Plasma electrolytic oxidation coatings were produced on a High Pressure Die Cast (HPDC) Al Si alloy using two types of electrolytes: a conventional silicate electrolyte (PEO coatings) and a silicate-based suspension electrolyte (PEOp coatings) containing strontium aluminate particles. The PEO process was applied to two types of initial surfaces: as-diecast (PEO-C/PEOp-C) and ground (PEO-G/PEOp-G). Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) were used to evaluate corrosion resistance and behavior in a 3.5 wt% NaCl solution at ambient temperature. The corrosion performance was monitored for up to 168 h of immersion. The results indicated that PEO coatings exhibited better corrosion resistance than PEOp coatings after 168 h of immersion. This behavior is primarily attributed to coating integrity, which plays a critical role in corrosion resistance. The use of suspension electrolyte led to higher instantaneous current density prior to the onset of the soft-sparking regime, which adversely affected the formation of the inner layer. Additionally, after 24 h of immersion, PEOp coatings formed on the as-diecast surfaces exhibited lower impedance values than those formed on ground surfaces. In contrast, PEO coatings maintained relatively stable corrosion performance regardless of the initial surface condition during the entire immersion period. The polarization resistance decreased by about 90% for PEOp-C coatings after 168 h of immersion, while PEO coatings experienced an increase of approximately 36% in polarization resistance for PEO-C coatings. • Composite PEO coatings are produced using SrAl 2 O 4 suspension electrolyte. • Suspension electrolyte modifies discharge behavior during the PEO process. • PEO coatings exhibited more stable impedance after 168 h of immersion. • Corrosion resistance is governed primarily by inner layer integrity. • The high instantaneous current density affects the inner layer microstructure.
Razzouk et al. (Fri,) studied this question.