Zur Korrosion des Hafnium – Oxide und Hydride

Corrosion of hafnium – oxides and hydrides Investigations concerning the growth and behaviour of hafnium as well as electrode kinetics have been performed on hafnium (Goodfellow and Material Research Company) in various electrolytes (acidic, alkaline and Cl^?-containing solutions). Conventional electrochemical methods and impedance spectroscopy were applied. Corrosion rates derived from electrochemical measurements were found to be significantly higher than the published data obtained from the loss weight observed usually over long of time. – The DK was evaluated to be 11.5. – SIMS profiles indicate that the oxide layer in not uniform, due to the nonstoichiometry of the Hf oxide at 22°C. The layer shows a very low n-conductivity – quasi insulator behaviour; the Mott-Schottky-plot leads to the flat band potential U = ? 10 mV. The initial oxide layer was determined by capacity measurements to 2.7 nm. Cl₂-evolution begins at U = 0.5 V in 0.5 M NaCl solution. In the cathodic potential region hydrogen is generated at Hf oxide at U = ? 1.69 ? 0.058 V. With respect to the hydrogen induced cracking of Hf the behaviour of hydrogen in Hf was investigated using the nuclear physics ¹⁵N-method. After cathodic polarisation hydrogen is absorbed by hafnium and the hydride HfH_1.6 is formed. The hydrogen diffusion coefficient is determined to D_20° = 3.8 × 10^?11 cm²s^?1. At 385°C hydrogen is released from HfH_1.6. The results are discussed and compared to those found for titanium and zirconium.     

Strom‐Potential‐korrelierte Rauschmessung (CorrElNoise) – Ein neues Verfahren zur elektrochemischen Rauschanalyse

Current‐potential correlated noise measurement (CorrElNoise) – a new technique for the evaluation of electrochemical noise analysis A new method for electrochemical noise measurements is presented. In contrast to most conventional noise measurement techniques which require an electrode arrangement of three “identical” electrodes, the new method is able to work with a two electrode set‐up. As a consequence, the recorded noise response of current and potential in this two electrode system is strongly correlated. This method is denoted as correlated noise measurement technique (CorrElNoise), therefore. It is shown by examples using aluminum as well as stainless steel electrode systems in corrosive environment that the CorrElNoise technique is more sensitive than the conventional method. This was checked by variation of particular experimental parameters during the noise measurements. Compared with the conventional set‐up, the resulting changes in the noise spectra become more clearly recognisable using CorrElNoise.     

Zur Frage des Mechanismus der Lochfraßkorrosion

The question of the pitting corrosion mechanism It is concluded from the time, concentration, temperature, PH and potential dependences of the corrosion current densities in the pits, that it is not justified always to suppose parallelity between the metal dissolution rates in the active pits and in the active region of the polarization curve. This is due to the fact, that in some cases diffusion is not the rate controlling factor of pitting corrosion, and that the metal dissolution in the pits and in the active states follows different mechanisms.     

EPR‐Messungen in warmgehenden Anlagen

EPR measurements in heat‐working plants This paper reports about experiences of sensitization behaviour of titanium and niob stabilized steels under practice conditions at operating temperature between 340 and 600°C. Pipes, flanges, welds including heat‐affected zones are examined for sensitization with help of the e lectrochemical‐p otentiodynamic‐r eactivation‐method. Measurements were carried out on‐site during plant stoppage at different pipes with diameter between 150 to 460 mm with a set‐up measuring cell.     

Untersuchung der Korrosionsneigung mit Hilfe des elektrochemischen Rauschens Teil 2: Meßtechnische Voraussetzungen

Examination of corrosion propensity by means of electrochemical noise Part 2: Requirements on measuring technique In this work the development of different electrochemical measuring instruments for the examination of the electrochemical noise for corrosion is presented. Furthermore a complete conception of a noise measuring setup is given. By means of different measurements the upper limits of the experimental setup were tested. Conclusions are drawn from these measurements for application of noise measurements for the examination of corrosion systems.     

Untersuchungen des Stofftransports und der chemischen Reaktionen im System Polyäthylen-weich/Schwefelsäure–Zur Frage der Spannungsrißkorrosion in Kunststoffen–

Investigations on mass transport and chemical reactions in the system LDPE/H₂SO₄–The question of stress corrosion cracking in plastic materials– Investigations on mass transport and chemical reactions in the system LDPE/H₂SO₄ reveal different kinds of damage and chemical attack both with bulk specimen containing internal stresses only and with pipes examined in creep tests under pressure. Evaluating light microscopy and electron scanning microscopy pictures it was found that the microstructure specific to the material as well as the macrostructure determined by the production process influence the character of damage. These structural parameters combined with internal or external stresses and a critical medium lead to a kind of failure which for polymers is called “stress corrosion cracking”.     

Korrosionsschutz – Antworten auf eine Herausforderung. Zur Gründung der Gesellschaft für Korrosionsschutz GfKORR

Corrosion Protection – Answers to a Challenge Corrosion based effects can be disastrous for safety and economy; early national surveys have estimated corrosion related annual costs of approximately 4% of the GNP for industrial states, or 4% of the turnover for chemical companies. When breaking corrosion costs down into costs of protection and costs of damage, both parts – described as a function of the protective effort — add up to a curve displaying a minimum. This value represents unavoidable costs due to the thermodynamic instability of technically used materials. Caused by insufficient application of available corrosion protection (CP) knowledge, in practice states as well as industry have to deal with approx. 25% additional (and avoidable) costs on top of the minimum. Industry therefore should strive for overall application of CP knowledge and parallel pursue technical R&D activities. States should promote research projects and initiate as well as maintain corrosion education on all technical levels. Responsibility for other important activities such as exchange of experience, initiation of projects, standardization, and certified education, however, can be tied neither to the state nor to industry. This area of activities now will be taken over by the newly founded Association for Corrosion and Corrosion Protection GfKORR, thus filling an existing gap in Germany.     

Netzwerke im Korrosionsschutz – Ein historischer Rückblick aus Anlass des 10‐jährigen Bestehens der GfKORR‐Gesellschaft für Korrosionsschutz e.V.

Networks for Corrosion Protection – A historical review on occasion of the 10^th anniversary of the GfKORR‐Society for Corrosion Protection, Germany The GfKORR‐Society for Corrosion Protection, Germany, was founded as a non‐for‐profit organisation in 1995 by amalgamation of the two former corrosion societies AGK‐Arbeitsgemeinschaft Korrosion e.V. and the VDKORR‐Verein Deutscher Korrosionsfachleute e.V.. Within 10 years GfKORR grew to a scientific‐technical society with far more than 500 members (including 70 legal members) which discuss problems of corrosion and corrosion protection within 20 different working groups. In 1998, GfKORR became a member of AiF‐Working Assosiation of Industrial Research Associations and together with 12 other AiF members formed the Curatorium for Corrosion Research. Since then GfKORR solicited from AiF 2.43 Mio EUR for corrosion research projects. GfKORR is active in supporting standardisation work and fosters education, continuing education and information transfer in the field of corrosion and corrosion protection, specifically by organizing national and international conferences, seminars and workshops.     

Vergleich des elektrochemischen Verhaltens der Stähle CrMnN 18 12 und CrNi 18 10 in Salpetersäure

Comparison of the electrochemical behaviour of the steels CrMnN 18 12 and CrNi 18 10 in nitric acid The electrochemical behaviour of the steels CrMnN 1812 and CrNi 18 10 in nitric acid has been studied by current-potential curves atomic absorption analysis of the solution and impedance spectroscopy. It has been found that in nitric acid solutions the corrosion resistance of the steel CrMnN 18 12 is comparable with that of the steel CrNi 18 10. Passivation of the steels starts at the corrosion potential. The two-step passivation range is probable attributable to a phase transformation in the oxidelayers formed on the steel surface. The analysis of the impedance curves allowed the establishment of an equivalent circuit for the passivation process. The double layer capacity, the polarization resistance, the Ohmic resistance and the capacity of the passive layer have been calculated.     

Untersuchung der Korrosionsneigung mit Hilfe des elektrochemischen Rauschens Teil 1: Phänomen,Entwicklungstrends und Probleme

Examination of corrosion propensity by means of electrochemical noise Part 1: Appearance, trends and problems A simple, sensitive and almost nondestructive measurement procedure for the examination of corrosion processes in corrosion research and assessment of corrosion propensity of metallic materials is available with the electrochemical noise diagnostic. The possibilities of application can be enlarged both on the solution of examining tasks in laboratory and on special problems for the corrosion monitoring in practice with today's knowledge about electrochemical noise at corrosion processes. So the various advantages of this measurement procedure can be used to solve a lot of problems and questions, often as a complementation of other test procedures. In this work the general bases of the understanding of the electrochemical noise, current trends and some difficulties concerning the use of electrochemical noise diagnostic are presented.     

Trinkwasser und seine “Verpackung” Werkstoff – Von der Antike bis heute –

Drinking water and its “packaging” material – From classical antiquity until today – Good drinking water and technical highly developed pipe materials for drinking water supply are the basic for a high standard of living. Today it is possible to draw off an hygienic, well tasting water at every time. The public water services have been developed during the millenniums. The reaction of water with the pipework had increasingly to be taken into account. The results of the research and development programme “Corrosion und Corrosion Protection” (FE‐KKS), during the last 20 years, supported the practical experience. Many of these results have been the basic for standardisation and technical rules. This report shows essential steps to a technical level of the public water supply.     

Grundlagen der Entwicklung korrosionsbeständiger Legierungen

Fundamental aspects of the development of corrosion proof alloys After considering the basic aspects determining the corrosion resistance of alloys, the possibilities of improving this resistance are discussed. Apart from changing the thermo-dynamic stability and the inhibition of anodic or cathodic processes, the article is mainly concerned with the possibilities of “cathodic alloying” which strengthens the influence of cathodic processes. Small quantities of Pd, Cu or Re have the effect of improving the resistance of steel with high Cr content in sulphuric acid and formic acid (provided that no halogen ions are present). Similarly favourable effects are obtained with Pd admixtures (up to 1 per cent.) to Ti or TiNb alloys. As the inhibition of corrosion is much stronger than might be expected from the alloy component, it must be assumed that the following conditions must be fulfilled for the reaction to be effective the base metal must, in principle, be sutiable for passivation; the passivation currents must be limited; there must be an adequate negative passivation potential (so that a shift into the positive range is possible); there must be no proneness to transpassivity and pit corrosion within the range of the passivation potential. As the enhanced corrosion resistance is only obtained after a certain exposure to the medium concerned, it must be assumed that the cathodically effective component must first be concentrated in the surface.     

Beurteilung spezieller Fragen zur Beständigkeit hochlegierter Werkstoffe mit dem EPR‐Versuch (Číhal‐Test)

The EPR‐technique to evaluate special questions on the corrosion resistance of high‐alloyed materials The electrochemical potentiokinetic reactivation (EPR‐)test is a very useful tool to reveal microstructural peculiarities of high‐alloyed materials and especially the sensibility to intergranular corrosion even if only very localised; it is possible to quantify the results referring to a standard. As this test does not only react on chromium depletion due to carbide formation a metallographic examination of the tested surface is indispensable. Due to it's very high sensitivity this technique is highly qualified not only for research tasks but also for the evaluation of components and for failure case analysis. To our opinion this method seems less recommendable or suitable for general purposes of acceptance testing as the results may be ambiguous and, in addition, generally accepted standards are missing up to now.     

Inhibitoren der Korrosion 20(1)–Über die zeitliche Abhängigkeit der analytischen Zusammensetzung der Korrosionsprodukte des Eisens von der Menge an verbrauchtem Sauerstoff

Analytical composition of the corrosion products of iron as a function of time and of the oxygen consumed Using a modified standard method of analysis it is possible to analyze mixtures containing Fe⁰, Fe²⁺ and Fe³⁺. This method is used to study corrosion systems catalyzed by chlorides. According to the results obtained the amount of Fe²⁺ is approx constant at pH 7, irrespective of the oxygen quantityt consumed, Fe⁰ being oxidized to give Fe²⁺ and the latter being oxidized, at a comparable rate, to give Fe³⁺. At pH 3, however, Fe²⁺ is obtained almost exclusively, and beyond pH Fe³⁺ becomes predominating.     

Lochkorrosionsverhalten von Auftragschweißungen – Schweißlagen‐ und Oberflächeneffekte –

Pitting corrosion behaviour of built‐up welds – Effects of welding layers and tarnish – The pitting corrosion resistance of nickel based deposition welds on a superduplex steel made by active‐gas metal pulsed‐arc welding was studied. Therefore the determination of the CPT (Critical Pitting Temperature) should be carried out corresponding to ASTM G 48 C. However an unexpectedly low resistance of the built‐up welds also at multilayer order was noticed. After visual assessment of the examined specimens a significant effect of the surface condition was assumed. Because the CPT determination according to ASTM does not allow any statement about the corrosion process, this method was not suitable to characterize the corrosion system. For this reason a new method was applied to clarify the causes of the low corrosion resistance. This method determines the CPT with the help of the electrochemical current noise under the same conditions demanded in ASTM G 48 C. The temperature is increased continuously and the characteristic parameters of the system are recorded and evaluated objectively within short time. So it was possible to see the influence of the surface condition on the pitting corrosion behaviour of the examined specimens. The required parameters to the post‐processing of the deposition welds were determined. The comparison of the results show that the surface tarnish formed after the shielded arc welding process influences the pitting corrosion resistance negatively. After its elimination the CPT could be determined in dependence of the welding layers.