Page 737 - Corrosion Engineering Principles and Practice

690 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 691

The alloy composition can influence metal penetration occurring
by subsurface oxidation along grain boundaries and within the alloy
grains, as schematically shown in Fig. 15.19 [11]. Most of the
commercial heat-resistant alloys are based upon combinations of Fe-
Ni-Cr. These alloys show about 80 to 95 percent of the total penetration
as subsurface oxidation. Some alloys change in how much of the total
penetration occurs by subsurface oxidation as time passes even
though the corrosion product morphologies may remain constant.
Alloys vary greatly in the extent of surface scaling and subsurface
oxidation. Tests were conducted in flowing air at 980, 1095, 1150, and
1250°C for 1008 hours. The results of these tests, in terms of metal loss
and average metal affected (metal loss and internal penetration), are
presented in Table 15.7 [12].

15.4.2 Sulfidation
Sulfidation is a common high-temperature corrosion-failure mecha-
nism. As the name implies, it is related to the presence of sulfur
compounds. When examining this form of damage microscopically,
a “front” of sulfidation is often seen to penetrate into the affected
alloy. Localized pitting-type attack is also possible. A distinction can
be made between sulfidation in gaseous environments and corro-
sion in the presence of salt deposits on corroding surfaces. Only the
former is considered in this section, the latter being discussed in

External scale

Total
penetration
Internal
penetration

Internal corrosion
products
Corroded
grain boundaries Uncorroded
alloy

FIGURE 15.19 Schematic view of total penetration measurement for a typical
corrosion product morphology.