Effects of three sealing methods of aluminum phosphate sealant on corrosion resistance of the Fe-based amorphous coating

Abstract

Porosity defects inevitably existed in the high-velocity oxy-fuel (HVOF)-sprayed Fe-based amorphous coatings, which directly affected the service lives of the coatings. Thus, sealing treatment became a key method to improve the coatings’ corrosion resistance and prolong their service lives. In this study, three sealing technologies including conventional impregnation sealing (CIS), ultrasonic excitation sealing (UES) and vacuum sealing (VS) were employed to seal the HVOF-sprayed Fe-based amorphous coatings with aluminum phosphate. The sealing effect and corrosion behaviour of these coatings were investigated by scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA) and electrochemical techniques. The results revealed that the aluminum phosphate sealant could block the structural defects of the coatings by forming a barrier layer which consisted of Al(PO3)3, H2(AlP3O10)·H2O, Al2P6O18 and AlPO4, independent of the sealing methods. The impedance values increased by one or two orders of magnitude compared with that of the unsealed coating indicating better corrosion resistance of the sealed coatings. Moreover, based on the long-term corrosion tests of the three sealed coatings, UES was the optimum sealing method. It was because that large amount of sealant was promoted to permeate into smaller cracks and holes and close the internal pores by ultrasonic energy.