Der Einfluß von Kriechverformung auf die Ausbildung der Oxidschicht auf der Hochtemperaturlegierung Ni20Cr

Influence of creep deformation on the formation of the oxide layer on the high temperature alloy Ni20Cr The formation of the Cr₂O₃-layer on Ni20Cr has been investigated at 850°C in H₂/H₂O (p(O₂) = 10^?19 bar) under simultaneous creep deformation with flat samples. The damage of the protecting oxide layer by cracks has been observed in dependence on deformation rate and strain. For additional information about the influence of the plastic deformation of the oxide layer and the healing of the cracks, preoxidized samples have been deformed in pure Ar-atmosphere. At strain rates below 10^?9s^?1 cracks cannot be observed. When strain rates < about 3 × 10^?8s^?1 are applied, cracks occur only above grain boundaries of the alloy, at higher strain rates they also lie in regions above the grains of the alloy. For > about 3 × 10^?8s^?1 the crack density depends no more on but only on strain . The different damages of the oxide layers in the two atmospheres allow the conclusion, that at from 10^?9s^?1 to 10^?7s^?1 beside the plasticity of the oxide layer in particular the crack healing influences the sum of the crack openings measurably. With increasing strain rates the contribution of plasticity can be neglected.     

Betrachtungen über das Eisen als mehrfache Mischelektrode

Iron—considered to be a multiple mixed electrode The system iron/aqueous solution is represented as a multiple mixed electrode with the cathodic potential-dependent electrode reactions The representation is made in a U_H (J_α, J_β, J) Diagram which turns out to be suitable for graphically representing the possible reactions of an iron electrode in the selected system. It is shown that only a limited number of these system can function as mixed electrodes.     

Über den grünen Rost von Messing

Green rust on brass In the electrical circuit fo a car formation of patina was observed on a brass plate in contact with a copper wire. The products formed were identified as orthorombic The sulfur-containing rubber sheath above the brass plate seems to be one of the causes of the formation of this corrosion product.     

Untersuchungen zum Einfluß des Stahlgefüges auf die stationäre Wasserstoffpermeation

Investigations on the influence of microstructure of steels on steady state hydrogen permeation The effect of microstructure of iron and of a low alloyed steel on steady state hydrogen permeation is studied by means of the electrochemical permeation method adapted to hydrogen gas phase charching at p = 1 bar in the temperature range of 15 to 80 °C. In case of pure annealed iron the permeation coefficient is given by Impurities, oxide inclusions and a high density of lattice defects do not affect steady state hydrogen permeation remarkably. In steel specimens of different microstructure (pearlitic, martensitic, bainitic) hydrogen permeability is decreased by a factor 4 to 8. Carbide precipitates in tempered martensite do not change the permeation coefficient. Also cold deformation by rolling to about 15% shows no effect on steady state permeation. Cold rolling to about 40% or higher degrees decreases the steady state hydrogen flux considerably. In all cases, no essential change in temperature dependence is observed.     

Zur Korrosion der Legierung AlMg 2 Mn 0,8 in hochkonzentrierter Salpetersäure

Corrosion of the alloy AlMg 2 Mn 0,8 in highly-concentrated nitric acid The purpose of the study was to investigate the corrosion of the alloy AlMg 2 Mn 0.8 in highly-concentrated nitric acid at temperatures between 5 °C and 50°C. The corrosion rates follow the Arrhenius equation the apparent activation energy was calculated to be ?60.8 kJ/mol. The ratio of the dissolved alloy components aluminum and magnesium indicates that magnesium had accumulated near the surface of the metal before the exposure to nitric acid. This finding is confirmed by surface analysis.     

Wirkung von schwefelhaltigen Inhibitoren auf die Korrosion von Messing 63/37 in Trichloressigsäure

Action of sulphur containing inhibitors on the corrosion of 63/37 brass in trichloroacetic acid Trichloroacetic acid is highly corrosive to brass. Sodium thioglycolate, sodium diethyl dithio carbamate and carbon disulphide have been studied as corrosion inhibitors for brass (63/37) in trichloroacetic acid. The inhibitive power of sulphur-containing organic compounds is due to chelate formation, the chelate adhering strongly to the metal surface. The efficiencies of above stated inhibitors were found in the following order: .     

SCC of X‐52 and X‐60 weldements in diluted NaHCO3 solutions with chloride and sulfate ions

Stress corrosion cracking tests were performed in both X‐52 and X‐60 weldments in sodium bicarbonate (NaHCO₃) solutions at 50°C using the Slow Strain Rate Testing (SSRT) technique. Solution concentrations varied between 0.1 to 0.0001 M, and to simulate the NS‐4 solution, chloride (Cl^?) and/or sulfate ( ) ions were added to the 0.01 M solution. Tests were complemented with hydrogen permeation measurements and polarization curves. It was found that the corrosion rate, taken as the corrosion current, I_corr, was maximum in 0.01 M NaHCO₃ and with additions of ions. Higher or lower solution concentrations or additions of Cl^? alone decreased the corrosion rate of the weldment. The SSC susceptibility, measured as the percentage reduction in area, was maximum in 0.01M NaHCO₃. Higher or lower solution concentrations of additions of Cl^? or decreased the SCC susceptibility of the weldment. The amount of hydrogen uptake for the weldment was also highest in 0.01 M NaHCO₃ solution, but it was minimum with the addition of Cl^? or ions. Thus, the most likely mechanism for the cracking susceptibility of X‐52 and X‐60 weldments in diluted NaHCO₃ solutions seems to be hydrogen‐assisted anodic dissolution.     

Atmospheric corrosion rates,time-of-wetness and relative humidity

The effect of relative humidity (RH) on atmospheric corrosion rates r has been studied in laboratory experiments for rust covered steel in air and air + 1 ppm SO₂. The expression: describes the experimental results, where r_crit is the corrosion rate at the critical relative humidity RH_crit and b″ is the normalized corrosion rate at RH = 100%. While a large effect of NaCl impurities in rust was found, no effect of SO₂ was observed. Time-of-wetness was determined in outdoor exposure using the previously described atmospheric corrosion monitor (ACM). It was found that for exposure at a site in Thousand Oaks, California, the time-of-wetness corresponds to RH > 40%.     

Einfluß von Korrosion und mechanischer Belastung auf das Rißwachstum niedriglegierter,ferritischer Stähle in sauerstoffhaltigem Hochtemperaturwasser

Influence of corrosion and mechanical loading on the crack growth of low-alloyed ferritic steels in oxygenated high temperature water The mathematical and with regards to the contents main features of the mostly developed analytical model for corrosion-assisted crack growth are presented and the crack growth velocities resulting for low-alloyed ferritic materials in high-temperature water are given. Experimentally determined crack growth velocities for the ferritic material 20 MnMoNi 5 5 with two different sulfur contents as well as for the similar material 22 NiMoCr 3 7 in deionized, oxygenated (0.4 and 8 ppm O₂) high temperature water at 240°C are compared with calculated ones. The constant load experiments at different level were performed on compact tension specimens with a thickness of 50 mm (2T-CT-specimens). The experimental results show, that up to a stress intensity factor K_I of 60 MPa $ \sqrt m $ the corrosion-assisted crack advance is neither dependent on the oxygen content of the medium and the K₁-value, nor on the sulfur content of the steel. A deviation up to 3 magnitudes compared to the calculated values exists. Furthermore, an increasing crack growth velocity with decreasing test duration is observed. Between 60 and 75 MPa $ \sqrt m $ the crack growth velocity increases by several magnitudes also independent of the above mentioned parameters. Above 75 MPa $ \sqrt m $ fracture of the specimen occurs soon after loading. In this region the experimentally derived crack growth velocity fits well with the analytical model. A possible explanation for the deviation between experimental and analytical results could be seen in low-temperature creep processes at the crack tip. Results of preliminary investigations on low-temperature creep processes of the material 20 MnMoNi 5 5 in air at 240°C are presented.     

Das anodische Verhalten des Zinns bei Selbstpassivierung

The anodic behaviour of tin in the case of auto-passivation The diagrams: electrochemical affinity(A): overvoltage (U) characterizing the behaviour of tin in the p_H range 4–14 occupy an intermediate position between the groups Ni, Co, Cu, and Ti, Zr, Nb, U. On passive tin the formation of a double layer electrode of the type sn|SnO₂|Sn(OH)₄ appears thermodynamically probable. The anodic Flade reference potential of tin, depending from the p_H value obeys the equation .     

4‐Chloro‐benzoic acid [1,2,4]triazol‐1‐ylmethyl ester as an effective inhibitor of mild steel corrosion in HCl solution

4‐Chloro‐benzoic acid [1,2,4]triazol‐1‐ylmethyl ester (CBT) was synthesized and its inhibiting action on the corrosion of mild steel in 1 M hydrochloric acid solutions was investigated by means of weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and scanning electron microscope (SEM). The results showed that CBT is an excellent inhibitor for mild steel in acid medium and its inhibition efficiency (IE%) is up to 90.2% at a concentration of 10^?3 M at 298 K. EIS showed that the charge transfer controls the corrosion process in the uninhibited and inhibited solutions. Potentiodynamic polarization studies clearly reveal that CBT acts essentially as mixed‐type inhibitor. Thermodynamic parameters such as adsorption heat ( ), adsorption entropy ( ), and adsorption free energy ( ) were obtained and discussed from experimental data of the temperature studies of the inhibition process at four temperatures ranging from 298 to 333 K. Kinetic parameters activation such as , , , and pre‐exponential factor have been calculated and discussed. Adsorption of the inhibitor on the mild steel surface followed Langmuir adsorption isotherm. The values of the free energy of adsorption indicated that the adsorption of CBT molecule was a spontaneous process, and was typical of chemisorption.     

Inhibierung der Lochbildung und des Lochwachstums bei Aluminium

Inhibition of pit initiation and pit growth on aluminum Under potentiostatic conditions over 40 organic and inorganic compounds have been investigated in order to determine their inhibiting effect on the pitting corrosion of aluminum in 0.01 molar NaC1 solutions. The concentration dependent inhibitor efficiency decreases in the order: Na-oleate, NO ? WO > IO, MoO, BrO, Na-benzoate > CrO > NO, Na-tartrate. SO, ClO and the other organic compounds tested accelerate the pit growth as well as too low concentrations of inhibiting ions. The accelerating effect depends directly on the conductivity of the electrolyte. The inhibitor efficiency depends on the potential and is deteriorated by non inhibiting ions such as ClO or SO. The minimum concentration required to inhibit completely the pit initiation and the pit growth is often not identical. This fact explains partly the inconsistent data found in the literature. The phenomena observed are explained on the basis of the Temkin isotherm.     

Untersuchungen der Korrosion von Aluminium in Wasser bei hohen Temperaturen und Drucken

Research on the corrosion of aluminium in water at high temperatures and pressures The stationary corrosion rate i_corr of aluminium is measured electrochemically in a 10^?3 m sodium bicarbonate solution at temperatures between 100°C and 200°C using a V4A high pressure loop, the result being Thus, the effective activation energy is 15 kcal/mole. The stationary thicknesses of the oxide layer on aluminium are calculated as a function of the temperature from the corrosion rates and the weight changes of the specimen. The results are compared to the thicknesses measured microscopically.     

Corrosion of pretreated aluminium after cathodic electrodeposition of paint

Cathodic deposition of paint (CDP) is Well introduced for the industrial coating of primers onto steel since nearly two decades. Epoxy resins provide optimum results. There is an increasing demand to apply the same technique for aluminium, especially for mixed constructions Fe/Al in motor car bodies. However, this metal may be attacked by the OH^?-ions, generated by the cathodic electrolysis of water according to: H₂O + e^? → ½ H₂ + OH^?. The Al₂O₃ · xH₂O protecting layer may dissolve slowly as aluminate and Al-metal then reacts rapidly with water to generate the threefold volume of hydrogen under the reestablishment of the oxide layer. Thus, the overall reaction for this cathodic corrosion of Al is given by: Al + 2H₂O + e^? → AlO + 2H₂. It can be foreseen, that the changes at the phase boundary Al, AlOOH/paint and the accumulation of hydrolysed aluminate in the coating may influence, among other, the corrosion protection behavior of the paint layers. A systematic study of the influence of four different industrial epoxy resins from BASF Lacks & Farben AG (1)–(4) with their individual pigment systems, the one for paint (3) to be free of lead silicate, was undertaken. Seven different aluminium (alloy) substrates were employed. Their pretreatment modes were mostly due to zincphosphatation. Three standard corrosion tests for conventional corrosion, (CC) and one for filiform corrosion (FFC) were employed and evaluated, as usual. The accelerated open air corrosion test lasted 360 days. It was found, that for CC the corrosion protection capability was predominantly influenced by the resin, and it decreased in the following order: The effect of the substrate was not very pronounced, but a relative optimum could be seen with Al Mg 0.4 Si 1.2-chromate pretreatment and Bonazinc 2000® and with Al Mg 1.5 Si 0.5 Cu 4.0-chromate pretreatment (with one exception). The ranking for FFC changed to: , and zincphosphated Al Mg 3 was superior over all the other substrates. The analytically determined rate of cathodic corrosion for unpigmented paints did not correlate to these results, and this may be indicative for specific pigment effects. In conclusion, this systematic study reveals, on the basis of practical systems and corrosion test methods, a way for the optimization of CDP on aluminium.     

Galvanic Corrosion of Al Alloys

In a systematic study of galvanic corrosion of Al alloys the effects of the dissimilar metal, the solution composition and area ratio have been studied using galvanic current and weight loss measurements, In 3.5% NaCl, galvanic corrosion rates of the Al alloys 1100, 20324,2219, 6061 and 7075 decrease with the nature of the dissimilar metal in the order AG>Cu> 4130 steel ?stainless steel ≈Ni>>Inconel 718?Ti-6A1-4V≈?Haynes 188>Sn>Cd. Coupling to zinc did not lead to cathodic protection of all A1 alloys. The potential difference of uncoupled dissimilar metals have been found to be a poor indication of galvanic corrosion rates. Dissolution rates of A1 alloys coupled to a given dissimilar material are higher in 3.5% NaCl than in tapwater and distilled water where they are found to be comparable. In assessing the galvanic corrosion behavior of a given A1 alloy as a function of environment, one has to consider the effect of the dissimilar metal. The dissolution rate of Al 6061 is, for example, higher in tapwater with Cu as cathode than in 3.5% NaCl with SS304L or Ti-6AI-4V as cathode. The effect of area ratio \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{{A^C }}{{A^A }} $\end{document} has been studied in 3.5% NaCl for area ratios of 0.1, 1.0 or 10. The galvanic current was found to be independent of the area of the anode, but directly proportional to the area of the cathode. The galvanic current density \documentclass{article}\pagestyle{empty}\begin{document}$ i_{^g }^A $\end{document} with respect to the anode has been found to be directly proportional to the area ratio (\documentclass{article}\pagestyle{empty}\begin{document}$ \frac{{A^C }}{{A^A }} $\end{document}), while the dissolution rate r_A of the anode was related to area ratio by \documentclass{article}\pagestyle{empty}\begin{document}$ r_A = k_{_2 } (1 + \frac{{A^C }}{{A^A }}) $\end{document}. The results obtained have been explained in terms of mixed potential theory.     

Untersuchung des Passivierungsmechanismus von Eisen in Phosphatelektrolyten an erosiv beanspruchten Oberflächen

Investigation into the passivation mechanism of iron in phosphate electrolytes on surfaces exposed to erosive attack On iron electrodes in neutral phosphate electrolytes by continuous solid particle impingement a reaction layer is formed within the transition potential region under anodic polarization. XPS and AES investigations show that the reaction layer formed under impingement will be replaced by an oxide layer during the transition into the passive state under simultaneous decrease of the layer thickness. The active/passive transition in phosphate electrolytes may be attached to the equilibrium potential of the reaction . According to this thermodynamic interpretation of both the active/passive transition and the passive/active transition, respectively on iron in presence of phosphate ions may be described as the Fe(II)/Fe(III)-redox reaction with two solid phases, the iron(II)-phosphate phase and the iron(III)-oxide phase. The increase of the interfacial dynamic processes at the solid/liquid interface causes in consequence of the solid particle impingement that thermodynamic laws govern the course of reactions.     

Investigation on lead corrosion products in sea water and in neutral saline solutions

A method was developed to characterize and quality lead corrosion products in sea water and in saline neutral solutions. This method is based on selective dissolution of various compounds, using suitable reagents (methanol, glycine, potassium nitrate etc.) and on subsequent chemical analysis of the various dissolved elements. The findings are then verified by X-ray diffractometer analysis. This method was used for an examination of the corrosion products adhering to a lead plate of a Roman ship wrecked in the Gulf of Toulon about two thousand years ago. The following corrosion products were determined: These products were compared with those obtained on sea water immersed lead specimen. In the latter case, the products were the following: The difference between the two test specimen is deemed to be due to the known formation caused by bacterial fouling processes (desulfovibrio desulfuricans) of hydrogen sulphide in marine sediments which, by altering the pH value, also alter the equilibrium of the CO₃^??-HCO₃^?-CO₂-SO₄^??? HSO₄^? systems thus affecting the differentiated formation of the corrosion products. Lead, despite its improved corrosion resistance in various environments as compared with other normally used metals (e.g. iron), is not so commonly employed because of its poor mechanical properties (deformation, grain coarsening, brittleness [1], etc.) so that it is only used for certain structures like pipings or coverings (roofs, chemical vats etc.) not exposed to strong mechanical stresses. These applications were common even in ancient times, when the Romans already covered their hulls with lead plates because they did not corrode easily and thus had a long life.     

Langzeitverhalten feuerverzinkter Stahlrohre in Trinkwasser unterschiedlichen pH-Wertes und unterschiedlicher Strömungsgeschwindigkeit

Long term performance of galvanized steel tubes in drinking water with different pH-value and flow velocity A 10 years' test program on galvanized steel tubes exposed to Berlin drinking water produced evidence that an undisturbed operation can only be guaranteed as long as the inner surface of the tubes is coated by metallic zinc. The zinc loss with time strongly depends on the pH-value and can be expressed by the mathematical equation . Due to the typical shape of the logarithmical function and its dependence on the pH-value a small increase of pH-value leads to an over-proportional increase in life-time of the zinc coating. pH = 7 leads to a total zinc loss within two years, whereas in the case of pH = 8 a zinc coating of 25–40 μm is still present after 10 year's time.     

Susceptibility of highly alloyed austenitic stainless steels to caustic stress corrosion cracking

Slow Strain Rate tests (5 × 10⁻⁶ to 4 × 10⁻⁸ s⁻¹) in 300 g/L sodium hydroxide at 200°C were conducted on highly alloyed austenitic stainless steels with various nickel and chromium concentrations: N08904 (20Cr‐25Ni‐4Mo), N8825 (22.5Cr‐40Ni‐3Mo), N08028 (27Cr‐30Ni‐3.5Mo), R20033 (32.5Cr‐31Ni‐1.5Mo). Stress Corrosion Cracking (SCC) resistance of studied alloys increases in the following order: N08904 → N8825 → N08028 → R20033 in accordance with increasing chromium content. The SCC susceptibility indexes decrease gradually with decreasing of strain rate. In materials exhibiting higher SCC resistance, tests should be conducted at very low strain rates ( < 2 × 10⁻⁷ s⁻¹) to observe indications of SCC. When sulphide ions are added the R20033 steel exhibiting an excellent corrosion behaviour in pure caustic solution, becomes highly susceptible to SCC, even at = 5 × 10⁻⁶ s⁻¹.     

Test of a dual electrode galvanic cell in binary carbonate melt

Basicity of the (Li_0.62K_0.38)₂CO₃, the current choice of electrolyte composition for molten carbonate fuel cells (MCFC's ), is defined as — log (a), where M represents an alkali metal and a is the net oxide ion activity. Net oxide ion activity is defined as the sum of the alkali oxides activities dissolved in the melt. To correlate measured cell e.m.f. values with basicity change in the (Li_0.62K_0.38)₂CO₃ melt, a dual electrode galvanic cell of the following arrangement was tested at 650°C with Pvarying above the melt: Au, A—B, CO₂, O₂ | mullite | A—B, CO₂, O₂ | ZrO₂ · Y₂O₃ | O₂, Au where A—B represents (Li_0.62K_0.38)₂CO₃. The response of the cell to P at constant P can be explained by thermodynamic model, which states that ion transference in the mullite tube is limited to Li* and/or K* and the dual electrode galvanic cell voltage is a direct measure of Δa or Δa for pure (Li_0.62K_0.38)₂CO₃ melt at constant P.