Resumen de Recubrimientos producidos por sol-gel con inhibidores de corrosión para la protección activa de aleaciones ligeras
Carolina Rosero Navarro
Aluminium, magnesium and their alloys present good mechanical properties, high conductivity and easy machining, but suffer localised corrosion when they are exposed to aggressive environments limiting their applications. High strength aluminium alloys contain a variety of alloying elements to promote the generation of multiphase alloy systems after appropriate thermo-mechanical treatments.
For such alloys, micro-galvanic coupling between different metallurgical phases in the presence of aggressive environments increases susceptibility to localised corrosion. Thus, active protection is strongly necessary to prevent the corrosion as well as any minor damage incurred in service. At present, the most efficient protection systems are chromium based coatings, like Chromium Conversion Coatings (CCC) and Chromium Acid Anodised (CAA), which offer a self-healing protection. However, chromates are considered as potential lung carcinogen responsible for DNA damage and make them environmentally hostile. A lot of research has been focused on the use of rare-earth metal compounds as alternative for chromates. Cerium accomplishes the requirements for alternative corrosion inhibitors: the ions form insoluble hydroxides, which enable them to be used as cathodic inhibitors; they have a low toxicity and are relatively abundant in nature.
In this PhD thesis it was evaluated the corrosion inhibition due to cerium ions when added to the electrolyte as well as the corrosion protection related to the inhibition action of the cerium ions incorporated in protective coatings. A split-cell technique and image-assisted electrochemical noise analysis, which provide minimal perturbation of the freely corroding system and good time resolution, are proposed as a tool for simultaneous investigation of the corrosion inhibition mechanism and assessment of performance. The results obtained are compared with those coming from electrochemical impedance spectroscopy, disclosing the advantages of these techniques in the evaluation of inhibitor performance.
In this thesis we have developed two different protecting systems for substituting chromate containing coatings (CCC or CCA): hybrid inorganic-organic coatings combining barrier effect and inhibition due to cerium salts, and pure glass-like CexOy coatings, able to provide active corrosion protection to aluminium and magnesium alloys.
The hybrid coatings developed onto aluminium substrates are based on cerium doped silicamethacrylate hybrid sol-gel coatings. These coatings combine barrier properties to delay the penetration of the corrosive agents with inhibition properties to hinder the corrosion process produced by the presence of pores, cracks or scratches. The coatings prepared using tetraethylorthosilicate (TEOS), 2-hydroxyethylmethacrylate (HEMA) and 3-methacryloxypropyltrimethoxysilane (MPS) provide small barrier functionality because of the low degree of cross-linking in the structure and the high hydrophilic nature. The incorporation of cerium ions in the sols originates the increase of defect concentration due to the disruption of the structure, reducing the barrier functionality of the coatings, but providing self-healing effect by precipitation of cerium oxides/hydroxides. Another signal of this inhibition mechanism is the increase of the impedance modulus at 0.01 Hz with immersion time in NaCl solutions, contrary to that observed in coatings without cerium.
Several modifications of these coatings, as removal of HEMA, and the incorporation of a commercial silica suspension and ethylene glycol dimethacrylate (EGDMA), produces an increase of the cross-linking and density of the films with a significant improvement of the barrier functionality.
However, the best way to combine barrier functionality and self-healing effect in this system is to develop a three-layer system where the external coatings provide barrier effect while the intermediate one contains cerium able to supply self-healing behaviour. The results present a very good corrosion protecting behaviour with clear signals of self-healing effect after immersion times of 3000 h in NaCl solution.
The corrosion behaviour of these systems has been studied through electrochemical techniques, mainly EIS that permitted to explain the corrosion and protection mechanisms by modelling with equivalent circuits. The best resulting systems were scaled up to 5 L of sols that were deposited by automatic spray on industrial demonstrators and tested in the companies.
Finally, a new type of cerium glass-like coatings is reported, developed from cerium nitrate and using different complexing compounds (citric acid, acetic acid, diethano-amine, etc.). These sols were scaled up and deposited by immersion and automatic spray onto aluminium and magnesium alloys to produce uniform and adherent coatings with amorphous structures (CexOy). Electrochemical measurements and standard corrosion tests (neutral salt spray, filiform corrosion test, immersion emission test and adhesion on embossing and T-bend test) were performed to study the corrosion behaviour of cerium glass-like coatings on aluminium and magnesium alloys. Electrochemical tests confirm the active protecting behaviour by healing or blocking the defects by the corrosion inhibition species. Excellent corrosion protection is provided by these cerium glass-like coatings, with a well adherence to primers and paints, satisfying the most exigent industrial requirements.
