A synergistic combination of tetraethylorthosilicate and multiphosphonic acid offers excellent corrosion protection to AA1100 aluminum alloy
Graphical abstract
Introduction
The 1xxx series aluminum alloys, which represent the purest commercially available metal, offer the highest corrosion resistance compared to other aluminum alloys. However, among the 1xxx series, the alloy AA1100 has the highest rate of corrosion due to the presence of a relatively high percentage of alloying elements, such as iron, silicon and copper [1]. Copper and some silicon are present in solid solution, while iron and the remaining silicon form intermetallic particles [2]. The main intermetallics are Al6Fe, Al3Fe and Al12Fe3Si2, all of which exhibit cathodic activity relative to the aluminum matrix [1]. In aqueous solutions, these intermetallic particles are preferential sites for the oxygen reduction reaction, while the alloying elements within the matrix undergo anodic dissolution [3], [4]. Alloy 1100 is employed in conventional engineering applications in which corrosion resistance is required but strength is relatively unimportant (e.g., architectural, appliance trim and low pressure tubing for carrying chemicals and foodstuffs). Therefore, the improvement of the AA1100 corrosion resistance is a topic of great importance.
Sol–gel processing using silane treatment has been reported to be one of the promising methods to replace chromate conversion coatings [5], [6], [7], [8], [9]. Sol–gel films exhibit good adherence to metallic surfaces due to bonds formed between the metallic surface and silanol groups, i.e., metalo-siloxane linkages. Many research works have evidenced that the deposition of silane compounds onto aluminum alloy substrates improve the corrosion resistance and the adhesion properties [5], [6], [7], [8], [9], even though results reported for the AA1100 series are scarce. For instance, Zand and Mahdavian [7] evaluated the effect of vinyltrimethoxysilane molecules (VTMS) on the corrosion resistance and adhesion strength of epoxy-coated AA1050. At pH 5.0, below the isoelectric point (IEP) of aluminum surfaces (IEP = 8.4), the surface is positively charged; therefore, the VTMS molecules in the form of δ+CH2CHSiOH are more prone to react with the aluminum hydroxide. Electrochemical impedance spectroscopy (EIS) measurements showed that the corrosion protection imparted by films formed at pH 5.0 is comparable to that observed in chromate samples. These results were similar to those previously obtained by Mohseni et al. [8] using amino- and vinyl-silane treatments or by Feng et al. [9] using glycidoxypropyltrimethoxysilane (GPTMS) and tetra-n-propoxyzirconium (TPOZ). The increase in the organic group content was also found to reduce the shrinkage of the coating and the tendency for crack formation.
Occasionally, pores and cracks in silane films allow the electrolyte to access to the metal surface, reducing the corrosion protection. To improve the corrosion resistance of these coatings, inhibitors can be added to the sol–gel matrix (Scheme 1a). Phosphonic acids have been reported to be effective corrosion inhibitors for aluminum alloys [10], [11] and may form self-assembled monolayers (SAMs) [12], [13], [14], [15], [16]. However, to the best of our knowledge, no previous work has been devoted to investigate the synergy between phosphonic acids and silane films, explaining why this combination is able to offer better protection to aluminum alloy systems. Lecollinet et al. [16] evaluated the behavior of six phosphonic acids as SAMs onto titanium, stainless steel and silicon substrates. The authors categorized phosphonic acids into two groups [16]. The first one contained molecules bearing simple alkyl chains, while the second, which was considered the most hydrophobic, included a perfluoropolyether linked to a bisphosphonate moiety with very interesting characteristics. According to these authors, linear alkyl chains are less voluminous than perfluoroalkyl ones, and lateral branched arms can increase the density of the packing. The use of organophosphorus coupling molecules in sol–gel processing is very attractive because their reactivity is quite different from that of silicon coupling molecules, which results in hybrid materials with different structures and stabilities [17]. Within this context, the protection against the corrosion of AA2024 imparted by phosphonic acid-containing tetraethylorthosilicate (TEOS) films has been recently investigated by our group [18], [19]. However, we have concluded that the improvement in the anticorrosion performance of silica-based coatings requires a better understanding about the influence of the phosphonic acid structure.
This work aims at comparing the sol–gel films obtained by combining the TEOS matrix with two different organophosphonic acids (Scheme 1b): 1,2-diaminoethanetetrakis methylenephosphonic acid (EDTPO) and aminotrimethyllenephosphonic acid (ATMP). The influence of the addition of EDTPO and ATMP on the morphological, chemical and anticorrosive properties of the TEOS films deposited onto AA1100 was evaluated using scanning electron microscopy (SEM), FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and quantum mechanical calculations. A mechanism for the metalphosphonate bond and silane interaction and deposition in the metal surface has also been proposed in this study (Scheme 1c).
Section snippets
Pretreatment of aluminum alloy
The surface of the AA1100 samples (area = 2 cm × 3 cm, thickness = 1 mm) was prepared by grinding with silicon carbide paper up to grade #1200, followed by washing with distilled water and drying under a hot air stream. The nominal composition of the AA1100 alloy is (units: mass %): Cu = 0.05–0.2; Zn = 0.1; Mn = 0.05; Fe and Si = 1.0; and Al = balance to 100%.
Functionalization of aluminum surface
A silanizing bath was prepared by mixing TEOS (Merck, ≥98%), ethanol (Nuclear, 99.5%) and deionized water (18.3 MΩ cm) in a 4:90:6 (v/v) ratio; the resulting
Evaluation of the organophosphonate and silane chemical interactions with AA1100
The stepwise organophosphonate-silane based procedure to modify the aluminum oxide surface is illustrated in Scheme 1a. Spectroscopic characterization of the AA1100 coated with TEOS, TEDTPO and TATMP (Fig. 1) revealed interesting features. Analysis of the spectrum of the substrate treated with TEOS (Fig. 1a) allowed us to detect Si–OH absorption bands from hydrolyzed TEOS molecules, Si–O–CH2 from not hydrolyzed groups and Si–O–Si linkages from silica network formation. However, the peak at 750 cm
Conclusions
The present results allow us to conclude that the addition of phosphonic acids (EDTPO or ATMP) improves the performance of TEOS in protecting the aluminum alloy AA1100 against corrosion. The FTIR-RA spectrum of the well-cleaned material evidences the existence of an effective process affecting silane adherence and furthermore phosphonic deposition. Thus, a well-defined spectrum with a lack of noise and strong absorption bands can only be achieved after TEOS treatment with phosphonic acids.
Acknowledgments
This work has been supported by MICINN and FEDER funds (MAT2009-09138 and MAT2012-34498) and by the DIUE of the Generalitat de Catalunya (2009SGR925). We thank Mrs. M. Dominguez and Dr. T. Trifonov for their assistance with XPS and SEM experiments, respectively. C.A. is also grateful to the “ICREA Academia” prize for excellence in research funded by the Generalitat de Catalunya. The authors are grateful to the CNPq and Capes Brazilian Agencies.
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