S. Yazdani, S. Javadpour, Sh. Mehdizadeh Naderi, M. Javidi,
Volume 13, Issue 2 (June 2016)
Abstract
The inherent reactivity of the Al–Cu alloys is such that their use for structural, marine, and aerospace components and structures would not be possible without prior application of a corrosion resistance system. Historically these corrosion resistance coatings were based on the use of chemicals containing Cr (VI) compounds. Silane coatings are of increasing interest in industry due to their potential application for the replacement of current toxic hexavalent chromate based treatments. In this study, hydrophobic coating sol was prepared with methyltriethoxysilane (MTES), methanol (MeOH), and water (as 7M NH4OH) at a molar ratio of 1:25:4.31 respectively. The coatings were applied by a dip-technique to 2024-T3 Al alloy, and subsequently cured at room temperature and there after heat treated in an oven at 150°C. The anticorrosion properties of the coatings within 3.5 wt% NaCl solution were studied by Tafel polarization technique. The sol–gel coating exhibited good anticorrosion properties providing an adherent protection film on the Al 2024-T3 substrate. The surface properties were characterized by water contact angle measurement, scanning electron microscopy (SEM), and the composition was studied by Fourier transform infrared spectroscopy (FTIR).
Yasin Mehdizadeh, Saeed Reza Allahkaram, Mohammad H.mohammad-Ebrahimi, Majid Shamsarjmand,
Volume 21, Issue 0 (IN PRESS 2024)
Abstract
The present work deals with the corrosion behavior and mechanical properties of a coted AZ31 magnesium alloy through plasma electrolyte oxidation (PEO) coating process in different alkaline electrolytes based on sodium silicate (Si-coating), sodium polyphosphate (P-coating) and sodium aluminate (Al-coating). The scanning electron microscopy (SEM) equipped with the energy dispersive x-ray spectroscopy (EDX) plus x-ray diffraction were recruited to investigate the morphology, chemical composition, and phase structure of coatings, respectively. Microscopic scrutiny revealed that the coating in the phosphate electrolyte was twice as thick and the relative porosity percentage was higher than those formed in the other electrolytes. The phase analysis indicated that the MgO was present as the prevailing phase in the Al-coating and P-coating. However, the dominant phase in the Si-coating was Mg2SiO4. Electrochemical testing was examined in a solution containing 3.5.wt% sodium chloride, showing improvements in corrosion resistance of coated alloys. These investigations confirmed that the corrosion resistance of Si-coating was dramatically higher than others which could be attributed to the presence of the dense and stable Mg2SiO4 phase as well as its relatively low porosity. According to the results of tensile tests, the coated samples had lower tensile strength and elongation than the uncoated one. The tensile strength and elongation diminished upon changing the electrolyte from Al-coating to P-coating, while the yield strength was almost similar. Further analyses indicated that the drop of tensile strength and elongation could be attributed to the presence of cracks and pores in the brittle ceramic PEO coating as stress concentration regions during deformation. Those areas are created due to thermal stress during the coating process and deformation in the elastic stage.
