A. Introduction

This chapter describes the approximately 40 nickels and high nickel (25% and above) alloys most commonly used for the construction of process plant, including heat exchangers, when superior corrosion resistance and/or high temperature mechanical properties are required to those provided by the lower nickel content stainless steels or cupronickels.

Corrosion can be a very complex phenomenon. Often certain constituents of the process fluid are present in such low quantities that their presence is not even stated (such as chlorides). They can make a more serious contribution to corrosion than the major constituents. This is particularly true when there are additional features present, such as heat transfer, mechanical crevices or deposits, etc., which can provide a mechanism for concentration of these apparently minor constituents to higher levels. For such reasons, in the space of one chapter, it is not easy and would almost certainly be misleading to attempt, to provide comprehensive data for all applications. Reputable alloy manufacturers have no wish that their alloys are put into unsuitable applications and are always willing to provide free advice about materials to combat specific corrosive environments. It is recommended that they should be consulted.

It is this author's experience that many potential users of nickel alloys are confused by the apparent jumble of seemingly expensive high nickel alloys presented to them, often by alloy manufacturers who produce only a portion of the available range under their own particular trade names. This is further compounded because, as in the case in many technologies, the chronological order of development of these alloys does not appear to be logical in itself. This is because constantly improving manufacturing methods, increasing diversification and understanding of corrosion resistance requirements and the need for cost-effectiveness have all led to continual optimising developments both at extreme and intermediate positions in the alloy range.