In Situ Determination of Corrosion Products Formed on Painted Galvanized Steel by Raman Spectroscopy
Résumé
Organic coatings are widely used to
protect metals against corrosion. In the
absence of active anticorrosive pigments,
the principal mechanism of corrosion protection
consists in blocking of diffusion of
corrosive agents like water or oxygen, (I).
However, due to the heterogeneous character
of paint films corrosion may occur. The
anodic reaction is generally initiated at
coating defects and drives the cathodic
reaction in an adjacent region. From this
follows that the knowledge of the composition
and structure of corrosion films
formed under the paint is of extreme importance
in order to better understand the
mechanisms of corrosion protection by
organic coatings.
During the last decade Raman spectroscopy
has been successfully used for
in-situ identification of corrosion products
formed on various metals and different
environments, (2-4). The main advantage of
this technique is the possibility of in-situ
structural and chemical identification of
corrosion products formed on metals under
potentiostatic control.
There have been numerous studies that
have tried to determine the nature of
cor rosi on products formed on iron and
galvanized steel after the deterioration of
organic coatings (see for instance 5).
However, most of these investigations are
done in ex-situ conditions from which may
follow changes in corrosion products when
the sample is exposed to air or vacuum.
Therefore, we report here for the f i r s t time
in-situ Raman spectra of corrosion products
formed after blistering of the paint which
were obtained without removing the organic
coating or the electrolyte. This work forms
part of an investigation that aims to better
understand the mechanisms of underfi I m
corrosion.