Aniline-based silane as a primer for corrosion inhibition of aluminium

https://doi.org/10.1016/j.porgcoat.2011.11.011Get rights and content

Abstract

Hybrid films based on polyaniline and polysiloxane moieties were successfully deposited on commercial aluminium alloy using N-(3-trimethoxysilylpropyl)aniline (AnSi) as primer. The spectroscopic studies conducted on both the silane solution and the resulting films, jointly with several corrosion tests in NaCl, strongly support the protection performance of the hybrid films. The overall study demonstrates that typical silane-based treatments with principally barrier action can gain in active properties if the silane compound contains monomers of conducting polymers as a functional group.

Highlights

► Hybrid aniline-based silane films on Al alloy AA1050. ► Optimal performance against AA1050 corrosion. ► Synergistic effect of combining monomers of conducting polymers and silane functionalities at a molecular level.

Introduction

Aluminium and its alloys constitute the most important material, after steel, for various different practical applications. The presence of an adherent and thin oxide layer onto aluminium, imparts high corrosion resistance in moderately corrosive media. However, in aggressive environments, the thin oxide film does not provide long-term corrosion resistance, resulting in localized attack of the substrate surface [1]. Therefore, different coating treatments of the aluminium surface are required to obtain major protection levels.

Varied conversion coatings were used for corrosion prevention. For example, chromating and chromium passivation provide good corrosion resistance for aluminium and its alloys [2], [3]. However, the toxicity associated with hexavalent chromium generated by the residues of the chromating process requires new chromium-free technologies. The corrosion inhibition of aluminium alloys has conditioned the development of several alternatives, among which conducting polymers (CPs) are intensively investigated as active coatings [4], [5], [6], [7], [8], [9], [10]. The protection capabilities of CPs are strongly influenced by the nature and surface preparation of the metal substrate, as well as by the CP characteristics and method of deposition. The low processability and poor adhesion to active metals, as well as the inherent porosity at molecular level with fast cation incorporation during the corrosion-induced CP reduction, limit their use as contiguous polymer films [11], [12], [13]. Our recent work dealing with polypyrrole electro-deposited on different Al alloys, namely, AA6082, AA5083-H111 and AA2024-T3, highlights also the role of the rate at which structural and conformational changes are driven within the polymer network [13]. Overall the suggested mechanism(s) for corrosion protection by CPs (anodic protection/passivation, ennobling, galvanic/barrier protection, galvanic/redox inhibition), still highly debated for aluminium alloys, galvanic coupling between metal and CP and the latter re-oxidation assisted by oxygen appear to play a key role in ensuring an effective corrosion inhibition [14], [15], [16], [17].

One of the recent treatments employed to replace chromium-based coatings consists in silane technology, where electrochemical participation of the metal in the silane-based film deposition mechanism is not necessary [18], [19], [20], [21]. Silanes are hybrid molecules containing functional organic groups attached to inorganic silicon atoms. By dipping the metal in a silane hydrolyzed solution, silanol groups Si–OH react with hydroxide groups on the substrate surface to form hydrogen bonds [22]. During subsequent thermal treatment, water elimination and covalent bonding at the film/metal interface (Si–O–Me) occur. Also, crosslinking and branching are favored thus obtaining a dense siloxane Si–O–Si network, that limits the access of aggressive species to the underlying metal. Nevertheless, for an effective barrier against corrosion, high hydrophobicity is necessary to limit water permeation. In contrast to conducting polymers, the most explored silanes do not provide active action and doping with substances having inhibiting properties is needed [21], [23].

A few years ago, we have developed a new approach that allows to obtain an hybrid film containing polypyrrole units and polysiloxane linkages in a simple way, using a pyrrole-based silane as a primer on as-received wrought Al alloys [24]. The outstanding performance was attributed to a synergistic effect in terms of adhesion, compactness and passive/active actions against metal degradation as a result of polypyrrole and polysiloxane interlinking in a single macromolecular network. Last but not least, hybrid coating deposition was carried out following the conventional steps for silanes deposition [23] consisting of dipping the metal in the hydrolysed primer solution and curing (thermal treatment). To acquire deeper knowledge on the protection capabilities of silane molecules functionalized with a monomer of a conducting polymer, aniline-based silane (AniSi) was investigated (Fig. 1a). Also, the distinct doping mechanism of aniline in relation to the limited oxidation/protonation of N-substituted aniline [25], [26], [27], could help to understand better the role of the conducting polymer moieties within the macromolecular hybrid network.

This work reports on the surface treatment of a commercial-purity aluminium alloy AA1050 with AniSi hydrolyzed solution, according to the above-mentioned procedure. AniSi solution and as-deposited PAniSi films were characterized by UV–visible and reflection-absorption IR spectroscopy, respectively. Corrosion protection was evaluated in a near neutral 0.6 M NaCl solution by several electrochemical experiments. In parallel, n-octyltrimethoxysilane (OcSi) of molecular weight comparable to that of AniSi, but with different organic functionality, was also investigated as a reference system (Fig. 1b).

Section snippets

Materials

The material used in this work was commercial-purity aluminium with 99.5% Al, denominated as AA 1050 H24, according to the ASTM-Standard [28]. The nominal composition is reported in Table 1. The material was purchased from AVIOMETAL S.p.A. in the form of sheets of 1.5 mm thickness, which were cut into 20 mm × 50 mm coupons. Prior to testing, the surface was degreased by sonication in three different solvents: n-hexane, acetone and methanol, respectively, 15 min each.

All chemicals were of analytical

Silane solutions and film characterization

UV–vis absorption spectra of AniSi and octylsilane (OcSi) as a function of time (up to 1 month) are shown in Fig. 2a and b. It can be appreciated that AniSi solution spectra differs importantly from those of OcSi. The main spectral features of OcSi are revealed at wavelengths (λ) below 400 nm, while AniSi exhibits a distinct broad absorption peak at 500 nm. In the case of OcSi (Fig. 2b), the batochromic shift and narrowing of the absorption bands with time indicate the reduction of HOMO–LUMO gap

Acknowledgments

The financial support of the Secretaría de Ciencia y Técnica (Universidad Nacional del Sur), the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) are gratefully acknowledged.

References (46)

  • Z. SzklarskaSmialowska

    Corros. Sci.

    (1999)
  • P. Campestrini et al.

    Electrochim. Acta

    (2001)
    P. Campestrini et al.

    Electrochim. Acta

    (2002)
  • J. Zhao et al.

    Surf. Coat. Technol.

    (2001)
  • D.E. Tallman et al.

    J. Solid State Electrochem.

    (2002)
    G. Spinks et al.

    J. Solid State Electrochem.

    (2002)
  • K.R.L. Castagno et al.

    J. Appl. Electrochem.

    (2009)
  • M. Rohwerder et al.

    Electrochim. Acta

    (2007)
    M. Rohwerder et al.

    Electrochim. Acta

    (2009)
  • M. Rizzi et al.

    Synth. Met.

    (2011)
  • S.F. Cogan et al.

    J. Electrochem. Soc.

    (2000)
  • M. Kendig et al.

    Corrosion

    (2004)
  • M.C. Yan et al.

    Electrochim. Acta

    (2008)
  • A.M. Beccaria et al.

    Corros. Sci.

    (1999)
  • W.J. van Ooij et al.

    Corrosion

    (2001)
  • E.P. Plueddemann

    Silane Coupling Agents

    (1990)
  • A.J. Heeger

    J. Phys. Chem. B

    (2001)
  • T. Lindfors et al.

    J. Electroanal. Chem.

    (2002)
    T. Lindfors et al.

    J. Electroanal. Chem.

    (2002)
  • J.F. Brown et al.

    J. Am. Chem. Soc.

    (1964)
  • T. Lin et al.

    J. Phys. Chem. B

    (2003)
  • K.G. Neo et al.

    J. Phys. Chem.

    (1992)
    K.G. Neo et al.

    Polymer

    (1993)
    E.T. Kang et al.

    Prog. Polym. Sci.

    (1998)
  • B. Zhao et al.

    Chem. Mater.

    (2000)
  • S.N. Kumar et al.

    Synth. Met.

    (1990)
  • T.D. Neguyen et al.

    Solid State Lett.

    (2003)
  • J. He et al.

    J. Electrochem. Soc.

    (2004)
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