Abstract
One of the most common ways of material degradation in metals iscorrosion. It leads to large economical losses due to premature failure of
diverse materials. Corrosion takes place when metals are in contact with an
aggressive medium, for example a high chloride contamination, acidic or
humid environment.
Carbon steel is an alloy widely used for structural purposes, in particular, it
is commonly used in marine applications due to its high yield stress and
relatively low cost. Of all the environments that could be in contact with a
metal, marine media stands out due to the presence of a high concentration
of chloride ions and potentially bacteria, that aid electrochemical processes
leading to metal loss, i.e., corrosion Usually, steel in such environments requires a combination of coatings to delay the onset of corrosion, which traditionally consisted of at least of one chromate-based conversion coating followed by several organic coatings on top: primer, intermediate and top coat. Chromates have now been banned due to environmental concerns and thus sol-gel based coatings have been considered a potential non-toxic alternative.
This work proposes a system to battle the natural occurring process of
corrosion on carbon steel in aggressive chloride environments. Hybrid
organic-inorganic coatings synthesized by the sol-gel method are proposed
as an alternative to chromates due to their good adhesion to metal via
siloxane bonds. They have also traditionally shown very good compatibility
to organic coatings. Therefore, the aim of this work is to develop a hybrid
organic-inorganic coating, which offers a passive barrier effect to the bare
steel alloy whilst also providing an active protection through the
incorporation of environmentally friendly corrosion inhibitors. This coating
could replace not only the conventionally chromate-based conversion
coatings, but also the primer coating.
To carry out this work, rare earth carboxylates such as lanthanum 4-hydroxy
cinnamate [La(4-OHCin)3·5H2O], have been directly incorporated to
different hybrid organic-inorganic coating matrixes (silicon and silicontitanium-based) synthesized by the sol-gel method.
The effect of La(4-OHCin)3 in hybrid silica-epoxy (Si-based) and silicatitania-epoxy (Si-Ti) coating formulations on the corrosion protection of the
coatings is studied through EIS, surface characterisation techniques such
as SEM-EDS and ATR/FT-IR. In addition, the influence of the corrosion inhibitor on the chemistry of the sol-gel process and the final structure and properties of the coatings is evaluated by NMR and thermal analysis such as DSC and TGA.
The silica-epoxy and silica-titania-epoxy coating formulations were also
modified with the aim of reducing the temperature or the time of the thermal
treatment given to the coatings, for this the use of amines as curing agents
is explored. Rare earth carboxylates are also added to these formulations
and their effect investigated. Finally, the best performing coatings were
evaluated through standard industry tests used to determine the corrosivity
category of the complete coating systems. They are also compared with
two commercially available paint systems.
The better corrosion protection of carbon steel of hybrid coatings containing
La(4-OHCin)3 has been demonstrated. Moreover, the concentration of the
corrosion inhibitor in the coating played an important role. In addition, the
different precursors used on the coating synthesis influenced the final
coating properties, i.e. when a curing agent was added to decrease the
temperature of the thermal treatment, an unwanted reaction between the
rare earth carboxylate and the curing agent led to poorer corrosion
properties.
| Date of Award | 2021 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Supervisor | Anthony Somers (Supervisor) & Fabiola Brusciotti (Supervisor) |