Investigation of passivity and its breakdown on Fe3Al-Si and Fe3Al-Ge intermetallics in chloride-containing solution

The passivity and passivity breakdown of Fe3Al-Si and Fe3Al-Ge iron aluminides were studied in borate buffer solution (pH 8.4) in the absence and presence of 100 mM KCl, performing potentiodynamic polarization, potentiostatic transients and electrochemical impedance spectroscopy (EIS) measurements complemented with scanning electron microscopy (SEM). In the absence of chloride ions Si and Ge exercise a beneficial role in the passivating characteristics of Fe3Al intermetallic. Addition of Si or Ge significantly modifies the electrochemical response of iron aluminide Fe3Al resulting in a more stable passive film. In the presence of chloride ions all the intermetallic compounds experience localized corrosion (pitting). However, Si and Ge alloying additions increase the breakdown potential and the extent of passivation domain, indicating improved resistance to initiation of pitting corrosion. Furthermore, EIS measurements performed at the breakdown state evidenced higher Rct and lower depression angle values for Fe3Al-Si and Fe3Al-Ge iron aluminides compared to Fe3Al intermetallic, confirming their better localized corrosion behavior. The improved resistance to pitting corrosion results from the enhanced protective function of passive film due to the presence of Si or Ge that inhibit pit initiation by hindering the adsorption of Cl− ions at the metal surface.