Effect of silicate on passive films anodically formed on iron in alkaline solution as studied by electrochemical impedance spectroscopy

Studies using electrochemical impedance spectroscopy (EIS) were carried out on an iron disk electrode in 0.01 mol/L sodium hydroxide (NaOH) + x ppm silica (SiO2) (x = 250, 500, and 750 ppm) polarized at 300 mV(SCE) to determine the effect of silicate on passive films anodically formed on iron in alkaline solutions. EIS proved silicate increased film resistance (R-f), decreased its capacitance (C), and affected the kinetics of interfacial electrochemical reactions, leading to an increase in the charge-transfer resistance (R-t), which depended upon silicate concentration and polarization time.