REPOSITORY > RESULTS

Abstract

This thesis mainly concerns with developing a technique using AFM to enable the in-situ
measurements of samples under different conditions in liquid. Not only the technique can
be applied to corrosion investigation, but can in principle be used in any electrochemical
study that requires real-time monitoring of the change of topography during exposure to
selected electrochemical conditions.. Unlike conventional electrochemical methods, which
give average information about surface properties the use of AFM, allows detection of
surface changes on nanoscale.
In order to study the corrosion or electrochemical reactions, an electrochemical cell was
designed. The new design enables easy access to the AFM scanner from the top. It also
allows using a different sample size of the working electrode as well as the usage of the
comparably large volume of liquid or electrolyte. Furthermore, the cell can be used as a
fluid flow cell and most importantly, allows locating the same spot in the air and after
injecting the liquid into the cell. Two designs having appropriate functionalities were
fabricated. Based on a simulation using the finite element approach, which revealed the
effect of electrode geometrical arrangement on the electric field distribution inside the cells,
one of the two possible cells was selected for further studies. The designed cells were used
to study the corrosion action of aluminium alloy AA2024 and copper, with different
coatings under different conditions, in NaCl (0.5M) solution as a corrosive environment.
The corrosion of aluminium alloy was studied under different conditions. First, AA2024
was studied in NaCl (0.5M) without applying any electrical potential to find the
spontaneous change of the surface. Then different chronoamperometric pulses were applied
on a different sample to find how the surface changes. Afterwards, AA2024 was gel-coated
by dip coating method such that different experiments were carried out while measuring
electrochemical impedance and open circuit potential, chronoamperometric and
chronopotentiometric tests were conducted.
Copper samples were tested in analogy to the aforementioned tests of aluminium. Bare
copper samples were tested for a spontaneous corrosion and dissolution of grains was
detected whereas electrochemical impedance and open circuit potential were measured at
the same time of corrosion to understand the relation between the dissolution of the copper
and the variation of electrochemical impedance.
A thin film of gel layer was used to coat copper samples by dip coating method.
Different experiments were carried out to test the coating resistance to corrosion. The first
experiment consisted of a long time immersion of sample without applying any external
potential and the electrochemical impedance as well as open circuit potential were
measured as a function of time. The next test involved application of a step potential from
the cathodic to anodic direction to monitor the dissolution of the film. At last, cyclic
voltammetry was carried out to check the possible destructive nature of this technique
with respect to tested samples.
In the last set of experiments, a thin film of graphene oxide was applied to copper
samples by drop casting and then partly reduced by annealing in argon atmosphere. The
reduced graphene oxide coated samples underwent several tests. By analogy to previous experiments, the first test comprised the immersion of coated sample for a long time in the
electrolyte and monitoring the surface properties with time. Then the step potential
excitation was carried out followed by extensive chronoamperometric testing in conjunction
with AFM measurements.