Environmental Engineering Reference
In-Depth Information
Figure 6.34
Weight change versus time data for the cyclic hot corrosion of Ni-30%
Cr and Ni-30% Cr-6% Al alloys exhibiting superior performance of the former at longer
times [58].
over certain composition ranges but can be deleterious over others. This is clearly
demonstrated in Fig. 6.34 for the corrosion of Ni-30% Cr-6% Al and Ni-30%
Cr alloys. In the hot corrosion resistance of Ni-based superalloys apart from the
beneficial effects shown by chromium; Co and Ta slightly improve the resistance;
Ti seems to do little, whereas Mo and W are detrimental, especially at higher
temperatures.
It must be recognized that all of the factors mentioned above and illustrated
in Fig. 6.33 have significance in preconditioning the alloy, which will ultimately
determine the type of propagation mode to be followed. Such preconditioning
may include (1) depletion of the element responsible for forming the initial oxide
layer (Al or Cr), (2) formation of sulfides of the alloy constituents due to sulfur
penetration through the scale, (3) dissolution of oxides into molten salt deposit,
(4) development of growth stresses, and (5) alteration in the salt composition for
more corrosive conditions.
The end of the initiation stage is marked by the propagation of attack, and
during this stage the characteristic hot corrosion morphology of the scale appears.
Divergent results have been reported in literature by numerous investigators
studying this phenomenon since their experimental conditions varied widely.
Generalization of the test results indicates that the degradation of alloys in the
