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Effects of porosity in a model of corrosion and passive layer growth
We introduce a stochastic lattice model to investigate the effects of pore
formation in a passive layer grown with products of metal corrosion. It
considers that an anionic species diffuses across that layer and reacts at the
corrosion front (metal-oxide interface), producing a random distribution of
compact regions and large pores, respectively represented by O (oxide) and P
(pore) sites. O sites are assumed to have very small pores, so that the
fraction of P sites is an estimate of the porosity, and the ratio
between anion diffusion coefficients in those regions is .
Simulation results without the large pores () are similar to those of
a formerly studied model of corrosion and passivation and are explained by a
scaling approach. If and , significant changes are
observed in passive layer growth and corrosion front roughness. For small
, a slowdown of the growth rate is observed, which is interpreted as a
consequence of the confinement of anions in isolated pores for long times.
However, the presence of large pores near the corrosion front increases the
frequency of reactions at those regions, which leads to an increase in the
roughness of that front. This model may be a first step to represent defects in
a passive layer which favor pitting corrosion.Comment: 8 pages, 6 figure
Roughness exponents and grain shapes
In surfaces with grainy features, the local roughness shows a crossover
at a characteristic length , with roughness exponent changing from
to a smaller . The grain shape, the choice of
or height-height correlation function (HHCF) , and the procedure to
calculate root mean-square averages are shown to have remarkable effects on
. With grains of pyramidal shape, can be as low as 0.71,
which is much lower than the previous prediction 0.85 for rounded grains. The
same crossover is observed in the HHCF, but with initial exponent
for flat grains, while for some conical grains it may
increase to . The universality class of the growth process
determines the exponents after the crossover, but has no
effect on the initial exponents and , supporting the
geometric interpretation of their values. For all grain shapes and different
definitions of surface roughness or HHCF, we still observe that the crossover
length is an accurate estimate of the grain size. The exponents obtained
in several recent experimental works on different materials are explained by
those models, with some surface images qualitatively similar to our model
films.Comment: 7 pages, 6 figures and 2 table
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