Monica Trueba

Anodic oxidation and silane treatment for corrosion protection of AM60B magnesium alloy

Oxide films have been produced on AM60B magnesium alloy using micro-arc oxidation process in an environmentally friendly alkaline solution with and without addition of different oxides nanoparticles (TiO2, ZrO2 and Al2O3). In order to seal the oxides porosity generated in the sparking process, a silane-based top coat has been applied. The surface morphology of samples was analyzed by Scanning Electron Microscopy (SEM). Scratch tests were performed for evaluating the adhesion strength of the anodic oxides. The corrosion resistance of the oxide- silane- based topcoat composite coatings was evaluated in 3.5% NaCl solution using anodic polarization tests. The anodizing in oxides nanoparticles rich solutions (ZrO2 or Al2O3), followed by a silane top coat treatment performed using OSi as precursor, is an interesting way to synthesize adherent corrosion resistant coatings on magnesium alloy AM60B.

Hybrid Coatings Based on Conducting Polymers and Polysiloxane Chains for Corrosion Protection of Al Alloys

It was previously demonstrated that the use of a pyrrole-based silane (PySi) for surface treatment of Al alloys provides both active and barrier protection due to the deposition of a hybrid coating, containing polypyrrole and polysiloxane chains. To further explore these features, a wider range of Al substrates and different silane-based formulations in terms of silane molecule, solvent nature, water amount and pH, were investigated. Also, some tests were carried out by using aniline-based silane (AniSi). Structural/morphological characterization of the coatings, as well as the investigation of PySi solutions by diverse spectroscopic techniques, in addition to corrosion tests in NaCl, strongly support the very promising protection performance of the hybrid film. This is indicated as well from the preliminary results obtained with the AniSi-based approach. Thus, typical silane-based treatments with principally barrier action can gain in active properties if the silane compound contains monomers of conducting polymers as a funtional group.

Electrochemical approach to repassivation kinetics of Al alloys: gaining insight into environmentally assisted cracking

Abstract This work provides a broad overview of the systematic experimental studies conducted in our group to understand the factors governing the transition to a more occluded corrosion front during the repassivation of Al alloys, manifested by the appearance of an inflection during anodic polarization into the active region in NaCl solutions. The collected data and the derived empirical relationships enabled us to discern the thermodynamic and kinetic aspects controlling the transition to more occluded local damage. From the thermodynamic standpoint, the potential at the inflection, namely, the pit transition potential Eptp, can be considered as the thermodynamic driving force of Al dissolution in the acidified pit-like solution in contact with a freshly created surface. The associated current density iptp and the steepness of the potential decrease with current below Eptp are determined by interfacial electrochemical kinetics. More importantly, all these properties are influenced by electrochemical and metallurgical events, including residual stresses. These findings point to the electrochemical approach in combination with external mechanical load as a promising tool for investigating environmentally assisted cracking, in particular, crack nucleation and non-steady crack tip processes.

Electrochemical investigation of corrosion and repassivation of structural aluminum alloys under permanent load in bending

Abstract This work reports an electrochemical investigation of corrosion and repassivation in NaCl solutions of structural Al 7075-T6 and Al 2024-T3 as a function of permanent load in bending by means of single-cycle anodic polarization. Experimental variables other than the load level were the extent of corrosion, chloride ion concentration and initial pH of the test solution, in addition to the pre-exposure in the aggressive environment and of the viscosity of the test solution for Al 7075-T6. The susceptibility to local stress during corrosion and repassivation depends on the alloy microstructure and corrosion mechanisms. In the case of Al 2024-T3, the mechanical activation is driven by cathodically controlled dissolution of this alloy at open circuit in concentrated NaCl, being indicative of a chemomechanical effect. Conversely, for Al 7075-T6, the effect of applied load is better discerned from the repassivation response. Film rupture/formation sequence and related interfacial (electro)chemical processes determine the stress-enhanced metastable repassivation of this alloy.

Properties of oxide-silane composite coating on AZ31 magnesium alloy

Abstract Various oxide films have been grown on AZ31 magnesium alloy by microarc oxidation in alkaline phosphate solution with addition of different compounds (Na2SiO3, NaAlO2). Because of the anodic oxide porosity, arising from the sparking treatment, organo-functional silane coatings have been applied to seal pores and cracks and achieve a more effective protective system. The surface and cross-section morphologies of the anodic films were analysed by scanning electron microscopy. The phase composition was determined by X-ray diffraction. Bare and anodised AZ31 specimens were post-treated with silanes, namely N-phenil-3-aminopropyltrimethoxysilane and Bis (trimethoxysilylethyl) benzene. The corrosion resistance of anodic oxides and oxide-silane composite coatings was evaluated in dilute Harrison solution by means of single-cycle anodic polarisation. The results showed that the addition of different salts to the alkaline phosphate solution affects the surface morphology and the phase composition of the coatings and, as a consequence, the corrosion resistance behaviour. Both silane post-treatments provide further corrosion protection to samples anodised in aluminate containing solution.

Pyrrole-Based Silane Primer for Corrosion Protection of Commercial AA 2024 T3

An alternative approach was developed for surface treatment of as-received commercial AA 2024 T3 by using a pyrrole-based silane (SiPy). For film deposition, just one immersion step is enough, followed by curing. SiPy layer structure contains both polysiloxane bonds and pyrrole oligomers, with some degree of doping, giving a highly coherent layer. The superior film quality with respect to simple polysiloxane, is probably the main reason for the better corrosion performance obtained for SiPy on 2024.