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- Electrochemical noise (7)
- Corrosion (6)
- Magnesium (4)
- Pitting corrosion (4)
- KorroPad (3)
- Magnesium alloys (3)
- Martensitic stainless steel (3)
- Biodegradation (2)
- C. Pitting corrosion (2)
- Heat treatment (2)
An unusual feature known as the negative difference effect (NDE) can be observed in magnesium when recording corrosion current density–potential curves. More hydrogen is evolved at a more positive potential which does not occur in conventional metals. Several models have been proposed in the literature in order to explain the phenomenon of NDE. They succeed in explaining some effects, and fail to deal with others. A new model, which explains the NDE by two electron consuming processes, is presented in this paper. By potentiostatic investigations of magnesium in a chloride electrolyte, measurements of hydrogen evolution and chemical analysis of the electrolyte the new model was experimentally verified.
The determination of critical pitting temperatures (CPT) in various test solutions like ferric chloride solution according to ASTM G48 or 'Green Death' solution is a common test method for the comparative assessment of the pitting corrosion resistance of highly alloyed steels and Ni–Cr–Mo alloys. In addition to the well-known disadvantages of standard methods, like long test times, subjective examination, and large scatter, for the highest alloyed Ni–Cr–Mo alloys no stable pitting corrosion can initiate even at the highest test temperatures. This paper describes the limitations of standard test methods and shows how these problems can be solved by an alternative test solution and an adjusted test method. By capturing and examining the current noise under potentiostatic conditions during continuous heating in a 4.5 M calcium chloride solution the transition from metastable to stable pitting corrosion as a criterion for CPT can be detected in a reproducible way.
Die Erzeugung geschliffener Oberflächen bei hochlegierten Chrom-Nickel-Stählen aus dekorativen Gründen z. B. beim Einsatz im Bereich Innenarchitektur kann sich u. U. problematisch hinsichtlich der Korrosionsbeständigkeit auswirken. Bei den Untersuchungen zum Einfluss der Verfahrensparameter beim industriellen Schleifen zur Erzeugung eines bestimmten optischen Erscheinungsbildes stellte sich heraus, dass die Rauheit allein kein Maß für die zu erwartende Korrosionsbeständigkeit darstellt. Die Variation verschiedener Schleifparameter, aber auch die Art des Schleifmittels können einen erheblichen Einfluss auf die Korrosionsbeständigkeit ausüben. Neben den Ergebnissen verschiedener Auslagerungsversuche werden elektrochemische Untersuchungen an den geschliffenen Oberflächen vorgestellt. Dabei konnte mittels elektrochemischer Rauschmessungen die unterschiedliche Korrosionsneigung der Oberflächen nachgewiesen werden.
Translated Abstract
The influence of grinding treatment of stainless steels on the corrosion behaviour
Mechanical grinding of stainless steels is often used for decoration purposes, for instance in the interior decoration. However, this treatment can cause problems regarding the corrosion resistance. During investigations of the influences of processing parameters of industrial grinding it was shown, that roughness only is not the expected crucial parameter for the corrosion resistance. The variation of different grinding parameters and also the type of abrasives have a substantial influence. Beside the long term tests electrochemical investigation at the ground surfaces are presented. The different corrosion susceptibility of the surfaces could be proven by means of electrochemical noise measurements.
Für Korrosionsprüfungen stehen eine ganze Reihe bewährter und standardisierter Verfahren zur Verfügung, die eine weite Verbreitung gefunden haben. Dazu gehören die Sprühnebelverfahren, Klimaprüfverfahren, Auslagerungsversuche in spezifisch wirkenden Prüfmedien und die elektrochemische Polarisationsmesstechnik. Dennoch bleiben eine ganze Reihe von Fragen und Wünschen offen. Insbesondere die Anfangsprozesse der Korrosion lassen sich häufig nur unzureichend untersuchen. Auch über den zeitlichen Verlauf der Korrosion geben die eingesetzten Verfahren keine ausreichenden Informationen. Weiterhin führt der Wunsch nach möglichst praxisnahen Prüfbedingungen bei vertretbarem Aufwand und die Suche nach Möglichkeiten zur Verkürzung von Prüfzeiten zur ständigen Weiterentwicklung der Prüf- und Untersuchungsmethoden.
In der Korrosionsforschung steht mit der Rauschdiagnostik bei der Untersuchung von Korrosionsprozessen ein einfaches, empfindliches und nahezu zerstörungsfreies Messverfahren zur Beurteilung der Korrosionsneigung metallischer Werkstoffe zur Verfügung. Mit dem heutigen Kenntnisstand über das elektrochemische Rauschen bei Korrosionsprozessen können die Einsatzmöglichkeiten sowohl auf die Lösung von Messaufgaben im Labor, als auch auf spezielle Probleme bei der Korrosionsüberwachung in der Praxis erweitert werden. Des Weiteren findet diese Methode auf Grund ihrer spezifischen Vorteile auch immer mehr Anwendung zur Unterstützung bzw. Verbesserung herkömmlicher Korrosionsprüfungen. Der Zugewinn wichtiger Erkenntnisse, die Verkürzung von Prüfzeiten sowie die damit verbundene Schonung von Ressourcen (Arbeitskraft, Energieaufwand, chemische Hilfsmittel etc.) sind einige der Vorteile, die eine moderne Korrosionsprüfung heute kennzeichnen.
Translated Abstract
Electrochemical noise measurements - Innovation in corrosion testing
There are a lot of approved and standardized methods available for corrosion testing which have found wide application. Salt-spray testing, electrochemical polarization techniques and exposure tests in specific test media belong to these. Nevertheless, some questions remain unanswered and requirements unsatisfied. In particular, both the start of corrosion and the way it proceeds with time can often be examined only to an insufficient extent. Furthermore, the requirements of practical test conditions, acceptable effort and expenditure and shorter test durations lead to continuous development of these test methods.
Within corrosion research, the analysis of electrochemical noise offers a simple, sensitive and virtually non-destructive measuring technique for assessment of the corrosion susceptibility of metallic materials and for the investigation of corrosion processes. The present status of knowledge concerning noise diagnostics in corrosion processes permits the application of this method not only to experimental tasks in the laboratory, but also to special problems in the context of practical corrosion monitoring. Furthermore, specific advantages of the technique enable its use to an increasing extent in supporting or improving conventional corrosion testing. The advantages here include obtaining additional information and shortening testing times (together with associated savings in resources such as manpower, energy, chemical reagents, etc.), thus resulting in state-of-the art corrosion testing.
Electrochemical noise measurements on anodically polarised type 304 stainless steel surfaces in contact with buffer solutions of neutral pH were performed to study the effect of chloride ions in the nucleation of pitting corrosion. Passive layer stability and susceptibility to pitting corrosion after pickling and passivation at different environmental conditions were also investigated by means of electrochemical current noise measurements under cathodic and anodic polarisation. According to the obtained experimental results pits nucleate independently on the presence of chloride ions. It has been also shown that protectiveness of stainless steel surfaces after pickling strongly depends on the relative humidity of the environment in which the surface is subsequently passivated.
Martensitic stainless steels are commonly used in cutlery fabrication requiring high hardness and sufficient corrosion resistance. The heat treatment process affects the mechanical and electrochemical behavior of martensitic stainless steels due to the precipitation of chromium carbides. Depending on the heat treatment the corrosion resistance of these steels can vary strongly, and improper heat treatment parameters can lead to a weak pitting corrosion resistance. The aim of this work is to identify heat treatment parameters influencing the corrosion resistance of martensitic stainless steels by using three different electrochemical testing methods. To this purpose, five different heat treatments were applied to the alloys 1.4116 and 1.4034. In addition to the determination of the critical pitting potentials and the modified double-loop electrochemical potentiodynamic reactivation tests (DL-EPR) a new KorroPad indicator test was used assessing the pitting corrosion behavior. The results showed that all methods used were in good agreement for verifying the influence of the various heat treatment parameters on the corrosion behavior and to identify the effect of heat treatment conditions on the pitting corrosion resistance.
As bio-absorbable implant material the magnesium alloy Mg-1Ca is able to degrade in-vivo. The mechanical properties of this alloy are similar to those of human bone; both Mg and Ca are essential elements in human body. The main problem is the high corrosion rate of this alloy. Two coating systems based on plasma-chemical oxidation and an organic dip coating are applied onto MgCa1.0 magnesium alloy in order to slow down the corrosion rate. The corrosion behaviour of the coated alloys was investigated with electrochemical noise measurements. The influence of hydrogen evolution and increasing pH-value on the cytotoxicity was examined. The results of these investigations suggest that a combination of both coating systems leads to promising degradation properties.
In the last years, new approaches for the evaluation of the corrosion resistance of stainless steels were developed which allow short term corrosion testing with increased information content. This work analyses the extensive influence of heat treatment on microstructure and the resulting corrosion resistance of the martensitic stainless steels 1.4034 and 1.4021 with advanced methods. Different heat treatments at various austenitization temperatures up to 1100°C and the effect of different cooling rates were evaluated; the last has not yet been studied in literature at all. The resulting corrosion behaviour in relation to the different carbon content in the two used alloys and the applied heat treatment will be presented and discussed. The applied methods of investigation were conventional evaluation of the critical pitting corrosion potential (PP), modified electrochemical potentiodynamic reactivation (mEPR) and the "KorroPad" (KP) technique. The performance of modified EPR and its interpretation were optimized to provide additional information about the general passivation ability and the extent of chromium depletion as result of applied heat treatment. The aim using PP and KP was the correlation between the Parameters of the mEPR with the changes in the pitting corrosion resistance. Furthermore the results will demonstrate the functionality and usability of the short term corrosion test methods mEPR and KP in order to increase their acceptance within the scientific community. The results indicate a surprisingly large influence of both austenitization temperature and cooling rate on the corrosion resistance within all three used test methods, which can explain the different corrosion behaviour of martensitic stainless steels in earlier investigations.
In the last years, new approaches for the evaluation of the corrosion resistance of martensitic stainless steels were developed, which allow nearly non-destructive as well as short-term corrosion testing. This work analyzes the sensitive influence of microstructure and surface treatment on the corrosion resistance. The effect of alloy composition and microstructure, which is always present in addition to the influence of various surface treatments will be evaluated and discussed. The corrosion behavior was investigated with conventional evaluation of the pitting potential, electrochemical potentiodynamic reactivation (EPR), electrochemical noise (EN), and the 'KorroPad' (KP) technique. EPR provided information about the passivation ability and the extent of chromium content reduction due to chromium carbide precipitation in the microstructure. The use of EN and KP aims to detect the influence of different surface treatment parameters on the corrosion resistance in a nearly non-destructive way. Furthermore the results shall demonstrate the functionality and convenience of the short-term corrosion test methods EN and KP in order to gain more acceptance within the scientific community. The results will allow the extraction of surface treatment effects on the corrosion behavior of the martensitic stainless steels 1.4116, 1.4034, and 1.4021.
In der industriellen Praxis ist die Sprühnebelprüfung eine häufig eingesetzte Prüfmethode zum Nachweis der Produktqualität von Halbzeugen und Zulieferteilen hinsichtlich ihrer Korrosionsbeständigkeit. Aus diesem Grund gehören die Klima- und Sprühnebelprüfungen zu den am weitesten verbreiteten Korrosionsprüfverfahren. Jedoch kann man über den zeitlichen Verlauf der Korrosion, ob linear, exponentiell, logarithmisch o.ä. keine Aussage treffen. Weiterhin sind die Prüfbedingungen bei den Sprühnebelprüfungen verschärft, um eine Verkürzung der Messzeit zu erreichen. Damit verändert man unter Umständen die Korrosionsmechanismen. Die Sprühnebelprüfung kann jedoch auch ohne Verwendung von zu scharfen, praxisfremden Prüfmedien deutlich an Erkennbarkeit gewinnen, indem man elektrochemische in-situ Messungen (Rauschmessungen) anwendet. Im Gegensatz zu herkömmlichen elektrochemischen Messungen, bei denen die untersuchten Proben von einem bulk-Elektrolyten umgeben sind, basieren die Messungen unter Sprühnebelbedingungen auf der Bildung eines Elektrolytfilms an den Probenoberflächen, sowie an den Innenwänden der Prüfkammer und der Anbauten (z. B. Probenhalter). Die Messeffekte weichen nicht grundlegend von konventionellen Messungen ab, die nutzbaren Signale sind jedoch von den Eigenschaften des Elektrolytfilms (Ausbreitung auf der Oberfläche, Leitfähigkeit, Dicke) abhängig. Die erfolgreiche Anwendung von Rauschmessungen unter Sprühnebelbedingungen wird für einige Beispiele der Korrosionsprüfung gezeigt.
Translated Abstract
Instrumentation of salt-spray test procedures
Salt-spray test procedures are frequently used in industrial practice as a test method for proof of product quality with semi-finished products and received parts with regard to their corrosion resistance. For this reason, salt-spray testing is one of the most well-established corrosion test procedure. However, no statement can be made about the way in which corrosion develops over time (linear, exponential or logarithmic). Furthermore, the test conditions for salt-spray tests are intensified in order to shorten the measuring times, which can lead to a change in the corrosion mechanism. Nevertheless, also without intensifying test conditions salt-spray testing can be brought to a qualitatively higher level and the clarity of the results can be improved by means of in-situ electrochemical noise measurements. In contrast to established electrochemical tests in which the specimens are surrounded by a common bulk electrolyte, electrochemical measurements in the spray chamber are based on the development of an electrolyte film on the surface of the specimen, but also on the inside walls of the testing chamber and on all fittings present (e. g. specimen holder). The measured effect thus has the same basis as in conventional electrochemical measurements. The exploitable signal is limited by the shortest connection route between the specimens, as well as by the thickness and the conducting ability of the electrolyte film. The successful application of noise measurements under salt-spray conditions is shown on various examples for corrosion testing.