Topic 1: Magnesium alloy design for improved corrosion resistance

　　Speaker 1: Dr. Carsten Blawert

　　　　　　　　Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Germany

　　Topic 2: Active protection coatings based on LDH: preparation, mechanisms, applications

　　Speaker 2: Dr. Maria Serdechnova

　　　　　　　　Institute of Materials Research, Helmholtz Zentrum Geesthacht, Germany

　　时间：2016年9月26日（周一） 9:00-11:00

　　地点：南区二楼学术报告厅

　　欢迎参加！

　　Abstract 1

　　The development of magnesium alloys with improved corrosion resistance requires additional concepts compared to standard alloys which obtain their corrosion resistance mainly by reducing the levels of impurities below certain alloy and process depending limits. A number of different approaches (e.g. use of corrosion barriers, reducing internal micro galvanic corrosion, embedding of impurities etc.) are available. The presentation will discuss how requirements of the intended application, knowledge about the corrosion mechanisms and of the electrochemical properties of the phases present as well as studies of the microstructure and prediction of phase fractions by thermodynamic calculations can be used to design modified magnesium alloys with improved corrosion resistance. Based on real examples such alloy developments will be demonstrated.

　　Abstract 2

　　For decades active corrosion protection for most technical metallic substrates was achieved using chromate-based surface treatments or Cr (IV) anticorrosion pigments integrated into organic polymer coatings. However, the application of chromates is banned in the most industries due to its high toxicity.

　　Two principal ways of LDH application are normally used: the direct growth of LDH architectures on metallic alloys (like Al, Mg and Zn based alloys or galvanized steel) and the addition of LDH powder in the polymer base protective coatings.

　　The first way has been recently studied in order to prepare LDH films in situ as a result of conversion process on bare aluminum, magnesium or zinc alloys in order to impart corrosion protection properties. The LDH forms a layer which contains either hydrophobic species in order to improve barrier properties and avoid the contact between substrate and aggressive electrolyte or inhibitive species near the metallic substrate in order to let them release and prevent corrosion when it starts. Being loaded with corrosion inhibitors (e.g. vanadate in the case of aluminum alloys), LDH acts as “smart” nanocontainers: the release of the corrosion inhibitor, formation of protective layer and catching of chloride ions from aggressive environment happen when the defect appears and corrosion starts.

　　A similar idea has been used in order to add self-healing properties to pre-treated (e.g. tartaric-sulfuric acid anodizing or plasma electrolytic oxidation (PEO)) aluminum alloys. Despite of the formation of hard and well adherent ceramic-like layer (in the case of PEO) its properties are compromised by the presence of defects (mainly different kinds of pores). The in-situ formation of LDH covers the surface, sealing the typical porous microstructure of PEO layers and improving the barrier properties and also provides the active protection as in the case of bare materials. The obtained results clearly demonstrate that such a treatment can significantly enhance the corrosion protection performance.

　　The second way is based on the preliminary synthesis of LDH powder and its intercalation with corrosion inhibitors. This powder loaded with anti-corrosion inhibitor can be added to polymer based coatings in order to combine barrier properties (polymer) and active corrosion protection (LDH loaded with inhibitor). In comparison with direct LDH growth on the metallic substrates, the preliminary synthesized LDH into the polymer coating can be applied to any substrate (e.g. non-metallic) and be also used for protection of galvanically coupled materials (like aluminum alloys coupled with carbon fiber reinforces plastic). In addition the inhibitors do not directly interact with polymer matrix avoiding its degradation.

　　In summary, different ways to develop LDH containers, loaded with corrosion inhibitors, can provide an effective protection for the substrate and be used in any industrial coating application for metallic products in automotive and aerospace industry.