Corrosion Inhibitor-Modified Plasma Electrolytic Oxidation Coatings on 6061 Aluminum Alloy

Abstract

There are many methods for incorporating organic corrosion inhibitors to oxide coatings formed on aluminum alloys. However, typically they require relatively concentrated solutions of inhibitors, possibly generating a problematic waste and/or are time-/energy-consuming (elevated temperature is usually needed). The authors propose a three-step method of oxide layer formation on 6061-T651 aluminum alloy (AAs) via alternating current (AC) plasma electrolytic oxidation (PEO), impregnation with an 8-hydroxyquinoline (8-HQ) solution, and final sealing by an additional direct current (DC) polarization in the original PEO electrolyte. The obtained coatings were characterized by scanning electron microscopy, roughness tests, contact angle measurements, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Additionally, corrosion resistance was assessed by potentiodynamic polarization in a NaCl solution. Two types of the coating were formed (A—thicker, more porous at 440 mA cm−2; B—thinner, more compact at 220 mA cm−2) on the AA substrate. The 8-HQ impregnation was successful as evidenced by XPS. It increased the contact angle only for the B coatings and improved the corrosion resistance of both coating systems. Additional DC treatment destroyed superficially adsorbed 8-HQ. However, it served to block the coating pores (contact angle ≈ 80°) which improved the corrosion resistance of the coating systems. DC sealing alone did not bring about the same anti-corrosion properties as the combined 8-HQ impregnation and DC treatment which dispels the notion that the provision of the inhibitor was a needless step in the procedure. The proposed method of AA surface treatment suffered from unsatisfactory uniformity of the sealing for the thicker coatings, which needs to be amended in future efforts for optimization of the procedure.

Surface Modification of 6xxx Series Aluminum Alloys

“…The performance parameters, such as hardness and wear resistance of PEO coatings, are influenced by the coating thickness, phase compositions, size, and amount of pores and microcracks [45]. Some of the characteristics of the PEO coatings could be improved by the incorporation of second phase particles, such as corrosion inhibitors, graphite, graphene, carbon nanotubes (CNT), or by an overlay of epoxy coatings [46][47][48][49]. Some of the advantages of PEO coatings are excellent adhesion to the substrate, even with complex geometry.…”

“…Some of the advantages of PEO coatings are excellent adhesion to the substrate, even with complex geometry. Growth rates are reasonably higher, and the possibility of adding species or modifying the composition of the coatings always opens new application domains [46,[50][51][52]. Under certain deposition conditions, the coatings could be porous.…”

“…Many researchers used various additives to improve different functional properties of the PEO coatings generated on the surface of the 6061 Al alloy. Sowa et al [46] added organic corrosion inhibitors to the PEO coatings developed on 6061 Al alloy. They reported that 8-hydroxyquinoline in ethanol could fill the pores produced during the direct current (DC) PEO process, and it exhibited improved corrosion resistance in the NaCl solution medium.…”

“…The profilometer measurements indicate a noticeable reduction in roughness, which is attributed to the filling of the pores with the 8-hydroxyquinoline. They also noted that thinner the coating, lesser the porosity, and sealing would be better [46]. Tran et al [61] added diamond powders into the electrolyte containing Na2SiO3 and H3BO3.…”

Development of an Oxide Layer on Al 6061 Using Plasma Arc Electrolytic Oxidation in Silicate-Based Electrolyte

“…Graphene is used as a scaffold in cell-tissue engineering, as an active electrode in supercapacitors to power implantable biomedical devices, and as detectors in biosensors [7]. High-temperature studies on graphene allow the researchers to understand the nanostructures' stability, behavior, and interactions with the substrate [8]. Analyzing these interactions aids in understanding the fundamental processes that control graphene development at high temperatures and thus can be explored to modify its properties [9].…”