No abstract

We reported the realization of superhydrophobic layered double hydroxide (LDH) coating with the double-anticorrosion mechanism on magnesium alloys. This coating was prepared via in-situ growth of LDHs on the etched AZ31 alloy and the modification was caused by 1H, 1H, 2H, 2Hperfluorooctyltrimethoxysilane. The water contact angle on the coating was as large as 161°, possessing the two-scale rough structure consisting of microislands and LDH crystal nanosheets. The excellent corrosion resistance properties of the superhydrophobic LDH coating can be attributed to the double-anticorrosion mechanism associated with LDH coating and superhydrophobic structure. In addition to the active protection of LDH coating, the passive protection of superhydrophobic coating can suppress the occurrence of ion-exchange reactions, leading to excellent corrosion resistance stability.

Corrosion rates of ideal Mg alloys used for sealing tools in petroleum industry should be able to be altered to adapt to different downhole environments, and corresponding mechanisms of modified corrosion rates need to be clarified. In this study, annealing treatments were carried out at 400 or 450 °C for 2 h to tune the microstructures and corrosion rates of Mg-2Gd-xCu (x = 0, 0.5 and 1 wt.%). Microstructures were characterized by scanning electron microscope, X-ray diffractometer and electron backscatter diffraction while corrosion behaviors were investigated by immersion and electrochemical tests. The results showed that after annealing, the growth in the average second phase size and the average grain size, the weakened basal texture and the eliminated residual stress contributed to reduced corrosion rates. As a result, corrosion rates of Mg-2Gd-0.5Cu and Mg-2Gd-1Cu decreased obviously but were still higher than 30 mm·y−1 after annealing, the recommended minimum corrosion rates. It proved that a gradient of corrosion rates of Mg-Gd-Cu can be achieved through annealing treatment, which were beneficial to further application of Mg-Gd-Cu in the field of oil and gas exploitation.

The effects of diffusion temperature and time on the mechanism of the surface layers formation during low activity vapour phase aluminizing process on a Ni-base superalloy CMSX4 single crystal was experimentally investigated. The coating morphology of as aluminized samples grown along the substrate [100] and [110] directions of the substrate crystal was analysed by means of SEM, EDS and EBSD techniques. The presence of three layers, where β-NiAl differently combines to γ’, γ and TCP phases, was always observed. The β-NiAl phase in particular displayed two different morphologies and textures depending on the interface movement mechanism generating this phase at its external/internal interface. The presence of secondary precipitated phases within the coating layer, their composition and morphology, helped to understand the coating development process. The surface layer formation mechanism was found to be slightly temperature-dependent.

A super-hydrophobic anti-corrosion film was facilely prepared via in situ growth of layered double hydroxides (LDHs) on the etched AZ31 magnesium alloy and then modification by 1H, 1H, 2H, 2H-perfluorooctyltrimethoxysilane (PFOTMS) in this work. The morphology, structure, composition, surface roughness and water contact angles (WCA), and the anti-corrosion performance of the samples were investigated. The results revealed that the micro/nano hierarchical surface morphology of the films was composed of island structures obtained after chemical etching and MgAl-LDH nanowalls grown in situ. The best hydrophobicity (CA = 163°) was obtained on the MgAl-LDHs with the maximum surface roughness. Additionally, the potentiodynamic polarization, electrochemical impedance spectroscopy, and immersion test indicated that the super-hydrophobic LDH films provided better corrosion resistance to AZ31 magnesium alloy due to the double-protection derived from the LDHs and super-hydrophobic properties. Furthermore, the contact angle could be kept at above 140° after dipped in 3.5 wt% NaCl solution for 6 days.