A. Burkert

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

In der Korrosionsforschung steht mit der Rauschdiagnostik bei der Untersuchung von Korrosionsprozessen ein einfaches, empfindliches und nahezu zerstorungsfreies Mesverfahren zur Beurteilung der Korrosionsneigung metallischer Werkstoffe zur Verfugung. Mit dem heutigen Kenntnisstand uber das elektrochemische Rauschen bei Korrosionsprozessen konnen die Einsatzmoglichkeiten sowohl auf die Losung von Mesaufgaben im Labor, als auch auf spezielle Probleme bei der Korrosionsuberwachung in der Praxis erweitert werden. So konnen die Vorteile dieses Mesverfahrens, haufig als Erganzung anderer Prufund Untersuchungsmethoden, zur Losung einer Reihe von Problemstellungen herangezogen werden. In dieser Arbeit wird neben den allgemeinen Grundlagen zum Verstandnis des elektrochemischen Rauschens, uber die aktuellen Entwicklungstrends, sowie uber einige Probleme beim Einsatz der Rauschdiagnostik berichtet. 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 todays 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.

Betrachtungen zur Auswertung von Rauschmessungen

The electrochemical noise diagnostic became a firm component of pretentious corrosion investigation in the last few years. This was, as well as on other scientific fields, finally enabled by the effort of digital measuring equipment and more efficient microelectronics and computer components. Thereby it became possible at all to evaluate the data with different methods, to represent and file them. The methods for evaluation are very versatile and should be choosen in dependance to the testing problem and the limiting conditions. In this case, an important context exists between the measuring methodology, the used measuring equipment and the digital data acquisition. Some of these interrelations are shown in this paper.

Elektrochemisches Rauschen bei der Rißinitiierung an niedriglegiertem Stahl in Hochtemperaturwasser

Taking as an example CERT-tests performed with a strain rate of 7.8 . 10 -6 /sec in oxygen containing (8 ppm) high temperature water on the low-alloy quenched and tempered steel 20 MnMoNi 55, the effect of mechanical stress on the different corrosion processes such as crack initiation and crack growth could be proved and they could be distinguished from each other by means of electrochemical noise. This was realized by the development of an experimental measuring device which allows to select the part which is important for the estimation of the corrosion susceptibility of the metal surfaces from the large bandwith of the electrochemical noise. Thereby the transients which occur at crack initiation and spalling of magnetite layers during the CERT-test are particularly obvious. Thus, new possibilities for the investigation and control of crack corrosion arise.

Rauschdiagnostik bei Korrosionsprozessen Teil 2: Problemanalyse

Obgleich das Phanomen des elektrochemischen Rauschens (ECR) bei der Korrosion seit fast 30 Jahren bekannt ist, trat es erst in den letzten Jahren als relativ neues Mesverfahren in das Blickfeld einer wachsenden Zahl von Interessenten. Einige Probleme, welche dem Einsatz elektrochemischer Rauschmessungen in der Korrosionsforschung entgegenstehen, werden aufgezeigt und diskutiert. Auf der Grundlage von Ergebnissen eines Ringversuches wird uber die aufgabenbezogene Gestaltung der Mestechnik, insbesondere die Auswahl eines geeigneten Frequenzbereiches, eine geeignete Mesdatenerfassung und verschiedene Auswertemethoden berichtet. Anhand experimenteller Untersuchungen an Aluminium zur Lochkeimbildung und Inhibition sowie an hochlegiertem Stahl unter Lochkorrosionsbedingungen werden die bei den Rauschmessungen auftretenden Probleme analysiert und Losungsvorschlage unterbreitet. Bei der Auswertung der Mesergebnisse zum Ringversuch wurde deutlich, das die einheitliche numerische Bewertung der Mesdaten fur die Vergleichbarkeit der Ergebnisse von entscheidender Bedeutung ist. Am Beispiel der Berechnung von Standardabweichungen fur unterschiedliche Bewertungszeitraume wird gezeigt, das selbst bei ein und derselben Auswertemethode durchaus sehr unterschiedliche Ergebnisse am gleichen Datensatz ermittelt werden konnen. Damit wird deutlich, das der Wechselwirkung zwischen der Mesdatenerfassung (Abtastrate, Filterung, Online- Bearbeitung, Storpegel) und der Auswertung stets entsprechende Beachtung zu schenken ist. Noise diagnostics at corrosion processes Part 2: Problem analysis Although the phenomenon of electrochemical noise (EN) is known in corrosion research for almost 30 years it only reaches attention of a growing number of prospective buyers as a relatively new measurement procedure within the last few years. Some problems concerning the use of electrochemical noise measurements in corrosion research are shown and discussed. Based on the results of a round robin test the task regarding arrangement of the EN measuring technique, particularly the choice of a suitable frequency range, a suitable data acquisition and different methods for evaluation of data are presented. On the basis of experimental investigations regarding pit initiation and inhibition of aluminium as well as pitting of stainless steel arising problems are analyzed and solutions are suggested. The evaluation of the measurement results of the round robin test makes it clear that a uniform numeric assessment of the readings is of decisive meaning for the comparability of the results. The example of calculation of standard deviations for various assessment time periods demonstrated that very different results can be determined from the same data set by the same evaluation method. It became clear that the complex interaction between the way in which EN data are recorded (sampling rate, filtering, online data manipulation, baseline noise level) and the subsequent data evaluation should be taken into account in any case.

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

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.

Rauschmessungen an einer Anlage zur Aufbereitung industrieller Abwässer

Damage by pitting corrosion occured at parts of a plant (duplex steel, material no. 1.4462) for conditioning of process and waste waters after some weeks. By examination of the electrochemical noise it should be analyzed, under which conditions increased corrosion propensity (pit initiation) appears and whether another material (Alloy C 4, material no. 2.4610) has a higher corrosion resistance under the given conditions. As example for practical use of electrochemical noise diagnostic under operating conditions it is shown, how several short time tests under different medium conditions allow to draw conclusions with respect to to the corrosion propensity of the materials examined. The comparison with results from accompanying laboratory examinations have shown good agreement. The great influence of ammonium concentration on the corrosion behaviour of the duplex steel was recognized in all investigations examined.

Elektrochemische untersuchungen zum Korrosionsverhalten von Duplexgusswerkstoffen in Rauchgasentschwefelungsanlagen

Der Beitrag berichtet uber Untersuchungsergebnisse zum Korrosionsverhalten von Pumpen- und Flugelwerkstoffen aus Duplexstahlguss in Rauchgasentschwefelungsanlagen (REA) unter Labor- und Betriebsbedingungen in den VEAG-Kraftwerken (KW) Boxberg III und Janschwalde. In der Praxis zeigen die genannten Bauteile in der REA des Kraftwerkes Boxberg (KWB) eine wesentlich bessere Korrosionsbestandigkeit. Zum gleichen Ergebnis kommen auch die Laboruntersuchungen, bei denen die Bestimmung kritischer Potentialwerte teilweise Unterschiede von mehreren hundert Millivolt ergab. Die im Bypass der REA durchgefuhrten Betriebsversuche bestatigen tendenziell die Ergebnisse der Laboruntersuchungen, jedoch waren an vorgeschadigten Originaloberflachen teilweise signifikante Unterschiede zu den Laborversuchen festzustellen. Diese konnen auf die verscharften Versuchsbedingungen in der REA, wie z. B. die Feststoffbelastung und die Stromung zuruckgefuhrt werden. Mit elektrochemischen Rauschmessungen wurde das Repassivierungsverhalten der Werkstoffe naher untersucht. Dabei zeigte die Lage der freien Korrosionspotentiale fur das Kraftwerk Janschwalde (KWJ) eine Uberschreitung der Potentialbereiche, die fur eine sichere Repassivierung entstandener Defektstellen erforderlich waren. Hierin liegt die Ursache fur die beobachteten Schaden an den Bauteilen. Abhilfemasnahmen werden im Rahmen des vorliegenden Beitrages ausfuhrlich diskutiert. Fur die REA des KWB liegen die freien Korrosionspotentiale etwa 200 mV unterhalb des Bereiches fur eine sichere Repassivierung entstandener Defektstellen, wodurch kurzzeitige Schwankungen der Systemparameter zu korrosionsverscharfenden Bedingungen toleriert werden konnen. Electrochemical investigations on the corrosion behaviour of duplex cast stainless steels in flue-gas desulphurisation plants This paper deals with results from corrosion examination on duplex cast stainless steels from original components in the flue-gas desulphurization (FGD) of the VEAG-power stations Boxberg III and Jaenschwalde under practice and laboratory conditions. In practice the mentioned components have a better corrosion resistance in the FGD medium of the Boxberg power station. The same results were obtained by laboratory examinations, getting a shift in critical potentials of a few hundred millivolts. The tests in a by-pass of the FGD confirm these results in tendency. However, partially significant differences to laboratory examinations were established on specimens with pre-damaged original surfaces. This can be traced back to the more severe test conditions in the FGD, like solids and flow. The repassivation behaviour of the materials under investigation was examined by means of electrochemical noise analysis. The values of the free corrosion potentials in the Jaenschwalde power station were higher than potentials, necessary for a safe repassivation of occurring pits. This cause the failures on the components. Remedy steps are discussed in this paper in detail. The free corrosion potential in the Boxberg power station is about 200 mV below the repassivation potential. Because of that, short-time fluctuations of system parameters leading to enhanced corrosion risk can be tolerated.

Rauschdiagnostik bei Korrosionsprozessen. Teil 1 : Ergebnisse eines ringversuches

The Institute of Materials Engineering and Materials Testing of the University of Magdeburg (IWW) organised a round robin test on the measurement of electrochemical noise (EN) and obtained the participation of 17 organisations (universities, commercial firms, R&D laboratories) from 5 countries. The experiment involved the investigation of EN signals from aluminium and stainless steel exposed under conditions in which pitting corrosion can occur. The data provided by the participants were analysed at IWW according to various criteria. Evaluation methods included visual inspection of the noise in the potential or current versus time curves, calculation of the standard deviation, counting of transients and transformation of the measurements into the frequency domain. Almost all of the participants were able to detect differences in EN behaviour during different stages of the experiment qualitatively. Quantitative evaluation, however, revealed a more complex situation. At times, differences in measured EN signal intensity of two orders of magnitude were obtained as a result of different apparatus used (sensitivity, frequency range, etc.). Nevertheless, the fact that more than half of the participants reported data within a relatively narrow scatterband is a positive result. The experience gained from the round robin test provides a basis for further development of EN as a tool both for the fundamental understanding of various aspects of corrosion and for the solution of practical problems of corrosion protection, in particular in the area of corrosion monitoring.