Plasma Electrolytic Oxidation Coatings of a 6061 Al Alloy in an Electrolyte with the Addition of K2ZrF6

Abstract

In this study, white thermal control coatings were produced on a 6061 Al alloy using plasma electrolytic oxidation (PEO). The coatings were mainly formed by incorporating K2ZrF6. The phase composition, microstructure, thickness, and roughness of the coatings were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), a surface roughness tester, and an eddy current thickness meter, respectively. The solar absorbance and infrared emissivity of the PEO coatings were measured using a UV–Vis–NIR spectrophotometer and FTIR spectrometer, respectively. The addition of K2ZrF6 to the trisodium phosphate electrolyte was found to significantly enhance the thickness of the white PEO coating on the Al alloy, with the coating thickness increasing in proportion to the concentration of K2ZrF6. Meanwhile, the surface roughness was observed to stabilize at a certain level as the K2ZrF6 concentration increased. At the same time, the addition of K2ZrF6 altered the growth mechanism of the coating. In the absence of K2ZrF6 in the electrolyte, the PEO coating on the Al alloy surface predominantly developed outwards. However, with the introduction of K2ZrF6, the coating’s growth mode transitioned to a combination of outward and inward growth, with the proportion of inward growth progressively increasing in proportion to the concentration of K2ZrF6. The addition of K2ZrF6 substantially enhanced the adhesion of the coating to the substrate and endowed it with exceptional thermal shock resistance, as the inward growth of the coating was facilitated by the presence of K2ZrF6. In addition, the phase composition of the aluminum alloy PEO coating in the electrolyte containing K2ZrF6 was dominated by tetragonal zirconia (t-ZrO2) and monoclinic zirconia (m-ZrO2). With the increase in K2ZrF6 concentration, the L* value of the coating increased from 71.69 to 90.53. Moreover, the coating absorbance α decreased, while the emissivity ε increased. Notably, at a K2ZrF6 concentration of 15 g/L, the coating exhibited the lowest absorbance (0.16) and the highest emissivity (0.72), which are attributed to the enhanced roughness resulting from the substantial increase in coating thickness caused by the addition of K2ZrF6, as well as the presence of ZrO2 with higher emissivity within the coating.

Influence of Additives on the Macroscopic Color and Corrosion Resistance of 6061 Aluminum Alloy Micro-Arc Oxidation Coatings

“…The F element did not exhibit a significant presence, potentially due to the short oxidation time. According to Tu’s study, F was found to enter the coating at the late stage of micro-arc oxidation [ 21 ]. The coating without Na 2 WO 4 and the coating without K 2 ZrF 6 share the same main elemental composition of alumina.…”

“…Materials 2024, 17, x FOR PEER REVIEW 9 of 12 found to enter the coating at the late stage of micro-arc oxidation [21]. The coating without Na2WO4 and the coating without K2ZrF6 share the same main elemental composition of alumina.…”

“…The addition of K2ZrF6 also led to a one-order-of-magnitude decrease in the corrosion current density (icorr), which is due to the increased coating thickness and the consequent slowing of the corrosion rate, resulting in better corrosion protection performance. However, the corrosion potential (Ecorr) decreased by 268 found to enter the coating at the late stage of micro-arc oxidation [21]. The coating without Na2WO4 and the coating without K2ZrF6 share the same main elemental composition of alumina.…”

The Tribological Properties of Plasma Electrolytic Oxidation Layers Synthesized on Arc Spray Coatings on Aluminum Alloys in Contact with Various Friction Materials

“…The expansion of the functionality of thermally sprayed aluminum coatings after their modification using PEO processing has already been demonstrated previously [42][43][44][45][46][47][48]. For the same purpose, plasma or cold spray deposition of aluminum coatings is also used [49,50], and aluminum, steel and titanium are used as the substrates [51][52][53][54][55][56][57][58].…”