland, the corrosion in the subtidal zone is mainly caused by marine growth resulting in the formation of shallow pits. In the tidal zone, Fe contents of more than 6.7% cause rust pittings at very low integral corrosion rates (≤0.002 mm/a). With integral Fe contents of between 0.5% and 3% in the final pass, the overlay welds have the same good corrosion resistance properties as commercial NiCu30Fe plate material. The compound butt welds exhibit slight pitting in the weld metal and the heat-affected zone which cannot be conclusively attributed to Fe pick-up but is rather due to the different free corrosion potentials of the NiCu30Mn(Ti) weld metal and the NiCu30Fe cladding material. In the tidal zone which is the preferential application of NiCu30Fe corrosion rates are less than 0.01 mm/a for all specimens welded by different welding methods. Iron contents of up to 8% which can be met by all welding methods when welded in two layers have no negative effect on the corrosion properties.     

Zum Korrosionsverhalten von metallischen Werkstoffen in chlorkohlenwasserstoffhaltigen Lösungsmittelgemischen

Corrosion behaviour of metallic materials in solvent mixtures containing chlorinated hydrocarbons The phenomenology of the corrosive attack experienced by a C-steel in trichloroethylene, tetrachloroethylene and 1,1,1-trichloroethane containing solvents was investigated. Mixtures with ethers, especially with tetrahydrofuran (THF), show increased corrosive properties. A surface layer of FeCl₂ · 4H₂O and β-FeOOH developes on C-steels in these organic solvents. The mixtures with THF attack stainless chromium-nickel-(molybdenum) steels. The presence of FeCl₃ increases this attack. Copper also is appreciably corroded.     

Korrosionsverhalten von Gasturbinenwerkstoffen unter strömenden Heißgasbedingungen

Corrosion behaviour of gas turbine alloys under high velocity burnt fuels The aim of alloy development in the field of nickel based superalloys for flying and land based gas turbines is to enhance significantly the mechanical properties at high temperatures thus leading to a higher temperature capability. The higher temperature capability of the structural elements of gas turbines results in an increased efficiency, a lowered fuel consumption and less emissions. To achieve an increased high temperature capability, however, surface degradation of the material must be adjusted adequately, hence corrosion resistance has to be improved. Additional to the isothermal and cyclic oxidation tests which are performed in stagnant air the oxidation behaviour of alloy 2100 GT and alloy C‐263 was investigated by means of burner‐rig‐experiments under high velocity burnt fuels. In the burner rig test facility the sample is exposed to a hot gas stream of burned natural gas with gas velocities in the range of 60 m/s to 150 m/s. The metal temperature of the sample can be adjusted in the range of 900°C to 1200°C. In the tests described in this paper the gas velocities were chosen to be 60 m/s, 100 m/s and 140 m/s. The test duration was 1 h and 10 h. The test temperature was kept constant at 1000°C. After 1 h of testing both alloys showed mass gain which was significantly higher for alloy C‐263. After 10 h of testing the mass loss of alloy C‐263 was enhanced with increasing gas velocity. Alloy 2100 GT showed only at the highest gas velocity a mass loss. The examinations by means of SEM and light‐optical microscopy of the oxide scale and of the microstructure showed that alloy 2100 GT has a dense adherent alumina scale and suffers no internal oxidation even under burner‐rig‐test conditions. Alloy C‐263 forms a mixed chromia and Cr‐Ti‐mixed oxide scale. The chromia is evaporated with increasing gas velocity, leaving (Cr‐Ti)O₂‐needles on the surface. In the isothermal and cyclic oxidation tests alloy 2100 GT shows an excellent oxidation behaviour up to 1200°C with a corrosion rate of less than 0.1 mm/a. The aluminium content of app. 3 wt.‐% which is remarkably high for a wrought alloy leads to the formation of a thin dense and adherent alumina scale. Alloy C‐263 is a chromia former which is not suitable for temperatures higher than 1000°C.     

Korrosionsverhalten von Bauteilen aus Nichteisenmetallen bei Berührung mit Baustoffen

Corrosion behaviour of non-ferrous metal structural elements in contact with building materials Aluminium-, copper-, lead- and zinc materials are commonly used in building projects. The corrosion strain on non-ferrous metals is small with buildings in comparison to other conditions, therefore corrosion damage is rare. The specific properties of these materials are explained in relation to their reaction force and their inhibition of chemical reactions. The possibility of corrosion damage is demonstrated and preventative measures are discussed.     

Korrosionsverhalten des Werkstoffs 1.4539 und von Nickelbasis-Legierungen in Gaswässern

Corrosion behaviour of material no. 1.4539 and nickel based alloys in gas waters Laboratory tests with synthetic gas waters containing the gases ammonia, carbon dioxide, hydrogen sulphide and hydrogen cyanide were carried out in order to examine the influence of medium components on the corrosion of material No. 1.4539 and nickel based alloys Hastelloy C-4, C-22 and C-276. Hydrogen sulfide was identified as the decisive component for corrosion. For stainless steel corrosion rates of about 2 mm ° a⁻¹ were already found at 50°C in a critical pH-range with sulfide concentrations > 2%. As cyanide stimulates corrosion by dissolving sulfide surface layers by complexation of the iron ions, an increased material loss rate per unit area was found in the critical range with increasing cyanide concentration. The much more stable nickel based alloys only revealed considerable weight losses after being exposed in the autoclave at 100°C. The graduation of the loss rates C-22 > C-4 > C-276 can be explained by the different contents of high grade alloy elements. The testing of nickel based alloys of the Hastelloy type and of material No. 1.4539 and 1.4571 by means of the dynamic tensile test (CERT-method) revealed no risks of stress corrosion cracking in the tested media. The mechanical data, such as energy at break, elongation at rupture and reduction of area when breaking, did not indicate any change in comparison to the data found in a neutral medium (glycerine). As a function of the aggressiveness of the liquid phase, the metallographic analysis showed some traces of local corrosion in the highly deformed area, ranging from pitting to limited surface cracking. These phenomena were found to nearly the same extent for all materials.