Hot Corrosion Behavior Of Co-Al-W Superalloy

In this thesis,a vacuum melting furnace is used to prepare Co-Al-W-based superalloys,and a salt-coated corrosion method is used to study the alloy’s hot corrosion opposition.The influences of varied contents of Ni,Ta,Cr,Si,re and Ru on the hot corrosion behavior of Co-Al-W superalloy were studied by means of SEM,XRD and EDS,and the hot corrosion mechanism of the alloy was analyzed.The main conclusions are as follows:Co-Al-W alloys with different Ni and Ta contents have been corroded for 64 hours,and it can be seen that the corrosion weight gain of 10Ni alloy is the smallest.With the increase of Ta content,the corrosion weight of the alloy raises and the hot corrosion resistance decreases.Co O is the main corrosion product of the alloy with varied Ni content after 2h.With the increase of time,the parimary corrosion products are Co O,Co₃O₄and Co Ni O₂.The corrosion products of alloys with different Ta content in the early stage of corrosion are mainly Co O and Co Ni O₂,and 10Ni-1Ta and 10Ni-2Ta alloys have a small amount of Ni Al₂O₄.In the later stage of corrosion,the corrosion products of alloys are mainly Co O and Co Ni O₂.The 10Ni alloy also has a large amount of Co₃O₄.The corrosion products of K417 alloy after 32h are mainly Ni O,Ti O and matrix,as well as Co Cr₂O₄and Co₃O₄.The corrosion products of Inconel738 alloy after 32h corrosion increased,mainly Cr₂O₃and matrix,and a small amount of Ni O and Ti O.After a certain period of corrosion,cracks appear on the surface of Co-Al-W based alloys with different Ni and Ta contents,which can accelerate the corrosion of the alloy.The outer corrosion layer of Co-Al-W-based alloys with different Ni and Ta contents is mainly mixed oxide after 32 hours of corrosion.Both K417 and Inconel738 alloys have formed thicker Al₂O₃and Cr₂O₃layers during the corrosion process.These thicker oxide layers can prevent the diffusion of O and S elements into the alloy,thereby improving the alloy’s thermal corrosion resistance.Therefore,the thermal corrosion resistance of K417 and Inconel738 alloy is better than that of Co-Al-W superalloy.Co-Al-W alloys with different Cr and Si contents are corroded for 64h,the corrosion process is divided into corrosion incubation period and accelerated corrosion period.The alloys with varied Cr and Si contents are in the incubation stage for the first 8 hours,and in the accelerated corrosion period after that.The corrosion rate of 0.1Si alloy is slow in the first 16h,but it is accelerated after corrosion for 16h.The weight gain of 0.5Si alloy does not change much before 32h of corrosion,but increases after 32h of corrosion.The rise of Cr and Si content can improve the hot corrosion resistance of the alloy,and the weight change of the alloy in the 0.5Si alloy is the smallest.At the early stage of corrosion,four kinds of alloys with different Cr content formed Co O based oxide。In the later stage of corrosion,the corrosion products mainly include Co₃O₄and Co Al₂O₄.Al Co is also formed in the 2Cr alloy,and in the 4Cr and 6Cr alloys.The presence of Al₂O₃is also detected.In the early stage of corrosion of alloys with different Si content,the corrosion products are parimary Co O.Co Al₂O4 was also discovered in 0Si and 0.1Si alloys,while Al₂O₃was discovered in 0.1Si and 0.5Si alloys.In the after phase of corrosion,the corrosion products of the alloy were mainly Co₃O₄and Co Al₂O₄.In addition,a small amount of Co O and Co₂Si O₄were also detected in the 0.1Si and 0.5Si alloys.Co-Al-W-based alloys with different Cr and Si contents will have a certain degree of cracks on the surface of the alloy after corrosion for a certain period of time,which can accelerate the corrosion of the alloy.Co-Al-W-based superalloys with different Si content formed a thick layer of Al₂O₃in the intermediate transition layer after 32 hours of corrosion,which can protect the alloy from hot corrosion.The corrosion weight of Co-Al-W alloys with different Re and Ru contents increases with the time in 64 hours of corrosion,0Re and 1Re alloys are in the corrosion incubation period in the first 8h corrosion,and are in accelerating corrsion stage.after that.1Re alloys tend to be flat after 32h corrosion.The kinetic curve of 0.5Re alloy is steady,and that of1Re1Ru alloy is steady before corrosion for 16h,Afterwards,its corrosion rate rapidly increased after corrosion for 32 hours.0Ru,0.5Ru and 1Ru alloys are in the corrosion incubation period before 8h corrosion,and the alloys are in the accelerated corrosion period after 8h corrosion.When the alloys with different Re contents were corroded for 2h,the corrosion products were Co O and Co Al₂O₄,and no sulfide is detected.After 32 hours of corrosion,the main corrosion products are Co₃O₄and Co Al₂O₄,and a small amount of Co O is also detected in the 1Re1Ru alloy.The corrosion products of alloys with different Ru content at the initial stage of corrosion are similar to those of alloys with different Re content at the initial stage of corrosion,are Co O and Co Al₂O₄,and the corrosion products at the later stage of corrosion are mainly Co₃O₄and Co Al₂O₄,The corrosion products in 1Ru and 1Re1Ru alloys are still a small amount of Co O was detected.Co-Al-W-based alloys with different Re and Ru contents have a certain degree of cracks and bulges on the surface of the alloy after corrosion for a certain period of time,which can accelerate the corrosion of the alloy.Co-Al-W-based alloys with varied Re contents presents a dense Al₂O₃layer in the corrosion layer after 32 hours of corrosion,which can prevent harmful impurity elements from diffusing into the alloy matrix.