Final Report - Strategic Environmental Research and Development ...

5.7. Task 7: Characterization of Local Environments in Coating Systems
5.7.1 Objectives and Background
The objective of this research is to develop a technique to supplement traditional investigation
approaches for characterizing corrosion of coated metals. Studies reported here focus on the
effects of the metallurgical aspects of thin films and the local environments in organic coatings
on pit formation, pit growth kinetics and pit morphology.
Much effort has been applied to characterizing the effect potential, pH and chloride
concentration on localized corrosion of bulk materials. This work has extended to studies of the
influence of these variables on pit current densities [1]. In such studies, the determination of
active pit surface area is critical for accurately calculating the anodic current density [2].
However, pit area is difficult or impossible to determine for 3D pit morphologies [3]. Most
investigators assume that pits are hemispherical. However Hisamatsu et al. studied the growth of
a single pit in stainless steel and cross-sectioned the pit after it grew for sometime. It was found
that the pit depth was much less than the pit mouth radius [4]. In order to develop precise models
of pit morphologies, some researchers have simulated pitting corrosion by using a 2D model that
involves deposition of thin films on inert substrates [1-3]. Since pits can rapidly penetrate the
thickness of the film and then grow outward in an easily measureable way, thin film pitting
provides a unique opportunity to study pit propagation in situ. Frankel found that under
potential-controlled conditions, both current density and pit morphology depended on the applied
potential [2]. At high potentials, pit growth is fast and rounds pits prevail as dissolution is
controlled by mass transport. Mass transport-controlled growth is reflected by constant current
density over a range of potentials. At low potentials, thin film pits exhibit convoluted
morphologies and dissolution occurred under ohmic control where the current density increases
linearly with potential.
Measurement of pit growth under open circuit conditions is very difficult to study because pit
growth rates are much slower [5]. Open circuit potential (OCP) pit growth is “natural” as the
cathodic current resource derives solely from the cathodic reaction that occurs on the sample
itself. This is an important aspect of experiments aimed at studying the effect of coatings and
inhibitors where inhibition of the cathodic reaction is one of the primary strategies for controlling
corrosion. Generally speaking, OCP pit morphology is more variable because it is constrained by
the rate of the supporting cathodic process [1]. Results show that pit morphologies range from
fractal-like patterns to smooth circular walls depending on the specific environmental conditions.
For example, high concentrations of Fe 3+ provided the additional cathodic reaction which lead to
a circular pit growth front in the pitting of iron. However, the anodic current density reaches a
limiting value when the cathodic reaction is controlled by mass transport. Fractal patterns arise
when the local convention of hydrogen bubbles dilutes the aggressiveness of the solution at the
vicinity of the active pit surface resulting in the passivation of that area.
In the case of Al alloys, additions of Cu to Al matrix are known to increase the susceptibility
pitting corrosion in thin films by affecting the oxide layer thickness and integrity [6-7]. The
formation of constituent θ phase (Al 2 Cu) also acts as cathodic reduction site, increasing the
corrosion rates and as a result, the distribution and microstructure of Cu in Al-Cu thin films has
been studied [8]. Conversely, when Cu is retained in solid solution, it acts as a pitting inhibitor.
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