ABSTRACT

This paper addresses basic issues germane to selecting materials for high-temperature aggressive environments. The overview provides first-choice options for dealing with corrosive conditions that extend beyond clean oxidation, including aggressive fluids and molten salts. Examples of material failures in various complex environments are included.

INTRODUCTION

Materials are selected on the basis of the service requirements, notably strength, so corrosion resistance (stability) may not be the primary design consideration. Assemblies need to be strong and resilient to the unique loads and stresses imparted thereon which can include significant temperature changes and thermal gradients for many high-temperature applications. Of particular benefit to many users is the spin-off from materials developed for the aerospace and gas turbine sectors. Many of these high-strength alloys are also very corrosion resistant and despite high direct costs these materials can often be economically justified against overall running costs and longevity of service.

In making a choice it is necessary to know what materials are available (and when) and to what extent they are suited to the specific application. The decision tree is quite involved (Figure 1) and choice is significantly affected by the circumstances of the environment and the intended use, (reactor vessel, tubes, supports, shields, springs, etc.). Further material factors include the fabricability and costs. The user (or designer) needs to properly define (or recognize) that the environment dictates the materials selection process at all stages in the process or application. For example, an alloy that performs well at the service temperature may corrode because of aqueous (dewpoint) corrosion at lower temperatures during off-load periods, or through some lack of design detail and/ or poor maintenance procedure that introduces local air draughts (or similar) that cool the system, (e.g., at access doors, inspection ports, etc.). The merits of using alloys that are considered both corrosion resistant and high-temperature resistant are obvious. Demand for strong, high-temperature materials has continued over the past twenty years, as has research focussed on improving performance and reliability. Improvements in casting (vacuum melting) and fabrication techniques, (powder metallurgy, mechanical alloying and the use of oxide dispersions to not only strengthen an alloy but also to improve scale adhesion (e.g., rare earth oxides such as yttria and ceria) can be noted. Not to be excluded from these developments are the continued use of composite materials (including co- extruded tubes for reactors or boiler tubes) and of overlay weld procedures. Also, advances in the use of surface coatings or modifications. There are many publications that herald the arrival of "new" alloys, some of which are capable of withstanding complex environments even at temperatures above 1100 C (2012 F), but probably not for too long in some cases.

A frustration in dealing with many end users is their expectation that a failing system can be made to last indefinitely by simply changing to another material. There are times when miracles occur, but it is important to recognize that there is no single panacea for all applications and typically each case has to be viewed on its own merits. In order to provide as optimum a performance as possible it is necessary for a supplier to be aware of the application, and for the user to be aware of the general range of available materials. In general, the oil and chemical industries are better aware of these parameters than in other sectors, including, general engineering, food-meat processing (broilers, ovens) and others. I

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