Effect of Molybdate on Corrosion Performance of Oxide Coating Produced on 7075 Al Alloy Using PEO

Abstract

In this research, plasma electrolytic oxidation (PEO) coatings were prepared on 7075 Al alloy in a silicate-based solution with Na2MoO4 additive using a unipolar waveform at constant current density. The coatings displayed micro-pores, micro-cracks, pancake-like and crater-like features, and also solidified molten oxide particles on the surface. The coatings were majorly composed of Al2O3 (γ, δ, and α), SiO2 (amorphous), and MoO3 phases, which confirms the incorporation of molybdenum in the case of additive-containing coatings. Molybdenum species were transported through cracks, channels, and micropores, as the ready access pathways into the coating and partly sealed the coating pores. The EIS technique was used to evaluate the long-term corrosion performance of the coatings up to 168 h of immersion in 3.5 wt.% NaCl solution. The results showed that the barrier action of the PEO coatings was highly enhanced by adding Na2MoO4 due to the higher resistance that alumina achieved to chlorine absorption and also its higher stability by the incorporation of MoO3. The coating formed in the presence of 5 g L−1 Na2MoO4 showed the highest thickness and the lowest porosity percent (15.15%), which provided the highest corrosion performance at long immersion times.

HVOF Deposition, comparative investigation and optimum selection of molybdenum, boron, chromium, and titanium in Iron amorphous composite coatings

“…Increase in the Vickers hardness with the addition of titanium was attributed to hard Fe 2 Ti on the layer of coating surface. Another reason for the increase in the mechanical properties of coating with the addition of titanium was decrease in laminar spacing and solid solution strengthening [33].…”

A Review on the Application‐Focused Assessment of Plasma Electrolytic Oxidation (PEO) Coatings Using Electrochemical Impedance Spectroscopy

“…[13][14][15][16][17][18][19][20] Also, several recent research papers discuss the EIS assessment of particle-incorporated PEO coatings. [21][22][23] To a large extent, the inclusion of additives occurs in the PEO coating's outer layer, while the inner layer is mainly metal oxide of the substrate metal. [8] The size of incorporated particles in PEO coatings is in either nano or micro ranges.…”