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.     

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 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.     

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.     

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.     

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.     

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.     

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.     

Korrosionsverhalten feuerverzinkter Baustähle in überwiegend sandhaltigen Böden

Corrosion behaviour of galvanized steel in mainly sandy grounds In mainly sandy grounds with different portions of fine parts < 0,06 mm (2,5–20,4%) specimens of ungalvanized and galvanized steel were stored outside and in the laboratory. Besides the composition of the ground, the salinity and the temperature of the ground were varied too. The corrosion rate and, for ungalvanized specimens, the behaviour to pitting corrosion were determined. Furthermore the factors characterising the corrosion behaviour such as specific resistance of soil and corrosion potential were investigated continuously. The loss in weight of metal was much greater for ungalvanized than for galvanized specimens and increased for ungalvanized specimens with an increasing portion of fine parts in the ground. Additions of salt at the beginning of the tests produced an increased amount of metal wastage, but for galvanized specimens they only had an influence upon initial corrosion. The increased removals of material started since contents of 3 · 10^?3 MolCl^? + SO/kg. If salts were added to the ground after 2 years (after the formation of a surface layer), they increased the wastage of material for ungalvanized but not for galvanized specimens. Apart from ungalvanized bars in the soil with a fineness portion of 20,4%, corrosion, after an acceleration at the beginning, slowed down owing to the formation of a surface layer. Ungalvanized specimens were attacked by a strong pitting corrosion and that more in aerated than in dense and, thus, water-containing grounds. The additions of salt accelerate more an uniform corrosion of material than a pitting corrosion. As for galvanized specimens after a local removal of zinc under extreme conditions the steel base had been hardly corroded away. The parts free of zinc were protected cathodically by the still existing zinc. The corrosion of steel depends upon the temperature: by increasing the temperature from 4 to 20°C increases of corrosion up to 100% were stated. As for galvanized surfaces temperature has only a small influence upon corrosion.     

Influence of alkali,silicate, and sulfate content of carbonated concrete pore solution on mild steel corrosion behavior

The increase in the rebar corrosion rate due to the concrete carbonation is the major cause of reinforced concrete degradation. The aim of this study was to investigate the corrosion behavior of mild steel rebars in simulated carbonated concrete solution. For this purpose, thermodynamic calculations, electrochemical techniques, gravimetric measurements, and surface analyses were used. Thermodynamic investigations of the nature of the interstitial solution provides an estimation of the influence of sulfate ( ) and alkali (Na⁺, K⁺) content on carbonate alkalinity of the CO₂/H₂O open system (pCO₂ = 0.3 mbar). In this system, calcium‐silicate hydrates (C–S–H) remain thermodynamically unstable and amorphous silica controls silicate aqueous content at 100 ppm. Electrochemical results highlight a decrease in the corrosion rate with increasing carbonate alkalinity and the introduction of silicate. The introduction of sulfate at fixed carbonate alkalinity shows a dual effect: at high carbonate alkalinity, the corrosion rate is increased whereas at low carbonate alkalinity, corrosion rate is decreased. Those results are supported by surface analysis. Authors conclude that silicate and sulfate release from cement hydrates and fixation of alkali on carbonated hydrates are key parameters to estimate mild steel corrosion in carbonated concrete.     

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.     

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.     

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.     

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.     

Covercrete with hybrid functions – A novel approach to durable reinforced concrete structures

Due to the corrosion of steel in reinforced concrete structures, the concrete with low water–cement ratio (w/c), high cement content, and large cover thickness is conventionally used for prolonging the passivation period of steel. Obviously, this conventional approach to durable concrete structures is at the sacrifice of more CO₂ emission and natural resources through consuming higher amount of cement and more constituent materials, which is against sustainability. By placing an economically affordable conductive mesh made of carbon fiber or conductive polymer fiber in the near surface zone of concrete acting as anode we can build up a cathodic prevention system with intermittent low current density supplied by, e.g., the solar cells. In such a way, the aggressive negative ions such as Cl^?, , and can be stopped near the cathodic (steel) zone. Thus the reinforcement steel is prevented from corrosion even in the concrete with relatively high w/c and small cover thickness. This conductive mesh functions not only as electrode, but also as surface reinforcement to prevent concrete surface from cracking. Therefore, this new type of covercrete has hybrid functions. This paper presents the theoretical analysis of feasibility of this approach and discusses the potential durability problems and possible solutions to the potential problems.     

Eigenschaften von Elektrolyt-Filmen bei der atmosphärischen Korrosion

Properties of electrolyte films formed through atmospheric corrosion An investigation has been carried out into the composition of the electrolyte films which are formed on non-metallic materials (glass) as well as on metals (Cu, Zn, Fe) in an atmosphere containing SO₂. Fresh as well as pre-corroded specimen were used. It was found that the SO₂ absorbed in the solution is very rapidly oxidized into SO if the electrolyte film contains dissolved particles of the corrosion products. With 1 to 55 ppm SO₂ in the atmosphere, the change in the pH value of the electrolyte is but small and does not vary with the SO₂ partial pressure.     

The corrosion behaviour in organic acid solutions—II. A steel electrode

The corrosion behaviour of steel in stagnant de-aerated citric and oxalic acid solutions (10^?5–10^?1M) in the pH range 2–6 was studied. It was found that the corrosion behaviour of steel in both acids was generally the same but it depended on the acid concentration and the pH value.In 10^?4–10^?1M, the corrosion rate (W) increased with the increase of acid concentration and decreased with the increase of pH value as follows:

$\text{log}\text{W = a+b}\text{log}\text{C, where b=0.485 and}\text{d}\text{log}\text{W}\text{d}\text{pH}\text{=?0.15?0.25.}$

At high concentration (10^?1 and 10^?2) and in the pH range 2–5, the steady state potential varied by 64 and 58 mV for a unit change of pH in oxalic and citric, respectively. The corrosion process was found to be cathodically controlled.In dilute solutions (10^?5–10^?3M) in the pH range of 2–5 and in all acid concentrations at pH 6, the steady state corrosion potential shifted in the negative direction with increase of acid concentration accompanied by an increase in the corrosion rate, indicating that the corrosion process became anodically controlled.     

Über das Verhalten einiger CrNiMo-Stähle beim Einsatz im chemischen Betrieb

The behaviour of some CrNiMo steels in use at chemical plants A report is given about the behaviour of some highly alloyed CrNiMo steels in use in inorganic-chemical plants. The observations are supported and supplemented by results of potentiostatic tests. In the presence of mixed acids, the corrosion resistance of the steels greatly depends, e.g., on the SO: Cl^? ion ratio. In the presence of Cl^? ions and at higher temperatures, exceeding about 70°C, the resistance is largely influenced by the specifically orientated analytical and structural pattern of the steels. Attention is drawn to the detrimental influence, especially in cast metals and welds, of the concomitant element, silicon, which — if encountered in increasing quantitities — favours the segregation of several corrosion-promoting phases. Examinations of case of damage in practical operation, supported by potentiostatic tests with CrNiMo steels, with and without copper contents, have shown that the presence of copper is apt to reduce the corrosion resistance in media containing hydrochloric acid or chlorine ions. On the other hand, a copper content may be beneficial in sulphate solutions free from chlorine ions.     

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.