Effect of electrolyte composition on morphology and corrosion resistance of plasma electrolytic oxidation coatings on aluminized steel

Abstract

Surface modification of the aluminized ultra-low carbon interstitial free steels by the process of plasma electrolytic oxidation (PEO) is reported in the current study. Coatings were formed by using a pulsating DC power source operating at a current density of 225 mA/cm2 over the period of 15 min. The effect of the electrolyte composition on the performance and properties of the coatings was studied. Three different electrolytes with varying concentrations of sodium silicate (Na2SiO3.9H2O) and potassium hydroxide (KOH) with their proportions as 2:1, 1:1 and 1:2 were used. The elemental composition, surface and cross section morphology and thickness of the coatings were analyzed by using scanning electron microscope (SEM) rigged with an energy dispersive X-ray spectroscope (EDS). The phase composition of the samples was assessed with X-ray diffraction (XRD). The surface profile and the roughness of the coatings were analyzed with an optical profilometer. The corrosion resistance of the substrate and the coated samples was determined by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) studies in a 3.5 wt% aqueous NaCl environment along with the immersion test in a 5 wt% aqueous NaCl environment. The practical adhesion strength of the PEO coating was assessed by the scratch testing. Among all the samples, the sample PEO treated with equal proportions of silicate and KOH concentration exhibited better corrosion resistance (icorr = 1.09 × 10−5 mA/cm2, inner barrier layer resistance of 1.32 × 108 Ω-cm2 and impedance (Ω-cm2) of 107 order) along with the better adhesion strength (Lc = 16.2 N). The better corrosion performance of the sample PEO treated with equal proportions of silicate and KOH concentrations can be imputed to the higher coating thickness along with the less porous surface morphology and higher inner barrier layer resistance. The higher adhesion strength of the sample, PEO treated with equi-concentrated silicate and KOH can be ascribed to the higher coating thickness along with the less porous surface morphology.