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Title: The corrosion and anodizing behaviour of Al-alloy/SiC composite.
Author: Shahid, Muhammad.
ISNI:       0000 0001 3401 6818
Awarding Body: University of Manchester, Institute of Science and Technology,
Current Institution: University of Manchester
Date of Award: 1990
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Literature concerning composite materials, including their physical and chemical properties, and their corrosion and anodizing behaviour, has been reviewed. Stemming from this it is evident that their detailed corrosion behaviour, with resultant protective methods, requires further elucidation. A composite material consisting of AI-alloy 8090 (AI, 2.4 Ll, 1.2 Cu, 0.7 Mg, 0.09 Si, 0.09 Fe, 0.12 wt % Zr) with 20 wt % silicon carbide particulates, fabricated by powder metallurgy, has been characterised in terms of structure and physical appearance, using optical and electron microscopy. The composite contains irregularly-shaped silicon carbide particles, up to 12 um in size, which are distributed non-uniformly within the matrix. Sharp and distinct interfaces between particles and the matrix, containing no readily visible reaction zone, have been revealed. The elemental chemistry of the composite has been investigated using EDX analysis associated with transmission electron microscopy, X-ray diffraction, Auger electron spectroscopy and secondary ion mass spectrometry. A reaction zone at the particle/matrix interface is not readily evident. However, alloying elements such as copper, magnesium, silicon and iron are revealed at the interfaces which indicates diffusion of these elements towards interfaces. This presumably implies the formation of a reaction zone or fine precipitates, however, such zones are not observed directly. Lithium is segregated and non-uniformly distributed in the matrix. Electropolishing and etching of the surface of the composite in acidic and alkaline solutions are unable to provide a smooth, particle-free surface; therefore, particle/matrix interfaces cannot be eliminated to improve corrosion resistance and a particle-free surface is not available for subsequent anodizing. Corrosion behaviour of the composite has been examined using immersion and salt spray tests, and with electrochemical techniques, in sodium chloride and sodium sulphate solutions; sodium chloride is an aggressive solution for the composite, whereas sodium sulphate is comparat ively mild. Corrosion of the composite in sodium chloride solution initiates by the dissolution of aluminium mainly adjacent to particle/matrix interface and is enhanced by crevice corrosion effects. The crevices are provided by the interfaces between the differently natured materials. Silicon carbide particles are chemically inert in the previously mentioned environments but enhance corrosion by providing crevices at the interfaces with the matrix. No galvanic corrosion is evident between silicon carbide and aluminium. The anodizing behaviour of the composite has been investigated in phosphoric and sulphuric acids. Anodizing of the composite in phosphoric acid is impractical since only the matrix is anodized and particles are lost during anodizing; due to the high chemical and relatively low field assisted dissolution rates in the acid. formation of a continuous and relatively thick anodic film is not possible. The compositeo however. is more readily anodized in sulphuric acid; the matrix 15 anodized and particles are occluded within the anodic film. The porous anodic film formed in sulphuric acid can afford corrosion resistance to the composite material during subsequent exposure to chloride-containing environments
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Material degradation & corrosion & fracture mechanics