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.     

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).

     

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.     

Effect of Si on the Corrosion behaviour of Fe-B and Fe-Ni-B glassy alloys

The effect of substituting Si for B on the electrochemical behaviour of the glassy alloy families Fe-B and Fe-Ni-B has been investigated in solutions with pH ranging from 4 up to 8.4. The influence of the aggressive ions (Cl^? and SOu) has also been studied. AES and XPS have been used to further characterize the alloys in order to interpret their behaviour in the various solutions.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Qualifizierung von hochlegierten Sonderedelstählen und Nickelbasislegierungen für den Transport von wassergefährdenden Stoffen und Abfallmedien

Qualification of high-alloy special stainless steels and nickel base alloys for the transportation of hazardous water polluting substances and waste media

1 Vortrag anläßlich der ACHEMA '94 5.–11. Juni 1994, Frankfurt/Main

High alloy special stainless steels and nickel base alloys have for the transportation of hazardous water polluting substances and waste media a wide range of applications. This has been proved in laboratory tests for the materials Cronifer 1925 hMo (1.4529), Nicrofer 3127 hMo (1.4562) and Nicrofer 5923 hMo (2.4605) as well as, in the case of Cronifer 1925 hMo, via evaluation of data from literature. Cronifer 1925 hMo possesses excellent resistance to halide containing solutions as well as to a large number of acids over a wide range of concentrations. While the advantages of Nicrofer 3127 hMo over Cronifer 1925 hMo were evident in only a few of the media under investigation, the corrosion resistance of Nicrofer 5923 hMo was clearly superior. Based on the evaluated literature data and the data of laboratory tests Cronifer 1925 hMo has been qualified as tank material for the transportation of over 1000 media.     

Inhibitoren der Korrosion 17(1) Die Bedeutung des „Hart-Weich-Konzepts”︁ von Pearson für die Erkennung von Struktur-Wirkungsbeziehungen von Inhibitoren bei der Korrosion des Eisens in NaCl-Lösungen bei Gegenwart von Sauerstoff

Corrosion inhibitors 17 The importance of pearson's “hard-soft-concept” with respect to the recongnition of relationship between structure and activity of inhibitors of the corrosion of iron in sodium chloride solutions in the presence of oxygen On the base of a previously published postulate according to which Pearson's hard-soft-concept may form the base for finding inhibitors for specific applications it is shown with the aid of model inhibitors with “hard” functional groups that it is indeed feasible to derive defined relations between the structure and the activity of inhibitors. The hard groups used were the phosphoryl ?P(O)O and the carbonyl ?C?O groups. It has been shown that such compounds are able to form very stable and protective surface layers on iron and that the efficiency of the inhibitors clearly increases with increasing length of the aliphatic chain. As has been shown by eletrochemical measurements these acids have a bearing on the rest potential as well as on the current-potential behaviour; this fact points to rather variable compositions of the surface layer. In the case of polyfunctional acids sterical factors may have a bearing on inhibitor efficiency, too.     

Oxidation of tungstate‐containing green rust (Cl–) in aqueous solution

The influence of tungstate on the oxidation of green rust [GR(Cl^–)], which contains both Fe(II) and Fe(III), was investigated by synthesizing suspensions of GR(Cl^–) containing tungstate and oxidizing them via injection of N₂ gas containing O₂. XRD and TEM analyses were used for characterizing the solid particles formed during synthesis and oxidation. The results showed that the formation of fine α‐FeOOH was enhanced by the addition of tungstate to the GR(Cl^–) suspensions, while GR(Cl^–) without tungstate was transformed primarily into γ‐FeOOH. The pH, oxidation‐reduction potential (ORP), and dissolved oxygen (DO) values of the aqueous solution were measured during oxidation of GR(Cl^–) with and without tungstate. The results showed that whereas the pH value of the solution was decreased and the ORP value was increased monotonically by oxidation of GR(Cl^–), the pH and ORP values during oxidation the GR(Cl^–) suspension containing tungstate revealed characteristic changes with time. XAS was also used for characterizing the chemical state and local structure of tungstate in the oxidized particles. The results indicated that the local structure of WO was essentially retained in the particles precipitated from GR(Cl^–) suspensions.     

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.     

Adsorption studies of water on copper,nickel, and iron using the quartz-crystal microbalance technique: Assessment of BET and FHH models of adsorption

In the atmospheric corrosion of copper, nickel, and iron, the adsorption of water affects the corrosion rates. Knowledge of water adsorption and metal oxyhydroxide formation is important in understanding the atmospheric corrosion process. The purposes of the present research were (1) to measure the adsorption of water on metal surfaces as a function of temperature and relative humidity (RH) and (2) to assess Brunauer-Emmett-Teller (BET) model and Frenkel-Halsey-Hill (FHH) model for water adsorption. In the present research, the quartz-crystal microbalance (QCM) technique was used to measure the mass changes of copper, nickel, and iron at 0 to 100% RH and 7–90°C under nitrogen environments. Less water was adsorbed on copper, nickel, and iron which formed oxides than on gold. BET and FHH models could not fit the data points with single functional relationships. ΔH values were calculated using modified BET method and they decreased with temperature.     

Electrochemical studies of Na2CrO4-Na2So4 melts at 1200 K

The electrochemical response of Na₂SO₄-Na₂CrO₄ solutions at a Pt working electrode was established by cyclic voltammetry and chrono-potentiometry measurements at 1200 K. Experiments were made under conditions of controlled and electrochemically measured values of melt basicity for basic, neutral, and acidic melts. Despite the thermodynamic stability of chromite ions under reducing conditions, a direct conversion of chromate to chromite upon cathodic polarization was not observed. Rather, the cathodic reduction of chromate ions proceeds by a reversible one-electron charge transfer to an electroactive intermediate species formed through an equilibrium between CrO and oxide ions in the solution. An irreversible chemical reaction reduces the chromium solute species further to precipitate a solid. As a solute in a corrosive fused salt film, a chromate to chromite transition would not be expected to stabilize the oxidation state and basicity. However, chromate ions to provide a reduction reaction more favorable than sulfate reduction to sulfide; chromate reduction leads to the precipitation of solid NaCrO₂ or Cr₂O₃ in basic or neutral solutions, respectively. A large shift in melt basicity can only occur for mildly acidic solutions with high chromate solute.     

Influence of the sulphate reducing bacteria on API‐X70 steel corrosion

The corrosion behaviour of API X70 immersed in a specific medium with a strain of thermophilic sulphate reducing bacteria (SRB) was analysed. Anaerobic corrosion test was carry out for 32 days at 50 °C. During the exposure time, pH, sulphate (SO) and hydrogen sulphide (H₂S) concentration were measured. Corrosion potential, linear polarization resistance and potentiodynamic polarization curve were used in order to get the influence of the SRB in the corrosion phenomenon. Scanning electron microscopy was used to determine corrosion morphology. Results show that the SRB activity influenced the overall corrosion process. The anodic branches in the polarization curves show a passivity feature, whereas, the cathodic branches were not affected. A localized corrosion attack was found.     

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.     

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.