Environmental Engineering Reference
In-Depth Information
Figure 6.33
Schematic illustration of the conditions that develop during the initiation
stage governing the time at which transition from the initiation to propagation stage occurs.
temperature cycles, test specimen geometry, and so forth. The effect of these
factors influencing the initiation of hot corrosion attack has been well documented
by Giggins and Pettit [58].
Hot corrosion process shows a strong temperature dependence. For most of
the superalloys, the corrosion rate is maximal within the temperature range 1123-
1173 K and decreases markedly at temperatures up to 1273 K. However, the time
required to initiate hot corrosion attack decreases as temperature is increased
(Fig. 6.32). For a fixed amount of sulfur in the gas, the SO
3
pressure decreases
as the temperature increases, resulting a lower hot corrosion rate. Moreover, for
the same ingestion rate of salt, less is deposited on test specimens as the tempera-
ture is increased. Accordingly, less attack takes place with smaller amount of
salt deposit at higher temperatures. The effect of alloy composition is still a matter
of debate. It is generally agreed that the content of chromium in the alloy is the
most important factor and for Ni-based alloys at least 15% Cr is required for
better resistance to such corrosion processes. Much of the disagreement concern-
ing the effects of other elements is possibly due to interactive effects within the
alloy scale and salt. Certain alloying elements are found to have beneficial effects
