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Organizado por Jose Ygnacio Pastor Caño

Frontiers in Materials 2018-10-03 Ewa Wierzbicka

  • Fechas:

    Del 03/10/18 al 03/10/18

  • Lugar:

    Sala Verde. C/ Profesor Aranguren 3 28040-Madrid, UPM-ETS de Ingenieros de Caminos, Canales y Puertos, Madrid, España (mapa)

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Light alloys and their application in transport:

Environmentally friendly protection systems

Ewa Wierzbicka

Departamento de Ingenieria Química y de Materiales, Facultad de Ciencias Quimicas

Universidad Complutense de Madrid, España

 

VISUALIZACIÓN EN: https://youtu.be/KB2TgDbdUKs

Since fossil fuels energy sources are being fast depleted, the further reduction of their utilization in the transport and aeronautic industry is a burning topic. One of the strategies to achieve this goal is a reduction the weight of the vehicles through the use of light metal alloys such as aluminium and magnesium.

Al and Mg alloys are very commonly used lightweight materials because their unique physical and chemical properties. Mg is the lightest of the metals that can be applied in the vehicles engineering, having a density of 1.74 g/cm3. It is 35% lighter than aluminium (2.7 g/cm3) and about four times lighter than steel (7.86 g/cm3). Both metals are active elements with low corrosion resistance predetermined by its atomic properties and its standard potentials: E˚ = -2.37 VSHE for Mg, and E˚ = −1.66 VSHE for Al. This makes those metals highly susceptible to atmospheric and galvanic corrosion caused by a presence of other intermetallic metals in alloys. In the presence of humidity and some ions like Cl- from sea-water, micro-galvanic couplings are formed between the metal matrix and intermetallic particles which fast leads to initiation of the corrosion process. The typical and the most effective way to prevent corrosion of metal surface is to isolate it from the corrosive environment by creating a barrier layer.

Currently, majority of corrosion protection surface treatments for Mg and Al employ the products containing hexavalent chromium as they possess high efficiency but unfortunately it is very harmful for human. According to REACH (EC 1907/2006) regulation it is required to abandon using of these compounds until September 2017. The urgency for their replacement is the main motivation for our studies. The challenge of such a replacement consists in meeting the combined requirements of excellent corrosion resistance, fatigue performance, adhesion to paints and primers and economically reasonable.

One of the approaches to provide the corrosion protection of metal surface is its electrochemical treatment in the solution, called anodizing. This electrochemical oxidation process, is an effective method to improve the corrosion resistance of Mg, Al and their alloys by forming a protective, typically 5-150 μm-thick layer on their surface. The formed coating is characterized by good adhesion to the substrate and comprise a barrier layer that isolates the base material from aggressive environmental agents. In general, it is possible to conduct anodizing process by two different approaches: below (typically in acidic solutions) and above breakdown potential (in basic solutions). Classical method in a low potentials region and acidic solution is well applicable for Al alloys but not very suitable for Mg alloy. It is because the value of Pilling-Bedworth ratio (molar Vox/Vm) for MgO/Mg system is only 0.81 which leads to a strong cracking of the oxide coating. Therefore it is necessary to introduce into Mg oxide layer a lot other types of chemical species, preferable with big volume and corrosion resistance, which is possible only if anodizing is carried out above breakdown potential by plasma electrolytic oxidation process (PEO). Under this conditions occur fast coting growth related with formation of plasma micro-discharges on the metal surface.

Anodizing has a greater variety of factors, such as voltage/current regime, electrolyte composition, sealings that can be easily modified and which affect the final composition and structure of the obtained coatings. The is also a novel idea which currently focus a lot of scientific attention, to combine the anodic film passive protection with active protection provided by the corrosion inhibitors by in-situ addition into electrolyte or post-treatment sealing. The other huge advantage of such system is related to porous structure of anodic films that offering a great reservoir for the corrosion inhibitors adsorbance and moreover excellent base for and adhesion of paint and primers which is the crucial parameter for industrial applications.

In summary, anodic coatings on light metals have a great potential to be an alternative to toxic chromium treatments that are currently in commercial use, and with great probability have a significant chance to reach very high performance in corrosion resistance.

 

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