Preparation And Performance Of Sealing Coating On Different Metal Substrates

In order to reduce the blade-tip chearances in compressors and turbines, toimprove the efficiency and reduce the fuel consumption, seal coating is widely used inaircraft engine as a functional coating. The powder of seal coating contains lubricationphase and thermal spray method is widely used, so it is easily to cause the coatingcontains some pores and oxidates. The environment around is corrosive, regardless ofthe aero-engine is in working or in parking, which can cause the seal coatings’corrosion. And the holes provides convenient channel for the permeating of corrodent,influence the coatings’ performance and the safety of the aircraft.The bond coating （NiAl） and the top coating （AlBN） were deposited onto thesurface of the GH4169substrate by air plasma spraying（APS） sequentially. The bondcoating （NiAl） and the top coating （NiCrAl-NiC） were deposited onto the surface ofthe GH907substrate by air plasma spraying （APS） and flame spraying, sequentially.The galvanic corrosion behavior of materials which are in touched with each other inusing has been studied, and combining the polarization curve and the open-potentialtest to analyse the galvanic corrosion. The salt spray corrosion have been studiedcombining the SEM and XRD to analyse the corrosion products, using thepolarization curve and EIS to analyse the coatings’ corrosion resistence.The galvanic corrosion shown that GH4169alloy and GH907alloy which haveslightly difference in composition, it can cause different galvanic corrosion behavior,the galvanic corrosion is more serious when the GH907alloy with lower Cr coupledwith NiAl coating. The galvanic corrosion grade is level A for NiCrAl-NiC coatingand NiAl coating which with similar composition and open potential, allowing totouch with each other. The other three galvanic couples with lower grade and can’ttouch with each other. The pores and oxide inclusions in seal coatings formed acomplex galvanic system. Oxide inclusions on the edge of thin coating can becomethe a priority place for galvanic corrosion, corrodent will permeate into the insidecoating through the pores on the edge of coating and cause local pitting.The neutral salt spray corrosion test of seal coatings shows that, the white corrosion product for the AlBN coating consists of Al₂O₃and it’s liable to flake off,the SEM shows the corrosion product loosely accumulates on the surface of AlBNcoating. The corrosion products are flocculent structure for NiCrAl-NiC coating andthey mainly consist of Al₂O₃and NiO. The micro morphology of the corrosionproduct for NiAl coating shows a great deal of cracks and the corrosion productconsists of NiO. The results of polarization curve test after salt spray corrosion showthat, the corrosion current for the AlBN coating is larger, because the corrosionproduct drop from the coating surface and it reflects the real corrosion situation. Thecorrosion currents decrease and polarization resistances increase for the NiCrAl-NiCand NiAl coating, because numerous corrosion products pile up on the surface ofcoatings and it would masquerade as the growth of corrosion resistance. EIS testshows that, the Rtand corrosion resistance decrease for the AlBN. The corrosionproducts pile up on the surface of NiCrAl-NiC and NiAl coatings and block theentrance for the corrosion media into the internal porosity, which cause the increase ofRt.