The corrosion and anodizing behaviour of Al-alloy/SiC composite.
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