Beitrag zur Wachstumskinetik der interkristallinen Korrosion von ausgehärteten Al-Cu-Legierungen – Teil I: Ergebnisse der Folien-Durchbruchsmethode

Contribution to the growth kinetics of the intergranular corrosion of age-hardened Al-Cu alloys – Part I: Results of the foil penetration technique The growth kinetics of the intergranular corrosion (IC) of age-hardened Al-Cu-alloys in aqueous chloride solutions under potentiostatic conditions have been investigated using the foil penetration technique originally developed for pit growth measurements. The two tested systems were a pure binary Al-4% Cu-alloy (sheet thicknesses 0.2 and 0.5 mm) in 0.01 m NaCl, pH 11, and a commercial AA 2024 type alloy (various sheet thicknesses ranging from 0.2 to 1.0 mm) in 0.1 m NaCl, pH 7, respectively. Both alloys were tempered to maximum IC susceptibility and have been tested at potentials where selective anodic dissolution of the grain boundary regions occurs. As a reference system, the pit growth kinetics in commercial pure aluminum (sheet thicknesses ranging from 0.2 to 1.0 mm) in 0.01 m NaCl + 0.01 m Na₂SO₄ (as pit initiation inhibitor), pH 11, have been investigated at an applied potential considerably higher than the pitting potential. The experimental results have shown that, for the IC in the finegrained commercial alloy, a uniform growth kinetic valid for the whole range of sheet thicknesses, as for the case of pitting, cannot be formulated. The correlation of the current-time-curves and the attack morphologies (after penetration) for different sheet thicknesses with the penetration times leads to the conclusion that the growth kinetics of intergranular attacks are related to the number of sites of active attack (cracks) per metal volume. This specific number of actively growing cracks depends on the grain size and on the electrochemical conditions and, for given parameters, on the exposure time and therefore on the sheet thickness. For the commercial alloy examined, the following three stages of attack with decreasing penetration velocity of the IC could be distinguished:

• activation stage
• transition stage
• stable, macroscopic grain boundary dissolution.

The coarse-grained pure binary alloy showed a markedly higher penetration velocity of the IC under milder electrochemical conditions. This alloy is suitable for a model investigation of the first stage of attack but no quantitative kinetic information could be obtained from the only two disposable sheet thicknesses. The pit growth measurements in pure aluminum showed that the square root-of-time growth law previously found for thin foils and sheets is valid for deep pits too. The discussion explains that the electrochemical mechanisms of pitting of aluminum and of the IC in Al-Cu-based alloys are identical and that the basic difference lies in the geometry of the sites of attack. The ohmic control of the aluminum dissolution and therefore of the growth kinetics of pits and intergranular cracks is governed by the total anodic area and its different time dependence during the growth of the sites of local attack.     

Über den Einfluß von Schwefelwasserstoff auf die Korrosion des Eisens in sauren Lösungen

The influence of hydrogen sulphide on the corrosion of iron in acid solutions A considerable change of the polarisation behaviour of iron electrodes in acidic perchlorate and sulphate solutions was observed if they where saturated with hydrogen sulfide gas. H₂S markedly increased the anodic current densities and increases the corrosion rates by a factor of ten. The anodic Tafel slopes d log i_A/dE are only about half as steep as those in the absence of hydrogen sulphide. It is probable that the SH^?-ions behave as catalysts of the iron dissolution reaction in a similar manner as the OH^?-ions, but are adsorbed almost to saturation.     

Untersuchungen Zur Inhibition Der Lochfraßkorrosion Von Nickel Und Stahl Mit Benzyl-n-propylsulfid

Measurements of pitting potentials of nickel single crystals and stainless steel in solutions with different concentrations of NaCl, pH 2–7, in the presence and absence of benzyl-n-propylsulphide (BPS) by potentiostatic polarisation measurements are described.Amperostatic polarisation curves in the cathodic and anodic active range have been measured in 0·5 m NaCl.The decomposition of BPS was studied by use of iodometrical measurements of SH-groups. The adsorption of BPS³⁵ has been investigated with the aid of tracer techniques.The results show an inhibiting effect on pitting corrosion at conditions allowing the formation of SH^? and BPSH⁺-ions, indicating secondary processes to play the most important role in inhibiting mechanism. Additions of Na₂S lead to increasing efficiency.     

Über den Einfluß der Strömungsgeschwindigkeit und der NaCl-Konzentration auf die Sauerstoffkorrosion unlegierten Stahls in Wässern

Influence of flow velocity and sodium chloride concentration on the oxygen corrosion of unalloyed steels in water Corrosion of mild steel in aerated water normally leads to pitting, and to an enrichment of anions within the pits. Even HCO₃^? ions can support localized corrosion. The intensity of this is remarkably increased by chloride ions. In equilibrium water with Ca²⁺ and HCO₃^? ions and about 40 ppm free CO₂ protective rust layers are formed after about 500 hs exposure not depending on flow rate and salt concentration. These layers decrease the mean corrosion rate to about 0.1 mm/a, but do not prevent localized corrosion. Protective passive layers with extremely low corrosion rates are formed in flowing water with Ca²⁺ and HCO₃^? ions and about 40 ppm free CO₂. The minimum flow rate can be estimated for a 3/4 inch pipe to lie between 0.35 and 1.5 m/s. Investigations with water containing 40 ppm free CO₂ without Ca²⁺ at pH 4.7 and with NaHCO₃ (pH 7.2) show at low pH high corrosion rates – as expected –, and at pH 7.2 formation of protective layers with poor reproducibility in comparison with the situation in equilibrium water. Thus, Ca²⁺ ions support the protective properties of rust layers. HCO₃^? ions are necessary for the formation of protective layers because these are not formed in pure NaCl solutions.     

Elektrochemische Prüfung der Wirksamkeit von Heizöl-Inhibitoren

Electrochemical testing of the effectiveness of fuel oil inhibitors The potential/time and current density/potential curves for unalloyed steel were obtained in NaCl solutions with different concentrations of a certain type of inhibitor. The results show that the inhibitor acts as a passivator. A comparison with the results of conventional static tests shows that even a 99 pC inhibiting effect against uniform corrosion does not constitute an effective protection against pit corrosion. An effective protection is only obtained if the steel specimen can, like a platinum electrode, be polarized up to oxygen formation without any penetration of the Cl^?-ions.     

Einfluß der Hydrodynamik auf die Korrosion von Eisen und Stahl in neutralen belüfteten NaCl- und Na2SO4-Lösungen

Influence of hydrodynamics on the corrosion of iron and steel in neutral aerated NaCl and Na₂SO₄ solutions The corrosion behaviour of iron and steel in aerated neutral NaCl and Na₂SO₄ solutions is mainly determined by the transport-controlled cathodic oxygen reduction. Its reaction zone depends on the physical and chemical properties of the 3-D layer formed on the corroding surface. The influence of hydrodynamics is restricted to the transport of the dissolved oxygen to the reaction zone. The obtained results are independent of laminar or turbulent flow conditions.     

Inhibitoren der Korrosion 25(1) – Über die Rolle der Kationen in Neutralsalzen bei der Korrosion von Eisenwerkstoffen in Gegenwart von Sauerstoff

Corrosion inhibitors 25(1) – The role of the cation in a neutral salt in the corrosion of iron in the presence of oxygen The rate of uptake of oxygen, under standard conditions, in the corrosion of 500 mg DAB6 iron powder is dependent on the nature of the cation in a neutral salt (chloride). In the presence of alkali, quaternary ammonium, quaternary phosphonium and alkali-earth chlorides, the rate of oxygen uptake is comparable. With zinc, cadmium, nickel and cobalt chlorides a marked decrease in the oxygen uptake is observed between pH5 and pH7. This is probably due to a block in the cathodic area in the iron boundary layer through precipitation of the rather insoluble basic hydroxides of zinc, cadmium, nickel and cobalt.     

The effect of molybdate anion on the ion-selectivity of hydrous ferric oxide films in chloride solutions

The ion-selective property of hydrous ferric oxide precipitate films has been investigated by measuring membrane potentials which arise across precipitate membranes of hydrous ferric oxide with and without adsorbed MoO₄^2? ions and of ferric molybdate in solutions of NaCl, KCl, MgCl₂, CaCl₂, BaCl₂, AlCl₃, and FeCl₃. The hydrous ferric oxide membrane was only permeable to Cl^? ions in chloride solutions, whereas the membrane with adsorbed MoO₄^2? ions was permeable to cations in NaCl and KCl solutions, and to both Cl^? and cations in the presence of multivalent cations. The ferric molybdate membrane was permeable to Cl^? and cations in NaCl and KCl solutions, and only to Cl^? ions in the presence of multivalent cations. It is suggested that in chloride solutions, the corrosion of iron covered with a precipitate film of hydrous ferric oxide is accelerated by enrichment of Cl^? ions under the film, which may decrease the local pH and introduce a positive diffusion potential in the film. The adsorption of MoO₄^2? ions on the oxide changes the ion-selectivity of the precipitate film from the anion-selective to the cation-selective in solutions of NaCl and KCl. This cation-selectivity of the film may inhibit the corrosion of iron, because of H⁺ ions diffusing out of the film. The inhibitive effect of MoO₄^2? ions would be reduced in the presence of multivalent cations.     

Die Ermittlung der Beständigkeit gegen Spannungsrißkorrosion (SpRK) von hochlegierten Sonderstählen in chloridhaltigen wäßrigen Medien

Determination of resistance to stress corrosion cracking (SCC) of high-alloy special steels in chloride-containing aqueous media The 18 Cr 10 Ni(Mo) based stainless steels have been continually improved by raising the Cr, Ni and Mo contents. The behavior of these high-alloy steels towards SCC was determined in test media generally used in practice since the question of the resistance to stress corrosion cracking (SCC) had still remained unanswered to a large extent. SCC tests on U-bend samples in boiling 62% CaCl₂ solution showed a good differentiation depending on the Ni and Mo contents. With increasing Ni content, the susceptibility of special high-alloy steels to SCC is shifted towards longer service lives, alloys containing ≧ 42% by weight of Ni being resistant. High-Mo special alloy steels are more resistant to SCC than low-Mo special alloy steels. These results could be confirmed by tests carried out on circular cross section samples in boiling 62% CaCl₂ solution under constant load and potentiostatic control. The free corrosion potentials recorded for 25% Ni special alloy steel and Ni-based alloys are within the potentiostatically determined range of insusceptibility to SCC. The high-Mo special alloy steel X 2 NiCrMoCu 25 20 6 (1.4529) shows the same critical SCC potential on the anodic side as the Ni alloy NiCr21 Mo (2.4858). Superferrit X 1 CrNiMoNb 28 4 2 (1.4575) and austenitic ferritic steel X 2 CrNiMoN 22 5 (1.4462) showed that the SCC behavior was unsatisfactory in both tests as in the case of steel X 10 CrNiMoTi 18 10 (1.4571). Tests in boiling 4 m NaCl showed no SCC, not even under the aggrevated test conditions in the test set-up. The great influence of the oxygen content was demonstrated in tests carried out in the autoclave with defined oxygen and chloride concentrations. The resistance of the steels to SCC decreases under air-saturated conditions (8 … 10 ppm O₂) whereas the chloride concentration (200 and 2000 ppm Cl^?) does not exercise an important influence. U-bend samples should be given preference to Erichsen samples for SCC tests. SCC break characteristics could be determined metallographically and by scanning electron microscope.     

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.     

Beitrag zur Wachstumskinetik der interkristallinen Korrosion von ausgehärteten Al?Cu-Legierungen. Teil II: Der Einfluß mechanischer Spannungen

Contribution to the growth kinetics of the intergranular corrosion of age-hardened Al? Cu alloys. Part II: Influence of mechanical stress The influence of constant uniaxial tensile stresses on the growth kinetics of the intergranular corrosion (IC) of age-hardened Al? Cu-alloys in aqueous chloride solutions under potentiostatic conditions has been investigated using the foil penetration technique. Two systems have been tested, a pure binary Al-4% Cu-alloy (sheet thickness 0.5 mm) in 0.01 m NaCl, pH 11, and a commercial AA 2024 type alloy (various sheet thicknesses from 0.2 to 1.0 mm) in 0.1 m NaCl, pH 7, respectively. Both alloys were tempered to maximum IC susceptibility and have been tested at potentials where selective anodic dissolution of the grain boundary regions occurs. For comparison, the influence of tensile stresses on the pit growth kinetics in commercial pure aluminum (sheet thicknesses ranging from 0.2 to 1.0 mm) in 0.01 m NaCl + 0.01 m Na₂SO₄ (as pit initiation inhibitor), pH 11, has been investigated at an applied potential considerably higher than the pitting potential. All measurements have been carried out under various tensile stress levels ranging from 25 to 88% of the 0.2% proof stress (R_p0.2). The results show a significant reduction of the penetration times by a factor of 2 to 10 for all applied stress levels when only a few parallel cracks are growing (coarse-grained pure binary alloy and thin sheets of the fine-grained commercial alloy). For systems with large numbers of cracks (thicker sheets of the fine-grained alloy), no influence of the tensile stresses on the growth kinetics of the IC could be found. The penetration times for pits growing in pure aluminium have been reduced by a factor of 1.2 to 1.7 in presence of tensile stresses. The basic assumption for the discussion of the results is that even the highest mean crack growth velocities observed in this work can be expained by pure anodic dissolution of the grain boundary regions at the crack tips. The increase of the dissolution current density at the crack tips is due to the mechanical widening of the cracks. This widening of the cracks may cause a change of the crack geometry and, therefore, of the integral electrical resistance of the system (primary current distribution). Additionally, the mass transport conditions between the cracks and the bulk electrolyte may als be improved (changes of the composition of the electrolyte and, therefore, of the ohmic potential drop inside the cracks). The widening of the cracks depends on the mechanical stress distribution in the whole specimen and not only on the stress intensity at the crack tips. This allows to explain why tensile stress does not have any influence when a large number of parallel cracks are present (mechanical stress relief). The comparatively small increase of pit growth velocity is based on the weak influence of tensile stresses on the geometry of circular pits.     

Zur Thermodynamik des Systems Mangan/wäßrige Lösung in der Abwesenheit und Anwesenheit von gelöstem Sauerstoff

Contribution to the thermodynamics of the system Mn/aqueous solution in the absence and presence of dissolved oxygen Using the thermodynamical criterion of the priority of certain reactions within a given system of possible reactions the authors consider the stability ranges of the different possible oxidation products of Mn in aqueous solutions. According to this study there is a limit at pH 8.5: below that value Mn²⁺-ions predominate, while above that value Mn(OH)₂ is the predominating substance. Under the influence of the oxygen dissolved in the water these primary oxidation products are transformed into products of increasing insolubility, the formation priorities of these products being different in alkaline and in acid media.     

Über die Inhibitorwirkung der Fettsäuren bei der Korrosion des Eisens in saurer Lösung

Inhibiting effect of fatty acids on the corrosion of iron in acid media Fatty acids (C₁–C₁₂) in acid media (0.01 M HClO₄ + 0.2 M NaClO₄) have partially inhibiting and partially stimulating effects on the corrosion of iron. Short chain acids (C₁ to C₇) in concentrations below 5·10^?5 M act as stimulators, in higher concentrations as inhibitors. Beyond C₈ the stimulating effect gradually disappears. As to the inhibition mechanism the fatty acids are of the mixed type (though the anodic control predominates); they are not chemisorbed but only physically adsorbed by van der Waals forces and become desorbed when the temperature is increased.     

Über den Einfluß des Sauerstoffgehaltes,des pH all und der Strömungsgeschwindigkeit auf die Korrosion feuerverzinkter und unverzinkter unlegierter Stahlrohre in Trinkwasser

Influence of oxygen content, pH and flow velocity under corrosion of hot-dip galvanized and bare piping of unalloyed steel in potable water Sections and weighed ring specimens from galvanized and nongalvanized tubes had been exposed to flowing tap water with different flow rates and oxygen concentrations of 0.01–4 mg/l at pH 7.2 and 8.0 for 3.5 ys. Specimens taken at different testing times had been investigated with respect to data of protective layers, corrosion manifestation and weight loss. The corrosion rate is nearly constant in the first weeks (initial corrosion), later it follows a power law (stationary corrosion). These two phases of corrosion are determined by the testing parameters differently. Thus, results from short-time experiments of some weeks cannot inform on the stationary behaviour. In oxygen free tap water remarkable corrosion occurs due to the acids H₂CO₃ and HCO₃^?. The corrosion resistance is supported by high oxygen content, high pH, flow rate and by galvanizing.     

Das anodische Verhalten des Thallium in alkalischen Elektrolyten

The anodic behaviour of thallium in alcaline electrolytes

• 1 Different surface layers are formed by anodic polarization of thallium in sodium hydroxide.
• 2 Electron-optical and X-ray examination have shown that the primary layer is formed Predominantly TlOH, the secondary layer of Tl₂O₃, and the tertiary layer which is obtained under stationary conditions consists essentially of a mixture of Tl₂O and Tl₂O₃.
• 3 The formation and reduction potentials of the Tl₂O₃, layer differ by 500 mV; this difference can be attributed to resistance polarization.
• 4 The quasi-stationary current-potential curve shows, that the reduction of Tl₂O₃ yields a monovalent thallium compound via a short-lived intermediate product.
• 5 The Tl₂O₃, layer has an electron conductance of 1.5 Ω^?l·cm^?1, and a ion conductance of 1.3· 10^?6Ω^?1·cm^?1.

     

Inhibitoren der Korrosion 6 (1) Vergleich der Schutzwerte einiger phosphor-, schwefel- und arsen-organischer Verbindungen gegenüber Eisenproben unterschiedlicher spezifischer Oberfläche

Corrosion inhibitors 6: Comparison of the protective values of some phosphorus, sulfur and arsenium organic compounds with respect to iron specimens of various specific surfaces Iron wire and carbonyl iron powder differ with respect to (1) composition (carbonyl iron powder is purer than iron wire 2) specific surface (0.5 grs of carbonyl iron powder have 23 times the surface of the iron wire of 3 mrs length used so far). The protective values of some representative phosphorus, sulfur and arsenium organic compounds are different with the two test materials; as a general rule they are lower at lower inhibitor concentrations with carbonyl iron than with iron wire. Exceptions are the biphosphonium salts tested. The protective action on iron is ascribed to a secundary inhibition. The molar concentrations required to obtain 50, 80 and 9O% protection are reported for 31 inhibitors with reference to 0.5 grs carbonyl iron in 250 ml HC1.     

Inhibitoren der Korrosion 30 (1,2) – Vergleichende Studien über das Verhalten bekannter und bisher unbekannter Inhibitoren der Korrosion des Kupfers unter Standardbedingungen (Sauerstoff,NaCl, pH 4,1, 22°C)

Corrosion Inhibitors 30 (1.2)-Comparative Studies on the Behaviour of Known and Unknown Corrosion Inhibitors of Copper Using Standard Conditions (Oxygen, NaCl, pH 4.1, 22°C) The following compounds were tested as inhibitors for the corrosion of copper samples under controlled (standard) conditions: benzotriazole (BTA); 5-aminotetrazole monohydrate (5-AT); 2,5-diphenyl-3-(4-chlorophenyl)-tetrazolium nitrate (Tetrazin-B); α- and β-naphthylamine; phenylthiourea; xanthanehydride; 2-mercaptothiazoline (2-MT) and cupferron (N-phenyl-N-nitrosohydroxylamine as its ammonium salt). The following measurements were applied: determination of the rate of O₂-uptake in agitation experiments and monitoring of the course of corrosion by the pH-stat method with variable: (a) inhibitor concentration; (b) structure of the copper sample (granules, dust, plate); (c) presence of added Cu²⁺-ions and (d) pH. Further information on the composition and structure of the surface coatings responsible for the inhibition was gathered from mass spectra, rest-potential/time and current/voltage curves and measurements of the polarisation resistance.     

Simulation atmosphärischer Korrosion durch dünne Elektrolytfilme

Simulation of atmospheric corrosion by thin films of electrolyte A method for investigation on atmospherical corrosion with the aid of thin electrolyte films is described, the special feature of this being the exact adjustability of the corrosion determining parameters. The investigations on the influence of pollutants showed that small additions of sodium chloride and sulphur dioxide into the water film cause two different types of corrosion of electrolytic copper: uniform growth of layer thickness and/or lateral surface growth of the corrosion products. In the initial phase the uniform growth of layer thickness on electrolytic copper depends on t² (t = time), later on proportional on t. The investigation of the corrosion in dependence on the thickness of electrolyte film showed new results regarding the transition of the bulk-electrolytical to thin film electrolyte corrosion (atmospheric corrosion). At electrolytic copper transition from uniform growth of layer thickness to lateral surface growth comes off, when reaching a film thickness of approx. 100 μm. The electrolyte was distilled water with an addition of 0,01%SO₂. The corrosion intensity of grey cast iron GG 25 under a film of pure water passes through a wide maximum of film thickness between 300 and 20 μm, with a weakly marked minimum on 100 μm, the corrosion intensity is decreasing at thinner films. It is herewith demonstrated that also under conditions excluding largely the convective oxygen transfer other corrosion types and -intensities show up when the films are only thin enough. The comparison of the effect of two vapour inhibitors, having been dissolved directly in the electrolyte films (dicyclohexyl-ammoniumnitrite and one usual in the trade of unknown composition) in different concentrations showed on grey cast iron GG 25 the superiority of the latter one.     

Influence of Bath Compositions and Some Operating Conditions on the Electroplating of Cobalt from Aqueous Sulphate Baths

A systematic study has been made of the influence of aqueous sulphate bath constituents on the cathodic polarization curves during cobalt electrodeposition. The optimum bath composition has been established and it contains: 0.30 mol l^?1 CoSO₄.7 H₃0, 0.30 mol^?1 H₃BO₃. 0.05 mol l^?1 (NH₄)₂SO₄ and 0.10 mole l^?1 Na₂SO₄.10 H₂o. Cobalt electroplating from the optimized bath has been performed under a variety of conditions viz: pH, 1.5 to 5.3; current density, 0.2 to 2.0 A dm^?2 and temperature 25 to 53°C. Under the most suitable conditions, the optimum bath is characterized by a high cathodic efficiency (~ 99%), moderate throwing power (21.6%) and throwing index (1.47). The bath produced smooth, adherent and semibright plates characterized by a high microhardness (411.4 kgf mm^?2) and their morphologies, as revealed by scanning electron microscopy, showed a dependence on the plating variables.     

Revised pourbaix diagrams for iron at 25–300 °C

The Pourbaix diagrams, potential pH diagrams, for iron at 25–300 °C have been revised. The selection of thermochemical data has taken into account the experimental solubility of magnetite at T = 100–300 °C. Temperature extrapolations of thermochemical data for aqueous species have been performed with the revised Helgeson-Kirkham-Flower model, which also allows uncharged aqueous complexes to be handled. The Pourbaix diagrams show a stability region for ferrous hydroxide, Fe(OH)₂(cr), at 10⁻⁶m and T ≤ 85 °C, implying that the Schikorr reaction is not thermodynamically possible above this temperature. A corrosion region between iron and magnetite exists at all temperatures in high purity water (10^∞ m), due to the hydrolysis products of iron(II).