High Temperature Corrosion Resistance of Pt-Based Superalloys in 0 . 2 % SO 2N 2 Gas
Pt-based superalloys are a new generation of materials intended to be used in various high temperature applications. Effluent gases emitted to the environment during combustion operations can lead to degradation of high temperature materials with deleterious effects on mechanical integrity and the corrosion of alloys. This paper focuses on sulphidation of one quaternary, and four ternary, Pt-based superalloys of various chemical compositions. Pt-based superalloys were exposed at 900°C in a reducing environment. The alloys were compared to coated and uncoated Ni-based superalloys (NBSAs). X-Ray diffraction (XRD), Scanning electron microscopy (SEM) and Raman spectroscopy were used to characterize corrosion products that formed. The Pt-based superalloys exhibited higher corrosion resistance than NBSAs as a result of the formation of a protective Al2O3 scale, whereas NBSAs suffered internal sulphidation and formed Cr and Ni sulphide corrosion compounds. The formation of brittle needle-like phases and the transformation of the NiAl phase to Ni3Al, led to early failure of the coated alloy. Pt-based superalloys alloyed with Co showed poor corrosion resistance as a result of depletion of alloying elements.
Overview of the development of new Pt-based alloys for high temperature application in aggressive environments
Pt-based alloys are being developed for high temperature applications in aggressive environments, with the aim to replace the currently used nickel-based superalloys (NBSAs) in at least some of the applications where higher density is less important. The Ptbased alloys have a similar structure to the NBSAs, and since Pt is more chemically inert and has a higher melting point, the Pt-based alloys can potentially be used at higher temperatures. Room temperature tensile tests were performed and results were variable, mainly due to microstructural differences. The alloy hardnesses were in the order of 300i?½350HV with ultimate tensile strengths of ~800 MPa and elongations of less than 10%. These results are comparable to other alloys used at high temperatures, Transmission electron microscopy (TEM) was done at NIMS, Japan, to investigate the changes in the microstructure of Pt-Al and Pt-Al-Ir alloys at elevated temperatures using in situ heating up to 1100i?½C. Starting from a promising Pt-Al-Cr alloy, the influence of Ni additions was investigated. After ageing finely dispersed precipitates are observed. Nb, Ta, Ti and Re increased the strength when added as a partial substitute for Al. Minor additions of B increased the creep resistance and rupture time remarkably, owing to strengthening of the grain boundaries. The evolution of the alloy development programme is presented, as well as selected properties of the multi-component Pt-base alloys. High temperature hot corrosion (HTHC) tests were conducted at 950i?½C for 540 hours. Analysis of the corrosion products was conducted using a Raman spectrometer, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Pt-based alloys displayed far superior corrosion properties compared to coated and uncoated benchmark Ni-based superalloys. Keywords: platinum, superalloys, hardness, tensile, creep, nickel, corrosion
Trends in kiln drives
The heart of the cement plant is the rotary kiln, and continuous quality production of cement clinker depends on the continuous accurate performance of the rotary kiln drive.