W. Richard Bowen and Nidal Hilal 4

134 4. INvEsTIgATINg MEMbRANEs ANd MEMbRANE PROCEssEs
Surface roughnes (nm)
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
First
stage
Second
stage
y = –0.4907x 2 + 2.2621x–1.1032
R 2 = 0.9906
0 0.5 1 1.5 2
Time (h)
FIgURE 4.29 Change in roughness of steel surface over time during exposure to NaCl
solution. Reproduced from [19].
It is apparent that the roughness of the surface increased in two stages.
In the first stage, no significant roughness increase was observed. After
about 35 min, the roughness increased at a rapid rate, which could be fitted
by a simple polynomial expression. The authors attributed the first stage to
the formation of a thin layer of corrosion products, whilst the second stage
was due to a selective attack of the ferrite phase of the steel.
Other studies have concentrated on the corrosion of surfaces due to
more aggressive corrosive agents. Wang et al. [20] studied the corrosion
of stainless steel over a number of days by sulphuric acid microdrops and
thin films using a combination of AFM and XPS and observed the formation
of pits and corrosion products. Solmaz and co-workers [21] examined
the corrosion of mild steel by HCl solutions with and without the pres-
ence of a corrosion inhibitory agent, using AFM to study changes in
surface morphology in combination with a number of other techniques.
When the surface was exposed to the HCl solution alone, corrosion pits
were observed to form, which became both wider and deeper as the exposure
time increased. When the surface was exposed to the HCl in combination
with 10 mM 2-mercapto thiazoline, the surface was more uniform than
surfaces exposed to HCl alone after 24 h exposure. For longer exposure
times (approximately 120 h), a smoother surface was still evident when
compared to treatment in the absence of 2-MT. A similar study by Qu et al.
[22] examined the efficacy of a different additive (EDTA) on reduction of
corrosion of steel surfaces by HCl.
An interesting study was carried out by Valtiner et al. [23] into the effect
of pH on acid dissolution of crystalline ZnO surfaces, which is often used
as a protective coating for steels. It is the stability of the thin oxide covering
that determines the corrosion resistance of zinc, and hence metal coated by
it. The single crystalline surfaces were observed to consist of large flat sections
several micrometres across, with step heights at the edges of 4–10 nm.
When immersed in electrolytes of pH 11–5.5, no dissolution was observed.
2.5