Development of Green Conversion Sol-Gel Coatings for Corrosion Protection of AZ61 Magnesium Alloy
Authors: Garcia-Galvan Federico R. , Páez-Pavón Alicia , Barranco Violeta , Llorente Irene , Romero Martin Miguel , Carmona Noemi , Feliu Sebastian , Fajardo Santiago , Galvan Juan Carlos 
Published: December 2024

The urgent need to address the challenges posed by climate change has led to increased research and development efforts aimed at creating lightweight materials to reduce greenhouse gas emissions. This study focuses on corrosion protection of the commercial magnesium alloy AZ61, valued for its wide-ranging applications in engineering and industry, including automotive, aeronautics, and biomedical engineering. Despite its interesting mechanical properties, the AZ61 alloy exhibits poor corrosion resistance in saline aqueous environments, prompting significant research into coatings to enhance its durability.

This research aims to design and prepare organic-inorganic hybrid sol-gel coatings for corrosion protection of the AZ61 alloy. The inorganic phase, mainly based on tetramethyl orthosilicate (TMOS), serves as a cross-linking agent, while four organofunctionalized silanes containing hydrolysable methoxy groups and functional organic groups into the same molecule are used as precursors for the organic component of these hybrid coatings. Specifically, the organofunctionalized silanes selected are γ-methacryloxypropyltrimethoxysilane (MAPTMS), γ -glycidoxypropyltrimethoxysilane (GPTMS), γ -aminopropyltrimethoxysilane (AMPTMS), and γ -mercaptopropyltrimethoxysilane (MPTMS). These silanes are chosen for their ability to create organopolysiloxane coatings with sufficient chemical compatibility and flexibility to accommodate other species without phase segregation or cracking. These properties are essential to create a suitable structural environment to encapsulate the selected ecofriendly corrosion inhibitors that are added in later stages of the study, thereby yielding a processable system that serves as a functional phase of active corrosion protection coatings for magnesium alloys.

Physicochemical characterization techniques are employed to understand the chemical environment within the hybrid networks and confirm the successful formation of organopolysiloxane networks. Thermogravimetry and differential thermal analysis (TG/DTA), Fourier transform infrared spectroscopy (FTIR), and high-resolution solid-state 13C and 29Si nuclear magnetic resonance spectroscopies are used for these purposes.

The performance of coated AZ61 samples is analysed using the aforementioned organic-inorganic hybrid gels. These sol-gel coatings are applied using immersion techniques (dip-coating) on unpolished (as-received) and polished AZ61 samples, to determine the influence of surface conditions on their behaviour during immersion tests in 0.6, 0.06 and 0.006 M NaCl aqueous solutions. Open circuit potential (OCP) measurements and global and localized electrochemical impedance spectroscopies (EIS, LEIS) are applied with this purpose. The microstructure and texture of the coatings, both before and after the corrosion tests, are observed by optical microscopy (OM) and scanning electron microscopy (SEM). Moreover, Energy dispersive X-ray (EDX) microanalyses are performed on several areas of the coated samples after the corrosion tests.

In the last phase of the study, modifications to the sol-gel formulations are explored by incorporating environmentally friendly corrosion inhibitors such as cysteine (L-Cys) and benzotriazole (BTA), as well as cross-linking agents like hexamethoxymethylmelamine (HMMM) in presence of an acid catalysts, specifically p-toluenesulfonic acid (p-TSA), to facilitate the cross-linking reaction within the organosilicon network. These modifications aim to enhance the protective properties of the sol-gel coatings. EIS and LEIS are used to assess their effectiveness. Films formulated with HMMM demonstrate robust passive protection against corrosion, while those doped with L-Cys and BTA exhibit self-healing and active protection properties, offering a promising alternative to traditional chemical conversion pretreatments based on hexavalent chromium.

Funding Sources

This work has been supported by the Project PID2022-139920OB-I00 (Ministry of Science, Innovation and Universities, MICINN, Spain).