The cross-sectional microstructure of the as-sprayed and corroded coatings are shown in Fig.1.The phase contents were determined using phase analysis software.In the as-sprayed coating，the ratio of metal to nonmetallic phase was 63 to 37.In the corroded coating，this ratio was 40 to60.Figure 1 clearly shows that the increase in nonmetallic phase（composed of pores filled by resin and boron nitride phase）resulted from the pore increase.Also，there are some corrosion products found in the section image（Fig.1b）.
Fig.1 SEM images of the as-sprayed coating section (a) and corroded coating section (b)
Surface Brinell hardness tests were conducted on the as-sprayed and corroded coatings.The average indentation diameter of the as-sprayed coating was 2.61 mm.After undergoing a 96-h salt spray test，the average indentation diameter was 2.68 mm.The corresponding Brinell hard-ness was 10.8 HB（SD 0.05）and 10.2 HB（SD 0.21），indicating that the hardness of the corroded coating decreased by approximately 6%in comparison with the as-sprayed coating.
The tensile bonding strengths of the as-sprayed coatings and coatings that underwent different salt-spray corrosion durations were tested according to the Chinese industry standard，HB20035-2013.Resin glue（FM-1000）was used to bind the coating surface to the holder.A tensile testing machine（AG-250KNE；INSTRON，USA）was used to test the bonding strength，and the extension rate was 1 mm/min.The average bonding strength of the as-sprayed coating was 9.2±1.46 MPa.After testing in salt spray for 48h，the average bonding strength of the coating decreased to4.4±1.08MPa；for 96 h，and the average bonding strength decreased to 2.69 MPa.Observations of the tested coating samples（Fig.2a）showed that the fracture occurred only in the surface layer of the coating that underwent 48h of salt spray，so only the binding layer was exposed.Whenthe coating experienced 96 h of salt spray（Fig.2b），frac-tures at the interface between the substrate and bonding layer were observed，so the corroded substrate was exposed.Corrosion had important effects on the coating bonding strength.
Fig.2 Fracture appearances of the tensile bonding strength test of the coating corroded for 48 h(a) and 96 h(b)
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