Abstract:
In this work, the effect of alloy surface pre-treatments on the corrosion behaviour of
sol–gel hybrid coated EN AW 6063 alloy was studied. Four pre-treatments were
used: solvent degreasing; alkaline degreasing followed by acid etching; alkaline
degreasing followed by alkaline plus acid etching and alkaline degreasing followed by
acid etching and oxide growth in boiling water. SEM/EDS revealed that the alloy
surface depends on the pre-treatment. The alkaline plus acid cleaned samples
exhibited a very smooth deoxidized surface. All the others showed a rougher surface
with an oxide layer which is more homogenous and denser in the samples subjected
to the oxide growth step. The sol-gel hybrid coating was synthesized from
glycidoxypropyltrimethoxysilane (GPTMS) and zirconium n-propoxide (TPOZ)
precursors and applied to metallic substrate by dip-coating. The corrosion behaviour
of the hybrid coated samples was evaluated by Electrochemical Impedance
Spectroscopy (EIS) and by an accelerated corrosion test. Complimentary to the
corrosion study, the adhesion of the hybrid sol-gel coating was evaluated by crosscut
and bend tests. The corrosion tests revealed that the pre-treatment that includes
the oxide growth step leads to the best protection performance, followed by the
alkaline degreasing and alkaline plus acid etching one. However, the latter exhibits
the worst adhesive behaviour, attributed to the smoother, deoxidized surface of the
substrate.
