| dc.description.abstract | Filiform corrosion (FFC) is a type of localized corrosion process that affects metals such as aluminium, magnesium and steel under certain environmental circumstances. FFC often occurs in environments combining marine and industrial atmospheres and it usually only degrades the aesthetic appearance of the structure. However, in certain cases FFC can lead ---in addition to the coating degradation--- to other types of more severe localized corrosion processes as pitting or intergranular corrosion (IGC). FFC is a concern mainly for automotive and building industries due to the increased use of coated aluminium. Several studies have demonstrated that two of the most important factors in decreasing the susceptibility to FFC of coated aluminium alloys are surface pretreatment and alloy composition.
In this work we review the current state-of-the-art understanding of surface pretreatments and FFC mechanism discussing open questions and possible solutions. For this reason, this work focuses on the mechanistic study of some common surface pretreatments, on the FFC mechanism, and on the effect of surface pretreatments and production site on FFC.
Analysis of surface pretreatments as alkaline etching and conversion coating allow a better understanding of the mechanism behind surface structure and composition induced by the exposure of aluminium to different solutions commonly used both in laboratory and in industrial lines.
The electrochemical dissolution of aluminium during alkaline etching is a transport limited process whose mechanism depends on the alloy composition. A defined pattern of enrichment of elements nobler than the base alloy is observed during alkaline etching.
During conversion coating --- if Cu2+ ions are used as additives in the conversion bath --- the formation of a complex multi-elemental conversion layer takes place and despite an increased cathodic activity of the surface the protection against FFC is improved hinting to an improved coating adhesion.
The comparison of the electrochemical properties and corrosion resistance of samples with different composition gives hints on the effect of different alloy elements on FFC. Dynamic electrochemical impedance spectroscopy and FFC tests reveal the beneficial effect of Mn on the FFC resistance of aluminium alloys.
The analysis focused on the filiform corrosion mechanism sheds light on the formation of reaction intermediates as γ-AlH3 and allows the observation of phenomena such as gas evolution and displacement of corrosion products during the filament propagation.
Finally, the evaluation of the FFC resistance of samples with different composition, surface pretreatment, and production site helps understanding the importance of these three parameters on the surface structure and composition and on turn on the FFC susceptibility. Etching of 0.5 g/m2 of surface is not sufficient to reduce the FFC susceptibility below industrially accepted levels if the surface enrichment of Mg of the as casted sample is too high. The presence of Mg (hydr)oxides can also hinder the formation of a thick and homogeneous conversion layer. Etching of 2 g/m2 of surface tend to reduce the difference in FFC susceptibility between samples with different composition, successive surface pretreatment, and production site allowing to reach FFC resistances well below the industrially accepted levels. | en_US |
