Pitting mechanism in a stainless steel-reinforced Fe-based amorphous coating

There is keen interest in developing Fe-based amorphous composite coatings with superior bonding strength and mechanical properties for load-bearing applications. However, the interfaces between the added second phase and the amorphous matrix always suffer from pitting corrosion in harsh Cl− solutions, leading to the degradation of performance of these composite coatings. The underlying pitting mechanism has remained elusive. In this study, the pits initiation behaviour of a Fe-based amorphous coating reinforced with stainless steel powders is systematically investigated in a 3.5% NaCl solution through polarization, electrochemical noise, scanning Kelvin probe and 'in-situ observation' measurements, as well as high-resolution transmission electron microscopy. The pitting resistance was found to be deteriorated obviously when stainless steel phase was added to the amorphous coating. The results indicated that pitting at the interfaces was not caused, as generally believed, by microgalvanic corrosion between the stainless steel phase and the amorphous matrix, but caused by the formation of Fe3O4 oxide at the interfaces due to the strong tendency towards oxidation of stainless steel particles in thermal spraying processes. This oxide is unstable in a Cl−-containing environment and acts as reactive sites for pits initiation. The present work emphasizes the effect of oxidation of the reinforcement on the pitting resistance of amorphous coatings and provides an alternative recommendation to the design of amorphous composite coatings for corrosion and load-bearing applications.