There has been steady progress in the development of wrought nickel?containing alloys for use in high temperature oxidizing environments . Contributing significantly to this progress is a growing knowledge base on the role of scales in enhancing oxidation resistance. Future improvements in oxidation resistance must build upon this understanding. This paper seeks to survey a portion of the wealth of information regarding scale characteristics of commercial wrought nickel?containing alloys and how scales are influenced by environment and alloy composition. Some suggestions as to the future direction of alloy development with regard to scale optimization and increased oxidation resistance are proposed.
Improving performance of wrought nickel-containing alloys for high temperature service has been a long standing challenge for materials engineers . In practice, alloy solutions for increasing corrosion resistance are no more complex for oxidizing environments than are solutions for high strength. Materials engineers quickly learn that maximizing performance for oxidizing atmospheres is an enigmatic interplay between the application and the corrosion resistance, mechanical properties, stability, weldability, repairability, and cost of the candidate materials for the application.
Experience has shown that the materials engineer cannot expect to alter corrosion resistance and not potentially compromise one or more of the other aspects of performance. For example, optimizing oxidation resistance through the use of aluminum additions may easily jeopardize phase stability. Among the variables at the disposal of the alloy developer for enhancing corrosion resistance, and most frequently resorted to, are the elements of the periodic table. It is of value for alloy developers to occasionally pause and attempt to assess the interplay that the various scale?forming elements have on oxidation resistance. One such assessment is the basis of this paper.
Mixed oxidant environments (oxygen plus one or more additional high temperature corrodents) are the most typical atmospheres to which heat resistant alloys are exposed, albeit, the oxygen potential may be low in certain instances as contrasted to that of other oxidants which may make up the same environment. Typical of such environments are those encountered in thermal processing, calcining, combustion, reforming, incineration, power generation and heat transfer. Ultimate alloy failure in these applications is frequently attributed, at least in part, to oxidation or mixed oxidant corrosion. It is interesting to note that oxidation is often deemed responsible for the loss of resistance to creep and stress rupture strength, and for the loss of resistance to thermal fatigue, while simultaneously, it is acknowledged that creep and thermal cycling can rupture and/or span protective scales and thus accelerate the degradation process. This interplay suggests that enhanced oxidation resistant scales will be developed only when attention is given to both inward and outward diffusion coefficients, scale compatibility with the substrate, growth stresses, and creep and thermal gradient stresses along with requirements and conditions of the ultimate application. Using information from the literature and the laboratory, it is possible to glean some notion of the role that alloy composition can play in solving some of these problems associated with scale integrity and performance.
