O. OGUNDARE ET AL. 917
Before exposure After 120 Hrs After 720 Hrs After 1200 Hrs
Figure 5. Microstructures of ASS before and after immersion in sodium chloride (400×).
The microstructures of DI (Figure 5) showed that the
corrosion produ cts gradually covered up the nodular ma-
trix as the immersion time increased. Initially, the cou-
pons revealed evenly distributed nodules in the pear-
lite/ferrite matrix of ductile iron.
Figure 5 shows the microstructures of ASS before and
after immersion in NaCl at times 0 hr, 120 hr, 720 hr and
1200 hr. It shows the morphologies of the surface corro-
sion product in which points of possible initial pitting
corrosion are revealed. The volume of pits formed on
each ASS coupon also increased progressively with time.
4. Conclusion
This paper has reported the study of corrosion behavior
of DI and ASS in table salt representing an extreme ma-
rine environment as the media. After 1200 hr, the corro-
sion rate of DI was 1.1 × 10–4 mpy compared to that of
ASS which was 6.8 × 10–7 mpy. The fact that ASS out-
performed DI has been established but what this work
has established is an almost 4 orders of magnitude dif-
ference in corrosion rate between the two materials. This
is not unconnected with the presence of chromium in
ASS coupon s which greatly adds to the overall cor rosion
resistance through a passivation process by forming a
complex spinel-type {(Fe,Ni)O(Fe,Cr)2O3} passive film.
While corrosion rate of DI increased progressively with
immersion time that of ASS decreased at comparable
immersion time periods.
5. Acknowledgements
The authors appreciate the support provided by the man-
agement of Engineering Materials Dev elopment Institu te,
Akure, Nigeria where the entire bench work was carried
out.
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