Influence of oxidation related structural defects on localized corrosion in HVAF-sprayed Fe-based metallic coatings

Oxidation occurred in thermal spray process is an important problem deteriorating corrosion performance of thermal-sprayed Fe-based metallic coatings due to the formation of Cr-depleted zones and structural defects around the spray formed oxides. To clarify the correlation between the oxidation related structural defects formed during thermal spray process (i.e. the interparticle regions and the connected pores) and the localized corrosion, a dense Fe-based metallic coating with high Cr-content was fabricated, the long term immersion tests which continued for 30 days combined with electrochemical impedance spectroscopy measurements in 3.5 wt% NaCl were performed. It was found that electrolyte solution diffused to the inner coating through the interparticle oxide layers, and subsequently the localized corrosion developed along the interparticle oxide layers. A number of corrosion pits and corrosion channels were observed along with the interparticle oxide layers for the coating after one month immersion. Through sealing the diffusion pathways in the as-sprayed coatings by using aluminum phosphate, the corrosion resistance of the coatings was enhanced with decreased corrosion current density and improved endurance in long term immersion tests.