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- Corrosion (7)
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The development of anti-corrosive pigments for the most diverse coating Systems is extremely time consuming and expensive, owing to the necessary climatic testing, such as testing under salt spray exposure. To accelerate work on a new zinc-free pigment with improved protection, modern, electrochemical investigation methods have been used successfully and verified using traditional tests.
The use of electrochemical noise (EN) measurements for the investigation and monitoring of corrosion has allowed many interesting advances in the corrosion science in recent years. A special advantage of EN measurements includes the possibility to detect and study the early stages of localized corrosion. Nevertheless, the understanding of the electrochemical information included in the EN signal is actually very limited. The role of the cathodic process on the EN signals remains uncertain and has not been sufficiently investigated to date. Thus, an accurate understanding of the influence of the cathodic process on the EN signal is still lacking. On the basis of different kinetics of the oxygen reduction it was established that the anodic amplitude of transients arising from pitting corrosion on stainless steel can be decreased by the corresponding electron consumption of the cathodic process. Therefore, the stronger the electron consumption, the weaker the anodic amplitude of the EN signal becomes. EN signals arising from pitting corrosion on stainless steel can be measured because the cathodic process is inhibited by the passive layer. This was confirmed by means of EN measurements under cathodic polarisation. Since the cathodic process plays a decisive role on the form of transients arising from pitting corrosion, its influence must be considered in the evaluation and interpretation of the EN signals.
Electrochemical noise (EN) enables corrosion research and monitoring in real time and with high sensitivity. In the case of submicroscopic nucleation events of pitting corrosion it has been observed that the cathodic process plays a decisive role in the mechanism as well as in the origin of EN signals, which depend not only on the metallic dissolution reaction but also on the electron-consuming process. EN signals arising from the nucleation process of localized corrosion on stainless steels can only be recorded due to the inhibition effect on the cathodic process achieved by the spontaneous formation of the passive layer. In consequence, passive layer stability becomes a significant factor in influencing EN signals. The way in which the passive layer stability affects the acquisition and analysis of EN signals arising from pitting corrosion on stainless steels is discussed in detail.
Development of anti-corrosive pigments for diverse coating systems is both time-consuming and costly, due to the necessary climatic testing, such as testing under salt spray exposure. To accelerate work on a new zinc-free pígment with improved protection, modern, electrochemical investigation methods have been used successfully and verified using traditional tests.
The main problem limiting the application of magnesium alloys as biodegradable implant material is its high degradation rate. In order to slow down the corrosion rate an extrusion process and specific coating systems based on plasma-chemical oxidation (PCO) and organic dip coating with poly(ʟ-lactid-co-caprolacton) (PLLC) were applied on Mg–1Ca magnesium alloy. The additional PLLC coating is used to delay the start of substrate corrosion, while the purpose of the PCO coating is to decrease the substrate corrosion rate. The corrosion behaviour was investigated in synthetic body fluid (SBF) through measurement of the hydrogen evolution rate in long term tests and polarisation and electrochemical noise measurements in short term tests. The results showed significant differences between the cast and extruded alloys and a decrease of the corrosion rate due to corrosion product formation. The combination of both coating systems resulted in a significant delay of metal substrate corrosion and all coating systems showed good correlation between short and long term tests. The combination of the three investigation methods provides the possibility to gain more information about the degradation behaviour and break down of protective coatings.
Sixteen laboratories have performed electrochemical noise (EN) measurements based on two systems. The first uses a series of dummy cells consisting of a 'star' arrangement of resistors in order to validate the EN measurement equipment and determine its baseline noise performance, while the second system, based on a previous round-robin in the literature, examines the corrosion of aluminium in three environments. All participants used the same measurement protocol and the data reporting and analysis were performed with automatic procedures to avoid errors. The measurement instruments used in the various laboratories include commercial general-purpose potentiostats and custom-built EN systems. The measurements on dummy cells have demonstrated that few systems are capable of achieving instrument noise levels comparable to the thermal noise of the resistors, because of its low level. However, it is of greater concern that some of the instruments exhibited significant artefacts in the measured data, mostly because of the absence of anti-aliasing filters in the equipment or because the way it is used. The measurements on the aluminium samples involve a much higher source noise level during pitting corrosion, and most (though not all) instruments were able to make reliable measurements. However, during passivation, the low level of noise could be measured by very few systems. The round-robin testing has clearly shown that improvements are necessary in the choice of EN measurement equipment and settings and in the way to validate EN data measured. The results emphasise the need to validate measurement systems by using dummy cells and the need to check systematically that the noise of the electrochemical cell to be measured is significantly higher than the instrument noise measured with dummy cells of similar impedance.
Unwanted corrosive phenomena on stainless steel surfaces like rouging had been observed for years occurring at certain service conditions. Rouging consists in the formation of a colored surface film containing mainly iron on metallic surfaces in contact with high-purity waters. In spite of numerous hypotheses and experimental examinations a clear explanation about the origin of rouging on stainless steels is currently still lacking. Because rouging occurs even at well passivated stainless steel surfaces the origin of this corrosive phenomenon should be related to the weakness of the stability of the passive film. In the present paper electrochemical noise measurements regarding the stability of the passive layer of alloy 904L (1.4539, X1NiCrMoCu25-20-5) in high-purity water at 70 °C depending on the cathodic process are presented. In addition, the importance of the cathodic reaction on the passivation and stability of the passive layer is discussed.
Auf dem Gebiet des mechanischen Fügens ist eine rege Entwicklung zu verzeichnen. Der Einsatz neuer Materialien, neuer Nietformen, neuer Schutzschichten, veränderter Fertigungstechnologien, die Notwendigkeit zur Mischbauweise, veränderte mediale Belastungen, geforderte Langzeitgarantien usw. lassen vielfältige alte und neue Fragen hinsichtlich der Korrosionsbeständigkeit auftreten. Herkömmliche Korrosionsprüfverfahren geben oft nur unzureichende Antworten auf diese Fragen. Mit dem Einsatz des elektrochemischen Rauschens ist es möglich, in kurzer Zeit aussagekräftige Ergebnisse über Material- und Bauteileigenschaften zu erhalten, da eine Metalloberfläche ständig Signale über ihren Zustand aussendet, auch wenn man nicht misst. Neben der Möglichkeit, die Korrosionsneigung zu bestimmen, lassen sich über die Rauschmessungen auch bestimmte Verarbeitungsfehler und Oberflächendefekte nachweisen. Die Anwendung dieser Methode bietet somit Möglichkeiten zur Prozessoptimierung und Qualitätssicherung mechanisch gefügter Bauteile, oder auch zur Einschätzung bestehender Fügeverbindungen.
Translated Abstract
Application of noise measurement for the early detection of corrosion property of mechanical joined metal sheets
In the area of mechanical joining an active development is to be registered. The use of new materials, new rivet forms, new protective layers, changed production technologies, use of mixed materials, changed medial loads, demanded long-term guarantees etc. let arise various old and new questions regarding the corrosion resistance. Conventional corrosion testing methods often give only insufficient answers to these questions. With the use of the electrochemical noise it is possible to receive meaningful results over material and construction characteristics within a short time since a metal surface constantly sends signals over its condition, even if one does not measure. Apart from the possibility of determining the probability of corrosion also certain processing errors and surface defects can be proven with noise measurements. Thus the application of this method offers possibilities for the process optimization and quality assurance of mechanically joined construction units, or also for the estimation of existing joining connections.
Auf dem Gebiet des mechanischen Fügens ist eine rege Entwicklung zu verzeichnen. Der Einsatz neuer Materialien, neuer Nietformen, neuer Schutzschichten, veränderter Fertigungstechnologien, die Notwendigkeit zur Mischbauweise, veränderte mediale Belastungen, geforderte Langzeitgarantien usw. lassen vielfältige alte und neue Fragen hinsichtlich der Korrosionsbeständigkeit auftreten. Herkömmliche Korrosionsprüfverfahren geben oft nur unzureichende Antworten auf diese Fragen. Mit einer neuen Methode (Rauschdiagnostik) ist es möglich, in kurzer Zeit aussagekräftige Ergebnisse über Material- und Bauteileigenschaften zu erhalten. Hierbei macht man sich die Tatsache zu Nutze, dass eine Oberfläche, gleichgültig ob man misst oder nicht misst, ständig kleinste Strom- und Spannungsimpulse generiert, die bei richtiger Messung und Interpretation wertvolle Aussagen liefern. Neben der Möglichkeit, die Korrosionsneigung zu bestimmen, lassen sich über diese Messungen auch bestimmte Verarbeitungsfehler und Oberflächendefekte nachweisen. Die Anwendung dieser Methode bietet somit Möglichkeiten zur Prozessoptimierung und Qualitätssicherung mechanisch gefügter Bauteile, oder auch zur Einschätzung des Zustandes von Fügeverbindungen in der Praxis.
Bei der Qualitätsbewertung von Schneidwaren aus nichtrostenden martensitischen Chrom-Stählen stellt die Überprüfung der Korrosionsbeständigkeit ein wichtiges Kriterium dar. Allgemein üblich sind Wechseltauchversuche mit relativ hohem zeitlichem und auswertungsbedingtem Aufwand, die noch dazu zerstörend sind und teilweise fragwürdige Ergebnisse liefern. In den letzten Jahren wurden neue Ansatze der Prüfung geschaffen, die eine nahezu zerstörungsfreie Kurzzeit-Korrosionsprüfung erlauben. Es wird gezeigt, dass mit dem elektrochemischen Rauschen sehr empfindlich Einflüsse des Härtens und der Oberflächenbehandlung auf die Korrosionsbeständigkeit nachgewiesen werden können, wodurch eine schnelle und objektive Qualitätsbewertung innerhalb des Fertigungsprozesses und der Endkontrolle von Produkten möglich ist. Das für die Prüfmethodik ausgenutzte Phänomen des elektrochemischen Rauschens zielt darauf ab, den Beginn der Korrosion empfindlich und schnell zu detektieren, ohne die Prüfbedingungen maßlos zu verschärfen und Produkte damit zu zerstören. Neben der Funktionalität der Prüfung stehen aber auch die Anwenderfreundlichkeit und somit die Akzeptanz bei den Herstellern im Vordergrund. Aus diesem Grund wurde zusätzlich ein Indikatortest (KorroPad) als praxistaugliche Alternative zum elektrochemischen Rauschen angewendet. Die Ergebnisse zeigen eine gute Übereinstimmung beider Methoden und ermöglichen eine effektive betriebliche Qualitätsüberwachung von Schneidwaren.