Abstract
The aim of this study was to develop new chrome-free surface pretreatments for AA2024-T3 aluminum alloy. These pretreatments were based on hybrid organic–inorganic sol–gel thin films prepared from mixtures of γ-methacryloxypropyltrimethoxysilane (MAPTMS) and tetramethylorthosilicate (TMOS). Different MAPTMS/TMOS molar ratios were used for optimizing the physical–chemical characteristics of the sol–gel films. The formulation of a set of these sols was modified by incorporating piperazine (1,4-diazacyclohexane) as a corrosion inhibitor. The resulting sol–gel films were characterized by using Fourier transform infrared spectroscopy (FTIR), liquid-state 29Si nuclear magnetic resonance spectroscopy (29Si-NMR) and viscosity measurements. The corrosion performance of the sol–gel films was analyzed by using electrochemical impedance spectroscopy (EIS) and local electrochemical impedance mapping (LEIM). The characterization techniques indicated that piperazine behaved as a catalyst for the condensation reaction during the formation of the MAPTMS/TMOS organopolysiloxane network and produces an increase of the crosslinking degree of the sol–gel films. EIS and LEIM results showed that piperazine is an effective corrosion inhibitor, which can be used to enhance the active corrosion protection performance of sol–gel films.
Highlights
The AA2024-T3 aluminum alloy is widely used in aerospace, automotive and transportation industries generally due to their high strength-to-weight ratio, age-hardening, increased damage tolerance and durability, excellent fracture toughness and fatigue crack propagation and low manufacturing costs [1,2]
The first step of the research was devoted to preparing different sol–gel formulations by adjusting the MAPTMS/TMOS molar ratio and optimizing the physical–chemical characteristics and
The first step of the research was devoted to preparing different sol–gel formulations by adjusting the MAPTMS/TMOS molar ratio and optimizing the physical–chemical characteristics and enhancing the electrochemical corrosion properties of methacrylate-SiO2 hybrid films deposited on AA2024-T3 aluminum alloy
Summary
The AA2024-T3 aluminum alloy is widely used in aerospace, automotive and transportation industries generally due to their high strength-to-weight ratio, age-hardening, increased damage tolerance and durability, excellent fracture toughness and fatigue crack propagation and low manufacturing costs [1,2]. AA2024-T3 presents high contents of copper making it more susceptible to corrosion related to the distribution and composition of intermetallic particles [2,3]. These particles are considered the initiation sites for localized corrosion acting either as anodic or cathodic regions [4,5]. Metals 2020, 10, 539 apply remedial measures such as, for example, the use of chemical conversion pretreatments. In this context, numerous researchers are making a considerable effort to look for alternatives to chromate conversion coatings (CCCs) due to the carcinogenic effect of the hexavalent chromium used on these formulations [6,7]. The hybrid sol–gel method consists of a combination of hydrolysis and condensation processes of mixtures of organic and inorganic constituents, which makes possible the production of coatings with good flexibility, reduced defects and hydrophobic in nature, keeping the adhesion to the metal substrate and improving the compatibility with polymeric top-coatings [12,13,14,15]
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