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

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Abstract

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.

Highlights

▸ 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.

Introduction

In automotive industry, the aluminium amount contained in the cars is in continual increase. This evolution can be explained by the research of a decreasing weight. The corrosion protection of parts of the car body is often obtained by a cataphoretic epoxy primer. Nevertheless, aluminium is recognized as a substrate difficult to cover by using the conventional phosphatized conversion layer applied before cataphoretic coating [1], [2]. Furthermore, aluminium alloys are also known to be particularly sensitive to a specific type of corrosion: filiform corrosion. It is produced between the coating and the substrate by formation of filaments [3]. The filaments generally follow irregularities on the surface or defects in the organic coating. Galvanic corrosion develops between the head front of the filament (anode) and the tail (cathode) [4], [5], [6]. At the anode, the most important reaction is the oxidation of the aluminium while at the cathode, the tail of the filament is the area where the reduction of O2 in OH ions occurs. This corrosion phenomenon can induce delamination of the coating and formation of corrosion products (aluminium oxides and hydroxides). Filiform corrosion is controlled by many parameters such as the metallic substrate [7], [8], surface preparation [9], [10], presence of defects, permeability of the coating to water and oxygen [11], adherence of the paint system and the presence of chlorides [12].

Aluminium surfaces are pre-treated prior the application of the paint coating in order to improve adherence and corrosion resistance properties of the coating system. Chromium conversion treatments involving the use of chromic acid containing Cr6+ species have been used for their corrosion protection and adhesion promotion performances. However hexavalent chromium presents very high toxicity and has a bad environmental impact [13]. The legal consequence is the prohibition of the chromium(VI) conversion. Numerous papers refer to the alternative of this conversion: pre-treatments based on cerium and lanthanium salts, zinc, Zr–Ti, Cr3+ conversion layers, phosphates [14], [15], [16], [17], [18], [19]. Among these alternatives organofunctional silane molecules are used as environmental friendly pretreatment and applied onto the metal via a sol–gel route. The sol–gel coatings may thus be used as coupling agent between metallic substrate and cataphoretic paint.

The aim of this work is to study the improvement of the barrier protection and filiform corrosion resistance of 6016 electrocoated aluminium by applying a new environmentally friendly silane sol–gel pretreatment. The pretreatment is an aqueous based silane mixture. The hybrid organic/inorganic silane sol–gel film was synthesized [20], [21] by mixing three different silanes compounds: glycidyl–oxypropyl–trimethoxysilane (GPS), tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES). γGPS is an organofunctionalized silane composed of a short carbon backbone with an epoxy functionalized tail and a Si atom substituted with (–O–CH3) groups. Due to the presence of the epoxy group, this molecule ensures an active interaction with the organic coating. The compatibility of this silane layer with electrocoating process was previously investigated [22]. The hydrolysis was performed in acidic conditions, modifying the natural pH of the solution by adding hydrochloric acid. The acidic conditions act as a catalyst, promoting the hydrolysis reactions. TEOS and MTES, after hydrolysis, form an inorganic network on metallic surfaces and give rise to the inorganic phase responsible of the barrier properties of the silane film [23].

In order to evaluate the hydrolysis conditions, two pH were tested: 2 and 3.5. The protection properties of the silane layers were determined by anodic and cathodic polarization curves, EIS measurements and FTIR spectra. The curing temperature of the cataphoretic coatings was varied in order to modify their mechanical properties. The filiform corrosion resistance of the complete system was determined by a normalised filiform corrosion test and electrochemical impedance measurements on intact coatings.

Section snippets

Materials

The tests were carried out on 6016 aluminium alloy samples. The composition of this alloy is summarized in Table 1.

The samples (100 mm × 50 mm × 1 mm) were degreased with acetone, etched for 10 min in a commercial acid bath (Henkel Ridoline® 124N + Novox Activator® 12B) and then rinsed with deionised water. The substrates were pre-treated using the following chemical treatments:

  • Without pretreatment.

  • A commercial silane called Oxsilan 9800 by Chemetall (H2O, H2ZrF2, organo-silanol and additives): samples

Silane layers

The presence of many hydroxyl groups on the aluminium surface induces a good wettability by the silane solution. The deposition of the silane layers on aluminium surfaces shows consequently a very good coverage. The thicknesses of the hybrid films deposited at pH 2 and 3.5 are 270 and 280 nm, respectively.

The comparison between the FTIR spectra of the sol–gel film prepared at pH 2 and 3.5, both cured at 90 °C during 15 min, is given on Fig. 1.

The FTIR spectra between the two sol–gel films are very

Conclusions

A mixture of three different silane molecules (GPS, TEOS, and MTES) was hydrolysed at two pH (2 and 3.5). The time and the temperature of the curing were optimized in order to obtain homogenous silane layers. FTIR spectra show that the two silane layers are polymerized in the sol–gel film. The pH of the silane mix solution induces an important modification of the sol–gel film construction. At pH 2, the layer is less dense than at pH 3.5 due to lower condensation of the silane. As a consequence

Acknowledgements

This study was also done in the framework of the Opti2mat “Programme Excellence” financed by the Région Wallonne (Belgium). The authors like to thank PPG (France) for supplying the cataphoretic coating bath, Chemetall for providing the Oxsilan 9800, Damien Cossement and Fabian Renaux for the ToF–SIMS analysis.

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