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

Rouhollah Ghorbani,Maryam Rahmati,Monica Santamaria,Amin Hakimizad,Keyvan Raeissi

doi:10.3390/coatings12020184

Rouhollah Ghorbani, Maryam Rahmati + Show 3 more

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https://doi.org/10.3390/coatings12020184

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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.

Highlights

7075 aluminum alloy is widely used for aerospace and automotive industries due to a combination of properties, such as high strength-to-weight ratio, high fracture toughness, and good shape-ability
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
This paper studies the PEO process of 7075 Al alloy in a silicate-based electrolyte containing Na2MoO4 to evaluate the change in microstructure, composition, and long-term corrosion performance of the prepared coatings

Summary

Introduction

7075 aluminum alloy is widely used for aerospace and automotive industries due to a combination of properties, such as high strength-to-weight ratio, high fracture toughness, and good shape-ability. The weak corrosion resistance, intergranular and pitting corrosion, represents a significant obstacle for its applications and has largely confined the broad use of 7075 aluminum alloy in some industries [1,2]. Plasma electrolytic oxidation (PEO) has been considered as one of the most suitable surface treatments for generating a conformal oxide layer on the surface of various valve metals to improve their mechanical and electrochemical properties for industrial applications. PEO process is an anodic oxidation treatment applied above the breakdown voltage, with the consequent onset of short-lived discharges on the coating surface melting the substrate and the oxides simultaneously, which provide thick, uniform, and very adhesive coatings. The most attractive advantage of the PEO process is it is environmentally friendly, providing hard oxide ceramic layers with excellent corrosion and wear resistance and good electrical and thermal properties

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