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.     

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.     

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.     

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.     

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      

Counteraction of pitting in copper water pipes by bicarbonate dosing

Copper pipes are extensively used for tap water installations and generally perform well. Exceptionally, however, copper pipes are perforated due to pitting. Three main types of pitting (I, II and III) have been identified, but as for the causes and the mechanisms these have not yet been fully clarified. Through case studies, model experiments, thermodynamic calculations, and service tests, evidence has been obtained that waters having a pitting propensity for copper pipes can be made less corrosive by an increase of the HCO content. On water treatment the following water composition should be aimed at:

• – a pH value of at least 7
• – an HCO content of at least 70 mg/l, preferably 100 mg/l
• – as low SO content as possible, or at least lower than the HCO content (both in mg/l).

     

Evaluation of localized corrosion phenomena with electrochemical impedance spectroscopy (EIS) and electrochemical noise analysis (ANA)

The use of electrochemical impedance spectroscopy (EIS) and electrochemical noise analysis (ENA) for non-destructive evaluation of corrosion processes is illustrated for three model systems. EIS can be used to detect and monitor localized corrosion of Al alloys and determine pit growth laws which can be used for lifetime prediction purposes. Electrochemical potential and current noise data can be analyzed in the time and the frequency domain. A comparison of noise data obtained for Pt and an Al 2009/SiC metal matrix composite (MMC) exposed to 0.5 N NaCl has shown that the use of potential noise data alone can lead to erroneous conclusions concerning corrosion kinetics and mechanisms. The electrochemical noise data have been evaluated using power spectral density (PSD) plots in an attempt to obtain mechanistic information. The system Fe/NaCl has been used to determine the relationship between the polarization resistance R_p obtained from EIS data and the noise resistance R_n determined by statistical analysis of potential and current noise data. Potential and current noise can be recorded simultaneously allowing construction of noise spectra from which the spectral noise resistance R can be obtained as the limit for zero frequency. Good agreement between R_P, R_n and R has been observed for iron exposed to NaCl solutions of different corrosivity. For polymer coated steel exposed to 0.5 N NaCl for five months analysis of EIS data allows to draw conclusions concerning the degree of disbonding of the coating and the decrease of the coating resistivity with exposure time. R_n and R obtained from electrochemical noise data for an alkyd coating on cold rolled steel agree with each other and show the same time dependence as R_p and the pore resistance R_po determined from EIS data, but are significantly lower than R_p and R_po. The relationships of derived noise parameters such as R_n and R to coating properties and to the remaining lifetime of a polymer coating are not clear at present.     

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.     

Korrosion und kathodischer Korrosionsschutz von unlegiertem Stahl in Sandböden

Corrosion and cathodic protection of unalloyed steel in sand soil Cathodic protection takes place if the potential criterion is satisfied. That means that the pipe-to-soil potential is less than the protection potential. This is to be controlled in the case of pipelines which are required to be inspected regularly by law. Protection potentials are described in technical standards (e.g. DIN 30676), and must be determined experimentally in doubtful cases. In general the protection potential is referred to as U = ?0.85 V for soils. In practical applications of this protection potential difficulties arise in the case of sand soils with a minute amount of salt and a high resistivity resulting from a very low humidity. These difficulties are related to a high protection current demand due to good aeration and a poor current distribution due to the high resistivity. On the other hand, sand soils are known to be less corrosive. Thus, one can think of using special protection potentials for sand soils which are more positive than ?0.85 V. The results of laboratory and field tests show that protection potentials in the range of ?0.75 V to ?0.65 V can be applied depending on soil resistivity. In these cases even micro cell action can be prevented.     

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.     

Electrochemical behaviour and hydride embrittlement evaluation of titanium during cathodic polarization in HNO3/UO/N2H solutions

In conjunction with reprocessing of nuclear fuels by electrochemical methods, the general electrochemical behaviour of titanium cathodes and the problem of their hydride embrittlement have been investigated in the absence of γ-radiation in HNO₃/UO/N₂H solutions of varying composition. The electrode potential of galvanostatically polarized titanium cathodes is shown to be located either in the passive range, within the passive-to-active transition, or in the regime of hydrogen absorption/hydride formation, depending on the current yield of the nitrate reduction reaction. Hydride growth was observed predominantly in the morphology of continuous layers and/or isolated platelets. Following a parabolic rate law it was restricted to the vicinity of the electrode surface, the maximum penetration for polarization times up to 1000 h being less than 100 μm. As a consequence, no significant loss of ductility of normal tensile specimens could be detected under these conditions.     

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.     

Fireside corrosion in dutch waste incinerations

The total installed capacity in the (eleven) Dutch waste incineration plants is 3.6 × 10⁶ t/y. About 70% of the waste is household, the remainder industrial and coarse waste. From the view point of fireside corrosion the furnace atmosphere may be characterized as follows: flue gas temperature 800–1000 °C, excess ait 50% P_HCl 10^?3–10^?5 bar, P 10^?4 bar. Two case histories concerning superheater failures are described. It has been found that fire side corrosion failures do occur in Dutch waste incinerators but they account for only part of the unforeseen plant unavailability. Improvements can be achieved by optimizing burning conditions, slag transport, gratings and, if necessary, by installation of more corrosion-resistant tube materials in (certain parts of) the boiler.     

Evaluation of corrosion inhibitors for cooling water systems operating at high concentration cycles

The present work aimed at evaluating AISI 1020 carbon steel corrosion resistance of a 6:4:1:1 (MoO/HEDP/PO/Zn²⁺) inhibitor mixture present in a solution which simulates an industrial cooling water system operating at high concentration cycles (1050 ppm Cl⁻ and 450 ppm Ca²⁺). High concentration cycles are desirable, because system purge and treated water consumption are decreased. On the other hand, a high number of concentration cycles can increase the concentration of salts and dissolved impurities, causing corrosion, incrustations, and deposits inside the pipes, heat exchangers, and cooling towers. Thus, the chloride (Cl⁻) and calcium (Ca²⁺) ions aggressiveness was studied on the proposed inhibiting mixture, at the temperatures of 40 and 60 °C, through electrochemical techniques like open circuit potential measurements, anodic and cathodic polarization, and weight loss. The results showed that the inhibitor mixture conferred adequate protection to carbon steel in low concentrations, even in high aggressive media.     

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.     

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.     

Die Bedeutung der pH-abhängigen Durchbruchsreaktionen bei NiCrMo-Werkstoffen in Medien der Chemie- und Umwelttechnik mit hohem Redoxpotential

The importance of pH-dependent break-through reactions of NiCrMo-materials in media of chemical and environmental engineering with a high redox potential Compared to CrNiMo-steels NiCrMo-alloys show a specific property at relatively positive potentials. As a result of breakthrough reaction which is not connected to the presence of Cl^?-ions Ni-alloys show a stable and uniform dissolution in media with pH-values between 4 and 9, whereas steels were not attacked. This phenomenon was studied for steels 1.4563 (Nicrofer 3127 LC), 1.4562 (Nicrofer 3127 hMo) and Ni-alloys 2.4856 (Nicrofer 6020 hMo) and 2.4605 (Nicrofer 5923 hMo) in Cl^? -and SO-solutions in the pH-range of pH = 3–10. In the break-through reaction Cr and Mo dissolve 6-valently and Ni 2-valently. The break-through potential is the same in Cl^?- and SO-solutions and is hardly influenced by the Mo-content. The reaction rate increases with increasing Mo-content of the alloy. This material property of Ni-alloys has to be considered if they should be used in slightly acid to slightly alkaline media having a high redox potential.     

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.     

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.