Silane sol–gel film as pretreatment for improvement of barrier properties and filiform corrosion resistance of 6016 aluminium alloy covered by cataphoretic coating

The aim of this study is to develop a newly silane sol–gel pretreatment on the barrier properties and filiform corrosion resistance of 6016 aluminium alloy covered by cataphoretic coating. The sol–gel coatings are used as coupling agent between aluminium substrate and cataphoretic paint. The pretreatment is an aqueous solution of three different silane compounds (glycidyloxypropyltrimethoxysilane (GPS), tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES)) hydrolysed at two different pH (2 and 3.5). A system without pretreatment was studied as reference. The electrocoatings were cured between 155 °C and 195 °C in order to modify their mechanical properties.Polarisation curves, EIS and FT-IR measurements were used in order to characterize the silane layers. EIS measurements were used to follow the barrier properties and the water uptake evolution on intact coatings.The filiform corrosion protection of the coating was also evaluated by a normalized filiform corrosion test.

► The pH value of a silane mixture induces an important modification of the sol–gel film construction.
► A cataphoretic electrocoating cured at different temperatures were applied on the sol–gel film.
► The electrocoated Al 6016 samples present very good barrier properties and weak water uptake.
► The sensitivity to filiform corrosion was evaluated by a normalised filiform corrosion test.
► The sol–gel film at pH 2 and an electrocoating at 155 °C is the best resistance to filiform corrosion.