Die aktive und passive Molybdän-Elektrode in sauren Lösungen

The active and passive molybdenum electrode in acid solution It follows from the anodic and cathodic thermodynamic polarization curves in the system molybdenum/aqueous solution (p_H 0 to 10) that the favourable behaviour of molybdenum in acid solutions is determined by the extension of the immunity region becomes smaller, but without simultaneous passivation of molybdenum; this fact explains the active behaviour of molybdenum in alcaline solution. The passivation of molybdenum starts as soon as the half cell reaction sets in, where The p_H-dependence of the cathodic passivation potential in acid solutions follows the equation      

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.     

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.     

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: .     

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.     

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.     

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.     

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.     

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.     

The oxidation of an 18/8 type stainless steel in CO2

The oxidation behaviour of an 18/8 type stainless steel has been studied in the temperature range 873-1258 K (600–985 °C) by continous weight gain measurements, metallography and scanning electron microscopy. An initial high oxidation rate was followed by a much slower rate. Duplex scales formed at an early stage of oxidation and the decrease in oxidation rate is probably due to the formation of an ordered chromium rich spinel at the oxide/metal interface. A ture parabolic rate constant (k_p) was obtained taking into account the formation of the “healing” phase using the Hales model. This gave: . where the activation energy is expressed in KJ · mole^?1. (55 Kcals mole^?1).     

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%.     

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.     

Anodische Korrosionsprozesse auf Stählen in inerten und in oxydierenden Atmosphären

Anodic corrosion processes on steels in inert and oxidising atmospheres The purpose of the investigation here described was to find out to what extent electrochemical techniques lend themselves to the examination of steel corrosion in the presence of molten sulphates and combustion gases. The measured equilibrium potentials of an inert metal electrode are intended to serve for the determination of the redox potential in the salt melt/combustion gases system. If no current is flowing, the stationary potential of a platinum electrode in a sulphate melt at 600° C can be expressed by the equation where E signifies the potential related to the silver/silver sulphate electrode, and p_SO2 p_O2 the partial pressures (in atmospheres) of the two gases in an inert gas (nitrogen + carbon dioxide). The formula permits the conclusion that the electrode reaction can be expressed by the equilibrium condition . The anode currents set up if the potential of a mild steel or pure iron electrode is kept at the above-mentioned temperature viz. ?0.3 V with N₂ + CO₂ + 5% O₂, or + 0.4 V with N₂ + CO₂ + 0.2% SO₂, show that under the test conditions, these metals would be greatly exposed to corrosion. Stainless steels become passive after a few hours although a residual corrosion current at + 0.4 V remains. These observations give rise to the expectation that electro-chemical examinations may well represent a useful means of examining corrosion phenomena caused by molten salts in the presence of combustion gases, so that they merit more detailed investigation.     

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 .     

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.     

Sauerstoffübertragung aus CO2?CO?und H2O?H2?Gemischen auf Metalle und Oxide

Oxygen transfer from CO₂-CO- and H₂O-H,-mixtures to metals and oxides In flowing CO₂-CO or H₂O-H,-mixtures oxygen is transferred from the gas phase to the solid, and equilibria are established according to the reactions These oxygen transfer reactions are rate determining for the linear oxidation of metals or for the linear reduction of oxides in these gases. With the aid of isotope exchange reactions the dependence of the oxygen transfer on the oxygen activity was measured on metals and oxides. The dependence on the oxygen activity is similar for reaction ( 1 ) and (2) on the same solid andit can be explained by oxygen adsorption according to the Freundlich-isotherm. From the measurement of the isotope exchange reactions during oxidation or reduction processes the stationary oxygen activity at the surface the solid can be determined. such measurements during the linear oxidation of iron to Wustite, during the linear reduction of magnetite to wustite and during the reduction of wustite to iron are described.     

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.