Electrochemical study of Aluminum-Fly Ash composites obtained by powder metallurgy

• Published in: Materials characterization Vol. 69; no. Complete; pp. 16 - 30
• Main Authors: Marin, E., Lekka, M., Andreatta, F., Fedrizzi, L., Itskos, G., Moutsatsou, A., Koukouzas, N., Kouloumbi, N.
• Format: Journal Article
• Language: English
• Published: New York, NY Elsevier Inc 01.07.2012; Elsevier
• Subjects: ALUMINIUM; Aluminum; Applied sciences; CHEMICAL COMPOSITION; Composites; Corrosion; Corrosion environments; Corrosion mechanisms; CORROSION RESISTANCE; CREVICE CORROSION; Cross-disciplinary physics: materials science; rheology; Dissolution; ELECTROCHEMISTRY; Exact sciences and technology; FLY ASH; HARDNESS; INTERMETALLIC COMPOUNDS; MATERIALS SCIENCE; MATRIX MATERIALS; Metal matrix composites; Metals. Metallurgy; MICROSTRUCTURE; MORPHOLOGY; PARTICLES; Particulate composites; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Phases; Physics; POLARIZATION; POWDER METALLURGY; SCANNING ELECTRON MICROSCOPY; Solidification; WEAR RESISTANCE; X-RAY SPECTROSCOPY; Fly ash; Composites; Corrosion mechanisms; Al; Corrosion mechanism; Composite materials; Corrosion; Powder metallurgy
• ISSN: 1044-5803; 1873-4189
• DOI: 10.1016/j.matchar.2012.04.004

In this paper, two different ASTM C 618 Class C fly ashes (FA) were used for the production of aluminum metal matrix composites (MMCs) using powder metallurgy (PM) technology. Calcareous FAs were sampled from the electrostatic precipitators of two different lignite-fired power stations: from Megalopolis, Southern Greece (MFA) and from Kardia, Northen Greece (KFA), under maximum electricity load. FAs were milled in order to reduce the mean particle diameter and Aluminum-FA composites containing 10% and 20% of FA were then prepared and compacted. The green products were sintered for 2h at 600°C. Sintered Al-FA MMCs showed increased hardness and wear resistance suggesting their possible use in industrial applications for example in covers, casings, brake rotors or engine blocks. As most possible industrial applications of MMCs not only require wear resistance, but also corrosion resistance in different mild aggressive medias, this paper aims to study the electrochemical behavior of FA MMCs in order to evaluate their corrosion resistance. The morphology and chemical composition of the phases in the Aluminum-FA composite samples were investigated using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDXS). Moreover, topographic and Volta potential maps were acquired by Scanning Kelvin Probe Force Microscopy (SKP-FM). Volta potential maps provide information about the electrochemical behavior of the different phases in absence of electrolyte. The electrochemical behavior was investigated by Open Circuit Potential measurements and potentiodynamic polarization, while the corrosion mechanisms were studied by SEM observations after different times of immersion in a mild corrosive medium. In all cases it could be stated that the addition of the FA particles into the Al matrix might cause an increase of the hardness and mechanical properties of the pure aluminum but deteriorates the corrosion resistance. The degradation phenomena occurring on the FA containing samples might be related to the following mechanisms: 1) Partial detachment or dissolution of the FA soluble phases, in particular based on Si, Fe and Ca; 2) dissolution of the Al matrix surrounding the FA particles due to crevice corrosion; 3) Al localized dissolution due to galvanic coupling between the Fe-rich intermetallics and the matrix. ► Aluminum metal matrix composites containing two types of fly ashes have been characterized. ► The microstructure and the electrochemical behavior have been studied using different techniques. ► The addition of FA deteriorates the corrosion resistance of the aluminum. ► Degradation mechanisms: galvanic coupling, crevice corrosion, detachment of FA particles.