Preparation of Phosphate Coatings on Ti-alloy and Their Corrosion Behavior Beneath Salt-mixture in Water Vapor Flow at 650℃
Phosphate coatings,composed of silica sol modified Al(H2PO4)3,ceramic fillers and addi-tives,are prepared and then applied on Ti-6Al-4V alloy,which are further characterized by means of XRD and Fourier transform infrared spectrometer,as well as corrosion test beneath deposits of salt-mixture in vapor flow at 650℃.The results show that silica sol can promote the formation of glass phase while the phosphate coating curing.When the mass ratio of silica sol to Al(H2PO4)3 reaches 1∶1,the mixture of sili-ca sol to Al(H2PO4)3 may produce a film of complete glass phase which ensures the formation a complete-ly dense phosphate coating.The modified phosphate coating provides excellent corrosion resistance to salt-mixture in vapor flow at 650℃,and the chemical stability of the film and filler is the vital factor of the corrosion resistance of the modified coating.The coating is still dense after 100 h of corrosion at 650℃ in such environment,and rare corrosion products of elements of the matrix alloy appears.As one filler of small amount,the Al flakes and their corrosion products are arranged parallel to the matrix in the coating,which can effectively prevent the diffusion of harmful ions such as Cl- to the matrix,meanwhile which can react with Na2SO4 so that inhibit the intrusion of molten salt into the coating and the corrosion of Ti-alloy matrix.
phosphate coatingmodifyTi-alloyhigh temperature corrosion
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