The effect of niobium on the corrosion resistance of nickel-base alloys

The role of niobium in enhancing the mechanical properties of nickel-base alloys is well established in the technical literature. Its effect in improving the corrosion behavior of nickelbase alloys is less well documented and understood. This paper reviews some of this literature pertaining to the corrosion resistance of niobium containing nickel-base alloys and presents some in-house data on corrosion performance of certain nickel-base alloys with varying additions of niobium. Primary focus will be on the contribution of niobium to the enhancement of sulfidation resistance in high-temperature sulfur-containing environments such as is found in chemical, petrochemical, power and petroleum refining plants. In addition to the use of niobium in high temperature alloys, the effect of increasing niobium content on aqueous corrosion resistance with an emphasis on localized corrosion resistance will be explored. To aid in explaining the mechanism(s) by which niobium is effective in enhancing corrosion resistance, key observations from a search of the literature will be summarized. The paper will report on two important factors that tend to explain the role of niobium. First, it has been suggested that niobium decreases the time required for an alloy to form its protective scale, such as Cr203, in an oxidizing/sulfidizing environment. Secondly, niobium additions may tend to form a blocking precipitate within the external scale that acts as a barrier for the inward diffusion of sulfur through scale defects. Then, corrosion data on several commercially significant Nb-containing alloys in typical industrial environments will be shown. These applications include petrochemical and chemical uses in both aqueous and high temperature environments and in both waste-to-energy and coalfired boilers. To complete the paper, niobium's role in Ni-base alloys for aqueous service and the development of a Nb enhanced alloy will be discussed. (AU)