Page 710 - Corrosion Engineering Principles and Practice

664 C h a p t e r 1 5 H i g h - Te m p e r a t u r e C o r r o s i o n 665

may be detrimental to the stability of an oxide scale. It is for this
reason that reducing industrial environments are generally considered
to be more corrosive than the oxidizing variety.
High-temperature materials are used for many critical components
in a wide number of industries, including power generation, chemical
processing, and gas turbine. With ever-continuing demands for
increased throughput and efficiency, there has been a trend toward
higher service temperatures and pressures. This has resulted in
continued corrosion problems, countered by continued improvements
in material compositions such as minimizing detrimental trace
elements, development of coating procedures, and improved
fabrication, notably casting, forging, and welding. For example, in
the gas turbine industry, alloys designed to cope with high stress-
bearing/elevated temperature scaling are presently used for service
temperatures in excess of 1100°C, compared with about 800°C
50 years ago [1].
Most common process temperatures are in the range 450 to
850°C or higher (Fig. 15.1). Materials of construction must withstand
excessive metal loss by scale formation from oxidation and from
penetration by internal oxidation products that could reduce the
remaining cross-sectional area to a level that cannot sustain the
load-bearing requirements. The component will then yield and may
swell or distort. In some cases the internal fluid pressures can be
sufficient to burst the component releasing hot, possibly toxic or
flammable fluids.
There are several ways of measuring the extent of high
temperature corrosion attack. Measurement of weight change per
unit area in a given time has been a popular procedure. However
the weight change/area information is not directly related to the
thickness (penetration) of corroded metal, which is often needed in
assessing the strength of equipment components. Corrosion is best
reported in penetration units that reveal the actual loss of sound
metal. A metallographic technique which is used to determine the
extent of damage is illustrated in Fig. 15.2 [3]. The parameters shown
in Fig. 15.2 relate to cylindrical specimens and provide information
about the load-bearing section (metal loss) and on the extent of
grain boundary attack that can also affect structural integrity.
Heating and cooling rates can also cause the buildup of invisible
damage due to thermal stress and other fatigue effects. The need for a
careful study of the properties of a heat-resistant alloy and its behavior
in the anticipated environment is therefore of considerable importance
in the selection of a suitable alloy for a particular service application.
New alloys and nonmetallic materials, which are continually being
made available to industry, are making it possible to make better
selections and to establish safe working limits within which the
material can be expected to give satisfactory performance over
a reasonable length of time.