Christine Blanc

AC Impedance Spectroscopy in Characterizing Time-Dependent Corrosion of AZ91 and AM50 Magnesium Alloys Characterization with Respect to Their Microstructures

The corrosion behavior of as-cast magnesium alloys (AM50, AZ91, and AZ91Si) was investigated in a 0.1 M sodium sulfate solution at the corrosion potential (E corr ) using electrochemical impedance spectroscopy. Transmission electron microscopy was used to analyze the corrosion product layer, and phase shifting interferometric microscopy was carried out to characterize the reactivity of intermetallic particles. Due to its microstructure, the AM50 alloy presented uniform corrosion during immersion, whereas corrosion of the AZ91 alloys began in the grain body and progressively spread to the eutectic areas. For the AZ91 alloys, the dissolution of the α-eutectic phase led to a strong aluminum enrichment of the corrosion product layer and, when a threshold was reached in the level of Al 2 O 3 in the magnesium oxide (or hydroxide) layer a change of phenomenology occurred in the impedance diagrams. In addition, electrochemical results revealed that an increase of silicon concentration for the AZ91 alloys decreased the corrosion resistance, This was attributed to an increase of the number of Mg 2 Si particles, accelerating the dissolution n of eutectic areas.

Local Electrochemical Impedance Spectroscopy Applied to the Corrosion Behavior of an AZ91 Magnesium Alloy

Local electrochemical impedance spectroscopy (LEIS) has been applied to study the corrosion behavior of an as-cast AZ91 magnesium alloy. The results are compared with those obtained from traditional electrochemical impedance spectroscopy and discussed in relation with previously published results. They confirm the specific time-dependent corrosion of the AZ91 magnesium alloy: due to the high aluminum content and to the presence of a eutectic constituent in the alloy, the corrosion begins in the grain body and then spreads to the eutectic areas. This leads to aluminum enrichment of the corrosion product layer. When an aluminum threshold concentration is reached, the properties of the oxide layer become predominantly determined by the properties of the alumina component and there is a change in the impedance diagrams for both global and local impedance spectroscopies. However, the space resolution of the local electrochemical impedance setup was not sufficient to give additional results on the influence of the alloy structure.

Quantitative characterization of protective films grown on copper in the presence of different triazole derivative inhibitors

Abstract The protective films developed on copper by anodic polarization in a borate-buffered solution containing benzotriazole (BTAH), 1-hydroxybenzotriazole (BTAOH) or 3-amino 1, 2, 4-triazole (ATA) have been characterized using coulometric experiments and an electrochemical quartz crystal microbalance (EQCM). The combination of these two techniques has allowed the CuO and Cu 2 O layers and the cuprous–organic layer to be analyzed quantitatively. In the presence of BTAOH, the oxide layers were very similar to those formed in inhibitor-free solution and BTAOH appeared to be adsorbed on the oxide film. In the presence of BTAH, a thick Cu 2 O film was covered by a Cu–BTA film containing 8% Cu + ions. Cupric oxide appeared on Cu 2 O areas uncovered by Cu–BTA. In the presence of ATA, the Cu 2 O layer was very thin and the greater part of Cu + ions (75%) was involved in a thick Cu–ATA film.

Statistical Study of the Corrosion Behavior of Al2CuMg Intermetallics in AA2024-T351 by SKPFM

A statistical study combining atomic force microscopy, scanning Kelvin probe force microscopy (SKPFM), and energy-dispersive spectroscopy was carried out on more than 300 Al2CuMg intermetallic particles of AA2024 alloy to determine their corrosion behavior in chloride-containing solutions. The combination of these three techniques allowed the correlation of the dissolution depth of the S-phase particles to their SKPFM potential and their chemical composition. This study also revealed that SKPFM measurements must be carried out with many precautions, but it is a powerful tool for the study of localized corrosion.

Characterisation of sealed anodic films on 7050 T74 and 2214 T6 aluminium alloys

Anodic films grown on 7050 T74 and 2214 T6 aluminium alloys by sulfuric acid anodising, and sealed in a Ni and Co acetate solution have been observed by scanning (SEM) and transmission (TEM) electron microscopy, and have been chemically analysed by X-ray photoelectron, secondary ion mass and glow-discharge optical emission spectroscopy. Corrosion behaviour has been investigated in a 0.1 M sodium chloride solution at the corrosion potential, using electrochemical impedance spectroscopy (EIS). The morphology of the oxide layer was dependent on the aluminium substrate. For the 7050 alloy, the surface presented a homogeneous structure, whereas for the 2214 alloy, the layer appeared porous and disorganised. For both alloys, surface enrichment of the oxide films in sealing bath elements has been observed. This enrichment was greater for the 2214 alloy. The protective efficiency of the sealed anodic film remained high for the 7050 alloy and decreased with immersion time for the 2214 alloy, in keeping with the disorganised structure of the oxide film on the latter.

Mechanistic Studies of the Corrosion of 2024 Aluminum Alloy in Nitrate Solutions

Nitrate ions, which are known as efficient inhibitors for the corrosion of aluminum, have been shown to become very aggressive in pitting of 2024 aluminum alloy in specific conditions. Plotting potentiokinetic curves and immersion tests revealed strong dissolution of 2024 alloy in nitrate solutions with or without chloride ions. Scanning electron microscopy observations showed that pitting of 2024 alloy mainly occurred on the intermetallic particles. Nitrate ions caused the dissolution of intermetallic particles whereas they acted as very good inhibitors for the aluminum matrix. However, local dissolution of the matrix has been found to occur around the Cu- and Mg-rich particles and has been quantified by using atomic force microscopy. Secondary ion mass spectroscopy and ultraviolet spectroscopy helped elucidate the mechanism of dissolution of the intermetallics. Reduction of nitrate ions to nitrite ions and then to ammonia on intermetallics induced pitting nucleation on these particles. Copper redeposition occurred and dissolution of the surrounding matrix was observed.

Galvanic Coupling Between Pure Copper and Pure Aluminum Experimental Approach and Mathematical Model

The corrosion behavior of a pure aluminum/pure copper couple in a weakly conductive sulfate solution was investigated. Potential and current distributions on the surface of the model couple at the beginning of immersion were obtained by solving the Laplace equation using a finite element method (FEM) algorithm. The potential distribution predicted by the calculations was checked using a Ag/AgCl microreference electrode. A good agreement was found between experimental and theoretical results. It was shown that the reaction occurring at the copper electrode was oxygen reduction, while aluminum remote from the Al/Cu interface remained in the passive state. Moreover, calculations predicted a large cathodic current, related to an increase in oxygen reduction, restricted to copper at the Al/Cu interface. This led to a local pH increase reaching values higher than 9, allowing the dissolution of aluminum to occur close to the interface. Combining these data with optical and scanning electron microscope observations after 24 h of immersion in the sodium sulfate solution allowed a three-step mechanism to be proposed to explain the corrosion damage, and particularly the presence of a copper deposit on the aluminum surface, some distance from the Al/Cu interface, a phenomenon currently observed in commercial copper-rich aluminum alloys.

Influence of Post-Welding Heat Treatment on the Corrosion Behavior of a 2050-T3 Aluminum-Copper-Lithium Alloy Friction Stir Welding Joint

The corrosion behavior of a Friction Stir Welding joint in 2050-T3 Al-Cu-Li alloy was studied in 1 M NaCl solution and the influence of T8 post-welding heat treatment on its corrosion susceptibility was analyzed. After exposure to 1 M NaCl solution, the heat affected zone (HAZ) of the weld without post-welding heat treatment was found to be the most extensively corroded zone with extended intergranular corrosion damage while, following T8 post-welding heat treatment, no intergranular corrosion was observed in the HAZ and the global corrosion behavior of the weld was significantly improved. The corrosion damage observed on the welded joints after immersion in 1 M NaCl solution was compared to that obtained after 750 h Mastmaasis Wet Bottom tests. The same corrosion damage was observed. Various stationary electrochemical tests were carried out on the global welded joint and/or each of the metallurgical zones of the welded joint to understand the corrosion damage observed. TEM observations helped in bringing meaningful elements to analyze the intrinsic electrochemical behavior of the different zones of the weld related to their microstructure. However, galvanic coupling tests showed that galvanic coupling effects between the different zones of the weld were at least partially responsible for its corrosion behavior.

Localized Approach to Galvanic Coupling in an Aluminum–Magnesium System

The corrosion behavior of a pure aluminum/pure magnesium couple in a weakly conductive sodium sulfate solution was investigated. Potential and current distributions on the surface of the model couple at the beginning of immersion were obtained by solving the Laplace equation using a finite element method algorithm. Magnesium acted as the anode of the system while oxygen and water were reduced on aluminum. Calculations predicted a large current peak at the Al/Mg interface related to a local increase in both Mg dissolution and oxygen and water reduction on aluminum, leading to a local pH increase. Optical and scanning electron microscope observations confirmed the strong dissolution of magnesium concomitantly with depassivation of aluminum at the Al/Mg interface. Local electrochemical impedance spectroscopy showed the detrimental effects of the galvanic coupling both on aluminum and magnesium.

Intergranular Corrosion of 2024 Alloy in Chloride Solutions

Experiments were performed to determine the propagation kinetics of intergranular corrosion on 2024 aluminum alloy immersed in 1 and 3 M chloride solutions. Tests consisting of immersion in a corrosive solution followed by optical observations on sectioned samples were carried out. This method was found to be time consuming and led to a lack of reproducibility due to the random nature of the corrosion attacks. Another method proved to be more efficient; it consisted of measuring the load to failure on precorroded tensile specimens vs preimmersion time in an aggressive environment. This method was found to allow the mean depth of the corrosion defects to be determined. Further, in 1 and 3 M chloride solution, intergranular corrosion led to the formation of a nonbearing zone, the thickness of which was equal to the mean depth of the corrosion defects. This corroded zone explained the premature failure of the specimens when a uniaxial tensile stress was applied.