Investigation of the corroded surface of SiCw/Al composite
Introduction
In recent years much attention has been paid to the scanning probe techniques, the best known and most widespread representatives of which are the scanning tunneling microscope (STM) and the atomic force microscopy (AFM). Surface manipulation by AFM is unfolding many interesting and promising new fields [1], [2].
SiCw/Al composite can be used in the aircraft industry and marine environments because of their excellence properties and low density [3], [4]. But it is susceptible to localized corrosion such as pitting and stress corrosion. These kinds of localized corrosion are dangerous to the safety since the crevices resulted from localized corrosion may lead to cracks of a structure. Thus the study on the corrosion properties of the composite is very importance for their practical applications.
To understand a corrosion process, it is essential to study the initiation of the corrosion process. For this purpose, observations of surface change during corrosion are useful because they provide a record of the corrosion process. However, it is difficult to observe the initial stages of corrosion by conventional microscopy because of its limited resolution. AFM has the potential to characterize the surface from the atomic lever to the nanometer scale.
The purpose of this article is to report the change in the surface morphology of SiCw/Al composite, focus the behavior in the early stages of the corrosion process, and study the influence of the interface between whisker and matrix and the matrix in the composite on the corrosion behavior.
Section snippets
Experimental
The composite used in this study is SiC whisker-reinforced pure aluminum composite. The composite was fabricated by squeeze casting. The size of the SiC whisker ranged from 0.1 to 1 μm in diameter and 30 to 100 μm in length. The volume fraction of the whisker was about 20%. Before testing, the investigated surfaces were first wet ground with grit paper to 2400 grit, and then polished with diamond paste, cleaned mechanically with pure ethanol and acetone and dried naturally in air. The specimens
Results and discussion
Typical AFM images of the surface of the SiCw/Al composite are shown in Fig. 1. Fig. 1(a) is the AFM image of the surface of the SiCw/Al composite before corrosion, it can be seen that some of whiskers presented on the surface. The interfaces between whisker and matrix in the composite are smooth, and holes are difficult to be found from these interfaces. Fig. 1(b) and (c) are the AFM images of the corroded surface of the composite exposed to 3.5% NaCl solution for 30 and 60 min, respectively.
Conclusions
The surface of the composite becomes rougher during corrosion; the average surface roughness increases with the corrosion time. Corrosion occurs preferentially at the interface between whisker and matrix, and pits formed at the end of the whisker and propagated along the interface. The matrix far away the whisker is corroded because of the presence of localized defects. Some of smooth peaks appear on the corroded surface with the progression of corrosion, but they disappear with increasing the
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