ABSTRACT
Several authors have suggested that hot corrosion of all susceptible alloys and coatings is characterized by a two-stage process: an incubation period with a slow rate of reaction, possibly with the alloy being protected by a stable Cr20 3 or Ah03 layer; and a propagation stage with rapid and often catastrophic material degradation as illustrated in Fig. 6.32. During the incubation period, elements in the alloy are oxidized, and electrons are considered to be transferred from metallic atoms to reducible substances in the salt deposit. When the reduced substances are the same as those that would have reacted with the alloy in the absence of the deposit, the reaction product barrier forms beneath the salt on the alloy surface. As the hot corrosion process is continued, certain features become apparent which indicate that the salt is affecting the corrosion process, and eventually the selective oxidation process is ineffective. The time, which may vary from a few hours to thousands of hours for which the most effective reaction product barrier is stable beneath the salt layer, is influenced by a number of factors as illustrated in Fig. 6.33. These factors include alloy composition and its microstructure, fabrication condition, pretreatment of alloys, environment composition, gas velocity, erosion, salt deposit condition and its composition, performance/test temperature,
Figure 6.32 Mass change versus time for IN-738 alloy coated with I mg/cm2 Na2S04 in I atm 0 2 at different temperatures [16).
