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.
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.
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.
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.
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.
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.
The degradation behaviour of an Mg-1Ca alloy is investigated in vitro to figure out the possibilities of influencing the degradation behaviour of such an alloy by applying heat treatment as well as the use of a coating system based on plasma-chemical oxidation. It is shown that an optimised solution annealing (T4-heat treatment) can reduce the degradation rate while an additional ageing between 240 and 300 °C (T6-heat treatment) increases it. A coating generated by plasma-chemical oxidation reduces the degradation rate in the immersion test. Its effect is depending on the former heat treatment of the Mg-1Ca alloy as well as on the parameter during plasma-chemical oxidation.