AN OVERVIEW OF Na 2 SO 4 AND/OR V 2 O 5 INDUCED HOT CORROSION OF Fe- AND Ni-BASED SUPERALLOYS

Abstract

Metals and alloys sometimes experience accelerated oxidation when their surfaces are covered with a thin film of fused salt in an oxidizing gas atmosphere at elevated temperatures. This is known as hot corrosion where a porous non-protective oxide scale is formed at the surfaces and sulphides in the substrate. Hot corrosion has been identified as a serious problem in high tem- perature applications such as in boilers, gas turbines, waste incinerations, diesel engines, coal gasification plants, chemical plants and other energy generation systems. It is basically induced by the impurities such as Na, V, S, K, and Cl, which are present in the coal or in fuel oil used for combustion in the abovementioned applications. The use of Ni-, Fe-, and Co- based superalloys in high temperature applications such as gas turbines, boilers, etc. is well known, and many more applications are still to be explored. Although the superalloys have adequate mechanical strength for such high temperature applications, they are prone to degradation by hot corrosion/high tem- perature oxidation during long term exposures. Therefore, the superalloys need to be protected, however the protection system must be practical, reliable, and economically viable. In this paper the hot corrosion phenomenon has been described in detail with a special reference to Fe- and Ni- based superalloys with the help of a survey of the available literature related to the hot corrosion of the superalloys, in the environments constituting mainly of Na 2 SO 4 and/or V 2 O 5 .