Fernanda Martins Queiroz

On the Effects of Hydrothermal Treatments on the Corrosion Resistance of the TSA Anodized AA7475-T761 Alloy

The effect of hydrothermal treatment time on sealing and corrosion resistance of the AA7475-T761 anodized aluminium alloy has been investigated in this study. The hydrothermal treatments tested are environmental compatible without chromium ions involved. Anodizing was carried out by a tartaric-sulphuric anodizing (TSA) process and this was followed by hydrothermal treatments for partial sealing, in various solutions. The effect of propyleneglycol (PRG) and/or cerium ions in the hydrothermal treatment solution was evaluated. Four treatment times were tested, specifically, 2.5, 5.0, 7.5 and 10 min. The corrosion resistance of the anodized and treated samples was evaluated by Electrochemical Impedance Spectroscopy (EIS) and the anodic layers formed were characterized by Scanning Electron Microscopy (SEM). The EIS results showed that the hydrothermal treatments in solutions with cerium ions resulted in similar impedances for periods of treatment from 5 to 10 min whereas in the solutions with PRG the impedance increased with time of treatment from 2.5 to 10 min showing a slower kinetics of anodic layer sealing. However, the fastest kinetics of sealing were associated to the treatments that combined two steps, one in PRG and other in cerium containing solutions with similar impedances obtained from 2.5 to 10 min of treatment. Surface evaluation by SEM showed that the porosities in the anodic layer were not sealed for the periods of hydrothermal treatments corresponding to 2.5 min. The presence of cerium in hydrothermal treatment had a beneficial effect on the stability of the anodic layer formed and provided a healing effect on the corroding sites.

Corrosion Protection of AA2524-T3 Anodized in Tartaric-Sulfuric Acid Bath and Protected with Hybrid Sol-Gel Coating

2xxx Al alloys are particularly sensitive to localized corrosion in chloride environments and in order to maintain their integrity, minimize maintenance needs and repairs, and to maximize component life, protective treatments are required.Anodizing is an electrochemical process based on the growth of the Al oxide layer by applying anodic potentials. One of the alternatives is tartaric/sulphuric acid (TSA) anodizing, which is environmentally compliant and provides corrosion resistance properties, compatible with the requirements of the aerospace industry with appropriate paint adhesion.In this study, AA2524-T3 specimens were anodized in a tartaric-sulfuric acid bath (TSA) and subsequently protected by application of a hybrid sol–gel coating. The sol–gel coating was prepared using a solution with high water content (58 %v/v) and obtained by the hydrolysis and condensation of tetraethoxysilane (TEOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS). The corrosion resistance evaluation of both unsealed and coated samples was carried out in a sodium chloride solution by EIS as a function of immersion time. The results were also fitted using electrical equivalent circuits.

Localized Corrosion Resistance of Dissimilar Aluminum Alloys Joined by Friction Stir Welding (FSW)

2XXX and 7XXX high strength aluminum alloys are the most used materials for structural parts of aircrafts due to their high strength/weight ratio. Their joining procedure is an engineering challenge since they present low weldability. Friction Stir Welding (FSW) is a joining technology developed in the early 90 ́s. It is a solid-state welding process, without the use of fillers or gas shield, that eliminates conventional welding defects and has been considered of great interest for application in the aircraft industry. FSW of aluminum alloys results in four regions of different microstructures, specifically: the base material (BM), the heat affected zone (HAZ), the thermo-mechanically affected zone (TMAZ), and the nugget zone (NZ). The complex microstructure of the weld region leads to higher susceptibility to localized corrosion as compared to the BM even when similar alloys are joined. The welding of dissimilar alloys in its turn results in even more complex microstructures as materials with intrinsically different composition, microstructures and electrochemical properties are put in close contact. Despite the great interest in FSW, up to now, only few corrosion studies have been carried out for characterization of the corrosion resistance of dissimilar Al alloys welded by FSW. The aim of this study is to investigate the corrosion behavior of aluminum alloy 2024-T3 (AA2024-T3) welded to aluminum alloy 7475-T761 (AA7475-T761) by FSW. The evaluation was performed in 0.01 mol.L-1 by means of open circuit potential measurements, polarization techniques and surface observation after corrosion tests.