Sealing post-treatments of coatings produced by plasma electrolytic oxidation (PEO) on aluminum 2024

A characteristic of ceramic coatings produced by plasma electrolytic oxidation (PEO) is the presence of surface features, like pores or cracks, whose formation is strictly related to the occurrence of energetic plasma events during the treatment. Such defects cause a reduction of the corrosion resistance of the treated component allowing the penetration of corrosive substances through the oxide layer. One of the most common solutions to this problem consists of performing a sealing post-treatment aimed at closing the porosity of the coating. One of the simplest sealing treatments is the hydrothermal one. It is based on partial hydration of the oxide which leads to the formation of hydration products with a volume larger than the original oxide. As a result, the porosity of the PEO coatings could be partially or completely closed. This process is simply performed by immersing the PEO samples into boiling water. A disadvantage of hydrothermal sealing is that it is run at high temperatures, thus requiring a huge amount of thermal power. To reduce the operating temperature, it is possible to modify the chemical composition of the sealing bath adding reagents able to produce at relatively low temperatures compounds insoluble or with low solubility which precipitate into pores and cracks of the oxide layer providing a clogging effect. In the present work, different types of sealing treatments are compared. The PEO treatment is performed on disks made of an aluminum alloy (AA2024) or a magnesium alloy (AZ31) using NaOH and KOH alkaline solutions containing Na2SiO3 and glycerin. The applied signal consists of an AC-pulsed duty cycle, 60 % anodic and 40 % cathodic with a cathodic peak of 7 %. The resulting PEO samples are post-processed by hydrothermal sealing, sealing with sodium acetate, layer double hydroxide (LDH) sealing, and sealing with cerium nitrite. The sealing duration varies from 15 to 60 min and the working temperatures range from the 100 °C of the hydrothermal process to 30-40 °C reached when sealing based on Ce(NO3)3 is performed. The SEM analysis shows the structure modification of the PEO coatings due to the sealing treatments. The major change is observed when the LDH sealing is carried out since the final surface of the coatings is characterized by the presence of lamellas mainly containing zinc coming from Zn(NO3)2 present in the sealing bath. It is found that all the sealing techniques lead to an increase in the corrosion resistance of the coatings as highlighted by the results of electrochemical tests: linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP) carried out in a 3.5 wt.% sodium chloride solution. In particular, the larger enhancement of the polarization resistance and the lower corrosion current density are obtained for LDH and cerium nitrite-sealed samples.