Investigation of electrochemically-induced repassivation of Al 7075-T6 and Al 2024-T3 as a function of applied stress and galvanic corrosion

The repassivation behavior of Al alloys 7075-T6 and 2024-T3 was investigated by means of pitting scan (PS) technique (Fig. 1) (Trueba, Trasatti, 2015). The effect of mechanical load and galvanic coupling was estimated by considering principally the electrochemical characteristics of the reverse curve, namely the pit transition potential (Eptp), the associated current density (iptp) and the steepness. The load levels explored were mostly below the elastic limit, using four point bent-beam (4PBB) stress-corrosion test specimens (ASTM G39-99). CRES 304 or Ti6Al4V alloy were physically joined with Al sheet for simulating stress-induced galvanic corrosion. Different experimental variables (e.g. irev, [Cl-], pH, scan rate, etc.) were also considered. The study was complemented with corrosion morphology analysis. Please click Additional Files below to see the full abstract

Investigation of SCC of high strength aluminum alloys by means of slow strain rate test and cyclic anodic polarization in combination

The stress corrosion cracking (SCC) behavior of high strength 7075-T6 and 2024-T3 Al alloys in NaCl solutions is investigated by means of slow strain rate test (SSRT) and cyclic anodic polarization in combination. Smooth, dog-bone shaped flat tension test specimens, having gage section areas of 40 mm2 and 32 mm2, respectively, and 90 mm of gage length, were machined in the longitudinal (rolling) direction from the commercial wrought sheets (Aviometal Spa). The tensile test was performed at a constant strain rate (ἐ = 10-7, 10-6 or 10-5 s-1) from a pre-load of about 5 kN until fracture. The electrochemical system consisted in non-connected two Plexiglas cylindrical cells that were fixed at the middle of the opposite surfaces of the tensile specimen (working electrode, surface area at each side of 2 cm2). The variation of the open circuit potential (OCP) during straining was measured with respect to saturated calomel reference electrode (SCE) by connecting the two electrode system to a Gamry potentiostat. Contemporarily, the opposite surface was electrochemically perturbed by imposing consecutive cyclic anodic polarizations with open circuit potential measurements in between (OCP/polarization sequences), using an Ir-coated Ti auxiliary electrode, another SCE and a second Gamry potentiostat. At least two combined experiments for each test condition were carried out for repeatability check. Experiments with no OCP/polarization sequence during straining, and vice versa, were performed for reference purposes. The stress-strain curves of Al 7075-T6 (Fig. 1a) show that the ultimate strength and failure strain decrease in aggressive environment as the strain rate is lowered, regardless the electrochemical perturbation, being in agreement with reported data [1]. More interestingly, quasi-periodic stress relaxation/recovery events above the elastic region in correspondence with the dissolution/repassivation cycle were detected for ἐ ≤ 10-6 s-1 and 0.1667 mV/s of potential scan rate (n). The resolved negative spikes in the stress time derivative curve and the related polarization curves (as log | I | - t) for ἐ = 10-7 s-1, 0.6 M NaCl and n = 0.1667 mV/s are reported in Figure 1b. The spike pattern along the time axe was dependent on ἐ and NaCl concentration. The results from ongoing combined experiments with Al 2024-T3 for verification of the above findings will be presented altogether with empirical data analysis for a quantitative insight into the environmentally assisted failure mechanisms. Please click Additional Files below to see the full abstract

Electrochemical Characterization of Multilayer Cr/CrN-Based Coatings

In this work, a series of mono-and multilayer coatings were considered. They consisted of CrN and Cr prepared by physical vapor deposition with a cathodic arc. The most common steels for molds of plastics were chosen as substrates: X37CrMoV5-1 (SMV3), X2NiCoMo18-8-5 (MARVAL M1), X105CrCoMo18-2 (N690) and X40CrMo15 (X13T6). The samples were made with surface state conditions reproducing the main finishes required for molding of plastics: mirror, electro-eroded, sandblasted and ground finish. The coatings were characterized morphologically and chemically. The corrosion behavior of bare and coated steels was evaluated by electrochemical methods

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.Fil: Trueba, Mónica. Università degli Studi di Milano; ItaliaFil: Trasatti, Stefano P.. Università degli Studi di Milano; ItaliaFil: Flamini, Daniel Omar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca; Argentina. Universidad Nacional del Sur. Departamento de Ingeniería Química; Argentin

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

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.Fil: Flamini, Daniel Omar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca; Argentina. Universidad Nacional del Sur. Departamento de Ingeniería; ArgentinaFil: Trueba, Mónica. Università degli Studi di Milano; ItaliaFil: Trasatti, Stefano P.. Università degli Studi di Milano; Itali

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

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

Effect of mechanical deformation on corrosion and repassivation of 2024-T3 and 7075-T6 aluminum alloys

Con il presente lavoro si riporta l’effetto dell’applicazione di una condizione di sollecitazione a flessione sui meccanismi di corrosione e ripassivazione delle leghe di alluminio 2024-T3 e 7075-T6 in soluzioni aggressive di NaCl, investigati tramite polarizzazione anodica a singolo ciclo ed esperimenti di corrosione galvanica. Sono stati applicati carichi fino al raggiungimento della tensione di snervamento di ogni specifica lega utilizzando provini di tensocorrosione in condizione di sollecitazione a flessione su 4 punti (4PBB). L’effetto del carico a flessione (a sollecitazione costante) sul comportamento elettrochimico risulta più evidente per la lega Al 7075-T6, anche in presenza di accoppiamento galvanico con CRES 304. Una condizione elettrochimica critica, associata all’insorgenza di microplasticità, è stata identificata dal comportamento a ripassivazione. L’esistenza di tale punto critico, sebbene meno evidente, può essere indicata anche per la lega Al 2024-T3. In questo caso, l’effetto della sollecitazione meccanica è probabilmente mascherato dalla corrosione microstrutturale tipica della lega in questione (microstructural pitting), derivante dalla presenza nella stessa di intermetallici a funzione catodica ricchi in rame. I risultati complessivi sottolineano l'interdipendenza nella tensocorrosione delle leghe di Al tra ambiente aggressivo, evento elettrochimico e stato meccanico.The present work reports on the effect of constant bending load on corrosion and repassivation of high strength 2024-T3 and 7075-T6 Al alloys in NaCl solutions, investigated by means of single cycle anodic polarization and galvanic corrosion experiments. Bending loads up to the yield strength of a given alloy were applied using four point bent beam stress corrosion test specimens. The effect of bending load (side in tension) on the electrochemical behavior is more evident for Al 7075-T6, also in the presence of galvanic coupling with CRES 304. A critical electrochemical condition associated to the onset of microplasticity is identified from the repassivation behavior. The existence of such critical condition though less noticeable is indicated for Al 2024-T3 also. In this case, the effect of stress is likely overwhelmed by microstructural corrosion. Overall results emphasize the interdependence between the local aggressive environment, the electrochemical event and the mechanical state in the stress corrosion cracking of Al alloys

Effect of applied and residual stresses on the electrochemically induced repassivation of sensitized 5xxx series Al alloys in NaCl solutions

This work highlights the connotation of the response to electrochemically induced repassivation by means of single cycle anodic polarization in the understanding of the susceptibility to IGC and IGSC of Al-Mg alloys in chloride-containing solutions. The delayed repassivation, manifested by the occurrence of an inflection during the reverse scan, is likely driven by the Al3Mg2 phase acting as anode and as local source of hydrogen. The effect of residual and applied (bending) stresses on the electrochemical properties of delayed repassivation is discussed. A possible correlation between microstructural, electrochemical and mechanical states is addressed

Pyrrolyl–Silicon Compounds as Precursors for Donor–Acceptor Systems Stabilized by Noncovalent Interactions

Pyrrolyl–silicon compounds were investigated by different theoretical approaches. Model monomers consisted of a pyrrole ring N-substituted with silylmethoxy and silylhydroxy end groups through a propyl chain spacer, designated as PySi and PySiOH. Geometrical, vibrational, and electronic properties, as well as chemical reactivity, are discussed and compared with pyrrole (Py) and <i>N</i>-propylpyrrole (<i>N</i>-PrPy) that were studied in parallel for reference purposes and methods validation. The electronic distribution between PySi and PySiOH differs importantly, the former being an electron donor, as Py and <i>N</i>-PrPy. Conversely, PySiOH presents donor–acceptor character with the LUMO energy level localized on the silanol end group. Global and local reactivity descriptors predict PySiOH more reactive than PySi with two preferential reactive sites: electron-rich Py ring and electron-deficient silanol group. On the basis of experimental studies, oligomers of PySiOH linked α–α′ via Py rings (α–α′Py_<i>n</i>SiOH, <i>n</i> = 2, 3) were considered as model molecules of hydrolyzed PySi. The most stable structures were derived from randomly generated α–α′Py_<i>n</i>SiOH that were optimized at semiempirical AM1 and refined with M05-2X/6-31G­(d,p). Conformational analysis of dimer and trimer structures points to stability enhanced by molecular packing. Nonetheless, NBO and RDG results indicate that oligomer stability is dictated by the cooperative contribution of hydrogen bonding between silanol end groups and dispersive vdW interactions between silanol and the π system of the Py ring. The latter interaction resulting from electron delocalization induced by an electron-deficient silanol group seems to determine the smaller gap energy of T-shaped OH−π arrangements. The theoretical findings support the peculiar chemical behavior revealed by experiment