Characterization of V-containing black plasma electrolytic oxide coatings on aluminium alloy: Impact of base electrolytes

Black oxide coatings on 6061 Al alloy were produced by plasma electrolytic oxidation (PEO) process in silicate- and phosphate-rich base electrolytes with addition of ammonium metavanadate (NH4VO3), respectively. The characteristics of PEO processes and resulting coatings were investigated. The thermal control properties of the coatings were measured. The corrosion protection performances of the coatings in an acidic environment were evaluated. Results show that the base electrolytes have significant influence on the PEO discharge behavior, coating characteristics and properties. There are more intensive discharges in silicate-rich electrolyte with addition of NH4VO3 (denoted as "Si-V") than that in phosphate-rich electrolyte with addition of NH4VO3 (denoted as "P-V"), leading to higher thickness and more defects for the Si-V coating. Both Si-V and P-V coatings are characterized by black color in appearance due to the formation of V-containing compounds. However, the P-V coating has higher contents of V-containing compounds and therefore lower lightness and higher solar absorptance (alpha s) than the Si-V coating. In addition, the P-V coating exhibits better corrosion protection performance and color durability compared with the Si-V coating in acidic NaCl solution (pH = 3.1 similar to 3.3) due to the formation of AlPO4 in the coating. It is demonstrated that phosphate-rich base electrolyte with addition of NH4VO3 is a good candidate for getting stable black therm control coatings on Al alloy for practical applications in harsh environment.