Search results

The purpose of this study is to research the effects of interrupted aging on the corrosion behavior of Al–Cu–Mg–Ag heat-resistant alloy by means of intergranular corrosion (IGC) testing, potentiodynamic polarization combined with optical microscopy and transmission electron microscopy.

The results show that the IGC began on the grain boundaries and continued along the grain boundary. The corrosion resistance property of Al–Cu–Mg–Ag alloy was enhanced by interrupted aging. The precipitations of the interrupted aged sample both in the grains and on the grain boundaries were fine, and the chain-like phases on the grain boundary were distributed nearly continuously.

The corrosion resistance of Al–Cu–Mg series Al alloy with equilibrium phase (Al₂Cu) is notably determined by precipitation-free zone (PFZ) as the self-corrosion potentials of (Al₂Cu), PFZ and the matrix satisfied the relation EPFZ < E_θ<EMatrix.

The connections of the PFZ on both sides of the grain boundary decreased the corrosion resistance of Al–Cu–Mg–Ag alloy treated by the single aging.

The effects of the thermo-mechanical treatment on the properties and microstructure of the Al–Cu–Mg–Ag alloy were investigated.

A short-duration preprecipitation process is designed prior to predeformation aging. The novel predeformation aging (solution treatment + holding at 185 °C for 15 min+ rolling deformation + aging at 185 °C, also named T8) was performed on a heat-resistant Al–Cu–Mg–Ag alloy.

The purpose of this study indicate that a short-duration heat treatment before predeformation is beneficial to the precipitation of O during the aging process. The precursors of O during this process might pin the dislocation and cause the grains to orient along some specific direction, which might be advantageous to the precipitation of O while disadvantageous to that of θ′. This novel thermal-mechanical process could result in an increase in the quantity and decrease in the size of the precipitation of O, which leads to a remarkable strength effect. The potential increases while the current density decreases with an increase in the deformation amount, which implies a smaller intergranular corrosion rate. The fine deformed structure leads to an opposite behavior in the exfoliation corrosion test compared with that for intergranular corrosion.

The intergranular corrosion resistance of the Al–Cu–Mg–Ag alloy is enhanced, whereas the exfoliation corrosion resistance is reduced by novel predeformation aging.

A chromate conversion coating was prepared on the surface of bare AA2024 aluminum alloy by direct immersion in the chromating treatment bath, and the corrosion behavior of chromated AA2024 aluminum alloy in 3.5 per cent NaCl solution was studied by electrochemical measurement and microstructural observation.

According to the polarization curve test and the scanning electron microscope observation, the corrosion evolution of chromated AA2024 in 3.5 per cent NaCl solution was divided into the following three stages: coating failure, pitting corrosion and intergranular corrosion (IGC).

In the first stage, the chromate coating degraded gradually due to the combined action of chloride anions and water molecules, resulting in the complete exposure of AA2024 substrate to 3.5 per cent NaCl solution. Subsequently, in the second stage, chloride anions adsorbed at the sites of θ phase (Al2Cu) and S phase (Al2CuMg) on the AA2024 surface preferentially, and some corrosion pits initiated at the above two sites and propagated towards the deep of crystal grains. However, the propagation of a pit terminated when the pit front arrived at the adjacent grain boundary, where the initiation of IGC occurred.

Finally, in the third stage, the corrosion proceeded along the continuous grain boundary net and penetrated the internal of AA2024 substrate, resulting in the propagation of IGC. The related corrosion mechanisms for the bare and the chromated AA2024 were also discussed.