BEM Analysis Of The Protective Mechanism Of
Patinas Against Atmospheric Corrosion
Author(s)
A. Peratta & E. Ayll´on
Abstract
Weathering steels are characterised by a low content of alloying elements such as
Cu, Cr, Ni, P, and Si, which contribute to the outdoor formation of a corrosion
product adherent film called a patina. Its attractive appearance and passivity power
towards the environment to which it is exposed makes paint or other coatings for
initial protection and subsequent maintenance unnecessary. Since its introduction
in 1933, these attributes have spread its use to a large variety of structural applications
such as buildings, bridges and electric power transmission line towers. Patinas
are made up of compounds of α, β, γ, and δ iron oxyhydroxides (FeOOH) and
magnetite in different proportions. Their composition, density and morphology
depend on the environmental conditions of the place they are formed in. At present,
no model explaining patina behaviour as an effective protection of steel against
atmospheric corrosion has gained general acceptance among researchers. The purpose
of this work is to propose a porous media model that shows the influence of
patinas compactness as a protector. The initial stages of the attack during exposure
without outdoor protection is in fact a selective one, being the non-metallic
inclusions the most remarkable points of corrosion. The controlling mechanism of
the patina protective attribute results from the compactness provided by the microalloying
elements which leads to a decrease of the pores and particle size. The
transport model of Fe2+ through the patina for weathering steels was analysed
by a 3D computational code based on the Boundary Element Method for solute
transport in porous media, and compared to the transport through non-weathering
steel, thus relating the electrochemical nature of the corrosion to the porosity of
the patina.
Keywords
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