No abstract

Yttria stabilized zirconia (YSZ) coating was formed on AA1050 Al alloys by aerosol deposition (AD), and its electrochemical corrosion properties were investigated in 3.5 wt% NaCl and 0.5 M H 2 SO 4 solutions. The crack-free, dense, and ~5 µm thick YSZ coating was successfully obtained by AD. The as-deposited coating was composed of cubic-YSZ nanocrystallites of ~10 nm size. The potentiodynamic test indicated that the YSZ coated Al alloy had much lower corrosion current densities (2 nA/cm 2 ) by comparison to uncoated sample and exhibited a passive behavior in anodic branch. Particularly, a pitting breakdown potential could not be identified in H 2 SO 4 . EIS tests revealed that the impedance of YSZ coated sample was ~106 Ωcm 2 in NaCl and ~10 7 Ωcm 2 in H 2 SO 4 , which was about 3 or 4 orders of magnitude higher than that of uncoated sample. Consequently, the corrosion resistance of Al alloy had been significantly enhanced by the YSZ coating.

The microstructure development during plasma electrolytic oxidation (PEO) and its influence on the electrochemical corrosion properties of AZ91D Mg alloy were investigated. PEO was conducted in a KF-NaAlO2 electrolyte, and the corrosion properties of PEO coatings were evaluated by potentiodynamic and potentiostatic tests and electrochemical impedance spectroscopy (EIS). At the breakdown voltage, a non-porous, fluoride-containing barrier-type layer with less-crystalline MgAl2O4 and MgF2 was formed. With the intense sparking, a porous layer appeared at the surface. The outer porous layer was a nearly amorphous oxide. The inner dense layer was composed of nano-crystalline MgAl2O4 and MgF2, and its fluoride content and crystallinity increased with PEO time. The existence of MgF2 phase in the innermost layer was clearly demonstrated by high resolution transmission electron microscopy (HRTEM). The potentiodynamic tests indicated that the corrosion potential (E corr) remained relatively constant, but the corrosion current density (i corr) significantly decreased with PEO time. The EIS results revealed that ∼2 orders of magnitude higher resistance of inner dense layer mainly contributed to the improved corrosion resistance of AZ91D Mg alloy. The enhanced corrosion resistance with the progress of PEO was discussed in terms of thickness, fluoride content, and crystallinity of inner dense layer.

Yttria stabilized zirconia (YSZ) coating was deposited on AA7075 aluminum alloy by aerosol deposition (AD) at room temperature, and its electrochemical corrosion properties were investigated in 0.5 M H2SO4, 3.5 wt% NaCl, and 0.1 M KOH solutions. A crack-free, dense, and ∼5 μm thick YSZ coating on Al alloy was successfully obtained by AD method. The YSZ coating was polycrystalline and composed of ∼10 nm nanocrystallites. Potentiodynamic and electrochemical impedance spectroscopy (EIS) tests indicated that the corrosion behavior in each solution was different, but the corrosion protection of the AA7075 Al alloy was significantly improved by the AD YSZ coating in all three solutions.

Titanium based alloys offer significant opportunities for weight reduction in aerospace design and more recently it has been considered that particle reiriforced titanium based composites will provide the most practical solutions in this field. In the present paper it is identified that it is possible to produce an in situ TiC- composite via a blended elemental reaction powder metallurgical route. The effect of process parameters on the microstructure and on the presence of possible contaminants and undesirable phases is also examined.MST/3417