Structure And Hot Corrosion Resistance Properties Of The Al-Si Coatings On K488Alloys

K488alloy is a new superalloy developed by our country which is used onheavy duty gas turbine blades for ship. Its working environment is poor and therequirement for the oxidation and corrosion resistance is high. In this experiment,the inorganic salt with Cr element was taken as the binding material and the Al-Sicoatings were prepared on the K488alloy. XRD, SEM and EDS were used toanalyze the structures of the coatings. The high temperature oxidation test and thesalt spray test were taken to evaluate the protective value of the coatings. And theeffect of the Al and Si elements were analyzed.The result show that the Ai-Si coating which has four sublayers is consist ofrich-Al NiAl phase, the Cr-Si compound second phase particles and the chromiumcarbide. With the increasing of the content of Si, the sublayers of the Cr-Sicompound and the chromium carbide thicken and the kinds of the Cr-Si compoundchange. But the thickness of the Ai-Si coating reduces. With the diffusion timeexpend, the distribution of the Si element because uniformity and the Cr-Sicompound second phase particles become tinier. But the thickness of the Ai-Sicoating do not changeThe result of the high temperature oxidation test show that the oxidationresistance of the coated K488alloys is excellent. With increasing of the content ofSi and the expending of the diffusion time, the performances of the coating becomebetter. With increasing of the content of Si, the oxidation products turn to thespherical and even α-Al₂O₃; the formation time of the compact oxide layer shorten;the interdiffusion between the coating and the matrix weak; the coarse second phaseparticles separate out less. With the expending of the diffusion time, the oxidationproducts becomes thick, even and spherical and more α-Al₂O₃forms and lessoxidation pits appear. With the prolonging of the oxidation time, the oxidationproducts turn to α-Al₂O₃from θ-Al₂O₃. After oxidation for four hours, lots ofα-Al₂O₃has formed which is much quicker than the aluminized coating.The result of the salt spray tests show that the corrosion resistance of the coatedK488alloys is good. There is a best content of Si in agent that more or less than itwould worsen the performance of the coating. When the Si content is low, long timediffusion will lead the performance of the coating to reduce. Over lowering of the Sicontent in the coating is disadvantageous to the formation of α-Al₂O₃. And the hardbrittle phase Al5Cr will appear with the high content of Si which can lead thecoating to be exfoliation. When the Si content is low, with the expending of thediffusion time, the distribution of Si element is more uniform and the content of Si reduces in the outside area. It is disadvantageous to the formation of α-Al₂O₃. Inanother hand, it is helpful to inhibit the interdiffusion.But it is hard to occur underlow temperature. When the Si content is high, prolonging the diffusion time affectlittle. But the second phase can be fine and dispersed in the long time diffusioncoating. In this way, the harmness of the hard brittle second phase can be reducedand the corrosion resistance property of the coat is improved.