Thermodynamische Gesichtspunkte zum elektrochemischen Auf- und Abbau von Oxydschichten auf edleren Metallen

Thermodynamical considerations concerning the electrochemical build-up and removal of oxide layers on nobler metals On the strength of thermodynamical considerations, the electrochemical behaviour of gold, platinum and palladium has been investigated with the aid of the A/ΔU diagram. The P_H relationship of the Flade reference potential was found to be: \documentclass{article}\pagestyle{empty}\begin{document}$ U_{H,\ F,\ K} = U^\circ \ _{H,\ F,\ K} - 0,059\ p_{H.} $\end{document} This corresponds:

• for Pt = 0.98 V to the removal of PtO
• for Au = 1.37 V to the removal of AuO
• for Pd = 1.10 V to the removal of PdO₂.

     

Dehnungsinduzierte Korrosionsprozesse

Strain-induced corrosion phenomena When investigating stress corrosion cracking phenomena in the past, application of a static load σ played a major role in the performance of experiments. A further influencing factor, to which until now was payed less attention, is the strain ? caused by a mechanical stress. Increasing attention is payed also to the variations with time of these influencing factors, \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \sigma $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ {\dot \varepsilon }$\end{document}. A strain, induced by a load may have the following effects: (i) break-up of a protective layer and/or (ii) mechanical stimulation of anodic metal dissolution. Both types of failures are discussed. The type of failure (i) is referred to as stress-induced, the type of failure (ii) as strain-induced corrosion. The authors discuss the question, whether type of failure (ii) is always involved in stress corrosion cracking. For some corroding systems, this question can clearly be answered in the negative, while in numerous systems the occurrence of stress corrosion cracking is markedly influenced by the strain rate. The relevant systems are described in detail. Essential for the authors' considerations is the existence of an upper and lower critical strain rate for the occurrence of stress corrosion cracking. Transition from strain-induced stress corrosion cracking to corrosion fatigue is given with relatively high upper critical strain rates. From the influence of strain rate upon stress corrosion cracking, conclusions are derived for the application of test methods for detecting susceptibility of metallic materials to stress corrosion cracking.     

Die Wasserstoffaufnahme von Eisen bei der Korrosion in neutralen bis schwach sauren Elektrolyten

Hydrogen uptake by iron during corrosion in neutral to weakly acid electrolytes During atmospheric corrosion and corrosion by aqueous solutions, hydrogen can enter into steel. The hydrogen activity built up in iron during corrosion by dilute aqueous solutions of hydrochloric acid, sulfuric acid and iron salts has been measured as a function of pH using a permeation technique. Below pH = 5 in oxygen free solutions and pH = 4 in air saturated solutions the hydrogen activity \documentclass{article}\pagestyle{empty}\begin{document}$ a_{\rm H} {\rm = }\sqrt {{\rm p}_{{\rm H}_{\rm 2} } {\rm /(}\mathop {{\rm p}_{{\rm H}_{\rm 2} } }\limits^{\rm o} {\rm = 1}\,{\rm bar)}} $\end{document} reaches values of more than 0.1 sufficient to cause delayed cracking of steels susceptible to hydrogen embrittlement. The anions and Na⁺-ions have no markable influence. The influence of Fe³⁺ and O₂ is discussed.     

Elektronenoptische und röntgenographische Untersuchung einer Eisenphosphat-Schutzschicht auf Eisen

Electron-optical and radiographical examination of a protective iron phosphate layer on iron The protective effect of a novel anticorrosive containing phosphoric acid has been examined for its dependence on mineralogical conditions. The base material contains, intro alia, phosphoric acid, potash mica, organic wetting agents and dyeing agents. Vivianite formation has been precluded by radiographic comparison with natural vivianite and with data quited in technical literature. The scraped protective layer was found to contain, apart from muscovite and α iron, dendritic formations which were observed under the electron microscope and identified, from their diffraction interferences, as cubic form of the potassium iron (III) phosphate leucophosphite \documentclass{article}\pagestyle{empty}\begin{document}$ KFe_2 [OH(PO_4)_2] \cdot 2\ H_2 O. $\end{document} KFe₂[OH(PO₄)₂] · 2 H₂O. Precision zone diffraction diagrams of individual dendrites showed a_o = 7.75 ± 0.05 Å and the transmission directions [100]^X [010]^X [202]^X, identical to those observed with natural pharmacosiderite, an iron arsenate corresponding to leucophosphite. Natural and synthetic leucophosphite are monoclinic; if Fe³⁺ is replaced by Al³⁺, and (PO₄)^3? by (AsO₄)^3?, a cubic alumopharmacosiderite is formed. The formation is thereofore interpreted as a cubic modification of leucophosphite, corresponding to pharmacosideriote. A structure comparison shows the epitactic growth of the leucophosphite with [110] and [333] on α iron [100] and [111]. The presence of leucophosphite directly on the iron surface is proved by electron diffraction with grazing incidence of the electron ray.     

Effects of water vapour and oxygen partial pressures on low carbon steel oxidation in N2–H2–H2O mixtures

A low carbon, low silicon steel was oxidised at temperatures of 900–1000 °C in flowing N₂–H₂–H₂O gas mixtures in which oxygen and water vapour partial pressures were varied independently. Scales of dense, single‐phase, coarse grained wüstite grew rapidly, according to parabolic kinetics. Both the scaling rate and the oxide grain growth increased with $p_{{\rm O}_{2} } $ at constant $p_{{\rm H}_{2} {\rm O}} $ , and also with $p_{{\rm H}_{2} {\rm O}} $ at constant $p_{{\rm O}_{2} } $ . An inert marker experiment showed that significant oxygen transport but majority metal transport supported scale growth. Gas composition effects are interpreted using point defect models involving formation of hydroxyl species on anion sites as well as cation vacancies.     

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.     

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.     

Investigation of the effect of CrO and MoO on carbon steel corrosion using AFM and electrochemical techniques

The effect of chromate ( ) and molybdate ( ) ions on the corrosion of carbon steel in 0.5 M NaCl solution has been studied using electrochemical measurements and atomic force microscopy (AFM) technique. Potentiodynamic polarization data suggest that both and have inhibition effect on carbon steel corrosion, and the inhibition efficiency increases with increase in concentrations of and ; at the same concentration, the inhibition efficiency of is higher than that of . The increase in concentrations of and anions causes a shift of the breakdown potential (E_b) in the positive direction, indicating the inhibitive effect of the added anions on the pitting attack. At the same concentrations, the breakdown potential of is higher than that of . Electrochemical impedance spectroscopy (EIS) tests reveal that the charge transfer resistance and passive film resistance increase with increase in concentrations of or ; at the same concentrations, as for the charge transfer resistance and passive film resistance were bigger than those of . AFM imaging technique shows that local corrosion was inhibited obviously after the addtion of or , and passive film of was much more compact than that of . AFM force–distance curves indicate that the passive film of is much stiffer than that of .     

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.     

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.     

Determination of the polarization resistance from impedance measurements

Four different methods for determining the polarization resistance R from impedance data are discussed. These methods are suitable for online corrosion monitoring. Their use is illustrated for iron in tapwater and in neutral, aerated Na₂SO₄ containing various inhibitors. R-values obtained with the CIRFIT-method are compared with R which is obtained from a linear sweep through E_corr. The integration method has the advantage of computation speed.     

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.     

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      

Reequilibrium kinetics of NiO-Cr2O3 solid solutions

Electrical conductivity has been measured to monitor the reequilibration kinetics for single crystals of NiO-Cr₂O₃ solid solutions. It has been found that the rate for the reduction process is higher than that for the oxidation runs, thus indicating that the obtained kinetic data are not purely bulk controlled. The following expressions for the apparent chemical diffusion coefficient have been obtained within the temperature range 900–1200°C and oxygen partial pressure range 1–10^?5 atm: $$\begin{gathered} \tilde D_{1 red} = 1.22 \times 10^{ - 2} exp \left( {\frac{{24,420 \pm 1210 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{1 oxid}^* = 1.44 \times 10^{ - 2} exp \left( {\frac{{27,340 \pm 700 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{2 red} = 2.29 \times 10^{ - 2} exp \left( {\frac{{25,340 \pm 2230 calmole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{2 oxid}^* = 0.109 exp \left( {\frac{{29,610 \pm 3200 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{3 red} = 3.16 \times 10^{ - 2} exp \left( {\frac{{26,020 \pm 2430 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{3 oxid}^* = 0.202 exp \left( {\frac{{31,500 \pm 2640 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \end{gathered} $$ .     

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.     

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.     

Structure/property relationships in sintered and thermally sprayed WC-Co

Thermally sprayed WC-Co is widely used as a wear-resistant coating for a variety of applications. Although it is well established that thermal spray processes significantly affect chemistry, microstructure, and the phase distribution of WC-Co coatings, little is known about how these changes influence wear resistance. In this study, the microstructure and wear behavior of sintered and thermally sprayed WC-Co materials are examined. Powders of WC-12 wt% Co and WC-17 wt% Co were pressed and sintered, as well as thermally sprayed by high-velocity oxy-fuel (HVOF), air plasma spray (APS), and vacuum plasma spray (VPS) techniques. Results indicated considerable differences in the resulting microstructures, mechanical properties, and wear resistance. The thermally sprayed coatings showed anisotropic fracture toughness, whereas the sintered materials did not. It was also shown that a combined mechanical property/microstructure parameter, based on considerations of indentation fracture mechanisms, can be used in most cases to describe abrasive and erosive wear resistance of thermally sprayed WC-Co materials as follows: Wear resistance a whereK _ic is the indentation fracture toughness,H is hardness, andV ^Co _f is the volume fraction of cobalt. This relationship provides a means for assessing wear resistance of WC-Co coatings intended for industrial applications requiring abrasion and/or erosion resistance.     

Reequilibration kinetics of the heterogeneous system NiO-NiCr2O4

Reequilibration kinetic data have been obtained for a model system consisting of an undoped NiO crystal covered with a thin layer of NiCr₂O₄. The data lead to following apparent chemical diffusion coefficients \(\tilde D^* \) measured within the temperature range 900–1200°C when varying oxygen partial pressure from 1 to 10^?5 atm: $$\begin{gathered} \tilde D^* _{red} = 5.28 \times 10^{ - 4} exp\left( {\frac{{19,160 \pm 870}}{{RT}}} \right); \hfill \\ \tilde D^* _{oxid} = 1.82 \times 10^{ - 2} exp\left( {\frac{{28,270 \pm 2,020}}{{RT}}} \right) \hfill \\ \end{gathered} $$ . Values of the apparent activation energies for the reequilibration processes were independent of the thickness of NiCr₂O₄ surface layer from about 0.01 to 0.05 mm. As determined by microprobe, the thickness of the NiCr₂O₄ surface layer changed only slightly during long reequilibration experiments. The change is in agreement with diffusion coefficients of Cr in NiO. Oxygen pressure dependence of electrical conductivity leads to the parameter 1/n = 1/5.55 being the same as those for undoped NiO crystals. This indicates the values of electrical conductivity taken at equilibrium are determined by the equilibrated undoped NiO crystal while the dissimilar reequilibration kinetic data result from the presence of the NiCr₂O₄ surface layer. The results indicate the slower oxidation kinetics to be surface-controlled and faster reduction kinetics to be bulk-diffusion-controlled.     

Thermodynamic Roles of Metallic Elements in Carburization and Metal Dusting

This paper addresses the thermodynamic aspects of metallic elements (typically Fe, Ni, and Cr) in carburization and metal-dusting processes in CH₄–H₂, CO–CO₂, and CO–H₂O–H₂ carburizing gas mixtures, and their possible contributions to understanding of carburization and metal-dusting phenomena. Carburization requires $a_C \left( {gas} \right) > a_C \left( {M_Z C/M} \right) \cdot a_C \left( {M_Z C/M} \right)$ is solely temperature-dependent, while a_C (gas) is dependent not only on temperature but also on gas chemistry and total pressure. In general, metallic elements tend to be carburized at higher temperatures in CH₄–H₂, but at lower temperatures in CO–CO₂ and CO–H₂O–H₂ carburizing gas mixtures. For metal dusting to occur, $a_C \left( {M_Z C/M} \right)$ (gas)> 1 (first-type) and $a_C \left( {gas} \right) > a_C \left( {M_Z C/M} \right)$ ; 1 (second-type) should be satisfied. Possible regimes for first- and second-type metal dusting are discussed for pure Fe and Ni, and the range for first-type metal dusting of Ni is considerably broader than that for Fe.     

Der Einfluß von CO2 auf die Korrosionskinetik von Zink in Modellwässern

Influence of CO₂ on the corrosion kinetics of zinc in water Mechanism and kinetics of Zn and Zn/Fe corrosion in water containing various contents of CO₂ is studied by impedance spectra completed by gravimetric corrosion measurements. Electrode impedance is clearly found to be a function of P, the diffusion parameter is of essential importance. A_d as a function of P yields the same curves as the rest potential dependence on CO₂ and moreover the gravimetrically measured corrosion velocity rises linearly with CO₂. –Determination of corrosion velocity according to STERN-GEARY cannot be realized in the systems under investigation. –Experimental results lead to the conclusion that the main corrosion reaction takes place at the metal/layer-phase. The corrosion mechanism is discussed.