Abstract
Given the corrosion tendency of the natural oxide film of aluminum alloys, micro arc oxidation (MAO) treatment is used as an efficient and economic method to enhance the corrosion resistance. However, irregular voids, pores, and micro cracks are easily formed during the MAO process, which are harmful to the anti-corrosion property of MAO coatings. In this paper, binary additives of electrolytes, including (NaPO3)6 and H3BO3, were used to obtain MAO coatings with improved thickness and compact microstructures on 6061 aluminum alloys. The as-prepared coatings were investigated using a thickness meter, scanning electron microscope (SEM), X-ray diffractometer (XRD), and electrochemical impedance spectroscope (EIS). The results showed that the coordinated influence of the binary additives could change the discharge behaviors and micro morphologies of the MAO coatings compared to the base silicate electrolyte. A thicker and stronger MAO coating could be achieved, which was mainly composed of Al2O3 phases. The EIS tests revealed that the corrosion current density of the obtained optimal MAO coating decreased by three orders of magnitude from 1.209 × 10−6 A·cm−2 to 2.981 × 10−9 A·cm−2. We believe that the binary additive-enhanced MAO coatings could provide a promising anti-corrosion solution in various applications.
Highlights
Aluminum (Al) is one of the most widely used metals for industrial and general applications
In this paper, (NaPO3 )6 and H3 BO3 were simultaneously added into the base electrolyte as binary additives to further improve the thickness, compactness, and corrosion resistance properties of the micro arc oxidation (MAO) coatings
The corrosion properties of the coatings were tested by electrochemical impedance spectroscopy (EIS, Ametek Parstat 4000+, Tennessee, USA) and potentiodynamic polarization tests after 1 h of immersion in 3.5 wt.% NaCl solution
Summary
Aluminum (Al) is one of the most widely used metals for industrial and general applications. Many factors can affect the thickness and porosity of MAO coatings, such as elements in the metal substrate [16,17], compositions of the electrolyte [18,19,20], processing time [21,22], processing temperature [23], current density [24,25,26], the type of power source [27,28,29], and other process assistance methods [30,31,32] Among these techniques, additive manufacturing is one of the simplest and most promising ways to achieve various enhancements of coating properties, such as thickness, wear resistance, and anti-corrosion property. In this paper, (NaPO3 ) and H3 BO3 were simultaneously added into the base electrolyte as binary additives to further improve the thickness, compactness, and corrosion resistance properties of the MAO coatings. This work explored a further way of optimizing binary additives in the processes so as to obtain high-quality MAO coatings on aluminum alloys
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