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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)
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.
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.
Thick-walled components made of duplex stainless steels are used in the semi-finished products as well as in machinery, apparatus and plant construction. Electron beam welding (EBW) of these components may be recommended for economic and quality reasons. To guarantee the necessary mechanical and technological properties and the corrosion resistance, the duplex stainless steels are welded with filler material and afterwards undergo a post-weld heat treatment. The present work shows interim results of investigations concerning the development of an electron beam multi-process technology for welding these steels without filler material and post-weld heat treatment. The studies were performed on standard duplex stainless steel of type 1.4462 (X2CrNiMoN22-5-3). When welding duplex stainless steels, the cooling rate and the chemical composition have a crucial influence on the final result. Based on fundamental investigations relating to the influence of the process parameters on the effusion of nitrogen and the cooling rates, the resulting austenite formation, mechanical properties and the corrosion resistance were taken into account to develop appropriate electron beam multi-process techniques. The ferrite content was measured metallographically and by magnetic induction, the impact toughness was measured at –40 °C and the determination of critical pitting temperatures was performed using electrochemical noise measurements.
Long term corrosion behavior of clad aluminium materials under different atmospheric conditions
(2013)
Aluminum materials roll-plated on both sides are combinations of metals, which consist of clad layers on a core. To document the long term behavior of roof profiles of Alclad 3004 after long term exposure of about 40 years, evaluations were made at three locations in Germany with different climatic conditions. Locations with typical rural-urban, coastal-urban, and industrial climates were selected. The corrosion progress as a function of time and climatic type could be characterized by cross-sections of roof specimens. To interpret the protective mechanism of the clad layer, the corrosion behavior of Alclad 6025 was characterized by electrochemical investigations. The results show that the passive clad material easily undergoes pitting in media containing specific amounts of chlorides. In case of contact of both cladding and core with an electrolyte the core alloy was protected because of the pitting corrosion of the cladding. This guarantees long-time corrosion protection of constructions like facades or roofs under different atmospheric conditions.
Zur vergleichenden Einschätzung der Lochkorrosionsbeständigkeit von verschiedenen, mehrlagigen MAGp-auftraggeschweißten Nickelbasis-Schweißgütern auf einem Superduplexstahl wurden kritische Lochkorrosionstemperaturen (critical pitting temperature, CPT) nach ASTM G 48 C ermittelt. Es zeigte sich eine unerwartet niedrige Beständigkeit der Auftragschweißungen, als dessen Ursache ein ungünstiger Oberflächenzustand angenommen wurde. Da die CPT-Bestimmung nach ASTM keine Aussage über den Korrosionsverlauf gestattet und Untersuchungen über einen weiten Temperaturbereich damit sehr aufwendig sind, wurde eine neue Prüfmethode angewandt. Diese ermittelt die CPT mit Hilfe des elektrochemischen Stromrauschens zwischen zwei gleichartigen Proben in der nach ASTM G 48 C geforderten Prüflösung bei kontinuierlicher Temperaturerhöhung. Dadurch war es möglich, den Einfluss des Oberflächenzustandes auf das Lochkorrosionsverhalten der untersuchten Proben schnell nachzuweisen und geeignete Maßnahmen zu ergreifen. Der beim MAGp-Schweißen entstehende oxidische Oberflächenbelag beeinflusst die Lochkorrosionsbeständigkeit negativ. Nach der mechanischen Beseitigung der Beläge durch Strahlen oder Schleifen weisen die Proben eine wesentlich höhere Lochkorrosionsbeständigkeit auf, und es konnte die Abhängigkeit von der Anzahl der Schweißlagen ermittelt werden.
Translated Abstract
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.
An important possibility to improve the corrosion behaviour of magnesium alloys is the application of protective coatings. The quality of such coatings depends mainly on the pretreatment and the exposure conditions after pretreatment. Since magnesium surfaces change much faster under atmospheric conditions than those of almost any other technical material, it is necessary to pay special attention to this particular feature. The activity of acid-pickled surfaces of the magnesium alloys AZ31 and AZ91 in dependence on the exposure time and the humidity conditions was investigated with electrochemical noise (EN) measurements. In addition to pickling, plasma chemical vapour deposition processes open new possibilities for an economical, as well as ecologically quite safe, pretreatment. The results of EN investigations after acid pickling as well as specific plasma oxidation treatments of the two magnesium alloys after exposure to air with different humidities are presented.