The overall aims of this thesis were to conduct a broad survey of possible organic
corrosion inhibitors in near-neutral chloride solutions and to elucidate the
mechanisms of such action.
Altogether, 130 organic compounds were studied as possible corrosion inhibitors for
pure iron, mild steel, copper and aluminium in aerated near-neutral (pH = 8.4)
solutions containing 500 ppm NaCl and 100 ppm NaHCO,, conditions often
encountered in water-based automotive engine coolants. Inhibitor behaviour was
investigated using steady-state electrochemical techniques including polarisation
curves, Stern-Geary and corrosion potential (Em,) measurements.
The organic compounds examined were found to be highly specific in their
inhibitive action toward the metals studied. Typical examples of highly effective
corrosion inhibitors were: sebacate and octanoate for pure iron; oleate and sebacate
for mild steel; benzotriazole and 2-mercaptobenzothiazole for copper; and laurate
and oleate for aluminium.
E, was found to provide a rapid and convenient screening test for evaluating the
inhibitor performance of organic compounds toward pure iron, mild steel and
aluminium but was less useful for copper.
Good organic inhibitors were found to act as anodic inhibitors toward pure iron and
mild steel but as anodic or mixed-type inhibitors toward copper. For aluminium,
the majority of the compounds studied were found to act as anodic inhibitors.
However,However, it was also found that only pit initiation was inhibited, i.e. existing pits
were not prevented from developing. Optical microscopy of pitted aluminium
surfaces indicated their nature varied considerably with inhibition efficiency.
The role of complex formation in organic corrosion inhibitors was found to vary
with the metal. Complexation of either iron(I1) or iron(II1) ions was found to have
an insignificant effect on mild steel. The corrosion rate of copper was found to
increase with the copper(LI) complex stability, thus indicating complex formation to
be the rate-determining step. For aluminium, the observed effects were found to
depend on complex stability. For weak to moderate complexants, inhibitor
efficiency (measured as E,,) increased with increasing complexation. However, very
strong complexing agents were sufficiently stable to dissolve the aluminium oxide
surface, leading to poor inhibition. Aluminium pit morphology was found, using
scanning electron microscopy, to change from hemispherical in the uninhibited
solution to irregular in the presence of complexing inhibitors.
No simple relationships between inhibitor efficiency and molecular structure were
found. However, carbon chain length, the nature of functional group(s) and their
location in the molecule were found to be important but varied according to the
metal.
The inhibiting ability of sebacate (a straight chain C, dicarboxylate) was found not
to be compromised by water movement (stirring) or pre-existing corrosion product
layers. Immersion tests showed that passive film formation on mild steel in
sebacate solution involved two stages and was complete only after -100 h
immersion.
The ion selective properties of several iron(II1) carboxylates and hydrated iron(II1)
oxide films were studied by membrane potential measurements in neutral sodium
chloride solutions. Some specimens were also studied by Mossbauer spectroscopy.
These results show that dicarboxylates are good inhibitors toward mild steel because
they form impermeable films. Poor inhibitor performance is associated with the
anion selectivity of the film which in turn appears to be related to the film purity.
A model is suggested for the inhibition mechanism of mild steel corrosion by
dicarboxylates in aerated near-neutral chloride solutions.
| Identifer | oai:union.ndltd.org:ADTP/221746 |
| Date | January 1991 |
| Creators | Swee Hain Tan |
| Publisher | Murdoch University |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.murdoch.edu.au/goto/CopyrightNotice, Copyright Swee Hain Tan |
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